NEW CERAMIC MATERIAL FOR THERMAL AND DIELECTRIC ISOLATION AT LOW-TEMPERATURES

A new perovskite material has been found, Pb (Mn 1 2 Ta 1 2)O3 which has the smallest thermal diffusivity of any known, densified solid at low temperatures (9 × 10-3 < k < 5 × 10-2 cm2 sec-1 between 4-10 K). This results from a large specific heat (∼ 0.1 J cm-3 K-1 at 10 K) and small thermal conductivity (∼ 0.7 mW cm-1 K-1 at 10 K). The large specific heat is due to the combination of a small Debye temperature, 149 K, a low-lying Einstein mode, 4.52 cm-1, and a large density, 9.68 gm cm-3. The 4.52 cm-1 mode is believed to be due to a small concentration of oxygen vacancies in the ceramic. The thermal conductivity is limited by phon scattering from localized excitations characterized by phonon mean free paths on the order of the lattice constant (4 Å). It is suggested that the local excitations may be associated with ferroelectric micro-regions. The dielectric constant and loss tangent at 4.2 K and 1 KHz are 351.7 and 0.0073, respectively, and show very small dispersion. The thermal expansion coefficient almost exactly matches that of Nb metal from 300 to 77 K (the lower limit of the measurement). © 1979.