Effect of sintering temperature on ferroelectric properties of 0.94(K0.5Na0.5)NbO3–0.06LNbO3 system

Lead free 0.94(K_0.5Na_0.5NbO₃)–0.06(LiNbO₃) (KNN–LN) system was synthesized by conventional solid state reaction route (CSSRR). The KNN–LN system was calcined at 850 °C for 6 h for the formation of single perovskite phase whereas the sintering was done at 1050 °C, 1080 °C and 1100 °C for 4 h, respectively. The KNN–LN samples sintered at 1080 °C show better properties: room temperature (RT) dielectric constant (?_r) ∼936, dielectric loss (tan δ) ∼0.016 at 1 MHz, a relatively high bulk density (ρ) ∼4.385 g/cm³, which is 97.5% of the theoretical density (TD ∼ 4.51), remnant polarization (P_r) ∼6.4 μC/cm² and coercive field (E_c) ∼9.6 kV/cm have been observed.     

Synthesis of mesoporous TiO2/γ-Al2O3 composite granules with different sol composition and calcination temperature

Mesoporous TiO₂/γ-Al₂O₃ composite granules were prepared by combining sol-gel method and oil-drop method. After calcination at 450 °C, the composite granules showed anatase and γ-Al₂O₃ phases with the specific surface area of 240-310 m²/g. The phase composition and pore structure of the granules can be controlled by calcination temperature and the mixing ratio of boehmite sol and titania sol. The product granules can be used as a photocatalyst or adsorbent in moving or fluidized bed reactors.     

Low temperature sintering and microwave dielectric properties of CaSiO3–Al2O3 ceramics for LTCC applications

The effects of CuO, Li₂CO₃ and CaTiO₃ additives on the densification, microstructure and microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics for low-temperature co-fired applications were investigated. With a single addition of 1 wt% Li₂CO₃, the CaSiO₃–1 wt% Al₂O₃ ceramic required a temperature of at least 975 °C to be dense enough. Large amount addition of Li₂CO₃ into the CaSiO₃–1 wt% Al₂O₃ ceramics led to the visible presence of Li₂Ca₃Si₆O₁₆ and Li₂Ca₄Si₄O₁₃ second phases. Fixing the Li₂CO₃ content at 1 wt%, a small amount of CuO addition significantly promoted the sintering process and lowered the densification temperature to 900 °C whereas its addition deteriorated the microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics. Based on 10 wt% CaTiO₃ compensation in temperature coefficient, good microwave dielectric properties of ε_r=8.92, Q×f=19,763 GHz and τ_f=−1.22 ppm/°C could be obtained for the 0.2 wt% CuO and 1.5 wt% Li₂CO₃ doped CaSiO₃–1 wt% Al₂O₃ ceramics sintered at 900 °C. The chemical compatibility of the above ceramics with silver during the cofiring process has also been investigated, and the result showed that there was no chemical reaction between silver and ceramics, indicating that the as-prepared composite ceramics were suitable for low-temperature co-fired ceramics applications.     

Low temperature sintering and microwave dielectric properties of Li2ZnTi3O8 ceramics doped with ZnO–La2O3–B2O3 glass

Li₂ZnTi₃O₈ ceramics doped with ZnO–La₂O₃–B₂O₃ glass were prepared by the conventional solid-state ceramic route. The effects of the ZnO–La₂O₃–B₂O₃ glass on the sintering temperature, phase composition, microstructure and microwave dielectric properties of Li₂ZnTi₃O₈ ceramics were investigated. The addition of ZLB glass can reduce the sintering temperature of Li₂ZnTi₃O₈ ceramic from 1075 °C to 925 °C without obvious degradation of the microwave dielectric properties. Only a single phase Li₂ZnTi₃O₈ with cubic spinel structure is formed in Li₂ZnTi₃O₈ ceramic with ZLB addition sintered at 925 °C. Typically, 1.0 wt% ZLB-doped Li₂ZnTi₃O₈ ceramic sintered at 925 °C can reach a maximum relative density of 95.8% and exhibits good microwave dielectric properties of ε_r=24.34, Q×f=41,360 GHz and τ_f=−13.4 ppm/°C. Moreover, this material is compatible with Ag electrode, which makes it a promising candidate for LTCC application.     

Effect of sintering temperature on the microstructure and mechanical properties of ZrO2-3 mol%Y2O3 sol–gel films

The microstructural evolution and mechanical properties of ZrO₂-3 mol%Y₂O₃ films were investigated as a function of the sintering temperature in the range from 100 °C to 1500 °C, using a battery of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and nanoindentation. It was found that the crystallization occurs at temperatures close to 300 °C. A gradual increase in the grain and crystallite sizes is observed as the sintering temperature increases up to 1000 °C, and above this sintering temperature the tendency changes abruptly with a rapid increase in these values. Although Young's modulus of the coatings did not change with sintering temperature, a slight decrease was observed in the hardness values above 1000 °C which is attributed to microstructure coarsening. Finally, a slight degradation of the films occurs above 1300 °C, which is due to the occurrence of a process of grain spheroidization.     

Enhanced pyrocatalysis of the pyroelectric BiFeO3/g-C3N4 heterostructure for dye decomposition driven by cold–hot temperature alternation

The BiFeO₃/g-C₃N₄ heterostructure,which is fabricated via a simple mixing–calcining method,benefits the significant enhancement of the pyrocatalytic performance.With the growth of g-C₃N₄ content in the heterostructure pyrocatalysts from 0 to 25%,the decomposition ratio of Rhodamine B(RhB)dye after 18 cold-hot temperature fluctuation(25-65℃)cycles increases at first and then decreases,reaching a maximum value of~94.2%at 10%while that of the pure BiFeO₃ is~67.7%.The enhanced dye decomposition may be due to the generation of the internal electric field which strengthens the separation of the positive and negative carriers and further accelerates their migrations.The intermediate products in the pyrocatalytic reaction also have been detected and confirmed,which proves the key role of the pyroelectric effect in realizing the dye decomposition using BiFeO₃/g-C₃N₄ heterostructure catalyst.The pyroelectric BiFeO₃/g-C₃N₄ heterostructure shows the potential application in pyrocatalytically degrading dye wastewater.     

Room temperature NO2 gas sensor based on PPy/α-Fe2O3 hybrid nanocomposites

Polypyrrole–iron oxide (PPy/α-Fe₂O₃) hybrid nanocomposite sensor films were prepared by spin coating method on glass substrate and characterized for structural and morphological properties by means of X-ray diffraction (XRD), Fourier transform infra red (FTIR) spectroscopy and scanning electron microscopy (SEM), which proved the strong interaction between polypyrrole and α-Fe₂O₃ nanoparticles. The gas-sensing properties of the hybrid nanocomposites were studied and compared with those of PPy and α-Fe₂O₃. It was found that PPy/α-Fe₂O₃ hybrid nanocomposites can complement the drawbacks of pure PPy and α-Fe₂O₃ to some extent. It was revealed that PPy/α-Fe₂O₃ (50%) hybrid sensor operating at room temperature could detect NO₂ at low concentration (10 ppm) with very high selectivity (18% compared to C₂H₅OH) and high sensitivity (56%), with better stability (85%). The sensing mechanism of PPy/α-Fe₂O₃ materials to NO₂ was presumed to be the synergism of PPy and α-Fe₂O₃ or the effect of p–n heterojunctions.     

Influence of the melting temperature and rate of cooling the melt to the glass making temperature on the redox equilibria Fe2+ ? Fe3+ and Cu+ ? Cu2+ in aluminum potassium barium phosphate glasses

This paper reports on the results of the investigation of aluminum potassium barium phosphate glasses that contain copper and iron additives and have compositions similar to the composition of the matrix of the KGSS 0180/35 neodymium phosphate glass used for fabricating large-sized active elements intended for high-power laser amplifiers with a high output energy. The redox equilibrium of iron ions has been studied as a function of the melting temperature of the glass (850, 1100, and 1300°C). The redox equilibrium of iron or copper ions and their contributions to the nonactive absorption coefficient of glasses prepared at the melting temperature (1100°C) or after cooling of the glass melt at different rates to the glass making temperature (850°C) have been investigated. It has been established that a decrease in the melting temperature of the glass leads to a shift in the redox equilibrium of iron ions toward the formation of Fe³⁺ ions. During cooling of the glass melt from 1100 to 850°C, the redox equilibrium of copper (iron) ions shifts toward the formation of Cu²⁺ (Fe³⁺) ions; in this case, the lower the rate of cooling the melt, the larger the shift. At the minimum rate of cooling the glass melt (250°C for 180 min), the contribution of copper ions to the nonactive absorption coefficient increases by 25%, whereas the corresponding contribution of iron ions decreases by 40%.     

High temperature calcined K–MoO3/γ-Al2O3 catalysts for mixed alcohols synthesis from syngas: Effects of Mo loadings

One series of oxidized K–MoO₃/γ-Al₂O₃ samples with different Mo loadings (MoO₃/Al₂O₃ (wt ratio)=0.05–0.45) was prepared by impregnating K and Mo compounds and successive calcination in air at 800°C. The oxidized samples were sulfided and then utilized for mixed alcohols synthesis from syngas. The structural information from laser Raman spectroscopy (LRS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ammonia saturation, temperature programmed desorption (TPD) and ethanol decomposition were studied to elucidate the reaction properties.

The results indicated that with Mo loading increased from MoO₃/Al₂O₃=0.05 to 0.25, the total yields of mixed alcohols and hydrocarbons decreased, but the selectivity to mixed alcohols was enhanced sharply from 3% to 50%. With Mo loading increased from MoO₃/Al₂O₃=0.25 to 0.45, the CO conversion was enhanced, but the selectivity to mixed alcohols leveled off. On these catalysts, Fischer–Tropsch (FT) synthesis to linear alcohols and the condensation reaction of low alcohols to form branched i-C₄OH occurred at the same time. With increased Mo loading, activity of the alcohols condensation became high.

Structural studies demonstrated that on oxidized samples with increased Mo loading the same K–Mo–O species was formed, but the dispersion of these K–Mo species decreased. The catalyst's acidity decreased remarkably with Mo loading up to MoO₃/Al₂O₃=0.25, and stayed unchanged as Mo loading was further increased to MoO₃/Al₂O₃=0.45. With increased Mo loading, the activity for ethanol dehydration changed parallel to the acidity. Results of the activity experiments for mixed alcohols' synthesis and the structural measurements indicated that the dispersion state of Mo species and the content of unreduced Mo species influenced the total CO conversion, and that the acidity of the catalyst controlled the selectivity to mixed alcohols.     

Paper derived SiC–Si3N4 ceramics for high temperature applications

Biomorphic Si₃N₄–SiC ceramics have been produced by chemical vapour infiltration and reaction technique (CVI-R) using paper preforms as template. The paper consisting mainly of cellulose fibres was first carbonized by pyrolysis in inert atmosphere to obtain carbon bio-template, which was infiltrated with methyltrichlorosilane (MTS) in excess of hydrogen depositing a silicon rich silicon carbide (Si/SiC) layer onto the carbon fibres. Finally, after thermal treatment of this Si/SiC precursor ceramic in nitrogen-containing atmosphere (N₂ or N₂/H₂), in the temperature range of 1300–1450 °C SiC–Si₃N₄ ceramics were obtained by reaction bonding silicon nitride (RBSN) process. They were mainly composed of SiC containing α-Si₃N₄ and/or β-Si₃N₄ phases depending on the nitridation conditions. The SiC–Si₃N₄ ceramics have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Raman spectroscopy. Thermal gravimetric analysis (TGA) was applied for the determination of the residual carbon as well as for the evaluation of the oxidation behaviour of the ceramics under cyclic conditions. The bending strength of the biomorphic ceramics was related to their different microstructures depending on the nitridation conditions.     

Dielectric characters of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 ceramics fabricated at ultra-low temperature by the spark-plasma-sintering method

Solid solution (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ shows high dielectric constant near room temperature and is an ideal capacitor material. The composition 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃, which is located near the morphotropic phase boundary, were densified by the spark-plasma-sintering method at an ultra-low temperature (700 °C). Dielectric constant measurement shows that the thus prepared sample shows higher dielectric constant at room temperature and good temperature stability in a wide temperature range. The behavior is much different from that of samples sintered by conventional method and could be ascribed to size effect.     

Depolarization temperature and piezoelectric properties of (Bi1/2Na1/2)TiO3–(Bi1/2Li1/2)TiO3–(Bi1/2K1/2)TiO3 lead-free piezoelectric ceramics

The phase transition temperature and piezoelectric properties of x(Bi_1/2Na_1/2)TiO₃–y(Bi_1/2Li_1/2)TiO₃–z(Bi_1/2K_1/2)TiO₃ [x + y + z = 1] (abbreviated as BNLKT100y–100z) ceramics were investigated. BNLKT100y–100z ceramics were prepared by conventional ceramic fabrication. The depolarization temperature T_d was determined by the temperature dependence of the dielectric and piezoelectric properties. This study focuses on the effect of Li¹⁺ and K¹⁺ ions on T_d and the piezoelectric properties of BNT ceramics. BNLKT100y–100z (y = 0–0.08) has a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases at z = 0.18–0.20, and high piezoelectric properties were obtained at the MPB composition. The piezoelectric constant d₃₃ increased with increasing y; however, T_d decreased above y = 0.06. The d₃₃ and T_d values of BNLKT4-20 and BNLKT8-20 were 176 pC/N and 171 °C, and 190 pC/N and 115 °C, respectively.     

Low temperature growth of nanocrystalline Fe2TiO5 perovskite thin films by sol–gel process assisted by microwave irradiation

Nanocrystalline Fe₂TiO₅ thin films have been grown on Si (1 0 0) at room temperature by using simple, cost effective sol–gel process assisted by microwave irradiation for thermal treatment. For comparison purpose the deposited films have been subjected to two kinds of annealing treatments: first set by using conventional annealing and second set by irradiating the deposited films at different microwave powers for 10 min. In both treated films, formation of orthorhombic phase of Fe₂TiO₅ structure has been observed. It is evident that there is a dramatic structural modification when the deposited films are exposed to microwave. There was slight stoichiometric change of Fe₂TiO₅ thin films treated by conventional annealing and microwave annealing. Microwave exposed films have shown 47% of Fe, 6% of Ti and 47% of O in the films of the Fe₂TiO₅, whereas annealed films have shown close to the stoichiometry in Fe₂TiO₅ with 30% of Fe, 14% of Ti and 56% of O. Plausible mechanism for the formation of nanocrystalline orthorhombic phase of Fe₂TiO₅ perovskite structure at low microwave powers has also been discussed. This new innovative microwave heating could open a door for the advanced nanotechnologies to cut down the process cost in post treatment of the nanomaterials.     

PrBa0.5Sr0.5Co2O5+δ layered perovskite cathode for intermediate temperature solid oxide fuel cells

Layered perovskite oxides have ordered A-cations localizing oxygen vacancies, and may potentially improve oxygen ion diffusivity and surface exchange coefficient. The A-site-ordered layered perovskite PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC) was evaluated as new cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The material was characterized using electrochemical impedance spectroscopy in a symmetrical cell system (PBSC/Ce_0.9Sm_0.1O_1.9 (SDC)/PBSC), exhibiting excellent performance in the intermediate temperature range of 500-700 °C. An area-specific-resistance (ASR) of 0.23 Ω cm² was achieved at 650 °C for cathode polarization. The low activation energy (Ea) 124 kJ mol⁻¹ is comparable to that of La_0.8Sr_0.2CoO_3−δ. A laboratory-scaled SDC-based tri-layer cell of Ni-SDC/SDC/PBSC was tested in intermediate temperature conditions of 550 to 700 °C. A maximum power density of 1045 mW cm⁻² was achieved at 700 °C. The interfacial polarization resistance is as low as 0.285, 0.145, 0.09 and 0.05 Ω cm² at 550, 600, 650 and 700 °C, respectively. Layered perovskite PBSC shows promising performance as cathode material for IT-SOFCs.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Effects of strain rate and temperature on compressive strength and fragment size of ZrB2–SiC–graphite composites

The quasi-static (strain rate of 10⁻⁴ s⁻¹) and dynamic compression experiments (strain rate of 200–1500 s⁻¹) of ZrB₂–SiC–graphite composites are conducted at 293 K and 1073 K. The initial compressive strength and Weibull modulus are calculated to handle the discrete quasi-static experimental data. Considering effects of strain rate and temperature, the compressions of ZrB₂–SiC–graphite composites are investigated. The results show that both compressive strength and fragment size are higher at 1073 K than those at room temperature. The compressive strengths increase with increasing strain rate at room temperature and 1073 K, whereas fragment sizes decrease. Moreover, a micromechanical model is utilized to characterize the effect of strain rate on the compressive strength. The predictions of this micromechanical model are good agreement with the experimental results. Meanwhile, the fragment sizes of dynamic compressive specimens are analyzed through analytical approaches.     

Bi(Zn1/2Ti1/2)O3 modified BiFeO3–BaTiO3 lead-free piezoelectric ceramics with high temperature stability

Lead-free high-temperature ceramics with compositions of 0.71BiFe_1−x(Zn_1/2Ti_1/2)_xO₃–0.29BaTiO₃ (BFZTx–BT, x=0–0.05 mol fraction) were fabricated by a conventional solid state reaction method. The effect of Bi(Zn_1/2Ti_1/2)O₃ (BZT) addition on the microstructure, electrical properties, relaxor behavior, and temperature stability has been studied. XRD patterns revealed that all compositions formed a single perovskite phase of pseudo-cubic crystal structure. The grain size was slightly affected by BZT addition. The diffuse phase transition and strong frequency dispersion of dielectric permittivity are observed for BZT modified ceramics. The addition of BZT into BFZTx–BT was also found to affect the piezoelectric properties and temperature stability of the ceramics with maximum values observed for x=0.5% and 1% BFZTx–BT compositions, respectively. The optimum piezoelectric properties with d₃₃=163 pC/N, together with high-temperature stability with a depolarization temperature T_d∼380 °C, reveal the BFZTx–BT ceramics to be promising high-temperature Pb-free piezoelectric materials.     

Study of AC electrical properties of V2O5–P2O5–B2O3–Dy2O3 glasses

The AC conductivity of glass samples of composition 60V₂O₅–5P₂O₅–(35−x)B₂O₃–xDy₂O₃, 0.4≤x≤1.2 has been analyzed. The samples were prepared by the usual melt-quench technique. The prepared compounds were analyzed by X-ray diffraction (XRD) and thermo gravimetric–differential thermal analysis (TG/DTA). The activation energies were evaluated using glass transition temperature (T_g) and peak temperature of crystallization (T_c) from TG/DTA. The dependence of activation energy on composition was discussed. The electrical conductance and capacitance were measured over a frequency range of 20 Hz to 1 MHz and a temperature range of 303–473 K; these reveal semiconducting features based predominantly on an ionic mechanism. The dielectric and complex-impedance response of the sample is discussed. The relaxation time was found to increase with increasing temperature. Jonscher's universal power law is applied to discuss the conductivity. The electrode polarization was found to be negligible and confirmed from electrical modulus.