why is thermal conductivity higher for cristalline than non-cristalline ceramics?
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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 phononsIn 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 anharmonicThe anharmonicity and defects impede the propagation of phononsIn 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) decreasesAs 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 loweredAn amorphous material is a material in which there is no long-range order to the atomsIn other words, there is no regular crystal structureOne can therefore think of an amorphous material as having an extremely high density of defects - so high that there is no ordering remainingThus, 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.
