Nowadays,thermoelectric materials have attracted a lot of attention as they can directly convert heat into electricity and vice versa.However,while strenuous efforts have been made,those conventional strategies are still inevitably going to meet their performance optimization limits.For this reason,brand new strategies are badly needed to achieve further enhancement.Here,the roles played by magnetism in recent advances of thermoelectric optimization are concluded.Firstly,magnetic thermoelectric materials can just be treated like other normal materials because the use of universal optimization strategies can still get good results.So,it is not a situation which is all or nothing and the tactics of using magnetism for thermoelectric optimization can coexist with other strategies.Besides,through magnetic doping,we can introduce and adjust magnetism in materials for further optimization.Magnetism provides more possibilities in thermoelectric optimization as it can directly influence the spin states in materials.Furthermore,in the form of magnetic secondphase nanoclusters,magnetism can be introduced to thermoelectric materials to conquer the dilemma that the solid solubility of many magnetic ions in thermoelectric materials is too low to have any significant effect on thermoelectric properties.Finally,when exposed to an external magnetic field,topological materials can rely on its unique band structures to optimize.
This paper introduces a feasible process to achieve the molybdenum disulfide atomic layers using chemical vapor deposition(CVD) method,with molybdenum thin film and solid sulfur as precursors.And some improvements were made to reduce the amount of metastable MoS_(2)-3 R.The morphology of the acquired MoS_(2) layers,existing as triangular flakes or large-area continuous films,can be controlled by adjusting the synthesis time and reacting temperature.The characterization results show that the monolayer MoS_(2) flakes reveal a(002)-oriented growth on SiO_(2)/Si substrates,and its crystalline domain size is approximately 30 μm,and the thickness is 0.65 nm.Since the synthesis of MoS_(2)-3 R is restrained,the electronic transport properties of MoS_(2) with different layers were investigated,revealing that those properties equal with those of MoS_(2) samples prepared by exfoliation methods.
Zhi-Tian ShiHong-Bin ZhaoXiao-Qiang ChenGe-Ming WuFeng WeiHai-Ling Tu
In microscale deformation, the magnitudes of specimen and grain sizes are usually identical, and size- dependent phenomena of deformation behavior occur, namely, size effects. In this study, size effects in micro- cylindrical compression were investigated experimentally. It was found that, with the increase of grain size and decrease of specimen size, flow stress decreases and inhomogeneous material flow increases. These size effects tend to be more distinct with miniaturization. Thereafter, a modified model considering orientation distribution of surface grains and continuity between surface grains and inner grains is developed to model size effects in micro- forming. Through finite element simulation, the effects of specimen size, grain size, and orientation of surface grains on the flow stress and inhomogeneous deformation were analyzed. There is a good agreement between experimental and simulation results.
By means of optical microscope （OM）, X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM） analyses, the microstructures of as-cast and heat-treated Mg--4Zn-IY （wt%） alloy containing quasi-crystal phase were studied. The microstructure of the as-cast alloy consists of a-Mg solid solution grains, intermetallic particles and eutectic phases （W-phase and 1-phase）, and huge grains with seri- ous dendritic segregation are clearly observed. After heat treatment, phase transformation and dissolution occur in the alloy and many phases remain. When the alloy was treated above 410 ~C, the eutectic phases transform into spherical shape as the I-phase turns to W-phase. After heat treatment for long time, the alloy is over burnt and the W-phase decomposes to Mg-Y binary phase.
Yang YangKui ZhangXing-Gang LiYong-Jun LiMing-Long MaGuo-Liang ShiJia-Wei Yuan
The miniaturization of products requires the mass production of microparts.The microforming can well meet this requirement.Due to the emergence of decreasing flow stress scale effect in the micro scale,the traditional forming process and theory may fail.Based on the crystal plasticity theory,upsetting tests of micro copper cylinders with different dimensions and grain sizes were simulated,and the decreasing flow stress scale effect was studied and discussed.Results show that with the decrease of billet dimensions,the flow stress is gradually decreased,and the decreasing flow stress scale effect is emerged;with the increase of grain size,the decreasing flow stress scale effect is more remarkable.It can also be seen that the decreasing flow stress scale effect can be well simulated with the crystal plasticity theory,and the necessary relevant information is provided for deeper understanding on this scale effect,as well as the design of processes and die structures in the microforming.
The microstructures and crystallization behavior of Ti-47 at% Ni-3 at% Fe shape memory alloy wire under the condition of severe cold drawing at room temperature and different post-deformation annealing processes were intensively investigated using transmission electron microscope（TEM） and differential scanning calorimetry（DSC）. It is indicated that the amorphous phase is dominant in the Ti50Ni47Fe3 wire after the cold drawing of 78 % areal reduction. The critical temperature for recrystalization is determined at about 300 °C. The average grain size grows from 7 up to 125 nm when annealing temperature rises from300 to 500 °C. Post-deformation annealing process exerts significant influence on the crystallization temperature which climbs up with the increase of annealing temperature.
