The effect of Al content on the wettability between liquid iron and MgOAl2O3 binary substrate

In the present study, the wettability between liquid iron with two different Al contents and MgOAl₂O₃ binary substrates was studied in reducing atmosphere. The contact angles between liquid iron with 18?ppm Al and MgO, MgO·Al₂O₃, Al₂O₃ were 133.5°, 113.7°, 126.9° respectively. With the variation of the substrate composition, the contact angles for the intermediate binary phases of the three components (MgO, MgO·Al₂O₃, Al₂O₃) obeyed the Cassie theory. In the experiment using iron with 370?ppm Al, all the contact angles were higher than that using low Al-containing iron. The surface of the iron drop was covered with an oxide layer, which mainly consisted of many small particles. With the variation of the substrate gradually from MgO to Al₂O₃, the composition of the oxide layer changed from MgO·Al₂O₃ to CaOAl₂O₃. The formation of the oxide layer prevented the spreading of liquid iron, leading to the increase of the contact angle.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Effect of La doping on microwave dielectric properties of translucent polycrystalline alumina ceramic

In this paper, the microstructure and microwave dielectric properties of translucent polycrystalline alumina (PCA) with different amounts of La₂O₃ (co-doped with 500 ppm MgO) were investigated. Compared with those doped only with 500 ppm MgO, the translucent PCA co-doped with 500 ppm La₂O₃ exhibited a higher Q×f value, which might be caused by the significantly larger ionic radius of La³⁺. As the La₂O₃ content was further increased, the presence of an increasing amount of impurities (LaAl₁₁O₁₈) would deteriorate the Q×f value significantly.     

Novel synthesis and thermal property analysis of MgO–Nd2Zr2O7 composite

MgO-Nd₂Zr₂O₇composites with ratios of 50–70 vol% MgO were produced via a one-pot combustion synthesis. A suite of characterization techniques, including X-ray diffraction, scanning and transmission electron microscopy were employed to investigate the structural properties while dilatometry, simultaneous thermal analysis and laser flash analysis were used to characterize the thermal properties of the composites. Dense pellets were produced after sintering at 1400 °C with grain sizes between 200 and 500 nm for both phases. The thermal properties of the composites are similar to those produced using standard methods. The composite with 70 vol% MgO was found to have the highest thermal conductivity below 1000 °C, while above this temperature the thermal conductivity was found to be similar and independent of MgO content. This novel synthesis route produces materials which show significant improvements in homogeneity with smaller particle sizes when compared to current standard synthesis techniques without significantly reducing thermal conductivity.     

The influence of silica nanoparticles addition on the physical,mechanical, thermo-mechanical as well as microstructure of Mag-Dol refractory composites

The high hydration potential of CaO and MgO phases restricted the application of Mag-Dol refractory composites. In this study, the impact of nano-silica (SiO₂) addition on the physical, mechanical, thermo-mechanical as well as microstructure of Mag-Dol refractory composites is investigated. Mag-Dol compositions were prepared by using calcined dolomite and magnesite particles (micron, 0–1, 1–3, 3–5, and 5–8 mm), liquid resin, and 0, 0.5, 1, 1.5, 2, and 2.5 wt% nano SiO₂ as additives. Specimens were heated up to 1650 °C for the 3 h soaking period. Fired specimens were characterized by physical (apparent porosity, bulk density, and hydration resistance), mechanical (cold crushing strength), and thermo-mechanical (flexural strength at 1200 °C) measurements. XRD and SEM/EDS analysis were done to study phases and microstructure analysis of the fired samples, respectively. Results showed that by adding up to 2.5 wt% nano-SiO₂, due to the formation of CaO·MgO·2SiO₂ (Diopside), 2CaO·MgO·2SiO₂ (Akermanite), and CaO·MgO·SiO₂ (Monticellite) phases, physical and mechanical properties were enhanced. But the highest flexural strength value is achieved for 1 wt% nano-SiO₂ containing sample.     

Effect of pressure on corrosion of Inconel 625 in supercritical water up to 100 MPa with acids or oxygen

The corrosion behavior of Inconel 625 in supercritical water was investigated under high pressure (30–100 MPa at 400 °C) with the addition of acids (5 mmol/kg CH₃COOH or 0.5 mmol/kg HNO₃) or O₂ (2.5–250 mmol/kg). Ni and Cr ions were the main dissolved metal ions in the effluent. The Ni ion concentration showed no clear dependence on the O₂ concentration or pressure in the experiment with O₂; the concentration was lower than 0.05 ppm. The concentration increased with pressure up to 0.09 ppm and 3 ppm (at 100 MPa) in the experiment with CH₃COOH and HNO₃, respectively. The Cr ion concentration increased with the O₂ concentration and pressure; the maximum value of the concentration was about 0.5 ppm. The Cr ion concentration was lower than 0.01 ppm in the experiment with CH₃COOH, while the concentration was considerably higher in the experiment with HNO₃: the concentration increased with pressure up to 0.87 ppm at 100 MPa. The effect of pressure on the corrosion behavior of Inconel 625 was discussed using potential-pH diagrams, metal oxide solubilities, pH, and equilibrium of ionic reactions. Pressure dependence of the metal ion concentrations was analyzed using a model with water density as a parameter; the log–log plots revealed a linear relationship.     

Enhancing the sinterability and fracture toughness of Al2O3–TiN0.3 composites

We introduce a new and effective method for improving the fracture toughness of Al₂O₃-based composites through the addition of a nonstoichiometric material. Al₂O₃–TiN_0.3 composites were sintered by spark plasma sintering with different TiN_0.3 content at temperatures between 1300 and 1600 °C for 10 min and a micro-region diffusion phenomenon was observed at the Al₂O₃–TiN_0.3 interface. Ti atoms from TiN_0.3 diffused into Al₂O₃ to occupy Al sites, which led to the formation of Al vacancies that enabled the transport of aluminum by a vacancy mechanism. The optimal densification temperature of the Al₂O₃–30vol% TiN_0.3 composite was approximately 1400 °C. The maximum fracture toughness measured was 6.91 MPa m^1/2, from the composite with 30 vol% TiN_0.3 sintered at 1500 °C.     

Degradation mechanisms of magnesia-carbon refractories by high-alumina stainless steel slags under vacuum

The corrosion behaviour of a pitch-bonded magnesia-carbon refractory by an Al₂O₃ rich (∼15 wt.%) stainless steelmaking slag was investigated by rotating finger tests in a vacuum induction furnace at high temperature (>1650 °C) and low oxygen partial pressure (1.5–4.3 × 10⁻¹⁰ atm). This study confirms the poor slagline behaviour of MgO-C bricks industrially observed in VOD ladles. Higher temperatures and longer exposure times lead to more severe slag infiltration and direct MgO dissolution. The intrinsic MgO-C reaction is the major decarburisation mechanism, while extrinsic decarburisation by oxygen from the atmosphere and/or reducible slag components (CrO_x, FeO_x) was limited. Three kinds of metallic particles with different size, shape, location, composition and origin were observed in the refractory specimens. Concurrently, the thermodynamic conditions for the formation of a protective Mg(Al,Cr)₂O₄ spinel layer at the slag/refractory interface are investigated. The industrial relevance of this spinel layer formation is discussed with respect to the chosen Al₂O₃ level. Guidelines are proposed to minimise MgO refractory dissolution in VOD slaglines.     

Effect of MgO/La2O3 additives on sintering behavior and microwave dielectric properties of (Sr,Ca)TiO3-(Sm,Nd)AlO3 ceramics

The effect of MgO/La₂O₃ additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 0.7(Sr_0.01Ca_0.99)TiO₃−0.3(Sm_0.75Nd_0.25)AlO₃ (7SCT-3SNA) ceramics prepared via conventional solid-state route were systematically investigated. MgO/La₂O₃ as additives showed no obvious influence on the phase composition of the 7SCT-3SNA ceramics and all the samples exhibited pure perovskite structures. The presence of MgO/La₂O₃ additives effectively reduced the sintering temperature of 7SCT-3SNA ceramics due to the formation of a liquid phase at a relatively low temperature during sintering progress. The 0.5 wt% MgO doped 7SCT-3SNA sample with 0.5 wt% of La₂O₃, sintered at 1320 °C for 4 h, was measured to show superior microwave dielectric properties, with an ε_r of 45.57, a Q×f value of 46205 GHz (at 5.5 GHz), and τ_f value of −0.32 ppm/°C, which showed dense and uniform microstructure as well as well-developed grain growth.     

Electrical resistivity of α-SiC ceramics sintered with Al2O3 or AlN additives

Highly resistive SiC ceramics were prepared by hot pressing α-SiC powders with Al₂O₃-Y₂O₃ additives with a 4:1 molar ratio. X-ray diffraction patterns, Raman spectra, electron probe microanalysis (EMPA), and scanning electron microscopy (SEM) images revealed that the bulk SiC ceramics consisted mostly of micron-sized 6H-SiC grains along with Y₂O₃ and Si clusters. As the additive content increased from 1 to 10 vol%, the electrical resistivity of the ceramics increased from 3.0 × 10⁶ to 1.3 × 10⁸ Ω cm at room temperature. Such high resistivity is ascribed to Al₂O₃ in which Al impurities substituting Si site act as deep acceptors for trapping carriers. More resistive α-SiC ceramics were produced by adding AlN instead of Al₂O₃. The highest resistivity (1.3 × 10¹⁰ Ω cm) was achieved by employing 3 vol% AlN-Y₃Al₅O₁₂ (yttrium aluminum garnet, YAG) as an additive.     

Improved mechanical properties of Al2O3 ceramic by in-suit generated Ti3SiC2 and TiC via hot pressing sintering

Al₂O₃ multi-phase composites with different volume fractions of SiC varying from 0 vol% to 30.0 vol% were fabricated by vacuum hot pressing sintering at 1600 °C under the pressure of 30 MPa for 2.0 h. The aim of this work was to investigate the effect of SiC content on the morphology and mechanical properties of the Al₂O₃ multi-phase composite. The results show that the addition of SiC and Ti can produce new strengthening and reinforcing phases include Ti₃SiC₂, TiC, Ti₅Si₃, which would hamper the migration of grain boundaries and promote sintering. The mechanical performances could reach the comprehensive optimal values for 20.0 vol% SiC, delamination and transgranular fracture being the major crack propagation energy dissipation mechanisms.     

Highest UV–vis transparency of MgAl2O4 spinel ceramics prepared by hot pressing with LiF

Reaction sintering of MgO and Al₂O₃ with addition of LiF as sintering additive was used to prepare MgAl₂O₄ spinel ceramic by hot pressing. The process parameter (temperature, pressure, dwell time), the stoichiometric ratio of MgO to Al₂O₃ and the selection of the alumina raw powder are equally important for highest transparency of the spinel ceramic. With this optimization highest transparency of 86% in the visible range at λ = 640 nm together with UV transmission of 62% at 200 nm for spinel ceramic with 4 mm thickness was reached.     

Low temperature catalytic steam gasification of HyperCoal to produce H2 and synthesis gas

HyperCoal is an ultra clean coal with ash content <0.05 wt%. Catalytic steam gasification of HyperCoal was carried out with K₂CO₃ at 775–650 °C for production of H₂ rich gas and synthesis gas. The catalytic gasification of HyperCoal showed nearly four times higher gasification rate than raw coal. The major gases evolved were H₂: 63 vol%, CO: 6 vol% and CO₂: 30 vol%. Catalyst was recycled for four times without any significant rate loss. The partial pressure of steam was varied from 0.5 atm to 0.05 atm in order to investigate the effect of steam pressure on H₂/CO ratio. The H₂/CO ratio decreased from 9.5 at 0.5 atm to 1.9 at 0.05 atm. No significant decrease in gasification rate was observed due to change in partial pressure of steam. Gasification rate decreased with decreasing temperature and become very slow at 650 °C. The preliminary results showed that HyperCoal, an ash less coal, could be a potential hydrocarbon resource for H₂ and synthesis gas production at low temperature by catalytic steam gasification process.     

Preparation of silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction and liquid phase sintering

Dense silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction method and liquid phase sintering was reported. Ammonium peroxydisulfate ((NH₄)₂S₂O₈) and ammonium carbonate ((NH₄)₂CO₃) were used as acid and base solutions to treat the silicon carbide powder, respectively. Influence of silicon carbide powder with chemical treatment on the preparation of silicon carbide suspension was studied. It was indicated that 50 vol% and 52 vol% silicon carbide suspensions with viscosities of 0.71 Pa s and 0.80 Pa s could be prepared using acid and base treated powders. Influence of silicon carbide powder with chemical treatment on the coagulation process and properties of green bodies and sintered ceramics were studied. It was indicated that silicon carbide green bodies with compressive strength of 1.13 MPa could be prepared using base treated powder. Dense silicon carbide ceramics with relative density above 99.3% and flexural strength of 697 ± 30 MPa had been prepared by DCC via dispersant reaction and liquid phase sintering using Al₂O₃ and Y₂O₃ as additives at 1950 °C for 2 h.     

Effect of post-sintering heat-treatment on thermal and mechanical properties of Si3N4 ceramics sintered with different additives

To improve the thermal conductivity of Si₃N₄ ceramics, elimination of grain-boundary glassy phase by post-sintering heat-treatment was examined. Si₃N₄ ceramics containing SiO₂–MgO–Y₂O₃-additives were sintered at 2123 K for 2 h under a nitrogen gas pressure of 1.0 MPa. After sintering, the SiO₂ and MgO could be eliminated from the ceramics by vaporization during post-sintering heat-treatment at 2223 K for 8 h under a nitrogen gas pressure of 1.0 MPa. Thermal conductivity of 3 mass% SiO₂, 3 mass% MgO and 1 mass% Y₂O₃-added Si₃N₄ ceramics increases from 44 to 89 Wm⁻¹ K⁻¹ by the decrease in glassy phase and lattice oxygen after the heat-treatment. Relatively higher fracture toughness (3.8 MPa m^1/2) and bending strength (675 MPa) with high hardness (19.2 GPa) after the heat-treatment were achieved in this specimen. Effects of heat-treatment on microstructure and chemical composition were also observed, and compared with those of Y₂O₃–SiO₂-added and Y₂O₃–Al₂O₃-added Si₃N₄ ceramics.     

Effect of graphite nanoplatelets on the mechanical properties of alumina-based composites

Al₂O₃-based composites using exfoliated graphite nanoplatelets (xGnPs) have been developed by powder metallurgy (PM) route using both conventional as well as spark plasma sintering (SPS) processes. Al₂O₃-0.2, 0.5, 0.8, 3 and 5 vol% xGnP composites have been developed, and the effect of the addition of xGnP on the density, hardness, fracture toughness and wear behaviour of the various Al₂O₃-xGnP composites have been analyzed. Conventional sintering was done at a temperature of 1650 °C for 2, 3 and 4 h in inert atmosphere, whereas SPS was carried out at 1450 °C under 50 MPa pressure for 5 min. A uniform dispersion of the xGnP in the Al₂O₃ matrix was observed in the composites upto the addition of 3 vol% xGnP. Results indicate that a significant improvement in hardness, wear resistance and fracture toughness of the composites could be achieved by using xGnP as nanofiller. The hardness and fracture toughness of the composites developed by both conventional sintering and SPS show an increase upto the addition of 3 and 0.8 vol% xGnP respectively. The wear resistance of the composites also shows significant improvement upto the addition of 3 vol% xGnP. The composites developed by SPS have been found to possess superior mechanical properties as compared to the composites developed by conventional sintering. The improvement in the mechanical properties can be attributed to the strong interaction between the xGnP and the Al₂O₃ matrix at the interfaces and to the toughening mechanisms such as crack bridging and crack deflection.     

Graphene reinforced alumina nano-composites

Graphene was prepared using liquid phase exfoliation and dispersed in an alumina matrix using an ultrasonication and powder processing route. Al₂O₃–graphene composites with up to 5 vol% content were densified (>99%) using SPS. The fracture toughness of the material increased by 40% with the addition of only 0.8 vol% graphene. However for higher graphene contents the improvement in fracture toughness was limited. Graphene changed the mechanism of crack propagation for the alumina matrix from inter-granular to trans-granular. The formation of an inter-connecting graphene network promoted easy fracture for concentration ⩾2 vol%. Elastic modulus remained nearly constant for up to 2 vol% and decreased significantly for 5 vol% due to the formation of the inter-connecting graphene network. Fracture toughness measured with the indentation and chevron notch methods were consistent up to 2 vol% and at 5 vol% the percolating network of graphene resulted in easy crack propagation with significant discrepancy between the results for the two methods.     

In-situ fabrication of Al2O3–SiC nanocomposites using B2O3 as sintering aid

Al₂O₃–SiC nanocomposites with 5 and 10 vol% SiC have been in-situ fabricated by sol-gel method followed by carbothermal reduction of alumina–silica gel using B₂O₃ as sintering aid. Green bodies were formed by cold isostatic pressing of calcined gel, which was prepared by an aqueous sol-containing aluminum chloride, TEOS, sucrose and boric acid. Pressureless sintering was carried out in Ar–12%H₂ atmosphere at 1700 °C. Addition of B₂O₃ (1 or 3 wt%) was an effective densification aid in the Al₂O₃–5 vol% SiC composites, while the densification of Al₂O₃–10 vol% SiC composites was not affected by adding B₂O₃. The composite material containing 5 vol% SiC doped with 3 wt% B₂O₃ reached 98.7% of full density. Nano-sized β-SiC particles were formed in-situ by means of a reaction between mullite and carbon at 1600 °C. Scanning electron microscopy revealed that the spherical in-situ synthesized SiC nanoparticles were well distributed through the composite and located predominantly to the interior of alumina matrix grains.     

Comparative 27Al NMR and LAXS studies on rapidly quenched aluminosilicate glasses

Aluminium site occupancies deduced from ²⁷Al NMR measurements of several aluminosilicate glasses of composition 35–60 mol% Al₂O₃ were used to calculate the mean coordination number, assuming two polyhedral models. In the models the 30 ppm Al NMR resonance was assigned to fivefold coordinated Al or to distorted tetrahedral units, respectively. Comparison of these mean coordination numbers with those derived from pair distribution functions (PDF) from X-ray scattering data of these glasses support the model in which the 30 ppm Al NMR peak is assigned to distorted tetrahedral units. This conclusion is also supported by simulations of the PDF line profiles using the NMR site occupancies and mean polyhedral bond lengths.     

Joining of Ti–Al–C ceramics by oxidation at low oxygen partial pressure

The joining of titanium aluminum carbides has been successfully performed at high temperature and low oxygen partial pressure. The mechanism of the bonding is attributed to the preferential oxidation of Al atoms in the titanium aluminum carbides at low oxygen partial pressure, which leads to the formation of an Al₂O₃ layer through the joint interface. The specimens joined at 1400 °C exhibit a high flexural strength of 315 ± 19.1 MPa for Ti₂AlC and 332 ± 2.83 MPa for Ti₃AlC₂, which is about 95% and 88% of the substrates, respectively, and the high flexural strength can be retained up to 1000 °C. The high mechanical performance of the joints is attributed to the similar density and thermal expansion coefficient values of Al₂O₃ to those of the Ti₂AlC and Ti₃AlC₂ substrates. It indicates that bonding via preferential oxidation at low oxygen partial pressure is a practical and efficient method for Ti₂AlC and Ti₃AlC₂.