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  • 1
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract The effects of synthesis conditions on the quantitative preparation of α-tricalcium phosphate (α-TCP) have been investigated. The following parameters of the synthesis were considered: nature of the starting material-Ca-deficient hydroxyapatite, DAP, versus hydroxyapatite-anhydrous dicalcium phosphate mixtures (HAP-DCPA); Ca/P atomic ratio of the mixture, calcination temperature and time, and cooling rate. The yield and crystallinity of the final product have been estimated using X-ray diffraction (XRD) and solid state 31P magic angle spinning NMR (MAS-NMR) techniques. The results show that pure, well-crystallized α-TCP powders exhibiting nearly ideal MAS-NMR spectra, can be obtained by reactive sintering of HAP-DCPA (Ca/P=1.50...1.52) mixtures, at 1400°C for 8 h. The broadening of MAS-NMR spectra can be used as an indicator of structural order in the final product. The α-TCP yield with DAP was always less than 50%.
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  • 2
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract The pH of a hydraulic calcium phosphate cement (HCPC) made of monocalcium phosphate monohydrate (Ca(H2PO4)2·H2O; MCPM), β-tricalcium phosphate (β-(Ca3(PO4)2; β-TCP) and water was measured as a function of reaction time and composition at room temperature. During setting, the cement pH varies from very acidic pH values, i.e., 2.5, to almost neutral pH values, i.e., 6. The cement pH profile significantly depends on the initial cement composition. However, all profiles are characterized by a sharp initial decrease of the pH due to the dissolution of MCPM crystals and the precipitation of dicalcium phosphate dihydrate (CaHPO4· 2H2O; DCPD) crystals. With an excess of MCPM, the final pH stays low, and its value can be predicted from the initial composition of the cement and solubility data. With an excess of β-TCP, the end pH is close to 5, which is much lower than 5.9, the value predicted by calculation. Results suggest that the difference may be due to the presence of impurities in the cement. Replacing MCPM by phosphoric acid renders the cement paste very acidic for the initial 30 s, but then the pH profile follows that obtained with MCPM. Adding pyrophosphate ions into the cement paste postpones the position of the pH minimum. The delay, which is proportional to the concentration of pyrophosphate ions, is thought to be due to the inhibiting action of pyrophosphate ions on the precipitation of DCPD crystals.
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  • 3
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract Combinations of citrate (C6H5O 7 3-− ), pyrophosphate (P2O 7 4− ) and sulfate (SO 4 2− ) ions were used to modify the physico-chemical properties of a calcium phosphate cement (CPC) composed of β-tricalcium phosphate (β-TCP) and phosphoric acid (PA) solution. The results obtained with only one additive at a time are similar to those previously published. New facts are: the positive effect of C6H5O 7 3− ions on cement failure strain and their negative effect on cement pH. The position of the setting time maximum measured at an SO 4 2− concentration of 0.09 M was not displaced by the addition of C6H5O 7 3− and P2O 7 4− ions. However, the effect of SO 4 2− ions on the setting time was depressed by C6H5O 7 3− ions. Moreover, no increase in tensile strength was observed when increasing amounts of SO 4 2− were added into a C6H5O 7 3− -containing cement. The latter results suggest a competitive effect of C6H5O 7 3− and SO 4 2− on setting time and tensile strength. Anhydrous dicalcium phosphate (DCP; CaHPO4) appeared in cement samples dried just after setting, but not in cement samples incubated for 24 h in deionized water before the drying step. It is believed that the setting reaction is stopped by the drying step, leaving a low internal pH in the sample, hence providing favorable conditions for the transformation of dicalcium phosphate dihydrate (DCPD) into DCP. Interestingly, even though C6H5O 7 3− ions dramatically lowered the equilibrium pH of the cement with 5 ml of deionized water, they still prevented the occurrence of the transformation of DCPD into DCP.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Journal of materials science 11 (2000), S. 155-162 
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract Cement samples made of β-tricalcium phoshate (β-TCP), phosphoric acid (PA) and water mixtures were incubated in several aqueous solutions to determine their stability over time. The effects of the cement composition and the incubating temperature were investigated in more detail. The cement samples contained mostly dicalcium phosphate dihydrate (DCPD) and remnants of β-TCP crystals. Depending on the initial cement composition, a certain amount of dicalcium phosphate (DCP) crystals were formed. The larger the initial PA concentration, the larger the DCP amount. After setting, the cement composition was stable for at least 16 days up to 60 °C. Above that temperature, the DCPD crystals decomposed into DCP crystals. The latter reaction provoked a decrease of the pH of the incubation solution, phenomenon expected for a cement sample containing an excess of PA. As the cement samples contained an excess of β-TCP, it was postulated that β-TCP crystals became so covered by DCP or DCPD crystals during setting that the setting reaction was stopped prematurely. The latter phenomenon gave a good explanation for the low pH values measured in the incubation solutions. ©©2000 Kluwer Academic Publishers
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