Grain-Boundary and Grain Electrical Resistances in CoxFe3−xO4

The complex impedance analysis technique was used to separate the grain-boundary and grain impedances of ferrites of the system Co_xFe_3−xO₄. All the samples, for x = 0.886, 0.960, 0.980, 0.995, 0.999, 1.005, 1.011, 1.021, 1.041, and 1.126, were fabricated by the traditional routine under the same conditions of preparation and sintering. The variations of both grain-boundary and grain resistances with temperature were evaluated in the frequency range 100 Hz < f < 40 MHz. The values of the activation energies for both conduction processes were determined from the Arrhenius plots, and the variations of these activation energies and of the resistances at 20° and 150°C were drawn as a function of x. A sharp change in these quantities was observed at the stoichiometric composition (x = 1). Explanations and suggestions for the observed behaviors have been made.     

Influence of Iron Doping on the Microstructure of YBa2(Cu1−xFex)3O7−δ Ceramics

Porosity, grain growth, phase composition, and microstructural defects were studied in sintered YBa₂ (Cu_1−x)₃O_7−x ceramics for x values up to 0.3. The porosity of the samples, related to the sintering mechanism, was independent of iron concentration. A linear dependence of the grain size with the inverse of the iron concentration was found, strongly suggesting grain boundary segregation of iron. The solubility limit was estimated to be x = 0.18 at 950°C in O₂. Beyond this limit, a new microstructural component was found consisting of YBa₂(Cu_1−xFe_x)₃O_7−δ, YBaCuFeO₅ and Ba(Cu,Fe)O₂. The transition from an orthorhombic twin to an orthorhombic tweed phase and a tetragonal phase was detected by polarized light microscopy.     

Microstructure and Piezoelectric Properties of (1−x)(Na0.5K0.5)NbO3–xLiNbO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃– x LiNbO₃ [(1− x )NKN– x LN] ceramics were produced by the conventional solid-state sintering method, and their microstructure and piezoelectric properties were investigated. The formation of the liquid phase and K₆Li₄Nb₁₀O₃₀ second phase that were observed in the (1− x )NKN– x LN ceramics was explained by the evaporation of Na₂O during the sintering. A morphotropic phase boundary (MPB) was observed in the specimens with 0.05< x <0.08. Promising piezoelectric properties were obtained for the specimens with x =0.07. Therefore, the piezoelectric properties of this 0.93NKN–0.07LN ceramic were further investigated and were found to be influenced by their relative density and grain size. In particular, grain size considerably affected the d ₃₃ value. Two-step sintering was conducted at different temperatures to increase the grain size. Piezoelectric properties of d ₃₃=240 (pC/N) and k _p=0.35 were obtained for the 0.93NKN–0.07LN ceramics sintered at 1030°C and subsequently annealed at 1050°C.     

Defect Structure and Electrical Properties of La1−ySryFe1−xAlxO3−δ

La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were studied as potential materials for solid-oxide fuel cell (SOFC) cathodes. The phase relations in the LaFeO₃–SrFeO_3−δ–LaAlO₃ system were investigated by X-ray powder diffraction analysis. The defect structure of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites was investigated by Mössbauer spectroscopy and weight-loss analysis. Relations between the nonstoichiometry and the conductivity of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were investigated. The incorporation of aluminum ( x ) into LaFe_{1− x} Al_xO₃ was found to have no influence on the defect structure but to decrease the conductivity. The incorporation of strontium ( y ) into La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ promotes the formation of anion vacancies and Fe⁴⁺ that lead to higher conductivity.     

High-K Dielectric Ceramics from Donor/Acceptor-Codoped (Ba1−xCax)(Ti1-yZry)O3 (BCTZ)

(Ba_{1− x} Ca _x )(Ti_{1− y} Zr _y )O₃ ceramics doped with various donors and acceptors were sintered in a reducing atmosphere and then annealed at different oxygen partial pressures. The dielectric maxima at the Curie point increased up to ∼30000 after reoxidation, depending on the grain size, the type of acceptor, and the annealing conditions. The increase of the dielectric maxima seems to be correlated with oxidation of the grain boundaries. The dielectric properties show relaxation effects with increased frequency.     

Water Corrosion of YBa2Cu3O7−δ Superconductors

Solid-state sintering was used to make YBa₂Cu₃O_7−δ superconducting bulk materials. Corrosion of the YBa₂Cu₃O_7−δ superconductor material was investigated in a humid environment. The superconducting materials exhibited significant corrosion after 4 h at 80° and 100% relative humidity. A grain-boundary phase was formed, and the percent superconducting phase in the material decreased by approximately 60%. The transition temperature (T_c) decreased with corrosion time. After 2 h of corrosion, T_c decreased from 87 to 81 K.     

Study of Electrical Conduction Behavior of the System La1−xNaxCo1−xNbxO3 (x 0.50)

Electrical behavior of a few compositions with x 0.50 in the system La_{1− x} Na _x Co_{1− x} Nb _x O₃ has been studied by complex-plane-impedance analysis in the temperature range 300–525 K. Three depressed semicircular arcs have been observed in the complex impedance plot of the composition with x = 0.50. These arcs represent the contributions of bulk, grain boundaries, and electrode polarization to the total observed resistance. Two semicircular arcs have been observed in the complex-impedance plot of the compositions with x = 0.70 and 0.80. They represent the bulk and the grain boundaries contribution to the total observed resistance. The values of the resistance for the bulk and grain boundaries in each sample indicate the formation of barrier layers in these materials.     

Phase Relations in the Na0.5Bi0.5TiO3–Li3xLa(2/3)−x□(1/3)−2xTiO3 System

The phase relations and the mechanism of solid-state synthesis for the Na_0.5Bi_0.5TiO₃–Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} TiO₃ system were investigated using X-ray powder diffraction, scanning electron microscopy, and thermal analysis. The study revealed that the extent of the homogeneity range—which is related to the A-site substitution between (Na_0.5Bi_0.5)²⁺ and (Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} )²⁺ pseudo cations of a perovskite structure—depends strongly on the ordering of the (Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} )²⁺ species. The solid-state reaction of the compounds in the homogeneity range is completed only after multiple high-temperature firings. However, the system is also subjected to a slow thermal decomposition; this is particularly so for the compounds with a high × value and an increased Li_{3 x} La_{(2/3)− x} □_{(1/3)−2 x} TiO₃ concentration.     

Superconductivity in the (Yba2Cu3)1−xNaxO7−δ System

The (YBa₂Cu₃)_1−xNa_xO_7–δ system in the range of x = 0–0.8 was investigated. Experimental data suggest that the sodium doping with x 0.26 does not affect the critical transition temperature T_c, and the crystal structure maintains the orthorhombic lattice with a slightly smaller unit cell. However, sodium doping increases the sintering and grain growth kinetics, resulting in a higher superconducting phase volume and an enhanced Meissner effect. It also lowers the processing temperaturel. The experimental data also suggest that the sodium atoms diffuse into the superconducting YBa₂Cu₃O_7−δ crystallites, which stabilizes the orthorhombic phase. The transition temperature (ortho-rhombic to tetragonal) in sodium-doped materials increases with the increasing concentration of sodium.     

Bismuth/Samarium Cosubstituted Ba6−3xNd8+2xTi18O54 Microwave Dielectric Ceramics

Modification of the microwave dielectric properties in Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ( x = 0.5) solid solutions by Bi/Sm cosubstitution for Nd was investigated. A large increase in the dielectric constant and near-zero temperature coefficient combined with high Qf values were obtained in modified Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ solid solutions where an enlarged solid solution limit of Bi in Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ was observed. Excellent microwave dielectric characteristics (ɛ= 105, Qf = 4110 GHz, and very low τ_f) were achieved in the composition Ba_{6−3 x} (Nd_0.7Bi_0.18Sm_0.12)_{8+2 x} Ti₁₈O₅₄.     

Nonisothermal Crystallization Kinetics of BixY3−xFe5O12 (0.25≤x≤1.00) Prepared from Coprecipitation Process

The nonisothermal crystallization kinetic of Bi _x Y_{3− x} Fe₅O₁₂ (0.25≤ x ≤1.00) powders prepared by coprecipitation process has been investigated. The activation energy of crystallization was calculated by differential scanning calorimetry at different heating rates. The activation energies of crystallization of Bi _x Y_{3− x} Fe₅O₁₂ system are 492, 447, 377, and 353 kJ/mol and the Avrami constant n are 3.49, 2.25, 2.48, and 2.33 for x =0.25, 0.50, 0.75, and 1.00, respectively. The Avrami exponent values (1< n <3) were consistent with surface and internal crystallizations occurring simultaneously for 0.50≤ x ≤1.00, the value ( n >3) for the Avrami exponent was consistent with bulk crystallization domination in Bi _x Y_{3− x} Fe₅O₁₂ system. The results reveal that increasing the substitution amount of bismuth for yttrium would significantly decrease activation energy in Bi _x Y_{3− x} Fe₅O₁₂ system.     

Methane Oxidation on Perovskite-Type Ca(Mn1−xTix)O3−δ

X-ray photoelectron spectroscopy was performed to elucidate the catalytic activity of CH₄ oxidation on perovskite-type Ca(Mn_{1− x} Ti _x )O_3−δ synthesized at 1173 K in a flow of oxygen from a gel with citric acid and ethylene glycol. The Mn ion content decreases and the ratio of the Mn³⁺ ion in the Mn ion increases with increases in x . Ca(Mn_{1− x} Ti _x )O_3−δ has a high catalytic activity of CH₄ oxidation at x =0.4. These results indicate that the catalytic activity strongly depends on the Mn³⁺ ion content of the surface.     

Superplastic Deformation in Fine-Grained YBa2Cu3O7−x

The superplastic behavior of YBa₂Cu₃O_{7− x} ceramic superconductors was studied. Large compressive deformation over 100% strain was measured in the temperature range of 775°–875°C, with a strain rate of 1 × 10⁻⁵ to 1 × 10⁻³/s, and a grain size of 0.5–1.4 μm. The nature of the deformation was investigated in terms of three deformation parameters: the stress exponent ( n ), the grain size exponent ( p ), and the activation energy ( Q ). The measured values of these parameters were n = 2 ± 0.3, p = 2.7 ± 0.7, and Q = 745 ± 100 kJ/mol. With the aid of the deformation map, the deformation mechanism was identified as grain boundary sliding accommodated by grain boundary diffusion. The conclusion is consistent with the microstructural observations made by SEM and TEM: the invariance of equiaxed grain shape, the absence of significant dislocation activity, no grain boundary second phases, and no significant texture development.     

The Li2O–Nb2O5–TiO2 Composite Microwave Dielectric Ceramics with Adjustable Permittivities

A group of new y M-phase/(1− y ) Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ composite ceramics with adjustable permittivities for low-temperature co-fired ceramic applications was initially investigated in the study. The 0.5 M-phase/0.5 Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ ( x =0.01, 0.02, 0.04, 0.06, 0.081) composite ceramics were first investigated to find the appropriate "Li₂TiO₃ss" composition ( x value). The best dielectric properties of ɛ_r=40.1, Q × f values up to 9318 GHz, τ_f=25 ppm/°C, were obtained for the ceramics composites at x =0.02. Based on the good dielectric properties, the suitable "Li₂TiO₃ss" composition with x =0.02 was mixed with the Li_1.0Nb_0.6Ti_0.5O₃ powder as the ratio of y "M-phase"/(1− y ) "Li₂TiO₃ss" ( y =0.2, 0.4, 0.5, 0.6, 0.8). By adjusting the y values, the group of composite ceramics could exhibit largely are adjustable permittivities varying from ∼20 to ∼60, while Q × f and τ_f values relatively good. Nevertheless, in this study, because there are interactions between the M-phase and Li₂TiO₃ss during sintering process, their microwave dielectric properties could not be predicted precisely by the empirical model.     

Enhanced Visible Light-Driven Photocatalytic Performance of La-Doped TiO2−xFx

La-doped TiO_{2− x} F _x (La–TiO_{2− x} F _x ) powders were prepared by the sol–gel method. X-ray diffraction results showed that La efficiently inhibited grain growth. X-ray photoelectron spectroscopy spectra revealed that La₂O₃ and O–Ti–F bonds have formed, the La₂O₃ maintained the high surface area of TiO_{2− x} F _x after calcination at a temperature above 500°C, while the O–Ti–F bonds increased the oxidation potential of the photogenerated hole in the valence band. The UV-vis spectroscopy of the La–TiO_{2− x} F _x showed that the presence of intraband gap states was likely responsible for its absorption of visible light. When the molar ratios of La and F to Ti were 1.5:100 and 5:100, respectively, and calcined at 500°C, the photocatalytic degradation rate of methylene blue over La–TiO_{2− x} F _x was about 1.5 times higher than that of F-doping TiO₂.     

Microstructure and Optical Properties of Cd1−x ZnxS Films Sintered with Cadmium Chloride

Polycrystalline Cd_1−xZn_xS films were prepared by coating (1 - x) CdS + xZnS slurry which contained various amounts of CdCl₂ on amorphous glass substrate and sintering in nitrogen to produce films with properties suitable for fabricating all-polycrystalline Cd_1−xZn_xS/CdTe heterojunction solar cells. Optical properties of these films have been correlated with composition and microstructure. The sintered film has undulating grain boundaries due to the occurrence of chemically induced grain-boundary migration. By optimizing the amount of CdCl₂ in the slurry and sintering conditions, it is possible to produce sintered Cd_1−xZn_xS films on glass substrates with high optical transmission and to produce Cd_1−xZn_xS/CdTe solar cells with efficiency higher than 11%.     

Glass-Free KxBa1−xGa2−xGe2+xO8 Ceramics for Low-Temperature Cofired Ceramics Technology: Synthesis, Phase Transitions, Sintering, and Microwave Dielectric Properties

K _x Ba_{1− x} Ga_{2− x} Ge_{2+ x} O₈ (0.6≤ x ≤1) polycrystalline ceramics are potential materials for glass-free low-temperature cofired ceramics (LTCC) substrates. We have made a comprehensive study of the kinetics of the monoclinic-to-monoclinic P 2₁/ a ⇔ C 2/ m phase transition. The low-temperature-stable P 2₁/ a phase with a high Q × f value was synthesized using a subsolidus method and was well sintered at the LTCC temperature with a H₃BO₃ additive. A good combination of low sintering temperature (910°–920°C), high Q × f values (96 700–104 500 GHz), low permittivities (5.6–6.0), and a small temperature coefficient of resonant frequency (∼−20 ppm/°C) was obtained for ceramics with x =0.67 and 0.9 and with 0.1 wt% of H₃BO₃.     

Ionic Conductivity Enhancement of La2Mo2−xWxO9 Nanocrystalline Films Deposited on Alumina Substrates by the Sol–Gel Method

Dense, crack-free, and uniform La₂Mo_{2− x} W _x O₉ ( x =0, 0.1, and 0.2) nanocrystalline films were successfully synthesized on poly-alumina substrates via a modified sol–gel method, with inorganic salt of La(NO₃)₃·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O, and (NH₄)₆H₂W₁₂O₂₄ as precursors. Pure La₂Mo₂O₉ phase was confirmed by X-ray diffractometer when the annealing temperature was >500°C. The average grain size of the La₂Mo_{2− x} W _x O₉ films is in the range of 90–400 nm, depending upon the conditions of thermal treatment, and the thickness of films can reach 1 μm by repetitive spin-coating. The electrical conductivity increases with decreasing grain size and reaches 0.074 S/cm at 600°C in the film with a grain size of 90 nm, which is one order of magnitude higher than that in the corresponding bulk materials. W-doping can suppress the phase transition that occurs at 580°C in pure La₂Mo₂O₉ and enhance the low-temperature ionic conductivity. Furthermore, the activation energy of conductivity in the nanocrystalline La₂Mo₂O₉ films decreases to about 0.6 eV in comparison with 1.0 eV in the bulk ones, which implies that the grain resistance prevails in the total resistance, when grain size reduces to nanometer domain.     

Effect of Grain Shape on Abnormal Grain Growth in Liquid-Phase-Sintered Nb1−xTixC–Co Alloys

The effect of titanium substitution for niobium on the grain shape change, grain growth inhibition, and abnormal grain growth during liquid-phase sintering of Nb_{1− x} Ti_xC–Co alloy was studied. With increased titanium substitution, the shape of the Nb_{1− x} Ti _x C grains in the liquid cobalt matrix was changed from a cube with round corners to a cube with angular corners, which implied increased edge energy. As the grain corners became more angular, the grain growth became markedly inhibited, and abnormal grain growth occurred. The results could be best explained by the increased edge energy of the interface of the Nb_{1− x} Ti _x C grains, which increased the barrier for the growth by two-dimensional nucleation.     

Microstructure, Density, and Dielectric Properties of Lead-Free (K0.44Na0.52Li0.04)(Nb0.96−xTaxSb0.04)O3 Piezoelectric Ceramics

Lead-free (K_0.44Na_0.52Li_0.04) (Nb_0.96−xTa_xSb_0.04)O₃ piezoelectric ceramics were prepared by the conventional solid-state sintering method. The grain growth of the ceramics was inhibited and the relative density was improved with Ta substituting for Nb. Increasing x led to different variations of dielectric properties before and after poling, and prevented the occurrence of orthorhombic–tetragonal phase transition (at T _{o − t} ). All the ceramics show an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics. Significantly enhanced dielectric and piezoelectric properties were obtained in the ceramics with x =0.20. The ceramics are very promising lead-free materials for electromechanical device applications.