Search This Blog

Thursday, February 5, 2015

Ozone


From Wikipedia, the free encyclopedia

Ozone
Skeletal formula of ozone with partial charges shown
Ball and stick model of ozone Spacefill model of ozone
Electrostatic potential map of ozone
Identifiers
CAS number 10028-15-6 YesY
PubChem 24823
ChemSpider 23208 YesY
UNII 66H7ZZK23N YesY
EC number 233–069–2
MeSH Ozone
ChEBI CHEBI:25812 YesY
RTECS number RS8225000
1101
Jmol-3D images Image 1
Image 2
Image 3
Properties
Molecular formula O3
Molar mass 48.00 g mol−1
Appearance Pale blue gas
Density 2.144 mg cm−3 (at 0 °C)
Melting point −192.2 °C; −313.9 °F; 81.0 K
Boiling point −112 °C; −170 °F; 161 K
Solubility in water 1.05 g L−1 (at 0 °C)
Solubility very soluble in CCl4, sulfuric acid
1.2226 (liquid)
Structure
Space group C2v
Digonal
Molecular shape Dihedral
Hybridisation sp2 for O1
Dipole moment 0.53 D
Thermochemistry
238.92 J K−1 mol−1
142.67 kJ mol−1
Hazards
EU classification Oxidizing Agent O Irritant Xi
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 4: Very short exposure could cause death or major residual injury. E.g., VX gas Reactivity code 4: Readily capable of detonation or explosive decomposition at normal temperatures and pressures. E.g., nitroglycerin Special hazard OX: Oxidizer. E.g., potassium perchlorateNFPA 704 four-colored diamond
0
4
4
Related compounds
Related compounds Sulfur dioxide
Trisulfur
Disulfur monoxide
Cyclic ozone
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
Infobox references

Ozone /ˈzn/ (systematically named 1,3λ1-trioxidane and catena-trioxygen), or trioxygen, is an inorganic molecule with the chemical formula OO
2
(also written [O
3
]
). It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O
2
, breaking down in the lower atmosphere to normal dioxygen. Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges, and is present in low concentrations throughout the Earth's atmosphere. In total, ozone makes up only 0.6 ppm of the atmosphere.

Ozone's odor is sharp, reminiscent of chlorine, and detectable by many people at concentrations of as little as 10 ppb in air. Ozone's O3 formula was determined in 1865. The molecule was later proven to have a bent structure and to be diamagnetic. In standard conditions, ozone is a pale blue gas that condenses at progressively cryogenic temperatures to a dark blue liquid and finally a violet-black solid. Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively.[1] It is therefore used commercially only in low concentrations.

Ozone is a powerful oxidant (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about 100 ppb. This makes ozone a potent respiratory hazard and pollutant near ground level. However, the so-called ozone layer (a portion of the stratosphere with a higher concentration of ozone, from two to eight ppm) is beneficial, preventing damaging ultraviolet light from reaching the Earth's surface, to the benefit of both plants and animals.

Nomenclature

The trivial name ozone is the most commonly used and preferred IUPAC name. The systematic names 1,3λ1-trioxidane and catena-trioxygen, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively. The name ozone derives from ozein (ὄζειν), the Greek word for smell (verb), referring to ozone's distinctive smell.

In appropriate contexts, ozone can be viewed as trioxidane with two hydrogen atoms removed, and as such, trioxidanylidene may be used as a context-specific systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named trioxidanediyl.

Trioxidanediyl (or ozonide) is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for ozone given above.

History


Christian Friedrich Schönbein (18 October 1799 – 29 August 1868)

A prototype ozonometer built by John Smyth in 1865

In 1785, Dutch chemist Martinus van Marum was conducting experiments involving electrical sparking above water when he noticed an unusual smell, which he attributed to the electrical reactions, failing to realize he had in fact created ozone.[2] A half century later, Christian Friedrich Schönbein noticed the same pungent odor and recognized it as the smell often following a bolt of lightning. In 1839 he succeeded in isolating the gaseous chemical and named it "ozone", from the Greek word ozein (ὄζειν) meaning "to smell".[3][4] For this reason, Schönbein is generally credited with the discovery of ozone.[2][5] The formula for ozone, O3, was not determined until 1865 by Jacques-Louis Soret[6] and confirmed by Schönbein in 1867.[3][7]

For much of the second half of the nineteenth century and well into the twentieth, ozone was considered a healthy component of the environment by naturalists and health-seekers. The City of Beaumont in California had as its official slogan "Beaumont: Zone of Ozone," as evidenced on postcards and Chamber of Commerce letterhead.[8] Naturalists working outdoors often considered the higher elevations beneficial because of their ozone content. "There is quite a different atmosphere [at higher elevation] with enough ozone to sustain the necessary energy [to work]," wrote naturalist Henry Henshaw, working in Hawaii.[9] Seaside air was considered to be healthy because of its "ozone" content but the smell giving rise to this belief is in reality that of rotting seaweed.[10]

Physical properties

Ozone is colourless or slightly bluish gas (blue when liquified), slightly soluble in water and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, where it forms a blue solution. At 161 K (−112 °C; −170 °F), it condenses to form a dark blue liquid. It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below 80 K (−193.2 °C; −315.7 °F), it forms a violet-black solid.[11]

Most people can detect about 0.01 μmol/mol of ozone in air where it has a very specific sharp odor somewhat resembling chlorine bleach. Exposure of 0.1 to 1 μmol/mol produces headaches, burning eyes and irritation to the respiratory passages.[12] Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics and animal lung tissue.

Ozone is diamagnetic, which means that its electrons are all paired. In contrast, O2 is paramagnetic, containing two unpaired electrons.

Structure

According to experimental evidence from microwave spectroscopy, ozone is a bent molecule, with C2v symmetry (similar to the water molecule). The O – O distances are 127.2 pm (1.272 Å). The O – O – O angle is 116.78°.[13] The central atom is sp² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.53 D.[14] The bonding can be expressed as a resonance hybrid with a single bond on one side and double bond on the other producing an overall bond order of 1.5 for each side.
Resonance Lewis structures of the ozone molecule

Reactions

Ozone is a powerful oxidizing agent, far stronger than O2. It is also unstable at high concentrations, decaying to ordinary diatomic oxygen. It has a varying half-life length, depending upon atmospheric conditions (temperature, humidity, and air movement). In a sealed chamber with a fan that moves the gas, ozone has a half-life of approximately a day at room temperature.[15] Some unverified claims imply that ozone can have a half life as short as a half an hour under atmospheric conditions.[16]
2 O
3
→ 3 O
2
This reaction proceeds more rapidly with increasing temperature and increased pressure. Deflagration of ozone can be triggered by a spark, and can occur in ozone concentrations of 10 wt% or higher.[17]

With metals

Ozone will oxidize most metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state. For example:
2 Cu+ + 2 H
3
O+
+ O
3
→ 2 Cu2+ + 3 H
2
O
+ O
2

With nitrogen and carbon compounds

Ozone also oxidizes nitric oxide to nitrogen dioxide:
NO + O
3
NO
2
+ O
2
This reaction is accompanied by chemiluminescence. The NO
2
can be further oxidized:
NO
2
+ O
3
→ NO3 + O
2
The NO
3
formed can react with NO
2
to form N
2
O
5
.

Solid nitronium perchlorate can be made from NO2, ClO2, and O
3
gases:
2 NO
2
+ 2 ClO2 + 2 O
3
→ 2 NO2ClO4 + O
2
Ozone does not react with ammonium salts, but it oxidizes ammonia to ammonium nitrate:
2 NH
3
+ 4 O
3
NH
4
NO
3
+ 4 O
2
+ H
2
O
Ozone reacts with carbon to form carbon dioxide, even at room temperature:
C + 2 O3 → CO2 + 2 O2

With sulfur compounds

Ozone oxidizes sulfides to sulfates. For example, lead(II) sulfide is oxidised to lead(II) sulfate:
PbS + 4 O3 → PbSO4 + 4 O2
Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide:
S + H2O + O3 → H2SO4
3 SO2 + 3 H2O + O3 → 3 H2SO4
In the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide:
H2S + O3 → SO2 + H2O
In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid:
H2S + O3 → S + O2 + H2O
3 H2S + 4 O3 → 3 H2SO4

With alkenes and alkynes

Alkenes can be oxidatively cloven by ozone, in a process called ozonolysis, giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.Ozonolysis scheme.svg

Usually ozonolysis is carried out in a solution of dichloromethane, at a temperature of −78oC. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g. zinc in acetic acid or dimethyl sulfide), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic hydrogen peroxide), carboxylic acids will be formed.[18]

Other substrates

All three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:
3 SnCl2 + 6 HCl + O
3
→ 3 SnCl4 + 3 H2O
Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:
I2 + 6 HClO4 + O3 → 2 I(ClO4)3 + 3 H2O

Combustion

Ozone can be used for combustion reactions and combustible gases; ozone provides higher temperatures than burning in dioxygen (O2). The following is a reaction for the combustion of carbon subnitride which can also cause higher temperatures:
3 C
4
N
2
+ 4 O
3
→ 12 CO + 3 N
2
Ozone can react at cryogenic temperatures. At 77 K (−196.2 °C; −321.1 °F), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical, which dimerizes:[19]
H + O
3
→ HO2 + O
2 HO2H
2
O
4

Reduction to ozonides

Reduction of ozone gives the ozonide anion, O
3
. Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO3, RbO3, and CsO3 can be prepared from their respective superoxides:
KO2 + O3 → KO3 + O2
Although KO3 can be formed as above, it can also be formed from potassium hydroxide and ozone:[20]
2 KOH + 5 O3 → 2 KO3 + 5 O2 + H2O
NaO3 and LiO3 must be prepared by action of CsO3 in liquid NH3 on an ion exchange resin containing Na+ or Li+ ions:[21]
CsO3 + Na+ → Cs+ + NaO3
A solution of calcium in ammonia reacts with ozone to give to ammonium ozonide and not calcium ozonide:[19]
3 Ca + 10 NH3 + 6 O
3
→ Ca·6NH3 + Ca(OH)2 + Ca(NO3)2 + 2 NH4O3 + 2 O2 + H2

Applications

Ozone can be used to remove manganese from water, forming a precipitate which can be filtered:
2 Mn2+ + 2 O
3
+ 4 H2O → 2 MnO(OH)2 (s) + 2 O2 + 4 H+
Ozone will also detoxify cyanides by converting them to cyanates, which are a thousand times less toxic.[citation needed]
CN + O3CNO + O2
Ozone will also completely decompose urea:[22]
(NH2)2CO + O3 → N2 + CO2 + 2 H2O

Ozone in Earth's atmosphere


The distribution of atmospheric ozone in partial pressure as a function of altitude

Concentration of ozone as measured by the Nimbus-7 satellite

Total ozone concentration in June 2000 as measured by EP-TOMS satellite instrument

The standard way to express total ozone levels (the amount of ozone in a vertical column) in the atmosphere is by using Dobson units. Point measurements are reported as mole fractions in nmol/mol (parts per billion, ppb) or as concentrations in μg/m3. The study of ozone concentration in the atmosphere started in the 1920s.[23]

Ozone layer

Location and production

The highest levels of ozone in the atmosphere are in the stratosphere, in a region also known as the ozone layer between about 10 km and 50 km above the surface (or between about 6 and 31 miles). However, even in this "layer" the ozone concentrations are only two to eight parts per million, so most of the oxygen there remains of the dioxygen type.

Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays (in the UVC band) but it can be also produced from x-rays reacting with oxygen:
O
2
+ photon (radiation λ < 240 nm) → 2 O
O + O
2
+ M → O
3
+ M
α + β + O
2
→ He + O
3
where "M" denotes the third body that carries off the excess energy of the reaction. The thus produced ozone is destroyed by the reaction with atomic oxygen:
O
3
+ O → 2 O
2
The latter reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In recent decades the amount of ozone in the stratosphere has been declining mostly because of emissions of chlorofluorocarbons (CFC) and similar chlorinated and brominated organic molecules, which have increased the concentration of ozone-depleting catalysts above the natural background.

Importance to surface-dwelling life on Earth


Levels of ozone at various altitudes and blocking of different bands of ultraviolet radiation. Essentially all UVC (100–280 nm) is blocked by dioxygen (at 100–200 nm) or by ozone (at 200–280 nm) in the atmosphere. The shorter portion of this band and even more energetic UV causes the formation of the ozone layer, when single oxygen atoms produced by UV photolysis of dioxygen (below 240 nm) react with more dioxygen. The ozone layer itself then blocks most, but not quite all, sunburn-producing UVB (280–315 nm). The band of UV closest to visible light, UVA (315–400 nm), is hardly affected by ozone, and most of it reaches the ground.

Ozone in the ozone layer filters out sunlight wavelengths from about 200 nm UV rays to 315 nm, with ozone peak absorption at about 250 nm.[24] This ozone UV absorption is important to life, since it extends the absorption of UV by ordinary oxygen and nitrogen in air (which absorb all wavelengths < 200 nm) through the lower UV-C (200–280 nm) and the entire UV-B band (280–315 nm). The small unabsorbed part that remains of UV-B after passage through ozone causes sunburn in humans, and direct DNA damage in living tissues in both plants and animals. Ozone's effect on mid-range UV-B rays is illustrated by its effect on UV-B at 290 nm, which has a radiation intensity 350 million times as powerful at the top of the atmosphere as at the surface. Nevertheless, enough of UV-B radiation at similar frequency reaches the ground to cause some sunburn, and these same wavelengths are also among those responsible for the production of vitamin D in humans.

The ozone layer has little effect on the longer UV wavelengths called UV-A (315–400 nm), but this radiation does not cause sunburn or direct DNA damage, and while it probably does cause long-term skin damage in certain humans, it is not as dangerous to plants and to the health of surface-dwelling organisms on Earth in general (see ultraviolet for more information on near ultraviolet).

Low level ozone

Low level ozone (or tropospheric ozone) is an atmospheric pollutant.[25] It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind.
Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog. Ozone photolysis by UV light leads to production of the hydroxyl radical HO• and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates, which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the above-mentioned reaction giving HO•, and by reactions with OH and the peroxy radical HO2•.[26]

There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species.[27][28] The United States Environmental Protection Agency is proposing a secondary regulation to reduce crop damage, in addition to the primary regulation designed for the protection of human health.

Certain examples of cities with elevated ozone readings are Houston, Texas, and Mexico City, Mexico. Houston has a reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125 nmol/mol.[28]

Ozone cracking


Ozone cracking in natural rubber tubing

Ozone gas attacks any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber, nitrile rubber, and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the rubber component and the concentration of ozone in the atmosphere. Such materials can be protected by adding antiozonants, such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tires for example, but the problem is now seen only in very old tires.[clarification needed][citation needed] On the other hand, many critical products like gaskets and O-rings may be attacked by ozone produced within compressed air systems. Fuel lines made of reinforced rubber are also susceptible to attack, especially within the engine compartment, where some ozone is produced by electrical components. Storing rubber products in close proximity to a DC electric motor can accelerate ozone cracking. The commutator of the motor generates sparks which in turn produce ozone.

Ozone as a greenhouse gas

Although ozone was present at ground level before the Industrial Revolution, peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher.[29][30] Ozone acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe. However, the most widely accepted scientific assessments relating to climate change (e.g. the Intergovernmental Panel on Climate Change Third Assessment Report)[31] suggest that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide.

The annual global warming potential of tropospheric ozone is between 918–1022 tons carbon dioxide equivalent/tons tropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a radiative forcing effect roughly 1,000 times as strong as carbon dioxide. However, tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide. This means that over a 20 year span, the global warming potential of tropospheric ozone is much less, roughly 62 to 69 tons carbon dioxide equivalent / ton tropospheric ozone.[32]

Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strong radiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has a radiative forcing up to 150% of carbon dioxide.[33]

Health effects

Ozone air pollution


Red Alder leaf, showing discolouration caused by ozone pollution[34]

Signboard in Gulfton, Houston indicating an ozone watch

Ozone precursors are a group of pollutants, predominantly those emitted during the combustion of fossil fuels. Ground-level ozone pollution (tropospheric ozone) is created near the Earth's surface by the action of daylight UV rays on these precursors. The ozone at ground level is primarily from fossil fuel precursors, but methane is a natural precursor, and the very low natural background level of ozone at ground level is considered safe. This section examines health impacts of fossil fuel burning, which raises ground level ozone far above background levels.

There is a great deal of evidence to show that ground level ozone can harm lung function and irritate the respiratory system.[25][35] Exposure to ozone and the pollutants that produce it is linked to premature death, asthma, bronchitis, heart attack, and other cardiopulmonary problems.[36][37]

Long-term exposure to ozone has been shown to increase risk of death from respiratory illness. A study of 450,000 people living in United States cities showed a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels such as Houston or Los Angeles had an over 30% increased risk of dying from lung disease.[38][39]

Air quality guidelines such as those from the World Health Organization, the United States Environmental Protection Agency (EPA) and the European Union are based on detailed studies designed to identify the levels that can cause measurable ill health effects.

According to scientists with the US EPA, susceptible people can be adversely affected by ozone levels as low as 40 nmol/mol.[37][40][41] In the EU, the current target value for ozone concentrations is 120 µg/m³ which is about 60 nmol/mol. This target applies to all member states in accordance with Directive 2008/50/EC.[42] Ozone concentration is measured as a maximum daily mean of 8 hour averages and the target should not be exceeded on more than 25 calendar days per year, starting from January 2010. Whilst the directive requires in the future a strict compliance with 120 µg/m³ limit (i.e. mean ozone concentration not to be exceeded on any day of the year), there is no date set for this requirement and this is treated as a long-term objective. [43]

In the USA, the Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, under a court order, the EPA lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. The move proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 nmol/mol.[37] Many public health and environmental groups also supported the 60 nmol/mol standard,[44] and the World Health Organization recommends 51 nmol/mol.

On January 7, 2010, the U.S. Environmental Protection Agency (EPA) announced proposed revisions to the National Ambient Air Quality Standard (NAAQS) for the pollutant ozone, the principal component of smog:
... EPA proposes that the level of the 8-hour primary standard, which was set at 0.075 μmol/mol in the 2008 final rule, should instead be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to provide increased protection for children and other ‘‘at risk’’ populations against an array of O
3
- related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity as well as total non- accidental and cardiopulmonary mortality....[45]
The EPA has developed an Air Quality Index (AQI) to help explain air pollution levels to the general public. Under the current standards, eight-hour average ozone mole fractions of 85 to 104 nmol/mol are described as "unhealthy for sensitive groups," 105 nmol/mol to 124 nmol/mol as "unhealthy," and 125 nmol/mol to 404 nmol/mol as "very unhealthy."[46]

Ozone can also be present in indoor air pollution, partly as a result of electronic equipment such as photocopiers. A connection has also been known to exist between the increased pollen, fungal spores, and ozone caused by thunderstorms and hospital admissions of asthma sufferers.[47]

In the Victorian era, one British folk myth held that the smell of the sea was caused by ozone. In fact, the characteristic "smell of the sea" is caused by dimethyl sulfide, a chemical generated by phytoplankton. Victorian British folk considered the resulting smell "bracing," but in high concentrations, dimethyl sulfide is actually toxic.[48]

Heat waves

Ozone production rises during heat waves, because plants absorb less ozone. It is estimated that curtailed ozone absorption by plants is responsible for the loss of 460 lives in the UK in the hot summer of 2006.[49] A similar investigation to assess the joint effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive.[50]

Physiology

Ozone, along with reactive forms of oxygen such as superoxide, singlet oxygen, hydrogen peroxide, and hypochlorite ions, is naturally produced by white blood cells and other biological systems (such as the roots of marigolds) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals, which are highly reactive and capable of damaging many organic molecules. Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation. The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. A team headed by Dr. Paul Wentworth Jr. of the Department of Chemistry at the Scripps Research Institute has shown evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen.[51]
When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed atheronals, generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol[52] as well as a secondary condensation product via aldolization.[53]

Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus."[54]

Safety regulations

Due to the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Center for Occupation Safety and Health reports that:
"Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both by the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations."[55]
To protect workers potentially exposed to ozone, U.S. Occupational Safety and Health Administration has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 table Z-1), calculated as an 8 hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol.[56] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.

Elevated ozone exposure can occur on passenger aircraft, with levels depending on altitude and atmospheric turbulence.[57] United States Federal Aviation Authority regulations set a limit of 250 nmol/mol with a maximum four-hour average of 100 nmol/mol.[58] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure.[57]

Production


Ozone production demonstration, Fixed Nitrogen Research Laboratory, 1926

Ozone generators are used to produce ozone for cleaning air or remove smoke odors[59] in unoccupied rooms. These ozone generators can produce over 3 g of ozone per hour. Ozone often forms in nature under conditions where O2 will not react.[12] Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m3, mg/h (milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1 to 5% in air and from 6 to 14% in oxygen for older generation methods. New electrolytic methods can achieve up 20 to 30% dissolved ozone concentrations in output water.

Temperature and humidity plays a large role in how much ozone is being produced using traditional generation methods such as corona discharge and ultraviolet light. Old generation methods will produce less than 50% its nominal capacity if operated with humid ambient air than when it operates in very dry air. New generators using electrolytic methods can achieve higher purity and dissolution through using water molecules as the source of ozone production.

Corona discharge method

This is the most common type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube.[60] They are typically cost-effective and do not require an oxygen source other than the ambient air to produce ozone concentrations of 3–6%. Fluctuations in ambient air, due to weather or other environmental conditions, cause variability in ozone production. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.

Ultraviolet light

UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates a narrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in the stratosphere of Earth.[61]

While standard UV ozone generators tend to be less expensive,[clarification needed] they usually produce ozone with a concentration of about 0.5% or lower. Another disadvantage of this method is that it requires the air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makes UV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization, for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation. VUV ozone generators are used in swimming pool and spa applications ranging to millions of gallons of water. VUV ozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlike corona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also not normally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which require extra costs and maintenance.

Cold plasma

In the cold plasma method, pure oxygen gas is exposed to a plasma created by dielectric barrier discharge. The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.

Cold plasma machines utilize pure oxygen as the input source and produce a maximum concentration of about 5% ozone. They produce far greater quantities of ozone in a given space of time compared to ultraviolet production. However, because cold plasma ozone generators are very expensive, they are found less frequently than the previous two types.

The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.

Some cold plasma units also have the capability of producing short-lived allotropes of oxygen which include O4, O5, O6, O7, etc. These species are even more reactive than ordinary O
3
.[citation needed]

Electrolytic

Electrolytic ozone generation (EOG) splits water molecules into H2, O2, and O3. In most EOG methods, the hydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achieve higher dissolution in water without other competing gases found in corona discharge method, such as nitrogen gases present in ambient air. This method of generation can achieve concentrations of 20–30% and is independent of air quality because water is used as the source material. Production of ozone electrolytically is typically unfavorable because of the high overpotential required to produce ozone as compared to oxygen. This is why ozone is not produced during typical water electrolysis. However, it is possible to increase the overpotential of oxygen by careful catalyst selection such that ozone is preferentially produced under electrolysis. Catalysts typically chosen for this approach are lead dioxide[62] or boron-doped diamond.[63]

Special considerations

Ozone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen) and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2 bars (200 kPa) absolute in oxygen and 3 bars (300 kPa) absolute in air. Several megawatts of electrical power may be installed in large facilities, applied as one phase AC current at 50 to 8000 Hz and peak voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.

The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by cooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower the gas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions, almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.

Because of the high reactivity of ozone, only a few materials may be used like stainless steel (quality 316L), titanium, aluminium (as long as no moisture is present), glass, polytetrafluorethylene, or polyvinylidene fluoride. Viton may be used with the restriction of constant mechanical forces and absence of humidity (humidity limitations apply depending on the formulation). Hypalon may be used with the restriction that no water come in contact with it, except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers with exposure to ozone. Ozone cracking is the common mode of failure of elastomer seals like O-rings.

Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.

Incidental production

Ozone may be formed from O
2
by electrical discharges and by action of high energy electromagnetic radiation. Unsuppressed arcing breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [O
2
→ 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [O
3
].[64] Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages, such as ionic air purifiers, laser printers, photocopiers, tasers and arc welders. Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors. Ozone is similarly formed in the Catatumbo lightning storms phenomenon on the Catatumbo River in Venezuela, which helps to replenish ozone in the upper troposphere. It is the world's largest single natural generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site.[65]

Laboratory production

In the laboratory, ozone can be produced by electrolysis using a 9 volt battery, a pencil graphite rod cathode, a platinum wire anode and a 3 molar sulfuric acid electrolyte.[66] The half cell reactions taking place are:
3 H2O → O3 + 6 H+ + 6 e (ΔEo = −1.53 V)
6 H+ + 6 e → 3 H2 (ΔEo = 0 V)
2 H2O → O2 + 4 H+ + 4 e (ΔEo = −1.23 V)
In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents of hydrogen. Oxygen formation is a competing reaction.

It can also be "prepared" by high voltage arc. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top, with in and out spigots at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. Dry O
2
should be run through the tube in one spigot. As the O
2
is run through one spigot into the apparatus and high voltage is applied to the foil leads, electricity will discharge between the dry dioxygen in the middle and form O
3
and O
2
out the other spigot. The reaction can be summarized as follows:[12]
3 O
2
electricity → 2 O
3

Applications

Industry

The largest use of ozone is in the preparation of pharmaceuticals, synthetic lubricants, and many other commercially useful organic compounds, where it is used to sever carbon-carbon bonds.[12] It can also be used for bleaching substances and for killing microorganisms in air and water sources.[67] Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine.[68] Ozone has a very high oxidation potential.[69] Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. Ozone can form the suspected carcinogen bromate in source water with high bromide concentrations. The Safe Drinking Water Act mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 μmol/mol residual free chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, since it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odor in drinking water.

Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency. Humidity control can vastly improve both the killing power of the ozone and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations where asthma patients start to have issues.

Industrially, ozone is used to:
  • Disinfect laundry in hospitals, food factories, care homes etc.;[70]
  • Disinfect water in place of chlorine[12]
  • Deodorize air and objects, such as after a fire. This process is extensively used in fabric restoration
  • Kill bacteria on food or on contact surfaces;[71]
  • Sanitize swimming pools and spas
  • Kill insects in stored grain[72]
  • Scrub yeast and mold spores from the air in food processing plants;
  • Wash fresh fruits and vegetables to kill yeast, mold and bacteria;[71]
  • Chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex organics lumped together as "colour");
  • Provide an aid to flocculation (agglomeration of molecules, which aids in filtration, where the iron and arsenic are removed);
  • Manufacture chemical compounds via chemical synthesis[73]
  • Clean and bleach fabrics (the former use is utilized in fabric restoration; the latter use is patented);[citation needed]
  • Assist in processing plastics to allow adhesion of inks;
  • Age rubber samples to determine the useful life of a batch of rubber;
  • Eradicate water borne parasites such as Giardia lamblia and Cryptosporidium in surface water treatment plants.
Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.

Many hospitals around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacteria.[74]

Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp.[75] It is often used in conjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in the manufacture of high-quality, white paper.[76]

Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining) by oxidizing cyanide to cyanate and eventually to carbon dioxide.[77]

Consumers

Devices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorize uninhabited buildings, rooms, ductwork, woodsheds, and boats and other vehicles.

One company has been successfully selling a CPAP sanitizer for the CPAP gear used by sleep apnea patients. This sanitizer works by pumping high concentration levels of electrically-generated ozone into the unit's humidification water tank (with or without water in it) and out through the hose into the mask, which is enclosed and sealed in an ozone-capturing receptacle (that also contains the ozone generator and pump that pushes it into the water tank), which completes a closed-loop system. This closed-loop system prevents the high levels of ozone from escaping while effectively sanitizing the CPAP equipment, as the CPAP equipment is prone to developing bacterial infestations and harboring viruses and other pathogens because of the constant moisture generated by the CPAP system's humidifier. The sanitizing unit has a two-hour cycle, it pumps the ozone for 6–10 minutes (user-designated) and then resting for two hours while maintaining the sealed closed-circuit loop as the ozone decays back into oxygen and finishes the sanitizing effect.[78]

In the U.S., air purifiers emitting low levels of ozone have been sold. This kind of air purifier is sometimes claimed to imitate nature's way of purifying the air without filters and to sanitize both it and household surfaces. The United States Environmental Protection Agency (EPA) has declared that there is "evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals" or "viruses, bacteria, mold, or other biological pollutants." Furthermore, its report states that "results of some controlled studies show that concentrations of ozone considerably higher than these [human safety] standards are possible even when a user follows the manufacturer’s operating instructions."[79] A couple kept repeating health claims for the generator they sold, without supporting scientific studies. In 1998 a federal jury convicted them, among others things, of illegally distributing an ozone generator and of wire fraud.[80]

Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University demonstrated that 0.3 μmol/mol levels of ozone dissolved in filtered tapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli 0157:H7 and Campylobacter. This quantity is 20,000 times the WHO-recommended limits stated above.[71][81] Ozone can be used to remove pesticide residues from fruits and vegetables.[82][83]

Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or bromine required by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone by itself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens. Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water.[84]

Ozone is also widely used in treatment of water in aquariums and fish ponds. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozone must not come in contact with fish's gill structures. Natural salt water (with life forms) provides enough "instantaneous demand" that controlled amounts of ozone activate bromide ion to hypobromous acid, and the ozone entirely decays in a few seconds to minutes. If oxygen fed ozone is used, the water will be higher in dissolved oxygen, fish's gill structures will atrophy and they will become dependent on higher dissolved oxygen levels.

Aquaculture

Ozonation - a process of infusing water with ozone - can be used in aquaculture to facilitate organic breakdown. Ozone is also added to recirculating systems to reduce nitrite levels[85] through conversion into nitrate. If nitrite levels in the water are high, nitrites will also accumulate in the blood and tissues of fish, where it interferes with oxygen transport (it causes oxidation of the heme-group of haemoglobin from ferrous (Fe2+) to ferric (Fe3+), making haemoglobin unable to bind O
2
).[86]
Despite these apparent positive effects, ozone use in recirculation systems has been linked to reducing the level of bioavailable iodine in salt water systems, resulting in iodine deficiency symptoms such as goitre and decreased growth in Senegalese sole (Solea senegalensis) larvae.[87]

Ozonate seawater is used for surface disinfection of haddock and Atlantic halibut eggs against nodavirus. Nodavirus is a lethal and vertically transmitted virus which causes severe mortality in fish. Haddock eggs should not be treated with high ozone level as eggs so treated did not hatch and died after 3–4 days.[88]

Agriculture

Ozone application on freshly cut pineapple and banana shows increase in flavonoids and total phenol contents when exposure is up to 20 minutes. Decrease in ascorbic acid (one form of vitamin C) content is observed but the positive effect on total phenol content and flavonoids can overcome the negative effect.[89] Tomatoes upon treatment with ozone shows an increase in β-carotene, lutein and lycopene.[90] However, ozone application on strawberries in pre-harvest period shows decrease in ascorbic acid content.[91]

Ozone facilitates the extraction of some heavy metals from soil using EDTA. EDTA forms strong, water-soluble coordination compounds with some heavy metals (Pb, Zn) thereby making it possible to dissolve them out from contaminated soil. If contaminated soil is pre-treated with ozone, the extraction efficacy of Pb, Am and Pu increases by 11.0–28.9%,[92] 43.5%[93] and 50.7%[93] respectively.

Ion thruster


From Wikipedia, the free encyclopedia


NASA's 2.3 kW NSTAR ion thruster for the Deep Space 1 spacecraft during a hot fire test at the Jet Propulsion Laboratory.

An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. The term is strictly used to refer to gridded electrostatic ion thrusters, but may often more loosely be applied to all electric propulsion systems that accelerate plasma, since plasma consists of ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic ion thrusters use the Lorentz force to accelerate the ions. In either case, when an ion passes through an electrostatic grid engine, the potential difference of the electric field converts to the ion's kinetic energy.

According to Edgar Choueiri ion thrusters have an input power spanning 1–7 kilowatts, exhaust velocity 20–50 kilometers per second, thrust 20–250 millinewtons and efficiency 60–80%.[1][2]

The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by 4.3 km/s while consuming less than 74 kilograms of xenon. The Dawn spacecraft has surpassed the record with 10 km/s.[1][2]

The applications of ion thrusters include control of the orientation and position of orbiting satellites (some satellites have dozens of low-power ion thrusters) and use as a main propulsion engine for low-mass robotic space vehicles (for example Deep Space 1 and Dawn).[1][2]

Ion thrusters are not the most prospective type of electrically powered spacecraft propulsion (although in practice they have worked out more than others).[2] Real ion engine on the technical characteristics (and especially on the thrust) is considerably inferior to his literary prototypes (according to Edgar Choueiri — «hardly the thundering rocket engine of sci-fi movies and more akin to a car that takes two days to accelerate from zero to 60 miles per hour»).[1][2] Technical capabilities of the ion engine are limited by the space charge created by ions, that limits the thrust density (force per cross-sectional area of the engine) to a very small level.[2] Therefore ion thrusters create very small levels of thrust (for example the thrust of Deep Space 1's engine approximately equals the weight of one sheet of paper[2]) compared to conventional chemical rockets but achieve very high specific impulse, or propellant mass efficiency, by accelerating their exhaust to very high speed.
However, ion thrusters carry a fundamental price: the power imparted to the exhaust increases with the square of its velocity while the thrust increases only linearly. Normal chemical rockets, on the other hand, can provide very high thrust but are limited in total impulse by the small amount of energy that can be stored chemically in the propellants.[3] Given the practical weight of suitable power sources, the accelerations given by ion thrusters are frequently less than one thousandth of standard gravity. However, since they operate essentially as electric (or electrostatic) motors, a greater fraction of the input power is converted into kinetic exhaust power than in a chemical rocket. Chemical rockets operate as heat engines, hence Carnot's theorem bounds their possible exhaust velocity.

Due to their relatively high power needs, given the specific power of power supplies, and the requirement of an environment void of other ionized particles, ion thrust propulsion is currently only practical on spacecraft that have already reached space, and are unable to take vehicles from Earth to space, relying on conventional chemical rockets to initially reach orbit.

Origins

The first person to publish mention of the idea was Konstantin Tsiolkovsky in 1911.[4] However, the first documented instance where the possibility of electric propulsion is considered is found in Robert H. Goddard's handwritten notebook in an entry dated 6 September 1906.[5] The first experiments with ion thrusters were carried out by Goddard at Clark University from 1916–1917.[6] The technique was recommended for near-vacuum conditions at high altitude, but thrust was demonstrated with ionized air streams at atmospheric pressure. The idea appeared again in Hermann Oberth's "Wege zur Raumschiffahrt” (Ways to Spaceflight), published in 1923, where he explained his thoughts on the mass savings of electric propulsion, predicted its use in spacecraft propulsion and attitude control, and advocated electrostatic acceleration of charged gases.[4]

A working ion thruster was built by Harold R. Kaufman in 1959 at the NASA Glenn Research Center facilities. It was similar to the general design of a gridded electrostatic ion thruster with mercury as its fuel. Suborbital tests of the engine followed during the 1960s and in 1964 the engine was sent into a suborbital flight aboard the Space Electric Rocket Test 1 (SERT 1). It successfully operated for the planned 31 minutes before falling back to Earth.[7] This test was followed by an orbital test, SERT-2, in 1970.

An alternate form of electric propulsion, the Hall effect thruster was studied independently in the U.S. and the Soviet Union in the 1950s and 1960s. Hall effect thrusters had operated on Soviet satellites since 1972. Until the 1990s they were mainly used for satellite stabilization in North-South and in East-West directions. Some 100-200 engines completed their mission on Soviet and Russian satellites until the late 1990s.[8] Soviet thruster design was introduced to the West in 1992 after a team of electric propulsion specialists, under the support of the Ballistic Missile Defense Organization, visited Soviet laboratories.

General description

Ion thrusters use beams of ions (electrically charged atoms or molecules) to create thrust in accordance with momentum conservation. The method of accelerating the ions varies, but all designs take advantage of the charge/mass ratio of the ions. This ratio means that relatively small potential differences can create very high exhaust velocities. This reduces the amount of reaction mass or fuel required, but increases the amount of specific power required compared to chemical rockets. Ion thrusters are therefore able to achieve extremely high specific impulses. The drawback of the low thrust is low spacecraft acceleration, because the mass of current electric power units is directly correlated with the amount of power given. This low thrust makes ion thrusters unsuited for launching spacecraft into orbit, but they are ideal for in-space propulsion applications.

Various ion thrusters have been designed and they all generally fit under two categories. The thrusters are categorized as either electrostatic or electromagnetic. The main difference is how the ions are accelerated.
  • Electrostatic ion thrusters use the Coulomb force and are categorized as accelerating the ions in the direction of the electric field.
  • Electromagnetic ion thrusters use the Lorentz force to accelerate the ions.
Power supplies for ion thrusters are usually solar panels but, at sufficiently large distances from the Sun, nuclear power is used. In each case the power supply mass is essentially proportional to the peak power that can be supplied, and they both essentially give, for this application, no limit to the energy.

Electric thrusters tend to produce low thrust, which results in low acceleration. Using 1 g is 9.81 m/s2; F = m a ⇒ a = F/m. An NSTAR thruster producing a thrust (force) of 92 mN[9] will accelerate a satellite with a mass of 1,000 kg by 0.092 N / 1,000 kg = 0.000092 m/s2 (or 9.38×10−6 g).

Electrostatic ion thrusters

Gridded electrostatic ion thrusters


Figure 2: A diagram of how a gridded electrostatic ion engine (multipole magnetic cusp type) works

Gridded electrostatic ion thrusters commonly utilize xenon gas. This gas has no charge and is ionized by bombarding it with energetic electrons. These electrons can be provided from a hot cathode filament and when accelerated in the electrical field of the cathode, fall to the anode. Alternatively, the electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge and omits any cathode (radio frequency ion thruster).

The positively charged ions are extracted by an extraction system consisting of 2 or 3 multi-aperture grids. After entering the grid system via the plasma sheath the ions are accelerated due to the potential difference between the first and second grid (named screen and accelerator grid) to the final ion energy of typically 1-2 keV, thereby generating the thrust.

Ion thrusters emit a beam of positive charged xenon ions only. To avoid charging up the spacecraft, another cathode is placed near the engine, which emits electrons (basically the electron current is the same as the ion current) into the ion beam.[7] This also prevents the beam of ions from returning to the spacecraft and cancelling the thrust.[citation needed]

Gridded electrostatic ion thruster research (past/present):
  • NASA Solar electric propulsion Technology Application Readiness (NSTAR) - 2.3 kW, used on two successful missions
  • NASA’s Evolutionary Xenon Thruster (NEXT) - 6.9 kW, flight qualification hardware built
  • Nuclear Electric Xenon Ion System (NEXIS)
  • High Power Electric Propulsion (HiPEP) - 25 kW, test example built and run briefly on the ground
  • EADS Radio-Frequency Ion Thruster (RIT)
  • Dual-Stage 4-Grid (DS4G)[10][11]

Schematic of a Hall Thruster

Hall effect thrusters

Hall effect thrusters accelerate ions with the use of an electric potential maintained between a cylindrical anode and a negatively charged plasma that forms the cathode. The bulk of the propellant (typically xenon gas) is introduced near the anode, where it becomes ionized, and the ions are attracted towards the cathode; they accelerate towards and through it, picking up electrons as they leave to neutralize the beam and leave the thruster at high velocity.

The anode is at one end of a cylindrical tube, and in the center is a spike that is wound to produce a radial magnetic field between it and the surrounding tube. The ions are largely unaffected by the magnetic field, since they are too massive. However, the electrons produced near the end of the spike to create the cathode are far more affected and are trapped by the magnetic field, and held in place by their attraction to the anode. Some of the electrons spiral down towards the anode, circulating around the spike in a Hall current. When they reach the anode they impact the uncharged propellant and cause it to be ionized, before finally reaching the anode and closing the circuit.[12]

Field-emission electric propulsion

Field-emission electric propulsion (FEEP) thrusters use a very simple system of accelerating ions to create thrust. Most designs use either caesium or indium as the propellant. The design comprises a small propellant reservoir that stores the liquid metal, a narrow tube or a system of parallel plates that the liquid flows through, and an accelerator (a ring or an elongated aperture in a metallic plate) about a millimetre past the tube end. Caesium and indium are used due to their high atomic weights, low ionization potentials, and low melting points. Once the liquid metal reaches the end of the tube, an electric field applied between the emitter and the accelerator causes the liquid surface to deform into a series of protruding cusps ("Taylor cones"). At a sufficiently high applied voltage, positive ions are extracted from the tips of the cones.[13][14][15] The electric field created by the emitter and the accelerator then accelerates the ions. An external source of electrons neutralizes the positively charged ion stream to prevent charging of the spacecraft.

Electromagnetic thrusters

Pulsed inductive thrusters (PIT)

Pulsed inductive thrusters (PIT) use pulses of thrust instead of one continuous thrust, and have the ability to run on power levels in the order of Megawatts (MW). PITs consist of a large coil encircling a cone shaped tube that emits the propellant gas. Ammonia is the gas commonly used in PIT engines. For each pulse of thrust the PIT gives, a large charge first builds up in a group of capacitors behind the coil and is then released. This creates a current that moves circularly in the direction of jθ. The current then creates a magnetic field in the outward radial direction (Br), which then creates a current in the ammonia gas that has just been released in the opposite direction of the original current. This opposite current ionizes the ammonia and these positively charged ions are accelerated away from the PIT engine due to the electric field jθ crossing with the magnetic field Br, which is due to the Lorentz Force.[16]

Magnetoplasmadynamic (MPD) / lithium Lorentz force accelerator (LiLFA)

Magnetoplasmadynamic (MPD) thrusters and lithium Lorentz force accelerator (LiLFA) thrusters use roughly the same idea with the LiLFA thruster building off of the MPD thruster. Hydrogen, argon, ammonia, and nitrogen gas can be used as propellant. In a certain configuration, the ambient gas in Low Earth Orbit (LEO) can be used as a propellant. The gas first enters the main chamber where it is ionized into plasma by the electric field between the anode and the cathode. This plasma then conducts electricity between the anode and the cathode. This new current creates a magnetic field around the cathode, which crosses with the electric field, thereby accelerating the plasma due to the Lorentz force. The LiLFA thruster uses the same general idea as the MPD thruster, except for two main differences. The first difference is that the LiLFA uses lithium vapor, which has the advantage of being able to be stored as a solid. The other difference is that the cathode is replaced by multiple smaller cathode rods packed into a hollow cathode tube. The cathode in the MPD thruster is easily corroded due to constant contact with the plasma. In the LiLFA thruster the lithium vapor is injected into the hollow cathode and is not ionized to its plasma form/corrode the cathode rods until it exits the tube. The plasma is then accelerated using the same Lorentz Force.[17][18][19]

Electrodeless plasma thrusters

Electrodeless plasma thrusters have two unique features: the removal of the anode and cathode electrodes and the ability to throttle the engine. The removal of the electrodes takes away the factor of erosion, which limits lifetime on other ion engines. Neutral gas is first ionized by electromagnetic waves and then transferred to another chamber where it is accelerated by an oscillating electric and magnetic field, also known as the ponderomotive force. This separation of the ionization and acceleration stage give the engine the ability to throttle the speed of propellant flow, which then changes the thrust magnitude and specific impulse values.[20]

Helicon double layer thruster

A helicon double layer thruster is a type of plasma thruster, which ejects high velocity ionized gas to provide thrust to a spacecraft. In this thruster design, gas is injected into a tubular chamber (the source tube) with one open end. Radio frequency AC power (at 13.56 MHz in the prototype design) is coupled into a specially shaped antenna wrapped around the chamber. The electromagnetic wave emitted by the antenna causes the gas to break down and form a plasma. The antenna then excites a helicon wave in the plasma, which further heats the plasma. The device has a roughly constant magnetic field in the source tube (supplied by solenoids in the prototype), but the magnetic field diverges and rapidly decreases in magnitude away from the source region, and might be thought of as a kind of magnetic nozzle. In operation, there is a sharp boundary between the high density plasma inside the source region, and the low density plasma in the exhaust, which is associated with a sharp change in electrical potential. The plasma properties change rapidly across this boundary, which is known as a current-free electric double layer. The electrical potential is much higher inside the source region than in the exhaust, and this serves both to confine most of the electrons, and to accelerate the ions away from the source region. Enough electrons escape the source region to ensure that the plasma in the exhaust is neutral overall.

Comparisons

The following table compares actual test data of some ion thrusters:

Engine Propellant Required power
(kW)
Specific impulse
(s)
Thrust
(mN)
Thruster mass
(kg)
NSTAR Xenon 2.3 3,300 to 1,700[21] 92 max.[9]
NEXT[9] Xenon 6.9[22] 4,300 [22][23][24] 236 max[9]
NEXIS[25] Xenon 20.5
HiPEP Xenon 20–50[26] 6,000–9,000[26] 460–670[26]
RIT 22[27] Xenon 5
Hall effect Bismuth 25[citation needed]
Hall effect Bismuth 140[citation needed]
Hall effect Xenon 25[citation needed] 3,250[citation needed] 950[citation needed]
Hall effect Xenon 75[citation needed]
FEEP Liquid Caesium 6×10−5–0.06 6,000–10,000[14] 0.001–1[14]
VASIMR Argon 200 3,000–12,000 ~5,000[28] 620 [1]
DS4G Xenon 250 19,300 2,500 max. 5
KLIMT Krypton 0.5[29] 4[29]

The following thrusters are highly experimental and have been tested only in pulse mode.

Engine Propellant Required power
(kW)
Specific impulse
(s)
Thrust
(mN)
Thruster mass
(kg)
MPDT Hydrogen 1,500 4,900[citation needed] 26,300[citation needed]
MPDT Hydrogen 3,750 3,500[citation needed] 88,500[citation needed]
MPDT Hydrogen 7,500[citation needed] 6,000[citation needed] 60,000[citation needed]
LiLFA Lithium Vapor 500 4,077[citation needed] 12,000[citation needed]

Lifetime

A major limiting factor of ion thrusters is their small thrust; however, it is generated at a high propellant efficiency (mass utilisation, specific impulse). The efficiency comes from the high exhaust velocity, which in turn demands high energy, and the performance is ultimately limited by the available spacecraft power.

The low thrust requires ion thrusters to provide continuous thrust for a long time to achieve the needed change in velocity (delta-v) for a particular mission. To cause enough change in momentum, ion thrusters are designed to last for periods of weeks to years.

In practice the lifetime of electrostatic ion thrusters is limited by several processes:
  • In electrostatic gridded ion thruster design, charge-exchange ions produced by the beam ions with the neutral gas flow can be accelerated towards the negatively biased accelerator grid and cause grid erosion. End-of-life is reached when either a structural failure of the grid occurs or the holes in the accelerator grid become so large that the ion extraction is largely affected; e.g., by the occurrence of electron backstreaming. Grid erosion cannot be avoided and is the major lifetime-limiting factor. By a thorough grid design and material selection, lifetimes of 20,000 hours and far beyond are reached, which is sufficient to fulfill current space missions.
A test of the NASA Solar electric propulsion Technology Application Readiness (NSTAR) electrostatic ion thruster resulted in 30,472 hours (roughly 3.5 years) of continuous thrust at maximum power. The test was concluded prior to any failure and examination indicated the engine was not approaching failure either.[30]

More recently, the NASA Evolutionary Xenon Thruster (NEXT) Project, conducted at NASA's Glenn Research Center in Cleveland, Ohio, operated continuously for more than 48,000 hours.[31] The test was conducted in a high vacuum test chamber at Glenn Research Center. Over the course of the 5 1/2 + year test, the engine consumed approximately 870 kilograms of xenon propellant. The total impulse provided by the engine would require over 10,000 kilograms of conventional rocket propellant for similar application. The engine was designed by Aerojet Rocketdyne of Sacramento, California.
  • Hall thrusters suffer from very strong erosion of the ceramic discharge chamber by impact of energetic ions: a test reported in 2010[32] showed erosion of around 1 mm per hundred hours of operation, though this is inconsistent with observed on-orbit lifetimes of a few thousand hours.
NASA's Jet Propulsion Laboratory has created ion drives with a time of continuous operation of more than 3 years.[1][2]

Propellants

Ionization energy represents a very large percentage of the energy needed to run ion drives. The ideal propellant for ion drives is thus a propellant molecule or atom that is easy to ionize, that has a high mass/ionization energy ratio. In addition, the propellant should not cause erosion of the thruster to any great degree to permit long life; and should not contaminate the vehicle.[33]

Many current designs use xenon gas, as it is easy to ionize, has a reasonably high atomic number, its inert nature, and low erosion. However, xenon is globally in short supply and very expensive.
Older designs used mercury, but this is toxic and expensive, tended to contaminate the vehicle with the metal and was difficult to feed accurately.

Other propellants, such as bismuth, show promise and are areas of research, particularly for gridless designs, such as Hall effect thrusters.

VASIMR design (and other plasma-based engines) are theoretically able to use practically any material for propellant. However, in current tests the most practical propellant is argon, which is a relatively abundant and inexpensive gas.

Energy efficiency


Plot of instantaneous propulsive efficiency (blue) and overall efficiency for a vehicle accelerating from rest (red) as percentages of the engine efficiency- note that peak vehicle efficiency occurs at about 1.6 times exhaust velocity.

Ion thrusters are frequently quoted with an efficiency metric. This efficiency is the kinetic energy of the exhaust jet emitted per second divided by the electrical power into the device.

The actual overall system energy efficiency in use is determined by the propulsive efficiency, which depends on vehicle speed and exhaust speed. Some thrusters can vary exhaust speed in operation, but all can be designed with different exhaust speeds. At the lower end of Isps the overall efficiency drops, because the ionization takes up a larger percentage energy, and at the high end propulsive efficiency is reduced.

Optimal efficiencies and exhaust velocities can thus be calculated for any given mission to give minimum overall cost.

Applications

Ion thrusters have many applications for in-space propulsion. The best applications of the thrusters make use of the long lifetime when significant thrust is not needed. Examples of this include orbit transfers, attitude adjustments, drag compensation for low Earth orbits, transporting cargo such as chemical fuels between propellant depots and ultra-fine adjustments for scientific missions. Ion thrusters can also be used for interplanetary and deep-space missions where time is not crucial. Continuous thrust over a very long time can build up a larger velocity than traditional chemical rockets.

Missions

Of all the electric thrusters, ion thrusters have been the most seriously considered commercially and academically in the quest for interplanetary missions and orbit raising maneuvers. Ion thrusters are seen as the best solution for these missions, as they require very high change in velocity overall that can be built up over long periods of time.

Pure demonstration vehicles

SERT
Ion propulsion systems were first demonstrated in space by the NASA Lewis (now Glenn Research Center) missions "Space Electric Rocket Test" (SERT) I and II.[34] The first was SERT-1, launched July 20, 1964, successfully proved that the technology operated as predicted in space. These were electrostatic ion thrusters using mercury and cesium as the reaction mass. The second test, SERT-II, launched on February 3, 1970,[35][36] verified the operation of two mercury ion engines for thousands of running hours.[37]

Operational missions

Ion thrusters are routinely used for station-keeping on commercial and military communication satellites in geosynchronous orbit, including satellites manufactured by Boeing and by Hughes Aerospace. The pioneers in this field were the Soviet Union, who used SPT thrusters on a variety of satellites starting in the early 1970s.

Two geostationary satellites (ESA's Artemis in 2001-2003[38] and the US military's AEHF-1 in 2010-2012[39]) have used the ion thruster for orbit raising after the failure of the chemical-propellant engine. Boeing[40] have been using ion thrusters for station-keeping since 1997, and plan in 2013-2014 to offer a variant on their 702 platform, which will have no chemical engine and use ion thrusters for orbit raising; this enables a significantly lower launch mass for a given satellite capability. AEHF-2 used a chemical engine to raise perigee to 10150 miles and is then proceeding to geosynchronous orbit using electric propulsion.[41]

In Earth orbit

GOCE
ESA's Gravity Field and Steady-State Ocean Circulation Explorer was launched on March 16, 2009. It used ion propulsion throughout its twenty month mission to combat the air-drag it experienced in its low orbit before intentionally deorbiting on November 11, 2013.

In deep space

Deep Space 1
NASA developed the NSTAR ion engine for use in their interplanetary science missions beginning in the late-1990s. This xenon-propelled ion thruster was first space-tested in the highly successful space probe Deep Space 1, launched in 1998. This was the first use of electric propulsion as the interplanetary propulsion system on a science mission.[34] Based on the NASA design criteria, Hughes Research Labs, developed the XIPS (Xenon Ion Propulsion System) for performing station keeping on geosynchronous satellites.[citation needed]. Hughes (EDD) manufactured the NSTAR thruster used on the spacecraft.
Hayabusa
The Japanese space agency's Hayabusa, which was launched in 2003 and successfully rendezvoused with the asteroid 25143 Itokawa and remained in close proximity for many months to collect samples and information, was powered by four xenon ion engines. It used xenon ions generated by microwave electron cyclotron resonance, and a carbon / carbon-composite material (which is resistant to erosion) for its acceleration grid.[42] Although the ion engines on Hayabusa had some technical difficulties, in-flight reconfiguration allowed one of the four engines to be repaired, and allowed the mission to successfully return to Earth.[43]
Smart 1
The European Space Agency's satellite SMART-1, launched in 2003, used a Snecma PPS-1350-G Hall thruster to get from GTO to lunar orbit. This satellite completed its mission on 3 September 2006, in a controlled collision on the Moon's surface, after a trajectory deviation so scientists could see the 3 meter crater the impact created on the visible side of the moon.
Dawn
Dawn was launched on 27 September 2007 to explore the asteroid Vesta and the dwarf planet Ceres. To cruise from Earth to its targets it uses three Deep Space 1 heritage xenon ion thrusters (firing only one at a time) to take it in a long outward spiral. An extended mission in which Dawn explores other asteroids after Ceres is also possible. Dawn's ion drive is capable of accelerating from 0 to 60 mph (97 km/h) in 4 days, firing continuously.[44]

Planned missions

In addition, several missions are planned to use ion thrusters in the next few years.
BepiColombo
ESA will launch the BepiColombo mission to Mercury in 2016. It uses ion thrusters in combination with swing-bys to get to Mercury, where a chemical rocket will be fired for orbit insertion.
LISA Pathfinder
LISA Pathfinder is an ESA spacecraft to be launched in 2015. It will not use ion thrusters as its primary propulsion system, but will use both colloid thrusters and FEEP for very precise attitude control -— the low thrusts of these propulsion devices make it possible to move the spacecraft incremental distances very accurately. It is a test for the possible LISA mission.
International Space Station
As of March 2011, a future launch of an Ad Astra VF-200 200 kW VASIMR electromagnetic thruster was being considered for placement and testing on the International Space Station.[45][46] The VF-200 is a flight version of the VX-200.[47][48] Since the available power from the ISS is less than 200 kW, the ISS VASIMR will include a trickle-charged battery system allowing for 15 min pulses of thrust. Testing of the engine on ISS is valuable, because ISS orbits at a relatively low altitude and experiences fairly high levels of atmospheric drag, making periodic boosts of altitude necessary.
Currently, altitude reboosting by chemical rockets fulfills this requirement. If the tests of VASIMR reboosting of the ISS goes according to plan, the increase in specific impulse could mean that the cost of fuel for altitude reboosting will be one-twentieth of the current $210 million annual cost.[45] Hydrogen is generated by the ISS as a by-product, which is currently vented into space.
NASA high-power SEP system demonstration mission
In June 2011, NASA launched a request-for-proposals[49] for a test mission (from context probably using the NEXT engine) capable of being extended to 300 kW electrical power; this was awarded to Northrop Grumman in February 2012.[50]

Future project

Geoffrey A. Landis proposed for interstellar travel future-technology project interstellar probe with supplying the energy from an external source (laser of base station) and ion thruster.[51][52]

Green-beard effect (Evolutionary Biology)


From Wikipedia, the free encyclopedia


The Green-beard effect is a form of sexual selection in which individuals with alleles that create unique observable traits tend to select people with similar trait. In this diagram note that the individuals with the trait for the same color head mate with each other and create offspring while those with different color heads do not mate.

The green-beard effect is a hypothesis used in evolutionary biology to explain selective altruism between designated individuals of a species. It is based on the gene-centered view of evolution, which emphasizes an interpretation of natural selection from the point of view of the gene which acts as an agent that has the metaphorical "goal" of maximizing its own propagation. A gene for (behavioral) selective altruism can be favored by (natural) selection if the altruism is primarily directed at other individuals who share the same allele.

A green-beard effect occurs when an allele, or a set of linked alleles, produce three phenotypic effects:
  • a perceptible trait, — the hypothetical "green beard"
  • recognition of this trait in others; and
  • preferential treatment to those recognized.
The carrier of the allele is essentialy recognizing copies of the same allele in other individuals. Whereas most alleles that are favored by kin selection spread by individuals who show family altruism toward others who share their alleles in a non-specific way, green-beard alleles would rise in frequency by promoting altruism toward individuals who share a specific phenotypic trait (allele).
This can have the effect of delineating a subset of organisms within a population that is characterized by members who show greater cooperation toward each other, this forming a "clique" that can be advantageous to it's members who are not necessarily kin.

Green-beard effect could increase altruism on green-beard phenotypes and therefore its presence in a population even if alleles are assisting other genes that are not exact copies of themselves in a molecular sense; all that is required is that they produce the three phenotypic characteristics described above. Green-beard alleles are vulnerable to mutant genes arising that produce the perceptible trait without the helping behaviour.

The idea of a green-beard allele was proposed by William D. Hamilton in his articles of 1964,[1][2] and named as "Green Beard" by Richard Dawkins in The Selfish Gene (1976).[3][4]

Examples

In the last several years, evolutionary biologists have questioned the potential validity of green-beard alleles, suggesting it would be extraordinarily rare for a single allele to produce three complex phenotypic effects. This criticism has led some to believe that they simply cannot exist or that they only can be present in less complex organisms, such as microorganisms. Several discoveries within the past ten years have illuminated the validity of this critique.

The concept remained a merely theoretical possibility under Dawkins' selfish gene model until 1998, when a green-beard allele was first found in nature, in the red imported fire ant (Solenopsis invicta).[4][5] Polygyne colony queens are heterozygous (Bb) at the Gp-9 gene locus. Their worker offspring can have both heterozygous (Bb) and homozygous (BB) genotypes. The investigators discovered that homozygous dominant (BB) queens, which in the wild form produce monogyne rather than polygyne colonies, are specifically killed when introduced into polygyne colonies, most often by heterozygous (Bb) and not homozygous (BB) workers. They concluded that the allele Gp-9b is linked to a greenbeard allele which induces workers bearing this allele to kill all queens that do not have it. A final conclusion notes that the workers are able to distinguish BB queens from Bb queens based on an odor cue.

The gene csA in the slime mould Dictyostelium discoideum, discovered in 2003,[6] codes for a cell adhesion protein which binds to gp80 proteins on other cells, allowing multicellular fruiting body formation on soil. Mixtures of csA knockout cells with wild-type cells yield spores, "born" from the fruiting bodies, which are 82% wild-type (WT). This is because the wild-type cells are better at adhering and more effectively combine into aggregates; knockout (KO) cells are left behind. On more adhesive but less natural substances, KO cells can adhere; WT cells, still better at adhering, sort preferentially into the stalk.

In 2006, green beard-like recognition was seen in the cooperative behavior among color morphs in side-blotched lizards, although the traits appear to be encoded by multiple loci across the genome.[7]

A more recent example, found in 2008, is a gene that makes brewer's yeast clump together in response to a toxin such as alcohol.[8] By investigating flocculation, a type of self-adherence generally present in asexual aggregations, Smukalla et al.[9] showed that S. cerevisiae is a model for cooperative behavior evolution. When this yeast expresses FLO1 in the laboratory, flocculation is restored. Flocculation is apparently protective for the FLO1+ cells, which are shielded from certain stresses (ethanol, for example). In addition FLO1+ cells preferentially adhere to each other. The authors therefore conclude that flocculation is driven by this greenbeard allele.

A mammalian example appears to be the reproductive strategy of the wood mouse, which shows cooperation among spermatozoa. Single sperms hook in each other to form sperm-trains, which are able to move faster together than single sperm would do.[10]

Polarization

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Polarization_(waves) Circular...