Joint Glass Centre

Two-step sintering (TSS) was applied to suppress the accelerated grain growth of sub-micron (∼150 nm) alumina powder. The application of an optimum TSS regime led to a remarkable decrease of grain size down to ∼500 nm; while the grain size of the full-dense structures produced by conventional sintering ranged between 1 and 2 m. To find how important the temperatures at sintering steps might be, several TSS regimes were conducted. The results showed that the temperatures at both sintering steps play vital roles in densification and grain growth of the alumina compacts. Based on the results, the optimum regime consisted of heating the green bodies up to 1250 • C (first step) and then holding at 1150 • C for more than 60 h (second step). This yielded the finest microstructure with no deterioration of the densification. Heating at 1300 • C (first step) and then at 1200 • C (second step) was not a successful procedure. Lowering the temperature of the second step down to 1100 • C resulted in exhaustion of the densification at 88%-theoretical density. A nearly full-dense structure with an average grain size of 850 nm was obtained when the temperature of the second step was increased to 1150 • C. Empirical results show that not only the first step temperature has to be high enough to reach a structure containing unstable pores, but the second sintering temperature must also be high enough to activate the densification mechanism without grain growth. This means that a considerable densification at the first step does not imply enough second-step densification.

This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.

Polycrystalline aluminas with various concentrations of oxide dopants CaO, MgO, and TiO 2 , ranging from 0.05 wt.% to 5 wt.% (3 wt.% in case of MgO), as well as pure alumina references were prepared by tape casting of aqueous suspensions and sintered in air at 1600 8C for 4 h for applications as low dielectric loss electroceramics. Loss tangents were measured at room temperature in the frequency range between 1 and 100 kHz as the key parameter for the intended application. The values of loss tangents of doped materials were influenced by the concentration of dopants. The addition of 0.5 and 5 wt.% of TiO 2 and 3 wt.% of MgO decreased the value of loss tangent in the whole frequency range. The addition of these dopants eliminated abnormal grain growth and decreased the amount of residual porosity. By doing so, the dopants compensated the negative influence of process impurities and decreased the loss tangent values. The cations (Ti 4+) with high solubility in the Al 2 O 3 crystal lattice were preferably built into the grain boundary glass, thus efficiently reducing the concentration of polarizable defects in corundum matrix; the formation of vitreous phase had a positive effect on the value of loss tangent in TiO 2 doped samples. The increased values of loss tangent were related to lower density of prepared materials, and the presence of residual porosity. Other contributing factors were especially the formation of calcium-containing secondary crystalline phases, and the increased concentration of lattice defects due to incorporation of atoms with different valencies to alumina crystal lattice.

Slip casting of stabilized aqueous suspensions followed by pressureless sintering was used for preparation of dense Al 2 O 3 /MWCNTs composites. The suspensions were stabilized by commercial polyelectrolyte dispersant Darvan C-N. In order to increase the stability, the pH value of the suspension was adjusted to $ 10. At this pH the highest z-potential values of the alumina powder and of the MWCNTs functionalised in boiling nitric acid were achieved. Two different agents, namely ammonium hydroxide and sodium hydroxide, were used for the pH adjustment. Their influence on the viscosity of suspensions and on consolidation and densification behaviour of the Al 2 O 3 /MWCNT composites was evaluated. The effect of ammonium hydroxide was more pronounced, as confirmed by lower viscosity of the suspension, higher sintered density, and fine-grained microstructure of the sintered composites. The Al 2 O 3 /t-MWCNTs composites with 0.1 wt% of the MWCNTs, with 99.9% relative density, the mean size of alumina grains $1 mm, and homogeneously distributed carbon nanotubes were prepared by the pressureless sintering at 1500 1C.

The present work deals with preparation of stable suspensions of a submicrometre alumina powder with different contents of solid for pressure filtration. The optimum dispersant content (2.2 wt.% of Darvan C-N) was determined by sedimentation tests and viscosity measurements. By modification of the solid loading and dispersant content two kinds of aggregation were observed. One type of aggregates is related to the use of excessive solid loading in suspension. In samples prepared from these suspensions only minor effect on sintered microstructure was observed, which increased with increasing volume fraction of hard aggregates. In case of excess dispersant addition weak aggregates formed as the result of depletion flocculation. Weak aggregates had stronger negative effect on green microstructure, with consequent negative impact on sinterability.

The paper deals with the synthesis and characterisation of binary aluminate glasses in the La 2 O 3-Al 2 O 3 system with Al 2 O 3 contents changing between 74.6 and 86.9 mol% (48-65 wt.%), and of ternary glasses with 75.7 mol% Al 2 O 3 doped with 1 mol% of Nd 2 O 3 or Er 2 O 3. Six binary and two ternary compositions were prepared. Flame synthesis facilitated the preparation of X-ray amorphous microspheres in the systems with 58 wt.% Al 2 O 3 , and with eutectic composition in the pseudobinary LaAlO 3-LaAl 11 O 18 system doped with Er. Other systems contained low fractions of crystalline LaAlO 3 perovskite, regardless of the composition. The diameter of prepared microspheres ranged between 2 and 10 mm. They were transparent for visible light, as well as in the IR wavenumber range from 1300 to 4000 cm À1 .

Conditions of green body preparation and consequently the conditions of sintering are prerequisite for the preparation of dense samples of Al 2 O 3 with superior optical and mechanical properties. The goal of this work was to determine the optimum forming conditions for preparation of green body, and to fi nd out the optimal sintering regime facilitating the preparation of ultra-fi ne grained high purity alumina with maximal density, fi ne microstructure and small pore size. Axial pressing followed by CIP was found to yield green body with the highest density and narrowest pore size distribution. In the two-stage sintering regime the temperature T 1 has to be higher or equal to 1300°C to obtain closed porosity unstable against shrinkage. The temperature T 2 ≥ 1130°C was found to be too high for suppression of grain growth in the fi nal stage of sintering and sintering trajectory was identical with that from standard sintering regime.

The Al2O3/SiC nanocomposites containing 3–8vol.% SiC were prepared through infiltration and in situ thermal decomposition of a preceramic polymer SiC precursor (poly(allyl)carbosilane) in pre-sintered alumina matrix. The volume fraction of SiC, and the microstructure of composites were adjusted by concentration of the polymer solution, and by the conditions of pyrolysis and sintering. The specimens were densified by pressureless sintering at

This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (14001-14501C) in order to close porosity without significant grain growth. The second step at temperatures around 11501C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 lm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 lm.

Multi-walled carbon nanotubes (MWCNTs) are often reported as additives improving mechanical and functional properties of ceramic composites. However, despite tremendous efforts in the field in the past 20 years, the results are still inconclusive. This paper studies room temperature properties of the composites with polycrystalline alumina matrix reinforced with 0.5-2 vol.% MWCNTs (composites AC) and zirconia toughened alumina with 5 vol.% of yttria partially stabilised zirconia (3Y-PSZ) containing 0.5-2 vol.% of MWCNTs (composites AZC). Dense composites were prepared through wet mixing of the respective powders with functionalised MWCNTs, followed by freeze granulation, and hot-pressing of granulated powders. Room temperature bending strength, Young's modulus, indentation fracture toughness, thermal and electrical conductivity of the composites were studied, and related to their composition and microstructure. Slight increase of Young's modulus, indentation fracture toughness, bending strength, and thermal conductivity was observed at the MWCNTs contents ≤1 vol.%. At higher MWCNTs contents the properties were impaired by agglomeration of the MWCNTs. The DC electrical conductivity increased with increasing volume fraction of the MWCNTs.

The Al 2 O 3 /SiC nanocomposites containing 3-8 vol.% SiC were prepared through infiltration and in situ thermal decomposition of a preceramic polymer SiC precursor (poly(allyl)carbosilane) in pre-sintered alumina matrix. The volume fraction of SiC, and the microstructure of composites were adjusted by concentration of the polymer solution, and by the conditions of pyrolysis and sintering. The specimens were densified by pressureless sintering at temperatures between 1550 and 1850 • C in flowing argon. The use of powder bed producing SiO, CO and other volatile species suppressed decomposition reactions in the composites and was vital for their successful densification. The experimental results are discussed against thermodynamic analysis of the system Al 2 O 3 /SiC/SiO 2 in an inert Ar atmosphere.

composites with more than 20 vol.% of SiC particles were produced using a multiple infiltration of porous bodies with a liquid polymer SiC precursor. The fully dense composites were successfully densified using a sinter-HIP process. Parameters of sintering and HIP steps are discussed with respect to both densification and microstructure evolution of the composites. The initial pressure during the sintering step plays an important role for the preparation of fully dense composites with a submicrometer alumina matrix at 1750°C. Optimized densification schedule of sinter-HIP represents a novel approach of densification at relatively mild conditions compared to previously reported or common densification methods of Al 2 O 3-SiC composites with high SiC content, such as pressureless sintering, hot pressing and post-HIPing. The method expands the possibilities for preparation of alumina based composites with SiC volume fraction N 20 vol.%, filling the gap in available literature data.

A study has been made of the mechanism of wet erosive wear of polycrystalline alumina. The aluminas were prepared with controlled grain size, and contained up to 10% by weight of magnesium silicate sintering aid. For materials of grain size-1 mm the dominant wear mechanism appeared to be tribochemical, giving polishing with a very low material removal rate. For coarser grain size materials the wear mechanism appeared to involve microfracture initiation and propagation, leading to partial or complete grain removal. For pure alumina materials fracture was predominantly intergranular with crack interlinking; for the liquid phase sintered materials fracture was mainly transgranular. The presence of the magnesium silicate sintering additive decreased the wear rates considerably, compared to pure alumina materials of the same mean grain size. No correlation was found between wear rates and Vickers indentation hardness and fracture toughness values. However, use of a depth sensing nanoindentation technique revealed differences between the pure and the magnesium silicate doped aluminas, with the pure alumina being stiffer and harder. Grain facetting was also a strong feature of the nanoindentation damage zones in the case of pure alumina, providing supporting evidence that crack development predominantly followed the grain boundaries. Magnesium silicate densified materials, in contrast, showed mainly intragranular fracture around the indentation crater. It is concluded that the wear process in alumina materials of mean grain size) 1 mm is at least partly dependent on the residual grain boundary stresses arising from the thermal expansion anisotropy of the alumina grains. The intergranular silicate film has two functions: it effectively strengthens the grain boundaries, and it increases the compliance of the material, so as to improve its ability to absorb and dissipate impact energy.

We report on plasma surface treatment of sub-micron alumina powder by the Diffuse Coplanar Surface Barrier Discharge (DCSBD) in ambient air. Even though the isoelectric point did not change after plasma treatment, the results indicated an enhanced colloidal stability of treated powder, especially when ultrasonication was used as homogenization method. Suspensions prepared from plasma treated powder also exhibit lower pH values compared to those of original powder. FTIR analysis confirmed enhanced sorption ability of treated powder. Polycrystalline alumina ceramics prepared from the treated powder possess significantly finer microstructure compared to those prepared from untreated powders. These findings may have a significant impact on ceramic engineering.

A range of silica-doped alumina ceramics with addition of 0, 0.5, 1, 5 and 10 wt.% of SiO 2 was prepared by hot pressing at 1450 • C. Controlled grain growth was achieved by high temperature annealing of hot pressed specimens at 1600 • C. Resistance against wet abrasive wear was tested using custom-built sample holder on a standard grinder/polisher machine. The wear rates increased with increasing rotation speed and loading force, and with size of alumina grains, and decreased with increasing content of SiO 2 addition. The measured wear rates were comparable to those of alumina-SiC nanocomposites with similar alumina matrix grain size, but inferior to pure alumina ceramics with submicrometre microstructure. The wear rates of silica doped specimens increased, similarly to monolithic alumina, with the time of high temperature isothermal treatment, which primarily resulted in the growth of alumina grains. The change of the phase composition (crystallisation of low thermal expansion phase mullite) and resulting change of the stress state of the annealed silica doped specimens had negative impact on both the hardness and wear resistance.

The addition of particulate carbon into oxide ceramics can significantly improve their durability and wear characteristics due to the lubricating properties of carbon. In the present work an alumina-carbon composite was prepared by pyrolytic cracking in inert atmosphere of the organic compounds added as lubricants, wetting agents and dispersants in the course of freeze granulation of an alumina powder. Carbon in the form of submicron-sized isometric graphitic inclusions was homogeneously distributed within the dense alumina matrix prepared by hot pressing. SiC was also formed by carbothermal reduction of the SiO 2-based sintering additive. The matrix consisted of submicron alumina grains. The Vickers hardness of the carbon-containing composites was higher than the hardness of their carbon-free counterparts (18 and 22 GPa respectively for carbon-free and carbon containing liquid phase sintered aluminas). The fracture toughness was not affected markedly by graphite incorporation.