Crystallization,mechanical, and optical properties of transparent,nanocrystalline gahnite glass‐ceramics

This paper describes the preparation of a transparent glass‐ceramic from the SiO₂‐K₂O‐ZnO‐Al₂O₃‐TiO₂ system containing a single crystalline phase, gahnite (ZnAl₂O₄). TiO₂ was used as a nucleating agent for the heat‐induced precipitation of gahnite crystals of 5‐10 nm. The evolution of the ZnAl₂O₄ spinel structure through the gradual formation of Al‐O bonds was examined by infrared spectroscopy. The dark brown color of the transparent precursor glass and glass‐ceramic was eliminated using CeO₂. The increase in transparency of the CeO₂‐doped glass and glass‐ceramics was demonstrated by UV‐visible absorption spectroscopy. EPR measurements confirmed the presence of Ce³⁺ ions, indicating that CeO₂ was effective in eliminating the brown color introduced by Ti³⁺ ions via oxidation to Ti⁺⁴. The hardness of the glass‐ceramic was 30% higher than that of the as‐prepared glasses. This work offers key guidelines to produce hard, transparent glass‐ceramics which may be potential candidates for a variety of technological applications, such as armor and display panels.     

Thermal Reaction of Cristobalite in Nano‐SiO2/α‐Al2O3 Powder Systems for Mullite Synthesis

Nanoscaled cristobalite and α‐Al₂O₃ powders were used as the starting materials for synthesizing mullite by solid‐state reaction. The thermal reaction of the cristobalite with α‐Al₂O₃ during the thermal treatment was examined. Cristobalite powder with a D₅₀ value of 430 nm was adopted to mix with α‐Al₂O₃ powders with a D₅₀ values of 230, 310, and 400 nm in a stoichiometric composition of 3Al₂O₃?2SiO₂ (71.8 wt% α‐Al₂O₃ and 28.2 wt% SiO₂). Samples for thermal reaction were prepared using uniaxial pressed from the three mixtures that showed various particle number ratios of SiO₂/Al₂O₃ due to the different particle sizes of α‐Al₂O_3. Examinations were performed by differential thermal analysis, X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy techniques. The results showed that cristobalite particles amorphized during the thermal treatment, and then reacted with the α‐Al₂O₃ particle to form mullite via nucleation and growth. The amorphization temperature can be reduced by using finer‐sized α‐Al₂O₃ powders, thus leading to a lower temperature for mullite formation. Mullite crystals with a multidomain structure were observed in the α‐Al₂O₃ particle matrixes. The crystal orientation of the mullite was controlled by the α‐Al₂O₃ matrix, that is, [001] α‐Al₂O₃ → [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO₂ with α‐Al₂O₃, initiating the nucleation of mullite. The α‐Al₂O₃ particles act as the hosts for mullite formation and determine the size of the mullite particles.     

Disperse Equiaxed α‐Al2O3 Nanoparticles Without Vermicular Microstructure

Nanocrystalline microstructure is regarded as a strategic approach to overcome the brittleness of alumina ceramics, and the preparation of disperse equiaxed α‐Al₂O₃ nanoparticles is an essential step for the preparation of nanocrystalline alumina ceramics. In this work, disperse equiaxed α‐Al₂O₃ nanoparticles were prepared using α‐Fe₂O₃ as seed and isolation phase. At first, the composite of α‐Al₂O₃ nanoparticles embedded in α‐Fe₂O₃ matrix was obtained by calcining the precursor powder containing γ‐AlOOH and Fe(OH)₃ (Fe³⁺/Al³⁺ mole ratio of 5) at 770°C for 2 h. Then disperse equiaxed α‐Al₂O₃ nanoparticles with a mean size of 12 nm and a size distribution from 2 to 40 nm without vermicular microstructure were obtained by removal of α‐Fe₂O₃ and other impurities in the composite through acid corrosion.     

Fabrication and Characterization of Near‐Net‐Shape In Situ Reaction‐Bonded Porous Cordierite/SiC Ceramics

Porous cordierite/SiC ceramics were fabricated by in situ reaction bonding using α‐SiC, α‐Al₂O_3, and MgO powders as the starting materials. During sintering, part SiC is oxidized to SiO₂ and then the latter reacts with Al₂O₃ and MgO to form cordierite. As a result, porous cordierite/SiC ceramics were obtained, and the ceramics are strengthened by the residual SiC. Due to the large volume expansion introduced by the oxidation of SiC, the ceramics exhibit small sintering‐induced dimension variations. In addition, a fine‐grained microstructure and good thermal and mechanical properties were obtained for the porous cordierite/SiC ceramics.     

Atomic Scale Mechanisms of the Reduction of Nickel–Magnesium Aluminate Spinels

The dynamics of the reduction reaction of Ni_xMg_1?xAl₂O₄ to form nickel metal and a remnant oxide was quantified to understand spinel behavior in catalysis applications. X‐ray diffraction, thermogravimetry, and pycnometry were employed to track the evolution of high‐Ni spinels to metastable nonstiochiometric spinels during reduction, but before the phase transformation to theta alumina. Rietveld refinements of X‐ray diffraction data were used to quantify structural changes in the spinel and the phase fraction, crystallite size, and microstrain of all phases during H₂ reduction. During reduction, one O^2? is lost for each Ni²⁺ reduced to Ni metal. Ni_0.25Mg_0.75Al₂O₄ and Ni_0.5Mg_0.5Al₂O₄ were shown to form Ni metal and a non‐stoichiometric spinel of the same Mg‐Al ratio as the starting composition. NiAl₂O₄ and Ni_0.75Mg_0.25Al₂O₄ were found to become unstable as full reduction was approached, and metastable spinel, Θ‐Al₂O₃, and α‐Al₂O₃ formed sequentially given sufficient time at temperature.     

Phase Evolution in the Transformation of Atomically Mixed Versus Ball‐Milled Mixtures of Nanopowders in the Formation of Composite MO·3Al2O3 Spinels: Bottom‐Up Processing is Not Always Optimal

Liquid‐feed flame spray pyrolysis (LF‐FSP) provides atomically homogeneous mixed metal powders with 30–40 nm average particle sizes, often producing kinetic phases due to the high quench rate As produced LF‐FSP Al₂O₃‐rich spinels, such as MgO·3Al₂O₃, form an Al₂O₃‐rich metastable single‐phase spinel. On heating, the powders phase separate to form MAl₂O₄ and α‐Al₂O₃. Compacts of MO·3Al₂O₃ (M = Co, Ni, Mg) were produced and sintered to evaluate the final duplex microstructure. The same composition was also approached from stoichiometric LF‐FSP MAl₂O₄ nanopowders ball‐milled with Al₂O₃ nanopowders in an attempt to evaluate how the initial length scale of mixing affected the final microstructure. Contrary to traditional sintering, we observe two distinct mechanisms. At 1000°C–1200°C, cation diffusion appears to control densification as a consequence of high vacancy concentrations and atomic mixing where traditionally expected site inversion plays less of a factor given the high quench rates. The second mechanism follows α‐Al₂O₃ exsolution and densification occurs via oxygen diffusion and α‐Al₂O₃ grain growth. When sintering the duplex MAl₂O₄/α‐Al₂O₃ compacts to at least 95% theoretical density, we find final microstructures that do not reflect the initial degrees of mixing. That is, the atomically mixed MgO·3Al₂O₃ does not does not offer an advantage over the submicron length scale of mixing in the ball‐milled samples.     

Selective Corrosion Preparation and Sintering of Disperse α‐Al2O3 Nanoparticles

Disperse fine equiaxed α‐Al₂O₃ nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe₂O₃ as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe₂O₃ acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al₂O₃ nanoparticles, resulting in disperse fine equiaxed α‐Al₂O₃ nanoparticles. These α‐Al₂O₃ nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al₂O₃ ceramics. The two‐step sintered bodies are nanocrystalline α‐Al₂O₃ with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al₂O₃ ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al₂O₃ nanoparticles have an excellent sintering activity.     

Influence of surface modification on the dispersion of nanoscale α‐Al2O3 particles in a thermoplastic polyurethane matrix

In this study, two types of nanoscale α‐Al₂O₃ particles were used for preparation of α‐Al₂O₃/thermoplastic polyurethane (TPU) composites. These α‐Al₂O₃ particles were either coated or uncoated with stearic acid. For the uncoated α‐Al₂O₃/TPU composite, the results of field‐emission scanning electron microscopy (FE‐SEM) and energy dispersive X‐ray spectrometry indicate that uncoated α‐Al₂O₃ particles are significantly aggregated together. This aggregation is due to the poor compatibility between the inorganic filler (α‐Al₂O₃) and the organic matrix (TPU). The size of clusters is in the range from 5 to 20 μm. For the coated α‐Al₂O₃/TPU composite, FE‐SEM results indicate that most coated α‐Al₂O₃ particles are well dispersed in the TPU matrix. This phenomenon results from the effect of surface modifier (i.e., stearic acid) on α‐Al₂O₃ particles. Stearic acid can act as a compatibilizer to bridge the boundary between the TPU matrix and the α‐Al₂O₃ particle. Stearic acid is not only a suitable surface modifier for the nanoscale α‐Al₂O₃ particle, but also a good dispersant for the dispersion of nanoscale α‐Al₂O₃ particles in the TPU matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

α‐Al2O3 improves the properties of gel polyacrylonitrile nanocomposite electrolytes used as electrolyte materials in rechargeable lithium batteries

In this investigation, a series of gel polyacrylonitrile (PAN)/α‐Al₂O₃ nanocomposite electrolyte materials that incorporate various fractions of PAN, α‐Al₂O₃ inorganic powders, propylene carbonate and ethylene carbonate as cosolvents, and LiClO₄ were prepared. X‐ray diffraction revealed that the gel nanocomposite electrolyte materials contained amorphous PAN in which was uniformly dispersed α‐Al₂O₃. The gel PAN/α‐Al₂O₃ nanocomposite electrolytes had lower glass‐transition temperatures (as determined by dynamic mechanical analysis) and higher conductivity than a similar electrolyte prepared in the absence of α‐Al₂O₃. The conductivity of the PAN/α‐Al₂O₃ nanocomposite films was inversely proportional to the size of the α‐Al₂O₃ particles and directly proportional to (I) the amount of α‐Al₂O₃ (up to 7 wt %), (II) the F value [LiClO₄/CH₂CH(CN) ratio], and (III) the amount of plasticizer (propylene carbonate/ethylene carbonate = 1 : 1). Cyclic voltammetry revealed that adding α‐Al₂O₃ significantly increased the electrochemical stability of the composite electrolyte system. A rechargeable lithium battery prepared using this gel nanocomposite electrolyte system exhibited good cyclability and a stable capacity. The coulombic efficiency for the recharge/discharge process was approximately 75%, even after 100 cycles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of physical vapor deposited Al2O3 film on TGO growth in YSZ/CoNiCrAlY coatings

Thermal barrier coatings (TBCs) comprising of yttria stabilized zirconia (YSZ) ceramic top coat and CoNiCrAlY metallic bond coat have been widely used in gas turbines. However, the developed oxides layer in the interface of the top and bond coats during thermal exposure of the TBCs always results in the destruction of the system. In order to restrain the growth of oxides layer and improve the thermal shock resistance of TBCs, a thin Al₂O₃ film was pre-deposited on CoNiCrAlY bond coat by physical vapor deposition (PVD) technology. After thermal exposure, morphologies and phase compositions of the thermal growth oxides (TGO) layer in the conventional and pre-deposited Al₂O₃ film TBCs were examined by scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). The residual stresses in the coatings were analyzed using micro-Raman spectroscopy (LabRam-1B). It was found that TGO layer formed in the conventional TBCs was mainly composed of Al₂O₃, (Cr,Al)₂O₃ + (Co,Ni)(Cr,Al)₂O₄ + NiO (CSN), and (Cr,Al)₂O₃ + (Co,Ni)(Cr,Al)₂O₄ (CS), while in the treated TBCs, the formed TGO layer appeared more uniform and compact. The CSN and CS clusters, which are normally considered as a weakness for TBCs, were greatly limited. The residual stresses in the TBCs after thermal shock were also reduced by the deposition of Al₂O₃ film.     

Crystallisation in SiO2 –Al 2 O 3 –ZnO– TiO 2 glass: process mechanism

Abstract

Devitrification of a frit based on the oxide system SiO₂–Al₂O₃–ZnO, using bulk glass as well as powdered glass samples with different particle sizes was studied. The following major crystalline phases were identified: gahnite (ZnAl₂O₄ ), α-quartz, cristobalite, and a solid solution exhibiting the β-quartz structure, which contained SiO₂ , Al₂O₃ , and ZnO and whose composition varied with temperature. Depending on the size of the glass (frit) samples, gahnite was found to devitrify directly from the initial glassy phase or from the solid solution with a β-quartz structure, which appeared to act as a precursor.     

Surface‐Functionalized White Sapphire α‐Al2O3 Platelets as Nanofillers for Vinylester Composites and Heavy Duty Anchoring Systems

Synthetic α‐Al₂O₃ platelets, also referred to as corundum and white sapphire, represent attractive fillers improving the mechanical properties of vinylester‐based chemical anchoring systems. Even in the absence of coupling agents, as verified by scanning electron microscopic (SEM) analyses of fracture surfaces, α‐Al₂O₃ platelets of 200 nm thickness and 5–10 µm size are uniformly dispersed in vinylester resins which are cured by free radical polymerization at room temperature. With increasing content of ultrahard α‐Al₂O₃ platelets (0–40 wt%) the Young's modulus of α‐Al₂O₃ platelet/vinylester composites increases from 3200 to 9000 MPa. However, 1–5 wt% 3‐methacryloyloxypropyl‐trimethoxysilane (MPS) as coupling agent, added to the vinylester resin or preferably used to functionalize α‐Al₂O₃ surfaces in a filler pretreatment step, improves elongation at break (+50%) without sacrificing high stiffness and strength. The X‐ray photoelectron spectroscopy (XPS) analysis confirms the successful surface‐functionalization of α‐Al₂O₃ platelets by using pretreatments with MPS in toluene, acidified ethanol/water or tetrahydrofuran, respectively. The MPS filler pretreatment simultaneously enhances tensile strength (+22%), elongation at break (+50%), and Young's modulus (+12%) as compared to composites containing unmodified filler. According to SEM analyses of composite fracture surfaces, MPS‐mediated functionalization affords significantly improved interfacial adhesion between α‐Al₂O₃ platelets and the polymer matrix.

     

Preparation of Fe3Si‐Al2O3 Nanocomposite Powders by Mechanochemical Reaction of Fe3O4‐Si‐Al Powder Mixtures

Phase evolution and morphology of Fe₃O₄‐Si‐Al powder mixtures during ball milling from 30 min to 20 h were investigated. A 3‐h critical milling was necessary for the occurrence of mechanically activated combustion reaction. The reaction results in the formation of Fe (Si), Fe₃Si, and α‐Al₂O₃. During ball milling from 3 to 20 h, Fe (Si) and Fe₃Si were combined into disordered Fe₃Si intermetallic and Fe₃Si‐Al₂O₃ composite powder was formed. The presence of in situ formed alumina leads to a decrease in crystallite and particle sizes. The Fe₃Si‐Al₂O₃ particles after milling for 20 h had a crystalline size of 10~12 nm.     

Lithium Disilicate Glass‐Ceramics by Heat Treatment of Lithium Metasilicate Glass‐Ceramics Obtained by Hot Pressing

Lithium disilicate glass‐ceramics are widely used as dental ceramics due to their machinability and translucency. In this study, lithium disilicate glass‐ceramic was fabricated through heat treatment of lithium metasilicate glass‐ceramics obtained by hot pressing of glass powder composed of SiO₂–Li₂O–P₂O₅–ZrO₂–Al₂O₃–K₂O–CeO₂ at low temperature. The crystalline phase, microstructure, and mechanical properties were investigated. The results indicated that lithium metasilicate glass‐ceramic with a relative density of higher than 99% was obtained after hot pressing, and glass‐ceramic with interlocked rod‐like Li₂Si₂O₅ crystals and good flexural strength (338 ± 20 MPa) was successfully obtained through heat treatment. The two‐step method was believed to be feasible in tailoring the microstructure and mechanical properties of lithium disilicate glass‐ceramics.     

Mechanical and optical properties in precipitated regions of alumina‐rich magnesium aluminate spinel

The toughening and strengthening of transparent ceramics is challenging because microstructural alterations typically lead to light scattering. Here, controlled precipitation of α‐Al₂O₃ from nonstoichiometric spinel is explored to demonstrate unique control over the evolution of second phase Al₂O₃ and how the microstructure might be altered to enhance fracture toughness while minimizing light scatter. Alumina‐rich magnesium aluminate spinel, MgO·nAl₂O₃, where n=2, was hot pressed and HIPed to produce fully dense, single‐phase material. The material was then heat treated in air at 1573 K for up to 20 hours to create a two‐phase spinel‐Al₂O₃ composite. The fracture toughness varies from 0.88 to 2.47 MPa√m depending on the microstructure; enhanced toughness at the surface was due to increased crack tortuosity at phase boundaries, but residual tensile stresses were observed in the interior of the material. Precipitation causes local volume contraction and the formation of porosity, decreasing optical transmission, especially for heat treatment times longer than 5 hours.     

Ethanol‐Water‐Derived Sucrose‐Coated‐Al2O3 for Submicrometer AlON Powder Synthesis

Fluffy and homogenous sucrose‐coated‐γ‐Al₂O₃ structured precursor was prepared by drying ethanol‐water sucrose/Al₂O₃ suspension, in which the ethanol content of 85 vol% was optimized. Using the C/Al₂O₃ mixture pyrolyzed from such precursor with 23.2 wt% sucrose, single‐phase AlON powder was synthesized by two‐step carbothermal reduction and nitridation method at 1550°C for 2 h and 1700°C for another 1.5 h. The particle size of the AlON powder was around 0.6–1.0 μm. Compared with those synthesized by the traditional approaches with mechanical C/Al₂O₃, Al/Al₂O₃, or AlN/Al₂O₃ mixtures, the synthesis temperature was reduced about 50°C, and the AlON powder was fine and exhibited good dispersity. Such superiority of this method was attributed to that the pyrolyzed carbon film on Al₂O₃ particle greatly restrained Al₂O₃ coalescence during the thermal treatment.     

Microstructure‐property relation in alumina ceramics during post‐annealing process after laser shock processing

Laser shock processing (LSP) is a new surface engineering approach to introduce significant compressive residual stress into ceramics to improve their mechanical properties. However, LSP of ceramics may induce microcracks, which limit the further improvement of mechanical properties of ceramics. In this research, the effect of a post‐LSP annealing process on α‐Al₂O₃ ceramics was investigated. The annealing treatment can cause thermal relaxation of compressive residual stress generated by LSP while still maintain the positive attribute of LSP. The compressive residual stress was stabilized after annealing after 10 hours at 1100‐1300°C. The healing of microcracks in α‐Al₂O₃ ceramics was observed during the post‐LSP annealing process, which is caused by diffusion bonding mechanisms and accompanied by dislocation and void formation. The combination of the stabilized compressive residual stress and microcrack healing can improve the cracking resistance of α‐Al₂O₃ ceramics to mechanical impact on the surface by 69%.     

Effect of Al Speciation on the Structure of High‐Al Steels Mold Fluxes Containing Fluoride

To design suitable mold fluxes for the casting of high‐Al steels, the structure of mold fluxes based on CaO–SiO₂, CaO–SiO₂–Al₂O₃, and CaO–Al₂O₃ was examined by Raman spectroscopy and magic‐angle spinning nuclear magnetic resonance. The results showed that Si atoms are replaced by Al atoms as the network formers with the increase in Al₂O₃ in the mold fluxes. This converts the silicate slags (CaO–SiO₂ mold fluxes) into aluminosilicates slags (CaO–SiO₂–Al₂O₃ or CaO–Al₂O₃ mold fluxes). The F^? ions in the mold flux containing Al₂O₃ are classified into three categories, according to function: Bridging F's, Nonbridging F's, and Free‐F's. The Al³⁺ ion holds three distinct coordination environments: ^IVAl, ^VAl, and ^VIAl. The addition of F affects the coordination environment of Al³⁺ to form AlO₃F and AlO₂F₂ that accommodate the network structure of slags. The network structure in the CaO–SiO₂ mold fluxes is mainly connected through Si–O–Si linkage. However, the network structure of the mold fluxes containing elevated content of Al₂O₃ is mainly connected through Si–O–Si, Al–O–Al, Al–O–Si, and Al–F–Al linkages. Hence, the structural characteristics of high‐Al steels mold fluxes must be considered during the designing step of the mold fluxes.     

Carbon nanotube/graphene oxide‐added CaO‐B2O3‐SiO2 glass/Al2O3 composite as substrate for chip‐type supercapacitor

A CaO‐B₂O₃‐SiO₂ (CBS) glass/40 wt% Al₂O₃ composite sintered at 900°C exhibited a dense microstructure with a low porosity of 0.21%. This composite contained Al₂O₃ and anorthite phases, but pure glass sintered at 900°C has small quantities of wollastonite and diopside phases. This composite was measured to have a high bending strength of 323 MPa and thermal conductivity of 3.75 W/(mK). The thermal conductivity increased when the composite was annealed at 850°C after sintering at 900°C, because of the increase in the amount of the anorthite phase. 0.25 wt% graphene oxide and 0.75 wt% multi‐wall carbon nanotubes were added to the CBS/40 wt% Al₂O₃ composite to further enhance the thermal conductivity and bending strength. The specimen sintered at 900°C and subsequently annealed at 850°C exhibited a large bending strength of 420 MPa and thermal conductivity of 5.51 W/(mK), indicating that it would be a highly effective substrate for a chip‐type supercapacitor.     

On the Mechanism of Low Temperature Oxidation for Aluminum Particles down to the Nano‐Scale

The oxidation of Al‐particles down to nano‐scale was investigated by TG, SEM and in‐situ X‐ray diffraction. Al particles are usually coated by a 2–4 nm layer of Al₂O₃ which can be derived from the degree of weight increase on complete oxidation by TG‐curves. The low temperature oxidation of Al particles occurs at least in two steps. The first step builds a layer of 6 to 10 nm thickness composed of crystallites of the same size independent on the initial particle size. This reaction is dominated by chemical kinetics and converts a substantial fraction of the particle if the particle sizes decrease below 1 μm, an effect carefully to be taken into account for nano‐particles because of safety reasons. The second step combines diffusion and chemical reaction and proceeds therefore slowly, the slower the bigger the particles are. The kinetic parameters of these two steps can be obtained by a model taking into account both reaction steps, chemical kinetics and diffusion for spherical particles when fitting it to TG‐curves. X‐ray diffraction shows that particles smaller than 1 μm build γ‐ and θ‐Al₂O₃ in the first step with nano‐crystalline structures which are then transformed to α‐Al₂O₃.