Search Results

You are looking at 1 - 10 of 97 items for :

  • "Calcium phosphates" x
  • Refine by Access: All Content x
Clear All
Journal of Thermal Analysis and Calorimetry
Authors: Ricardo Simões, Ana Aleixo, Ana Lagoa, Manuel Minas da Piedade, João Leal, Thorsten Peitsch, and Matthias Epple

Abstract  

A thermoanalytical, morphological, and structural study of fluoridated calcium phosphates that were prepared by different variants of a synthesis in anhydrous alcohols is reported. The obtained materials were neither fully amorphous nor single-phased crystalline, and their nature considerably depended on the synthesis conditions. In all cases, the retention of significant amounts of solvent in the solid product was observed. A complete removal of the solvent was only possible by heating to temperatures above ~573–673 K which resulted in variations in the elemental composition, phase changes, and an increase of the crystallinity. Consequently, this synthesis in anhydrous alcohols is not a viable route to obtain materials with a defined crystallinity and stoichiometry.

Restricted access

Abstract  

TL-glow curves of calcium phosphate ceramic powders prepared either from natural or synthetic phosphates have been investigated. The TL-samples from synthetic phosphates, particularly those having Ca/P ratio=2.8, show higher TL-sensitivity, about 1/10 times that of LiF TLD-100 powders, with glow curves having maxima (peaks) around 110, 175 and 325 °C. The TL-response of all phosphate ceramic samples showed a dependence on -dose, well described by a power function in a range from about 1 to 104 Gy, which is useful for therapeutic and radiation processing levels. For regeneration of irradiated TL-samples, annealing around 350 °C for 20 min was found suitable. A low Ca/P ratio can be recommended for high dose measurements, while higher ratio gives greater TL-sensitivity, hence allowing measurement of lower -doses.

Restricted access

Abstract  

A microcalorimeter (Setaram c-80) was used to study the thermokinetics of the hydration process of calcium phosphate cement (CPC), a biocompatible biomaterial used in bone repair. The hydration enthalpy was determined to be 35.8 J g–1 at 37.0°C when up to 80 mg CPC was dissolved in 2 mL of citric buffer. In the present study, parameters related to time constants of the calorimeter were obtained by fitting the recorded thermal curves with the function θ=Ae–?t(1– e–?2t). The real thermogenetic curves were then retrieved with Tian function and the transformation rate of the hydration process of CPC was found to follow the equation α=1–[1–(0.0075t)3]3. The microstructures of the hydrated CPC were examined by scanning electron microscopy. The nano-scale flake microstructures are due to crystallization of calcium phosphate and they could contribute to the good biocompatibility and high bioactivity.

Restricted access

Abstract  

Porous ceramic materials based on calcium phosphate compounds (CFC) have been studied and developed for several biomedical applications such as implants, controlled drug delivery, and radioactive sources for brachytherapy. Two kinds of hydroxyapatite (HAp) powders and their ceramic bodies were characterized. In this study, non-radioactive iodine was incorporated in two types of biodegradable hydroxyapatite-based porous matrices (HA and HACL). The results reveal that both systems present a high capacity of incorporating iodine. The quantity of incorporated iodine was measured by neutron activation analysis (NAA). The porous ceramic matrices based on hydroxyapatite demonstrated a great potential for uses in low dose rate (LDR) brachytherapy.

Restricted access

Abstract  

The coprecipitation of strontium by a calcium phosphate phase formed at an elevated pH 10.8 was investigated. The first phase obtained under these conditions in the amorphous calcium phosphate (ACP) which is transformed into crystalline hydroxyapatite (HA) after the induction period. Is has been shown that this transformation together with morphological changes of the precipitated solid phase, influences the amount of the sorbed strontium significantly. The possible consequences of this finding on practical application of coprecipitation of strontium by calcium phosphate have been discussed.

Restricted access

Abstract  

The effects of Al2O3 and SiO2 additives on the crystallization of calcium phosphate glasses were studied. When the Al2O3 content was higher than 7 mol%, surface devitrification occurred in the glasses. However, for glasses with Al2O3 contents higher than 10 mol%, bulk devitrification predominanted. For the glasses with SiO2, a surface devitrification mechanism predominanted. Non-isothermal DTA techniques were applied in order to establish the devitrification mechanism, and the kinetic parameters of crystal growth were obtained. The parameter m depends on the mechanism and morphology of devitrification of calcium phosphate, glass containing SiO2 as additive, the values of m being lower than 1.2. These results indicate that the devitrification is controlled by the reaction at the glass-crystal interface, or occurs from surface nuclei.

Restricted access

Abstract  

By heating a residue attained by the evaporation of a solution containing calcium phosphate, ruthenium and nitric acid, ruthenium volatilizes. The amount of ruthenium volatilized /both carrier-free106Ru and milligram amounts of ruthenium/ depends on the composition of the solution before evaporation, on the temperature and time of residue heating. At 500 °C up to 90% of ruthenium can volatilize within six min.

Restricted access

Abstract  

The sorption of radioactive strontium by poorly crystalline hydroxyapatite (HA) obtained by the transformation of amorphous calcium phosphate (ACP) at initial pH 10.8, was investigated. Morphological analysis, performed by transmission electron microscopy, has demonstrated that the solid phase consists of nanometer size ultra-micro particles connected into spherical aggregates. A maximum sorption capacity, determined as the asymptote of the sorption isotherm was =(3.8±0.3)·10–2mol Sr/mol P. Based on this result and the structural conditions, a possible theoretical explanation for the incorporation of a strontium ion into the apatite lattice has been proposed.

Restricted access

Abstract  

Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies on uranium doped calcium phosphate yielded mechanistic information on the observed glow peaks at 365, 410 and 450 K. TSL spectral studies of the glow peaks showed that UO2 2+ acts as the luminescent center. Electron paramagnetic resonance studies on gamma-irradiated samples revealed that the predominant radiation induced centers are H0, PO4 2-, PO3 2- and O- ion. Studies on the temperature dependence studies of the EPR spectra of samples annealed to different temperatures indicate the role of H0 and PO4 2- ions in the main glow peak at 410 K.

Restricted access

Abstract  

The precipitation of calcium phosphate was studied using a heat flow twin calorimeter (C80, Setaram, France). The process was carried out using two identical membrane vessels. In the lower parts of both vessels 2 mL of a supersaturated solution (solution A) containing Ca(H2PO4)·2H2O (0.054 M) and CaCl2·H2O (0.125 M) with a molar ratio Ca/P = 1.67 were added. Then 0.05 mL of an ammonium solution (25% w/w) (solution B) and 0.05 mL of distilled water were transferred in the upper parts of sample and reference vessels, respectively. After temperature had been maintained at 303, 313, 323 and 333 K the membranes of both sample and reference vessels were broken simultaneously. The precipitation process also repeated with the same conditions for periods of 15, 60 and 120 min in a bath. The first two calorimetric curves (303, 313 K) show a single exothermic step during the process as a sharp peak in the initial stage. On the contrary at the experimental temperature of 323 K except of the sharp peak in the initial stage, a steadily exothermic tendency appears after 100 min time. In higher temperature (333 K) the sharp peak appears in the initial stage followed by a broad exothermic step between 75 and 320 min time. The XRD analyses show that the solids in the initial experimental stages are mainly consisted of dicalcium phosphate dihydrate (DCPD) for the lower temperature and a biphasic or triphasic system consisted of hydroxyapatite (HA), dicalcium phosphate anhydrous (DCPA) and octacalcium phosphate (OCP) for the rest temperatures. The XRD analyses show also that during the solution aging the initial products are transformed into the more stable thermal forms of HA and octacalcium phosphate (OCP).

Restricted access