Dielectric

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Dielectric

A polarised dielectric material

In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor, because they have no loosely bound, or free, electrons that may drift through the material, but instead they shift, only slightly, from their average equilibrium positions, causing dielectric polarisation. Because of dielectric polarisation, positive charges are displaced in the direction of the field and negative charges shift in the direction opposite to the field (for example, if the field is moving parallel to the positive x axis, the negative charges will shift in the negative x direction). This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarised, but also reorient so that their symmetry axes align to the field.

The study of dielectric properties concerns storage and dissipation of electric and magnetic energy in materials. Dielectrics are important for explaining various phenomena in electronics, optics, solid-state physics and cell biophysics.

Terminology

Although the term insulator implies low electrical conduction, dielectric typically means materials with a high polarisability. The latter is expressed by a number called the relative permittivity. The term insulator is generally used to indicate electrical obstruction while the term dielectric is used to indicate the energy storing capacity of the material (by means of polarisation). A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor. The polarisation of the dielectric by the applied electric field increases the capacitor's surface charge for the given electric field strength.

The term dielectric was coined by William Whewell (from dia + electric) in response to a request from Michael Faraday. A perfect dielectric is a material with zero electrical conductivity (cf. perfect conductor infinite electrical conductivity), thus exhibiting only a displacement current; therefore it stores and returns electrical energy as if it were an ideal capacitor.

Electric susceptibility

Main articles: Electric susceptibility and Permittivity

The electric susceptibility χ_e of a dielectric material is a measure of how easily it polarises in response to an electric field. This, in turn, determines the electric permittivity of the material and thus influences many other phenomena in that medium, from the capacitance of capacitors to the speed of light.

It is defined as the constant of proportionality (which may be a tensor) relating an electric field E to the induced dielectric polarisation density P such that

$${\displaystyle \mathbf{P}={\epsilon }_0{\chi }_e\mathbf{E},}$$

where ε₀ is the electric permittivity of free space.

The susceptibility of a medium is related to its relative permittivity ε_r by

$${\displaystyle {\chi }_e={\epsilon }_r-1.}$$

So in the case of a vacuum,

$${\displaystyle {\chi }_e=0.}$$

The electric displacement D is related to the polarisation density P by

$${\displaystyle \mathbf{D}={\epsilon }_0\mathbf{E}+\mathbf{P}={\epsilon }_0\left(1+{\chi }_e\right)\mathbf{E}={\epsilon }_0{\epsilon }_r\mathbf{E}.}$$

Dispersion and causality

In general, a material cannot polarise instantaneously in response to an applied field. The more general formulation as a function of time is

$${\displaystyle \mathbf{P}(t)={\epsilon }_0{\int }_{-\mathrm{\infty }}^t{\chi }_e\left(t-t^{\prime }\right)\mathbf{E}(t^{\prime })d{t}^{\prime }.}$$

That is, the polarisation is a convolution of the electric field at previous times with time-dependent susceptibility given by χ_e(Δt). The upper limit of this integral can be extended to infinity as well if one defines χ_e(Δt) = 0 for Δt < 0. An instantaneous response corresponds to Dirac delta function susceptibility χ_e(Δt) = χ_eδ(Δt).

It is more convenient in a linear system to take the Fourier transform and write this relationship as a function of frequency. Due to the convolution theorem, the integral becomes a simple product,

$${\displaystyle \mathbf{P}(\omega )={\epsilon }_0{\chi }_e(\omega )\mathbf{E}(\omega ).}$$

The susceptibility (or equivalently the permittivity) is frequency dependent. The change of susceptibility with respect to frequency characterises the dispersion properties of the material.

Moreover, the fact that the polarisation can only depend on the electric field at previous times (i.e., χ_e(Δt) = 0 for Δt < 0), a consequence of causality, imposes Kramers–Kronig constraints on the real and imaginary parts of the susceptibility χ_e(ω).

Dielectric polarisation

Basic atomic model

Electric field interaction with an atom under the classical dielectric model

In the classical approach to the dielectric, the material is made up of atoms. Each atom consists of a cloud of negative charge (electrons) bound to and surrounding a positive point charge at its center. In the presence of an electric field, the charge cloud is distorted, as shown in the top right of the figure.

This can be reduced to a simple dipole using the superposition principle. A dipole is characterised by its dipole moment, a vector quantity shown in the figure as the blue arrow labeled M. It is the relationship between the electric field and the dipole moment that gives rise to the behaviour of the dielectric. (Note that the dipole moment points in the same direction as the electric field in the figure. This isn't always the case, and is a major simplification, but is true for many materials.)

When the electric field is removed the atom returns to its original state. The time required to do so is called relaxation time; an exponential decay.

This is the essence of the model in physics. The behaviour of the dielectric now depends on the situation. The more complicated the situation, the richer the model must be to accurately describe the behaviour. Important questions are:

• Is the electric field constant or does it vary with time? At what rate?
• Does the response depend on the direction of the applied field (isotropy of the material)?
• Is the response the same everywhere (homogeneity of the material)?
• Do any boundaries or interfaces have to be taken into account?
• Is the response linear with respect to the field, or are there nonlinearities?

The relationship between the electric field E and the dipole moment M gives rise to the behaviour of the dielectric, which, for a given material, can be characterised by the function F defined by the equation:

$${\displaystyle \mathbf{M}=\mathbf{F}(\mathbf{E}).}$$

When both the type of electric field and the type of material have been defined, one then chooses the simplest function F that correctly predicts the phenomena of interest. Examples of phenomena that can be so modelled include:

• Refractive index
• Group velocity dispersion
• Birefringence
• Self-focusing
• Harmonic generation

Dipolar polarisation

Dipolar polarisation is a polarisation that is either inherent to polar molecules (orientation polarisation), or can be induced in any molecule in which the asymmetric distortion of the nuclei is possible (distortion polarisation). Orientation polarisation results from a permanent dipole, e.g., that arising from the 104.45° angle between the asymmetric bonds between oxygen and hydrogen atoms in the water molecule, which retains polarisation in the absence of an external electric field. The assembly of these dipoles forms a macroscopic polarisation.

When an external electric field is applied, the distance between charges within each permanent dipole, which is related to chemical bonding, remains constant in orientation polarisation; however, the direction of polarisation itself rotates. This rotation occurs on a timescale that depends on the torque and surrounding local viscosity of the molecules. Because the rotation is not instantaneous, dipolar polarisations lose the response to electric fields at the highest frequencies. A molecule rotates about 1 radian per picosecond in a fluid, thus this loss occurs at about 10¹¹ Hz (in the microwave region). The delay of the response to the change of the electric field causes friction and heat.

When an external electric field is applied at infrared frequencies or less, the molecules are bent and stretched by the field and the molecular dipole moment changes. The molecular vibration frequency is roughly the inverse of the time it takes for the molecules to bend, and this distortion polarisation disappears above the infrared.

Ionic polarisation

Ionic polarisation is polarisation caused by relative displacements between positive and negative ions in ionic crystals (for example, NaCl).

If a crystal or molecule consists of atoms of more than one kind, the distribution of charges around an atom in the crystal or molecule leans to positive or negative. As a result, when lattice vibrations or molecular vibrations induce relative displacements of the atoms, the centers of positive and negative charges are also displaced. The locations of these centers are affected by the symmetry of the displacements. When the centers don't correspond, polarisation arises in molecules or crystals. This polarisation is called ionic polarisation.

Ionic polarisation causes the ferroelectric effect as well as dipolar polarisation. The ferroelectric transition, which is caused by the lining up of the orientations of permanent dipoles along a particular direction, is called an order-disorder phase transition. The transition caused by ionic polarisations in crystals is called a displacive phase transition.

In cells

Ionic polarisation enables the production of energy-rich compounds in cells (the proton pump in mitochondria) and, at the plasma membrane, the establishment of the resting potential, energetically unfavourable transport of ions, and cell-to-cell communication (the Na+/K+-ATPase).

All cells in animal body tissues are electrically polarised – in other words, they maintain a voltage difference across the cell's plasma membrane, known as the membrane potential. This electrical polarisation results from a complex interplay between ion transporters and ion channels.

In neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving the dendrites, axon, and cell body different electrical properties. As a result, some parts of the membrane of a neuron may be excitable (capable of generating action potentials), whereas others are not.

Dielectric dispersion

In physics, dielectric dispersion is the dependence of the permittivity of a dielectric material on the frequency of an applied electric field. Because there is a lag between changes in polarisation and changes in the electric field, the permittivity of the dielectric is a complex function of the frequency of the electric field. Dielectric dispersion is very important for the applications of dielectric materials and the analysis of polarisation systems.

This is one instance of a general phenomenon known as material dispersion: a frequency-dependent response of a medium for wave propagation.

When the frequency becomes higher:

1. The dipolar polarisation can no longer follow the oscillations of the electric field in the microwave region around 10¹⁰ Hz,
2. The ionic polarisation and molecular distortion polarisation can no longer track the electric field past the infrared or far-infrared region around 10¹³ Hz,
3. The electronic polarisation loses its response in the ultraviolet region around 10¹⁵ Hz.

In the frequency region above ultraviolet, permittivity approaches the constant ε₀ in every substance, where ε₀ is the permittivity of the free space. Because permittivity indicates the strength of the relation between an electric field and polarisation, if a polarisation process loses its response, permittivity decreases.

Dielectric relaxation

Dielectric relaxation is the momentary delay (or lag) in the dielectric constant of a material. This is usually caused by the delay in molecular polarisation with respect to a changing electric field in a dielectric medium (e.g., inside capacitors or between two large conducting surfaces). Dielectric relaxation in changing electric fields could be considered analogous to hysteresis in changing magnetic fields (e.g., in inductor or transformer cores). Relaxation in general is a delay or lag in the response of a linear system, and therefore dielectric relaxation is measured relative to the expected linear steady state (equilibrium) dielectric values. The time lag between electrical field and polarisation implies an irreversible degradation of Gibbs free energy.

In physics, dielectric relaxation refers to the relaxation response of a dielectric medium to an external, oscillating electric field. This relaxation is often described in terms of permittivity as a function of frequency, which can, for ideal systems, be described by the Debye equation. On the other hand, the distortion related to ionic and electronic polarisation shows behaviour of the resonance or oscillator type. The character of the distortion process depends on the structure, composition, and surroundings of the sample.

Debye relaxation

Debye relaxation is the dielectric relaxation response of an ideal, noninteracting population of dipoles to an alternating external electric field. It is usually expressed in the complex permittivity ε of a medium as a function of the field's angular frequency ω:

$${\displaystyle \hat{\epsilon }(\omega )={\epsilon }_{\mathrm{\infty }}+\frac{\mathrm{\Delta }\epsilon }{1+i\omega \tau },}$$

where ε_∞ is the permittivity at the high frequency limit, Δε = ε_s − ε_∞ where ε_s is the static, low frequency permittivity, and τ is the characteristic relaxation time of the medium. Separating into the real part ${\displaystyle {\epsilon }^{\prime }}$ and the imaginary part ${\displaystyle {\epsilon }^{″}}$ of the complex dielectric permittivity yields:

$${\displaystyle \begin{array}{rl}{\epsilon }^{\prime }& ={\epsilon }_{\mathrm{\infty }}+\frac{{\epsilon }_s-{\epsilon }_{\mathrm{\infty }}}{1+{\omega }^2{\tau }^2}\\ {\epsilon }^{″}& =\frac{({\epsilon }_s-{\epsilon }_{\mathrm{\infty }})\omega \tau }{1+{\omega }^2{\tau }^2}\end{array}}$$

Note that the above equation for ${\displaystyle \hat{\epsilon }(\omega )}$is sometimes written with ${\displaystyle 1-i\omega \tau }$ in the denominator due to an ongoing sign convention ambiguity whereby many sources represent the time dependence of the complex electric field with ${\displaystyle \exp (-i\omega t)}$ whereas others use ${\displaystyle \exp (+i\omega t)}$. In the former convention, the functions ${\displaystyle {\epsilon }^{\prime }}$ and ${\displaystyle {\epsilon }^{″}}$ representing real and imaginary parts are given by ${\displaystyle \hat{\epsilon }(\omega )={\epsilon }^{\prime }+i{\epsilon }^{″}}$ whereas in the latter convention ${\displaystyle \hat{\epsilon }(\omega )={\epsilon }^{\prime }-i{\epsilon }^{″}}$. The above equation uses the latter convention.

The dielectric loss is also represented by the loss tangent:

$${\displaystyle \tan (\delta )=\frac{{\epsilon }^{″}}{{\epsilon }^{\prime }}=\frac{\left({\epsilon }_s-{\epsilon }_{\mathrm{\infty }}\right)\omega \tau }{{\epsilon }_s+{\epsilon }_{\mathrm{\infty }}{\omega }^2{\tau }^2}}$$

This relaxation model was introduced by and named after the physicist Peter Debye (1913). It is characteristic for dynamic polarisation with only one relaxation time.

Variants of the Debye equation

Cole–Cole equation

This equation is used when the dielectric loss peak shows symmetric broadening.

Cole–Davidson equation

This equation is used when the dielectric loss peak shows asymmetric broadening.

Havriliak–Negami relaxation

This equation considers both symmetric and asymmetric broadening.

Kohlrausch–Williams–Watts function

Fourier transform of stretched exponential function.

Curie–von Schweidler law

This shows the response of dielectrics to an applied DC field to behave according to a power law, which can be expressed as an integral over weighted exponential functions.

Djordjevic-Sarkar approximation

This is used when the dielectric loss is approximately constant for a wide range of frequencies.

Paraelectricity

See also: Ferroelectricity

Paraelectricity is the nominal behaviour of dielectrics when the dielectric permittivity tensor is proportional to the unit matrix, i.e., an applied electric field causes polarisation and/or alignment of dipoles only parallel to the applied electric field. Contrary to the analogy with a paramagnetic material, no permanent electric dipole needs to exist in a paraelectric material. Removal of the fields results in the dipolar polarisation returning to zero. The mechanisms that causes paraelectric behaviour are distortion of individual ions (displacement of the electron cloud from the nucleus) and polarisation of molecules or combinations of ions or defects.

Paraelectricity can occur in crystal phases where electric dipoles are unaligned and thus have the potential to align in an external electric field and weaken it.

Most dielectric materials are paraelectrics. A specific example of a paraelectric material of high dielectric constant is strontium titanate.

The LiNbO₃ crystal is ferroelectric below 1430 K, and above this temperature it transforms into a disordered paraelectric phase. Similarly, other perovskites also exhibit paraelectricity at high temperatures.

Paraelectricity has been explored as a possible refrigeration mechanism; polarising a paraelectric by applying an electric field under adiabatic process conditions raises the temperature, while removing the field lowers the temperature. A heat pump that operates by polarising the paraelectric, allowing it to return to ambient temperature (by dissipating the extra heat), bringing it into contact with the object to be cooled, and finally depolarising it, would result in refrigeration.

Tunability

Tunable dielectrics are insulators whose ability to store electrical charge changes when a voltage is applied.

Generally, strontium titanate (SrTiO
₃) is used for devices operating at low temperatures, while barium strontium titanate (Ba
_1−xSr
_xTiO
₃) substitutes for room temperature devices. Other potential materials include microwave dielectrics and carbon nanotube (CNT) composites.

In 2013, multi-sheet layers of strontium titanate interleaved with single layers of strontium oxide produced a dielectric capable of operating at up to 125 GHz. The material was created via molecular beam epitaxy. The two have mismatched crystal spacing that produces strain within the strontium titanate layer that makes it less stable and tunable.

Systems such as Ba
_1−xSr
_xTiO
₃ have a paraelectric–ferroelectric transition just below ambient temperature, providing high tunability. Films suffer significant losses arising from defects.

Applications

Capacitors

Main article: Capacitor

Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric (orange) reduces the field and increases the capacitance.

Commercially manufactured capacitors typically use a solid dielectric material with high permittivity as the intervening medium between the stored positive and negative charges. This material is often referred to in technical contexts as the capacitor dielectric.

The most obvious advantage to using such a dielectric material is that it prevents the conducting plates, on which the charges are stored, from coming into direct electrical contact. More significantly, however, a high permittivity allows a greater stored charge at a given voltage. This can be seen by treating the case of a linear dielectric with permittivity ε and thickness d between two conducting plates with uniform charge density σ_ε. In this case the charge density is given by

$${\displaystyle {\sigma }_{\epsilon }=\epsilon \frac{V}{d}}$$

and the capacitance per unit area by

$${\displaystyle c=\frac{{\sigma }_{\epsilon }}{V}=\frac{\epsilon }{d}}$$

From this, it can easily be seen that a larger ε leads to greater charge stored and thus greater capacitance.

Dielectric materials used for capacitors are also chosen such that they are resistant to ionisation. This allows the capacitor to operate at higher voltages before the insulating dielectric ionises and begins to allow undesirable current.

Dielectric resonator

Main article: Dielectric resonator

A dielectric resonator oscillator (DRO) is an electronic component that exhibits resonance of the polarisation response for a narrow range of frequencies, generally in the microwave band. It consists of a "puck" of ceramic that has a large dielectric constant and a low dissipation factor. Such resonators are often used to provide a frequency reference in an oscillator circuit. An unshielded dielectric resonator can be used as a dielectric resonator antenna (DRA).

BST thin films

From 2002 to 2004, the United States Army Research Laboratory (ARL) conducted research on thin film technology. Barium strontium titanate (BST), a ferroelectric thin film, was studied for the fabrication of radio frequency and microwave components, such as voltage-controlled oscillators, tunable filters and phase shifters.

The research was part of an effort to provide the Army with highly-tunable, microwave-compatible materials for broadband electric-field tunable devices, which operate consistently in extreme temperatures. This work improved tunability of bulk barium strontium titanate, which is a thin film enabler for electronics components.

In a 2004 research paper, U.S. ARL researchers explored how small concentrations of acceptor dopants can dramatically modify the properties of ferroelectric materials such as BST.

Researchers "doped" BST thin films with magnesium, analyzing the "structure, microstructure, surface morphology and film/substrate compositional quality" of the result. The Mg doped BST films showed "improved dielectric properties, low leakage current, and good tunability", meriting potential for use in microwave tunable devices.

Some practical dielectrics

Dielectric materials can be solids, liquids, or gases. (A high vacuum can also be a useful, nearly lossless dielectric even though its relative dielectric constant is only unity.)

Solid dielectrics are perhaps the most commonly used dielectrics in electrical engineering, and many solids are very good insulators. Some examples include porcelain, glass, and most plastics. Air, nitrogen and sulfur hexafluoride are the three most commonly used gaseous dielectrics.

• Industrial coatings such as Parylene provide a dielectric barrier between the substrate and its environment.
• Mineral oil is used extensively inside electrical transformers as a fluid dielectric and to assist in cooling. Dielectric fluids with higher dielectric constants, such as electrical grade castor oil, are often used in high voltage capacitors to help prevent corona discharge and increase capacitance.
• Because dielectrics resist the flow of electricity, the surface of a dielectric may retain stranded excess electrical charges. This may occur accidentally when the dielectric is rubbed (the triboelectric effect). This can be useful, as in a Van de Graaff generator or electrophorus, or it can be potentially destructive as in the case of electrostatic discharge.
• Specially processed dielectrics, called electrets (which should not be confused with ferroelectrics), may retain excess internal charge or "frozen in" polarisation. Electrets have a semi-permanent electric field, and are the electrostatic equivalent to magnets. Electrets have numerous practical applications in the home and industry.
• Some dielectrics can generate a potential difference when subjected to mechanical stress, or (equivalently) change physical shape if an external voltage is applied across the material. This property is called piezoelectricity. Piezoelectric materials are another class of very useful dielectrics.
• Some ionic crystals and polymer dielectrics exhibit a spontaneous dipole moment, which can be reversed by an externally applied electric field. This behaviour is called the ferroelectric effect. These materials are analogous to the way ferromagnetic materials behave within an externally applied magnetic field. Ferroelectric materials often have very high dielectric constants, making them quite useful for capacitors.

Management of obesity

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Management_of_obesity

Management of obesity can include lifestyle changes, medications, or surgery. Although many studies have sought effective interventions, there is currently no evidence-based, well-defined, and efficient intervention to prevent obesity.

The main treatment for obesity consists of weight loss via healthy nutrition and increasing physical exercise. A 2007 review concluded that certain subgroups, such as those with type 2 diabetes and women who undergo weight loss, show long-term benefits in all-cause mortality, while long‐term outcomes for men are "not clear and need further investigation."

The most effective treatment for obesity is bariatric surgery. Surgery for severe obesity is associated with long-term weight loss and decreased overall mortality. One study found a weight loss of between 14% and 25% (depending on the type of procedure performed) at 10 years, and a 29% reduction in all cause mortality when compared to standard weight loss measures. Another study also found reduced mortality in those who underwent bariatric surgery for severe obesity.

In June 2021, the US Food and Drug Administration (FDA) approved semaglutide injection sold under the brand name Wegovy for long-term weight management in adults. It is associated with a loss of 6-12% body weight along with mild gastrointestinal side effects.

Another medication, orlistat, is widely available and approved for long-term use. Its use produces modest weight loss, with an average of 2.9 kg (6.4 lb) at 1 to 4 years, but there is little information on how these medications affect longer-term complications of obesity. Its use is associated with high rates of gastrointestinal side effects.

Diet programs can produce short-term weight loss and, to a lesser extent, over the long-term. Greater weight loss results, including amongst underserved populations, are achieved when proper nutrition is regularly combined with physical exercise and counseling. Dietary and lifestyle changes are effective in limiting excessive weight gain in pregnancy and improve outcomes for both the mother and the child.

Dieting

Main article: Dieting

 

Treatment selection based on BMI

Treatment	25-26.9	27-29.9	30-34.9	35-39.9	≥40
Lifestyle intervention (diet, physical activity, behavior)	Yes	Yes	Yes	Yes	Yes
Pharmacotherapy	Not appropriate	With co-morbidities	Yes	Yes	Yes
Surgery	Not appropriate	Not appropriate	Not appropriate	With co-morbidities	Yes

Diets to promote weight loss can be divided into four categories: low-fat, low-carbohydrate, low-calorie, and very low calorie. Many dietary patterns are effective. A meta-analysis of six randomized controlled trials found no difference between three of the main diet types (low calorie, low carbohydrate and low fat), with a 2–4 kilograms (4.4–8.8 lb) weight loss in all studies. At two years these three methods resulted in similar weight loss irrespective of the macronutrients emphasized. High protein diets do not appear to make any difference. A diet high in added sugars such as those in soft drinks increases weight. There is evidence that dieting alone can be effective for weight loss and improving health for obese individuals. However, a large study of adults found that obesity was associated with differences in brain structure, largely due to shared genetic factors, suggesting that interventions for obesity should not focus solely on energy content, but also take into account the neurobehavioral profile that obesity is genetically associated with.

Dieting for calorie restriction is advised for overweight individuals by the Dietary Guidelines for Americans and United Kingdom's NICE.

Exercise

See also: Physical exercise

With use, muscles consume energy derived from both fat and glycogen. Due to the large size of leg muscles, walking, running and cycling are the most effective means of exercise to reduce body fat. Exercise affects macronutrient balance. During moderate exercise, equivalent to a brisk walk, there is a shift to greater use of fat as a fuel. To maintain health, the American Heart Association recommends a minimum of 30 minutes of moderate exercise at least 5 days a week.

The Cochrane Collaboration found that exercising alone led to limited weight loss. In combination with diet, however, it resulted in a 1 kilogram weight loss over dieting alone. A 1.5 kilograms (3.3 pounds) loss was observed with a greater degree of exercise. Even though exercise as carried out in the general population has only modest effects, a dose response curve is found and very intense exercise can lead to substantial weight loss. During 20 weeks of basic military training with no dietary restriction, obese military recruits lost 12.5 kg (28 lb). High levels of physical activity seem to be necessary to maintain weight loss. A pedometer appears useful for motivation. Over an average of 18-weeks of use, physical activity increased by 27% resulting in a 0.38 decrease in BMI.

Signs that encourage the use of stairs as well as community campaigns have been shown to be effective in increasing exercise in a population. The city of Bogota, Colombia, for example, blocks off 113 kilometers (70 mi) of roads every Sunday and on holidays to make it easier for its citizens to get exercise. These pedestrian zones are part of an effort to combat chronic diseases, including obesity.

In an effort to combat the issue, a primary school in Australia instituted a standing classroom in 2013.

There is evidence that exercise alone is not sufficient to produce meaningful weight loss, but combining dieting and exercise provide the greatest health benefits and weight loss on the long term.

Weight loss programs

Weight loss programs involve lifestyle changes including diet modifications, physical activity and behavior therapy. This may involve eating smaller meals, cutting down on certain types of food and making a conscious effort to exercise more. These programs also enable people to connect with a group of others who are attempting to lose weight, in the hopes that participants will form mutually motivating and encouraging relationships. Since 2013, the United States guidelines recommend treating obesity as a disease and actively treat obese people for weight loss.

A number of popular programs exist including Weight Watchers, Overeaters Anonymous and Jenny Craig. These appear to provide modest weight loss (2.9 kg; 6.4 lb) over dieting on one's own (0.2 kg; 0.44 lb) over a two-year period, similarly to non-commercial diets. As of 2005, there was insufficient scientific evidence to determine whether Internet-based programs produce effective weight loss. The Chinese government has introduced a number of "fat farms" where obese children go for reinforced exercise and has passed a law which requires students to exercise or play sports for an hour a day at school (see Obesity in China).

In a structured setting with a trained therapist, these interventions produce an average weight loss of up to 8 kg in 6 months to 1 year, and 67% of people who lost greater than 10% of their body mass maintained or continued to lose weight one year later. There is a gradual weight regain after the first year of about 1 to 2 kg per year, but on the long-term this still results in weight loss.

Attending group meetings for weight reduction programmes rather than receiving one-on-one support may increase the likelihood that obese people will lose weight. Those who participated in groups had more treatment time and were more likely to lose enough weight to improve their health. Study authors suggested that one explanation for the difference is that group participants spent more time with the clinician (or whoever delivered the programme) than those receiving one-on-one support.

Comprehensive diet programs, providing counseling, targets for calorie intake and exercise, may be more efficient than dieting without guidance ("self-help"), although the evidence is very limited. Following comprehensive lifestyle modifications, the average maintained weight loss is more than 3 kg (6.6 lb) or 3% of total body mass, and could be sustained for five years, and up to 20% of the individuals maintain a weight loss of at least 10% (average of 33 kg). There is some evidence that fast weight loss produce greater long-term weight loss than gradual weight loss. Moderate on-site comprehensive lifestyle changes produce a greater weight loss than usual care, of 2 to 4 kg on average in 6 to 12 months. High-intensity comprehensive programs usually yield more weight loss than moderate or low-intensity, with about 35% to 60% of overweight individuals maintaining more than 5 kg weight loss after 2 years.

The NICE devised a set of essential criteria to be met by commercial weight management organizations to be approved.

The Transtheoretical Model (TTM) has been used as a framework to assist the design of lifestyle modification programmes, including weight management. A systematic review found that there is insufficient evidence to draw conclusions regarding the effects of TTM-based programs targeting weight loss that included dietary or physical activity interventions, or both (and also combined with other interventions), on sustainable weight loss (one year or longer) in overweight and obese adults. However, very low quality evidence points that this approach may induce positive changes in physical activity and dietary habits, such as increased in exercise duration and frequency, improvement in fruits and vegetables consumption, and reduced dietary fat intake.

Medication

Main article: Anti-obesity medication

Orlistat (Xenical), the most commonly used medication to treat obesity and sibutramine (Meridia), a withdrawn medication due to cardiovascular side effects

Anti-obesity medications currently approved by the FDA for weight loss

Several anti-obesity medications are currently approved by the FDA for long term use.

• Semaglutide (Wegovy) is currently approved by the FDA for long-term use, being associated with a 6-12% loss in body weight compared to placebo.
• The combination drug phentermine/topiramate (Qsymia) is approved by the FDA as an addition to a reduced-calorie diet and exercise for chronic weight management.
• Orlistat reduces intestinal fat absorption by inhibiting pancreatic lipase. Over the longer term, average weight loss on orlistat is 2.9 kg (6.4 lb). It leads to a reduced incidence of diabetes, and has some effect on cholesterol. However, there is little information on how it affects the longer-term complications or outcomes of obesity.
• Racemic amphetamine, phendimetrazine, diethylpropion, and phentermine are approved by the FDA for short term use.

Other medications

• Bupropion, topiramate, and zonisamide are sometimes used off-label for weight loss.
• The usefulness of certain drugs depends upon the comorbidities present. Metformin is preferred in overweight diabetics and for those gaining weight because taking clozapine for schizophrenia, as it may lead to mild weight loss in comparison to sulfonylureas or insulin. The thiazolidinediones, on the other hand, may cause weight gain, but decrease central obesity. Diabetics also achieve modest weight loss with fluoxetine and orlistat over 12–57 weeks.
• Rimonabant (Acomplia), another drug, had been withdrawn from the market. It worked via a specific blockade of the endocannabinoid system. It has been developed from the knowledge that cannabis smokers often experience hunger, which is often referred to as "the munchies". It had been approved in Europe for the treatment of obesity but has not received approval in the United States or Canada due to safety concerns. European Medicines Agency in October 2008 recommended the suspension of the sale of rimonabant as the risk seem to be greater than the benefits.
• Sibutramine (Meridia), which acts in the brain to inhibit deactivation of the neurotransmitters, thereby decreasing appetite was withdrawn from the UK market in January 2010 and United States and Canadian markets in October 2010 due to cardiovascular concerns. In 2010 it was found that sibutramine increases the risk of heart attacks and strokes in people with a history of cardiovascular disease.
• Fenfluramine and dexfenfluramine were withdrawn from the market in 1997, while ephedrine (found in the traditional Chinese herbal medicine má huáng made from the Ephedra sinica) was removed from the market in 2004.
• Lorcaserin used to be approved by the Food and Drug Administration for use in the treatment of obesity before being withdrawn due to cancer risk.
• Recombinant human leptin is very effective in those with obesity due to congenital complete leptin deficiency via decreasing energy intake and possibly increases energy expenditure. This condition is, however, rare and this treatment is not effective for inducing weight loss in the majority of people with obesity. It is being investigated to determine whether or not it helps with weight loss maintenance.
• Though hypothesized that supplementation of vitamin D may be an effective treatment for obesity, studies do not support this. There is also no strong evidence to recommend herbal medicines for weight loss.

Surgery

Main article: Bariatric surgery

Bariatric surgery ("weight loss surgery") is the use of surgical intervention in the treatment of obesity. As every operation may have complications, surgery is only recommended for severely obese people (BMI > 40) who have failed to lose weight following dietary modification and pharmacological treatment. Weight loss surgery relies on various principles: the two most common approaches are reducing the volume of the stomach (e.g. by adjustable gastric banding and vertical banded gastroplasty), which produces an earlier sense of satiation, and reducing the length of bowel that comes into contact with food (e.g. by gastric bypass surgery or endoscopic duodenal-jejunal bypass surgery), which directly reduces absorption. Band surgery is reversible, while bowel shortening operations are not. Some procedures can be performed laparoscopically. Complications from weight loss surgery are frequent.

Surgery for severe obesity is associated with long-term weight loss and decreased overall mortality. One study found a weight loss of between 14% and 25% (depending on the type of procedure performed) at 10 years, and a 29% reduction in all cause mortality when compared to standard weight loss measures. A marked decrease in the risk of diabetes mellitus, cardiovascular disease and cancer has also been found after bariatric surgery. Marked weight loss occurs during the first few months after surgery, and the loss is sustained in the long term. In one study there was an unexplained increase in deaths from accidents and suicide, but this did not outweigh the benefit in terms of disease prevention. When the two main techniques are compared, gastric bypass procedures are found to lead to 30% more weight loss than banding procedures one year after surgery. For obese individuals with non-alcoholic fatty liver disease (NAFLD), bariatric surgery improves or cures the liver.^[

A preoperative diet such as low-calorie diets or very-low-calorie diet, is usually recommended to reduce liver volume by 16-20%, and preoperative weight loss is the only factor associated with postoperative weight loss. Preoperative weight loss can reduce operative time and hospital stay. Although there is insufficient evidence whether preoperative weight loss may be beneficial to reduce long-term morbidity or complications. Weight loss and decreases in liver size may be independent from the amount of calorie restriction.

Ileojejunal bypass, in which the digestive tract is rerouted to bypass the small intestine, was an experimental surgery designed as a remedy for morbid obesity.

The effects of liposuction on obesity are less well determined. Some small studies show benefits while others show none. A treatment involving the placement of an intragastric balloon via gastroscopy has shown promise. One type of balloon led to a weight loss of 5.7 BMI units over 6 months or 14.7 kg (32 lb). Regaining lost weight is common after removal, however, and 4.2% of people were intolerant of the device.

An implantable nerve simulator which improves the feeling of fullness was approved by the FDA in 2015.

In 2016 the FDA approved an aspiration therapy device that siphons food from the stomach to the outside and decreases caloric intake. As of 2015 one trial shows promising results.

Health policy

Obesity is a complex public health and policy problem because of its prevalence, costs, and health effects. As such, managing it requires changes in the wider societal context and effort by communities, local authorities, and governments. Public health efforts seek to understand and correct the environmental factors responsible for the increasing prevalence of obesity in the population. Solutions look at changing the factors that cause excess food energy consumption and inhibit physical activity. Efforts include federally reimbursed meal programs in schools, limiting direct junk food marketing to children, and decreasing access to sugar-sweetened beverages in schools. The World Health Organization recommends the taxing of sugary drinks. When constructing urban environments, efforts have been made to increase access to parks and to develop pedestrian routes.

Mass media campaigns seem to have limited effectiveness in changing behaviors that influence obesity. At the same time they can increase knowledge and awareness regarding physical activity and diet, which might lead to changes in the long term. Campaigns might also be able to reduce the amount of time spent sitting or lying down and positively affect the intention to be active physically. Nutritional labelling with energy information on menus might be able to help reducing energy intake while dining in restaurants.

Clinical protocols

Much of the Western world has created clinical practice guidelines in an attempt to address rising rates of obesity. Australia, Canada, the European Union, and the United States have all published statements since 2004.

In a clinical practice guideline by the American College of Physicians, the following five recommendations are made:

1. People with a BMI of over 30 should be counseled on diet, exercise and other relevant behavioral interventions, and set a realistic goal for weight loss.
2. If these goals are not achieved, pharmacotherapy can be offered. The person needs to be informed of the possibility of side-effects and the unavailability of long-term safety and efficacy data.
3. Drug therapy may consist of sibutramine, orlistat, phentermine, diethylpropion, fluoxetine, and bupropion. Evidence is not sufficient to recommend sertraline, topiramate, or zonisamide.
4. In people with a BMI over 40 who fail to achieve their weight loss goals (with or without medication) and who develop obesity-related complications, referral for bariatric surgery may be indicated. The person needs to be aware of the potential complications.
5. Those requiring bariatric surgery should be referred to high-volume referral centers, as the evidence suggests that surgeons who frequently perform these procedures have fewer complications.

A clinical practice guideline by the US Preventive Services Task Force (USPSTF) concluded that the evidence is insufficient to recommend for or against routine behavioral counseling to promote a healthy diet in unselected people in primary care settings, but that intensive behavioral dietary counseling is recommended in those with hyperlipidemia and other known risk factors for cardiovascular and diet-related chronic disease. Intensive counseling can be delivered by primary care clinicians or by referral to other specialists, such as nutritionists or dietitians. A survey of primary care physicians in the United States found that although clinical guidelines do not consider overweight to be a risk factor that increases mortality, physicians often report believing that being overweight increases all-cause mortality.

Canada developed and published evidence-based practice guidelines in 2006. The guidelines attempt to address the prevention and management of obesity at both the individual and population levels in both children and adults. The European Union published clinical practice guidelines in 2008 in an effort to address the rising rates of obesity in Europe. Australia came out with practice guidelines in 2004.

Nuclear chemistry

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Nuclear_chemistry

Alpha decay is one type of radioactive decay, in which an atomic nucleus emits an alpha particle, and thereby transforms (or "decays") into an atom with a mass number decreased by 4 and atomic number decreased by 2.

Nuclear chemistry is the sub-field of chemistry dealing with radioactivity, nuclear processes, and transformations in the nuclei of atoms, such as nuclear transmutation and nuclear properties.

It is the chemistry of radioactive elements such as the actinides, radium and radon together with the chemistry associated with equipment (such as nuclear reactors) which are designed to perform nuclear processes. This includes the corrosion of surfaces and the behavior under conditions of both normal and abnormal operation (such as during an accident). An important area is the behavior of objects and materials after being placed into a nuclear waste storage or disposal site.

It includes the study of the chemical effects resulting from the absorption of radiation within living animals, plants, and other materials. The radiation chemistry controls much of radiation biology as radiation has an effect on living things at the molecular scale. To explain it another way, the radiation alters the biochemicals within an organism, the alteration of the bio-molecules then changes the chemistry which occurs within the organism; this change in chemistry then can lead to a biological outcome. As a result, nuclear chemistry greatly assists the understanding of medical treatments (such as cancer radiotherapy) and has enabled these treatments to improve.

It includes the study of the production and use of radioactive sources for a range of processes. These include radiotherapy in medical applications; the use of radioactive tracers within industry, science and the environment, and the use of radiation to modify materials such as polymers.

It also includes the study and use of nuclear processes in non-radioactive areas of human activity. For instance, nuclear magnetic resonance (NMR) spectroscopy is commonly used in synthetic organic chemistry and physical chemistry and for structural analysis in macro-molecular chemistry.

History

After Wilhelm Röntgen discovered X-rays in 1895, many scientists began to work on ionizing radiation. One of these was Henri Becquerel, who investigated the relationship between phosphorescence and the blackening of photographic plates. When Becquerel (working in France) discovered that, with no external source of energy, the uranium generated rays which could blacken (or fog) the photographic plate, radioactivity was discovered. Marie Skłodowska-Curie (working in Paris) and her husband Pierre Curie isolated two new radioactive elements from uranium ore. They used radiometric methods to identify which stream the radioactivity was in after each chemical separation; they separated the uranium ore into each of the different chemical elements that were known at the time, and measured the radioactivity of each fraction. They then attempted to separate these radioactive fractions further, to isolate a smaller fraction with a higher specific activity (radioactivity divided by mass). In this way, they isolated polonium and radium. It was noticed in about 1901 that high doses of radiation could cause an injury in humans. Henri Becquerel had carried a sample of radium in his pocket and as a result he suffered a highly localized dose which resulted in a radiation burn. This injury resulted in the biological properties of radiation being investigated, which in time resulted in the development of medical treatment.

Ernest Rutherford, working in Canada and England, showed that radioactive decay can be described by a simple equation (a linear first degree derivative equation, now called first order kinetics), implying that a given radioactive substance has a characteristic "half-life" (the time taken for the amount of radioactivity present in a source to diminish by half). He also coined the terms alpha, beta and gamma rays, he converted nitrogen into oxygen, and most importantly he supervised the students who conducted the Geiger–Marsden experiment (gold foil experiment) which showed that the 'plum pudding model' of the atom was wrong. In the plum pudding model, proposed by J. J. Thomson in 1904, the atom is composed of electrons surrounded by a 'cloud' of positive charge to balance the electrons' negative charge. To Rutherford, the gold foil experiment implied that the positive charge was confined to a very small nucleus leading first to the Rutherford model, and eventually to the Bohr model of the atom, where the positive nucleus is surrounded by the negative electrons.

In 1934, Marie Curie's daughter (Irène Joliot-Curie) and son-in-law (Frédéric Joliot-Curie) were the first to create artificial radioactivity: they bombarded boron with alpha particles to make the neutron-poor isotope nitrogen-13; this isotope emitted positrons. In addition, they bombarded aluminium and magnesium with neutrons to make new radioisotopes.

In the early 1920s Otto Hahn created a new line of research. Using the "emanation method", which he had recently developed, and the "emanation ability", he founded what became known as "applied radiochemistry" for the researching of general chemical and physical-chemical questions. In 1936 Cornell University Press published a book in English (and later in Russian) titled Applied Radiochemistry, which contained the lectures given by Hahn when he was a visiting professor at Cornell University in Ithaca, New York, in 1933. This important publication had a major influence on almost all nuclear chemists and physicists in the United States, the United Kingdom, France, and the Soviet Union during the 1930s and 1940s, laying the foundation for modern nuclear chemistry. Hahn and Lise Meitner discovered radioactive isotopes of radium, thorium, protactinium and uranium. He also discovered the phenomena of radioactive recoil and nuclear isomerism, and pioneered rubidium–strontium dating. In 1938, Hahn, Lise Meitner and Fritz Strassmann discovered nuclear fission, for which Hahn received the 1944 Nobel Prize for Chemistry. Nuclear fission was the basis for nuclear reactors and nuclear weapons. Hahn is referred to as the father of nuclear chemistry and godfather of nuclear fission.

Main areas

Radiochemistry is the chemistry of radioactive materials, in which radioactive isotopes of elements are used to study the properties and chemical reactions of non-radioactive isotopes (often within radiochemistry the absence of radioactivity leads to a substance being described as being inactive as the isotopes are stable).

For further details please see the page on radiochemistry.

Radiation chemistry

Radiation chemistry is the study of the chemical effects of radiation on matter; this is very different from radiochemistry as no radioactivity needs to be present in the material which is being chemically changed by the radiation. An example is the conversion of water into hydrogen gas and hydrogen peroxide. Prior to radiation chemistry, it was commonly believed that pure water could not be destroyed.

Initial experiments were focused on understanding the effects of radiation on matter. Using a X-ray generator, Hugo Fricke studied the biological effects of radiation as it became a common treatment option and diagnostic method. Fricke proposed and subsequently proved that the energy from X - rays were able to convert water into activated water, allowing it to react with dissolved species.

Chemistry for nuclear power

Radiochemistry, radiation chemistry and nuclear chemical engineering play a very important role for uranium and thorium fuel precursors synthesis, starting from ores of these elements, fuel fabrication, coolant chemistry, fuel reprocessing, radioactive waste treatment and storage, monitoring of radioactive elements release during reactor operation and radioactive geological storage, etc.

Study of nuclear reactions

See also: nuclear physics and nuclear reactions

A combination of radiochemistry and radiation chemistry is used to study nuclear reactions such as fission and fusion. Some early evidence for nuclear fission was the formation of a short-lived radioisotope of barium which was isolated from neutron irradiated uranium (¹³⁹Ba, with a half-life of 83 minutes and ¹⁴⁰Ba, with a half-life of 12.8 days, are major fission products of uranium). At the time, it was thought that this was a new radium isotope, as it was then standard radiochemical practice to use a barium sulfate carrier precipitate to assist in the isolation of radium. More recently, a combination of radiochemical methods and nuclear physics has been used to try to make new 'superheavy' elements; it is thought that islands of relative stability exist where the nuclides have half-lives of years, thus enabling weighable amounts of the new elements to be isolated. For more details of the original discovery of nuclear fission see the work of Otto Hahn.

The nuclear fuel cycle

This is the chemistry associated with any part of the nuclear fuel cycle, including nuclear reprocessing. The fuel cycle includes all the operations involved in producing fuel, from mining, ore processing and enrichment to fuel production (Front-end of the cycle). It also includes the 'in-pile' behavior (use of the fuel in a reactor) before the back end of the cycle. The back end includes the management of the used nuclear fuel in either a spent fuel pool or dry storage, before it is disposed of into an underground waste store or reprocessed.

Normal and abnormal conditions

The nuclear chemistry associated with the nuclear fuel cycle can be divided into two main areas, one area is concerned with operation under the intended conditions while the other area is concerned with maloperation conditions where some alteration from the normal operating conditions has occurred or (more rarely) an accident is occurring. Without this process, none of this would be true.

Reprocessing

Law

In the United States, it is normal to use fuel once in a power reactor before placing it in a waste store. The long-term plan is currently to place the used civilian reactor fuel in a deep store. This non-reprocessing policy was started in March 1977 because of concerns about nuclear weapons proliferation. President Jimmy Carter issued a Presidential directive which indefinitely suspended the commercial reprocessing and recycling of plutonium in the United States. This directive was likely an attempt by the United States to lead other countries by example, but many other nations continue to reprocess spent nuclear fuels. The Russian government under President Vladimir Putin repealed a law which had banned the import of used nuclear fuel, which makes it possible for Russians to offer a reprocessing service for clients outside Russia (similar to that offered by BNFL).

PUREX chemistry

The current method of choice is to use the PUREX liquid-liquid extraction process which uses a tributyl phosphate/hydrocarbon mixture to extract both uranium and plutonium from nitric acid. This extraction is of the nitrate salts and is classed as being of a solvation mechanism. For example, the extraction of plutonium by an extraction agent (S) in a nitrate medium occurs by the following reaction.

Pu⁴⁺_aq + 4NO₃⁻_aq + 2S_organic → [Pu(NO₃)₄S₂]_organic

A complex bond is formed between the metal cation, the nitrates and the tributyl phosphate, and a model compound of a dioxouranium(VI) complex with two nitrate anions and two triethyl phosphate ligands has been characterised by X-ray crystallography.

When the nitric acid concentration is high the extraction into the organic phase is favored, and when the nitric acid concentration is low the extraction is reversed (the organic phase is stripped of the metal). It is normal to dissolve the used fuel in nitric acid, after the removal of the insoluble matter the uranium and plutonium are extracted from the highly active liquor. It is normal to then back extract the loaded organic phase to create a medium active liquor which contains mostly uranium and plutonium with only small traces of fission products. This medium active aqueous mixture is then extracted again by tributyl phosphate/hydrocarbon to form a new organic phase, the metal bearing organic phase is then stripped of the metals to form an aqueous mixture of only uranium and plutonium. The two stages of extraction are used to improve the purity of the actinide product, the organic phase used for the first extraction will suffer a far greater dose of radiation. The radiation can degrade the tributyl phosphate into dibutyl hydrogen phosphate. The dibutyl hydrogen phosphate can act as an extraction agent for both the actinides and other metals such as ruthenium. The dibutyl hydrogen phosphate can make the system behave in a more complex manner as it tends to extract metals by an ion exchange mechanism (extraction favoured by low acid concentration), to reduce the effect of the dibutyl hydrogen phosphate it is common for the used organic phase to be washed with sodium carbonate solution to remove the acidic degradation products of the tributyl phosphatioloporus.

New methods being considered for future use

The PUREX process can be modified to make a UREX (URanium EXtraction) process which could be used to save space inside high level nuclear waste disposal sites, such as Yucca Mountain nuclear waste repository, by removing the uranium which makes up the vast majority of the mass and volume of used fuel and recycling it as reprocessed uranium.

The UREX process is a PUREX process which has been modified to prevent the plutonium being extracted. This can be done by adding a plutonium reductant before the first metal extraction step. In the UREX process, ~99.9% of the uranium and >95% of technetium are separated from each other and the other fission products and actinides. The key is the addition of acetohydroxamic acid (AHA) to the extraction and scrubs sections of the process. The addition of AHA greatly diminishes the extractability of plutonium and neptunium, providing greater proliferation resistance than with the plutonium extraction stage of the PUREX process.

Adding a second extraction agent, octyl(phenyl)-N,N-dibutyl carbamoylmethyl phosphine oxide (CMPO) in combination with tributylphosphate, (TBP), the PUREX process can be turned into the TRUEX (TRansUranic EXtraction) process this is a process which was invented in the US by Argonne National Laboratory, and is designed to remove the transuranic metals (Am/Cm) from waste. The idea is that by lowering the alpha activity of the waste, the majority of the waste can then be disposed of with greater ease. In common with PUREX this process operates by a solvation mechanism.

As an alternative to TRUEX, an extraction process using a malondiamide has been devised. The DIAMEX (DIAMideEXtraction) process has the advantage of avoiding the formation of organic waste which contains elements other than carbon, hydrogen, nitrogen, and oxygen. Such an organic waste can be burned without the formation of acidic gases which could contribute to acid rain. The DIAMEX process is being worked on in Europe by the French CEA. The process is sufficiently mature that an industrial plant could be constructed with the existing knowledge of the process. In common with PUREX this process operates by a solvation mechanism.

Selective Actinide Extraction (SANEX). As part of the management of minor actinides, it has been proposed that the lanthanides and trivalent minor actinides should be removed from the PUREX raffinate by a process such as DIAMEX or TRUEX. In order to allow the actinides such as americium to be either reused in industrial sources or used as fuel the lanthanides must be removed. The lanthanides have large neutron cross sections and hence they would poison a neutron-driven nuclear reaction. To date, the extraction system for the SANEX process has not been defined, but currently, several different research groups are working towards a process. For instance, the French CEA is working on a bis-triazinyl pyridine (BTP) based process.

Other systems such as the dithiophosphinic acids are being worked on by some other workers.

This is the UNiversal EXtraction process which was developed in Russia and the Czech Republic, it is a process designed to remove all of the most troublesome (Sr, Cs and minor actinides) radioisotopes from the raffinates left after the extraction of uranium and plutonium from used nuclear fuel. The chemistry is based upon the interaction of caesium and strontium with poly ethylene oxide (poly ethylene glycol) and a cobalt carborane anion (known as chlorinated cobalt dicarbollide). The actinides are extracted by CMPO, and the diluent is a polar aromatic such as nitrobenzene. Other diluents such as meta-nitrobenzotrifluoride and phenyl trifluoromethyl sulfone have been suggested as well.

Absorption of fission products on surfaces

Another important area of nuclear chemistry is the study of how fission products interact with surfaces; this is thought to control the rate of release and migration of fission products both from waste containers under normal conditions and from power reactors under accident conditions. Like chromate and molybdate, the ⁹⁹TcO₄ anion can react with steel surfaces to form a corrosion resistant layer. In this way, these metaloxo anions act as anodic corrosion inhibitors. The formation of ⁹⁹TcO₂ on steel surfaces is one effect which will retard the release of ⁹⁹Tc from nuclear waste drums and nuclear equipment which has been lost before decontamination (e.g. submarine reactors lost at sea). This ⁹⁹TcO₂ layer renders the steel surface passive, inhibiting the anodic corrosion reaction. The radioactive nature of technetium makes this corrosion protection impractical in almost all situations. It has also been shown that ⁹⁹TcO₄ anions react to form a layer on the surface of activated carbon (charcoal) or aluminium. A short review of the biochemical properties of a series of key long lived radioisotopes can be read on line.

⁹⁹Tc in nuclear waste may exist in chemical forms other than the ⁹⁹TcO₄ anion, these other forms have different chemical properties. Similarly, the release of iodine-131 in a serious power reactor accident could be retarded by absorption on metal surfaces within the nuclear plant.

Education

Despite the growing use of nuclear medicine, the potential expansion of nuclear power plants, and worries about protection against nuclear threats and the management of the nuclear waste generated in past decades, the number of students opting to specialize in nuclear and radiochemistry has decreased significantly over the past few decades. Now, with many experts in these fields approaching retirement age, action is needed to avoid a workforce gap in these critical fields, for example by building student interest in these careers, expanding the educational capacity of universities and colleges, and providing more specific on-the-job training.

Nuclear and Radiochemistry (NRC) is mostly being taught at university level, usually first at the Master- and PhD-degree level. In Europe, as substantial effort is being done to harmonize and prepare the NRC education for the industry's and society's future needs. This effort is being coordinated in a project funded by the Coordinated Action supported by the European Atomic Energy Community's 7th Framework Program. Although NucWik is primarily aimed at teachers, anyone interested in nuclear and radiochemistry is welcome and can find a lot of information and material explaining topics related to NRC.

Spinout areas

Some methods first developed within nuclear chemistry and physics have become so widely used within chemistry and other physical sciences that they may be best thought of as separate from normal nuclear chemistry. For example, the isotope effect is used so extensively to investigate chemical mechanisms and the use of cosmogenic isotopes and long-lived unstable isotopes in geology that it is best to consider much of isotopic chemistry as separate from nuclear chemistry.

Kinetics (use within mechanistic chemistry)

The mechanisms of chemical reactions can be investigated by observing how the kinetics of a reaction is changed by making an isotopic modification of a substrate, known as the kinetic isotope effect. This is now a standard method in organic chemistry. Briefly, replacing normal hydrogen (protons) by deuterium within a molecule causes the molecular vibrational frequency of X-H (for example C-H, N-H and O-H) bonds to decrease, which leads to a decrease in vibrational zero-point energy. This can lead to a decrease in the reaction rate if the rate-determining step involves breaking a bond between hydrogen and another atom. Thus, if the reaction changes in rate when protons are replaced by deuteriums, it is reasonable to assume that the breaking of the bond to hydrogen is part of the step which determines the rate.

Uses within geology, biology and forensic science

Cosmogenic isotopes are formed by the interaction of cosmic rays with the nucleus of an atom. These can be used for dating purposes and for use as natural tracers. In addition, by careful measurement of some ratios of stable isotopes it is possible to obtain new insights into the origin of bullets, ages of ice samples, ages of rocks, and the diet of a person can be identified from a hair or other tissue sample. (See Isotope geochemistry and Isotopic signature for further details).

Biology

Within living things, isotopic labels (both radioactive and nonradioactive) can be used to probe how the complex web of reactions which makes up the metabolism of an organism converts one substance to another. For instance a green plant uses light energy to convert water and carbon dioxide into glucose by photosynthesis. If the oxygen in the water is labeled, then the label appears in the oxygen gas formed by the plant and not in the glucose formed in the chloroplasts within the plant cells.

For biochemical and physiological experiments and medical methods, a number of specific isotopes have important applications.

• Stable isotopes have the advantage of not delivering a radiation dose to the system being studied; however, a significant excess of them in the organ or organism might still interfere with its functionality, and the availability of sufficient amounts for whole-animal studies is limited for many isotopes. Measurement is also difficult, and usually requires mass spectrometry to determine how much of the isotope is present in particular compounds, and there is no means of localizing measurements within the cell.
• ²H (deuterium), the stable isotope of hydrogen, is a stable tracer, the concentration of which can be measured by mass spectrometry or NMR. It is incorporated into all cellular structures. Specific deuterated compounds can also be produced.
• ¹⁵N, a stable isotope of nitrogen, has also been used. It is incorporated mainly into proteins.
• Radioactive isotopes have the advantages of being detectable in very low quantities, in being easily measured by scintillation counting or other radiochemical methods, and in being localizable to particular regions of a cell, and quantifiable by autoradiography. Many compounds with the radioactive atoms in specific positions can be prepared, and are widely available commercially. In high quantities they require precautions to guard the workers from the effects of radiation—and they can easily contaminate laboratory glassware and other equipment. For some isotopes the half-life is so short that preparation and measurement is difficult.

By organic synthesis it is possible to create a complex molecule with a radioactive label that can be confined to a small area of the molecule. For short-lived isotopes such as ¹¹C, very rapid synthetic methods have been developed to permit the rapid addition of the radioactive isotope to the molecule. For instance a palladium catalysed carbonylation reaction in a microfluidic device has been used to rapidly form amides and it might be possible to use this method to form radioactive imaging agents for PET imaging.

• ³H (tritium), the radioisotope of hydrogen, is available at very high specific activities, and compounds with this isotope in particular positions are easily prepared by standard chemical reactions such as hydrogenation of unsaturated precursors. The isotope emits very soft beta radiation, and can be detected by scintillation counting.
• ¹¹C, carbon-11 is usually produced by cyclotron bombardment of ¹⁴N with protons. The resulting nuclear reaction is ¹⁴N(p,α)¹¹C. Additionally, carbon-11 can also be made using a cyclotron; boron in the form of boric oxide is reacted with protons in a (p,n) reaction. Another alternative route is to react ¹⁰B with deuterons. By rapid organic synthesis, the ¹¹C compound formed in the cyclotron is converted into the imaging agent which is then used for PET.
• ¹⁴C, carbon-14 can be made (as above), and it is possible to convert the target material into simple inorganic and organic compounds. In most organic synthesis work it is normal to try to create a product out of two approximately equal sized fragments and to use a convergent route, but when a radioactive label is added, it is normal to try to add the label late in the synthesis in the form of a very small fragment to the molecule to enable the radioactivity to be localised in a single group. Late addition of the label also reduces the number of synthetic stages where radioactive material is used.
• ¹⁸F, fluorine-18 can be made by the reaction of neon with deuterons, ²⁰Ne reacts in a (d,⁴He) reaction. It is normal to use neon gas with a trace of stable fluorine (¹⁹F₂). The ¹⁹F₂ acts as a carrier which increases the yield of radioactivity from the cyclotron target by reducing the amount of radioactivity lost by absorption on surfaces. However, this reduction in loss is at the cost of the specific activity of the final product.

Nuclear spectroscopy

Nuclear spectroscopy are methods that use the nucleus to obtain information of the local structure in matter. Important methods are NMR (see below), Mössbauer spectroscopy and Perturbed angular correlation. These methods use the interaction of the hyperfine field with the nucleus' spin. The field can be magnetic or/and electric and are created by the electrons of the atom and its surrounding neighbours. Thus, these methods investigate the local structure in matter, mainly condensed matter in condensed matter physics and solid state chemistry.

Nuclear magnetic resonance (NMR)

NMR spectroscopy uses the net spin of nuclei in a substance upon energy absorption to identify molecules. This has now become a standard spectroscopic tool within synthetic chemistry. One major use of NMR is to determine the bond connectivity within an organic molecule.

NMR imaging also uses the net spin of nuclei (commonly protons) for imaging. This is widely used for diagnostic purposes in medicine, and can provide detailed images of the inside of a person without inflicting any radiation upon them. In a medical setting, NMR is often known simply as "magnetic resonance" imaging, as the word 'nuclear' has negative connotations for many people.