Growth of AlN Crystals on SiC Substrates by Thermal Nitridation of Al2O3

The growth of AlN crystals on c‐plane 6H–SiC substrates by thermal nitridation of Al₂O₃ pellets in the presence of graphite and ZrO₂ was demonstrated. Addition of graphite and ZrO₂ effectively accelerated the evaporation of Al₂O₃, yielding c‐axis oriented AlN films on SiC substrates. The SiC substrate was severely deteriorated at 2173 K, which produced a porous interface between the AlN film and substrate, resulting in low‐quality AlN crystals. The deterioration of SiC was successfully suppressed by introducing a pre‐deposited homo‐buffer layer, allowing two‐dimensional‐like growth of AlN. The buffer layer promoted the formation of a high‐quality AlN film. At 2173 K, the full‐width at half maximum of the X‐ray rocking curves of the (0002) and (10–10) planes of the AlN film was 360 and 425 arcsec, respectively.     

Growth mechanism of AlN crystals via thermal nitridation of sintered Al2O3–ZrO2 plates

The crystal growth of AlN from aluminum oxides was studied using a thermal nitridation method. Four types of aluminum oxides, sintered Al₂O₃, ZrO₂-containing sintered Al₂O₃, and a- and c-plane sapphires, were used as a source material. As observed, millimeter-sized AlN crystal grains were successfully grown from the ZrO₂-containing sintered Al₂O₃ only at temperatures ranging from 2223 to 2323 K. The growth mechanism, including the role of ZrO₂ additive, was discussed from a thermodynamic viewpoint. The following growth model was proposed: predominant nitridation of ZrO₂ in Al₂O₃ suppresses Al₂O₃ nitridation, and the ZrO₂–Al₂O₃ liquid phase forms, which promotes the formation of Al₂O(g) and Al(g) from Al₂O₃. These Al-based gases react with CN(g) and/or N₂(g) to form AlN crystals on the Al₂O₃–ZrO₂ plate.     

Fabrication of Al2O3/AlN composite ceramics with enhanced performance via a heterogeneous precipitation coating process

To improve the thermal and mechanical properties of Al₂O₃/AlN composite ceramics, a novel heterogeneous precipitation coating (HPC) approach was introduced into the fabrication of Al₂O₃/AlN ceramics. For this approach, Al₂O₃ and AlN powders were coated with a layer of amorphous Y₂O₃, with the coated Al₂O₃ and AlN powders found to favor the formation of an interconnected YAG second phase along the grain boundaries. The interconnected YAG phase was designed to act as a diffusion barrier layer to minimize the detrimental interdiffusion between Al₂O₃ and AlN particles. Compared with samples prepared by a conventional ball-milling method, the HPC Al₂O₃/AlN composites exhibited less AlON formation, a higher relative density, a smaller grain size and a more homogeneous microstructure. The thermal conductivity, bending strength, fracture toughness and Weibull modulus of the HPC Al₂O₃/AlN composite ceramics were found to reach 34.21 ± 0.34 W m⁻¹ K⁻¹, 475.61 ± 21.56 MPa, 5.53 ± 0.29 MPa m^1/2 and 25.61, respectively, which are much higher than those for the Al₂O₃ and Al₂O₃/AlN samples prepared by the conventional ball-milling method. These results suggest that HPC is a more effective technique for preparing Al₂O₃/AlN composites with enhanced thermal and mechanical properties, and is probably applicable to other composite material systems as well.     

Fabrication of pressureless sintered dense β-SiAlON via a reaction-bonding route with ZrO2 addition

A reaction-bonding and post-sintering process was applied to fabricate pressureless sintered β-Si_6?ZAl_ZO_ZN_8?Z (Z = 1–3) ceramics with monoclinic ZrO₂ added to the starting powder. Samples with ZrO₂ showed enhanced nitridation and achieved near theoretical density following post-sintering, although this could not be achieved in the samples produced without ZrO₂. Thus the ZrO₂ is effective in both enhancing the nitridation of Si and as a sintering additive for densification of β-SiAlON. Part of the added monoclinic ZrO₂ was transformed to tetragonal ZrO₂, and the ZrN phase was also formed due to reaction with nitrogen during the reaction-bonding process. After the post-sintering process, the ZrN phase remained only in the Z = 3 composition. In the other compositions the ZrN reacted with SiO₂ to form both tetragonal and monoclinic ZrO₂ phases. These differences are explained in terms of the increasing densification and grain growth for Al₂O₃ rich compositions and the ZrN being trapped inside such grains in the case of the Z = 3 sample.     

Microstructure and improved mechanical properties of Al2O3/Ba-β-Al2O3/ZrO2 composites with YSZ addition

Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites were fabricated by solid-state reaction sintering of Al₂O₃, BaZrO₃, and yttria stabilized zirconia (YSZ) powders. The effects of YSZ addition on microstructure and mechanical properties have been investigated. The incorporation of YSZ promoted the densification of the composites and formation of tetragonal ZrO₂ phase. The microstructure of the composites was characterized by elongated Ba-β-Al₂O₃ phase and equiaxed ZrO₂ particles including added YSZ and reaction-formed ZrO₂. The Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites with YSZ addition exhibited improved fracture toughness, as a result of multiple toughening effects including crack deflection, crack bridging, crack branching, and martensitic transformation of ZrO₂ formed by the reactions between Al₂O₃ and BaZrO₃. Moreover, owing to the grain refinement of Al₂O₃ matrix, dispersion strengthening of the added YSZ particles, and an increase in density of the composites, the Vickers hardness and flexural strength of Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites were dramatically enhanced in comparison with the composites without YSZ addition.     

Production of Al2O3‐Stabilized Tetragonal ZrO2 Nanoparticles for Thermal Barrier Coating

Al₂O₃‐stabilized tetragonal ZrO₂ nanoparticles were obtained through hot‐air spray pyrolysis and characterized after postsynthesized treatments. The produced nanoparticles were 26 nm in size with surface area of 59 m²/g. A multilayer thermal barrier coating of nanostructured Al₂O₃‐ZrO₂‐embedded silicate was applied to the mild steel (EN3) specimen using spin‐coating technique and characterized comprehensively employing X‐ray diffraction and scanning electron microscope. The Al₂O₃‐stabilized ZrO₂ with silicate matrix facilitates the formation of zirconium silicate nanostructured surface‐protective coating on EN3 specimen. The Al₂O₃‐ZrO₂/SiO₂ matrix‐based hybrid inorganic coating shows effective thermal barrier for EN3 after firing at a high temperature of 600°C.     

Effect of Si3N4 mesophase on the formation of Al2OC-AlNss in resin-bonded Al–Al2O3 composites

In this study, we developed a novel method for synthesising Al₂OC-AlNss using a solid nitrogen source: a Si₃N₄ mesophase. The two-step sintered Al–Al₂O₃ and Si₃N₄–Al–Al₂O₃ samples were prepared under an atmosphere of nitrogen to investigate the effect of Si₃N₄ on the formation of Al₂OC-AlNss in resin-bonded Al–Al₂O₃ composites. The samples were investigated via XRD and SEM. The results indicated that the synthesis of Al₂OC-AlNss with different morphologies was achieved via the Si₃N₄ mesophase, and its morphology was influenced by the source of AlN. Both Al₂OC-AlNss and Al₄O₄C were formed in the two-step sintered Al–Al₂O₃ sample, whereas only Al₂OC-AlNss was formed in the two-step sintered Si₃N₄–Al–Al₂O₃ sample. Induced by the AlN formed by the nitridation of Al, needle-like Al₂OC-AlNss was generated. Compared to that formed by the nitridation of Al, more AlN nuclei were provided by the reaction between Si₃N₄ and Al. Subsequently, columnar and granular Al₂OC-AlNss were formed. Furthermore, fibre-like Al₂OC-AlNss was also generated via the VS and VLS mechanism. The reaction model was established in this study.     

Enhanced performance of low-carbon MgO–C refractories with nano-sized ZrO2–Al2O3 composite powder

Low-carbon MgO–C refractories are facing great challenges with severe thermal shock and slag corrosion in service. Here, a new approach, based on the incorporation of nano-sized ZrO₂–Al₂O₃ composite powder, is proposed to enhance the thermal shock resistance and slag resistance of such refractories in this work. The results showed that addition of ZrO₂–Al₂O₃ composite powder was helpful for improving their comprehensive performances. Particularly, the thermal shock resistance of the specimen containing 0.5 wt% composite powder was enhanced significantly which was related to the transformation toughening of zirconia and in-situ formation of more spinel phases in the matrix; also, the slag resistance of the corresponding specimen was significantly improved, which was attributed to the optimization of pore structure and formation of much thicker MgO dense layer.     

Effect of CeO2 and Al2O3 contents on Ce‐ZrO2/Al2O3 composites

Effect of CeO₂ and Al₂O₃ contents on phase composition, microstructures, and mechanical properties of Ce–ZrO₂/Al₂O₃ composites was studied. The CeO₂ content in CeO₂–ZrO₂ varied from 7 to 16 mol%, and the Al₂O₃ content in Ce‐ZrO₂/Al₂O₃ composites were 7 and 22 wt%. When CeO₂ content was ≤10 mol%, high Al₂O₃ content contributed to hinder the tetragonal‐to‐monoclinic ZrO₂ phase transformation during cooling and decrease the density of microcracks in the composites. Tetragonal ZrO₂ single‐phase was obtained in the composites with ≥12 mol% CeO₂, regardless of the Al₂O₃ content. Hardness, flexural strength, and toughness were dependent on CeO₂ and Al₂O₃ contents which were related to the microcracks, grain size, and phase transformation. The high flexural strength and toughness of the composites with 7wt% Al₂O₃ could be obtained at an optimum CeO₂ content of 12 mol%, whereas those of the composites with 22 wt% Al₂O₃ could be achieved in the wide CeO₂ content range of 8.5‐12 mol%.     

Role of scanning speed on the microstructure and mechanical properties of additively manufactured Al2O3‒ZrO2

Melt-grown Al₂O₃–ZrO₂ eutectic (AZ eutectic) ceramics have attracted extensive attention for harsh environment applications. In this work, AZ eutectic ceramic is additively manufactured via one-step laser powder bed fusion (LPBF). The role of scanning speed on phase formation, crystallographic characteristics, microstructure evolution, and mechanical properties were systematically investigated. The as-fabricated specimens are mainly composed of α-Al₂O₃ and t-ZrO₂. Lower scanning speeds induced the formation of cellular structures consisting of randomly oriented ZrO₂. In contrast, nanometer eutectic lamellar structure with well-defined multiple crystallographic orientation relationships, for example, {10$\overline{1}$0} Al₂O₃ || {100} ZrO₂ and {0001} Al₂O₃ || {001} ZrO₂, occurred at higher scanning speeds. Both the cell size and lamellar spacing decreased with increasing scanning speed. With the microstructure refinement, the crack propagation mode changes from intergranular to transgranular fracture, leading to progressively enhanced fracture toughness with a maximum value of 7.76 MPa·m^1/2. The present work could shed light on tailoring the microstructure of LPBF AZ eutectic ceramic via varying processing parameters.     

Thermal shock resistance of Cr2O3–Al2O3–ZrO2 bricks with the microstructure of Cr2O3 aggregates coated by ZrO2

Zirconia-coated Cr₂O₃ aggregates synthesized by mixing ZrO₂ powders and Cr₂O₃ aggregates with a phenolic resin binder followed by thermosetting treatment were employed as modified Cr₂O₃ aggregates to obtain Cr₂O₃–Al₂O₃–ZrO₂ bricks (high-chromia bricks). The elastic modulus (E) and cold modulus of rupture (CMOR) of these high-chromia bricks before and after thermal shock cycles were systematically investigated, and the residual ratios of CMOR and E were calculated. The thermal shock resistance of the high-chromia bricks was significantly improved by the factor of modification of Cr₂O₃ aggregates. The mechanism of the improved thermal shock resistance of these high-chromia bricks was studied via microstructure analysis, and the crack propagation behavior was analyzed by scanning electron microscopy (SEM). The fracture work (γ_WOF), thermal shock damage factor (R′′′′), and thermal stress crack stability parameter (R_st) were measured and calculated using the wedge splitting test (WST). The results indicate that the porous ZrO₂ coating layer wrapped the Cr₂O₃ aggregates, forming modified Cr₂O₃ aggregates that can increase crack deflection, free path of crack propagation, and fracture work, thus improving the thermal shock resistance of high-chromia bricks. The thermal shock resistance of the fabricated high-chromia bricks was highly correlated with the thickness of the ZrO₂ coating layer surrounding the Cr₂O₃ aggregates. The variation trend of R_st is well consistent with the experimental results, which is suitable to evaluate the thermal shock resistance of high-chromia bricks.     

Coating Titania Aerosol Particles with ZrO2, Al2O3/ZrO2 and SiO2/ZrO2 in a Gas-Phase Process

The formation of ZrO₂, Al₂O₃/ZrO₂, and SiO₂/ZrO₂ coatings on TiO₂ particles by a continuous gas-phase process was studied. Titania particles were formed by the reaction of TiCl₄ vapor with O₂ in a hot-wall tubular reactor at 1300°C and were mixed with ZrCl₄ and AlCl₃ or SiCl₄ vapors near the end of the reactor. The ZrCl₄/TiCl₄ molar ratio was varied from 6.7 x 10^-4 to 5 x 10^-3 while the AlCl₃/TiCl₄and SiCl₄/TiCl₄ molar ratios varied in the ranges 8 x 10^-3-8 x 10^-2 and 2 x 10^-2-8 x 10^-2, respectively. Discrete tetragonal ZrO₂ nanoparticles of average diameter 20 nm were formed on the surfaces of the titania particles at a surface concentration that increased with the ZrCl₄ gas-phase concentration. The sequential introduction of AlCl₃ and ZrCl₄ vapors resulted in composite coatings. These consisted of dense, coherent, amorphous, and smooth Al₂O₃ layers approximately 10 nm thick, on top of which ZrO₂ nanocrystalline particles were dispersed in a similar pattern as in the absence of Al₂O₃. Concentration depth profiles of these powders were obtained by Auger Electron Spectroscopy and supported the TEM observations. Particles coated by sequentially introducing SiCl₄ and ZrCl₄ vapors had amorphous rough SiO₂ coatings, 5-10 nm thick, and ZrO₂ particles sporadically dispersed on their surfaces. Increasing the SiCl₄/ZrCl₄ molar ratio from 5 to 19 increased the thickness and roughness of the silica layers and resulted in encapsulation of the ZrO₂particles inside the silica coatings. The different coating morphologies obtained were attributed to different coating formation mechanisms for the metal oxides used in this study.     

Optimization of microstructure and mechanical properties of oscillatory pressure sintered ZrO2–Al2O3-SiCp ceramics

Zirconia ceramic is a significant structural material, but its use under some extreme circumstances is limited by its mechanical properties. In this work, SiC particles (SiCp) were added into alumina toughened zirconia ceramics to prepare ZrO₂–Al₂O₃-SiCp ceramics with high performance by using oscillatory pressure sintering (OPS). Results showed that the best OPS temperature of 1600 °C was obtained, and the optimal SiCp particle size and content were 200 nm and 10 vol% respectively. Under these conditions, the specimen exhibited higher mechanical properties including Vickers hardness of 15.43 GPa, bending strength of 1162 MPa and fracture toughness of 6.36 MPa m^1/2. Moreover, it was found that the atomic matching between ZrO₂/SiCp, Al₂O₃/SiCp, and ZrO₂/Al₂O₃ was much higher, showing the coherent interface relationship. Therefore, it was favorable for enhanced mechanical properties of as-prepared ZrO₂–Al₂O₃-SiCp ceramics.     

The microstructure,formation mechanism and sintering characteristics of Al2O3/ZrO2 supersaturated solid solution powders

In this study, the Al₂O₃/ZrO₂ supersaturated solid solution powders with different ZrO₂ contents were successfully synthesized by a novel combustion synthesis combined with water cooling (CS-WC) method. The solid solubility and formation mechanism of solid solution under the extremely non-equilibrium solidification condition were discussed in details. The ultra-high cooling rate greatly improves the solubility limit of Al₂O₃ in ZrO₂. When ZrO₂ content is 30 mol%, the Al₂O₃ has been almost dissolved into the ZrO₂ lattice. The formation mechanism of solid solution can be attributed to solute interception caused by the huge degree of supercooling. During the sintering process, the solid solution powders precipitate ZrO₂ particles and the Al₂O₃ matrix, which forms a fine and uniform nanostructure. Due to the synergistic effect of t-m phase transformation toughening and ZrO₂ nanoparticles toughening, the Al₂O₃/ZrO₂ nanoceramics exhibit excellent mechanical properties when ZrO₂ contents are at the range of 25–37 mol%.     

Performance enhancement in InZnO thin-film transistors with compounded ZrO2–Al2O3 nanolaminate as gate insulators

We fabricated compounded ZrO₂–Al₂O₃ nanolaminate dielectrics by the atomic layer deposition (ALD) and used them to successfully integrate the high-performance InZnO (IZO) thin-film transistors (TFTs). It is found that nanolaminate dielectrics combine the advantages of constituent dielectrics and produce TFTs with improved performance and stability compared to single-layer gate insulators. The mobility in IZO-TFT was enhanced about 22% by using ZrO₂–Al₂O₃ gate insulators and the stability was also improved. The transfer characteristics of IZO-TFTs at different temperatures were also investigated and temperature stability enhancement was observed for the TFT with ZrO₂–Al₂O₃ nanolaminates as gate insulators. A larger falling rate (∼1.45 eV/V), a lower activation energy (Ea, ∼1.38 eV) and a smaller density-of-states (DOS) were obtained based on the temperature-dependent transfer curves. The results showed that temperature stability enhancement in InZnO thin-film transistors with ZrO₂–Al₂O₃ nanolaminate as gate insulators was attributed to the smaller DOS.     

Laser stereolithography of ZrO2 toughened Al2O3

Ceramic laser stereolithography is a manufacturing process suitable candidate for the production of complex shape technical ceramics. The green ceramic is produced layer by layer through laser polymerisation of UV curable ceramic suspensions. A number of critical issues deserve attention: high solid loading and low viscosity of the suspensions, high UV reactivity, prevention of interlayer delamination in the green and in the sintered body, good mechanical performance. In this work, ZrO₂ reinforced Al₂O₃ components have been obtained from an acrylic modified zircon loaded with alumina powders. The zircon compound is effective as organic photoactivated resin and allows the dispersion of a high volume fraction of Al₂O₃ powder (up to 50 vol.%) while keeping viscosity at reasonable low values. The zircon compound also represents a liquid ceramic precursor that converts to oxide after burning out of the binder. Thanks to the good dispersion of the alumina powder in the zircon acrylate, a uniform dispersion of ZrO₂ submicron particles is obtained after pyrolysis. These are located at the grain boundaries between alumina grains. Formation of both monoclinic and tetragonal ZrO₂ occurs as evidenced by XRD. No delamination occurs in bending tests as evidenced by SEM fractography, satisfactory modulus and strength values were concurrently found.     

Microwave hybrid and conventional sintering of Al2O3 and Al2O3/ZrO2 multilayers fabricated by aqueous tape casting

In this study, microwave hybrid sintering and conventional sintering of Al₂O₃- and Al₂O₃/ZrO₂-laminated structures fabricated via aqueous tape casting were investigated. A combination of process temperature control rings and thermocouples was used to measure the sample surface temperatures more accurately. Microwave hybrid sintering caused higher densification and resulted in higher hardness in Al₂O₃ and Al₂O₃/ZrO₂ than in their conventionally sintered counterparts. The flexural strength of microwave-hybrid-sintered Al₂O₃/ZrO₂ was 70.9% higher than that of the conventionally sintered composite, despite a lower sintering temperature. The fracture toughness of the microwave-hybrid-sintered Al₂O₃ increased remarkably by 107.8% despite a decrease in the relative density when only 3 wt.% t-ZrO₂ was added. The fracture toughness of the microwave-hybrid-sintered Al₂O₃/ZrO₂ was significantly higher (247.7%) than that of the conventionally sintered composite. A higher particle coordination and voids elimination due to the tape casting and the lamination processes, the microwave effect, the stress-induced martensitic phase transformation, and the grain refinement phenomenon are regarded as the main reasons for the mentioned outcomes.     

Microstructure of the directionally solidified ternary eutectic ceramic Al2O3/MgAl2O4/ZrO2

The directionally solidified Al₂O₃/MgAl₂O₄/ZrO₂ ternary eutectic ceramic was prepared via induction heating zone melting. Smooth Al₂O₃/MgAl₂O₄/ZrO₂ eutectic ceramic rods with diameters of 10 mm were successfully obtained. The results demonstrate that the eutectic rods consist of Al₂O₃, MgAl₂O₄ and ZrO₂ phases. In the eutectic microstructure, the MgAl₂O₄ and Al₂O₃ phases form the matrix, the ZrO₂ phase with a fibre or shuttle shape is embedded in the matrix, and a quasi-regular eutectic microstructure formed, presenting a typical in situ composite pattern. During the eutectic growth, the ZrO₂ phase grew on non-faceted phases ahead of the matrix growing on the faceted phase. The hardness and fracture toughness of the eutectic ceramics reached 12 GPa and 6.1 MPa·m ^1/2, respectively, i.e., two times and 1.7 times the values of the pre-sintered ceramic, respectively. In addition, the ZrO₂ phase in the matrix reinforced the matrix, acting as crystal whiskers to reinforce the sintered ceramic.     

Optimization of corrosion resistance and mechanical properties of Al2O3-Cr2O3 refractories for waste incinerator

The corrosion resistance and mechanical properties directly affects the operation and service life of Al₂O₃-Cr₂O₃ refractories used in waste incinerators. In this study, ZrO₂ particles were introduced via vacuum impregnation to adjust microstructure and properties of Al₂O₃-Cr₂O₃ refractories. The results showed that the impregnated ZrO₂ particles and increasing impregnation times resulted in the decreased median pore size and increased compactness, and mechanical strengths of refractories were elevated from the inhibited cracks propagation by ZrO₂ particles. The decreased amounts of large pores and increased amounts of small pores from the filled ZrO₂ particles inhibited penetration of low melting point phases, and the formed CaZrO₃ phase from the reactions between corrosion reagent and ZrO₂ particles increased the viscosity of penetrated corrosion reagent, resulting in the decreased penetration index from 8.57% to 2.58%. Meanwhile, the filled ZrO₂ particles around alumina particles prevented reactions between molten corrosion reagent and alumina, leading to the decreased corrosion index from 3.78% to .74%. The decreased pore size and formation of CaZrO₃ phase were primary factors that enhanced the penetration resistance. And formation of wrapped layers from ZrO₂ particles around alumina particles presented prominent effects on the strengthened corrosion resistance of Al₂O₃-Cr₂O₃ refractories.     

Microstructural evolution of supra-nanostructure Al2O3/ZrO2 eutectic powders by combustion synthesis-spray cooling

Eutectic powders with fine microstructure are difficult to synthesize by crushing eutectic bulk because of the damage of crystal structure and the introduction of milling media during preparation process. In this work, a novel combustion synthesis-spray cooling (CSSC) method is developed to fabricate supra-nanostructure Al₂O₃/ZrO₂ eutectic powders. CSSC is a kind of self-heating technique, which simplifies operations, reduces costs and supplies ultra-high cooling rate. The phase composition and the microstructural evolution are investigated using experiment studies and ANSYS simulation. During the process, the t-ZrO₂ are stabilized at room temperature because of the solubility of Al₂O₃ in ZrO₂. The ultra-high cooling rate greatly refined eutectic structure. Although the eutectic structure coarsens with increases in particle size, the interphase spacing of all particles reaches supra nanoscale. The work provides a route for preparing supra-nanostructure Al₂O₃/ZrO₂ eutectic powders and for better understanding the microstructural evolution.