Powder quality requirements

The majority of Si3N4 ceramics are made from powders and therefore depend to a large extent on the quality of the starting powders. Si3N4 ceramics of superior quality require well characterized and often extremely pure powders. The powders determine the processing, the sintering behaviour and the... 

Powder preparation by solid-state reactions generally has an advantage in terms of production cost, but as outlined earlier, the powder quality is also an important consideration for advanced ceramics. The powders are normally agglomerated and a grinding step is almost always required to produce powders with better characteristics. Grinding in ball mills leads to the contamination of the powder with impmities. Incomplete reactions, especially in poorly mixed powders, may produce undesirable phases. Furthermore, the particle shape of ground powders is usually difficult to control. 

The choice given to the production engineer or research powder technologist is thus dependent upon the level of information required. In the design of mass flow hoppers there tends to be a greater need for knowledge on yield loci, stresses and failure criteria than that possibly needed for routine powder quality control. 

Phthalide. In a 1 litre bolt-head flask stir 90 g. of a high quality zinc powder to a thick paste with a solution of 0 5 g. of crystallised copper sulphate in 20 ml. of water (this serves to activate the zinc), and then add 165 ml. of 20 per cent, sodium hydroxide solution. Cool the flask in an ice bath to 5°, stir the contents mechanically, and add 73-5 g. of phthalimide in small portions at such a rate that the temperature does not rise above 8° (about 30 minutes are required for the addition). Continue the stirring for half an hour, dilute with 200 ml. of water, warm on a water bath imtil the evolution of ammonia ceases (about 3 hours), and concentrate to a volume of about 200 ml. by distillation vmder reduced pressure (tig. 11,37, 1). Filter, and render the flltrate acid to Congo red paper with concentrated hydrochloric acid (about 75 ml. are required). Much of the phthalide separates as an oil, but, in order to complete the lactonisation of the hydroxymethylbenzoic acid, boil for an hour transfer while hot to a beaker. The oil solidifles on cooling to a hard red-brown cake. Leave overnight in an ice chest or refrigerator, and than filter at the pump. The crude phthalide contains much sodium chloride. RecrystaUise it in 10 g. portions from 750 ml. of water use the mother liquor from the first crop for the recrystaUisation of the subsequent portion. Filter each portion while hot, cool in ice below 5°, filter and wash with small quantities of ice-cold water. Dry in the air upon filter paper. The yield of phthalide (transparent plates), m.p. 72-73°, is 47 g. 

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