Investigations on synthesis of HfB2 and development of a new composite with TiSi2

This paper presents the results of investigation carried out on synthesis and densification of monolithic HfB₂ and the effect of TiSi₂ as sinter additive. Pure phase HfB₂ was prepared by boron carbide reduction of HfO₂ and hot pressed to full density with the addition of TiSi₂. Isothermal oxidation study of this composite was carried out at 850 °C up to 64 h. Formation of HfB₂ was seen at 1200 °C but pure HfB₂ was formed at a much higher temperature of 1875 °C in vacuum. Hot pressing of HfB₂ at 1850 °C and 35 MPa pressure gave a compact of 80% TD. Addition of TiSi₂ helped in achieving a much higher density at a lower temperature of 1600 °C and a pressure of 20 MPa. A fully dense composite of HfB₂ and TiSi₂ was obtained with 15% TiSi₂. Hardness and fracture toughness of this composite were 27.4 ± 1.9 GPa and 6.6 ± 0.2 MPa m^1/2, respectively. Considerable deflection was observed in the crack propagation in composites. Oxidation studies indicated the formation of HfO₂, SiO₂, TiO₂ and HfSiO₄ with some glassy phase and the composite with 15% TiSi₂ was seen to be completely covered with a protective glassy layer.     

Microstructure and mechanical properties of laminated ZrB2-SiC ceramics with ZrO2 interface layers

Laminated ZrB₂-SiC ceramics with ZrO₂ interface layers were successfully prepared by tape casting, laminating and hot pressing. The flexural strength and fracture toughness are 561 ± 20 MPa and 14.4 ± 0.3 MPa m^1/2 for parallel direction, and 432 ± 18 MPa and 5.8 ± 0.3 MPa m^1/2 for perpendicular direction. The fracture toughness for parallel direction is improved significantly compared to monolithic ZrB₂-SiC ceramics. The toughening mechanism was attributed to the deflection and branch of the crack and the new microcracks, which would increase the propagation path and fracture work.     

Bulk WC-Al2O3 composites prepared by spark plasma sintering

WC and WC-Al₂O₃ materials without metallic binder addition were densified by spark plasma sintering in the range of 1800-1900 °C. The densification behavior, phase constitution, microstructure and mechanical properties of pure WC and WC-Al₂O₃ composite were investigated. The addition of Al₂O₃ facilitates sintering and increases the fracture toughness of the composites to a certain extent. An interesting phenomenon is found that a proper content of Al₂O₃ additive helps to limit the formation of W₂C phase in sintered WC materials. The pure WC specimen possesses a hardness (HV₁₀) of 25.71 GPa, fracture toughness of 4.54 MPa·m^1/2, and transverse fracture strength of 862 MPa, while those of WC-6.8 vol.% Al₂O₃ composites are 24.48 GPa, 6.01 MPa·m^1/2, and 1245 MPa respectively. The higher fracture toughness and transverse fracture strength of WC-6.8 vol.% Al₂O₃ are thought to result from the reduction of W₂C phase, the crack-bridging by Al₂O₃ particles and the local change in fracture mode from intergranular to transgranular.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Microstructure and mechanical properties of ZrB2–Nb composite

The high relative density of the ZrB₂-based composite toughened by 25 vol.%Nb (ZN) was hot-pressed at reduced temperatures with low pressure of 30 MPa. Compared with the toughness of 2.3–3.5 MPa m^1/2 and strength of 350 MPa of the monolithic ZrB₂, the toughness and strength of the ZN composite were improved to 6.7 MPa m^1/2 and 773 MPa, respectively, due to the addition of ductile Nb. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. Furthermore, the densification mechanism was analyzed and discussed. The results here pointed to a potential method for improving fracture toughness and strength of ZrB₂-based ceramics.     

Synthesis, microstructure and mechanical properties of reaction-infiltrated TiB2-SiC-Si composites

TiB₂-C preforms formed with different compositions and processing parameters were reactively infiltrated by Si melts at 1450 °C to fabricate TiB₂-SiC-Si composites. Phase constituent and microstructure of these composites were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The resulting composites are generally composed of TiB₂ and reaction-formed β-SiC major phases, together with a quantity of residual Si. Unreacted carbon is detected in the samples with a starting composition of 2TiB₂ + 1C formed at higher pressure and in all of the ones at the composition of 1TiB₂ + 1C. The distribution of these phases is fairly homogeneous in microstructure. TiB₂-SiC-Si composites show good mechanical properties, with representative values of 19.9 GPa in hardness, 395 GPa in elastic modulus, 3.5 MPa m^1/2 in fracture toughness and 604 MPa in bending strength. The primary toughening and strengthening mechanism is attributed to the crack deflection of TiB₂ particles.     

Preparation and characterization of high-toughness ZrB2/Mo composites by hot-pressing process

ZrB₂/Mo composites with various volume fractions of Mo (0–10%) were prepared by a hot-pressing process at 1950 °C. The addition of Mo both facilitated the densification and improved the mechanical properties of the ZrB₂/Mo composites. Compared with the pure ZrB₂ ceramics, the fracture toughness of the ZrB₂/Mo (10%) composite increased from 4.52 to 7.98 MPa m^1/2 and its bending strength increased from 424 to 450 MPa. The enhancement in mechanical properties of the ZrB₂/Mo composites could be attributed to the reduced grains and the MoB phase formed in the reaction between ZrB₂ and Mo.     

An evaluation of Al2O3-ZrO2 composites produced by coprecipitation method

The objective of this work is to produce Al₂O₃-ZrO₂ composite from nano-sized powders processed by coprecipitation method. Al₂O₃ and mixture of Al₂O₃ + 10 wt.% ZrO₂ precipitated successfully by chemical route from aluminum sulfate and zirconium sulfate were pressed under uniaxial compression of 170 MPa and sintered at 1600 °C for 1 h. SEM investigations revealed that, pure alumina sample has a microstructure with coarse grains which anisotropically grown up to 30-40 μm in size. In alumina-zirconia composite, the structure consists of very fine equiaxed grains of typically 2 μm in which zirconia precipitates were uniformly dispersed. By adding zirconia to alumina, hardness and indentation fracture toughness were increased from 11.6 GPa to 16.8 GPa and from 3.2 MPa m^1/2 to 4.9 MPa m^1/2, respectively. Improvement in fracture toughness was attributed to bridging effects of zirconia particles as well as transformation toughening.     

Mechanochemical and volume combustion synthesis of ZrB2

Formation of ZrB₂ by volume combustion synthesis (VCS) and mechanochemical process (MCP) from ZrO₂-Mg-B₂O₃ was studied. Production of ZrB₂ by VCS in air occurred with the formation of side products, Zr₂ON₂ and Mg₃B₂O₆ in addition to MgO and ZrB₂. Zr₂ON₂ formation was prevented by conducting VCS experiments under argon. Wet ball milling was applied to the VCS products before leaching for easier removal of Mg₃B₂O₆ phase. MgO and Mg₃B₂O₆ were removed from wet ball-milled products by leaching in 5 M HCl for 2.5 h. In MCP, 30-hour ball milling was found to be sufficient for the formation of ZrB₂ with no minor phase formation. Leaching of MCP products in 1 M HCl for 30 min was sufficient to remove MgO. Complete conversion of ZrO₂ to ZrB₂ did not take place in both production methods, even with excess amounts of Mg and B₂O₃. Therefore, formed ZrB₂ contained residual ZrO₂.     

The thermal stability in air of hot-pressed diboride matrix composites for uses at ultra-high temperatures

The resistance to oxidation in ambient air at a temperature up to 1600 °C of two hot-pressed diborides matrix composites, both containing 19.5% v/o SiC and 3 v/o HfN (as sintering aid), was investigated. The diboride matrix was based on HfB₂ or a ZrB₂/HfB₂ mixture (volume ratio ≈ 1). Both the materials were subjected to repeated heating-cooling cycles at 1600 °C, and a 20 h exposure at 1450 °C in flowing dry air. Modest weight gains and limited corrosion depths highlighted a rather good thermal stability. In accordance with the thermo-gravimetric test at 1450 °C, the oxidation kinetics for both the composites superbly fit a para-linear law. The introduction of the SiC particles provided tangible benefits for the resistance to oxidation. One of the oxidation products, a borosilicate glass, sealed pores and coated the exposed faces, greatly limiting the inward transport of oxygen towards the internal oxide/diboride interfaces.     

Dielectric and microwave properties of ZrO2 doped CaO-SiO2-B2O3 ceramic matrix composites

The behavior of dielectric and microwave properties against sintering temperature has been carried out on CaO-SiO₂-B₂O₃ ceramic matrix composites with ZrO₂ addition. The results indicated that ZrO₂ addition was advantageous to improve the dielectric and microwave properties. X-ray diffraction (XRD) patterns show that the major crystalline β-CaSiO₃ and a little SiO₂ phase existed at the temperature ranging from 950 °C to 1050 °C. At 0.5 wt% ZrO₂, CaO-SiO₂-B₂O₃ ceramic matrix composites sintered at 1000 °C possess good dielectric properties: ?_r = 5.85, tan δ = 1.59 × 10⁻⁴ (1 MHz) and excellent microwave properties: ?_r = 5.52, Q · f = 28,487 GHz (11.11 GHz). The permittivity of Zr-doped CaO-SiO₂-B₂O₃ ceramic matrix composites exhibited very little temperature dependence, which was less than ±2% over the temperature range of −50 to 150 °C. Moreover, the ZrO₂-doped CaO-SiO₂-B₂O₃ ceramic matrix composites have low permittivity below 5.5 over a wide frequency range from 20 Hz to 1 MHz.     

Effect of Cr7C3 on the mechanical, thermal, and electrical properties of Cr2AlC

In this work, phase pure Cr₂AlC and impure Cr₂AlC with Cr₇C₃ have been fabricated to investigate the mechanical, thermal, and electrical properties. The thermal expansion coefficient is determined as 1.25 × 10⁻⁵ K⁻¹ in the temperature range of 25-1200 °C. The thermal conductivity of the Cr₂AlC is 15.73 W/m K when it is measured at 200 °C. With increasing temperature from 25 °C to 900 °C, the electrical conductivity of Cr₂AlC decreases from 1.8 × 10⁶ Ω⁻¹ m⁻¹ to 5.6 × 10⁵ Ω⁻¹ m⁻¹. For the impure phase of Cr₇C₃, it has a strengthening and embrittlement effect on the bulk Cr₂AlC. And the Cr₂AlC with Cr₇C₃ would result in a lower high-temperature thermal expansion coefficient, thermal conductivity, specific heat capacity and electrical conductivity.     

Synthesis and characterization of in situ carbon-coated Li2FeSiO4 cathode materials for lithium ion battery

Li₂FeSiO₄/C composites with in situ carbon coating were synthesized via sol-gel method based on acid-catalyzed hydrolysis/condensation of tetraethoxysilane (TEOS) with sucrose and l-ascorbic acid as carbon additives, respectively. As-obtained Li₂FeSiO₄/C composites prepared with l-ascorbic acid as a carbon additive are composed of nanoparticulate Li₂FeSiO₄ in an intimate contact with a continuous thin layer of residual carbon and exhibit large specific surface area up to 395.7 m² g⁻¹. The results indicate that structure of the residual carbon is graphene-rich with obviously lower disordered/graphene (D/G) ratio. These as-obtained Li₂FeSiO₄/C composites exhibit first discharge capacity of 135.3 mAh g⁻¹ at C/16 and perform cycling stability, which are superior to those of Li₂FeSiO₄/C composites synthesized with sucrose as a carbon additive.     

In situ synthesized (ZrB2 + ZrC) hybrid short fibers reinforced Zr matrix composites for nuclear applications

In the present work, novel zirconium matrix composites reinforced with ZrB₂ or (ZrB₂ + ZrC) hybrid short fibers were designed and prepared for the first time, through the reaction from Zr, B and B₄C with non-consumable vacuum arc melting. The result shows that the ZrB₂ particles grow in large short-fiber shape while the ZrC particles grow in thin short-fiber shape or near equiaxed shape. The morphological characters are related to their crystal structure and growth mechanism. Homogeneous distribution of both two in situ reinforcements can be found in the as-cast composites. As both the Zr matrix and the in situ reinforcements have good nuclear and mechanical properties, these Zr matrix composites are considered to be good candidates for applied as reactor core components.     

Modification of TiO2 nanorods by Bi2MoO6 nanoparticles for high performance visible-light photocatalysis

In this work, TiO₂ nanorods were prepared by a hydrothermal process and then Bi₂MoO₆ nanoparticles were deposited onto the TiO₂ nanorods by a solvothermal process. The nanostructured Bi₂MoO₆/TiO₂ composites were extensively characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis diffuse reflectance spectroscopy. The photocatalytic activity of the Bi₂MoO₆/TiO₂ composites was evaluated by degradation of methylene blue. The Bi₂MoO₆/TiO₂ composites exhibit higher catalytic activity than pure Bi₂MoO₆ and TiO₂ for degradation of methylene blue under visible light irradiation (λ > 420 nm). Further investigation revealed that the ratio of Bi₂MoO₆ to TiO₂ in the composites greatly influenced their photocatalytic activity. The experimental results indicated that the composite with Bi₂MoO₆:TiO₂ = 1:3 exhibited the highest photocatalytic activity. The enhancement mechanism of the composite catalysts was also discussed.     

Low temperature growth of nanocrystalline TiO2 films with Ar/O2 low-field helicon plasma

TiO₂ thin films were deposited on silicon wafer substrates by low-field (1 < B < 5 mT) helicon plasma assisted reactive sputtering in a mixture of pure argon and oxygen. The influence of the positive ion density on the substrate and the post-annealing treatment on the films density, refractive index, chemical composition and crystalline structure was analysed by reflectometry, Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). Amorphous TiO₂ was obtained for ion density on the substrate below 7 × 10¹⁶ m^− 3. Increasing the ion density over 7 × 10¹⁶ m^− 3 led to the formation of nanocrystalline (~ 15 nm) rutile phase TiO₂. The post-annealing treatment of the films in air at 300 °C induced the complete crystallisation of the amorphous films to nanocrystals of anatase (~ 40 nm) while the rutile films shows no significant change meaning that they were already fully crystallised by the plasma process. All these results show an efficient process by low-field helicon plasma sputtering process to fabricate stoichiometric TiO₂ thin films with amorphous or nanocrystalline rutile structure directly from low temperature plasma processing conditions and nanocrystalline anatase structure with a moderate annealing treatment.     

In situ template polymerization of aniline on the surface of negatively charged TiO2 nanoparticles

The in situ oxidative template polymerization of aniline was performed successfully on the surface of negatively charged titania (TiO₂) nanoparticles with a mean diameter of 40 nm using ammonium persulfate and a Chem-Solv solution at pH 1 and 25 °C. SEM showed that the resulting polyaniline (PANI)/TiO₂ composites were well dispersed in solution due to the electrostatic repulsion force. Ultraviolet/visible spectroscopy, thermogravimetric analysis, Fourier-transform infrared spectroscopy, and cyclic voltammetry showed that the optical, thermal, and electrical properties of PANI/TiO₂ composites were quite different from those of pure PANI or TiO₂, which was attributed to the strong interaction between the two components. The conductivity of the PANI/TiO₂ composite was estimated to be 0.91 × 10⁻¹ S/cm at 25 °C in the range of semiconductor.     

Optimization of processing conditions and characterization investigations of ceramic injection molded and sintered ZrO2/WC composites

In this investigation, 3 mol% Y₂O₃ stabilized ZrO₂-based composites reinforced with 10 vol.%, 20 vol.% and 40 vol.% WC (named as 3Y-TZP/10WC, 3Y-TZP/20WC and 3Y-TZP/40WC) were fabricated by using injection molding and sintering. Mechanical properties of these composites varied due to WC addition and dwelling time. Density, strength and toughness decreased with shorter dwelling time and increasing WC content however a significant enhancement in fracture toughness was obtained by 3Y-TZP/20WC composite which had 9.2 MPa m^1/2 toughness. Severe unlubricated wear tests which were performed under 55 N normal load and 45 km sliding distance showed that 3Y-TZP/20WC composite had the lowest wear rate and wear volume values which are 2 × 10⁻⁸ mm³/(N m⁻¹) and 0.05 mm³, respectively.     

Synthesis and consolidation of TiN/TiB2 ceramic composites via reactive spark plasma sintering

The simultaneous synthesis and densification of TiN/TiB₂ ceramic composites via reactive spark plasma sintering (RSPS) was investigated. Different component ratios (TiH₂/BN (TiN, B)) and heating rates (112.5-300 °C/min) were used to initiate the chemical reaction for TiN/TiB₂ synthesis. The omit RSPS process was revealed to have three stages, which are described separately. The relationships between the RSPS conditions, the microstructure and the properties of sintered ceramic composites were established. A Vickers hardness of 16-25 GPa and a fracture toughness of 4-6.5 MPa m^1/2 were measured for various compositions. Sintered ceramic composites containing 36 wt% TiB₂ with the highest relative density of 97.4 ± 0.4% and an average grain size of 150-550 nm have been obtained.     

Piezoelectric and dielectric properties of Dy2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ + x wt% Dy₂O₃ with x = 0-0.3 ceramics were synthesized by conventional solid-state processes. The effects of Dy₂O₃ on the microstructure, the piezoelectric and dielectric properties were investigated. X-ray diffraction pattern confirmed that the coexistence of tetragonal and rhombohedral phases in the (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ composition was not changed by adding 0.05-0.3 wt% Dy₂O₃. SEM images indicate that all the ceramics have pore-free microstructures with high density, and that doping of Dy₂O₃ inhibits the grain growth of the ceramics. The addition of Dy₂O₃ shows the double effects on decreasing the piezoelectric and dielectric properties for 0 < x < 0.15 when Dy³⁺ ions substitute B-site Ti⁴⁺ ions, and increasing the properties for 0.15 < x < 0.3 when Dy³⁺ ions enters into A-site of the perovskite structure. The optimum electric properties of piezoelectric constant d₃₃ = 170 pC/N and the dielectric constant ?_r = 1900 (at a frequency of 1 kHz) are obtained at x = 0.3.