Unified facility for low-temperature measurements

Abstract

A highly automated facility for investigating the properties ofmaterials at low temperatures is described, which contains a BKT-IO cryostat, operating over the 1.5-370 K temperature range with changeable modules for calibrating resistance thermometers and for measuring heat capacity and thermal conductivity. The AK6.25 data acquisition system controlled by an IBM personal computer is used for the measurements.The purpose of this research was to construct a modem highly automated and economic facility for investigating the properties of structural materials at low temperatures: the heat capacity, thermal conductivity, and electrical conductivity. We investigated a method of making measurements on standard samples over a wide temperature range (and also on different types of samples), and also the choice of testing of standard samples based on the results of these investigations.Neither in this country nor abroad is standard equipment available for malting wide-range measurements of thermal quantities. The equipment available in this country, as a rule, is designed to measure a single q~)antity, for example, heat capacity or thermal conductivity. The unified automated facility we have comtructed enables various thermal q~l~ntities to be measured on samples with different dimensions and different properties. To adjust the equipment to make another type of measurement or for another sample one simply replaces a sm,ll module, which is connected to the cryostat by a joint with a single removable nut, which is unscrewed by hand.In the first version of the facility in 1993-1996 we developed modules for calibrating resistance thermometers over the 0.6-370 K range ar~_ for measuring heat capacity and thermal conductivity over the 1.5-370 K range.We designed the basic BKT-10 cryostat covering the 1.5-370 K temperature range, based on previously developed immersion cryostats. It is extremely economical: over the range of liquid-helium temperatures the consumption of liquid helium does not exceed 2.5 liters per day, and the consumption of liquid helium over the whole series of measurements amounts to 7-10 liters. Unlike the previous BKT-3 model, designed to be immersed in a Dewar helium vessel with a straight-through opening of 25 ram, the BKT-10 cryostat and its modifications have a maximum diameter of 44 mm and are designed for a Dewar vessel with a standard Pneurop connection. Vessels of this type are widely used in Europe. All the gas and vacuum connections of the cryostat also correspond to the Pneurop standard.The gas and electron circuits of the cryostat with the connected heat-capacity cell is shown in Fig. 1. The cryostat is shown schematically, its actual length being about 1200 mm with a diameter of the immersed part of 44 ram.The cryostat has a built-in sorption pump 2, which enables one, when using He 4, to obtain a minimum temperature of 1.3 K. When using He 3 as the working gas the minimum temperature reaches about 0.5 K. The amount of working gas needed to operate the cryostat is stored in the small tank 1, pl...