Advanced Materials Research Vols. 11-12

Abstract: Crystal structures of SrAl2O4, BaAl2O4 and their solid solutions have been reviewed in terms of the linkage pattern of [AlO4] tetrahedra. With SrAl2O4 the hexagonal-to-monoclinic phase transformation occurs at 950K during cooling. The space group change from P63 to its subgroup P21 eliminates the triad axis of the former phase, which involves a reduction in the symmetry of the trigonally distorted rings. The hexagonal structures of SrAl2O4 and BaAl2O4 differ distinctly in the linkage pattern of the [AlO4] tetrahedra. In the former structure, all of the tetrahedral rings are equivalent. In the latter, there are two types of tetrahedral rings; trigonal rings and asymmetrical ones. The trigonal rings, comprising 25% of the total number of rings, contain in their centers the Ba atoms with the special position. This implies that the triad axes exist in the centers of the rings, and hence they are distorted trigonally as in the hexagonal SrAl2O4. On the other hand, the Ba atoms in the asymmetrical rings are located at the general position site. The structural disorder in Ba0.6Sr0.4Al2O4 (space group P6322) was investigated by the combined use of Rietveld method and maximum-entropy method. The electron density distribution was satisfactorily expressed by the split-atom model, in which the strontium/barium and oxygen atoms were split to occupy the lower symmetry sites.

Abstract: Normal and abnormal grain growth has been observed in 70NbC-30Co with varying B concentrations at 1450°C and in alumina with varying impurity and additive concentrations at 1600°C -1650°C as typical systems with and without liquid matrix. The grain growth behavior depends on the roughening of the interfaces as indicated by the grain and grain boundary shapes. When 4% B is added to 70NbC-30Co, the NbC grains in Co-rich liquid matrix are spherical and undergo diffusion controlled normal growth, because the grain-liquid interface is rough. As the B concentration is decreased to 3, 2, 1, and 0%, the NbC grains become more cubic and the tendency for abnormal grain growth increases because of the step growth mechanism of the flat singular surface segments. When compacts of high purity alumina powder are sintered at 1650°C, the grain boundaries are smoothly curved, indicating their atomically rough structures. With increasing impurity content—in particular SiO2—in the alumina powder, abnormal grain growth becomes more pronounced with increasing number of flat grain boundaries. These singular grain boundaries are expected to move by a step mechanism and thus cause the abnormal grain growth. These results show that the interface roughening and hence the grain growth mode changes gradually with the additive or impurity concentrations. Therefore, the abnormal grain growth cannot be sharply distinguished from the normal grain growth as has been previously suggested in general and for alumina in particular.

Abstract: Investigations for space-selected structure ordering from nano-particles to single-crystal patterning in glasses will be described. Transparent crystallization in glass must be the best material solution to obtain novel functional glasses with a permanent second-order optical nonlinearity. We focus on the new functions created by structure ordering in glass by means of laser micro-fabrication for space-selected crystallization. Two topics in our recent experimental results of space-selected structure ordering in glass will be presented as follows: 1) Single crystalline patterning by atom heat laser processing in Sm-doped glasses for optical waveguides with second-order optical nonlinearity, 2) structure ordering of domains in crystallized glass fibers for possible photonic fiber-type devices with active signal processing.

Abstract: Based on the results in glass scientific research fields such as the National Natural Science Foundation project—forming and structure of heavy metal oxide glass, national defense new material projects and various other research projects, the new achievement and the present progress on the investigation in non-crystalline science exploration and the development for new kinds of glass materials in the school of Materials Science and Engineering of Central South University are briefly introduced in this paper.

Abstract: Glasses in the Bi2O3-B2O3-BaO-ZnO system were prepared using the normal melt quenching technology. The structure of the glasses was analyzed by using of the infrared spectra within the range 2000∼400cm-1. The relation among the properties such as density (D), thermal expansion coefficient (α) and transition temperature (Tg), Bi2O3 introducing content and structure of the glass have been discussed.

Abstract: The R2O-MO- Al2O3-SiO2 system glasses were prepared by conventional melt quenching technology. The composition mass fraction range of the glass is SiO2 (55%∼65%), MgO (0%∼15.2%), CaO (0%∼15.2%), SrO (0%∼15.2%), BaO (0%∼15.2%), Na2O (0%∼15.6%), K2O (0%∼15.6%). The relationship between the composition and the thermal expansion coefficient of the glass was investigated by comparing the thermal expansion coefficients of the glasses with different chemical composition. The results show that the thermal expansion coefficient of the glass increases sharply with the increase of alkali-metal oxide content and when K+, Na+ and Li+ exist simultaneously in the structure of the glass, the complex “mixed alkali effect” can be observed from the composition–thermal expansion coefficient curve. When introducing different kind but same quantity alkaline-earth metal oxide, the thermal expansion coefficient of the glasses increased obviously with the rising of the radius of alkaline-earth metal ions but the “mixed alkali effects” can also be observed for the glasses containing a few kinds of alkaline-earth metal oxides.

Abstract: The heterogeneous 12-tungstophosphoric acid (HPW) catalyst is becoming important in industrial processes for example in esterification reaction. A novel solid acid catalyst of HPW entrapped on mesoporous silica was synthesized by sol gel technique. Neutral template dodecylamine was introduced to obtain mesopores structure catalyst. The physical and chemical properties of the catalyst were characterized by XRD, nitrogen sorption and FTIR. In conclusion, this new type of mesoporous solid acid catalyst is a very promising heterogeneous acid catalyst for esterification reaction involving bulky molecules such as fatty acid.

Abstract: Thermo-chemical reactions of modified PAN fibers during heat-treatment process were systematically studied by DSC, FT-IR, EA, XPS, etc. Comparing with original PAN fibers, chemical reactions, structures and elemental compositions of fibers modified with potassium permanganate (KMnO4) were all changed at a certain extent. KMnO4 had reduced the activation energy of cyclization reactions, decreased the area and widened the peak of exothermic curve, decreased the velocity of cyclization reaction, increased the oxygen content about 67%, hence increased C-O-C and C=O groups and the core/shell ratio.

Abstract: Symmetric three-layer alumina/zirconia/mullite composites have been prepared by tape casting and hot-pressing (20 MPa) at 1500°C for 2 h in argon. The hardness and Young’s modulus of the layered materials were evaluated using low load continuous depth-sensing indentation equipment (nanoindentation), appear to be affected by the formation of processing defects and residual stress.

Abstract: Three dimensional textile carbon fiber reinforced silicon carbide (3D textile C/SiC) composites with pyrolytic carbon interfacial layer were fabricated by chemical vapor infiltration. The microstructure and mechanical property of 3D textile C/SiC composites were investigated. A thin pyrolysis carbon layer (0.2 ± μm) was firstly deposited on the surface of carbon fiber as the interfacial layer with C3H6 at 850°C and 0.1 MPa. Methyltrichlorosilane (CH3SiCl3 or MTS) was used for the deposition of the silicon carbide matrix. The conditions used for SiC deposition were 1100°C, a hydrogen to MTS ratio of 10 and a pressure of 0.1 MPa. The density of the composites was 2.1 g cm-3. The flexural strength of the 3D textile C/SiC composites was 438 MPa. The 3D textile C/SiC composites with pyrolytic carbon interfacial layer exhibit good mechanical properties and a typical failure behavior involving fibers pull-out and brittle fracture of sub-bundle. The real part (ε′) and imaginary part (ε″) of the complex permittivity of the 3D-C/SiC composites are 51.53-52.44 and 41.18-42.08 respectively in the frequency range from 8.2 to 12.4 GHz. The 3D-C/SiC composites would be a good candidate for microwave absorber.