Papers by Keyword: Formation Mechanism

Abstract: The carbon inclusions in SiC bulk crystals were studied by using optical microscope. The carbon inclusions were classified into three types, namely Type Ⅰ (100~1000 μm), Type Ⅱ (20~50 μm), Type Ⅲ (~5 μm) carbon inclusions based on their different morphologies. In addition, the evolution of these three type carbon inclusions with crystal growth process was traced. It was found that the number of Type Ⅰ and Type Ⅱ carbon inclusions showed declined tendency with crystal growth, while the number of Type Ⅲ carbon inclusions exhibited less dependence on crystal growth. Furthermore, the formation mechanism of carbon inclusions was clarified. This study would give out a better understanding of carbon inclusions, thus contribute to reducing or eliminating carbon inclusions in SiC crystals.

Abstract: Competitive reactions and formation mechanism of microstructures in Mg/Cu super-laminate composites (SLCs) during initial hydrogenation were studied. During initial hydrogenation of Mg/Cu SLCs, hydrogenation of Mg and alloying of Mg with Cu followed by hydrogenation of Mg₂Cu are competitive. It is found that microstructures of Mg/Cu SLCs during initial hydrogenation have changed drastically depending on the order of hydrogenation of Mg and Mg₂Cu. The microstructures of Mg/Cu SLCs after initial hydrogenation can be categorized in 3 types.

Abstract: In the present work, the Mg/Al bimetallic composites were successfully prepared by the lost foam casting (LFC) process, and the characteristics and formation mechanism of the interface of the Mg/Al bimetallic composites were investigated. The results show that a uniform and compact metallurgical interface with an average thickness of about 1400 μm was formed between magnesium alloy and aluminum alloy. The interface layer of the Mg/Al bimetallic composites was composed of three different reaction layers, namely the Al₁₂Mg₁₇+δ(Mg) eutectic layer adjacent to the magnesium matrix, the Al₁₂Mg₁₇+Mg₂Si interlayer and the Al₃Mg₂+Mg₂Si layer close to the aluminum matrix. The microhardnesses of the interface layer were remarkably higher than those of the magnesium and aluminum matrixes. The stress strength of the Mg/Al bimetallic composites was up to 47.67 MPa. The fractograph of the push out sample mainly showed a brittle fracture nature. The formation of the interface of the Mg/Al bimetallic composites was attributed to the fusion and diffusion bonding. With the variations of the concentrations of the different elements at the interface, the Al₃Mg₂ intermetallic phase first formed near to the aluminum matrix, and then the Al₁₂Mg₁₇ and Mg₂Si successively generated toward the magnesium matrix, finally obtaining the interface layer of the Mg/Al bimetallic composites.

Abstract: Zirconia (ZrO₂) ceramic and Nb were brazed using Ag-Cu-Ti powder without and with Mo-particle-reinforced. The effects of the brazing temperature, holding time and Mo content of the composite filler on the interfacial microstructures and joining mechanism of ZrO₂/Nb brazed joints were investigated. By increasing the brazing temperature and holding time, the thickness of Ti₃Cu₃O layer increased and the thickness of TiO layer decreased, while the total thickness of the reaction layers increased slightly with the sufficient interfacial reaction. Meanwhile, the blocky Ti-Cu compounds gradually accumulate and grow up in the brazing seam. The calculated Ti activity increased first and then decreased as the Mo content was increased from 5 to 40 wt%. When the Mo content was 5wt%, only single Ti₃Cu₃O reaction layers formed adjacent to the ceramic substrate. By increasing of Mo particles, TiO layer became thicker. When 40 wt% Mo particles were added to the composite filler, Mo particles aggregated into larger clumps damaged the shear strength of brazing joint.

Abstract: The current research status of reheat crack in welded joint of low alloy heat-resistant steels are outlined. Some new phenomena discovered in engineering in recent years are summarized, and related problems that remain to be studied are also presented. Reheat crack in welded joint is occurred frequently during postweld heat treatment (PWHT) or in service at elevated temperature. Mainly four mechanisms leading to reheat crack, respectively, impurity segregation grain boundary weakening, precipitation hardening, creep rupture and no-precipitation zone weakening have been generally accepted now. Several essential factors consist of parent material chemical composition, microstructure and thickness of welded joint, filler material, preheat and postheat procedures, welding energy input, PWHT parameter, and stress and strain and so on. Theoretical calculation methods, criterion of RoA (Reduction of Area) and several experimental methods are put forward to evaluate reheat crack susceptibility. Based on new issues emerged in engineering, problems of concerning reheat crack are necessary to be researched, which include at least mechanism and influence factors of new multi-element composite strengthening heat-resistant steels, the mitigation measures for increasing wall thickness weldment, the root cause analysis and prevention methods of conventional low alloy heat-resistant steels for long term service.

Abstract: (NH₄)₂V₄O₉ thin films composed of vertically aligned single-crystal nanosheets were directly fabricated on quartz glass by a one-step hydrothermal method. In order to study the formation mechanism and to explore the potential applications of the thin films, process experiments were carried out and the band gap was analyzed. An oriented-attachment exfoliation–recrystallization–crystal growth mechanism was proposed for the formation of the (NH₄)₂V₄O₉ thin films. In addition, the band gap varied from 2.13 to 2.87 eV with variations in the thickness from 40.21 to 78.64 μm. The move of the band gap main due to the decreasing bond lengths and defect densities with increasing reaction time and which is significant for practical applications.

Abstract: SiO₂/TiO₂ nano-hybrid particles were successfully prepared by hydrothermal method by using titanium tetrafluoride (TiF₄) as the titanium source, silicon dioxide as template. The factors of hydrothermal temperature and hydrothermal time on the morphologies and photocatalytic activity of SiO₂/TiO₂ nano-spheres were systematically discussed in this paper. The structures and properties of the SiO₂/TiO₂ nano-hybrid particles were studied by some techniques such as SEM, TEM, XRD and BET analysis. The results indicated that TiO₂ prepared by hydrothermal method were anatase crystal, and in the form of polycrystalline. The formation mechanism of SiO₂/TiO₂ nano-hybrid particles was also discussed.

Abstract: H₂S gas would pose a threat to coal mine safety. This article from the biodegradation, microbial sulfate reduction, thermal decomposition chemistry, chemical sulfate reduction and magmatic 5 aspects to analyze the formation mechanism of hydrogen sulfide gas in coal mine, a coal mine combined with field data, analysis of hydrogen sulfide gas in the coal mining the transport mechanisms. Research for the comprehensive management of coal mine hydrogen sulfide gas has certain significance.

Abstract: The special geological conditions of talus are often encountered in southwest China, it brings a lot of difficulties to tunnel construction. The formation mechanism and developmental properties of talus are analyzed in this paper. Based on field investigation on talus diseases, the hazard to tunnel engineering induced by talus and its corresponding measures are also analyzed. The results can be used for the safe and fast construction of tunnels which go through talus deposit.

Abstract: Mesoporous sulfated TiO₂-graphene oxide composites were prepared by hydrothermal method. The samples were analyzed by using scanning electron microscope (SEM), X-ray diffraction (XRD), FT-IR spectroscopy (IR) and Brunauer-Emmett-Teller (BET) method. photocatalytic activities of sulfated titania-GO samples were evaluated using the photooxidation of methylene blue in aqueous solutions by ultraviolet-visible (UV-via) spectroscopy. The results indicated that oxygen-containing carbonaceous groups in the GO are incorporated into the network of C\O\Ti and coordinated to titania in bidentate model, Sulfur species in the form of sulfate are incorporated into the network of Ti\O\Ti, resulting in the strong inductive effect, large specific surface area, and mesoporous structure. All these are beneficial to improve the photocatalytic activities of the mesoporous sulfated TiO₂-graphene oxide composites. The formation mechanism of sulfated mesoporous titania-graphene oxide composites was explored. Joint electronic system of reduced graphene oxide, SO₄²⁻ and TiO₂ was also predicted.