maryam shojaie-bahaabad

The current study aims to fabricate HfB2-30 vol. % SiC and HfB2-30 vol. % SiC-2 vol. % Nd2O3 composites through Spark Plasma Sintering (SPS) method at 1950 °C for 10 min. The oxidation behavior of the prepared composites was investigated at 1400 °C and different times namely 4, 8, 12, and 16 hours. The relative density, hardness, toughness, and strength of the HfB2-30 vol. % SiC composite increased from 98.5 %, 20.19 GPa, 414.9 MPa, and 4.36 MPa.m0.5 up to 99.1 % , 24.47 GPa, 485 .5 MPa, and 4.93MPa.m0.5 for HfB2-30 vol. % SiC-2 vol. % Nd2O3 composite, respectively. After 16 hours of oxidation, SiO2 layer, which was extremely thick, was produced locally on the oxidized HfB2-30 vol. % SiC composite surface. The thinckness of the SiO2 layer was calculated to be around 25 μm. The thickness measurement revealed the SiO2 produced layer on the surface of the HfB2-30 vol. % SiC-2 vol. % Nd2O3 composite to be 5 μm. The oxidation kinetic results of the composite exhibited linear-parabolic behavior. The chemical reaction during the oxidation process controlled the oxidation rate after eight hours. After 16 hours of performing the oxidation procedure at 1400 °C, HfB2-30 vol. % SiC-2 vol. % Nd2O3 composite exhibited parabolic behavior, while HfB2-30 SiC exhibited linear behavior. This composite's improved oxidation resistance was attributed to Nd(Hf,Si)OxCy phases and decreased porosity, resulting in the generation of thin, dense, adherent, and protective layers. Therefore, it was concluded that the oxygen diffusion rate could control the oxidation process in HfB2-30 vol. % SiC-2 vol. % Nd2O3 composite.

The current study aims to fabricate the HfB2-SiC-graphite composite through Spark Plasma Sintering (SPS) method at 1950 °C for 10 min. The oxidation behavior of the prepared composites was investigated at 1400 °C and different times of 4, 8, 12, and 16 h. In addition, the weight changes and thickness of the generated oxide layer were measured. The relative density, hardness, toughness, and strength of the composite made through the SPS method were calculated as 99.39 %, 10.16 GPa, 4.73 MPa.m1/2, and 464.12 MPa, respectively. The oxidation kinetic results of the composite exhibited linear-parabolic behavior. The chemical reaction during the oxidation process controlled the oxidation rate after 8 h. Followed by oxidation for more 12 h, the thickness of the oxide scale slowly increased, thus following a parabolic trend as a result of a decrease in the oxygen diffusion when HfOxCy and SiOxCy phases were formed. Therefore, it was concluded that the oxygen diffusion rate could control the oxidation process.

Boron carbide is one of the hardest materials. The combustion method was used to synthesize B4C-TiB2 nanocomposite powder in a B2O3-Mg-TiO2-C system. An experimental study of the formation of B4C–TiB2 nanoparticles was conducted in the thermal explosion mode. A mixture of B2O3:TiO2:Mg:C at a molecular ratio of 3:1:12:1 was chosen to obtain the B4C–TiB2. This powder mixture was milled for different times. The combustion reaction of powders milled for up to 12 h was done in an inert atmosphere at 900°C. The synthesized powders were characterized by X-Ray diffraction, Scanning and Transmission Electeron Microscopies (SEM and TEM). It was demonstrated that mechanical activation is a useful method for obtaining B4C and TiB2 powders simultaneously; without milling, the B4C composition did not form. B4C formed after 3 h of milling of the reactants. Crystallite size of products obtained from reactant powders milled for 6 and 12 hour was less than 100 nanometer.

In this work, SiO2 –ZrO2 mixed oxides was prepared by the polymeric sol–gel route. The characterization of pore structure, which determines the permeation process of membrane, is of great importance. So far, most investigations have focused on such pore structure as specific surface area and pore size distribution, but the surface fractal, the important parameter reflecting the roughness of pore surface. Pore surface roughness change in SiO2-ZrO2 unsupported membranes induced by chemical composition and heating process has been investigated by the analysis of surface fractal dimension. Fractal features are analyzed from N2 adsorption–desorption measurements. It was found that a decrease in the surface fractal dimension occurs while zirconia content increases at the unsupported membranes with different molar ratio of zirconia to silica heating at 500 ºC. The surface fractal dimension of membrane with 30 mol% silica content slightly increases while heating from 200 to 500 ºC due to shrinkage and increase of mass fractal dimension of silica clusters.

The 70SiO2-15TiO2-15ZrO2 membrane was prepared by a sol-gel procedure. The corrosion behavior of microporous toplayers along with the membrane characterization in terms of pore size, surface area, pore volume and weight loss is described. The final ceramic membrane with a thickness of 400 nm and uniform surface was obtained. This membrane confirmed the fine microporous characteristic with mean pore size < 2 nm. After corrosion test, the corroded membrane revealed a surface with non-uniform coverage of the toplayer. The heated ceramic membrane after and before corrosion test was amorphous. Dissolution of ions was increased in acidic and basic solutions. The weight less of samples increased when pH increased at RT. Porosity of samples increased after the corrosion test. The pore size of membrane increased as compared to the pore size of the original membrane after corrosion conditions. Surface area of the membrane increased in basic solution whereas decreased in acidic solution.