Coarse Alumina Particles Research Articles

Microstructure-based discrete simulations of the compaction of refractory powder composites

This work describes an original methodology to simulate the compaction behavior of refractories by taking explicitly into account the microstructure of each composite phase. The Discrete Element Method (DEM) is used to model the composite as a mixture of fine and coarse alumina particles and graphite flakes. The binder phase is accounted for by a specific contact law which adjusts the contact rigidity according to the indentation between the coated particles. Small indentations lead to a soft response while above a critical indentation, the contact becomes stiffer. This simple model incorporates the granular behavior of the composite as well as the continuous matrix of the binder. Composites with various compositions are numerically submitted to closed-die compaction. The densification behavior during compaction depends primarily on the binder phase. The contribution of each phase to the total axial stress is investigated, thus providing new insights into the complex behavior of these composites.

The Influence of the Chemical Composition on the Microstructural Changes and on the Resistance to Abrasion of Low-Alloy Casting Steels

This study analyzes the influence of boron in the presence of chromium, vanadium, and titanium on changes in the tribological properties of low-alloy casting steels caused by structural modifications brought about by the above elements. Eight grades of cast steels were manufactured based on the designed chemical composition. The wear behavior of these materials was investigated with the GOST (Russian) standard wear test equipment, in which the steel samples were worn by coarse alumina particles (grit size #90). The wear of the cast steels was evaluated by weight loss and their wear mechanisms were investigated with scanning electron microscopy. The two predominant abrasive wear mechanisms for the cast steels were microploughing and microcutting. The worn surfaces also showed traces of pitting originating from the removal of larger particles. The results obtained during the wheel–rubber wear laboratory test may be used to compare the abrasive wear resistance of materials with a ferrite–pearlite microstructure because the test is sensitive to changes in abrasion resistance caused by microstructural changes due to alloying additives of non-heat-treated cast steels.

Overcoming the Trade-off between Water Permeation and Mechanical Strength of Ceramic Membrane Supports by Interfacial Engineering.

Porous ceramic membrane supports with high mechanical strength and permeation are required for highly permeable ceramic membranes. The water permeation of a ceramic membrane support is largely dependent on its level of open porosity, which can be however generally detrimental to the mechanical strength. In this work, low-cost kaolin nanoflakes were rationally composited with coarse alumina particles, and multichannel flat-sheet ceramic supports were successfully fabricated by extrusion and subsequent partial sintering. The macroscopic properties, microstructure characteristics, permeability, and mechanical strength of the ceramic membrane supports were systematically established and comprehensively studied. The incorporation of kaolin nanoflakes effectively reduced the sintering temperature to about 1200 °C. An interesting evolution of the pore structure was evidenced with the increase in sintering temperature. Interestingly, the porous ceramic supports prepared at 1400 °C with a nominal pore size of 1.47 μm showed the highest water permeability of 9911.9 ± 357.5 LMHB, and at the same time the flexural strength reached 109.6 ± 4.6 MPa. The much improved permeability was attributed to the unique multilevel pore structures, and the enhanced flexural strength mainly originated from the strongly interfacial bonding, as evidenced by the trans-granular fracture behavior. Also, the ceramic membrane supports exhibited excellent chemical resistance and good removal efficiency for oily wastewater. This work highlights the significant role of interfacial engineering in simultaneously improving the water permeation and mechanical strength, thereby overcoming their trade-off in porous ceramic membrane supports.

Highly porous hybrid particle-fibre ceramic composite materials for use as diesel particulate filters

This paper describes an investigation into the potential of a novel type of ceramic composite for use as Diesel Particulate Filters (DPFs). Materials for this application need to be highly permeable to gas flow, capable of filtering and retaining fine particulate and be stable under demanding thermo-mechanical conditions. Novel materials have been produced, via a conventional blending, cold pressing and sintering route, containing coarse alumina particles and/or fine alumina fibres. Several thermal and mechanical properties are measured, for a range of fibre contents. Thermal Shock Resistance (TSR) has been investigated both experimentally and via evaluation of a TSR merit index, based on a combination of properties. This type of material shows promise in terms of thermo-mechanical stability, particularly with high fibre content. There are also indications that such composites would be suitable for DPF usage in terms of permeability levels and the filtration of fine particulate.

Effect of Surface Modification on In-Depth Transformations and Flexural Strength of Zirconia Ceramics.

Chairside surface adjustments of zirconia dental restorations enhance the toughening stress-induced tetragonal-to-monoclinic phase transformation and domain reorientation by ferro-elastic domain switching (FDS), but also trigger subsurface damage, which could compromise long-term clinical performance. The purpose of this study was to assess the depth of phase transformation, associated FDS, and flexural strength of dental zirconia (BruxZir HT 2.0), after chairside surface treatments. Square specimens were sectioned from CAD/CAM blocks and sintered according to manufacturer's recommendations (n = 30). They were left as-sintered (AS; control), air abraded with fine (AAF) or coarse (AAC) alumina particles, ground (G) or ground and polished (GP). Roughness was measured by profilometry. Crystalline phases were investigated by grazing incidence X-ray diffraction (GIXRD) (n = 3). GIXRD data were fit using semi-log regression protocols to assess transformation depth and extent of FDS. The mean biaxial flexural strength was measured according to ISO 6872. Subsurface damage was assessed from SEM images using a bonded polished interface configuration. Flaw distribution was assessed by Weibull analysis. Results were analyzed by Kruskal-Wallis with Tukey's adjustment for multiple comparisons (p < 0.05). Air-abraded and ground groups exhibited higher mean surface roughness than control. AAF group exhibited the highest flexural strength (1662.6 ± 202.6 MPa) with flaw size (5.9 ± 1.8 μm) smaller than transformation (14.5 ± 1.2 μm) or FDS depth (19.3 ± 1.1 μm), followed by GP group (1567.2 ± 209.7 MPa) with smallest FDS depth (9.3 ± 2.0 μm) and flaw size (2.6 ± 1.8 μm), but without m-phase. AAC group (1371.4 ± 147.6 MPa) had the largest flaw size (40.3 ± 20.3 μm), transformation depth (47.2 ± 3.0 μm) and FDS depth (41.2 ± 2.2 μm). G group (1357.0 ± 196.7 MPa) had the smallest transformation depth (8.6 ± 1.5 μm), and mean FDS depth (19.8 ± 3.7 μm) and flaw size (18.6 ± 3.1 μm). AAC and AAF exhibited the highest Weibull modulus (11.2 ± 0.4 and 9.8 ± 0.3 μm, respectively). Variations in mean biaxial flexural strength were explained by the balance between the depth of toughening mechanisms (phase transformation and FDS) and subsurface damage. AAF and GP groups were the most efficient surface adjustments in promoting the highest mean biaxial flexural strength.

The influence of austenitizing temperature on prior austenite grain size and resistance to abrasion wear of selected low-alloy boron steel

This study forms part of the current research on modern steel groups with higher resistance to abrasive wear. In order to reduce the intensity of wear processes, and also to minimize their impact, the immediate priority seems to be a search for a correlation between the chemical composition and structure of these materials and their properties. In this paper, the correlation between prior austenite grain size and wear resistance to abrasion were researched. The growth of austenite grains is an important factor in the analysis of the microstructure, as the grain size has an effect on the kinetics of phase transformation. The wear behavior has been investigated using GOST (Russian) standard T-07 wear test equipment in which the steel samples were worn by coarse alumina particles (grit size #90). The wear of the steels was evaluated by weight loss and their wear mechanisms were investigated using scanning electron microscopy. The abrasive wear mechanisms for analyzed low-alloy steels with boron additions were micro-cutting, micro-chips removal and micro-plowing. After the test it was stated that with an increase in austenite grain size, the intensity of wear increases, and the resistance to abrasive wear reduces, both in a linear manner.

Mechanical properties of rolled A356 based composites reinforced by Cu-coated bimodal ceramic particles

Abstract Three kinds of A356 based composites reinforced with 3 wt.% Al2O3 (average particle size: 170 μm), 3 wt.% SiC (average particle size: 15 μm), and 3 wt.% of mixed Al2O3–SiC powders (a novel composite with equal weights of reinforcement) were fabricated in this study via a two-step approach. This first process step was semi-solid stir casting, which was followed by rolling as the second process step. Electroless deposition of a copper coating onto the reinforcement was used to improve the wettability of the ceramic particles by the molten A356 alloy. From microstructural characterization, it was found that coarse alumina particles were most effective as obstacles for grain growth during solidification. The rolling process broke the otherwise present fine silicon platelets, which were mostly present around the Al2O3 particles. The rolling process was also found to cause fracture of silicon particles, improve the distribution of fine SiC particles, and eliminate porosity remaining after the first casting process step. Examination of the mechanical properties of the obtained composites revealed that samples which contained a bimodal ceramic reinforecment of fine SiC and coarse Al2O3 particles had the highest strength and hardness.

Abrasive wear characteristics of selected low-alloy boron steels as measured in both field experiments and laboratory tests

This investigation compares the results of field tests of plowshares with results from a dry sand-rubber wheel laboratory abrasion test of selected low-alloy boron steels. After field test of plowshares made of selected low alloy boron steels the wear behavior of these steels has been investigated using a GOST (Russian) standard T-07 wear test equipment in which the steel samples were worn by coarse alumina particles (grit size #90). The wear of the steels was evaluated by weight loss and their wear mechanisms were investigated using scanning electron microscopy. The two predominant abrasive wear mechanisms for the low-alloy steels with boron additions were micro-cutting and micro-plowing. Worn surfaces showed traces of pitting originating from the removal of larger particles. Considering both field tests and dry sand-rubber wheel tests, the highest and lowest resistance to abrasive wear occurred within the same material grades. The results obtained during the wheel-rubber wear laboratory test may be used on the basis of a comparison of abrasive wear resistance of materials with similar structure. In this case such tests can replace expensive and time consuming field tests.

Effect of particle size distribution of alumina on strength of glass-infiltrated alumina

Glass-infiltrated alumina composites were prepared by infiltrating glass into a pre-sintered alumina. Three different alumina preforms were obtained from various combinations of fine and coarse alumina particles. After infiltration of glass into the porous alumina preforms, their microstructure and strength were studied. The highest bending strength of 510MPa was observed when the composite was made by mixing coarse and fine alumina powders at a ratio of 6:4. The infiltrated glass corroded the alumina preform, and the dissolved aluminum ions reprecipitated on the alumina grains during the heat-treatment for infiltration.

Temperature sensitive polymers as efficient and selective flotation collectors

This study investigates both the efficacy and selectivity temperature responsive of poly(N-isopropylacrylamide) PNIPAM as a flotation collector. The results of this study showed that charged poly(N-isopropylacrylamide) polymers selectively increased both the probability particle/bubble attachment as well as the floatability of coarse alumina and quartz particles respectively. These results provide a basis for considering PNIPAM as both an effective and selective collector in a flotation system. Coupled with previous studies that demonstrate the efficacy of PNIPAM as a flocculant in mineral suspensions, these results clearly demonstrate the potential use of PNIPAM as a dual function reagent which acts as both selective flotation collector and flocculant in the flotation of ultrafine particles.

Permeation and Blockage of Fine Particles Transported by Updraft through a Packed Bed

The fines generated in iron making blast furnace bring about the unsteady or unstable gas/liquid permeability and solid motion. Fundamental experiment was carried out to investigate the inhomogeneous phenomena such as flow channel blockage by fine particles transferred by updraft through a fixed packed bed. The alumina sphere and coke were used for packed bed and fine particles transferred, respectively. Conclusion of this study is as follows. The principal factor controlling a criterion for the blockage is a particle diameter ratio of coke to alumina particle, and fines concentration is not a primary factor. Consideration on a hydraulic equivalent diameter of a regular packing model of spheres is effective to predict the critical diameter ratio and it resulted in good agreement with the experimentally determined. Locally blocked mass, that is, a cluster consisting of coarse alumina particles and fines through which gas is incapable of flowing is dispersed in the bed. The void fraction of packed bed being equivalent to the pressure drop, calculated from Ergun's equation, is considerably small compared with the initial value when the local blockage is in existence. Fraction of the local blockage calculated from the void fraction and static holdup of fines is 15–20% as far as the present experimental condition.

Tensile and fatigue fracture of nanometric alumina reinforced copper with bimodal grain size distribution

Alumina dispersion-strengthened copper was produced by an internal oxidation process and hot powder extrusion method. The microstructure of the composite consisted of fine-grained region with an average grain size of 1.1 ± 0.1 μm, coarse-grained region with an average grain size of 5.6 ± 0.1 μm, nanometric alumina particles (γ-type) with an average diameter of 30 nm, and coarse alumina particles (350 nm) at the boundaries of the large grains. The tensile and fatigue fracture of the composite was studied in the extruded condition and after 11% cold working. The low cycle fatigue behavior was examined in strain control mode ( ɛ = 0.5%) under fully reverse tension-compression cycle at 1 Hz up to 1000 cycles. High cycle fatigue was conducted using rotate-bending test up to 10 7 cycles. To reveal the role of alumina particles and the bimodal grain size structure, the tensile and fatigue fracture of extruded copper with normal grain structure was examined. It is shown that: (1) the strength and hardness of the nanocomposite are about three folders of the magnitude of the unreinforced copper; (2) fatigue strength of the nanocomposite at 10 7 cycles is about 40% higher than that of the copper; (3) both extruded copper and the nanocomposite exhibit cyclic hardening during the early stages of low cycle fatigue test which eventually approaches to a saturation stress; (4) dimple-type fracture is realized on the fracture surface of the tensile tests where dimple size decreases in the presence of the alumina particles; (5) the fatigue fracture mechanism is locally ductile deformation, microscopic void formation and coalescence.

Non-Equilibrium Hydrogen Production from Ethanol Using CO2 Absorption Ceramic and Precious Metal Catalysts

Non-equilibrium H2 production from ethanol was achieved with precious metal catalysts using a CO2 absorption ceramic. The ceramic consisted of lithium silicate, which was granulated and coated with coarse alumina particles. H2 production in steam reforming of ethanol was enhanced and CO production was suppressed significantly in the presence of the CO2 absorption ceramic. The optimum temperature region for non-equilibrium H2 production was 500–550°C. For example, at a temperature of 500°C, at atmospheric pressure and with a 1 wt% Rh/CeO2 catalyst, the H2 concentration was 91 mol%-dry, which was considerably higher than that at the chemical equilibrium of 63 mol%-dry, and it was 1.4 times higher than that obtained without the CO2 absorption ceramic. A highest H2 concentration of 96 mol%-dry was obtained at 550°C. The CO concentration decreased dramatically at 500°C from 1.5 mol%-dry to less than the detection limit, which was about 1 × 10–2 mol%-dry. It was markedly lower than that of the chemical equilibrium. Similar results were also obtained with a 1 wt% Pt/CeO2 catalyst. At 500°C in particular, the concentration of H2 was fairly high at 89 mol%-dry and the CO concentration was lower than the detection limit. Carbon deposition was not observed visually in each case. It is concluded that non-equilibrium H2 production from ethanol with a precious metal catalyst in the presence of the CO2 absorption ceramic has a high potential for application to a non-equilibrium reactor for proton exchange membrane fuel cell (PEMFC) systems. A markedly low concentration of CO and a fairly high concentration of H2 will contribute to simplifying the complicated system configuration and to improving PEMFC performance, respectively.

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.

Hydrogen Production from Ethanol Using a CO2 Absorption Ceramic and Base Metal Catalysts

With the aim of developing a non-equilibrium reactor with a CO 2 recovery function for proton exchange membrane fuel cell (PEMFC) systems and other applications, steam reforming of ethanol was performed in a preliminary study in the presence of a CO 2 absorption ceramic at atmospheric pressure in a temperature range of 450-650°C using a plug flow reactor and commercially available reforming catalysts. The CO 2 absorption ceramic consisted of lithium silicate powder, which was granulated and coated with coarse alumina particles. Reaction products were analyzed by gas chromatography. The effects of the CO 2 absorption ceramic on hydrogen selectivity and species concentrations were estimated. It was concluded that steam reforming ethanol in the presence of the CO 2 absorption ceramic has high potential for application to a non-equilibrium reactor. Hydrogen production was enhanced and CO and methane production was suppressed significantly in the presence of the CO 2 absorption ceramic. For example, at a temperature of 500°C and with a commercial FCR-4-02 catalyst, hydrogen selectivity was 1.3 times higher than that in the absence of the CO 2 absorption ceramic, and the hydrogen concentration was 95 mol%-dry. This hydrogen concentration was considerably higher than that at chemical equilibrium, which was 63 mol%-dry. The methane concentration decreased from 10 to 4.2 mol%-dry, and the CO concentration decreased dramatically from 2.0 mol%-dry to less than the detection limit, which was about 100 ppm. The CO and methane concentrations were considerably lower than those at chemical equilibrium, which were 3.4 and 11 mol%-dry, respectively. These results indicate the potential for constructing a fuel processor system, which will not need either a high-temperature shift reactor (HTS) or a low-temperature shift reactor (LTS), for PEMFC applications.

Particle design and evaluation of dry [formula omitted] recovery sorbent with a liquid holding capability

For CO 2 capture at high-temperature/high CO 2 partial pressure conditions a new dry CO 2 removal sorbent was developed based on lithium silicate with additives to accelerate CO 2 sorption. To avoid adhesion and defluidization associated with the formation of the liquid carbonate phase in the temperature range of 500 – 600 ∘ C , the sorbent particles were designed whereby the granulated lithium silicate core was coated with a layer of coarse alumina particles. This prevents the liquid lithium carbonate from wetting the sorbent surface. Several different types of sorbents were manufactured by granulation and coating with different ceramic sol binders and different prescriptions in a tumbling fluidized bed. Minimum fluidization velocity, fluidizing behavior and adsorption/desorption properties of the sorbent particles were determined in a laboratory scale fluidized bed reactor (i.d., 0.03 m) in the temperature range up to 1073 K. During a repeated cyclic adsorption/desorption, a gradual decrease in both the overall conversion and maximum adsorption capacity took place. Nevertheless, the performance obtained for the sorbent manufactured with alumina sol was found to be the most successful one characterized by no defluidization and a high adsorption capacity of 50% in conversion (10 wt% including the coat layer) over 60 cycles.

Recrystallized grain size prediction in a particulate reinforced metal matrix composite

The recrystallized grain size following cold rolling and annealing of an Al alloy (AA2014) and a particulate reinforced AA2014 composite was investigated. The composite contained 20 vol% alumina particulates of average diameter 15 μm. The recrystallized grain size in the composite was finer than in the alloy, for a given set of conditions; this was most notable for material strained less than 50% cold-work. This behaviour was attributable to a higher nucleation efficiency in the matrix adjacent to coarse alumina particles in the composite. A model was presented for the composite to predict the recrystallized grain size as a function of strain, with respect to the size distribution and number density of alumina particles. This model predicted the strain dependence of the recrystallized grain size and, in particular, the grain size insensitivity to strain at moderate-to-high levels of cold-work.

Influence of Aluminum Titanate Formation on Sintering of Bimodal Size‐Distributed Alumina Powder Mixtures

The sintering behavior of green compacts, in which coarse alumina particles formed a skeletal structure and fine alumina and/or fine titania particles filled the voids of the skeletal structure, was investigated. Sinterability of the green compacts changed according to the titania content in the fine powder. The titania content of 33 mol% was the most effective for the densification. The volume expansion due to the aluminum titanate formation occurred in the voids of the skeletal structure, and the densification of the skeletal structure progressed more because the grain growth between the fine and coarse alumina particles did not proceed. As the titania content decreased, the densification did not progress more than that of the compact with 33 mol% titania content because the grain growth proceeded more. As the titania content increased, the expansion of the compacts was larger, and large grains were formed by the reaction between the titania and coarse alumina particles. Therefore, densification became difficult.

Porous ceramic lamellae for orthodontic ceramic brackets: part I: fabrication and characterization.

This study was undertaken to test a new and original orthodontic bracket base, consisting of a porous lamella, which was designed to facilitate removal of ceramic brackets from the enamel surface after treatment. In the phase of the study presented here, porous pellets were made by bonding coarse alumina particles (calcined or fused) with mullite, formed by their reaction with fine quartz particles during firing of the mixture at 1700 degrees C. After machining the pellets to the desired shape, nine types of lamellae with different porosities were attached to the brackets using two different adhesive resins, and also bonded to bovine enamel using the same adhesives. The tensile bond strengths for the assemblies were determined so that the lamellae and the bonding adhesive that might be suitable for clinical application could be selected for testing in vitro in the second phase of the study.

Influence of Zirconia Addition on the Sintering Behavior of Bimodal Size Distributed Alumina Powder Mixtures

Fine zirconia powder was added to the bimodal size distributed alumina powder mixtures and the influence of zirconia addition on the sintering behavior of the powder mixtures was investigated. At low firing temperature, that is, in the initial stage of sintering, zirconia retarded the densification of the powder mixtures and also enlarged the pore size of the compacts. At high firing temperature, the influence of zirconia addition was different depending on the packing structure of the green compact. When the coarse alumina particles were dispersed in the matrix of the fine particles, zirconia promoted the densification of the mixtures. This result was explained by the promotion of the densification between the fine and the coarse alumina particles in the intermediate stage of sintering and by the migration of zirconia and the hindrance effect of zirconia on the grain growth of alumina in the final stage of sintering. When the coarse alumina particles formed a skeletal structure, zirconia did not promote the densification of the mixtures. This result was explained by the dependence of the densification of the compacts only on the densification among the coarse alumina particles.