Solid-Sample 11B Nuclear Magnetic Resonance and X-ray Photoelectron Spectroscopy Study of Boron-Doped β-Silicon Carbide

Solid-sample magic angle spinning (MAS) nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS), in conjunction with scanning electron microscopy (SEM), were used to investigate the fate of boron used as a sintering aid for silicon carbide. The results of the NMR studies indicated that the boron penetrated the silicon carbide grain boundaries during sintering, and was incorporated in a tetrahedral form in the bulk, regardless of the gas used during the process. The NMR spectrum of a sample sintered under nitrogen indicated the formation of a trigonal form of boron as well. XPS identified this trigonal boron as boron nitride; however, no boron was detected by XPS in any form on the fracture surface of the silicon carbide sintered under argon, even though the NMR results confirmed the presence of tetrahedral boron in the bulk sample. The SEM results indicated that the fracture process for these materials was predominantly intergranular. This suggested that the boron in the silicon carbide sintered under argon penetrated the grains and left the grain boundaries depleted of boron.     

Tribological Performance of Bamboo Fabric Reinforced Epoxy Composites

Bamboo fiber is one of the strongest natural fibers with high strength-to-weight and stiffness-to-weight ratios and can be used economically for manufacturing fiber-reinforced composites. In this paper, bamboo fabric-reinforced epoxy composite is manufactured and its tribological properties for load-bearing applications are investigated. Sliding wear tests are conducted using a linear reciprocating tribometer and the effect of dry and lubricated contact conditions, applied load, sliding speed, temperature, and woven fabric direction on the coefficient of friction and wear rate are investigated. A scanning electron microscope is used to define the wear mechanisms at room and elevated temperatures. It is observed that the fabric orientation influences the mechanical and tribological performances of the composite material. Wear rate increases at higher loads and working temperatures; however, the effect of sliding speed is not remarkable, especially under lubricated contact conditions. The results present in this paper can be used for designing bamboo-reinforced epoxy composites for load-bearing applications, under different working conditions.     

Investigation of the strain‐rate‐dependent mechanical behavior of a photopolymer matrix composite with fumed nano‐silica filler

With the evolution of additive manufacturing, there is an increasing demand to produce high strength and stiffness polymers. Photopolymers are very commonly used in stereolithography and fused deposition modeling processes, but their application is limited due to their low strength and stiffness values. Nano‐sized fibers or particles are generally embedded in the polymer matrix to enhance their properties. In this study, we have studied the effect of fumed nano‐sized silica filler on the elastic and viscoelastic properties of the photopolymer. The uniaxial testing coupons with different concentrations of silica filler have been fabricated via casting. We observed improvement in mechanical properties by the addition of the nano‐sized filler. As polymers exhibit time‐dependent mechanical response, we have conducted tensile tests at different strain rates as it is one of the most common modes of deformation, and is commonly used to characterize the parameters of the rate‐dependent material. We noticed significant dependence of the mechanical properties on the strain rate. quasi‐linear viscoelastic (QLV) model, which combines hyperelastic and viscoelastic phenomena, has been employed to capture the response of the material at different strain rates. We found out that the QLV model with Yeoh strain energy density function adequately describes the rate‐dependent behavior of the material and has reasonable agreement with the experimental results. POLYM. ENG. SCI., 59:1695–1700 2019. © 2019 Society of Plastics Engineers     

Experimental study of the flow structure in a counter flow Ranque–Hilsch vortex tube

The mechanism of the temperature separation in a Ranque–Hilsch vortex tube has been investigated since the discovery of this phenomenon. In spite of being investigated by many researchers, no consensus has yet been reached regarding the mechanism’s hypothesis.This paper reports on a study in progress exploring the temperature separation in a counter-flow vortex tube. The effects of the geometrical parameters, including inlet nozzles, cold exit, hot exit and length of the tube, were investigated, which indicated the settings for the best performance of the vortex tube. Flow properties in the vortex tube were measured and used to understand the flow structure inside the tube. Accurate measurements of the three-dimensional velocity distribution in the tube were conducted. The results provided enough evidence that the flow in the tube consists of a forced vortex formed near the inlet gradually transforming to a free vortex at the hot end. Experimental results found in this research show the vortex transformation along the tube and support the hypothesis proposed in previous study.     

A static friction model for tube bulge forming using a solid bulging medium

In a metal working process, the friction between the material and the tools influences the process by modifying the strain distribution of the workpiece. From a numerical point of view, a constant coefficient of friction (Coulomb’s friction) is commonly used in finite element simulations to model the frictional behaviour of contacting solids. However, friction coefficient varies in time and space with many parameters. We presented here a theoretical model of static friction in rubber/metal contact which allows the determination of the static coefficient of friction as a function of local contact conditions. Simulations using finite element software ABAQUS/Explicit were carried out for an axisymmetric tube bulging operation using the defined friction model. We compared the computed tube thickness related to the constant coefficient of static friction with the defined friction model. The results clearly showed that the new friction model provides better agreement between experiments (Girard, Grenier, Mac Donald, J Mater Process Technol 172:346–355, 2006) and results of numerical simulations.     

Reciprocating Sliding Wear Behavior of 60NiTi As Compared to 440C Steel under Lubricated and Unlubricated Conditions

ABSTRACT

60NiTi is a hard (～60 HRC) and highly corrosion-resistant intermetallic with a relatively low elastic modulus (～100 GPa). In addition, this alloy exhibits a high compressive strength (～2,500 MPa) and a high elastic compressive strain of over 5%. These attributes make this alloy an attractive candidate to be employed in structural and mechanical component applications. However, sliding wear behavior of this intermetallic has not yet been studied in a systematic way. In this study, lubricated and unlubricated reciprocating sliding wear behavior of 60NiTi is compared to 440 C steel as a conventional bearing and wear-resistant alloy. Results of experiments carried out under different loads show that 60NiTi, despite having a higher hardness, exhibits a significantly inferior wear behavior under dry conditions in comparison to 440 C steel. These unexpected results indicate that 60NiTi does not follow conventional wear theories where the wear of materials has an inverse relationship to their hardness. On the other hand, under lubricated conditions with castor oil and a synthetic gear oil, 60NiTi exhibits low specific wear rates. These results exhibit the importance of proper lubrication in sliding mode applications where 60NiTi is exploited as a wear-resistant alloy.     

A new approach to verify and derive a transverse-field-dependentmobility model for electrons in MOS inversion layers

A modeling approach is described that extracts the functional dependence of carrier mobility on local transverse and longitudinal fields, channel doping, fixed interface charge, and temperature in MOS inversion and accumulation layers directly from the experimentally measured effective (or average) mobility. This approach does not require a priori detailed knowledge of the experimental variation of mobility within the inversion or accumulation layer, and it can be used to evaluate the validity of other models described in the literature. Also, an improved transverse-field dependent mobility model is presented for electrons in MOS inversion layers that was developed using this new modeling approach. This model has been implemented in the PISCES 2-D device simulation program. Comparisons of the calculated versus measured data show excellent agreement for I_D-V_G and I_D-V_D curves for devices with L_eff=0.5 to 1.2 μm     

Models for electron and hole mobilities in MOS accumulation layers

We present new physically based effective mobility models for both electrons and holes in MOS accumulation layers. These models take into account carrier-carrier scattering, in addition to surface roughness scattering, phonon and fixed interface charge scattering, and screened Coulomb scattering. The newly developed effective mobility models show excellent agreement with experimental data over the range 1×10¹⁶-4×10¹⁷ cm^-3 for which experimental data are available. Local-field dependent mobility models have also been developed for both electrons and holes, and they have been implemented in the two-dimensional (2-D) device simulators, PISCES and MINIMOS, thus providing for more accurate prediction of the terminal characteristics in deep submicron CMOS devices. In addition, transition region mobility models have been developed to account for the transition in the mobility in going from the accumulation layer in the gate-to-source overlap region to the inversion layer region in the channel     

Influence of humidity on creep response of sandwich beam with a viscoelastic soft core

Polymer foam cored sandwich beams are widely used in load-bearing components due to their high strength to weight ratio. To improve the reliability in using sandwich beams, it is essential to understand their long-term creep response in terms of variation of stresses and deformations with time under external mechanical and environmental stimuli. This paper presents an analytical model for investigating the creep response of sandwich beams made with a viscoelastic soft core, including the effect of the variable ambient humidity under the sustained load and its influence on the creep behavior. The model is based on a high-order viscoelastic structural modeling. The soft core is modeled as a viscoelastic material using differential-type constitutive relations that are based on the linear Boltzman’s principle of superposition and accounting for the deformability of the core in shear and through its thickness. Several numerical examples are presented in order to show the capability of the model and to investigate the effect of moisture on the creep behavior of sandwich beams. Finite element simulations of the creep response of sandwich beams are also performed using ABAQUS software to validate the proposed theoretical model. The results show the concentrations of shear and transverse normal stresses near the edges and their variation in time and with the change of humidity.