A cryostat and temperature control system optimized for measuring relaxations of glass-forming liquids

Abstract

An experimental setup, including a cryostat and a temperature control system, has been constructed to meet the demands of measuring linear and nonlinear macroscopic relaxation properties of glass-forming liquids in the extremely viscous state approaching the glass transition. In order to be able to measure such frequency-dependent response functions accurately (including dielectric permittivity, specific heat, thermal expansivity, and shear and bulk moduli), as well as nonlinear relaxations following a temperature jump, one must have the ability to hold temperatures of liquids steady over the span of several days or even several weeks. To maximize temperature stability, special care is taken to thermally isolate the sample chamber of the cryostat. The main temperature control system is capable of maintaining temperatures within a few millikelvins. If liquid is deposited into a special transducer assembly that includes a subcryostat unit, the temperature of liquids can be maintained even more precisely, within a few tenths of a millikelvin. This subcryostat unit is more responsive to temperature changes because (i) it is equipped with a Peltier element that provides secondary heating and cooling, (ii) the transducer contains a layer of liquid that is only 50 micfom thick, and (iii) feedback proportional-integral-derivative temperature control is implemented by a fully analog circuit. The subcryostat permits us to change and stabilize temperatures quickly; it takes only 10 s to stabilize the temperature within tenths of a millikelvin after a jump of 1 K, for example, a capability that is highly advantageous for accurately observing relaxation processes following a temperature step.