No abstract

Calcium oxide samples differing widely in surface activity (specific surface 1 t o 100 m.2g.-l) have been hydrated by exposure t o water vapour at room temperature.Although the rates of hydration differ considerably, each sample takes up in chemical combination a stoicheiometric quantity of water but, in further uptake of water by the nealv formed calcium hydroxide by physical adsorption, the isotherm gives no 'stepwise' evidence of formation of the alleged hemi-hydrate, Ca(OH),.O.5H2O. Changes in surface area and lattice structure during hydration indicate that the most active limes become aged (agglomerated) rapidly in the presence of water vapour. and must be hydrated at very low relative pressures t o maintain the same number of crystallites; limes of moderate activity can be hydrated without ageing; the least active limes undergo hydration only when the number of crystallites increases, the necessary fission of the original crystallites being accompanied by slow rates of hydration. Progressive changes in surface area during hydration are related to the volumes of the oxide and hydroxide on the basis that the reaction proceeds inwards from the outside of each particle by an advancing interface mechanism.Methods are described for the preparation of polymer-modified rubbers by bulk polymerization in natural rubber swollen by methyl methacrylate. The course of the reaction, using different catalyst systems, is followed by recording the temperature rise under conditions of low heat loss, and some suggestions are made as to the molecular weight of polymer formed at different stages of the reaction.The polymer-modified rubbers are characterized by separation into their 'free' and 'graft' components, and their physical properties are described. A large-scale preparation is described where initiation of polymerization is sustained under conditions of low heat loss by the use of a mixed catalyst system.

Methods of nitride production are summarised and their thermodynamics surveyed. Crystal structures and types of bonding in binary and ternary nitride compounds are classified and discussed. Kinetics of nitride formation are related to structural changes in the materials, which control diffusion of metals and nitrogen and cause nitride scaling. Metal nitridation with ammonia and nitride formation during ammonia synthesis are discussed.Information so far available on the sintering of nitrides and its effect on their chemical reactivity is reviewed. This effect is influenced by additives or impurities such as oxides formed by partial nitride hydrolysis and oxidation. Sintering and hot pressing increase the resistance of nitrides to hydrolysis and oxidation, so that they become more suitable for use as refractories. Often, corrosion resistance of nitride layers on metal surfaces is impaired by poor scaling resistance in air or oxygen at comparatively low temperatures. The kinetics and products of oxidation of nitrides so far studied, notably AIN, TiN and UN, depend mainly on the intrinsic reactivity of the material and the available surface at which oxidation can occur.Subsequent work will be concerned with changes in phase composition, surface area and crystallite and aggregate sizes and their correlation with production, sintering, hydrolysis and oxidation conditions for single and mixed nitrides. Suitable experimental techniques are summarked in this paper.

The calcination of calcium hydroxide and of calcium carbonate (precipitated or prepared from calcium oxalate monohydrate) has been studied by determining the changes occurring in the specific surface and lattice structure during each decomposition in uacuo or in air a t a series of temperatures up to 900" or 1000"~.In parallel with earlier observations on calcium hydroxide, the decomposition of the calcium carbonate results in a several-fold increase in specific surface which is ascribed to the activation of the newly-formed calcium oxide when it recrystallizes from a pseudo-lattice of calcium carbonate t o very small individual crystals having a more stable lattice structure. The preliminary decomposition of the calcium oxalate, particularly i n uacuo a t about 450", produces samples of calcium carbonate of much higher specific surface than that of the precipitated form, but they tend to sinter considerably at higher temperatures before decomposing to calcium oxide. Further calcination after complete decomposition always causes sintenng of the calcium oxide, the extent of which is enhanced by increased temperature whether in uacuo or in air.

No abstract