Dielectric Properties of the Bi(1.6−0.8x)YxTi2O(6.4+0.3x) (0.03<x<2) Pyrochlore Solid Solution

We have studied the dielectric properties of a new, cubic pyrochlore-type solid solution Bi_{(1.6−0.8 x )}Y _x Ti₂O_{(6.4+0.3 x )} (0.03< x <2). The single-phase nature of the pyrochlore solid solution was determined using X-ray diffraction and scanning electron microscopy. We found that at room temperature and 1 MHz the pyrochlore solid solution exhibited values of the dielectric constant ɛ between 127 and 64, and low dielectric losses, tan δ, of <6 × 10⁻³. The temperature coefficient of the dielectric constant τ_k is, however, strongly dependent on the composition. Below room temperature dielectric relaxation phenomena were observed for the compositions with low x values.     

Synthesis and Thermal Expansion Properties of the Ca(1+x)/2Sr(1+x)/2Zr4P6−2xSi2xO24 System

Ca_{(1+ x )/2}Sr_{(1+ x )/2}Zr₄P_{6−2 x} Si_{2 x} O₂₄( x ≤ 0.37) compositions, which belong to the [NZP] family of low-thermal-expansion materials, were synthesized using the solid-state reaction method. The lattice thermal expansion of members of this system was determined up to 1000°C by high-temperature X-ray diffractometry. The bulk thermal expansion and the microcracking during cooling also were investigated. These properties depended on the composition, on the synthesis method, and on the sintering conditions.     

Electrical Conduction Behavior of La1+xSr1−xGa3O7–δ Melilite-Type Ceramics

Ceramics of the melilite-type compound La_{1+ x} Sr_{1− x} Ga₃O_7−δ were prepared by conventional ceramic processing. Samples prepared represented the entire homogeneity region of the phase (i.e., x =−0.15 to 0.60). Electrochemical characterization under variable temperature and atmospheric conditions in the vicinity of air entailed four-point direct-current conductivity measurements and electromotive force measurements. La_{1+ x} Sr_{1− x} Ga₃O_7−δ samples exhibited a p -type behavior with generally increased conductivity with increased substitution of lanthanum for strontium, which reached a saturation value of ∼10⁻¹ S·cm⁻¹ at 950°C.     

Low-Loss Microwave Dielectrics Using Mg2(Ti1−xSnx)O4 (x=0.01–0.09) Solid Solution

Low-loss ceramics having the chemical formula Mg₂(Ti_{1− x} Sn _x )O₄ for x ranging from 0.01 to 0.09 have been prepared by the conventional mixed oxide route and their microwave dielectric properties have been investigated. X-ray powder diffraction patterns indicate the corundum-structured solid solutions for the prepared compounds. In addition, lattice parameters, which linearly increase from 8.4414 to 8.4441 Å with the rise of x from 0.01 to 0.09, also confirm the forming of solid solutions. By increasing x from 0.01 to 0.05, the Q × f of the specimen can be tremendously boosted from 173 000 GHz to a maximum 318 000 GHz. A fine combination of microwave dielectric properties (ɛ_r∼15.57, Q × f ∼318 000 GHz at 10.8 GHz, τ_f∼−45.1 ppm/°C) was achieved for Mg₂(Ti_0.95Sn_0.05)O₄ ceramics sintered at 1390°C for 4 h. Ilmenite-structured Mg(Ti_0.95Sn_0.05)O₃ (ɛ_r∼16.67, Q × f ∼275 000 GHz at 10.3 GHz, τ_f∼−53.2 ppm/°C) was detected as a second phase. The presence of the second phase, however, would cause no significant variation in the dielectric properties of the specimen, because the second phase properties are very similar to the primary phase. These unique properties, in particular, low ɛ_r and high Q × f , can be utilized as a very promising dielectric material for ultra-high-frequency applications.     

(Bi1/2Na1/2)TiO3–Ba(Cu1/2W1/2)O3 Lead-Free Piezoelectric Ceramics

(Bi_1/2Na_1/2)TiO₃ with 0–6 mol% Ba(Cu_1/2W_1/2)O₃ (BNT-BCW), a new member of the BNT-based group, has been prepared following the conventional mixed oxide route. The compacted bodies were sintered at 1130°C for 2 h to get dense ceramics. The addition of BCW into BNT ceramics facilitated the poling process because of a reduction in leakage current. 0.995BNT·0.005BCW ceramics exhibit a relatively high piezoelectric constant ( d ₃₃= 80 × 10⁻¹² C/N) and a relatively low dielectric loss (tan δ= 1.5%). Increased amount of BCW was found to increase the dielectric constant and loss of BNT-BCW ceramics and to suppress the grain growth. During sintering, some BCW diffuses into the lattice of BNT to form a solid solution and some remains on the grain boundaries.     

Oxygen Sensors Based on the Electrical Conductivity of the Solid Solutions (Mg1–xFex)O and(Mg1–xCox)O

Solid solutions of general compositions (Mg_{1- x}Fe_x)O and (Mg_1-xCo_x)O (0.05 ≤ x ≤ 0.25) were investigated with regard to the dependence of their specific electrical conductivity on the oxygen partial pressure (10⁻¹⁷≤ po₂≤ 10⁵ Pa) at 900^c and 800°C. The experimental results, especially the slopes of plots of log σ vs log po₂ and the stability of the solid solutions studied, indicate that some of these compositions could be used as oxygen sensors at high temperatures.     

Compact Swelling in Bi1.4Pb0.6Sr2Ca2Cu3.6Oy During the Formation of High-Tc Superconducting Phase

Compact swelling in Pb-doped Bi-Sr-Ca-Cu-O superconductor has been studied by observing the effects of the size of calcined powders, volatilization of materials, and sintering of high- T _c (2223) powders. The bulk density increases at the early stage of sintering, for about 20 h, and then decreases. Densification occurs when the low- T _c (2212) phase and a liquid phase exist, whereas dedensification occurs with the formation of the 2223 phase regardless of the presence of the liquid. Gas evolution from specimens does not appear to be responsible for compact swelling. Compact swelling is explained by anisotropic growth of thin, platelike 2223 grains in random orientation. When 2223 grains grow in a preferred direction, compact swelling is suppressed.     

La1+xSr1–xGa3O7–δ Melilite-Type Ceramics - Preparation, Composition, and Structure

Ceramic samples of the melilite-type La_{1+ x} Sr_{1– x} Ga₃O_7–δ ( x =−0.15 to 0.60) compound were prepared by conventional ceramic processing. Sintering characteristics and microstructural evolution were studied. A phase diagram study was performed to establish the solid solubility limits as a function of the La:Sr ratio. Structural investigations of the Dalton composition as well as strontium- and lanthanum-rich samples entailed X-ray, neutron, and electron diffraction techniques at ambient and elevated temperatures. The homogeneity region was remarkably broad ( x =−0.15 to 0.60) with no changes in space group observed. A small shrinkage of the unit cell was found with increased lanthanum content. Phase transitions at ambient and intermediate temperatures did not occur.     

Dielectric Properties of the Fluorite-like Bi2O3–Nb2O5 Solid Solution and the Tetragonal Bi3NbO7

Our analysis of the microwave dielectric properties of the δ-Bi₂O₃–Nb₂O₅ solid solution (δ-BN_ss) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb₂O₅. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb₂O₅. At the same composition there is a discontinuity in the grain size of the δ-BN_ss ceramics. For the sample containing 25 mol% Nb₂O₅ two structural modifications were observed. A single-phase tetragonal Bi₃NbO₇, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ-BN_ss homogeneity range. Compared with the δ-BN_ss the Bi₃NbO₇ ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.     

Solid-Solution Ranges of the n= 2 and n= 3 Superconducting Phases in Bi2(SrxCa1 −x)n+1CunOy and the Effect on Tc

The cation solubility limits of the n = 2 and n = 3 superconducting phases in the Bi₂(Sr _x Ca_{1 − x} ) _{n +1}Cu _n O _y system were established along tie lines with compatible phases via electron probe microanalysis on bi- (or poly-) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n = 3 phase. In each case football-shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi:Sr:Ca:Cu) with the long axis parallel to the Sr-Ca side of the quaternary (i.e., Sr-to-Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline-earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. T _c variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c -axis parameter.     

Electrical Conductivities of (CeO2)1−x(Y2O3)x Thin Films

Electrical properties of CeO₂ thin films of different Y₂O₃ dopant concentration as prepared earlier were studied using impedance spectroscopy. The ionic conductivities of the films were found to be dominated by grain boundaries of high conductivity as compared with that of the bulk ceramic of the same dopant concentration sintered at 1500°C. The film grain-boundary conductivities were investigated with regard to grain size, grain-boundary impurity segregation, space charge at grain boundaries, and grain-boundary microstructures. Because of the large grain boundary and surface area in thin films, the impurity concentration is insufficient to form a continuous highly resistive Si-rich glassy phase at grain boundaries, such that the resistivity associated with space-charge layers becomes important. The grain-boundary resistance may originate from oxygen-vacancy-trapping near grain boundaries from space-charge layers. High-resolution transmission electron microscopy coupled with a trans-boundary profile of electron energy loss spectroscopy gives strong credence to the space-charged layers. Since the conductivities of the films were observed to be independent of crystallographic texture, the interface misorientation contribution to the grain-boundary resistance is considered to be negligible with respect to those of the impurity layer and space-charge layers.     

Retrograde Densification in Bi2Sr2CaCu2O8 Superconductors

Bi₂Sr₂CaCu₂O₈ was prepared using the mixed oxide-carbonate method and sintered at temperatures ranging from 850° to 911°C. The samples were characterized for density, mechanical strength, phase composition, microstructure, and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated in the temperature range 850° to 890°C whereby the material first becomes less dense as the sintering temperature is raised, and only in a narrow temperature range from 900° to 905°C does the material densify then with the formation of a liquid phase. The retrograde densification mechanism is shown to be that of the formation of thin platelike crystallites which grow in a randomly oriented fashion, thus pushing the structure apart. This retrograde densification, coupled with a narrow sintering range overlapping the melting temperature, makes this compound a difficult one to process.     

Transitional Phase of Ca2SiO4 Solid Solution with Incommensurate Superstructure

Crystals of Ca₂SiO₄ doped with K, P, Al, and Fe were grown and examined by means of both X-ray and electronbeam diffraction. They are made up of domains 120° different in orientation around the c axis of the previous α phase. Each of the domains gives an orthorhombic superstructure whose cell is incommensurate with the underlying orthorhombic subcell with a=0.540, b=0.936, and c=0.681 nm. This orthorhombic phase, characterized by the cell edge length of 3.8a, is termed 3.8aC₂S(ss). Both the domain and the incommensurate structure strongly suggest that this is the precursor to a phase thermodynamically more advantageous at lower temperatures.     

High-Temperature Mechanical Properties and Chemical Stability of Ba1+xZr4P6–2xSi2XO24 Low-Thermal-Expansion Ceramics

The NaZr₂P₃O₁₂ (NZP) family of materials is attracting increasing attention due to its low-thermal-expansion behavior. The system Ba_1+xZr₄P_6–2xSi_2xO₂₄ (0 ≤ x ≤ 1), belonging to the NZP family, shows ultralow thermal expansion over a wide temperature range. It also shows anisotropy in its lattice thermal expansion. This causes microcracking as the sintered specimens are cooled, which results in degradation of the mechanical properties. In this work, the chemical stability, strength, and Young's modulus of Ba_1+xZr₄P_6–2xO₂₄ ( X = 0.25 and 0.5) ceramics at high temperatures have been determined. An attempt has been made to correlate the mechanical properties to the thermal expansion anisotropy.     

Structural and Electrical Properties of W6+-Doped Bi3TiNbO9 High-Temperature Piezoceramics

High-temperature piezoceramics based on W⁶⁺-doped Bi₃TiNbO₉ (BTNO) were prepared by solid-state reaction sintering. A study of the donor doping effects on the structure, microstructure, and electrical properties was conducted. The Curie point ( T _c) decreased in the range of 860°–910°C with increasing amount of W⁶⁺doping. W⁶⁺ donor dopant decreased the conductivity in BTNO by as much as two orders of magnitude. The increased resistivity in W⁶⁺-doped BTNO induced the materials to be polarized under a high enough field. The highest piezoelectric constant, d ₃₃, of 12.0 pC/N was achieved with the composition of Bi₃Ti_0.99W_0.01NbO₉. Also, this composition had a good thermal depoling resistance property.     

High-Temperature Stability of Ca2ZrSi4O12

The standard Gibbs free energy of formation of orthorhombic Ca₂ZrSi₄O₁₂ from component oxides ZrO₂ (monoclinic), CaO (rock salt), and SiO₂ (quartz) has been determined in the temperature range 973 to 1273 K using a solid-state cell incorporating single-crystal CaF₂ as the electrolyte: This is the only quantitative information now available on the stability Ca₂ZrSi₄O₁₂.     

Influence of Ca2PbO4 on Phase Formation and Electrical Properties of (Bi,Pb)2Sr2Ca2Cu3O10/Ag Superconducting Composites

(Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) precursor powders with large and small amounts of Ca₂PbO₄ phase were prepared and used to make superconductor/silver composite tapes. The melting behavior of the powders and tapes was examined by differential thermal analysis (DTA). The influence of Ca₂PbO₄ on the formation and microstructure of Bi2223, and electrical properties of the tapes, was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and critical current measurements. It was found that the melting onset temperature ( T _m,onset) of precursor powders and composite tapes was strongly dependent on the amount of Ca₂PbO₄. Tapes with a small amount of Ca₂PbO₄ had a higher T _m,onset and a higher optimum sintering temperature compared with tapes with a large amount of Ca₂PbO₄. Also because of the higher sintering temperature, the total sintering time required for the former tapes was drastically shortened compared with the latter ones (250 vs 110 h). Furthermore, the microstructure and the current-carrying capacity of the tapes were significantly improved by reducing the Ca₂PbO₄ content of the precursor powders. These results are of practical significance for the commercialization of Bi-based high-temperature superconductors.     

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.     

High-Temperature Creep Behavior of Mixed Conducting La0.5Sr0.5Fe1−xCoxO3-δ (0.5≤x≤1) Materials

Steady-state compressive creep rate of La_0.5Sr_0.5Fe_0.5Co_0.5O_3−δ (LSFC) and La_0.5Sr_0.5CoO_3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N₂ compared with air. Relaxation by creep of chemical-induced stresses in oxygen-permeable membranes is addressed, especially at low partial pressure of oxygen.