Thermoelectric properties of p-type (Bi_(0.15)Sb_(0.85))_2Te_3-PbTe graded thermoelectric materials with different barriers

The p-type (Bi0.15Sb0.85)2Te3 and PbTe are typical thermoelectric materials used for low and middle temperature range and functional graded materials (FGM) is an inevitable way to widen the working temperature range. Here two segments graded thermoelectric materials (GTM) consisting of (Bi0.15Sb0.85)2Te3, PbTe and different barriers were fabricated by the common hot pressure method. Metals Fe, Mg and Ni were used as barriers between the two segments. The diffusion of different barriers between the barriers and bases were analyzed by electron microprobe analysis (EMA). The phase and crystal structures were determined by X-ray diffraction analysis (XRD). The thermoelectric properties were measured at 303 K along the direction parallel to the pressing direction. The results show that the compositional diffusion occurs when there is no barrier at the interface of the two segments, and the diffusion of Pb is most obvious; as the barrier material, the diffusion of metals Fe, Mg and Ni between different bases is not very obvious, and the thermoelectric properties of GTM is much better than that of the original segment.     

Thermoelectric properties of p—type Bi—Sb—Te Compositionally Graded thermodelectric materials with different barriers

In order to find more suitable materials as barriers and to improve the thermoelectric properties,p-type(Bi1-xSbx)2Te3(x=0.85,0.9) two segments compositionally graded thermoelectric materials(CGTM) with different barriers were fabricated by conventional hot pressure method.Metals Fe,Co,Cu and Al were used as barriers between two segments.The effects of different barriers on thermoelectric properties of CGTM were investigated.The results show that metal Fe is more stable and suitable as the barrier.     

Thermoelectric Properties of N-typer Bi_2Te_3-PbTe Graded Thermoelectric Materials with Different Barriers

In order to investigate the adaptability of thermoelectric materials system with different barriers to functional graded thermoelectric materials, n-type Bi2Te3 and PbTe two segments graded thermoelectric materials (GTM) with different barriers were fabricated by conventional hot pressing method. Metals Cu, Al, Fe, Co and Ni were used as barriers between two segments. The effects of different barriers on thermoelectric properties of GTM were investigated. The phase and crystal structures were determined by x-ray diffraction analysis (XRD). The distributions of different compositions were analyzed by electron microprobe analysis (EMA). The thermoelectric properties were measured at 303 K along the direction parallel to the pressing direction. The electric conductivity of samples was measured at 303 K by the four-probe technique. To measure the Seebeck coefficient, heat was applied to the samples, which were placed between two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature differences (△T< 275 K) between the both ends of the samples. The Seebeck coefficient of the samples was determined from the E/△T.     

Thermoelectric properties of p-type Bi-Sb-Te compositionally graded thermoelectric materials with different barriers

In order to find more suitable materials as barriers and to improve the thermoelectric properties, p-type (Bi_(l-x)Sb_x)_2 Te_3 (x = 0.85, 0.9) two segments compositionally graded thermoelectric materials (CGTM) with different barriers were fabricated by conventional hot pressure method. Metals Fe, Co, Cu and Al were used as barriers between two segments. The effects of different barriers on thermoelectric properties of CGTM were investigated. The results show that metal Fe is more stable and suitable as the barrier.     

The Quantum Chemistry Calculation and Thermoelectrics of Bi-Sb-Te Series

The density function theory and discrete variation method ( DFT- DVM ) was used to study correlation between composition, structure, chemical bond, and property of thermoelectrics of Bi-Sb-Te series. 8 models of Bi2o-xSbxTe32( x = 0,2,6,8,12, 14,18 and 20) were ca/zulated. The resu/ts show that there is less difference in the ionic bonds between Te(I)-Bi( Sb ) and Te(Ⅱ)-Bi( Sb ) , but the covalent bond of Te( I )-Bi( Sb ) is stronger than that of Te(Ⅱ)-Bi( Sb ). The interaction between Te( I ) and Te( I ) in different layers is the weakest and the interaction should be Van Der Waals power. The charge of Sb is lower than that of Bi , and the ionic bond of Te- Sb is weaker than that of Te- Bi . 7he covalent bond of Te- Sb is also weaker than that of Te-Bi. Therefore, the thermoelectric property may be improved by adjusting the electrical conduaidty and thermal conductivity through changing the composition in the compounds of Bi-Sb- Te. The calculated results are consistent with the experiments.     

Numerical simulation on heat transfer performance of vertical U-tube with different borehole fill materials

Heat exchange performance of vertical U-tube heat exchanger was studied with two different borehole fill materials and CFD software. Borehole surface temperature and water temperature distribution were simulated on the condition of continuous operation for 8 h in winter with inlet water temperature being 10 ℃. The results show that there is no obvious difference on heat exchanger performance between the two different borehole fill materials.     

Wind tunnel test on aerodynamic effect of wind barriers on train-bridge system

To investigate the aerodynamic effect of wind barriers on a high-speed train-bridge system,a sectional model test was conducted in a closed-circuit-type wind tunnel.Several different cases,including with and without barriers,with different barrier heights and porosity rates,and with different train arrangements on the bridge were taken into consideration;in addition,the aerodynamic coefficients of the train-bridge system were measured.It is found that the side force and rolling moment coefficients of the vehicle are efficiently reduced by a single-side wind barrier,but for the bridge deck these values are increased.The height and porosity rate of the barrier are two important factors that influence the windbreak effect.Train arrangement on the bridge will considerably influence the aerodynamic properties of the train-bridge system.The side force and rolling moment coefficients of the vehicle at the windward side are larger than at the leeward side.     

Effect of relative density on cyclic oxidation resistance properties of MoSi2

MoSi2 powders were fabricated respectively by mechanical alloying technique and sintering at different temperatures to prepare materials with different relative densities. The relative oxidation behavior of all MoSi2 materials at 1 473 K was investigated by TGA,SEM and XRD. The results show that the ＂pesting＂ is not found in all materials after being oxidized for 480 h. The density has no essential relation to the ＂pesting＂. The oxidation curve of specimens with lower density shows two-step oxidation kinetics. Both the first stage （0-1 h） and the second stage （1-480 h） nearly obey linear kinetics,but the oxidation rates are obviously different. The oxidation kinetics of MoSi2 with higher relative density nearly follows parabolic law. The mass gains of MoSi2 with the lowest relative density （78.6%） and the highest relative density （94.8%） are increased by 10.390 and 0.135 mg/cm^2,respectively. The oxide scale of materials with lower densities is non-protective and makes the oxygen diffusion easy. A dense scale in the material with higher density is formed,which acts as a diffusion barrier to the oxygen atoms to penetrate into the matrix,showing much better high temperature oxidation resistance. The phases distribution of oxidation scale from the outside to the inside is SiO2→Mo5Si3→MoSi2.     

Design and optimization of W/Cu divertor mock-ups

Tungsten is a promising candidate for plasma-facing materials to cover the surface of the divertor plate in the design of an international thermonuclear experimental reactor （ITER）. Copper as a heat sink material serves to transfer heat excellently. Divertor mock-ups with W/Cu graded interlayers were designed to reduce thermal stresses. Thermally induced stresses and temperature in a W/Cu divertor mock-up were analyzed using the finite element method. The graded structures with different exponents p and thick- nesses were designed and discussed. The conclusions drawn from these analyses are that thermal stresses reach the minimum and the temperature is suitable when exponent p is 1.5 and the thickness of five graded interlayers is 5 mm.     

Thermoelectric properties of BiSb_x (x=0.6-0.8) thermoelectric materials fabricated by different processing

In order to improve the thermoelectric properties, hot-pressing sintering and ultra high pressure sintering methods were adopted to fabricate BiSb.,. The phase and crystal structures were determined by X-ray diffraction analysis (XRD). The thermoelectric properties were measured at 303 K along the direction parallel to the pressing direction. The electric conductivity of the samples was measured at 303 K by the four-probe technique. To measure the Seebeck coefficient, heat was applied to the samples placed between two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature differences ( △T < 2℃) between the both ends of the samples. The Seebeck coefficient of the samples was determined from the value of E/△T. The results indicate that the thermoelectric properties of the samples fabricated by UHPS (ultra high pressure sintering) method are much higher than that by HPS (hot pressing sintering) method and have the highest values at x=0.7.     

Fabrication of metal/metal functionally graded materials with a high melting point difference

Three kinds of full compositional distribution （from 0 to 100wt%W） W/Cu FGMs （functionally graded materials） with high density is fabricated by resistance sintering under ultra-high pressure. Microstructure analysis showed that the good grading composition of all FGMs has been obtained. The sintering mechanism of W is mainly solid state sintering. Thermal shock test in air demonstrated that the grading at the interface between W and Cu is effective for the reduction of thermal stress, but obvious transver- se and vertical cracks occur in the pure W layer. The oxidation of the W60Cu40 layer and the W40Cu60 layer is heavier than that of the other layers.     

Cyclic oxidation behaviors of MoSi<Subscript>2</Subscript> with different relative density

The influence of different relative density on the cyclic oxidation behaviors of MoSi2 at 1 273 K were studied. ＂Pesting＂ was not found in all MoSi2 materials after being oxidized for 480 h. All samples exhibited continuous mass gain during the oxidation process. The mass gains of MoSi2 with the lowest relative density （78.6%） and the highest relative density （94.8%） are increased by 8.15 mg·cm^-2 and 3.48 mg·cm^-2, respectively. The surface of the material with lower relative density formed a loose, porous and discontinuous oxidation scale, which accelerated oxygen diffusion and aggravated the oxidation process. However, a dense scale in the material with higher relative density is formed, which acts a diffusion barrier to the oxygen atoms penetrating into the matrix. The high temperature oxidation resistance of MoSi2 can be improved by increasing its relative density.     

Elements inter—diffusion in the turning of wear—resistance aluminum bronze

Inter-diffusion of elements between the tool and the workpiece during the turning of aluminum bronze using high-speed steel and cemented carbide tools have been studied.The tool wear samples were prepared by using M2 high-speed steel and YW1 cemented carbide tools to turn a novel high strength,wear-resistance aluminum bronze without coolant and lubriant,Adhesion of workpiece materials was found on all tools‘ surface.The diffusion couples made of tool materials and alumium bronze were prepared to simulate the inter-diffusionn during the machining,The results obtained from tool wear samples were compared with those obtained from diffusion couples.Strong inter-diffusion between the tool materials and the aluminum bronze was observed in all samples.It is concluded that diffusion plays a significant role in the tool wear mechanism.     

Effect of temperature on microstructure ofmolybdenum diffusion coating on titanium substrate

The halide-activated pack cementation process was used to form molybdenum diffusion coating on titanium substrate. The morphology, structure, elements diffusion distribution and microhardness of the coatings formed at different diffusion temperatures were studied. The results indicate that the coating is made up of deposition layer and diffusion layer, and the surface roughness of specimens is increased after diffusion. In the diffusion layer, the major phases are Mo and β-Ti phase with addition of α′-Ti phase and α″-Ti phase. And the phase composition of Mo →β→α″→α′ is formed for different Mo contents in the diffusion layer from outside to inside. The diffusion of Ti element is very obvious as well as Mo element. With increasing the diffusion temperature, the thickness of diffusion layer is increased rapidly, and the microhardness is changed more smoothly with diffusion depth, which shows the same distribution rules as the Mo content.     

Influence of stress path change on the resistance to plastic deformation of cold rolled sheets

Flat workpieces have been tested in order to investigate the influence of stress path change （loading mode） while keeping strain path unchanged. These investigations are pertinent to the testing of cold rolled strips and to subsequent forming. The workpieces which first compressed by plane strain compression in thickness direction were then tested in perpendicular direction in order to measure the influence of strain and stress path. The tension workpieces came from flat die compression test at different deformation histories. Two different materials were investigated： 18/8 Ti stainless steel and AW-1050 aluminium. The results show that the plastic flow by tension in lengthwise direction after pre-strain by compression in thickness direction will begin at an appreciably lower stress than that of the workpieces unloaded after pre-compression. Comparing with two materials, it can be seen that both 18/8 Ti stainless steel and AW-1050 aluminium behave similarly. The drop in yield stress is lower for AW-1050 aluminium than that for 18/8 Ti stainless steel. However, reloading in different directions than in the precious step results in significantly higher strain hardening.     

Electronic Structures and Chemical Bonds of Cobaltite and Ni-Doped

The relation among electronic structure, chemical bond and thermoelectric property of Ca3 Co2 O6 and Ni-doped was studied by density function theory and discrete variation method（DFT-DVM）. The results indicate that the highest valence band（ HVB ）attd the lowest conduction band（ LCB ） are mainly attribuled to Co3d, Ni3d and O2p atomic orbitals. The property of a semiconductor is shown from the gap between HVB and LCB. The gap of Ni-doped one is less than that of Ca3 Co2 O6. The non-metal bond or ceramic characteristic of Ni-doped one is weaker than that of Ca3 Co2 O6, but the metal characteristics of Ni-doped one are stronger than those of Ca3 Co2 O6. The thermoelectric property should be improved by adding Ni element into the system of Ca3 Co2 O6 .     

Surface resistivity of carbonaceous fiber/PTFE antistatic coatings

Abstract： PAN （Polyacrylonitrile）-based carbonaceous fibers were prepared at the heat treatment temperature （HTT） range of 650 to 900 ℃. The relationships among HTT, carbon content and volume resistivity of the carbonaceous fibers were investigated. The carbonaceous fibers/PTFE （Polytetrafluoroethylene） antistatic coatings were prepared by the spraying technology and the effects of carbonaceous fibers and pigments on surface resistivity of the coatings were systematically discussed. Micrographs provide insight into the antistatic mechanism of the coating. The results show that carbon content of the carbonaceous fibers increases from 68.8% to 74.8% （mass fraction） and the volume resistivity decreases drastically from 1.94× 10^-3 to 8.27× 10 ^-2.cm. The surface resistivity of the antistatic coating is adjustable between 10^5 and 10^8Ω2 to fit the different antistatic materials. Static is dissipated by a conductive network of short fibers and the tunneling effect between the neighboring fibers and conductive pigments. Conductive pigments make the conductive network more perfect and improve the antistatic ability, but insulating pigments acting as barriers for the formation of conductive channel increases the surface resistivity of the coatings. The influence of pigments on the surface resistivity drops gradually with the decrease of the carbonaceous fibers volume resistivity.     

Effect of plastic deformation on diffusion-rolling bonding of steel sandwich plates

Diffusion bonding is one of the most important techniques for composite materials, while bonding temperature, holding time, and rolling reduction are the key parameters that affect the bonding strength of sandwich plates. To study the effect of plastic deformation on the bonding strength, laboratory experiments were carried on a Gleeble Thermal Simulator to imitate the diffusion-rolling bonding under different reductions for steel sandwich plates. The bonding strength and interlayer film thickness were measured, and the element diffusion was analyzed using line scanning. The relationship between the bonding strength and ＂diffused interlayer＂ thickness was investigated. It has been found that the bonding strength increases with reduction, whereas the interlayer film thickness decreases gradually as the reduction increases. The diffusion under plastic deformation is obviously enhanced in comparison with that of nil reduction. The mechanism of plastic deformation effect on the diffusion bonding and related models have been discussed.     

Effects of cryogenic treatment on mechanical properties and microstructure of Fe-Cr-Mo-Ni-C-Co alloy

Fe-Cr-Mo-Ni-C-Co alloy was quenched in liquid nitrogen and held for 24 h. Hardness tester, OM, XRD, SEM were used to investigate the mechanical properties and microstructures of the alloy. The results show that the hardness increases by 1-2 （HRC） and the compressive strength decreases slightly after cryogenic treatment. The increase in hardness is attributed to the transformation from austenite to martensite and the precipitation of the very tiny carbide η-Fe2C. The decrease in compressive strength is caused by residual stress. The great amount of carbides, such as Cr7C3 and Fe2MoC, in the alloy and the obvious difference in thermal expansion coefficient between these carbides and the matrix at the cryogenic temperatures lead to this residual stress. The microscopy of cryogenic martensite is different from that of the non-cryogenic martensite. The cryogenic martensite is long and fine; while the non-cryogenic martensite is short and coarse. There is obvious surface relief of the cryogenic martensite transformation. It is not orientational of this kind surface relief and the boundary of this surface relief is smooth and in a shape of butterfly. The surface relief in the non-cryogenic martensite is wide and arranged in parallel, and the boundary of surface relief is not smooth. These characteristics may imply different growth ways of the two kinds of martensite.     

Synthesis,characterization and biological activity in vitro of FeCrAl（f）/HA asymmetrical biological functionally gradient materials

FeCrAl（f）/HA biological functionally gradient materials（FGMs） were successfully fabricated by the hot pressing technique.Scanning electron microscope（SEM）,energy dispersive spectrometer（EDS） and bending strength test machine were utilized to characterize the microstructure,component,mechanical properties and the formation of the Ca-deficient apatite on the surface of these materials.The results indicate that an asymmetrical FeCrAl（f）/HA FGM,consolidating powders prepared by mixing HA with 3%–15%（volume fraction） is successfully prepared.Both of the matrix and FeCrAl fiber are integrated very tightly and bite into each other very deeply.And counter diffusion takes place to some extent in two phase interfaces.The elemental compositions of the FeCrAl（f）/HA FGM change progressively.Ca and P contents increase gradually with immersion time increasing,and thereafter approach equilibrium.The bone-like apatite layer forms on the materials surface,which possesses benign bioactivity,and the favorable biocompatibility can provide potential firm fixation between FeCrAl（f）/HA asymmetrical FGM implants and human bone.