Coarse Alumina Powder Research Articles

EVALUATION OF INTERNAL COHESION OF MULTIPHASE PLASMA-SPRAYED COATINGS BY CAVITATION TEST: FEASIBILITY STUDY

Mechanical characterization of plasma-sprayed coatings at microscopic level represents a major challenge due to the presence of numerous inherent microstructural features such as cracks, pores, or splat boundaries, which complicate coatings characterization by conventional testing methods. Need for reliable testing of structural integrity of newly developed multiphase plasma-sprayed coatings introduced even more complexity to the testing. In this study, applicability of indirect vibratory cavitation test (adapted from ASTM G32 standard) for such testing was evaluated. Three plasmasprayed coatings having distinctive microstructures were tested: i) conventional alumina coating deposited from coarse powder, ii) hybrid coating deposited by co-spraying of coarse alumina powder and fine yttria-stabilized zirconia (YSZ) suspension, and iii) compact alumina coating deposited from fine ethanol-based suspension. Differences in the coatings internal cohesion were reflected in different failure mechanisms observed within the cavitation crater by scanning electron microscopy and mean erosion rates being i) 280 μm/hour, ii) 97 μm/hour and iii) 14 μm/hour, 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 5wt% TiO2–CuO were fully densified and exhibited sufficient flexural strength (342±21MPa) when sintered at a temperature of 1450°C for 2h.

Fabrication and characterization of ceramic coatings with alumina–silica sol-incorporated α-alumina powder coated on woven quartz fiber fabrics

Abstract The formation and properties of dense silica/α-alumina coatings derived from alumina–silica sol-incorporated coarse alumina powder (median particle size d 50 =0.509 μm) using Al(NO 3 ) 3 ·9H 2 O and tetraethyl orthosilicate as precursors have been investigated. Phase compositions and thermal evolution analysis of the coatings as well as interface microstructure between the coatings and the matrix were characterized by means of XRD, XRF, DSC-TG, TEM, SEM supplemented with EDS, and tensile strength tester. Silica/α-alumina coatings combined with quartz fiber matrix are homogeneously integrated with distinct inter-diffusion near the interface. The heat conduction mechanism of the quartz fiber matrix was influenced by the dense silica/α-alumina ceramic coatings, and the prepared coatings provide practical thermodynamic stability and desired mechanical strength for the woven quartz fiber fabrics.

Combined effect of chemical nature and fineness of mineral powders on Portland cement hydration

This paper focuses on the influence of the chemical nature and the fineness of the fillers on the hydration process and on the compressive strength development. Four different types of fillers are considered in combination with Portland cement: quartzite filler, alumina filler, limestone filler, and silica fume. The study deals with blended mortars having a 0.45 water to powder (cement and filler) ratio with a 10% substitution of cement by filler. Quartzite fillers do not seem to accelerate the hydration process in a significant way. No positive effect is noticed on the strength development either. The presence of a fine inert alumina powder increases the rate of early hydration of Portland cement. The greater the fineness, the faster the rate of hydration heat development. This reactivity leads to an increase in the compressive strength at early age for mortar containing the finest alumina powders. In case of coarse alumina powder, no acceleration effect is obtained. Finely ground limestone (calcite) fillers promote heterogeneous nucleation of hydrates which significantly accelerates hydration. At early age, this also results in an increased mortar compressive strength in comparison with the control mortar. From the obtained results, it is clear that both chemical natures as well as fineness are important with regard to the accelerating effect of the hydration process. With increasing fineness, the accelerating effect increases. For powders with comparable fineness, it is clear that limestone powder has a more significant accelerating effect than silica fume and alumina filler. Quartzite filler seems to have no significant effect.

Preparation and Sintering of Macroporous Ceramic Membrane Support from Titania Sol‐Coated Alumina Powder

The preparation of ceramic support at relatively low sintering temperature of 1350°–1500°C was investigated with a powder processing route, namely, titania sol coated on coarse alumina powder (median particle size d50, 27.5 μm). The sintering kinetics analysis indicates that the microstructure control of support is closely related to the sol coating and the sintering conditions. The mechanisms of sintering process were discussed in detail with the corresponding kinetics parameter, i.e., activation energy Ea and exponent n as well as the microstructure characterization. The prepared support provides practical applications with tunable pore size and desired mechanical strength.

Effect of Ceramic Powders on the Reaction Sintering of Ceramic/Metal Mixed Powders

The reaction-sintered alumina and zirconia-alumina ceramics with low firing shrinkage were prepared from Al 2 O 3 /Al and ZrO 2 (C a -PSZ)/Al powder mixture by attrition milling and the effect of the characteristics of used ceramic powders was investigated. Flaky Al powders mixed with coarse alumina powders could be effectively pulverized due to high hardness of alumina powders. On the contrary fused Ca-PSZ/Al powder mixture could not be effectively comminuted due to lower hardness than alumina powders. So by using hard alumina ball media or coarse Al 2 O 3 powders rather than Al, the milling efficiency could be much more increased. After attrition milling the Al 2 O 3 /Al specimens were oxidized at 1200°C for 8 hours and followed by sintering at 1550°C for 3 hours. Because mixed powders of Al with alumina were much more effectively comminuted, more than 97% theoretical density of sintered body could be achieved. In the case of fused Ca-PSZ powder reaction-sintered with Al at 1550°C for 3 hours, the reaction-sintering and densification were a little more difficult than Al 2 O 3 /Al specimens because of the insufficient milling. And the unstabilization of Ca-PSZ body which caused the cracks in the specimens was brought by the diffusion of Ca ion from Ca-PSZ grains to alumina.

Silicon CVD on powders in fluidized bed: Experimental and multifluid Eulerian modelling study

The Computational Fluid Dynamics code MFIX was used for transient simulations of silicon Fluidized Bed Chemical Vapor Deposition (FBCVD) from silane (SiH 4) on coarse alumina powders. FBCVD experiments were first performed to obtain a reference database for modelling. Experimental thermal profiles existing along the bed were considered in the model. 3D simulations provide better results than 2D ones and predict silane conversion rate with a mean deviation of 9% compared to experimental values. The model can predict the temporal and spatial evolutions of local void fractions, gas and particle velocities, species gas fractions and silicon deposition rate. We aim at mid term to model FBCVD treatments of submicronic powders in a vibrated reactor since we have performed experiments proving the efficacy of the process to treat submicronic particles.

Milling and Sintering Behavior of the Metal-Ceramic Mixed Powders

The reaction-sintered alumina ceramics with low firing shrinkage were prepared from Al/Al2O3 powder mixture by attrition milling and the effect of milling characteristics of raw powders on reaction sintering was investigated. Powder mixtures of flaky shape Al with coarse alumina was much more effectively comminuted by the attrition milling than the mixtures of globular shape Al with coarse alumina powders. Furthermore the coarse alumina was much more useful in pulverizing and grinding the ductile Al particles than fine alumina. After attrition milling and isopressing at 400MPa, the Al/Al2O3 specimen was oxidized at 1200°C for 8 hours followed by sintering at 1550°C for 3 hours. Because mixed powder of coarse alumina with flaky Al was much more effectively comminuted than the globular Al, sintered body of more than 97% theoretical density was achieved, but low contents of Al leads to relatively higher shrinkage of about 8%. As the coarse alumina particles are much more useful in cutting and reducing the ductile Al particles, the use of the coarse alumina powder was much more effective in reaction-sintering.

Preparation of a coal surface for contact angle measurements

Coal specimens of different ranks were polished using silicon carbide abrasive papers (with a grit from #60 to #1200) and alumina powder of varying size (from 5 to 0.05 μm). The coal surface roughness and contamination (by alumina powder) were examined with both scanning electron microscopy and atomic force microscopy. The water advancing and receding contact angles were measured on such surfaces by varying the bubble size, using the captive-bubble technique. It was found that silicon carbide paper abraded all components of the coal surface, i.e. both organic and inorganic matter, to a similar depth. The roughness of the coal surface due to polishing with silicon carbide abrasive papers affected the contact angle hysteresis and the contact angle vs. bubble size relationship. Polishing of coal specimens with alumina powder reduced the microroughness of the coal surface but produced rough features at the macro level and caused mineral inclusions rising above the smooth organic matter. This phenomenon results from the heterogeneity of coal specimens consisting of minerals and macerals with different hardness values. The roughness at the macro level was easily distinguishable and had a significant impact on the measured contact angles when the coal surface was polished with coarse alumina powders, 5 and 1 μm in diameter. The effect of surface roughness on the advancing and receding water contact angles was significantly reduced (if not completely eliminated) when the coal surface was polished with a fibrous cloth (CHEMOMET) in the final step, after having been polished with 0.05 (0.06) μm alumina powder. Microscopic observation of the coal surfaces revealed that an appropriate ultrasonic treatment (8-10 min in an ultrasonic bath filled with water) and mechanical cleaning (polishing with a CHEMOMET cloth) of coal samples were required to remove the alumina particles left on the surface due to the previous polishing procedure. An improved methodology for coal surface preparation, prior to contact angle measurements, as proposed in this paper, includes polishing with a series of abrasive papers and 0.05 (0.06) μm alumina powder, polishing and cleaning with a fibrous cloth (e.g. CHEMOMET), and, finally, an extended cleaning in an ultrasonic bath filled with water.

Liquid phase sintering of bimodal size distributed alumina powder mixtures

Fine and coarse alumina powder mixtures (non-additive specimen) and those containing the additive formed liquid phase during firing (additive specimen) were compacted and fired at 1400–1600°C. Liquid phase sintering proceeded markedly at 1400–1500°C and additive specimens had much higher relative density than non-additive specimens at 1500°C. As the liquid phase sintering proceeded, the open pore volume decreased abruptly, but the open pore size changed depending on the packing structure. The open pore size decreased in the specimens where the fine particles formed matrix structure, while it increased in the specimens where the coarse particles formed skeletal structure. At 1600°C all additive specimens having different mixing ratios of fine and coarse powders had similar microstructure and the same relative density of 97%. However, spherical large pores were formed and remained in all additive specimens even at 1600°C. The bending strength of those specimens was about 400 MPa.

Micropore Distribution Change of Heated Powder Compacts Using Binary Alumina Powder Mixtures

The sintering behavior of bimodal alumina powders was investigated with reference to the change of open pore size distributions. Avarage particle sizes of fine and coarse alumina powders were 0.52μm (f particle) and 6.3μm (c particle). The coarse powder was spherical and polycrystalline. The open pore size distributions in green compacts were narrow for specimens with mixing ratios c:f=4:6-1:9, but were broad for c:f=9:1-5:5. When sintered at 1400°-1600°C, the open pore size increased with c:f=9:1-4:6, remained unchanged with c:f=3:7-1:9, and decreased with c:f=0:10. It is considered that the growth of pores was caused by the difference of sintering rates between tight and loose-packed parts, and also between c and f particles.