Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 THE SOLID-SOLID INTERFACE 26 1et ~ 1 . ~ have ~ ' studied the formation of spinels from cadmium oxidelferric oxidemixtures. A valve voltmeter was used to follow the conductivity changes and theresults correlated with a number of other measurements.4.6 MAGNETIC MEASUREMENTSExperimental details for the techniques of magnetic susceptibility and magneticresonance are described in Section 2.1.6. Experiments for solid state reactions arepotentially simpler than adsorption studies if a controlled atmosphere is not required.Standard methods' 34 for magnetic susceptibility determination can beused. Greatest sensitivity is achieved when a non ferromagnetic substance reactsto form a ferromagnetic substance or vice oersa. If both are ferromagnetic themethod becomes insensitive, but if the Curie points are sufficiently different it maybe possible to choose a temperature above the Curie point of one of the reactants.Hofer et ~ 1 . ~ have ~ ' studied the decomposition of cobalt carbide into a cobaltand carbon; both are ferromagnetic but the specific magnetisation increasessharply during the decomposition. The disproportionation of ferrous oxide359 canbe followed by observing susceptibility changes. By quenching the stoichiometriccomposition, a homogeneous substance can be obtained. Disproportionationoccurs above 300" C according to the equation4Fe0 + Fe3 O4 + Fecausing a rapid rise in the intensity of magnetisation. Many further referencesare to be found in the literature360.Turkevich et ~1.'~' have used EPR techniques to follow the interaction ofmanganous halides with zinc sulphide. Zinc sulphide shows no resonance, buton heating with manganous salts at 600" C the resonance lines appear. Measurementswere made on a Varian spectrometer operating at 9.5 kMc . sec-', with themagnetic field modulated to 100 kc. sec-'. A Varian multipurpose cavity wasused with the sample extending beyond the confines of the cavity. The spin concentrationwas determined with an internal ruby standard.4.7 DILATOMETRYThe measurement of dimensional or volume changes can be used in appropriateinstances to observe the progress of a solid state reaction. Linear expansion canbe measured by dial gauges; micrometers, interferometer, telescopes, lineardifferential transformers and from X-ray patterns. Except for the X-ray techniques,the reaction can be studied in situ. Non-isotropic materials probably requiremeasurements in several orientations.References pp. 27&278