Influence of processing atmosphere on the microstructural evolution of submicron alumina powder during sintering

Investigations into the sintering of submicron oxide powders have revealed interesting behavior, particularly insofar as it concerns their microstructural evolution in the early, low temperature transformations during heating. In this work, experiments were conducted on a submicron alumina powder, whose microstructural evolution and densification were characterized after sintering from 900 °C to 1400 °C in air, dry air and high vacuum (10⁻⁸ atm). The results indicated that the processing atmosphere strongly influences the particle size distribution at low temperatures before shrinkage occurs. Shrinkage began concomitantly with grain growth and the sintering atmosphere influenced the sintering kinetics. This factor, which is associated with previous narrowing of the particle size distribution, may affect grain growth and densification during the final stage of sintering.     

A model for initial-stage sintering thermodynamics of an alumina powder

A model was proposed to calculate several thermodynamic parameters for the initial-stage sintering of an alumina powder obtained after calcinations at 900 °C for 2 h of a precursor. The precursor was synthesized by an alumina sulphate-excess urea reaction in boiling aqueous solution. The cylindrical compacts of the powder with a diameter of 14 mm were prepared under 32 MPa by uniaxial pressing using oleic acid (12% by mass) as binder. The compacts were fired at various temperatures between 900 and 1400 °C for 2 h. The diameter (D) of the compacts before and after firing was measured by a micrometer. The D value after firing was taken as a sintering equilibrium parameter. An arbitrary sintering equilibrium constant (K_a) was calculated for each firing temperature by assuming K_a = (D_i − D) / (D − D_f), where D_i is the largest value before sintering and D_f is the smallest value after firing at 1400 °C. Also, an arbitrary change in Gibbs energy (ΔG _a°) was calculated for each temperature using the K_a value. The graphs of ln K_avs. 1 / T and ΔG _a° vs. T were plotted, and the real change in enthalpy (ΔH°) and the real change in entropy (ΔS°) were calculated from the slopes of the obtained straight lines, respectively. Inversely, real ΔG° and K values were calculated using the real ΔH° and ΔS° values in the ΔG° = − RT ln K = ΔH° − TΔS° relation. The best fitting ΔH° and ΔS° values satisfying this relation were found to be 157,301 J mol^− 1 and 107.6 J K^− 1 mol^− 1, respectively.     

Densification of fine-grained alumina ceramics doped by magnesia,yttria and zirconia evaluated by two different sintering models

The influence of various dopants (500 ppm MgO and Y₂O_3; 250 ppm ZrO₂) on sintering of fine-grained alumina ceramics was evaluated by high-temperature dilatometry. The apparent activation energy of sintering was estimated with the help of Master Sintering Curve and a model proposed by Wang and Raj. The densification kinetics was controlled by at least two mechanisms operating at low (higher activation energy) and high (lower activation energy) densities. Good agreement between the activation energies calculated with both models was observed for low as well as for high densities. The lowest value of activation energy exhibited undoped alumina; the addition of MgO resulted in slight increase of the activation energy. Y₂O₃ and ZrO₂ significantly inhibited the densification, which was reflected in the higher sintering activation energies. The low activation energies in the final sintering step indicates the importance of proper choice of sintering temperature, namely in the two-step sintering process.     

The sintering kinetics of ultrafine tungsten carbide powders

The sintering kinetics of nano grained tungsten carbide (n-WC) powders has been analyzed by non isothermal and isothermal sintering. Non isothermal sintering experiments reveal a multi staged sintering process in which at least three major sub-stages can be distinguished. The isothermal shrinkage strain also exhibits an asymptotic behavior with time indicating an end point density phenomenon in most of the temperature ranges. Combined microstructural and kinetic data analyses suggest that differences in the sinterability of inter and intra agglomerate pore phases introduce sub-stages in the sintering process which manifest as stagnant density regions in both the isothermal and non isothermal experiments. Kinetic analysis of the data reveals very low activation energies for sintering suggesting that particle rearrangement and agglomeration at low temperatures may be brought about by surface diffusion leading to neck growth and grain rotation. At higher temperatures rapid grain boundary diffusion by overheating along inter particle boundaries induced by sparking may be a dominant sintering mechanism. Although grain growth and densification in conventional WC powders generally obey an inverse relation to each other, in n-WC powders both can act synergistically to increase the net densification rate. In fact, complete densification cannot be achieved in n-WC powders without grain growth as one abets the other.     

Slurry-based selective laser sintering of polymer-coated ceramic powders to fabricate high strength alumina parts

Instead of conventional powder-based selective laser sintering, a novel slurry-based process to fabricate high strength ceramic parts is proposed. A slurry which was composed of alumina powder coated with water-insoluble semi-crystalline polyvinyl alcohol (PVA) as a structure material, water-soluble PVA as an organic binder, ammonium polymethacrylate (DARVAN C-N) as a dispersant, and deionized water as a solution, could be prepared with colloidal processing. A rigid green block could be built with a self-made rapid prototyping apparatus. The polymers contained in the scanned region were melted to connect the alumina powders, but transformed to be water-insoluble. However, the un-scanned region remained water-soluble. Due to dissolving of the polymers in water, the un-scanned region could collapse to obtain the green part. After binder removing and sintering, an alumina ceramic part could be obtained. An average flexural strength of 363.5 MPa and a relative density of 98% were achieved.     

The influence of ageing on consolidation and sinterability of a sub-micron alumina powder

A commercial sub-micron alumina powder was used for investigation of the influence of exposure to atmospheric humidity on powder characteristics, consolidation behaviour, densification and final microstructure of alumina ceramics. A significant uptake of atmospheric water by inadequate storage confirmed by thermal analysis, XPS, and FTIR, resulted in decreased sinterability of the powder, although no significant influence on the mean size of alumina grains was observed. In addition, the effect of low temperature heat treatment (drying at 120 °C and calcination at 700 °C) was also studied. The sinterability of pre-dried powder increased if a wet consolidation method (pressure filtration) was used, but a negative effect of pre-drying was observed in case of dry forming (axial pressing). The calcination decreased the ability of the powder to adsorb water. The presence of aggregates formed by calcination markedly decreased the green and sintered densities in compacts consolidated by axial pressing.     

红柱石粒度对氧化铝纤维增强红柱石基复合材料烧结性能的影响

以南非红柱石和多晶氧化铝纤维为原料,在纤维加入量(w)分别为5%、10%、15%和20%,烧成温度分别为1350℃和1500℃的条件下,研究了红柱石原料粒度为0.2～101.5μm和0.1～34.7μm时对传统无压烧结工艺制备的氧化铝纤维增强红柱石基复合材料烧结性能的影响。结果表明:随着红柱石粒度的减小,基体材料的莫来石化温度和烧结温度明显降低,但纤维团聚现象加剧;由于纤维与基体界面结合力较强,纤维的增强作用以纤维的脱粘和断裂为主;在材料烧结后,红柱石粒度的变化对其常温耐压强度影响不大。     

Effect of alumina addition on initial sintering of cubic ZrO2 (8YSZ)

The effect of 0–10 wt% alumina addition on the initial sintering of 8 mol% Y₂O₃ cubic ZrO₂ (8YSZ) was studied. Activation energy and initial stage of sintering mechanism were analyzed in order to understand the effect of the alumina in the sintering process. The analysis was carried out using the analytical method for constant rate heating (CRH). The activation energy decreased from 716 to 599 kJ/mol for undoped 8YSZ to 2.16 wt% of alumina–8YSZ, respectively. The mechanism for the initial stage of sintering for <2.16% Alumina–8YSZ changed from grain boundary diffusion (GBD) to volumetric diffusion (VD). With 10 wt% of alumina, the activation energy increased to 854 kJ/mol which was thought due to the change in the initial stage of sintering mechanism from VD to GBD.     

Effect of powder morphology on sintering kinetics,microstructure and mechanical properties of monazite ceramics

This work focuses on the effect of precursor morphology on the microstructural evolution of monazite-type lanthanum-europium phosphate ceramics during sintering, including grain growth rate, as well as correlations between microstructure, texture effects and their mechanical properties. Sintering kinetics of La_0.5Eu_0.5PO₄ powders with two different grain morphologies (needle-shaped and spherical) was studied by an in situ HT-ESEM method at 1340°C. La_0.5Eu_0.5PO₄ pellets with high density (99% of the theoretical density) were obtained for both precursor powders by hot pressing. Analysis of XRD data collected for the hot pressed pellets obtained from needle-shaped precursors revealed preferential orientation of the grains towards the (100) direction. Mechanical properties of the hot pressed pellets were studied by the Vickers indentation method. The dependence of microhardness and fracture toughness on microstructure and texture was investigated.     

Preparation of hydrophobic mesoporous silica powder with a high specific surface area by surface modification of a wet-gel slurry and spray-drying

A hydrophobic mesoporous silica powder was prepared by surface modification of a sodium silicate-based wet-gel slurry. The effects of the volume percentage (%V) of trimethylchlorosilane (TMCS), used as surface-modifying agent, on the physicochemical properties of the silica powder were investigated. We observed that as the %V of TMCS in the simultaneous solvent exchange and surface modification process increased, so did the specific surface area and cumulative pore volume of the resulting silica powder. Hydrophobic silica powder with low tapping density (0.27 g/cm³), high specific surface area (870 m²/g), and a large cumulative pore volume (2.2 cm³/g) was obtained at 10%V TMCS. Surface silanol groups of the wet-gel slurry were replaced by non-hydrolysable methyl groups (-CH₃), resulting in a hydrophobic silica powder as confirmed by FT-IR spectroscopy and contact angle measurements. We also employed FE-SEM, EDS, TG-DTA, and nitrogen physisorption studies to characterize the silica powders produced and to compare the properties of modified and unmodified silica powders. Moreover, we used a spray-dying technique in the present study, which significantly reduced the overall processing time, making our method suitable for economic and large-scale industrial production of silica powder.     

Preparation of microfiltration membrane supports using coarse alumina grains coated by nano TiO2 as raw materials

To increase the mixing uniformity of coarse alumina grains with a small amount of nano TiO₂ particles, TiO₂ particles were prepared on the surface of coarse Al₂O₃ grains by in-situ hydrolysis of TiCl₄. The coated coarse Al₂O₃ powder was used to prepare microfiltration membranes supports. The effects of TiO₂ content and sintering temperatures on the bending strength, porosity and pore size distribution of the obtained supports were studied. The results show that the melted nano TiO₂ grains locate mainly at the neck of Al₂O₃ grains, which increases the bending strength of the support by increases the neck area. However, the bending strength is weakened if the TiO₂ content is excessive. No aggregated nano TiO₂ grainsare found. The resulting supports sintered at 1650 °C for 2 h yields a bending strength of 55.4 MPa, a porosity of 38% with a mean pore size of 8.0 μm.     

Influence of the gas atmosphere on restructuring and sintering kinetics of nickel and platinum aerosol nanoparticle agglomerates

The kinetics of the primary particle growth and the restructuring of nanoparticle agglomerates of Ni and Pt were studied under variation of temperature and gas composition. The aerosol particles used in the study were produced by spark discharge into nitrogen as carrier gas. Restructuring was monitored by measuring the mobility equivalent diameter by SMPS after different residence times in a tempered volume. To determine the kinetics of primary particle growth, samples were taken over a range of residence times for TEM analysis. These experiments were conducted in reducing atmospheres and in pure nitrogen to study the effect of surface state on the kinetics of the processes. To quantify sintering kinetics, a multi-stage sintering model based on the reduction of the surface energy was fitted to the experimental results.

A strong dependence of the primary particle growth and the agglomerate restructuring on the surface state of the particles was found. Both processes were accelerated strongly with increasing purity of the particle surfaces. The model yielded an activation energy for the primary particle growth of Ni-agglomerates in pure nitrogen (99.99%) of about 80 kJ/mol which was decreased for reduced Ni and Pt particles to a value of about 50 kJ/mol. The kinetics of restructuring was much faster than the one for primary particle growth. This enables the seperation of the manipulations of agglomerate structure and primary particle size.     

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al₂O₃ and ZrO₂ monoliths as well as layered Al₂O₃/ZrO₂ composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.     

Role of microstructure reactivity and surface diffusion in explaining flash (ultra-rapid) sintering kinetics

The competition between sintering and coarsening is cited by numerous authors as one of the potential factors for explaining the ultra-rapid sintering kinetics of flash sintering. In particular, surface diffusion is a mechanism decreasing the driving force of sintering by changing the initial highly reactive microstructures (particle contact) into poorly reactive porous skeleton structures (spherical porosity). We show by finite element simulations that flash SPS experiments high specimen temperatures close to 2000 °C. These high temperatures are not sufficient to explain the ultra-rapid sintering kinetics if typical spherical pore theoretical moduli are employed. On the contrary, reactive experimentally determined moduli succeed in explaining the ultra-rapid sintering kinetics. Mesoscale simulations evidenced that the origin of such reactive experimental moduli is a porous skeleton geometry with a significant delay in surface diffusion and particle rearrangement. This highlights the important role of the surface diffusion negation (favoring higher stress intensification factor) in flash sintering.     

氧化铝-莫来石复合粉对莫来石烧结行为的影响

将莫来石先驱体溶胶预先引入到硫酸铝水溶液中 ,干燥后经 12 0 0℃煅烧获得氧化铝 -莫来石复合粉料。研究了该粉料与硅溶胶混合获得的混合粉的烧结行为 ,并与氧化铝、莫来石晶种和硅溶胶三相混合获得的混合粉的烧结行为进行了分析比较。其中 ,两种混合粉料均是以理论莫来石组分进行配比 (Al2 O3∶SiO2 =72∶2 8) ,并且两种混合粉中莫来石晶种的质量分数均为 5%。实验结果表明 :前者在 1450℃烧结 2 0min即实现完全莫来石化 ,其显微结构为晶须状莫来石 ;后者在 150 0℃烧结 2 0min实现完全莫来石化 ,其显微结构为针状莫来石     

The sintering temperature of ash, agglomeration, and defluidisation in a bench scale PFBC

Five Australian black coals were studied in a bench scale pressurised fluidised bed combustor (PFBC) to investigate the agglomeration propensity. It was found that coals with higher proportions of calcium aluminosilicate showed higher propensity for agglomeration and defluidisation. The pressure-drop sintering technique can predict the agglomeration propensity for coals. Samples with a sintering temperature lower than the operating temperature of the PFBC showed agglomeration. The laboratory ash can be a good representative of the PFBC ash when studying agglomeration and defluidisation.     

The sintering kinetics of porcelain bodies made from waste glass and fly ash

Porcelain is a material produced from kaoline, quartz and potassium-feldspar. Recently, research of new materials, for example non-hazardous wastes, that are able to replace traditional fluxing agents without changing the process or quality of the final products has been realized. The aim of this work is to study the possibility of the use of glass powder waste and fly ash together for manufacturing porcelain. Instead of quartz, fly ash was used at the selected porcelain composition. The waste glass was added partially and fully in replacement of potassium-feldspar. Samples were fired in an electric furnace with a heating rate of 10 °C/min at 1100, 1150 and 1200 °C for a period of 1, 2, 3 and 5 h. The sintered samples were characterised by XRD (X-ray diffraction) and SEM (scanning electron microscopy). Sintering activation energies were determined based on the bulk density result. At 10, 15, 20 and 25 wt.% glass waste addition, the apparent activation energies were calculated to be 145, 113.5, 70.4 and 53.74 kJ/mol, respectively. It was found that the sintering activation energy decreased with increasing waste glass addition.     

Novel fabrication of pressure-less sintering of translucent powder injection molded (PIM) alumina blocks

A new method of fabricating translucent alumina brackets using powder injection molding (PIM) is reported. Alumina powder was mixed with MgO, La₂O₃, and Y₂O₃ to control grain size and porosity. The powders were mixed with a binder consisting of a mixture of paraffin wax and polyethylene in a 1:1 ratio to make feedstock for injection molding. The total amount of binder was limited to 14 wt% to minimize shrinkage and cracking after sintering. After injection molding, debinding was performed using the wicking method and samples were sintered in a vacuum at 1700 °C to achieve high density. Ultimately, translucent corundum was fabricated. The sintering additives resulted in a decrease in porosity and an improvement in translucency by promoting grain growth during pressure-less sintering. After sintering, Vickers hardness, bending strength, density, and transmittance of the fabricated parts were measured to show that those values were comparable to those of the commercially available dental brackets. Therefore, the translucent alumina block was successfully fabricated using PIM method to be potentially used as a dental bracket.     

Effect of surface characteristics of different alumina on metal-support interaction and hydrodesulfurization activity

Effect of surface characteristics of two species of commercial γ-alumina (alumina-A and alumina-B) on metal-support interaction and hydrodesulfurization (HDS) activity of the corresponding CoMo catalysts was studied systematically. The HDS activity tests were carried out in a continuous flow fixed-bed micro-reactor. Compared with alumina-A, alumina-B with better crystallinity and less hydroxyl groups can partly inhibit inactive cobalt spinel formation and moderate the metal-support interaction, causing a notable increase in the average slab length and stacking number of MoS₂. It stimulates the promoting effect of cobalt on molybdenum and contributes to a noticeable increase in HDS activity.     

Ball-mill grinding kinetics of master alloys for steel powder metallurgy applications

The grinding kinetics of three newly developed master alloys for steel powder metallurgy applications were investigated using a laboratory ball-mill. Non-first order grinding kinetic is observed for the three master alloys as the breakage rate increases with grinding time due to the work hardening of the ductile phase in the microstructure. Agglomeration of fine particles is observed after a critical time at which d₉₀ reaches its lowest value (~ 30 μm). Critical times are related to the hardness and the microstructure of the different master alloys. Agglomeration of fine particles can be overcome with the use of a process control agent. In this study, the addition of stearic acid to master alloy powders prior to grinding successfully eliminates agglomeration for long grinding times (d₉₀ ~ 16 μm after grinding for 270 min).