why is thermal conductivity higher for cristalline than non-cristalline ceramics?

Answer 1

In electrically insulating materials, heat transfer occurs through the propagation of vibrations of the atoms in the material. (In conductors, heat is also transferred by electrons, which are also subject to the sorts of scattering processes discussed below.) Atomic vibrations can be described as particle-like entities known as "phonons". In an idealized perfect crystalline material that has an infinite, regular, crystal lattice with no defects, and in which the vibrations of the atoms are perfectly harmonic, phonons will propagate indefinitely, and the heat energy they carry will likewise propagate quickly.

Real crystals, however, contain defects (grain boundaries, lattice defects, vacancies, etc.). Furthermore, the atoms in real materials are not perfect harmonic oscillators; their vibrations are anharmonic. The anharmonicity and defects impede the propagation of phonons. In particular, as the defect density increases, the mean free path (the distance a phonon can propagate before it gets absorbed or scattered by a defect) decreases. As the mean free path decreases, the effective speed at which phonons can propagate decreases, and this means that the propagation of the heat energy they carry is also slowed down, i.e., the thermal conductivity is lowered.

An amorphous material is a material in which there is no long-range order to the atoms. In other words, there is no regular crystal structure. One can therefore think of an amorphous material as having an extremely high density of defects -- so high that there is no ordering remaining. Thus, for two materials with the same chemical composition, one crystalline and one amorphous, the amorphous form will have lower thermal conductivity because of its high density of defects.

Answer 2

wikipedia: The silica firebricks that line metallic-making furnaces are used at temperatures as much as 1650°C (3000°F), which might soften many different varieties of ceramic, and rather element of the silica firebrick liquefies. HRSI, a textile with the comparable composition, is used to make the insulating tiles of the gap holiday. yet they have low thermal conductivity, as maximum ceramics do. for metals, there are a number of that soften at intense temperatures, yet they do no longer seem to be low-priced. Platinum melts at 1772ºC , to illustrate. Titanium melts at 1668ºC (a 2d reference says 1725ºC) so it would desire to be the proper determination. .

Answer 3

Positive thermal coefficient
Increases in temperature can cause grain boundaries to suddenly become insulating in some semiconducting ceramic materials, mostly mixtures of heavy metal titanates. The critical transition temperature can be adjusted over a wide range by variations in chemistry. In such materials, current will pass through the material until joule heating brings it to the transition temperature, at which point the circuit will be broken and current flow will cease. Such ceramics are used as self-controlled heating elements in, for example, the rear-window defrost circuits of automobiles.

At the transition temperature, the material's dielectric response becomes theoretically infinite. While a lack of temperature control would rule out any practical use of the material near its critical temperature, the dielectric effect remains exceptionally strong even at much higher temperatures. Titanates with critical temperatures far below room temperature have become synonymous with "ceramic" in the context of ceramic capacitors for just this reason.


Processing of ceramic materials
Non-crystalline ceramics, being glasses, tend to be formed from melts. The glass is shaped when either fully molten, by casting, or when in a state of toffee-like viscosity, by methods such as blowing to a mould. If later heat-treatments cause this class to become partly crystalline, the resulting material is known as a glass-ceramic.

Crystalline ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories - either make the ceramic in the desired shape, by reaction in situ, or by "forming" powders into the desired shape, and then sintering to form a solid body. Ceramic forming techniques include shaping by hand (sometimes including a rotation process called "throwing"), slip casting, tape casting (used for making very thin ceramic capacitors, etc.), injection molding, dry pressing, and other variations. (See also Ceramic forming techniques. Details of these processes are described in the two books listed below.) A few methods use a hybrid between the two approaches.