Alumina Samples Research Articles

Measuring the Concentration of Freestream Species on a Hypersonic Transpiration-Cooled Stagnation Point

This paper presents direct surface concentration measurements of a transpiration-cooled stagnation point in hypersonic flow. Pressure-sensitive paint is employed on a porous alumina sample to measure the concentration of freestream species and thus how well the coolant mitigates mass diffusion from the freestream to the surface. Experiments are conducted at Mach 6.9 at three different pitot pressures: 10, 20, and 30 kPa. Porous alumina is chosen due to its ability to bond pressure-sensitive paint and its similar microstructure to porous ultra-high-temperature ceramics. Nitrogen, argon, and krypton are used as injection gases at mass flow rates ranging from 0.01 to , in order to displace up to 99% of the freestream gas at the surface. The experimental data show that transpiration cooling is more effective in displacing freestream gas than predicted by analytical models and numerical solutions. The microheterogeneous surface with recessed pores means that there is an additional pressure gradient within the first layer of pores.

Transparent Alumina Nanoceramics Prepared under High Pressure and Low Temperature

A transparent alumina ceramic with a transmittance of 71.7% at 640 nm is fabricated by sintering mixed metastable Al2O3 powder at 5.0 GPa/600 °C. The spherical starting particles are deformed and highly densified during compression, and transformed into α‐Al2O3 at 5.0 GPa/500 °C. High pressure suppresses the growth of transformed grains to produce alumina bulks with an average grain size of approximately 52 nm at 600 °C. In addition, the Hall–Petch effects combined with the pressure‐induced curvilinear grain boundary and Moiré fringe‐like defects enhance the transparent alumina sample to yield a Vickers hardness of 26.4 GPa (0.5 kgf). However, these microstructures do not enhance the fracture toughness of the transparent alumina ceramics.

Modelling textural and mass transfer properties for gamma‐alumina catalysts using randomly generated pore networks

AbstractA Monte Carlo approach is used to generate 2D and 3D networks of randomly connected cylindrical pores with a variety of configurations. These networks are created to represent the gamma‐alumina supports of hydrotreating catalysts. Textural properties from generated pore networks are compared with experimental values of porosity, specific surface area, and specific pore volume. The experimental properties were estimated using a helium pycnometer and nitrogen sorption isotherms for five gamma‐alumina samples. Simulated and experimental textural properties concur. Internal diffusion is simulated by 1D Fick diffusion within each pore of the network. A macroscopic diffusion parameter for vacuum distillate type molecules, previously obtained by inverse liquid chromatography and by pulsed‐field gradient nuclear magnetic resonance experiments on alumina samples, is predicted and confronted with experimental values. Diffusional properties are in good agreement when considering two hierarchically organized porous domains.

Ceramic paste for space stereolithography 3D printing technology in microgravity environment

Additive manufacturing techniques have demonstrated great potential in space manufacturing for long-duration human spaceflight and colonization on alien planets. Herein, we present the design and manufacturing of previously inaccessible complex ceramic components with high precision in a microgravity environment. The proposed approach is based on controlling the rheological properties of ceramic slurry in reduced-gravity. Briefly, HE-cellulose and carbomer 940 are added as thickening agents to transform the ceramic slurry into a ceramic paste(soft matter), which exhibits Bingham pseudoplastic behavior and is not sensitive to gravity variations during parabolic flight. Additionally, X-ray computed tomography (XCT) results reveal that the as-prepared alumina samples, 3D printed in a microgravity environment, render a higher density of 99.3% after sintering. The proposed route can be applied to fabricate complex-shaped ceramic components in space, enabling various possibilities of manufacturing structural and functional materials, including gradient materials, solid fuel cells, active insulation ceramic structures and piezoelectric sensors.

Size effects on the formation of LaAlO3 phase in La-doped γ-Al2O3 after hydrothermal treatment

A series of La-doped alumina ceramics were prepared by a strong electrostatic adsorption technique in comparison with a conventional incipient wetness impregnation method. The commercial La-doped alumina support was used as a reference sample. The samples were calcined in a temperature range of 800–1200 °C. The characterization of the ceramics was performed using low-temperature nitrogen adsorption, transmission electron microscopy, X-ray diffraction analysis, and photoluminescence. A strong electrostatic interaction between alumina and lanthanum was found to affect the stabilization of alumina and the resistance of La2O3 towards agglomeration during the wet procedures. The size effects related to the formation of LaAlO3 of ∼10 nm in size from the amorphous oxide phase take place on the surface of alumina doped with lanthanum. In cases of the commercial La-modified alumina sample and the sample prepared by wetness impregnation with lanthanum nitrate, the hydrothermal treatment under mild conditions has two effects. Firstly, the La2O3 particles weakly bonded with the support grow in size. Secondly, the high-temperature calcination of the samples leads to the formation of the LaAlO3 phase. Oppositely, the samples prepared by the adsorption of La(EDTA)- complexes showed no formation of the LaAlO3 phase neither before nor after the hydrothermal treatment procedure. The key factor accelerating the dissolution of the La2O3 particles was found to be the regime of the suspension stirring.

Angular-Based Mueller Matrix Polarimetry Parameters for Subwavelength Pore Size Differentiation

Mueller matrix polarimetry is exploited to find a potential polarization feature sensitive to subwavelength pore size variation in porous alumina samples. After careful analysis using standard machine learning methods, it is observed that existing Mueller matrix decomposition methods and parameters are insufficient to distinguish areas with different pore sizes. Thus, a new angular-based Mueller matrix polarimetry parameter capable of linearly separating areas with varying pore sizes is proposed. Such an angular-based parameter is novel because it is based on angular parameters, it utilizes multi-angle measurements, and it extracts physical information independent of existing decomposition methods or parameters. Hopefully this work should inspire future research on the angular parameters in Mueller matrix polarimetry and their relationships to microstructure information.

Graphene-reinforced ceramics obtained by slip casting and pressureless sintering: Interactions and stability of particles in aqueous environment

Ceramic-graphene composites had been considered very promising for both their conductivity and superior fracture toughness compared to nonreinforced materials. However, agglomeration of graphene flakes occurring during the shaping process causes the properties of obtained composites to differ from theoretical values. Authors investigate slip casting as the alternative to the granulation and powder metallurgy route. Slip casting as a near-net-shape technique results in fewer steps compared to the standard routes of ceramic-graphene composites preparation. To enable slip casting of such materials stable ceramic-graphene suspensions of low viscosity are required. The influence of dispersing agents on zeta potential and viscosity of obtained suspensions had been investigated. Authors define possible interactions between graphene and alumina powder within suspension and their influence on the properties of suspensions. The composite materials have been well densified by pressureless sintering in a reductive atmosphere. The composites exhibited much higher fracture toughness in comparison to pure alumina samples.

Studying the Effect of Hydroxyapatite Coating on the Properties of Alumina

The expansion in the application of bio-materials leads to a wide variety of forms. The current study includes the preparation of a biological material in the form of a bio-coating that is represented by the, combination of a substrate with biological properties and at the same time high mechanical properties represented by alumina and the use of a coating from a material that provides high biological properties and low mechanical properties if it compared with the substrate, which is the hydroxyapeptite . Hydroxyapeptite was prepared from the biological waste, which is the egg shells after chemically cleaning by acetone solution and drying them in air , grind for 14 hours then calcination at 850 Co for 2hours, and reacting with phosphoric acid ,heat treatment by sintering at 1100Co, for 3 hours to obtain the pure hydroxyapatite phase. After preparing cylindrical samples of alumina and sintering them at a temperature of 1300°C and coating them by immersion with a solution of hydroxyapeptite prepared from mixing of hydroxyapeptite with of distilled water, the immersion is done at intervals ranging from (10,30,and 50 ) minutes, sintering process for soaking sample has been done at 1200 Co for one hours. The XRD and EDS technique showed an increase in the proportion of hydroxyapatite coatings formed and appears in calcium and phosphorous ions with an increase in the immersion time. Degradation test has been done after soaked the samples in Buffer solution for 7 days which proved increment in degradation as the soaking time increase .

Hard x-ray photoelectron spectroscopy of Al2O3 with Cr Kα excitation

A thick alumina sample was analyzed by hard x-ray photoelectron spectroscopy by means of a lab-scale spectrometer equipped with a Cr Kα (5414.8 eV) excitation source. The reported spectra include a survey scan as well as Al 1s, Al 2s, Al 2p, O 1s, and C 1s core-level spectra.

Powder Bed Selective Laser Processing of Alumina: Scanning Strategies Investigation

Powder Bed Selective Laser Processing (PBSLP) is a promising technique for the additive manufacturing of alumina. For the method’s success, PBSLP process parameters such as laser power, scanning speed, hatching distance, and scanning strategies need to be investigated. This paper focuses on studying the scanning strategies’ effects on the PBSLP of alumina numerically and experimentally. Scanning strategies such as linear with different orientation, concentric, and islands were investigated. A numerical model was developed in which the PBSLP parameters, scanning strategy effects, and interpreting the experimental results could be observed. The numerical model proved its ability to reach the proper process parameters instead of using experimental trails which are time and cost consuming. For relative density, the island strategy succeeded to print alumina samples with a high relative density reaching 87.8%. However, there are round passages formed inside the samples that remain a barrier for the island strategy to be effectively used in PBSLP of alumina. Both linear and concentric strategies achieved a relative density of 75% and 67%, respectively. Considering the top surface roughness, samples printed with linear strategies gave low top surface roughness compared to the island and concentric strategies. Linear-45° is considered the effective strategy among the studied strategies as it achieved good relative density and low roughness at top and side surfaces. For PBSLP of alumina, new scanning strategies should be considered, and this study presents a new scanning strategy that is mainly based on space filling mathematical curves and should be studied in future work.

Surface characteristics of adsorbents based on aluminum oxide by means of X-ray photoelectron spectroscopy and pH-measurement

Alumina samples, containing potassium and sodium cations, have been studied by the pH-measurement and XPS methods.

Study and performance analysis of graphite and aluminium oxide coating on heat spreader application

To sustain in a global Market, the electronic industries need to compete continuously in the areas such as quality, production rate and continuous improvement in manufacturing. Also Electronic devices are continuously used in day-to-day applications induce an excessive amount of heat which will indirectly affect the working condition of the system. The main issue lies in the increase in chip temperature and causes failure in performance, reliability and life span of the process. To avoid these limitations, the excessive amount of heat is to be transferred onto the outside environment. In the present situation, it is important to find the better thermal conductivity material for heat dissipation in heat spreader than the old one. Among various material, graphite and Aluminium Oxide are chosen, as they show best thermal conductivity nano materials in nature. The proposed research work analysis the heat transfer through graphite and aluminum oxide coating in electronic applications. The heat transfer rate of coated graphite and aluminum oxide were compared with uncoated material through a heat transfer measurements. Initially, graphite and aluminum oxide is mixed with the required solvents separately through a magnetic stirrer. The prepared solutions are then coated with the substrate through spin coating with two different specimens. Surface roughness measurements are also measured on the coated and uncoated specimen. The scanning electron microscopic (SEM) is captured on the specimen which confirmed the presence of graphite and aluminum oxide coating on the aluminum substrate. The overall heat transfer efficiency was improved by 30 % and 46 % respectively for Aluminium Oxide and graphite samples.

Влияние добавок графена на динамическую прочность и разрушение оксида алюминия при ударном нагружении

The results of dynamic testing of aluminum oxide samples with different graphene content are presented. The tests were carried out using a split Hopkinson rod according to the "Brazilian test" method at an impact speed of 10 m/s. The optimal value of the graphene content was obtained. The strength characteristics of the aluminum oxide nanocomposite with graphene reach a maximum at this value.

Fabrication of Al2O3-Nb2O5-LiF-ZrO2 FGMs by SPS method: Microstructural evaluation, dynamic and sintering behaviour

For the first time, alumina functionally graded materials (FGMs) with additions of niobium oxide, lithium fluoride and zirconia were produced by spark plasma sintering (SPS) and their sintering behaviour and dynamic properties were studied aiming to evaluate possibility of their use as ballistic shielding. Six groups of alumina samples with different layer compositions were produced by SPS at 1400 ?C/5min. The samples were characterized by dilatometry, scanning electron microscopy (SEM) and Hopkinson split bar method. The composition with the zirconia addition exhibited lower shrinkage rates at higher temperatures than the groups without zirconia, which promoted small sample shrinkage, resulting in lower density and higher porosity. The dynamic test showed that the alumina FGMs with layer containing LiF had the highest strain and strain rate values, exhibiting that the presence of continuous gradients in the composition positively affects the ceramic properties. Densification, layer change and cracks propagating through the material layers were also analysed by SEM analyses.

Influence of graphene additives on dynamic strength and failure of alumina under shock loading

The results of dynamic testing of alumina samples with different graphene content are presented. The tests were carried out using a split Hopkinson rod according to the "Brazilian test" method at an impact speed of 10 m/s. The optimal value of the graphene content was obtained. The strength characteristics of the alumina nanocomposite with graphene reach a maximum at this value. Keywords: ceramic composite, graphene, split Hopkinson rod, Brazilian test.

Study on the effect of ZrO2 and SiO2 additions type on the properties of high alumina crucibles for melting heat-resistant alloys and steels

The technology of vibrocasting high-quality refractories from granular masses of different chemical composition, size, configuration and weight of products, in particular crucibles for induction melting of heat-resistant alloys was developed and introduced by JSC “URIR named after A. S. Berezhnoy”. According to the developed technology, mullite-alumina zircon-containing crucibles contain ZrO2 ~ 8 %, zirconia-containing crucibles — ~ 17 %, alumina zircon-containing crucibles — ~ 8 %, ~ 8—10 % and ~ 25 %, have open porosity < 20 %, cold crushing strength depending on the used raw materials and firing temperature within ~ 50—230 N/mm2, thermal shock resistance> 20 thermal cycles (1300 °C — water).
 The article studies the effect of ZrO2 and SiO2 additives type introduced with zircon or zirconia in combination with a silica-containing additive on the main properties of vibrocasting samples after firing at 1580 °C.
 As a result of the studies the high alumina samples with 10 % ZrO2, introduced by zircon or zirconia powder, were produced, which are characterized by apparent density of 3.12 and 3.08 g/cm3, open porosity of 14.4 and 15.5 %, cold crushing strength 145 and 210 N/mm2, thermal shock resistance > 20 thermal cycles and 6 thermal cycles (1300 °C — water) respectively. That is, with practically the same chemical composition, samples with zircon have better indicators of apparent density and especially thermal shock resistance than samples with zirconia in combination with silica. The high thermal shock resistance of samples with zircon can be explained by a nature of the pores which long irregular in configuration and connected to each and, as well as microfractured structure due to the decomposition of zircon and the formation of microcrystalline monoclinic zirconia while in the structure of the samples, which included zirconia powder and silica and a mixture of joint grinding of alumina and a silica-containing additive, the pores are predominantly round isolated. Considering also that zircon is a domestic material and is cheaper than imported ZrO2, it is more expedient to use zircon for crucibles.

Effect of particle size on mechanical properties of alumina ceramic processed by photosensitive binder jetting with powder spattering technique

As an Additive Manufacturing technique, Binder Jetting enables the fabrication of customized and complex ceramic parts. However, the insufficiency of powder packing in green parts restricts the final products’ densifications and strengths. To form a layer of ceramic green parts, Photosensitive Binder Jetting with powder spattering technique provides entrapment of ceramic powder by printing of photo-curable resin and recoating by releasing the powder through a vibrating mesh. This recoating technique enables the processing of fine alumina powders, the average particle sizes of which are 3 μm, 1.65 μm, and their mixtures. Apparent densities, porosities, mechanical properties, and microstructures of the sintered parts were investigated. The alumina sample with the apparent density of 3.70 g/cm3, the compressive strength of 94.87 MPa, the biaxial strength of 50.06 MPa, and the porosity of 41.01 % was attained by the mixture with 70:30 wt.% of the 3 μm and 1.65 μm powders respectively.

Pressure-sensitive paint diagnostic to measure species concentration on transpiration-cooled walls

This paper presents the performance of pressure-sensitive paint (PSP) for the direct measurement of species concentration on a porous surface with mass injection. It is used to measure the ability of an injected gas to reduce the mass transfer of freestream species to the surface. A porous alumina sample was sprayed with a PSP luminophore solution. The sample was installed into a flat plate model and exposed to hypersonic cross-flows in the Oxford High-Density Tunnel. Tests were conducted with no coolant injection, air injection, and nitrogen injection at increasing blowing ratios. Oxygen partial pressure maps on the transpiration-cooled surface were obtained for several conditions at unit Reynolds numbers between 2.58{-}5.0 times 10^7/ mathrm{m} and blowing ratios between 0.016{-}0.078%. The oxygen pressure decreases as the unit Reynolds number decreases and the blowing ratio increases.Graphic abstract

A comparative study on the thermal stability, textural, and structural properties of mesostructured γ-Al2O3 granules in the presence of La, Sn, and B additives

Mesoporous γ-alumina is widely used as catalyst support in various catalytic reactions of industrial interest. However, due to the instability of γ-alumina at elevated temperatures, many efforts have been reported to inhibit the α-alumina phase transition through doping with suitable metalloids, as well as transition, post-transition, or rare-earth elements. In the present study, undoped and La-, Sn-, and B-doped alumina granules were synthesized via sol-gel/oil drop method with the aim to clarify the role of the additives and their content on the porous structure as well as on the chemical, structural, and microstructural behavior of γ-alumina. XRD and DTA/TG results demonstrated that thermal stability of transition aluminas increases more than 100 °C by 3 wt% lanthanum and tin doping; however, boron doping seems to have only negligible effect on the thermal stability. On the other hand, based on nitrogen adsorption-desorption analysis, tin and boron-doped aluminas showed a higher surface area at 750 °C (between 214.74 m2/g to 245.97 m2/g) but higher loss in the surface area after calcination at 1200 °C (between 25.45 m2/g to 8.57 m2/g). On the contrary, the 3 wt% La-doped alumina sample, with a relatively high surface area at 750 °C (227.17 m2/g), exhibited the highest surface area after calcination at 1200 °C (53.07 m2/g). 27Al MAS NMR and HRTEM studies indicated that the presence of 3 wt% La in alumina structure leads to thermal and mesoporous structure stability up to 1200 °C by inhibiting oxygen lattice restructuring. These results provide a comparative perspective of La, B, and Sn additives' behavior in γ-alumina.

Afterglow luminescence phenomena in the porous anodic alumina

A series of mesoporous alumina layers obtained by anodic oxidation in solutions of H 2 SO 4 , H 3 PO 4 and H 2 C 2 O 4 were subjected to spectroscopic studies. Their photoluminescence was investigated mainly by the afterglow luminescence (A-Lum) method using a low-pressure mercury lamp emitting at 254 nm as the source of excitation. All mesoporous alumina samples (PAA) showed short-time emission (a few to a few 10 s) of biexponential decay. The intensity and shape of the emission spectra were proved to depend on the thermal treatment of Al/AL 2 O 3 samples, type of electrolyte used in their synthesis and excitation time. On the basis of the determined kinetic parameters of emission decay curves and emission spectra, significant differences were found between the A-Lum and photoluminescence phenomena for all systems studied. • Detection of afterglow luminescence of porous structures of amorphous alumina. • Analysis of the emission spectrum and kinetic curves of the A-Lum decay.