Phosphorous compounds soluble reactive phosphorus

In the past, many variations of the sol-gel process have been developed and used to produce powders with different Ca/P ratios, by altering not only the quantity and composition of precursors but also the processing variables. As the synthesis of HA requires a calcium to phosphorus molar ratio of 1.67 1 in the final product, a number of caldum/phosphorus precursor combinations have been used in the preparation of HA powders. For example, calcium nitrate or different calcium alkoxides and 2-ethyl-hexyl phosphate, triethyl phosphate or orthophos-phoric acid, have been used as the calcium and phosphoras precursors, respectively. The major limitation for appUcation of the sol-gel process was shown to be the very poor solubility of the calcium alkoxides in organic solvents, and the poor reactivity of the phosphorous compounds. The effective control of stoichiometry due to the volatility of the phosphorous compounds used also represented a challenge. 

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