Papers by Keyword: Defect

Abstract: Magnetism of reduced graphene oxide/rGO prepared by a green synthesis method from coconut shells (rGO-s) and the commercial product (rGO-c, ®Graphenea) have been investigated. Similar magnetic feature of a weak ferromagnetism concomitant with diamagnetic nature was observed in both samples. At 300 K, the saturation magnetization (M_S) of rGO-s is approximately 14×10^-3 emu/g, which is about 3 times of that observed in rGO-c (~5×10^-3 emu/g). The noticeable difference in the M_S is suggested due to the different concentration of oxygen-functional groups and other defects presented in the rGO sheets. The samples have similar structure and contains similar functional groups, yet rGO-s contains higher concentration of oxygen-functional groups and defects than rGO-c. A paramagnetic behavior was also indicated at low temperature. This study supports an indication of the defect-induced-magnetism in rGO and confirms that various magnetic features, such as ferromagnetic, diamagnetic and paramagnetic, can coexist in rGO.

Abstract: The effect of point defects on the shear rupture resistance in titanium aluminide is investigated by the density functional theory and pseudopotential methods. Vacancies, as well as substitution atoms – tungsten and chromium were considered as points defects. The shear was simulated in the(111) slip plane for two directions, namely [110] and [11-2]. It is shown that for a {111}<110> sliding system, vacancies significantly reduce the shear resistance. However, when alloying element occupies a titanium vacancy, it can partially compensate for this negative effect.

Abstract: The model for sapphire crystals growth parameters estimation has been suggested. It can be served as a basis for the analysis of heat transfer processes during sapphire crystals obtaining. It allows to predict the bubbles, stresses and cracks appearing during sapphire crystals growth. We describe the results of the analysis and parameters estimation of sapphire crystals growth on all stages by horizontal directed crystallization method.

Abstract: The research and commercialization of SiC based power device have been burgeoning over the last decade worldwide, which is bringing about an increasing demand on lost-cost and low-defect SiC wafers. To meet this challenge, we have been continuously making efforts on improving the crystal growth and wafer processing techniques. Now, the mass-production of high quality 4-inch, 6-inch n-type and semi-insulating SiC wafers has been realized. Statistically, the micropipe density is lower than 0.5 cm^-2. The resistivity of the wafers is lower than 0.02 Ω·cm and up to 10⁸ Ω·cm for n-type and semi-insulating SiC single crystals, respectively. A state of the art processing technique has been developed to control wafer deformation and thickness within the desired values for subsequent epitaxy. The total defect number of the epitaxial layers grown on the "epi-ready" 4-inch SiC wafer is 63, and the usable area is 97.6%, indicating the high quality of our SiC substrates.

Abstract: 70-um thick homoepitaxial layers with very low defect density were grown on 6-inch 4° off-axis wafers using hot-wall chemical vapor deposition (CVD). Process optimization resulted in reduction of the density of triangular defects from 1.01 cm^-2 to 0.14 cm^-2. The treatment of wafer (CMP or selection) was essential. The in-situ etch process was optimized prior to the epitaxial growth. Junction Barrier Schottky diodes fabricated on the epitaxial films presented a typical I–V characteristic and a block voltage of 6500 V.

Abstract: Addictive manufacturing (AM) allows for the layer-by-layer fabrication of components via sequential material deposition and it is of immediate interest in many applications, in particularly aviation field. This work is tackling the issue that the influence of the inner-defects and building orientations on the fatigue behavior of Ti-6Al-4V Alloys produced by Selective Laser Melting (SLM). Specimens were built in two orientations (horizontal and vertical to the substrate) in order to evaluate the impact of the induced anisotropy of fatigue properties. A series of fatigue tests at five stress ratios ranged from-1 to 0.8 are conducted at 400°C. Scanning Electron Microscopy (SEM) is used to examine the fracture surfaces of fatigue specimens to qualify the failure mechanism and crack initiation sites, which are most likely attached to the surface defects. The fracture surface analysis of HCF specimen tested at two temperatures reveals that near 85% of the crack initiates from the defect under subsurface. The results of this study imply that the fatigue properties at 400°C are highly dependent on the specimen orientations relative to build directions, as the defects close to surface are the mainly cause of the crack initiations.

Abstract: In this study, the assessment and calculation methods for the crack propagation life of steam turbine rotor shafts containing defects are presented. The analytic methods for estimating the average stress and the alternating stress amplitude of the steam turbine rotor shafts are introduced. The defects on/in the rotor shafts were regularized by the method of fracture mechanics, and the high cycle fatigue crack propagation life and low cycle fatigue crack propagation life of the rotor shafts are estimated from Paris formula. Taking the 60MW turbine rotor shafts containing an initial surface defect and an initial internal defect as the examples respectively, the crack propagation life of them were calculated. The results indicated that the assessment method for the crack propagation life can preliminarily be both used to estimate the safety-operating life and to analyze the fracture reason of a steam turbine rotor shaft containing defects. This paper can provide reference for periodic maintenance and safety evaluation of turbine rotor shafts.

Abstract: For critical structures in the oil and gas refining, petrochemical and chemical industries, it is necessary to determine the value of mechanical characteristics. In the present paper we consider issues related to the determination of the mechanical characteristics of structural materials using compact samples. Basically, current values ​​of the mechanical properties of materials are determined by measuring hardness using existing empirical dependences that combine hardness values ​​with yield limit and tensile strength values. But such dependencies are not universal and require experimental verification. The need for compact samples application occurs during determination of the reasons of equipment failure, when the size of a fragment of broken down equipment does not allow producing standard samples for traditional testing performance. The application of empirical dependencies for determination of hardness mechanical characteristics is complicated by the fact that it is necessary to take into account the degree of the material degradation. This, in its turn, requires additional studies. The key issue in determining the accuracy of measurements on compact samples is the role of the surface in the formation of fracture focal points. The experiments were carried out on thin steel samples and the mechanical characteristics were determined on Instron 8801 dynamometer unit. It was shown that in case of increase of thickness of the samples that underwent tension testing, the fracture work varies according to the power law.

Abstract: Based eddy current transducer (ECT), a probe has been designed to research composite materials. Defects inspection of composite materials is performed to determine the following standard defects: defect of the metallic and (or) polymer layer uniformity. The subminiature ECT of the original design is used as a sensor in this device, it is made according to a differential scheme of switching on of the coils of a transformer ECT and allowing to localize the control area up to 0.1-0.5 mm. The measurement procedure allowing one to detect defects in composite materials with a high accuracy is described. The sensor was tested on the composite material consisting of paper or low-density polyethylene and aluminum layers in which the model defect was placed. The dependences of the ECT signal on the defect in this structure are given. The determined dependence of electrical conductivity of composite materials on model defects make it possible to carry out defects inspection of composite materials.

Abstract: The search for effective methods of management is one of the strategic directions of economic development. To obtain high-quality products, organizations should manage nonconformities and causes of product defects. The paper focuses on the analysis of groups of defects of welded joints using statistical methods. Welding process was considered as one of the processes of management. The possible causes of nonconformities were indicated. The method of failure modes and effects analysis was used to quantify the level of risk. A matrix of consequences and probabilities was presented, and critical risks and risk situations were selected. The proposed measures to eliminate or minimize negative effects enable identification of problem areas of the life cycle of welded structures.