A series of SiC-AlN compositions of 0, 10, 25, 50, 75, 90, and 100 mol% AlN were hot pressed at 2100°C for a 1 h soak at a pressure of 35 MPa under vacuum. 2H-wurtzite SiC-AlN solid-solution structures were formed for compositions with 25-100 mol% AlN. The associated lattice parameters for these solid solutions followed Vegard's law. The microstructures varied with composition; the number of needlelike grains decreased for compositions up to 25 mol% AlN and the amount of equiaxed grains increased for compositions with 25-100 mol% AlN. Densities for all the specimens were >99% of the theoretical density. Coefficients of thermal expansion varied from 4.80 × 10 −6 /°C to 6.25 × 10 −6 /°C in the 20°-1400°C range. Young's moduli varied from 451 GPa to 320 GPa at room temperature (RT) and retained 98%, 96%, and 94% of their RT values at 500°, 1000°, and 1250°C, respectively. These three properties correlated linearly with composition. RT microhardness varied from 21.6 GPa to 11.2 GPa and correlated linearly with composition within the solid-solution range. Flexural strengths increased from 487 MPa to 604 MPa from 0 mol% AlN to 25 mol% AlN and then decreased to 284 MPa for 100 mol% AlN. At 1250°C, flexural strengths decreased from 90% to 65% of the RT values. Fracture toughness increased from 3.6 MPaؒm 1/2 to 4.2 MPaؒm 1/2 from 0 mol% AlN to 10 mol% AlN and then decreased to 2.5 MPaؒm 1/2 for 100 mol% AlN.
Nanophase-structured composites were fabricated by heat treating hot-pressed 2H-wurtzite SiC-AlN solid-solution specimens of 25, 50, and 75 mol% AlN within the spinodal decomposition zone. Heat-treatment conditions were 1750°C for 150 h, in flowing nitrogen gas. The hot-pressed specimens contained 2H-wurtzite equiaxed grains, and the grain size increased with AlN content. Lattice parameters followed Vegard's law. Nanoprecipitates with typical modulated tweed-type structures were observed along the [21 1 0] zone axis and were orthogonal to the {011 2} planes that make angles of 46.70°, 46.90°, and 47.11°to the [0001] for the three compositions. The microhardness, flexural strength, and fracture-toughness values of the heat-treated specimens were not significantly different from the hot-pressed values.
Photocatalytic hydrogen generation holds promise as the future source of environmentally friendly and economically feasible energy source. In order to conduct more efficient photocatalytic reaction, anatase TiO2 doped with transition metals is proposed as catalyst. Investigation was conducted by using density functional theory (DFT) augmented with Hubbard U treatment to correct the band gap of TiO2. Emergence of new states inside the band gap of doped anatase TiO2 can lead to a material with a better photocatalytic property, i.e., able to work at visible light than that of pristine TiO2 which is sensitive to UV light only. The investigated materials comply with standard hydrogen electrode (SHE), thus can be used as photocatalyst in water splitting reaction. Out of the two options tested, TiO2 doped with Fe produces a material with the better photocatalytic properties.Keywords: DFT + U, anatase TiO2, photocatalytic, water splitting, band gap
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