Segregation of anion (Cl−) impurities at transparent polycrystalline α-alumina interfaces

Small amounts of anion impurities (e.g. Cl), which are incorporated during the synthesis of ceramic powders, can affect the properties and microstructure of the final sintered ceramic. The effect of anion impurities is a little studied and poorly understood phenomenon. In this work a combination of STEM-EDX analysis and atomistic modeling approach was used to understand the segregation of Cl in transparent alumina ceramics. A high resolution analytical electron microscopy study showed the presence of Cl at the grain boundaries and especially at triple points. Atomistic simulations were carried out to understand the origins and consequences of such segregation. Segregation energy calculations predict a strong segregation of Cl at the different surfaces and grain boundaries of alumina. A higher coordination number of Cl at surfaces was observed, which indicates strong ionic bonds making it difficult to remove at low temperature, which explains the presence of Cl at triple points.     

Optical and mechanical characterization of transparent α-alumina fabricated by spark plasma sintering

The aim of this work was the analysis of the experimental results of a transparent alumina (BMA15) ceramic which was fabricated by Spark Plasma Sintering (SPS) from nanopowder (BMA15, Baikowski Chimie, France), at different temperatures (1200°C, 1250°C, 1300°C). With the application of a maximum uniaxial pressure of 73 MPa during all the fabrication-cycle (more than 3 hours). We sought an optimal sintering temperature combining better optical and mechanical properties of our pellets. The sintered alumina (BMA15) has a crystalline and dense microstructure. The samples sintered at 1200°C exhibit the best optical properties, in particular: good real inline transmission (RIT) and an optical gap greater than those of the samples sintered at 1250°C and 1300°C. Due to their low density, the Young modulus of alumina sintered at 1200 °C, deduced by ultrasound, has a low value which is about 385 GPa. Similarly, its small grain size gives it a better Vickers hardness ~ 21 GPa. Therefore, the value of the coefficient of friction μ stabilizes around the mean value of 0.21.     

Flexural strength of glass–ceramic for structural applications

In order to characterise the design strength of an innovative type of glass–ceramic with a view to structural applications, 4-point-bending and Ring-on-Ring biaxial bending tests have been performed. Strength is comparable with that of tempered glass but remarkably, the material breaks into large pieces like annealed glass. Interpreted through Weibull-type probability distributions, the data show much lower dispersion with respect to glass. Additional tests performed at different load rates have allowed an evaluation of the effects of static fatigue. Using a phenomenological model of equivalent-crack growth, a method is presented to calculate the decay of strength with loading time. As expected, this material is proven to be less sensitive than glass to this type of degradation.     

Microreactor-assisted synthesis of α-alumina nanoparticles

In this paper, nanosized α-Al₂O₃ powders were synthesized with the aid of a high throughput impinging stream microreactor. It was demonstrated that the as-prepared powders exhibited better dispersity and more homogeneous distribution of particle size than that prepared by conventional parallel flow precipitation method due to the drastic collisions and homogeneous explosive nucleation in microchannel during the precipitation process. The effects of calcinating temperature and time on the morphology, phase composition and particle size of α-Al₂O₃ were systematically studied. Comparatively well dispersed and crystallized α-Al₂O₃ powders with higher sintering activity were obtained by calcinating the as-prepared precursors at 1200 °C for 2 h in air. The specific surface area of α-Al₂O₃ powders was above 20 m² g⁻¹ and average particle size was about 110 nm with higher sintering behavior. From room temperature to 1520 °C, the green compact exhibited shrinkage of 19.34%. This work opens a door for developing ultrafine α-Al₂O₃ powders with uniform size distribution, high crystallinity, and excellent thermal expansion property.     

Plastic forming and microstructural development of α-alumina ceramics from highly compacted green bodies using extrusion

High density (>99% TD) and microstructurally controlled α-alumina ceramics were produced from seeded nano-size boehmite (γ-AlOOH) sols with a very fine crystallite size (2–3 nm). A totally wet processing technique comprising vacuum filtering and pressure filtration (PF) was applied in order to increase the solids-loading of the sol and hence form an extrudable paste suitable for plastic forming using extrusion. High packing densities (>68% TD in the green state) are achieved by PF starting from the slurry state resulting in the formation of a consolidated paste which is further consolidated by extrusion. This combined processing technique was successfully applied, in an attempt to reduce the γ-Al₂O₃ formation temperature, and hence lower the θ- to α-Al₂O₃ transition temperature. The microstructure of dense α-Al₂O₃ bodies derived from seeded boehmite sol contains very fine (250 nm) alumina grains after sintering at 1200 °C for 2 h. Although the DTA evidence points to a θ- to α-Al₂O₃ transition temperature of 1208 °C for a seeded (with 30 nm TiO₂) sample, X-ray analysis indicates that a seeded, pressure filtrated and extruded sample is transformed to α-Al₂O₃ phase after sintering at 1100 °C for 2 h.     

Fabrication and dielectric properties of transparent PZN–BT ceramic

Transparent ceramic of 0.85Pb(Mg_1/3Nb_2/3)O₃–0.15BaTiO₃ has been successfully prepared by a two-stage sintering method using conventional raw materials. The ceramics exhibited an excellent crystallinity, high density and clean grain boundary. The transmittance keeps about4 0% from visible to near infrared regions. The frequency dependence of T_m and relaxor behavior has also been investigated using Vogel–Fulcher model and Power model.     

The activation mechanism of oxalic acid on γ-alumina and the formation of α-alumina

Converting the γ phase into the α phase completely is necessary in the presintering stage of industrial alumina (Al₂O₃), which requires high temperature and energy consumption. To reduce the presintering temperature, γ-Al₂O₃ was activated by oxalic acid. XRD, ²⁷Al-MAS-NMR and TG-DSC were used to characterize the γ - alumina before and after activation, and the phase transformation was studied. The formation temperature of α-Al₂O₃ decreased to 1029 °C for oxalic acid activated γ-Al₂O₃, and the α-fraction was 100% for activated γ-Al₂O₃ at 1300 °C. After oxalic acid activation, the diffraction peak intensity of γ-Al₂O₃ decreased significantly; the results of ²⁷Al-MAS-NMR suggested that octahedral [AlO₆] in γ-Al₂O₃ was easier than tetrahedral [AlO₄] to be attacked by oxalic acid, and the formation of pentavalent [AlO₅] with higher reaction activity, which was in favour of the lowering formation temperature of α-Al₂O₃. The dissolution concentration of Al increased after oxalic acid activation, and the dissolution process was controlled by surface reactions. Oxalic acid mainly attacked the octahedral aluminium in γ-Al₂O₃ and extracted Al as three complexes of [Al(C₂O₄)]⁺, [Al(C₂O₄)₂]^- and [Al(C₂O₄)₃]^3-. Oxalic acid activated γ - Al₂O₃ with a lower phase transformation temperature has broad application prospects in the alumina industry.     

Sintering densification behavior of ZnO–SnO2 binary ceramic targets

In this work, high-performance ZnO–SnO₂ binary ceramic targets for magnetron sputtering of transparent conductive oxide (TCO) films were prepared by pressureless oxygen atmosphere sintering. The sintering behavior and densification mechanism of the ZnO–SnO₂ binary targets were analyzed by systematically studying the oxide powder state, formation process of the solid reaction phase, and evolution of the target microstructure. The data revealed that the ZnO–SnO₂ powder treatment improved the sintering activity and the powder dispersion; furthermore, it promoted a mutual reaction between the different components during sintering and the homogeneity of the target composition. The densification of the pure SnO₂ ceramic target was difficult to achieve, and the addition of ZnO led to a continuous interaction between the ZnO and SnO₂ components. The Zn₂SnO₄ phase started to form, and a temporary shrinkage of the target occurred above 800°C. After formation of the stable Zn₂SnO₄ and SnO₂ phases, the target shrunk rapidly with increasing temperature, densification occurred during growth, and the two phases started to interact. The sintering temperature provided the driving force for the target densification, with the densification activation energy of the ZnO–SnO₂ binary ceramic target estimated to be 580 kJ/mol based on the master sintering curve. A binary ceramic target with a high density (99.78% relative density), a fine grain size, and a homogeneous phase structure was achieved at a temperature of 1600°C. These findings are promising for the further improvement and performance enhancement of SnO₂-based materials.     

Shock activation of α-alumina from calcinated Al-rich sludge

Enormous quantities of Al-sludge from aluminum surface treatments are yearly generated all over the world. Despite of being known as important supplier of alumina, the Al-sludge also have some other components that need to be removed. Calcination has revealed to be an inevitable treatment to achieve such objective. It was also found that conventional densification processes are not efficient for this kind of powders and, to overcome that difficulty, a dynamic compaction approach, using detonation generated shock waves have been followed. The calcinated powders compacted by this way have shown up values of density and hardness, after sintering, comparable to the ones of commercial alumina processed by the conventional methods.     

Effects of alumina sols on the sintering of α-alumina ceramics

The effects of two kinds of alumina sols on the densification behavior of sub-micron grain sized α-alumina ceramics have been investigated. Composition of the sol-derived gels was investigated by energy dispersive spectra and Fourier transform infrared spectra. Structural evolution of the gels at different temperatures was characterized by a combination of X-ray diffraction and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy. Results showed that the gel containing chlorine and carbon transformed to α-alumina at about 950 °C, significantly lower than the other gel which transformed at about 1050 °C. Density measurements and scanning electron microscopy analyses were used to investigate the sintering of alumina ceramics with or without alumina sols. It was found that the alumina sols had profound effects on the densification of alumina ceramics. The ceramic displayed the best densification behavior when the sol containing chlorine and carbon was added.     

Synthesis of transparent and thermally stable polyimide-aramid nanocomposites – Prospective materials for high-temperature electronic manufacture applications

Herein we report a method that improves the optical properties of polyimide-aramid (PIA) by the addition of a large quantity of silica. High silica loading was successful due to the good compatibility of synthesized unique silane end-capped polyimide-aramid (PIA) with in-situ formed silica nanoparticles. The prepared polymer films exhibited excellent optical properties, stability at 400 °C, a high decomposition and glass transition temperature, and a relatively low coefficient of thermal expansion (CTE) close to that of a glass substrate. These films may have potential applications in high-temperature (over 400 °C) electronic and optoelectronic manufacturing processes as a flexible transparent substrate.     

Aligning α-alumina platelets via uniaxial pressing of ceramic-filled polymer blends for improved sintered transparency

Uniaxial warm pressing was used to align alumina platelet-filled polyethylene-based copolymer blends. The solids loading (30-40 vol.%) and platelet diameter (1.2 and 11 μm) were varied to compare effects on viscosity, percent reduction, and final alignment. All ceramic-filled thermoplastic polymer blends exhibited pseudoplastic behavior. Crystallographic alignment of green body samples was quantified by the orientation parameter (r) and grain misalignment angle (full width at half maximum, FWHM) obtained from rocking curve analysis. Blends with 11 μm diameter platelets displayed a higher temperature sensitivity constant, better flow properties, and higher alignment compared to blends with 1.2 μm diameter platelets. Optimal samples produced with blends containing 30 vol.% of 11 μm diameter platelets demonstrated an alignment of r = .251 ± .017; FWHM = 11.16° ± 1.16°. A sample with optimal alignment was hot-pressed to transparency and obtained an in-line transmission of 70.0% at 645 nm. The final alignment of this pre-aligned hot-pressed sample (r = .254 ± .008; FWHM = 11.38° ± 0.54°) improved when compared to a non-pre-aligned sample (r = .283 ± .005; FWHM = 13.40° ± 0.38°).     

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

The formation and properties of dense silica/α-alumina coatings derived from alumina–silica sol-incorporated coarse alumina powder (median particle size d₅₀=0.509 μm) using Al(NO₃)₃·9H₂O and tetraethyl orthosilicate as precursors have been investigated. Phase compositions and thermal evolution analysis of the coatings as well as interface microstructure between the coatings and the matrix were characterized by means of XRD, XRF, DSC-TG, TEM, SEM supplemented with EDS, and tensile strength tester. Silica/α-alumina coatings combined with quartz fiber matrix are homogeneously integrated with distinct inter-diffusion near the interface. The heat conduction mechanism of the quartz fiber matrix was influenced by the dense silica/α-alumina ceramic coatings, and the prepared coatings provide practical thermodynamic stability and desired mechanical strength for the woven quartz fiber fabrics.     

Rationalizing the role of magnesia and titania on sintering of α-alumina

The rationalization of selection of sintering additives for α-alumina was investigated using two oxides (MgO and TiO₂) to discern their individual roles. Using both dynamic heating study in a thermomechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700 °C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700 °C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO₂ significantly enhanced the densification and promoted grain growth in α-alumina. At 1700 °C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO₂ was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at ~120° indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO₂ doped samples.     

A comparison of the tape casting of α- and β-alumina

This work concerns the manufacture of planar cell configurations for molten sodium batteries in energy storage devices such as vehicle batteries and stationary storage cells. Tape casting of beta-alumina electrolyte could provide a low-cost mass production route but intriguingly there are only a few reports of tape casting using beta-alumina directly as the raw powder. We first compared tape casting of α-alumina and beta-alumina using polyvinyl butyral (PVB) in conventional formulations. While it is relatively easy to obtain homogeneous α-alumina tape cast sheet, beta-alumina resulted in adhesion to the substrate and cracking. These problems were shown to be attributable to particle characteristics. When the binder was changed to polymethylmethacrylate (PMMA), tape casting of beta-alumina and three different α-alumina powders was facilitated, producing a formulation that was more tolerant to different powder types. Screening of several commercial dispersants provided two which were effective with PMMA and a conventional MEK/ethanol dual solvent.     

Synthesis of MFI films on α-alumina at neutral pH

Oriented films of MFI were prepared on α-alumina substrates by a method including substrate seeding and subsequent growth of the seeds in a synthesis gel containing fluoride as mineralizing agent. The films were characterized by scanning electron microscopy and X-ray diffraction. The preparation procedure presented here is promising for the synthesis of highly siliceous MFI films on alumina substrates.     

Optimization of nano spray drying parameters for production of α-amylase nanopowder for biotheraputic applications using factorial design

Abstract

This study was designed to optimize the effect of operating conditions and formulation parameters using various additives to develop α-amylase nanoparticles. α-Amylase was chosen due to its importance in the substantial number of industrial processing with emphasis on pharmaceutical industry. Factorial statistical design was adopted to effectively optimize the size, yield value, residual enzyme activity, and morphology of α-amylase nanoparticles using Nano Spray Dryer BÜCHI B-90. The physicochemical characterization of the prepared nanopowder was carried out using zetasizer and scanning electron microscopy (SEM) and enzyme activity assay. Results showed that the type of additive and mesh size significantly influenced the particles size and yield value. SEM images showed three different structure patterns where particle morphology was influenced by Tween^® 80 or sucrose at low concentration (0.05%). Optimized spherical nanoparticles (600?nm) was obtained using 7 µm mesh cap size, sucrose (0.15%), 95% yield value, drying flow rate (100?L/min), and inlet temperature of 80?°C. Higher storage stability was detected for enzyme spray-dried using larger cap size. It was concluded that nano spray drying of aqueous enzyme solution under determined operating conditions produced stable α-amylase powders. This would extend the application of the enzyme in a variety of pharmaceutical products.     

Preparation and characterisation of α-methylstyrene–butadiene latexes for paper coating applications

The purpose of this work is to demonstrate how α-methylstyrene (AMS) can replace styrene in preparing styrene–butadiene (SB) type latexes and to compare the properties of the paper coating of the prepared α-methylstyrene–butadiene emulsion with the commercial styrene–butadiene latex reference sample. A lot of work is nowadays being conducted on different biorefinery concepts replacing fossil oil with biomass based raw materials due to the expected rise of the fossil oil cost. Aromatics can in principle be produced from renewable raw materials, such as lignin, sugars and terpenes for example. The potential methods include thermochemical conversions, catalytic fast pyrolysis, metabolic engineering, catalytic aromatisation and dehydrogenation among others. Terpenes, such as α-limonene and pinene, are possible sources of aromatics, and they can indeed be catalytically converted to p-cymene. Industrial hydrodealkylation and disproportionation processes developed by major petrochemical companies can further convert p-cymene to BTX aromatics or simultaneously dehydrogenate the alkyl chain of p-cymene to styrenic monomers such as α-methylstyrene. Based on the measured paper properties for uncalendered and calendered coated samples, AMS proved to be adequate to replace the oil based styrene in commercial reference SB latexes. Even though the emulsion polymerisation for the α-methylstyrene–butadiene latex was not optimised, almost all tested properties were at least equally good as in the commercial reference sample. α-Methylstyrene containing coating colours had slightly higher viscosity than the other coating colours. Coating colours containing α-methylstyrene seems to have an improved water retention compared to the commercial reference styrene–butadiene latex coating colour and the laboratory prepared styrene–butadiene coating colour. The paper coated with the commercial reference latex containing coating colour was less porous than the other coated papers. Despite of that, both dry and wet surface strength were at least equally good as in the case of the commercial reference latex. The results are promising when thinking of the future development of the bio-based latexes.     

Microstructure,strength and CO2 separation characteristics of α-alumina supported γ-alumina thin film membrane

Abstract

Abstract

In this work, a mesoporous γ-Al₂O₃ membrane was synthesised on an α-Al₂O₃ substrate by a sol–gel dip coating process. The membranes were characterised using SEM, field emission SEM, X-ray diffractometry and N₂ adsorption/desorption measurements (Brunauer–Emmett–Teller analyses). The characterisation results revealed that a flawless γ-alumina membrane layer with 3·5 μm thickness and 2·1 nm average pore size was achieved. Subsequent separation tests indicated that CO₂ could be separated from N₂ by the mesoporous γ-Al₂O₃ membrane. In spite of the difficulties of the separation, the optimised microstructure achieved for the prepared membrane led to desirable and regular permeation behaviour. It was also observed that separation could be more efficient in high pressure permeation conditions. The prepared substrate fulfilled the required strength and permeability for the thin γ-Al₂O₃ membrane film under these conditions. Accordingly, an optimised completely ceramic membrane structure, applicable for CO₂ separation applications, was achieved.     

Formation of porous α-alumina from ammonium aluminum carbonate hydroxide whiskers

Ammonium aluminum carbonate hydroxide (AACH) whiskers prepared by hydrothermal technique were employed as precursor material for development of porous alumina. After compaction of AACH whiskers at 8 bars, calcination was performed at 650?°C followed by sintering at different temperatures. The sintered samples were characterized by XRD, FTIR, SEM and mercury intrusion porosimetry. Mechanical strength was determined by compression testing. At sintering temperatures of 1200?°C to 1400?°C, the % age porosity was around 80%. At 1500?°C, the percentage porosity decreased to 71%. The as-prepared AACH consisted of bundles of whiskers with diameters as thick as 0.7?µm, while an individual whisker had a diameter of about 100?nm with an aspect ratio of about 33. A two-phase mixture consisting of θ- and α-alumina was obtained at 1100?°C, while at 1200?°C and above, single phase α-alumina was formed. θ-alumina retained the bundle-like morphology. However, transformation to α-alumina was accompanied by formation of bead-like morphology. These beads were joined together through necks/stems within the whiskers as well as across the parallel-lying whiskers. These necks grew at 1300?°C to form aggregates with smooth surfaces. At 1400?°C, these aggregates started joining with each other by neck formation and at 1500?°C, a three-dimensional network was formed. For sintering temperatures of up to 1400?°C, pores with sizes around 260?nm were very stable. At 1500?°C, significant pore growth took place along with an overall densification. Therefore, number of pores with sizes of around 260?nm decreased and those with sizes around 10?µm, 1?µm and 5?nm increased. The compression strength of samples sintered at 1100?°C to 1300?°C was in the range of 3.4–4.3?MPa. At 1400?°C, the strength increased to 5.2?MPa, while at 1500?°C, it jumped to 10.8?MPa due to the formation of three-dimensional network.