Reactions and phase transformations in Monocalcium Phosphate Monohydrate (MCPM) systems

Abstract

This research project mainly focuses on the characteristic reaction processes and phase transformation (transition) in the system of monocalcium phosphate monohydrate (MCPM, Ca(H2PO4)2∙H2O) – base and covers two types of reactions: aqueous reactions and solid-state reactions. Firstly, a preliminary research, which deals with the influences of the reaction conditions, such as pH, pH buffer reagent, ageing and stirring, in the MCPM – base system has been carried out. It is found that pH is a key factor determining the phase formation. As the pH level increases, the sequence of the phase formation from precipitation reaction is from dicalcium phosphate dihydrate (DCPD, CaHPO4∙2H2O) to calcium-deficient (hydroxy) apatite (Ca10-x(HPO4)x(PO4)6-x(OH)2-x, 0 < x ≤ 1) and finally to stoichiometric apatite. The pH buffer reagent, such as NaOH or NH3∙H2O, is also an important factor with respect to the determination of the precipitated phase. A strong base, for example NaOH, promotes the formation of apatite with higher stoichiometry, in comparison with a weak base, such as NH3∙H2O. The micro-morphology of apatite precipitated from NaOH is more homogeneous than that of apatite formed from NH3∙H2O. The MCPM – NaOH is therefore set as the research system onwards. In addition, ageing is essential to the phase maturation of apatite, which is then extensively explored in the MCPM – NaOH system. Also stirring is deemed to accelerate the reaction process. Based on the preliminary results, the MCPM – NaOH system, both aqueous reactions and the subsequent solid-state reactions have been separately investigated. In the study of aqueous reactions, a continual change of Ca:P ratio in monocalcium phosphate monohydrate (MCPM) by increasing the amount of NaOH added in to the MCPM is reported. The Ca:P ratio is observed to gradually change from 1.0, which corresponds to dicalcium phosphate dehydrate (DCPD), to stoichiometric 1.67 of apatite. It is proposed that the high solubility of MCPM results in fast dissolution and reprecipitation. A multi-step chemical reaction is proposed to elucidate the reaction sequence in the aqueous MCPM-NaOH system. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation is applied to the kinetic study. The phase transformation of MCPM to apatite has been accelerated by increasing the NaOH molarity. The high solubility gives MCPM the ability of fast dissolution and reprecipitation. Chemical reactivity and sensibility of MCPM towards base (NaOH) has been verified. Based on the dissolution-reprecipitation mechanism, a model to elucidate the microstructural evolution is proposed. Depending on the attained Ca:P ratio in the powder synthesized in the aqueous solutions, subsequent calcination of these chemically formed powders leads to the formation of various single phasic calcium phosphates or biphasic compounds. It is observed that β-calcium pyrophosphate (β-CPP, β-Ca2P2O7) is produced from calcination of DCPD, while β-tricalcium phosphate (β-TCP, β-Ca3(PO4)2) results from thermal reactions between DCPD and apatite. The biphasic mixtures of β-TCP and hydroxyapatite (HA, Ca10(PO4)6(OH)2) can be achieved from thermal decomposition of calcium-deficient apatite. The phase composition of β-TCP/HA biphasic calcium phosphate is determined by the calcium-deficiency of apatite. The more calcium-deficient the apatite is, the more β-TCP exists in the biphasic compound. The kinetics of the thermal decomposition process of calcium-deficient apatite is characterised using the JMAK equations. The thermal behaviours of calcium-deficient apatite with different calcium-deficiencies have been compared. It is discovered that the more calcium-deficient the apatite is, the faster the decomposition. This is explained by the vacancy mechanism of diffusion in solids. The sinter-ability of β-TCP/HA biphasic ceramics with two different phase compositions has been examined. It was revealed that the formation of more β-TCP is detrimental to the densification of the biphasic ceramics during sintering. Several characterization techniques have been used throughout the project. They include powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), high angle annular dark field (HAADF) imaging under scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM) and cryogenic transmission electron microscopy (Cryo-TEM).