Enhanced properties of pure alumina ceramics by oscillatory pressure sintering

A novel oscillatory pressure sintering (OPS) process to consolidate high-quality pure alumina ceramics is reported. The microstructure of the ceramics prepared by OPS develops into a higher final density, a smaller and a narrower distribution of grain sizes compared with those prepared by conventional pressureless sintering (PS) and hot-pressing (HP) processes. Enhanced mechanical properties of alumina ceramics were investigated by OPS process. The bending strength, hardness and elastic modulus of the OPS specimen reached about 546 MPa, 19.1 GPa and 374 GPa, respectively, i.e values significantly higher than that of the specimens by PS and HP. XRD analysis indicates the strengthening of atomic bonds aided by oscillatory pressure. The results suggest OPS to be an effective technique for preparing high-quality pure alumina ceramics.     

Effect of fluoride mineralizers on alumina sintering

Conclusions Additions to ground commercial alumina of fluorides (AlF₃ or CaF₂) in amounts of 1% exert a retarding and refining action during the sintering of alumina fired in the temperature range 1200–1700°C. The AlF₃ is the most effective additive. Retardation of sintering is explained by the healing of the surface defects in the grains of Al₂O₃ under the influence of the mineralizers.Translated from Ogneupory, No. 1, pp. 37–40, January, 1970.     

Transparent alumina by vacuum sintering

Alumina windows transparent in IR range and translucent in UV range were obtained by high vacuum sintering (～10^–6 mbar). To determine the influence of the atmosphere on grain growth, a sintering kinetics and intragranular porosity studies have been performed for samples sintered in air and in high vacuum atmospheres. It has been established that by using high vacuum sintering instead of the conventional one in air, alumina compacts with lower grain size, less impurities in grain boundaries and less quantity of intragranular pores were obtained. It has been proven that high vacuum sintering allows to reach high densities (>99%th) without exaggerated grain growth (d₅₀ < 1 μm). Finally, strong correlation between microstructure and light transparency has been found and consequently it can be concluded that vacuum sintering clearly enhances the optical properties of alumina.     

Influence of magnesia on sintering stress of alumina

The sintering stress and the densification of MgO-doped Al₂O₃ were measured with a self-loading apparatus and a thermomechanical analyzer, respectively. The densification started at 950 °C and finished at about 1450 °C. The measured surface tensions were 0.7–0.8 N m⁻¹ in the intermediate sintering stage but drastically decreased to 0.2 N m⁻¹ in the final stage of sintering.     

氟化铝对氧化铝烧结性能的影响

介绍了ＡｌＦ３的加入对烧结Ａｌ２Ｏ３的体积密度、气孔率的影响,以及ＡｌＦ３对板状氧化铝形成的作用     

氧化铝陶瓷低温烧结的研究现状和发展前景

本文综述了氧化铝陶瓷低温烧结的研究现状,包括粉体的制备,处理,成型及烧结工艺,并对此领域做了相应的设想和展望。     

烧结合成致密尖晶石的试验研究

采用粒度≤0.058mm、MgO质量分数为96.28%的轻烧镁砂和粒度≤0.043mm的αAl2O3为原料,分别在50MPa、100MPa、150MPa和200MPa的成型压力以及1600℃、1650℃、1700℃、1750℃和1800℃的烧成温度下进行合成尖晶石的试验研究。结果表明:通过控制合理的工艺参数,特别是m(MgO):m(Al2O3)的比约为1:1,成型压力约100MPa,烧成温度>1700℃时,可一步煅烧制得体积密度>3.30g·cm-3、尖晶石含量较高的致密镁铝尖晶石。     

Plastic flow in the sintering of alumina

The occurrence of plastic flow during sintering is no acknowledged in the classical theory of sintering densification. However, study of sintered -Al₂O₃ by transmission electron microscopy showed that significant plastic flow occurred during the sintering of the material. It is concluded that evidence of similar plastic flow in the sintering of fine ceramic powders is probably destroyed by the practice of prolonged firing at high temperatures. Dislocations activated by neck ‘capillarity’ stress can aid densification by a glide-climb mechanism.     

复合烧结剂对高铝质干式料烧结性能的影响

以粒度3～5mm,1～3mm,≤1mm,≤0.088mm的高铝矾土为主要原料,以粒度≤0.1mm的硼酸(H3BO3>99.6%)、粒度≤0.05mm的粘土、粒度≤0.1mm的钾长石和粒度≤0.1μm的硅灰(SiO2>90%)为复合烧结剂,按m(骨料)∶m(细粉)=65∶35的配比配料。将混合料在陶瓷模具中手工捣打成型,将成型好的试样分别在600℃、700℃、800℃、900℃和1000℃下均保温2h后脱模。测量热处理后各试样的耐压强度和显气孔率;采用XRD分析了试样的物相组成。结果表明:复合烧结剂中钾长石和硼酸在中温、低温下具有良好的烧结作用,在700～800℃热处理后,试样耐压强度和显气孔率明显增加。硼酸含量为2%的试样,在800～1000℃热处理后,显气孔率增幅较大。添加硅灰可以降低钾长石烧结温度;而复合烧结剂中的粘土在中温、低温下不利于干式料的烧结,低于800℃热处理后的试样,耐压强度和显气孔率没有随粘土含量增加而变化;900～1000℃热处理后的试样,耐压强度随粘土含量的增加而降低,显气孔率增加不大。     

Effect of sintering temperature on functional properties of alumina membranes

Supported membranes were prepared from different submicron alumina powders. The evolution of pore size, hardness and permeability were monitored after sintering the films at temperatures ranging from 1000 to 1400 °C. These functional properties and the microstructure of the films were compared with the free-standing membranes. Sintering at temperature range from 1000 to 1200 °C maintained the narrow, monomodal pore size distribution of the supported membranes. The effect of sintering temperature on the hardness of the membranes was weak. The permeability was also independent on the sintering temperature. When sintering temperature was raised up to 1300 and 1400 °C, the pore size increased significantly and distribution was changed to bimodal containing fraction of large pores. The hardness of the membranes increased while significant densification was not observed. Permeability increased due to the large pore size and the high porosity. In sintering of the free-standing membranes pore size remained almost unchanged, density increased when sintering temperature was raised, hardness was dependent on the density and permeability decreased continuously. The substrate did not have effect on the grain growth, which was dependent on the sintering temperature. Evolution of the properties of the free-standing membranes suggests local densification. The rigid substrate restricts the sintering shrinkage leading to densification of small areas. This local densification opens large flow channels between agglomerates. This increases the pore size, broadens the pore size distribution and increases the permeability. The macroscopic densification of the film is small.     

Effect of carbon contamination on the sintering of alumina ceramics

The present work aimed with the carbon contamination in alumina ceramics and its influence on sinterability of alumina in low vacuum and atmospheres of argon and nitrogen. The commercially available alumina was coated with carbon and sintered at different atmospheres to investigate the effect of carbon presence on alumina sintering behaviour. The sintering conditions were: heating/cooling rates 5 °C/min and 1.7 °C/min until the maximum temperature of 1400 °C and a dwell time of 2 h. The microstructure of the samples was investigated from fracture and surface, prior to polishing, chemical or thermal etching. The non-densified (porous) surface layer was found in the samples sintered in nitrogen and vacuum, however, sintering in argon atmosphere showed a negligible effect on the surface. The core of investigated specimens exposes a transgranular/intergranular fracture mode and is dense in all cases. In the case of vacuum sintering, the strong carbon diffusivity was also noticeable by the dark grey color of the samples. Interestingly, the formation of aluminium nitride took place during sintering of carbon coated alumina samples in a nitrogen atmosphere at 1400 °C. The thickness of the reactive porous layer was approximately 15 μm beneath the surface. Such a porous layer is inappropriate to the desired features of final ceramic products. Presented results lead to better understanding of the sintering behaviour of ceramic and to suitable selecting of the set-up by densification conditions.     

Influence of thickness on the constrained sintering of alumina films

Alumina films prepared by tape casting were sintered freely and under geometrical constraint at 1350 °C. The effect of film thickness on sintering kinetics and microstructure development was investigated. A decrease in film thickness in the constrained case leads to enhanced retardation of densification and increased orientation of anisometric pores.     

Effect of green compact structure on the sintering of alumina

The effect of particle agglomeration on sintering has been studied by slipcasting suspensions with pH values ranging from 2 to 11. The rate of densification has been found to depend on the degree of agglomeration. Complete dispersion of alumina primary particles has not been attained through adjustment of pH of suspensions, and agglomerates also remained in the best-dispersed suspension. Elimination of the agglomerates by sedimentation lowered the densification temperature of slip-cast compacts. The grain size-density curve is a function of temperature. High-density and small-grained sintered bodies were obtained by low-temperature long-time firing.     

High strength porous alumina by spark plasma sintering

High strength porous alumina was fabricated by spark plasma sintering (SPS) at temperatures between 1000 and 1200 °C with nanocrystalline Al(OH)₃ as the starting powder without any seeds, dopants or inclusions. Decomposition of the Al(OH)₃ produced a series of transitional alumina phases depending on sintering temperature and pressure and finally the stable α-alumina phase was obtained. A network of continuous pores with unimodal pore size distribution was estimated by mercury porosimetry and BET surface area measurements, with the porosity ranging between 20% and 60% based on sintering conditions. Predominance of fine grains and extensive necking between them led to better strength in the sintered samples. The bending strength of the sintered compacts rapidly increased with sintering temperature while retaining reasonable porosity suitable for practical applications. The results clearly indicate that in situ phase formation of α-Al₂O₃ and θ-Al₂O₃ provides strength and porosity, respectively. Phase transformation, pore morphology and microstructure evolution were also studied.     

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.     

Ultrafast high-temperature sintering of dense and textured alumina

Crystallographic texture engineering in ceramics is essential to achieve direction-specific properties. Current texture engineering methods are time-consuming, energy extensive, or can lead to unnecessary diffusion of added dopants. Herein, we explore ultrafast high-temperature sintering (UHS) to prepare dense and textured alumina using templated grain growth (TGG). From a slurry containing alumina microplatelets coated with Fe₃O₄ nanoparticles dispersed in a matrix of alumina nanoparticles, green bodies with oriented microplatelets were prepared using magnetic assisted slip casting (MASC). The effects of the sintering temperature, time and heating rate on the density and microstructure of the obtained ceramics were then studied. We found that TGG occurs for a temperature range between 1640 and 1780 °C and 10 s sintering time. Sintering at 1700 °C for 10 s led to dense and textured alumina with anisotropic grains thanks to the Fe₃O₄ coating, which did not have the time to diffuse. The highest texture and relative density were obtained with a heating rate of ~5500 °C/min, leading to texture-dependent anisotropic mechanical properties. This study opens new avenues for fabricating textured ceramics in ultra-short times.     

Effect of MgO addition and sintering parameters on mullite formation through reaction sintering kaolin and alumina

Abstract

In the present work, the influence of MgO addition and sintering parameters on the formation and densification of mullite was investigated. The morphology of powders and the microstructure of the sintered samples were characterised by means of a scanning electron microscope. X-ray diffraction was used to characterise phases formed in sintered samples. The density of sintered samples was measured using a densimeter and quantified according to the Archimedes principle. MgO was added at 1, 2, 3, 4, 5 and 6 wt-% to kaolin and alumina and the powders were ball milled for 5 h then uniaxially compacted at 75 MPa and finally sintered at 1500, 1550, 1600 and 1650°C for 2, 4, 6 and 8 h. It was found that addition of MgO not only affected mullite formation but also promoted grain growth. For samples containing 0, 1 and 2 wt-%MgO only mullite was formed. While, in addition to mullite, Al₂O₃ was present in sample containing 3 wt-%MgO. At higher MgO content (4, 5 and 6 wt-%), three phases, i.e. mullite, Al₂O₃ and spinel, were formed. Addition of 1 wt-%MgO increased the density of all samples for all sintering times and higher densities corresponded to higher sintering temperatures. At higher MgO content, higher temperatures led to lower densities and lower temperatures led to higher densities for almost all sintering times.     

Explosive compaction and low-temperature sintering of alumina nanopowders

Physical aspects of explosive compaction of alumina nanopowders with different phase compositions are studied experimentally. Physical processes that occur during consolidation of nanoparticles under pulsed loading are considered. Conditions of retaining of the material nanostructure after compaction and subsequent low-temperature sintering are determined. Physicomechanical properties of explosive compacts and ceramics on the basis of these compacts are studied. A ceramic material characterized by a nanostructure (grain size of ≈200 nm) and high values of density (97% of the theoretical value) and microhardness (up to 23.5 GPa) is obtained. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 114–126, November–December, 2008.     

Investigation of sintering mechanisms of alumina using kinetic field and master sintering diagrams

Strain rates during sintering of alumina at different heating rates between 0.2 and 20 K/min have been measured non-contact using a thermooptical measuring technique. The shrinkage data were represented in a kinetic field diagram. It was shown that coarsening effects can be indirectly derived from the position and slope of iso-strain lines identified in the kinetic field. In addition temperature gradients which frequently lead to an erroneous interpretation of shrinkage kinetics were identified. Uni-axial loads up to 6 MPa were applied during sintering. The strain rates were measured parallel and perpendicular to the load. They were processed in separate kinetic field diagrams. It was shown that creep leads to a parallel shift of the iso-strain lines either to lower temperatures, for the strain rates parallel to the load, or to higher temperatures, for the strain rates perpendicular to the load. The individual kinetic field diagrams for a specific load were assembled in 3d, with the load used as z-axis. They formed a master sintering diagram (MSD) where the iso-strain lines were replaced by iso-strain surfaces. The parallel shift of the iso-strain lines with load enabled a simple representation of sintering kinetics for various loads during the entire range of densification, temperatures and heating rates.