Thermal Expansion (redirected from Dedensification)

thermal expansion

Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change. When the material is a solid, thermal expansion is usually described in terms of change in length, height, or thickness. If a crystalline solid has the same structural configuration throughout, the expansion will be uniform in all dimensions. Otherwise, there may be different expansion coefficients and the solid will change shape as the temperature increases. If the material is a fluid, it is more useful to describe the expansion in terms of a change in volume. Because the bonding forces among atoms and molecules vary from material to material, expansion coefficients are characteristic of elements and compounds.

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thermal expansion [′thər·məl ik′span·chən]

(physics)

The dimensional changes exhibited by solids, liquids, and gases for changes in temperature while pressure is held constant.

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Thermal expansion

Solids, liquids, and gases all exhibit dimensional changes for changes in temperature while pressure is held constant. The molecular mechanisms at work and the methods of data presentation are quite different for the three cases.

The temperature coefficient of linear expansion α_l is defined by Eq. (1),

(1) 

where l is the length of the specimen, t is the temperature, and p is the pressure. For each solid there is a Debye characteristic temperature &THgr;, below which α_l is strongly dependent upon temperature and above which α_l is practically constant. Many common substances are near or above &THgr; at room temperature and follow approximate equation (2),

(2) 

where l₀ is the length at 0°C and t is the temperature in °C. The total change in length from absolute zero to the melting point has a range of approximately 2% for most substances.

So-called perfect gases follow the relation in Eq. (3),

(3) 

where p is absolute pressure, v is specific volume, T is absolute temperature, and R is the so-called gas constant. Real gases often follow this equation closely. See Gas constant

The coefficient of cubic expansion α_v is defined by Eq.(4)

(4) 

, and for a perfect gas this is found to be 1/T. The behavior of real gases is largely accounted for by the van der Waals equation. See Kinetic theory of matter

For liquids, α_v is somewhat a function of pressure but is largely determined by temperature. Though α_v may often be taken as constant over a sizable range of temperature (as in the liquid expansion thermometer), generally some variation must be accounted for. For example, water contracts with temperature rise from 32 to 39°F (0 to 4°C), above which it expands at an increasing rate. See Thermometer

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thermal expansion

The change in length or volume which a material or body undergoes on being heated.

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Thermal Expansion 

the dimensional changes exhibited by a substance when it is heated.

A quantitative characterization of thermal expansion at constant pressure is provided by the isobaric thermal expansion coefficient

which is often called the coefficient of volume, or cubical, expansion. In practice the value of α is determined from the formula

Here, Vʹ is the volume of the gas, liquid, or solid at the temperature T₂ > T₁; V is the initial volume of the substance; and the temperature difference T₂ – T₁ is assumed to be small.

Table 1. Isobaric coefficients of volume expansion of some gases and liquids at atmospheric pressure
Substance	Temperature (°C)	α [10–3(°C)–1]
Gases
Helium ...............	0–100	3.658
Hydrogen ...............	0–100	3.661
Oxygen ...............	0–100	3.665
Nitrogen ...............	0–100	3.674
Air (without CO2) ...............	0–100	3.671
Liquids
Water ...............	10	0.0879
	20	0.2066
	80	0.6413
Mercury ...............	20	0.182
Glycerol ...............	20	0.500
Benzene ...............	20	1.060
Acetone ...............	20	1.430
Ethyl alcohol ...............	20	1.659

The thermal expansion of solids is characterized by, in addition to α, the coefficient of linear expansion

where l is the initial length of the solid in some chosen direction. In the general case of anisotropic solids, α = α_x + α_y + α_z, where the linear expansion coefficients α_x, α_y, and α_z along the x, y, and z crystallographic axes, respectively, are equal or unequal depending on the symmetry of the crystal. For crystals with cubic symmetry, for example, as for isotropic solids, α_x = α_y = α_z and α ≈ 3α₁.

For most substances, α > 0. Water, on the other hand, contracts when it is heated from 0° to 4°C at atmospheric pressure. The dependence of α on T is most pronounced in the cases of gases; for an ideal gas, α = 1/T. The dependence is less marked for liquids. For a number of substances, such as quartz and Invar, a is small and is virtually constant over a broad range of temperatures. As T → 0, α → 0. Tables 1 and 2 give the isobaric coefficients of volume and linear expansion of a number of substances at atmospheric pressure.

Table 2. Isobaric coefficients of linear expansion of some solids at atmospheric pressure
Substance	Temperature (°C)	α1 [10–6(°C)–1
Carbon
diamond ...............	20	1.2
graphite ...............	20	79
Silicon ...............	3–18	25
Quartz
parallel to axis ...............	40	78
perpendicular to axis ...............	40	14.1
fused ...............	0–100	0.384
Glass
crown ...............	0–100	∼9
flint ...............	0–100	∼7
Tungsten ...............	25	4.5
Copper ...............	25	16.6
Brass ...............	20	18.9
Aluminum ...............	25	25
Iron ...............	25	12

The thermal expansion of a gas is due to the increase in the kinetic energy of the gas particles as the gas is heated; this energy is used to perform work against the external pressure. In the case of solids and liquids, thermal expansion is associated with the asymmetry (anharmonicity) of the thermal vibrations of the atoms; as a result of this asymmetry, the interatomic distances increase with increasing T. The experimental determination of α and α₁ is carried out by the methods of dilatometry. The thermal expansion of substances is taken into account in the designing of all installations, devices, and machines that operate under variable temperature conditions.

REFERENCES

Novikova, S. I. Teplovoe rasshirenie tverdykh tel. Moscow, 1974.
Hirschfelder, J., C. Curtiss, and R. Bird. Molekuliarnaia teoriia gazov i zhidkostei. Moscow, 1961. (Translated from English.)
Perry, J. Spravochnik inzhenera-khimika, vol. 1. Leningrad, 1969. (Translated from English.)