Study of neck formation and densification in porous hydroxyapatite ceramics using thermal conductivity measurements

Neck formation and densification during sintering have strong effects on the thermal conductivity of a porous ceramic body. This has been described by an analytical model using grain conductivity, grain size, pore fraction and particle - particle contact area as input parameters. It has been tested on hydroxyapatite ceramics sintered with conventional, microwave and spark plasma techniques. The green bodies containing at least 40% porosity yield conductivity values in the range 0.24-0.29 Wm- 1K-1. Neck formation in the initial stage of sintering increases the values to above 0.5 Wm- 1K-1. Further increase is achieved by densification, well described by Landauer's relation as part of the model with close agreement to experiment for hydroxyapatite ceramics containing 40 to 5% porosity. An evaluation of thermal conductivity for 100% dense hydroxyapatite gives a value of 1.5 Wm- 1K-1 which is almost constant between room temperature and 900 degrees C.