Low temperature firing of Co2Y–NiCuZn ferrite composites

Hexagonal structure magnetoplumbite ferrites have revealed a higher dispersion frequency than that of nickel ferrites because of the magnetoplumbite's magnetic anisotropy. The magnetoplumbite ferrite densification temperature always exceeds 1000 °C and the initial low temperature firing permeability of magnetoplumbite ferrites with added glass is too low (μ_i = 2–4). Therefore, it is desirable to develop a material that has a higher permeability at above 300 MHz and can be densified at temperatures below 900 °C. The Bi₂O₃–B₂O₃–ZnO–SiO₂ (BBSZ) glass addition effects on the densification and magnetic properties of Co₂Y–NiCuZn ferrite composites with various Co₂Y/NiCuZn ferrite ratios were investigated. The densification of Co₂Y–NiCuZn ferrite composites was enhanced by the addition of glass at low sintering temperatures (<900 °C) due to the liquid phase sintering. Co₂Y–NiCuZn ferrite composites with 4 wt% BBSZ glass sintered at 900 °C show a relative density above 90%, a high-initial-permeability of 5–6, a quality factor of above 30 in the 200–300 MHz frequency and a resonance frequency above 1 GHz, which can be used in high frequency multilayer chip inductors.     

Charge–discharge curves and discharge capacities of LiNi1−yCoyO2 synthesized from lithium carbonate and nickel and cobalt oxides

LiNi_1?yCo_yO₂ (y=0.1, 0.3, and 0.5) were synthesized by a solid-state reaction method at 800 °C and 850 °C using Li₂CO₃, NiO, and Co₃O₄ as the starting materials. The electrochemical properties of the synthesized LiNi_1?yCo_yO₂ were then investigated. For samples with the same composition, the particles synthesized at 850 °C were larger than those synthesized at 800 °C. The particles of all the samples synthesized at 850 °C were larger than those synthesized at 800 °C. LiNi_0.5Co_0.5O₂ synthesized at 850 °C had the largest first discharge capacity (159 mA h/g), followed in order by LiNi_0.7Co_0.3O₂ synthesized at 800 °C (158 mA h/g) and LiNi_0.9Co_0.1O₂ synthesized at 850 °C (151 mA h/g). LiNi_0.9Co_0.1O₂ synthesized at 850 °C had the best cycling performance with discharge capacities of 151 mA h/g at n=1 and 156 mA h/g at n=5.     

Synthesis of lithium LiNi1−yCoyO2 from lithium carbonate,nickel oxide and cobalt carbonate and their electrochemical properties

LiNi_1?yCo_yO₂ (y = 0.1, 0.3 and 0.5) were synthesized by solid state reaction method at 800 °C and 850 °C from Li₂CO₃, NiO and CoCO₃ as starting materials. The electrochemical properties of the synthesized LiNi_1?yCo_yO₂ were investigated. As the content of Co decreases, particle size decreases rapidly and particle size gets more homogeneous. When the particle size is compared at the same composition, the particles synthesized at 850 °C are larger than those synthesized at 800 °C. Among LiNi_1?yCo_yO₂ (y = 0.1, 0.3 and 0.5) synthesized at 850 °C, LiNi_0.7Co_0.3O₂ has the largest intercalated and deintercalated Li quantity Δx at the first charge–discharge cycle, followed in order by LiNi_0.9Co_0.1O₂ and LiNi_0.5Co_0.5O₂. LiNi_0.7Co_0.3O₂ synthesized at 850 °C has the largest first discharge capacity (142 mAh/g), followed in order by LiNi_0.9Co_0.1O₂ synthesized at 850 °C (113 mAh/g), and LiNi_0.5Co_0.5O₂ synthesized at 800 °C (109 mAh/g).     

Microwave dielectric properties of LBBS glass added (Zn0.95Co0.05)2SiO4 for LTCC technology

The effects of Li₂O–B₂O₃–Bi₂O₃–SiO₂ (LBBS) glass on the sintering characteristics and microwave dielectric properties of (Zn_0.95Co_0.05)₂SiO₄ were investigated in this study. (Zn_0.95Co_0.05)₂SiO₄ powders were fabricated by traditional solid-state preparation, and LBBS glass was synthesised by quenching method. The LBBS glass can effectively reduce the sintering temperature of (Zn_0.95Co_0.05)₂SiO₄ from 1300 °C to 900 °C and thus promote the densification and uniformity of the specimens. XRD patterns indicated that no other secondary phases existed in our doping range (0–2 wt%). To obtain the highest sintering density and a uniform microstructure when the samples were sintered at 900 °C, the optimal doping content was set to be 1.5 wt%. The sample also demonstrated the following excellent microwave dielectric properties: ɛ_r=6.16, Qf=33,000 GHz and τ_f=−59 ppm/°C.     

Effects of La doping on electrical conductivity,thermal expansion and electrochemical performance in LaxSr1–xCo0.9Sb0.1O3–δ cathodes for IT–SOFCs

Perovskite oxides La_xSr_1–xCo_0.9Sb_0.1O_3–δ (LSCSbx, x=0.0–0.8) are investigated as IT–SOFC cathodes supported with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ (LSGM) electrolyte. All LSCSbx oxides have a tetragonal distorted perovskite structure with s.g. P4/mmm, while a La₂Co₂O₅ impurity phase was observed within La doping levels at x=0.6–0.8. The LSCSb0.4 has a good chemical compatibility with LSGM electrolyte for temperatures up to 1050 °C. XPS examinations indicate the existence of Co³⁺/Co⁴⁺ mixed valence states in LSCSbx. The conductivity increases with La doping and the LSCSbx with x=0.4 exhibits the highest electrical conductivity (e.g., 673–1637 S cm⁻¹ at 300–850 °C). The thermal expansion coefficient (TEC) decreases from 25.89×10^–6 K^–1 for x=0.0 to 18.5×10^–6 K^–1 for x=0.6 at 30–900 °C. Among the LSCSbx compositions, the LSCSb0.2 exhibits the lowest polarization resistance (R_p), which is merely 0.069 Ω cm² at 700 °C. The maximum power density of the cell with LSCSb0.2 cathode on 300 µm thick LSGM electrolyte attains 564 mW cm^–2 at 850 °C, which is higher than that of SrCo_0.9Sb_0.1O_3–δ (SCSb) cathode. All of the results indicate that LSCSb0.2 is a promising material for application in IT–SOFCs cathodes.     

Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells

Nanocrystalline SOFC cathode materials of perovskite family, La_1?xSr_xM_1?yCo_yO₃, where 0 < x ≤ 0.5, 0 < y ≤ 0.8 (M is transitional metal = Mn or Fe), have been synthesized at a relatively low temperature by combustion synthesis using alanine as a novel fuel. Detailed X-ray powder diffraction analyses show 47–96% phase purity in the as-synthesized powder and upon calcination at ～825 °C single-phase material is obtained wherein the nanocrystallinity (crystallite size ～19–24 nm) is retained. Densification studies of the materials are carried out within 900–1100 °C. The coefficient of thermal expansion (CTE) of these cathodes is measured. Electrical conductivity of the cathodes sintered at different temperatures are measured in the temperature range 700–900 °C and correlated with the density of the sintered materials. The electrochemical performances of Ni-YSZ anode-supported SOFC having YSZ electrolyte (～10 μm) with CGO interlayer (～15 μm) are studied with the developed cathodes in the temperature range 700–800 °C using H₂ as fuel and oxygen as oxidant. Highest current density of ～1.7 A/cm² is achieved during testing at 800 °C measured at 0.7 V with a cathode composition of La_0.5Sr_0.5Co_0.8Fe_0.2O₃. Precipitation of nanocrystalline grains over the core grains in porous microstructure of this cathode might be one of the reasons for such high cell performance.     

A rapid synthesis of cobalt cyclotetraphosphate Co2P4O12 at low temperature

The cobalt cyclotetraphosphate, Co₂P₄O_12, was synthesized by evaporating a mixture of cobalt chloride hexahydrate, phosphoric acid and water at 70 °C, with further calcinations at 400 °C, 500 °C and 600 °C for 3 h. XRD results indicated that the Co₂P₄O₁₂ compounds obtained had only monoclinic phase with space group C_2h⁶ (Z=4). From FT-IR spectra, vibrational modes corresponding to internal vibrations of the P₄O₁₂^4? anion were identified. The morphologies and crystallite sizes for the Co₂P₄O₁₂ obtained from SEM data and X-ray line broadening showed non-uniform particles and 37–68 nm, respectively. All characterization methods showed that the monoclinic phase of Co₂P₄O₁₂ could be synthesized at the low temperature of 400 °C with short time consumption. The temperature was about 400 °C lower than that used in the previous synthesis method with CoCl₂ as reactant, and resulted in a cost-, energy-, and time-saving method.     

Low-temperature sintering and microwave dielectric properties of Nd(Co1/2Ti1/2)O3 ceramics using glass addition of oxides

The microstructures and microwave dielectric characteristics of complex perovskite Nd(Co_1/2Ti_1/2)O₃ ceramics with 60P₂O₅–15ZnO–5La₂O₃–5Al₂O₃–5Na₂O–5MgO–5Yb₂O₃ (PZLANMY) additions (1–4 wt%) prepared through the conventional solid-state route were investigated. It was found that Nd(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1210 °C owing to the sintering aid of PZLANMY-glass addition. At 1300 °C, Nd(Co_1/2Ti_1/2)O₃ ceramics with 1 wt% of PZLANMY-glass addition possess a dielectric constant (ε_r) of 27, a Q×f value of 64,000 GHz and a temperature coefficient of resonant frequency (τ_f) of ?29 ppm/°C. The PZLANMY-glass doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices that require low sintering temperature.     

Coarsening of nano-crystalline SiC in amorphous Si–B–C–N

The formation of nano-crystalline SiC is studied in various amorphous precursor derived Si–B–C–N bulk ceramics at temperatures between 1600 and 1800 °C. The formation process of SiC can be described by a very rapid crystallization (<15 min) of nano-sized particles with diameters between 2 and 7 nm which are embedded in an amorphous Si–B–C–N matrix. During further annealing of the material up to 40 h, particle growth due to coarsening takes place, which leads to maximum crystallite diameters of 30 nm. The kinetics of coarsening can be described by the Lifshitz–Slyozov–Wagner model. The product of the rate constant of coarsening, k_c, and of the temperature, T, follows an Arrhenius behaviour with an activation enthalpy of about 8 eV (770 kJ/mol), which is approximately the activation enthalpy of self-diffusion in Si–B–C–N, indicating diffusion controlled crystallite growth. The kinetics of coarsening is fastest for the ceramics with a low concentration of Si and N in the amorphous matrix.     

Characterisation of SnO2–CoO–MnO–Nb2O5 ceramics

Varistors based on SnO₂ have attracted increasing interest in recent years. However, the combined effect of CoO–MnO on SnO₂ ceramics is still unclear. In this study, the non-Ohmic behaviour of the 98.95 mol%SnO₂–0.5 mol%CoO–0.5 mol%MnO–0.05 mol%Nb₂O₅ system, the microstructures and the influence of sintering temperature were investigated. The samples were prepared by the mixed oxide route, and were sintered at temperatures in the range 1250–1450 °C. SEM observation and EDS analysis revealed that the ceramics have a two-phase microstructure comprising SnO₂ primary grains and a Mn, Co rich secondary phase of small particles. The sintered density of the samples increased with the increase in sintering temperature. The maximum non-linear coefficient (α = 10) was obtained at a sintering temperature of 1350 °C.     

Effect of B2O3 and CuO additives on the sintering temperature and microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

The B₂O₃ added Ba(Zn_1/3Nb_2/3)O₃ (BBZN) ceramic was sintered at 900 °C. BaB₄O₇, BaB₂O₄, and BaNb₂O₆ second phases were found in the BBZN ceramic. Since BaB₄O₇ and BaB₂O₄ second phases have an eutectic temperature around 900 °C, they might exist as the liquid phase during sintering at 900 °C and assist the densification of the BZN ceramics. Microwave dielectric properties of dielectric constant (ɛ_r) = 32, Q × f = 3500 GHz, and temperature coefficient of resonance frequency (τ_f) = 20 ppm/°C were obtained for the BZN with 5.0 mol% B₂O₃ sintered at 900 °C for 2 h. The BBZN ceramics were not sintered below 900 °C and the microwave dielectric properties of the BBZN ceramics sintered at 900 °C were very low. However, when CuO was added, BBZN ceramic was well sintered even at 875 °C. The liquid phase related to the BaCu(B₂O₅) second phase could be responsible for the decrease of sintering temperature. Good microwave dielectric properties of ɛ_r = 36, Q × f = 19,000 GHz and τ_f = 21 ppm/°C can be obtained for CuO doped BBZN ceramics sintered at 875 °C for 2 h.     

Low firing and microwave dielectric properties of BaTi4O9 with B2O3–ZnO–La2O3 glass addition

A high-Q low firing ceramics material was fabricated from a composite of 80 wt% of BaTi₄O₉ powder and 20 wt% of B₂O₃–ZnO–La₂O₃ glass flux. The sample sintered at 900 °C for 3 h had the maximum bulk density. B₂O₃–ZnO–La₂O₃ glass was found to act as the sintering aid. The material sintered at 900 °C for 3 h consists of BaTi₄O₉, LaBO₃, an unidentified crystal phase and residual glass phase, and possesses excellent microwave dielectric properties: permittivity k ≈ 27, quality factor Q × f ≈ 20,000 GHz, temperature coefficient of resonant frequency τ_f ≈ 6.5 ppm/°C.     

Microstructure and dielectric properties of the Ti-rich BaO–TiO2 thin films for microwave devices

Ti-rich BaO–TiO₂ thin films were grown on a Pt/Ti/SiO₂/Si substrate using rf sputtering and the structural and dielectric properties of the films were investigated. For the film grown at room temperature and rapidly thermal annealed (RTA) at 900 °C for 3 min, an amorphous phase with a small BaTi₅O₁₃ crystalline phase was formed. As the growth temperature increased, the amount of the BaTi₅O₁₁ crystalline phase increased. For the film grown at 350 °C and RTA at 900 °C for 3 min, the homogeneous BaTi₅O₁₁ phase was formed. The BaTi₄O₉ phase was developed when the growth temperature exceeded 450 °C. The thin film with the homogeneous BaTi₄O₉ phase was obtained when the film was grown at 550 °C and RTA at 900 °C for 3 min. The dielectric properties of the films were measured at 1–6 GHz range. The dielectric constant (ϵ_r) of the BaTi₅O₁₁ film was about 33 and the dissipation factor was about 0.01. The ϵ_r and the dissipation factor of the BaTi₄O₉ film were about 37 and 0.005, respectively.     

Crystal structure,thermal expansion and electrical conductivity of perovskite oxides BaxSr1−xCo0.8Fe0.2O3−δ (0.3 ≤ x ≤ 0.7)

Ba_xSr_1−xCo_0.8Fe_0.2O_3−δ (0.3 ≤ x ≤ 0.7) composite oxides were prepared and characterized. The crystal structure, thermal expansion and electrical conductivity were studied by X-ray diffraction, dilatometer and four-point DC, respectively. For x ≤ 0.6 compositions, cubic perovskite structure was obtained and the lattice constant increased with increasing Ba content. Large amount of lattice oxygen was lost below 550 °C, which had significant effects on thermal and electrical properties. All the dilatometric curves had an inflection at about 350–500 °C, and thermal expansion coefficients were very high between 50 and 1000 °C with the value larger than 20 × 10⁻⁶ °C⁻¹. The conductivity were larger than 30 S cm⁻¹ above 500 °C except for x > 0.5 compositions. Furthermore, conductivity relaxation behaviors were also investigated at temperature 400–550 °C. Generally, Ba_0.4Sr_0.6Co_0.8Fe_0‘2O_3−δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ are potential cathode materials.     

Electrical conductivity and thermoelectric power studies of solution-combustion-processed Ca2.76Cu0.24Co4O9

Nanocrystalline Ca_2.76Cu_0.24Co₄O₉ powders (25 nm in crystallite size) are synthesized by the solution combustion method, using aspartic acid as the combustion fuel. In this study, we discuss the effect of sintering temperature on the microstructure and thermoelectric properties of Ca_2.76Cu_0.24Co₄O₉. The density and grain size increase with an increase in sintering temperature. The Ca_2.76Cu_0.24Co₄O₉ sintered at 900 °C shows the largest value of electrical conductivity and Seebeck coefficient, resulting in the largest power factor (3.8×10^?4 W m^?1 K^?2 at 800 °C). This value is more than 22 times larger than that of the Ca_2.76Cu_0.24Co₄O₉ sintered at 940 °C (1.7×10^?5 W m^?1 K^?2 at 800 °C).     

Sintering process optimization for multi-layer CGO membranes by in situ techniques

The sintering of asymmetric CGO bi-layers (thin dense membrane on a porous support; Ce_0.9Gd_0.1O_1.95?δ = CGO) with Co₃O₄ as sintering additive has been optimized by combination of two in situ techniques. Optical dilatometry revealed that bi-layer shape and microstructure are dramatically changing in a narrow temperature range of less than 100 °C. Below 1030 °C, a higher densification rate in the dense membrane layer than in the porous support leads to concave shape, whereas the densification rate of the support is dominant above 1030 °C, leading to convex shape. A flat bi-layer could be prepared at 1030 °C, when shrinkage rates were similar. In situ van der Pauw measurements on tape cast layers during sintering allowed following the conductivity during sintering. A strong increase in conductivity and in activation energy E_a for conduction was observed between 900 and 1030 °C indicating an activation of the reactive sintering process and phase transformation of cobalt oxide.     

Microwave dielectric properties of the BaTi5O11 thin film grown on the poly-Si substrate using rf magnetron sputtering

BaTi₅O₁₁ thin films were grown on the poly-Si/SiO₂/Si substrate using rf magnetron sputtering. The BaO-TiO₂ thin film deposited on the poly-Si substrate had an amorphous phase even though the growth temperature was high at 550 °C. The amorphous film was crystallized into the BaTi₅O₁₁ phase when the film was post annealed above 800 °C. The post annealing temperature is one of the most important factors for the formation of the crystalline BaTi₅O₁₁ thin film. The homogeneous BaTi₅O₁₁ thin film was obtained when the film was grown at 550 °C and rapid thermal annealed (RTA) at 900 °C for 3 min. The dielectric constant (ɛ_r) of the BaTi₅O₁₁ film measured at 100 kHz was about 35 and the dissipation factors of all the films were smaller than 4.0%. The dielectric properties of the BaTi₅O₁₁ thin film were also measured at microwave frequencies. For the BaTi₅O₁₁ thin film grown at 550 °C and RTA at 900 °C for 3 min, the ɛ_r of 34–30 and dielectric loss of 0.025 ± 0.005 were obtained at 1–6 GHz.     

Microwave dielectric properties of BiNbO4 ceramics with CuO–V2O5 addition

BiNbO₄ ceramics were developed by using CuO–V₂O₅ as a liquid phase sintering agent. The resultant dielectric properties were analyzed in terms of the densification and the amount of CuO–V₂O₅ sintering agent. The addition of 0.8 wt.% CuO–V₂O₅ as its sintering agent was observed to perform most satisfactory. At 850 °C, uniform and enhanced microstructure was observed for the BiNbO₄ specimen with 0.8 wt.% CuO–V₂O₅ addition. Furthermore, the effect of CuO–V₂O₅ addition on the microwave dielectric properties of BiNbO₄ was also investigated. As the sintering temperature increased to 900 °C, the dielectric constant increased but nearly constant and the quality factor (QF) showed a maximum at 850 °C and then decreased for all compositions of the 900 °C sintered specimens. With an increase in CuO–V₂O₅ content, the temperature coefficient of frequency (TCF) increased in accordance with the dielectric mixing rule and microstructural behavior.     

Low-dielectric-constant LiAlO2 ceramics combined with LBSCA glass for LTCC applications

This study used Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) glass to reduce the sintering temperature of LiAlO₂ ceramics and to realise the low dielectric constants (ɛ_r<5) of low-temperature co-fired ceramic (LTCC) materials. LBSCA glass remarkably enhanced the densification of LiAlO₂ ceramics. X-ray diffraction patterns indicated that only the γ-LiAlO₂ phase occurred within the doping range of 1 wt% to 3.5 wt%. Scanning electron microscopy images showed dense and uniform grains in samples with 3.0 wt% LBSCA glass. These samples also exhibited low dielectric constants and low dielectric loss when sintered at 900 °C and 950 °C (i.e., ɛ_r=4.48, Qf=35,540 GHz and τ_f=−53 ppm/°C at 900 °C; ɛ_r=4.50, Qf=38,979 GHz and τ_f=−55 ppm/°C at 950 °C, respectively). The material prepared was chemically compatible with silver and showed potential in applications of high-frequency LTCC microwave substrates.     

Oxidation behavior characteristics of an aluminized Ni-based single crystal superalloy CM186LC between 900 °C and 1100 °C in air

Oxidation behavior of an aluminized Ni-based single crystal superalloy CM186LC was performed between 900 °C and 1100 °C in air. The oxidation kinetics approximately followed a parabolic oxidation law at 900 °C and 1100 °C. The mass gains were significantly increased owing to the formation of θ-Al₂O₃ during initial oxidation stage. After 100 h oxidation, the mass gain rates were then decreased due to the transformation from θ-Al₂O₃ to α-Al₂O₃. The microstructures after 500 h oxidation at all temperatures generally consisted of scale, coating layer, interdiffusion zone (IDZ), substrate diffusion zone (SDZ) accompanied with the topologically close-packed (TCP) and substrate.