Papers by Author: Marc Bohner

Abstract: Phase-pure α-TCP powder was milled using a high-energy planetary mill to obtain a partially X-ray amorphous material. Calcination at temperatures between 350 and 600 °C was employed to recrystallize the powder. The phase composition as a function of calcination time and temperature was determined in-situ using high-temperature XRD equipment. It was found that the amorphous fraction recrystallized mainly to α-TCP, with only small amounts of β-TCP formed. At low temperatures (≤ 450 °C), a stable composition with approximately 85 wt-% α-TCP was found once 100% crystallinity was reached. The time required to reach full crystallinity depended on the calcination temperature. For temperatures > 450 °C a slow transformation to β-TCP was observed. The transformation rate depended on the calcination temperature and on the milling intensity. A moderately milled powder recrystallized to α-TCP, followed by a slow transformation to β-TCP at 600 °C, whereas an intensely milled powder also recrystallized to α-TCP, followed by a fast transformation to β-TCP at the same temperature.

Abstract: Tests were performed to assess the parameters influencing the injectability of cement pastes loaded with large particles, such as porogens or drug-delivery agents. The use of non-setting model pastes permitted to demonstrate that two phase separation mechanisms occurred simultaneously, i.e. the separation between liquid and powder, known as filter-pressing phenomenon, and the separation between larger and smaller particles.

Abstract: X-ray amorphous tricalcium-phosphate nanoparticles (ATCP) produced by flame spray synthesis were heat-treated at temperatures between 500 and 1000 °C and analyzed in situ by X-ray powder diffraction. The main phase occurring after crystallisation at 525 °C was α-TCP, minor phases were identified as β-TCP and hydroxyapatite. More elevated temperatures induced crystallite growth and the transformation of α-TCP into β-TCP. Above 900 °C no α-TCP was traceable anymore. α’-TCP was not observed in the experiment. This study shows that nanoparticulate α-TCP can be obtained by thermal treatment of an amorphous TCP nanoparticle in a temperature range where sintering effects such as particle growth and densification are moderate or nearly negligible.

Abstract: The effect of composition on the reactivity of a calcium phosphate cement (CPC) made of tricalcium phosphate (TCP) – water mixtures was investigated by isothermal calorimetry at 37°C. The parameters of interest were the mean particle size of the powder, the use of small amounts of nanosized hydroxyapatite powder, and the phosphate concentration and the pH of the aqueous solution. The results could be well explained by theoretical considerations. The main parameter controlling CPC reactivity was TCP particle size.