双掺杂BaxAgyCa3-x-yCo4O9氧化物的织构与电输运性能

采用柠檬酸溶胶凝胶和放电等离子烧结制备了Ca位双掺杂型的Ba_xAg_yCa_3-x-yCo₄O₉烧结体,利用X射线衍射仪、扫描电子显微镜、电阻率测试仪等研究了烧结体相组成、取向度、织构及电性能.结果表明：含Ag的掺杂试样中出现了偏离化学计量比分布的Ag单质,掺杂试样取向度随Ba与Ag掺杂量之比x/y的增大而提高,含Ag的掺杂试样取向度低于未掺杂试样,不含Ag的掺杂试样取向度高于未掺杂试样.x=y=01的试样导电机理发生变化.Ba,Ag掺杂量相等的试样保持较低取向度的同时具有较低的电阻率,在973 K时达到最低值（73 mΩcm）,而取向度最低的Ag单掺杂试样电阻率在所有试样中最低,在973 K时为63 mΩcm. 关键词： 3Co₄O₉')" href="#">Ca₃Co₄O₉ 双掺杂 织构 电输运性能     

Transport coefficients and defect structure of Sb2−xAgxTe3 single crystals

Incorporation of Ag in the crystal lattice of Sb₂Te₃ creates structural defects that have a strong influence on the transport properties. Single crystals of Sb_2−xAg_xTe₃ (x=0.0; 0.014; 0.018 and 0.022) were characterized by measurements of the temperature dependence of the electrical resistivity, Hall coefficient, Seebeck coefficient and thermal conductivity in the temperature range of 5-300 K. With an increasing content of Ag the electrical resistance, the Hall coefficient and the Seebeck coefficient all decrease. This implies that the incorporation of Ag atoms in the Sb₂Te₃ crystal structure results in an increasing concentration of holes. However, the doping efficiency of Ag appears to be only about 50% of the expected value. We explain this discrepancy by a model based on the interaction of Ag impurity with the native defects in the Sb_2−xAg_xTe₃ crystal lattice. Defects have a particularly strong influence on the thermal conductivity. We analyze the temperature dependence of the lattice thermal conductivity in the context of the Debye model. Of the various phonon scattering contributions, the dominant influence of Ag incorporation in the crystal lattice of Sb₂Te₃ is revealed to be point-defect scattering where both the mass defect and elastic strain play a pivotal role.     

Structure and low-temperature properties of Ba8Ni6Ge40

Structural, magnetic, heat capacity, electrical and thermal transport properties are reported on polycrystalline Ba₈Ni₆Ge₄₀. Ba₈Ni₆Ge₄₀ crystallizes in a cubic type I clathrate structure with unit cell a=10.5179 (4) Å. It is diamagnetic with susceptibility χ_dia=−1.71×10^-6 emu/g Oe. An Einstein temperature 75 K and a Debye temperature 307 K are estimated from heat capacity data. It exhibits n-type conducting behavior below 300 K. It shows high Seebeck coefficients (−111×10^-6 V/K), low thermal conductivity (2.25 W/K m), and low electrical resistivity (8.8 mΩ cm) at 300 K.     

Structural,magnetic and electrical properties of Co1−xZnxFe2O4 synthesized by co-precipitation method

Nanoparticles of Co_1−xZn_xFe₂O₄ with stoichiometric proportion (x) varying from 0.0 to 0.6 were prepared by the chemical co-precipitation method. The samples were sintered at 600 °C for 2 h and were characterized by X-ray diffraction (XRD), low field AC magnetic susceptibility, DC electrical resistivity and dielectric constant measurements. From the analysis of XRD patterns, the nanocrystalline ferrite had been obtained at pH=12.5–13 and reaction time of 45 min. The particle size was calculated from the most intense peak (3 1 1) using the Scherrer formula. The size of precipitated particles lies within the range 12–16 nm, obtained at reaction temperature of 70 °C. The Curie temperature was obtained from AC magnetic susceptibility measurements in the range 77–850 K. It is observed that Curie temperature decreases with the increase of Zn concentration. DC electrical resistivity measurements were carried out by two-probe method from 370 to 580 K. Temperature-dependent DC electrical resistivity decreases with increase in temperature ensuring the semiconductor nature of the samples. DC electrical resistivity results are discussed in terms of polaron hopping model. Activation energy calculated from the DC electrical resistivity versus temperature for all the samples ranges from 0.658 to 0.849 eV. The drift mobility increases by increasing temperature due to decrease in DC electrical resisitivity. The dielectric constants are studied as a function of frequency in the range 100 Hz–1 MHz at room temperature. The dielectric constant decreases with increasing frequency for all the samples and follow the Maxwell–Wagner's interfacial polarization.     

Ca3－x AgxCo4O9(x=0—0.05)氧化物的电输运性能

Ca位掺杂可以优化Ca₃Co₄O₉复合氧化物的电输运性能. 采用柠檬酸溶胶凝胶结合放电等离子烧结制备了Ca位微量掺杂Ag的Ca_3-xAg_xCo₄O₉(x=0—005)氧化物块体试样,通过X射线衍射、扫描电子显微镜、电参数测试仪分析了所得试样. 试验结果表明: 产物呈单一物相,Ca位微量掺杂Ag降低了Ca_{关键词： 3}Co₄O₉')" href="#">Ca₃Co₄O₉ 掺杂 电输运     

High temperature transport properties of Ag-added (Ca0.975La0.025)3Co4O9 ceramics

Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics were fabricated using spark plasma sintering from the precursor powder synthesized by a polyacrylamide gel method. The results indicated that Ag precipitated as a second phase in Ca₃Co₄O₉ matrix. The addition of Ag was effective in enhancing the electrical conductivity and had a slight effect on Seebeck coefficient. In addition, the temperature dependence of electrical conductivity showed that the hole hopping conduction mechanism was dominant for the Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics. The activation energy remained unchanged with the increasing Ag content. The thermoelectric power factor of Ag-added (Ca_0.975La_0.025)₃Co₄O₉ ceramics reached about 5×10⁻⁴ Wm⁻¹ K⁻² at 700 °C, suggesting a promising thermoelectric oxide candidate at high temperatures.     

Large enhancement of room temperature magnetoresistance in Ag-added La0.67(Ca0.65Ba0.35)0.33MnO3

The electrical and magnetoresistant properties of La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃/Ag_x (abbreviated by LCBMO/Ag_x) have been studied. The results show that Ag addition causes a decrease of resistivity dramatically and especially induces a large enhancement of room temperature magnetoresistance (MR). The room temperature MR ratio for x=0.27 sample in 10 kOe magnetic field is 41%, almost 20 times larger than that for x=0 sample. This enhancement is related to that the Curie temperature (T_c) of the sample is near room temperature, as well as the significant reduction of resistivity. The good fits of experimental results for x=0.27 sample to Brillouin function indicate that the MR behavior in the Ag added LCBMO is induced by the spin-dependent hopping of the electrons between the spin clusters, which is an intrinsic property of the CMR materials.     

Studies on fabrication of Ag/Tl2Ba2Ca2Cu3O10/CdSe heterostructures with electrochemical technique

The Ag/Tl₂Ba₂Ca₂Cu₃O₁₀/CdSe heterostructure was fabricated at room temperature by soft electrochemical processing technique for the first time. The formation of the heterostructure with non-diffusive interfaces was confirmed by X-ray diffraction. The crystallite sizes determined for Tl-2223 and CdSe films were 33 nm and 25 nm, respectively. The Tl₂Ba₂Ca₂Cu₃O₁₀ film electrodeposited onto Ag-substrate has shown the superconducting transition temperature T_c at 116.5 K and J_c = 2.1 × 10³ A/cm². These values were found to improve after the deposition of CdSe onto Ag/Tl-2223 films. The effect of red He-Ne laser irradiation on the superconducting properties of heterostructure are studied and discussed at length in this paper.     

Thermoelectric properties of sol-gel derived cobaltite Bi2Ca2.4Co2Oy

Layered misfit cobaltite Bi₂Ca_2.4Co₂O_y has been synthesized by a sol-gel method. This compound exhibits large thermoelectric (TE) power (S_300 K∼170 μV K⁻¹), low resistivity (ρ_300 K∼42 mΩ cm) and relatively small thermal conductivity (κ_300 K∼2.8 W K⁻¹ m⁻¹) at room temperature. Furthermore, the resistivity of this compound displays a metallic behavior above T^?∼150 K with a semiconducting behavior below this temperature. This abnormal behavior in resistivity is analogous to those observed in Sr and Ba based misfit cobaltites. The observed features of the TE have been discussed based on the narrow band model.     

High-temperature thermoelectric properties of Ca3Co4O9+δ with Eu substitution

We have prepared polycrystalline Ca_3−xEu_xCo₄O_9+δ (x=0, 0.15, 0.3 and 0.45) samples using a sol-gel process followed by SPS sintering and investigated the Eu substitution effects on their high-temperature thermoelectric properties. With the Eu substitution, both the electrical resistivity and thermopower increase monotonously. This could be attributed to the decrease of hole concentrations by substitution of trivalent Eu³⁺ for divalent Ca²⁺. The Eu substituted samples (x=0.15, x=0.3) have lower thermal conductivity than Ca₃Co₄O_9+δ due to their lower electronic and lattice thermal conductivity. The dimensionless figure of merit ZT reaches 0.3 at 1000 K for the sample of Ca_2.7Eu_0.3Co₄O_9+δ.     

Magnetotransport properties of (La0.7Pb0.3MnO3)1−xAgx composites

Magnetic and transport properties of (La_0.7Pb_0.3MnO₃)_1−xAg_x composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution with increase of Ag content percentage. Clearly irreversible behavior in the zero-field cooling and field cooling curves at a low field caused by the competition between the magnetization and magnetic domain orientation processes has been observed as x increases. Saturation magnetization decreases as x increases, while ferromagnetic transition temperature remains around 346 K for all composites. The resistivity decreases significantly for (La_0.7Pb_0.3MnO₃)_1−xAg_x composites. It is suggested that introduction of Ag into the niche of grain boundaries forms artificial conducting network and improves the carriers to transport. However, enhancement of magnetoresistance has been observed for the system.     

Effects of double filling of Pb and Ba on thermoelectric properties of PbxBayCo4Sb11.5Te0.5 compounds by HPHT

Skutterudite compounds Pb_xBa_yCo₄Sb_11.5Te_0.5 (x≤0.23,y≤0.27) with bcc crystal structure have been prepared by the high pressure and high temperature (HPHT) method. The study explored a chemical method for filling Pb and Ba atoms into the voids of CoSb₃ to optimize the thermoelectric figure of merit ZT in the system of Pb_yBa_xCo₄Sb_11.5Te_0.5. The structure of Pb_xBa_yCo₄Sb_11.5Te_0.5 skutterudites was evaluated by means of X-ray diffraction. The Seebeck coefficient, electrical resistivity and power factor were performed from room temperature to 710 K. Compared with Co₄Sb_11.5Te_0.5, the thermal conductivity of Pb and Ba double-filled samples was reduced evidently. Among all filled samples, Pb_0.03Ba_0.27Co₄Sb_11.5Te_0.5 showed the highest power factor of 31.64 μW cm⁻¹ K⁻² at 663 K. Pb_0.05Ba_0.25Co₄Sb_11.5Te_0.5 showed the lowest thermal conductivity of 2.73 W m⁻¹ K⁻¹ at 663 K, and its maximum ZT value reached 0.63 at 673 K.     

Thermal, electrical, and electrochemical properties of Lanthanum-doped Ba0.5Sr0.5 Co0.8Fe0.2O3-δ

Crystal structure, thermogravimetry (TG), thermal expansion coefficient (TEC), electrical conductivity and AC impedance of (Ba_0.5Sr_0.5)_1-xLa_xCo_0.8Fe_0.2O_3-δ (BSLCF; 0.05?x?0.20) were studied in relation to their potential use as intermediate temperature solid oxide fuel cell (IT-SOFC) cathode. A single cubic pervoskite was observed by X-ray diffraction (XRD). The TEC of BSLCF was increasing slightly with the increasing content of La, and all the compounds showed abnormal expansion at high temperature. Proved by the TG result, it was associated with the loss of lattice oxygen. The electrical conductivity, which is the main defect of Ba_0.5Sr_0.5 Co_0.8Fe_0.2O_3-δ (BSCF), was improved by La doping, e.g., the compound of x=0.20 demonstrated a conductivity of σ=376 S cm⁻¹ at 392 °C. The increase of electrical conductivity resulted from the increased concentration of charge carrier induced by La doping. In addition, the AC impedance revealed the better electrochemical performance of BSLCF. For example, at 500 °C, the sample with composition x=0.15 yielded the resistance values of 2.12 Ω cm², which was only 46% of BSCF.     

Structural, AC, and DC magnetic properties of Zn1−xCoxFe2O4

Structural, AC and DC magnetic properties of polycrystalline Zn_1−xCo_xFe₂O₄ (x=0.2, 0.4) samples sintered at various temperatures (1100-1300 °C), and various dwell times (0.2-15 h) have been investigated thoroughly. The bulk density of the Zn_0.60Co_0.40Fe₂O₄ samples increases as the sintering temperature (T_s) increases from 1100 to 1250 °C, and above 1250 °C the bulk density decreases slightly. The Zn_0.80Co_0.20Fe₂O₄ samples show similar behavior of changes to that of Zn_0.60Co_0.40Fe₂O₄ samples except that the bulk density is found to be highest at 1200 °C. The DC magnetization as a function of temperature curves show that the Zn_0.60Co_0.40Fe₂O₄ sample is ferrimagnetic at room temperature while the Zn_0.80Co_0.20Fe₂O₄ sample is paramagnetic at room temperature. The T_c of Zn_0.80Co_0.20Fe₂O₄ sample is found to be 170 K from DC magnetization measurement. Separate measurement (AC magnetization), initial permeability as a function of temperature shows that the T_c of the Zn_0.60Co_0.40Fe₂O₄ sample is 353 K. Slight variation of T_c is observed depending on sintering condition. The initial permeability for the Zn_0.60Co_0.40Fe₂O₄ composition sintered at 1250 °C is found to be maximum.     

Effects of Ag addition on electrical transport and magnetic properties of La0.67Ba0.33Mn0.88Cr0.12O3

We have prepared a series of polycrystalline manganites with the nominal compositions, La_0.67Ba_0.33Mn_0.88Cr_0.12O₃/Ag_x (LBMCO/Ag_x) (x   is the mole fraction) with x=0$x=0$, 0.05, 0.1, 0.15, 0.2, 0.23, 0.27, 0.3, 0.35. The X-ray diffraction patterns show that the samples with x>0.05$x>0.05$ are two-phase composites. The Ag addition in LBMCO improves the properties of grain surfaces/boundaries and reduces the resistivity of the composites. For x=0.30$x=0.30$ sample, a minimum resistivity is obtained and a maximum room temperature magnetoresistance up to −54.5% was observed at 288 K, 1 T field. The room temperature T_C and the reduced resistivity are responsible for the enhancement of room temperature MR.     

Temperature dependence of electrical resistivity for Ca-doped perovskite-type Y1−xCaxCoO3 prepared by sol-gel process

The temperature dependences of DC electrical resistivity for perovskite-type oxides Y_1−xCa_xCoO₃ (0?x?0.1), prepared by sol-gel process, were investigated in the temperature range from 20 K up to 305 K. The results indicated that with increase of doping content of Ca the resistivity of Y_1−xCa_xCoO₃ decreased remarkably, which was found to be caused mainly by increase of carrier (hole) concentration. In the whole temperature range investigated the temperature dependence of resistivity ρ(T) for the un-doped (x=0) sample decreased exponentially with decreasing temperature (i.e. ln ρ∝1/T), with a conduction activation energy ; the resisitivity of lightly doped oxide (x=0.01) possessed a similar temperature behavior but has a reduced E_a (0.155 eV). Moreover, experiments showed that the relationship ln ρ∝1/T existed only in high-temperature regime for the heavily doped samples (T?82 and ∼89 K for x=0.05 and 0.1, respectively); at low temperatures Mott's ln ρ∝T^−1/4 law was observed, indicating that heavy doping produced strong random potential, which led to formation of considerable localized states. By fitting of the experimental data to Mott's T^−1/4 law, we estimated the density of localized states N(E_F) at the Fermi level, which was found to increase with increasing doping content.     

Magnetoresistive properties of Zn1−xCoxFe2O4 ferrites

The magnetic and magnetoresistive properties of spinel-type Zn_1−xCo_xFe₂O₄ (x=0, 0.2 and 0.4) ferrites are extensively investigated in this study. A large negative magnetoresistance (MR) effect is observed in Zn_1−xCo_xFe₂O₄ ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature, and show a spin-(cluster) glass transition at low temperatures, depending on the chemical compositions. The MR curves as a function of magnetic fields, MR(H), are parabolic at all temperatures for paramagnetic polycrystalline ZnFe₂O₄. The MR for ZnFe₂O₄ at 110 K in the presence of 9 T applied magnetic field is 30%. On the other hand, MR(H) are linear for x=0.2 and 0.4 ferrimagnetic Zn_1−xCo_xFe₂O₄ samples up to 9 T. The MR effect is independent of the sintering temperatures, and can be explained with the help of the spin-dependent scattering and the Yafet–Kittel angle of Zn_1−xCo_xFe₂O₄ mixed ferrites.     

On the existence of a homologous series of BamCum+nOy oxides with the cubic structure of the BaCuO2 oxide

Phase relations have been studied in the BaO–CuO_x system in the range of 42.0–83.0 mol.% CuO at P(O₂) = 21 kPa (air) by visual polythermal analysis (VPA), powder X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), chemical analysis (CA), and electron diffraction (ED) with simultaneous elemental analysis (EA) in a transmission electron microscope (TEM). The existence of discrete crystallization fields of barium–copper oxides of cation compositions Ba₄Cu₅O_y, Ba₅Cu₆O_y, Ba₇Cu₈O_y, Ba₁₂Cu₁₃O_y, and Ba₂₄Cu₂₅O_y, which have the cubic structure of the BaCuO₂ oxide, is revealed in the studied region of the system. The oxides may be represented as members of a Ba_mCu_m+nO_y homologous series. The BaCuO₂ oxide does not exist in the subsolidus region and does not have its own crystallization field. The oxygen-deficient oxide BaCuO_1.78 of the cation composition (Ba:Cu) 1:1 with the BaCuO₂ cubic structure is found in melted samples of the 50.0 mol.% CuO composition quenched at 1020–1060 °С.     

Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis

Uniform and adherent cobalt oxide thin films have been deposited on glass substrates from aqueous cobalt chloride solution, using the solution spray pyrolysis technique. Their structural, optical and electrical properties were investigated by means of X-ray diffraction (XRD), scanning electron micrograph (SEM), optical absorption and electrical resistivity measurements. Along with this, to propose Co₃O₄ for possible application in energy storage devices, its electrochemical supercapacitor properties have been studied in aqueous KOH electrolyte. The structural analysis from XRD pattern showed the oriented growth of Co₃O₄ of cubic structure. The surface morphological studies from scanning electron micrographs revealed the nanocrystalline grains alongwith some overgrown clusters of cobalt oxide. The optical studies showed direct and indirect band gaps of 2.10 and 1.60 eV, respectively. The electrical resistivity measurement of cobalt oxide films depicted a semiconducting behavior with the room temperature electrical resistivity of the order of 1.5 × 10³ Ω cm. The supercapacitor properties depicted that spray-deposited Co₃O₄ film is capable of exhibiting specific capacitance of 74 F/g.     

Effect of Zn substitution for Co on the transport properties of Y BaCo4O7

Electrical resistivity and Seebeck coefficients of Y BaCo_4−xZn_xO₇ (x=0.0,0.5,1.0,2.0) were investigated in the temperature range 350-1000 K. It was found that the electrical resistivity and activation energy increase with increasing Zn concentration, while Seebeck coefficients do not increase but decrease when electrical resistivity increases. We explained the increase of electrical resistivity and the drop of Seebeck coefficients for Zn-substituted samples by the decrease of carrier mobility, rather than of carrier concentration. The effect of oxygen absorption and desorption on the electrical resistivity and Seebeck coefficients was also investigated. An abrupt change of transport properties happens at about 650 K for x=0.0 and 0.5 samples measured in oxygen. For x=1.0 and 2.0 samples, however, such change disappears and the transport behavior in oxygen is almost same as that in nitrogen due to the significant suppression of oxygen diffusion caused by the higher Zn concentration in these samples.