Preparation and properties of nano-SiC strengthening Al2O3 composite ceramics

The processing and mechanical behaviors of Al₂O₃-xwt.%SiC (x = 1, 2, 5, ASx) nano-composites prepared by the in situ synthesis of SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The microstructure and mechanical properties of the sintered composites were analyzed. The results showed that a fully dense structure was obtained when a few nano-SiC were doped and that the fracture toughness and strength were highly improved compared with those of monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in AS2 composite. The maximum flexural strength was 516 MPa obtained in AS1 composite.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Hot-pressed A12O3–SiC(–C) composites with polycarbosilane as the precursor

The processing and mechanical behavior of Al₂O₃–xSiC (–C) (x = 1, 2, 5, 10 wt.%, ASx and ASCx) composites prepared by in situ reaction synthesis SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The pyrolysis process of PCS, microstructure, phase structure and mechanical properties of sintered composites were analyzed. Fully dense structure was obtained, and it was found that the fracture toughness and strength were highly improved compared with monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in 1 wt.%SiC composite ASC1. AS1 showed 516 MPa of flexural strength.     

Synthesis and mechanical properties of high-purity Cr2AlC ceramic

High-purity and dense Cr₂AlC has been successfully fabricated by hot-pressing, using Cr, Al and graphite as raw materials. Delamination, kink bands, monolamellar kink, transgranular crack and transgranular fracture of bulk Cr₂AlC are found during the room-temperature test. The density, Vickers hardness, flexural strength, Young's modulus, compressive strength and fracture toughness of the Cr₂AlC are 5.17 g/cm³, 4.9 GPa, 469 ± 27 MPa, 282 GPa, 949 ± 22 MPa and 6.22 ± 0.26 MPa m^1/2, respectively. The strength of Cr₂AlC could be greatly improved by second phase of Cr₇C₃. And the slipping of basal planes and slip system cold be hindered by Cr₇C₃, thus resulting in a lower toughness.     

Mechanical properties of 3D KD-I SiCf/SiC composites with engineered fibre-matrix interfaces

Three-dimensional (3D) silicon carbide (SiC) matrix composites reinforced with KD-I SiC fibres were fabricated by precursor impregnation and pyrolysis (PIP) process. The fibre-matrix interfaces were tailored by pre-coating the as-received KD-I SiC fibres with PyC layers of different thicknesses or a layer of SiC. Interfacial characteristics and their effects on the composite mechanical properties were evaluated. The results indicate that the composite reinforced with as-received fibre possessed an interfacial shear strength of 72.1 MPa while the composite reinforced with SiC layer coated fibres had a much higher interfacial shear strength of 135.2 MPa. However, both composites showed inferior flexural strength and fracture toughness. With optimised PyC coating thickness, the interface coating led to much improved mechanical properties, i.e. a flexural strength of 420.6 MPa was achieved when the interlayer thickness is 0.1 μm, and a fracture toughness of 23.1 MPa m^1/2 was obtained for the interlayer thickness of 0.53 μm. In addition, the composites prepared by the PIP process exhibited superior mechanical properties over the composites prepared by the chemical vapour infiltration and vapour silicon infiltration (CVI-VSI) process.     

Fabrication of SiCf/SiC composites by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites, employing KD-1 SiC fibers (from National University of Defense Technology, China) as reinforcements, were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and properties of the as prepared SiC_f/SiC composites were studied. The results show that the density and open porosity of the as prepared SiC_f/SiC composites are 2.1 g/cm³ and 7.7%, respectively. The SiC fibers are not severely damaged during the VSI process. And the SiC fibers adhere to the matrix with a weak interface, therefore the SiC_f/SiC composites exhibit non-catastrophic failure behavior with the flexural strength of 270 MPa, fracture toughness of 11.4 MPa·m^1/2 and shear strength of 25.7 MPa at ambient conditions. Moreover, the flexural strength decreases sharply at the temperature higher than 1200 °C. In addition, the thermal conductivity is 10.6 W/mk at room temperature.     

Mechanical properties and microstructure of Al2O3/TiAl in situ composites doped with Cr2O3

Al₂O₃/TiAl composites were successfully fabricated from powder mixtures of Ti, Al, TiO₂ and Cr₂O₃ by a hot-press-assisted exothermic dispersion method. The effect of the Cr₂O₃ addition on the microstructures and mechanical properties of Al₂O₃/TiAl composites was characterized, and the results showed that the Rockwell hardness, flexural strength and fracture toughness of the composites increased as the Cr₂O₃ content increased. When the Cr₂O₃ content was 2.5 wt%, the flexural strength and the fracture toughness attained peak values of 925 MPa and 8.55 MPa m^1/2, respectively. This improvement of mechanical properties was due to the more homogeneous and finer microstructure developed from the addition of Cr₂O₃ and an increase in the ratio of α₂-Ti₃Al to γ-TiAl matrix phases.     

SiCf/O′-SiAlON composite: properties and oxidation retained properties

A composite of O′SiAlON (Si_2-xAl_xN_2-xO_1+x, with x 0.14) reinforced with 20 vol.% SiC monofilaments was fabricated by hot-pressing, at 1600°C, for 2 h under 34 MPa pressure. The mechanical and interfacial properties of the composites, as-fabricated as well as post-oxidized, were, investigated. The composite exhibited a significant improvement in ultimate flexure strength (640 MPa) and work of fracture (42 kJ m⁻²) compared with that (350 MPa and 1.8 kJ m⁻², respectively) of the monolithic material. These mechanical properties were slightly increased after the composite was heat treated for 24 h in air at 1200 and 1300°C. However, the composite exhibited a significant degradation in ultimate strength, while the work of fracture (WOF) remained unchanged after exposure in air at temperatures beyond 1400°C. The as-fabricated composite revealed a low interfacial shear strength (6.2 MPa) and a frictional sliding stress (3.2 MPa). After the composite was oxidized at elevated temperatures, the interfacial bonding and sliding stresses were reduced to noticeable extents, resulting from the degradation of the carbon coating layer of the SiC monofilaments.     

Structure and thermal conductivity of Gd2(TixZr1 − x)2O7 ceramics

The Gd₂(Ti_xZr_1 − x)₂O₇ (x = 0, 0.25, 0.50, 0.75, 1.00) ceramics were synthesized by solid state reaction at 1650 °C for 10 h in air. The relative density and structure of Gd₂(Ti_xZr_1 − x)₂O₇ were analyzed by the Archimedes method and X-ray diffraction. The thermal diffusivity of Gd₂(Ti_xZr_1 − x)₂O₇ from room temperature to 1400 °C was measured by a laser-flash method. The Gd₂Zr₂O₇ has a defect fluorite-type structure; however, Gd₂(Ti_xZr_1 − x)₂O₇ (0.25 ≤ x ≤ 1.00) compositions exhibit an ordered pyrochlore-type structure. Gd₂Zr₂O₇ and Gd₂Ti₂O₇ are infinitely soluable. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ increases with increasing Ti content under identical temperature conditions. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ first decreases gradually with the increase of temperature below 1000 °C and then increases slightly above 1000 °C. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ is within the range of 1.33 to 2.86 W m^− 1 K^− 1 from room temperature to 1400 °C.     

Improved thermoelectric properties by adding Al for Zn in (ZnO)mIn2O3

The grain size and density of the sintered (Zn_1 − xAl_xO)_mIn₂O₃ bodies decreased with the small Al₂O₃ content (≤ 0.012), and then increased gradually by further increasing the Al₂O₃ content. The addition of Al for Zn in the (ZnO)_mIn₂O₃ led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient. This indicates that the power factor was significantly enhanced by adding Al for Zn. The thermoelectric power factor was maximized to 1.67 × 10^− 3 W m^− 1 K^− 2 at 1073 K for the (Zn_0.992Al_0.008O)_mIn₂O₃ sample.     

Structure and negative thermal expansion of Pb1 −xBixTiO3

Pb_1-xBi_xTiO₃ (x = 0.0-0.1) compounds were prepared to study the unique dopant effect of bismuth in PbTiO₃. Their thermal expansions and structures were investigated by high-temperature X-ray diffraction and X-ray Rietveld method. The results indicated that Bismuth substitution evidently weakened the tetragonality of PbTiO₃ solid solution, but increased the spontaneous polarization. Both the enhanced spontaneous polarization and the decreased tetragonality led to small volume shrinkage with temperature rising, where the average volumetric thermal expansion coefficient changed from − 1.99 × 10^− 5/°C for pure PbTiO₃ to − 0.56 × 10^− 5/°C for Pb_0.90Bi_0.10TiO₃. The Curie point of Pb_1 − xBi_xTiO₃ was slightly raised compared to PbTiO₃ and permitted one to use it in a wide temperature range.     

The thermal shock resistance of the ZrB2–SiC–ZrC ceramic

In the present work, the thermal shock resistance of the ZrB₂–SiC–ZrC ceramic was estimated by the water quenching method and the flexural strength of the quenched specimen was measured. The measured critical temperature difference of the ZrB₂–SiC–ZrC ceramic was significantly greater than that of the ZrB₂–15 vol.% SiC ceramic. The improvement in thermal shock resistance was attributed to its higher fracture toughness (6.7 MPa m^1/2) and lower flexural strength (526 MPa) relative to the ZrB₂–15 vol.% SiC ceramic (4.1 MPa m^1/2 and 795 MPa) based on Griffith fracture criterion. Furthermore, the temperature and thermal stress distributions in the specimen during instantaneous water quenching were simulated by Finite element analysis.     

Electrode effect on resistive switching of Ti-added amorphous SiOx films

Ti-added amorphous SiO_x films were sputter-deposited into stacks of Pt/SiO_x/Pt and Cu/SiO_x/Pt. Optimally prepared Pt/SiO_x/Pt exhibits unipolar resistive switching over 10² cycles, resistance ratio ∼ 10³, yet wide voltage distribution (2 ∼ 7 V for SET, 0.5 ∼ 1.5 V for RESET). Cu/SiO_x/Pt exhibit similar endurance, resistance ratio up to 10⁷, and SET and RESET voltages reduced to 1.8 ∼ 4.2 V and 0.5 ∼ 1 V, respectively. Cu diffusion into SiO_x at the virgin state may play a role in resistive switching of Cu/SiO_x/Pt stack besides of filament conduction. Ti-added amorphous SiO_x films incorporating Cu electrode shows potential for resistive memory.     

Synthesis and electrochemical properties of Ti doped Li3 − xFe2 − xTix(PO4)3/C cathode materials

Li_3 − xFe_2 − xTi_x(PO₄)₃/C (x = 0-0.4) cathodes designed with Fe doped by Ti was studied. Both Li₃Fe₂(PO₄)₃/C (x = 0) and Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C (x = 0.2) possess two plateau potentials of Fe³⁺/Fe²⁺ couple (around 2.8 V and 2.7 V vs. Li⁺/Li) upon discharge observed from galvanostatic charge/discharge and cyclic voltammetry. Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C has higher reversibility and better capacity retention than that of the undoped Li₃Fe₂(PO₄)₃/C. A much higher specific capacity of 122.3 mAh/g was obtained at C/20 in the first cycle, approaching the theoretical capacity of 128 mAh/g, and a capacity of 100.1 mAh/g was held at C/2 after the 20th cycle.     

Effect of graphite flake on the mechanical properties of hot pressed ZrB2-SiC ceramics

A ZrB₂ ceramic containing 20 vol.% SiC and 10 vol.% graphite flake (ZrB₂-SiC-G) was fabricated by hot pressing. It was shown that the fracture toughness was improved due to the introduction of graphite flake, whereas the flexure strength and hardness decreased slightly. The fracture toughness of ZrB₂-SiC-G composite was 6.1 ± 0.3 MPa·m^1/2, which was much higher than that of monolithic ZrB₂, ZrB₂-SiC composite and similar ZrB₂-SiC-C composite. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. The results here pointed to a potential method for improving fracture toughness of ZrB₂-based ceramics.     

Optical and electrical properties of vapour phase grown Zn1 − xCrxTe crystals

The optical and electrical properties of vapour phase grown crystals of diluted magnetic semiconductor Zn_1 − xCr_xTe were investigated for 0 ≤ x ≤ 0.005. The diffuse reflectance spectra exhibited an increase in the fundamental absorption edge (E₀) with composition x and were also dominated by a broad absorption band around 5200 cm^− 1 arising from ⁵T₂ → ⁵E transition. The ⁵T₂ and ⁵E levels originate from the crystal field splitting of the ⁵D free ion in the ground state. The electrical resistivity measurements revealed semiconducting behaviour of the samples with p-type conductivity in the temperature range of 200-450 K.     

Microstructure–mechanical properties relations in SiC–TiB2 composite

Densification and mechanical properties (fracture toughness, flexural strength and hardness) of SiC–TiB₂ composite were studied. Pressureless sintering experiments were carried out on samples containing 0–50 vol% of TiB₂ created by in situ reaction between TiO₂, B₄C and carbon. Al₂O₃ and Y₂O₃ were used as sintering additives to create liquid phase and promote densification at sintering temperature of 1940 °C. The sintered samples were subsequently heat treated at 1970 °C. It was found that the presence of TiB₂ serves as an effective obstacle to SiC grain growth as well as crack propagation thus increasing both strength and fracture toughness of sintered SiC–TiB₂ composite. The subsequent heat treatment of sintered samples promoted the elongation of SiC matrix and further improved mechanical properties of the composite. The best mechanical properties were measured in heat-treated samples containing 12–24 vol% TiB₂. The maximum flexural strength of ∼600 MPa was obtained in samples with 12 vol% TiB₂ whereas the maximum fracture toughness of 6.6 MPa m^1/2 was obtained in samples with 24 vol% TiB₂. Typical microstructures of samples with the mentioned volume fractions of TiB₂ consist of TiB₂ particles (<5 μm) uniformly dispersed in a matrix of elongated SiC plates.     

Composition dependent thermoelectric properties of sintered Mg2Si1−xGex (x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg₂Si_1−xGe_x (x = 0-1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg₂Si_1−xGe_x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg₂Si_1−xGe_x with higher Ge content (x = 0.6-1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10^− 5 Wcm^− 1K^− 2 was observed at 420 K for Mg₂Si and Mg₂Si_0.6Ge_0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg₂Si_1−xGe_x. The values close to 0.02 Wcm^− 1K^− 1 were obtained for Mg₂Si_1−xGe_x (x = 0.4-0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm^− 1K^− 1 at 773 K for Mg₂Si_0.4Ge_0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg₂Si_0.6Ge_0.4.     

Electrical properties of lead-free thick film NTC thermistors based on perovskite-type BaCoxCo2xBi1 − 3xO3

Lead-free thick film negative temperature coefficient (NTC) thermistors based on perovskite-type BaCo^II_xCo^III_2xBi_1 − 3xO₃ (x ≤ 0.1) were prepared by mature screen-printing technology. The microstructures of the thick films sintered at 720 °C were examined by X-ray diffraction and scanning electron microscopy. The electrical properties were analyzed by measuring the resistance-temperature characteristics. For the BaBiO₃ thick films, the room-temperature resistivity is 0.22 MΩ cm, while the room-temperature resistivity is sharply decreased to about 3 Ω cm by replacing of Bi with a small amount of Co. For compositions 0.02 ≤ x ≤ 0.1, the values of room-temperature resistivity (ρ₂₃), thermistor constant (B_25/85) and activation energy are in the range of 1.995-2.975 Ω cm, 1140-1234 K and 0.102-0.111 eV, respectively.     

The microwave-assisted synthesis and characterization of Zn1 − xCoxO nanopowders

In this paper, we present a simple microwave-assisted synthesis of Zn_1 − xCo_xO nanopowders. With the advantages of the microwave-assisted method, we have successfully synthesized good crystalline quality and good surface morphology Zn_1 − xCo_xO nanopowders. The nanopowders are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-VIS absorption, and micro-Raman spectroscopy. We found, in the synthesis process, the surfactant Triethanolamine (TEA) plays an important role on the morphology of Zn_1 − xCo_xO nanoparticles. The XRD study shows that for Co doping up to 5%, Co²⁺ ions are successfully incorporated into the ZnO host matrix. The absorption spectra of Zn_1 − xCo_xO (x = 1-5%) nanopowders show several peaks at 660, 611 and 565 nm, indicating the presence of Co²⁺ ions in the tetrahedral sites. The Raman study shows that the linewidth of E₂^low mode increases with Co concentration, which further indicates the incorporation of Co²⁺ ions into the ZnO host matrix.