The effect of manganese oxide on the sinterability of hydroxyapatite

The sinterability of manganese oxide (MnO₂) doped hydroxyapatite (HA) ranging from 0.05 to 1 wt% was investigated. Green samples were prepared and sintered in air at temperatures ranging from 1000 to 1400°C. Sintered bodies were characterized to determine the phase stability, grain size, bulk density, hardness, fracture toughness and Young’s modulus. XRD analysis revealed that the HA phase stability was not disrupted throughout the sintering regime employed. In general, samples containing less than 0.5 wt% MnO₂ and when sintered at lower temperatures exhibited higher mechanical properties than the undoped HA. The study revealed that all the MnO₂-doped HA achieved > 99% relative density when sintered at 1100–1250 °C as compared to the undoped HA which could only attained highest value of 98.9% at 1150 °C. The addition of 0.05 wt% MnO₂ was found to be most beneficial as the samples exhibited the highest hardness of 7.58 GPa and fracture toughness of 1.65 MPam^1/2 as compared to 5.72 GPa and 1.22 MPam^1/2 for the undoped HA when sintered at 1000 °C. Additionally, it was found that the MnO₂-doped samples attained E values above 110 GPa when sintered at temperature as low as 1000 °C if compared to 1050 °C for the undoped HA.     

The effect of manganese oxide on the sinterability of hydroxyapatite

The sinterability of manganese oxide (MnO₂) doped hydroxyapatite (HA) ranging from 0.05 to 1 wt% was investigated. Green samples were prepared and sintered in air at temperatures ranging from 1000 to 1400 °C. Sintered bodies were characterized to determine the phase stability, grain size, bulk density, hardness, fracture toughness and Young's modulus. XRD analysis revealed that the HA phase stability was not disrupted throughout the sintering regime employed. In general, samples containing less than 0.5 wt% MnO₂ and when sintered at lower temperatures exhibited higher mechanical properties than the undoped HA. The study revealed that all the MnO₂-doped HA achieved >99% relative density when sintered at 1100-1250 °C as compared to the undoped HA which could only attained highest value of 98.9% at 1150 °C. The addition of 0.05 wt% MnO₂ was found to be most beneficial as the samples exhibited the highest hardness of 7.58 GPa and fracture toughness of 1.65 MPam^1/2 as compared to 5.72 GPa and 1.22 MPam^1/2 for the undoped HA when sintered at 1000 °C. Additionally, it was found that the MnO₂-doped samples attained E values above 110 GPa when sintered at temperature as low as 1000 °C if compared to 1050 °C for the undoped HA.     

Influence of powder characteristics on the rheological behaviour of hydroxyapatite slurries

A study of the influence of several dispersants (nature and concentration), of the calcination temperature and of the concentration of hydroxyapatite (HAP), on the rheological behaviour of HAP slurries, made it possible to obtain low viscosity slurries containing 73 wt% HAP and 3.5 wt% dispersant. The calcination of the HAP powder affected both the morphology and the chemical nature of the surface of particles. This last effect was clearly shown by the decrease of the zeta potential with increasing calcination temperature. The optimal calcination temperature was chosen beyond 800 °C. Parts sintered at 1200 °C for 30 min were obtained with a density higher than 96% of theoretical by casting optimized suspensions.     

Effect of drying conditions during synthesis on the properties of hydroxyapatite powders

The effect of the drying conditions during the hydroxyapatite (HAp) powder synthesis on the size and microstructure was studied. The starting materials were agitated in water, dried at 60–150 °C, and heat-treated at 720 °C. The heat-treated HAp powders were crystalline, and their particle sizes decreased with an increase in the drying time, but were independent of the drying temperature. For a 3 day drying period, it was 1.61 μm, and 0.55 μm for 21 days. The surface zeta potential of the HAp powder with a long drying period was more negatively charged than that from the short drying period. The average primary particle sizes of the HAp powders seem to be almost equivalent, and so the drying period may be related to the dispersibility of the primary particles, and may lead to a small HAp particle size.     

Preparation and characterization of titanate powders of different sinterability

BaTiO₃, SrTiO₃ and CaTiO₃ sintering powders were synthesized using precursors such as BaTiO(C₂O₄) · 4H₂O and BaTiO₂(O₂) · 3H₂O, the analogous Sr and Ca compounds, respectively. The titanate powders obtained were characterized with respect to their analytic and granulometric properties. Sintering behaviour was examined in relation to that of BaTiO₃, SrTiO₃ and CaTiO₃ synthesized by the conventional route from carbonates and TiO₂.     

Improvement of sinterability of barium dititanate powders by ball-milling

In the present study, high-energy ball-milling technique was applied to barium dititanate (BaTi₂O₅) powders. The improvement in sinterability of the ball-milled powders was evidenced, as compared with the sintering behavior of unmilled powders. After ball-milling, BaTi₂O₅ coarse powders were effectively ground into fine ones, whose maximum particle size was decreased from ~400 to ~40 μm and about 20 vol% nano-scaled particles were produced. After further consolidation by spark plasma sintering at 1,323 K, almost fully dense BaTi₂O₅ ceramics (97.7% theoretical density) with a compact structure was obtained. But for BaTi₂O₅ coarse powders, the ceramics with high densification was difficult to obtain even after being sintered at a high temperature of 1,423 K, whose relative density was only 90.5%. The room temperature permittivities and dielectric losses at 1 MHz of the BaTi₂O₅ ceramics derived from fine powders and coarse powders were 32.9, 0.034 and 26.7, 0.053, respectively. The permittivity of BaTi₂O₅ ceramics sintered from fine powders reached a maximum value (ε′ = 240, 100 kHz) at the Curie temperature of ~730 K,which was larger than that sintered from coarse powders (ε′ = 175, 100 kHz) due to lower porosity. The present work proved that high-energy ball-milling is a feasible way to improve the sinterability of BaTi₂O₅ powders.     

Effect of external energy on atomic,crystalline and powder characteristics of antimony and bismuth powders

Next to atoms and molecules the powders are the smallest state of matter available in high purities and large quantities. The effect of any external energy on the shape, morphology and structure can thus be studied with relative ease. The present investigation deals with the effect of a non-contact external energy on the powders of antimony and bismuth. The characteristics of powders treated by external energy are compared with the as received powders (control). The average particle sizes, d ₅₀ and d ₉₉, the sizes below which 99% of the particles are present showed significant increase and decrease indicating that the energy had caused deformation and fracture as if the powders have been subjected to high energy milling. To be able to understand the reasons for these changes the powders are characterized by techniques such as X-ray diffraction (XRD), surface area determination (BET), thermal analytical techniques such as DTA-DTG, DSC-TGA and SDTA and scanning electron microscopy (SEM).     

Crystallization and sinterability of cordierite-based glass powders containing CeO2

Dense glass-ceramics for low firing temperature substrates were prepared by the addition of CeO₂ flux to a glass of the MgO-Al₂O₃-SiO₂ system. The glass powders were fabricated by melting at 1500°C and ball milling. Glass powder compacts prepared by dry pressing were heated at 800–1000°C for 0.5–4 h and sintered at 900–1000°C for 3 h. The crystallization behaviour and sinterability of the glass powder compacts were analysed by thermal and thermomechanical techniques. X-ray diffractometry and scanning electron microscopy. The addition of CeO₂ prevents the formation of -cordierite phase in the glass-ceramics and improves the formation of -cordierite phase. The activation energy of the glass containing CeO₂ for crystallization was lower than that of the CeO₂-free glass. Therefore, crystallization properties were enhanced. Because the crystallization onset temperature increased and the softening temperature decreased on the addition of CeO₂, the sinterability increased and dense glass-ceramics were fabricated below 1000°C. The properties of the glass-ceramics containing CeO₂ appeared to be correct for low firing temperature substrates.     

Si-substituted hydroxyapatite nanopowders: Synthesis, thermal stability and sinterability

Synthetic hydroxyapatites incorporating small amounts of Si have shown improved biological performances in terms of enhanced bone apposition, bone in-growth and cell-mediated degradation.This paper reports a systematic investigation on Si-substituted hydroxyapatite (Si 1.40 wt%) nanopowders produced following two different conventional wet methodologies: (a) precipitation of Ca(NO₃)₂·4H₂O and (b) titration of Ca(OH)₂. The influence of the synthesis process on composition, thermal behaviour and sinterability of the resulting nanopowders is studied.Samples were characterised by electron microscopy, induced coupled plasma atomic emission spectroscopy, thermal analysis, infrared spectroscopy, N₂ adsorption measurements, X-ray diffraction and dilatometry. Semicrystalline Si-substituted hydroxyapatite powders made up of needle-like nanoparticles were obtained, the specific surface area ranged between 84 and 110 m²/g. Pure and Si-substituted hydroxyapatite nanopowders derived from Ca(NO₃)₂·4H₂O decomposed around 1000 °C. Si-substituted hydroxyapatite nanopowders obtained from Ca(OH)₂ were thermally stable up to 1200 °C and showed a distinct decreased thermal stability with respect to the homologous pure sample. Si-substituted hydroxyapatites exhibited higher sintering temperature and increased total shrinkage with respect to pure powders. Nanostructured dense ceramics were obtained by sintering at 1100 °C Si-substituted hydroxyapatites derived from Ca(OH)₂.     

溶胶-凝胶法制备羟基磷灰石粉末的研究

对以硝酸钙和磷酸作为前驱体，采用溶胶－凝胶法制备羟基磷灰石粉体进行了研究。研究发现硝酸钙溶液的初始浓度、煅烧温度对所制得的粉体颗粒大小有较大的影响。通过XRD、FTIR及TG－DTA分析，最终探索出了较为容易操作的工艺条件。     

Calcining influence on the powder properties of hydroxyapatite

The effect of different calcination temperatures on the powder characteristics and the sintered density of synthetic hydroxyapatite (HA) powders, produced using two different processing routes, was examined. Powders were produced by either drying, milling and sieving an as-precipitated HA or by spray-drying a slurry of precipitated HA. Calcining the two powders at temperatures between 400 and 1000 °C did not significantly affect the powder particle size. The specific surface areas of the two powders, however, were reduced from 70–80 m²/g for a calcination temperature of 400 °C to approximately 5–7 m²/g for 1000 °C. Analysis of the surfaces of the HA powders using scanning electron microscopy (SEM) illustrated the coarsening and subsequent sintering of the sub-micron crystallites that constitute a powder particle as the calcination temperature increased, corresponding to the decrease in surface area of the powders. The sintered densities of the final ceramics were not significantly affected by calcining the powders. Microhardness measurements of ceramics prepared from powders calcined at different temperatures showed no significant variations with calcination temperature or powder processing method. The results of this study have illustrated that for applications where HA may be used in powder form, for example in plasma-spraying and for the production of HA-polymer composites, calcining the HA will significantly affect the powder properties, namely the surface area and morphology of the powders. For applications requiring HA in a dense ceramic form, for example as granules or blocks, calcining the powders does not significantly affect the properties of the final ceramic.     

纳米羟基磷灰石粉体的水热合成

采用Ca(NO3)2·4H2O和(NH4)3PO4·3H2O作为反应前驱物,通过水热合成颗粒尺寸在100nm以下的短棒状或针状HA晶体.X射线衍射(XRD)、透射电镜(TEM)和红外光谱(FTIR)分析讨论水热温度、反应时间、表面活性剂和烧结与物相组成、晶粒尺寸和晶体形貌的关系.实验结果表明升高反应温度和延长反应时间有利于HA的生成;表面活性剂有助于改善粉体的分散性能;烧结能提高晶体的结晶程度,但粉体易团聚,当温度高于800℃时HA发生分解.     

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.     

The sinterability of ultrafine WC powders obtained by a CVD method

The sintering behaviour of ultrafine WC powders produced by a CVD method (particle size <0.3m) and commercial WC powders (particle size ~ 1m) is investigated in hydrogen and in vacuum. It has been found that ultrafine WC powders have an extremely high sinterability and give a sintered body with a relative density of 100% by sintering at a considerably lower temperature than normal, such as 1750° C. Also WC powders with a large particle size and a wide-size distribution have a high sinterability caused by the presence of fine particles and the sinterability of WC powders is influenced significantly by the sintering atmosphere. The atmospheric effect is discussed in connection with the heating behaviour of a surface oxide layer and free carbon.     

Effect of carbonate substitution on the ultrastructural characteristics of hydroxyapatite implants

Carbonate ion substitution has been shown to be beneficial for increasing the amount of in vivoosseointegration to hydroxyapatite (HA). Nevertheless, mechanisms by which carbonate ions increase in vivo bioactivity are not fully understood. Sintered granules of HA and carbonate-substituted hydroxyapatite (CHA) were implanted for 6 and 12 weeks in an ovine model. Samples containing the bone-implant interface were prepared for transmission electron microscopy (TEM) and TEM was used to compare the in vivo reactivity of sintered granules of HA and CHA.The current findings demonstrated that CHA (1.2 and 2.05 wt.%) is more soluble than pure HA in vivo. More dissolution was observed from the CHA, at the bone-implant interface and within the implant, when compared to pure HA. A less crystalline phase was formed between the 2.05 wt.% CHA and bone at 12 weeks in vivo. Bone surrounding both the pure HA and 1.2 wt.% CHA was relatively disorganised at 12 weeks. In comparison, bone surrounding the 2.05 wt.% CHA was considerably more organised and in many regions collagen fibrils were present. Despite increased quality of bone surrounding 2.05 wt.% CHA, compared to 1.2 wt.% CHA, the amount of dissolution from both materials was similar.     

Consolidation of nanocrystalline hydroxyapatite powder

The effect of sintering temperature on the sinterability of synthesized nanocrystalline hydroxyapatite (HA) was investigated. The starting powder was synthesized via a novel wet chemical route. HA green compacts were prepared and sintered in atmospheric condition at various temperatures ranging from 900–1300°1C. The results revealed that the thermal stability of HA phase was not disrupted throughout the sintering regime employed. In general, the results showed that above 98% of theoretical density coupled with hardness of 7.21 GPa, fracture toughness of 1.17 MPa m^1/2 and Young’s modulus of above 110 GPa were obtained for HA sintered at temperature as low as 1050 1C. Although the Young’s modulus increased with increasing bulk density, the hardness and fracture toughness of the sintered material started to decline when the temperature was increased beyond 1000–1050 °C despite exhibiting high densities > 98% of theoretical value. The occurrence of this phenomenon is believed to be associated with a thermal-activated grain growth process.     

Y2O3添加剂对ZrO2超微粉体性质的影响

添加0-6mol%Y2O3的ZrO2超微粉体由化学共沉淀法制备，系统地研究了添加剂对粉体粒子粒径，表面性质和团聚状态的影响，结果表明，添加Y2O3组元，使制备的ZrO2粉体粒子粒径减小，对粒子的长大趋势可起到抑制作用，但加剧了粉体粒子的团聚，团聚体颗粒变大，团聚强度升高，随着组元添加量的增多，作用加剧。     

Effect of calcination on characteristics,surface texture and sinterability of chemically prepared barium titanate

Highly pure and finely divided barium titanate powders have been obtained by the pyrolysis of barium titanyl oxalate. The effect of time and temperature of calcination on the degree of crystallinity, fineness, and surface texture of the powders obtained have been demonstrated. The densification properties of the sintered bodies are discussed in relation to the characteristics of the calcined materials.     

Preparation of porous scaffold from hydroxyapatite powders

Hydroxyapatite (HA) powder was prepared by wet chemical method. The hydroxyapatite phase was stable up to 1250 °C without decomposition to beta-tricalcium phosphate. Interconnected porous hydroxyapatite scaffold resembling trabecular bone structure was developed from polymeric replica sponge method. The prepared scaffold has 60 vol.% porosity having a major fraction of ~ 50–125 μm pore diameter. The pore content, pore morphology, pore interconnectivity of scaffold and their compressive strength were dependent on the solid loading and binder content. In-vitro bioactivity and bioresorbability confirmed the feasibility of the developed scaffolds.