Open Access Research Article Issue
High temperature mechanical retention characteristics and oxidation behaviors of the MoSi2(Cr5Si3)-RSiC composites prepared via a PIP-AAMI combined process
Journal of Advanced Ceramics 2019, 8 (2): 196-208
Published: 13 June 2019

In the present paper, MoSi2(Cr5Si3)-RSiC composites were prepared via a combination of precursor impregnation pyrolysis (PIP) and MoSi2-Si-Cr alloy active melt infiltration (AAMI) process. Composition, microstructure, mechanical retention characteristics, and oxidation behaviors of the composites at elevated temperature were studied. X-ray diffraction (XRD) pattern confirms that the composites mainly compose of 6H-SiC, hexagonal MoSi2, and tetragonal Cr5Si3. Scanning electron microscopy (SEM) image reveals that nearly dense MoSi2(Cr5Si3)-RSiC composites exhibiting three-dimensionally (3D) interpenetrated network structure are obtained when infiltrated at 2173 K, and the interface combination of the composites mainly depends on the composition ratio of infiltrated phases. Oxidation weight gain rate of the composites is much lower than that of RSiC matrix, where MoSiCr2 possesses the lowest value of 0.1630 mg•cm-2, about 78% lower than that of RSiC after oxidation at 1773 K for 100 h. Also, it possesses the highest mechanical values of 139.54 MPa (flexural strength σf and RT) and 276.77 GPa (elastic modulus Ef and RT), improvement of 73.73% and 29.77% as compared with that of RSiC, respectively. Mechanical properties of the composites increase first and then decrease with the extension of oxidation time at 1773 K, due to the cooperation effect of surface defect reduction via oxidation reaction and thermal stress relaxation in the composites, crystal growth, and thickness increase of the oxide film. Fracture toughness of MoSiCr2 reaches 2.24 MPa·m1/2 (1673 K), showing the highest improvement of 31.70% as compared to the RT value.

Open Access Research Article Issue
A study of morphological properties of SiO2 aerogels obtained at different temperatures
Journal of Advanced Ceramics 2018, 7 (4): 307-316
Published: 18 December 2018

In this paper, temperature dependence of nanoporous framework evolution process and variety of pore properties (pore volume, specific surface area (BET), and pore size) of SiO2 aerogels were characterized by FTIR, XPS, XRD, SEM, TEM, BET, BJH, etc. Results show that SiO2 aerogels treated at different temperatures all possess amorphous structure. With the increase of treated temperatures, BET values of SiO2 aerogels increase initially and then decrease, and it reaches the maximum value of 882.81 m2/g when treated at 600 ℃ for 2 h due to the addition of the nanopores and shrinkage skeleton of SiO2 aerogels. Higher temperatures may result in a framework transformation and particle growth; both factors could reduce the BET values of the aerogels. Nanoporous skeleton of SiO2 aerogels at room temperatures is composed of tetrahedron with a pore size of about 22.28 nm. Higher treated temperatures result in an increase of octahedron amount in nanoporous framework and a decrease of pore size. When treated at 1000 ℃, an approximate dense SiO2 bulk via the framework collapse and particle growth is obtained. These varieties are derived from the formed extra bonds of Si–O–Si, higher local stress, and liquid phase between particles during heat treatment process.

total 2