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.     

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     

Hydrothermal precipitation of hydroxyapatite on anodic titanium oxide films containing Ca and P

Highly-crystallized hydroxyapatite (HA) can be precipitated during heat treatment in high-pressure steam at 300 °C on an anodic titanium oxide film containing Ca and P (AOFCP), which has been electrochemically formed on a titanium substrate prior to the hydrothermal treatment. Factors affecting the precipitation, such as a percentage of distilled water in the autoclave and additives in the AOFCP, were evaluated by scanning electron microscopy. Ca²⁺ and PO^3– ₄ ions were leached from the AOFCP into a water layer covering the film surface, and nucleate HA heterogeneously on the porous TiO₂ matrix of the AOFCP which was made by the ion leaching. The morphology of the precipitated crystals was significantly affected by the water volume ratio because the concentrations of the Ca²⁺ and PO₄ ^3– ions varied depending on the thickness of the water layer. The amount of the precipitation decreased on the AOFCP which was formed in the solution containing a small amount of Mg²⁺ ions or formed on Ti-6Al-4V alloy instead of titanium.     

Growth of calcium phosphate on ion-exchange resins pre-saturated with calcium or hydrogenphosphate ions: an SEM/EDX and XPS study

Calcium phosphate formed on the surfaces of ion-exchange resins pre-saturated with either Ca²⁺ or HPO₄ ^2- ions has been studied using a combination of scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). Calcium phosphate was formed at a temperature of 36.5°C via two methods. On Ca²⁺ or HPO₄ ^2--saturated resins, 1.5xSBF (simulated body fluid) solution was used while on Ca²⁺-saturated resins only, a novel biomimetic growth medium using the alkaline phosphatase-catalysed hydrolysis reaction of disodium p-nitrophenylphosphate as a source of inorganic phosphate was employed. SEM micrographs showed that the use of 1.5xSBF growth medium solution led to extensive coverage of the resins with calcium phosphate. In contrast, calcium phosphate coatings formed via the alkaline phosphatase-catalysed reaction were of a more variable quality whose morphology could be influenced by adding albumin and collagen to the growth medium. Average Ca:P ratios determined by EDX for coatings formed from the 1.5xSBF growth medium were in the range 1.62–1.74 suggesting that hydroxyapatite had formed. In contrast, Ca:P ratios for the calcium phosphate compounds formed on resins from the alkaline phosphatase reaction were lower at 1.50 suggesting that calcium-deficient hydroxyapatite had formed which was confirmed by inductively coupled plasma (ICP) analysis and X-ray diffraction of isolated amorphous and crystallized powder samples, respectively. Evidence from X-ray photoelectron studies supports a mechanism of formation of the coatings which involves diffusion of the ion out of the interior of the resin to create a high local concentration at the surface thus stimulating precipitation of the coating material on the resin beads.     

Development of bioactive zirconium–tin alloy by combination of micropores formation and apatite nuclei deposition

In previous studies, Zr gained apatite‐forming ability by various methods; however, it took more than 7 days in simulated body fluid (SBF) to gain apatite‐forming ability. In this study, the authors developed the method to achieve apatite‐forming ability in Zr alloy within 1 day in SBF by a combination with apatite nuclei that promote apatite formation in SBF. First, Zr–Sn alloy was soaked in concentrated sulphuric acid, and pores in micro‐level were formed on the surface of Zr–Sn alloy. To attain apatite forming ability in Zr–Sn alloy, second, apatite nuclei were formed in the micropores. To evaluate apatite‐forming ability, thus‐obtained Zr–Sn alloy with apatite nuclei was soaked in SBF; hydroxyapatite formation was observed on the whole surface of the Zr–Sn alloy plates. From this result, it was clarified that higher apatite‐forming ability was attained on the apatite nuclei‐treated Zr–Sn alloy with micropores in comparison with that without micropores. When adhesive strength of formed hydroxyapatite film with respect to Zr–Sn alloy plates was measured, high‐adhesive strength of the formed apatite film was attained by forming micropores and subsequently precipitating apatite nuclei in the fabrication process because of an interlocking effect caused by hydroxyapatite formed in the micropores.Inspec keywords: precipitation, zirconium alloys, calcium compounds, bioceramics, tin alloys, adhesion, thin filmsOther keywords: apatite forming ability, micropore formation, hydroxyapatite film, bioactive zirconium‐tin alloy, apatite nuclei‐treated zirconium‐tin alloy, zirconium‐tin alloy plates, simulated body fluid, concentrated sulphuric acid, hydroxyapatite formation, adhesive strength, precipitating apatite nuclei, time 1.0 d, ZrSn, Ca₁₀ (PO₄)₆ (OH)₂      

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.     

Encapsulation of apatite particles for improvement in bone regeneration

The layering of fluorapatite on hydroxyapatite bodies provides a means of decreasing the solubility of hydroxyapatite, providing fluoride for possible stimulation of bone formation and delaying the release of calcium and phosphate from the more soluble hydroxyapatite. The purpose of this work was to encapsulate hydroxyapatite particles with fluorapatite spanning a thickness more than several crystallites deep. A three-step procedure was employed. Hydroxyapatite powder was immersed in an electrolyte solution until an equilibrium was established between the solid and the dissolved calcium at pH 4.67 and 37 °C. Equilibrium was determined by measurement of dissolved calcium with a calcium-specific ion-specific electrode. A 5×10^–2 M ammonium fluoride added to the suspension resulted in a rapid decrease of both calcium and fluoride in the solution. Analysis with X-ray diffraction indicated that a fluoride rich layer containing calcium fluoride deposited onto the particle surface. Scanning electron microscopy revealed submicron spherical precipitate clusters on the hydroxyapatite particles. These clusters transformed to fluorapatite by soaking in a 0.1 M K₂HPO₄ solution at pH 8 and 70 °C. A total time of 10 h was necessary for complete transformation of CaF_{_2} into fluorapatite.     

Rapid electroplating of photocatalytically highly active TiO2-Zn nanocomposite films on steel

Nanocomposite films consisting of TiO₂ and Zn with thickness of 10–15 m (TiO₂-Zn) have been electrodeposited on steel plates by rapid plating from a ZnSO₄-based bath (I _d > 10 A dm^–2). Upon addition of NH₄NO₃ to the bath (<0.3 g L^–1), the uptake of TiO₂ in the film significantly increased. Glow discharge optical emission spectrometry clarified that TiO₂ particles were incorporated throughout the film and the loaded amount increased near the surface. The first-order rate constant (k/h^–1) for gas-phase CH₃CHO oxidation was employed as an indicator of the photocatalytic activity. The k value for the TiO₂-Zn film prepared at I _d = 12 A dm^–2 (0.20 h^–1) was comparable to that for the sample from a ZnCl₂-based bath at I _d = 4 A dm^–2 (0.27 h^–1). X-ray diffraction measurements indicated that a TiO₂-ZnO nanocomposite layer was generated on the surface by the heat treatment in air at 673 K for 6 h. Consequently, the photocatalytic activity was further improved (k = 0.29 h^–1); this effect was explained in terms of the synergy of TiO₂ and ZnO in photocatalysis.     

Preparation of cobalt oxide films by plasma-enhanced metalorganic chemical vapour deposition

Co oxide films were prepared on glass substrates at 150–400°C by plasma-enhanced metalorganic chemical vapour deposition using cobalt (II) acetylacetonate as a source material. NaCl-type CoO films were formed at low O₂ flow rate of 7cm³ min^–1 and at a substrate temperature of 150–400°C. The CoO films possessed (100) orientation, independent of substrate temperature. Deposition rates of the CoO films were 40–47 nm min^–1. The CoO film deposited at 400 °C was composed of closely packed columnar grains and average diameter size at film surface was 60 nm. At high O₂ flow rate of 20–50 cm³ min^–1, high crystalline spinel-type Co₃O₄ films were formed at a substrate temperature of 150–400°C. The Co₃O₄ film deposited at 400°C possessed (100) preferred orientation and the film deposited at 150°C possessed (111) preferred orientation. Deposition rates of the Co₃O₄ films were 20–41 nm min^–1. Both Co₃O₄ films with (100) and (111) orientation had columnar structure. The shape and average size of the columnar grains at the film surface were different; a square shape and 35 nm for (100)-oriented Co₃O₄ film and a hexagonal shape and 60 nm for (111)-oriented film, respectively.     

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).     

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.     

Chemistry and Kinetics of ZnO Growth from Alkaline Hydrothermal Solutions

Special features (polar growth and nonstoichiometry) of ZnO single crystals grown on seed plates of various crystallographic orientations in the ZnO–KOH–H₂O and ZnO–KOH–LiOH–H₂O hydrothermal systems are analyzed. The growth proceeds via the interaction of ZnO^2- ₂ anions with crystal surfaces, and its rate depends on the atomic structure and electric charge of the surface. The mechanism underlying the influence of Li⁺ ions on polar ZnO growth is considered. Partial replacement of Zn²⁺ by Li⁺ decreases the positive charge on the (0001) face and hinders the attachment of ZnO^2- ₂ anions. The incorporation of Li ions into the (0001¯) face decreases its negative charge and accelerates growth in the [0001¯] direction.     

Photo-induced formation of hydroxyapatite on TiO2 synthesized by a chemical–hydrothermal treatment

The formation of hydroxyapatite (HAp) on TiO₂ surfaces under continuous ultraviolet (UV) irradiation was investigated. Pure Ti substrates were chemically treated with H₂O₂/HNO₃ at 353 K for 20 min to form a TiO₂ gel layer. The specimens were then hydrothermally treated with an aqueous NH₃ solution in an autoclave at 453 K for 12 h. An adhesive and sufficiently crystallized anatase-type TiO₂ film could be synthesized on the Ti surface. The specimens were immersed in simulated body fluid in darkness or under UV irradiation with a centered wavelength of λ = 365 nm. Under dark conditions, a thin homogeneous HAp film was formed, with just a few spherical clusters of HAp. The UV illumination promoted the formation of HAp clusters, which may be due to the generation of functional Ti–OH or Ti–O groups on the TiO₂ surface. On the other hand, the UV light produced electron-hole pairs in the TiO₂, and the photogenerated holes that migrated to the surface repelled the Ca²⁺ ions in the solution. As a consequence, the UV irradiation suppressed the formation of a HAp thin film.     

Effect of urea on formation of hydroxyapatite through double-step hydrothermal processing

The effect of urea on the formation of hydroxyapatite (HAp) was studied by employing the double-step hydrothermal processing of a powder mixture of beta-tricalcium phosphate (β-TCP) and dicalcium phosphate dihydrate (DCPD). Co-existence of urea was found to sustain morphology of HAp crystals in the compacts under an initial concentration of 2 mol dm^-3 and less. Homogenous morphology of needle-like crystals was observed on the compacts carbonated owing to decomposition of urea. Carbonate ions (CO₃^2-) was found to be substituted in both the phosphate and hydroxide sites of HAp lattice. The synthesized HAp was calcium deficient, as it had a Ca/P atomic ratio of 1.62 and the phase was identified as calcium deficient hydroxyapatite (CDHA). The release of CO₃^2- ions from urea during the hydrothermal treatment determined the morphology of the CDHA in the compacts. The usage of urea in the morphological control of carbonate-substituted HAp (CHAp) employing the double-step hydrothermal method is established.     

Synthesis of carbon nanotube films by thermal CVD in the presence of supported catalyst particles. Part I: The silicon substrate/nanotube film interface

The interface between the silicon substrate and a carbon nanotube film grown by thermal CVD with acetylene (C₂H₂) and hydrogen at 750 or 900 °C has been characterized by high resolution and analytical transmission electron microscopy, including electron spectroscopic imaging. Silicon (0 0 2) substrates coated with a thin (2.8 nm) iron film were heat treated in the CVD furnace at the deposition temperature in a mixture of flowing argon and hydrogen whereby nanosized particles of (Fe,Si)₃O₄ formed. These particles were reduced to catalytic iron silicides with the –(Fe, Si), ₂–Fe₂Si and ₁–Fe₂Si structures during CVD at 900 °C, and multi-wall carbon nanotubes grew from supported particles via a base-growth mechanism. A limited number of intermediate iron carbides, hexagonal and orthorhombic Fe₇C₃, were also present on the substrate surface after CVD at 900 °C. The reduction of the preformed (Fe, Si)₃O₄ particles during thermal CVD at 750 °C was accompanied by disintegration leading to the formation of a number of smaller (<5 and up to 10 nm) iron and silicon containing particles. It is believed that the formation of these small particles is a prerequisite for the growth of aligned multi-wall carbon nanotube films.     

Microwave accelerated synthesis of nanosized calcium deficient hydroxyapatite

Rapid synthesis of calcium deficient hydroxyapatite (CDHA, Ca_10–x(HPO₄)_x(PO₄)_6–x(OH)_2–x) with Ca/P ratio 1.5 was done by precipitation using calcium nitrate tetra-hydrate and phosphoric acid and subsequently subjecting to microwave irradiation in a domestic microwave oven for 15 min. Transmission electron microscopy analysis shows needle like morphology of CDHA having length 16–39 nm and width 7–16 nm. The synthesized CDHA has the characteristic HPO^2–₄ vibration band at 875 cm^–1 in Fourier transform infrared (FT-IR) spectra. The X-ray powder diffraction (XRD) analysis shows a pattern corresponding to stoichiometric hydroxyapatite (HA) with broad peaks suggesting that CDHA particles were nanosized. Fourier transform Raman spectroscopy (FT-Raman) do not indicate any fluorescence band that is characteristic of non-stoichiometric HA. The thermal decomposition of CDHA to beta tricalcium phosphate (-TCP) was also studied for the additional confirmation. The nanosized CDHA was found to be stable up to 600°C.     

Chemical vapour deposition of Ca(Ti,Fe)O3 thin film by thermal decomposition of organocomplexes

A thin film of Ca(Ti, Fe)O₃, which is a mixed conductor of oxide ions and electrons, was prepared on various substrates by chemical vapour deposition using organocomplexes Ca(C₁₁H₁₉O₂)₂, Ti(O-ⁱC₃H₇)₄ and Fe(C₅H₇O₂)₃ as starting materials. These complexes were evaporated at temperatures of 250, 115 and 45 °C, respectively, and transported to the substrate surface at an almost steady state. Homogeneous films of single-phase Ca(Ti, Fe)O₃ were obtained at deposition temperatures of 750–800 °C under the total pressure of 30 torr for the reaction time of 60–90 min on silica glass substrate. The amount of Ca(Ti, Fe)₃ films formed and their microstructure were found to be greatly affected by the compositions and surface structures of substrate materials.     

Synthesis and properties of Fe3C film by r.f. magnetron sputtering

Fe₃C film, which is a promising new magnetic recording material, can be synthesized by r.f. magnetron sputtering. Several graphite plates were attached to an iron plate to adjust the area of a composite target for the control of film composition. The crystalline phases in a film changed from Fe-C solid solution to Fe₃C with increasing substrate temperature from 350 °C and above. Sputtering at an argon pressure of 5 Pa was favourable for the formation of crystalline Fe₃C film. All Fe₃C films showed in-plane magnetization. The saturation magnetization of the film was around 100–120 e.m.u. g^–1 regardless of the deposition conditions. The coercivity of the films increased from 1 Oe to 250 Oe with increasing substrate temperature, and the coercivity remained constant at 250 Oe at 350 °C, regardless of argon pressure.     

BCP coatings on pure titanium plates by CD method

Ca–P coatings on pure titanium plates were precipitated in this work by a cathode deposition (CD) method and showed several differences from other reported works. The fast calcification solution (FCS) and revised simulation body fluid (R-SBF) were used as electrolytes. A significant difference in sizes of crystals and thickness of the precipitated coatings was observed between the bioactive calcium phosphate (BCP) coatings precipitated from FCS and from R-SBF. The possible reason of this difference was ascribed to that the ion concentrations of Ca²⁺ and HPO₄²⁻ and some inhibitors of Ca–P crystals growth such as Mg²⁺ and CO₃²⁻ ions and pH of electrolytes. The crystalline structure and composition of BCP coatings are of special importance to applications of the BCP coatings. The characterization has been fulfilled by the use of XRD, FTIR, SEM and TEM. The results of this study made it clear that the precipitates on the Ti plates by cathode deposition method in different electrolytes were not the hydroxyapatite (HA) but octacalcium phosphate (OCP) and some carbonate-containing amorphous calcium phosphate apatite, and a few of precipitates changed to needle-like HA after immerged in a 0.1 M NaOH solution at 60 °C for 48 h. The present study confirms that CD method is a more convenient and fast way to prepare BCP coatings on titanium implants than the reported works.