Synthesis and crystallographic study of Pb-Sr hydroxyapatite solid solutions by high temperature mixing method under hydrothermal conditions

The solid solutions in the system of Pb and Sr hydroxyapatite, Sr_10−xPb_xHAp (x = 0-10), were successfully synthesized by high-temperature mixing method (HTMM) at 160 °C for 12 h under hydrothermal conditions. The samples were characterized by X-ray diffraction, chemical analysis and electron microscopic observation, and the site of the metal ions in the solid solutions was analyzed with the Rietveld method. The lattice constants, both a and c, of the solid solutions varied linearly with Pb content. It was found that Pb ions in the solid solutions preferentially occupied the M(2) site in the apatite structure. HTMM gives Sr-Pb HAp solid solutions much better crystallization. However, due to the formation of intermediate compound of Pb₃O₂(OH)₂ in the Pb(NO₃)₂·4H₂O solution before mixing with (NH₄)₂HPO₄ solution at 160 °C, HTMM causes the decrease of crystallization of the samples with high Pb content.     

Coating of hydroxyapatite on various substrates via hydrothermal reactions of Ca(edta)2- and phosphate

Hydroxyapatite was coated on various substrates such as 12 mol % ceria-doped tetragonal zirconia (12Ce-TZP), 3 mol % yttria-doped tetragonal zirconia (3Y-TZP), alumina, monetite coated titanium (Ti/CaHPO₄) and calcium titanate coated titanium (Ti/CaTiO₃) via hydrothermal reactions of Ca(edta)^2- and 0.05 M NaH₂PO₄ at initial pH 6 and 160–200 °C for 0.5–6 h. Rod-like particles of hydroxyapatite precipitated to form film on the substrates above 160 °C. The morphology of the film changed significantly depending on the characteristics of substrate, i.e. hydroxyapatite entirely coated the surfaces of 12Ce-TZP, Ti/CaHPO₄ and Ti/CaTiO₃ plates, but sparsely deposited on 3Y-TZP and Al₂O₃ plates. Film thickness increased with time (ca. 20 and 90 m on 12Ce-TZP plates for 0.5 and 6 h, respectively, at pH 6 and 200 °C). The adhesive strength of the film for the substrate was in the order, 12Ce-TZP/hydroxyapatite(28 MPa) > Ti/CaTiO₃/hydroxyapatite (22 MPa) > Ti/CaHPO₄/hydroxyapatite (9 MPa). © 2001 Kluwer Academic Publishers     

Formation of filamentous carbon through dissociation of chromium carbide under hydrothermal conditions

Synthesis of filamentous carbon through the decomposition of chromium carbide was studied employing hydrothermal technique in the pressure and temperature range of 100–200 MPa and 350–800 °C respectively. It was found that chromium carbide dissociates into chromium oxide in the presence of water at temperature <400 °C. But, the formation of free elemental carbon as filamentous particles was noticed in the presence of organic compounds at temperatures above 600 °C. The organic compounds are known to dissociate to C–O–H supercritical fluids under hydrothermal condition. The supercritical fluids generated by the dissociation of organic compounds have great influence on the decomposition of chromium carbide. The scanning electron microscopic (SEM) studies of the experimental run products show that the fibrous or filamentous form of carbon was found with a few spherical shaped carbons, in the chromium carbide—organic compound runs. These carbon particles were solid curved filaments with a mean diameter of 50–100 nm. Micro Raman spectroscopic studies show that the filaments have sp ² -hybridized carbon atoms.     

Coating of CaTiO3 on titanium substrates by hydrothermal reactions using calcium-ethylene diamine tetra acetic acid chelate

Titanium plates were treated in [Ti(O₂)EDTA]^2-– -Ca(EDTA)^2- mixed solutions and/or Ca(EDTA)^2- solutions (where EDTA is ethylene diamine tetra acetic acid) at pH 9–13 and 150–250 °C for 0.5–12 h. The film, about 50 m thick, and consisting of mixtures of CaTiO₃ and TiO₂ was formed in 0.01 M [Ti(O₂)EDTA]^2- – 0.01 M Ca(EDTA)^2- mixed solution at pH 13 and 250 °C for 6 h. The film consisted of large icosahedral and hexagonal particles, of about 10 m diameter, and small aggregated particles, of about 1 m diameter. On the other hand, the film, about 20 m thick, consisted of hexagonal plate-like CaTiO₃ particles, of about 1 m diameter, was formed in 0.01 M Ca(EDTA)^2- solution at pH 13 and 250 °C for 6 h. The thickness of both films increased with time, where the film formation rate in 0.01 M [Ti(O₂)EDTA]^2- – 0.01 M Ca(EDTA)^2- mixed solution was much faster. The CaTiO₃ film formed on the surface of titanium promoted the precipitation of hydroxyapatite on the substrate by the hydrothermal reactions in Ca(EDTA)^2-–PO ₄ ^3- mixed solutions.     

Direct fabrication of TiO2 nanoparticles deposited on hydroxyapatite crystals under mild hydrothermal conditions

TiO2 nanoparticles-deposited on hydroxyapatite (HAp) have been successfully synthesized by direct (single step) hydrothermal treatments of a CaCO3 suspension in a H3PO4 solution with 10 vol% TAS-FINE (titanium amine complex) at 150 degrees C for 6 h or 120 degrees C for 12-24 h under nearly neutral pH conditions. The obtained products were characterized by XRD, SEM-EDX, visible, Raman, and TEM. The XRD and Raman results showed the formation of HAp and TiO2 anatase phases under these hydrothermal conditions. SEM and TEM observations revealed that anatase TiO2 nanoparticles with the size of about 10 nm were deposited on the surfaces of the HAp crystals.     

Effect of pH on crystallization of sputtered hydroxyapatite film under hydrothermal conditions at low temperature

Hydroxyapatite (HA) was coated onto titanium substrates using radio frequency sputtering, and the as-sputtered films were placed under hydrothermal conditions in distilled water solutions at pH 5.0, 7.0 and 9.0 and 110°C. The crystallinity, the Ca/P ratio, thickness, and the surface of the films were observed using XRD, EDS, and SEM, respectively.The as-sputtered film was crystallized in distilled water at varying pH after the hydrothermal treatment, and the crystallinity of the film increased with treatment time. The HA crystal size increased with pH. At pH 5.0, β-TCP was produced as well as HA. The Ca/P ratio of the film decreased with increasing treatment, and the ratio at pH 9.0 was 1.74 after 48 h, while in pH 5.0 and 7.0 it was 1.63. After hydrothermal treatment, the film remnant ratio increased with pH, with 75.9, 82.4 and 91.7% of the film remaining after 48 h at pH 5.0, 7.0 and 9.0, respectively.     

An optical technique forin situ density determination in electrolyte solutions under hydrothermal conditions

We report on a technique which determines the density of hydrothermal solutions through the relationship between density and refractive index given by the Lorentz Lorenz equation. An optical cell is described which allows the refractive index of fluids to be measured at temperatures up to 823 K and pressures up to 1200 bar. The validity of the Lorentz-Lorenz equation is demonstrated for pure water and water/NaCl solutions. New experimental results are presented for the density of water/NaNO₃ solutions at high temperature and pressure. The use of density data to derive other properties such as compressibility, thermal expansion coefficient, and apparent molar volume is demonstrated.     

The thermal stability of hydroxyapatite in biphasic calcium phosphate ceramics

Biphasic calcium phosphate ceramics (BCP) comprising a mix of non-resorbable hydroxyapatite (HA) and resorbable β-tricalcium phosphate (β-TCP) are particularly suitable materials for synthetic bone substitute applications. In this study, HA synthesised by solid state reaction was mechanically mixed with β-TCP, then sintered to form a suite of BCP materials with a wide range of HA/β-TCP phase content ratios. The influence of sintering temperature and composition on the HA thermal stability was quantified by X-ray diffraction (XRD). The pre-sinter β-TCP content was found to strongly affect the post-sinter HA/β-TCP ratio by promoting the thermal decomposition of HA to β-TCP, even at sintering temperatures as low as 850 °C. For BCP material with pre-sinter HA/β-TCP = 40/60 wt%, approximately 80% of the HA decomposed to β-TCP during sintering at 1000 °C. Furthermore, the HA content appeared to influence the reverse transformation of α-TCP to β-TCP expected upon gradual cooling from sintering temperatures greater than 1125 °C. Because the HA/β-TCP ratio dominantly determines the rate and extent of BCP resorption in vivo, the possible thermal decomposition of HA during BCP synthesis must be considered, particularly if high temperature treatments are involved.     

Synthesis and thermal behaviour of nanostructured ZrO2 powders obtained under hydrothermal conditions

Nanocrystalline zircone (ZrO2) powders were synthesized from zirconium oxyacetate aqueous solution treated up to 240 degrees C under autogeneous pressure. Depending on experimental conditions, colloidal particles of 26-150 nm are formed by aggregation of zirconia crystallites. The effect of some experimental conditions over the colloidal particle size or zirconia phase composition was studied. The powder sintering behaviour is also presented.     

Characterization of macroporous carbonate-substituted hydroxyapatite bodies prepared in different phosphate solutions

Bone mineral of human is different in composition from the stoichiometric hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) in that it contains additional ions, of which CO₃²⁻ is the most abundant species. Carbonate-substituted hydroxyapatite (CHA) bodies were prepared by the hydrothermal treatment of highly porous calcium carbonate (CaCO₃) body at 120 °C in 1 M M₂HPO₄ and M₃PO₄ solutions (M = NH₄ or K). It was found that CaCO₃ body was almost transformed into CHA body after hydrothermal treatment for 24 h irrespective of type of phosphate solution. However, a small amount of CaCO₃ still remained after the treatment in K₃PO₄ for 48 h. Crystal shape of CHA bodies prepared in those solutions except for K₂HPO₄ was flake-like, which was different from that (stick-like) of original CaCO₃ body used for the preparation of CHA body. CHA prepared in the K₂HPO₄ showed globule-like crystal. Average pore size and hole size of the CHA bodies were 150, 70 μm and their porosities were about 89% irrespective of the solution. Carbonate content was slightly higher in the CHA bodies obtained from potassium phosphate solutions than in those obtained from ammonium phosphate solutions. Mostly B-type CHA was obtained after the hydrothermal treatment in the potassium phosphate solutions. On the other hand, mixed A- and B-type CHA (ca. 1–2 in molar ratio) was obtained in the ammonium phosphate solutions. The content of CO₃²⁻ in the CHA body depended on the type of phosphate solution and was slightly larger in the potassium phosphate solutions.     

Intercalation of kaolinite under hydrothermal conditions

Intercalation of kaolinite with dimethylsulphoxide (DMSO) was carried out under hydrothermal conditions using a Parr bomb. The intercalated complex has been characterized by X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR) and thermogravimetric analysis techniques. It was found that the intercalation reaction is completed in 20 min and a high yield, 90%, of kaolinite/DMSO intercalate was obtained. An expansion of the original c-axis spacing of 0.714 nm in kaolinite to 1·124 nm, as a result of incorporation of DMSO molecules into the kaolinite interlayers, has been observed by XRD studies. FT-IR analyses of the intercalated complex have established the nature of interactions between the host structures and the incoming DMSO molecules. The hydrothermal method of intercalation developed in this study can also be used for other systems.     

Oxygen self-diffusion in rutile under hydrothermal conditions

Oxygen self-diffusion coefficients have been determined for synthetic and natural rutile single crystals under hydrothermal conditions at 100 MPa total pressure and in the temperature range 873–1373 K. The diffusion coefficients are lower than the results from dry gas exchange studies would predict. Between 973 and 1373 K the results can be characterized by two linear Arrhenius relationships. D=1.14×10^–11 exp(–168.8 kJ mol^–1/RT) m^–2s^–1 for the natural rutile, and D=2.41×10^–12 exp(–172.5 kJ mol^–1/RT) m²s^–1 for the synthetic crystal. The results have been interpreted in terms of a defect model involving the dissolution of water in rutile as substitutional hydroxyl defects on oxygen lattice sites, with a solution enthalpy in the range 81–106 kJmol^–1.     

工艺条件对低温燃烧/水热法合成纳米羟基磷灰石的影响

以Ca(NO3)2·4H2O、(NH4)2HPO4和柠檬酸为原料,浓硝酸为溶剂,NH4NO3为助燃剂,采用低温燃烧/水热法能够快速合成羟基磷灰石纳米粉体.通过对不同工艺条件下得到的试样进行XRD、SEM等分析,研究了柠檬酸和NH4NO3的加入量对燃烧反应的影响,以及水热处理的温度和时间对水热反应的影响.研究结果表明,选择合理的工艺条件能够快速合成高纯度的羟基磷灰石纳米粉体.     

Topotactic crystallisation of calcite under hydrothermal conditions

It is shown that control of the phase of calcium carbonate produced during hydrothermal crystallisation can be achieved by the use of titania as a catalyst. In this work it was demonstrated that if either amorphous or the rutile phase of titania is added the aragonite polymorph of calcium carbonate is produced to the exclusion of all other phases. When anatase is the major titania form present then the calcite phase of calcium carbonate is favoured though some aragonite is still produced. The selectivity towards calcite is enhanced by increasing the hydrothermal processing temperature from 120°C to 135°C. It is suggested that the titania phase present catalyses the crystallisation by providing nucleation sites for the calcium carbonate to grow from.