The influence of high sintering temperatures on the mechanical properties of hydroxylapatite

The kinetics of the thermal decomposition of stoichiometric hydroxylapatite (HA) has been studied up to 1500°C for the purpose of determining the maximum admissible combinations of temperature and time for sintering HA. The influence of the sintering temperature on shrinkage, density and grain growth is then investigated in the temperature range from 1000 to 1450°C. Nearly theoretical density was achieved above 1300°C. A maximum fracture toughness is obtained for the samples sintered at 1300°C whereas hardness increases up to a sintering temperature of 1400°C. These results are discussed in terms of the roles of porosity and grain size.     

Pore evolution model of ceramic membrane during constrained sintering

Pore size has been found to strongly depend on the sintering program in the preparation of porous ceramic membranes. In this paper, a model was developed to predict the variation in pore size and porosity of membranes during the sintering process. A comparison of shrinkage characteristics was made between the sintering processes of supported membranes and unsupported membranes. For supported membranes, the effect of restriction coming from a rigid substrate on the sintering behavior has been taken into account in the calculation. It is predicted that the pore size increases in supported membranes and decreases in unsupported membranes as the sintering temperature is increased. Calculations also showed that the loss of porosity in the supported membranes was less than that in the unsupported membranes. In order to verify reliability of this model, unsupported and supported membranes were prepared with α-Al₂O₃ powders at the sintering temperatures ranging from 1125 °C to 1325 °C. The pore size and porosity were measured by gas permeation technique and Archimedes’s method. The experimental results for the unsupported and supported α-Al₂O₃ membranes showed a good agreement with those calculated from the model. Therefore, this model provides an effective tool in predicting the porosity and the pore size of ceramic membranes at the different sintering temperatures.     

Electrical conductivity and sintering in iron oxides at high temperatures

Two raw materials were chiefly used, namely laboratory ferric oxide and a high grade hematite. All experiments were carried out in an oxygen atmosphere, in the range 600 to 1300° C.The conductivity/temperature determinations described are of two kinds,viz those on loose powders and those across fractures in previously compacted and sintered material. In the former, indications were obtained of differences in electrical behaviour above 1100° C which could be correlated with the appearance (red or black) of the powder. In the latter, unstable electrical behaviour was noted above the Tammann temperature in the hematite, but not in the ferric oxide.Additionally, isothermal sintering experiments were carried out, at 975 to 1200° C on red ferric oxide, and at 1350 to 1400° C on black ferric oxide. The temperature dependence was approximately the same for both materials and the results are compatible with the electrical data, on the assumption that conductivity in general reflected particle surface conditions, whereas shrinkage was dependent on internal lattice movements.     

Sintering and electromagnetic properties of BaFe12O19 ferrite prepared by co-precipitation method

BaFe₁₂O₁₉ (BaM) was synthesized through the co-precipitation route. Pure phase BaM was formed after calcination of precipitated powder at 900 °C. BaM was sintered at three different temperatures; 1100, 1200, and 1300 °C to study the sintering kinetics by varying the sintering time from 1 to 4 h. Apparent porosity decreased, and bulk density increased with increasing sintering temperature and period. A bulk density of about 4.6 g/cm³ was achieved after sintering at 1300 °C/4 h. The rate-controlling mechanism of BaM densification was the diffusion of oxygen, and the activation energy for the sintering process was 274 kJ/mol. The grain size of BaM increased with rising sintering temperatures. Permittivity increased from about 11 to 17 and the permeability increased from about 10 to 16 with the increase in sintering temperature from 1100 to 1300 °C. Saturation magnetization was also enhanced to about 69 emu/g after sintering at 1300 °C/4 h. Therefore, BaM ferrite synthesized through the co-precipitation route can be effectively used for high-frequency applications after sintering at 1300 °C.

     

Reaction-sintered porous mineral-based mullite ceramic membrane supports made from recycled materials

Bulk porous mullite supports for ceramic membranes were prepared directly using a mixture of industrial waste fly ash and bauxite by dry-pressing, followed by sintering between 1200 and 1550 °C. The effects of sintering temperature on the phase composition and shrinkage percent of porous mullite were studied. The XRD results indicate that secondary mullitization reaction took place above 1200 °C, and completed at 1450 °C. During sintering, the mixture samples first shrunk, then expanded abnormally between 1326 and 1477 °C, and finally shrunk again above 1477 °C. This unique volume self-expansion is ascribed to the secondary mullitization reaction between bauxite and fly ash. More especially, the micro-structural variations induced by this self-expansion sintering were verified by SEM, porosity, pore size distribution and nitrogen gas permeation flux. During self-expansion sintering, with increasing temperature, an abnormal increase in both open porosity and pore size is observed, which also results in the increase of nitrogen gas flux. The mineral-based mullite supports with increased open porosity were obtained. Furthermore, the sintered porous mullite membrane supports were characterized in terms of thermal expansion co-efficient and mechanical strength.     

Effect of sintering temperature on the microstructure and high-frequency magnetic properties of Ni0.467Zn0.07Co0.015Fe0.511O4 ferrite

In this work, an attempt is made to study the effects of sintering temperature on the microstructure and high-frequency (HF) magnetic properties of a nickel zinc ferrite compound of very low ZnO content of Ni_0.467Zn_0.07Co_0.015Fe_0.511O₄ composition. Samples were prepared by a conventional ceramic route and sintered for 2 h at 1150, 1200, 1250, and 1300 °C. It was shown that the higher the sintering temperature the higher the saturation magnetization and the measured initial permeabilities, and the lower was the H _c of the samples. This was related to the increased sintered densities and grain sizes. The magnitudes of the electrical resistivity of the samples sintered at 1300 °C compared to those of the samples sintered at 1150 °C and 1200 °C showed four orders decrease. This is thought to be due to the grain-size increase and possibly the formation of higher Fe²⁺/Fe³⁺ concentration. The lowest measured quality factor (Q-factor) obtained in the range of 60–210 MHz, corresponds to the samples sintered at 1300 °C. The highest Q-value in the frequency range of 125–210 MHz was obtained for the samples sintered at 1150 °C, which has also shown the highest electrical resistivity.     

Thermal stability and its improvement of the alumina membrane top-layers prepared by sol-gel methods

The thermal stability of unsupported alumina membrane top-layers was studied by determining the pore structure (mainly pore size) change of alumina gels, prepared by sol-gel methods, after sintering at different temperatures ranging from 450 to 1200 °C. The average pore size of the pure alumina membranes and PVA-added membranes increased sharply after sintering at temperatures higher than 1000 °C. Addition of 3% lanthanum, either by mixing lanthanum nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by a second heat treatment, can considerably stabilize the pore structure of the alumina membrane top-layers. The pore diameter for the lanthanum-doped membranes was stabilized within 25 nm after sintering at 1200 °C for 30 h, about one-sixth of that for the pure alumina membranes after sintering at 1200 °C for 30 h. The substantial increase in the pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200 °C was accompanied by the phase transformation from -to -alumina. The addition of lanthanum can raise this phase transformation temperature by about 200 °C.     

Dilatometry of attrition milled nanocrystalline titanium powders

The sintering behavior of nanosized titanium powders was investigated by dilatometry. The nanosized Ti powders (40 nm) were produced by the attrition milling of micron sized Ti powders (12 μm) in Ar atmosphere. Sintering was carried out in Ar atmosphere in the temperature range of 450–1250 °C for nanosized Ti and 650–1250 °C for micron sized Ti by heating at 10 °C/min, up to the sintering temperature followed by isothermal holding for 1 h. The nanosized Ti powders exhibited a lower sintering onset temperature, larger shrinkage, larger shrinkage rate, and lower activation energy for sintering as compared to the micron sized Ti powders. The sintered micron sized Ti specimens exhibited both intraagglomerate and interagglomerate porosity while the nanosized Ti specimens exhibited well densified agglomerates (almost no interagglomerate porosity) and large intraagglomerate porosity. In nanosized Ti grain growth was found to take place beyond 700 °C and reached a maximum of 66 nm in samples sintered at 1100 °C.     

Synthesis of <alpha>-cordierite nanoparticles from bentonite using thermal shock assisted solid-state reaction method

For the first time, we believe, nanosized <alpha>-cordierite glass-ceramics are produced using bentonite, talc, alumina, and kaolin as the raw materials and applying thermal shock to the precursor powders and sintered at 1100, 1200, and 1300?°C. A combination of a furnace at about 800?°C and liquid nitrogen was used for the applied thermal shock with a total temperature difference of about 1000?°C. The effects of thermal shock process and sintering temperature on <alpha>-cordierite formation and microstructure have been investigated. The results show that <alpha>-cordierite was formed above 1160?°C and its weight ratio increased continuously as sintered temperature increased to 1300?°C. By increasing the temperature, <alpha>-cordierite nanoparticles grain sizes and the intensity of FTIR peaks started to increase. Applying thermal shock to precursor powders reduced the grain size of each consisting mineral and resulted in nanosized <alpha>-cordierite powder.     

Thermal behavior of SrSO4–SrCO3 and SrSO4–SrCO3–Al2O3 mixtures

The high temperature behavior of SrSO₄, SrCO₃ and Al₂O₃ mixtures was studied. A mixture of 1:1 mole of SrSO₄ and mechanically activated SrCO₃ was mixed and characterized using thermal gravimetric analysis. Some samples were uniaxially pressed and sintered at 1100, 1200 and 1300 °C for 8 h and then analyzed using X-ray diffraction and scanning electron microscopy. Additionally, a mixture of SrSO₄:SrCO₃:Al₂O₃ was uniaxially pressed and sintered at 1500 °C. The decomposition temperature of SrCO₃ was decreased 18° by milling for 180 min. Samples sintered at 1300 °C showed a microstructure free of porosity. X-ray diffraction analysis showed the presence of SrO and SrSO₄ after sintering at 1100, 1200 and 1300 °C. The mixture containing alumina showed the formation of a strontium aluminum oxide sulfate compound in addition to strontium aluminate.     

Processing of porous Ti and Ti5Mn foams by spark plasma sintering

Titanium and its alloys are one of the best metallic biomaterials to be used for implant application. In this study, porous Ti and Ti5Mn alloy with different porosities were successfully synthesized by powder metallurgy process with the addition of NH₄HCO₃ as space holder and TiH₂ as foaming agent. The consolidation of powder was achieved by spark plasma sintering process (SPS) at 16 MPa and pressureless conditions. The morphology of porous structure was investigated by using scanning electron microscopy (SEM) and X-ray micro-tomography (μ-CT). Nano-indentation tester was used to evaluate Young’s modulus of the porous Ti and Ti5Mn alloy. Experimental results showed that pure Ti sample, which sintered under pressure of 16 MPa, full relative density was achieved even at a relative low sintering temperature 750 °C; however, in the case of pressureless condition at sintering temperature 1000 °C the porosity was 53% and Young’s modulus was 40 GPa. The Ti5Mn alloy indicated a good pore distribution, and the porosity decreased from 56% to 21% by increasing the sintering temperature from 950 °C to 1100 °C. Young’s modulus was increased from 35 GPa to 51.83 GPa with increasing of the sintering temperatures from 950 °C to 1100 °C.     

Influence of the sintering temperature on Young's modulus and the shear modulus of tricalcium phosphate – fluorapatite composites evaluated by ultrasound techniques

The mechanical properties of the tricalcium phosphate sintered between 1100 °C and 1450 °C for 1 h with different percentages of fluorapatite (13.26 wt%; 19.9 wt%; 26.52 wt%; 33.16 wt% and 40 wt%) have been characterized and evaluated using the ultrasound techniques. Young's modulus and the shear modulus were calculated from the point of the longitudinal and the transversal ultrasonic velocities. Young's modulus and the shear modulus of tricalcium phosphate increased with the sintering temperature and with the addition of the fluorapatite additive into the tricalcium phosphate matrix. At 1300 °C, the shear modulus and Young's modulus of the tricalcium phosphate – 40 wt% fluorapatite composites registered optimum values: 26 GPa and 66.2 GPa, respectively. Above 1300 °C, the mechanical properties of the tricalcium phosphate – fluorapatite composites were hindered by the tricalcium phosphate allotropic transformation and the formation of both the intragranular porosity and the cracks.     

Sintering behaviour and mechanical properties of hydroxyapatite and dicalcium phosphate

The sintering behaviour of powders of two calcium phosphates, namely hydroxyapatite (HA) and dicalcium phosphate (DCP), were studied at various temperatures and in various environments. The density, flexural strength and Knoop hardness of HA sintered in air for 4 h initially increased with the sintering temperature, reaching maxima at around 1150°C, and then decreased due to decomposition of HA into tri- (TCP) and tetracalcium phosphates. Sintering in vacuum caused decomposition of HA at lower temperatures, and consequently the mechanical properties were poorer than those of HA sintered in air. The densification and mechanical properties of DCP sintered in air and vacuum showed similar behaviour to those of HA. In air DCP underwent phase transformation from - to - and to -phases. In vacuum DCP started to decompose into tricalcium phosphate at 1000°C. To reduce dehydroxylation, HA powder was sintered in moisture at various temperatures up to 1350°C and X-ray diffraction study did not indicate any decomposition at the highest sintering temperature. The density, flexural strength and hardness of HA sintered in moisture increased with the sintering temperature and eventually reached plateaux at about 1300°C, but below 1200°C they were lower than those of HA sintered in air at corresponding temperatures. Thus, it is seen that dehydroxylation did not hinder sintering of HA. On the other hand, decomposition obstructed sintering of both HA and DCP.     

Silica Glass from Aerosil by Sol-Gel Process: Densification and Textural Properties

Sol-gel processes allow the synthesis of inorganic materials from solutions of molecular precursors. These precursors can be either organic, inorganic, or a mixture of the two. The aim of this work is the preparation of aqueous solutions consisting of Aerosil OX 50 (fumed silica) using small amount of additives like ammonium fluoride NH₄F (less than 2%), which are dried at room temperature in order to obtain monolithic gels. These are then densified at temperatures below 1200°C and sintered at 1300°C in order to obtain pure silica glass. The textural properties evolution of these gels is investigated as a function of temperature by thermal analysis (mass loss, shrinkage, and density) and scanning electron microscopy.     

Characterization of the metatitanate systems (1−x)BaTiO3xNiTiO3 and (1−x)BaTiO3xZnTiO3 obtained by the peroxide method

It is known that metatitanates are prospective materials for the production of electronic components. In this work, two compositions of metatitanates are investigated: (1–x)BaTiO₃xNiTiO₃ and (1–x)BaTiO₃xZnTiO₃ with x=0.05–0.20 mol and at four sintering temperatures in the range 1100–1300 °C. The starting metatitanate powders are obtained by the peroxide method. The dependence of the dielectric constant and the loss tangent on the mole ratio x is investigated. The dependence of both parameters on the sintering temperature T_s is also investigated and opposite behavior is found for the two compounds. Due to the depressor character of NiTiO₃ and ZnTiO₃ a significant smoothing of the dependence of the dielectric constant on the ambient temperature T_a is obtained with respect to the same dependence for pure BaTiO₃. It is found that compounds 0.80BaTiO₃0.20NiTiO₃ sintered at 1300 °C and 0.80BaTiO₃0.20ZnTiO₃ sintered at 1200 °C are prospective materials for production of high-frequency capacitors with low losses.     

Far infrared reflectance of sintered nickel manganite samples for negative temperature coefficient thermistors

Single phase complex spinel (Mn, Ni, Co, Fe)₃O₄ samples were sintered at 1050, 1200 and 1300 °C for 30 min and at 1200 °C for 120 min. Morphological changes of the obtained samples with the sintering temperature and time were analyzed by X-ray diffraction and scanning electron microscope (SEM). Room temperature far infrared reflectivity spectra for all samples were measured in the frequency range between 50 and 1200 cm⁻¹. The obtained spectra for all samples showed the presence of the same oscillators, but their intensities increased with the sintering temperature and time in correlation with the increase in sample density and microstructure changes during sintering. The measured spectra were numerically analyzed using the Kramers-Krönig method and the four-parameter model of coupled oscillators. Optical modes were calculated for six observed ionic oscillators belonging to the spinel structure of (Mn, Ni, Co, Fe)₃O₄ of which four were strong and two were weak.     

Structural and Phase Transformations in Alloys during Spark Plasma Sintering of Ti + 23.5 at % Al + 21 at % Nb Powder Mixtures

We have studied the effect of sintering temperature on the structural and phase transformations of alloys produced by the spark plasma sintering of Ti + 23.5 at % Al + 21 at % Nb powder mixtures at temperatures in the range 1100–1550°C. The sintered alloys have been characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy (elemental X-ray mapping). The alloys sintered at temperatures of 1100 and 1200°C have been shown to have a nonuniform microstructure. According to electron microscopy results, the alloys consist of grains of the α₂ and Nb₂Al phases and small precipitates of the O-phase (intermetallic compound Ti₂AlNb). In addition, there are particles of unreacted niobium and titanium. The alloys sintered at a temperature of 1300°C have a uniform lamellar structure.     

Nanosized hydroxyapatite powders derived from coprecipitation process

Nanosized hydoxyapatite (Ca₁₀(PO₄)₆(OH)₂ or HA) powders were prepared by a coprecipitation process using calcium nitrate and phosphoric acid as starting materials. The synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific area measurment techniques. Single phase HA, with an average grain size of about 60 nm and a BET surface area of 62 m²/g, was obtained. No grain coarsening was observed when the HA powders were heated at 600°C for 4 hours. HA ceramics were obtained by sintering the powders at temperatures from 1000°C to 1200°C. Dense HA ceramics with a theoretical density of 98% and grain size of 6.5 m were achieved after sintering the HA powders at 1200°C for 2 hours. HA phase was observed to decompose into tricalcium phosphate when sintered at 1300°C. The microstructure development of the sintered HA ceramics with sintering temperature was also characterized and discussed.     

Influence of glass addition and sintering temperature on the structure,mechanical properties and dielectric strength of high-voltage insulators

Soda-lime glass as a substituent for the feldspar was used to prepare high-tension electrical porcelain by standard chemical solid reaction technique. The effect of glass substitution and sintering temperature on the physical properties, microstructure, hardness, modulus of rupture, flexural strength and Dielectric breakdown strength were examined. Zero water absorption (WA %) and apparent porosity (AP %) were achieved for the samples with glass content >15 wt.% sintered at 1100 °C. The apparent density was found to increase with sintering temperature. The Vicker’s micro-hardness increased with both glass addition and sintering temperature. Both of the modulus of rupture (MOR) and flexural strength (σ_f) had maxima values at 15 wt.% glass addition. The structure and morphology were characterized by X-ray diffraction and scanning electron microscope (SEM). It showed the formation of mullite needles at sintering temperature of 1100 °C, which enhanced the mechanical and electrical properties of the porcelain. The dielectric breakdown strength increased with sintering temperature and glass addition. The highest dielectric strength was found at 10 wt.% of glass addition depending on the Na₂O and Fe₂O₃ content.     

Effect of temperature on nonlinear electrical behavior and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO₂ ceramics doped with 0.75 mol% Ca and 2.5 mol% Ta were sintered at different temperatures ranging from 1300 to 1450°C. The effects of sintering temperature on the microstructure, nonlinear electrical behavior, and dielectric properties of the ceramics were studied. The sample sintered at 1300°C exhibits the highest nonlinear coefficient (5.5) and a comparatively lower relative dielectric constant.