Fabrication of reaction-bonded Cr2O3 ceramics

Reaction-bonding to form Cr₂O₃ can be achieved by gaseous oxidation of a Cr phase. The reaction-bonding process is best conducted by complete oxidation before sintering. Below ≈800 °C, the activation energy for oxidation is 220 kJ mol⁻¹, indicating the predominance of Cr³⁺ outward diffusion along high diffusivity paths, e.g., grain boundaries and dislocations. At higher temperatures, the activation energy is reduced to 52 kJ mol⁻¹ as a result of oxygen transport along lower-energy paths, e.g., along microcracks, and the internal and external surfaces. In spite of the decrease in activation energy, the access of oxygen to the inside of the powder compact is hindered by the progressive densification of the oxidizing powder compacts. Maximum densification is achieved for fully oxidized Cr/Cr₂O₃ compacts when the oxygen partial pressure is close to that of the Cr-Cr₂O₃ equilibrium. 0.1 wt% MgO addition increases the density and reduces the grain size of the reaction-bonded Cr₂O₃ samples due to the possible formation of the spinel phase MgCr₂O₄. ZrO₂ and MgO additions improve the fracture strength and toughness of conventionally sintered Cr₂O₃ and change its fracture mode from intergranular to intragranular. For reaction-bonded Cr₂O₃ samples with or without MgO addition, their fracture strength and toughness data are roughly the same as those of sintered Cr₂O₃ doped with ZrO₂ and MgO and their fracture surfaces are predominantly intragranular.     

Microstructure and grain size dependence of ferroelectric properties of BaTiO3 thin films on LaNiO3 buffered Si

BaTiO₃ films were prepared by radio frequency sputtering on the LaNiO₃/Si substrates and then annealed at different temperatures. The films exhibit a highly (1 0 0)-oriented structure and their grain size range from 14 to 55 nm after annealing. Polarization-reversal characteristics for different BaTiO₃ films were measured. The results show that while obvious ferroelectricity is obtained for films with grain size larger than 22 nm, a weak ferroelectricity is still observed in BaTiO₃ film of 14 nm grains, indicating that if a critical grain size exists for ferroelectricity it is less than 14 nm for polycrystalline BaTiO₃ on LaNiO₃/Si. The suppression of macroscopic ferroelectricity for BaTiO₃ with finest grain size and the grain size dependence of remnant polarization and coercive field are also discussed in detail.     

Processing and dielectric characterization of Ba0.3Sr0.7TiO3 thin films on alumina substrates

Ba_0.3Sr_0.7TiO₃ (BST) thin films were prepared from the sols based on alkaline earth acetates and titanium propoxide in 2-methoxyethanol–acetic acid solvents and deposited on polished alumina substrates by spin coating. The perovskite phase crystallizes upon heating at/above 700 °C. By increasing the annealing temperature from 700 to 900 °C the grain size increases from 40 to 80 nm, due to the increased driving force for crystallization. The annealing time has got only a minor influence on grain size as a consequence of constrained conditions of the film. The dielectric permittivity and tunability (ɛ_0 V/ɛ_200 V) of BST films, measured at 1 MHz, strongly depend on the grain size, exhibiting the values of 345 and 1.47, and 722 and 1.93 for the films with 40 and 80 nm-sized grains, respectively.     

The effect of La2O3 on the microstructure and room temperature mechanical properties of t-ZrO2

Structural and mechanical properties of La₂O₃ added (up to 5 wt%) t-ZrO₂ compacts were examined with the aim of optimizing hardness and fracture toughness for room temperature applications. Structural examinations were performed using an X-ray diffractometer and a scanning electron microscope. Mechanical properties of the compacts were determined as modulus of elasticity, hardness and fracture toughness by conducting Vickers indentation tests under test loads of 0.5, 1 and 10 kg. Addition of 0.5 wt% La₂O₃ deteriorated the room temperature stability of t-ZrO₂ by forming m-ZrO₂ and coarse polygonal grains in the matrix. Higher concentrations (2 and 5 wt% La₂O₃) caused precipitation of La₂Zr₂O₇ at the grain boundaries, which also accompanied by a reduction in hardness. Fracture toughness increased with increasing La₂O₃ content of the compact. Finally, an optimum combination between hardness and fracture toughness was obtained for the 0.5 wt% La₂O₃ containing compact.     

Liquid flame spray fabrication of WO3-reduced graphene oxide nanocomposites for enhanced O3-sensing performances

WO₃ is one of the inspiring sensing materials that show high response to O₃; an efficient fabrication of WO₃ film with incorporation of complementary additives is essential for enhanced sensitivity. Here we report film deposition by liquid flame spraying, characterization of nanostructured WO₃-reduced graphene oxide (rGO) composites and their gas-sensing activities to O₃. The starting feedstock was prepared from WC_l6 and rGO for pyrolysis synthesis by flame spraying. Nano-porous WO₃-rGO films were successfully fabricated and characterized by transmission electron microscopy, field emission scanning electron microscopy, Raman spectrometry, thermal analyses and X-ray diffraction. Nanosized WO₃ grains exhibited oriented nucleation on rGO flakes whereas rGO retained intact its nano-structural features after spraying. Constrained grain growth of WO₃ of 60–70 nm in size was realized in the rGO-containing films with as compared to ~220 nm in the pure WO₃ film. The WO₃-rGO film sensors showed quicker response to O₃ and faster recovery than rGO-free WO₃ film sensors. Addition of rGO in 1.0 wt% or 3.0 wt% in the films caused a significantly reduced effective working temperature of the film sensors from ~ 250 °C to ~ 150 °C.     

Mechanical properties of low temperature synthesized dense and fine-grained Cr2AlC ceramics

Mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr₂AlC ceramic at relatively low temperatures. A mixture of Cr, Al and C powders with a molar ratio of 2:1.2:1 was mechanically alloyed for 3 h, and then subjected to hot pressing at 30 MPa and different temperatures for 1 h in Ar atmosphere. The results show that a dense Cr₂AlC ceramic with a grain size of about 2 μm can be synthesized at a relatively low temperature of 1100 °C. The synthesized fine-grained Cr₂AlC has a high density of 99%, which is higher than the 95% density for the coarse-grained Cr₂AlC (grain size of about 35 μm) as synthesized by hot pressing unmilled Cr, Al and C. The flexural strength, fracture toughness and Vickers hardness of the fine-grained Cr₂AlC were determined and compared with the values for the coarse-grained Cr₂AlC.     

Low-temperature crystallization of tantalum pentoxide films under elevated pressure

Ta₂O₅ thin films deposited via a metal-organic decomposition method were crystallized via atmospheric pressure annealing and a high-pressure crystallization (HPC) process. Ta₂O₅ thin films started to become crystallized at 700 °C as subjected to atmospheric pressure annealing. When the HPC process was adopted and annealing at 16.5 MPa was performed, the crystallization temperature of Ta₂O₅ films was greatly dropped to as low as 350 °C. The developed HPC process considerably reduced the thermal budget and energy consumption during film processing. The crystallized Ta₂O₅ phase was found to be homogeneously distributed within the HPC-derived films. With annealing at 700 °C under atmospheric pressure, the silicon species diffused from the substrates into the Ta₂O₅ layers, thereby leading to reduced dielectric constants. The HPC process effectively suppressed the interdiffusion between the substrates and dielectric layers by lowering the required heating temperature, and also significantly increased the dielectric constants of Ta₂O₅ thin films. The HPC process was confirmed to effectively lower the crystallization temperature and improve the dielectric properties of Ta₂O₅ thin films.     

Thermal-induced blister cracking behavior of annealed sandwich-structured TiN/CrAlN films

Sandwich-structured TiN/CrAlN films were rationally designed using metallic Ti and Al-Cr alloy targets by RF-pulsed magnetron sputtering. After obtained films were annealed at diverse temperatures at atmospheric pressure for 1 h, the hardness reveals an apparent decrease evolution from 29.2 to 15.7 GPa and H/E* ratio declines below 0.1 with increasing annealing temperature. Meanwhile, the grain size gradually becomes larger from 16.3 to 130.0 nm with increasing annealing temperature. Interestingly, it is observed that cracking behavior of sandwich-structured composite TiN/CrAlN films at elevated temperature is originated from the top of the blisters where main component is alumina on the surface, in virtue of intrinsically induced stress during oxidation, thermal expansion mismatch and phase transformation of the oxide layer. No cracks, nevertheless, are yielded in the film between any two blisters. Herein, these findings provide some beneficial references for preparing high quality films and coatings in high temperature service.     

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y₂O₃ powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol^?1 and associated diffusion coefficients higher by 10³ than expected for Y³⁺ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.     

Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

Constrained sintering kinetics of 8 mol% Y₂O₃/92 mol% ZrO₂ (8YSZ) films approximately 10–15 μm thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 °C. The results are compared with those reported earlier for 3YSZ films.Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol^?1) despite all probably being mainly controlled by grain boundary cation diffusion.     

Synthesis,microstructure and mechanical properties of Cr2AlC/Al2O3 in situ composites by reactive hot pressing

Al₂O₃ particle-reinforced Cr₂AlC in situ composites were successfully fabricated from powder mixtures of Cr₃C₂, Cr, Al, and Cr₂O₃ by a reactive hot-pressing method at 1400 °C. A possible synthesis mechanism was proposed to explain the formation of the composites in which Al₂O₃ was formed by the aluminothermic reaction between Al and Cr₂O₃, meanwhile, Cr₃C₂, Al, together with Cr reacted to form Cr₂AlC in a shortened reaction route. The effect of Al₂O₃ addition on the microstructure and mechanical properties of Cr₂AlC/Al₂O₃ composites was investigated. The results indicated that the as-sintered products consisted of Cr₂AlC matrix and Al₂O₃ reinforcement, and the in situ formed fine Al₂O₃ particles dispersed at the matrix grain boundaries. The flexural strength and Vickers hardness of the composites increased gradually with increasing Al₂O₃ content. But the fracture toughness peaked at 6.0 MPa m^1/2 when the Al₂O₃ content reached 11 vol.%. The strengthening and toughening mechanism was also discussed.     

Sintering of nanocrystalline Ta2O5 and ZrO2 films compared to that of TiO2 films

Thin (d = 60 nm/140 nm) nanocrystalline Ta₂O₅ and ZrO₂ films were deposited onto SiO₂ flakes, using a liquid route synthesis. Their sintering behaviour was characterized and compared to that of the corresponding powders and the known equivalent TiO₂ film in terms of grain size, grain growth and layer porosity. The effect of the substrate was noticeable on crystallisation process but not on grain growth. The sintering behaviour was actually dictated by the initial size and the packing of the precipitated grains related to the synthesis of the film.     

Particle size effect of starting SiC on processing,microstructures and mechanical properties of liquid phase-sintered SiC

Dense SiC (97.3–99.2% relative density) of 1.1–3.5 μm average grain size was prepared by the combination of colloidal processing of bimodal SiC particles with sintering additives (Al₂O₃ plus Y₂O₃, 2–4 vol%) and subsequent hot-pressing at 1900–1950 °C. The fracture toughness of SiC was sensitive to the grain boundary thickness which was controlled by grain size and amount of oxide additives. A maximum fracture toughness (6.2 MPa m^1/2) was measured at 20 nm of grain boundary thickness. The mixing of 30 nm SiC (25 vol%) with 800 nm SiC (75 vol%) was effective to reduce the flaw size of fracture origin, in addition to a high fracture toughness, leading to the increase of flexural strength. However, the processing of a mixture of 30 nm SiC (25 vol%)–330 nm SiC (75 vol%) provided too small grains (1.1 μm average grain size), resultant thin grain boundaries (12 nm), decreased fracture toughness, and relatively large defect of fracture origin, resulting in the decreased strength.     

Coating of nano-sized ionically conductive Sr and Ca doped LaMnO3 films by sol–gel route

In this study, Sr and Ca doped LaMnO₃ thin ceramic films were coated on Al₂O₃ substrates by using a sol–gel route as the cathode material for SOFC. Nitrate precursors were used for the preparation of the thin film coating solution, and methanol and acetyl acetone were also used as the solvent and chelating agent, respectively. After the solution was prepared, an Al₂O₃ single crystal substrate was dipped into the solution. Then it was fired at 500 °C and annealed at 1025 °C for the crystallization. Coated films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), focused ion beam (FIB) and atomic force microscopy (AFM). Conductivity of the coated films was measured by the four probe Van der Pauw method. XRD, SEM, AFM and FIB characterizations of the coated film showed that the LaMnO₃ phase was formed, surface of the films was uniform and had homogenously distributed pores sized about 10 nm, mean grain size was about 60–80 nm and the film thickness was about 180 nm. The specific resistivity of the film was calculated to be 0.524 Ω m.     

Structure and properties of nanostructured Fe(AlCr)2O4–Cr–(AlCr)2O3–Fe composite coating prepared by plasma spraying

In-situ nanostructured Fe(AlCr)₂O₄-based composite coating (FACr_52.5 coating) was prepared by reactive plasma spraying with micro-sized Al–Fe₂O₃–Cr₂O₃ powders. The microstructure, toughness and Vickers hardness, and adhesive strength of the coating were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that the interlamellar spacing of the FACr_52.5 coating is only 1 μm. The coating exhibited nanostructured microstructure. The in-situ Cr (20 nm) and Fe (50–200 nm) particles were uniformly distributed in an Fe(AlCr)₂O₄ matrix, while the grain size of the Fe(AlCr)₂O₄ matrix is about 60 nm. The FACr_52.5 composite nano-coating exhibited much higher hardness, better wear resistance, stronger adhesive strength and toughness as compared to those of the composite nano-coating sprayed with Fe₂O₃–Al powders. Excellent mechanical properties of the FACr_52.5 coating were attributed to the uniform distribution of the in-situ nano-sized Cr particles in the coating matrix.     

Structural,electrical and optical properties of stoichiometric In2Te3 thin films

In₂Te₃ thin films were grown by thermal evaporation technique. The annealing of films played a major role to obtain stoichiometry, regardless of substrate temperature. Annealing at 300 ⁰C resulted in well oriented, mono-phased and nearly stoichiometric In₂Te₃ thin films. The variation in grain size of In₂Te₃ films associated with the substrate temperatures provides a significant control over the resistivity of the films, and the resistivity decreased with an increase in the grain size. The activation energy and optical band gap of stoichiometric In₂Te₃ films were found to be 0.01±0.005 eV and 0.99±0.02 eV, respectively. The absorption co-efficient of these films was found to be of the order of 10⁵ cm⁻¹.     

Synthesis and characterization of dense and fine nickel ferrite ceramics through two-step sintering

Two-step sintering (TSS) has been employed in the current study to suppress the accelerated grain growth of NiFe₂O₄ nanopowder compacts in the final sintering stage. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range from 1200 °C to 1300 °C is effective for the first-step sintering (T₁) due to its highest densification rate. The second-step sintering temperature (T₂) should be within the kinetic window, where grain boundary diffusion is maintained but grain boundary migration is suppressed. The grain sizes of high density (≥98% theoretical density) NiFe₂O₄ compacts produced by TSS are smaller than 700 nm, while that of those formed by CS are over 2.5 μm. The evidence indicates that the saturation magnetization of nearly full NiFe₂O₄ ceramics is independent of grain size and likewise high, with the corresponding values of approximately 54 emu/g. The Vickers hardness and fracture toughness both increase with the decrease of grain size and porosity.     

Sintering,microstructure and mechanical properties of Al2O3–Y2O3–ZrO2 (AYZ) eutectic composition ceramic microcomposites

We report on how the mechanical properties of sintered ceramics (i.e., a random mixture of equiaxed grains) with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition compare with those of rapidly or directionally solidified Al₂O₃–Y₂O₃–ZrO₂ eutectic melts. Ceramic microcomposites with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition were fabricated by sintering in air at 1400–1500 °C, or hot pressing at 1300–1400 °C. Fully dense, three phase composites of Al₂O₃, Y₂O₃-stabilized ZrO₂ and YAG with grain sizes ranging from 0.4 to 0.8 μm were obtained. The grain size of the three phases was controlled by the size of the initial powders. Annealing at 1500 °C for 96 h resulted in grain sizes of 0.5–1.8 μm. The finest scale microcomposite had a maximum hardness of 19 GPa and a four-point bend strength of 282 MPa. The fracture toughness, as determined by Vickers indentation and indented four-point bending methods, ranged from 2.3 to 4.7 MPa m^1/2. Although strengths and fracture toughnesses are lower than some directionally or rapidly solidified eutectic composites, the intergranular fracture patterns in the sintered ceramic suggest that ceramic microcomposites have the potential to be tailored to yield stronger, tougher composites that may be comparable with melt solidified eutectic composites.     

Microstructure,mechanical properties and machining performance of spark plasma sintered Al2O3–ZrO2–TiCN nanocomposites

The effect of addition of nanocrystalline ZrO₂ and TiCN to ultrafine Al₂O₃ on mechanical properties and microstructure of the composites developed by spark plasma sintering (SPS) was investigated. The distribution of the nanoparticles was dependent on their overall concentration. Maximum hardness (21 GPa) and indentation toughness (5.5 MPa m^1/2) was obtained with 23 vol% nanoparticles, which was considered as the optimum composition. The Zener pinning criteria were also satisfied at this composition with grain size of the restraining nanoparticles ～63–65 nm. Hardness of the composites follows the rule of mixtures; crack deflection and crack arrest by nanoparticles at grain boundaries along with mixed fracture mode led to high toughness in the nanocomposites. Cutting tool inserts were developed by SPS with the optimized composition and their machining performance was compared with commercial alumina based inserts. Increased toughness in the nanocomposite inserts reflects in the machining performance as the tool life improves drastically compared to that of the commercial inserts at high cutting speeds ≥500 m min^?1. This was attributed to differences in their failure modes; the commercial inserts fail catastrophically by fracture due to their low toughness whereas the nanocomposite inserts reach the tool failure criteria by crater wear at all machining conditions.     

The effect of silica doping on neodymium diffusion in yttrium aluminum garnet ceramics: implications for sintering mechanisms

The Nd³⁺ cation diffusion into transparent polycrystalline YAG (Y₃Al₅O₁₂) was investigated as a function of temperature and silica content. Thin neodymium oxide layers were deposited on sintered YAG substrates prior to annealing under air at temperatures from 1400 to 1600 °C. Bulk and grain boundary neodymium diffusion coefficients were measured by secondary ion mass spectrometry. The experimental results show that silica addition increases the diffusivity of Nd³⁺ by a factor 10 whatever the diffusion path, probably as a result of extrinsic point defects formation, especially rare-earth vacancies.The experimental diffusion data were used to elucidate the sintering mechanism of Nd:YAG ceramics in the temperature range 1450–1550 °C. Firstly, it appeared that the intermediate stage of solid-state sintering should be controlled by the rare-earth diffusion along the grain boundary with an activation energy of about 600 kJ mol^?1. Secondly, grain growth mechanism at the final stage of liquid-phase sintering was investigated for silica-doped Nd:YAG samples. Thus, the grain growth should be limited by the reaction at interfaces at a temperature lower than 1500 °C, with an activation energy of about 880 kJ mol^?1. At higher temperature, it seems to be limited by the ionic diffusion through the intergranular liquid phase, with an activation energy of 250 kJ mol^?1.