The preparation of core–shell Al2O3/Ni composite powders by electroless plating

Core–shell nanostructured Ni-coated Al₂O₃ composite powders were synthesised by using the electroless plating method. The influence of the chemical components and powder concentration in the Ni coating was investigated by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction techniques. The results show that the concentration of the plating components plays an important role in the formation of core–shell Al₂O₃/Ni composite powders. The nickel content in the composite powders could be effectively controlled by adjusting the nickel chloride content and the concentration of NaH₂PO₂·H₂O in the plating solution. The nanostructure of the crystalline Ni coatings was observed to be very attractive for achieving good bonding between ceramic particles and matrices for composite production.     

Synthesis of highly-infrared transparent Y2O3–MgO nanocomposites by colloidal technique and SPS

Infrared (IR) transparent Y₂O₃–MgO nanocomposites with a volume ratio of 50:50 were synthesized by combining colloidal and spark-plasma-sintering (SPS) techniques. In order to attain well-dispersed and homogeneous starting Y₂O₃–MgO nanopowder mixture, the effects of the pH value and the amount of polyetherimide (PEI) dispersant on the suspension stability were studied. Rheological measurement reveals that highly-dispersed and stable suspension was obtained at 7 wt% of PEI dispersant under pH = 10.6. The obtained nanopowders with particle size of 20–30 nm were densified using SPS at several sintering temperatures. The sintered composites show fine grains, narrow grain size distribution and uniform microstructure. The nanocomposite sintered at 1250 °C showed the maximum IR transmittance of 84% at a wavelength range of 2.5–6 μm. The Vickers hardness of the nanocomposite was about 11.9 ± 0.3 GPa, which is significantly higher than those of single phase MgO or Y₂O₃. Successful fabrication of the high-performance Y₂O₃–MgO nanocomposite indicates that i) the colloidal technique is an effect method to obtain highly dispersed and homogeneous nanopowders and ii) the SPS technique is a powerful tool to fabricate fine-grained dense transparent ceramics, which are suitable for fabricating IR transparent Y₂O₃–MgO composite ceramics from commercial starting powders.     

Synthesis of SiC/SiO2 core–shell powder by rotary chemical vapor deposition and its consolidation by spark plasma sintering

SiC (core) and SiO₂ (shell) powders were synthesized via rotary chemical vapor deposition (RCVD). The SiC particles (3C, <1 μm in diameter) were coated with a layer of SiO₂ (10–15 nm in thickness). Using spark plasma sintering, the SiC/SiO₂ nanopowders were then synthesized into SiC/SiO₂ composite bodies. Although a phase transformation from 3C to 6H was observed at above 2123 K in the sintered monolithic SiC bodies, sintered SiC/SiO₂ bodies did not display such phase transformation. In addition, SiC/SiO₂ bodies did not exhibited grain growth until the sintering temperature reached 2223 K. The density and Vickers hardness of the sintered SiC/SiO₂ bodies increased with increasing sintering temperature. The highest density and hardness of SiC/SiO₂ composite bodies were 98.1% and 24.4 GPa at 2223 K, respectively, which were higher than the corresponding values of 90% and 14 GPa for monolithic SiC bodies.     

Severe wear mechanisms in Al2O3–AlON ceramic composites

Severe wear mechanisms in Al₂O₃-AlON ceramic composites during their friction against a bearing steel were investigated and analysed by different techniques, mainly transmission electron microscopy (TEM). It was shown that ceramic damages correspond not to a classical intergranular cracking but to a breakdown of alumina–alumina grain boundaries leading to their pull off. Consequently, a new model of the severe wear of ceramics, based on a dielectric approach is proposed. Moreover, the existence of AlON located at such boundaries induces a delaying effect of this damage and seems to participate in the forming and the stability in the contact of a third body essentially constituted by iron oxides.     

In situ synthesis of NbB2–Al2O3 nanocomposite powder by mechanochemical processing

This research is based on the production of NbB₂–Al₂O₃ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of niobium, aluminium and boron oxide powders was subjected to high-energy ball milling. The structural evaluation of powder particles after different milling times was conducted by the X-ray diffractometry (XRD), scanning electron microscopy, and transmission electron microscopy. The results showed that during ball milling the Nb/Al/B₂O₃ reacted with a combustion mode producing NbB₂–Al₂O₃ nanocomposite. The XRD analyses exhibited that the NbB₂–Al₂O₃ nanocomposite was formed after 10?h milling time and increasing milling time up to 30?h had no significant effect other than refining the crystallite size. In the final stage of milling, the crystallite sizes of NbB₂ and Al₂O₃ were estimated to be less than 50?nm.     

Al2O3–B4C–Al Composite Material Systems via Pressureless Infiltration Methods

Modification of the Al₂O₃–Al system's chemistry via the addition of B₄C is described and is shown to result in fully dense structures via wetting techniques at high temperatures, without the need for pressure-assisted infiltration. The relationships between the surface area of boron carbide and alumina powders, the effectiveness of infiltration, the material chemistry following infiltration, and the resulting mechanical properties of Al₂O₃–B₄C–Al composites are evaluated. Additional approaches, including the incorporation of aluminum metal powder as an additional wetting agent before infiltration, are described in conjunction with a variation of both the surface areas and the volumetric ratios of inert Al₂O₃ to reactive B₄C phases. These methods can provide the means to achieve low-cost metal matrix composites in both vacuum and argon infiltration environments, and represent an approach that enables the generation of articles with complex geometries, requiring minimal secondary finishing treatment.     

Preparation of α-Fe2O3/Ag core/shell nanocomposites and SERS properties

用籽晶法,以甲醛为还原剂、3-氨丙基三乙氧基硅烷(APS)为改性剂,在Ag［(NH₃)₂］⁺溶液中制备α-Fe₂O₃/Ag核壳结构复合粉体。采用XRD、TEM和EDX对样品进行表征,系统研究了APS改性剂、醇水比等对复合纳米颗粒包覆效果及性能的影响;并用吡啶(Py)为探针,研究了α-Fe₂O₃/Ag核壳纳米颗粒作为拉曼衬底时的拉曼增强性能相似文献   

Antibacterial and photocatalytic active transparent TiO2 crystallized CaO–BaO–B2O3–Al2O3–TiO2–ZnO glass nanocomposites

Transparent TiO₂ crystallized 5CaO–10BaO–65B₂O₃–Al₂O₃–20TiO₂–10ZnO (CBBATZ) glass nanocomposites were fabricated using melt-quenching technique followed by specific heat treatments. As-quenched glass samples were provided three different heat treatments at 630°C for 3, 5, and 10 hours in order to obtain different amounts of TiO₂ nanocrystals in the glass. The presence of rutile phase of TiO₂ nanocrystals in glass was confirmed by X-ray diffraction. The glass nanocomposite heat treated for 10 hours showed a hydrophobic nature with contact angle of 90.90°. Contact angle decreased from 90.90 to 22.20°, when irradiated under ultraviolet (UV) radiation for 45 minutes. This photoinduced hydrophilicity showed a photocatalytic and self-cleaning properties of glass nanocomposite. During photocatalytic ink test, the maximum change in color of Resurin (Rz) ink and 60% degradation in absorbance of ink within 150 minutes under UV radiation were found for glass nanocomposite heat treated at 10 hours. Also, 78% degradation in absorbance of methylene blue dye (pollutant) within 180 minutes under UV irradiation was found for glass naocomposite heat-treated at 10 hours. Antibacterial performance of transparent glass nanocomposite against Escherichia coli was evaluated as well. More than 95% of the bacterial cells were degraded with glass nanocomposite heat-treated at 10 hours. CBBATZ glass nanocomposite found to impart the antibacterial effect through generation of reactive oxygen species (ROS) in aqueous medium. ROS species which was confirmed in the bacterial cell through intracellular ROS generation kit. During evaluation of mechanical properties using nanoindentation technique, the values of hardness and reduced modulus increased by ~26% and 10%, respectively, for glass nanocomposite heat-treated at 10 hours as compared to as-quenched glass.     

Synthesis and characterization of monodispersed BaSO4/Y2O3:Eu3+ core–shell submicrospheres

Spherical BaSO₄ particles have been coated with Y₂O₃:Eu³⁺ phosphor layers (BaSO₄/Y₂O₃:Eu³⁺) by the wet chemical method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dipersive spectroscopy (EDS), photoluminescence spectra were utilized to characterize the BaSO₄/Y₂O₃:Eu³⁺ core–shell-structured phosphor particles. The obtained core–shell phosphors consist of well dispersed submicron spherical particles with narrow size distribution. XRD result shows that no reaction occurred between the BaSO₄ cores and the Y₂O₃:Eu³⁺ shells even after annealing at 1400 °C. TEM and EDS results show that BaSO₄ particles are well coated with the shell of Y₂O₃:Eu³⁺. The BaSO₄/Y₂O₃:Eu³⁺ core–shell particles show a red emission corresponding to ⁵D₀?⁷F₂ of Eu³⁺ under the excitation of ultraviolet.     

Preparation of Al2O3–SiC composite powder from kyanite tailings via carbothermal reduction process

ABSTRACT

Al₂O₃–SiC composite powders were prepared from kyanite tailings mixed with 20% excess carbon coke via carbothermal reduction (CR) reaction. The optimised synthesis condition for synthesising Al₂O₃–SiC composite powders was at 1600°C for 6?h. The equilibrium relationship curves of the condensed phases were presented and the temperature dependence of the phase composition was also studied. The results show that irregular Al₂O₃ and SiC grains first formed at 1500°C, and the elements C, O, Al, and Si randomly distributed in the each crystal particles. The amount of Al₂O₃–SiC composites increased with the increasing synthesis temperature and reaction time. Finally, Al₂O₃–SiC composite bulk materials were further prepared by pressureless sintering using the synthesised Al₂O₃–SiC composite powders as raw materials, and their mechanical properties were investigated in detail. All these results indicate that the CR method can offer a niche application for the development of kyanite tailings.     

Single-phase γ-Fe2O3 nanoparticles synthesized by green ionothermal method and their magnetic characterization

The γ-Fe₂O₃ nanoparticles were prepared by ionothermal method in ionic liquids. A wide range of maghemite particle sizes within the nanometer scale are obtained by this method. The structural and magnetic properties of these γ-Fe₂O₃ particles were studied by using XRD, SEM and IR. Magnetic properties of the prepared nanoparticles were evaluated on a vibrating sample magnometer (VSM). By changing the ionic liquids, particles with an average diameter from 5 to 10 nm were prepared. The size of the final particle decreases with the longer alkyl chain of ionic liquid in this method while the template is bounded to the particle surface. Also magnetic properties of the products change with different ionic liquids. A longer alkyl chain in ionic liquid causes an increase in the magnetic properties in this method.     

Phase evolution and lightweight application of dolomite@Al2O3 spherical particle with core–shell structure

Dolomite@Al₂O₃ spherical particles with core–shell structure were fabricated by using the dolomite powders as the core and wrapping corundum powders on their surface. The phase evolution and microstructure of the dolomite@Al₂O₃ spherical particles with different firing temperatures were studied. As the spherical particles fired at different temperatures, the dolomite in the spherical core decomposed into MgO and CaO. Due to the Kirkendall effect, MgO and CaO diffused into the shell of the spherical particle and left Kirkendall holes in the core. The holes produced by the Kirkendall effect and the decomposition of dolomite act together to form a hollow structure in the center. And during the diffusion process, MgO and CaO reacted with corundum to generate spinel and calcium hexaaluminate. A small amount of spinel and calcium hexaaluminate can form a ternary compound Ca₂Mg₂Al₂₈O₄₆. Moreover, simulation software Factsage and Abaqus were used separately to help prove the phase evolution and the introduction of spherical particles to the properties of refractories. To test the lightweight effect of dolomite@Al₂O₃ spherical particles on refractories, it was introduced into spinel–corundum refractories as medium particles in different content and fired at different temperatures. The results show that after firing at 1650°C for 3 h with introducing 30% dolomite@Al₂O₃ spherical particles, the samples can reach a high compressive strength (128 MPa), high refractoriness under load (1683°C), and low thermal conductivity (1.79 W (m K)⁻¹).     

Effect of solid loading on properties of Y2O3–Al2O3–Nd2O3 powder mixtures obtained by planetary ball milling and ceramics based on them

The effect of the solid loading (41–50 wt%) of the slurry on granulometric composition and physico-chemical characteristics of Y₂O₃–Al₂O₃–Nd₂O₃ powder mixtures obtained by planetary ball milling has been studied for the first time. It was shown that the particle size distribution of powder, its Zeta potential, and specific surface area depend on the solid loading of the milled slurry and, consequently, on the interparticle distance during milling. The interparticle distance decreases from 200 nm to 142 nm with an increase of solid loading in the range of 41–50 wt%. It was shown that for the solid loading of 47 wt%, the convergence of particles to a distance comparable to their median diameter promotes subsequent clustering of particles. This facilitates the sintering of highly-homogenous ceramics. It was found that solid loadings in the 46–50 wt% range is useful for obtaining high-quality Nd:YAG transparent ceramics. The lowest optical losses optical losses of 1 × 10^?3 cm^?1 and the highest in-line transmittance of 84.1%@1064 nm were obtained for 1 at.% Nd:YAG transparent ceramics (22 × 3 × 4 mm³) prepared from slurries with 47 wt% solid loading (taking all other ball milling parameters fixed). If the interparticle distance in the powder is higher (solid loading of 41 wt%) than the median particle diameter, the ceramics are characterized by significant residual porosity due to the survival of large particles (insufficient milling).     

Mechanical performance and microstructure of 3Y-TZP/Al2O3/GNPs medical ceramic surgical scalpel material prepared through SPS–HF dual sintering method

In this study, 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP)/Al₂O₃/graphene nanoplatelets (GNPs) medical ceramic materials for manufacturing surgical scalpels were sintered in vacuum in an SPS–625HF furnace. The mechanical performances and microstructures of the composites were investigated, and the influence mechanisms of the sintering temperature and amount of added GNPs were studied. During the sintering process at 1400°C and 30 MPa for 5 min, the added GNPs enhanced the mechanical properties of the 3Y-TZP/Al₂O₃ composites. The results showed that the composite with .1 wt.% GNPs had 6.4% (910 ± 11 MPa) higher flexural strength than 3Y-TZP/Al₂O₃. The composite with .4 wt.% GNPs had 38.7% (12.95 ± .22 MPa m^1/2) greater fracture toughness than 3Y-TZP/Al₂O₃. The main toughening mechanisms of 3Y-TZP/Al₂O₃/GNPs were crack bridging, crack deflection, crack branching, GNPs bridging, transgranular fracture structures, and phase transformation of t-ZrO_1.95. The two-stage densification displacement curve appeared at the optimal sintering temperature of the materials, and the 3Y-TZP/Al₂O₃/GNPs composites with a two-stage densification displacement curve had excellent mechanical properties. The added GNPs can inhibit the grain growth during the sintering process, thereby refining the zirconia grains. With the increase in GNPs content, the grain size and flexural strength of the composites decreased gradually. However, higher content of GNPs was beneficial to improve the relative density and thermal diffusivity of 3Y-TZP/Al₂O₃/GNPs composite material.     

Syngas production via dry reforming of methane over Ni/Al2O3–MgO nanocatalyst synthesized using ultrasound energy

The Ni/Al₂O₃–MgO nanocatalyst with Al/Mg ratio of 1.5 was prepared successfully using sonochemistry method and shown high activity and stability in dry reforming of methane. XRD, BET, FESEM, TEM and EDAX-dot mapping techniques have been used for nanocatalyst characterization. XRD analysis confirmed the formation of MgO and NiO cubic phases. According to the FESEM micrographs, nanostructure grains with uniform surface size distribution have been observed in of the synthesized nanocatalyst. The TEM micrographs showed that ultrasound-assisted preparation method induced uniform morphology without agglomeration of particles. The activity of synthesized nanocatalyst could reach thermodynamic equilibrium conversions and H₂/CO ratios.     

Fabrication of WSi2–Al2O3 and W5Si3–Al2O3 composites by combustion synthesis involving thermite reduction

Formation of WSi₂–Al₂O₃ and W₅Si₃–Al₂O₃ composites was studied by thermite-based combustion synthesis. The addition of two thermite combinations composed of WO₃+2Al and 0.6WO₃+0.6SiO₂+2Al into the W-Si reaction systems facilitated the combustion wave propagating in a self-sustaining manner and contributed to the in situ formation of tungsten silicides along with Al₂O₃. Experimental results showed that the former thermite mixture is more exothermic than the latter, and a decrease in the combustion temperature and flame-front velocity with increasing silicide phase formed in the composite. Depending on the reaction stoichiometry, the combustion wave velocity varied from 9.5 to 3.7 mm/s and temperature from 1650 to 1280 °C. A complete phase conversion and a broad range of the molar ratio of WSi₂/Al₂O₃ from 0.8 to 4.0 were achieved for the production of the WSi₂–Al₂O₃ composites. Due to the lower formation exothermicity, the W₅Si₃–Al₂O₃ composites were produced with a narrower range of W₅Si₃/Al₂O₃ from 0.4 to 2.0, beyond which combustion failed to proceed. Moreover, there exist WSi₂ and unreacted W in the as-synthesized W₅Si₃–Al₂O₃ composites.     

The formation mechanism of pores and unbonding in the Al2O3/Al2O3 joints brazed by 50Bi2O3–35B2O3–15ZnO glass

50Bi₂O₃–35B₂O₃–15ZnO (mol. %) glass referred to as Bi50 glass, was used to braze Al₂O₃ ceramics. The phase transformations and wettability of the Bi50 glass on Al₂O₃ substrates at different temperatures were investigated. The results showed that the chemical compatibility of Bi50 glass and Al₂O₃ substrates at 650 °C was excellent. However, Al₂O₃/Al₂O₃ joints having a considerable volume fraction of pores and unbonding were obtained when the joining procedures were carried out by a one-step brazing method. Based on the experiments and simulation results, the prime determinants responsible for the presence of the pores and unbonding within the brazing joints can be divided into two aspects: (i) the intrinsic causes leading to the formation of closed pores (ii) the external factors causing the failure of pores and glass separation. Ultimately, an advanced joining procedure named two-step brazing was proposed, and joints nearly free of defects were acquired.     

Highly transparent cubic γ-Al2O3 ceramic prepared by high-pressure sintering of home-made nanopowders

Highly transparent gamma-aluminum oxide (γ-Al₂O₃) ceramics were fabricated for the first time, by combining homogeneous precipitation and high-pressure sintering in the absence of exogenous dopants. The resulting cubic γ-Al₂O₃ transparent ceramic material exhibits a promising replacement for single-crystal sapphire. The optimum optical properties are achieved in response to sintering at 5 GPa and a temperature of 300 °C and include maximum transmittance of 86% in the range of 0.6–1.2 µm which are properties that are comparable to those of single-crystal sapphire (∼86%). Vickers hardness (16 GPa) and compressive strength (350 MPa) in response to high-pressure sintering are also similar to those of a conventional sapphire single crystal. Meanwhile, the dielectric constant (9.46) is comparable to that of sapphire in the C-axis direction. These findings will facilitate further development of transparent Al₂O₃ ceramics for use in a wider range of optical applications.     

Investigation on properties of Al2O3–SiO2 complex shaped refractory fabricated by layered extrusion forming

As one of the 3D printing methods, layered extrusion forming (LEF) has distinct advantages to form complex configuration ceramics directly. The feasibility of using LEF to make refractory products with complex shapes was explored by this work, using water-based Al₂O₃–SiO₂ ceramic slurry and specially equipped device. By measuring rheological parameters, the effects of binder addition, dispersant addition and volume proportion of the solid portion composed of α-Al₂O₃ ultrafine powder (92 wt%) and silica fume (8 wt%) on rheological behavior of the slurry were investigated. The green body specimens prepared by the LEF were fired at 1400°C–1600 °C for 3h. The influence of firing temperature on phase composition, microstructure, sintering degree and comprehensive properties of the specimens was investigated. At 2.5 wt% addition of aluminum dihydrogen phosphate as binder, 0.2 wt% addition of sodium hexametaphosphate as dispersant and with solid portion between 56 vol% and 58 vol%, required pseudoplastic behavior of the slurry can be achieved, suitable for the LEF. With the increase of heating temperature, mullitization by the reaction between the α-Al₂O₃ ultrafine powder and silica fume becomes stronger and sintering gets enhanced, leading to improved comprehensive properties of the specimens. Fired at 1600 °C, properties in terms of bulk density 3.03g/cm³, cold compressive strength 190.5 MPa and refractoriness under load 1598 °C are achieved. Crucible slag test shows a good resistance to the glass melt corrosion. Good feasibility of fabricating some complex shaped refractory products by LEF as a novel forming approach has been confirmed by the present work.     

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass was prepared from Y₂O₃, Al₂O₃, and SiO₂ micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10⁻⁶, matching-well with that of SiC_f/SiC composites. SiC_f/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiC_f/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.