Rongjun Liu

Zirconium carbide, hafnium carbide and their ternary carbide nanoparticles by an in situ polymerization route

An in situ polymerizable complex method to produce zirconium carbide, hafnium carbide and their ternary carbides at a relatively low temperature (1300 °C) using simple and mainly nontoxic starting reagents is presented. In this aqueous process, citric acid (CA) was used to chelate the metal ion and ethylene glycol (EG) to form a polymerized complex resin. We suggest that, based on the results of FT-IR and 13C NMR spectroscopies, a very stable metal–CA chelate complex formed in the starting solution, which was thermally stable upon gelation even up to 350 °C. Immobilization of the metal ion in a rigid polymer can largely guarantee the in situ charring, resulting in carbon adjacent to the metal oxide in the pyrolysed product. The contiguous carbon and metal oxide led to in situ reaction (1100 °C) with a minimum of diffusion, which involved the formation of large numbers of metastable phases. Afterwards, well-defined binary and ternary carbide nanoparticles (∼100 nm) were formed through localized particle coarsening by Ostwald ripening.

Synthesis and formation mechanism of submicrometer ZrB2 powders via the Pechini-type polymerizable complex route

ZrB2 powders were synthesized by a polymerizable complex method based on the Pechini-type reaction route, wherein a precursor solution of citric acid, glycerol, boric acid, and zirconium ions was prepared, and polymerized to form a semitransparent resin without any precipitation at 150 °C. The precursor solutions and the resulting resins were characterized by FT-IR and 13C NMR spectroscopy. The results show the formation of a hybrid polymer with zirconium and boron arrested within the polymeric chain by complexation. The submicrometer ZrB2 powders (200–600 nm) are formed after pyrolysis of the polymeric precursor with 4 B/metal molar ratios at 1400 °C. Investigation of the formation mechanism of ZrB2 powders indicates that ZrC is the intermediary phase and two reduction reactions determine the specific pathway leading to ZrB2 formation: (1) ZrC formation, (2) the formed ZrC directly reacts with B2O3 to form ZrO2 and ZrB2. In the whole conversion process, ZrC formation by carbothermal reduction is a fast reaction, while the direct reaction of ZrC with B2O3 to form ZrO2 and ZrB2 is the rate-limiting step.

Study on water vapor corrosion resistance of rare earth monosilicates RE2SiO5 (RE = Lu, Yb, Tm, Er, Ho, Dy, Y, and Sc) from first-principles calculations

Corrosion resistance of rare earth monosilicates (RE2SiO5, RE = Lu, Yb, Tm, Er, Ho, Dy, Y, and Sc) in water vapor has been studied using the first-principles calculations. The results show that the water vapor corrosion resistance of RE2SiO5 demonstrates the following order: Sc2SiO5 > Dy2SiO5 > Y2SiO5 > Ho2SiO5 > Er2SiO5 > Yb2SiO5 > Tm2SiO5 > Lu2SiO5. To further improve their water vapor resistance, a doping strategy has been employed for the first time. Two scenarios have been investigated: one is a half mole proportion of substitution of various rare earth elements for Yb in the Yb2SiO5 lattice; the other is a half mole fraction substitution of rare earth elements in RE2SiO5 (RE = Lu, Yb, Er and Y) by scandium. It is unveiled that the water vapor resistance of YbScSiO5 and YScSiO5 has been greatly improved in contrast to other rare earth monosilicates. The current study provides guidelines for the selection of environmental barrier coatings with a better water vapor corrosion resistance.

Effect of 3D-Braided Structure on Thermal Expansion of PIP–Cf/SiC Composites

3D3d, 3D4d, 3D5d-braided Cf/SiC composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor. The coefficient of thermal expansion (CTE) of Cf/SiC composites was measured in longitudinal and transversal directions in the temperature range from –150°C to 25°C. The longitudinal CTE varies in the range (0.09–0.68) × 10–6/°C, and the transversal CTE varies in the range (0.21–1.95) × 10–6/°C. Various CTEs of 3D-braided Cf/SiC composites were mainly determined by different braided structures of carbon fibers. The longitudinal CTE is lower than transversal CTE for the negative axial expansion of carbon fibers at cryogenic temperature. Microcracks were examined to understand the effect of structure on the thermal expansion of composites.