Comparative and Efficient Ammonia Gas Sensing Study with Self-assembly-Synthesized Metal Oxide-SiC Fiber-Based Mesoporous SiO2 Composites.

Self-assembled-assisted ternary nanocomposite In2O3-SiC, CuO2-SiC, and MnO2-SiC semiconductors were mixed with SiO2 to enable gas sensing using cyclic voltammetry. The results of TEM (transm In2O3-SiC-SiO2 ion electron microscopy), X-ray diffraction spectroscopy, and Raman spectra analysis affirm the closeness of few layers between SiO2 and SiC in In2O3-SiC, MnO2-SiC, and CuO2-SiC. Among the electrochemical impedance spectra curves of the nanocomposites, none of the samples had a semicircle profile, which indicates the existence of a higher charge-transfer resistivity behavior between the electrolyte and the sample electrode with charge carrier and transport effects, which is related to the well-developed porous structure of synthesized composites. CuO2-SiC-SiO2 and MnO2-SiC-SiO2 showed high resistivity and a quite significant response for NH3 gas at room temperature. While there was a response for NH3 gas for In2O3-SiC-SiO2, the sensor showed a low response for the gas. From the sensing test, correspondences between the chemical structure of the sensor and the molecular structure of the gases have been found. The surface reactions between the sensor surface and the gas with a pore structure, along with the electron receiver/donor phase are observed from the results of gas sensor tests, and all factors are determining the precise state. Finally, the adsorption of NH3 molecules and the alteration of the electronic resistance of In2O3-SiC-SiO2, MnO2-SiC-SiO2, and CuO2-SiC-SiO2 were presented that include various thicknesses of charge to represent which are achieved by the connection with the substrates and the particles.

Effect of Impurities Control on the Crystallization and Densification of Polymer-Derived SiC Fibers.

The polymer-derived SiC fibers are mainly used as reinforcing materials for ceramic matrix composites (CMCs) because of their excellent mechanical properties at high temperature. However, decomposition reactions such as release of SiO and CO gases and the formation of pores proceed above 1400 °C because of impurities introduced during the curing process. In this study, polycrystalline SiC fibers were fabricated by applying iodine-curing method and using controlled pyrolysis conditions to investigate crystallization and densification behavior. Oxygen and iodine impurities in amorphous SiC fibers were reduced without pores by diffusion and release to the fiber surface depending on the pyrolysis time. In addition, the reduction of the impurity content had a positive effect on the densification and crystallization of polymer-derived SiC fibers without a sintering aid above the sintering temperature. Consequently, dense Si-Al-C-O polycrystalline fibers containing ß-SiC crystal grains of 50~100 nm were easily fabricated through the blending method and controlled pyrolysis conditions.