Relaxor ferroelectric and photocatalytic properties of BaBi4Ti4O15

ABSTRACT

Aurivillius phase BaBi₄Ti₄O₁₅ micro-sized powders were produced by solid-state reaction and their photocatalytic properties were reported for the first time. X-ray diffraction revealed the polar orthorhombic structure. BaBi₄Ti₄O₁₅ ceramics exhibited diffuse phase transition at ～ 410°C. The freezing temperature of 274°C was obtained by fitting the Vogel–Fulcher law. The distinct ferroelectric domain switching current peaks in current – electric field (I-E) loop and piezoelectric coefficient d₃₃ value of 7.0?±?0.1 pC/N at room temperature further demonstrated relaxor ferroelectric behaviour of BaBi₄Ti₄O₁₅. UV-vis absorption spectra indicated that BaBi₄Ti₄O₁₅ had a direct band gap of 3.2?eV. The photocatalytic study showed 15% degradation of Rhodamine B (RhB) solution by BaBi₄Ti₄O₁₅ powders after 3.5?h UV-vis irradiation. The RhB degradation rate was further enhanced by depositing Ag nanoparticles on the BaBi₄Ti₄O₁₅ powders surface. This work suggested that the relaxor ferroelectric BaBi₄Ti₄O₁₅ is promising for photocatalytic applications.     

Exceptionally High Dielectric Constant in Ceramic Pr2CuO4

The pristine layered cuprate Pr₂CuO₄ samples of >95% density were fabricated as thin disks. The samples, analyzed by X‐ray diffraction and Scanning electron microscopy, showed clean T′‐type phase with Rietveld refined lattice parameters a = b = 3.95805(±5) Å and c = 12.2262(±5) Å. The measured dielectric properties of the Pr₂CuO₄ ceramics, in the temperature range ?100°C–150°C and frequencies (ν) 0.1 Hz–1 MHz, showed extremely high ε_r′ > 10⁴ (above ?30°C), and dissipation (tan δ = ε_r′′/ε_r′) between 0.1 and 5 (for 500 Hz ≤ ν ≤ 1 MHz, and ?100 ≤ T ≤ 150°C). The ac conductivity of Pr₂CuO₄ ceramics ranged between 10^?6 and 10^?3 Scm^?1 for the measured frequencies and temperatures, and showed frequency‐dependent double power law behavior akin to a modified Jonscher's power law.     

Good thermal stability,giant permittivity,and low dielectric loss for X9R-type (Ag1/4Nb3/4)0.005Ti0.995O2 ceramics

With the intense demand of the developing microelectronics market, the study of giant permittivity dielectric materials is being promoted. However, it is difficult to obtain suitable dielectric materials for such applications, especially due to high dielectric loss at low frequencies. In this work, Ag+Nb codoped TiO₂ ceramics were designed and fabricated in a conventional solid reaction by sintering at 1290-1340°C for 5-10 hours. The issue of how the microstructure and dielectric properties of (Ag_1/4Nb_3/4)_0.005Ti_0.995O₂ ceramics are affected by the sintering conditions was discussed. By optimizing sintering conditions, a dense microstructure, a high dielectric constant (ε_r ≈ 9410), and a low dielectric loss (tanδ ≈ 0.037) at 1 kHz were achieved. Most importantly, the temperature coefficient value of ε_r at different frequencies remained stable between −14.3% and 13.7% within the temperature range from −190 to 200°C, which has potential applications in X9R capacitor.     

Effective dielectric loss (tan δ) reduction of CaCu3Ti4O12 (CCTO) via various addition of glasses

Addition of various glasses has successfully reduced tan δ of CCTO-based ceramics. Less amounts (0-1.0 wt%) of glasses (BaO-SrO-Nb₂O₅-B₂O₃-SiO₂ [BSNBS] and SrO-B₂O₃-SiO₂ [SBS], respectively) were added into pure CCTO and sintered at 1040°C for 10 hours. The phase formation and microstructure of each sample were characterized using X-ray diffraction and scanning electron microscopy, respectively. The dielectric behavior of the samples was measured at 1 MHz. The addition of BSNBS and SBS glasses (≤0.5 wt%) successfully reduced tan δ of CCTO from 0.5 until 0.4 and 0.39, respectively, and simultaneously increased ε_r. Smaller tan δ and ɛ_r were further obtained when the added glasses were more than 0.5 wt%. These behaviors were due to segregation of glasses together with precipitation of CuO which decreased the grain size, and caused the presence of pores at grain boundaries. Therefore, small amount of various additions of glasses could modify the CCTO dielectric properties.     

Effects of four isocyanates and four plasticizers on the thermomechanical and tensile properties of hydroxyl-terminated polybutadiene elastomers and the effect of solid particle filling

The aim of this study is to analyze effect of four different isocyanates and four different plasticizers in hydroxyl-terminated polybutadiene (HTPB) based elastomers by quantitative analysis of the shape of the loss factor (tan δ), tensile strength, deformation frequency shift of the maximum temperatures of loss moduli G" and of tan δ. The first part of the study shows intensities of the tan δ curves with the four isocyanates follow the order HDI > IPDI > H12MDI > Desmodur™ E305. By molecular modeling of the isocyanates and the corresponding polyurethane parts the influence of molecular geometry on tan δ are discussed. The second part of the study analyzes HTPB-IPDI elastomers with the four different plasticizers DOA, DOS, DOZ, and IDP. The IDP provides lowest T_g at about −83°C, while the others provide at about −78°C. In the third part, aluminum (Al-18 μm) and ammonium perchlorate (AP-200 μm) are added to HTPB-IPDI+DOA to analyze the effect of particle size, wt% content and particle type on the shape and intensity of the tan δ curves. From the frequency shift of the two maximum temperatures one receives activation energies Eaf. Their average values without and with plasticizer are with tan δ 178 and 165 kJ/mol and with G" 274 and 248 kJ/mol, respectively.     

Surface characterization of sulfated zirconia and its catalytic activity for epoxidation reaction of castor oil

The solid acid catalysts SO₄^2?/ZrO₂ were prepared by impregnation technique at different calcination temperatures. The surface characterizations were carried out by using scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), temperature programed desorption of NH₃ (NH₃-TPD), and N₂-BET. The SEM results showed that the size of the SO₄^2?/ZrO₂ was not uniform and varied from about 1 to 20?µm. The characteristic peaks in FTIR spectra were essentially the same within the calcination temperature range of 400–700?°C. The XRD results indicated that the transition temperature from amorphous to tetragonal phase was up to 500?°C. The strong acid and superacid sites of the samples could be observed by the NH₃-TPD results. The largest BET surface area was 140 m²/g, when the calcination temperature was at 500?°C, and all the pore size distributions belong to mesoporous range. The solid acid SO₄^2?/ZrO₂ was used for the epoxidation of castor oil. When the calcination temperature of SO₄^2?/ZrO₂ was 600?°C, reaction temperature 45?°C, and reaction time 8?h, the reaction effect was better with an iodine value of 33.0?±?1.6?g/100?g and an epoxy value of 2.45?±?0.11?mol/100?g.     

A green method for solvent-free conversion of epoxides to thiiranes using NH4SCN in the presence of NiFe2O4 and MgFe2O4 magnetic nanocatalysts

Nickel and magnesium ferrite magnetic nanoparticles were fabricated and applied as efficient and reusable catalysts in the solvent-free conversion of various epoxides to the corresponding thiiranes with ammonium thiocyanate under oil bath (60°C) conditions. NiFe₂O₄ and MgFe₂O₄ nanoparticles can catalyze the reactions at short times in high to excellent yields. The catalysts can also be recovered easily using an external magnetic field and be reused four times without any significant loss of activity.     

Thickness dependent dielectric properties of calcium copper titanate ceramics measured in a controlled atmosphere

Phase pure CaCu₃Ti₄O₁₂ CCTO ceramics are prepared by solid-state synthesis route. The effect of measuring atmospheres (air and dry N₂) on the stability and reproducibility of electrical properties of CCTO as a function of sample thickness (as-prepared to thinned down) is investigated. As-sintered CCTO prepared at 1080 °C for 5 h with an initial thickness of 2.31–2.32 mm is reduced subsequently by fine grinding to 1.835 mm and then to 1.65–1.5 mm. Large inconsistency in the impedance spectra is observed when the samples are measured in air despite the thickness variations. Stable and reproducible dielectric properties are obtained in dry N₂. A relatively closer resistivity (∼2 × 10⁸ Ωcm at 23 °C in N₂) regardless of the sample thickness suggests the absence of any barrier layer at the sample surface. Increased space charge accumulation at grain boundaries (GBs) leading to much larger dielectric constant (ε′) was observed in air at 23 °C. A temperature (from 23 °C to 225 °C) and frequency (from 1 Hz to 1 MHz) independent and stable ε′ is observed when samples are tested in N₂. Much lower tan δ values with large ε′ are observed for both as prepared (0.010 ± 0.001 with ε′ of 9,663 ± 4 at 1.4 kHz) and the thinned down (0.015 ± 0.000 with ε′ of 9,352 ± 5 and 4.5 kHz) samples at 23 °C in N₂.     

Preparation and catalytic performance of Co3O4 catalysts for low-temperature CO oxidation

Without use of any surfactant or oxidant, a series of Co₃O₄ catalysts have been prepared from cobalt nitrate aqueous solution via a very simple liquid-precipitation method with ammonium acid carbonate followed by calcination at various temperatures. The catalytic performance of the Co₃O₄ for CO oxidation has been studied with a continuous flowing laboratory microreactor system. The results show that the CO conversion of all the samples can reach 100% at ambient temperature. The catalyst calcined at 300 °C is able to maintain its activity for CO complete oxidation more than 500 min at 25 °C and about 240 min even at −78 °C. High reaction temperature results in a high catalytic stability, while the catalytic stability decreases with further increasing the reaction temperature. Characterizations with X-ray powder diffraction and transmission electron microscopy suggest that all the samples exist as a pure Co₃O₄ phase with the spinel structure, the samples are apt to aggregate and the specific surface area gradually decreases with increasing the calcination temperature, which directly leads to the decrease of catalytic stability. Furthermore, the amount of active oxygen species measured by CO titration experiments appears to be critical for catalytic performance.     

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.     

Electrical properties of Co-doped CaCu3Ti4O12

Co-doped CaCu₃Ti₄O₁₂ samples were synthesized by solid-phase reaction. Electrical properties were studied by impedance spectroscopy in wide temperature (25–450 °C) and frequency (10 Hz–10 MHz) intervals. It was shown that the presence of the copper oxide interlayer significantly reduces the value of the dielectric constant. The amount of impurity copper in the CaCu₃Ti_4-хCo_хO_12-δ samples (x = 0.06; 0.12; 0.24) rise with an increase in the cobalt content. The samples are characterized by a granular microstructure, with an average grain size ranging from 2 to 10 μm. The impedance of the samples was simulated at a temperature of 25 °C and in the range of 100–450 °C. It was found that the samples are characterized by low- and high-frequency polarization. The conductivity activation energy varied from 0.94 to 0.87 eV depending on the cobalt content. The CaCu₃Ti_3.94Co_0.06O_12-δ sample are characterized by the best values of the dielectric permittivity and the dielectric loss tangent, ε = 400 and tanδ = 0.2 (at 1 MHz and room temperature), respectively.     

Structure and properties of ethylene—methacrylic acid copolymers and their sodium salts: Dielectric and proton magnetic relaxation studies

Dielectric and proton magnetic relaxation data have been obtained for an ethylene-methacrylic acid copolymer (containing about 4 mole% methacrylic acid units) and its 53% ionised sodium salt. The degrees of crystallinity and percentage ionisation of the samples investigated were estimated by infra-red methods. The dielectric results were obtained principally in the frequency range 100 Hz to 10 kHz and at temperatures ranging from 80° to – 120°. A few results are also reported at frequencies down to 0·1 Hz and up to 100 MHz. For the acid copolymer, two dielectric loss regions are observed and these are correlated with the reported mechanical β′- and γa-processes respectively. The partly ionised copolymer exhibits three dielectric relaxation regions which correlate with the mechanical α-, β- and γa-relaxations respectively. In addition, a dielectric peak appears at about —40° in the presence of absorbed water, a result similar to that found in the polyamides. The proton magnetic relaxation results were obtained by pulse methods which yielded the spin-lattice relaxation times T₁ (at 30 MHz) and T_1p (at kilohertz frequencies) as a function of temperature from ?180° to 100°. Two components were generally observed for T_1p. For the acid copolymer the β′- and γ-processes have been observed from these results, as well as a lower-temperature (δ) process which has not been detected by the mechanical or dielectric methods. For the sodium salt the γ- and δ-processes are also found, in addition to a high-temperature process in the region of the merged α- and β-processes. The present data are consistent with previous assignments for the β′-, α-, β- and γ-processes. The ‘water’ relaxation appears to involve some rotation of water molecules, or of ionic segments to which water molecules are attached, in the proposed ionic domains. The δ-process is ascribed to the rotations of methyl groups present in the methacrylic acid units.     

Effect of Heat Treatment on Micromechanical Properties of Sprayed TiO2 Film on Ce-Doped Glass

Abstract

Anatase TiO₂ films were deposited on unheated Ce-doped soda–lime–silicate glass substrates by a spray technique from an anatase sol made in the laboratory. In order to investigate the micromechanical properties of TiO₂ films, the deposited films were heated treated at: 350, 500 and 550°C, each for 1 h. X-ray diffraction spectra revealed a crystalline structure with an anatase phase and the average diameter of the grains increased from 21.4 to 31.2 nm as the temperature in the heat treatment rose from 20 to 550°C. The films as-deposited and heat-treated at 350°C were found to be smooth and relatively dense. Cracks appeared in TiO₂ films when the heating temperature increased to 550°C. The results of nano-indentation test showed that when the heating temperature rose to 500°C, the TiO₂ films were found to have nano-hardness and elastic modulus values of 1.1 and 30.8 GPa, respectively. These were the highest values recorded in this work. When the temperature reached 550°C, the nano-hardness and elastic modulus decreased due to the presence of cracks in the films.     

Structural and electrical properties of magnetic ceramics of Co1−xCuxFe2O4 spinel nanoferrites

Abstract

Magnetic ceramics of the type of spinel nanoferrites of Co_1?xCu_xFe₂O₄ with Cu concentrations of x?=?0·00, 0·25, 0·50, 0·75 and 1·00 were prepared by chemical co-precipitation method. X-ray diffraction results confirmed the formation of a single spinel ferrite structure with crystallite size in the range of 20–63 nm. Scanning electron microscopy and EDS were used to study the morphological and compositional changes taking place with varying Cu concentration. DC electrical resistivity, activation energy and drift mobility are strongly influenced by both Cu concentration and temperature. Resistivities of the prepared magnetic ceramics were found to decrease with the increase in Cu concentration to follow Verwey mechanism. The semiconductor behaviour of the prepared nanoparticles was confirmed from the standard Arrhenius relation of resistivity versus temperature. The dielectric constants were measured in the frequency range of 100 Hz–3·0 MHz and are explained by the Maxwell–Wagner interfacial type of polarisation.     

Relaxation behavior and activation energy of relaxation for polyimide and polyimide–graphene nanocomposite

The relaxation behavior of polyimide and its nanocomposite containing 10 wt % of graphene was studied by using the dynamic mechanical spectrometer. Dynamic mechanical analysis of polyimide and its composite was performed as a function of temperature and frequency in the temperature range of 25–480 °C and frequency range between 0.05 and 100 Hz. The effect of increasing frequency of testing from 0.05 to 100 Hz is a significant shift from the glass transition temperature, T_g, to higher temperature from 360 °C at 0.05 Hz to 420 °C at 100 Hz. The tan δ peak height for both α and β transitions decreased with increasing test frequency from 0.24 at 0.05 Hz to 0.08 at 100 Hz, due to increasing restriction to chain motion. At any given testing frequency, the T_g for the composite was shown to be higher than that for the matrix by about 5–10 °C. The Arrhenius equation was used to calculate the activation energy for both α and β transitions. The activation for α and β transitions for the composite and polyimide matrix were determined to be 688 and 537 kJ/mol and 313 and 309 kJ/mol, respectively, indicating that a significant increase in the energy barrier to chain relaxation occurred as a result of reinforcement of polyimide with low weight fraction of graphene. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43684.     

Solid–Liquid Equilibrium (SLE) for Polyhydric Alcohol + Cs2SO4 + H2O Ternary Systems at Different Temperatures

The solubility, density, and refractive index data for 1,2-propanediol + Cs₂SO₄ + H₂O, ethylene glycol + Cs₂SO₄ + H₂O, and glycerin + Cs₂SO₄ + H₂O ternary systems were determined at 15°C, 25°C, and 35°C. In all cases, the solubility of Cs₂SO₄ in aqueous solutions decreased significantly due to the presence of the polyhydric alcohol. The experimental density, refractive index, and solubility data of saturated solutions for these systems were correlated using polynomial equations. Furthermore, the refractive index and density of unsaturated ternary solutions were also determined and correlated with salt concentrations and proportions of polyhydric alcohol in these systems.     

Monomer concentration effect on the phase behavior of poly(propyl acrylate) and poly(propyl methacrylate) with supercritical CO2 and C2H4

Experimental cloud-point data to temperature of 186 °C and pressure of ~2,500 bar are presented for ternary mixtures of poly(propyl acrylate)(PPA)-CO₂-propyl acrylate (PA) PPA-C₂H₄-PA and poly(propyl methacrylate) (PPMA)-CO₂-propyl methacrylate (PMA) systems. Cloud-point pressures of PPA-CO₂-PA system were measured in the temperature range of 32 °C to 175 dgC and to pressures as high as 2,070 bar with PA concentrations of 0.0, 5.0, 11.7 and 30.4 wt%. Adding 34.1 wt% PA to the PPA-CO₂ mixture significantly changes the phase behavior. This system changes the pressure-temperature slope of the phase behavior curves from U-LCST region to LCST region as the PA concentration increases. Cloud-point data to 170 °C and 1,400 bar are presented for PPA-C₂H₄-PA mixtures and with PA concentration of 0.0, 5.7, 15.5 and 22.2 wt%. The cloud-point curve of PPA-C₂H₄ system shows relatively flat at 730 bar for temperatures between 41 and 150 °C. With 15.5 and 22.2 wt% PA the cloud-point curve exhibits a positive slope that extends to 35 °C and ~180 bar. Also, the ternary PPMA-CO2-PMA system was measured below 186 °C and 2,484 bar, and with cosolvent of 5.2-20.1 wt%. PPMA does not dissolve in pure CO₂ to 233 °C and 2,500 bar. Also, when 41.5 wt% PMA is added to the PPMA-CO₂ solution, the cloud-point curve shows the typical appearance of a lower critical solution temperature (LCST) boundary.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

Characterization of Supported NiSO4 and Effect of TiO2–ZrO2 Composition on Catalytic Activity for Acid Catalysis

A series of catalysts, NiSO₄/TiO₂–ZrO₂ having different TiO₂–ZrO₂ composition, for acid catalysis was prepared by the impregnation method using an aqueous solution of nickel sulfate. The addition of TiO₂ to ZrO₂ improved the surface area of the catalyst and enhanced its acidity remarkably because of the formation of new acid sites through the charge imbalance of Ti–O–Zr bonding. The binary oxide, TiO₂–ZrO₂ calcined above 600 °C resulted in the formation of crystalline orthorhombic phase of ZrTiO₄. Therefore, NiSO₄/TiO₂–ZrO₂ calcined at 500 °C exhibited a maximum catalytic activity for acid catalysis, and then the catalytic activity decreased with the calcination temperature. The correlation between catalytic activity and acidity held for both reaction, 2-propanol dehydration and cumene dealkylation. NiSO₄ supported on 50TiO₂–50ZrO₂ (TiO₂/ZrO₂ ratio = 1) among TiO₂–ZrO₂ binary oxides exhibited the highest catalytic activity for acid catalysis.     

Enhanced dielectric performance of (Ag1/4Nb3/4)0.01Ti0.99O2 ceramic prepared by a wet-chemistry method

The (Ag_1/4Nb_3/4)_0.01Ti_0.99O₂ ceramic, with ultra-high permittivity and relatively low dielectric loss, was synthetized by a sol-seed method. Significant influences of Ti concentration on the phase structure, microstructure, and dielectric performances were observed. By optimizing the Ti concentration, high active powders and effectively controlled grain uniformity were obtained, which is great benefit for depressing the calcining temperature. The sample shows an ultra-high permittivity (ε_r ~43271 at 1 kHz) and relatively low dielectric loss (tanδ ~ 0.048 at 1 kHz) at room temperature, meanwhile, the temperature coefficient of ε_r keeps in ±15% within the temperature range from −78 to 130°C, fortunately for X7R capacitor. Based on the XPS results, the defect cluster structure was revealed and giant permittivity is closely associated with the electron pinned defect dipole.