Microstructure Of Alumina Ceramics Research Articles

Influence research of temperature heat treatment on the microstructure of alumina ceramics

The JSC "URIR named after A. S. Berezhnoy" developed and introduced the technology of highly refractory alumina extra-dense ceramics. It is known that, the structure and properties of alumina ceramics in many cases depend on the temperature of its heat treatment, the holding time at the final temperature, the thermal unit, and a number of other factors. In order to study the processes of structure formation during of heat treatment the microstructure of alumina ceramic samples of two compositions (composition 1 with a moisture content of 30 %, as well as with dispersing and hardening additives, and composition 2 with a moisture content of 20 % only with dispersing additive) which were made by slip casting aqueous alumina slips based on reactive alumina α–from with Al2O3 content over 99 %, specific surface ~ 9 m2/g by electron microscopic analysis was studied. It was established that, after heat treatment at 1000 °C the beginning of sintering and recrystallization of corundum grains are observed, after heat treatment at 1200 °C take place packing of structure and recrystallization of α–Al2O3. After firing at 1400 °C the microstructure of samples composition 1 is represented to a large extent by crystals of tabular corundum with a particle size ~ 0.5—5.5 μm, predominant — ~ 2.5—3.0 μm. After firing at 1400 °C the microstructure of samples composition 2 is represented — by individual particles of a rounded and oval form with a size of ~ 0.5—3.5 μm, predominant — ~ 1.5—2.0 μm which are interconnected by intercrystalline layers. The microstructure of alumina ceramic samples of both compositions after firing at 1500 °C is similar and is composed of recrystallized corundum grains with a size of ~ 0.5—5.5 μm, predominant — ~ 1.5—2.0 μm, among which there are also larger α–Al2O3 grains, with a size of ~ 2—8 μm. After firing at 1580 °C the microstructure of samples is represented by a dense fine-grained structure of well-formed practically defect-free corundum grains, with a size of ~ 0.5—3.0 μm, among which grains of ~ 5—10 μm.

Influence of rare earth Tb4O7 addition on the densification, abrasion resistance and microstructure of alumina ceramics

This study aimed to improve the purity and performance of alumina ceramics used as ball milling media. High-alumina ceramics (> 96 wt% Al2O3), with high densification and excellent abrasion resistance, were fabricated by the cold isostatic pressing method. The effects of adding the rare earth Tb4O7 on the densification, abrasion resistance, crystalline phase, micro-morphology and grain size of the ceramics were studied. The experiment results showed that the densification and abrasion resistance of the samples increased with Tb4O7 addition. The sample with 0.8 wt% Tb4O7 sintered at 1625 °C exhibited the best performance, with a linear shrinkage, relative density and abrasion rate of 22.28%, 95.70% and 0.103‰, respectively. The abrasion resistance improved by 27.5% compared with the sample without Tb4O7. X-ray diffraction analysis indicated that the primary phases of the samples were corundum, spinel, CaAl12O19 and α-quartz, and a small quantity of Tb3Al5O12 was generated when more than 0.4 wt% Tb4O7 was added. Furthermore, some Mg2+ and Ca2+ ions in the liquid phases dissolved into Tb3Al5O12 crystalline grains during the sintering process, which enhanced the grain boundary cohesion of the materials. Scanning electron microscopy indicated that the existence of Tb3Al5O12 at grain boundaries reduced the average size of the corundum grains. This helped transform inter-granular fractures into trans-granular fractures, thereby improving the abrasion resistance of the ceramic materials.

The effect of nano-copper additives on the porosity, mechanical properties, and microstructure of alumina ceramics using commercial rice husk ash as a pore former

The aim of the present research is to examine the effect of Cu metal addition in nano-scale particle size on the mechanical properties and porosity of porous alumina ceramics using commercial rice husk ash as pore forming agent and silica (SiO2) source. Porous alumina ceramics reinforced were prepared using nano-scale Cu metal particles as their strengthening phase. Solid-state and sacrificial techniques were used to prepare the porous alumina reinforced ceramics. A field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), and transmission and electron microscope (TEM) were used to analyze the microstructure and ceramic phases. Different ratios of Cu metal were added (3, 6, 9, and 12 wt%) at different ratios of commercial rice husk ash. The results of this investigation show that with increasing ratios of Cu metal, the porosity decreased and the mechanical properties increased. The increase in the mechanical properties could be attributed to the decrease in the porosity, the toughening mechanism, increase density of porous alumina ceramics, and formation of the tenorite (CuO) phase due to sintering at high temperature (1600 °C). Some potential applications include purging of gas filtration and thermal insulation.

The Effect of Commercial Rice Husk Ash Additives on the Porosity, Mechanical Properties, and Microstructure of Alumina Ceramics

A porous ceramic is made from composite materials which consist of alumina and commercial rice husk ash. This type of ceramics is obtained by mixing the commercial rice husk ash as a source of silica (SiO2) and a pore forming agent with alumina (Al2O3) powder. To obtain this type of ceramic, a solid-state technique is used with sintering at high temperature. This study also investigated the effects of the rice husk ash ratios on the mechanical properties, porosity, and microstructure. The results showed that, by increasing the content of the rice husk ash from 10 to 50 wt%, there is an increase in the porosity from 42.92% to 49.04%, while the mechanical properties decreased initially followed by an increase at 30 wt% and 50 wt%; the hardness at 20 wt% of the ash content was recorded at 101.90 HV1. When the ash content was increased to 30 wt% and 50 wt%, the hardness was raised to 150.92 HV1and 158.93 HV1, respectively. The findings also revealed that the tensile and compressive strengths experienced a decrease at 10 wt% of the ash content and after that increase at 30 wt% and 50 wt% of rice husk ash. XRD analysis found multiple phases of ceramic formation after sintering for the different rice husk ash content.

Dielectric Performance of Alumina Ceramics Sintered at Different Temperatures

This paper investigated the influence of sintering temperature on the dielectric performance of alumina ceramic. Three different kinds of alumina ceramic samples were prepared using the same formula with three different sintering temperatures. The flashover voltage and surface charge distribution of the test samples were measured with a finger type electrode system under a positive pulse voltage application in a 1×10-4Pa vacuum circumstance. The experimental results showed that the samples with higher sintering temperature have worse dielectric performance. Based on the SEM observation, the change of dielectric performance could be attributed to the change of microstructure of alumina ceramics.

Influence of different seeds on transformation of aluminum hydroxides and morphology of alumina grains by hot-pressing

The influence of different seeds on transformation of aluminum hydroxides and morphology of alumina grains was investigated. There are two kinds of alumina seeds used separately, alumina milling ball abrasive seeds and ultra-fine alumina powder seeds. Abrasive seeds were introduced to the starting materials by wet-grinding of high-purity alumina milling balls. The alumina powder seeds were directly added into raw materials. First, the introduction of different seeds can lower the transformation temperature from κ phase to α phase. The phase transformation from aluminum hydroxide to α-Al2O3 was completed at 1100 °C in 2 h when a certain amount of seeds were employed. Second, seeds also changed the sintering behaviour and microstructure of alumina ceramics. For the hot-pressed sample with alumina powder seeds added directly, the microstructure with platelets was observed. However, the anisotropic growth of alumina grains is favored in the presence of alumina abrasives as seeds. The elongated grains were developed in the sample with abrasive seeds introduced.

アルミナのち密化，機械的性質及び微細構造に対する放電プラズマ焼結の影響

アルミナのち密化，機械的性質及び微細構造に対する放電プラズマ焼結の影響

Effect of SiO<sub>2</sub> and TiO<sub>2</sub> on the Sinterability and Microstructure of Alumina Ceramics

Effect of SiO2 and TiO2 additions on the phase formation, sinterability, and microstructure of alumina ceramics was studied by differential thermal analysis, X-ray powder diffractometry, SEM, TEM, and dilatometry. Alumina-based mixture powders were prepared from starting materials of α-Al2O3 powder, Si(OC2H5)4 and Ti(OC3H7)4. The silica gel obtained from the hydrolysis of Si(OC2H5)4 was coated on Al2O3 particles to form a thin surface layer. During the firing process, the silica gel didn't crystallize to cristobalite but reacted with Al2O3 to form mullite above 1500°C. The silica gel, mullite and/or Al2O3-SiO2 liquid phase retarded the densification and the grain growth of alumina. As TiO2 was added to the alumina-silica gel mixture, the sinterability was enhanced and the formation temperature of mullite was lowered.

Effects of TiO 2 on sintering of alumina ceramics

The effects of TiO2 addition on the sintering process and microstructure of alumina ceramics were studied. In air, the solid solubility of TiO2 in α-Al2O3 was too small to be determined by the lattice parameter shift of α-Al2O3. Then, the relative amounts of titanium compounds remaining in fired bodies were measured by X-ray diffractometry using a step-scanning technique which can detect less than 0.1wt% rutile or Al2TiO5 in α-Al2O3, and were compared with the amount of TiO2. The solid solution of TiO2 in α-Al2O3 was found above 1150°C, and the solubility was estimated to be 0.27wt% at the temperature range from 1300° to 1700°C. Beyond the solubility limit, excess TiO2 coexisted with α-Al2O3 as rutile below 1350°C and as Al2TiO5 above 1450°C. The sintering of α-Al2O3 was markedly promoted when TiO2 was added up to the solubility limit and the fired density higher than 97% of the theoretical was obtained at 1400°C. The addition of TiO2 also promoted the grain growth of α-Al2O3. But beyond the solubility limit, the grain size decreased with an increase of Al2TiO5. Therefore it is inferred that Al2TiO5 existing as a second phase retards the grain growth of α-Al2O3. The lattice parameters of Al2TiO5 in fired bodies considerably differed from the those of a single crystal. It is explained as due to the difference of thermal expansion coefficient between Al2TiO5 and α-Al2O3.

Effect of Environmental Impurities on the Microstructure and Sinterability of Alumina Ceramics

The effect of volatiles present in the kiln atmosphere on the sinterability and microstructure of alumina ceramics was studied. The experiments were so designed that the volatile impurities generated from the respective salts of Na, Cu, Zn, Ni, Cr and Fe were allowed to act on pure alumina ceramics.The properties studied included porosity, density, fired shrinkage, microhardness, power factor, loss factor, volume resistivity and dielectric strength. Scanning Electron Microscopy was used for the characterization of the resulting microstructures. The results when compared with the corresponding properties of alumina ceramics fired under a pure environment have been presented.

Effects of TiO<sub>2</sub> on Sintering of Alumina Ceramics

The effects of TiO2 addition on the sintering process and microstructure of alumina ceramics were studied. In air, the solid solubility of TiO2 in α-Al2O3 was too small to be determined by the lattice parameter shift of α-Al2O3. Then, the relative amounts of titanium compounds remaining in fired bodies were measured by X-ray diffractometry using a step-scanning technique which can detect less than 0.1wt% rutile or Al2TiO5 in α-Al2O3, and were compared with the amount of TiO2. The solid solution of TiO2 in α-Al2O3 was found above 1150°C, and the solubility was estimated to be 0.27wt% at the temperature range from 1300° to 1700°C. Beyond the solubility limit, excess TiO2 coexisted with α-Al2O3 as rutile below 1350°C and as Al2TiO5 above 1450°C. The sintering of α-Al2O3 was markedly promoted when TiO2 was added up to the solubility limit and the fired density higher than 97% of the theoretical was obtained at 1400°C. The addition of TiO2 also promoted the grain growth of α-Al2O3. But beyond the solubility limit, the grain size decreased with an increase of Al2TiO5. Therefore it is inferred that Al2TiO5 existing as a second phase retards the grain growth of α-Al2O3. The lattice parameters of Al2TiO5 in fired bodies considerably differed from the those of a single crystal. It is explained as due to the difference of thermal expansion coefficient between Al2TiO5 and α-Al2O3.