Three application areas for SrTiO3 glass-ceramics

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Abstract
The perovskite SrTiO3 can be controllably crystallized in situ in a glass matrix, and the resulting glass-ceramic displays a large peak in the dielectric constant at about 75 K (shifted from 35 K by doping and glass-chemistry techniques) and a T-1 increase in the dielectric constant below 0.1 K. This high degree of dielectric activity at cryogenic temperatures makes possible three potential applications: (1) A capacitance thermometer with a range from 0.1 to 70K which is unaffected by magnetic fields: (2) A 77 K capacitance energy-storage device; and (3) An adiabatic-depolarization cooling device below 0.1 K.Actual thermometers fabricated from this glass-ceramic (1 × 2 × 5 mm) had sensitivities ⋍ 250pF/K at 4.2 K and a linear C-T range from 1.1 to 5.2 K. It is demonstrated that this thermometer is unaffected by an intense magnetic field of 140 kG between 1.5 and 4.2 K.The field dependence of the dielectric constant at 77 K. was measured up to 180kV/cm, and these data were used to evaluate the Helmholtz energy-storage integral for fields up to 1.2mV/cm. Breakdown strength measurements indicate possible energy-storage values at 77 K of 1.5 to 4.3 joules/cm3 (24 to 72 joules/in3) and 0.39 to 1.12 joules/gm (177 to 509 joules/lb.).An adiabatic-depolarization cooling experiment is described wherein 38.5 volts were removed from an array of capacitors (0.025 mm thick dielectric layers) at 32 mfC. It was estimated that the array cooled to at least 20 mK upon depolarization, based on capacitance measurements made on the array itself. The experiment was duplicated, but efforts to verify the effect with an independent thermometer have not yet been successful.