Synthesis, characterization and optical properties of sheet-like ZnO

Sheet-like ZnO with regular hexagon shape and uniform diameter has been successfully synthesized through a two-step method without any metal catalyst. First, the sheet-like ZnO precursor was synthesized in a weak alkaline carbamide environment with stirring in a constant temperature water-bath by the homogeneous precipitation method, then sheet-like ZnO was obtained by calcining at 600 °C for 2 h. The structures and optical properties of sheet-like ZnO have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and UV-vis-NIR spectrophotometer. The results reveal that the product is highly crystalline with hexagonal wurtzite phase and has appearance of hexagon at (0 0 0 1) plane. The HRTEM images confirm that the individual sheet-like ZnO is single crystal. The PL spectrum exhibits a narrow ultraviolet emission at 397 nm and a broad visible emission centering at 502 nm. The band gap of sheet-like ZnO is about 3.15 eV.     

Microstructural evolution and dielectric properties of Cu-deficient and Cu-excess CaCu3Ti4O12 ceramics

The microstructural evolution and dielectric properties of CaCu_3−xTi₄O_12−x (3 − x = 2.8-3.05) ceramics were investigated. Normal grain growth behavior was observed at Cu/Ca ≤ 2.9, while abnormal grain growth was observed at Cu/Ca ≥ 2.95. A CuO-rich intergranular liquid phase at Cu/Ca ≥ 2.95 and angular grain morphology were the main reasons for abnormal grain growth. However, the abundant intergranular liquid at Cu/Ca = 3.05 significantly affected the relative dielectric permittivity and dielectric loss. The CuO composition is the key parameter that determines the microstructure and dielectric properties of CCTO ceramics.     

Synthesis and anomalous magnetic properties of hexagonal CoO nanoparticles

CoO nanoparticles in the 38-93 nm range have been prepared by thermal decomposition. The particles were characterized to be pyramid shape with a hexagonal close-packed structure. Their anomalous magnetic behavior includes: (i) vanishing of antiferromagnetic transition around 300 K; (ii) creation of hysteresis below a blocking temperature of 6-11 K; (iii) presence of relatively large moments and coercivities accompany with specific loop shifts at 5 K; and (iv) appearance of an additional small peak located in low field in the electron spin resonance spectrum. Further, the present results provide evidence for the existence of uncompensated surface spins. The coercivity and exchange bias decrease with increasing particle size, indicating a distinct size effect. These observations can be explained by the multisublattice model, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the total CoO particle.     

Synthesis, characterization and humidity sensing properties of Cu-Sr-Al mixed metal oxide composites

Copper-strontium-aluminum mixed metal oxide composites prepared by sol-gel technique was investigated for humidity sensing properties. X-ray diffraction, scanning electron microscopy (SEM), FT-IR spectroscopy and nitrogen adsorption/desorption isotherm at 77 K was employed, respectively, to identify the structural phases, surface morphology, vibrational stretching frequencies and BET surface area of the composites. The composites were prepared with the mole ratios of Cu:Sr as (0.0:1.0, 0.2:0.8, 0.4:06, 0.6:0.4, 0.8:0.2 and 1.0:0.0) keeping the aluminium mole ratio as constant and were labeled as CuSA1, CuSA2, CuSA3, CuSA4, CuSA5 and CuSA6, respectively. The samples sintered at 900 °C for 5 h, were subjected to dc resistance measurements as a function of relative humidity (RH) in the range 5-98% and the results revealed that the sensitivity factor of CuSA5 possessed the highest humidity sensitivity of 42,000 while the pure composite copper aluminate (CuSA1) and strontium aluminate (CuSA6) possessed comparatively low sensitivities 10,000 and 20,000, respectively. The response and recovery characteristics of the composites CuSA1, CuSA5 and CuSA6 were studied.     

Investigations on structural evolution and dielectric characteristics of high performance Bi-based dielectrics

A new system of (Bi_1.5Zn_1−x/3Ti_xNb_1.5−2x/3)O₇ (0 ≤x ≤ 1.5) ceramics have been successfully developed and the dielectric properties have been systematically studied. The results showed the formation of temperature compensation dielectrics over the wide range of Ti concentrations. The incorporation of Ti⁴⁺ into the bismuth zinc niobate ceramics induce the decrease of lattice constant linear while remaining cubic pyrochlore phase. The IR spectra confirm the formation of pyrochlore structure and give information about the distribution of ions between the A and B sites. It has been found that the effect of Ti substitution on dielectric properties of sintered ceramics intensifies with a higher x. The system exhibits the novel high permittivity and low dielectric loss: the permittivity values (?_r) saturate at 160-210, and loss values remain at low values (∼1 × 10⁻⁴). The temperature dependence of permittivity is strongly dependent with the compositions. Dielectric relaxation phenomena have been observed during low temperature. New high frequency and MW materials, differed by Ti content, with temperature compensation and controllable dielectric constant of 150-210 have been developed.     

Electrodeposited ZnO films with high UV emission properties

We report here our results in the preparation of ZnO films with high UV band to band characteristic luminescence emission by potentiostatic electrodeposition. Zinc nitrate aqueous baths with different concentration and additives were employed for the preparation of the films on platinum substrates. We focused our research in determining how the electrodeposition bath composition, i.e. zinc nitrate concentration and addition of KCl or polyvinyl pyrolidone and applied overpotential influence the morphological and optical properties of the oxide films. Scanning electron microscopy was employed for characterizing the films in terms of morphology. Optical reflection, photoluminescence spectroscopy and cathodoluminescence were used for determining the optical characteristics of the samples. The morphology of the deposit varies from hexagonal prisms to platelets as a function of the deposition rate. This experimental parameter also influences the luminescence properties. We found that at low deposition rates high UV luminescent material is obtained.     

Study of the dielectric properties in the NaNbO3 -KNbO3-In2O3 system using AC impedance spectroscopy

We report a comparative study of the dielectric properties of solid-state (ceramic method) synthesized NaNbO₃ (NN), Na_0.75K_0.25NbO₃ (K25NN), K_0.5Na_0.5NbO₃ (KNN) and some composite materials containing In₂O₃ and NN or KNN using an AC impedance method. Powder X-ray diffraction (PXRD) was employed to investigate the phase purity. No significant amount of impurity phase was observed for NN, K25NN, and KNN. Substitutions of 10, 15 and 25 mol% In³⁺ for Nb⁵⁺ in KNN and NN using solid-state reactions at 1150 °C resulted in composite materials. AC impedance studies of NN, KNN and K25NN in the temperature range of 500-800 °C showed a single semicircle (attributed to the bulk property) in the high-frequency range of 10³ to 10⁶ Hz. The individual contributions from the bulk and grain boundary on the dielectric properties were resolved and quantified from the impedance data. The calculated dielectric values for NN were consistent with previously reported in the literature. 10% Indium based KNN composite materials had the lowest dielectric loss 0.585 and the dielectric constant of 233 at 100 kHz at the temperature of 650 °C.     

Synthesis, structure and dielectric characterization of Ln2Ti2−2xM2xO7(Ln=Gd, Er; M=Zr, Sn, Si)

The progress in wireless communications and information access has demanded the use of electronic ceramics exhibiting desired properties. To further our understanding of these properties, compounds in the Ln₂Ti_2-2xM_2xO₇ (Ln=Gd, Er; M=Zr, Sn, Si) systems were synthesized by ceramic methods and characterized by powder X-ray diffraction. The ZrO₂-doped Gd₂Ti_2−2xZr_2xO₇ compounds adopt the pyrochlore structure type and form a complete solid solution. Er₂Ti_2−2xZr_2xO₇ forms a pyrochlore solid solution for x<0.1. However, stoichiometric Er₂Zr₂O₇ does not form; instead Er₄Zr₃O₁₂ forms a with defect fluorite structure. The Sn-doped Ln₂Ti_2−2xSn_2xO₇ (Ln=Gd, Er) compounds form complete solid solutions, and the Si compounds adopt the pyrochlore structure up to x=0.05. At ambient temperature, dielectric constants range from 10 to 61 for Er₂Ti_2−2xZr_2xO₇ and 16-31 for Gd₂Ti_2−2xZr_2xO₇ with low dielectric loss (1×10⁻³) at 1 GHz.     

Low temperature sintering and microwave dielectric properties of SmAlO3 ceramics

The effects of sintering aids on the microstructures and microwave dielectric properties of SmAlO₃ ceramics were investigated. CuO and ZnO were selected as sintering aids to lower the sintering temperature of SmAlO₃ ceramics. With the additions, the sintering temperature of SmAlO₃ can be effectively reduced from 1650 to 1430°C. The crystalline phase exhibited no phase differences at low addition level while Sm₄Al₂O₉ appeared as a second phase as the doping level was over 0.5 wt.%. In spite of the additions, the dielectric constants showed no significant change and ranged 19-21. However, the quality factor Q×f was strongly dependent upon the type and amount of additions. The Q×f values of 51,000 and 41,000 GHz could be obtained at 1430°C with 0.25 wt.% CuO and ZnO additions, respectively. The temperature coefficients depended on the additions and varied from −40 to −65 ppm/°C. Results of X-ray diffractions, EDS analysis and scanning electron microscopy were also presented.     

The preparation of zinc silicate/ZnO particles and their use as an efficient UV absorber

The formation of zinc silicate/ZnO particles synthesized by a two-step method and their incorporation into PMMA is presented. In the first step a segmented-flow tubular reactor was used for the continuous room-temperature preparation of a zinc silicate/Zn(OH)₂ gel that was thermally treated after rinsing and drying in the second step. The same preparation procedure was also employed for the synthesis of pure ZnO and pure zinc silicate particles. It was found that the presence of the zinc silicate phase significantly influenced the final particle size, decreased the degree of crystallization and reduced the particles’ UV absorption capabilities. The reduced photocatalytic activity of the zinc silicate/ZnO particles indicated that the majority of ZnO crystallites were formed inside the zinc silicate matrix. The nanocomposite prepared from zinc silicate/ZnO particles (0.04 wt.%) and PMMA showed high UV shielding and at the same time sufficient transmittance in the visible-light region.     

Rapid synthesis, characterization and optical properties of TiO2 coated ZnO nanocomposite particles by a novel microwave irradiation method

A novel and rapid microwave method was used to prepare TiO₂ coated ZnO nanocomposite particles. The resulted particles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). Results show that ZnO nanoparticles were coated with 6-10 nm amorphous TiO₂ layers. In addition, zeta potential analysis demonstrated the presence of TiO₂ layer on the surface of ZnO nanoparticles. Photoluminescence (PL) spectroscopy and UV-visible spectroscopy were used to investigate the optical properties of the nanoparticles. Compared to uncoated ZnO nanoparticles, the TiO₂ coated ZnO nanoparticles showed enhanced UV emission. The UV-visible diffuse reflectance study revealed the significant UV shielding characteristics of the nanocomposite particles. Moreover, amorphous TiO₂ coating effectively reduced the photocatalytic activity of ZnO nanoparticles as evidenced by the photodegradation of Orange G with uncoated and TiO₂ coated ZnO nanoparticles under UV radiation.     

Synthesis and characterization of novel hyperbranched polyimides/attapulgite nanocomposites

Novel hyperbranched polyimides/attapulgite (HBPI/AT) nanocomposites were successfully synthesized by in situ polymerization. HBPI derived from novel 2,4,6-tri[3-(4-aminophenoxy)phenyl]pyridine (TAPP) and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA). 4,4′-diphenylmethane diisocyanate (MDI) modified AT copolymerized with HBPI and the nanocomposites formed multilinked network. Chemical structure, morphology, thermal behavior, and mechanical properties of nanocomposites were investigated by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), and tensile testing et.al. Results indicated that modified AT was homogeneously dispersed in matrix and resulted in an improvement of thermal stability, mechanical properties and water resistance of HBPI/AT nanocomposites.     

Optimized microwave dielectric properties of Co- and Ca-substituted Mg0.6Zn0.4TiO3

The system of (1 − y)(Mg_0.6Zn_0.4)_1−xCo_xTiO₃-yCaTiO₃ was investigated to optimize its microwave dielectric properties by adopting appropriate contents of Co and Ca and by controlling sintering conditions. The effect of Co substitution was to enhance densification and Qf value, while the addition of CaTiO₃ resulted in increases of dielectric constant and TCF. As an optimal compositions, 0.93(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃-0.07CaTiO₃ successfully demonstrated a dielectric constant of 23.04, a Qf of 79,460 GHz and a TCF value of +1.4 ppm/°C after firing at a relatively lower sintering temperature of 1200 °C. The increase of sintering temperature beyond 1200 °C tended to degrade overall microwave dielectric properties presumably due to Zn volatilization as evidenced by the presence of a Zn-deficient phase (MgTi₂O₅) at 1400 °C. An attempt to establish the correlation between microstructure characteristics and dielectric properties was made in this dielectric system where the extensive range of firing temperature up to 1400 °C was evaluated.     

Microwave dielectric properties and sintering behavior of nano-scaled (α + θ)-Al2O3 ceramics

The dielectric properties at microwave frequencies and the microstructures of nano (α + θ)-Al₂O₃ ceramics were investigated. Using the high-purity nano (α + θ)-Al₂O₃ powders can effectively increase the value of the quality factor and lower the sintering temperature of the ceramic samples. Grain growth can be limited with θ-phase Al₂O₃ addition and high-density alumina ceramics can be obtained with smaller grain size comparing to pure α-Al₂O₃. Relative density of sintered samples can be as high as 99.49% at 1400 °C for 8 h. The unloaded quality factors Q × f are strongly dependent on the sintering time. Further improvement of the Q × f value can be achieved by extending the sintering time to 8 h. A dielectric constant (_r) of 10, a high Q × f value of 634,000 GHz (measured at 14 GHz) and a temperature coefficient of resonant frequency (τ_f) of −39.88 ppm/°C were obtained for specimen sintered at 1400 °C for 8 h. Sintered ceramic samples were also characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

Effect of temperature on nonlinear electrical behavior and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO₂ ceramics doped with 0.75 mol% Ca and 2.5 mol% Ta were sintered at different temperatures ranging from 1300 to 1450°C. The effects of sintering temperature on the microstructure, nonlinear electrical behavior, and dielectric properties of the ceramics were studied. The sample sintered at 1300°C exhibits the highest nonlinear coefficient (5.5) and a comparatively lower relative dielectric constant.     

Synthesis of ZnO flowers and their photoluminescence properties

Flower-like ZnO nano/microstructures have been synthesized by thermal treatment of Zn(NH₃)₄²⁺ precursor in aqueous solvent, using ammonia as the structure directing agent. A number of techniques, including X-ray diffraction (XRD), field emission scan electron microscopy (FESEM), transmission electron microscopy (TEM), thermal analysis, and photoluminescence (PL) were used to characterize the obtained ZnO structures. The photoluminescence (PL) measurements indicated that the as-synthesized ZnO structures showed UV (∼375 nm), blue (∼465 nm), and yellow (∼585 nm) emission bands when they were excited by a He-Gd laser using 320 nm as the excitation source. Furthermore, it has been interestingly found that the intensity of light emission at ∼585 nm remarkably decreased when the obtained ZnO nanocrystals were annealed at 600 °C for 3 h in air. The reason might be the possible oxygen vacancies and interstitials in the sample decreased at high temperature.     

Synthesis,electrical and dielectric characterization of cerium doped nano copper ferrites

The nanosized CuFe_2−xCe_xO₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8) ferrites doped with cerium are synthesized by chemical co-precipitation method. The synthesized materials are characterized by XRD, FTIR, TGA and SEM. XRD analysis of cerium substituted copper ferrites confirms the cubic spinel structure. The average crystallite size calculated by using Scherrer's formula ranges from 37 to 53 nm. The values of cell constant and cell volume vary with the dopant concentration. These variations can be explained in terms of their ionic radii. The DC electrical resistivity, measured by two point probe method, increases with increase in dopant concentration while it decreases with rise in temperature exhibiting semiconductor behaviour. Energy of activation of these ferrites is calculated by using Arrhenius type resistivity plots. Dielectric measurements of the synthesized compounds show exponential decrease in dielectric constant and dielectric loss factor with increase in frequency. This indicates the normal dielectric behaviour of ferrites.     

Synthesis of nanocrystalline magnesium titanate by an auto-igniting combustion technique and its structural, spectroscopic and dielectric properties

Nanocrystalline magnesium titanate was synthesized through an auto-ignited combustion method. The phase purity of the powder was examined using X-ray diffraction, thermo gravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy and Raman spectroscopy. The transmission electron microscopy study showed that the particle size of the as-prepared powder was in between 20 and 40 nm. The nanopowder could be sintered to 98% of the theoretical density at 1200 °C for 3 h. The microstructure of the sintered surface was examined using scanning electron microscopy. The dielectric constant (?_r) of 16.7 and loss factor (tan δ) of the order of 10⁻⁴ were obtained at 5 MHz when measured using LCR meter. The quality factor (Q_u × f) 73,700 and temperature coefficient of resonant frequency (τ_f) −44.3 ppm/°C, at 6.5 GHz are the best reported values for sintered pellets obtained from phase pure nanocrystalline MgTiO₃ powder.     

Crystallographic and dielectric properties of barium-substituted Pb(Mg1/3Ta2/3)O3 ceramics

Ceramic powders of (Ba,Pb)Pb(Mg_1/3Ta_2/3)O₃ were prepared via a B-site precursor route. Crystal symmetries and lattice parameters were determined. Monophasic perovskite was developed after the two-step reaction process, in which the lattice parameters showed linear changes in the entire composition range. Dielectric responses of the ceramics with compositional and frequency changes were investigated. The results were also compared with the (Ba,Pb)(Zn_1/3Ta_2/3)O₃ data.     

Microwave dielectric properties and microstructures of CuO- and ZnO-doped LaAlO3 ceramics

The microwave dielectric properties and the microstructures of 0.25 wt.% CuO-doped LaAlO₃ ceramics with ZnO additions have been investigated. The sintered LaAlO₃ ceramics are characterized by X-ray diffraction spectra and scanning electron microscopy (SEM). Tremendous reduction in sintering temperature can be achieved with the addition of sintering aids CuO and ZnO. The ceramic samples show that dielectric constants (ε_r) of 22−24 and Q×f values of 33,000−57,000 (at 9.7 GHz) can be obtained at low sintering temperatures 1340−1460°C. The temperature coefficient of resonant frequency varies from −24 to −48 ppm/°C. At the level of 0.25 wt.% CuO and 1 wt.% ZnO additions, LaAlO₃ ceramics possesses a dielectric constant (ε_r) of 23.4, a Q×f value of 57,000 (at 9.7 GHz) and a τ_f value of −38 ppm/°C at 1400°C for 2 h.