Thermal expansion

From Wikipedia, the free encyclopedia

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

 

Thermodynamics
The classical Carnot heat engine

Expansion joint in a road bridge used to avoid damage from thermal expansion.

Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.

Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, molecules begin to vibrate and move more, usually creating more distance between themselves. Substances which contract with increasing temperature are unusual, and only occur within limited temperature ranges (see examples below). The relative expansion (also called strain) divided by the change in temperature is called the material's coefficient of linear thermal expansion and generally varies with temperature. As energy in particles increases, they start moving faster and faster weakening the intermolecular forces between them, therefore expanding the substance.

Overview

Predicting expansion

If an equation of state is available, it can be used to predict the values of the thermal expansion at all the required temperatures and pressures, along with many other state functions.

Contraction effects (negative thermal expansion)

A number of materials contract on heating within certain temperature ranges; this is usually called negative thermal expansion, rather than "thermal contraction". For example, the coefficient of thermal expansion of water drops to zero as it is cooled to 3.983 °C and then becomes negative below this temperature; this means that water has a maximum density at this temperature, and this leads to bodies of water maintaining this temperature at their lower depths during extended periods of sub-zero weather. Also, fairly pure silicon has a negative coefficient of thermal expansion for temperatures between about 18 and 120 kelvin.

Factors affecting thermal expansion

Unlike gases or liquids, solid materials tend to keep their shape when undergoing thermal expansion.

Thermal expansion generally decreases with increasing bond energy, which also has an effect on the melting point of solids, so, high melting point materials are more likely to have lower thermal expansion. In general, liquids expand slightly more than solids. The thermal expansion of glasses is higher compared to that of crystals. At the glass transition temperature, rearrangements that occur in an amorphous material lead to characteristic discontinuities of coefficient of thermal expansion and specific heat. These discontinuities allow detection of the glass transition temperature where a supercooled liquid transforms to a glass.

Absorption or desorption of water (or other solvents) can change the size of many common materials; many organic materials change size much more due to this effect than due to thermal expansion. Common plastics exposed to water can, in the long term, expand by many percent.

Effect on density

Thermal expansion changes the space between particles of a substance, which changes the volume of the substance while negligibly changing its mass (the negligible amount comes from energy-mass equivalence), thus changing its density, which has an effect on any buoyant forces acting on it. This plays a crucial role in convection of unevenly heated fluid masses, notably making thermal expansion partly responsible for wind and ocean currents.

Coefficient of thermal expansion

The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure, such that lower coefficients describe lower propensity for change in size. Several types of coefficients have been developed: volumetric, area, and linear. The choice of coefficient depends on the particular application and which dimensions are considered important. For solids, one might only be concerned with the change along a length, or over some area.

The volumetric thermal expansion coefficient is the most basic thermal expansion coefficient, and the most relevant for fluids. In general, substances expand or contract when their temperature changes, with expansion or contraction occurring in all directions. Substances that expand at the same rate in every direction are called isotropic. For isotropic materials, the area and volumetric thermal expansion coefficient are, respectively, approximately twice and three times larger than the linear thermal expansion coefficient.

Mathematical definitions of these coefficients are defined below for solids, liquids, and gases.

General thermal expansion coefficient

In the general case of a gas, liquid, or solid, the volumetric coefficient of thermal expansion is given by

${\displaystyle \alpha =\alpha _{\text{V}}={\frac {1}{V}}\,\left({\frac {\partial V}{\partial T}}\right)_{p}}$

The subscript "p" to the derivative indicates that the pressure is held constant during the expansion, and the subscript V stresses that it is the volumetric (not linear) expansion that enters this general definition. In the case of a gas, the fact that the pressure is held constant is important, because the volume of a gas will vary appreciably with pressure as well as temperature. For a gas of low density this can be seen from the ideal gas

Expansion in solids

When calculating thermal expansion it is necessary to consider whether the body is free to expand or is constrained. If the body is free to expand, the expansion or strain resulting from an increase in temperature can be simply calculated by using the applicable coefficient of Thermal Expansion.

If the body is constrained so that it cannot expand, then internal stress will be caused (or changed) by a change in temperature. This stress can be calculated by considering the strain that would occur if the body were free to expand and the stress required to reduce that strain to zero, through the stress/strain relationship characterised by the elastic or Young's modulus. In the special case of solid materials, external ambient pressure does not usually appreciably affect the size of an object and so it is not usually necessary to consider the effect of pressure changes.

Common engineering solids usually have coefficients of thermal expansion that do not vary significantly over the range of temperatures where they are designed to be used, so where extremely high accuracy is not required, practical calculations can be based on a constant, average, value of the coefficient of expansion.

Linear expansion

Change in length of a rod due to thermal expansion.

Linear expansion means change in one dimension (length) as opposed to change in volume (volumetric expansion). To a first approximation, the change in length measurements of an object due to thermal expansion is related to temperature change by a coefficient of linear thermal expansion (CLTE). It is the fractional change in length per degree of temperature change. Assuming negligible effect of pressure, we may write:

${\displaystyle \alpha _{L}={\frac {1}{L}}\,{\frac {dL}{dT}}}$

where ${\displaystyle L}$ is a particular length measurement and ${\displaystyle dL/dT}$ is the rate of change of that linear dimension per unit change in temperature.

The change in the linear dimension can be estimated to be:

${\displaystyle {\frac {\Delta L}{L}}=\alpha _{L}\Delta T}$

This estimation works well as long as the linear-expansion coefficient does not change much over the change in temperature ${\displaystyle \Delta T}$, and the fractional change in length is small ${\displaystyle \Delta L/L\ll 1}$. If either of these conditions does not hold, the exact differential equation (using ${\displaystyle dL/dT}$) must be integrated.

Effects on strain

For solid materials with a significant length, like rods or cables, an estimate of the amount of thermal expansion can be described by the material strain, given by ${\displaystyle \epsilon _{\mathrm {thermal} }}$ and defined as:

${\displaystyle \epsilon _{\mathrm {thermal} }={\frac {(L_{\mathrm {final} }-L_{\mathrm {initial} })}{L_{\mathrm {initial} }}}}$

where ${\displaystyle L_{\mathrm {initial} }}$ is the length before the change of temperature and ${\displaystyle L_{\mathrm {final} }}$ is the length after the change of temperature.

For most solids, thermal expansion is proportional to the change in temperature:

${\displaystyle \epsilon _{\mathrm {thermal} }\propto \Delta T}$

Thus, the change in either the strain or temperature can be estimated by:

${\displaystyle \epsilon _{\mathrm {thermal} }=\alpha _{L}\Delta T}$

where

${\displaystyle \Delta T=(T_{\mathrm {final} }-T_{\mathrm {initial} })}$

is the difference of the temperature between the two recorded strains, measured in degrees Fahrenheit, degrees Rankine, degrees Celsius, or kelvin, and ${\displaystyle \alpha _{L}}$ is the linear coefficient of thermal expansion in "per degree Fahrenheit", "per degree Rankine", “per degree Celsius”, or “per kelvin”, denoted by °F⁻¹, R⁻¹, °C⁻¹, or K⁻¹, respectively. In the field of continuum mechanics, the thermal expansion and its effects are treated as eigenstrain and eigenstress.

Area expansion

The area thermal expansion coefficient relates the change in a material's area dimensions to a change in temperature. It is the fractional change in area per degree of temperature change. Ignoring pressure, we may write:

${\displaystyle \alpha _{A}={\frac {1}{A}}\,{\frac {dA}{dT}}}$

where ${\displaystyle A}$ is some area of interest on the object, and ${\displaystyle dA/dT}$ is the rate of change of that area per unit change in temperature.

The change in the area can be estimated as:

${\displaystyle {\frac {\Delta A}{A}}=\alpha _{A}\Delta T}$

This equation works well as long as the area expansion coefficient does not change much over the change in temperature ${\displaystyle \Delta T}$, and the fractional change in area is small ${\displaystyle \Delta A/A\ll 1}$. If either of these conditions does not hold, the equation must be integrated.

Volume expansion

For a solid, we can ignore the effects of pressure on the material, and the volumetric thermal expansion coefficient can be written:

${\displaystyle \alpha _{V}={\frac {1}{V}}\,{\frac {dV}{dT}}}$

where ${\displaystyle V}$ is the volume of the material, and ${\displaystyle dV/dT}$ is the rate of change of that volume with temperature.

This means that the volume of a material changes by some fixed fractional amount. For example, a steel block with a volume of 1 cubic meter might expand to 1.002 cubic meters when the temperature is raised by 50 K. This is an expansion of 0.2%. If we had a block of steel with a volume of 2 cubic meters, then under the same conditions, it would expand to 2.004 cubic meters, again an expansion of 0.2%. The volumetric expansion coefficient would be 0.2% for 50 K, or 0.004% K⁻¹.

If we already know the expansion coefficient, then we can calculate the change in volume

${\displaystyle {\frac {\Delta V}{V}}=\alpha _{V}\Delta T}$

where ${\displaystyle \Delta V/V}$ is the fractional change in volume (e.g., 0.002) and ${\displaystyle \Delta T}$ is the change in temperature (50 °C).

The above example assumes that the expansion coefficient did not change as the temperature changed and the increase in volume is small compared to the original volume. This is not always true, but for small changes in temperature, it is a good approximation. If the volumetric expansion coefficient does change appreciably with temperature, or the increase in volume is significant, then the above equation will have to be integrated:

${\displaystyle \ln \left({\frac {V+\Delta V}{V}}\right)=\int _{T_{i}}^{T_{f}}\alpha _{V}(T)\,dT}$

${\displaystyle {\frac {\Delta V}{V}}=\exp \left(\int _{T_{i}}^{T_{f}}\alpha _{V}(T)\,dT\right)-1}$

where ${\displaystyle \alpha _{V}(T)}$ is the volumetric expansion coefficient as a function of temperature T, and ${\displaystyle T_{i}}$,${\displaystyle T_{f}}$ are the initial and final temperatures respectively.

Isotropic materials

For isotropic materials the volumetric thermal expansion coefficient is three times the linear coefficient:

${\displaystyle \alpha _{V}=3\alpha _{L}}$

This ratio arises because volume is composed of three mutually orthogonal directions. Thus, in an isotropic material, for small differential changes, one-third of the volumetric expansion is in a single axis. As an example, take a cube of steel that has sides of length L. The original volume will be ${\displaystyle V=L^{3}}$ and the new volume, after a temperature increase, will be

${\displaystyle V+\Delta V=(L+\Delta L)^{3}=L^{3}+3L^{2}\Delta L+3L\Delta L^{2}+\Delta L^{3}\approx L^{3}+3L^{2}\Delta L=V+3V{\Delta L \over L}.}$

We can easily ignore the terms as change in L is a small quantity which on squaring gets much smaller.

So

${\displaystyle {\frac {\Delta V}{V}}=3{\Delta L \over L}=3\alpha _{L}\Delta T.}$

The above approximation holds for small temperature and dimensional changes (that is, when ${\displaystyle \Delta T}$ and ${\displaystyle \Delta L}$ are small); but it does not hold if we are trying to go back and forth between volumetric and linear coefficients using larger values of ${\displaystyle \Delta T}$. In this case, the third term (and sometimes even the fourth term) in the expression above must be taken into account.

Similarly, the area thermal expansion coefficient is two times the linear coefficient:

${\displaystyle \alpha _{A}=2\alpha _{L}}$

This ratio can be found in a way similar to that in the linear example above, noting that the area of a face on the cube is just ${\displaystyle L^{2}}$. Also, the same considerations must be made when dealing with large values of ${\displaystyle \Delta T}$.

Put more simply, if the length of a solid expands from 1 m to 1.01 m then the area expands from 1 m² to 1.0201 m² and the volume expands from 1 m³ to 1.030301 m³.

Anisotropic materials

Materials with anisotropic structures, such as crystals (with less than cubic symmetry, for example martensitic phases) and many composites, will generally have different linear expansion coefficients ${\displaystyle \alpha _{L}}$ in different directions. As a result, the total volumetric expansion is distributed unequally among the three axes. If the crystal symmetry is monoclinic or triclinic, even the angles between these axes are subject to thermal changes. In such cases it is necessary to treat the coefficient of thermal expansion as a tensor with up to six independent elements. A good way to determine the elements of the tensor is to study the expansion by x-ray powder diffraction. The thermal expansion coefficient tensor for the materials possessing cubic symmetry (for e.g. FCC, BCC) is isotropic.

Isobaric expansion in gases

For an ideal gas, the volumetric thermal expansion (i.e., relative change in volume due to temperature change) depends on the type of process in which temperature is changed. Two simple cases are constant pressure (an isobaric process) and constant volume (an isochoric process).

The derivative of the ideal gas law, ${\displaystyle pV=T}$, is

${\displaystyle pdV+Vdp=dT}$

where ${\displaystyle p}$ is the pressure, ${\displaystyle V}$ is the specific volume, and ${\displaystyle T}$ is temperature measured in energy units.

By the definition of an isobaric thermal expansion, we have ${\displaystyle dp=0}$, so that ${\displaystyle pdV=dT}$, and the isobaric thermal expansion coefficient is:

${\displaystyle \alpha _{p}\equiv {\frac {1}{V}}\left({\frac {dV}{dT}}\right)={\frac {1}{V}}\left({\frac {1}{p}}\right)={\frac {1}{pV}}={\frac {1}{T}}}$.

Similarly, if the volume is held constant, that is if ${\displaystyle dV=0}$, we have ${\displaystyle Vdp=dT}$, so that the isochoric thermal expansion coefficient is

${\displaystyle \alpha _{V}\equiv {\frac {1}{p}}\left({\frac {dp}{dT}}\right)={\frac {1}{p}}\left({\frac {1}{V}}\right)={\frac {1}{pV}}={\frac {1}{T}}}$.

Expansion in liquids

Theoretically, the coefficient of linear expansion can be found from the coefficient of volumetric expansion (α_V ≈ 3α_L). For liquids, α_L is calculated through the experimental determination of α_V. Liquids, unlike solids have no definite shape and they take the shape of the container. Consequently, liquids have no definite length and area, so linear and areal expansions of liquids have no significance.

Liquids in general, expand on heating. However water is an exception to this general behaviour: below 4 °C it contracts on heating. For higher temperature it shows the normal positive thermal expansion. The thermal expansion of liquids is usually higher than in solids because of weak intermolecular forces present in liquids.

Thermal expansion of solids usually shows little dependence on temperature, except at low temperatures, whereas liquids expand at different rates at different temperatures.

Apparent and absolute expansion of a liquid

The expansion of liquids is usually measured in a container. When a liquid expands in a vessel, the vessel expands along with the liquid. Hence the observed increase in volume of the liquid level is not actual increase in its volume. The expansion of the liquid relative to the container is called its apparent expansion, while the actual expansion of the liquid is called real expansion or absolute expansion. The ratio of apparent increase in volume of the liquid per unit rise of temperature to the original volume is called its coefficient of apparent expansion.

For small and equal rises in temperature, the increase in volume (real expansion) of a liquid is equal to the sum of the apparent increase in volume (apparent expansion) of the liquid and the increase in volume of the containing vessel. Thus a liquid has two coefficients of expansion.

Measurement of the expansion of a liquid must account for the expansion of the container as well. For example, when a flask with a long narrow stem, containing enough liquid to partially fill the stem itself, is placed in a heat bath, the height of the liquid column in the stem will initially drop, followed immediately by a rise of that height until the whole system of flask, liquid and heat bath has warmed through. The initial drop in the height of the liquid column is not due to an initial contraction of the liquid, but rather to the expansion of the flask as it contacts the heat bath first. Soon after, the liquid in the flask is heated by the flask itself and begins to expand. Since liquids typically have a greater expansion over solids, the expansion of the liquid in the flask eventually exceeds that of the flask, causing the level of liquid in the flask to rise. A direct measurement of the height of the liquid column is a measurement of the apparent expansion of the liquid. The absolute expansion of the liquid is the apparent expansion corrected for the expansion of the containing vessel.

Examples and applications

Thermal expansion of long continuous sections of rail tracks is the driving force for rail buckling. This phenomenon resulted in 190 train derailments during 1998–2002 in the US alone.

The expansion and contraction of the materials must be considered when designing large structures, when using tape or chain to measure distances for land surveys, when designing molds for casting hot material, and in other engineering applications when large changes in dimension due to temperature are expected.

Thermal expansion is also used in mechanical applications to fit parts over one another, e.g. a bushing can be fitted over a shaft by making its inner diameter slightly smaller than the diameter of the shaft, then heating it until it fits over the shaft, and allowing it to cool after it has been pushed over the shaft, thus achieving a 'shrink fit'. Induction shrink fitting is a common industrial method to pre-heat metal components between 150 °C and 300 °C thereby causing them to expand and allow for the insertion or removal of another component.

There exist some alloys with a very small linear expansion coefficient, used in applications that demand very small changes in physical dimension over a range of temperatures. One of these is Invar 36, with expansion approximately equal to 0.6×10⁻⁶ K⁻¹. These alloys are useful in aerospace applications where wide temperature swings may occur.

Pullinger's apparatus is used to determine the linear expansion of a metallic rod in the laboratory. The apparatus consists of a metal cylinder closed at both ends (called a steam jacket). It is provided with an inlet and outlet for the steam. The steam for heating the rod is supplied by a boiler which is connected by a rubber tube to the inlet. The center of the cylinder contains a hole to insert a thermometer. The rod under investigation is enclosed in a steam jacket. One of its ends is free, but the other end is pressed against a fixed screw. The position of the rod is determined by a micrometer screw gauge or spherometer.

To determine the coefficient of linear thermal expansion of a metal, a pipe made of that metal is heated by passing steam through it. One end of the pipe is fixed securely and the other rests on a rotating shaft, the motion of which is indicated by a pointer. A suitable thermometer records the pipe's temperature. This enables calculation of the relative change in length per degree temperature change.

Drinking glass with fracture due to uneven thermal expansion after pouring of hot liquid into the otherwise cool glass

The control of thermal expansion in brittle materials is a key concern for a wide range of reasons. For example, both glass and ceramics are brittle and uneven temperature causes uneven expansion which again causes thermal stress and this might lead to fracture. Ceramics need to be joined or work in concert with a wide range of materials and therefore their expansion must be matched to the application. Because glazes need to be firmly attached to the underlying porcelain (or other body type) their thermal expansion must be tuned to 'fit' the body so that crazing or shivering do not occur. Good example of products whose thermal expansion is the key to their success are CorningWare and the spark plug. The thermal expansion of ceramic bodies can be controlled by firing to create crystalline species that will influence the overall expansion of the material in the desired direction. In addition or instead the formulation of the body can employ materials delivering particles of the desired expansion to the matrix. The thermal expansion of glazes is controlled by their chemical composition and the firing schedule to which they were subjected. In most cases there are complex issues involved in controlling body and glaze expansion, so that adjusting for thermal expansion must be done with an eye to other properties that will be affected, and generally trade-offs are necessary.

Thermal expansion can have a noticeable effect on gasoline stored in above-ground storage tanks, which can cause gasoline pumps to dispense gasoline which may be more compressed than gasoline held in underground storage tanks in winter, or less compressed than gasoline held in underground storage tanks in summer.

Expansion loop on heating pipeline

Heat-induced expansion has to be taken into account in most areas of engineering. A few examples are:

• Metal-framed windows need rubber spacers.
• Rubber tires need to perform well over a range of temperatures, being passively heated or cooled by road surfaces and weather, and actively heated by mechanical flexing and friction.
• Metal hot water heating pipes should not be used in long straight lengths.
• Large structures such as railways and bridges need expansion joints in the structures to avoid sun kink.
• One of the reasons for the poor performance of cold car engines is that parts have inefficiently large spacings until the normal operating temperature is achieved.
• A gridiron pendulum uses an arrangement of different metals to maintain a more temperature stable pendulum length.
• A power line on a hot day is droopy, but on a cold day it is tight. This is because the metals expand under heat.
• Expansion joints absorb the thermal expansion in a piping system.
• Precision engineering nearly always requires the engineer to pay attention to the thermal expansion of the product. For example, when using a scanning electron microscope small changes in temperature such as 1 degree can cause a sample to change its position relative to the focus point.
• Liquid thermometers contain a liquid (usually mercury or alcohol) in a tube, which constrains it to flow in only one direction when its volume expands due to changes in temperature.
• A bi-metal mechanical thermometer uses a bimetallic strip and bends due to the differing thermal expansion of the two metals.

Thermal expansion coefficients for various materials

Volumetric thermal expansion coefficient for a semicrystalline polypropylene.

 

Linear thermal expansion coefficient for some steel grades.

This section summarizes the coefficients for some common materials.

For isotropic materials the coefficients linear thermal expansion α and volumetric thermal expansion α_V are related by α_V = 3α. For liquids usually the coefficient of volumetric expansion is listed and linear expansion is calculated here for comparison.

For common materials like many metals and compounds, the thermal expansion coefficient is inversely proportional to the melting point. In particular, for metals the relation is:

${\displaystyle \alpha \approx {\frac {0.020}{T_{m}}}}$

for halides and oxides

${\displaystyle \alpha \approx {\frac {0.038}{T_{m}}}-7.0\cdot 10^{-6}\,\mathrm {K} ^{-1}}$

In the table below, the range for α is from 10⁻⁷ K⁻¹ for hard solids to 10⁻³ K⁻¹ for organic liquids. The coefficient α varies with the temperature and some materials have a very high variation; see for example the variation vs. temperature of the volumetric coefficient for a semicrystalline polypropylene (PP) at different pressure, and the variation of the linear coefficient vs. temperature for some steel grades (from bottom to top: ferritic stainless steel, martensitic stainless steel, carbon steel, duplex stainless steel, austenitic steel). The highest linear coefficient in a solid has been reported for a Ti-Nb alloy.

(The formula α_V ≈ 3α is usually used for solids.)

Material	Linear coefficient CLTE α at 20 °C (x10−6 K−1)	Volumetric coefficient αV at 20 °C (x10−6 K−1)	Notes
Aluminium	23.1	69
Brass	19	57
Carbon steel	10.8	32.4
CFRP	– 0.8	Anisotropic	Fiber direction
Concrete	12	36
Copper	17	51
Diamond	1	3
Ethanol	250	750
Gasoline	317	950
Glass	8.5	25.5
Glass, borosilicate	3.3	9.9	matched sealing partner for tungsten, molybdenum and kovar.
Glycerine		485
Gold	14	42
Ice	51
Invar	1.2	3.6
Iron	11.8	35.4
Kapton	20	60	DuPont Kapton 200EN
Lead	29	87
Macor	9.3
Nickel	13	39
Oak	54		Perpendicular to the grain
Douglas-fir	27	75	radial
Douglas-fir	45	75	tangential
Douglas-fir	3.5	75	parallel to grain
Platinum	9	27
Polypropylene (PP)	150	450
PVC	52	156
Fused quartz	0.59	1.77
alpha-Quartz	12-16/6-9		Parallel to a-axis/c-axis T = -50 to 150 C
Rubber	disputed	disputed
Sapphire	5.3		Parallel to C axis, or [001]
Silicon Carbide	2.77	8.31
Silicon	2.56	9
Silver	18	54
Glass-ceramic "Sitall"	0±0.15	0±0.45	average for −60 °C to 60 °C
Stainless steel	10.1 ~ 17.3	30.3 ~ 51.9
Steel	11.0 ~ 13.0	33.0 ~ 39.0	Depends on composition
Titanium	8.6	26
Tungsten	4.5	13.5
Water	69	207
Glass-ceramic "Zerodur"	≈0.007-0.1		at 0...50 °C
ALLVAR Alloy 30	−30	anisotropic	at 20 °C

Effects of climate change on marine mammals

From Wikipedia, the free encyclopedia

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

The effect of climate change on marine life and mammals is a growing concern. Many of the effects of global warming are currently unknown due to unpredictability, but many are becoming increasingly evident today. Some effects are very direct such as loss of habitat, temperature stress, and exposure to severe weather. Other effects are more indirect, such as changes in host pathogen associations, changes in body condition because of predator–prey interaction, changes in exposure to toxins and CO
2 emissions, and increased human interactions. Despite the large potential impacts of ocean warming on marine mammals, the global vulnerability of marine mammals to global warming is still poorly understood.

It has been generally assumed that the Arctic marine mammals were the most vulnerable in the face of climate change given the substantial observed and projected decline in Arctic sea ice cover. However, the implementation of a trait-based approach on assessment of the vulnerability of all marine mammals under future global warming has suggested that the North Pacific Ocean, the Greenland Sea and the Barents Sea host the species that are most vulnerable to global warming. The North Pacific has already been identified as a hotspot for human threats for marine mammals and now is also a hotspot of vulnerability to global warming. This emphasizes that marine mammals in this region will face double jeopardy from both human activities (e.g., marine traffic, pollution and offshore oil and gas development) and global warming, with potential additive or synergetic effect and as a result, these ecosystems face irreversible consequences for marine ecosystem functioning. Consequently the future conservation plans should therefore focus on these regions.

Potential effects

Marine mammals have evolved to live in oceans, but climate change is affecting their natural habitat. Some species may not adapt fast enough, which might lead to their extinction.

Ocean warming

The illustration of temperature changes from 1960 to 2019 across each ocean starting at the Southern Ocean around Antarctica (Cheng et. al., 2020)

During the last century, the global average land and sea surface temperature has increased due to an increased greenhouse effect from human activities. From 1960 to through 2019, the average temperature for the upper 2000 meters of the oceans has increased by 0.12 degree Celsius, whereas the ocean surface has warmed up to 1.2 degree Celsius from the pre-industrial era.

Marine organisms usually tend to encounter relatively stable temperatures compared with terrestrial species and thus are likely to be more sensitive to temperature change than terrestrial organisms. Therefore, the ocean warming will lead to increased species migration, as endangered species look for a more suitable habitat. If sea temperatures continue to rise, then some fauna may move to cooler water and some range-edge species may disappear from regional waters or experienced a reduced global range. Change in the abundance of some species will alter the food resources available to marine mammals, which then results in marine mammals’ biogeographic shifts. Additionally, if a species cannot successfully migrate to a suitable environment, unless it learns to adapt to rising ocean temperatures, it will face extinction.

Sea level rise is also important when assessing the impacts of global warming on marine mammals, since it affects coastal environments that marine mammals species rely.

Primary productivity

Changes in temperatures will impact the location of areas with high primary productivity. Primary producers, such as plankton, are the main food source for marine mammals such as some whales. Species migration will therefore be directly affected by locations of high primary productivity. Water temperature changes also affect ocean turbulence, which has a major impact on the dispersion of plankton and other primary producers. Due to global warming and increased glacier melt, thermohaline circulation patterns may be altered by increasing amounts of freshwater released into oceans and, therefore, changing ocean salinity. Thermohaline circulation is responsible for bringing up cold, nutrient-rich water from the depths of the ocean, a process known as upwelling.

Ocean acidification

Change in pH since the beginning of the industrial revolution. RCP 2.6 scenario is "low CO₂ emissions" . RCP 8.5 scenario is "high CO₂ emissions", the path we are currently on. Source: J. P. Gattuso et al., 2015

About a quarter of the emitted CO_2, about 26 million tons is absorbed by the ocean every day. Consequently, the dissolution of anthropogenic carbon dioxide (CO₂) in seawater causes a decrease in pH which is corresponding to an increase in acidity of the oceans with consequences for marine biota.  Since the beginning of the industrial revolution, ocean acidity has increased by 30% (the pH decreased from 8.2 to 8.1). It is projected that the ocean will experience severe acidification under RCP 8.5, high CO₂ emission scenario, and less intense acidification under RCP 2.6, low CO₂ emission scenario. Ocean acidification will impact marine organisms (corals, mussels, oysters) in producing their limestone skeleton or shell. When CO₂ dissolves in seawater, it increases protons (H+ ions) but reduces certain molecules, such as carbonate ions in which many oysters needed to produce their limestone skeleton or shell. The shell and the skeleton of these species may become less dense or strong. This also may make coral reefs become more vulnerable to storm damage, and slow down its recovery. In addition, marine organisms may experience changes in growth, development, abundance, and survival in response to ocean acidification

Sea ice changes

Sea ice, a defining characteristic of polar marine environment, is changing rapidly which has impacts on marine mammals. Climate change models predict changes to the sea ice leading to loss of the sea ice habitat, elevations of water and air temperature, and increased occurrence of severe weather. The loss of sea ice habitat will reduced the abundance of seal prey for marine mammals, particularly polar bears. Initially, polar bears may be favored by an increase in leads in the ice that make more suitable seal habitat available but, as the ice thins further, they will have to travel more, using energy to keep in contact with favored habitat. There also may be some indirect effect of sea ice changes on animal heath due to alterations in pathogen transmission, effect on animals on body condition caused by shift in the prey based/food web, changes in toxicant exposure associated with increased human habitation in the Arctic habitat.

Increase frequency of Hypoxia Occurrence in the entire Baltic Sea calculated as the number of profiles with recorded hypoxia relative to the total number of profiles (Conley et. al., 2011)

Hypoxia

Hypoxia occurs in the variety of coastal environment when the dissolved of oxygen (DO) is depleted to a certain low level, where aquatic organisms, especially benthic fauna, become stressed or die due to the lack of oxygen.  Hypoxia occurs when the coastal region enhance Phosphorus release from sediment and increase Nitrate (N) loss. This chemical scenario supports favorable growth for cyanobacteria which contribute to the hypoxia and ultimately sustain eutrophication.  Hypoxia degrades an ecosystem by damaging the bottom fauna habitats, altering the food web, changing the nitrogen and phosphate cycling, decreasing fishery catch, and enhancing the water acidification.  There were 500 areas in the world with reported coastal hypoxia in 2011, with Baltic Sea contains the largest hypoxia zone in the world. These numbers are expected to increase due to the worsening condition of coastal areas caused by the excessive anthropogenic nutrient loads that stimulate intensified eutrophication.  The rapidly changing climate in particularly, global warming, also contributes to the increase of Hypoxia occurrence that damaging marine mammals and marine/coastal ecosystem.

Species impacted

Polar bears

A polar bear waiting in the Fall for the sea ice to form.

Polar bears are one of many Arctic marine mammals at risk of population decline due to climate change. When carbon dioxide is released into the atmosphere, a greenhouse like effect occurs, warming the climate. For polar bears and other Arctic marine mammals, rising temperature is the changing the sea ice formations that they rely on to survive. In the circumpolar north, the Arctic sea ice is a dynamic ecosystem. The levels of sea ice extent varies by season. While some areas maintain year-round ice, others only have ice on a seasonal basis. The amount of permanent sea ice is decreasing with global temperature increases. Climate change is causing slower formations of sea ice, quicker decline and thinner ice sheets. Polar bears and other Arctic marine mammals are losing their habitat and food sources in result of the sea ice decline.

Polar bears rely on seals as their main food source. Although polar bears are strong swimmers, they are not successful at catching seal underwater, therefore polar bears are ambush predators. When they hunt seals, they wait at seal breathing hole to ambush and haul out their prey onto the sea ice for feeding. With slower sea ice formations, thinner ice sheets and shorter winter seasons, polar bears are having less opportunity for optimal hunting grounds. Polar bears are facing pressures to swim further to gain access to food. This requires more calories spent to obtain calories to sustain their body conditions for reproduction and survival. Researchers use body condition charts to track polar bear population health and reproductive potential. Trends suggest 12 out of 19 sub populations of polar bears are declining or data deficient.

Polar bears also rely on sea ice to travel, mate and female polar bears usually choose to den up on the sea ice during denning season. The sea ice is becoming less stable, forcing pregnant female polar bears to choose less optimal locations for denning. These aspects are known to result in lower reproduction rates and smaller cub years.

Dolphins

Dolphins are marine mammals with broad geographic extent, making them susceptible to climate change in various ways. The most common effect of climate change on dolphins is the increasing water temperatures across the globe. This has caused a large variety of dolphin species to experience range shifts, in which the species move from their typical geographic region to warmer waters.

In California, the 1982-83 El Niño warming event caused the near-bottom spawning market squid to leave southern California, which caused their predator, the pilot whale, to also leave. As the market squid returned six years later, Risso's dolphins came to feed on the squid. Bottlenose dolphins expanded their range from southern to central California, and stayed even after the warming event subsided. The Pacific white-sided dolphin has had a decline in population in the southwest Gulf of California, the southern boundary of their distribution. In the 1980s they were abundant with group sizes up to 200 across the entire cool season. Then, in the 2000s, only two groups were recorded with sizes of 20 and 30, and only across the central cool season. This decline was not related to a decline of other marine mammals or prey, so it was concluded to have been caused by climate change as it occurred during a period of warming. Additionally, the Pacific white-sided dolphin had an increase in occurrence on the west coast of Canada from 1984 to 1998. 

In the Mediterranean, sea surface temperatures have increased, as well as salinity, upwelling intensity, and sea levels. Because of this, prey resources have been reduced causing a steep decline in the short-beaked common dolphin Mediterranean subpopulation, which was deemed endangered in 2003. This species now only exists in the Alboran Sea, due to its high productivity, distinct ecosystem, and differing conditions from the rest of the Mediterranean.

In northwest Europe, many dolphin species have experienced range shifts from the region’s typically colder waters. Warm water dolphins, like the short-beaked common dolphin and striped dolphin, have expanded north of western Britain and into the northern North Sea, even in the winter, which may displace the white-beaked and Atlantic white-sided dolphin that are in that region. The white-beaked dolphin has shown an increase in the southern North Sea since the 1960s because of this. The rough-toothed dolphin and Atlantic spotted dolphin may move to northwest Europe. In northwest Scotland, white-beaked dolphins (local to the colder waters of the North Atlantic) have decreased while common dolphins (local to warmer waters) have increased from 1992-2003. Additionally, Fraser’s dolphin, found in tropical waters, was recorded in the UK for the first time in 1996.

River dolphins are highly affected by climate change as high evaporation rates, increased water temperatures, decreased precipitation, and increased acidification occur. River dolphins typically have a higher densities when rivers have a lox index of freshwater degradation and better water quality. Specifically looking at the Ganges river dolphin, the high evaporation rates and increased flooding on the plains may lead to more human river regulation, decreasing the dolphin population.

As warmer waters lead to a decrease in dolphin prey, this led to other causes of dolphin population decrease. In the case of bottlenose dolphins, mullet populations decrease due to increasing water temperatures, which leads to a decrease in the dolphins’ health and thus their population. At the Shark Bay World Heritage Area in Western Australia, the local Indo-Pacific bottlenose dolphin population had a significant decline after a marine heatwave in 2011. This heatwave caused a decrease in prey, which led to a decline in dolphin reproductive rates as female dolphins could not get enough nutrients to sustain a calf. The resultant decrease in fish population due to warming waters has also influenced humans to see dolphins as fishing competitors or even bait. Humans use dusky dolphins as bait or are killed off because they consume the same fish humans eat and sell for profit. In the central Brazilian Amazon alone, approximately 600 pink river dolphins are killed each year to be used as bait. Another side effect of increasing water temperatures is the increase in toxic algae blooms, which has caused a mass die-off of bottlenose dolphins.

Sabatier reaction

From Wikipedia, the free encyclopedia

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

 

Paul Sabatier (1854-1941) winner of the Nobel Prize in Chemistry in 1912 and discoverer of the reaction in 1897

The Sabatier reaction or Sabatier process produces methane and water from a reaction of hydrogen with carbon dioxide at elevated temperatures (optimally 300–400 °C) and pressures (perhaps 30 bar) in the presence of a nickel catalyst. It was discovered by the French chemists Paul Sabatier and Jean-Baptiste Senderens in 1897. Optionally, ruthenium on alumina (aluminium oxide) makes a more efficient catalyst. It is described by the following exothermic reaction.

${\displaystyle {\ce {CO2{}+4H2->[{} \atop 400\ ^{\circ }{\ce {C}}][{\ce {pressure}}]CH4{}+2H2O}}}$ ∆H = −165.0 kJ/mol

There is disagreement on whether the CO₂ methanation occurs by first associatively adsorbing an adatom hydrogen and forming oxygen intermediates before hydrogenation or dissociating and forming a carbonyl before being hydrogenated.

${\displaystyle {\ce {{CO}+ 3H2 -> {CH4}+ H2O}}}$ ∆H = −206 kJ/mol

CO methanation is believed to occur through a dissociative mechanism where the carbon oxygen bond is broken before hydrogenation with an associative mechanism only being observed at high H₂ concentrations.

Methanation reaction over different carried metal catalysts including Ni, Ru and Rh has been widely investigated for the production of CH₄ from syngas and other power to gas initiatives. Nickel is the most widely used catalyst due to its high selectivity and low cost.

Applications

Creation of synthetic natural gas

Methanation is an important step in the creation of synthetic or substitute natural gas (SNG). Coal or wood undergo gasification which creates a producer gas that must undergo methaneation in order to produce a usable gas that just needs to undergo a final purification step.

The first commercial synthetic gas plant opened in 1984 and is the Great Plains Synfuel plant in Beulah, North Dakota. It is still operational and produces 1500 MW worth of SNG using coal as the carbon source. In the years since its opening, other commercial facilities have been opened using other carbon sources such as wood chips.

In France, the AFUL Chantrerie, located in Nantes, started in November 2017 the demonstrator MINERVE. This methanation unit of 14 Nm3 / day was carried out by Top Industrie, with the support of Leaf. This installation is used to feed a CNG station and to inject methane into the natural gas boiler.

It has been seen in a renewable-energy-dominated energy system to use the excess electricity generated by wind, solar photovoltaic, hydro, marine current, etc. to make hydrogen via water electrolysis and the subsequent application of the Sabatier reaction to make methane In contrast to a direct usage of hydrogen for transport or energy storage applications, the methane can be injected into the existing gas network, which in many countries has one to two years of storage capacity. The methane can then be used on demand to generate electricity (and heat—combined heat and power) overcoming low points of renewable energy production. The process is electrolysis of water by electricity to create hydrogen (which can partly be used directly in fuel cells) and the addition of carbon dioxide CO₂ (Sabatier process) to create methane. The CO₂ can be extracted from the air or fossil fuel waste gases by the amine process, amongst many others. It is a low-CO₂ system, and has similar efficiencies of today's energy system.

A 6 MW power-to-gas plant went into production in Germany in 2013, and powered a fleet of 1500 Audi A3s.

Ammonia synthesis

In ammonia production CO and CO₂ are considered poisons to most commonly used catalysts. Methanation catalysts are added after several hydrogen producing steps to prevent carbon oxide buildup in the ammonia synthesis loop as methane does not have similar adverse effects on ammonia synthesis rates.

International Space Station life support

Oxygen generators on board the International Space Station produce oxygen from water using electrolysis; the hydrogen produced was previously discarded into space. As astronauts consume oxygen, carbon dioxide is produced, which must then be removed from the air and discarded as well. This approach required copious amounts of water to be regularly transported to the space station for oxygen generation in addition to that used for human consumption, hygiene, and other uses—a luxury that will not be available to future long-duration missions beyond low Earth orbit.

NASA is using the Sabatier reaction to recover water from exhaled carbon dioxide and the hydrogen previously discarded from electrolysis on the International Space Station and possibly for future missions. The other resulting chemical, methane, is released into space. As half of the input hydrogen becomes wasted as methane, additional hydrogen is supplied from Earth to make up the difference. However, this creates a nearly-closed cycle between water, oxygen, and carbon dioxide which only requires a relatively modest amount of imported hydrogen to maintain.

Ignoring other results of respiration, this cycle looks like:

${\displaystyle {\ce {2H2O->[{\text{electrolysis}}]O2{}+2H2->[{\text{respiration}}]CO2{}+2H2{}+{\overset {added}{2H2}}->2H2O{}+{\overset {discarded}{CH4}}}}}$

The loop could be further closed if the waste methane was separated into its component parts by pyrolysis, the high efficiency (up to 95% conversion) of which can be achieved at 1200 °C:

${\displaystyle {\ce {CH4->[{\text{heat}}]C{}+2H2}}}$

The released hydrogen would then be recycled back into the Sabatier reactor, leaving an easily removed deposit of pyrolytic graphite. The reactor would be little more than a steel pipe, and could be periodically serviced by an astronaut where the deposit is chiselled out.

Alternatively, the loop could be partially closed (75% of H₂ from CH₄ recovered) by incomplete pyrolysis of the waste methane while keeping the carbon locked up in gaseous form as acetylene:

${\displaystyle {\ce {2CH4->[{\text{heat}}]C2H2{}+3H2}}}$

The Bosch reaction is also being investigated by NASA for this purpose and is:

${\displaystyle {\ce {CO2 + 2H2 -> C + 2H2O}}}$

The Bosch reaction would present a completely closed hydrogen and oxygen cycle which only produces atomic carbon as waste. However, difficulties maintaining its temperature of up to 600 °C and properly handling carbon deposits mean significantly more research will be required before a Bosch reactor could become a reality. One problem is that the production of elemental carbon tends to foul the catalyst's surface (coking), which is detrimental to the reaction's efficiency.

Manufacturing propellant on Mars

The Sabatier reaction has been proposed as a key step in reducing the cost of human mission to Mars (Mars Direct, SpaceX Starship) through in-situ resource utilization. Hydrogen is combined with CO₂ from the atmosphere, with methane then stored as fuel and the water side product electrolyzed yielding oxygen to be liquefied and stored as oxidizer and hydrogen to be recycled back into the reactor. The original hydrogen could be transported from Earth or separated from Martian sources of water.

Importing hydrogen

Importing a small amount of hydrogen avoids searching for water and just uses CO₂ from the atmosphere.

"A variation of the basic Sabatier methanation reaction may be used via a mixed catalyst bed and a reverse water gas shift in a single reactor to produce methane from the raw materials available on Mars, utilising carbon dioxide in the Martian atmosphere. A 2011 prototype test operation that harvested CO₂ from a simulated Martian atmosphere and reacted it with H₂, produced methane rocket propellant at a rate of 1 kg/day, operating autonomously for 5 consecutive days, maintaining a nearly 100% conversion rate. An optimised system of this design massing 50 kg "is projected to produce 1 kg/day of O₂:CH₄ propellant ... with a methane purity of 98+% while consuming ~17 kWh per day of electrical power (at a continuous power of 700 W). Overall unit conversion rate expected from the optimised system is one tonne of propellant per 17 MWh energy input."

Stoichiometry issue with importing hydrogen

The stoichiometric ratio of oxidiser and fuel is 2:1, for an oxygen:methane engine:

${\displaystyle {\ce {CH4 + 2O2 -> CO2 + 2H2O}}}$

However, one pass through the Sabatier reactor produces a ratio of only 1:1. More oxygen may be produced by running the water-gas shift reaction (WGSR) in reverse (RWGS), effectively extracting oxygen from the atmosphere by reducing carbon dioxide to carbon monoxide.

Another option is to make more methane than needed and pyrolyze the excess of it into carbon and hydrogen (see above section), where the hydrogen is recycled back into the reactor to produce further methane and water. In an automated system, the carbon deposit may be removed by blasting with hot Martian CO₂, oxidizing the carbon into carbon monoxide (via the Boudouard reaction), which is vented.

A fourth solution to the stoichiometry problem would be to combine the Sabatier reaction with the reverse water-gas shift (RWGS) reaction in a single reactor as follows:

${\displaystyle {\ce {3CO2 + 6H2 -> CH4 + 2CO + 4H2O}}}$

This reaction is slightly exothermic, and when the water is electrolyzed, an oxygen to methane ratio of 2:1 is obtained.

Regardless of which method of oxygen fixation is utilized, the overall process can be summarized by the following equation:

${\displaystyle {\ce {2H2 + 3CO2 -> CH4 + 2O2 + 2CO}}}$

Looking at molecular masses, we have produced 16 grams of methane and 64 grams of oxygen using 4 grams of hydrogen (which would have to be imported from Earth, unless Martian water was electrolysed), for a mass gain of 20:1; and the methane and oxygen are in the right stoichiometric ratio to be burned in a rocket engine. This kind of in-situ resource utilization would result in massive weight and cost savings to any proposed manned Mars or sample-return missions.