High-Temperature Electrical Transport Behavior Observed in the La1.96M0.04CuO4 (M: Mg, Ca, Sr) Polycrystalline Ceramics

We prepared high-density La_1.96M_0.04CuO₄ (M: Mg, Ca, Sr) polycrystalline ceramics by a spark plasma sintering method, and investigated the temperature dependence of the electrical conductivity, Seebeck coefficient, and power factor. Our results indicate that the relative density of these sintered ceramic samples can reach nearly 100%, and alkaline earth metal ions doping can increase the activation energy (from 0.021 to 0.128 eV). All ceramic samples show high positive Seebeck coefficient (∼100–510 μV/K). The obtained Sr-doped La₂CuO₄ ceramics show better thermoelectric properties, which potentially make them for thermoelectric applications.     

Low-Temperature Sintering of La(Ca)CrO3 Powder Prepared through the Combustion Process

Ultrafine La(Ca)CrO₃ (LCC) powder was prepared through the glycine–nitrate gel combustion process. It was shown for the first time that the use of relatively inexpensive CrO₃ as a starting material for chromium has potential for the bulk preparation of sinter-active LCC powder. As-prepared powder, when calcined at 700°C, resulted in LCC along with a small amount of CaCrO₄. The calcined powder was found to be composed of soft agglomerates with a particle size of ≈70–290 nm. The cold pressing and sintering of the calcined powder at 1200°C resulted in the mono-phasic La_0.7Ca_0.3CrO₃ with density ≈98% of its theoretical value. This is the lowest sintering temperature ever reported for La_0.7Ca_0.3CrO₃. The conductivity of the sintered La_0.7Ca_0.3CrO₃ at 1000°C was found to be ≈57 S/cm in air. The sintering and electrical behavior achieved for La_0.7Ca_0.3CrO₃ may find application as an interconnect material for high-temperature solid oxide fuel cells if problems with chemical expansion and poor conductivity in fuel can be overcome.     

Thermoelectric Properties of Homologous Compounds in the ZnO–In2O3 System

Homologous compounds of (ZnO) _m In₂O₃ (m = integer) with layered structures were synthesized by reaction-sintering a mixed powder of ZnO and In₂O₃ at 1823 K for 2 h in air, and their thermoelectric properties, i.e., electrical conductivity, Seebeck coefficient, and thermal conductivity, were measured at 500 to 1100 K. Their thermoelectric figure of merit depended on the composition, and an optimum value of m apparently existed giving the largest figure of merit.     

Preparation, Electrical Conductivity, and Thermal Expansion Behavior of Dense Nd1−xCaxCrO3 Solid Solutions

Nd_{1− x} Ca _x CrO₃ (0≤ x ≤0.25) solid solutions are synthesized using a two-step calcination solid-state reaction method and can potentially be used as interconnect applications in solid oxide fuel cells (SOFCs). The lattice parameters and unit cell volume decrease with an increase of the Ca²⁺ dopant concentration at the A-site. On the other hand, the relative density, electrical conductivity, and thermal expansion coefficient (TEC) increase with an increase of Ca²⁺ contents. Calcium doping significantly depresses the chromium vaporization of NdCrO₃, thus accelerating the sintering and densification of the oxides. Nd_{1− x} Ca _x CrO₃ with x =0.25 shows the best electrical conductivity of 28.8 and 1.1 S/cm at 850°C in air and hydrogen, respectively, with a high relative density of 98.0% and a good TEC of 9.19 × 10⁻⁶ K⁻¹ in the temperature range from 30° to 1000°C in air. This indicates that Nd_0.75Ca_0.25CrO₃ is a promising candidate as an interconnect material for application in SOFCs.     

Effect of Cation Substitution and Non-Stoichiometry on the Microwave Dielectric Properties of Sr(B'0.5Ta0.5)O3 [B'=Lanthanides] Perovskites

The Sr(B'_0.5Ta_0.5)O₃ ceramics where B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, and Yb have been prepared by the conventional solid-state ceramic route and their microwave dielectric properties have been investigated. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscope techniques. The relative permittiviy (ɛ_r) varies linearly with B'-site ionic radii, except for La, and the temperature coefficient of resonant frequency (τ_f) varies linearly with the tolerance factor. The Sr(B'_0.5Ta_0.5)O₃ ceramics have ɛ_r in the range 25.9–32, Q _u× f =4500–54 300 GHz, and τ_f=−79 to −42 ppm/°C. A slight deviation from stoichiometry affects the dielectric properties of these double perovskites. Partial substitution of Ba for Sr could tune the dielectric properties. Addition of rutile (TiO₂) lowered the sintering temperature and improved the dielectric properties of Sr(B'_0.5Ta_0.5)O₃ ceramics.     

Electrical Conduction in β-Bi2O3 Doped with Sb2O3

Electrical conduction in tetragonal β-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, ionic transference number, and Seebeck coefficient. The β-Bi₂O₃ doped with 1 to 10 mol% Sb₂O₃ was stable up to 600°C and showed an oxygen ionic and electronic mixed conduction, where the electron conduction was predominant at low oxygen pressures. The oxygen-ion conductivity showed a maximum at 4 mol% Sb₂O₃, whereas the activation energy for the ionic conduction remained unchanged for 4 to 10 mol% Sb₂O₃-doped specimens. These results were interpreted in terms of the oxygen vacancy concentration and the distortion of the tetragonal structure. The electron conductivity and its oxygen pressure dependence decreased with increasing Sb₂O₃ content. The fact that Sb⁵⁺ is partially reduced by excess electrons in heavily doped β specimens at low oxygen pressures is explained.     

Electronic Conductivity, Seebeck Coefficient, and Defect Structure of La1-xSrxFeO3 (x=0.l, 0.25)

The electrical conductivity and Seebeck coefficients of La_1-xSr _x FeO₃ ( x =0.1, 0.25) were measured as a function of oxygen partial pressure, P_O2, at T =900° to 1300°C. The electrical conduction was p type in the higher P_O2 range, and n type in the lower P _O2 range. The Seebeck coefficient indicated that the conduction was due to electron hopping between Fe^×_Fe and Fe_Fe, in the higher P_O2 range and electron hopping between Fe'_Fe and Fe^×_Fe in the lower range. The carrier concentrations were calculated from the values of electrical conductivity and Seebeck coefficient. From the P _O2 dependences of the carrier concentrations, the defect structure of La_1-xSr _x FeO₃ was determined. It was found that the electrical properties can be described by considering the imperfections Sr_La, Vo , Fe_Fe, and Fe'_Fe.     

Microwave Dielectric Properties of Doped BaTi4O9

Polycrystalline BaTi₄O₉ doped with Mn, Sn, Zr, Ca, Sr, and Pb was prepared from carbonates and oxides. Single-phase ceramics with densities exceeding 97% of theoretical were made with up to 3 mol% Mn; 6 mol% Sn, Zr, and Ca; and 8 mol% Sr and Pb. Dielectric constant, k , quality factor, Q , and temperature coefficient of frequency, τ _f , of ∼37, 5675, and 15 ppm/°C, respectively, were determined at 4 GHz for undoped BaTi₄O₉. Doping did not significantly affect k and τ _f . However, doping with Mn, Sn, and Pb lowered Q , whereas doping with Zr, Ca, and Sr increased Q by up to 2000. Additions of 0.5 mol% MnO₂ as a second phase improved Q from 3675 to 7600.     

Piezoelectric Properties and Phase Relations of Pb(Mg1/3Nb2/3)O3-PbTiO3−PbZrO3 Ceramics with Barium or Strontium Substitutions

When a small amount of Ba or Sr is substituted for Pb in Pb(Mg_1/3 Nb_2/3)O₃-PbTiO₃-Pb2rO 3 , the morphotropic boundary and the compositions which show the highest planar coupling coefficient and dielectric constant shift slightly toward the decreasing PbTiO₃ content. The tetragonality of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ and Pb(Mg_1/2 Nb_2/3)-O₃-PbTiO₃-PbZrO₃ ceramics decreased with increasing Ba or Sr content. The lattice parameter (α axis) in the rhombohedral or pseudocubic phase increased with the increase of Ba but decreased with the increase of Sr substitution. Although the Curie temperature was lowered with the increase of Ba or Sr, the dielectric constants of the ceramics were increased. The dielectric and piezoelectric properties of the ternary compositions near the morphotropic boundary were improved through selection of sub-stituent and base composition. A planar coupling coefficient of 0.66 and a low Young's modulus were obtained with substitution of 5 mole % Ba. A dielectric constant greater than 3500 and a planar coupling of 0.63 can be obtained by substituting 5 mole % Sr.     

Transport Properties of the Binary Type I Clathrate K8Ge44□2

We report the synthesis and low-temperature electrical resistivity, Seebeck coefficient, and thermal conductivity of the binary type I clathrate K₈Ge₄₄□₂ (□=Ge framework vacancy). Electrical resistivity measurements indicated metallic or heavily doped semiconductor behavior, while the Seebeck was relatively high at −77 μV/K at room temperature. The thermal conductivity was very low, on the order of 1 W/m K at room temperature. This work is part of a continuing effort to investigate new compositions in open-structured and guest-framework materials for potential thermoelectric applications.     

Electronic, Magnetic, and Structural Properties of Sr2MnRuO6 and LaSrMnRuO6 Double Perovskites

The perovskites Sr₂MnRuO₆ and LaSrMnRuO₆ have been investigated and their properties compared with those of the broader family of A₂MRuO₆ double perovskites (A=Sr, La; M=Cr, Mn, Fe, Co, Ni). Neutron powder diffraction shows that both phases lack long-range Mn/Ru order. Sr₂MnRuO₆ exhibits a cooperative Jahn–Teller distortion, conductivity that proceeds via variable-range hopping, and antiferromagnetic (AFM) order (C-type structure, T _N≅200K, 2.05(1) μ_B per transition metal). LaSrMnRuO₆ exhibits nearly undistorted octahedra, conductivity that proceeds via diffusion-assisted small-polaron hopping, and ferromagnetic (FM) order ( T _C≅220K, 1.60(4) μ_B transition metal). Band structure calculations show that AFM Mn–O–Ru coupling optimizes polarization of the Ru t _{2 g} orbitals, while FM coupling favors delocalization of the Ru t _{2 g} electrons. The transition from antiferromagnetism to ferromagnetism is linked to a loss of orbital order and stabilized over competing glassy states by delocalization of Ru t _{2 g} electrons.     

Electrical Conductivity in the ZrO2-Rich Region of Several M2O3—ZrO2 Systems

The electrical conductivity of M₂O₃-ZrO₂ compositions containing 6 to 24 mole % M₂O₃, where M represents La, Sm, Y, Yb, or Sc, was examined. Only Sm₂O₃, Y₂O₃, and Yb₂O₃ formed cubic solid solutions with ZrO₂ over most of this substitutional range. Scandia forms a wide cubic solid solution region with ZrO₂ at temperatures above 130°C whereas the cubic solid solution region at room temperature is narrow (6 to 8 mole % Sc₂O₃). Lanthana additions to ZrO₂produced no fluorite-type cubic solid solutions within the compositional range investigated. Generally, the electrical conductivity of these cubic solid solutions increased as the size of the substituted cation decreased and the electrical conductivity for each binary system attained a maximum at about 10 to 12 mole % M₂O₃.     

Synthesis and Evaluation of Doped Y2O3-Stabilized ZrO2 for the Production of Hydrogen

The preparation and characterization of single-phase sintered ceramic materials based on Y₂O₃-stabilized ZrO₂ with oxides of Ti, Zn, Ce, Cr, Mn, and U/Fe as third-component additives are described. These materials were prepared for a process to produce high-purity hydrogen and were evaluated for this process via measurements of electrical conductivity, stability, and transport number. Compositions with oxides of Ce and Cr in Y₂O₃-stabilized ZrO₂ are shown to be the most promising candidates.     

Low Loss Dielectrics in the Ca(B'1/2Nb1/2)O3 [B'=Lanthanides, Y] System

The Ca(B'_1/2Nb_1/2)O₃ [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] complex perovskites have been prepared by conventional solid-state ceramic route. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscopy methods. The ceramics have dielectric constant (ɛ_r) in the range 23–32, normalized Q -factor ( Q _u× f ) 11 000–38 000 GHz and temperature coefficient of resonant frequency (τ_f) −43–5.2 ppm/°C. The microwave dielectric properties of Ca(B'_1/2Nb_1/2)O₃ ceramics are found to depend on the ionic radii of B'-site elements and tolerance factor ( t ). The substitution of Ba²⁺ and Sr²⁺ for Ca²⁺ resulted a phase transition in Ca(B'_1/2Nb_1/2)O₃ ceramics. The (Ca_0.05Ba_0.95) (Y_1/2Nb_1/2)O₃ has τ_f close to zero (1.2 ppm/°C) with ɛ_r=35 and Q _u× f =48 500 GHz and is proposed as a useful material for base station applications. Dielectric properties of the Ca(B'_1/2Nb_1/2)O₃ ceramics were tailored by the addition of TiO₂ and CaTiO₃.     

Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.     

Superconducting Properties of Barium-Doped (Bi,Pb)2Sr2Ca2Cu3Ox Glass-Ceramics

The Ba-doped superconducting (Bi,Pb)₂Sr_{2- x} Ba _x Ca₂Cu₃O _y and (Bi,Pb)₂Sr₂Ca_{2- x} Ba _x Cu₃O _y (0 ≦ x ≦ 1.0) were prepared by using a melt-quenching method, and the effect of Ba additions on the glass-forming ability and the crystalline phase was examined. The glass-forming ability was not improved by substitution of Ba for Sr or Ca, and particularly BaPbO₃ as well as CaO was observed in the melt-quenched sample of (Bi,Pb)₂SrBaCa₂Cu₃O _y . BaPbO₃ crystals were precipitated in all glass-ceramics with Ba substituted for Sr or Ca. The partial substitution of Ba substituted for Sr was effective for the formation of the high- T _c phase, and (Bi,Pb)₂Sr_1.4Ba_0.6Ca₂Cu₃O _y glass-ceramics obtained by annealing at 830°C for 100 h exhibited superconductivity with a T _c of 103 K, although BaPbO₃ and the low- T _c phase were still largely present.     

Electronic Conductivity, Seebeck Coefficient, and Defect Structure of LaFeO3

Electronic conductivity and Seebeck coefficients of LaFeO₃ were measured as a function of temperature (1000° to 1400°C) and P ( O ₂) (10⁵ to 10⁻¹³ Pa). Electronic conduction was found to be n-type in the lower P ( O₂ ) range, and p -type in a higher P(O₂) range. The calculated carrier mobilities suggest a hopping-type conduction mechanism. The carrier concentrations were calculated as a function of P ( O₂ ) and the defect structure was described. It was found that the electrical properties of LaFeO₃ are determined not only by the concentration of oxygen vacancies, but also by the La/Fe ratio.     

Microwave Dielectric Properties of Sr(B'1/2Nb1/2)O3 [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] Ceramics

Sr(B'_1/2Nb_1/2)O₃ [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] ceramics were prepared by the conventional solid-state ceramic route. The crystal structure and microstructure of the ceramics were characterized by X-ray diffraction and scanning electron microscopic methods, and the dielectric properties were measured in the microwave range. Addition of a small amount of CeO₂ as a sintering aid improved the densification and dielectric properties of the ceramics. The effects of cation substitution and glass addition on the microwave dielectric properties of the ceramics were also investigated. Glass addition resulted in the lowering of the processing temperature of the materials without much affecting the dielectric properties, whereas cation substitution resulted in the variation of the temperature coefficient of resonant frequencies. A correlation of dielectric properties with the tolerance factor and ionic radii of B'-site elements of the ceramics has been observed.     

Effects of Cd-Substitution Site on the Electrical Conductivities in Pb(Ni1/3Nb2/3)O3-PbZrO3-PbTiO3 Ceramics

The effects of Cd²⁺ substitution for Pb²⁺ and Ni²⁺ ions (on the A-site and B-site of the ABO₃ perovskite structure, respectively) on the electrical conductivities in Pb(Ni_1/3-Nb_2/3)O₃-PbZrO₃-PbTiO₃ (PNN-PZ-PT) ceramics have been investigated. Generally, the compounds that contain PbO show p -type conductivity, because of PbO evaporation. Thus, the conductivities are known to be proportional to the PbO evaporation. However, PNN-PZ-PT ceramics exhibit a reciprocal relationship between the conductivities and the evaporation of PbO. Generally, no vacancy change is observed with the substitution of the same-charge valence ion. However, in the PNN-PZ-PT ceramic, lead vacancies could be created or annihilated by replacing Pb²⁺ and Ni²⁺ ions with Cd²⁺ ions, because of incomplete substitution. The electrical conductivities are influenced by this incomplete substitution.     

Electrical Conductivity of A12O3: Fe + Y

High-temperature dc electrical conductivity and emf of oxygen concentration cells with A1₂O₃ as the electrolyte were studied. The defect structure of α-Al₂O₃ doped with Fe and Y was investigated to test an explanation proposed for the favorable effect of Y addition to super alloys (Fe, Cr, Ni, Al) which leads to well-adherent and nonconvoluted A1₂O₃ oxide scales. Results indicate that Y is a singly ionizable donor in Al₂O₃ and Y additions are effective in compensating Fe acceptors in A1₂O₃ at iron concentrations up to the solubility limit of Y. At higher acceptor concentrations leading to a decrease in [ V "'_Al] and an increase in [Al^…_i], incorporation of Y leads to a small increase in the concentration of V'" _Al. This suggests that the mechanism proposed for the prevention of oxide scale spallation based on the Y donor action to suppress the bulk diffusion of aluminum interstitials created by the Fe acceptors cannot explain the effect. Energy level positions of Fe and Y in A1₂O₃ are estimated, and values for electron and hole mobility at 1500°C are derived.