A Low Temperature Glass-Ceramic Capacitance Thermometer

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Abstract
Dielectric measurements are reported on capacitance thermometers based on SrTiO3 crystallized from glass at 1100 and 1200°C. Multilayer experimental units, approximately 5×2×1 mm, were fabricated using pilot plant facilities and measured from 1.7 to 300 K. If both the capacitance and loss tangent are measured, the range of the 1100°C crystallized thermometer is from at least 1.7 to 300 K. For capacitance measurement alone, the thermometer range is below 72 K, and in this range the capacitance varies smoothly and monotonically with temperature, becoming linear below 5.2 K. The sensitivity is largest in this linear region and averages 250 pF·K−1 for the 1100°C crystallized thermometers. The self-heating at 4.2 K is about 70 pW (5 kHz, 7 mV) and decreases with decreasing temperature. The effect of a magnetic field on the thermometer was measured in an open Dewar of liquid helium at four frequencies in fields up to 80 kG. The measured changes were ≤±1 mK and are interpreted as fluctuations in the bath temperature rather than an intrinsic magnetocapacitance effect. The response time between 4.2 and 300 K is at least 70 K·sec−1. The main disadvantage of the thermometer is a transient (∼30 min) capacitance instability which is equivalent to a ≤30 mK change before stabilization. Once stabilized, the thermometer repeats to about ±2 mK when compared to a germanium thermometer, and repeats to ±13 mK on cyclically plunging into liquid helium (15 cycles) in the unstabilized case. This instability is apparently due to helium migration into the unencapsulated sensor body. The polar properties of glass crystallized SrTiO3, on which the thermometer is based, are briefly discussed, and comparisons are made with two other capacitance thermometers reported in the literature.