Zur Kenntnis der Reaktionen in festem Zustande. VII

Investigation of exchange reactions between alkaline earth metal oxides and some transition metal halides

“…Thermal Anal. 4,1972 action products appear. On the further increase of temperature, the electrical conductivity again increases, due to the melting of the reaction product BaBr~_ at 730 ~ in the presence of unreacted starting reagents and the second product, Cu20.…”

“…On the further increase of temperature, the electrical conductivity again increases, due to the melting of the reaction product BaBr~_ at 730 ~ in the presence of unreacted starting reagents and the second product, Cu20. The appearance of the liquid phase at 360 ~ is due to the melting of the Ba(OH)zBan eutectic [4,7]. According to the available data [13] the Ba(OH)2-BaBr 2 euteetic melts at 345 ~ The appearance of the liquid phase exerts no decisive effect on the reaction rate since it improves the contact between the solid reagent grains, whereas the melting of the BaO-Ba(OH)2 eutectic results in the transition of one of the initial reagents to a liquid phase and the reaction proceeds between the solid CuBr and liquid Ban.…”

“…This phenomenon of the constancy of the temperature at which one reagent begins to interact with others is referred to as the Hedvall effect. Since the alkaline earth metal oxides are strongly hygroscopic, this constancy of the temperature at which interaction starts led some investigators to suggest that Hedvall and Tamman had used hydroxide-contaminated oxides whose melting points were rather low, and the interaction had been brought about by the presence of a liquid phase [4,5]. A number of attempts were made [6][7][8] to reproduce the Hedvall effect.…”