Thermometric Properties of Carbon-Impregnated Porous Glass at Low Temperatures

Abstract

Samples (4.8×1.6×1.0 mm) were cut from a plate of carbon‐impregnated porous glass, encapsulated in He‐filled Pt cans as two‐lead devices, and their thermometric properties measured from 2 to 400 K. This material has a large sensitivity (R 77/R 300≅2.0, R 4.2/R 300≅130) combined with a relatively small resistivity (R 4.2≃2100 Ω, R 77≃34 Ω, R 300≃17 Ω, for the samples' size), and d lnR/d lnT is smooth and monotonic over this temperature range. Repeatability of the two‐lead device is ≃±0.8 mK at 4.2 K and <±60 mK at 77 K, and no evidence of isothermal aging was observed. The magnetoresistance was measured at 4.2 K in fields up to 30 kG and found to be approximately twice as large as for carbon radio resistors at 4.2 K. Curve‐fitting studies were made over increasingly larger temperature intervals (2–5, 2–10,…, 2–200 K) based on the equation log R=A+BT−P , and it was found that in each interval the R−T data could be fitted to better than 1% of the upper temperature. An interesting scaling relation was found between the R−T curves of the units, such that at each temperature,Ri =bRj m for the ith and jth units, where b and m are constants. This scaling could permit a two‐point calibration transfer. It is postulated that the broad‐range thermometric properties of this material derive from the high‐purity carbon filaments deposited in the ∼40 Å pore‐size porous glass.