A Low Temperature (4.2 to 350 K) Differential Dilatometer

Abstract

A new tube-type vitreous silica vertical differential dilato-meter for the measurement of the coefficient of thermal expansion of solids is described. Thermal dilatation is measured by means of an inductive displacement tranducer (LVDT) and its electronic equipment. The present detection limit of the apparatus, due to the analog recorder, is 1 × 10⁻⁴ mm (Δℓ/ℓ = 2 × 10⁻⁶) for a maximum differential expansion Δℓ (Δℓ = Δℓ_sample − Δℓ_reference) of ±3 × 10⁻¹ mm, or 3.33 × 10⁻⁴ mm for a maximum Δℓ of ±1 mm (useful range). The maximum recorded drift rate was 2.5 × 10⁻⁴ mm/24 hours. The force applied by a vitreous silica push rod on the sample can be modified from 0 to 1N even during an experiment. A liquid helium circulation cryostat and a furnace with its associated electronic equipment are employed concurrently during experiments in order to achieve linear cooling and heating at rates as low as 0.04 K/min and as high as 5 K/min, as well as isothermal maintenance of the sample and the reference in the 4.2 to 350 K temperature range. The above specifications make this apparatus especially useful to study low temperature phase transformations. In fact, the apparatus is presently being used to study the austenite to martensite transformation of Fe-Ni-C alloys which have a low Ms temperature (below 240 K), as well as connected phenomena. Results on a Fe-24.2%Ni-0.41%C (weight percent) alloy are presented.