Thermal Properties:

The most important thermal properties of ceramic materials are heat capacity, thermal expansion coefficient, and thermal conductivity. Many applications of ceramics, such as their use as insulating materials, are related to these properties.

Thermal energy can be either stored or transmitted by a solid. The ability of a material to absorb heat from its surrounding is its heat capacity. In solid materials at T > 0 K, atoms are constantly vibrating. The atomic vibrations are also affected by the vibrations of adjacent atoms through bonding. Hence, vibrations can be transmitted across the solid. The higher the temperature, the higher the frequency of vibration and the shorter the wavelength of the associated elastic deformation.

The potential energy between two bonded atoms can be schematically represented by a diagram:

Figure 2: Graph depicting the potential energy between two bonded atoms

The distance at which there is minimum energy (potential well) represents what is usually described as the bond length. A good analogy is a sphere attached to a spring, with the equilibrium position of the spring corresponding to the atom at the bond length (potential well). When the spring is either compressed or stretched from its equilibrium position, the force pulling it back to the equilibrium position is directly proportional to the displacement (Hooke's law). Once displaced, the frequency of oscillation is greatest when there is a large spring constant and low mass ball. Ceramics generally have strong bonds and light atoms. Thus, they can have high frequency vibrations of the atoms with small disturbances in the crystal lattice. The result is that they typically have both high heat capacities and high melting temperatures.

As temperature increases, the vibrational amplitude of the bonds increases. The asymmetry of the curve shows that the interatomic distance also increases with temperature, and this is observed as thermal expansion. Compared to other materials, ceramics with strong bonds have potential energy curves that are deep and narrow and correspondingly small thermal expansion coefficients.

The conduction of heat through a solid involves the transfer of energy between vibrating atoms. Extending the analogy, consider each sphere (atom) to be connected to its neighbors by a network of springs (bonds). The vibration of each atom affects the motion of neighboring atoms, and the result is elastic waves that propagate through the solid. At low temperatures (up to about 400˚C), energy travels through the material predominantly via phonons, elastic waves that travel at the speed of sound. Phonons are the result of particle vibrations which increase in frequency and amplitude as temperature increases.

Phonons travel through the material until they are scattered, either through phonon-phonon interactions* or at lattice imperfections. Phonon conductivity generally decreases with increasing temperature in crystalline materials as the amount of scattering increases. Amorphous ceramics which lack the ordered lattice undergo even greater scattering, and therefore are poor conductors. Those ceramic materials that are composed of particles of similar size and mass with simple structures (such as diamond or BeO) undergo the smallest amount of scattering and therefore have the greatest conductivity.

At higher temperatures, photon conductivity (radiation) becomes the predominant mechanism of energy transfer. This is a rapid sequence of absorptions and emissions of photons that travel at the speed of light. This mode of conduction is especially important in glass, transparent crystalline ceramics, and porous ceramics. In these materials, thermal conductivity increases with increased temperature.

Although the thermal conductivity is affected by faults or defects in the crystal structure, the insulating properties of ceramics essentially depend on microscopic imperfections. The transmission of either type of wave (phonon or photon) is interrupted by grain boundaries and pores, so that more porous materials are better insulators. The use of ceramic insulating materials to line kilns and industrial furnaces are one application of the insulating properties of ceramic materials.

The electron mechanism of heat transport is relatively unimportant in ceramics because charge is localized. This mechanism is very important, however, in metals which have large numbers of free (delocalized) electrons.

*Phonon-phonon interactions are another consequence of the asymmetry in the interaction potential between atoms. When different phonons overlap at the location of a particular atom, the vibrational amplitudes superimpose. In the asymmetrical potential well, the curvature varies as a function of the displacement. This means that the spring constant by which the atom is retained also changes. Hence the atom has the tendency to vibrate with a different frequency, which produces a different phonon.

Table 2: Comparison of thermal properties of different ceramic materials.

Material	Melting Temp.(oC)	Heat Capacity (J/kg.K)	Coefficient of Linear Expansion 1/oCx10-6	Thermal Conductiv-ity (W/m.K)
Aluminum metal	660	900	23.6	247
Copper metal	1063	386	16.5	398
Alumina	2050	775	8.8	30.1
Fused silica	1650	740	0.5	2.0
Soda-lime glass	700	840	9.0	1.7
Polyethylene	120	2100	60-220	0.38
Polystyrene	65-75	1360	50-85	0.13

One of the most interesting high-temperature applications of ceramic materials is their use on the space shuttle. Almost the entire exterior of the shuttle is covered with ceramic tiles made from high purity amorphous silica fibers. Those exposed to the highest temperatures have an added layer of high-emittance glass. These tiles can tolerate temperatures up to 1480˚ C for a limited amount of time. Some of the high temperatures experienced by the shuttle during entry and ascent are shown in Figure 3.

Figure 3: Diagram of space shuttle's ascent and descent temperatures

The melting point of aluminum is 660˚C. The tiles keep the temperature of the aluminum shell of the shuttle at or below 175˚C while the exterior temperatures can exceed 1400˚ C. The tiles cool off rapidly, so that after exposure to such high temperatures they are cool enough to be held in the bare hand in about 10 seconds. Surprisingly, the thickness of these ceramic tiles varies from only 0.5 inches to 3.5 inches.

Figure 4: Graph of inner temperature of tile versus tile thickness.

The shuttle also uses ceramic applications in fabrics for gap fillers and thermal barriers, reinforced carbon-carbon composites for the nose cone and wing leading edges, and high temperature glass windows.