Key Engineering Materials Vol. 633

Abstract: Equilibrium compositions of chemical vapor deposition progress for silicon carbide (CVD-SiC) coatings with MTS/H₂ mixture system were calculated by means of HSC Chemistry5.0 code, and influences of the reaction temperature (T), the system pressure (P) and the composition of raw materials (molar ratio of H₂ to SiCH₃Cl₃, β) were investigated. Thermodynamic analysis showed that the value of n_C /n_SiC in reaction product decreased to a minimum and then increased with the increase of the reaction temperature between 700-1600°C, which meant an optimal reaction temperature existed theoretically for CVD-SiC coatings with highest purity. Furthermore, CVD-SiC coatings with high purity were obtained by experiments under the reaction pressure of 10kPa, H₂/SiCH₃Cl₃ value of 10 and reacton temperature of 1100°C, which was in accordance with the theoretical prediction.

Abstract: ZrB₂, YAG and Al₂O₃ are widely applied because of some excellent performances, but ZrB₂ is easily oxidized in the high-temperature air. To make the ZrB₂ ceramics obtain better oxidation resistance, high-density ZrB₂-YAG-Al₂O₃ ceramics were prepared. The oxidation mechanism of ZrB₂-YAG-Al₂O₃ ceramics is investigated. When YAG-ZrB₂ ceramics are exposed to air at temperatures from 1100°C¡æ to 1600°C¡æ, the weight change rate of YAG-ZrB₂ ceramics is fleetly increased, the reacted speed of ZrB₂ and O to form B₂O₃ and ZrO₂ is more faster. The quantity of melted B₂O₃ is increased and carried to the surface, B₂O₃ is reacted with Al₂O₃ to form Al₁₈B₄O₃₃. Al₁₈B₄O₃₃ is melted and coated on the surface of ceramics to form protection layer for the oxidation resistance of ceramics at high temperature, which shows the similar with the influence of reinforcing phase on the oxidation model of ZrB₂ based multi-phase ceramics.

Abstract: ZrO₂ doped with 7.5% (volume percent) nanoalumina ceramics were prepared by microwave sintering processes. The effects of nanoalumina additions and various sintering temperature on densification, phase composition, microstructure and mechanical properties of Al₂O₃-ZrO₂ ceramics were investigated. The results show that the m-ZrO₂ phase transformed into t-ZrO₂ during the process of microwave sintering. Relative densities between 95% and 99% were attained in the different conditions. In any cases the grain size was maintained at a submicron scale at a processing microwave sintering. The presence of Al₂O₃ grains had an effect of hindering grain growth of ZrO₂ grains. When the microwave sintering temperature was 1500°C, 7.5Al₂O₃-ZrO₂ composite ceramics presented excellent mechanical properties: HV=12.0 GPa, σ_f=715.7 Mpa, K_IC=11.9 MPa·m^1/2. Compared with that of pure ZrO₂ ceramic, the bending strength and the fracture toughness were improved 45% and 23% at least, respectively. The fracture mode was associated with sintering temperature: when the sintering temperature was 1350°C~1450°C, the intergranular fracture and transgranular fracture coexisted; when the sintering temperature was 1500°C, intergranular fracture was the main fracture mode.

Abstract: By taking Ti-B₄C and CrO₃-Al as the primary system and the subsystem respectively, the curve dependence of CrO₃-Al subsystem on the adiabatic temperature of the reactive system was calculated in chemical dynamics, and laminated composite with TiB₂-based ceramic to stainless steel was achieved without Al₂O₃ inclusions and microcracks at the interface, and the intermediate was clearly presented between the ceramic and the stainless steel through liquid fusion and liquid diffusion of the ceramic liquid and the molten steel. Because of the differences in constitutional diffusion and solid precipitation, the hybrid microstructures was presented in the intermediate, i.e. within the intermediate the ceramic and metallic phases in different size were alternately distributed to form 3-D net ceramic-metal microstructure, while the continuously-graded microstructure from the TiB₂ matrix ceramic to stainless steel was also presented in both volume fraction and size of the TiB₂ and TiC phases.

Abstract: Layered rare-earth hydroxide nanosheets (3-8 nm thick) of Y₂(OH)₅NO₃·nH₂O (NO₃^--LYH) were successfully synthesized in one step via chemical precipitation at ~4 °C and pH ~8, using yttrium nitrate and ammonium hydroxide as reagents. The interlayer NO₃^- was found to be free ions and can be completely replaced with SO₄^2- to yield Y₂(OH)₅(SO₄)_0.5·nH₂O (SO₄^2--LYH). Thermal decomposition behavior of the SO₄^2--LYH was studied in detail, and the phase and morphology evolutions upon calcination in the temperature range 500-1100 °C were also investigated. Characterizations via XRD, FE-SEM, TEM, and FT-IR found that anion exchange did not bring about any appreciable change to the 2-dimensional crystallite morphology but the basal spacing of the crystal structure shrank from ~0.884 to 0.840 nm owing to the indirect coordination of SO₄^2- to the Y³⁺ ions. DTA/TG and XRD analysis found that the NO₃^--LYH converts to Y₂O₃ at ~600 °C, but the SO₄^2--LYH decomposes to oxide at a higher temperature of ~1000 °C via monoclinic Y₂O₂SO₄ in the range of 800-900 °C. The resultant Y₂O₃ particles have an average size of ~60 nm.

Abstract: Boron and boron-rich boride has attracted considerable attention in the past few years for their wide varieties of structures and property associated with their unusual three-center electron-deficient bonds. Boron film also exhibits many unique and fascinating properties, such as high melting point (~ 2500K) with low density, high harness close to diamond, and excellent thermoelectric property. In this paper, boron (¹⁰B) films were prepared on (100) silicon substrate by radio frequency (r. f.) magnetron sputtering method under the different working pressure and power with a target of boron and boron oxide (B:B₂O₃ 40wt%). After 3 hours sputtering deposition, the substrate was covered with boron films tightly. The morphology of deposited films under different temperature was characterized by high resolution scanning electron microscopy (HRSEM), FTIR spectrum (FTIR), Raman spectrum (Raman). The results show that the film contains boron and little oxygen. At last, the growth mechanism of B film was analyzed.

Abstract: Cracking and chipping of MnZn Ferrites usually occur during the grinding process although proper additives such as V₂O₅ can restrain development of cracks. To study the mechanisms, the effects of adding V₂O₅ or ZrO₂ to MnZn ferrite were investigated by the rattler test. It was confirmed that adding V₂O₅ could improve the toughness of MnZn ferrite while addition of ZrO₂ showed opposite result. Based on micro-structure analysis, it was found that adding V₂O₅ increased ratio of intergranular fracture mode to the transgranular but adding ZrO₂ caused more transgranular cracks. Besides, V₂O₅ addition changed its grain size distribution and resulted in some larger grains. Possible mechanisms of the toughness improvement were proposed according the above findings.

Abstract: This paper studies about the boric acid affect on magnesium slag powder properties. The certain content of 0.05% to 3.0%(weight ratio) boric acid and magnesium slag was sufficiently mixed, the mixed powder is putted into a 20 × 20 × 20mm mold, and using 13 tf pressure to press the powder. The molded sample was placed in a muffle furnace, sintering at different temperatures (1000 °C ~ 1200 °C) and different holding times (1h ~ 6h), when holding time is reached, the sample was cooled naturally. Samples were analyzed. The sintered samples using XRD to study phase analysis and using EDS to research elemental composition and doing SEM for microstructure analysis. The results showed that: the main components of magnesium slag CaO, MgO, Fe₂O₃, Al2O3, β-C2S, γ-C2S and so on. Magnesium slag with certain content of boric acid, with the sintering temperature, the acid content and the holding time increases, the conversion rate of the β-dicalcium silicate change into γ-dicalcium silicate in magnesium slag is decreases. Without adding boric acid the suppress square material magnesium slags are mess into powder, magnesium slag joined with certain content of boric acid still be a square material, and suppression of the sample becomes more dense.

Abstract: After laser ablation treatment, there are grooves on the surfaces of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3δ (BSCFO) membranes. These grooves with width about 150μm and depth about 25μm increase the specific surface of these membranes. The oxygen permeation fluxes through these membranes, which sides were treatment by laser ablation, were measured. Comparing with the membrane without laser ablation treatment, the oxygen permeation fluxes through the membranes with cross grooves pattern decorated on both sides after laser ablation treatment can increase by 34.7%.

Abstract: The microcline powder, which collected from the hornblende syenite from Anhui, China, was treated by NaOH solution at 250 °C for 2-6h. The solid samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The XRD shows that the main solid sample is hydroxycancrinite. The images observed by SEM indicate that the crystal morphology of the hydroxycancrinite can be presented as hexagonal cylindrical when it reacted with 4.17M NaOH at 250 °C for 6h.