Mechanical-conduction calorimeters

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). 

In well-designed isoperibol calorimeters, the heat transfer between the calorimeter proper and the jacket takes place according to Newton s law, with conduction being the dominant mechanism [3,21,35-38]. In this case, the rate of temperature change during the initial and final periods, g, is given by... 

The excess heats of mixing experiments were performed using a modified McGlashan calorimeter. All the dynamic mechanical analyses were conducted using a Polymer Laboratories instrument at a frequency of 10 Hz and a heating rate of 2 C per minute. 

The determination of quantities of heat is important in a large number of physical and chemical problems. One of the most important of these is the determination of specific heats. The specific heat of a body is closely connected with its chemical nature, and is an exceedingly important quantity. Change of state (melting and evaporation with the reverse processes of sohdification and condensation), all forms of chemical transformation, the creation and the disappearance of mechanical work, the conduction of electricity, the absorption of light and other forms of radiation, are all processes which involve transference of heat. The apparatus with w hich quantities of heat are determined are called calorimeters. 

Heat transfer between the heat source (furnace wall or heater) and the block calorimeter by conductive, convective, and radiative mechanisms. 

The essentia] point for classification of this type of calorimeter is that the local temperature difference, which invariably exists wherever a heat exchange takes place, is measured. The heat released or consumed in the calorimeter (measuring system) initially causes a change of temperature with regard to the surroundings. This causes a relaxation process the heat exchange with the surroundings continues until the reestablishment of isothermal or steady-state conditions. The possible mechanisms of heat transport - thermal conduction, convection, and radiation - are discussed in Chapter 4. 

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