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A novel quaternary 413-type Zr 3 TiSiC 3 MAX phase has been synthesized in 97% yield by the vacuum carbosilicothermic reduction (VCSTR) method using mixtures of TiO 2 , ZrO 2 , SiC, and C powders as starting materials. The crystal structure of the synthesized Zr 3 TiSiC 3 was studied by X-ray diffraction Rietveld analysis. The lattice constants of Zr 3 TiSiC 3 MAX phase were a = 0.328292(5) nm and c = 2.39651(6) nm. It was revealed that the distribution of Zr and Ti atoms was fairly uniform in the basal plane of lattice, but was markedly inhomogeneous along the c -axis, with Zr atoms preferentially occupying the inner layers in the [M 4 X 3 ] blocks rather than outer layers. The occupations of the outer and inner layers by Zr atoms were found to be approximately 67.4% and 82.6%, respectively, indicating a rather poor chemical ordering of Zr and Ti atoms in metal sublattice of Zr 3 TiSiC 3 MAX phase. [ABSTRACT FROM AUTHOR]

A novel 312-type MAX-phase solid solution series in the Zr-Ti-Si-C system has been synthesized by the vacuum carbosilicothermic reduction method using mixtures of TiO 2 , ZrO 2 , SiC, and Si powders as starting materials. The upper limit for Zr content in metal sublattice of the synthesized (Zr,Ti) 3 SiC 2 MAX phase solid solutions was found to be as much as approximately 66 at%, closely corresponding to a hypothetical quaternary Zr 2 TiSiC 2 MAX phase. A wide miscibility gap inside the interval of Zr content in metal sublattice ranging between 22 at% and 55 at% was found. Crystal structure of the synthesized MAX-phase solid solutions was studied by HR-STEM/HAADF and XRD Rietveld analyses. The lattice constants were determined to be linearly correlated with Zr content as predicted by Vegard's law. A significant inhomogeneity in distribution of metal atoms similar to that of out-of-plane ordered quaternary MAX phases has been established for both Ti-rich and Zr-rich MAX-phase solid solutions. [ABSTRACT FROM AUTHOR]

SiC particles coated with Ti-Si-C layer were prepared by the molten salt synthesis method using NaCl-KCl eutectic mixture as the reaction medium and Ti metal powder as the Ti source. The synthesis was conducted under static argon atmosphere at 900 °C for 1–3 h. The post-synthesis treatment included dissolution of salts in hot water, followed by ultrasonic dispersion and sedimentation procedures. The thickness of the Ti-Si-C layer varied from 0.1 to 0.9 μm as the ratio of Ti to SiC components in the starting mixture changed from 0.05 to 0.4. The layer contained mainly nanocrystalline Ti 5 Si 3 , minor phases were TiC and Ti 3 SiC 2. The as-prepared powders were hot pressed under 30 MPa at 1700 °C. The densification behavior of the samples during hot pressing as well as changes both in phase composition and in microstructure were studied. The flexural strength of the hot-pressed samples increased with an increase in the Ti:SiC molar ratio, giving the value as high as 213 ± 20 MPa for the sample with Ti/SiC = 0.4. [ABSTRACT FROM AUTHOR]

Novel C/SiC core-shell fibers have been synthesized through incomplete conversion of carbon fibers by their siliconization with SiO gas. The synthesis was performed in the laboratory-made semi-closed batch-type reactor at 1380 °C for 3 h using a 9:1 M ratio mixture of Si and SiO 2 powders as a solid source of SiO gas. The conversion rate of carbon into SiC was 34.0%. All synthesized fibers had a distinct C/SiC core-shell composite structure. The fiber product was of fairly good uniformity in respect of the shell thickness which varied approximately from 0.6 μm to 0.8 μm depending on the location of fibers inside the reactor. It was revealed that the formation of the shell was the result of inward growth of the SiC product layer. The effectiveness of the proposed semi-closed reactor for the synthesis of C/SiC core-shell fibers has been demonstrated. [Display omitted] [ABSTRACT FROM AUTHOR]

Dense Ti 3 SiC 2 –TiB 2 -(TiC)–SiC composites have been fabricated from leucoxene concentrate , a product of preliminary processing of titanium-containing sandstones, by a three-step technique. In the first step, Ti 3 SiC 2 –TiB 2 –SiC agglomerated powders substantially differing in SiC content were prepared by the method of the vacuum carbosilicothermic reduction (VCSTR) synthesis using SiC as a reductant and B 4 C additive as a solid boriding agent. The unwanted impurities were removed from the products of the VCSTR synthesis by leaching with hydrofluoric acid in the second step. In the third step, the purified Ti 3 SiC 2 –TiB 2 –SiC powders were hot-pressed in a graphite die under 30 MPa at 1500–1550 °C, resulting in nearly fully dense ceramics. The values of flexural strength and fracture toughness measured for the prepared leucoxene-derived ceramics fell in the ranges 450–600 MPa and 5.6–7.0 MPa m1/2, respectively, with the best results obtained for the sample with low SiC content. It was also shown that when SiC content was quite low, a part of Ti 3 SiC 2 may have decomposed during the hot pressing step, resulting in the formation of the corresponding amount of TiC. These observations thus suggest that the presence of SiC particles in the composites play an ambivalent role, on the one hand, negatively affecting strength properties, but on the other hand, preventing the decomposition of Ti 3 SiC 2 during the hot pressing step. According to the results obtained from analyses of microstructure and strength properties of the leucoxene-derived Ti 3 SiC 2 –TiB 2 -(TiC)–SiC composites fabricated through the VCSTR synthesis, it was concluded that these materials have high potential to be applied as a promising structural ceramics. • Dense Ti 3 SiC 2 –TiB 2 -(TiC)–SiC composites were fabricated through VCSTR synthesis. • Leucoxene concentrate from titania sandstones was used as starting material. • Leaching with HF was applied to remove impurities from products of VCSTR synthesis. • Presence of SiC particles prevented the decomposition of Ti 3 SiC 2 during hot pressing. • The best strength properties were obtained for the sample with lower SiC content. [ABSTRACT FROM AUTHOR]