Alumina Grains Research Articles

Leveraging Grain Size Effects on Hydrogen Generated via Doped Aluminum–Water Reactions Enabled by a Liquid Metal

Through the aluminum–water reaction, hydrogen is densely stored and generated on-demand. To enable the reaction, we harness eutectic gallium–indium which permeates through aluminum’s grain boundaries, disrupting aluminum’s oxide layer and inhibiting further passivation of the aluminum grain surfaces. We find that, in addition to doping, grain refining and grain coarsening offer a latitude in engineering aluminum microstructures to tune hydrogen generation rates and reaction efficiency. In Mg-doped, Mg2Si-doped, and pure aluminum, reducing the grain size from an as-cast structure to around 30 μm increases the rate of hydrogen flux by an order of magnitude. Mild silicon doping at 0.6 wt % significantly increases the reaction rate in which the same grain size reduction increases hydrogen flux rates by two orders of magnitude. Additionally, the byproduct of the aluminum–water reaction varies with both doping and grain size. Results of combined grain size and doping effects on hydrogen evolution rates allow for further exploration into the underlying reaction mechanism in the presence of a liquid metal.

Graphene doped spray dried ceramic nano oxides for high capacity battery electrodes

Electric vehicles or portable electronic devices have come to rely heavily upon electrochemical devices, such as rechargeable batteries with optimum charge discharge characteristics, current ratings, charge-discharge rate (rate capability), cyclability etc. to perform under the expected service conditions. One of the goals of a rechargeable battery materials researcher is to fabricate materials to realize solid-state batteries with high reliability and lithium–air batteries with ultimate capacities. Most of the materials although possess high theoretical energy density values: invariably suffer from inferior cyclic performance. The performance of these batteries is guided by the electrodes within these devices which in turn depend upon the materials used to fabricate them. Chemical composition and its uniformity, consistency in microstructural features, and adequate choice of various layers that may be in the form of coatings to be overlaid on the base materials mostly comprised of ceramic oxides such as oxides of Li doped with niobates, manganates, vanadate etc. with carbon or graphene coated over layers to provide with the suitable interfacial conductivity as electrode materials in Li-ion batteries. The interfacial layers and the mechanism of interfacial phenomena encompassing the grains play a significant role in determining the performance. Optimum microstructure is obtained by choosing the right processing equipment and spray drying the composition in slurry form provides the most optimum solution. Further, spray drying offers high potential for a transfer from a lab scale technology to industrial level extrapolation. In this paper, nano graphene has been spray dried along with nano alumina grains in water media and polyvinyl alcohol binder to ascertain the free flowability, consistency in formation of graphene over layer on alumina grains as well as uniformity in graphene on alumina composition. The free flowing spray dried graphene coated alumina powders were analysed via SEM, EDS and XRD and results are presented. Additional information based on a review conducted on published information on most popular compositions in terms of electrode materials such as in Li-ion, sodium-ion etc have also been included. In the review section the rapidly increasing literature on spray drying of solutions and suspensions are also included.

Revealing the Mechanisms for the Nucleation and Formation of Equiaxed Grains in Commercial Purity Aluminum by Fluid-Solid Coupling Induced by a Pulsed Magnetic Field

This study considers the factors controlling the grain structure of commercially pure aluminium when a Pulsed Magnetic Field (PMF) is applied during solidification. It is revealed that PMF of pure metal forms equiaxed grains by two different nucleation and growth mechanisms depending on the casting conditions. One mechanism is nucleation of the columnar grains that grow into a shell on the walls of the mold before PMF is applied and once applied fluid-solid coupling develops due to the Lorenz force. Depending on the shell thickness, this fluid-solid coupling detaches the columnar shell or, for thicker shells, only the top few millimeters of the shell from the mold wall. The detached shell is then fragmented into large blocky grains due to a lower melting point iron-rich liquid phase on the grain boundaries. The other mechanism occurs on the mold wall when PMF is applied from above the melting point or on the exposed mold wall after detachment of the initially formed solidified shell as in the case above. In these conditions copious nucleation occurs on the mold wall and these fine grains are detached by pulses occurring every few milliseconds creating a bimodal structure of refined grains between the much fewer large blocky grains. The optimal condition for uniform refinement is when PMF is applied from above the melting point ensuring that a refined equiaxed grain structure forms throughout the casting.

Strontium hexaaluminate formation in alumina and alumina–zirconia matrixes

In this study, the formation mechanism of strontium hexaaluminate (SrAl12O19) in Al2O3–SrO and Al2O3–SrO–ZrO2 composites was investigated. Strontium oxide (SrO) powder was added to initial suspensions in various quantities. High-purity submicron powders were used during the experiments. Powder compositions were thermally treated in the temperature range of 900°С–1500°С for 5 h. The stepwise character of SrAl12O19 formation was established by X-ray phase analysis. The SrAl2O4 intermediate compound formed in the temperature range of 900°С–1200°С. The onset of SrAl12O19 formation corresponded to 1200 °C. Formation of SrAl12O19 was completed in the temperature range of 1400°C-1500°С. After sintering at 1300°С–1500°С for 1 h, the samples were analysed. The density of materials indicated a decreasing trend with an increase in the amount of SrO. The density of alumina ceramics sintered at 1500 °C was 3.65 ± 0.2 g/cm3. However, alumina ceramics comprised 15 wt% SrAl12O19 and their density was 3.35 ± 0.4 g/cm3. A decrease in the density level is explained by the platelet shape of SrAl12O19 grains, which prevented additional contact points from generating between the matrix grains and led to porosity formation in the materials. The effect of a reduction in the alumina grain size was established with an increase in the content of the initial SrO and SrAl12O19 powders. Scanning electron microscopy found that the SrAl12O19 platelets comprised one to three layers fused together along the basal planes. Zirconia grains inhibited the lateral growth of the SrAl12O19 platelets.

Development and evolution of a novel (Zr1-xSnx)O2 toughened alumina-mullite slip cast refractory: Effect of SnO2

Zr1-xSnxO2 reinforced alumina-mullite refractory was manufactured by slip cast methods using SnO2 as a sintering agent. SnO2 had considerable influence in the enhanced reaction kinetics of zircon dissociation and subsequent reaction sintering of alumina and zircon. Presence of pro-eutectic transient Al2O3-SnO2 liquid and SnO2-SiO2-ZrO2 amorphous phases, enhanced densification and mullitisation through liquid phase sintering and, mitigated SnO2 volatilisation by (Zr1-xSnx)O2 solid-solution formation. Based on their morphology and aspect ratios, three types of mullite crystals, MI, MII and MIII, were evolved across the matrix microstructure. Characterization of the evolved microstructure revealed coalescence and grain growth of matrix alumina grains including the presence of an acicular tertiary MIII mullite which acts as both a reinforcement and bridging network of matrix to aggregate grains. SnO2 had the effect of lowering the monoclinic – tetragonal phase transformation temperature. Hot flexural strength values remained nearly unchanged (±2%) compared with the AZS composition without SnO2 doping.

Investigation of strain hardening near grain boundaries of an aluminum oligocrystal: Experiments and crystal based finite element method

Grain boundary mechanics plays a key role in the strengthening of polycrystals. In this study, deformation of aluminum grains is investigated to better understand the origins of strengthening mechanisms near the grain boundaries. For this reason a sample with nearly columnar structure was prepared and fully characterized by electron back scattered diffraction measurements using the two side surfaces of the sample before loading. The total strain distribution on one surface of the sample is measured during tensile loading using the digital image correlation technique. The numerical studies include implementation of crystal based finite-element model to understand the origin of grain boundary strengthening using both local phenomenological, local dislocation density based and non-local strain gradient based constitutive laws. The effects of mesh refinement, through thickness discretization, boundary condition, and orientation scatter are investigated. Comparison of experimental and simulated total strain distributions near grain boundaries suggests the existence of strain gradient hardening. For this reason, a novel non-local flux based model is developed and implemented with a consistent time integration scheme. Dislocation density based models reveal strain distributions that are in better agreement with the experimental results than the phenomenological model that was not able to capture grain boundary strengthening and experimental strain distributions. The non-local strain gradient based strain hardening law, which does not require separate definition of grain boundary elements, was successful in capturing the strengthening effect near grain boundaries.

Structure and mechanical properties of ceramic materials based on alumina and zirconia with strontium hexaaluminate additives

In this study, the effect of a small amount of strontium hexaaluminate (3 wt %) additives on the properties of alumina- and zirconia-based ceramics is discussed. α-Al2O3, 3Y-ZrO2 and SrO submicron powders are used as the starting materials. Specimens are prepared by water suspension dispersion in a bead mill, spray drying granulation, isostatic pressing and pressureless sintering. Structural studies are conducted using a scanning electron microscope. The Archimedes approach is employed to determine the density and porosity of the specimens. Microhardness, flexural strength at three-point bending and fracture toughness are determined by the single-edge V-notch beam (SEVNB) method and indentation. A material containing 50-wt. % Al2O3 and 50-wt. % ZrO2 exhibited the most rational combination of characteristics such as strength (950 ± 50 MPa), microhardness (1800 HV) and fracture toughness (6.5 MPa m1/2). The SEVNB fracture toughness tests showed that the highest recorded value of a critical stress intensity factor was 9 MPa m1/2 for specimens containing 47-wt. % Al2O3, 3-wt. % SrAl12O19 and 50-wt. % ZrO2. The length of strontium hexaaluminate platelets decreased with an increase in the zirconia content in the materials and reached 0.8–2.6 μm. A pronounced fragmentary structure is characteristic of the strontium hexaaluminate platelets. The platelets under study comprised one or two boundary-separated blocks with a thickness of ~150–200 nm. Each block is divided into smaller fragments. The structure of strontium hexaaluminate platelets affects the crack propagation trajectory. The identified mechanisms of crack propagation are as follows: transcrystalline and intercrystalline fractures of alumina grains, crack deflection from the propagation trajectory, crack branching and failure of SrAl12O19 platelets.

Novel Alumina Compounds with Niobia, Silica and Magnesia for Ballistic Armor

Compounds of alumina APC-2011 SG with niobia, silica and magnesia were sintered at 1400°C/3h, in order to evaluate their ballistic performance. The content of niobia (Nb2O5) was 4.0 to 8.0 wt.%; silica (SiO2) was 0.8 wt.%; and magnesia (MgO) at 0.15 wt.%. Sintered samples were characterized by hardness, densification and energy absorption at impact. Ballistic tests were performed by ceramic discs firmly glued to steel plates and then subjecting the target to impact using 7.62 mm projectile shot from a rifle with a throw distance of 5 m. The energy absorbed by the disintegration of the ceramic discs was estimated by the residual velocity. The fracture surfaces of the samples were observed by scanning electron microscopy. It was verified for the first that the addition of high amounts of Nb2O5 to the alumina with the presence of SiO2 and MgO increases the presence of vitreous phase in the contours of alumina grains and improved the properties of the material for use in ballistic protection.

Effect of post-weld heat treatment on double-sided friction stir welded joint of 120 mm ultra-thick SiCp/Al composite plates

Friction stir welding (FSW) is a very promising and efficient technique in joining aluminum matrix composites (AMCs) without clear drawbacks, and post-weld heat treatment (PWHT) is a viable option to improve the mechanical properties of friction stir welded joint for heat treatable AMCs. In this work, 120 mm ultra-thick plates of 16 vol% SiCp/2014Al composites were successfully joined by double-sided FSW and subsequently friction stir welded joints were subjected to four different PWHTs (T6–25, T6–70, T4–25 and T4–70, denoting T6 or T4 heat treatment and being quenched in water at 25 °C or 70 °C after solution during heat treatment), respectively. The effect of PWHT on the distributions of microstructures and mechanical properties along the thickness direction in the nugget zone (NZ) of friction stir welded joint is investigated. For as-FSW joint, the tensile strength first decreases gradually along the thickness direction in the shoulder-affected and pin-affected regions, and then increases in the overlapping region. For T6–25 joint, the tensile strength does not vary a great deal in the shoulder-affected and pin-affected regions, and then increases slightly in the overlapping region. After T6–25 PWHT, the finer precipitates θ' are detected, aluminum grains grow up, and the tensile strength is obviously improved both in the NZ and base material attributing to the precipitation strengthening effect from finer precipitates θ'. The distributions of microstructures and mechanical properties along the thickness direction in the NZ are changed from heterogeneous distribution for as-FSW joint to nearly homogeneous distribution for T6–25 joint except in the overlapping region. After different PWHTs, the T6–25 joint shows the highest Vickers hardness in the NZ, followed by T6–70, T4–25 and T4–70 joints, and the as-FSW joint shows the lowest Vickers hardness. A comprehensive understanding can be provided for the effect of PWHT on the double-sided friction stir welded joint of thick or ultra-thick metal matrix composite plates.

Influence of codoping with Hf and La on grain‐boundary transport in alumina

AbstractThe role of reactive elements (RE) is an important topic in understanding the oxidation behavior of high‐temperature alloys. In this work, the influence of codoping alumina with two different RE elements (500 ppm Hf + 500 ppm La) was studied. The kinetics of oxygen grain‐boundary (GB) transport were studied at 1400°C using metallic nickel particles as markers. The results were compared with data obtained on the corresponding singly doped compositions; alumina‐500 ppm La, and alumina‐500 ppm Hf. The results showed that singly doping with La did not have any benefit compared to undoped alumina, whereas singly doping with Hf resulted in a slowing of transport by a factor of ~7. The behavior of the codoped sample was very similar to that of the singly doped Hf composition. For all the studied compositions, atomic scale characterization using high‐angle annular dark‐field scanning transmission electron microscopy and atom probe tomography (APT) revealed strong segregation of the dopant ions to the alumina grain boundaries. In the codoped sample, APT revealed evidence of oxygen excess and aluminum depletion at the GB core.

Investigation on microstructure and fracture mechanism of n-SiCp/7075 Al composites

In the present study, the composites have been prepared using vacuum hot pressing process technique. The influence of 2.0 wt.% nanometric SiC particles (n-SiCp) on the microstructure and fracture mechanism of 7075 aluminum alloy was studied by means of metallographic microscope, scanning electron microscope (SEM), energy-dispersive spectrometer (EDS), X-ray diffraction (XRD), laser scanning confocal (LSC). The results indicate that SiCp fill at grain boundaries dispersing evenly and nail to aluminum grains which sizes have been reduced successfully, that making the organization tighter, the gaps fewer and the surface rougher. Meanwhile, phases (MgZn2) precipitated from the 7075 aluminum alloy were been detected. The grain bonding states of matrix alloys are strengthened by the combination of fine grains strengthening and dispersion strengthening and thus affect the fracture morphology. The micro-fracture mechanism of 7075 aluminum alloy is micro-porous aggregation fracture, however, the 7075 aluminum matrix composites (AMCs) are quasi cleavage fracture.

Coating of Ca(OH)2 / γ-Al2O3 pellets with mesoporous Al2O3 and its application in thermochemical heat storage for CSP plants

Thermochemical heat storage using the Ca(OH)2 dehydration/hydration reaction for CSP plants requires the development of advanced materials that avoids the breakage shown by conventional CaO pellets. In this work, spherical cores made up of pelletized 60 wt % Ca(OH)2 and 40% γ-Al2O3 powders were coated with different mesoporous alumina coverings by means of a dip-coating route, in order to enhance its structural integrity for dehydration/hydration cycles. Three strategies of preparation were devised using as coatings alumina gels containing cetyltrimethylammonium bromide as surfactant in neutral (A2 CS) and acid pH (A4 CS) and without surfactant in neutral medium (A3 CS). In both A2 CS and A4 CS materials, denser alumina coatings were attained originating higher crushing strengths, pointing out the positive role played by the surfactant in the gel makeup. SEM images indicated that A2 presented small alumina grains (0.3–3 μm) highly dispersed over their surface, while over A4 CS, long sheets were appreciated. All the materials exhibited high hydration capacities after 10 cycles (>80%, >800 kJ/kg) at 250 °C, but only A2 CS hold its structural integrity that was preserved even after 20 cycles without loss of its hydration capacity.

Tensile deformation behavior and strengthening mechanism in graphene nanoplatelet reinforced bimodal grained aluminum nanocomposite synthesized by spark plasma sintering and hot rolling

Present research work demonstrates utilization of a unique combination of consolidation techniques, spark plasma sintering and hot rolling, to fabricate bimodal Al-graphene nanoplatelet (GNP) nanocomposite, possessing excellent amalgamation of strength and ductility. Bimodal Al-GNP nanocomposites comprised of nanocrystalline and microcrystalline Al (10, 20, 30, 40, 50 wt%) with 0.5 wt% functionalized GNPs were synthesized. Rolled Al-GNP nanocomposite containing 30 wt% microcrystalline Al exhibited 54% higher yield strength than rolled microcrystalline Al-GNP and 103% higher elongation than rolled nanocrystalline Al-GNP. A theoretical calculation of yield strength of the bimodal nanocomposite was carried out considering the different strengthening mechanisms viz. load transfer mechanism, grain refinement, and coefficient of thermal expansion mismatch. The load transfer mechanism contributed maximum (75%) towards strengthening in rolled 30 wt% microcrystalline bimodal Al-GNP nanocomposite. Microcrystalline Al compact fractured in ductile mode and the nanocrystalline counterpart exhibited brittle mode of failure. On the other hand, the bimodal nanocomposite showed mixed mode of fracture, ductile in microcrystalline Al regions and brittle in nanocrystalline Al regions.

Behaviors of ZTA (zirconia toughened alumina) reinforced iron composites under impact abrasive wear conditions

This paper reports our investigation of the behaviours of ZTA (zirconia toughened alumina) particle reinforced iron composite materials (ZTAp/Fe45) during impact abrasive wear tests. The samples were produced by vacuum sintering and tested on an MLD-10 Impact Wear Rig using Fe45 as comparative material. The results show that most ZTAp/Fe45 composites have higher impact wear resistance than Fe45 matrix material due to the impeding role of ZTA particles against the impact of SiO2 abrasives. The wear resistance of ZTAp/Fe45 composites firstly increases and then decreases with the increase of particle size of ZTA, achieving best performance when the ratio between average sizes of the ZTA particles and SiO2 abrasives approaches 0.5. The wear mechanisms of Fe45 matrix material are mainly micro ploughing, impact plastic deformation and fatigue induced flaking. The failure process and mechanism of the ZTA particles are that cracks nucleate and propagate preferentially along the phase boundaries between the alumina and zirconia, resulting in micro peeling-off in the double phase mixture zone and eventually the delamination of alumina grains.

Mechanism of Variation in High-Temperature Grain Stability of Aluminum in Dissimilar Friction Stir Welds

Abstract In the dissimilar Friction Stir Welding (FSW) of aluminum to titanium, a large fraction of titanium particles is inhomogeneously distributed in the weld nugget and their distribution is highly complex. Such a distribution can have an immense influence on the grain stability of the weld nugget, which decides its mechanical properties at the high temperatures experienced in critical applications. The present investigation highlights the variation in grain structure at the top surface and center of the weld nugget. The results show that the microstructure at the surface of the weld contains a higher fraction of fine titanium particles, refined grains of aluminum and high-angle grain boundaries, and a lower intensity of shear texture components when compared to the center of the weld nugget. The variation in the grain stability of the weld was correlated with the qualitative variation in the strain rate and temperature in the weld. It is proposed that the formation and distribution of a high fraction of fine titanium particles results in superior grain stability of aluminum at the surface of the weld due to arrest of the grain boundary mobility against grain growth. This mechanism and methodology can be applied in developing metal matrix composites with superior mechanical properties as well.

On the Analysis of High-Temperature Mechanical Properties and Dry Sliding Tribological Behavior of Ultra-Fine Grained Boron Carbide-Reinforced Al5083-Based Trimodal Composites

Numerous research studies have been done on ultra-fine grained (UFG) aluminum and aluminum-based composites materials due to their higher tensile strength in comparison with coarse-grained (CG) bulks. However, the relationship between the mechanical behavior and dry sliding wear performance of ultra-fine grained Al5083-based boron carbide-reinforced trimodal composite bulks at room and high temperatures have been neglected. Furthermore, the effect of temperature rise on these properties seems critical since these materials have been used in several working and operating conditions. In the present paper, it is intended to study the impact of temperature rise on the mechanical behavior and wear resistance of ultra-fine grained Al5083-5wt%B4C trimodal composite bulks. Results revealed that increasing the temperature to 200 °C, did not have any significant effect on the hardness and strength of the Al5083-5wt%B4C bulk. However, significant changes occurred in the hardness and wear resistance of the composite samples by adding Al5083 coarse grains. In this case, severe plastic deformation and surface damage occurred in the Al5083-5wt%B4C-50%CG sample and the wear rate reached 9.7 × 10–7(mm3/m–N).

Wafer-scale high anti-reflective nano/micro hybrid interface structures via aluminum grain dependent self-organization

Surface anti-reflection (AR) with micrometer or nanometer scaled surface texture has been studied extensively and showed excellent light extraction (or harvest) performance in corresponding opto-electronic devices. In this paper, a 2-inch anodized aluminum oxide (AAO) template with nano/micro hybrid structure was obtained directly through a simple lithography-free aluminum anodization process for the first time. This new nano/micro hybrid structure combines the advantages of sub-100 nm sub-wavelength structure (SWS) and micrometer scaled surface roughening. Aluminum grains were found to rearrange the distribution of the penetrated surface current during the anodization and eventually induced the micrometer scaled fluctuation underneath. The ultra-thin AAO replication scheme was introduced to fabricate and study the spectrum (300 nm to 800 nm) dependent AR performance of the nano/micro hybrid PDMS structure, which obtained an AR enhancement fraction of more than 42% higher than that of the nano-patterned one. Thin film and GaN based light-emitting diode (LED, 450 nm) models were built and studied. Compared to the flat LED, the AR enhancement of the nano/micro structured one can reach as high as more than 3 folds. These results suggest the nano/micro hybrid structure could be a good alternative for the applications of surface AR structure.

Optical method for evaluation of shrinkage in two dimensions during drying of ceramic green bodies

The evolution of shrinkage and mass during drying of kaolin and alumina green bodies was characterized. Standard practise uses a position sensor with simultaneous mass measurement. In this work an optical method was developed to follow shrinkage in two orthogonal directions. Green body samples were made by pressing of ceramic pastes. Results have been compared through the well known Bigot curves which show two distinct stages. First a linear shrinkage with the evaporated water loss is observed. Then in the second stage with further water loss, negligible dimension changes occur. For each material, the overall shrinkage values are identical in the two directions parallel to the pressing axis, but higher in the pressing direction. This difference is related to the shaping method yielding a preferential orientation of grains. Anisotropy in shrinkage is more significant for kaolin with tabular shaped grains compared to the more isometric grains of alumina.

Finite element analysis of the interface defect in ceramic-metal assemblies: Alumina-Silver

The realization of a connection between a ceramic and a metal is always accompanied by the creation of a multi-axial stress field. Different physical and mechanical origins explain the genesis of stresses at the bonding interface in the ceramic and metal at the bond formation of ceramic-metal. In this study, a finite element model is developed to analyze numerically the effect of the sites of alumina grains snatches on the distribution and stresses level of the interface with the silver. The analysis has been extended to the effect of alumina grains-alumina grains interaction, alumina grains size and form.

Corrosion Resistance Properties of Porous Alumina–Mullite Ceramic Membrane Supports

Cost‐effective macroporous alumina–mullite membrane supports are fabricated by sintering coarse α‐alumina and colloidal silica, using cornstarch as the foaming agent at 1550 °C for 2 h in air atmosphere. To evaluate the corrosion resistance of such membrane supports, the corrosion behaviors of the supports before and after corroding in the H2SO4 and NaOH solutions (80 °C) are systematically investigated. Interestingly, with increasing Al2O3/SiO2 ratio, the support presents numerous alumina grains and little glass phases, resulting in excellent corrosion resistance especially in acidic media. Benefiting from the alumina grains and little glass phases among the grain boundaries, lower mass loss, less variations in pore size and porosity, high residual strength, and an almost unchanged gas flux are obtained for a higher Al2O3/SiO2 ratio support. The porous support, herein, is suitable for microfiltration membranes used in harsh environments.