新型电解质材料CaZr0.1Ti0.9O3的制备与电性能研究

采用溶胶-燃烧法合成了可用于固体氧化物燃料电池(SOFC)的新型固体电解质材料CaZr_0.1Ti_0.9O₃。通过XRD、交流复阻抗等电化学方法对样品的结构、电导性能进行了表征,并考察了材料的烧结性能。结果表明,溶胶-燃烧法可以成功制备出具有良好烧结性能的CaZr_0.1Ti_0.9O₃电解质粉末,1400℃下得到的烧结体的相对密度可达到95%。电性能测试表明CaZr_0.1Ti_0.9O₃烧结体在中温范围内具有较高的氧离子电导率（σ^_800℃＝2.24×10^-3 S/cm）、低的电导活化能（0.89 eV）;样品的导电性能受烧结温度的影响,合理的控制烧结温度对于获得导电性能优良的CaZr_0.1Ti_0.9O₃电解质材料具有重要作用。     

Dielectric and magnetic properties of multiferroic BiFeO3 ceramics sintered with the powders prepared by hydrothermal method

BiFeO₃ ceramics were sintered in the temperature range of 700–900 °C by using the pure BiFeO₃ powders hydrothermally synthesized at 250 °C. The low reaction temperature and low sintering temperature prevent the element volatilization and phase decomposition. The ceramics sintered at 800 and 850 °C exhibit much dense microstructure with clear grains and grain boundaries. They also show high dielectric constant, dielectric dispersion and low loss tangent. At room temperature, the dielectric behaviors of BiFeO₃ ceramics are mainly attributed to the transition of localized charge carriers and the microstructure of grains and grain boundaries. The temperature dependence of dielectric constant and loss tangent confirms that the localized charge carriers are a main contribution to the dielectric permittivity. Activation energy E_α of relaxation process for the BiFeO₃ ceramic sintered at 850 °C is 0.397 eV. The obtained BiFeO₃ ceramics show magnetic responses, which are relative to the grain size.     

Thermophysical properties of gamma-irradiated LaFeO3 and YFeO3 orthoferrites

In the present work, LaFeO₃ and YFeO₃ orthoferrites were carefully prepared using the solid state high temperature ceramic and sintering technique. On these two materials, numerous measurements were applied before and after γ-absorbed dose (1.24 Mrad) namely, X-ray diffraction (XRD), DC-electrical conductivity, UV/Vis, IR, Mössbauer effect and EPR. Results obtained were explained, interpreted, and discussed in detail on the basis of lattice structure of the new perovskite investigated and the interaction of γ-radiation with the investigated materials. Finally the γ-induced functions are evaluated for electrical conduction, Mössbauer effect and EPR for the first time.     

Influence of sintering method on microstructure,electrical and magnetic properties of BiFeO3–BaTiO3 solid solution ceramics

The (1-x)BiFeO₃-xBaTiO₃ (short for (1-x)BFO-xBTO, x = 0.2–0.4) ceramics were prepared by solid-state reaction with microwave sintering (MWS) and conventional sintering (CS) methods. The crystal structure, microstructures, dielectric properties, ferroelectric properties, and magnetic properties of BFO-BTO ceramics sintered by MWS and CS were systematically investigated. It is found that the MWS can effectively decrease the grain size and enhance the compactness of BFO-BTO ceramics. The X-ray diffraction (XRD) results confirm that all ceramics exhibit a single perovskite structure, and the phase transforms from rhombohedral in BiFeO₃-rich compositions to pseudo-cubic phase gradually as x increases. Introducing BTO into BFO can strengthen its dielectric relaxation behavior. Compared with CS, the MWS samples have a lower remanent polarization (P_r) and a smaller coercive field (E_c) under the same electric field. Therefore, MWS contributes to the decrease of dielectric loss. Addition of BTO can contribute to the reduction of the coercive force (H_c) of BFO-based ceramic, and so decrease the hysteresisloss. At the same time, its remanent magnetization (M_r) value can be decreased by introducing BTO into BFO and using MWS method. The present research provides a route for decreasing the dielectric loss and hysteresis loss of BiFeO₃-based ceramics using the MWS method.     

Correlation of microstructural and physical properties in bulk BiFeO3 prepared by rapid liquid-phase sintering

BiFeO₃ sample prepared by rapid liquid phase sintering (RLPS) technique is found to posses better dielectric properties than the subsequently annealed sample. These samples also show ferromagnetism at room temperature. Detailed X-ray diffraction and Rietveld refinement of the data show that oxygen–iron–oxygen (O–Fe–O) bond angle is considerably more distorted in samples prepared by rapid liquid-phase sintering technique than in the annealed sample. In addition, synchrotron X-ray diffraction as well as high resolution transmission electron microscopy studies show higher concentration of impurity phases in the annealed samples, which is the likely reason for inferior dielectric properties. Further transmission electron microscopy results indicate that high crystalline order is not necessary for better dielectric properties. However weak ferromagnetism is observed in all these samples and appears to be the intrinsic property of samples prepared by rapid liquid phase sintering technique.     

Synthesis and characterization of LiCo3/5Mn2/5VO4 ceramics

Dielectric and a.c. conductivity properties of LiCo_3/5Mn_2/5VO₄ ceramic are investigated. This compound is prepared by solution-based chemical method and the formation is checked by X-ray diffraction (XRD) study. XRD analysis at room temperature shows an orthorhombic phase. Frequency dependence of dielectric constant (?_r) at different temperatures shows a dispersive behavior at low frequencies. Temperature dependence of ?_r at different frequencies indicates the transition temperature (T_c) = 235 °C, 245 °C, 257 °C and 265 °C with (?_r)_max. ～3689, 1373, 750 and 386 for 10, 50, 100 and 200 kHz respectively. A.c. conductivity analysis indicates that electrical conduction in the material is a thermally activated process.     

Further investigations on the thermophysical properties of gamma-irradiated urania-doped orthoferrites

(Y_1.85?U_0.15) FeO₃ orthoferrite was carefully prepared using the solid-state high temperature ceramic and sintering technique. On this orthoferrite, numerous measurements have been undertaken before and after 1.24 Mrad γ-absorbed dose, namely X-ray diffraction (XRD), DC-electrical conductivity versus temperature, UV/Vis. absorption, IR absorption, Mössbauer effect (ME) and EPR. In the present work the results obtained were explained, interpreted and discussed in details on the basis of lattice structure of the new perovskite investigated and the interaction of γ-radiation with the orthoferrite lattice.     

Size-induced diffuse behavior in Pb0.89La0.11Zr0.40Ti0.60O3 nanocrystalline ferroelectric ceramics

The Pb_1−xLa_xZr_yTi_1−yO₃ system is a perovskite ABO₃ structured material which presents ferroelectric properties and has been used as capacitors, actuators, transducers and electro-optic devices. In this paper, we describe the synthesis and the characterization of Pb_0.89La_0.11Zr_0.40Ti_0.60O₃ (PLZT11) nanostructured material. The precursor polymeric method and the spark plasma sintering technique were respectively used to prepare ceramic samples. In order to compare the effect of grain size, microcrystalline PLZT11 ceramic samples were also prepared. PLZT11 samples were characterized by X-ray diffraction technique which results show a reduction on the degree of tetragonality as the average grain size decreases. Moreover, the grain size decrease to a nanometer range induces a diffuse behavior on the dielectric permittivity curves as a function of the temperature and a reduction on the dielectric permittivity magnitude. Furthermore, the large number of grain boundaries due to the nanometer size gives rise to a dielectric anomaly.     

Magneto-thermal and dielectric properties of biferroic YCrO3 prepared by combustion synthesis

Microstructural, magnetothermal and dielectric properties of YCrO₃ powders prepared by combustion and solid state methods have been studied by a combination of XRD, specific heat, magnetization and permittivity measurements. The TEM and XRD characterization confirm that the combustion powders are amorphous plate-like agglomerates of nano-sized crystalline particles. A more uniform grain size along with an increase of the relative density is observed by SEM in the sintered samples prepared by combustion route with respect to those produced by solid state reaction. Similar to the material obtained through solid state synthesis, the material prepared by the combustion method also shows spin canted antiferromagnetic ordering of Cr⁺³ (S=3/2) at ∼140 K, which is shown by magnetization as well as λ-type anomaly in the total specific heat. Furthermore, the magnetic contribution to the total specific heat reveals spin fluctuations above T_N and a spin reorientation transition at about 60 K. Both YCrO₃ compounds show a diffuse phase transition at about 450 K, typical of a relaxor ferroelectric, which is characterized by a broad peak in the real part of the dielectric permittivity as a function of temperature, with the peak decreasing in magnitude and shifting to higher temperature as the frequency increases. The relaxor dipoles are due to the local non-centrosymmetric structure. Furthermore, the high loss tangent in a broad range of temperature as well as conductivity analysis indicates a hopping mechanism for the electronic conductivity as we believe it is a consequence of the outer d³-shell, which have detrimental effects on the polarization and the pooling process in the YCrO₃ bulk material. The more uniform particle size and higher density material synthesized through the combustion process leads to an improvement in the dielectric Properties.     

Evidence for transition from polaron to bipolaron conduction in electroactive LixCr0.11V2O5.16 powders: A dynamic study from 10 to 10 Hz

This paper presents a study on the electrical transport properties of lithiated Cr_0.11V₂O_5.16, which can be used as a rechargeable cathodic material in lithium batteries. Dielectric and conductivity spectra of Li_xCr_0.11V₂O_5.16 powders (x=0, 0.05, 0.40 and 1.20) were recorded in a broad frequency range of 10-10¹⁰ Hz at temperature varying between 300 and 400 K. Complex resistivity diagrams have enabled to obtain thermal behaviors of bulk dc-conductivity. Dielectric relaxations were found, attributed to small polarons and (intersite) bipolarons hopping. The transport properties are shown to be consistent with small polaron and bipolaron conduction models. The change from polaronic to bipolaronic conduction has been evidenced with the increase of the lithium content x from 0.40 to 1.20. This work opens up new prospects for a more fundamental understanding of the electronic transport in relation with the electrochemical properties of Cr_0.11V₂O_5.16.     

Ionic conductivity of double vanadate Ag3Sc2(VO4)3

Ionic conductivity of double vanadate Ag₃Sc₂(VO₄)₃ with the NASICON structure is studied by the method of impedance spectroscopy in the frequency range from 5 to 5 × 10⁵ Hz and in the temperature range of 300–827 K. The vanadate Ag₃Sc₂(VO₄)₃ is prepared in the form of fine crystalline powder by solid-state synthesis from V₂O₅, Sc₂O₃, and AgNO₃ at 1173 K. The conductivity of Ag₃Sc₂(VO₄)₃ ceramic samples σ = 8 × 10^?3 S/cm (at 563 K). The σ vs. T curve demonstrates an anomaly at 563–623 K which corresponds to thermal effects in this temperature range. The values of enthalpy of ion transport activation are ΔH = 0.40 ± 0.05 eV (T < 563 K) and ΔH = 0.30 ± 0.05 eV (T > 623 K). The ionic conductivity of Ag₃Sc₂(VO₄)₃ is due to Ag⁺ ions localized in channels of the framework structure of the NASICON type.     

Study of dielectric and ac impedance properties of citrate-gel synthesized Li0.35Zn0.3Fe2.35O4 ferrite

Nano-crystals of Li_0.35Zn_0.3Fe_2.35O₄ ferrite have been synthesized using citrate precursor method. The sample synthesized was sintered at different temperatures in order to vary their crystallite size. The average crystallite size was found in the range 24?C57?nm by varying the temperature from 300 to 1,100?°C. X-ray diffraction measurements confirmed the formation of cubic spinel structure at all the sintering temperatures in this work. The high frequency performance of the ferrite samples were estimated by measuring the frequency dispersion of the dielectric constant, dielectric loss and ac electrical conductivity. The dielectric constant has been observed to show normal behavior with frequency and decreases with the decrease in crystallite size. It is also observed that decrease in dielectric constant depends on sintering temperature because of lithium evaporation at higher temperature. A low value of dielectric constant and dielectric loss has been found, which makes them applicable for high frequency applications by decreasing the skin effect. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of Li_0.35Zn_0.3Fe_2.35O₄ ferrite. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the prepared samples.     

Preparation and characterization of proton conducting terbium doped strontium cerate membranes

SrCe_0.95Tb_0.05O_3−δ (SCT) membranes with perovskite-type structure were prepared by the citrate method. Both gas-permeable and hermetic membranes can be obtained through the control of sintering steps. Membranes made from powders treated by an intermediate sintering step tend to be porous and gas-permeable, while hermetic membranes can be formed at lower final sintering temperatures by eliminating the intermediate sintering step. The membranes can be gas-tight even at a relative density of 71%. The density of the membrane increases with the increase of final sintering temperature. The oxalic acid method does not produce SCT with desired perovskite structure due to deficiency of strontium in the solid precipitates. The SCT membranes show appreciable proton conduction in a hydrogen-containing atmosphere.     

Effect of precursor calcination temperature on the microstructure and thermoelectric properties of Ca3Co4O9 ceramics

Ca₃Co₄O₉ (CCO) powder precursors were prepared by the chemical sol–gel route and calcined at various temperatures between 923?K (CCO-923?K) and 1,073?K (CCO-1,073?K). The calcination temperature was found to be a critical factor affecting the microstructure and thermoelectric properties of CCO ceramic bulk samples. The grain size increases with calcination temperature. The nano-crystals with size about 100?nm in the powders calcined at 923?K promote large crystal growth and texture development during sintering. Bulk pellets made from CCO-923?K powder have large crystal grains, uniform grain size distribution, and a high degree of crystal alignment. By contrast, pellets made from CCO powders at higher calcination temperatures have a bimodal distribution of large and small grains and a large amount of randomly oriented grains. Transmission electron microscopy analysis shows that each crystal grain (identified in SEM images) consists of bundles of CCO nano-lamellas. The nano-lamellas within one bundle share the same c-axis orientation and have fiber texture. The electrical resistivity of CCO-923?K is weakly dependent on operating temperature. Compared to the CCO-1,073?K sample, the CCO-923?K sample has the highest power factor, a lower thermal conductivity, and higher electrical conductivity.     

Synthesis, Characterization, and Conductivity Studies of Poly-o-Methoxyaniline Intercalated into V2O5 Xerogel

In this work we study the conductivity properties of poly-o-methoxyaniline/V₂O₅ intercalation compounds obtained through intercalative polymerization of o-methoxyaniline with V₂O₅·nH₂O in hydrogel form and by reacting directly with V₂O₅ film in de-hydrated form (xerogel). These new compounds were characterized using Fourier-transform infrared and ultra-violet/visible spectroscopies, electron paramagnetic resonance, elemental analysis (C, N, H), thermogravimetric analysis, scanning electron microscopy, dc-conductivity and powder X-ray diffraction. For samples formed from V₂O₅ xerogel in film form, an increase in dc-conductivity and a decrease in the thermal activation energy in comparison with pure matrix was observed. The increase in conductivity is attributed to an increase of carrier density in the vanadium oxide lattice and the contribution of polarons from the polymer. On the other hand, for the intercalation compound obtained with V₂O₅·nH₂O in hydrogel form presents a very low room temperature conductivity value. The decrease in conductivity is due to the lack of connectivity of the various parts that compose the material.     

Improvement of lithium-ion conductivity in A-site-disordered lithium lanthanum titanates by V5+/Nb5+ substitution

The V⁵⁺/Nb⁵⁺-substituted lithium lanthaum titanates are synthesized by a conventional solid-state reaction method at high temperature in air. The structural and conductivity studies of the obtained perovskite oxide samples are investigated by x-ray diffraction (XRD), SEM, and impedance spectroscopy. From the powder XRD patterns, it is clearly observed that the synthesized samples exhibit a well-defined cubic structure with the Pm3m (Z = 1) space group. The lattice parameter is decreased with increasing vanadium content in Li_0.5?x La_0.5Ti_1?x V _x O₃, but increased with the increasing niobium content in Li_0.5?x La_0.5Ti_1?x Nb _x O₃. The scanning electron microscope measurements confirmed that these materials consist of fairly ordered grains throughout the surface area. The conductivity variations with the substitution of vanadium/niobium are also reported. The bulk ionic conductivity measured in the temperature range from room temperature to 150 °C is about the same as reported earlier for the related lithium lanthanum titanate. However, the low activation energies for ionic conduction observed for these samples encourage further investigations for better conductors in this system.     

Thermoelectric properties of Ru2Si2 prepared by spark plasma sintering method

Electrical conductivities, Seebeck coefficients and thermal diffusivities were measured for Ru₂Si₃ samples with different densities prepared by spark plasma sintering method and an Rh-doped sample prepared by the floating zone melting and spark plasma sintering methods. Electrical conductivities, Seebeck coefficients and thermal diffusivities increased with sample density for undoped Ru₂Si₃. The thermoelectrical figure of merit also increased with sample density. Electrical conductivity of the 4% Rh-doped sample was larger than that of our previous samples prepared by floating zone melting method. Values of Seebeck coefficients were similar for samples prepared by either method. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electrical conductivity of titanium pyrophosphate between 100 and 400 °C: effect of sintering temperature and phosphorus content

The synthesis of titanium pyrophosphate is carried out, and the material is sintered at different temperatures between 370 and 970 °C. Yttrium is added during the synthesis to act as acceptor dopant, but it is mainly present in the material in secondary phases. The conductivity is studied systematically as a function of sintering temperature, pH₂O, pO₂, and temperature (100–400 °C). Loss of phosphorus upon sintering above 580–600 °C is confirmed by energy dispersive spectroscopy and combined thermogravimetry and mass spectrometry. The conductivity decreases with increasing sintering temperature and decreasing phosphorus content. The highest conductivity is 5.3?×?10^?4 S cm^?1 at 140 °C in wet air (pH₂O?=?0.22 atm) after sintering at 370 °C. The conductivity is higher in wet atmospheres than in dry atmospheres. The proton conduction mechanism is discussed, and the conductivity is attributed to an amorphous secondary phase at the grain boundaries, associated with the presence of excess phosphorus in the samples. A contribution to the conductivity by point defects in the bulk may explain the conductivity trend in dry air and the difference in conductivity between oxidizing and reducing atmospheres at 300–390 °C. Slow loss of phosphorus by evaporation over time and changes in the distribution of the amorphous phase during testing are suggested as causes of conductivity degradation above 220 °C.     

Sol–gel route to prepare well-ordered nanowires with anodic aluminum oxide template

In this work, we report a sol?Cgel route to fabricate oxide nanowires of BiFeO₃ using two-step anodic aluminum oxide as template. We prepared oxide nanowires with uniform dimensions that can vary from 35 to 100?nm in diameter and with 4???m in length, as confirmed by scanning electron microscopy and transmission electron microscopy measurements. Magnetization measurements, performed in a vibrating sample magnetometer, show that this nanostructures present ferromagnetism at room temperature.     

Size confinement and magnetization improvement by La3+ doping in BiFeO3 quantum dots

Nanometric multiferroic samples Bi_1−xLa_xFeO₃; 0.05 ≤ x ≤ 0.40 were prepared using ceramic method. Structural and magnetic properties were investigated using XRD, TEM, magnetic susceptibility and M–H loop. The decrease in the lattice parameters is due to the difference between the ionic radii of Bi and La and this effect is compensated by the change in the atomic weight of the two elements which is reflected as a decrease in the density. The obtained results showed that all samples were antiferromagnetic in character. The small values of remnant and saturation magnetization indicated the canted type antiferromagnetism. Maximum coercivity H_c = 5265 Oe was obtained at x = 0.25. The magnetic susceptibility measurements show its size dependence due to long range spin arrangement. Improvement of the magnetization of BiFeO₃ is achieved by La³⁺ at different doping levels. The obtained quantum dot size of the crystallites enhances their use in spintronic devices.