Effects of Y2O3 and La2O3 addition on the crystallization of Li2O·Al2O3·4SiO2 glass-ceramic

Effects of adding Y₂O₃ and La₂O₃ on the crystallization of -quartz solid solution (ss) and the subsequent -quartz ss to -spodumene transformation of Li₂O·Al₂O₃·4SiO₂ glass-ceramic were investigated. Adding 4 mol% YO_3/2 or 8 mol % LaO_3/2 effectively improved the control of the crystallization process of the glass. Y₂O₃ did not effectively induce bulk crystallization of -quartz ss, but can reduce the rate of surface crystallization. La₂O₃ completely suppressed the surface crystallization and promoted a uniform, bulk crystallization of -quartz ss. For both the Y₂O₃- and La₂O₃-doped glasses, the kinetics for glass crystallization to -quartz ss was delayed as the doping level increased. Except for the 8 mol % LaO_3/2-doped glass in which no -spodumene was formed, the kinetics for the -quartz ss to -spodumene transformation for the doped glasses was enhanced compared with that for the undoped glass. For the 4 and 8 mol % YO_3/2-doped compositions, the relative amount of -spodumene to -quartz revealed an anomalous decrease trend with heating temperature in a particular temperature range. This can be explained by the surface crystallization characteristic, which induced an overlap of crystallization and -quartz ss to -spodumene transformation. Glass doped with 8 mol % LaO_3/2 exhibited an Avrami exponent of about 2.4 and an activation energy for crystal growth of -quartz ss of about 418 kJ mol^–1.     

Dielectric properties of Lanxide™ Al2O3/Al composites

The dielectric properties of unreinforced Lanxide Al₂O₃/Al composites have been investigated over a wide range of temperatures and frequencies. These composites were formed by the directed oxidation of suitably doped aluminium-based alloy melts, with no filler or reinforcing material in the reaction path. As-grown composite materials were good electrical conductors in all directions owing to the presence of an interconnected metallic constituent. As the metallic phases were partially removed (in favour of porosity) by continuing the oxidation reaction to completion, the composites remained electrically conducting parallel to, and became insulating transverse to, the original growth direction of the composite. This anisotropy apparently was caused by different connectivity of the metal phase between the two directions. Thermal treatments at 1600°C in argon resulted in volatilization of the residual metal in the composite, thus further increasing the porosity. As the metal content was decreased, the composites changed from conducting to insulating along the growth direction. When the metallic phase was removed completely, the porous alumina ceramic maintained anisotropic dielectric properties, due to c-axis alignment of the alumina (corundum) phase along the growth direction. The dielectric constants were 8.0 and 6.4, respectively, parallel and perpendicular to the c-axis aligned directions of the porous alumina ceramic. A dielectric relaxation phenomenon was observed in some samples of both as-grown and thermally treated material, and was attributed to an unidentified impurity effect.     

Processing and properties of Al–Li–SiCp composites

Al–Li–SiC_p composites were fabricated by a modified version of the conventional stir casting technique. Composites containing 8, 12 and 18 vol% SiC particles (40 mm) were fabricated. Hardness, tensile and compressive strengths of the unreinforced alloy and composites were determined. Ageing kinetics and effect of ageing on properties were also investigated. Additions of SiC particles increase the hardness, 0.2% proof stress, ultimate tensile strength and elastic modulus of Al–Li–8%SiC and Al–Li–12%SiC composites. In case of the composite reinforced with 18% SiC particles, although the elastic modulus increases the 0.2% proof stress and compressive strength were only marginally higher than the unreinforced alloy and lower than those of Al–Li–8%SiC and Al–Li–12%SiC composites. Clustering of SiC particles appears to be responsible for reduced the strength of Al–Li–18%SiC composite. The fracture surface of unreinforced 8090 Al-Li alloy (8090Al) shows a dimpled structure, indicating ductile mode of failure. Fracture in composites occurs by a mixed mode, giving rise to a bimodal distribution of dimples in the fracture surface. Cleavage of SiC particles was also observed in the fracture surface of composites. Composites show higher peak hardness and lower peak ageing time compared with unreinforced 8090Al alloy. Macroand microhardness increase significantly after peak ageing. Ageing also results in considerable improvement in strength of the unreinforced 8090Al alloy and its composites. This is attributed to formation of δ^' (Al₃Li) and S^' (Al₂CuMg) precipitates during ageing. Per cent elongation, however, decreases due to age hardening. Al–Li–12%SiC, which shows marginally lower UTS and compressive strength than the Al–Li–8%SiC composite in extruded condition, exhibits higher strength than Al–Li–8%SiC in peak-aged condition.     

Microstructures and properties of in-situ NiAl–Al2O3 functionally gradient composites

NiAl–Al₂O₃ functionally gradient composites (FGCs) have been fabricated by reactive hot compaction of Ni and Al powders in which one or both were partially preoxdized. The FGCs consisted of four or five layers with alumina content increasing up to about 35 vol.%. The gradient in the composition was obtained by stacking powder mixtures of desired compositions. It was found that the technique resulted in a gradual transition of the microstructure and microhardness from NiAl to NiAl–35 vol.%Al₂O₃. In contrast, the NiAl– (NiAl–35 vol.%Al₂O₃) bilayer compact showed a much steeper microhardness change with pronounced residual stress at the boundary. The FGCs were found to have higher fracture toughness values than the corresponding composites.     

Fabrication of bioglass infiltrated Al2O3–(m-ZrO2) composites

Using 80 vol.% of poly methyl methacrylate (PMMA) as a pore-forming agent to obtain interconnected porous bodies, porous Al₂O₃–(m-ZrO₂) bodies were successfully fabricated. The pores were about 200 μm in diameter and were homogeneously dispersed in the Al₂O₃–25 vol.% (m-ZrO₂) matrix. To obtain Al₂O₃–(m-ZrO₂)/bioglass composites, the molten bioglass was infiltrated into porous Al₂O₃–(m-ZrO₂) bodies at 1400°C. The material properties of the Al₂O₃–(m-ZrO₂)/bioglass composites, such as relative density, hardness, compressive strength, fracture toughness and elastic modulus were investigated.     

Microstructure and mechanical properties of SiC whisker reinforced ZrO2–6 mol.-%Y2O3 composites

Abstract

Microstructure, mechanical properties, fracture behaviour, and toughening mechanisms of hot pressed SiC whisker (SiC_w)

reinforced ZrO₂–6 mol.-%Y₂O₃ composites were investigated via transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and mechanical testing. The experimental results show that there is a continuous increase in the Vickers hardness, elastic modulus, and fracture toughness of the composites with increasing SiC_w content, and an addition of 30 vol.-%SiC_w increases the fracture toughness from 3·42 MN m^?3/2 for the unreinforced matrix to 5·83 MN m^?3/2. The flexural strength is increased from 293 MN m^?2 for the unreinforced matrix to a maximum of 372 MN m^?2 by an addition of 10 vol.-%SiC_w, then it is significantly decreased by further increasing the SiC_w content. Observations via transmission electron microscopy show that no distinct second phase or intermediate layers form at the SiC_w/ZrO₂ interface. Diffusional separation of tetragonal phase from the cubic matrix occurred during cooling after hot pressing. Whisker bridging and crack deflection are the main toughening mechanisms, but whisker pull-out, crack branching, and refinement of the matrix particles also contribute to the improvement in the fracture toughness.

MST/1747     

Sinter-HIP of polymer-derived Al2O3–SiC composites with high SiC contents

Submicrometer Al₂O₃ composites with more than 20 vol.% of SiC particles were produced using a multiple infiltration of porous bodies with a liquid polymer SiC precursor. The fully dense composites were successfully densified using a sinter-HIP process. Parameters of sintering and HIP steps are discussed with respect to both densification and microstructure evolution of the composites. The initial pressure during the sintering step plays an important role for the preparation of fully dense composites with a submicrometer alumina matrix at 1750 °C. Optimized densification schedule of sinter-HIP represents a novel approach of densification at relatively mild conditions compared to previously reported or common densification methods of Al₂O₃–SiC composites with high SiC content, such as pressureless sintering, hot pressing and post-HIPing. The method expands the possibilities for preparation of alumina based composites with SiC volume fraction > 20 vol.%, filling the gap in available literature data.     

Fabrication and mechanical properties of α-Al2O3/β-Al2O3/Al/Si composites by liquid displacement reaction

Al₂O₃/metal composites were fabricated by heating three kinds of commercial mullite refractories in contact with Al, and their mechanical properties were investigated. Aluminum reacted with the mullite and SiO₂-glass constituting the mullite refractories and changed them into -Al₂O₃ and Si. Simultaneously, -Al₂O₃ was formed by the reactions among -Al₂O₃ and sodium and potassium oxides in the glassy phase. Also, Al penetrated into the -Al₂O₃/-Al₂O₃/Si composite by partly dissolving Si. Finally, the mullite refractories were changed into -Al₂O₃/-Al₂O₃/Al/Si composites. The phase contents, microstructures and mechanical properties of the resulting composites varied with the composition of the refractories. The content of -Al₂O₃ in the composite was lowest at the lowest Na₂O and K₂O contents in the refractories. Silicon in the composite had its highest content at the highest SiO₂ content. The composite fabricated from SiO₂/Al₂O₃ (in mol) (SAR)=1.85 consisted of 2–5 m Al₂O₃ grains embedded in metal, but that from SAR=1.05 showed a complicated microstructure with small and large grains. The bending strength of the composites fabricated from the refractories of SAR=1.85, 1.24, and 1.05 were 327, 405 and 421 MPa, respectively. Also, the corresponding fracture toughness values were 5.2, 6.1, and 5.5 MPam , respectively.     

Microstructure and mechanical properties of SiC Whisker reinforced ZrO2–Y2O3 composites

Abstract

The mechanical properties of 20 vol.-%SiC whisker reinforced ZrO₂?V₂O₃ composites containing 2 and 6 mol.-% Y₂O₃ were measured at room temperature and the fracture surface was examined. The results indicate that the mechanical behaviour of the composites is strongly influenced by the Y₂O₃ content. The magnitude of the enhancement of the toughness in composites containing 2 mol.-% Y₂O₃ compared with unreinforced ZrO₂?Y₂O₃ matrix is larger than that for the composites containing 6 mol.-% Y₂O₃. Crack propagation modes were characterised by crack deflection, whisker bridging, and whisker pullout. High resolution electron microscopic observations show that in composites containing 2 mol.-% Y₂O₃ the whiskers are directly bonded to the matrix. However, in composites containing 6 mol.-% Y₂O₃ there is always a thick amorphous layer at the interface, indicating that the high Y₂O₃ content has promoted the formation of interfacial amorphous layers. These interfacial amorphous layers strengthen the interfacial bonding, resulting in a composite with a low fracture toughness.

MST/2043     

Fabrication and mechanical properties of woven Al2O3 fibre – ZrO2 matrix minicomposite reinforced Al2O3 matrix composites by slurry infiltration – sintering process

Abstract

A plain woven Al₂O₃ fibre - ZrO₂ minicomposite reinforced Al₂O₃ matrix composite ((Al₂O₃)_f/ZrO₂)_mc/Al₂O₃) has been fabricated by a simple, pressureless, multiple slurry infiltration - sintering process. The fabrication process consisted of two major steps: fabrication of the woven Al₂O₃ fibre - ZrO₂ minicomposite ((Al₂O₃)_f/ZrO₂)_mc) and fabrication of the composite ((Al₂O₃)_f/ZrO₂)_mc/Al₂O₃. The woven fibre form of ((Al₂O₃)_f/ZrO₂) _mc was made by dipping a plain woven Al₂O₃ fabric into colloidal ZrO₂ solution. Then Al₂O₃ powder dispersed slurry was infiltrated into the open spaces of the woven fabric ((Al₂O₃) /ZrO₂)_mc. The infiltrated minicomposites were stacked, pressed, and sintered in ambient air to form a plate of ((Al₂O₃)_f/ZrO₂)_mc/Al₂O₃. The optimum sintering temperature of ((Al₂O₃)_f/ZrO₂)_mc/Al₂O₃ was investigated via tensile strength and matrix density. Tensile behaviour and fracture resistance of the composite fabricated under the optimum processing condition were evaluated. The tensile stress - strain behaviour of the optimised composite showed non-catastrophic fracture behaviour with bundle bridging and extensive fibre bundle pullouts. The maximum tensile stress revealed that almost full strength expected from the bare bundle strength was exploited by the proposed process.     

Effect of Bi2O3 on structural and optical properties of Li2O·PbO·Bi2O3·B2O3 glasses

Journal of Materials Science: Materials in Electronics - The quaternary glass system has a composition of 30Li2O·20PbO·xBi2O3·(50-x)B2O3 (where x?=?0, 10, 20, 30, and...     

Sintering and crystallization of off-stoichiometric BaO·Al2O3·2SiO2 glasses

Glass-ceramics with off-stoichiometric celsian composition of 50 wt% BaO·2SiO₂ – 50 wt% BaO·Al₂O₃·2SiO₂, (B2S-BA2S) were fabricated and investigated for their sintering and crystallization characteristics. (B2S-BA2S) glass powder showed a melting temperature much lowered compared to that of stoichiometric BaO·Al₂O₃·2SiO₂ (BA2S) glass powder and high sintering ability. (B2S-BA2S) glass powder containing B₂O₃, (B2S-BA2S)B and that containing B₂O₃ and TiO₂, (B2S-BA2S)BT revealed much lowered crystallization peak temperatures, but rather low sintered density. By applying Kissiger analysis to differential thermal analysis (DTA) data activation energy values for crystallization were determined as 265, 195 and 242 kJ/mol, respectively for (B2S-BA2S), (B2S-BA2S)B and (B2S-BA2S)BT glasses. X-ray diffraction (XRD) patterns from all the glass-ceramics crystallized at 1100°C for 4 h revealed formation of crystalline phases of -BaO·2SiO₂, monocelsian and hexacelsian. (B2S-BA2S) glass-ceramics crystallized at 1400°C for 4 h showed formation of -BaO·2SiO₂ and monocelsian phases with only trace of metastable hexacelsian phase.     

Sintering and crystallization of off-stoichiometric SrO·Al2O3·2SiO2 glasses

Glass-ceramics with the celsian-corundum binary join composition of 88.8 wt% SrO·Al₂O₃·2SiO₂ – 11.2 wt% Al₂O₃, (SA2S-A), were fabricated by pressureless sintering and investigated for their sintering and crystallization behaviors. The (SA2S-A) glass powder showed crystallization peak and melting temperatures of 1059 and 1550 °C, respectively and high sintering ability. The (SA2S-A) glass powders containing B₂O₃, (SA2S-A)B and those containing B₂O₃ and TiO₂, (SA2S-A)BT showed lowered crystallization peak temperatures of 1033 and 997 °C, respectively. By applying Kissiger analyses to the DTA data of the (SA2S-A), (SA2S-A)B and (SA2S-A)BT glass powders, the activation energy values for crystallization were determined as 488, 370 and 333 kJ/mol, respectively. The Ozawa analyses on the DTA data gave the Avrami parameter values at 1.2, 1.1 and 1.9, respectively for the (SA2S-A), (SA2S-A)B and (SA2S-A)BT glass powders. The x-ray diffraction (XRD) patterns of the (SA2S-A) glass-ceramics, crystallized at 1100 °C for 4 h, showed formation of both the monocelsian and hexacelsian phases. The (SA2S-A)B and (SA2S-A)BT glass-ceramics crystallized at 1100 °C for 1 h, showed formation of the phase-pure monocelsian and did not show any evidence of the hexacelsian formation prior to the monocelsian formation.     

Effects of MgO on properties of Li2O–Al2O3–SiO2 glass–ceramics for LTCC applications

Li₂O–Al₂O₃–SiO₂ (LAS) glass–ceramics for low temperature co-fired ceramics (LTCC) application were prepared by melting method, and the effects of MgO on the sinterability, microstructure, dielectric property, thermal expansion coefficient (CTE) and mechanical character of this glass–ceramics have been studied. The X-ray diffraction images represent that the main phase is β-spodumene solid solutions. And some ZrO₂ and CaMgSi₂O₆ phases in LAS glass–ceramics are detected. The LAS glass–ceramics without additive (MgO) sintered at 800° had the dielectric properties: dielectric constant (ε_r) of 5.3, dielectric loss (tanδ) of 2.97 × 10^?3 at 1 MHz, CTE value of 1.06 × 10^?6 K^?1, bulk density of 2.17 g/cm³, and flexural strength of 73 MPa. 5.5 wt% MgO-added LAS glass–ceramic achieves densification at 800° exhibited excellent properties: low dielectric constant and loss (ε_r = 7.1, tanδ = 2.02 × 10^?3 at 1 MHz), low CTE (2.89 × 10^?6 K^?1), bulk density = 2.65 g/cm³ as well as high flexural strength (145 MPa). The results indicate that the addition of MgO is helpful to improve the dielectric and mechanical properties. The formation of CaMgSi₂O₆ crystal phase with higher CTE leads to the increase of CTE value of LAS glass–ceramics due to the increasing MgO content, and the increase of CTE is favourable for matching with silicon (3.1 × 10^?6 K^?1). The prepared LAS glass–ceramics have the potential for LTCC application.     

Influence of Al2O3/SiO2 ratio on the microstructure and properties of low temperature co-fired CaO–Al2O3–SiO2 based ceramics

In this work, in order to obtain the materials for low temperature co-fired ceramics applications, CaO–Al₂O₃–SiO₂ (CAS) based ceramics were synthesized at a low sintering temperature of 900 °C. The influences of Al₂O₃/SiO₂ ratio on the microstructure, mechanical, electrical and thermal properties were studied. According to the X-ray diffractomer and scanning electron microscopy results, the addition of the Al₂O₃ is advantageous for the formation of the desired materials. Anorthite(CaAl₂Si₂O₈) is the major crystal phase of the ceramics, and the SiO₂ phase is identified as the secondary crystal phase. No new crystal phase appears in the ceramics with the increasing Al₂O₃ content. More or less Al₂O₃ addition would all worsen the sintering, mechanical and dielectric properties of CAS based ceramics. The ceramic specimen (Al₂O₃/SiO₂ = 20/18.5) sintered at 900 °C shows good properties: high bending strength = 145 MPa, low dielectric constant = 5.8, low dielectric loss = 1.3 × 10^?3 and low coefficient of thermal expansion value = 5.3 × 10^?6 K^?1. The results indicate that the prepared CAS based ceramic is one of the candidates for low temperature co-fired ceramic applications.     

Fabrication and properties of novel composites in the system Al–Zr–C

Composite bodies in the system Al–Zr–C, with about 95% relative density, were obtained by heating the compact body of powder mixture consisting of Al and ZrC (5 : 1 mol %) in Ar at 1100–1500°C for various lengths of time. Components of the material heated at more than 1200°C were Al, Al3Zr, ZrC and AlZrC2. The Al3Zr exhibited plate-like aggregation, and its size increased with increasing temperature. In the material heated at 1500°C for 1 h, the largest plate-like Al3Zr aggregation was 2000 m long and 133 m thick. Then the AlZrC2 was present as well-proportioned hexagonal platelet particles with a 8–9 m diameter and a 1–2 m thickness in the interior of the plate-like Al3Zr aggregation and Al matrix phase. The average three-point bending strength of the bodies was 140–190 MPa, and the maximum strength was 203 MPa in the body heated at 1300°C for 1 h. The body heated at 1500°C for 1 h showed high oxidation resistivity to air up to 1000°C.     

Fabrication,superhydrophobicity, and microwave absorbing properties of the magnetic γ-Fe2O3/bamboo composites

Magnetic γ-Fe₂O₃ nanoparticles were successfully deposited on the surface of the bamboo via a coprecipitation process at room temperature. Spherical-like magnetic γ-Fe₂O₃ nanoparticles with a diameter of about 17 nm displayed well superparamagnetic behavior and were chemically bonded to the bamboo surface through the combination of hydrogen groups. With further modification by 1H,1H,2H,2H-perfluorodecyltri ethoxysilane (FAS-17), magnetic γ-Fe₂O₃/bamboo composites (MBCs) expressed superhydrophobic performances to not only water but also common liquids like coffee, milk, ink, tea, and coke. When immersed into the corrosive solutions including strong acid (pH = 1), heavy alkaline (pH = 14), and salt with high concentration (5 M) for 24 h, superhydrophobic magnetic γ-Fe₂O₃/bamboo composites (SMBC) still remained magnetism as well as superhydrophobicity. Also, under harsh conditions like boiled at 100 °C for 4 h, frozen at − 40 °C for 24 h, SMBCs were kept a robust magnetism and superhydrophobicity. Additionally, SMBC was a typical ferromagnet and exhibited some microwave absorbabilities.