Thermal Decomposition of Hydroxyapatite During Plasma Spraying

2 Thermal Decomposition of Hydroxyapatite During Plasma Spraying 

The extremely high temperature in a plasma jet leads, even during the very short residence time (hundreds of microseconds to few milliseconds, depending on particle density and size) of the hydroxyapatite particles, to dehydroxylation and finally thermal decomposition by incongruent melting. This thermal decomposition of hydroxyapatite in the hot plasma jet occurs in four consecutive steps as shown in Table 6.7. 

Ca0-P205 in the absence of water (Kreidler and Hummel, 1967). The inset shows the region of interest, that is, the incongruent thermal decomposition of hydroxyapatite (Ca/P = 1.67) to a -tricalcium phosphate (Ca/P = 1.5) and tetracalcium phosphate 

The second shell has been heated to a temperature above the incongruent melting point of hydroxyapatite (1570 °C), and hence consists of a molten mixture of TCP and TTCP (Table 6.7, step 3). The outermost spherical shell of the particle comprises solid CaO + melt because evaporation of P2Os shifts the composition along the liquidus towards CaO-richer phases (Table 6.7, step 4). The temperature increases to well beyond 1730 °C, and the only unmelted composition is CaO. 

The existence of OHAp as products of partial dehydroxylation of hydroxyapatite was postulated earlier (Bredig, Franck and Fiildner, 1933 Trombe and Montel, 1978) but the existence of OAp was subject to controversy for many years until it has now been well established thanks to modern analytical techniques (e.g., Gross and Pluduma, 2012 see Chapter 6.2.1.4). Liao etal. (1999) reported that (i) a small amount of OH- ions is always present in the structure of OAp and (ii) even a loss of 75% of the chemically bound water maintains the apatite channel structure. The limiting composition is then Ca10(PO4)6(OH)0 5O0 75 that corresponds to a water loss of 1.34% (Trombe and Montel, 1971). Hence, partially dehydroxylated hydroxyapatite (designated OHAp) could be described either as a non-stoichiometric 

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