α-Al2O3 Powders Research Articles

Effects of Al<sub>2</sub>O<sub>3</sub> Species and Particle Sizes on Properties of Al<sub>2</sub>O<sub>3</sub>-Cr<sub>2</sub>O<sub>3</sub> Refractories

The microstructure of Al2O3-Cr2O3 refractories is an important factor to affect its high temperature performance. The Al2O3-Cr2O3 green bodies were prepared by hot pressing using aluminum oxide and Cr2O3 powder mixture by adding a binder. And then the specimens were sintered at 1650 for 4h in the electric muffle furnace. Properties of specimens with different species of Al2O3 powders were studied, including α-Al2O3 powder, ρ-Al2O3 powder, and fused corundum powder. Moreover, three sizes of α-Al2O3 powder (D50=0.8 μm, 1.4 μm, 4.0 μm) were used as additive. Properties of specimens, including apparent porosity, bulk density, cold modulus of rupture, pore size distribution were tested. The morphology of sintered specimens was analyzed by the Scanning Electron Microscope. The results showed that the specimens adding α-Al2O3 powder had the better properties since α-Al2O3 powder has higher sintering activity, and it was more efficient for Al2O3-Cr2O3 solid solution. The specimens with α-Al2O3 had lower porosity, higher bulk density and cold modulus of rupture, and more uniform pore distribution. There were great differences in sintering activity for specimens with different particle size of α-Al2O3 powder. And the microstructure of Al2O3-Cr2O3 was significantly dissimilar. The specimen with the addition of the activated alumina powder of D50 with the size of 1.4 μm and bimodal size distribution showed the perfect performance, including high density and high flexural strength. The experimental results showed that the microstructure of this specimen was uniform and its pore size was homogeneous. This special microstructure is beneficial for improving the slag resistance and thermal shock resistance of Al2O3-Cr2O3 refractories.

Preparation of sol-gel derived microcrystalline corundum abrasives with hexagonal platelets

The effects of different additives on the mechanical properties, microstructures, and wear behavior of corundum abrasives were investigated. When the number of additive phases increases, the sintering temperature and wear rate decrease, while the densification and mechanical properties increase. The additive SiO2 is responsible for the development of equiaxed grains, whereas both CaO and MgO promote the development of platelike grains. By controlling the molar ratio of additives, it is possible to obtain different microstructures. With SiO2-MgO-CaO (molar ratio, 2:1:1) as the additives and nano α-Al2O3 powders as the seed, microcrystalline corundum abrasives with hexagonal platelets were obtained using sol-gel process by sintering at 1300°C for 0.5 h. The average diameter and thickness of hexagonal platelets are 1.38 μm and 360 nm respectively, the single-particle compressive strength is 26.44 N, and the wear rate is (3.06±0.21)×10−7 mm3/(N·m).

Properties of Low-Carbon Al<sub>2</sub>O<sub>3</sub>-C Refractories with Different Critical Particle Size

Low-carbon Al2O3-C refractories were prepared using white fused corundum, α-Al2O3 powders and flake graphite as main raw materials. The critical particle sizes of corundum selected in this experiment were 0.5mm, 1mm and 2mm. The effects of corundum critical particle size on physical, mechanical and thermo-mechanical properties of low-carbon Al2O3-C refractories were investigated. The results show that the increase of critical particle size is conducive to the improvement of thermal shock resistance and fracture energy, but little effect on thermal expansion. The cold modulus of rupture after thermal shock test of samples using 0.5mm critical particle size corundum was 2.09MPa, while using 2mm critical particle size corundum was 2.98MPa. And the fracture energy increased from 265N/mm to 588N/mm when the critical particle size increased from 0.5mm to 2mm. The effects of critical particle size on apparent porosity, bulk density and modulus of rupture were insignificant.

Effect of surfactants on preparation of nanoscale α-Al2O3 powders by oil-in-water microemulsion

The nanoscale α-Al2O3 powders have been synthesized by the O/W microemulsion system using cyclohexane as the oil phase, ultrapure water as the water phase, OP-10 and alcohol as the surfactant and co-surfactant. It has been found that the nature of surfactants played an important role to regulate the size and morphologies of the α-Al2O3 nanoparticles. Three different nonionic surfactants (OP-10, Triton X-100 and Tween-80) have been used for the preparation of microemulsions. The microemulsions were characterized by Zeta Potential Analyzer. The results showed that the OP-10 system possesses wide and stable microemulsion phase regions. The synthesized α- Al2O3 powders have been comprehensively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD). The powders calcined at 900–1150°C showed the presence of alumina phase with crystal structure, The SEM images showed that the powders was an average diameter of 30–100nm.

Enhanced performance of a macroporous ceramic support for nanofiltration by using α-Al2O3 with narrow size distribution

Enhanced performance of a macroporous disk alumina support was fabricated through colloidal filtration route, by using α-Al2O3 powder with an average particle size of 1.1μm. The support, sintered at 1250°C, showed relative high permeances towards water (101Lh−1m−2bar−1) and nitrogen (∼2×10−6molm−2s−1Pa−1), with an average surface roughness of ∼175nm and a high mechanical strength of 61.1MPa. Titania supported γ-Al2O3 mesoporous layers were deposited onto this promising disk α-Al2O3 support through dip-coating. The disk membrane A1100/TiO2/γ-Al2O3, with pore size of ca. 4.4nm, showed a pure water flux as high as 4.5Lm−2h−1bar−1, which is four times higher than that of γ-Al2O3 membrane reported in literature. This mesoporous membrane showed relative high retention rate (∼80%) towards di-valent cations like Ca2+, Mg2+, but not for the mono-valent cation (Na+).

Influence of temperature on AC conductivity of nanocrystalline CuAlO2

Nanocrystalline CuAlO2 was synthesized by mechanical alloying of Cu2O and α-Al2O3 powders in the molar ratio of 1:1 for 20 h in toluene medium with tungsten carbide balls and vials using planetary ball mill. The ball milling was carried out at 300 rpm with a ball to powder weight ratio of 10:1 and then annealed at 1373 K in a platinum crucible for 20 h to get CuAlO2 phase with average crystallite size 45 nm. Complex impedance spectroscopic measurement in the frequency region 1 Hz to 10 MHz between the temperatures 333 to 473 K was carried out for nanocrystalline CuAlO2 sample. The obtained complex impedance data was analyzed for AC conductivities, DC and AC conductivities correlations and crossover frequencies (f co ). The BNN (Barton, Nakajima and Namikawa) relation was applied to understand the correlation between DC and AC conductivities. The observed experimental results were discussed in the paper.

Fabrication and Properties of CaAl<sub>12</sub>O<sub>19</sub>-Al<sub>2</sub>O<sub>3</sub> Composite Ceramics

CaAl12O19-Al2O3 composite ceramics were synthesized by in situ solid state reaction using CaCO3 and industrial α-Al2O3 powders. The effects of sintering temperature and ratio of CA6 (CaAl12O19) content on phases and microstructure of the final composite ceramics were studied by XRD and SEM. The result showed that sheet-like CA6 appeared in the materials when sintering temperature was higher than 1400°C and grew up with the increased of the sintering temperature. The optimum sintering temperature was 1600°C, when the sample containing 20wt% CA6 was sintered at 1600°C, the bulk density was 3.21g/cm3, the apparent porosity achieved 9.94% and the bending strength gained 149.88MPa. With the increasing of CA6 content in the final composite ceramics, the bulk density and the bending strength increased firstly and then decreased. The lowest apparent porosity 1.42%, a density of 3.60g/cm3 and the highest bending strength of 265.10MPa were achieved for the sample containing 10wt% CA6 sintered at 1600°C.

Effects of mechanical properties of polymer on ceramic-polymer composite thick films fabricated by aerosol deposition

Two types of ceramic-polymer composite thick films were deposited on Cu substrates by an aerosol deposition process, and their properties were investigated to fabricate optimized ceramic-based polymer composite thick films for application onto integrated substrates with the advantage of plasticity. When polymers with different mechanical properties, such as polyimide (PI) and poly(methyl methacrylate) (PMMA), are used as starting powders together with α-Al2O3 powder, two types of composite films are formed with different characteristics - surface morphologies, deposition rates, and crystallite size of α-Al2O3. Through the results of micro-Vickers hardness testing, it was confirmed that the mechanical properties of the polymer itself are associated with the performances of the ceramic-polymer composite films. To support and explain these results, the microstructures of the two types of polymer powders were observed after planetary milling and an additional modeling test was carried out. As a result, we could conclude that the PMMA powder is distorted by the impact of the Al2O3 powder, so that the resulting Al2O3-PMMA composite film had a very small amount of PMMA and a low deposition rate. In contrast, when using PI powder, the Al2O3-PI composite film had a high deposition rate due to the cracking of PI particles. Consequently, it was revealed that the mechanical properties of polymers have a considerable effect on the properties of the resulting ceramic-polymer composite thick films.

Electron paramagnetic resonance and photoluminescence properties of α-Al2O3:Cr3+ phosphors

The red emitting Cr3+ activated α-Al2O3 powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al2O3 phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to 4A2g (F)→4T1g (F) and 4A2g (F)→4T2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to 2E g →4A2g transition of Cr3+ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr3+ ions are occupying in Al3+ sites in octahedral coordination.

Analysis of the crystal phases of Na+-beta-aluminas synthesized by infiltration of alkali molten salts

Na+-beta-aluminas in ternary Na2O–Al2O3–Li2O systems were synthesized by infiltration of alkali molten salts into porous α-Al2O3 pre-form. The effects of calcination temperature, infiltration time, and starting materials on phase formation were investigated. Pre-forms were fabricated by pressureless powder packing (PLPP), slip casting, and cold isostatic pressing (CIP) with fused and calcined α-Al2O3 powder. They were heated and infiltrated by alkali molten salts at [Na2O]:[Al2O3]=1:5 (Li2O=0.4 w/o). Each infiltrated specimen was calcined at 1100–1300°C to form Na+-beta-aluminas, whose crystalline formations, phase fractions, and alkali ion contents were analyzed. The pre-form prepared by PLPP with 1μm-calcined α-Al2O3 powder showed the highest Na-β″-Al2O3 phase fraction (59.5%), without residual α-Al2O3 phase.

Synthesis and sintering of pure nanocrystalline α-Al2O3 powder

When 25-nm α-Al2O3 powder is injected into an aluminum nitrate solution of ≥0.5 wt %, the kinetics of formation of the stable α phase is significantly accelerated upon subsequent heating of the precursor. The total transformation into α-Al2O3 occurs at temperatures varying from 800 to 930°C in accordance with the amount and the method of seed injection. The resulting powder consists of crystallites 50–60 nm in size. They form porous agglomerates 10–20 μm in size, and their size does not depend on the amount of injected seed. Milling under soft conditions breaks the agglomerates and makes it possible to obtain a nanopowder which sinters into a material of 95–99% density in the range of 1300–1400°C.

Preparation and properties of crystallizable Glass/Al2O3 composites for LTCC material

The investigated low temperature Co fired ceramics(LTCC) composite of 60wt% CaO-Al2O3-B2O3-SiO2 glass and 40wt% α-Al2O3 as a filler is a non-reactive system, which is a critical part of the low temperature Co fired ceramics process. Through a study on densification process, the phase transformation and microstructure can be revealed. Its composites typically consist of CaO-Al2O3-B2O3-SiO2 glass and α-Al2O3 powders of average particle size (D50=3.49 μm). The sintering behavior, phase evaluation, sintered morphology, and microwave dielectric properties were investigated. In the fire range of 800 to 900 °C, the composites were crystallized after completion of densification. It is found that the composites start to densify at 825 °C, simultaneously, the dielectric constant (ɛr) reaches its maximum. With increasing heat-treatment temperatures, due to the loose microstructure of the material, tanδ increases slightly. The last of the sintered samples were identified as partly Anorthite at 850 °C. At that temperature it has ɛr of 7.9 and tanδ less than 1×10−3, and can be used as a promising LTCC material.

Spark Plasma Sintering of Two Commercial Al<sub>2</sub>O<sub>3</sub> Powders

Al2O3 ceramic is prepared by spark plasma sintering (SPS) using two commercial α-Al2O3 powders at elevated sintering temperature. The relative density and average grain size of the prepared Al2O3 ceramics are measured and compared. One α-Al2O3 powder has good sintering property because the relative density of the prepared α-Al2O3 ceramic is higher than 97% while another α-Al2O3 powder has poor sintering property.

Fabrication and Properties of Plasma-Sprayed Al2O3/ZrO2 Composite Coatings

Al2O3/xZrO2 (where x = 0, 3, 13, and 20 wt.%) composite coatings were deposited onto mild steel substrates by atmospheric plasma spraying of mixed α-Al2O3 and nano-sized monoclinic-ZrO2 powders. Microstructural investigation showed that the coatings comprised well-separated Al2O3 and ZrO2 lamellae, pores, and partially molten particles. The coating comprised mainly of metastable γ-Al2O3 and tetragonal-ZrO2 with trace of original α-Al2O3 and monoclinic-ZrO2 phases. The effect of ZrO2 addition on the properties of coatings were investigated in terms of microhardness, fracture toughness, and wear behavior. It was found that ZrO2 improved the fracture toughness, reduced friction coefficient, and wear rate of the coatings.

Influence of Z Value and Sintering Temperature on Properties of β-Sialon Bonded Corundum Composite

Using corundum particles and fine powder, α-Al2O3 powder, silicon powder, aluminum powder and some additives as raw materials, β-Sialon bonded corundum composite was synthesized in-situ at the nitrogen atmosphere. The influence of Z value in sialon phase and sintering temperature on physical properties, mechanical properties, thermal shock resistance and anti-oxidation performance of β-Sialon bonded corundum composite was investigated. The results indicate that the comprehensive properties of β-Sialon bonded corundum composite is excellent on condition that Z=2.75, sintering temperature is 1500°C at nitrogen atmosphere.

Research on Al<sub>2</sub>TiO<sub>5</sub>-Si<sub>3</sub>N<sub>4</sub> Composites Fabricated by Reaction Method

Al2TiO5-Si3N4 composites were fabricated by mixing 75-95% premixed powder, composed of titanium dioxide, α-Al2O3 powder, and suitable amount of stabilizing agents such as MgO, SiO2 and ZrO2, and 5-25% α-Si3N4 and being sintering at 1450-1550°C in nitrogen atmosphere. The effects of Si3N4 content, sintering temperature and soaking temperature on the properties of Al2TiO5-Si3N4 composites were investigated. The results showed that the optimum formula for Al2TiO5-Si3N4 composites was 90% premixed powder and 10% α-Si3N4. The optimum technological parameter was that sintering time was 1550°C for 2h.

Spark plasma sintering of alumina: Study of parameters, formal sintering analysis and hypotheses on the mechanism(s) involved in densification and grain growth

The spark plasma sintering (SPS) of an undoped commercial α-Al2O3 powder (0.14μm) was investigated. The SPS parameters such as the dwell temperature, applied external pressure, temperature of pressure application, dwell time and pulse pattern were varied. A sintering path (relative density vs. grain size) showing two regimes has been brought to the fore: densification without grain growth occurring at the lower temperatures and grain growth without much further densification taking place at the higher temperatures, with a threshold between 1100 and 1200°C. In addition, a formal sintering analysis was performed in order to identify the mechanism(s) involved in densification and grain growth.

Research on Molding for Al<sub>2</sub>O<sub>3</sub>Ceramic Substrate and its Processing Technic

By adopting technique for forming colloid to manufacture Al2O3 Ceramic Substrate, the effect of aggregation reaction which is caused by pH value, temperature for shaping, evocating agent and some other factors of slurry has been researched in the processes for shaping Substrate. The result addresses that the substrate with well-proportioned in density, contractibility and over 78MPa intensity is able to be made while the pH is about 9, the temperature is between 60 and 70°, the shaping duration is no more than 30 minutes, in glass mold, and with 120 item undersize α-Al2O3 powder. To dunk the desiccating substrate for 20 minutes, it would gain the good flexibility, moreover, it is able to be cut. After it is desiccated again, it becomes firm again, this kind of characteristic can be repeated numbers of times.

Fabrication and optical properties of a highly transparent Nd:YAG ceramic

A neodymium doped yttrium aluminum garnet (Nd:YAG) transparent ceramic was fabricated by a solid state reaction method using commercial α-Al2O3, Y2O3, and Nd2O3 powders as starting materials and tetraethyl orthosilicate (TEOS) as sintering aid. The morphology and microstructure of the nanopowders and the Nd:YAG transparent ceramic were investigated. The fully dense Nd:YAG ceramic with an average grain size of ∼20 μm was obtained by vacuum sintering at 1720°C for 12 h. Few pores and grain-boundary phases were observed. The in-line transmittance of the ceramic was 81.5% at 1064 nm.

Solid-phase processes in a ceramic matrix in the system SiO2 – Al2O3 – TiO2 With addition of fibrous nanostructural aluminum oxide

The formation of a composite ceramic based on the system SiO2 – Al2O3 – TiO2 with the addition of γ- and α-Al2O3 nanostructural powders during heat treatment in the temperature range 1300 – 1400°C is investigated. It is established that the introduction of γ-Al2O3 reactive nanostructural powder stimulated a solid-phase reaction between the silica and alumina with mullite being formed, while the addition of α-Al2O3 powder led to the formation of aluminum titanate. These processes altered the phase composition and the porous and crystalline structure of the composites and increased their physical – chemical and thermophysical properties.