The effect of the concentration of citric acid and pH values on the preparation of MgAl2O4 ultrafine powder by citrate sol-gel process

Ultrafine MgAl₂O₄ was synthesized by citrate sol-gel process. A model was presented to evaluate the concentration of species in a citric solution for preparing MgAl₂O₄ ultrafine powder. The evaluated concentration of species can provide valuable information and help in selecting the optimal condition for preparation of MgAl₂O₄ powder by citrate sol-gel process. The influence of molar ratio of cations, citric acid and pH on the formation of MgAl₂O₄ was studied. The spinel precursor gel and the ultrafine MgAl₂O₄ spinel were characterized by X-ray diffraction (XRD), differential thermal analysis, thermogravimetric (TG-DTA) and scanning electron microscope (SEM). The results show that the MgAl₂O₄ spinel phase begins to form at 600 °C, and most of MgAl₂O₄ crystals are spherical with a crystal size about 30-50 nm.     

Synthesis and characterization of (Na0.85K0.15)0.5Bi0.5TiO3 ceramics by different methods

The (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (BNKT) powders were synthesized by solid-state method, sol-gel method and stearic acid method. Microstructure, piezoelectric and dielectric properties of the ceramics were investigated. Attempts had been made to understand the reaction processes by using thermo gravimetric (TG) and differential scanning calorimetry (DSC). The BNKT powders have a perovskite structure with average crystallite sizes of 168 nm, 85 nm and 79 nm, corresponding to the solid-state method, the sol-gel method and the stearic acid method, respectively. The ceramics derived from the powder synthesized by sol-gel method presents the most homogeneous microstructure and largest grain size (5-7 μm). The effects of average crystallite size on microstructures and electric properties of the BNKT ceramics were investigated. Both the piezoelectric properties and dielectric properties were enhanced with the increase of grain size.     

Structure and electrical properties of (1 − x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3 ceramics near morphotropic phase boundary

The binary lead-free piezoelectric ceramics with the composition of (1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ were synthesized by conventional mixed-oxide method. The phase structure transformed from rhombohedral to tetragonal phase in the range of 0.16 ≤ x ≤ 0.20. The grain sizes varied with increasing the Bi_0.5K_0.5TiO₃ content. Electrical properties of ceramics are significantly influenced by the Bi_0.5K_0.5TiO₃ content. Two phase transitions at T_t (the temperature at which the phase transition from rhombohedral to tetragonal occurs) and T_c (the Curie temperature) were observed in all the ceramics. Adding Bi_0.5K_0.5TiO₃ content caused the variations of T_t and T_c. A diffuse character was proved by the linear fitting of the modified Curie-Weiss law. Besides, the ceramics with homogeneous microstructure and excellent electrical properties were obtained at x = 0.18 and sintered at 1170 °C. The piezoelectric constant d₃₃, the electromechanical coupling factor K_p and the dielectric constant ?_r reached 144 pC/N, 0.29 and 893, respectively. The dissipation factor tan δ was 0.037.     

Combustion synthesis of La0.7Sr0.3Co0.5Fe0.5O3 (LSCF) porous materials for application as cathode in IT-SOFC

La_0.7Sr_0.3Co_0.5Fe_0.5O₃ (LSCF) porous materials have attracted a substantial interest for application as cathode in solid oxide fuel cells of intermediate temperature (IT-SOFC). This work investigates the effect of different propellants (urea, glycine, citric acid and sucrose) in the preparation of LSCF powders by the combustion method and also the influence of the sintering temperature on the porosity and electrical conductivity. TGA profiles of the as-prepared samples showed a lower weight loss for the sample prepared with glycine, associated with the higher combustion temperature. XRD patterns presented characteristic reflections of LSFC perovskite and a small formation of secondary phases, with nanometric crystallite sizes (9-20 nm). SEM analysis revealed the loose and porous structure of the powder materials. Densification studies were carried within 950-1100 °C, showing that porosity decreased with increasing sintering temperature. Electrical conductivity was measured in the temperature range 300-800 °C and correlated with the sintering temperature.     

Combustion synthesis of Sr0.5Ba0.5Nb2O6 and effect of fuel on its microstructure and dielectric properties

Nano-sized Sr_0.5Ba_0.5Nb₂O₆ (SBN50) powder has been synthesized, at very short reaction time, for the first time by a novel combustion method. Ba(NO₃)₂ and Sr(NO₃)₂ were used as source of Sr and Ba, respectively, while Nb-oxalate was used as the source of niobium. Urea, hexamethyltetramine (HMT) and glycine were used as fuel. The crystallite sizes in the powder ranged between 14-125 nm. X-ray diffraction analysis showed complete SBN50 phase formation at 700 °C, when urea/HMT was used as fuel, and at 800 °C when glycine was used as fuel. Ferroelectric-paraelectric phase transition temperature (T_c) close to 40 °C was observed when urea and HMT were used and the T_c was −49 °C when glycine was used. When urea was used as fuel highest dielectric constant was observed for the pellets sintered at 1250 °C for 4 h. Low dielectric loss was observed when HMT was used as fuel. Larger grain sizes in the sintered pellets were observed when glycine was used as fuel.     

Synthesis and high-pressure sintering of (W0.5Al0.5)C

A new solid solution of Al in WC, which can be expressed by the chemical formula (W_0.5Al_0.5)C, has been synthesized directly by reaction milling (RM) of a W_0.5Al_0.5 alloy and the proper amount of carbon. The total reaction time is about 50 h. The ESEM photograph shows that the prepared (W_0.5Al_0.5)C powders are spherical, and the average particle size is about 40 nm. (W_0.5Al_0.5)C has been identified to crystallize in the hexagonal space group P-6m2 (No.187) and belongs to the WC structure type. The lattice parameter of (W_0.5Al_0.5)C is calculated to be a = 2.908(1) Å, c = 2.836(1) Å. This nanocrystalline powder can be well sintered at the high temperature (1600 °C) under the high pressure (4.5 GPa), and the relative density reaches 99.1%. The hardness of the sintered (W_0.5Al_0.5)C is tested to be 1500 ± 50 kg mm⁻², while the density is about 9.417 ± 0.003 g cm⁻³, which is far lower than that of WC.     

Synthesis and characterization of nanosized SrBi2Ta2O9 powder by a novel sol-gel process

Chemically homogeneous SrBi₂Ta₂O₉ (SBT) sol was synthesized using ethoxy tantalum, strontium acetate, and bismuth subnitrate as starting materials, methoxyethylene as a solvent and acetic acid (HOAc) as a catalyst. Single-phased perovskite phase SBT ferroelectric ultrafine powder was obtained after the dried gel was treated at 350°C for 30 min and calcined at 800°C for 1 h. FT-IR, XRD, TEM and TG-DTA were employed to investigate the transformation processes of sol to gel and gel to ultrafine SBT powder. Acetic acid not only acts as an acid catalyst, but also changes the alkoxide precursor as a ligand at a molecular level. Bidentate acetates replace OR groups and are directly bounded to the tantalum, leading to the formation of Ta (OR)_x(OAc)_5−x. The perovskite SBT phase formed via intermediate phase Bi₃TaO₇ and a Bi-deficient pyrocholore phase.     

Processing, dielectric properties and impedance characteristics of Na0.5Bi0.5Cu3Ti4O12 ceramics

Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were prepared by conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of NBCTO ceramics sintered at various temperatures with different soaking time were investigated. Pure NBCTO phase could be obtained with increasing the temperature and prolonging the soaking time. High dielectric permittivity (13,495) and low dielectric loss (0.031) could be obtained when the ceramics were sintered at 1000 °C for 7.5 h. The ceramics sintered at 1000 °C for 7.5 h also showed good temperature stability (−4.00 to −0.69%) over a large temperature range from −50 to 150 °C. Complex impedances results revealed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (R_g) was 12.10 Ω cm and the grain boundary resistance (R_gb) was 2.009 × 10⁵ Ω cm when the ceramics were sintered at 1000 °C for 7.5 h.     

Fine-sized LiCoO2 particles prepared by spray pyrolysis from polymeric precursor solution

Fine-sized LiCoO₂ particles with high discharge capacities and good cycle properties were prepared by spray pyrolysis from a spray solution with citric acid and ethylene glycol. The precursor particles obtained from the spray solution with citric acid and ethylene glycol transformed into fine-sized LiCoO₂ particles with regular morphology after post-treatment at temperatures between 700 and 900 °C. The mean size and aggregation degree of the primary particles were affected by the concentrations of polymeric precursors added into spray solutions. The LiCoO₂ particles obtained from the spray solution without polymeric precursors had irregular and aggregated morphology. The discharge capacities of the LiCoO₂ particles changed from 132 to 151 mAh/g when the concentrations of the citric acid and ethylene glycol changed from 0 to 1 M. The particles prepared from the spray solution containing 0.3 M each of citric acid and ethylene glycol had the maximum discharge intensity.     

Synthesis of SrAl2O4 from a mixed-metal citrate precursor

A mixed-metal citrate precursor method was used to synthesize SrAl₂O₄. The effects of the pH of the starting solutions and the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of SrAl₂O₄ were studied. DTA, TG, FT-IR, XRD and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. XRD analysis showed that single-phase SrAl₂O₄ was synthesized from CA/M = 2 precursors at a temperature of 900 °C for 2 h, without the formation of any intermediate phase.     

Crystal and magnetic study of the disordered perovskites Ca(Mn0.5Sb0.5)O3 and Ca(Fe0.5Sb0.5)O3

We have investigated the double perovskites Ca₂MSbO₆ (M = Mn, Fe) that have been prepared by solid-state reaction (M = Fe) and wet chemistry procedures (M = Mn). The crystal and magnetic structures have been studied from X-ray (XRD) and neutron powder diffraction (NPD) data. Rietveld refinements show that the crystal structures are orthorhombic (space group Pbnm) with complete disorder of M and Sb cations, so the formula should be rewritten as Ca(M_0.5Sb_0.5)O₃. Due to this disorder no evidences of Jahn-Teller distortion can be observed in the MnO₆ octahedra of Ca(Mn_0.5Sb_0.5)O₃, in contrast with the ordered double perovskite Sr₂MnSbO₆. Ca(Fe_0.5Sb_0.5)O₃ behaves as an antiferromagnet with an ordered magnetic moment for Fe³⁺ of 1.53(4)μ_B and a propagation vector k = 0, as investigated by low-temperature NPD. The antiferromagnetic ordering is a result of the high degree of Fe/Sb anti-site disorder of the sample, which originates the spontaneous formation of Fe-rich islands, characterized by the presence of strong Fe-O-Fe antiferromagnetic couplings with enough long-range coherence to produce a magnetic contribution perceptible by NPD. By contrast, the magnetic structure of Ca(Mn_0.5Sb_0.5)O₃ cannot be observed by low-temperature NPD because the magnitude of the ordered magnetic moments is below the detection threshold for neutrons.     

Synthesis of La0.5A0.5MnO3 (A = Sr, Ba) by a hydrothermal method at low temperature

A mild hydrothermal method has been adopted to prepare La_0.5Sr_0.5MnO₃ and La_0.5Ba_0.5MnO₃, which is of interest for a number of possible applications. The results from X-ray diffraction (XRD) indicate that in the present work the temperature of 200 and 240 °C are sufficient to prepare phase pure La_0.5Sr_0.5MnO₃ and La_0.5Ba_0.5MnO₃ crystals. At 200 °C, La_0.5Sr_0.5MnO₃ nanowires are obtained. The average width and length of the nanowires are 40 nm and 4 μm, respectively. At 240 °C, La_0.5Ba_0.5MnO₃ powders obtained have a cubic structure with the average size of 3-5 μm.     

Preparation, characterization and microwave absorption properties of polyaniline/Co0.5Zn0.5Fe2O4 nanocomposite

The polyaniline (PAni)/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was prepared by an in situ polymerization in an aqueous solution. The products were characterized by Fourier transform infrared (FT-IR) spectrometer, ultraviolet-visible (UV-vis) spectrometer, X-ray diffraction (XRD) and transmission electron microscope (TEM). The average particle size of the PAni/Co_0.5Zn_0.5Fe₂O₄ was estimated to be about 70 nm by TEM. The reflection loss (dB) of the nanocomposite was measured at different microwave frequencies in X-band (8.2-12.4 GHz), U-band (12.4-18 GHz) and K-band (18-26.5 GHz) by radar cross-section (RCS) method according to the national standard GJB-2038-94. The results showed the reflection loss of the PAni/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was higher than that of the PAni. The maximum reflection loss of the PAni/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was about −39.9 dB at 22.4 GHz with a bandwidth of 5 GHz (full frequency width at about a half of the peak response). In conclusion, this sample is a good microwave shielding and absorbing materials at higher frequency.     

Effect of MgWO4 content on properties of Ba0.5Sr0.5TiO3 composite ceramics for tunable microwave applications

xMgWO₄-(1 − x) Ba_0.5Sr_0.5TiO₃ (x = 0.0, 5.0, 15.0, 25.0 and 35.0 wt%) composite ceramics were prepared via solid state reaction processing. Their structural and dielectric properties were systematically characterized. A significant increase in grain size was observed with increasing MgWO₄ content, which was accompanied by obvious variations in dielectric properties of the composite ceramics. It is found that the permittivity peaks of the samples gradually shifted to low temperatures with increasing MgWO₄ content. At the same time, tunabilities of the composite ceramics decreased, but their Q values increased. The sample with 35 wt% MgWO₄ possesses a high tunability of 16.8% (∼10 kHz), a low permittivity of 65 and an appropriate Q value of 309 (∼4.303 GHz), which meet the requirements of high power and impedance matching, thus making it a promising candidate for applications as electrically tunable microwave devices.     

Synthesis and characterisation of the double perovskite Ba2(Zn0.5Ti0.5X)O6 (X = Nb, Ta) ceramics

Ba₂(Zn_0.5Ti_0.5X)O₆ compounds from the general ABO₃ perovskite family were synthesized by the classical solid-state route for X = Nb and Ta with various A/B ratios (1.005, 1 and 0.995). After the calcination step at 1100 °C, both compounds (X = Nb and Ta) contain mainly the cubic disordered ‘Ba₂(Zn_0.5Ti_0.5X)O₆’ phase but traces of BaTiO₃ and secondary phases are often detectable. Nevertheless, after the sintering stage at higher temperature (from 1300 to 1500 °C) and for all A/B ratios investigated, Ti enters into the cubic perovskite structure, resulting in the formation of a unique ‘Ba₂(Zn_0.5Ti_0.5X)O₆’ phase. Attractive dielectric properties have been measured on the tantalum-based compound for A/B = 0.995 (Q ∼2000 at 7.4 GHz and ? = 39.6) as well as on the niobium-based phase for A/B = 1.005 (Q ∼2200 at 6.1 GHz and ? = 54.8). All these characteristics were confirmed at 1 MHz and a linear dependence of the permittivity versus temperature from −60 to 180 °C has also been evidenced for both formulations. Sinterability, dielectric properties and microstructure of such compounds are discussed with respect to the stoichiometry.     

Tunable and microwave dielectric properties of Ba0.5Sr0.5TiO3-BaWO4 composite ceramics doped with Co2O3

Co₂O₃ doped BaWO₄-Ba_0.5Sr_0.5TiO₃ composite ceramics, prepared by solid-state route, were characterized systematically, in terms of their phase compositions, microstructure and microwave dielectric properties. Doping of Co₂O₃ promoted grain growth, reduced Curie temperature and broadened phase-transition temperature range of BaWO₄-Ba_0.5Sr_0.5TiO₃, which were attributed mainly to the substitution of Co³⁺ for Ti⁴⁺ at B site in the perovskite lattice. Dielectric diffusion behaviors of the composite ceramics were discussed. The composite ceramics all had dielectric tunability of higher than 10% at 30 kV/cm and 10 kHz, with promising microwave dielectric properties. Specifically, the sample doped with 0.2 wt.% Co₂O₃ exhibited a tunability of 20%, permittivity of 225 and Q of 292 (at 1.986 GHz), making it a suitable candidate for applications in electrically tunable microwave devices.     

Sol-gel auto-combustion synthesis of Ni0.5Zn0.5Fe2O4/(SiO2)x (x = 10, 20, 30 wt.%) nanocomposites and their characterizations

A nitrate-citrate-silica gel was prepared from metallic nitrates, citric acid and tetraethoxysilane (TEOS) by sol-gel process, and it was further used to synthesize Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites by auto-combustion. The obtained Ni_0.5Zn_0.5Fe₂O₄/(SiO₂)_x (x = 10, 20, 30 wt.%) samples were characterized by IR, ²⁹Si CP/MAS NMR, XRD, TEM, EPR and impedance analyzer measurements. Particle size of these composites was calculated from Scherrer's formula, and that decreased with increasing SiO₂ content. The content of TEOS in the starting solution affects the interaction between NiZn ferrite and silica, and then determines the particle size, dielectric properties and the EPR properties (ΔH_PP, g factor, N_S and T₂) of the as-synthesized powder.     

Materials properties of electrodeposited SnS0.5Se0.5 films and characterization of photoelectrochemical solar cells

Thin films of tin sulfoselenide (SnS_0.5Se_0.5) have been electrodeposited from an aqueous solution on tin oxide coated glass substrates by potentiostatic technique. XRD pattern of SnS_0.5Se_0.5 films showed polycrystalline nature and orthorhombic structure. The presence of Sn, S, and Se of the films were confirmed by XPS analysis. All films showed an indirect band gap. Surface morphological studies were carried out using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analyses. Mott-Schottky plots have been drawn (in the dark condition) to evaluate the semiconductor parameters which confirmed the p-type nature of the films. The photoelectrochemical behavior of the SnS_0.5Se_0.5 films was studied in the electrolyte containing 0.1 M (FeCl₂/FeCl₃) + 0.05 M H₂SO₄ and the results are reported.     

Surface characterization of LaCoO3 synthesized using citric acid

In preparation for synthesizing LaCoO₃, gel was prepared by adding citric acid to the aqueous solution of La(NO₃)₃·6H₂O and Co(NO₃)₂·6H₂O. The infrared spectrum indicates that the gel prepared with 0.007 mol of citric acid is the mixture of LaCo(C₆H₅O₇)(NO₃)₃, lanthanum nitrate, and cobalt nitrate, while the gel prepared with 0.011 mol of citric acid is LaCo(C₆H₅O₇)₂. Perovskite-type LaCoO₃ was obtained by firing the gel above 600°C. The LaCoO₃ surface was characterized by X-ray photoelectron spectroscopy and with respect to the catalytic activity of CO oxidation.     

Giant strain in Nb-doped Bi0.5(Na0.82K0.18)0.5TiO3 lead-free electromechanical ceramics

The effect of Nb substitution on the crystal structure, ferroelectric, and electric field induced strain properties of Bi_0.5(Na₈₂K_0.18)_0.5TiO₃ (BNKT) ceramics has been investigated. The coexistence of rhombohedral and tetragonal phases was found in undoped BNKT ceramics, however, Nb doping induced a phase transition to a pseudocubic phase with high electrostriction coefficients. When 3 mol% Nb was substituted on Ti ions, the electric field induced strain was markedly enhanced up to S_max/E_max = 641 pm/V, which is higher than those previously reported on non-textured lead-free electromechanical ceramics.