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.     

Electrical conductivity of the Al-stabilized La<Subscript>2/3</Subscript>TiO<Subscript>3</Subscript>

A-site deficient lanthanum titanate (La_2/3TiO₃) materials with perovskite structure are attractive due to their electrical applications such as ion conductors and dielectrics. However, its stability at room temperature in air is obtained only if Na or Li etc. is incorporated into La site or Al into Ti site. In this study, the electrical conductivities of La_0.683(Ti_0.95Al_0.05)O₃ have been measured in oxygen partial pressure (Po₂) between 1 and 10⁻¹⁸ atm at 1000~1400°C. The electrical conductivity exhibited −1/4, −1/6 and −1/5 dependence (log σ ∝ log , n = −1/4, −1/6, −1/5) depending upon temperature and Po₂. The defect model explaining the observation was proposed and discussed. The chemical diffusion coefficient was estimated from the electrical conductivity relaxation.     

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.     

Properties of Cu,Ni, and V doped-LaCrO <Subscript>3</Subscript> interconnect materials prepared by pechini,ultrasonic spray pyrolysis and glycine nitrate processes for SOFC

The ceramic interconnect,La_0.8Sr_0.05Ca_0.15(Cr_{1 −x} , B_x)O₃ (B = Cu, Ni, V, x = 0.02, 0.1, 0.5) (LSCCB) powders were prepared by Pechini method, Ultrasonic Spray Pyrolysis (USP) and Glycine Nitrate Process (GNP). Nano sized powders were synthesized by GNP and their chemical compositions were confirmed by ICP analysis. The electrical conductivities of LSCCCu, LSCCNi, and LSCCV samples were 34 S/cm, 48 S/cm, and 22 S/cm at 800^∘C in air, respectively. Among the LSCCB powders, the LSCCNi sample shows highest relative density and electrical conductivity. In a low oxygen partial pressure, however, LSCCV sample was more stable. The perovskite phase of the composition LSCCV sample is of large practical interest for interconnects in SOFC because of the stability in low oxygen partial pressure.     

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.     

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.     

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.     

Preparation and characterization of Z-type hexaferrites, Ba3(1−x)Sr3x Co2Fe24O41 with x = 0–0.5, via a two-step calcination with an intermediate wet milling

Z-type hexaferrites (Ba_{3(1 − x)}Sr_3x Co₂Fe₂₄O₄₁with x = 0–0.5, Z-hex) were prepared in a nearly phase pure state by a two-step calcination with an intermediate wet milling. The first calcination of the starting mixture comprising oxides or hydroxides at temperatures below 1100^∘C brought about a mixture of layer-structured M and Y-type hexaferrite phases, together with a spinel phase of Co ferrite. Z-hex was fully crystallized after the second calcination up to 1230^∘C. Wet milling between the two calcination steps was decisive for the phase purity. Emphasis was laid on the quantitative analyses of Z-hex, together with the evaluation of anisotropic growth of the crystallites. Sr addition stabilizes Z-hex, while decreases degree of anisotropy simultaneously.     

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.     

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.     

Oxygen exchange and transport properties of yttria-stabilized zirconia coated with LaCrO3

The oxygen permeation flux through YSZ (Yttria-Stabilized Zirconia) in reducing Po ₂ is mostly controlled by the surface-exchange kinetics in spite of very high temperature [1]. In order to study the kinetics, the YSZ surface was coated with LaCrO₃ on feed side, permeate side, or two sides, and the oxygen fluxes were measured under controlled Po ₂ gradient generated by different CO/CO₂ mixtures (permeate side: ∼3 × 10⁻¹² atm, feed side: 2 × 10⁻¹⁰∼2 × 10⁻⁸ atm) at 1600^∘C. The oxygen flux was determined by measuring the change in CO₂ content of the permeate-side gas. When both feed and permeate surfaces were coated, the oxygen flux increased by ∼6 times. For either permeate- or feed-side coating, the increases were ∼4 times and ∼1.5 times, respectively. A model was proposed in order to estimate the surface-exchange coefficient ( ) of feed and permeate side with or without coating.     

Crystal Growth and High Temperature Piezoelectricity of La3Ta0.5Ga5.5 − x Al x O14 Crystals

For the application of high temperature piezoelectric devices, Al^{3 +} substituted La₃Ta_0.5Ga_5.5O₁₄ (LTG) crystals with chemical formula of La₃Ta_0.5Ga_{5.5 – x}Al_xO₁₄ (LTGA) were synthesized and grown by -PD (Micro-pulling-down) and Czochralski technique. LTGA compound has been shown to exhibit congruent melting in a wide compositional range without destabilization of the melt during growth, while distribution coefficient of the substituted Al^{3 +} ion was close to unity. LTGA crystals have shown preferable effects for temperature stability on piezoelectric properties in the range from R.T (room temperature) to 500_C.     

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.     

Conductivity and expansion at high temperature in Sr0.7La0.3TiO3−α prepared under reducing atmosphere

Sr_0.7La_0.3TiO_3−α specimens were prepared in reducing atmosphere, and the structural and electrical properties were studied. The lattice parameter of Sr_0.7La_0.3TiO_3−α at room temperature was larger than that expected from Vegard’s law between SrTiO₃ and LaTiO₃ due to the reductive expansion. The conductivity of this specimen was 100 S cm⁻¹ at 1000°C, p_O2 = 10⁻¹³ Pa. However, the conductivity was not preserved after an oxidation-reduction cycle. Over p_O2 = 10² Pa, the conductivity drastically dropped with increasing p_O2. The thermal expansion coefficient of Sr_0.7La_0.3TiO_3−α was 11.8 × 10⁻⁶ K⁻¹ in 9% H₂/N₂ (room temperature – 1000°C). In this Sr_0.7La_0.3TiO_3−α, the chemical expansion on oxidation reached Δl/l_o = 0.51%, when changing p_O2 from 10⁻¹¹ Pa to 2 × 10⁴ Pa (air) at 1000°C.     

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.     

Ni-Cu-Zn Ferrites for Low Temperature Firing: I. Ferrite Composition and its Effect on Sintering Behavior and Permeability

Ni-Cu-Zn ferrites of composition Ni_{1 − x − y}Cu_yZn_xFe₂O₄ with 0.4 ≰ x ≰ 0.6 and 0 ≰ y ≰ 0.25 were prepared by standard ceramic processing routes. The density of samples sintered at 900^∘C increases with copper concentration y. Dilatometry reveals a significant decrease of the temperature of maximum shrinkage with y. The permeability has maximum values of μ = 500–1000 for x = 0.6. The Curie temperature is sensitive to composition and changes form about 150^∘C for x = 0.6 to T_c > 250^∘C for x = 0.4, almost independent on the Cu-content. A small iron deficiency in Ni_0.20Cu_0.20Zn_{0.60 + z}Fe_{2 − z}O_{4 − (z/2)} with 0 ≰ z ≰ 0.06 significantly enhances the density of samples sintered at 900^∘C. The maximum shrinkage rate is shifted to T < 900^∘C. These compositions are therefore appropriate for application in low temperature co-firing processes. The permeability is reduced with z, hence a small z = 0.02 seems to be the optimum ferrite composition for high sintering activity and permeability.     

Piezoelectric properties of SrBi2M2− xVxO9 (M = Nb and Ta) ceramics

Vanadium-substituted strontium bismuth tantalate, Sr_0.8Bi_2.2Ta_{2− x}V_xO₉ (SBTVx), and strontium bismuth niobate, SrBi₂Nb_{2− x}V_xO₉ (SBNVx), ceramics were synthesized by a low-temperature processing, and their dielectric, ferroelectric and piezoelectric properties were characterized. With the partial substitution of tantalum or niobium by vanadium cations, the single phase of the ABi₂M₂O₉-type structure was preserved and the sintering temperature was significantly decreased. For the SBNV ceramics, the T _c of 437^∘C for x = 0.0, the vanadium content hardly changed. On the other hand, the T _c of the SBTV ceramics increased from 408^∘C for the non-substituted SBTV to 414^∘C for x = 0.05 and then with the increasing vanadium content, the T _c decreased to 379^∘C for x = 0.20. The remanent polarizations, P _r, of SBTV and SBNV at room temperature were 4.9 and 5.4 μC/cm², respectively. All the obtained independent electromechanical coupling factors of the SBTV0.05 ceramics were as follows: k _p = 0.119, k ₃₁ = 0.073, k ₃₃ = 0.165, k ₁₅ = 0.051 and k _t = 0.134, and the SBNV0.05 ceramics were as follows: k _p = 0.074, k ₃₁ = 0.045, k ₃₃ = 0.175, k ₁₅ = 0.106 and k _t = 0.140. These coupling factors were higher than those of the non-substituted materials. From these results, the vanadium-substituted SBT and SBN-based materials can be expected to be lead-free piezoelectric resonator materials that can be prepared at low sintering temperatures.     

Synthesis and properties of ferroelectric Si-doped (Bi, Nd)4Ti3O12 thin films by chemical solution deposition

Ferroelectric Si-doped (Bi,Nd)₄Ti₃O₁₂ thin films have been prepared on Pt/TiO_x/SiO₂/Si substrates through metal-organic compounds by the chemical solution deposition. The Bi_3.25Nd_0.75Ti_2.9Si_0.1O₁₂ (BNTS) precursor films were found to crystallize into the Bi-layered perovskite Bi₄Ti₃O₁₂ single-phase above 600^∘C. The synthesized BNTS films revealed a random orientation having a strong 117 reflection. The BNTS thin films prepared between 600^∘C and 700^∘C showed well-saturated P-E hysteresis loops with P _r of 13–14 μ C/cm² and E _c of 100–110 kV/cm at an applied voltage of 5 V. The surface roughness of the BNTS thin films was improved by Si doping compared with that of undoped Bi_3.35Nd_0.75Ti₃O₁₂ films.     

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.