Sintering and oxygen permeation studies of La0.6Sr0.4Co0.2Fe0.8O3−δ ceramic membranes with improved purity

High purity raw materials are used for synthesizing La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF6428) powders to reduce the effect of impurity phases on oxygen permeability of the corresponding membranes. The as-synthesized LSCF6428 powders require a sintering temperature above 1180 °C to achieve membrane density over 90%. Ball milling of the powders increases the membrane sintering. It also increases oxygen permeation flux from 0.37 to 0.43 ml cm⁻² min⁻¹ at 950 °C for the membranes sintered at 1100 °C. A decrease in oxygen permeation fluxes with the further increase in sintering temperature is observed for the membranes with ball-milled starting powders, accompanied by an obvious increase in grain size. It suggests, at low level of impurity phases, the grain boundaries facilitate the oxygen diffusion. The combination of ball milling of the starting powders and a sintering temperature of 1100 °C is optimal to achieve high oxygen permeability of LSCF6428 membranes with improved purity.     

Effect of Bi2O3 additives on sintering and microwave dielectric behavior of La(Mg0.5Ti0.5)O3 ceramics

Bi₂O₃ was selected as liquid phase sintering aid to lower the sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics. The sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics is generally high, about 1600 °C. However, the sintering temperature was significantly lowered about 275 °C from 1600 °C to 1325 °C by incorporating in 15 mol% Bi₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (?_r) value of 40.1, a quality factor (Q × f) value of 60,231 GHz, and the temperature coefficient (τ_f) value of 70.1 ppm/°C. During all addition ranges, the relative dielectric constants (?_r) were different and ranged from 32.0 to 41.9, the quality factors (Q × f) were distributed in the range of 928–60,231 GHz, and the temperature coefficient (τ_f) varies from 0.3 ppm/°C to 70.3 ppm/°C. Noticeably, a nearly zero τ_f can be found for doping 5 mol% Bi₂O₃ sintering at 1325 °C. It implies that nearly zero τ_f can be achieved by appropriately adjusting the amount of Bi₂O₃ additions and sintering temperature for La(Mg_0.5Ti_0.5)O₃ ceramics.     

Preparation and characterization of the lead-free piezoelectric ceramic of Bi0.5Na0.5TiO3 doped with CuO

This study investigates the effects of copper oxide (CuO) addition, calcining temperature, and sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of lead-free piezoelectric ceramic of bismuth sodium titanate (Bi_0.5Na_0.5TiO₃), BNT, which was prepared using the mixed oxide method. Three kinds of starting powders (Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined BNT powder and a certain weight percentage of CuO were mixed, calcined, and compressed into a green compact of BNT-CuO. This green compact of BNT-CuO was sintered to be a disk doped with CuO, and its characteristics were measured. In this study, the calcining temperature ranged from 700 to 1000 °C, the sintering temperature ranged from 950 to 1050 °C, and the weight percentages of CuO doping included 2, 4, 6, and 8 wt.%. The largest relative density of the BNT-CuO disk obtained in this study was 96.7% at the calcining temperature of 700 °C, the sintering temperature of 950 °C, and 4 wt.% of CuO addition. The corresponding dielectric constant and loss tangent were 494 and 0.181%, respectively. This study shows that adding CuO to the BNT not only improves the relative density and the dielectric constant of the BNT disk, but it also lowers the sintering temperature.     

Electrochemical properties of nano- and micro-sized LiNi0.5Mn1.5O4 synthesized via thermal decomposition of a ternary eutectic Li-Ni-Mn acetate

Nano- and micro-sized LiNi_0.5Mn_1.5O₄ particles are prepared via the thermal decomposition of a ternary eutectic Li-Ni-Mn acetate. Lithium acetate, nickel acetate and manganese acetate can form a ternary eutectic Li-Ni-Mn acetate below 80 °C. After further calcination, nano-sized LiNi_0.5Mn_1.5O₄ particles can be obtained at an extremely low temperature (500 °C). When the sintering temperature goes above 700 °C, the particle size increases, and at 900 °C micro-sized LiNi_0.5Mn_1.5O₄ particles (with a diameter of about 4 μm) are obtained. Electrochemical tests show that the micro-sized LiNi_0.5Mn_1.5O₄ powders (sintered at 900 °C) exhibit the best capacity retention at 25 °C, and after 100 cycles, 97% of initial discharge capacity can still be reached. Nano-sized LiNi_0.5Mn_1.5O₄ powders (sintered at 700 °C) perform the best at low temperatures; when cycled at −10 °C and charged and discharged at a rate of 1 C, nano-sized LiNi_0.5Mn_1.5O₄ powders can deliver a capacity as high as 110 mAh g⁻¹.     

Low-temperature solution combustion synthesis of high performance LiNi0.5Mn1.5O4

High-voltage LiNi_0.5Mn_1.5O₄ spinels were synthesized by a low temperature solution combustion method at 400 °C, 600 °C and 800 °C for 3 h. The phase composition, structural disordering, micro-morphologies and electrochemical properties of the products were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and constant current charge–discharge test. XRD analysis indicated that single phase LiNi_0.5Mn_1.5O₄ powders with disordered Fd-3m structures were obtained by the method at 400 °C, 600 °C and 800 °C. The crystallinity increased with increasing preparation temperatures. XRD and FTIR data indicated that the degree of structural disordering in the product prepared at 800 °C was the largest and in the product prepared at 600 °C was the least. SEM investigation demonstrated that the particle size and the crystal perfection of the products were increased with increasing temperatures. The particles of the product prepared at 600 °C with ~200 nm in size are well developed and homogeneously distributed. Charge/discharge curves and cycling performance tests at different current density indicated that the product prepared at 600 °C had the largest specific capacity and the best cycling performance, due to its high purity, high crystallinity, small particle size as well as moderate amount of Mn³⁺ ions.     

Mechanochemical synthesis and characterisation of BaTa2O6 ceramic powders

Mechanochemical synthesis was used to prepare BaTa₂O₆ powders from BaCO₃ and Ta₂O₅ precursors in a planetary ball mill. Effect of milling time and heat treatment temperature on the formation of BaTa₂O₆ and on the microstructure was investigated. Intensive milling of starting materials resulted in crystallization of BaTa₂O₆ even after 1 h of milling time and single phase BaTa₂O₆ was obtained after 10 h of milling under optimal conditions. The powder derived from 10 h of mechanical activation had crystallite size of 22 nm. But the increase in milling time did not decrease the crystallite size further. High energy milling activated the powders that although 1 h of milling led to formation of single phase BaTa₂O₆ at 1200 °C, this temperature decreased to 900 °C after 5 h of milling. No significant grain growth was observed when the milled powders were heat treated below 900 °C. However, annealing at 1100 and 1200 °C gave an average BaTa₂O₆ grain size of 180 and 650 nm, respectively. An unidentified phase started to form at 1100 °C increasing to high amounts at 1200 °C and they had different shapes and sizes than BaTa₂O₆ grains. These elongated large grains were thought to be due to liquid phase formation caused by iron contamination.     

Densification, microstructural evolution and microwave dielectric properties of fluxed sintered 12R-Ba(Ti0.5Mn0.5)O3 ceramics

Doped hexagonal BaTiO₃ (h-BaTiO₃) ceramics have recently been identified as potential candidates for use in microwave dielectric resonators. However, similar to other common microwave ceramics, doped h-BaTiO₃ ceramics require a sintering temperature higher than 1400 °C. In this study, the effects of Bi₂O₃ and Li₂CO₃ on the densification, microstructural evolution and microwave properties of hexagonal 12R-Ba(Ti_0.5Mn_0.5)O₃ ceramics were examined. Results indicate that Bi₂O₃ and Li₂CO₃ are able to effectively reduce the sintering temperature of 12R-Ba(Ti₀₅Mn_0.5)O₃ ceramics through liquid phase sintering while retaining the hexagonal structure and the microwave dielectric properties. The best results were obtained for the 12R-Ba(Ti_0.5Mn_0.5)O₃ with the additions of 5 wt% Bi₂O₃ sintered at 1200 °C (?_r: 36.0, Qf_r: 6779 GHz, and τ_f: 25.3 ppm/°C), and 5 wt% Li₂CO₃ sintered at 1200 °C (?_r: 28.1, Qf_r: 5304 GHz, and τ_f: 35.3 ppm/°C).     

Dielectric properties of CuO-doped La2.98/3Ba0.01(Mg0.5Sn0.5)O3 ceramics at microwave frequency

The microwave dielectric properties of La_2.98/3Ba_0.01(Mg_0.5Sn_0.5)O₃ ceramics prepared by the conventional solid-state method were investigated for application in mobile communication. A 100 °C reduction of the sintering temperature was obtained by using CuO as a sintering aid. A dielectric constant of 20.0, a quality factor (Q × f) of 50,100 GHz and a temperature coefficient of resonant frequency τ_f of −78.3 ppm/°C were obtained when La_2.98/3Ba_0.01(Mg_0.5Sn_0.5)O₃ ceramics with 0.25 wt.% CuO were sintered at 1500 °C for 4 h.     

Analysis of structural and thermal properties of Li2TiO3 ceramic powders

Li₂TiO₃ ceramic powders have been developed by a solid state reaction method and those have been sintered at four different temperatures (600 °C, 700 °C, 800 °C and 900 °C) towards the optimization of sintering temperature that has been found to be at 800 °C based on the nature of the XRD profiles. The sample sintered at 800 °C has shown a good crystallinity situation from its XRD peaks and the sample is found to be in monoclinic structure which is in accordance with the reported data of JCPDS 33-0831. The SEM images for samples sintered at 600 °C, 700 °C, 800 and 900 °C, EDAX peaks, FTIR profile have been measured for the temperature optimized (800 °C) sample for understanding the structural details of Li₂TiO₃ ceramic powders. Besides these, dielectric constant, dielectric loss and a.c. conductivities have been measured for the temperature optimized sample. In order to strengthen the observations made in the XRD profiles at four different temperatures, Raman spectra of those four sintered ceramic powders have also been studied. In respect of the thermal properties, only for the as synthesized (precursor) sample, simultaneous measurement of TG-DTA profiles has been carried out for analysis.     

Mechanosynthesis,crystal structure and magnetic characterization of M-type SrFe12O19

M-type strontium hexaferrite was prepared by mechanosynthesis using high-energy ball milling. The influence of milling parameters, hematite excess and annealing temperature on magnetic properties of SrFe₁₂O₁₉ were investigated. Commercial iron and strontium oxides were used as starting materials. It was found that mechanical milling followed by an annealing treatment at low temperature (700 °C) promotes the complete structural transformation to Sr-hexaferrite phase. For samples annealed at temperatures from 700 to 1000 °C, saturation magnetization values (M_s) are more sensitive to annealing temperature than coercivity values (H_c). The maximum M_s of 60 emu/g and H_c of 5.2 kOe were obtained in mixtures of powders milled for 5 h and subsequently annealed at 700 °C. An increase in the annealing temperature produces negligible changes in magnetic saturation and coercivity. An excess of hematite as a second phase produces a slight decrease in the saturation magnetization but leads to a significant increase in coercive field, reaching 6.6 kOe.     

Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Synthesis of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 powders through the molten salt method

The Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ powder synthesis through molten salt method was investigated in the temperature range of 650–700 °C for 2–4 h. The XRD results indicated that the optimal synthesizing temperature for molten salt method was 700 °C, significantly lower than that for conventional processing route of solid state reaction method, where a calcining temperature of 850 °C was needed. The SEM results revealed better crystallization of the powders obtained through molten salt method, compared with those through the conventional processing route of solid state reaction method.     

Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.     

Synthesis of LaB6 powders from La2O3, B2O3 and Mg blends via a mechanochemical route

This study reports a room temperature mechanochemical route for the synthesis of LaB₆ powders from La₂O₃–B₂O₃–Mg blends. The synthesis reaction was driven by high-energy ball milling and was gradually examined in terms of milling duration and process control agent. Following the mechanochemical synthesis, unwanted MgO phase and Fe contamination worn off from the milling vial/balls were removed with HCl acid leaching under the effect of ultrasonic stirring. Pure LaB₆ powders were obtained after repeated centrifuging, repeated washing and drying. Subsequent annealing was performed in a tube furnace at 800 °C for 5 h under Ar atmosphere in order to reveal residual elements. Phase and microstructural characterizations of the milled, leached and annealed powders were performed using X-ray diffractometry (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. A novel route for producing fine-grained LaB₆ powders was accomplished with shorter reaction times resulting in higher purity.     

Characterization and low-temperature sintering of Ni0.5Zn0.5Fe2O4 nano-powders prepared by refluxing method

The as-prepared Ni_0.5Zn_0.5Fe₂O₄ powders fabricated directly from the solution of metal nitrates by the refluxing method were testified by the analysis of XRD, TEM, SAED and HRTEM. XRD pattern indicated that obtained Ni_0.5Zn_0.5Fe₂O₄ powders were single phase with spinel structure, TEM analysis showed that the powders with cubic shape were uniform in particle size of about 10-20 nm. Ceramics prepared by the as-synthesized Ni_0.5Zn_0.5Fe₂O₄ powders sintered at various temperatures between 950 °C and 1150 °C for 2 h were observed by SEM technique, which indicated that the Ni_0.5Zn_0.5Fe₂O₄ ferrites can almost be sintered to theoretic density at 1100 °C for 2 h, lower by at least about 200 °C compared with those ferrites prepared by the conventional oxide method. The relative magnetic loss tanδ/μ_i of the ceramic samples sintered at the temperature 1050 °C was measured to be of the order of 10^− 4-10^− 5 in the frequency range from 1 MHz to 10 MHz, and the threshold frequency of the ferrites was 77.2 MHz.     

Preparation, characterization and dielectric properties of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ nano-sized powders were successfully prepared by sol-gel technique and calcination at 600-900 °C. The thermal decomposition process, phase structures and morphology of synthesized powders were characterized by IR, DSC-TG, XRD, TEM, respectively. It was found that the main weight-loss and decomposition of precursors occurred below 450 °C and the complex perovskite phase appeared when the calcination temperature was higher than 700 °C. Using above synthesized powders as starting materials, CCTO-based ceramics with excellent dielectric properties (?₂₅ = 5.9 × 10⁴, tan δ = 0.06 at 1.0 kHz) were prepared by sintering at 1125 °C. According to the results, a conduction mechanism was proposed to explain the origin of giant dielectric constant in CCTO system.     

Molten salt synthesis of LiNi0.5Mn1.5O4 spinel for 5 V class cathode material of Li-ion secondary battery

Well-ordered high crystalline LiNi_0.5Mn_1.5O₄ spinel has been readily synthesized by a molten salt method using a mixture of LiCl and LiOH salts. Synthetic variables on the synthesis of LiNi_0.5Mn_1.5O₄, such as synthetic atmosphere, LiCl salt amount, synthetic temperature, and synthetic time, were intensively investigated. X-ray diffraction (XRD) patterns and scanning electron microscopic (SEM) images showed that LiNi_0.5Mn_1.5O₄ synthesized at 900 and 950 °C have cubic spinel structure () with clear octahedral dimension. LiNi_0.5Mn_1.5O₄ spinel phase began to decompose at around 1000 °C accompanied with structural and morphological degradation. LiNi_0.5Mn_1.5O₄ powders synthesized at 900 °C for 3 h delivered an initial discharge capacity of 139 mAh/g with excellent capacity retention rate more than 99% after 50 cycles.     

Fabrication and electrical properties of textured (Na,K)0.5Bi0.5TiO3 ceramics by reactive-templated grain growth

Textured (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) ceramics with a relative density of >94% were fabricated by reactive-templated grain growth. Plated-like Bi₄Ti₃O₁₂ template particles synthesized by the NaCl–KCl molten salt process were aligned by tape casting in a mixture of original oxide powders. The effect of sintering temperature on the grain orientation and electrical properties of textured NKBT ceramics were investigated. The results show that the textured ceramics have a microstructure with plated-like grains aligning in the direction parallel to the casting plane. The degree of grain orientation increased at increasing sintering temperature. The textured ceramics show anisotropic electrical properties in the directions parallel and perpendicular to the casting plane. The dielectric constant parallel to {h 0 0} plane is three times higher than that of the perpendicular direction in textured NKBT ceramics. The optimized sintering temperature is 1150 °C where the maximum dielectric constant is 2041, the remnant polarization is 68.7 μC/cm², the electromechanical coupling factor (k₃₁) and the piezoelectric constant (d₃₃) amount to 0.31 and 134 pC/N, respectively.     

Synthesis and thermal behavior of Cu/Y2W3O12 composite

Cu metal matrix composite with Y₂W₃O₁₂ as a thermal expansion compensator was fabricated by high energy ball milling followed by compaction and sintering, and its thermal properties were explored for the potential applications as heat sinks in electronic industries, high precision optics, and space structures. The volume fraction of reinforcement was varied from 40% to 70% in order to tailor the composite for the simultaneous accomplishment of low thermal expansion and high thermal conductivity. The synthesis technique was optimized by varying the parameters like milling time from 1 to 20 h and sintering temperature from 600 to 1000 °C in order to achieve densified composites. The relative density of the composites is found to be around 90% for the 10 h milled powders followed by compaction at a pressure of 700 MPa and sintering at a temperature of 1000 °C. The thermal expansion of the composites exhibits linear behavior in the temperature range 200 to 800 °C and the low coefficient of thermal expansion (CTE) is found to be for Cu–70%Y₂W₃O₁₂ composite whose value, 4.32±0.75×10⁻⁶/°C, matches with that of Si substrate. The thermal conductivities are found to increase with a decrease in the volume fraction of the reinforcement and decrease with an increase in the temperature for all the samples. The experimentally determined CTE and thermal conductivity values are found to be comparable to those predicted by the thermal expansion based Kerner and Turner model and the thermal conductivity based Maxwell model, respectively.     

Fabrication and electrical properties of Pb(Co1/3Nb2/3)O3 ceramics

Pb(Co_1/3Nb_2/3)O₃ (PCN) ceramics have been produced by sintering PCN powders synthesized from lead oxide (PbO) and cobalt niobate (CoNb₂O₆) with an effective method developed for minimizing the level of PbO loss during sintering. Attention has been focused on relationships between sintering conditions, phase formation, density, microstructural development, dielectric and ferroelectric properties of the sintered ceramics. From X-ray diffraction analysis, the optimum sintering temperature for the high purity PCN phase was found at approximately 1050 and 1100 °C. The densities of sintered PCN ceramics increased with increasing sintering temperature. However, it is also observed that at very high temperature the density began to decrease. PCN ceramic sintered at 1050 °C has small grain size with variation in grain shape. There is insignificant change of dielectric properties with sintering temperature. The P–E hysteresis loops observed at −70 °C are of slim-loop type with small remanent polarization values, which confirmed relaxor ferroelectric behavior of PCN ceramics.