Transparent Hydroxyapatite Ceramics through Gelcasting and Low‐Temperature Sintering

Abstract

Transparent hydroxyapatite (HAP) was prepared by sintering gel-cast powder compacts at 1000°C for 2 h; the resultant HAP material was studied using X-ray diffractometry, transmission electron microscopy, scanning electron microscopy, and microhardness measurement. Nanoscale HAP crystallites were prepared using a precipitation method that involved calcium nitrate and ammonium dihydrogen orthophosphate solutions; the preparation was conducted at a temperature of 0°C. The precipitate was gel-cast and sintered at 1000°C in the form of a transparent ceramic that had a uniform grain size of 250 m. The maximum Vickers microhardness obtained for a sample sintered at 1000°C was 6.57 GPa. The sintering behavior of gel-cast samples prepared from high-temperature-precipitated HAP was compared with that of material prepared at 0°C.

Morphology‐Enhanced Low‐Temperature Sintering of Nanocrystalline Hydroxyapatite

“…[24][25][26][27][28][29] The lowest sintering temperature ever reported in the literature is 1000°C with the use of uncalcined nanometer-sized powder particles. [30,31] However, using HA nanorods, we demonstrate in this study that dense (> 99 % of the theoretical) HA nanocrystalline bodies can be attained at 850 or 900°C through morphology-enhanced diffusion and driving force for densification. This new mechanism for increasing the diffusion rate and driving force opens up possibilities to obtain advanced ceramics and composites with enhanced properties or new functionalities via low-temperature sintering.…”

“…The current strategy in the scientific community for lowtemperature sintering of HA is through the use of uncalcined nanometer-sized powder particles, which results in dense HA bodies (> 99 %) at temperatures as low as 1000°C. [30,31] To further reduce the sintering temperature, we report here the use of HA nanorods (Fig. 1a) as the starting powder to attain dense nanocrystalline bodies (> 99 %) at 900°C with holding for 2 h or at 850°C with holding for 24 h. The HA nanorods are synthesized via a wet precipitation process using reagentgrade calcium nitrate 4-hydrate, Ca(NO 3 ) 2 · 4 H 2 O, and diammonium phosphate, (NH 4 ) 2 HPO 4 .…”

Medical Application of Calcium Orthophosphate Bioceramics

“…They can exhibit an optical transmittance of ~ 66 % at a wavelength of 645 nm [370]. The preparation techniques, for example, include a hot isostatic pressing [204,206], an ambient-pressure sintering [362], gel casting coupled with a low-temperature sintering [365,368], a pulse electric current sintering [366], as well as a spark plasma sintering [289,292]. Fully dense, transparent calcium orthophosphate bioceramics was obtained at temperatures above ~ 800 °C.…”

“…Fully dense, transparent calcium orthophosphate bioceramics was obtained at temperatures above ~ 800 °C. Depending on the preparation technique, the transparent bioceramics has a uniform grain sizes ranging from ~ 81 nm [370] to ~ 250 μm [365] and always is pore-free. Furthermore, a translucent calcium orthophosphate bioceramics is also known [371][372][373].…”

“…Thus, transparent calcium orthophosphate bioceramics (Fig. 5) [361] have been prepared and investigated [204,206,289,292,[361][362][363][364][365][366][367][368][369][370]. They can exhibit an optical transmittance of ~ 66 % at a wavelength of 645 nm [370].…”