Synthesis of negative thermal expansion materials ZrW2−xMoxO8 (0 ≤ x ≤ 2) using hydrothermal method

Negative thermal expansion materials ZrW_2−xMo_xO₈ (0 ≤ x ≤ 2) have been successfully synthesized by the reaction of a mixture of ammonium tungstate and ammonium molybdate with zirconium oxynitrate using a hydrothermal method. Effect of substituted ion Mo on the microstructure, α-to-β and cubic to trigonal phase transition in resulting ZrW_2−xMo_xO₈ powders was examined by the XRD experiments. It was found that the structural phase transition temperature decreased slightly with increasing substituted content. The cubic to trigonal phase transition was also influenced by substituted content. The resulting products decomposed to WO₃/MoO₃ and ZrO₂ as temperature increasing when x ≤ 0.5 and while x > 0.5, the cubic phase transited to trigonal phase. The effect of substituted Mo on the morphology of resulting products was also investigated by SEM experiments.     

Microwave dielectric properties of BaO-2(1 − x)ZnO-xNd2O3-4TiO2 (x = 0-1.0) ceramics

Microwave dielectric properties of (1 − x)BaZn₂Ti₄O₁₁-xBaNd₂Ti₄O₁₂ (x = 0-1.0) ceramics were investigated by the solid-state reaction with the purpose of finding a microwave ceramics with high dielectric constant (?_r), high quality factor (Q × f) and low temperature coefficient of resonant frequency (τ_f). A two phase system BaZn₂Ti₄O₁₁-BaNd₂Ti₄O₁₂ was formed and SEM photographs show equiaxed BaZn₂Ti₄O₁₁ grains and columnar BaNd₂Ti₄O₁₂ grains. The microwave dielectric properties were strongly determined by the chemical composition. As increasing x from 0 to 1.0, the phase composition varied from pure BaZn₂Ti₄O₁₁, to the two phase system BaZn₂Ti₄O₁₁-BaNd₂Ti₄O₁₂ and then to pure BaNd₂Ti₄O₁₂. Therefore, the ?_r raised from 29.1 to 82.0 and the Q × f values decreased from 54,630 GHz to 8110 GHz, and the τ_f values increased from −29 ppm/°C to 94 ppm/°C. 0.8BaZn₂Ti₄O₁₁-0.2BaNd₂Ti₄O₁₂ ceramics sintered at 1250 °C for 2.5 h had ?_r = 39.1, Q × f = 37,850 GHz and τ_f = −9 ppm/ °C.     

Preparation, characterization and electrical conductivity studies of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics

Several compositions of NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb_1−xGd_xZr₂O₇ ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb_0.5Gd_0.5Zr₂O₇ composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10⁻² S cm⁻¹ at 1173 K for NdYb_0.3Gd_0.7Zr₂O₇ composition.     

Phase equilibria and glass formation studies in the (1 − x)TeO2-xCdO (0.05 ≤ x ≤ 0.33 mol) system

Phase equilibria and glass formation studies of the (1 − x)TeO₂-xCdO system (0.05 ≤ x ≤ 0.33 mol) were realized by using differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The samples were prepared by applying a conventional melt-quenching technique at 800 °C. The glass formation range of the system was determined as 0.05 ≤ x < 0.15 and the sample containing 10 mol% CdO showed the highest glass stability. Crystallization behavior of the TeO₂-CdO glasses was investigated and formation and/or transformation of different phases were detected for each crystallization reaction. In order to obtain thermal stability of the system, as-cast samples were heat-treated above all crystallization reaction temperatures at 550 °C for 24 h. A binary eutectic: liquid → TeO₂ + CdTe₂O₅ was detected at 638 ± 4 °C. Crystallization behavior of the TeO₂-CdO glasses and microstructural characterization of the TeO₂-CdTe₂O₅ system was realized.     

New binary (1 − x)Ba(Lu1/2Nb1/2)O3-xPbTiO3 solid solution with morphotropic phase boundary

A new ferroelectric solid solution of (1 − x)Ba(Lu_1/2Nb_1/2)O₃-xPbTiO₃ (BLN-PT) (0 ≤ x ≤ 1) has been synthesized by solid state reactions. Its structure and electric properties have been studied by X-ray diffraction and di-/ferro-electric measurements. Based on the investigation, a partial solid state phase diagram of the binary BLN-PT ceramics system has been established, which exhibits a morphotropic phase boundary (MPB) region in the composition range of 0.64 ≤ x ≤ 0.68. The Curie temperature is measured to be around 250 °C in the vicinity of the MPB region, which is much higher than that of PMNT or PZNT system. The dielectric behavior has been discussed based on Curie-Weiss Law and Lorentz-type quadratic relationship. With increasing PT content, a transformation from relaxor to ferroelectric phase has been demonstrated in the solid solution system.     

Adjustable dielectric properties of Li2CuxZn1−xTi3O8 (x=0 to 1) ceramics with low sintering temperature

Single-phase dielectric ceramics Li₂Cu_xZn_1−xTi₃O₈ (x=0–1) were synthesized by the conventional solid-state ceramic route. All the solid solutions adopted Li₂MTi₃O₈ cubic spinel structure in which Li/M and Ti show 1:3 order in octahedral sites whereas Li and M are distributed randomly in tetrahedral sites with the degree of Li/M cation mixing varying from 0.5 to 0.3. The substitution of Cu for Zn effectively lowered the sintering temperatures of the ceramics from 1050 to 850 °C and significantly affected the dielectric properties. As x increased from 0 to 0.5, τ_f gradually increased while the dielectric constant (ε_r) and quality factor value (Q×f) gradually decreased, and a near-zero τ_f of 1.6 ppm/°C with ε_r of 25.2, Q×f of 32,100 GHz could be achieved for Li₂Cu_0.1Zn_0.9Ti₃O₈ ceramic sintered at 950 °C, which make it become an attractive promising candidate for LTCC application. As x increases from 0.5 to 1, the dielectric loss significantly increases with AC conductivity increasing up to 2.3×10⁻⁴ S/cm (at 1 MHz).     

Perovskite-type La2−xSrxNiO4 (0 ≤ x ≤ 1) as active anode materials for methanol oxidation in alkaline solutions

Perovskite-type ternary oxides with molecular formulae, La_2−xSr_xNiO₄ (0 ≤ x ≤ 1), were prepared by a modified citric acid sol-gel route at 600 °C for their possible use in a direct methanol fuel cell (DMFC). The study was conducted by cyclic voltammetry, chronoamperometry, impedance and anodic Tafel polarization techniques. The results showed that the electrocatalytic activity of the base oxide (x = 0) in 1 M KOH plus 1 M CH₃OH at 25 °C increases with x, the observed current densities being 23.6, 47.3, 43.2 and 50.9 mA cm⁻² at a scan rate of 10 mV s⁻¹ and E = 0.6 V versus Hg/HgO for oxides with x = 0, 0.25, 0.5 and 1.0, respectively. All the four perovskite anodes used in this study did not indicate any poisoning by the methanol oxidation intermediates/products. The methanol electro-oxidation reaction followed a Tafel slope of ∼2 × 2.303RT/3F (=40 mV decade⁻¹) on each oxide catalyst, regardless of Sr content.     

Microstructural and electrical behavior of Bi4V2−xCuxO11−δ (0 ≤ x ≤ 0.4)

Cation substituted bismuth vanadate possesses high oxygen ion conductivity at lower temperatures. The ionic conductivity of this material at 300 °C is 50–100 times more than any other solid electrolyte. Three phases (α, β, γ) are observed in the substituted compound; α and γ are low and high conducting phase, respectively. Samples of Bi₄V_2−xCu_xO_11−δ (x = 0–0.4) were prepared by solid-state reaction technique. Impedance spectroscopy measurements were carried out in the frequency range of 100 Hz to 100 kHz using gold sputtered cylindrical shaped pellets to obtain bulk ionic conductivities as a function of the substitution and temperature. The change of slopes observed in the Arrhenius plots is in agreement with the phase transitions for all the compositions. The highest ionic conductivity of the Cu-substituted compound was observed in Bi₄V_1.8Cu_0.2O_11−δ which is attributed to its lower activation energy. Microstructural studies indicated the stabilization of high temperature γ-phase at low temperature in those samples whose ionic conductivity observed was higher.     

Microwave dielectric properties of the (1 − x)Mg2TiO4-xCaTiO3-y wt.% ZnNb2O6 ceramics system

Composite ceramics based on (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12-0.16, y = 0-8) were prepared by a conventional mixed-oxide route. Zn²⁺ partially replaced Mg²⁺ in Mg₂TiO₄ and formed the spinel-structured (Mg_1−δZn_δ)₂TiO₄ phase. Nb²⁺, is known to be solid soluble in CaTiO₃, was found to change its shape from cubic to pliable. A bi-phase system (Mg_1−δZn_δ)₂TiO₄ and CaTiO₃ exhibited in all samples, where a small amount of second phase Mg_1−δZn_δTiO₃ was also detected. The microwave dielectric properties of specimens were strongly related to ZnNb₂O₆ and CaTiO₃ content. As y increased, ?_r and τ_f decreased, however, Q × f decreased to a minimum value and started to increase thereafter. It was also found that ?_r and τ_f increased and Q × f decreased with increasing x. The optimized microwave dielectric properties with ?_r = 18.37, Q × f = 31,027 GHz (at 6 GHz), and τ_f = 0.51 ppm/°C were achieved for (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12, y = 4) sintered at 1360 °C for 6 h.     

Characterization of MgxM2 − xP2O7 (M = Cu and Ni) solid solutions

In this study, Mg_xM_2 − xP₂O₇ (M = Cu, Ni; 0 ≤ x ≤ 2) and Mg_3 − yNi_y(PO₄)₂ (0 ≤ y ≤ 3) compositions were synthesized by the chemical coprecipitation method and characterized by X-ray diffraction, UV-vis-NIR spectroscopy and CIE L* a* b* (Commission Internationale de l’Eclairage L* a* b*) parameters measurements.Solid solutions with α-Cu₂P₂O₇ and α-Ni₂P₂O₇ structures and solid solutions with Ni₃(PO₄)₂ structure were obtained from diphosphate and orthophosphate compositions respectively. Isostructurality of α-Ni₂P₂O₇ and α-Mg₂P₂O₇ structures enlarges the compositional range of solid solution formation respect to the Mg_xCu_2 − xP₂O₇ solid solutions one.The CIE L* a* b* parameters in Mg_xNi_2 − xP₂O₇ samples were obtained comparable with these parameters in others yellow materials suitable for ceramic pigments. Mg_0.5Ni_1.5P₂O₇ composition fired at 800 °C or 1000 °C is the optimal composition to obtain yellow materials with α-diphosphate structure in conditions of this study.     

Double channel electrode flow cell application to the study of HO2 production on MnxCo3−xO4 (0 ≤ x ≤ 1) spinel films

We conducted a study on the electroreduction of O₂ in alkaline solution at room temperature on pure thin oxide electrodes of composition Mn_xCo_3−xO₄ (0 ≤ x ≤ 1) using the double channel electrode flow cell (DCEFC). The oxides were prepared at 150 °C and deposited by spray pyrolysis onto titanium substrates. The oxygen reduction reaction (orr) occurs through “interactive” and “parallel” pathways, and the ratio of O₂ molecules reduced to OH⁻ ions with respect to those reduced to HO₂⁻ ions depends on the oxide stoichiometry and on the applied overpotential. The formation of HO₂⁻ increases when the manganese concentration increases. The results obtained for the orr show that the number of electrons transferred per O₂ molecule decreases from 3 to 2 and the ratio k₁/k₂ (the rate constants for direct reduction to OH⁻ and indirect reduction to HO₂⁻) increases, respectively, in the overpotential studied range (−0.05 to −0.6 V). The Mn³⁺ ions placed in the B-sites of the spinel structure seem to be the active centres, where hydrogen peroxide is formed.     

Synthesis and negative thermal expansion property of Y2−xLaxW3O12 (0≤x≤2)

Y_2−xLa_xW₃O₁₂ solid solutions were successfully synthesized by the solid state reaction method. The microstructure, hygroscopicity and thermal expansion property of the resulting samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM) and thermal mechanical analysis (TMA). Results indicate that the structural phase transition of the Y_2−xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phase can form for 0≤x≤0.4 with orthorhombic structure and for 1.5≤x≤2 with monoclinic one. High lanthanum content leads to a low relative density of Y_2−xLa_xW₃O₁₂ ceramic. Thermal expansion coefficients of the Y_2−xLa_xW₃O₁₂ (0≤x≤2) ceramics also vary from −9.59×10⁻⁶ K⁻¹ to 2.06×10⁻⁶ K⁻¹ with increasing substituted content of lanthanum. The obtained Y_0.25La_1.75W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.66×10⁻⁶ K⁻¹ from 103 °C to 700 °C.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Raman and dielectric investigation of (Ba0.9−xSrxCa0.1)(Ti0.8Zr0.2)O3 ferroelectric ceramics

Solid solutions of (Ba_0.9−xSr_xCa_0.1)(Ti_0.8Zr_0.2)O₃ (BSCTZ) (0.1≤x≤0.4) were prepared using the conventional solid state reaction method. The effects of the substitution content on the crystallographic structure, phase transition and dielectric properties of the samples were investigated by dielectric and Raman spectroscopy over a wide temperature range from 100 to 500 K. All the samples were noted to undergo a diffuse phase transition from the tetragonal to the cubic phase and to exhibit a relaxor ferroelectric behavior.     

Polypyrrole and La1−xSrxMnO3 (0≤ x ≤0.4) composite electrodes for electroreduction of oxygen in alkaline medium

Composite G/PPy/PPy(La_1−xSr_xMnO₃)/PPy electrodes made of the perovskite La_1−xSr_xMnO₃ embedded into a polypyrrole (PPy) layer, sandwiched between two pure PPy films, electrodeposited on a graphite support were investigated for electrocatalysis of the oxygen reduction reaction (ORR). PPy and PPy(La_1−xSr_xMnO₃) (0≤ x ≤0.4) successive layers have been obtained on polished and pretreated graphite electrodes following sequential electrodeposition technique. The electrolytes used in the electrodeposition process were Ar saturated 0.1 mol dm⁻³ pyrrole (Py) plus 0.05 mol dm⁻³ K₂SO₄ with and without containing a suspension of 8.33 g L⁻¹ oxide powder. Films were characterized by XRD, SEM, linear sweep voltammetry, cyclic voltammetry (CV) and electrochemical impedance (EI) spectroscopy. Electrochemical investigations were carried out at pH 12 in a 0.5 mol dm⁻³ K₂SO₄ plus 5 mmol dm⁻³ KOH, under both oxygenated and deoxygenated conditions. Results indicate that the porosity of the PPy matrix is considerably enhanced in presence of oxide particles. Sr substitution is found to have little influence on the electrocatalytic activity of the composite electrode towards the ORR. However, the rate of oxygen reduction decreases with decreasing pH of the electrolyte from pH 12 to pH 6. It is noteworthy that in contrast to a non-composite electrode of the same oxide in film form, the composite electrode exhibits much better electrocatalytic activity for the ORR.     

Properties of Ni1−xFex (0.1 < x < 0.9) and Invar (x = 0.64) alloys obtained by electrodeposition

Electrodeposition of Ni_1−xFe_x (x = 0.1-0.9) films was carried out from a chloride plating solution containing saccharin as an organic additive at a constant current density (5 mA/cm²) and a controlled pH of 2.5. X-ray diffraction studies revealed the existence of an fcc, or γ phase, in the range of 10-58 wt.% Fe, a mixed fcc/bcc phase in the range of 59-60 wt.% Fe, and a bcc, or α phase in the range of 64-90 wt.% Fe. The saturation magnetization, B_s, of electrodeposited Ni_1−xFe_x alloys at the room temperature was found to increase with the increase of Fe-content and follows the Slater-Pauling curve, but deviates from as-cast bulk NiFe alloys. The coefficient of thermal expansion, CTE, of electrodeposited alloys at room temperature also deviates from as-cast bulk NiFe alloys. Annealing of α-Ni₃₆Fe₆₄ alloy results in a martensitic α → γ phase transformation, which takes place between 300 and 400 °C. It was demonstrated that thermal treatment above 400 °C was necessary to obtain magnetic and mechanical properties similar to those to conventional Invar alloy. Annealing of α-Ni₃₆Fe₆₄ alloy at 700 °C brings about a decrease of B_s from 1.75 to 0.45 T. By controlling the annealing conditions of α → γ martensitic transformation, it is possible to adjust the CTE of Ni₃₆Fe₆₄ alloy over the broad limits from 2.7 to 8.7 × 10⁻⁶/°C.     

Structural and magnetic characterization of MnxZn1 − xFe2O4 (x = 0.2; 0.35; 0.65; 0.8; 1.0) ferrites obtained by the citrate precursor method

In this work the microstructure and magnetic properties of Mn-Zn ferrites powders were investigated. Mn_xZn1 _− xFe₂O₄ powders where x = 0.2; 0.35; 0.5; 0.65; 0.8 and 1.0 were obtained by citrate precursor method. Citrate resin precursor was burned on air atmosphere at 400 °C for 3 h. Mn-Zn powders were calcined at 950 °C during 150 min under inert atmospheres: N₂ and rarefied atmosphere. Thermal analysis of precursor resin, phase evolution and microstructure of Mn-Zn ferrites powders were investigated by TG, DTA, XRD and SEM techniques. The powders calcined under rarefied atmosphere show spinel cubic structure and contamination of α-Fe₂O₃, while powders calcined under N₂ presents only the spinel cubic structure. Particle size was observed by SEM ranging from 80 to 150 nm. The magnetic properties were measured employing a vibrating sample magnetometer (VSM). It was observed that the saturation magnetization M_s increased with the increase of Mn content. The M_s of Mn_0.8Zn_0.2Fe₂O₄ calcined on rarefied atmosphere and Mn_0.8Zn_0.2Fe₂O₄ calcined on N₂ was 23.31 emu g⁻¹ and 56.23 emu g⁻¹, respectively.     

Uncommon potential hysteresis in the Li/Li2xVO(H2−xPO4)2 (0 ≤ x ≤ 2) system

Physical and electrochemical investigations of vanadium phosphates, Li_2xVO(H_2−xPO₄)₂ (0 < x < 2), have been undertaken. H⁺/Li⁺ ionic exchange from VO(H₂PO₄)₂ to Li₂VO(HPO₄)₂ leads to grain decrepitation. Further ionic exchange toward formation of Li₄VO(PO₄)₂ lowers the symmetry. As inferred from potentiodynamic cycling correlated to ex situ and in situ X-ray diffraction (XRD), the system Li/Li₄VO(PO₄)₂ shows several phase transformations that are associated with thermodynamical potential hysteresis that span from roughly 15 mV to more than 1.8 V. Small hysteresis are associated with topotactic reactions and with V^V/V^IV and V^III/V^II redox couples. Large potential hysteresis values (>1 V) were observed when oxidation of V^III to V^IV is involved.     

Structure and electrochemical characteristics of TiV1.1Mn0.9Nix (x = 0.1-0.7) alloys

The structure and electrochemical properties of TiV_1.1Mn_0.9Ni_x (x = 0.1-0.7) solid solution electrode alloys have been investigated. It is found that these alloys mainly consist of a solid solution phase with body centered cubic (bcc) structure and a C14 Laves secondary phase. The solid solution alloys show easy activation behavior, high temperature dischargeability, high discharge capacity and favorable high-rate dischargeability as a negative electrode material in Ni-MH battery. The maximum discharge capacity is 502 mAh g⁻¹ at 303 K when x = 0.4. Electrochemical impedance spectroscopy (EIS) test shows that the charge-transfer resistance at the surface of the alloy electrodes decreases obviously with increasing Ni content.     

Influence of A-site Ba substitution on microwave dielectric properties of (BaxMg1−x)(A0.05Ti0.95)O3 (A=Zr,Sn) ceramics

The microwave dielectric properties of (Ba_xMg_1−x)(A_0.05Ti_0.95)TiO₃ (A=Zr, Sn) ceramics were investigated with regard to substitution of Ba for Mg of A-site. The microwave dielectric properties were correlated with the Ba content. With an increase in Ba content from 0.01 to 0.1, the dielectric constant and the τ_f value increased, but the Q×f value decreased. The sintered (Ba_xMg_1−x)(Zr_0.05Ti_0.95)TiO₃ (called B_xMZT) ceramics had a permittivity in the range of 19.1−20.6, quality factor from 180,000 to 25,000 GHz, and variation in temperature coefficient of resonant frequency from −35 to −39 ppm/°C with increasing composition x. For sintered (Ba_xMg_1−x)(Sn_0.05Ti_0.95)TiO₃ (called B_xMST) ceramics, the dielectric constant increased from 19 to 20.5, Q×f value increased from 120,000 to 37,000 (GHz), and the τ_f value increased from −50 to −3.3 ppm/°C as the x increased from 0.01 to 0.1. When A=Sn and x=0.1, (Ba_0.1Mg_0.9)(Sn_0.05Ti_0.95)TiO₃ ceramics exhibited dielectric constant of 20.5, Q×f value of 37,000 (GHz), and a near-zero τ_f value of −3.3 ppm/°C sintered at 1210 °C for 4 h.