In Situ Reaction Synthesis, Electrical and Thermal, and Mechanical Properties of Nb4AlC3

In this work, a bulk Nb₄AlC₃ ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb₄AlC₃ were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10⁻⁶ K⁻¹ in the temperature range of 200°–1100°C. The thermal conductivity of Nb₄AlC₃ increased from 13.5 W·(m·K)⁻¹ at room temperature to 21.2 W·(m·K)⁻¹ at 1227°C, and the electrical conductivity decreased from 3.35 × 10⁶ to 1.13 × 10⁶Ω⁻¹·m⁻¹ in a temperature range of 5–300 K. Nb₄AlC₃ possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m^1/2. Most significantly, Nb₄AlC₃ could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C.     

Mechanical and Thermal Properties of Antiperovskite Ti3AlC Prepared by an In Situ Reaction/Hot-Pressing Route

Bulk Ti₃AlC ceramic containing 2.68 wt% TiC was prepared by an in situ reaction/hot-pressing route. The reaction path, microstructure, mechanical and thermal properties were systematically investigated. At room temperature Vickers hardness of Ti₃AlC ceramic is 7.8 GPa. The flexural strength, compressive strength, and fracture toughness are 182, 708 MPa, and 2.6 MPa·m^1/2, respectively. Its apparent Young's modulus, shear modulus, bulk modulus and Possion's ratio are 208.9, 83.4, 140.4 GPa, and 0.25 at room temperature. Apparent Young's modulus decreases slowly with the increasing temperature, and at 1210°C the modulus is 170 GPa. The average coefficient of thermal expansion of Ti₃AlC ceramic is about 10.1 × 10⁻⁶ K⁻¹ in the temperature range of 150°–1200°C. Both the molar heat capacity and thermal conductivity increase with an increase in the temperature. At 300 and 1373 K, the molar heat capacities are 87 and 143·J·(mol·K)⁻¹, while the thermal conductivities are 8.19 and 15.6 W·(m·K)⁻¹, respectively.     

Synthesis, Physical, and Mechanical Properties of Bulk Zr3Al3C5 Ceramic

An in situ reactive hot-pressing process using zirconium (zirconium hydride), aluminum, and graphite as staring materials and Si and Y₂O₃ as additives was used to synthesize bulk Zr₃Al₃C₅ ceramics. This method demonstrates the advantages of easy synthesis, lower sintering temperature, high purity and density, and improved mechanical properties of synthesized Zr₃Al₃C₅. Its electrical and thermal properties were measured. Compared with ZrC, Zr₃Al₃C₅ has a relatively low hardness (Vickers hardness of 12.5 GPa), comparable stiffness (Young's modulus of 374 GPa), but superior strength (flexural strength of 488 GPa) and toughness (fracture toughness of 4.68 MPa·m^1/2). In addition, the stiffness decreases slowly with increasing temperature and at 1600°C remains 78% of that at ambient temperature, indicating that Zr₃Al₃C₅ is a potential high-temperature structural ceramic.     

Tribophysical Properties of Polycrystalline Bulk Ti3AlC2

A nearly pure, dense, polycrystalline bulk Ti₃AlC₂ sample was prepared by reactively hot pressing the element titanium, aluminum, and graphite powders. The tribophysical properties were investigated by sliding a Ti₃AlC₂ block dryly against a low-carbon steel disk. It was found that the friction coefficient is as low as ∼0.1, and the wear rate of Ti₃AlC₂ is only ∼2.5 × 10⁻⁶ mm³/N·m for the highest sliding speed of 60 m/s and the largest normal pressure of 0.8 MPa. These unusual properties are attributable to the presence of a compact self-generating film, which covers uniformly over the friction surface of Ti₃AlC₂ with a thickness of ∼0.5 μm.     

Pressure-Assisted Combustion Synthesis of Dense Layered Ti3AlC2 and its Mechanical Properties

A near-single-phase Ti₃AlC₂ ternary carbide was synthesized from 3Ti–1.1Al–1.8C powder blend, both by the wave propagation and thermal explosion (TE) modes of self-propagating high-temperature synthesis. The application of a moderate (28 MPa) pressure immediately after TE at 800°C (reactive forging) yielded a 95% dense material containing, in addition to Ti₃AlC₂, an appreciable amount of TiC_{1− x} . By adjusting the starting composition, a 99% dense material containing up to 90 vol.% Ti₃AlC₂ was obtained. The material had a fine-layered microstructure with Ti₃AlC₂ grain size not exceeding 10 μm. The samples were readily machinable and had a high compressive strength of ∼800 MPa up to 700°C.     

Mechanical Properties of Unidirectionally Oriented SiC-Whisker-Reinforced Si3N4 Fabricated by Extrusion and Hot-Pressing

SiC-whisker-reinforced Si₃N₄ was fabricated by extrusion and hot-pressing. A unidirectional alignment of the whiskers was achieved through sheet forming by extrusion. The degree of whisker orientation changed with the thickness of the green sheets. Unidirectionally oriented whiskers increased fracture strength and toughness compared to samples with more randomly oriented whiskers. Anisotropy of fracture strength was observed. Bridging by whiskers impeded crack propagation when the whisker orientation was perpendicular to the crack plane.     

Hot-Pressing and Mechanical Properties of Al2O3 with an Mo-Dispersed Phase

Compositions of alumina with a molybdenum dispersed phase were investigated in the 0 to 5 vol% Mo range. These compositions were also prepared with a 0.5 wt% MgO addition. All specimens were fabricated by hot-pressing, and near theoretical densities were achieved. Specimens were characterized by metallographic and X-ray diffraction analyses, and microhardness, elastic moduli, tensile strength, and fracture energy were determined. Results revealed that Mo additions did not affect grain growth; in contrast, MgO additions significantly inhibited grain growth. However, Mo additions did reduce the elastic moduli and microhardness but did not measurably affect the tensile strength. Tensile strength was dependent on grain size and fitted the G^−1/3 relation. The fracture energy of Al₂O₃+5% Mo was 50% greater than that of Al₂O₃. Specimens were successfully hot-pressed with a micro-structure graded from that of Al₂O₃ to that of the 5% Mo composition.     

Synthesis and Characterization of Bulk Zr2Al3C4 Ceramic

Polycrystalline Zr₂Al₃C₄ was fabricated by an in situ reactive hot-pressing process using zirconium (zirconium hydrides), aluminum, and graphite as starting materials. The investigation on reaction path revealed that the liquid Al played an important role in triggering the formation of ternary zirconium aluminum carbides. The mechanical properties of Zr₂Al₃C₄ at room temperature were measured (Vickers hardness of 10.1 GPa, Young's modulus of 362 GPa, flexural strength of 405 MPa, and fracture toughness of 4.2 MPa·m^1/2). The electrical resistivity and thermal expansion coefficient were determined as 1.10 μΩ·m and 8.1 × 10⁻⁶ K⁻¹, respectively.     

Hot-Pressing Behavior of Si3N4

The densification behavior of Si₃N₄ containing MgO was studied in detail. It was concluded that MgO forms a liquid phase (most likely a magnesium silicate). This liquid wets and allows atomic transfer of Si₃N₄. Evidence of a second-phase material between the Si₃N₄ grains was obtained through etching studies. Transformation of α- to β-Si₃N₄ during hot-pressing is not necessary for densification.     

Synthesis of Ti3AlC2 Powders Using Sn as an Additive

Nearly pure Ti₃AlC₂ powders have been synthesized by calcining a mixture of titanium, aluminum, and graphite powders using tin powders as additives. Four recipes with different mole ratios of Ti:Al:C:Sn were examined at calcining temperatures from 1300° to 1500°C. The addition of Sn effectively inhibited the generation of thermal explosion when the volume of the starting materials is larger, and considerably reduced the lower-limit calcining temperature. The nearly pure Ti₃AlC₂ powders can be obtained reliably on a large scale by calcining the starting materials with a mole ratio of 3Ti:1Al:1.8C:0.2Sn at temperatures from 1350° to 1500°C.     

Microstructure and Mechanical Properties of In Situ Produced TiC/TiB2/MoSi2 Composites

A novel microstructure of in situ produced TiC/TiB₂/MoSi₂ composite and its mechanical properties were investigated. The results indicate that TiC/TiB₂/MoSi₂ composites can be fabricated by reactive hot pressing the mixed powders of MoSi₂, B₄C, and Ti. A novel microstructure consisting of hollow particles of TiC and TiB₂ grains in an MoSi₂ matrix was obtained. Grains of in situ produced TiC and TiB₂ were much finer, from 100 to 400 nm. During the fracture process, hollow particles relieved crack tip stress, encouraging crack branching and changing the original direction of the main crack. The highest bending strength of this composite achieved was 480 MPa, twice that of monolithic MoSi₂, and the greatest fracture toughness of the composite reached 5.2 MPa·m^1/2.     

In Situ Reaction of B2O3 with AlN and/or Si3N4 to Form BN-Toughened Composites

BN-toughened oxide matrix composites were formed by the in situ reaction of B₂O₃ with Si₃N₄ and/or AlN. A lowtemperature transient liquid phase aids densification at <1000°C, and the process tends to produce an intrinsically homogeneous microstructure. Mechanical properties and microstructure of the composites formed in situ were compared to those of composites prepared by conventional means from oxide and BN powders. Fracture toughness and flexural strength of the nearly isotropic in situ formed composites ranged from 2.82 to 3.66 MPa · m^1/2 and 130 to 320 MPa, respectively, with Young's moduli of 100 to 110 GPa. Densities achieved ranged from 90% to 97% of estimated theoretical densities. The strength and toughness values are intermediate to the extreme values for the anisotropic composites formed by hot-pressing mixed powders.     

Insulating Properties of Lanthanum-Doped BaTiO3 Ceramics Prepared by Low-Temperature Synthesis

Samples of composition Ba_{1− x} La _x Ti_{1− x /4}O₃, x = 0, 0.003, 0.03, and 0.10, were prepared by an alkoxide sol–gel route with final firing of ceramics at 1100°C, 2 h in air. All samples showed bulk insulating behavior with no evidence of semiconductivity caused by either direct donor doping or oxygen loss.     

Synthesis of Mg2SiO4 Whiskers by an Oxidation-Reduction Reaction

Magnesium orthosilicate (forsterite) whiskers were synthesized by an oxidation-reduction reaction in the present investigation. These whiskers were rectangular parallelepipeds, with long sides in a cross-sectional view from two to ten times as long as the short sides, and measuring from several micrometers to 200 μm wide and ∼15 mm in the elongated direction. The Mg₂SiO₄ elongation was on the c-axis. The growth mechanism of the whiskers was investigated on the basis of chemical thermodynamics, and the present study revealed that the Mg₂SiO₄ whiskers grew by a VS (vapor-solid) mechanism .     

Characterization of the Thermally Contracting Tungstates Ta22W4O67, Ta2WO8, and Ta16W18O94

An investigation of the properties of high-purity (>99 wt%) tantalum tungstates (Ta₂₂W₄O₆₇, Ta, WO₈, and Ta₁₆W₁₈O₉₄) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, micro-hardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to ∼ 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm³, are relatively soft (120 to 655 kg/mm² hardnesses), and are electrical insulators (6× 10³ to 2× 10⁸Ω^.cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from + 0.6×10⁻⁸/K to −5.1× 10⁻⁶/K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity     

Synthesis, Microstructure, and Mechanical Properties of Al3BC3

In situ synthesis of bulk Al₃BC₃ was achieved via a reactive hot-pressing method using Al, B₄C, and graphite powders at 1800°C for 2 h. The reaction path for synthesizing Al₃BC₃ was investigated. It was found that Al₃BC₃ formed via the reaction of C, B₄C, and Al₄C₃ above 1180°C. Dense Al₃BC₃ was prepared with a little B₄C and graphite remained. Microstructure observations revealed the plate-like morphology of Al₃BC₃ grains. Moreover, the mechanical properties of Al₃BC₃ were characterized (Vickers hardness of 11.1 GPa, bending strength of 185 MPa, fracture toughness of 2.3 MPa·m^1/2, and Young's modulus of 163 GPa). Young's modulus decreased slowly with increasing temperature, and at 1600°C remained 79% of that at ambient temperature. These results show that Al₃BC₃ is a promising lightweight high temperature structural material.