Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

256 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONShas been studied with an optical microscope343. After irradiation with 0.5 MradX-rays, the polymerisation proceeded at a measurable rate between 20-60” C.Single crystals were irradiated and placed on the microscope stage in a constanttemperature bath of polybutene oil to follow the rate of polymerisation. Thereaction was quite complex, but four or five distinct stages could be recognisedas the temperature was raised.Optical microscopy has been used to observe the rate of sintering of NiO-Fe,O,and MgO-Fe,O, systems344. Oxide spheres were placed in contact with a polycrystallineplate of the other component and given varied heat treatments. Thespecimens were then mounted in a lucite block and sectioned for microscopicexamination. Temperature gradients are a problem, since the temperature sensingdevice is usually situated in the heating block and the sample is heated by conductionthrough the microscope slide. By selecting a relatively small area and usinga slow heating rate, the area under observation will be nearly isothermal, but theerror between the measured temperature and actual temperature of reaction stillexists. A possible way round this objection is calibration of the temperaturedifference at various temperatures with substances whose transformation temperatureis already known. Reese et ~ 1 . ~ have ~ ’ developed a mechanical method,which permits detection of phase transformations by the recording of changes inlight intensity. Polarized light is used to illuminate the specimen and this passesthrough the microscope and is directed by a beam splitter to a recording cell.The use of polarised light has the advantage that melting is distinguished by a largedrop in the transmitted light intensity.4.4 DIFFERENTIAL THERMAL ANALYSISThe concept of “reaction temperature”, when the heat produced in a solidstate reaction causes an appreciable temperature rise, was used in early studiesand is related to the kinetics of the reaction. This method has evolved into thetechnique of differential thermal analysis (DTA), which has developed rapidly inrecent years. It is undoubtedly of great value as a qualitative tool for the study ofsolid state reactions. Several excellent comprehensive reviews of the subject havebeen made which cover every aspect of DTA technique^^^^-^^'. The review bySmothers and Chiang349 contains an indexed list of over four thousand publicationsOn DTA Up to 1965.The basic principle of DTA is the measurement of the temperature differencebetween a sample and a reference material, as they are simultaneously heated at auniform rate. A solid state reaction will cause an evolution of heat, which will beshown as a temperature difference (AT) between the sample and the referencematerial. As this heat is dissipated to the surroundings, AT reduces to zero again.Measurement of temperature and AT over a suitable range will give a thermogram