Microstructure and properties of p-type （Bi0.25Sb0.75）2Te3 fabricated by spark plasma sintering

P-type thermoelectric material （Bi0.25Sb0.75）2Te3 was sintered by spark plasma sintering（SPS） process in the temperature range of 320-420℃. The microstructures of sintered materials were found to be well aligned, particularly when sintered at lower sintering temperatures. The electrical conductivity of the material became larger as the sintering temperature increased. The Seebeck coefficient showed a general decreasing tendency with an increase in sintering temperature. In terms of the power factor, the optimum sintering temperature was found to be 380 ℃ for a maximum value of around 2.6 mW/K.     

Enhancement of room-temperature magnetoresistance in La0.5Sm0.2Sr0.3MnO3/（Ag2O）x/2

La0.5Sm0.2Sr0.3MnO3/(Ag2O)x/2 (x = 0.00, 0.04, 0.08, 0.25, 0.30) samples were prepared by the solid-state reaction method, and their transport behaviors, transport mechanism, and magnetoresistance effect were studied through the measurement and fitting of ρ-T curves. The results show that the element Ag takes part in reaction when the doping amount is small. Ag is mainly distributed at the grain boundary of the host material and is in metallic state when the doping amount is relatively large; then the system becomes a two-phase composite. A small amount of Ag doping can apparently increase grain-boundary magnetoresistance induced by the spin-dependent scattering. The resistivity of the sample doped with 30 mol% Ag is one order of magnitude smaller than that of low-doped samples, and its magnetoresistance in the magnetic field of 0.5 T and at 300 K is strengthened apparently reaching 9.4%, which is connected not only with the improvement of the grain-boundary structure of the host material but also with the decrease of material resistivity.     

Temperature stability of magnetoresistance （MR） and MR enhancement in Lal-x（Srl-yAgy）xMn03 system.

Polycrystalline samples of La12x(Sr12yAgy)x MnO3 (y = 0.0, 0.2, 0.4, 0.6, 1.0) were prepared by the solid-state reaction method. The temperature stability of magnetoresistance and magnetoresistance enhancement in La1-x(Sr1-yAgy)x MnO 3 system with both univalent and bivalent elements doped at A site and with unchanged value of Mn3+ /Mn4+ ratio were explored through the measurements of X-ray diffraction patterns, magnetiza-tion-temperature (M-T) curves, resistivity-temperature (q-T) curves and magnetoresistance-temperature (MR-T) curves. The results are as follows: there are two peaks in the q-T curves of the samples with Ag doping, one is caused by resistance change during the paramagnetism-ferromagnetism transition, and the other is from boundary-dependent scattering of conduction electrons on the boundaries of grains. The peak value of MR increases with increasing Ag doping content, and it increases from 8.2 % for y = 0.2 to 29.6 % for y = 1.0 under the magnetic field of B = 0.8 T; MR remains a constant of 12 % in the temperature range of 218-168 K for the sample with y = 1.0, and the temperature stability of MR is in favor of the practical application of MR.     

Magnetoresistance enhancement and temperature stability of magnetoresistance in La1-x(Sr1-yNay)xMnO3

A series of the samples La1-x(Sr1-yNay)xMnO3(y=0.0,0.2,0.4,0.6,0.8,1.0)were prepared by the solid-state reaction method.Magnetoresistance enhancement and temperature stability of magnetoresistance in the system La1-x(Sr1-yNay)xMnO3 with unchanged Mn3+/Mn4+ ratio through the doping of both monovalent and divalent elements at A site were studied through the measurements of X-ray diffraction(XRD)patterns,resistivity-temperature(p-T)curves and magnetoresistance-temperature(MR-T)curves.The results indicate that with the increase of Na doping amount,the peak value of MR increases,and it increases from 12.4％ for y=0.2 to 50.6％ for y=1.0 in the magnetic field B=0.8 T;p-T curves exhibit the double-peak phenomenon,which comes from the competition between the resistivity of surface phase and that of body phase; for the sample of y=0.8,MR increases slowly from 8.3％ to 9.4％ in the temperature range from 259 to 179 K,and MR is so stable in such a wide temperature range,which provides reference for the research on the temperature stability of MR.     

Research Progress on AgSbTe2-based Thermoelectric Materials

Thermoelectric power generation represents a class of energy conversion technology,which has been used in power supply of aeronautic and astronautic exploring missions,now showing notable advantages to harvest the widely distributed waste heat and convert the abundant solar energy into electricity at lower cost than Si based photovoltaic technology.Thermoelectric dimensionless figure of merit ZT plays a key role in the conversion efficiency from thermal to electrical energy.Low thermal conductivity and large Seebeck coefficient make the AgSbTe 2 compound a very promising candidate for high efficiency p-type thermoelectric applications.The AgSbTe2-based thermoelectric system has been repeatedly studied as prospective thermoelectric materials.In this review,we firstly clarify some fundamental tradeoffs dictating the ZT value through the relationship ZT =S 2 σT/κ.We also pay special attentions to the recent advances in AgSbTe2 based thermoelectric materials.Finally,we provide an outlook of new directions in this filed.     

Elastic properties of double layered manganites R_（1.2）Sr_（1.8）Mn_2O_7（R=La,Pr, Nd,Sm）

The polycrystalline colossal magnetoresistive double-layered manganite samples R1.2Sr1.8Mn2O7(R = La Pr, Nd, Sm) were prepared by the sol–gel method and their room temperature elastic behavior was investigated by ultrasonic pulse transmission technique at 1 MHz. The values of elastic constants were calculated from longitudinal and shear velocities and they were corrected to zero porosity using Hasselman and Fulrath's formulae. The elastic constants of the samples were also estimated by Modi's heterogeneous metal-mixture rule which is based on the metal ions present in the samples. The measured,corrected, and estimated values of elastic moduli are found to increase with decreasing rare earth ion size. The variation of elastic moduli with the size of the rare earth ion is interpreted in terms of strength of interatomic bonding.     

Low-field magnetoresistance of (1-x)La_(0.6)Dy_(0.1)Sr_(0.3)MnO_3/0.5x (Sb_2O_3) composite system under different sintering temperatures of matrix

A series of (1-x)La0.6Dy0.1Sr0.3MnO3/0.5x(Sb2O3)(x=0.15) samples were prepared by the solid-state reaction method, and the influence of sintering temperature of the matrix on low-field magnetoresistance of (1-x)La0.6Dy0.1Sr0.3MnO3/0.5x (Sb2O3) was studied through the measurements of X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) image, resistivity-temperature (ρ-T) curves, and magnetoresistance-temperature (MR-T) curves. The results indicate that for the samples with low sintering temp...     

Effects of RE^3＋（RE=La,Ce,Pr,Sm,Dy,Ho,Er,and Tm） on the luminescence of Sr2MgSiO5：Eu^2＋ phosphors

Sr2MgSiO5:Eu2+phosphors were synthesized through a high temperature solid state reaction method.The phase and luminescence properties of the samples were characterized by X-ray diffraction(XRD) and a luminescence spectrometer.The effects of RE3+(RE = La,Ce,Pr,Sm,Dy,Ho,Er,and Tm) on the luminescent properties of the samples were studied.The results show that the luminescence spectrum of Sr2MgSiO5:Eu2+ is a broad band composed of two emission peaks,at 460 and at 530 nm under near-ultraviolet excitation.Rare earth ions Re3+(RE = La,Ce,Pr,Sm,Dy,Ho,Er,and Tm) have great influence on the emission of Sr2MgSiO5:Eu2+ phosphors.The La3+ ion in Sr2MgSiO5:Eu2+ phosphors greatly enhances the emission at 460 and 530 nm.     

Abnormal electric transport property and magnetoresistance stability of La–Sr–K–Mn–O system

The perovskite samples La1-x (Sr1-yKy )xMnO3 (y = 0.0, 0.2, 04, 0.6, 0.8) were prepared by the solid-state reaction method with comparatively low sintering temperature and with comparatively short sintering time, and the electric transport property and temperature stability of MR of this system were studied. The q-T curves show the abnormal phenomenon that with the increase of K doping amount, resistivity increases, and the insulator-metal transition temperature decreases, which is because the influence of the occupation disorder degree of A-site ions σ2 on the electric transport property of perovskite manganites is larger than that of the radius of A-site ions. In the temperature range below 225 K, MR increases continuously with the decrease of temperature, which is the characteristic of low-field magnetoresistance; in the comparatively wide temperature range near 250 K, the MR-T curves of all the samples are comparatively flat, and the value of MR almost does not change with temperature, which shows the temperature stability of magnetoresistance, and can be explained by the competition between the low-field magnetoresistance induced by spin-dependent tunneling of surface phase and the intrinsic magnetoresistance of grain phase. The magnetoresistance value of the sample with y = 0.8 keeps at (7.92 ± 0.36) % in the very wide temperature range of 225-275 K, and this is a good reference for the preparation of this kind of sample with practical application value in the future.     

Microstructure and thermoelectric properties of Bi_(0.5)Na_(0.02)Sb_(1.48-x)In_xTe_3 alloys fabricated by vacuum melting and hot pressing

The Bi_(0.5)Na_(0.02)Sb_(1.48-x)In_xTe_3 alloys(x =0.02-0.20) were synthesized by vacuum melting and hot pressing methods at 753 K,60 MPa for 30 min.Effects of Na and In dual partial substitutions for Sb on the thermoelectric properties were investigated from 300 to 500 K.Substituting Sb with Na and In can enhance the Seebeck coefficient effectively near room temperature.The electrical resistivity of the Na and In dual-doping samples is higher within the whole test temperature range.The Bi_(0.5)Na_(0.02)Sb_(1.48-x)In_xTe_3 samples(x = 0.02,0.06) play a great role in optimizing the thermal conductivity.As for the Bi_(0.5)Na_(0.02)Sb_(1.46)In_(0.02)Te_3 alloy,the minimum value of thermal conductivity reaches 0.53 W·m~(-1)·K~(-1) at 320 K.The thermoelectric performance of the Na and In dualdoped samples is greatly improved,and a figure of merit ZT of 1.26 is achieved at 300 K for the Bi_(0.5)Na_(0.02)Sb_(1.42)In_(0.06)Te_3,representing 26%enhancement with respect to ZT = 1.0 of the undoped sample.     

Structural Features of（Ce,La or Sr）（Mn or Co）O_3 NanoPerovskites as a Catalyst for Carbon Monoxide Oxidation

The collection of different cations in the A and B sites of ABO3 was explored for the regularity of perovskites phase formability.Here,Sr2？,La3？,and Ce4？are selected as the cations of site A.The site B is considered to be Mn or Co cations.XRD analysis and Fourier transform infrared spectroscopy results confirm the formation of perovskite structure for catalysts in which La3？and Sr2？are considered as the cations of site A.Ceria is detected as the main crystalline phase when Ce4？is selected to be cation of site A.It is found that the octahedral factor（rB/rO） takes the same important role as the tolerance factor to form cubic perovskite.Average crystallite size of the products was calculated by data of the XRD and measured by the TEM analysis.Results of the XRD and TEM studies were supported by the study of the particles size distribution,which was carried out in a particle size analyzer.The perovskite samples were also used for stoichiometric oxidation of carbon monoxide with air.     

Temperature-induced liquid state change and its effects on solidification of thermoelectric alloy Bi_（0.3）Sb_（1.7）Te_3

The behaviors of electrical resistivity vs temperature（ρ-T） of the molten p-type thermoelectric alloy Bi0.3Sb1.7Te3（at.%） were explored in heating and cooling processes. An obvious hump appeared on the ρ-T curve from 932 ℃ to 1,020 ℃ at the heating process, while the curve became smooth in the following cooling, which suggests an irreversible temperature-induced liquid-liquid structure transition（TI-LLST） occurred in the liquid alloy. Based on this judgment, solidification experiments were carried out to find out the effects of the different liquid states. It was verified that, for the melt experiencing the presumed TI-LLST, both the nucleation and growth undercooling degrees were elevated and the solidification time was remarkably prolonged. On the other hand, the configuration of Bi0.3Sb1.7Te3 phase was refined, and its preferential orientation was weakened.     

Rapid fabrication and thermoelectric performance of SnTe via non-equilibrium laser 3D printing

Thermoelectric technologies based on Seebeck and Peltier effects,as energy techniques able to directly convert heat into electricity and vice versa, hold promise for addressing the global energy and environmental problems. The development of efficient and low-cost thermoelectric modules is the key to their large-scale commercial applications. In this paper, using a non-equilibrium laser3 D printing technique, we focus an attention on the fabrication of mid-temperature p-type SnTe thermoelectric materials. The influence of laser power, scanning speed and layer thickness on the macro-defects, chemical and phase composition, micro structure and thermoelectric performance was systematically investigated. First and foremost,the processing parameter window for printing a highquality layer is determined. This is followed by the finite element method used to simulate and verify the influence of the laser-induced molten pool temperature distribution on the final composition and microstructure. Finally, the high-performance SnTe layer with 10 mm × 10 mm in area is produced within seconds with room temperature Seebeck coefficient close to that of SnTe manufactured by the traditional methods. Consequently,this work lays a solid foundation for the future fabrication of thermoelectric modules using laser non-equilibrium printing techniques.     

Electric transport properties and temperature stability of magnetoresistance of composite system between La8/9Sr1/45Na4/45MnO3 and Sb2O3

The samples ofLa8/9Sr1/45Na4/45MnO3 （LSNMO）/x/2（Sb2O3） were prepared by the solid-state reaction method. The electric transport properties and the temperature stabil-ity of magnetoresistance （MR） of the samples were studied through the measurements of X-ray diffraction patterns, resistivity-temperature （ρ-T） curves, mass magnetization-temperature （σ-T） curves, and magnetoresistance-temper-ature （MR-T） curves. The results indicate that the p-Tcurves of the original material LSNMO show two peaks, and the phenomenon of two peaks of ρ-T curves disappears for the composite samples, which can be explained by a competition between surface-phase resistivity induced by boundary-dependent scattering and body-phase resistivity induced by paramagnetism-ferromagnetism transition. For all the sam-ples in the low temperature range, MR increases continu-ously with the decrease of temperature, which shows a characteristic of low-field magnetoresistance. However, MR basically keeps the same in the high temperature range. The paramagnetism-ferromagnetism transition is observed in the high temperature range due to a composite between perov-skite manganite and insulator, which can enhance the tem-perature of MR appearance in the high temperature range and make it to appear near room temperature. For the sample with x = 0.12, MR remains constant at the value of 7.5 % in the temperature range of 300-260 K, which achieves a tem-perature stability of MR near room temperature. In addition,for the sample with x = 0.16, MR is above 6.8 % in the high temperature range of 318-252 K （△T = 66 K）. MR almost remains constant in this temperature range, which favors the practical application of MR.     

Enhanced thermoelectric property and stability of NiCr–NiSi thin film thermocouple on superalloy substrate

Thin film thermocouples(TFTCs) can provide fast and accurate surface temperature measurement with minimal impact on the physical characteristics of the supporting components. In this study, NiCr and NiSi films were prepared with radio frequency(RF) magnetron sputtering and the influences of vacuum annealing on the resistivity of the films were investigated. Afterward, NiCr–NiSi films were deposited on Ni-based superalloy substrates to form TFTCs. The overall dimension of the thermocouple is 64 mm in length, 8 mm in width and 30 lm in thickness. Compared with those of as-deposited sample, the thermoelectric property and stability of the TFTC are significantly improved by vacuum annealing of NiCr and NiSi films. The variation of the Seebeck coefficient of TFTC was discussed based on the size effect of NiCr and NiSi films. And a lower Seebeck coefficient of TFTC of 38.4 μV·℃~(-1) is obtained.     

Microwave synthesis of La_（1-x）Sr_xCo_（1-y）Fe_yO_3 cathode materials for solid oxide fuel cells

La1-xSrxCo1-yFeyO3 cathode materials for solid oxide fuel cells were prepared by the microwave synthesis method.The reaction mechanism was studied using a thermal analysis method.The influences of microwave synthesis conditions were examined and the characteristics of the samples were determined by X-ray diffraction and scanning electron microscopy.It was found that the optimum conditions were the microwave power of 800 W and the reaction time of 40 min.Samples with perosvkite-type crystal structure were obtained.The samples consist of uniformly distributed internal particles and the particle size is less than 1 μm.     

Electrochemical behaviour of commercially pure titanium and Co—Cr alloy in Ringer‘s solution

The electrochemical behaviors of commercially pure titanium(CP Ti) and Co-Cr alloy in Ringer‘s solution have been investigated.The results indicate that the electric potential of passive region for CP Ti is up to 3000mV,and its passive current density is 3.078 μA/cm^2,The excellent corrosion resistance of CP Ti can be attributed to the formation of TiO2 oxide film.The passive region of Co-Cr alloy is 770mV,which is narrower than that of CP Ti.However,no hystersis loops are found in the reverse scanning curves of Cu-Cr alloy.A complex oxide film of Co3O4,Co2O3,and Cr2O3 formed on the surface provides Co-Cr alloy with a stable electrochemistry property.The corrosion rates of the crevice samples increase with the pH value of medium decreasing.The electron probe microanalyzer(EPMA) analysis indicates that Ti in CP Ti and Co,Cr in Co-Cr alloy dissolve in crevice area due to the Sealed-Cell effect.     

Electric transport mechanism and magnetoresistance of La_(0.80)Sr_(0.15)Ag_(0.05)MnO_3/x(CuO)

The samples of La0.80Sr0.15Ag0.05 Mn O3/x(Cu O)(x = 0, 0.05, 0.10, 0.15, 0.20) were prepared by the solid-state reaction method, and the structure of the samples was detected by X-ray diffraction(XRD), scanning electron microscope(SEM) equipped with energy dispersive spectroscopy(EDS), electric transport mechanism, and magnetoresistance enhancement, and the temperature stability of magnetoresistance of the samples was studied through resistivity–temperature(q-T) curves, q-T fitted curves, and magnetoresistance–temperature(MR-T) curves.The results indicate that q-T data can be fitted by the formula q = q0 AT2 very well, and the electric transport mechanism of all the samples in metal-like area is the scattering of single magneton upon spin electron; the magnetoresistance of composite samples is far larger than that of the original material, and the MR peak value of the sample with x = 0.20 is nearly 4 times as large as that of the sample with x = 0; composite samples have comparatively good temperature stability of magnetoresistance in the temperature range of 200–260 K, and the magnetoresistance of the sample with x = 0.15 almost does not change with temperature and keeps at(5.03 ± 0.20) % in the temperature range of 210–260 K.     

La2O3-Gd2O3-Mo secondary emission material

A new kind of materials La2O3-Gd2O3-Mo has been produced by powder metallurgy method.The composition and microstructure of the material were studied by XRD and SEM.It shows that no chemical reaction takes place among La2O3,Gd2O3,Mo and the rare earth oxides exist along molybdenum grain boundaries and in the pores.The emission property measurement results of this material show that adding rare earth oxide into molybdenum can improve the secondary emission coefficient of the emitter,and the emission property depends on the activation temperature.After La2O3-Gd2O3-Mo was activated at 1360℃,the maximum secondary emission coefficient can be high to 2.62,which has exceeded that for practical uses(2.0).     

Preparation and microwave dielectric properties of Csf/Si3N4 composites

The CJSi3N4 composites were prepared by hot-press sintering method using α-Si3N4 power, short carbon fibers and La2O3-Y2O3 sintering additives. The mechanical and microwave dielectric properties of CjSi3N4 composites were studied and discussed. The results show that the addition of the short carbon fibers can not destroy the relative density of the sintered samples, but it deteriorates the flexural strength of the sintered samples, so the flexural strength of the silicon nitride matrix is the highest among the samples. The real part （ε3 and the imaginary part （ε＇3 of the permittivity of CsfCSi3N4 composites greatly increase with increasing voltmae fraction of the short carbon fibers, achieve the maximum 73.1 and 101.5, respectively. A strong frequency dependence of the imaginary part （ε″） of the permittivity is observed.