Micrometric Alumina Research Articles

Study of powders for sintering SiC in the presence of a liquid phase

In this study, nanometric powders of silicon carbide (SiC), micrometric alumina (Al₂O₃) and yttria (Y₂O₃), with specific characteristics provided by the manufacturers, were used to investigate the compaction behavior and the resulting properties after sintering. The SiC used has a purity of over 99% and a particle size distribution with a d50 of 40 nm, while Al₂O₃ and Y₂O₃ have purities of 99.8% and 99.995%, respectively. The morphological evaluation of the powders was carried out using scanning electron microscopy (SEM), while the apparent density analysis was conducted using gas pycnometry. SiC mixtures with 1% and 5% Al₂O₃ and Y₂O₃ were prepared and subjected to uniaxial compaction tests to determine densification behavior under different pressures. The results of this study are relevant to the development of ceramic materials with optimized properties for a wide range of industrial applications. The addition of small amounts of oxides to SiC does not compromise its compaction properties, maintaining the effectiveness of the sintering process and resulting in materials with high density and improved mechanical properties.

Construction of micro-nano hybrid structure based on carbon nanotube whisker and alumina for thermally conductive yet electrically insulating silicone rubber composites

High-performance electronics urgently need more effective thermally conductive rubber composites to solve interfacial heat transfer problems in the thermal management systems. Tiny amounts nanocarbon materials (NCM) can significantly improve the thermal conductivity of conventional ceramic-filled rubber composites, but the volume exclusion effect of micrometer ceramic fillers makes NCM highly susceptible to the formation of the conductive pathways, which inevitably leads to the substantial decrease in the volume resistivity of the materials, posing a safety hazard, such as short circuits, to electronic devices. Here, we report an electrostatic self-assembly method to prepare CNW@n-Al2O3 hybrids by loading nano-alumina (n-Al2O3) onto carbon nanotube whiskers (CNW) and co-filling them with micrometer alumina (m-Al2O3) to silicone rubber, constructing a micro-nano-multi-level hybrid network structure, which can fully utilize the high thermal conductivity while shielding the electrical conductivity of CNW. The resulting composite filled with 2 phr of CNW@n-Al2O3 exhibits a significantly enhanced thermal conductivity of 1.137 W/(m·K) and a high volume resistance of 1.323 × 109 Ω cm, and is proved to be used as an excellent thermal interface material to assist the heat dissipation of the microelectronic chip. This study provides a facile and effective strategy for the design of thermally conductive yet electrically insulating rubber composites filled with CNW, which shows a bright application prospect in the thermal management of high-performance electronic devices.

Study on the Tribological Properties of Micro-Al2O3 Modified Carbon Fiber Hybrid-Reinforced Polymer

Micro-sized Al2O3-modified carbon fiber hybrid reinforced polymer composites (CFHRP) were prepared using the vacuum-assisted resin transfer molding (VARTM) process, and friction and wear tests were performed on an MRTR-1 multifunctional friction and wear tester to study the effect of carbon fiber surface doping with trace amounts of Al2O3 and its content on the tribological properties of carbon fiber reinforced polymer composites. The surface morphology of the composite material was characterized using a three-dimensional confocal laser scanning microscope (LSM). The results showed that under dry friction conditions, the wear behavior of the composite material with micro-Al2O3 added was mainly abrasive wear and adhesive wear. Under the condition of water lubrication, the friction coefficient of the composite material with micro-Al2O3 added was higher than that of the carbon fiber/epoxy resin-based composite material without Al2O3, and the abrasive wear and adhesive wear were significantly reduced. In addition, under the condition of water lubrication, the cooling and boundary lubrication of water played a dominant role in the tribological behavior, so the influence of transfer film on the tribological behavior of the composite material was smaller than that under dry friction conditions. When the micro alumina content is 6%, the friction and wear reduction of CFHRP composite material are improved under water containing conditions. Under dry friction conditions, the content of micrometer alumina has a minimal effect on the change in friction coefficient.

Preparation and characterization of Nextel 720/alumina ceramic matrix composites via an improved prepreg process

AbstractIn this work, the Nextel 720 continuous fiber reinforced alumina ceramic matrix composites (CMCs) were prepared by an improved prepreg process. The alumina matrix was derived from aqueous slurry, which consisted of organic glue, alumina sol, nanometer alumina powders, and micrometer alumina powders. This design provided a densely packed matrix for the CMC, and made the whole process relatively simple. The ratio of different alumina components in aqueous slurry was optimized to obtain good sintering activity, high thermal resistance, and excellent mechanical properties simultaneously. Furthermore, a preceramic polymer of mullite was used to strengthen the ceramic matrix through a multiple infiltration process. The final CMC sample achieved a high flexural strength of 255 MPa and a good high‐temperature stability. After 24 h of heat treatment at 1100°C, 85% of the maximum flexural strength still had been retained.

Obtainment of the alumina polyamide-coated spherical particles

The indirect selective laser sintering (SLS) process consists of a powder-based additive manufacturing (AM) technique. Recently, an attempt was carried out using the indirect-SLS method on ceramics to obtain complex geometric parts without the need to fabricate tools or molds. In this context, the ceramic particles employed in indirect-SLS must be coated with a polymer layer and retain spherical format. Currently, several processes of encapsulation and balling up of ceramic particles are available, among which thermally induced phase separation (TIPS) is noteworthy. The present study describes the synthesis, via TIPS, of spherical micrometric alumina particles coated with polyamide 12 to be employed in indirect-SLS. Dimethyl sulfoxide (DMSO) was used as a solvent to obtain these particles. After encapsulation, the agglomerates were characterized according to granulometry, microstructure, thermal behavior, and flowability. The obtained results delineated the ideal conditions to synthesize agglomerates with adequate flowability, particle size profile, and similar thermal behavior to that verified for PA-12, a currently used material in SLS printing. Based on the results of the present study, it is possible to confirm the effectiveness of the TIPS technique in obtaining ceramic polymer-coated spherical particles that are suitable for use in AM processes via indirect-SLS.

Influence of Al2O3 Particle Size on Microstructure, Mechanical Properties and Abrasive Wear Behavior of Flame-Sprayed and Remelted NiCrBSi Coatings

The influence of micrometric alumina (low surface area-to-volume ratio) and nanometric alumina (high surface area-to-volume ratio) on microstructure, hardness and abrasive wear of a NiCrBSi hardfacing alloy coating applied to an AISI 304 substrate using flame spraying (FS) combined with surface flame melting (SFM) is studied. Remelting after spraying improved the mechanical and tribological properties of the coatings. Microstructural characterization using XRD, SEM and EDS indicated that alumina additions produced similar phases (NiSi, Ni3B, CrC and Ni31Si12) regardless of the alumina size, but the phases differed in morphology, size distribution and relative proportions from one coating to another. The addition of 12 wt.% nanometric Al2O3 increased the phases concentration more than five- to sixfold and reduced the hard phases size about four-to threefold compared with NiCrBSi + 12 wt.% micrometric Al2O3. Nanoalumina led to reduced mass loss during abrasive wear compared to micrometric alumina and greater improvement in hardness.

Continuous-Flow Hydrogenation of D-Xylose with Bimetallic Ruthenium Catalysts on Micrometric Alumina

Continuous-flow hydrogenation of D-xylose to xylitol was evaluated with pristine and silver modified ruthenium supported on micrometric alumina. The original size of alumina support is required for direct use in flow hydrogenation since it prevents setup clogging. The parent ruthenium catalyst shows high activity and selectivity to the desired product. However, there was an evident competition between adsorption of substrate and product desorption, and ruthenium nanoparticles aggregation. In situ modification of the parent catalyst with silver improved catalysts stability and minimize the competitive adsorption behavior between reactant and product.

Multi-Scale-Structured Composite Coatings by Plasma-Transferred Arc for Nuclear Applications

In nuclear plants, the replacement of hardfacing Stellite, a cobalt-based alloy, on parts of the piping system in connection with the reactor has been investigated since the late 60’s. Various Fe-based or Ni-based alloys, Co-free or with a low content of Co, have been developed but with mechanical properties generally lower than that of Stellites. The 4th generation nuclear plants impose additional or more stringent requirements for hardfacing materials. Plasma-transferred arc (PTA) coatings of cobalt-free nickel-based alloys with the addition of sub-micrometric or micrometric alumina particles are thought to be a potential solution for tribological applications in the primary system of sodium-cooled fast reactors. In this study, PTA coatings of nickel-based alloys reinforced with alumina particles were deposited on 316L stainless steel substrates. Under the conditions of this study, the addition of alumina particles resulted in a refinement of coating microstructure and the improvement of their resistance to abrasive wear. However, it does not bring about any change in coating micro-hardness.

Comparison of Different Alumina Powders for Aqueous Tape Casting Process

Tape casting process was used to produce Al2O3 substrates in an aqueous system with acrylic latex emulsion as binder. The present work studied the slurry formulations in aqueous medium of Al2O3 powders with different particle size distribution and made correlation to the green and sintered tapes. Two commercial alumina powders, one sub-micrometric and other micrometric were used. Compositions of Al2O3 slurries with 80 and 83 wt% of solids were prepared by dispersing the powders in water with a dispersant with subsequently additions of 7 and 10 wt% of binder. Sub-micrometric Al2O3 resulted in a high densification tapes regardless solid concentration and binder amount in the slurry, though green density was affected. For micrometric alumina, increasing the solid concentration resulted in a little higher final density.

Flow field measurements of pulverized coal combustion using optical diagnostic techniques

This paper demonstrates the application of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) techniques to a particle-laden reacting flow of pulverized coal. A laboratory-scale open-type annular burner is utilized to generate velocity profiles of coal particles and micrometric alumina particles. Pair-wise two-component LDV measurements and high-speed stereo PIV measurements provide three-dimensional velocity components of the flow field. A detailed comparison of velocities for alumina and coal particle seeding revealed differences attributed to the wide size distribution of coal particles. In addition, the non-spherical shape and high flame luminosity associated with coal particle combustion introduces noise to the Mie scatter images. The comparison of mean and RMS velocities measured by LDV and PIV techniques showed that PIV measurements are affected by the wide size distribution of coal particles, whereas LDV measurements become biased toward the velocity of small particles, as signals from large particles are rejected. This small-particle bias is also reflected in the spectral characteristics for both techniques, which are in good agreement within the range of frequencies accessible. PIV measurements showed an expected lack of response of large coal particles to the turbulence fluctuations. The overall good agreement between LDV and PIV measurements demonstrates the applicability of the high-speed PIV technique to a particle-laden, high luminosity coal flame while highlighting some of its limitations.

How effective is the addition of nanoscaled particles to alumina–magnesia refractory castables?

The addition of nanoscaled alumina and magnesia particles to the matrix of alumina–magnesia refractory castables drastically reduces the residual expansion related to the in situ spinel formation. Nonetheless, as their benefits on other relevant properties have not been reported so far, the effectiveness of such nanoengineering design for castables applied in steel ladles is still uncertain. In the present work, not only the expansion level, but also the corrosion resistance, the hot modulus of rupture and the creep deformation of different nanoparticle-containing castables were evaluated and compared with the results attained by refractory materials designed only by micrometric-scaled Al2O3 and MgO. Although the addition of a nanoalumina and nanomagnesia mixture ensured the best results regarding to the expansive behavior, thermo-mechanical and thermo-chemical properties, its performance was only slightly superior to the castable containing micrometric alumina and magnesia particles. Therefore, as the cost–benefit ratio is one of the main requirements for the end users, the nanotechnology use in the refractory production must be previously carefully analyzed.

A comparison study of the agglomeration mechanism of nano- and micrometer aluminum particles

The agglomeration mechanism of micro- and nanosize aluminum particles with a primary mean particle diameter of 4.5 {mu}m and 75 nm, respectively, was comparatively investigated under an incident shock wave. The morphology, particle size, and agglomeration process of micro- and nanometer alumina particles were comprehensibly compared by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. Images of X-ray diffraction reveal that a varied of phases of alumina ({gamma}-, {delta}-, {epsilon}-, and {alpha}-Al{sub 2}O{sub 3}) were simultaneously found in the nanosize alumina products, which may give some detail information of the wide variety of reacting temperature of aluminum nanoparticles, while Al{sub 4}C{sub 3} was detected in micrometer alumina products, which also gives some dynamic information of aluminum to alumina, i.e., aluminas have actually reacted with the free active carbon atoms to produce their intermediates. The microstructure of aluminas induced by the incident shock waves was detected and analyzed by using transmission electron microscopy combined with X-ray photoelectron spectroscopy spectrum. These results are an additive evidence to support that the initial stage sintering of the alumina nanosize powders is dominated by grain boundary diffusion, while the volume diffusion is the main character for the initial stage sintering ofmore » the micrometer alumina powders.« less

Diameter- and length-dependent self-organizations of multi-walled carbon nanotubes on spherical alumina microparticles

Multi-scale hybrid structures of multi-walled carbon nanotubes (MWCNTs) and micrometric alumina particles (μAl 2O 3) have been produced. The hybrid structures are obtained by in situ grafting carbon nanotubes (CNTs) on spherical μAl 2O 3 particles using an easy chemical vapor deposition method, without any pre-patterned catalyst treatment. The study of the influence of temperature and hydrogen ratio shows three regular hybrid structures defined according to CNT arrangement on μAl 2O 3. Furthermore, the organization modes demonstrate that the hybrid structures are strongly dependent on the diameter, length and area number density of CNTs. This dependency has been explained using a proposed nano-cantilever beam model. It evaluates the maximum deflection of one CNT due to weak van der Waals interactions. The model analysis shows that the MWCNT organizations are a result of varying competitive interactions between MWCNT rigidity and their attractive forces on the large curvature surface of μAl 2O 3. In addition, the influence of specific characteristics of μAl 2O 3 on hybrid structures is also discussed.

Tribological behaviors of surface-coated serpentine ultrafine powders as lubricant additive

The effect of surface-coated ultrafine powders (UFPs) of serpentine suspended in lubricants on the tribological behaviors of a mated 1045 steel contact was investigated. Through the addition of serpentine UFPs to oil, the wear resistance ability was improved and the friction coefficient was decreased. The addition of 1.5 wt% serpentine to oil is found most efficient in reducing friction and wear. The nano-hardness and the ratio of hardness to modulus of friction surface are observably increased. Such effects can be attributed to the formation of a tribofilm of multi-apertured oxide layer, on which the micrometric alumina particles embedded and serpentine nano-particles adsorbed.

Processing of high performance silicon carbide

Liquid phase sintering based on the dissolution-precipitation mechanism was applied to densify a 0.8 micrometer SiC powder with alumina (1.2 vol%)-yttria (0.9-3.3 vol%) additives. To uniformly distribute the sintering additives around the SiC particles, a heterocoagulated particle network was formed among negatively charged SiC particles, positively charged 0.2 micrometer alumina and yttrium ions in an aqueous suspension at pH 5. Yttrium ions were electrostatically adsorbed on the negatively charged SiC surfaces. The consolidated green compacts were highly sintered to 97-99% of theoretical density by hot-pressing at 1950°C. The mechanical properties (four-point strength, fracture toughness and Weibull modulus) were highly enhanced when a bimodal particle size system of SiC (0.8 micrometer-30 nanometer SiC) was sintered. The maximum strength of 75 vol% 0.8 micrometer SiC-25 vol% 30 nanometer SiC reached 1.1 GPa at room temperature. The fracture toughness was about 6 MPa·m 1/2 and the Weibull modulus was 5.9. When a small amount of SiC precursor polymer was infiltrated in the green compact, the strength and Weibull modulus were further improved.

High temperature mechanical behavior of polycrystalline alumina from mixed nanometer and micrometer powders

Sintered aluminum oxide materials were formed using commercial methods from mechanically mixed powders of nano- and micrometer alumina. The powders were consolidated at 1500 and 1600°C with 3.2 and 7.2 ksi applied stress in argon. The conventional micrometer sized powders failed to consolidate. While 100% nanometer-sized alumina and its mixture with the micrometer powders achieved >99% density. Preliminary high temperature creep behavior indicates no super-plastic strains. However high strains (>0.65%) were generated in the nanometer powder, due to cracks and linked voids initiated by cavitation.

Thick slurry bevelling: a new technique for bevelling extremely fine microelectrodes and micropipettes.

A simple, inexpensive and rapid bevelling method is described. A settled slurry of 0.05 micrometer alumina powder in saline is used as the grinding surface. The bevelling process is continuous and reproducible over a wide range of electrode resistances.