Electrical transport properties of YCo_(1-x)Mn_xO_3(0 ≤x≤ 0.2)prepared by sol–gel process

Mn substitution compounds YCo1-xMnx O3(0 ≤ x ≤ 0.2) are synthesized by using the sol–gel process. Electrical transport properties of YCo1-xMnx O3 are investigated in the temperature range from 200 K to 780 K. The experimental results show that after Mn substitution the electrical resistivity of YCo1-xMnx O3 first increases, then decreases, which is due to the electrons introduced by Mn doping. The sign of Seebeck coefficient for YCo1-xMnx O3(x = 0) is positive or negative, which is also proved by the Hall coefficient measurement. Moreover, at about room temperature, the Seebeck coefficient of YCo1-xMnx O3 with 1% doping Mn content becomes a negative value, whose absolute value is maximum;furthermore, the absolute value gradually decreases with increasing the Mn substitution content, which can be explained by the double carrier model.     

Cu Doping Effect on Electrical Resistivity and Seebeck Coefficient of Perovskite-Type LaFeO3 Ceramics

Perovskite-type LaFe_1-xCu_xO₃ (x=0.10$, 0.14, 0.18) solid solution is prepared with the conventional solid-state reaction technique. The electrical resistivity and the Seebeck coefficient are measured in the temperature range 473-1073K to elucidate the Cu doping effect on the thermoelectric properties of the LaFeO₃. The electrical resisitivity of LaFe_1-xCu_xO₃ shows semiconducting behavior. The temperature dependence of the electrical resistivity indicates that the adiabatic small-polaron hopping mechanism is dominant for their electric transportations. The activation energy decreases with the increasing Cu content as well as the increasing temperature. The Seebeck coefficient changes from a negative value to a positive value around 510K, and increases with rising temperature up to 710K, then becomes saturated around 200μV/K. The Seebeck coefficient decreases with the substitution of Cu atoms in the temperature range of 573-1073K, while the electrical resistivity decreases with the substitution of Cu atoms in the whole measured temperature. Overall the power factor increases with rising temperature, and the highest value of power factor is 54μW/K²m for x=0.10 of Cu doping.     

Thermoelectric properties of Sr_(0.61)Ba_(0.39)Nb_2O_(6-δ) ceramics in different oxygen-reduction conditions

The thermoelectric properties of Sr0.61Ba0.39Nb2O6-δceramics, reduced in different conditions, are investigated in the temperature range from 323 K to 1073 K. The electrical transport behaviors of the samples are dominated by the thermal-activated polaron hopping in the low temperature range, the Fermi glass behavior in the middle temperature range,and the Anderson localized behavior in the high temperature range. The thermal conductivity presents a plateau at hightemperatures, indicating a glass-like thermal conduction behavior. Both the thermoelectric power factor and the thermal conductivity increase with the increase of the degree of oxygen-reduction. Taking these two factors into account, the oxygen-reduction can still contribute to promoting the thermoelectric figure of merit. The highest ZT value is obtained to be ～0.19 at 1073 K in the heaviest oxygen reduced sample.     

Metal--Insulator Transition in Ca-Doped Sr14-xCaxCu24O41 Systems Probed by Thermopower Measurements

High quality Sr14-xCaxCu24O41 single-crystals are successfully grown by floating-zone technique, and the trans- port properties are studied. The temperature dependence of resistivity along the c-axis direction is semiconductor- like for x ≤ 10 and it can be fitted by the thermal activation equation p = po exp（ △ /kBT） with kB being the Boltzmann constant and A the activation energy. A break in the slope of thermopower （S） versus the inverse temperature （1 IT） corresponding to the formation of charge-density waves （CD W） is first observed for x ≤ 6. The temperature dependence of thermopower becomes metallic for x ≥ 8 while the resistivity is still semiconductorlike. We propose that the insulation behaviour of the resistivity in the Ca doping range 8 ≤ x ≤ 11 could result from the localization of the charge carriers due to the disorder induced by Ca doping and a revised electronic phase diagram is derived based on our observations.     

Tuning the Heavy Fermion State of CeFeGe_3 by Ru Doping

We successfully synthesize a series of polycrystalline CeRu_xFe_(1-x)Ce_3(0≤x≤0.5)samples,which are characterized using powder x-ray diffraction,resistivity and specific heat measurements.The expansion of the lattice constants with increasing x demonstrates the successful doping of Ru into the CeFeGe_3 lattice.Upon doping,it is found that the temperature up to which Landau-Fermi liquid behavior is observed in the resistivity is reduced.Meanwhile,there is also a pronounced increase in the resistivity coefficient and residual resistivity,as well as a clear upturn in C/T at low temperatures,suggesting that Ru doping may tune the system towards a quantum critical point.     

Analysis of Local Structures around Ni Atoms Doped in ZnO-Based Diluted Magnetic Semiconductors by Fluorescence EXAFS

Zn1-xNixO （x = 0.001, 0.01, 0.02, 0.05 and 0.20） powders are prepared by sol-gel method. An extended x-ray absorption fine structure technique （EXAFS） for the Ni K.edge is employed to probe the local structures around Ni atoms doped in ZnO powders by fluorescence mode. The near edge EXAFS of the samples does not change in the range of Ni concentration from x = 0.001 to 0.05, which is consistent with the results of x-ray diffraction of the samples. The simulation results for the first shell EXAFS signals indicated that Ni atoms are substituted in Zn sites.     

Effect of Nb doping on microstructures and thermoelectric properties of SrTiO_3 ceramics

Nb-doped SrTiO_3 thermoelectric ceramics with different niobium concentrations,sintering temperatures and Sr-site vacancies are successfully prepared by high energy ball milling combined with carbon burial sintering.For fully understanding the effect of niobium doping on SrTiO_3,thermoelectric transport properties are systematically investigated in a temperature range from 300 K to 1100 K.The carrier mobility can be significantly enhanced,and the electrical conductivity is quadrupled,when the sintering temperature rises from 1673 K to 1773 K(beyond the eutectic temperature(1713 K) of SrTiO_3–TiO_2).The lattice vibration can be suppressed by the lattice distortion introduced by the doped niobium atoms.However,Sr-site vacancies compensate for the lattice distortion and increase the lattice thermal conductivity more or less.Finally,we achieve a maximum value of figure-of-merit z T of 0.21 at 1100 K for Sr Ti_(0.9)Nb_(0.1)O_3 ceramic sintered at1773 K.     

Thermal Conductivity Anomalies Related to the Double-Bump of Resistivity in Nd0.7Sr0.3Mn1-xCrxO3

The temperature dependence of the thermal conductivity κ(T) and electrical resistivity ρ(T) has been measured for perovskite Nd0.7Sr0.3Mn1-xCrxO3 (0.01≤x≤0.15) in the temperature range of 10-300 K. The double-bump feature in p(T) is observed for x = 0.08 and 0.10 samples. Below the insulating-metallic transition temperature TIM, κ(T) shows a sharp increase and such an increase is depressed gradually by the increasing Cr content.While κ(T) displays an anomalous decrease around the temperature where the second bump of ρ(T) emerges.We suggest that the enhancement of κ(T) below TIM is related to both phonons and magnons, the ferromagnetic interaction of Mn^3 -O-Cr^3 should be super-exchange interaction rather than double-exchange interaction, and the Cr doping just weakens the DE interaction in this system.     

Improvement of Thermoelectric Performance in BiCuSeO Oxide by Ho Doping and Band Modulation

We try to use Ho doping combined with band modulation to adjust the thermoelectric properties for BiCuSeO.The results show that Ho doping can increase the carrier concentration and increase the electrical conductivity in the whole temperature range.Although Seebeck coefficient decreases due to the increase of carrier concentration,it still keeps relatively high values,especially in the middle and high temperature range.On this basis,the band-modulation sample can maintain relatively higher carrier concentration while maintaining relatively higher mobility,and further improve the electrical transporting performance.In addition,due to the introduction of a large number of interfaces in the band-modulation samples,the phonon scattering is enhanced effectively and the lattice thermal conductivity is reduced.Finally,the maximal power factor(PF) of 5.18μW·cm~(-1)K~(-2) and the dimensionless thermoelectric figure of merits(ZT) of 0.81 are obtained from the 10% Ho modulation doped sample at 873 K.     

Magneto-resistance and vortex phase diagram of BaNi_（0.1）Fe_（1.9）As_2 single crystal

The transition from vortex glass to a liquid phase is studied in Ba Ni0.1Fe1.9As2 single crystal with Tc = 19.4 K by magneto-resistance measurements. The resistivity curves are measured in magnetic fields in a range of 0 T–13 T for H‖c and H⊥c. Good scalings for all values of resistivity ρ（H, T） and the effective pinning potential U0（H, T） are obtained with the modified vortex glass theory by using the critical exponents s and H0. Phase diagrams for H‖c and H⊥c are determined based on the obtained vortex glass temperature Tg, the vortex dimensionality crossover temperature T＊, and the upper critical magnetic field Hc2. Our results suggest that both below and above 5 T, single vortex pinning co-exists with collective creep, and collective creep is dominant. There is a narrower vortex liquid region for H⊥c than for H‖c in the vortex phase diagram, which may originate from a stronger pinning force.     

Electron Transport Behavior of Multiferroic Perovskite BiMnO_3 Prepared by Co-Precipitation Method

Perovskite BiMnO_3 samples are successfully synthesized by the co-precipitation method at relatively low pressure and moderate temperature.The temperature dependences of resistivity are measured and systematically investigated.It is shown that the electrical resistivity increases sharply with the decrease of temperature above 210 K and the fitted results demonstrate that the thermally activated conduction model is the dominant conduction mechanism for the electron transport behaviors in this temperature region.A dual conducting mechanism,i.e.,the variable range hopping and thermal activated conduction,is suggested to be responsible for the transport behaviors of BiMnO_3 in the region of 180–200 K.Moreover,the resistivity increases slightly with the decrease of temperature below 180 K and the transport is governed by the variable range hopping mechanism.     

Preparation and the physical properties of antiperovskite-type compounds Cd1-xInxNNi3 (0≤x≤0.2) and Cd1-yCuyNNi3 (0≤y≤0.2)

Two series of Cd1-xInx NNi3(0 ≤x≤ 0.2) and Cd1-yCuyNNi3(0≤y≤0.2) samples were prepared from CdO, In2O3 , CuO, and nickel powders under NH3 atmosphere at 773K. The structural and physical properties were investigated by means of X-ray powder diffraction temperature-dependent resistivity and magnetic measurements. X-ray powder diffraction results showed that the Cd 1 x In x NNi 3 and Cd 1 y Cuy NNi 3 compounds have a typical antiperovskite structure, and the CdNNi3, Cd0.9 In 0.1 NNi3 , and Cd0.9Cu0.1NNi3 compounds show metallic temperature-dependent resistivity and exhibit a Fermi liquid behavior at low temperature. In contrast to the paramagnetism previously reported, the CdNNi 3 sample exhibits very soft and weak ferromagnetism, and no superconductivity was found in the Cd 1 x In x NNi 3 and Cd 1 y Cu y NNi 3 samples down to 2 K. Each sample exhibited very soft and weak ferromagnetism, and the temperature dependence of the magnetization of the Cd 1-xInx NNi 3 and Cd1-y Cu y NNi 3 samples can be well fitted to the combination of a Bloch term and a Curie–Weiss term.     

Low temperature properties of pnictide CrAs single crystal

High quality single crystal CrAs was grown by Sn flux method.The results of magnetic susceptibility and electrical resistivity are reported in a temperature range of 2 to 800 K.At low temperatures,a T2 dependence of resistivity is observed showing a Fermi-liquid behavior.The Kadowaki-Woods ratio is found to be 1×10-5 μΩ cm mol2 K2 mJ-2,which fits well to the universal value for many correlated electron systems.At about 270 K,a clear magnetic transition is observed with sharp changes of resistivity and susceptibility.Above 270 K,a linear-temperature dependence of the magnetic susceptibility is observed up to 700 K,which resembles the T-dependent magnetic susceptibility of parents of iron-pnictides superconductors.     

Effects of Ni doping on the structural properties and collapse of magnetic ordering in NdFe_(1-x)Ni_xO_3(0.1 ≤x≤ 0.7) orthoferrites

Nd Fe_(1-x)Ni_xO_3(0.1 ≤ x ≤ 0.7) orthoferrites are synthesized by solid state reaction method, and the structural properties of these materials are investigated by employing x-ray diffraction(XRD), scanning electron microscopy(SEM) and M o¨ssbauer spectroscopy. The orthorhombic structure is observed in all systems; however, with the increase in Ni doping,the increase in tolerance factor and the decrease in the cell volume are observed. Orthorhombic distortion decreases with Ni content increasing up to 50%, while above 50% Ni doping it increases. SEM examination indicates the increases in grain size and intermixing of grains with increase in Ni concentration. Comparison between bulk and theoretical densities shows that in each of all samples porosity is less than 2%. M?ssbauer spectroscopic investigations are performed to explain local structure, Fe oxidation states and collapse of the magnetic ordering. In these samples the Fe oxidation state remains+3 and there is no considerable increase in hole states observed; however due to mismatch of the ionic radii between Fe~(3+) and Ni~(3+), octahedral distortions, sagging and distribution of hyperfine parameters increase with increase in Ni concentration. The major factors behind the collapse of magnetic ordering in the Ni-doped systems are the weakening of the super-exchange interactions, decrease in the Neel temperature, increase in spin–spin relaxation frequency and high spin to low spin transition.     

Improved thermoelectric property of cation-substituted CaMnO_3

Single-phase pristine and cation-substituted calcium manganite(Ca1-xBixMn1-yVyO3-δ) polycrystalline samples were synthesized by the solid state reaction technique. Their thermoelectric properties were measured by a set up that was designed and assembled in the laboratory. The Ca1-x BixMn1-yVyO3-δsample with x = y = 0.04 has shown a power factor(S2σ) of 176 μW/m/K2 at 423 K, which is nearly two orders of magnitude higher than that of the pristine sample(2.1 μW/m/K2). The power factor of the substituted oxide remains almost temperature independent as the Seebeck coefficient increases monotonically with temperature, along with the simultaneous decrease in electrical resistivity which is attributed to enhanced electron density due to co-doping of bismuth and vanadium and grain boundary scattering. These cation-substituted calcium manganites can be used as a potential candidate for an n-type leg in a thermoelectric generator(module).     

Phase transition and high temperature thermoelectric properties of copper selenide Cu2-x Se （0 ≤ x ≤ 0.25）

With good electrical properties and an inherently complex crystal structure, Cu_2-xSe is a potential “phonon glass electron crystal” thermoelectric material that has previously not attracted much interest. In this study, Cu_2-xSe (0 ≤ x ≤ 0.25) compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu_2-xSe were investigated in the temperature range of 300 K—750 K. The results of X-ray diffraction at room temperature show that Cu_2-xSe compounds possess a cubic structure with a space group of Fm3m (#225) when 0.15 < x le 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 ≤ x ≤ 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu₂Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.     

Large reversible magnetocaloric effect induced by metamagnetic transition in antiferromagnetic HoNiGa compound

The magnetic properties and magnetocaloric effects(MCE) of Ho Ni Ga compound are investigated systematically.The Ho Ni Ga exhibits a weak antiferromagnetic(AFM) ground state below the Neel temperature TNof 10 K, and the AFM ordering could be converted into ferromagnetic(FM) ordering by external magnetic field. Moreover, the field-induced FM phase exhibits a high saturation magnetic moment and a large change of magnetization around the transition temperature,which then result in a large MCE. A large-?S_M of 22.0 J/kg K and a high RC value of 279 J/kg without magnetic hysteresis are obtained for a magnetic field change of 5 T, which are comparable to or even larger than those of some other magnetic refrigerant materials in the same temperature range. Besides, the μ_0H~(2/3)dependence of |?S_M~(pk)| well follows the linear fitting according to the mean-field approximation, suggesting the nature of second-order FM–PM magnetic transition under high magnetic fields. The large reversible MCE induced by metamagnetic transition suggests that Ho Ni Ga compound could be a promising material for magnetic refrigeration in low temperature range.     

Magnetic Properties and Magnetoresistance Effect of YMn6 Sn6—x Crx （x=0—0.8） Compounds

The magnetic properties and magnetoresistance effect of YMn6Sn6-x Crx(x=0-0.8) compounds have been experimentally studied by magnetic properties and resistivity measurements in the applied field range 0-5T.The compound (x=0.8) displays a ferromagnetic behaviour,while the compounds (x=0-0.4) display an antiferromagnetic behaviour in the whole ordering temperature range.The compounds(x=0.5,0.6) experienced a transition from an antiferromagnetic state to a ferromagnetic state with increasing temperature.The compound with x=0.8 is rapidly saturated in the lower magnetic field with saturation magnetization of 35.92emu/g.The compounds(x=0-0.6) display a field-induced metamagnetic transition,and the threshold fields decrease with increasing Cr content.The cell-volume V of compounds(x=0-0.8) increases,and the ordering temperature decreases with the increasing Cr content.A large magnetoresistance effect was observed for the compounds (x=0.4,0.5),and the maximum absolute value at 5K are 32% and 24% under 5T for x=0.4 and x=0.5,respectively.     

Magnetizations and magneto-transport properties of Ni-doped PrFeO_3 thin films

The present study reports the magnetizations and magneto-transport properties of PrFe1-xNixO3 thin films grown by pulsed laser ablation technique on LaAlO3substrates. From DC M/H plots of these films, weak ferromagnetism or ferrimagnetism behaviors are observed. With Ni substitution, reduction in saturation magnetization is also seen. With Ni doping, variations in saturation field(Hs), coercive field(Hc), Weiss temperature(θ), and effective magnetic moment(peff)are seen. A small change of magnetoresitance with application of higher field is observed. Various essential parameters like density of state(Nf) at Fermi level, Mott's characteristic temperature(T0), and activation energy(Ea) in the presence of and in the absence of magnetic field are calculated. The present observed magnetic properties are related to the change of Fe–O bond length(causing an overlap between the oxygen p orbital and iron d orbital) and the deviation of the Fe–O–Fe angle from 180?. Reduction of magnetic domain after Ni doping is also explored to explain the present observed magnetic behavior of the system. The influence of doping on various transport properties in these thin films indicates a distortion in the lattice structure and single particle band width, owing to stress-induced reduction in unit cell volume.     

Synthesis and thermoelectric properties of Mn-doped AgSbTe₂ compounds

Polycrystalline p-type Ag 0.9 Sb 1.1 x Mn x Te 2.05(x = 0.05,0.10,and 0.20) compounds have been prepared by a combined process of melt-quenching and spark plasma sintering.The sample composition of Ag 0.9 Sb 1.1 x Mn x Te 2.05 has been specially designed in order to achieve the doping effect by replacing part of Sb with Mn and to present the uniformly dispersed Ag 2 Te phase in the matrix by adding insufficient Te,which is beneficial for optimizing the electrical transport properties and enhancing the phonon scattering effect.All the samples have the NaCl-type structure according to our X-ray powder diffraction analysis.After the treatment of spark plasma sintering,only the sample with x = 0.20 has a small amount of MnTe 2 impurities.The thermal analysis indicates that a tiny amount of Ag 2 Te phase exists in all these samples.The presence of the MnTe 2 impurity with high resistance and high thermal conductivity leads to the deteriorative thermoelectric performance of the sample with x = 0.20 due to the decreased electrical transport properties and the increased thermal conductivity.In contrast,the sample with x = 0.10 exhibits enhanced thermoeletric properties due to the Mn-doping effect.A dimensionless thermoelectric figure of merit of 1.2 is attained for the sample with x = 0.10 at 573 K,showing promising thermoelectric properties in the medium temperature range.