Densification behavior of hydroxyapatite green pellets prepared by different methods

Abstract

The densification behavior of the green pellets with different densities prepared from hydroxyapatite powders consisting of plate-like particles was investigated. The green pellets were prepared by two methods, hydrothermal hot-pressing and uniaxial pressing, at different compression pressures. The hydrothermal hot-pressing gave the green pellets with densities higher than those using with uniaxial pressing. The shrinkage of the green pellets was observed by calcination at temperature above 800°C. The green pellets with higher bulk density gave the sintered compacts with higher density. When the calcination temperature was increased, densification proceeded with the increase in pore diameter and the decrease of pore volume. Even at temperature below 700°C, the size of pores increased with the increase in calcination temperature without shrinkage of the green pellets. Thus, the green compacts of the same density but with different pore sizes distribution prepared by hydrothermal hot-pressing and uniaxial pressing, had similar pore size distribution after calcination at 700°C, As a result, they had the same densification behavior by calcination at high temperature above 800°C. The densification behavior was found to be dependent on the density of green pellets, but not on the method of preparation of green pellets.

Low-temperature densification of Mg-doped hydroxyapatite fine powders under hydrothermal hot processing conditions

“…The present hypothesis is experimentally supported by the results reported at the beginning of the current decade by one of the present authors [22]. Who firstly proposed the processing technology for accelerating the solid-state sintering stage of calcium hydroxyapatite powders, which involves a preliminary particle consolidation step conducted under constant loading and assisted by a hydrothermal reaction at very low temperatures (150 -200 ºC).…”

“…The first approach involved the in-situ formation of HA particles obtained by the chemical reaction between the precursor reactants and water [18][19][20][21][22][23][24][25]. The second approach consisted of the densification of chemically coprecipitated HA powders with water under hydrothermal hot pressing conditions.…”

“…Previous studies have investigated the preparation of highly packed structures to obtain dense and controlled porous HA monolithic specimens for different potential applications using the hydrothermal hot pressing (HHP) technique. Dense specimens with high mechanical strength can be obtained under continuous loading at low temperatures (150-300 ºC), which is achieved according to interactions between the particles and water (used as the solvent medium) [18][19][20][21][22][23][24][25]. Over the last two decades, several researchers have studied the densification of pure HA powders under HHP conditions.…”

“…An alternative processing route to obtain denser HA monolithic specimens has recently been proposed. This method involves HA powder densification at low temperatures (200 ºC) by HHP and a subsequent sintering treatment conducted at high temperature [22]. Hence, hydrothermal hot-pressing has proved to be effective for producing dense pellets by accelerating the sintering kinetics and therefore constitutes an emerging new processing route for the production of high-density bioceramic materials [22].…”

“…In recent years, many efforts have been made to improve the densification of HA particles [18][19][20][21][22][23][24][25]27]. …”