Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Comparison between microwave and conventional sintering on the properties and microstructural evolution of tetragonal zirconia

In this research, the comparison between microwave sintering and conventional sintering on the mechanical properties and microstructural evolution of 3?mol% yttria-stabilised zirconia were studied. Green bodies were compacted and sintered at various temperatures ranging from 1200?°C to 1500?°C. The results showed that microwave assisted sintering was beneficial in enhancing the densification and mechanical properties of zirconia, particularly when sintered at 1200?°C. It was revealed that as the sintering temperature was increased to 1400?°C and beyond, the grain size and mechanical properties for both microwave- and conventional-sintered ceramics were comparable thus suggesting that the sintering temperature where densification mechanism was activated, grain size was strongly influenced by the sintering temperature and not the sintering mode.     

The sintering behaviour,mechanical properties and creep resistance of aligned polycrystalline yttrium aluminium garnet (YAG) fibres,produced from an aqueous sol–Gel precursor

Continuous ceramic fibres are finding applications as reinforcements in ceramic matrix composites, and yttrium aluminium garnet (YAG) is a particularly attractive candidate material on account of its creep resistance at high temperatures. A continuous, aligned, 5·5 μm diameter polycrystalline YAG fibre was manufactured from an aqueous sol–gel precursor which contained chlorine, and compared to a similar nitrate containing YAG precursor fibre we have reported previously. The precursor sol was found to be stable at a higher concentration than the nitrate containing sol, and this resulted in denser gel fibres which demonstrated better sintering at equivalent temperatures, giving a 98·5% sintered YAG fibre at 1550°C with a grain size of only 1 μm. However, on firing in air, the fibres formed fully crystalline YAG between 800 and 900°C, a temperature 100°C higher than the fibres containing nitrate, and they were weakened by the presence of many hemispherical faults. It was shown that both of these features were due to the retention of chlorine until the onset of formation of the crystalline YAG phase, and a series of steaming experiments were devised to remove the halide before this process could occur. It was found that steaming the precursor fibre from 200 to 500°C over 3 h, followed by firing to the required temperature in air, removed the chlorine and the problems it caused in the formation of the YAG phase without any change in the sintering characteristics or grain size. The steamed fibres were of a strength and quality comparable to fibres drawn from organometallic precursors. Empirical friability measurements showed the strength was maintained after firing to 1550°C, although there was a deterioration in apparent strain to break of the aligned blanket product above 1200°C. Conversely, the creep resistance, measured using the BSR test, improved with increase in temperature. The fibres fired to 1550°C were fully relaxed at temperatures 100–150°C below that of coarser, larger YAG fibres previously reported with a 3 μm grain size and 120 μm diameter. However, when allowance was made for grain size, the difference in creep rates was within the range obtained by extrapolating previous data using lattice diffusion and grain boundary effect models. Fibres fired to 1400°C were much finer grained but only slightly inferior to the 1550°C fibre in terms of creep. The alumina sol used in this work contained a significant level of sodium, and this suggests that the creep rates are effected by grain boundary impurities, especially sodium. A sodium free sol has been procured and further work is recommended to clarify the effect of impurities and improve fibre properties.     

Fabrication and microstructure development of Yb:YAG transparent ceramics from co-precipitated powders without additives

Sintering additives are generally considered to be important for improving densification in fabrication of transparent ceramics. However, the sintering aids as impurities doped in the laser materials would decrease the laser output power and produce additional heat during laser operation. In this work, Yb:YAG ceramics were vacuum-sintered without additives at different temperatures for various soaking time through using ball-milled powders synthesized by co-precipitation route. The densification behavior and grain growth kinetics of Yb:YAG ceramics were systematically investigated through densification curves and microstructural characterizations. It was determined that the densification in the 1500°C-1600°C temperature range was controlled by a grain-boundary diffusion. It is revealed that the volume diffusion is the main mechanism controlling the grain growth between 1600°C and 1750°C. Although SiO₂ additives can promote densification during low-temperature sintering, the optical transmittance of Yb:YAG ceramic with no additives, sintered at 1800°C for 15 hours, reaches a maximum of 83.4% at 1064 nm, very close to the measured transmittance value of Yb:YAG single crystal. The optical attenuation loss was measured at 1064 nm in Yb:YAG transparent ceramic, to be 0.0035 cm⁻¹, a value close to that observed for single crystals.     

Sintering of zirconia ceramics by intense high-energy electron beam

A comparative analysis of the efficiency of zirconia ceramics sintering by thermal method and high-energy electron beam sintering was performed for compacts prepared from commercial TZ-3Y-E grade powder. The electron energy was 1.4 MeV. The samples were sintered in the temperature range of 1200–1400 °C. Sintering of zirconia ceramics by high-energy accelerated electron beam is shown to reduce the firing temperature by about 200 °C compared to that in conventional heating technique. Ceramics sintered by accelerated electron beam at 1200 °C is of high density, microhardness and smaller grain size compared to that produced by thermal firing at 1400 °C. Electron beam sintering at higher temperature causes deterioration of ceramics properties due to radiation-induced acceleration of high-temperature recrystallization at higher temperatures.     

Fe-doped YSZ electrolyte for the fabrication of metal supported-SOFC by co-sintering

Considering Fe as sintering aid for Yttria Stabilized Zirconia (YSZ), studies have been conducted under conditions necessary for the fabrication of MS-SOFC by co-sintering. Pure and Fe doped YSZ was sintered at 1350 °C in air and in argon atmosphere and a comparative study has been performed. Structural characterizations were carried out using X-ray diffraction (XRD) techniques, while ionic conductivity was measured in air using four-probe impedance spectroscopy. It is found that Fe enhances the sintering rate of YSZ in both air and argon atmosphere. All samples sintered in argon atmosphere are characterized by better ‘sinterability’, larger lattice parameter, higher density, larger grain size and lower conductivity, as compared to samples sintered in air. Ionic conductivity is found to decrease with increase in Fe concentration for all sintered samples.     

Evolution of the microstructural and mechanical properties of hydroxyapatite bioceramics with varying sintering temperature

This study highlights the effects of sintering temperature on the microstructures, densification, grain sizes/boundaries, calcium/phosphorus (Ca/P) ion ratios, mechanical and bioactive properties of biocompatible hydroxyapatite (HA) ceramics prepared via cold isostatic pressing. X-ray diffraction refinement analysis revealed that the phase ratios of hexagonal HA and secondary phases were sensitive to the sintering temperature. Grain sizes, densities, and shrinkages of the as-sintered HA ceramics increased with increasing sintering temperature. The Ca/P ratios of the as-sintered HA specimens ranged from 1.63 to 1.71 for sintering temperatures of 900–1300 °C. The maximum mechanical hardness was achieved in the specimen sintered at 1200 °C due to the dense matrix formed with a smaller grain size and fewer flawed grain boundaries, as determined by high-resolution transmission electron microscopy. Ion release analysis (in a simulated body fluid solution) indicated that phosphorus ions were absorbed and rapid deposition of calcium ions occurred after immersion periods of above 4 days.     

Continuous aluminum oxide-mullite-hafnium oxide composite ceramic fibers with high strength and thermal stability by melt-spinning from polymer precursor

Continuous aluminum oxide-mullite-hafnium oxide (AMH) composite ceramic fibers were obtained by melt-spinning and calcination from polymer precursor that synthesized by hydrolysis of the aluminum isopropoxide, dimethoxydimethylsilane and hafnium alkoxide. Due to the fine diameter of 8–9 µm, small grain size of less than 50 nm and the composite crystal texture, the highest tensile strength of AMH ceramic fibers was 2.01 GPa. And the AMH ceramic fibers presented good thermal stability. The tensile strength retention was 75.48% and 71.49% after heat treatment at 1100 °C and 1200 °C for 0.5 h respectively, and was 61.57% after heat treatment at 1100 °C for 5 h. And the grain size of AMH ceramic fibers after heat treatment was much smaller than that of commercial alumina fibers even when the heat treatment temperature was elevated to 1500 °C, benefited by the grain size inhibition of monoclinic-HfO₂ (m-HfO₂) grains distributed on the boundary of alumina and mullite grains.     

Effect of Yttria and Yttrium-Aluminum Garnet on Densification and Grain Growth of Alumina At 1200°–1300°C

Densification and grain growth of alumina were studied with yttria or yttrium-aluminum garnet (YAG) additives at the relatively low temperatures of 1200°–1300°C. Yttria doping was found to inhibit densification and grain growth of alumina at 1200°C and, depending on dopant level, had a lesser effect at 1300°C. At 1200°C, yttria inhibits densification more than it hinders grain growth. The rate of grain growth increases faster with temperature than the rate of densification. Alumina-YAG particulate composites were difficult to sinter, yielding relative densities of only 65% and 72% after 100 h at 1200° and 1300°C, respectively. Pure YAG compacts exhibited essentially no densification for times up to 100 h at 1300°C.     

In-Situ Chemical Vapor Growth of Carbon-Rich Silicon Carbide Fibers Upon Pyrolysis of Polyzirconosilane

Carbon-rich SiC fibers were prepared in a cost-effective way using in-situ chemical vapor growth upon pyrolysis of polyzirconosilane at 1200 °C under nitrogen atmosphere, with 0.5-1 μm in diameter and a few milimeters in length and a yield of about 30 %. The empirical formula of the fibers is Si₁C_6.6O_0.1, and both silicon and carbon atoms inside the fibers are found to be uniformly distributed, and the free carbon atoms show a turbostratic state. The carbon-rich SiC fibers show ideal thermo-oxidation resistance at 1200 °C in air. Chemical vapor growth of ceramic fibers upon preceramic pyrolysis can be used to prepare high performance ceramic fibers on a large scale cost- effectively.     

Effects of CuO on constrained sintering of a polycrystalline TiO2 ceramics

Effects of CuO on constrained sintering of a polycrystalline TiO₂ ceramics have been investigated. The densification temperature of TiO₂ is reduced from 1100-1200°C for pure TiO₂ to 900°C with the presence of 0.5-3 mol% CuO under free sintering. For the samples with 1 mol% CuO, the constrained densification is slowed down, but a high sintered density of >95% at 950°C, which is close to that sintered freely, is still obtained. The above results are caused by the formation of CuO-rich film at the grain boundaries, which reduces grain-boundary energy and enhances grain-boundary migration kinetics of TiO₂. To confirm the above findings, molecular dynamics simulation, at which the ratio of grain boundary energy of TiO₂ between with and without CuO agrees well with that obtained experimentally, is conducted.     

Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering

Due to the sensitivity of nanopowders and the challenges in controlling the grain size and the density during the sintering of ceramics, a systematic study was proposed to evaluate the densification and the microstructure of ZnO ceramics using spark plasma sintering technique. Commercially available ZnO powder was dried and sintered at various parameters (temperature (400–900?°C), pressure (250–850?MPa), atmosphere (Air/Vacuum) etc.). High pressure sintering is desirable for maintaining the nanostructure, though it brings a difficulty in obtaining a fully dense ceramic. Whereas, increasing the temperature from 600 to 900?°C results in fully densified ceramics of about 99% which shows to have big impact on the grain size. However, a high relative density of 92% is obtained at a temperature as low as 400?°C under a pressure of 850?MPa. The application of pressure during the holding time seems to lower the grain size as compared to ceramics pressed during initial stage (room temperature).     

Effect of two-step sintering on the hydrothermal ageing resistance of tetragonal zirconia polycrystals

The effects of two-step sintering (TSS) on the mechanical properties and hydrothermal ageing resistance of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were investigated. In TSS, the first step involved heating the samples up to 1400 °C at a heating rate of 10 °C/min and holding the samples at this temperature for 1 min. The second step involved sintering by cooling the samples down to 1200 °C and holding the samples at this temperature for various holding times (t) ranging from 0 to 30 h before cooling to room temperature. Moreover, TSS promoted densification with increasing holding time without sacrificing the mechanical properties of the sintered body and causing abnormal grain growth. The average grain size was found not to be affected by the long holding times, and the final microstructure composed of a uniformly distributed tetragonal grain having sizes ranging from 0.24 to 0.26 µm. The beneficial effect of TSS in suppressing the hydrothermal ageing of Y-TZP has been revealed in the present work. In particular, samples sintered at t=20 and 30 h exhibited excellent resistance to low-temperature degradation when exposed to superheated steam at 180 °C, attributed mainly to the enhance densification of the sintered bodies.     

Effects of microstructure on fracture strength and conductivity of sintered NMC333

Sintering of LiNi_0.33Mn_0.33Co_0.33O₂ cathode material was investigated for potential application in all-electric aerospace propulsion systems utilizing new architectural concepts. All-solid-state batteries, while inherently safe, may not reach the high energy density required for next generation propulsion systems. To meet this performance requirement, multifunctionality of sintered active material may achieve systems level weight savings through simultaneous load bearing and electrochemical energy storage performance. The effects of sintering conditions on structural stability, chemistry, densification, grain size, fracture strength and electrical conductivity were quantified for the active cathode material. X-ray diffraction and inductively coupled plasma results indicated the structure and stoichiometry were maintained across the range of processing conditions to facilitate intercalation. Densification was achieved by sintering at 1050°C in ambient atmosphere, but grain coarsening was observed for higher temperatures and longer hold times. Mechanical strength was improved with reduction in porosity, but excessive grain growth decreased strength, providing a maximum of 50 MPa for samples sintered at 1050°C for 10 hours. Electrical conductivity initially improved with densification, but significantly diminished as the microstructure coarsened. The optimal sintering condition of 1050°C maximized mechanical fracture strength and electrical conductivity, with shorter sintering times preferred.     

Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

In this study, calcium lanthanum sulfide (CaLa₂S₄, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth.     

Electrospun mullite nanofibers derived from diphasic mullite sol

Fine‐grained mullite nanofibers derived from the diphasic mullite sol were successfully fabricated by electrospinning and subsequent pyrolysis at 1500°C. Polymethylsiloxane and aluminum tri‐sec‐butoxide were selected as the silicon and aluminum source to synthesize the diphasic sol. Results show that the weight loss of mullite precursor fibers in our work was about 60 wt.%, which is similar with that of fibers fabricated using the monophasic sol. This low weight loss was mainly attributed to the high ceramic yield of polymethylsiloxane and low introduced polyvinylpyrrolidone content, which ensures the integrity of fiber morphology during the sintering process. Mullite fibers with 216 nm average diameter were fabricated after sintered at 1500°C and the corresponding grain size was only ~100 nm, much smaller than that in mullite fibers derived from monophasic sols. Therefore, it can be predicated that mullite fibers in this work should possess a higher mechanical strength than those derived from monophasic sols when the sintering temperature was higher than 1400°C and therefore was an ideal starting materials for the fabrication of mullite nanofibrous ceramics used as the high‐temperature thermal insulation materials.     

Transparent yttrium aluminum garnet (YAG) ceramics by spark plasma sintering

Commercial nanocrystalline yttrium aluminum garnet (nc-YAG) powders were used for fabrication of dense and transparent YAG by spark plasma sintering (SPS). Spherical 34 nm size particles were densified by SPS between 1200 and 1500 °C using 50 and 100 MPa pressures for 3, 6, and 9 min durations. Fully dense and transparent polycrystalline cubic YAG with micrometer grain size were fabricated at very moderate SPS conditions, i.e. 1375 °C, 100 MPa for 3 min. Increase in the SPS duration and pressure significantly increased the density especially at the lower temperature range. The observed microstructure is in agreement with densification by nano-grain rotation and sliding at lower densities, followed by curvature driven grain boundary migration and normal grain growth at higher densities. Residual nanosize pores at the grain boundary junctions are an inherent microstructure feature due to the SPS process.     

Preparation of Nitrogen‐Doped Mullite Fiber by Nitridation of Silica–Alumina Gel Fiber in Ammonia

Nitrogen‐doped mullite fibers were first synthesized through the nitridation of Al₂O₃–SiO₂ gel fibers in NH₃. The results showed that nitrogen take‐up began at 800°C, reached the maximum at 900°C, and then decreased with increasing temperature. The ceramic fibers nitridated at 900°C were essentially amorphous, but contained a small amount of nano‐sized Al–Si spinel crystals. Mullite was formed after nitridation at 1200°C, accompanied by crystallization of χ‐SiAlON and δ‐Al₂O₃. The incorporation of nitrogen resulted in the formation of a variety of nitrogen‐containing crystalline phases. The grain size of the mullite fibers can be adjusted by changing of the nitrogen content.     

Effect of Yttrium Aluminum Garnet Additions on Alumina-Fiber-Reinforced Porous-Alumina-Matrix Composites

The effects of incorporating yttrium aluminum garnet (YAG) into a porous alumina matrix reinforced with Nextel 610 alumina fibers were investigated. Composites with various amounts of YAG added to the matrix were prepared to determine its effect on retained tensile strengths after heating to 1100° and 1200°C. Strengths of YAG-containing composites were slightly lower than those of an all-alumina-matrix composite after heating for 5 h to 1100°C. However, after heating for 5 or 100 h at 1200°C, all the YAG-containing composites displayed greater strengths and greater strains to failure than the all-alumina composite. At the higher temperature, the presence of YAG is believed to inhibit the densification of the matrix, which helps to maintain higher levels of porosity and weaker interparticle bonding that allows for crack-energy dissipation within the matrix. A reduction in grain growth of the fibers by the presence of segregated Y was also observed, which may also contribute to higher fiber strength, thereby increasing the retained strengths of the YAG-containing composites.     

Densification,microstructure and mechanical properties of Ta4HfC5-based ceramics obtained from synthesized nanoscale powder

Solvothermal treatment was used to synthesize nanoscale 4TaC-HfC (Ta₄HfC₅) powder at a relatively low calcination temperature and in a short period (1400 °C, 2 h). The obtained powder had uniform size distribution and dispersion. Ta₄HfC₅ ceramics were then consolidated via spark plasma sintering at 2100 °C. Ceramics had a better densification and smaller mean grain size at a shorter sintering time compared with that of materials sintered using mechanical ball milling method. The densification behavior of ceramics deriving from synthesized or ball milled powders was analyzed and the mechanical properties of different samples were investigated. To further increase the mechanical properties, a nearly fully dense Ta₄HfC₅-MoSi₂ ceramic was sintered using the synthesized powder. The mechanical properties of the ceramic composite doubled the strength values. This processing route demonstrated to be a viable approach to synthesize nanoscale Ta₄HfC₅ powder with high purity and uniformity, and obtain higher performances ceramics once sintered.