Thermal decomposition of synthesised carbonate hydroxyapatite

Heat treatments are used when sintering hydroxyapatite to make porous blocks and granules and during plasma spraying of coatings. Calcium : phosphorus ratio is known to affect the thermal decomposition behavior of hydroxyapatite. Hydroxyapatite with carbonate ions substituted for phosphate ions is more similar in composition to bone mineral. While it has been shown that carbonate apatite may be sintered, relatively little is known about its high temperature stability. Various atmospheres have been used in investigations into the thermal stability of hydroxyapatites and carbonate hydroxyapatites, including nitrogen, wet carbon dioxide air, water vapor and wet oxygen, but few of these studies were directly comparable. Previous work has shown that loss of carbonate from CHA at high temperature is time dependent, which suggests that rapid high temperature treatment may prevent carbonate loss during processing. This study investigated the effect of dry carbon dioxide, carbon dioxide containing 3% water, nitrogen and nitrogen containing 3% water on the phase composition of hydroxyapatite containing between 1.0 and 11.5 wt % carbonate rapidly heated to temperatures of between 700 and 1400 °C. Carbonate ion substitution was observed to decrease the temperature at which crystallisation occurred to a minimum of 700 °C for 11.8 wt % carbonate apatite heated in wet atmospheres. Atmosphere was found to appreciably affect the crystallization temperature and phase transformations of carbonate apatite containing 7.8 wt % carbonate. In wet and dry carbon dioxide atmospheres, crystallisation began in this material at 1100 and 900 °C, TCP was formed at 1500 and 1300 °C respectively. The high temperature decomposition of carbonate hydroxyapatite would appear to depend on the composition of the apatite and the atmosphere in which it is heated.     

Effect of the concentration of carbonate groups in a carbonate hydroxyapatite ceramic on its in vivo behavior

Carbonate-substituted hydroxyapatites containing up to 9 wt % of carbonate groups were synthesized and fabricated in the form of porous granules with a view to developing materials for use in bone tissue repairs. The use of sintering additives forming a liquid phase allowed the granule sintering temperature to be reduced by 400–450°C. It was found that the carbonate groups enter into the structure of the ceramic by a mixed AB-type substitution; the microstructure of the granules depends substantially on the concentration of the carbonate groups; introduction of 6 wt % of carbonate groups into apatite ensures high biological properties of the granules in experiments in vivo.     

The influence of unstable signals for electron spin resonance dosimetry with synthetic A-type carbonated apatite

Synthetic A-type carbonated apatite prepared in controlled conditions was irradiated at room temperature with 60Co gamma rays. The ESR spectrum was associated to axial CO2- and orthorhombic CO3- species. Radicals used as dose markers in biological apatites are long-lived paramagnetic species. The stability of the post-irradiation signal of A-type apatite was investigated for almost 2 years. Measurements showed variations in the spectra attributed to unstable CO3- species, which can be eliminated by thermal treatments at 100 degrees C for 24 h. Results indicated the potential use of an A-type carbonated apatite as a dosemeter.     

A coprecipitation technique to prepare NiNb2O6

A mixture of ammonium carbonate and ammonium hydroxide was used to coprecipitate nickel and niobium ions as nickel carbonate and niobium hydroxide under basic conditions. This precursor yielded NiNb₂O₆ (NN) ceramics on calcining at 700 °C (at a temperature lower than 800 °C which is necessary for the formation of NiNb₂O₆ when prepared by the traditional solid state method). The average particle size and morphology of these powders were investigated by transmission electron microscope (TEM).     

Formation of translucent hydroxyapatite ceramics by sintering in carbon dioxide atmospheres

Hydroxyapatite is used in a variety of clinical applications as a result of the apparent adherence to and mild reaction of bone and soft tissue to it owing to its structural similarity with bone mineral. Transparent hydroxyapatite has previously been fabricated by either or both of two methods; namely the application of pressure during sintering and/or the use of fine particle sized apatite prepared by either a sol-gel process or aqueous precipitation. Recently it has been shown that translucent carbonate hydroxyapatite may be formed by sintering nanocrystalline gels of carbonate hydroxyapatite in a wet carbon dioxide atmosphere. In this study we report for the first time that this atmosphere can be used to sinter microcrystalline powder compacts of hydroxyapatite to form translucent ceramics at ambient pressure. The effect of water partial pressure and sintering time at 1300°C on the optical transmission and microstructure of the ceramic was investigated. It was found that translucent ceramics were formed in all carbon dioxide atmospheres and that optical transmission varied with sintering time. Maximum transmission (13%) of 2 mm thick ceramic was obtained in materials sintered for four hours at 1300°C in a mixture of carbon dioxide containing water at a partial pressure of 4.6 kPa.     

Material Problems in Using High-Temperature Thermocouples

The material compatibility and thermal stability of ceramic-composite coatings of different oxide ceramics deposited on alumina tubes to prevent the reduction of the alumina were investigated in the high-temperature range between 1750 °C and 1850 °C. It turned out that the coatings were thermally unstable and did not provide adequate protection against the reduction of the alumina tubes. The oxide ceramics formed eutectic compositions with low melting temperatures and were also prone to reduction to elementary metals by carbon. A new type of high-temperature thermocouple on the basis of refractory and noble metals was tested in the temperature range between 1325 °C and 1800 °C. Two metal-sheathed prototypes were constructed. The thermoelectric behavior of the tungsten5%rhenium/iridium thermocouples (W5%Re/Ir) was investigated by different high-temperature exposures, and the thermoelectric stability was checked by repeated measurements at the ice point.     

Effect of sintering parameters on the density and microstructure of carbonate hydroxyapatite

This work documents an investigation into the effect of water on the density and microstructure of carbonate hydroxyapatite in carbon dioxide sintering atmospheres. Carbonate apatites with carbonate contents of between 3.2 and 7.8 wt % were precipitated and the precipitates were formed into dry gels. Isothermal and isochronal sintering experiments were performed under dry carbon dioxide and wet carbon dioxide (containing 3 wt % water) atmospheres. The effect of carbonate content was studied by using two gels both with a green density of 37% and with carbonate contents of 5.8 and 7.8 wt %. Both isothermal and isochronal experiments demonstrated that bloating of the apatite occurred and this behavior was associated with the loss of carbonate from the apatite. It was found that only in wet carbon dioxide atmospheres fully dense translucent carbonate apatite could be formed. 93% dense carbonate apatite was formed after 4 h sintering at temperatures as low as 700 °C. © 2000 Kluwer Academic Publishers     

A novel cost-effective approach to fabricate diopside bioceramics: A promising ceramics for orthopedic applications

The objective of the present study is to prepare low temperature diopside (CaMgSi₂O₆) ceramics from natural waste (Rice husk ash & eggshells) and study the physico-mechanical and in vitro biological properties. X-ray powder diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), Fourier transforms infrared (FTIR) and energy-dispersive spectrometry (EDS) were used to assess the crystalline phase, thermal behavior, microstructure, functional groups and composition, respectively. Degradation as well as mechanical stability was studied by testing the weight loss and compressive strength in dynamic mode of simulated body fluid (SBF) according to ISO 10993-14. The bioactivity of diopside samples was tested by means of ability and rate of apatite mineralization on the surface in static mode of SBF. Cytocompatibility by human osteoblast-like cells and their proliferation were studied using MTT assay. Results revealed that the pure phase of diopside was successfully attained at significantly low temperature (800 °C) with good mechanical properties, which were nearly similar to that of human cortical bone, and with enhanced mechanical stability. Diopside ceramics possessed apatite growth on its surface in SBF and exhibits excellent biocompatibility with MG-63 cells. These results suggested that prepared diopside can be a cost-effective bioceramics for potential orthopedic applications.     

Regulation of bone-related genes expression by bone-like apatite in MC3T3-E1 cells

Bone-like apatite on HA/TCP ceramics sintered at 1,100 °C (HT1) and 1,200 °C (HT2) could be obtained via immersing substrates into simulated body fluid (SBF) for 3 days. When MC3T3-E1 preosteoblastic cells cultured on the surface of the bone-like apatite for 3 days, SEM observations revealed cell membrane features with secreted crystals very similar to in vivo bone formation during intramembranous ossification with a direct bone apposition on the ceramics. According to semi-quantitative RT-PCR method, mRNA expressions of osteocalcin (marker of late-stage differentiation) and type 1 collagen were increased in cultures with HT1S and HT2S when compared to HT1 and HT2 after cultured for 6 days. The results indicated that bone-like apatite had the ability to support the growth of osteoblast-like cells in vitro and to promote osteoblast differentiation by stimulating the expression of major phenotypic markers. Taken together, our findings will be helpful in understanding the mechanism of osteoinductivity of calcium phosphate ceramics and in constructing more appropriate biomimetic substrate.     

Physiochemical characterizations of hydroxyapatite extracted from bovine bones by three different methods: Extraction of biologically desirable HAp

In the present study, subcritical water and alkaline hydrolysis methods are proposed methodologies for extraction of natural hydroxyapatite bioceramic from bovine bone. In these processes, the bovine bones powder were treated by high pressure water at 250 °C for 1 h and 25% (wt) sodium hydroxide at 250 °C for 5 h, respectively. Also the conventional calcination methodology has been utilized as well (T = 850 °C for 1 h). The obtained apatites from the three treatment processes have been characterized by powder X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT IR), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), electron scanning microscopy (SEM), energy dispersive X-ray analysis (EDX) and field emission scanning electron microscopy (FE SEM). FT IR and XRD results affirmed that both the proposed methods and the traditional one can eliminate the collagen and other organic materials present in the bovine bones. The physiochemical characterizations for the obtained apatites have proved that the subcritical water and the alkaline hydrolysis relatively preserve the carbonate content present in the biological apatite, so they yield carbonated hydroxyapatite which is medically preferable. While, the thermal process produces almost hydroxyapatite carbonate-free.     

Apatite formation from simulated body fluid on various phases of TiO2 thin films prepared by filtered cathodic vacuum arc deposition

Hydroxyl (OH⁻)-free TiO₂ thin films with amorphous and crystalline phases were deposited onto (100) silicon substrates using filtered cathodic vacuum arc deposition in order to investigate the in vitro apatite formation in simulated body fluid (SBF). The surface morphology, composition and structure of the TiO₂ thin films were characterized. The X-ray photoelectron spectroscopy results confirmed the presence of calcium and phosphorus on all TiO₂ thin film surfaces after immersion in SBF at 37 °C. Fourier transform infra red results showed the presence of carbonated apatite on the surface of these films. Amorphous structured TiO₂ thin film showed poor ability to form apatite on its surface in SBF. Apatite formation was more pronounced on the surfaces of the anatase films in comparison to those of rutile. The carbonated apatite deposition rate increased significantly when the TiO₂ film was illuminated with UV light prior to immersing in the SBF. In particular, the UV-treated anatase and rutile films showed increased rates of carbonated apatite formation on their surfaces in comparison to samples not treated with radiation. The increase in hydrophilicity due to UV treatment appears beneficial for the apatite growth on these surfaces.     

Formation of carbonated apatite particles from a supersaturated inorganic blood serum model

Pathological calcification is common among for instance dialysis patients, and this causes an increase in mortality risk. An elevated serum phosphate concentration among those patients strongly correlates to this increase. In this work investigations of the conditions, composition, crystallinity and morphology of in vitro calcification are performed and related to results from in vivo studies. The study was performed under conditions mimicking physiological ones, i.e. a pH around 7.40, a temperature of 37°C, an ionic strength of 150 mM and ion concentrations close to those in human serum including the effects of elevated phosphate concentrations. The course of precipitation involves an initial precipitate that subsequently re-dissolves to give another precipitate, in accordance with the well-known Ostwald ripening theory. The final bulk precipitate consists of a macroscopically amorphous carbonated apatite. The amorphous apatite is formed from assemblies of spherical particles in the μm range, in turn composed of nano-crystalline needles of about 10 × 100 nm. Even the initially formed precipitate, as well as a small amount of precipitate that occurs on the liquid surface, consist of a carbonated calcium phosphate. The in vitro observed carbonated apatite bears strong resemblance to in vivo cardiovascular calcification known from literature.     

Carbonate loss from two magnesium-substituted carbonated apatites prepared by different synthesis techniques

This study was aimed at the investigation of the thermal stability of Mg-substituted carbonated apatites over the wide temperature range. Two different apatites were studied, which were prepared by either precipitation from aqueous solution or by solid–liquid interaction. The following methods were employed: FTIR spectroscopy of the condensed gas phase to evaluate the CO and CO₂ release with increasing temperature, FTIR of the solid residue after heating, XRD analysis, thermogravimetry and scanning electron microscopy. Decomposition behavior was shown to depend significantly on the synthesis method. Wet-synthesized powders are significantly less thermally stable compared with those prepared by solid–liquid interaction. Intensive release of carbon oxides from the former was observed at 300 °C, whereas the latter powder was relatively stable up to temperature about 1000 °C.     

Thermal decomposition of natural dolomite

We have studied thermal modification of natural dolomite chips, which has allowed us to optimize conditions for the preparation of supports for manganese oxide catalysts with appropriate physicochemical properties (elevated carbon dioxide partial pressure and calcination temperature no higher than 800°C).     

微孔致孔剂对钙磷陶瓷表面类骨磷灰石形成的研究

钙磷陶瓷表面类骨磷灰石层的形成对瓷诱导新骨生成起非常重要的作用. 本文利用体外模拟装置首次研究了微孔致孔剂对新工艺制备的含CO₃^2-的双相HA/β-TCP多孔陶瓷表面类?骨磷灰石形成的影响. 结果表明, 该陶瓷因CO₃^2-的掺入, 导致类骨磷灰石晶体的形成时间大大缩短(从14天缩短至6天). 此外还有缺钙羟基磷灰石晶体?的生成. 并且微孔致孔剂以聚乙烯醇缩丁醛(PVB)优于硬脂酸(SA)粉末, 它更有?利于类骨磷灰石的形成. 在同时加入PVB作微孔致孔剂时, 类骨磷灰石晶体的形成情况随PVB加入量的增大而越来越好. 综合其相关性能来看, PVB的加入量以m_钙磷粉:m _致孔剂=1:0.3为宜. 微孔致孔剂的优化有利于该陶瓷材料骨诱导性的提高.     

Low-temperature stages in the mechanochemical synthesis of gamma-lithium monoaluminate

Low-temperature (<500°C) stages in the synthesis of gamma-lithium monoaluminate through heat treatment of a mechanically activated mixture of aluminum hydroxide and lithium carbonate have been studied by thermogravimetry and differential scanning calorimetry in combination with mass spectrometry. The results demonstrate that heating the mixture to above 80–100°C is accompanied by the release of not only water (resulting from the decomposition of X-ray amorphous aluminum hydroxide) but also carbon dioxide. The carbon dioxide originates from the reaction of the lithium carbonate with the products of X-ray amorphous aluminum hydroxide thermolysis.     

Mineralization of calcium carbonate by controlled release of carbonate in aqueous solution

The generation of carbon dioxide from organic precursors in aqueous media in presence of calcium chloride and under conditions where calcium carbonate is formed was investigated. Photochemical decomposition of pyruvic acid, 2-nitro-phenylacetic acid and 4-benzoylphenyl acetic acid gave ill-defined morphologies of calcium carbonate contaminated by organic by-products. Alkaline hydrolysis of dimethyl- and diethyl-carbonate yielded amorphous calcium carbonate in form nanospheres of narrow distribution of radii if the precipitation occurred under quiescent conditions. Addition of a polymer surfactant gave spherical particles as well, however consisting of vaterite.The formation of spherical particles of amorphous calcium carbonate is rationalized in terms of a liquid–liquid phase separation followed by rapid gelation of the droplet phase of high concentration of calcium carbonate. A lower critical solution temperature is postulated and its value was estimated as 10 °C.     

Synthesis and characterization of bioactive hydroxyapatite/fluoroapatite solid solutions using corals

Hydroxyapatite/fluoropatite solid solutions were prepared by two different methods using Indian coast corals. In the ‘low temperature method’ a known volume (50%) of hydrofluoric acid was added to the coral and an exchange reaction with diammonium phosphate was carried out to form apatite at 250°C under hydrothermal conditions. The coralline derived hydroxyapatite by the above method was reacted with 50 mol% sodium fluoride at 900°C in the ‘high temperature method’. The X-ray powder diffraction, infrared spectroscopy and thermal analysis techniques were employed to characterize the fluorination of the hydroxyapatite.In vitro solubility study in phosphate buffer solution of pH 7·2 indicate the stability of the apatites. The low temperature method of fluorination seems to be advantageous due to betterin vitro stability and absence of impurity phases. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Hydrolysis of monetite/chitosan composites in α-MEM and SBF solutions

There are two objectives of this work. The first objective is to study the hydrolysis behavior of monetite cements formed in the presence and absence of the chitosan in cell culture media (α-MEM) and simulated body fluid (SBF) solutions at 37°C. During hydrolysis, monetite transformed to carbonated apatite. Therefore, the second objective is to examine how addition of chitosan affects on the formation of carbonated apatite phases. The changes in the phase structure of monetite after hydrolysis reactions were characterized using XRD, FTIR and SEM. Pure monetite and monetite/chitosan composite were soaked in α-MEM and SBF solution for 4 and 7 days. In α-MEM solution, the monetite particles started to transform into carbonated apatite with a slow rate. However, in SBF, the rate of monetite transformation to carbonated apatite was more rapid. The presence of the chitosan had no significant effect on the precipitation of carbonated apatite on the monetite particles.     

Carbonate-containing apatite (CAP) synthesis under moderate conditions starting from calcium carbonate and orthophosphoric acid

The synthesis of carbonate-containing apatite (CAP) from calcium carbonate and orthophosphoric acid under moderate conditions was investigated. In all cases, complete precipitation of orthophosphate species was observed. The reaction temperature influenced strongly the decomposition of calcium carbonate and therefore the composition of formed products. The reaction temperature of 80 °C was found to be effective for the complete decomposition of calcium carbonate particles after 48 h of reaction. Infra-red spectroscopy (IR), nuclear magnetic resonance (NMR), thermogravimetry/mass spectroscopy (TG–MS) coupling, and X-ray diffraction (XRD) characterizations allowed the identification of the composition of formed products. By increasing the reaction temperature from 20 °C to 80 °C, the content of A-type CAP increased and that of B-type CAP decreased, according to the favorable effect of temperature on the formation of A-type CAP. The total amount of carbonate content incorporated in CAP's structure, which was determined by TG–MS analysis, increased with the reaction temperature and reached up to 4.1% at 80 °C. At this temperature, the solid product was mainly composed of apatitic components and showed the typical flat-needle-like structure of CAP particles obtained in hydrothermal conditions. These results show an interesting one-step synthesis of CAP from calcium carbonate and orthophosphoric acid as low cost but high purity starting materials.