The SI unit of volumetric heat capacity is joule per kelvin per cubic meter, J⋅K−1⋅m−3.
This quantity may be convenient for materials that are commonly measured by volume rather than mass, as is often the case in engineering and other technical disciplines.
This quantity is used almost exclusively for liquids and solids, since for gases it may be confused with the "specific heat capacity at constant volume", which generally has very different values.
Eventually it became clear that heat capacities per particle for all substances in all states are the same, to within a factor of two, so long as temperatures are not in the cryogenic range.
The volumetric heat capacity of solid materials at room temperatures and above varies widely, from about 1.2 MJ⋅K−1⋅m−3 (for example bismuth[3]) to 3.4 MJ⋅K−1⋅m−3 (for example iron[4]).
Atoms vary greatly in density, with the heaviest often being more dense, and thus are closer to taking up the same average volume in solids than their mass alone would predict.
This reflects the modest loss of degrees of freedom for particles in liquids as compared with solids.
Thus, in the limit of ideal gas behavior (which many gases approximate except at low temperatures and/or extremes of pressure) this property reduces differences in gas volumetric heat capacity to simple differences in the heat capacities of individual molecules.
This factor of two represents vibrational degrees of freedom available in solids vs. gas molecules of various complexities.
In monatomic gases (like argon) at room temperature and constant volume, volumetric heat capacities are all very close to 0.5 kJ⋅K−1⋅m−3, which is the same as the theoretical value of 3/2RT per kelvin per mole of gas molecules (where R is the gas constant and T is temperature).
Monatomic gas heat capacities per atom (not per molecule) are decreased by a factor of 2 with regard to solids, due to loss of half of the potential degrees of freedom per atom for storing energy in a monatomic gas, as compared with regard to an ideal solid.
Since the bulk density of a solid chemical element is strongly related to its molar mass (usually about 3R per mole, as noted above), there exists noticeable inverse correlation between a solid's density and its specific heat capacity on a per-mass basis.
These two factors determine the volumetric heat capacity, which as a bulk property may be striking in consistency.
For instance, arsenic, which is only 14.5% less dense than antimony, has nearly 59% more specific heat capacity on a mass basis.
In other words; even though an ingot of arsenic is only about 17% larger than an antimony one of the same mass, it absorbs about 59% more heat for a given temperature rise.
Thermal inertia is a term commonly used to describe the observed delays in a body's temperature response during heat transfers.
The phenomenon exists because of a body's ability to both store and transport heat relative to its environment.
Larger values of volumetric heat capacity, as may occur in association with thermal effusivity, typically yield slower temperature responses.