Microstructural characterization and crystallization kinetics of (1 − x)TeO2–xK2O (x = 0.05, 0.10, 0.15, 0.20 mol) glasses

Microstructural characterization and crystallization kinetics of (1 − x)TeO₂–xK₂O (x = 0.05, 0.10, 0.15, and 0.20 in molar ratio) glasses were investigated using DTA, XRD, Raman spectroscopy, optical microscopy and SEM techniques. Whereas only one exothermic peak was observed for the 0.95TeO₂–0.05K₂O and 0.90TeO₂–0.10K₂O glasses, two crystallization peaks were present on the DTA plots of the 0.85TeO₂–0.15K₂O and 0.80TeO₂–0.20K₂O glasses. On the basis of the XRD and Raman spectrophometry investigations, α-TeO₂, γ-TeO₂ and K₂Te₄O₉ crystal phases were present in the (1 − x)TeO₂–xK₂O (x = 0.05, 0.10, 0.15, and 0.20 in molar ratio) glass samples heated above the peak crystallization temperatures, T_p. SEM/EDS investigations of (1 − x)TeO₂–xK₂O (x = 0.05, 0.10, 0.15, and 0.20 in molar ratio) glasses heated above T_p revealed the presence of distinct TeO₂-rich and K₂Te₄O₉ in the 0.95TeO₂–0.05K₂O and triangular wedge-shaped crystalline regions in the 0.90TeO₂–0.10K₂O, 0.85TeO₂–0.15K₂O and 0.80TeO₂–0.20K₂O glasses. DTA analyses were carried out at different heating rates and the Avrami constant for the 0.95TeO₂–0.05K₂O glass was calculated as 0.94, an indication of surface crystallization also confirming SEM results. On the other hand, the n values were between 1.7 and 1.87 for the exothermic peaks of the 0.80TeO₂–0.10K₂O, 0.85TeO₂–0.15K₂O and 0.80TeO₂–0.20K₂O glasses, indicating one-dimensional crystalline growth mechanisms for these glasses. Activation energies for one-dimensional crystal growth mechanisms in these crystals determined from the modified Kissinger plots were found to vary between 550 and 650 kJ/mol.     

Microstructural characterization and crystallization behaviour of (1 − x)TeO2–xWO3 (x = 0.15, 0.25, 0.3 mol) glasses

DTA, XRD and SEM investigations were conducted on the (1 − x)TeO₂–xWO₃ glasses (where x = 0.15, 0.25 and 0.3). Whereas the 0.75TeO₂–0.25WO₃ and 0.7TeO₂–0.3WO₃ glasses show no exothermic peaks, an indication of no crystallization in their glassy matrices, two crystallization peaks were observed on the DTA plot of the 0.85TeO₂–0.15WO₃ glass. On the basis of the XRD measurements of the 0.85TeO₂–0.15WO₃ glass samples heated to 510 °C and 550 °C (above the peak crystallization temperatures), α-TeO₂ (paratellurite), γ-TeO₂ and WO₃ phases were detected in the sample heated to 510 °C and the α-TeO₂ and WO₃ phases were present in the sample heated to 550 °C. SEM micrographs taken from the 0.85TeO₂–0.15WO₃ glass heated to 510 °C showed that centrosymmetrical crystals were formed as a result of surface crystallization and were between 3 μm and 15 μm in width and 12 μm and 30 μm in length. On the other hand, SEM investigations of the 0.85TeO₂–0.15WO₃ glass heated to 550 °C revealed the evidence of bulk massive crystallization resulting in lamellar crystals between 1 μm and 3 μm in width and 5 μm and 30 μm in length. DTA analyses were carried out at different heating rates and the Avrami constants for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C were calculated as 1.2 and 3.9, respectively. Using the modified Kissinger equation, activation energies for crystallization were determined as 265.5 kJ/mol and 258.6 kJ/mol for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C, respectively.     

Non-isothermal crystallization kinetics of MgO–BaO–B2O3–Al2O3–SiO2 glass

5MgO–9BaO–33B₂O₃–33Al₂O₃–20SiO₂ (mol%) glass was prepared by the melt quenching method at 1823 K for 2 h. Dilatometry and differential scanning calorimetry (DSC) curves of the glass have been investigated. Fragility index F was used to estimate glass formability. The crystallization kinetics of the glass was described by the activation energy (E) for crystallization and numerical factors (n, m) depending on the nucleation process and growth morphology. XRD and SEM analysis were also used to describe the crystals’ types and morphology precipitated from the MgO–BaO–B₂O₃–Al₂O₃–SiO₂ glass. The results show that the effective activation energy of the crystallization process E was 45.19 kJ/mol, and n up to 4.05. Two crystals phases, i.e. Al₄B₂O₉ and Al₂₀B₄O₃₆ were observed in the crystallized samples. SEM results were consistent with crystallization kinetics.     

Microwave dielectric properties of rutile (Zn1/3Nb2/3)0.40(Ti1−xSnx)0.60O2 (0.15 ≤ x ≤ 0.30) ceramics

Microwave dielectric properties of (Zn_1/3Nb_2/3)_0.40(Ti_1−xSn_x)_0.60O₂ ceramics were investigated as a function of SnO₂ content (0.15 ≤ x ≤ 0.30). A single phase with tetragonal rutile structure was obtained through the entire composition. The unit-cell volume of the specimens was increased with SnO₂ content, due to the larger ionic radius of Sn⁴⁺ (0.69 Å) than that of Ti⁴⁺ (0.605 Å) for octahedral site. Dielectric constant (K) of the sintered specimens was affected by the dielectric polarizability. Quality factor (Qf) was dependent on the degree of reduction of Ti⁴⁺ ion. With an increase of SnO₂ content, the temperature coefficient of resonant frequency (TCF) of the specimens decreased due to the decrease of the octahedral distortion of rutile structure.     

Effect of glass additions on the sintering and microwave properties of composite dielectric ceramics based on BaO–Ln2O3–TiO2 (Ln = Nd, La)

Ten weight percent BBZS (Bi₂O₃, B₂O₃, ZnO and SiO₂) glass was added to x(Ba₄Nd_9.333Ti₁₈O₅₄) − (1 − x)(BaLa₄Ti₄O₁₅) (BNLT, 0 ≤ x ≤ 1) composite dielectric ceramics to lower their sintering temperature whilst retaining microwave properties useful for low temperature co-fired ceramic and antenna core technology. With the addition of 10 wt% BBZS glass, dense BNLT composite ceramics were produced at temperatures between 950 and 1140 °C, depending on composition (x), an average reduction of sintering temperature by 350 °C. X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy studies revealed that there was limited inter-reaction between BLT/BNT and the BBZS glass. Microwave property measurement showed that the addition of BBZS glass to BNLT ceramics had a negligible effect on _r and τ_f, although deterioration in the measured quality factor (Qf) was observed. The optimised composition (xBNT − (1 − x)BLT)/0.1BBZS (x = 0.75) had _r  61, τ_f  38 ppm/°C and Qf  2305 GHz.     

Preparation of CexZr1−xO2 (x = 0.75, 0.62) solid solution and its application in Pd-only three-way catalysts

Nanoparticles of Ce_xZr_1−xO₂ (x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H₂O₂ as an oxidant, and characterized by N₂ adsorption, XRD and H₂-TPR. Ce_xZr_1−xO₂ prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of Ce_xZr_1−xO₂ increased, but thermal stability of Ce_xZr_1−xO₂ decreased. The surface area of Ce_0.62Zr_0.38O₂ was 41.2 m²/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO₂, and the reduction properties of Ce_xZr_1−xO₂ were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce_0.75Zr_0.25O₂ was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C₃H₈ was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C₃H₈ was induced by the steam reform and dissociation adsorption reaction of C₃H₈. Pd-only catalyst using Ce_0.75Zr_0.25O₂ as active washcoat showed high light off activity, the reaction temperatures (T₅₀) of 50% conversion of CO, C₃H₈ and NO were 180, 200 and 205 °C, respectively. However, the conversions of C₃H₈ and NO showed oscillation with continuously increasing the reaction temperature. The presence of La₂O₃ in washcoat decreased the light off activity and suppressed the oscillation of C₃H₈ and NO conversion. After being aged at 900 °C for 4 h, the operation windows of catalysts shifted slightly to rich burn. The presence of La₂O₃ in active washcoat can enhance the thermal stability of catalyst significantly.     

Phase transition behavior and electrical properties of lead-free (1 − x)(0.98K0.5Na0.5NbO3–0.02LiTaO3)–x(0.96Bi0.5Na0.5TiO3–0.04BaTiO3) piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(0.98K_0.5Na_0.5NbO₃–0.02LiTaO₃)–x(0.96Bi_0.5Na_0.5TiO₃–0.04BaTiO₃) (KNN–LT–BNT–BT) with x = 0–0.10 have been synthesized by a conventional sintering technique. All samples possess pure perovskite structure, showing room temperature symmetries of orthorhombic at x < 0.02, and tetragonal at 0.05 ≤ x ≤ 0.10. A coexistence of orthorhombic and tetragonal phases in the composition range of 0.02 ≤ x < 0.05 in this system is caused by the temperature of the polymorphic phase transition (PPT) decreasing to around room temperature but not the behavior of the morphotropic phase boundary (MPB). The samples near the coexistence region exhibit improved properties, which are as follows: piezoelectric constant d₃₃ = 155 pC/N, remnant polarization P_r = 24.2 μC/cm², and coercive electric field E_c = 2 kV/mm. The results indicate that although this kind of ceramics displays good properties, further study is needed to promote the stabilities of the ceramics in order to utilize them in varying temperature environments.     

Fabrication and microstructure of directionally solidified SrCe1 − xYxO3 − δ (x = 0.1, 0.2) high temperature proton conductors

SrCe_1 − xY_xO_3 − δ (x = 0.1, 0.2) high temperature proton conductors (HTPC) have been fabricated by directional solidification using a laser-heated float zone (LHFZ) method. The resulting microstructures have been studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The LHFZ method produces a cellular microstructure consisting of crystalline cells embedded in an amorphous matrix, showing a strong biaxial texture. EBSD studies show that all the crystalline cells have their <0 0 1> axis as described in the cubic Pm3m prototype unit cell parallel to the growth direction. The observed microstructural features are explained in terms of the fabrication process.     

Comparative study of Li(Li1/3Ti5/3)O4 and Li(Ni1/2−xLi2x/3Tix/3)Ti3/2O4 (x = 1/3) anodes for Li rechargeable batteries

A solid solution of spinel (2/3)Li(Li_1/3Ti_5/3)O₄–(1/3)Li(Ni_1/2Ti_3/2)O₄ was prepared, and its structural/electrochemical properties were compared with Li(Li_1/3Ti_5/3)O₄ to identify the effect of doping to the structural invariance of Li(Li_1/3Ti_5/3)O₄. The solid solution retained the zero strain characteristic of Li(Li_1/3Ti_5/3)O₄ during discharge/charge with an excellent cycle stability, while the rate capability was notably improved. However, a reversible broadening of the XRD peak was observed at the end of discharge, indicating some structural changes. XANES measurements showed that the oxidation state of Ti was +4 and that of Ni was +2 in the solid solution.     

Synthesis, characterization, redox and photocatalytic properties of Ce1−xPdxVO4 (0 ≤ x ≤ 0.1)

Zircon-type CeVO₄ and Ce_1−xPd_xVO₄ (0.02 ≤ x ≤ 0.1) were synthesized by a single step solution combustion method. The materials were characterized by powder X-ray diffraction, magnetic measurement and temperature-programmed reduction (TPR) with H₂. The redox properties of CeVO₄ and Ce_0.98Pd_0.02VO₄ have been investigated by temperature-programmed reduction. The gas-phase and liquid-phase catalytic properties of CeVO₄ and Ce_0.98Pd_0.02VO₄ were investigated. Ce_0.98Pd_0.02VO₄ shows better catalytic activity than CeVO₄ towards CO oxidation by O₂, NO reduction by CO and the photocatalytic degradation of various dyes. This higher catalytic activity of Ce_0.98Pd_0.02VO₄ can be attributed to the lattice labile oxygen and high surface area compared to that of CeVO₄.     

Nanosized perovskite-type oxides La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) for the catalytic removal of ethylacetate

Nanometer perovskite-type oxides La_1−xSr_xMO_3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m² g⁻¹, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn⁴⁺ and Mn³⁺ in La_1−xSr_xMnO_3−δ and Co³⁺ and Co²⁺ in La_1−xSr_xCoO_3−δ, Sr substitution induced the rises in Mn⁴⁺ and Co³⁺ concentrations; adsorbed oxygen species (O⁻, O₂⁻, or O₂²⁻) were detected on the catalyst surfaces. The O₂-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H₂-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h⁻¹, the catalytic activity (as reflected by the temperature (T_100%) for EA complete conversion) increased in the order of LaCoO_2.91 (T_100% = 230 °C) ≈ LaMnO_3.12 (T_100% = 235 °C) < La_0.6Sr_0.4MnO_3.02 (T_100% = 190 °C) < La_0.6Sr_0.4CoO_2.78 (T_100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios), and rich lattice defects of the nanostructured La_1−xSr_xMO_3−δ materials.     

Double perovskite Pr2−xBixSr2O6 (x = 0.533) in ketonization of 1-butanol: Effect of water vapor addition

An effective ketonization of 1-butanol mixed with water vapor over the catalyst Pr_1.467Bi_0.533Sr₂O_5.928 of monoclinically distorted perovskite structure is reported. The catalyst is characteristic of 1.2 at.% of oxygen vacancies, supposed to act as active centers of Lewis type, and of a high oxidative ability reflected by large change in the catalyst's effective valence factor (V-3). The ketonization performed without water addition is accompanied with total carbonization of 1-butanol, caused by the catalyst's oxygen, the latter easily releasable beginning from low temperatures. This negative effect is practically removable if the process is performed under the presence of water vapor. It is also found that water addition influences the formation of C₃H₇–CHO aldehyde and of C₃H₇–CO–C₃H₇ ketone. Nevertheless, water addition leads to a steady destruction of the catalyst's crystal structure because of CO₂, appearing in course of the WGSR process, and of its subsequent reaction with the structure component SrO to SrCO₃. The catalyst is found easily reproducible by its re-oxidation in air at 850–900 °C.     

Ho-doped BaTiO3: Polymorphism, phase equilibria and dielectric properties of BaTi1−xHoxO3−x/2: 0 ≤ x ≤ 0.17

An extensive range of Ho-doped BaTiO₃ solid solution forms in which Ho substitutes for Ti with creation of oxygen vacancies. The effect of Ho substitution is to destabilise thermodynamically the high-temperature hexagonal polymorph of BaTiO₃. Nevertheless, at high Ho contents, the hexagonal polymorph forms as a kinetically stable intermediate before transforming to the thermodynamically stable cubic polymorph; its formation represents an example of Ostwald's rule of successive reactions. Samples fired at 1400 °C and cooled in air are insulating and transform from ferroelectric to relaxor ferroelectric behaviour with increasing x.     

Structure and physicochemical properties of the (AsSI)1? x (As2S3) x and (AsSeI)1? x (As2Se3) x glasses

The structure and physicochemical properties of glasses in the (AsSI)_1−x (As₂S₃) _x and (AsSeI)_1−x (As₂Se₃) _x systems are investigated. It is demonstrated using ¹²⁷I M?ssbauer spectroscopy that the structure of the glasses under study is built up of the structural units AsS_3/2 (AsSe_3/2), which correspond to the compounds As₂S₃ (As₂Se₃), and the ternary structural units, which correspond to the compounds AsSI (AsSeI). In turn, these structural units are connected into a three-dimensional network. The proposed model accounts for the observed linear dependences of the physicochemical properties (density, microhardness, glass transition temperature) of the glasses on the composition. Original Russian Text ? V.A. Bordovskii, R.A. Castro, A.V. Marchenko, P.P. Seregin, 2007, published in Fizika i Khimiya Stekla.     

The effects of La2O3 doping on the photosensitivity,crystallization behavior and dielectric properties of Li2O-Al2O3-SiO2 photostructurable glass

Photostructurable Li₂O-Al₂O₃-SiO₂ glass is a promising material to fabricate complex three-dimensional structure with a high aspect ratio. However, its high dielectric loss at high frequencies has restrained its application in the field of integrated circuits packaging. In this research, La₂O₃, which has a large ionic radius, as well as strong polarization and bonding strength, was used to obstruct mobile ion migration to reduce the dielectric loss. The results indicated that moderate doping with La₂O₃ could effectively reduce the dielectric loss. When the dopant amount was 3%, the dielectric loss was successfully reduced to a minimum of 4?×?10^?3 with a dielectric constant of 6.6 at 1?GHz, and this sample also possessed the optimal dielectric-temperature stability. Additionally, the effects of doping on the photosensitivity and crystallization behavior were also analysed. The results suggested that La₂O₃ doping did not affect the photosensitivity and selective crystallization characteristics. However, La₂O₃ restrained the precipitation of silicate from the [SiO₄] tetrahedron, resulting in a decrease of nucleation rate and a delay of crystallization.     

Low temperature synthesis of MgxAl2(1−x)Ti(1+x)O5 films by sol–gel processing

Aluminium titanate films thicker than 0.5 μm have been synthesized by sol–gel methods. The films have been deposited via repetitive dip-coating on silicon wafers and their thermal stability has been tested as a function of the annealing time and temperature. The sol–gel approach has allowed the formation of the aluminium titanate phase at temperatures (700 °C) much lower than those necessary for solid-state reactions (1450 °C). Magnesium oxide has been used to improve the thermal stability of the films at high temperatures. The behavior of samples prepared with two different Mg content, i.e. Mg_0.2Al_1.6Ti_0.8O₅ and Mg_0.6Al_0.8Ti_1.6O₅, has been studied. The films have proven to be stable at 1150 °C, for up to 90 h. X-ray photoelectron spectroscopy has shown that after firing at 500 °C the surface chemical composition of the films is in accordance with the nominal one, whilst at higher annealing temperatures some differences, attributed to diffusion effects, have been observed.     

Simultaneous desorption–crystallization of CO2–CaCO3 in multi-stage flash (MSF) distillers

In this work, the desorption–crystallization of CO₂–CaCO₃ in MSF distillers was simulated by coupling mass transfer with chemical reaction and correlating the CaCO₃ crystallization rates to the CO₂ release rates. The model was applied to two 20-stage reference MSF once-through and recycles distillers. The CO₂ release rates decreased exponentially from the first stage to the last stage. The CO₂ release rates increased with increasing top brine temperature (TBT) and so CaCO₃ deposition rates did. The CaCO₃ deposition rates increased by 76.9, 102.5 and 123.0 g per ton distillate at 90, 100 and 110 °C, respectively. This corresponded to fouling resistance of 0.64, 0.83 and 1.00 m²K/kW, respectively. The results were fully discussed and interpreted.     

Sinterability, crystallization and properties of glass–ceramic tiles belonging to CaF2–CaO–MgO–Al2O3–SiO2 system

The effect of CaO substitution by different amounts of MgO on crystallization and various properties of CaF₂–CaO–Al₂O₃–SiO₂ glass system were investigated. It was shown that the properties of obtained glass–ceramics are greatly influenced by MgO contents. The Avrami exponent and activation energy for crystallization of the most promising and the MgO-free specimens were determined. Results suggested that surface crystallization was the main precipitation mechanism of both samples and while the activation energy for crystallization of MgO containing sample was less than MgO-free one, its ability to crystallize was diminished. SEM results confirm occurrence of surface crystallization of samples and depicts the phenomenon of microstructure coarsening by increasing MgO content and also reduced densification of specimen with magnesium oxide more than 9 wt.%.     

Relationships between crystal structure and microwave dielectric properties of (Zn1/3B2/3)xTi1 − xO2 (B = Nb, Ta) ceramics

Dependence of microwave dielectric properties on the crystal structure of (Zn_1/3B_2/3⁵⁺)_xTi_1 − xO₂ (B⁵⁺ = Nb, Ta) ceramics was investigated as a function of Zn_1/3B_2/3⁵⁺O₂ (B⁵⁺ = Nb, Ta) content (0.4 ≤ x ≤ 0.7). Dielectric constant (K) and the temperature coefficient of resonant frequency (TCF) of sintered specimens were strongly dependent on the structural characteristics of oxygen octahedra in rutile structure. Cation rattling and the distortion of oxygen octahedra were dependent on the bond length ratio of apical (d_apical)/equatorial (d_equatorial) of oxygen octahedra. The quality factor (Qf) was dependent on the reduction of Ti ion as well as the microstructure of the sintered specimens.     

High-performance Li4Ti5−xVxO12 (0 ≤ x ≤ 0.3) as an anode material for secondary lithium-ion battery

Powders of spinel Li₄Ti_5−xV_xO₁₂ (0 ≤ x ≤ 0.3) were successfully synthesized by solid-state method. The structure and properties of Li₄Ti_5−xV_xO₁₂ (0 ≤ x ≤ 0.3) were examined by X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electronic microscope (SEM), galvanostatic charge–discharge test and cyclic voltammetry (CV). XRD shows that the V⁵⁺ can partially replace Ti⁴⁺ and Li⁺ in the spinel and the doping V⁵⁺ ion does almost not affect the lattice parameter of Li₄Ti₅O₁₂. Raman spectra indicate that the Raman bands corresponding to the Li–O and Ti–O vibrations have a blue shift due to the doping vanadium ions, respectively. SEM exhibits that Li₄Ti_5−xV_xO₁₂ (0.05 ≤ x ≤ 0.25) samples have a relative uniform morphology with narrow size distribution. Charge–discharge test reveals that Li₄Ti_4.95V_0.05O₁₂ has the highest initial discharge capacity and cycling performance among all samples cycled between 1.0 and 2.0 V; Li₄Ti_4.9V_0.1O₁₂ has the highest initial discharge capacity and cycling performance among all samples cycled between 0.0 and 2.0 V or between 0.5 and 2.0 V. This excellent cycling capability is mainly due to the doping vanadium. CV reveals that electrolyte starts to decompose irreversibly below 1.0 V, and SEI film of Li₄Ti₅O₁₂ was formed at 0.7 V in the first discharge process; the Li₄Ti_4.9V_0.1O₁₂ sample has a good reversibility and its structure is very advantageous for the transportation of lithium-ions.