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.     

Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength

Coarse alumina powder compacts doped with various amounts of titania and copper oxide were pressurelessly sintered from 900 °C to 1600 °C. Their phase assemblages and microstructural evolution, as well as their properties, were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry/thermogravimetric (DSC/TG) analysis, and three-point bending and wetting test. The role of TiO₂ and CuO during the sintering is discussed in detail. The experimental results show that the liquid phase from the copper oxide appeared at approximately 1200 °C, so the solid-state reaction between alumina and titania took place at a lower temperature. Such solid state-reaction sintering had a strong impact on the grain growth and greatly promoted the densification of the alumina compact. In addition, the liquid phase inhibited the abnormal grain growth and microcracking. As a result, the coarse alumina powder compacts doped with 5 wt% TiO₂–CuO were fully densified and exhibited sufficient flexural strength (342±21 MPa) when sintered at a temperature of 1450 °C for 2 h.     

Mechanical modelling of microwave sintering and experimental validation on an alumina powder

Microwave sintering (MW) allows fast heating (≤30 min) and densification of ceramic materials, like alumina Al₂O₃. In order to predict the final material properties (density, size and grain size) the mechanical SOVS (Skorohold Olevsky Viscous Sintering) model is adapted and validated for conventional sintering of alumina. The model is implemented on ABAQUS with UMAT subroutine. Secondly, the SOVS model is modified for the microwave sintering by adapting the shear viscosity Arrhenius type law. Pre-exponential and exponential coefficients are modified for MW sintering. The calculated relative densities are compared to experimental results from conventional and microwave sintering and the relative difference remains under 3%. The coefficients identified for the MW sintering reveal a decrease in the shear viscosity by around 10 and an increase by up to 50 times in the grain boundaries diffusion coefficient.     

Surface area reduction during the sintering of alumina

Surface area reduction trajectories of two well characterized calcined alumina powders were analyzed over a range of thermal profiles. Surface area reduction trajectories were confirmed to be independent of initial green density. Variations in trajectory were explained through experimental demonstrations of blended coarse and fine particle alumina systems. This work demonstrates that (1) nanoparticles do not appear to assist conventional sintering, because the nanoparticle surface area drops precipitously with minimal increase in density; and (2) the initial compact density does not contribute, nor control, surface area reduction during alumina sintering.     

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.     

Preparation of tabular corundum aggregate from different sources alumina raw powder: Sintering kinetics and establishment of kinetics model

In this paper, we report the use of four types of commercial alumina raw powders as raw materials for the preparation of tabular corundum aggregates under the same conditions. The influence of the transition phases of alumina raw powder on the sintering kinetics of tabular corundum is discussed, the sintering model of materials with pseudomorphic structure is established, and the mechanisms underlying the different performances of various commercial tabular corundum samples are evaluated. The following conclusions were drawn based on the results of the study. (1) A double tetrakaidecahedron model was established and was shown to satisfactorily describes the sintering mechanism of alumina raw powder with pseudomorphic structure, which accords with the porosity change trend of sintered body and provides a basis for perfecting the sintering theory. (2) Compared with the other transition phases, γ-Al₂O₃ shows the largest phase transformation volume contraction, which provides the driving force for the sintering process via an increase in surface energy and mainly acts in the densification and grain growth stages. Thus, high-quality refractory raw materials are prepared with optimized physical properties and Intracrystalline pores or pore clusters in the crystal structure. The preparation of these high-quality refractory products is of importance for prolonging the life of these materials and also meeting rising energy demands.     

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.     

Role of fabrication route and sintering on wear and mechanical properties of liquid-phase-sintered alumina

Liquid-phase-sintered Al₂O₃ (LPS) fabricated by slip casting, tape casting, isopressing, uniaxial pressing, piston and auger extrusion showed substantial differences in wear due to differences in morphology as observed in image analyses of SEM micrographs. The abrasive wear was low in the case of uniaxial pressing and high in the case of tape casting in the ‘dry sand and rubber wheel’ test. The wear surface of the tape cast specimen exhibited extensive microcracking possibly due to orientation of Al₂O₃ platelet (major face) parallel to the abraded surface whereas some degree of perpendicular orientation in extruded surface resulted in lower wear loss. In wet-milling wear test, the isopressed balls of a 95–97 wt% LPS derived from reactive powder (<1 μm) showed 25% lower wear loss than that of the extruded balls of a 91–94 wt% LPS derived from coarse powder (70–100 μm). Sintering at a lower temperature with longer duration and batch milling of the composition in between 12 and 16 h resulted in low wear loss. Flexural strength also improved by longer sintering time but did not show any improvement by increasing milling time. However, the variation in flexural strength was minimized by isopressing the extruded specimen. A high indentation fracture toughness at 49.03 N test load was associated with (i) large elongated reinforcement grains in a fine-grained microstructure with overall elongated morphology and (ii) with an intergranular fracture.     

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.     

Effects of particle size distribution and sintering temperature on properties of alumina mold material prepared by stereolithography

Alumina mold materials prepared by stereolithography usually have considerable sintering shrinkage, and their properties related to casting have been rarely studied. In this study, alumina molds materials were prepared by stereolithography, and the effects of particle size distribution and sintering temperature on the properties of the materials were investigated. Results show that the viscosity of the slurries decreases as the fraction of fine powder increases, and the particle size distribution affects the curing behaviors slightly. Sintering shrinkage increases as the fraction of fine powder or the sintering temperature increases. Although lower sintering shrinkage can be achieved by sintering at 1350 °C or 1450 °C, the mold materials sintered at lower temperatures would continue to shrink under the service temperature of 1550 °C, and thus 1550 °C is determined as the optimal sintering temperature. As the fraction of fine powder increases, the creep resistance first increases and then decreases, and specimens prepared with 0.1 fraction of fine powder exhibit the best creep resistance with the droop distance of 4.44 ± 0.45 mm. Specimens prepared with 0.1 fraction of fine powder and sintered at 1550 °C exhibit linear shrinkage of 6.36% along the X/Y direction and 11.39% along the Z direction, and have a flexural strength of 78.15 ± 3.50 MPa and porosity of 30.12 ± 0.08%. The resulting material possesses relatively low sintering shrinkage, proper mechanical strength, porosity and high-temperature properties that meet the requirements for casting purposes.     

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.     

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.     

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.     

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.     

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.