Solid State Phenomena Vols. 124-126

Abstract: The Si3N4/SiC and Si3N4/SiC/TiO2 composite ceramics were sintered at 1850 °C, for 2 h, in air by Hot-Press. A semi-elliptical surface crack was made at the center of the tensile surface of the test specimen using a Vickers indenter at a load of 24.5 N. By this method, the crack length on surface was made the semi-elliptical cracks of about 100 ㎛. Crack-healing behavior depends on healing temperature. The optimum healing condition of Si3N4/SiC and Si3N4/SiC/TiO2 composite ceramics are well known for 1,300 °C, 1h in air environment. So Smooth and as-cracked specimens were heated in this condition. To investigate temperature-dependence of the SiO2 gel coated specimen, four kinds of temperatures (800, 900, 1000, 1,300 °C) were tested. And to investigate SiO2 gel coating time-dependent, one and three times were coated. SiO2 gel coated specimens indicated increase of bending strength as the temperature goes up. Moreover the bending strength of SiO2 gel coating specimens was similar to the bending strength of healed smooth specimen at 900 °C.

Abstract: Highly crystalline titania nano-particles were synthesized by hydrothermal method using novel stable water-soluble titanium complexes. It was confirmed that single phase anatase, rutile and brookite, which can be rarely synthesized as a single phase, can be obtained by varying the ligand in the complex and pH of the aqueous solution. TEM observations and BET specific surface area measurements had shown that these samples consisted of nanosized particles of 5~200 nm and had high specific surface areas of 25~150 m2/g. According to UV-visible diffuse reflectance spectra, these titania samples absorbed light in the visible region (λ > 400 nm). Photocatalytic activities in NO oxidation reaction exhibited by synthesized titania powders under the irradiation by UV- visible light were higher than the activity of the commercial TiO2 photocatalyst P25 (Degussa). Especially, under illumination by only visible light of above 510 nm wavelength, photocatalytic activity of the obtained specimens exceeded that of P25 more than four times. We also clearly demonstrated that single phase brookite had high photocatalytic activity for NO oxidation.

Abstract: Non-oxide ceramic SiCN possesses unique physical and chemical properties, which make it a suitable material for various applications such as catalyst support, high temperature semiconducting devices, and metal or polymer matrix composites. In this work, SiCN foams with pore size of 500 and 1000 nm and high BET surface area of 184.5 and 71 m2/g were prepared using preceramic polymer, polyvinylsilazane, as a precursor and polystyrene sphere packed bed as a self-sacrificial template. The polystyrene sphere packed bed was prepared by the natural sedimentation of spheres. After the infiltration and curing of viscous preceramic polymer, the sample was pyrolyzed at 1000 °C under N2 atmosphere to produce the ceramic foams. It is a very simple synthesis because in the process of pyrolysing preceramic polymer, the polystyrene sphere template was removed simultaneously by pyrolysis with no additional etching step. SEM, N2 adsorption-desorption measurement and TGA were used to characterize the obtained samples. It was concluded from the TGA measurement that the obtained SiCN foam possessed very high thermal stability at 1000 °C under air.

Abstract: Bi2V0.9Co0.1O5.5-δ with a pure Aurivillius phase was synthesized by a sol-gel method using EDTA and citric as mixed complexing agents. It was found that homogeneous and fine powder (100-200 nm) with a pure Aurivillius phase can be produced by calcining the complex precursor at 450 °C for 1h in air. The sintering properties of Bi2V0.9Co0.1O5.5-δ were investigated in the range of 560-680 °C with respect to relativity density. Sintering at 640 °C was ascertained to be preferred for Bi2V0.9Co0.1O5.5-δ, producing a dense microstructure with uniform grains around 3-5 μm. The Bi2V0.9Co0.1O5.5-δ ceramic exhibits an oxygen ionic conductivity of 1.0×10-1 S·cm-1 at 600 °C. The difference between the activation energies for the ionic conducting in low and high temperature regions is qualitatively interpreted in terms of an order-disorder phase transition. This research demonstrates the advantage of the EDTA-citrate method in preparing Bi2V0.9Co0.1O5.5-δ with respect to the simplicity of synthesis process, desired morphology of synthesized powder, low sintering temperature and superior ionic conducting properties of ceramic specimen.

Abstract: Na2O excess 95(Na0.5K0.5)NbO3-5LiTaO3 (NKN-5LT) ceramics were developed by conventional sintering process. Sintering temperature was lowered by adding Na2O as a sintering aid. Grain growth behaviors with Na2O addition were explained in terms of the effect of Na2O on interface reaction - controlled grain growth and the critical driving force. The electrical properties of NKN-5LT ceramics were investigated as a function of Na2O concentration. In the 1 mol% Na2O excess NKN-5LT samples sintered at 1000oC for 4h in air, electromechanical coupling factor (kP) and piezoelectric coefficient (d33) of NKN-5LT ceramics were found to reach the highest values of 0.43 and 190 pC/N, respectively.

Abstract: Ion-sieve type manganese oxide spinels for the selective adsorption of lithium(Li) from seawater were prepared and their properties were examined. LiM0.5Mn1.5O4 (M=Mg, Zn) precursors, in which part of manganese(Mn) was substituted with magnesium(Mg) or zinc(Zn), were synthesized through the solid-state reaction. The adsorbents, HM0.5Mn1.5O4 (M=Mg, Zn) were derived from LiM0.5Mn1.5O4 (M=Mg, Zn) by acid treatment. The optimum acid treatment was obtained at hydrochloric acid concentration of 0.5M and 0.3M for LiMg0.5Mn1.5O4 and LiZn0.5Mn1.5O4 precursors, respectively. Both cases required thrice-conducted acid treatments for the best results. The adsorption of Li by HMg0.5Mn1.5O4 and HZn0.5Mn1.5O4 adsorbents followed the Freundlich equation and the maximum adsorption of Li in artificial seawater was 30.3 mg/g and 33.1 mg/g, respectively. The adsorption efficiency of Li by HMg0.5Mn1.5O4 and HZn0.5Mn1.5O4 adsorbents in artificial seawater, in which Li concentration was adjusted to 0.2 mg/L, was as high as 88% and 89%.

Abstract: The dielectric properties of alumina ceramics have been measured using a free-space time-domain technique from room temperature to 1400 oC in the frequency range 8.2 – 12.4 GHz. The effects of thickness and lateral size of specimen were investigated with comparing the measured values to the calculated ones based on the half-wavelength and the Gaussian beam focusing. From these results the optimum specimen dimension for the high temperature dielectric measurement was suggested with experimental verification.

Abstract: Cordierite-bonded silicon carbide porous ceramics were prepared by a reactive process, in which kaolin, talc, and alumina powders were mixed with silicon carbide powders and graphite powder was used as a pore former. The mixture was heated in air so that graphite was burned out and silicon carbide particles were bonded by reaction-derived cordierite. Open porosity and strength of porous ceramics were strongly dependent on the volume fraction of graphite and cordierite. A three-point bending strength of 13.9 MPa was achieved at a open porosity of 38.5 %, exhibiting coefficient of thermal expansion of 7.28 × 10-6/oC at the range of 25 – 800oC. Increase of cordierite volume in SiC matrix indicated mechanical improvement of the porous ceramics due to the increase of neck area among the SiC particles. Porous ceramics sintered at 1450oC possessed higher open porosity and median pore diameter than those of samples sintered at 1250 and 1350oC, since pore generated by the transport of dissolved talc materials as increasing temperature.

Abstract: Bulk Ti3(Al1-nGen)C2 (n=0, 0.3, 0.5, 0.7, 1.0)were synthesized by a reactive hot pressing of TiCX (x=0.6), Al and Ge powder mixture at 1400 °C for 1 h under 25 MPa. A series of Ti3(Al1-nGen)C2 was successfully synthesised by a hot pressing through a direct liquid-solid reaction between Al-Ge melt and TiCx, and simultaneous densification. Synthesized bulk Ti3(Al1-nGen)C2 was maintained a typical layered structure as shown in other ternary carbide such as Ti3AlC2 and Ti3GeC2. With increasing Al or Ge contents in Ti3(Al1-nGen)C2, both of Vickers hardness and flexural strength of Ti3(Al1-nGen)C2 were increased compared to those of pure Ti3AlC2 and Ti3GeC2 . The maximum flexural strength of Ti3(Al1-nGen)C2 (n=0.3) was about 600 MPa, which was about 3 times higher than that of pure Ti3GeC2. The Vickers hardnesses of Ti3(Al1-nGen)C2 and Ti3AlC2 were slightly decreased with increasing an indentation load.

Abstract: Thermal behavior and microstructural characterization of the CNTs/Al2O3 nanocomposites with different relative densities were studied by TGA and SEM. The onset temperature for weight loss corresponded to a decomposition of CNTs in TGA increased with an increase of relative density. The activation energy for CNTs decomposition obtained by the Kissinger method increased with increasing relative density. The difference in thermal behavior was explained by the porosity effect on the oxidation and decomposition of CNTs.