Alumina-glass Composites Research Articles

Synthesis of bioinspired ice-templated bulk metallic glass-alumina composites with intertwined dendritic structure

We have successfully developed bulk-metallic glass (BMG) composites reinforced with intertwined dendritic alumina, inspired by the structure of nacre (Abalone shells). We harness the dendritic growth of ice crystals in a thermal gradient to assemble an alumina with dendritic pore architecture, which is then infiltrated with BMG-forming melt to make a bioinspired, micrometer-scale, intertwined BMG-alumina composite. The composite exhibits ultra-high strength with exceptional relative compressive strength, far better than ceramic particulate BMG composites. The method presented here can be further exploited to produce novel BMG composites with excellent mechanical properties.

Glass alumina composites for functional and structural applications

Abstract A SiO2-Al2O3-CaO based glass was mixed with Al2O3 at various ratios to fabricate composites for functional and structural applications such as substrate, low temperature co-fired ceramics and radomes, which necessitates a thorough characterization of mechanical, thermal and dielectric properties. The optimum densification temperatures decreased from 1350 °C (20 wt% glass) to 1200 °C (40 wt% glass), and to 850 °C (50–60 wt% glass) due to liquid phase sintering for the samples with 20 and 40 wt% glass, and viscous sintering for the samples with 50-60 wt% glass. Main phase was Al2O3 as well as anorthite crystallized after 825 °C. Glass addition decreased the mechanical properties such as Young's modulus from 231 GPa (20 wt% glass) to 105 GPa (60 wt% glass), flexural strength from 387 MPa (20 wt% glass) and 213 MPa (60 wt% glass). Dielectric constant and dielectric loss of the composites with 20-60 wt% glass were 8.76–7.32 and 0.0022–0.0081 at 5 MHz, respectively. Thermal expansion coefficient also decreased with increasing glass content from 7.72 ppm/°C (20 wt% glass) to 6.66 ppm/°C (60 wt% glass). The composites with 50-60 wt% glass were suitable for substrate and low temperature co-fired ceramic applications due to lower densification temperatures (

The size-effect of Al2O3 on the sinterability, microstructure and properties of glass-alumina composites

The glass-ceramic composites were prepared from low-softening glass CaO–B2O3–SiO2, micrometer and nanometer alumina. The influences of nano-Al2O3 on the sinterability, microstructure, dielectric property, mechanical strength, and thermal expansion of glass-alumina composites were investigated. The densification mechanism of nano-Al2O3 modified glass-alumina composites was revealed further. The results show that nano-Al2O3 effectively lowers the densification temperature of glass-alumina ≤900°C. The density gradually increases with increasing nano-Al2O3 to 15 wt %, and then decreases. Dielectric constant, dielectric loss, and flexural strength respectively versus nano-Al2O3 exhibit a similar change trend to the density. The sample with 15 wt % nano-Al2O3 sintered at 900°C could form the dense microstructure (~2.95 g/cm3) and achieve the high performance: low dielectric constant (6.13), low dielectric loss (5.2 × 10−4), low thermal expansion (4.21 × 10−6/°C), and high mechanical strength (178 MPa).

Lanthanum glass infiltrated alumina/alumina composites for dental prosthetic applications

Lanthanum glass frits suitable for infiltration in pre-sintered alumina were developed in order to obtain innovative alumina–glass composites, which are promising materials for all-ceramic dental prosthetic restorations, such as single crowns and fixed partial dentures. Glass compositions belonging to the lanthanum-silicate-borate system were synthesized in order to reinforce and give an improved esthetic appearance to the pre-sintered Al2O3.Fifteen formulations, defined by using a mixture design approach, were melted and quenched in water at room temperature in order to obtain the frits. Those exhibiting the lower melting temperatures were tested for infiltration into Al2O3 to obtain Al2O3/glass composites. The frits and the composites were characterized by X-Ray diffraction, differential thermal analyses, hot stage microscopy, colorimetry (Hunter color scale) and scanning electron microscopy. The mechanical properties of the composites, in terms of flexure strength, were evaluated using a three-point bending tests.

Effect of alumina nanofiller on properties of heat-treated glass composite sealants.

Alkali/alkaline-earth borosilicate glass-alumina composites containing 10 vol% Al2O3 were prepared for use as solid oxide fuel cell sealants. The effect of heat treatment and Al2O3, addition on the viscosities and electrical conductivities was investigated to improve cyclic sealing performance. Upon a 48-h heat treatment, the viscosity of the glass-alumina composites at 750 degrees C was approximately four orders of magnitude higher than that of the base glass owing to the crystallization of the glass in the presence of Al2O3. Heat treatment increased the electrical conductivities of both the base glass and the glass-alumina composites. The electrical conductivities of glass-alumina composites in the range from 400 degrees C to 550 degrees C were three times higher than those of the base glass regardless of heat treatment. This increase in the conductivities and viscosities by heat treatment was attributed to the devitrification and structural densification of the sealing glass and the partial dissolution of the Al2O3 filler in alkali/alkaline-earth borosilicate sealing glass.

Structural, electrical and thermal properties of borosilicate glass–alumina composites

Borosilicate glass–alumina composites with (1−x) Glass+x Al2O3 (x=0, 5, 10, 25vol.%) were prepared and the effect of Al2O3 addition on the structural, electrical and thermal characteristics was investigated. XRD patterns revealed the presence of cristobalite (SiO2) in sintered borosilicate glass and that the addition of Al2O3 hinders cristobalite formation. This behavior is due to the diffusion of some Al3+ ions from alumina to glass, which leads to changes in glass structure and composition as identified by SEM/EDS. Cristobalite was undetected in composites containing 10% Al2O3 that attained the lowest thermal expansion coefficient value (∼4.6×10−6°C−1). Conductivity (dc and ac) increased with the amount of Al3+ ions present in the glass structure as modifiers and formers. Dielectric constant values, in the range 5.0–7.2, increased with Al2O3 addition and the values of loss tanδ (1.5–2.1×10−2) indicate that these materials are good insulators.

Glass–alumina composite coatings for high temperature corrosion protection. Part I: Effect of crystallization and interfacial reaction on the thermo-physical properties

The effect of crystallization and interfacial reaction on the thermo-physical properties of glass–alumina composites, which show good prospects for use as coating materials for high-temperature corrosion protection, has been investigated by using post-annealing processes at 850 °C. Zirconolite precipitated from the glass matrix and the alumina inclusions were steadily transformed into gahnite and feldspar by interfacial reactions. This phase evolution resulted in an overall decrease in strength, Young's modulus, Vickers hardness and thermal expansion coefficient while increasing the indentation toughness. The composites became more refractory and thus better prospects for use in high temperature applications.

Kinetics of integrated liquid phase sintering of glass-alumina composite: Activated pores evolution

Activated pores in the glass-alumina composite during isothermal sintering at 710?C by colloidal processing were extracted by the threshold conversion of the micrograph obtained with a field emission scanning microscope. The relative ratio of the activated pores in the final stage of sintering to that in the initial stage of sintering was calculated. The kinetics equation of the evolution of the activated pores during isothermal sintering was established. The pores evolution confirms the viscous flow of the glass matrix, indicating the validity of the predominant liquid-phase sintering of the composite.

Thermal analysis and in vitro bioactivity of bioactive glass–alumina composites

Bioactive glass–alumina composite (BA) pellets were fabricated in the range 95/5–60/40wt.% respectively and were heat-treated under a specific thermal treatment up to 950°C. Control (unheated) and heat-treated pellets were immersed in Simulated Body Fluid (SBF) for bioactivity testing. All pellets before and after immersion in SBF were studied by Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM-EDS) and X-ray Diffraction (XRD) analysis. All composite pellets presented bioactive response. On the surface of the heat-treated pellets the development of a rich biological hydroxyapatite (HAp) layer was delayed for one day, compared to the respective control pellets. Independent of the proportion of the two components, all composites of each group (control and heat-treated) presented the same bioactive response as a function of immersion time in SBF. It was found that by the applied methodology, Al2O3 can be successfully applied in bioactive glass composites without obstructing their bioactive response.

Threshold conversion for field emission scanning electron micrograph of glass-alumina composites in determining the activated interfaces

Activated alumina interfaces in glass-alumina composites during isothermal sintering at 710?C by a novel colloidal processing was extracted by the threshold conversion of the micrograph obtained by a field emission scanning microscope. The relative ratio of the activated alumina interfaces in the final stage of sintering to that in the initial stage of sintering was calculated. The kinetics equations of both the evolution for the activated alumina interfaces during isothermal sintering and its effect on the linear shrinkage of the glass-alumina composites were established. The results show that the activated alumina interfaces in the final stage of sintering are 1.256 times more than that in the initial stage of sintering. The increased alumina interfaces decreases the total shrinkage rate of the glassalumina composites, in which a particular obstruction of the viscous flow of the glass matrix is suggested.

Bond Strengths of Alumina-Glass and Zirconia-Glass Composites to Opaque-Dentine Porcelain

This study was conducted to evaluate shear bond strength between self-made alumina glass composites (AGC), zirconia glass composites (ZGC) and Vita alpha opaque-dentine porcelain, and also evaluate the effects of different surface treating methods on the bond strength. The AGC and ZGC specimens were treated differently and then bonded with Vita alpha opaque-dentine porcelain. The bond strength was measured by shear test and the surface treating methods included no-treatment, sandblasting and etching. The results showed that the values of bond strength for AGC groups were 30.1, 41.1 and 41.9 MPa respectively, and the bond strength of both sandblasting group and etching group were significantly higher than that of the untreated group; the bond strength of ZGC groups were 60.2, 63.6 and 35.5MPa respectively, and the value of sandblasting group was significantly higher while etching group was lower than that of untreated group. These results indicated that Vita alpha opaque-dentine porcelain can be well sintered to self-made AGC and ZGC. Sandblasting can greatly improve the bond strength for both AGC and ZGC, while the etching treating method had different effect on their bond strengths.

Near net-shape fabrication of alumina glass composites

The purpose of the present study is to fabricate alumina glass composites by melt infiltration with better dimensional control through reducing both the presintering and infiltration temperature. Main efforts were put to develop glasses that are chemically compatible with alumina. After extensive investigations, a glass of 21SiO 2–24B 2O 3–35Al 2O 3–15Li 2O–5CaO wt.% was successfully developed. The glass shows good chemical compatibility with alumina at elevated temperatures and low viscosity above 900 °C. Dense alumina glass composites can be fabricated by the melt infiltration process at 950 °C, which is 150 °C lower than the current state-of-art. Investigations showed improved net-shape capability for the newly developed composites, where the total linear shrinkage for the sintering and infiltration at 950 °C is less than 0.1%, as compared with the shrinkage of 0.5% induced by the presintering and infiltration at 1100 °C. Preliminary mechanical tests showed that the fracture strength and toughness of the composites are 303 MPa and 3.4 MPa m 1/2, respectively. The lower processing temperature and the better dimensional control are the major advantages for the newly developed alumina glass composites.

용융침투법으로 제조한 알루미나-유리 복합체의 내마모 특성

용융침투법으로 제조한 알루미나-유리 복합체의 내마모 특성

Effect of alumina particle size and distribution on infiltration rate and fracture toughness of alumina–glass composites prepared by melt infiltration

Four commercial alumina powders having different particle sizes (0.44, 2.85, 7.08 and 39.81 μm) were presintered for 2 h at 1120 °C and then lanthanum–aluminosilicate glass was infiltrated for up to 4 h at 1100 °C to prepare the densified alumina–glass composites. Alumina having a bimodal and broader particle size distribution was the most effective to the packing. The penetration rate constant increased with raising the alumina particle size having a parabolic dependence of infiltration distance on time described by the Washburn equation. However, the optimum mechanical properties were observed for the composite containing the 2.85 μm alumina. Although the higher fracture toughness of the composites may be achieved by increasing the alumina particle size probably due to the dispersion toughening, the larger alumina particles reduce the strength.

Strength and fracture toughness of MgO-modified glass infiltrated alumina for CAD/CAM

Objective: This study was conducted to investigate the effect of MgO additive to Al 2O 3 on the flexural strength, fracture toughness of glass infiltrated alumina for CAD/CAM application. Methods: Alumina blanks with additive of 0.5 wt% MgO were prepared via isostatic pressing and sintering at 1400°C for 2 h, and then alumina–glass composites were fabricated by infiltrating the molten glass into the partially sintered alumina compact. Flexural strength and fracture toughness were determined using three point bending methods and a single edge notched beam method. The mechanism of crack propagation was observed under a field emission scanning electron microscope. Results: The three-point flexural strength and fracture toughness of partially sintered alumina and alumina–glass composite were 210 MPa, 1.86 MPam 1/2, and 432.2 MPa, 5.12 MPam 1/2, respectively, and they were free of shrinkage during the processing of glass infiltration. The field emission SEM micrograph indicated that indentation caused a non-planar crack propagation including crack deflection and crack bowing. Significance: MgO was used as an additive to alumina to improve the strength and fracture toughness of partially sintered alumina and alumina–glass composite. The high strength and toughness are related to toughening by the distribution of alumina with uniform particle sizes, crack bowing, crack deflection and the beneficial wetting properties of the particle surface.

Compaction and sintering behaviour of glass–alumina composites

Abstract The effects of Al2O3 additions on the compaction and sintering behaviour of a leadborosilicate glass (LG) have been investigated. LG powder was prepared by melting, fritting and milling a glass of the composition: 77PbO, 10B2O3, 10SiO2, 2Al2O3 and 1P2O5 (wt.%). The mean particle sizes of the powders were: LG, 6.5 μm and Al2O3, 3.3 μm. The compaction behaviour of LG–Al2O3 powder mixtures can be represented by a new compaction equation: [(Dg−D0)/(1−D0)]=(P/Pf)n, where Dg is the relative green density, D0 the relative tap density and n and Pf are material constants. The exponent n decreases from 0.192 to 0.065 as the Al2O3 content is increased from 0 to 100 vol.%. The Frenkel equation for isothermal shrinkage has been found to be valid. It is shown that in the glass matrix composites the minimum sintering temperature can be determined by measuring the dilatometric deformation temperature. The presence of Al2O3 in excess of 15 vol.% has been found to strongly retard the sintering kinetics. An addition of 45 vol.% Al2O3 increases the activation energy for sintering from 67 to 112 kcal mol−1. The presence of Al2O3 particles also induced a partial crystallisation in LG matrix.

Mesoporous glass films supported on α-Al 2O 3

Glass–alumina composites were developed for possible use as membrane supports. Preparation involved dip-coating of α-Al 2O 3 tubes with a suspension of borosilicate glass particles (9.1% Na 2O–29.7% B 2O 3–61.2% SiO 2), sintering to convert the particle layer into a nonporous layer of phase separated glass, and leaching the glass with a strong acid to remove the soluble phase and obtain the final porous layer of about 10 μm thickness. In some preparations a 10% α-Al 2O 3 powder was added to the initial suspension. The composite supports were characterized by SEM, EDAX and EPMA for elemental composition, XRD for crystalline phase content, nitrogen adsorption for surface area and pore size distribution, and by permeation measurements with single gases and mixtures. After leaching the glass layers had pore size 1–4 nm, and contained varying amounts of boron, sodium, and aluminum oxides, in addition to silica. The nitrogen permeance of the composite supports was 10–100 times higher than that of standard porous Vycor tubing.

Mechanical Properties and Microstructure of Alumina‐Glass Composites

Alumina blanks were prepared via isostatic pressing and sintering at 1400°C for 2 h, whereas alumina‐glass composites were prepared by infiltrating the molten glass into the partially sintered alumina compacts. The samples had a high bending strength (340 MPa) and high fracture toughness (3.91 MPa·m1/2) and were free of shrinkage. Concurrently, zirconia (5 wt%) was used as an additive to alumina to improve the mechanical properties of both partially sintered alumina and alumina‐glass composites. Results show that zirconia notably improves the mechanical properties of the partially sintered alumina but increases that of the composites by no more than 10%.

Rapid Full Densification of Alumina-glass Composites Fabricated by a Selective Laser Sintering Process

Rapid Full Densification of Alumina-glass Composites Fabricated by a Selective Laser Sintering Process

Shrinkage‐Free, Alumina–Glass Dental Composites via Aluminum Oxidation

Alumina–glass composites that contain oxidized aluminum were investigated to make shrinkage‐free, all‐ceramic dental porcelain. This was achieved by control of the volume expansion and shrinkage that occur during the aluminum oxidation and densification processes. The dimensional change and strength of the alumina‐glass composites were dependent on the aluminum content and particle size. A shrinkage‐free sample with a strength of 330 MPa and transmittance of 1.21 was fabricated by using 4‐5 vol% of aluminum powder with an average particle size of 6 μ m in the alumina‐glass composite made by the In‐Ceram method.