Synthesis and sintering properties of (La0.8Ca0.2−x Sr x )CrO3 perovskite materials for SOFC interconnect

Synthesis and sintering properties of the (La_0.8Ca_0.2−x Sr _x )CrO₃ samples doped by two alkaline earth metals in comparison to the doped only by one alkaline earth metal were evaluated by phase analysis, sintering properties, thermal expansion behaviors, and electrical conductivity. The sintered (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) and (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0.2) were found to have orthorhombic and rhombohedral symmetries, respectively. Relative density of the (La_0.8Sr_0.2)CrO₃ sample sintered at 1500^∘C for 5 h was lower than that of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) sample. TECs of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, 0.1, and 0.2) in air were 11 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, and 11.3 × 10⁻⁶/^∘C, respectively. The electric conductivity of the (La_0.8Ca_0.2−x Sr _x )CrO₃ sample was determined.     

The effect of alumina addition on the electrical conductivity of Gd-doped ceria

Acceptor doped-ceria is a possible electrolyte material for the IT-SOFC (intermediate temperature solid oxide fuel cell) due to its high oxygen-ion conductivity. However, its use has been limited by its mechanical weakness and the appearance of electronic conductivity in reducing condition. In this study, alumina was selected as an additive in the doped-ceria to see if it increases the oxygen-ion conductivity and mechanical strength. Effects of alumina addition in doped ceria were studied as a function of alumina content and acceptor (Gd) content. The electrical conductivity of (Ce_1−x Gd _x O_2−δ)_1−y + (Al₂O₃) _y (x = 0–0.35, y = 0–0.10) was measured by using impedance spectroscopy. The grain conductivity of Ce_0.8Gd_0.2O_2-δ (GDC20) with 5 mol% alumina increased ∼3 times from that of GDC20 at 300^∘C. The grain conductivity was even ∼2 times higher than that of Ce_0.9Gd_0.1O_2−δ (GDC10) at 300^∘C. The electrical conductivity of GDC20 without alumina addition, measured at 500^∘C in air, rapidly decreased after exposure to reducing condition (Po₂∼10⁻²² atm) at 800^∘C. However, the decrease was much slower in GDC20 with alumina addition, indicating the improved mechanical strength. Among the examined compositions, (Ce_0.75Gd_0.25 O_2-δ)_0.95 + (Al₂O₃)_0.05 (GDC25A5) showed the highest conductivity at most temperatures.     

Low-temperature sintering and electrical properties of (1−x)Pb(Zr0.5Ti0.5)O3-xPb(Cu0.33Nb0.67)O3 ceramics

Electrical properties and sintering behaviors of (1 − x)Pb(Zr_0.5Ti_0.5)O₃-xPb(Cu_0.33Nb_0.67)O₃ ((1 − x)PZT-xPCN, 0.04 ≤ x ≤ 0.32) ceramics were investigated as a function of PCN content and sintering temperature. For the specimens sintered at 1050^∘C for 2 h, a single phase of perovskite structure was obtained up to x = 0.16, and the pyrochlore phase, Pb₂Nb₂O₇ was detected for further substitution. The dielectric constant (ε _r), electromechanical coupling factor (K_p) and the piezoelectric coefficient (d ₃₃) increased up to x = 0.08 and then decreased. These results were due to the coexistence of tetragonal and rhombohedral phases in the composition of x = 0.08. With an increasing of PCN content, Curie temperature (T_c) decreased and the dielectric loss (tanδ) increased. Typically, ε_r of 1636, K_p of 64% and d₃₃ of 473pC/N were obtained for the 0.92PZT-0.08PCN ceramics sintered at 950^∘C for 2 h.     

Ferroelectric properties of (Bi3.15 Nd 0.85 )Ti 3 O 12 with decreasing film thickness

Film texture and ferroelectric behaviors of (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNdT) of layered-perovskite structure deposited on Pt/TiO₂/Si substrate are dependent on the film thickness. When the film thickness is reduced from ∼240 to ∼120 nm, BNdT grains evolve from a rod-like morphology to a spherical morphology, accompanied by a decrease in average grain size. At the same time, P-E hysteresis transforms from a square-shaped hysteresis loop (2Pr ∼24.1 μC/cm² at 240 nm) to a relative slimmer hysteresis loop (with a lower 2Pr = 19.8 μC/cm² at 120 nm). The nonvolatile polarization (Δ P) shows a maximum at the film thickness of 160 nm, where Δ P was measured to be 14.7 μC/cm² and 6.8 μC/cm² at 5 V and 3 V, respectively. A small amount of excess bismuth in the film thickness of 130 nm, introduced by co-sputtering, can lead to a much enhanced remanent polarization (2Pr of 21.3 μC/cm² at 5 V and 15.2 μC/cm²at 3 V), which is promising for low voltage FRAM applications.     

Thermoelectric properties of p-type Bi0.5Sb1.5Te3 compounds fabricated by spark plasma sintering

P-type thermoelectric Bi_0.5Sb_1.5Te₃ compounds were prepared by the spark plasma sintering method with temperature ranges of 300–420^∘C and powder sizes of ∼75 μm, 76–150 μm, 151–250 μm. As the sintering temperature increased, the electrical resistivity and thermal conductivity of the compound were greatly changed due to an increase in the relative density. The Seebeck coefficient and electrical resistivity were varied largely with decreasing the powder size. Subsequently, the compound sintered at 380^∘C with the powders of ∼75 μm showed the maximum figure-of-merit of 2.65 × 10⁻³K⁻¹ and the bending strength of 73 MPa.     

Investigation on the design and synthesis of new systems of BNT-based lead-free piezoelectric ceramics

Three primary differences between BNT- and PZT-based ceramics were analyzed from the composition and the active component of the materials. Based on the analysis the authors’ group developed the new idea of the design of the multiple complex in the A-site ions of BNT compounds. (Bi_0.5Na_0.5)²⁺, Bi³⁺ and Na⁺ in the ABO₃ structure are defined as A-site, A₁-site and A₂-site ions, respectively, and A, A₁ and A₂-site ions can be simultaneously or singly substituted partially by alkaline-earth metal ions, metal ions with +3 valence and metal ions with +1 valence, respectively. Under this consideration, Several new systems of Bi_0.5Na_0.5TiO₃ (abbreviated as BNT)-based lead-free piezoelectric ceramics were proposed. These ceramics can be prepared by conventional ceramic techniques and have excellent piezoelectric performance. Among these materials, Bi_0.5(Na_1−x−y K _x Li _y )_0.5TiO₃ possesses higher piezoelectric constant (d ₃₃ = 230 pC/N), higher electromechanical couple factor (k _p = 0.40), larger remanent polarization (P _r = 38.9 μC/cm²) and a better P-E hysteresis loop until about 200^∘C. This work was supported by the projects of NSFC (50410179), (50572066), and (59972020), and NAMMC (2001-AA325060).     

Electrical conductivity of Nd1−x Ca x Sc1−y Mg y O3 perovskite-type oxides

The electrical conductivities of Nd_1−x Ca _x Sc_1−y Mg _y O₃ were measured in the temperature range from 673 to 1273 K under both wet and dry conditions. The optimum Ca²⁺ doping concentration to the Nd³⁺ site in NdScO₃ was found to be the range from 5 to 10%. Hole conduction was predominant under dry and highly oxidized conditions of P(O₂) > 10⁻² kPa in the temperature range from 973 to 1273 K, and proton conduction was predominant under wet and reduced conditions in the temperature range from 673 to 973 K. Oxide ion conduction was predominant under wet and reduced conditions in the temperature range from 1073 to 1273 K. The Mg²⁺ doping concentration limit for the Sc³⁺ site to hold a single crystal phase was 2%, and this doping enhanced hole conduction.     

Temperature Sensitive Properties of the La(Ti<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript> System

The electric mechanisms of perovskite-type LaMnO₃ was investigated with B-site substitution in this paper. Samples of La(Ti_xMn_{1 − x})O₃ (0.1 ≰ x ≰ 0.7) were sintered at different temperature. The voltage-temperature (V-T) curves of the samples were tested from room temperature (25^∘C) to 300^∘C, then the electric properties were measured and analyzed. The experimental results showed that the resistivity-temperature (ρ-T) curves of the samples matched NTC characteristic. The resistivity increased slightly with the increase of Ti amount as x was less than 0.5, however, it rose greatly after x exceeded 0.5; The sintering temperatures have a little influence on the resistivity, except for the sample with x = 0.7.     

The effect of Co addition on the electrical conductivity of Sr- and Mg-doped LaAlO3

Co was added to see its effect on the electrical conductivity of Sr- and Mg-doped LaAlO₃ (La_0.9Sr_0.1 Al_0.9Mg_0.1O₃, LSAM). Electrical conductivities of La_0.9Sr_0.1(Al_0.9Mg_0.1)_{1− x}Co_xO₃ (LSAMC) for x = 0–0.20 were measured using 2-probe a.c. and 4-probe d.c. method at temperature between 300 and 1300^∘C in air, and as a function of Po ₂ (1–10⁻¹⁵ atm) at 1200^∘C. Electrical conductivities in air increased with increasing Co content, while their activation energy decreased. From the impedance spectroscopy analysis, it was found that both the grain and the grain boundary conductivities of LSAMC samples increased rapidly with Co-addition. LSAMC samples were oxygen ion conductors in low Po₂ and mixed conductors in high Po ₂ up to x = 0.1 just like LSAM. With Co-doping, p-type conductivities increased, however, ionic conductivities remained nearly constant.     

Preparation and characterization of LiNi0.80Co0.20– x Al x O2 as cathode materials for lithium ion batteries

LiNi_0.80Co_{0.20− x} Al _x O₂ samples (x = 0.025, 0.050 and 0.100) were prepared by a solid-state reaction at 725^∘C for 24 h from LiOH⋅H₂O, Ni₂O₃, Co₂O₃ and Al(OH)₃ under oxygen flow. LiNiO₂ simultaneously doped by Co-Al has been tried to improve the cathode performance. The results showed that substitution of optimum amount Al and Co at the Ni-site in LiNiO₂ improved cycling performance. As a consequence, LiNi_0.80Co_0.15Al_0.05O₂ has 178.2 mAh/g of the first discharge capacity and 174.0 mAh/g after 10 cycles. Differential capacity vs. voltage curves indicated that the Co-Al co-doped LiNiO₂ showed suppression of the phase transitions compared with pure LiNiO₂.     

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>—BaO ⋅ Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>

BaO ⋅ Nd₂O₃ ⋅ 4TiO₂—based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200–1450^∘C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al₂O₃ to the starting mixtures reduced the sintering temperatures by at least 100^∘C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε_r) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ_f) towards zero. The addition of 0.5 wt% Al₂O₃ with 8 wt% Bi₂O₃ improved densification, increased both ε_r (to 88) and Q× f (to 8000 GHz) and moved τ_f closer to zero. Ceramics in the system (1 − x)BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + xBaO ⋅ Al₂O₃ ⋅ 4TiO₂ exhibited very limited solid solubility. The end member BaO ⋅ Al₂O₃ ⋅ 4TiO₂ was tetragonal in structure with the following dielectric properties: ε_r = 35; Q× f = 5000 GHz; τ_f = −15ppm/^∘C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ_f were achieved at compositions of approximately 0.14BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + 0.86BaO ⋅ Al₂O₃ ⋅ 4TiO₂, where Q× f = 8200 GHz and ε_r = 71.     

Effect of partial substitution of Mg for Co in Mn1.4Ni1.2Co0.4O4 NTC thermistors on electrical stability

Sintered Mn_1.4Ni_1.2Co_{0.4 − x} Mg_xO₄ samples were thermal constrained by heating at 850^∘C followed by air cooling. As-sintered and thermal constraint samples were composed of Mn- and Ni-rich phases with a cubic spinel structure. The substituted Mg suppressed the separation of Ni-rich phase in a Mn_1.4Ni_1.2Co_{0.4 − x} Mg_xO₄ solid solution and resulted in a more stable spinel structure. In particular, the substituted Mg led to a significant decrease in the resistance drift of the Mn_1.4Ni_1.2Co_{0.4 − x} Mg_xO₄ NTC thermistors after thermal constraint. This indicates that the Mg substituted Mn_1.4Ni_1.2Co_{0.4 − x} Mg_xO₄ NTC thermistors have good electrical stability in comparison with the Mg-free Mn_1.4Ni_1.2Co_0.4O₄ thermistors. We strongly recommended the substitution of Mg for Co in Mn-Ni-Co-O NTC thermistors for stabilizing their resistivity drift, i.e., ageing.     

Electromechanical Properties and Morphotropic Phase Boundary of Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3-BaTiO3 Lead-free Piezoelectric Ceramics

The crystal structure and electromechanical properties of two ternary ceramic Na_0.5Bi_0.5TiO₃- K_0.5Bi_0.5TiO₃-BaTiO₃ (NBT-KBT-BT) systems were investigated. A gradual change in crystalline structure and microstructure with the increase of KBT and BT concentrations were observed. It was ascertained that the rhombohedral-tetragonal morphtropic phase boundary (MPB) lies in the range of 0.024 ≰ x ≰ 0.030 for (1–5x) NBT-4x KBT-x BT system and 0.025 ≰ y ≰ 0.035 for (1 − 3y) NBT—2y KBT—y BT system at room temperature. The piezoelectric constant d₃₃ and electromechanical coupling factor k_p of the ceramics attain a maximum value of 150 pC/N and 0.298, respectively. The MPB phase diagram of NBT-KBT-BT ternary system was determined by phase analysis of XRD patterns from calcined specimens. The ferroelectric properties of the (1 − 5x) NBT—4x KBT—x BT system have been characterized. The ternary system ceramics have relatively high Curie temperature T_c.     

Effects of Mn-doping on TSDC and degradation of BaTiO3

The effects of Mn-doping on TSDC (Thermally Stimulated Depolarization Current) and electrical degradation of BaTiO₃ have been investigated. TSDCs of un-doped BaTiO₃ and Ba(Ti_1−x Mn_x)O_3−δ exhibited the three sharp TSDC peaks around phase transition temperatures. TSDC of Ba(Ti_0.995Mg_0.005)O_2.995 increased gradually from 50^∘C and this anomalous depolarization current kept going up well above the Curie temperature (∼130^∘C). TSDCs of un-doped BaTiO₃ and Ba(Ti_0.995Mn_0.005)O_3−δ decreased in the temperature range above the Curie point, whereas a slight increase in TSDC was confirmed at the specimen of Ba(Ti_0.99Mn_0.01)O_3−δ. TSDCs of Ba(Ti_0.995−y Mg_0.005Mn_y)O_3−δ (y = 0.005, 0.01) were lower than that of Ba(Ti_0.995Mg_0.005)O_2.995.     

Preparation and characteristics of LiCoO2 paste electrodes for lithium ion micro-batteries

The thick film electrodes (cathode) have been developed via screen printing using LiCoO₂ paste to improve the discharge capacity of lithium ion micro-batteries. The LiCoO₂ pastes were formulated using the mixtures of LiCoO₂ powder as a functional material, carbon black as a conducting agent, ethyl cellulose and terpineol as a vehicle, and Emphos PS-21A as a dispersant. Depending on the amount of carbon black, the average and maximum surface roughnesses varied from 0.54 to 1.00 μm and 6.2 to 18.7 μm respectively. The internal resistance in the paste electrodes could also be controlled to 120 KΩcm⁻² by the addition of carbon black in the pastes. The thickness of the printed film is independent of the paste composition, but it decreased from 15 to 6 μm with the increase in screen mesh number from 250 to 500. The initial specific discharge capacity of the printed cathode which was prepared using the mixture Ag-coated LiCoO₂ powder improved to 180 μAhcm⁻².     

Characteristics of thick-film CO2 sensors based on NASICON with Na2CO3-CaCO3 auxiliary phases

Potentiometric CO₂ sensors were fabricated using a NASICON (Na_1+x Zr₂Si_xP_3−x O₁₂) thick film and auxiliary layers. The powder of a precursor of NASICON with high purity was synthesized using the sol-gel method. Using the NASICON paste, an electrolyte was prepared on the alumina substrate through screen printing and then sintered at 1000^∘C for 4 h. In the present study, as new auxiliary phases, a series of Na₂CO₃-CaCO₃ system was deposited on the Pt sensing electrode. The electromotive force (EMF) values were found to be linearly dependent on the logarithm of the CO₂ concentration in the range of 1000–10000 ppm. The device to which Na₂CO₃-CaCO₃ (1:2) was attached showed good sensing properties at low temperatures.     

Impedance spectroscopy of acceptor-doped CaZrO3 with cation nonstoichiometry

The electrical conductivities of acceptor-doped Ca_1−x Zr_0.99M_0.01O_3−δ (M = Mg²⁺, In³⁺) systems have been investigated as a function of cation nonstoichiometry (0 ≤ x ≤ 0.05). The electrical characterization was carried out using impedance spectroscopy at different temperatures in dry air. The contributions of grain (R _g) and grain boundary resistance (R _gb ) to the total resistance (R _t) were examined with the impedance and/or modulus plane representation. For the stoichiometric composition, the R _t of the specimen decreased with acceptor doping. When Ca deficiency was small, both the R _g and the R _gb slightly increased with nonstoichiometry (x). However, when Ca deficiency was large and excess zirconia was found as a second phase, the R _gb and the contribution of the R _gb to the R _t significantly increased with x.     

Control of 0.2CaTiO3-0.8Li0.5Nd0.5TiO3 microwave dielectric ceramics by additions of Bi2Ti2O7

Ceramics of 0.2CaTiO₃-0.8Li_0.5Nd_0.5TiO₃) have been prepared by the mixed oxide route using additions of Bi₂O₃-2TiO₂ (up to 15 wt%). Powders were calcined 1100^∘C; cylindrical specimens were fired at temperatures in the range 1250–1325^∘C. Sintered products were typically 95% dense. The microstructures were dominated by angular grains 1–2 μm in size. With increasing levels of Bi₂O₃-2TiO₂ additions, needle and lath shaped second phases developed. For Bi₂Ti₂O₇ additions up to 5 wt%, the relative permittivity increased from 95 to 131, the product of dielectric Q value and measurement frequency increased from 2150 to 2450 GHz and the temperature coefficient of resonant frequency (τ _f ) increased from −28pp/^∘C to +22pp/^∘C. A product with temperature stable τ _f could be obtained at ∼2 wt% Bi₂Ti₂O₇ additions. For high levels of additives, there is minimal change in relative permittivity, the Qxf values degrade and τ _f becomes increasingly negative.     

Effect of heating rate on the structure evolution of (K0.5Na0.5)NbO3–LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃–xLiNbO₃ (abbreviated an KNLN) have been synthesized by traditional ceramics process. Effects of heating rate on the phase structure, microstructure evolution and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃–xLiNbO₃ were investigated. Results show that the heating rate has no effects on the phase structures. However, the fracture surface of the 0.94(K_0.5Na_0.5)NbO₃ −0.06 LiNbO₃ ceramics transforms from intergranular fracture mode to a typical transgranular fracture mode with the increasing of the heating rate. This result is ascribed to the presence of agglomerates of grains which exhibit different sintering behavior at diverse heating rates. The 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ ceramic sintered at 1080°C with heating rate of 5°C/min shows the optimum piezoelectric properties(d ₃₃ = 210 pC/N, k _p = 0.403 and k _t = 0.498).     

Dielectric Properties of Yttrium Vanadate Crystals from 15 K to 295 K

Yttrium Vanadate (YVO₄) is a birefringent crystal, which has similar dielectric constant as that of Sapphire. In this paper we have reported the measurement of the real part of permittivity and loss tangent of YVO₄ crystal in the temperature range 15–295 K at a frequency of 16.3 GHz. We have used the dielectric post resonator technique for the microwave characterisation of the YVO₄ dielectric rod. The multifrequency Transmission Mode Q-Factor (TMQF) technique has been used for data processing and hence precise values of permittivity and loss tangent are achieved. Easily machineable YVO₄ is characterized by low losses at microwave frequencies. At temperature of 15 K and frequency of 16.3 GHz the permittivity was 9.23 and loss tangent was 2 × 10^{− 5}. YVO₄ is identified as a potential candidate to replace expensive Sapphire in many microwave applications.