High reliability temperature apparatus

As mentioned by Mathias et al. [9], reliable methods to measure the thermal conductivity of diffusion layers as a function of compression pressures are very scarce in the open literature. Khandelwal and Mench [112] designed an ex situ method to measure accurately the thermal conductivities of different components used in a fuel cell. In their apparatus, the sample materials were placed between two cylindrical rods made out of aluminum bronze (see Figure 4.28). Three thermocouples were located equidistantly in each of the upper and lower cylinders to monitor the temperatures along these components. Two plates located at each end compressed both cylinders together. The temperatures of each plate were maintained by flowing coolant fluids at a high flow rate through channels located inside each of the plates. A load cell was located between two plates at one end so that the compression pressure could be measured. 

An apparatus with high sensitivity is the heat-flow microcalorimeter originally developed by Calvet and Prat [139] based on the design of Tian [140]. Several Tian-Calvet type microcalorimeters have been designed [141-144]. In the Calvet microcalorimeter, heat flow is measured between the system and the heat block itself. The principles and theory of heat-flow microcalorimetry, the analysis of calorimetric data, as well as the merits and limitations of the various applications of adsorption calorimetry to the study of heterogeneous catalysis have been discussed in several reviews [61,118,134,135,141,145]. The Tian-Calvet type calorimeters are preferred because they have been shown to be reliable, can be used with a wide variety of solids, can follow both slow and fast processes, and can be operated over a reasonably broad temperature range [118,135]. The apparatus is composed by an experimental vessel, where the system is located, which is contained into a calorimetric block (Figure 13.3 [146]). 

The sample is filled into the vial which then is attached to the suspension device. Then the high-pressure chamber is firmly closed and heated or cooled to system temperature. More of the sample material is placed within the view cell. The apparatus then is evacuated to remove air and to obtain a reliable value for initial mass. The thermostated dense gas is charged into the chamber and the view cell until system pressure is attained. Pressure build-up only takes a few seconds because of the small volume of the chamber. The weight of the sample rises as gas dissolves in or is being adsorbed to the sample and assymptotically nears its equilibrium value. At the same time, the dissolution of the gas in the solid leads to volume changes, a process observed with the sample particles enclosed in the view cell. Their size is recorded with the camera and evaluated with the help of a PC. The apparatus is evacuated again at the end of each experiment to remove the entire gas. 

An apparatus specially developed to meet these conditions which has proved to work reliably and with high sensitivity for a wide range of polymers, from LDPE to highly rigid short fibre reinforced thermoplastics, over a wide temperature range is that described by Darlington and Saunders and Darlington (1971, unpublished). 

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