Revealing thermal ablation mechanisms of C/SiC with in situ optical observation and numerical simulation

Structural ceramics such as C/SiC (carbon fiber reinforced silicon carbide) are of great importance for high temperature applications, where thermal ablation could occur and impair the structural integrity. Here we adopt in situ and real time optical visualization technique and numerical simulation to study the thermal ablation mechanisms of C/SiC subjected to oxyacetylene flame. The in situ optical setup captures the real time surface evolution at temperatures up to 1800℃, demonstrating clearly the formation of surface gas bubble, the flowing and merging process of liquid SiO₂, which are later well reproduced and explained by the numerical simulation. Four different surface ablation regions on the sample subjected to thermal ablation have been identified by the captured images and flow field streamlines, which reveal the distribution and flow mechanism of SiO₂ droplets on the C/SiC surface.     

Microstructure evolution and grain growth mechanisms of h-BN ceramics during hot-pressing

Pure h-BN ceramic specimens were prepared by hot-pressing under different sintering temperatures and pressures using ball milled h-BN powders composed of amorphous and nanocrystalline BN. Microstructures and thermal conductivities of these h-BN ceramic specimens were characterized and measured. Higher sintering pressure is more favorable to the preferred orientation growth of plate-like h-BN grains along the pressure direction, forming microstructures where the c-axes of h-BN grains are preferentially oriented perpendicular to the pressure direction. However, such microstructures can only be obtained at appropriate sintering temperature. Thermal conductivities of h-BN ceramic specimens are strongly related to their microstructures, especially the grain orientation. Growth mechanisms of h-BN grains were investigated. There is multi-area co-growth phenomenon around the grain boundaries composed of the basal planes of h-BN grains, which results in the formation of stacking faults in the as-grown h-BN grains.     

VO2-ZnO composite films with enhanced thermochromic properties for smart windows

VO₂ film is a promising thermochromic material in smart windows due to its reversible metal to insulator transition (MIT) accompanied with an abrupt change of transmittance in near-infrared region at around 68 °C (T>68 °C, translucent; T < 68 °C, transparent), but which has not been widely applied because its low luminous transmittance (<60%) and solar modulation efficiency (<10%) are difficult to be improved simultaneously. In order to solve this problem, the ZnO-VO₂ composite film was prepared by a facile method, in which commercial ZnO nanoparticles (NPs) and VO₂ micro-particles were mixed by ball milling method to form the composites. By introducing ZnO NPs into the composite film, the luminous transmittance (T_lum) of the composite film was increased by 16.9% (from 54.9% to 63.9%) and the solar modulation efficiency (ΔT_sol) was increased by 14.1% (from 9.9% to 11.3%) compared to the pure VO₂ composite film. This was because ZnO NPs not only played the role of antireflection, but also prevented VO₂ particles from agglomeration by dispersing around VO₂ particles. Furthermore, the two-layered film based on ZnO-VO₂ composites exhibited an astonishing ΔT_sol of 18.8%, while maintaining excellent T_lum of 54.3%. This work could provide a simple and novel idea for us to improve the thermochromic properties of VO₂ films and simultaneously to promote their practical application.     

Fabrication of porous (Ba,Sr)(Co,Fe)O3-δ (BSCF) ceramics using gelatinization and retrogradation phenomena of starch as pore-forming agent

Porous (Ba,Sr)(Co,Fe)O_3-δ (BSCF) ceramics with high open porosity and good electrical conductivity was fabricated using Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), which shows a high mixed ionic-electronic conductivity. In general, during the fabrication of porous ceramics by the sacrificial template method using pore former particles, closed pores are easily formed unless sufficient pore former particles are added. In this study, we have devised a method using the gelatinization-retrogradation phenomena of starch for producing a porous body with an excellent percolated pore network structure. By dispersing BSCF and starch in an aqueous slurry (0–50% by weight) and heating, gelatinization of the starch occurred and the starch particles adhered to each other. Furthermore, in order to retain the percolated structure, the water solvent was removed by freeze-drying without heating to obtain a dried green body. The sintering behavior of the porous BSCF bodies prepared under various conditions was characterized by microstructural observations and relative density measurements. By optimizing the process conditions of the gelatinization and retrogradation, a porous body having an open porosity of 48.3%, and with 99% of the total pores open, was obtained. The matrix was also well connected and showed a sufficiently high conductivity which was similar to the porous bodies made by the traditional sacrificial template method.     

Decomposition process of bastnaesite concentrate in NaOH-CaO-H_2O system

A novel process of calcification-leaching for bastnaesite concentrate(REFCO_3) was proposed. The prior calcification was carried out in the system of NaOH-CaO-H_2O and the lgC-pH pattern for Ce-F-Ca-C-H_2O system was drawn. The thermodynamics result indicates that decomposition for bastnaesite requires certain alkaline condition, but excessive alkalinity also causes decomposition of CaF_2. XRD and SEM-EDS analyses on the calcification-leaching process reveal that bastnaesite first decomposes into RE(OH)_3 and CaF_2. Then, by HCl leaching rare earths were extracted,while CaF_2 was left in the leaching residue. In addition, effects of temperature, time, NaOH and CaO on the calcification were investigated. The results show that the leaching rate of rare earths(REs)reaches 72.5 wt%, at the same time 99.2 wt% of F is left in leaching residue with 20 wt% NaOH and 38 wt% CaO at 493 K for 180 min.     

Control of phase transition in TaSe3

Stabilization factors of the high and low temperature phases of TaSe₃ are investigated. The low temperature phase is stabilized by zirconium or titanium impurity. The high temperature phase is stabilized by oxygen impurity. Without oxygen impurity, the low temperature phase is stable up to decomposition temperature and the phase transition is completely suppressed by zirconium impurity.     

Transmission electron microscopy investigation of the transient oxides formed on the aluminide coating on Inconel 625 superalloy

A high activity aluminide coating was formed on the γ phase Inconel 625 superalloy by the pack cementation process. Transmission electron microscopy was used to investigate the structure of the transient oxides formed on the aluminide coating. The transient oxides formed on the coating after exposure in air at 1200 °C for 250 s were found to consist of highly oriented NiO, Ni(Al, Cr)₂O₄ and α-(Al, Cr)₂O₃.