Thermal expansion - Citizendia

Material Properties
Specific heat	$c=\frac{T}{N}\left(\frac{\partial S}{\partial T}\right)$
Compressibility	$\beta=-\frac{1}{V}\left(\frac{\partial V}{\partial p}\right)$
Thermal expansion	$\alpha=\frac{1}{V}\left(\frac{\partial V}{\partial T}\right)$

Thermal Expansion is the tendency of matter to change in volume in response to a change in temperature. The thermodynamic properties of materials are intensive thermodynamic parameters which are specific to a given material Specific heat capacity, also known simply as specific heat, is the measure of the heat energy required to increase the Temperature of a unit quantity In Thermodynamics and Fluid mechanics, compressibility is a measure of the relative volume change of a Fluid or Solid as a response The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically When a substance is heated, its constituent particles move around more vigorously and by doing so generally maintain a greater average separation. Materials that contract with an increase in temperature are very uncommon; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature. When the Temperature of a substance changes the energy that is stored in the Intermolecular bonds between atoms changes

Common engineering solids usually have thermal expansion coefficients 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, calculations can be based on a constant, average, value of the coefficient of expansion.

Materials with anisotropic structures, such as crystals and composites, will generally have different expansion coefficients in different orientations. Anisotropy (pronounced with stress on the third syllable ˌænaɪˈsɒtrəpi is the property of being directionally dependent as opposed to Isotropy, which means homogeneity In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating Composite materials (or composites for short are engineered Materials made from two or more constituent materials with significantly different physical or chemical

To more accurately calculate thermal expansion of a substance a more advanced Equation of state must be used, which will then predict the values of the thermal expansion at all the required temperatures and pressures, along with many other state functions. In Physics and Thermodynamics, an equation of state is a relation between state variables More specifically an equation of state is a thermodynamic In Thermodynamics, a state function, state quantity, or a function of state, is a property of a system that depends only on the current

For solid materials with a significant length, like rods or cables, an estimate of the amount of thermal expansion can be described by the $\frac{}{}\epsilon_{thermal}$ ratio of strain:

$\epsilon_{thermal} = \frac{(L_{final} - L_{initial})} {L_{initial}}$

$\frac{}{}L_{initial}$ is the initial length before the change of temperature and

$\frac{}{}L_{final}$ the final length recorded after the change of temperature.

For most solids, thermal expansion relates directly with temperature:

$\epsilon_{thermal} \propto {\Delta T }$

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

$\frac{}{} \epsilon_{thermal} = \alpha \Delta T$

where

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

and

$\frac{}{}\alpha$ is the coefficient of thermal expansion in inverse kelvins. When the Temperature of a substance changes the energy that is stored in the Intermolecular bonds between atoms changes

$\frac{}{}\Delta T$ is the difference of the temperature between the two recorded strains, measured in celsius or kelvin. The Celsius Temperature scale was previously known as the centigrade scale. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic

A number of materials contract on heating within certain temperature ranges; we usually speak of negative thermal expansion, rather than thermal contraction, in such cases. Negative Thermal Expansion ( NTE) is a physicochemical process in which some materials contract upon heating rather than expanding as most materials do For example, the coefficient of thermal expansion of water drops to zero as it is cooled to roughly 4 °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.

Common polymers expand roughly 4 times more than metals, which expand more than ceramics. A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across The word ceramic is derived from the Greek word κεραμικός ( keramikos) Thermal expansion generally decreases with increasing bond energy, which also has an effect on the hardness of solids, so, harder materials are more likely to have lower thermal expansion. Hardness refers to various properties of Matter in the Solid phase that give it high resistance to various kinds of shape change when Force In general, liquids expand slightly more than solids.

In many common materials, changes in size can also be due to water (or other solvents) being absorbed/desorbed, and many organic materials change size much more due to this effect than they do to thermal expansion. Common plastics exposed to water can, in the long term, expand many percent.

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

Metal framed windows need rubber spacers
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
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. expansion joint is an assembly designed to safely absorb the heat-induced expansion and contraction of various construction materials
a Gridiron pendulum uses an arrangement of different metals to maintain a more temperature stable pendulum length. The gridiron pendulum was an improvement of Clock Pendulums invented by John Harrison, consisting of alternating Zinc and Iron rods

This phenomenon can also be put to good use, for example in the process of thermal shrink-fitting parts are assembled with each at a different temperature, and sized such that when they reach the same temperature, the thermal expansion of the parts forces them together to form a stable joint. Shrink-fitting is a technique in which pieces of a structure are heated or cooled employing the phenomenon of Thermal expansion, to make a joint

Thermometers are another example of an application of thermal expansion — most contain a liquid which is constrained to flow in only one direction (along the tube) due to changes in volume brought about by changes in temperature. The thermometer is a device that measures Temperature or Temperature gradient using a variety of different principles it comes from the Greek roots A bi-metal mechanical thermometer uses a bi-metal strip and registers changes based on the differing coefficient of thermal expansion between the two materials. A bi-metallic strip is used to convert a temperature change into mechanical displacement When the Temperature of a substance changes the energy that is stored in the Intermolecular bonds between atoms changes

Anisotropy

Many solid materials will expand evenly in all three directions, but this is not true for all. Graphite for example has a pronounced layer structure and the expansion in the direction perpendicular to the layers is quite different from that in the layers. In general the proper description of the thermal expansion of a solid must therefore include its symmetry. For cubic materials a single expansion coefficient suffices, but for a material with triclinic symmetry six parameters must be distinguished, three for each of the three axes (a,b,c) and three for the change in the angles (α,β,γ) between them. An excellent way of measuring the entire expansion tensor is to perform powder diffraction on the material during a heating or cooling run and monitor the position of its diffraction peaks. Powder diffraction is a scientific technique using X-ray, Neutron, or Electron Diffraction on powder or Microcrystalline samples for

External links

Glass Thermal Expansion Thermal expansion measurement, definitions, thermal expansion calculation from the glass composition

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