纳米羟基磷灰石的常压合成与表征

以乙二醇为氢氧化钙的溶剂在常压下合成了一种纳米羟基磷灰石晶体。分析结果表明，以乙二醇为氢氧化钙(Ca(OH)2)的溶剂反应合成的羟基磷灰石为细针状的纳米晶体，具有弱结晶结构，为非化学计量，其钙磷摩尔比约为1．66。这种纳米磷灰石晶体和自然骨中的磷灰石晶体在很多方面有相似之处。     

Electrolytic depositions of calcium phosphates on substrate

Calcium phosphate deposition layers on cathode substrates were prepared by electrolysing acidic Ca(H₂PO₄)₂·H₂O (MCP) solutions of 0.02–0.21 mol dm^–3 added with and without NaNO₃ at cathode currents of 2–200 mAcm^–2 at 20–90 °C. At relatively high MCP concentrations and low currents, CaHPO₄·2H₂O and CaHPO₄ plate-like crystals were deposited below and above 30 °C, respectively. With increasing current, the deposits became fine. On further increasing the current and/or decreasing the MCP concentration, apatite deposition occurred. The addition of NaNO₃ facilitated the formation of apatite. Deposited apatites were very fine grains and had a calcium-deficiency of 1.50 in the Ca/P molar ratio.     

Hardness and fracture toughness of dense calcium–phosphate-based materials

Hardness, H, and fracture toughness, KIc, have been determined as well as fracture energy and embrittlement index, H/KIc for six calcium–phosphate ceramics differing in phase composition. These materials were produced from initial calcium–phosphate precipitates with Ca–P molar ratios ranging between 1.50 and 1.73, synthesized by wet methods. After uniaxial or isostatic moulding and thermal treatment at 1250°C, the obtained dense sinters constituted mono-, bi- or triphase ceramic materials containing hydroxyapatite (HAp), -TCP, -TCP and CaO. When comparing the investigated materials, the best parameters, i.e. relatively high hardness accompanied by high KIc, were observed in the case of a HAp–TCP composite, containing 15 wt% HAp. It has been stated that free CaO occurring on the surface of the HAp samples obtained from powders with Ca–P ratios exceeding 1.67, transforms partially to CaCO3 due to contact with the surrounding atmosphere. The well shaped calcite crystals existing on the surfaces of such sinters significantly reduce hardness and increase fracture energy of the material when comparing both with the monophase HAp and the biphase HAp–TCP ceramics. © 1998 Chapman & Hall     

Sintering Behavior of Hot-Pressed Ca–αSialon Ceramics

On the basis of phase relationships in the Ca–Si–Al–O–N system, a Ca––sialon ceramic was synthesized using the hot-pressing technique. The reaction sequences and densifications of the Ca––sialon vs. firing temperatures have been characterized in detail. The present experiments reveal a reaction sequence as follows: at 1250°C the reactant mixture started to soften, at 1300°C a gehlenite phase was produced, at 1500°C the gehlenite phase was resolved into a liquid phase and a Ca––sialon started to form, and at 1600°C the formation of Ca––sialon was complete. The product was stable and almost entirely single phase Ca––sialon. Accompanying to the above sequences, densification also proceeded via a liquid-phase sintering, particle rearrangement, solution–reprecipitation, and grain growth process. In the final microstructure elongated grains of Ca––sialon were obtained, improving the fracture toughness of this Ca––sialon ceramic.     

Shape change and phase transition of needle-like non-stoichiometric apatite crystals

Nanometre-size needle-like non-stoichiometric apatite crystals are made in our laboratory by hydrothermal treatment at 140°C at a pressure of 0.3 MPa for 2 h. The shape change and phase transition of these crystals are studied using transmission electron microscopy (TEM), X-ray diffractometer (XRD) and infrared spectroscopy (IR). The results show that water molecules are present in the crystal structure of non-stoichiometric apatite. The condensation of HPO₄ ^2- ions can happen over a wide temperature range more than 600°C and is followed closely by the reaction of P₂O₇ ^4- ions with OH^- ions. The obvious TCP phase at 750°C is the outcome of the fusion and recrystallization of the needle-like crystals at 650°C and 750°C. Generally, stoichiometric apatite cannot contain water in its crystal structure while non-stoichiometric apatite can. Maybe this is the reason why bone contains non-stoichiometric apatite.     

Sol-gel-derived hydroxyapatite powders and coatings

Hydroxyapatite (HAP) and tri-calcium phosphate (TCP) powders and coatings with a Ca/P molar ratio from 1.56 to 1.77 were prepared by the sol-gel technique using calcium 2-ethylhexanoate (Ca(O₂C₈H₁₅)₂) and 2-ethyl-hexyl-phosphate as calcium and phosphorus precursors, respectively. The structural evolution and phase formation mechanisms of HAP and tri-calcium phosphate in calcined powders and coatings on Si wafer and Ti-alloy substrates (Ti-30Nb-3Al and Ti-5Al-2.5Fe) were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The elimination of organics was studied by differential thermal analysis (DTA) and thermogravimetry (TGA). Two different formation mechanisms of crystallization are proposed. In sols with Ca/P 1.67, -tricalcium phosphate is formed as the major phase and hydroxyapatite as a minor phase by calcination at 700°C. At 900°C these phases react to form AB-type carbonated hydroxyapatite (Ca_10–2x/3[(PO₄)_6–x (CO₃) _x ][(OH)_2–x/3–2y (CO₃) _y ]). A release of CO₂ substituting PO₄ ^3– occurs between 900°C and 1100°C yielding carbonate apatite, Ca₁₀(PO₄)₆[(OH)_2–2y (CO₃) _y ], whereas CO₂ substituting OH^– groups in the apatite structure is released above 1200°C. In sols with Ca/P 1.70, rather than carbonate apatite, B-carbonated hydroxyapatite Ca_10–2x/3[(PO₄)_6–x (CO₃) _x ](OH)₂ is formed, which subsequently decomposes into HAP and CaO above 1200°C. The optimum sintering conditions for coatings on Ti-alloys are found to be 600°C for 10 minutes, since, at higher temperature, oxidation of titanium and the formation of rutile (TiO₂) occur. Dip coating and sintering in two cycles yielded a homogeneous and dense coated film with a thickness of 250 nm.     

Fibrous growth of tricalcium phosphate ceramics

Structural transformation and sintering processes of tricalcium phosphate (TCP) ceramics prepared from defective hydroxyapatite (Ca9HPO4(PO4)5OH) were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). Starting powders with Ca/P ratio 1.5 were obtained by adding 0.5 l of 0.3 M H3PO4 solution to an equal volume of 0.45 M Ca(OH)2. In the prepared ceramics, the onset temperature for transformation of defective hydroxyapatite into TCP (witlokite) agrees with the onset temperature for sintering (800 °C). Sintering occurs through the formation of a fibrous structure, which resembles biological hard tissue. In the 1000–1200°C range, these fibres coalesce into grains of up to 0.6 m in size with a fibrous-laminar morphology. At the end of this sintering stage witlokite transforms into TCP. At about 1450°C, partial decomposition of TCP into Ca2P2O7+Ca4P2O9 is observed. AFM observations suggest that Ca2P2O7 is segregated in the liquid state and increases the velocity of grain growth (up to 12 m).     

Characteristics of the γ′ precipitates at high temperatures in Ni-base polycrystalline superalloy IN738LC

IN738LC is a Ni-base cast superalloy used in land-base gas turbinesand aerospace applications. As in other superalloys precipitates contribute to strengthening of this alloy at hightemperatures. In this study, the authors investigate thecharacteristics and mechanisms of precipitate dissolution into thematrix solid solution. The precipitates grow in cuboidal shape up to1130°C, above that a duplex-size precipitate microstructure sets inupon quenching from the temperature range 1140–1150°C. Theduplex-size precipitate microstructure consists of two very distinctsizes of precipitates (fine and coarse). Holding for longer times inthe temperature range 1140–1150°C does not coarsen the fineprecipitates of the duplex microstructure. The source for theformation of the fine precipitates in the duplex microstructure isthe dissolution of the newly grown smaller-sized precipitates whenthe agings start from fine size precipitates and the corner dissolution of coarse precipitates when the startingmicrostructure consists of the maximum-sized cuboidal precipitates.At and above 1160°C, the duplex as well as the coarse precipitatemicrostructures dissolve to form a single-size fine precipitatemicrostructure upon quenching from any temperature up to 1225°C. A single-phase solid solution with no precipitates is obtained onlyupon quenching from 1235°C or above. The dissolution of coarseprecipitates and formation of the fine ones are found to be very fastprocesses in the corresponding temperature ranges. The fineprecipitates are postulated to form during quenching from thetemperature range 1140–1225°C and are considered to be of the cooling type.     

Dissolution and adhesion behaviour of radio-frequency magnetron-sputtered Ca–P coatings

Radio-frequency magnetron sputtering deposition was used to produce calcium phosphate sputter coatings with three different thicknesses (0.1, 1 and 4 m) on titanium discs. Half of the as-sputtered coatings were subjected to an additional heat treatment for 2 h at 500°C. X-ray diffraction demonstrated that annealing at 500°C changed the amorphous 1 and 4 m sputtered coatings into an amorphous–crystalline structure, while the amorphous 0.1 m changed in a crystalline apatite structure. Further, scanning electron microscopy (SEM) inspection demonstrated that annealing of the 1 and 4 m coatings resulted in the appearance of some cracks. The dissolution behaviour of these Ca–P coatings was determined in a simulated body fluid. It was found that after incubation for 4 weeks the dissolution was determined by the crystallinity of the deposited coating. SEM and Fourier transform infrared evaluation showed that all the heat-treated sputter coating appeared to be stable under the test conditions and a Ca–P precipitate was always deposited on the coating surface. On the other hand, the amorphous 0.1 and 1 m coatings dissolved completely within 4 weeks, while the amorphous 4 m coating showed only signs of surface dissolution. Scratch testing demonstrated that there is a linear correlation between the critical load, L _c, and the coating thickness. A heat treatment for the CaP-4 coating resulted in an additional decrease in the critical load. On the basis of these findings, we conclude that already a 0.1 m heat-treated Ca-P sputter coating is of sufficient thickness to show in-vitro adequate bioactive and adhesive properties.     

Study of a hydraulic calcium phosphate cement for dental applications

Calcium phosphate-based cements (CPCs) have attracted much interest because of their good osteoconductivity for bone reconstruction. We obtained CPCs by mixing calcium bis-dihydrogenophosphate monohydrate (MCPM) and calcium oxide with water or sodium phosphate buffers (NaP) as liquid phase. Cement samples with different calcium-to-phosphate ratios (Ca/P), liquid-to-powder ratios (L/P) and liquid phases were analyzed by X-rays diffraction (XRD), pH-metry, extensometry and calorimetry. Antibacterial activity on two bacterial strains (Streptococcus mutans, Lactobacillus acidophilus) and a polycontaminated bacterial inoculum was also studied using the agar diffusion method. The best mechanical properties (25 MPa) corresponded to Ca/P ratios between 1.67 and 2.5, a 1 M sodium phosphate buffer pH 7, as liquid phase and a L/P ratio of 0.6 ml g^-1. The final setting time increased with the Ca/P ratio. The setting expansion, around 1–2%, depended on the Ca/P and L/P ratios. The inner temperature of the cements rose to 45° during setting then decreased rapidly. The injectability was 100% up to 3.5 min and then decreased. It increased with increasing the L/P ratio but to the detriment of the compressive strength and setting time. XRD analysis indicated that the setting reaction led to a mixture of calcium hydroxide and calcium-deficient hydroxyapatite even for a Ca/P ratio of 1.67. Consequently, the pH of the surrounding fluids rose to 11.5–12 during their dissolution. Bacterial growth inhibition was only clearly observed for Ca/P2. This bioactive calcium phosphate cement can potentially be employed for pulp capping and cavity lining as classical calcium hydroxide-based cements, but it is not usable, in the present formulation, for root canal filling because of its short setting time.     

A metastable phase in thermal decomposition of Ca-deficient hydroxyapatite

We investigated the microstructural changes on an atomic length scale during thermal decomposition process of Ca-deficient hydroxyapatite (Ca-def HAp) by high-resolution transmission electron microscopy (HRTEM). Ca-def HAp was prepared by hydrolysis of -tricalcium phosphate. The Ca-def HAp had a whisker-like morphology 2–5 m in length and 0.1 m in diameter that was elongated along c-axis. Thicker planer defects parallel to the (100) plane of the HAp matrix were observed as precipitation in the sample annealed at 700 and 800 °C by HRTEM observation. Thickness of the precipitation was about 10 nm and the boundaries between the precipitation and HAp matrix was coincident. The periodicity in the precipitation was parallel to the (100) plane of the HAp matrix and measured to be 1.42 nm. Since the precipitation was observed only in the sample annealed at a narrow temperature range of 700–800 °C, it was regarded as a metastable phase formed on the thermal decomposition process. Absorption peaks in IR spectra of annealed Ca-def HAp containing the metastable phase appeared at 744 and 3538 cm^–1 due to non-stoichiometric HAp with high Ca/P molar ratio. Furthermore, the results of energy dispersive X-ray spectroscopy showed that the metastable phase had higher Ca/P molar ratio than that of the matrix and stoichiometric HAp. Therefore, the metastable phase could be identified as Ca-rich metastable phase. The presence of Ca-rich metastable phase was confirmed to be associated with the thermal decomposition process.     

The influence of discharge power and heat treatment on calcium phosphate coatings prepared by RF magnetron sputtering deposition

Ca–P coatings with different Ca/P ratio and composition were successfully prepared by RF magnetron sputtering deposition. The Ca/P ratio, phase composition, structure and morphological properties were characterized by XRD, FTIR, EDS and SEM analyses. All the as-sputtered coatings were amorphous and after IR-irradiation the coatings altered into a crystalline phase. The obtained coatings had a Ca/P ratio that varied from 0.55 to 2.10 and different phase compositions or mixtures of apatite, beta-pyrophosphate and beta-tricalciumphosphate structures were formed. Evidently, the phase compositions of the sputtered coatings are determined not only by the discharge power ratio of the hydroxylapatite and calcium pyrophosphate targets but also by the annealing temperature.     

Mechanochemical Synthesis and Thermal Behavior of Metastable Mixed Oxides in the CaO–Sb2O3–Bi2O3 System

The phase composition of crystalline mechanochemical synthesis products in the CaO–Sb₂O₃–Bi₂O₃ system was determined. Of the known phases in this system, only three could be prepared mechanochemically: Ca₂Sb₂O₅, CaSb₂O₄, and CaBiO_2.5 (fcc). A new metastable phase, "-Bi₂O₃, with an orthorhombic structure close to that of the high-temperature, fluorite phase -Bi₂O₃, was obtained by mechanical processing at 30°C. A number of new metastable fluorite solid solutions of binary and ternary oxides were obtained as single-phase powders by mechanochemical synthesis. The mechanochemical yield of primary crystalline products was shown to be several times higher than that of secondary products. A broad composition range was revealed in which perovskite and fluorite phases are in mechanochemical equilibrium. The composition dependence of the lattice parameter of the metastable fluorite phase Bi_{2 – x} Sb _x O₃ was found to be the opposite of the one predicted by Vegard's law. Metastable mixed oxides undergo phase transformations during heating (starting at 280°C in the case of the ternary perovskite phase). Bi_{2 – x} Ca _x O_{3 – 0.5x} fluorite solid solutions experience a transformation at 400°C, accompanied by oxygen loss. During heating in air, Sb₂O₃-containing fluorite phases partially stabilize owing to oxidation but, nevertheless, undergo structural transformations above 480°C. The transformation of Sb_{2 – x} Ca _x O_{3 – 0.5x} metastable fluorite solid solutions near 500°C in air is accompanied by the formation of needle-like Sb₂O₃ crystals. A mechanism is proposed for the extremely rapid growth of such crystals: extrusion of the Sb₂O₃ resulting from fluorite decomposition in agglomerates through triple junctions of aggregates and through cracks in the surface layer of agglomerates.     

A simple method to prepare calcium phosphate coatings on Ti6Al4V

A two-step chemical treatment followed by immersion in a supersaturated calcification solution (SCS) was found to be a simple way to prepare calcium phosphate (Ca–P) coatings on Ti6Al4V. The Ca–P deposition on the treated metallic surfaces could be accelerated by employing a pre-calcification (Pre-Ca) procedure prior to immersion in SCS. The two-step treatment was performed by etching the metallic plates with a mixture of HCl and H2SO4 followed by ageing in boiling diluted NaOH solution at 140°C. Pre-Ca was carried out by incubating the two-step treated plates in Na2HPO4 solution and then in saturated Ca(OH)2 solution. The formation of a bioactive microporous surface oxide layer on Ti6Al4V by the two-step treatment was most probably responsible for the induction of Ca–P precipitation. The deposition rates and compositions of Ca–P coatings in two different SCSs were investigated by means of scanning electron microscopy, X-ray diffraction and infrared spectrophotometry.     

Synthesis and characterization of nano-HA/PA66 composites

Based on the bioactivity and biocompatibility of hydroxyapatite (HA) and the excellent mechanical performance of polyamide 66 (PA66), a composite of nanograde HA with PA66 was designed and fabricated to mimic the structure of biological bone which exhibits a composite of nanograde apatite crystals and natural polymer. The HA/PA66 composite combines the bioactivity of HA and the mechanical property of PA66. This study focused on the preparation method of HA/PA66 composite and the influence of HA crystals on the characterization of the composite. HA slurry was used directly to prepare HA/PA66 composite by a solution method, in which HA is able to form hydrogen bond, i.e. chemical bonding with PA66. The nano-HA needle-like crystals treated by hydrothermal method are better in the particle size distribution and the particle dispersion. The morphology, crystal structure and crystallinity as well as crystal size of these needle-like crystals are similar to bone apatite. The nano-HA needle-like crystals dispersed uniformly in PA66 matrix with reinforcement effect and can prevent the micro-crackle spreading into cleft and fracture during the deformation process. The mechanical testing shows that the nano-HA/PA66 composite has a good mechanical property, and may be a promising bone replacement material.     

Comparative study of apatite formation on CaSiO<Subscript>3</Subscript> ceramics in simulated body fluids with different carbonate concentrations

Apatite formation on CaSiO₃ ceramics was investigated using two different simulated body fluids (SBF) proposed by Kokubo (1990) and Tas (2000) and three sample/SBF (S/S) ratios (1.0, 2.5 and 8.3 mg/ml) at 36.5°C for 1–25 days. The CaSiO₃ ceramic was prepared by firing coprecipitated gel with Ca/Si = 0.91 at 1400°C. The bulk density was 2.14 g/cm³ and the relative density about 76%. The two SBF solutions contain different concentrations of HCO₃^– and Cl^– ions, the concentrations of which are closer to human blood plasma in the Tas SBF formulation than in the Kokubo formulation. The pH values in the former solution are also more realistic. The CaSiO₃ ceramics show apatite formation in SBF (Kokubo) after soaking for only 1 day at all S/S ratios whereas different phases were formed at each S/S ratio in SBF (Tas). The crystalline phases formed were mainly apatite at S/S = 1.0 mg/ml, carbonate-type apatite at 2.5 mg/ml and calcite at 8.3 mg/ml. At higher S/S ratios the increase in the Ca concentration became higher while the P concentration became lower in the reacted SBF. These changes in SBF concentrations and increasing pH occurred at higher S/S ratios, producing more favorable conditions in the SBF for the formation of carbonate bearing phases, finally leading to the formation of calcite instead of apatite in the higher HCO₃^– ion concentration SBF (Tas). Apatite is, however, formed in the lower HCO₃^– ion concentration SBF (Kokubo) even though the Ca and P concentrations change in a similar manner to SBF (Tas).     

Formation of hydroxyapatite on low Young's modulus Ti–30Nb–1Fe–1Hf alloy via anodic oxidation and hydrothermal treatment

Hydroxyapatite (HA) on the Ti–30Nb–1Fe–1Hf alloy has been fabricated via anodic oxidation followed with the hydrothermal treatment. The anodic oxide film (AOF) containing Ca and P was formed by anodic oxidation in a solution consisting of β-glycerophosphate disodium pentahydrate(β-GP), and calcium acetate monohydrate(CA). The AOF was formed by a 2-stage growth: (1) a thin oxide film that intimately contacted the substrate formed prior to sparking; (2) after sparking, the thickness of AOF increased rapidly, accompanying with the formation of numerous craters in the AOF. When anodizing to 300 V, the AOF had a glassy amorphous structure. Increasing anodizing potential increased the Ca/P ratio and contents of Ca and P, but decreased the adhesion strength between the AOF and the substrate. After 6 h of hydrothermal treatment at 250 °C, a great number of crystalline HA precipitated on the surface of AOF anodized to 300 V. The shapes and population density of HA crystals can be controlled by modifying the anodizing potential and the solution pH of hydrothermal treatment. Increasing the pH of the solution in hydrothermal treatment enhanced the precipitation of HA crystals. Numerous needle-like HA crystals that nearly covered the surface of AOF were obtained when hydrothermally treated in the pH 13 solution.     

Preparation and characterization of nanograde osteoapatite-like rod crystals

In this paper, nanograde osteoapatite-like rod crystals are made from wet synthesized calcium phosphate precipitates by hydrothermal treatment at 140°C under 0.3 MPa pressure for 2 h. The morphology, crystal structure, crystallinity and phase composition of these nanograde rod crystals are similar to those of thin apatite crystals in bony tissues of the body. This analogy provides an opportunity in the near future to build bone-like substitutes which consist of the nanograde rod crystals and special organic matrices and cells.     

Chemical vapour growth of HfP whiskers and their properties

Whiskers and ribbon-like single crystals of -HfP (hexagonal) have been prepared from HfCl₄+PCl₃+H₂+Ar gas mixtures at 1100–1200 °C using a metal impurity-activated chemical vapour deposition process. The growth conditions, morphology and chemical properties were examined. The 3.5–6.5 mm (average 4 mm) long HfP whiskers were obtained at 1200 °C using Si+Pt or Si+Pd mixed impurities. The HfP whiskers were very stable against oxidation up to 3 h exposure at 1000 °C and for 80 min immersion in concentrated HCl solution at 50 °C.     

Effects of environment and temperature on ceramic tensile strength–grain size relations

Overall strength ()–grain size (G), i.e. –G-1/2, relations retain the same basic two-branched character to at least 1200–1300°C. However, some polycrystalline as well as single crystal strength shifts or deviations are seen relative to each other, and especially relative to Young's moduli versus temperature for poly- and single crystals. The variety and complexity of these deviations are illustrated mainly by Al2O3, BeO, MgO and ZrO2 for which there is considerable data. At 22°C, Al2O3 polycrystals show substantial strength decrease due to H2O while MgO, ZrO2 and BeO polycrystals have limited, variable decreases. Al2O3 single crystals (sapphire) also show substantial strength decreases, but ZrO2 and MgO single crystals show little or none. Sapphire's strength markedly decreases from at least –196°C to a minimum in the 400–600°C range, then rises to a maximum at1000°C, followed by an accelerating decrease with further temperature increase. Polycrystalline Al2O3 shows similar (but less pronounced) strength minima and maxima, or alternatively an approximate strength plateau from 22 to 1000°C interrupting the normally expected strength decreases with increasing temperature at suitably large grain size and absence of defects (e.g. pores) dominating failure. BeO crystals show a linear strength decrease with increasing temperature (T) similar to that of Young's modulus. BeO polycrystals often show a significant strength (apparently grain size and impurity dependent) maximum (at 500–800°C) or plateau (from 22 to 1000°C) interrupting an otherwise continuous decrease. MgO shows similar temperature behaviour to BeO, but more pronounced crystal strength decrease and less pronounced polycrystalline strength maxima. Polycrystalline ZrO2 shows more rapid Young's modulus (E), and especially strength, decreases at 200–500°C than single crystals. More limited data for other materials also shows greater, variable –T versus E–T trends, e.g. MgAl2O4 has a similar, but less pronounced decrease than ZrO2. Collectively these deviations suggest variable impacts on primarily flaw controlled –G-1/2 behaviour due to factors such as microplasticity, machining stresses, and thermal expansion and elastic anisotropies requiring more comprehensive testing and evaluation to better sort out these effects.