| Recently,a novel sol-gel technique which uses calcium nitrate tetrahydrate and phosphoric pentoxide as Ca and P precursors has been developed to prepare nano HA powder.Compared with other sol-gel processes, it is a simple way which does not require controlling pH value and long time. However,the reaction mechanism of this sol-gel process has not known.In this paper,the phase transformation and reaction mechanism of the sol-gel approach were studied systematically.The further applicatin of the sol-gel process was explored to prepare for pure HA,dense biphasicβ-TCP/HA ceramics,high-temperature stabilizedβ-tricalcium phosphate and porous calcium phosphate ceramics.In order to achieve pure HA or TCP,the phase transformation and reaction mechanism of the sol-gel approach were firstly studied via a series of experiments associated with the the decomposition of HA at high temperatures. The experimental results indicated that a non-uniform mixing of the reactants at molecular-level could result in the decomposition of HA at high temperatures even though an average Ca/P ratio was 1.67 for the reactants. Introduction of ethylene glycol into the the gel could improve stability of the sol-gel system and ensure a molecular-level mixing of the calcium and phosphorus so that pure HA or TCP was finally obtained.In order to prepare denseβ-TCP/HA ceramic,a novel method was applied.The above mentioned sol-gel process was engaged to synthesize a sol with a Ca/P molar ratio of 0.56(Ca0.56P) and then it was introduced into pure HA powder not only to be used to create liquid phase to enhance densification but also to allow it to react with HA to in-situ form biphasic HA/β-TCP ceramics during sintering.The effect of the content of Ca0.56P on biphasicβ-TCP/HA components and densification during the sintering process was investigated.The results indicated that biphasic ceramic with higher density could be obtained at lower sintering temperatures by using this method compared with the conventional sintering approach and that the ratio of HA andβ-TCP could be controlled by the amount of Ca0.56P introduced into HA powder.And in addition,a novelty method based on the sol-gel process was used for the preparation of high-temperature stabilizedβ-tricalcium phosphate. Firstly,HA powder was mixed with the ethanol solutions of phosphorous pentoxide homogeneously to obtain a composition powder.And thenβ-TCP ceramic was in-situ formed during the sintering process.The experimental results demonstrated that the phase transformation temperature ofβ-TCP toα-TCP could be elevated to 1300℃via this technique.Porous calcium phosphate ceramics were prepared by using the sol-gel process.The significant advantage associated with this approach is mainly reflected by one-step in situ synthesis and the synthesis procedure of powder was eliminated in this study compared with conventional methods.The porous calcium phosphate ceramics obtained via this approach exhibited an open, interconnected and sponge-like structure with uniform big pores and small pores distributed on the big pore walls.The sizes of big and small pores were within 300～800μm and 20～300μm,respectively.The pore size and porosity of ceramic foams could be controlled via temperature,the amount of ethanol,and foam stabilizers during the formation of porous gel.Different calcium phosphate ceramics could be obtained only by controlling the ratio of calcium nitrate tetrahydrate and phosphoric pentoxide.
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