Effect of processing conditions on the characteristics of pores in hot isostatically pressed alumina

Effect of processing conditions on the characteristics of residual pores was studied with an optical microscope in hot isostatically pressed translucent alumina ceramics. Green bodies formed by isostatic pressing were sintered at 1300, 1400 and 1600°C and then hot isostatically pressed at a temperature 50°C below the respective sintering temperature for 1 h at 100 MPa. All specimens were fully dense within experimental accuracy (±0.1%), and the grain size increased with increasing sintering/hot isostatic pressing temperatures. A variety of pores were found in all specimens. The distribution of pores was uniform at various locations within the specimen. The pore population decreased with increasing pore size, but was finite in the size range exceeding 84 m. The pores in this range increased with increasing sintering/hot isostatic pressing temperature. Except for these large pores, the pore population was similar under all processing conditions.     

Sintering and microstructural studies in the system ZrO2 · TiO2 · CeO2

Tetragonal zirconia polycrystals (TZP) in the system ZrO₂ · TiO₂ · CeO₂ have been prepared from titanium and zirconium alkoxides and cerium nitrate precursors. The change in microstructure with sintering temperature in the range 1300 to 1600° C has been characterized. A fully tetragonal structure with theoretical final density has been achieved after liquid-phase sintering in the range 1350 to 1400° C for 2h. Sintering at temperatures above 1450° C resulted in a loss of stabilizer from the matrix, by the formation of zirconium titanate and to a cerium-, titanium-rich liquid. The loss of stabilizer was such that in the temperature range 1500 to 1600° C, extensive transformation to monoclinic zirconia occurred spontaneously on cooling. The tetragonal zirconia formed after sintering at 1350° C was found to be very stable. Thec/a ratio of the tetragonal phase in this system is higher than in any of the binary TZP systems reported in the literature. The stability of the tetragonal phase is believed to be associated with this highc/a ratio.     

Temperature induced structural changes in sapphire whiskers

The effect of high temperature annealing (800 to 1500° C) on the structure of individual sapphire whiskers has been determined with an electron microscope technique. Two types of whiskers, grown by a similar process, but containing different levels of silicon impurity (6 and 0.2% respectively) were studied. Discrete second phase particles were observed within and at the surface of many of the whiskers with 6% silicon. After heat treatments at 1000 to 1300° C in high purity argon, these particles coarsened and coalesced in the larger whiskers and spheroidised on the surface of the smaller whiskers. In addition, a dispersion of fine particles was formed in some whiskers free from grown in particles.Some melting of the second phase occurred between 1000 and 1400° C, with an attendant disintegration of the whiskers. Although the sapphire whiskers with 0.2 silicon % did not contain second particles, some breakdown of the whiskers also occurred at about 1300° C, a process which is attributed to the presence, and melting, of a surface coating.Surface pits were formed at temperatures above 1000° C, and became extensive at 1400 to 1500° C, particularly in the 6% silicon whiskers. It is considered that the surface pitting is a consequence of impurity diffusion and internal stress in the sapphire whiskers.     

Fracture strength and damage resistance of slip cast Y-TZP/Ce-TZP layered composites

Asymmetric, three- and symmetric five-layer Y-TZP/Ce-TZP composites have been prepared by sequential slip casting and pressureless sintering at 1400–1600°C in air. The three-layer material sintered at 1500°C showed the maximum fracture strength (534 MPa), measured by a diametral compression test and failed by the triple-cleft mode of fracture. Contact damage resistance was superior in three-layer composite compared with five-layer, possibly due to the development of relatively large residual compressive stress.     

Nano-precipitation in hot-pressed silicon carbide

Heat treatments at 1300°C, 1400°C, 1500°C, and 1600°C in Ar were found to produce nanoscale precipitates in hot-pressed silicon carbide containing aluminum, boron, and carbon sintering additives (ABC-SiC). The precipitates were studied by transmission electron microscopy (TEM) and nano-probe energy-dispersive X-ray spectroscopy (nEDS). The precipitates were plate-like in shape, with a thickness, length and separation of only a few nanometers, and their size coarsened with increasing annealing temperature, accompanied by reduced number density. The distribution of the precipitates was uniform inside the SiC grains, but depleted zones were observed in the vicinity of the SiC grain boundaries. A coherent orientation relationship between the precipitates and the SiC matrix was found. Combined high-resolution electron microscopy, computer simulation, and nEDS identified an Al₄C₃-based structure and composition for the nano-precipitates. Most Al ions in SiC lattice exsolved as precipitates during the annealing at 1400 to 1500°C. Formation mechanism and possible influences of the nanoscale precipitates on mechanical properties are discussed.     

Synthesis of TiB2 powder from a mixture of TiN and amorphous boron

TiB₂ powder was synthesized by solid state reaction using amorphous boron and TiN as a source of titanium. The TiB₂ formation did not occur at all in a nitrogen atmosphere even at 1400° C. TiB₂ formed above 1100° C in argon and hydrogen atmospheres. The only crystalline phase of TiB₂ powder was favourably synthesized at 1400° C for 360 min in an argon atmosphere from a starting powder with a composition containing excess boron (B/Ti = 2.2). The synthesized powder was well dispersed and had a particle size of 0.5 to 2 µm. The powder activity was evaluated by sintering at 4 G Pa and 1300 to 1600° C for 15 min.     

Effect of the sintering temperature on the properties of Ce0.85La0.10Ca0.05O2−δ electrolyte material

Rare earth and alkaline earth co-doped Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte material with the powder obtained by solid-state reaction method was sintered at 1300, 1400, 1500 and 1600 °C respectively. The results showed that the ionic conductivity of the sample sintered at 1400 °C was slightly lower compared to that sintered at 1500 °C in the temperature range of 300-550 °C, while the sample sintered at 1400 °C showed the highest ionic conductivity in all the samples above 550 °C. The ionic conductivity of ∼0.021 S/cm at 600 °C and the relative density of 98.2% were observed for the sample sintered at 1400 °C. In addition, the highest flexural strength with 145 MPa was also obtained for the sample sintered at 1400 °C. It suggested that the sintering temperature for Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte may be reduced to as low as 1400 °C with desired properties.     

Preparation and phase development of yttria-doped ceria coated TZP powder

This paper presents a simple technique for preparation of yttria-doped ceria (YDC) coated tetragonal zirconia polycrystal (3Y-TZP) powder and its phase development upon firing. The coating solution was prepared using yttrium nitrate hexahydrate and cerium nitrate hexahydrate as starting reagents. Thermochemical reactions of the coated powder were studied using TGA and FTIR while phase development upon firing was examined using XRD. Inward diffusion of the coating YDC into the TZP particles was monitored by observing the change of crystal structure and lattice parameter as a function of sintering temperature and time. At sintering temperature of 1300 °C for 1 h, crystal structure of the sample was still tetragonal (t-ZrO₂). Increasing sintering time to 5 h at 1300 °C, diffusion of YDC into TZP particles occurred drastically and the structure was changed to cubic (c-ZrO₂) as indicated by the disappearance of (002)/(200) peak splitting. Increasing sintering temperature to 1400 and 1500 °C, however, resulted in the co-existence of tetragonal and cubic phases as indicated by the appearance of triples around 72.5–74° 2θ and also the decrease of cubic lattice parameter. When the sintering temperature was further increased to 1600 °C, lattice parameter was only slightly changed, suggesting that inward diffusion of YDC reached saturation point around this temperature.     

Effect of preparation temperature on the characteristics of Eu-doped borate phosphor particles in the spray pyrolysis

Eu-doped gadolinium borate (GdBO₃:Eu) phosphor particles with fine size and uniform morphology were prepared by spray pyrolysis. The effects of the preparation temperatures on the characteristics of the GdBO₃:Eu phosphor particles prepared by spray pyrolysis were analyzed. The precursor particles obtained at preparation temperatures below 1400 °C had hollow and porous inner structures. On the other hand, the precursor particles obtained at preparation temperature of 1600 °C had dense structure and uniform morphologies. The precursor particles with glass phases were formed at high-preparation temperatures above 1400 °C. The precursor particles prepared at preparation temperature of 1400 °C turned into GdBO₃:Eu phosphor particles with fine size and regular morphology after post-treatment. The GdBO₃:Eu phosphor particles prepared from the precursor particles obtained at a preparation temperature of 1400 °C had the maximum photoluminescence intensity, which was similar to that of the commercial (Y,Gd)BO₃:Eu phosphor particles.     

Effect of sintering on TiO2-impregnated alumina foams

The effect of sintering on the bulk properties, morphology and phase composition of ultralight Al₂O₃ foams impregnated with TiO₂ was investigated in comparison with pure alumina foam in the temperature range of 900–1600°C in air. Impregnation was carried out by immersion of pre-sintered alumina foam in a sol of titanium isopropoxide-acetylacetone complex. The changes of the foam linear shrinkage, effective density and porosity were studied along with morphological evolution and relationship between these properties was demonstrated. Titania impregnation increased the linear shrinkage (LS) during sintering by a maximum of 5% relative to pure alumina foams. The change of LS and weight loss of TiO₂/Al₂O₃ foams lead to a final density of 0.19 g/cm³ and porosity of 95%. The initial coating was found to develop a mosaic structure due to early shrinkage of the coating. After sintering at 1600°C the coating reacted with the underlying Al₂O₃ surface and became uniformly distributed. Finally, it was shown that the reacting TiO₂ layer formed the tialite (Al₂TiO₅) phase below 1400°C. This Tialite coating remained intact under 1200°C without stabilizers.     

Joining of Silicon Carbide Using MgO-Al2O3-SiO2 Filler

Pressureless sintered SiC specimens were joined using MgO-Al2O3-SiO2 (MAS) filler. MAS filler showed excellent behaviour of wetting on SiC substrate above 1480 °C, and the wettability was much influenced by the joining atmosphere. The joining was carried out at 1500 and 1600 °C for 30 min in Ar atmosphere. The flexural strength of the joined specimen showed 342–380 MPa up to 800 °C. However, the flexural strength of the joined specimen decreased to about 80 MPa at 900 °C due to softening of the joint interlayer. The results of the XRD and WDS showed that the reaction between SiC and the MAS filler produced the oxycarbide glass.     

Sintering behaviour of the diamond-cobalt system at high temperature and pressure

A powder mixture of diamond—8.9 vol % Co was consolidatedin situ on a WC-10 wt % Co base at temperatures of 1300 to 1500° C under a pressure of 5.8 GPa. The sintered body obtained at 1300° C, which is below the diamond—cobalt eutectic point, was not hard, and the surface of the diamond particle was partially graphitized. On the other hand, the sintered body obtained at 1400 to 1500° C was fairly hard. A strong correlation was also observed between hardness and the cobalt content found in the sintered body. The cobalt content in the harder sintered body was clearly lower compared with that of the softer one. The surface graphitization of the diamond particles is necessary to the transfer of cobalt during the sintering of diamond. In sintering the diamond-cobalt system, the sinterability of diamond was closely related to the feasibility of transformation from diamond to graphite.     

Effects of particle size on thermal durability and microporous characteristics for sintered bodies of alumina-pillared fluorine micas

Powder compacts of alumina-pillared fluorine micas having different particle sizes were fired at various temperatures in the range of 500–800 °C. Sintered bodies of the alumina-pillared micas were obtained at 500–700 °C with retaining their pillared structure. The pillared structure collapsed at 800 °C, losing microporous characters. Specific surface area and micropore volume of the sintered bodies obtained from coarse particles are larger than those obtained from fine particles. Thermal durability of the pillared structure depends on the particle size of alumina-pillared fluorine micas, especially on the c-dimension of the host mica crystals; thermal durability of the sintered bodies obtained from coarse particles is higher than that from fine particles. The sintered bodies are machinable due to the interlocking microstructure of the flaky mica crystals.     

Synthesis and phase evolution of Si-C-Ti powder derived from poly(methylsilaacetylene) and Ti

Si-C-Ti powder was synthesized by reactive pyrolysis of poly(methylsilaacetylene)(PSCC) precursor mixed with metal Ti powder. Pyrolysis of PSCC/Ti mixture with certain atomic ratio was carried out in argon atmosphere between 1300 °C and 1500 °C. The metal-precursor reactions, and phase evolution were studied using X-ray diffraction and scanning electron microscopy equipped with EDX. Ti₃SiC₂ phase was obtained from reaction of PSCC and Ti for the first time. Ti₃SiC₂ formation started at 1300 °C and its amount increased significantly at 1400 °C. In addition, liquid formed by additive CaF₂ could promote the formation of Ti₃SiC₂ phase.     

Rapidly sintering nanosized SiC particle reinforced TiC composites by the spark plasma sintering (SPS) technique

Different content of nanosized SiC reinforced TiC matrix composites were fabricated at 1600°C by spark plasma sintering (SPS) without any aids. It was found that the materials could be sintered in a relatively short time (12 min) and low sintering temperature (1600°C) to satisfactory relative density (99%). The phase distribution and microstructure of composites have been investigated by optical microscopy and scanning electron microscope. Fracture toughness and Vickers hardness at room temperature were also measured by indentation tests. The results showed that nanosized SiC particles addition could inhibit the coalescence of TiC grains and increase fracture toughness of composites due to the crack deflections.     

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.     

Effect of additives on sintering of α-cristobalite

Sintering is performed for the pure α-cristobalite, with TiO₂ additive, and with CaO additive powder compacts. The sintering temperatures were 1500, 1515, and 1525 °C for different dwell times. The master sintering curve theory is applied to the densification data and the activation energies are obtained. The splitting strength is measured for the sintered specimens. It is found that CaO decreases the activation energy due to the formation of liquid phase at the used sintering temperatures. The TiO₂ increases the activation energy because it exists as solid particles at the used sintering temperatures. The measured splitting strengths of the sintered specimens are shown to be best expressed by a sigmoid function of the work of sintering.     

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.     

Sintering behaviour of ultrafine NbC and TaC powders

Compositional and microstructural changes upon firing ultrafine (300 to 400 Å) stoichiometric NbC and TaC powder lots have been studied up to a temperature of 1600° C. Substantial amounts of oxygen impurities, mostly oxide particles or layers are eliminated by reductions with hydrogen, free carbon or the carbides themselves. TGA showed these reactions to take place at 700 to 1400° C with maxima around 1000 to 1100° C. Low temperature sintering is inhibited by this impurity and its removal is thus essential. Other impurities (Ni, Cr, Fe) were also found in the starting powders in total concentration 0.5 to 1%. They give rise to a liquid phase located at grain edges at temperatures as low as 1100° C which then controls microstructure development. It dissolves to some extent in the carbide matrix at high temperature, and has a tendency to rise to the free surface of the samples. Compositional and structural heterogeneities are thus produced between bulk and surface at high temperatures. Owing to these impurity effects, it was not possible to clearly evaluate the influence of powder granulometry.     

Influence of starch content and sintering temperature on the microstructure of porous yttria-stabilized zirconia tapes

Porous yttria-stabilized zirconia (YSZ) substrates with volume fractions of porosity ranging from 28.9 to 53 vol.% were developed using starch as a fugitive additive. Concentrated aqueous YSZ slips with different amounts of starch and an acrylic latex binder were prepared. The influence of the volume fraction of starch and sintering temperature on the sintering behavior and final microstructure were investigated. Two kinds of pores were observed in the sintered tapes: large pores created by the starch particles with lengths between 15 and 80 μm and smaller pores in the matrix with lengths between 0.6 and 3.8 μm. The porosities were above those predicted for each of the starch contents. However, larger deviations from the predicted porosity were found as more starch was added. The top surface of the sintered tapes had a greater porosity than the bottom one for all the starch contents examined. The total porosity and the percentage of open porosity in the sintered tapes could be controlled by the volume fraction of added starch as well as by the sintering temperature. The open pores between the YSZ particles were removed by sintering at 1600 °C. As the volume fraction of starch increased from 17.6 to 37.8 vol.%, there was a gradual increase in the interconnectivity of the pore structure.