Electrodeposition of cobalt on monolayer graphene synthesized by chemical vapor deposition produces Co–CoO/graphene composite structures, which is accompanied by increases in the electrical ...resistance and magnetoresistance. We show that the observed magnetoresistance effect is caused by two competing contributions: negative (NMR) and positive (PMR) magnetoresistance. In weak magnetic fields, the NMR is described by quantum localization correction to the Drude model of conductivity in graphene. The enhancement of PMR observed in strong magnetic fields is related to the Lorentz mechanism in Co–CoO particles.
The goal of the paper is to present selected, untypical, and intuitively unexpected phenomena from nonlinear mechanics. Particular attention is paid to the dynamics of self-, parametric and external ...excited systems. Interactions between these various vibration types lead mainly to quasi-periodic responses. However, in the selected domains of system parameters, the effect of frequency locking is observed. Furthermore, external harmonic force imposed on such a system produces a specific internal loop inside a resonance zone. As an example of nonlinear autoparametric systems, a structure (oscillator) with an attached pendulum is presented. The nonlinear terms introduced by pendulum motion cause instabilities in the resonance region. This instability transits the pendulum to rotation or chaotic motion. An application of nonlinear couplings for the reduction of unwanted vibrations is also studied. In order to reduce vibrations, the main structure is coupled to an electrical oscillator by a quadratic term. It has been shown that such a coupling leads to the amplitude saturation phenomenon which can then be used to design a nonlinear control strategy.
Several samples were cast using thixoforming method. The feedstock used was a hot rolled commercial bar of the steel X210CrW12 with the following chemical composition: 2,11%C, 10,5%Cr, 0,7%W, 0,4%Si. ...An apparatus was constructed allowing the thixoforming of steel parts in a protective argon atmosphere and at a high piston velocity powered by a high pressure air. It was designed and built in such a way to allow first pumping air out and argon in, fast heating of a feedstock using high frequency generator, stamping of semi liquid sample at a high piston rate of a few m/s with in earlier resistance heated mould and simultaneous control of sample and mould temperature to ±1°C. The liquid fraction was determined using Differential Scanning Calorimetry (DSC). Observation of the microstructure was performed using optical and transmission electron microscopy. It showed a globular structure in the material, when pre-heated to the temperature of 1230°C at a liquid fraction of 30%. After several experiments with various sample and mould temperatures, a desired globular microstructure after a thixoforming process was obtained. Basic technological parameters which permit the complete filling of the die and a useful microstructure, such as piston velocity, temperature of the die, were elaborated. The microstructure after the thixoforming process showed globular structures with a size of 50 to 200 µm of the oval particles of the austenite within an eutectic matrix. The crystal structure of globules was mainly austenite and that of the eutectic ferrite and M7C3 carbide.
Summary
Silicon vicinal surfaces can be successfully used as substrates for the preparation of one‐dimensional nanostructures. The quality of the structures prepared may be controlled using scanning ...tunnelling microscopy, as shown in this work. Additionally, it is possible to obtain valuable information using reflection high‐energy electron diffraction. A typical way of employing reflection high‐energy electron diffraction is to observe patterns of scattered electrons on a screen. However, it is possible to obtain more detailed information on the arrangement of atoms at the surface if azimuthal plots are collected. Azimuthal plots are measured by recording the intensity of specularly reflected electrons during the rotation of the sample around an axis perpendicular to its surface. So far, only flat surfaces have been examined in such a way. In this work, it is shown that such data, containing interesting features, can also be collected for vicinal surfaces.
Thixoforming of spray formed M2 tool steel Dutkiewicz, J.; Rogal, Ł.; Sołek, K. ...
International journal of material forming,
04/2010, Letnik:
3, Številka:
Suppl 1
Journal Article
Recenzirano
Using SW7M (M2) tool steel as a feedstock material, several thixoforming tests were performed at various starting temperatures in the semisolid range between 1310-1350°C corresponding to 25 - 50 % of ...the liquid phase as indicated by the results of DSC studies. The chemical composition of the investigated steel is following: 0.85 %C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. Specially constructed apparatus allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. Fast heating of the feedstock was achieved using induction heating through a high frequency generator, followed by the stamping of semi liquid sample to a graphite mould pre-heated by resistance heating. Observations of the microstructure before and after semisolid processing were performed using optical, scanning and transmission electron microscopy. The initial material has a very fine grain size of ∼2-5 μm. This material after thixoforming at 1315°C shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W
2
C, Fe
3
C, Mo
2
C, M
23
C
6
and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M
23
C
6
was identified. The hardness of spray formed steel is around 304 HV significantly increases up to 780 HV after thixo-forming due to formation of martensite. At the end of paper the basic results of numerical simulations were shown.
Nowadays semisolid processing of materials attracts much research interest because both scientists and industrial engineers believe that such processing may reduce significantly the number of ...operations to obtain the final product, compared to classical manufacturing. One semisolid technique under current extensive investigation is casting of materials displaying a globular (thixotropic) microstructure. This paper reports a detailed study of this method. First, the experimental preparation of Al-Si alloy billets and test parts with a complicated shape are described. Then, an approach to numerical modelling the casting of test parts is described briefly. Computer software FORGE3 is used to reproduce the moving front of the material with high accuracy. Finally, three dimensional numerical simulations of low temperature (incomplete) casts are discussed in detail. It is concluded that by comparing low temperature simulated casts with experimental casts one may learn the important information needed to develop detailed theoretical models of thixocasting.
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