Structural,spectroscopic and dielectric properties of KVO3

Polycrystalline samples of KVO₃, a member of the pyroxene structural family have been synthesised with high-temperature solid-state reaction technique. Preliminary structural and spectroscopic and detailed dielectric properties have been studied in different conditions. No dielectric anomaly or hysteresis loop has been observed in a wide temperature range (30°C to 450°C). Basic structural and spectroscopic (IR and Raman) studies suggest that at room temperature, KVO₃ is centrosymmetric as reported earlier. This and other observations do not support the recent report of ferroelectric phase transition in the compound.     

Heat treatment effects on dielectric and physico-chemical properties of an epoxy polymer

Infrared (IR) spectroscopy, dielectric spectroscopy (DS), and thermally stimulated depolarization current (TSDC) have been used to study heat treatment effects on an epoxy-based polymer. Variations in physico-chemical and dielectric properties were examined for annealing temperatures between 55 and 170 °C. IR results have shown that heating causes both chain scission and thermal oxidation of the polymer, increasing thus the amount of trapped charges. The complex dielectric permittivity and the dielectric modulus have been analyzed, by means of DS, to highlight and separate charge relaxation phenomena from conduction contributions. Results indicate structural rearrangements, leading to a decrease of dipolar relaxation frequency (from 16 to 13.5 kHz) and an increase of the relaxation strength (around 20%). TSDC measurements have shown a current peak shift towards higher temperatures, and a significant intensity decrease, which is proportional to the quantity of released charges.     

Study of dielectric behaviour of Mn-Zn nano ferrites

Dielectric properties of Mn_0.4Zn_0.6Fe₂O₄ ferrites synthesised by co-precipitation method have been investigated as a function of frequency (up to 30 MHz) at different temperatures. Dispersion in dielectric constant has been observed between temperatures 450-500 K. DC resistivity was found to increase up to 100 times greater than those for the samples prepared by the conventional ceramic methods. Resistivity variation with temperature is also reported in the present work. The particle size is calculated using Scherrer equation for Lorentzian peak, which comes out between 9 and 19 nm. Possible mechanisms contributing to these processes have been discussed.     

Preparation, structural analysis and dielectric properties of Bi x La1−x FeO3 perovskite

Rare earth element substituted bismuth ferrites (BiFeO₃) are of enormous importance as magnetoelectric materials. The polycrystalline samples of Bi _x La_1−x FeO₃ (x=0, 0.2, 0.4, 0.6, 0.8) were prepared by solid-state reaction using standard ceramic method. The single-phase formation of these compounds was confirmed by X-ray diffraction (XRD) studies. The samples with x=0, 0.2, 0.4, 0.6 are found to be orthorhombic while the sample with x=0.8 is triclinic. The dielectric constant (ε′) and dissipation factor (tan δ) were measured in the frequency range 100 Hz to 1 MHz at room temperature and as a function of temperature at certain fixed frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz). All the samples showed dielectric dispersion. The dielectric constant with temperature shows a broad peak; the peak temperature shifts with frequency which reflects the relaxor-type behavior. The peak above 600 K in the measured temperature range corresponds to antiferromagnetic ordering temperature (Néel temperature). The broadness of the peak changes with composition. The ac conductivity as well as ε′ are found to be maximum for the sample x=0.2 at room temperature.     

Influence of Ca additives on the optical and dielectric studies of sol-gel derived PbTiO3 ceramics

Sintered ceramic powders of calcium-doped lead titanate [Pb_1−xCa_xTiO₃] ceramics with different Ca dopant concentration in the range (x=0-0.35) have been prepared using a sol-gel chemical route. The sol-gel technique is known to offer better purity and homogeneity, and can yield stoichiometric powders with improved properties at relatively lower processing temperature in comparison to conventional solid-state reaction. X-ray diffraction (XRD) and Raman spectroscopy studies have been carried out to identify the crystallographic structure and phase formation. The infrared absorption spectra in the mid-IR region (400-4000 cm⁻¹) show the band corresponding to the Ti-O bond at ∼576 cm⁻¹ and is found to shift to a higher wave number 592 cm⁻¹ with increasing Ca content. The dielectric properties as a function of frequency, and phase transition studies on sintered ceramic Pb_0.65Ca_0.35TiO₃ has been investigated in detail over a wide temperature range 30-600 °C and the results are discussed.     

Role of manganese ions on the stability of ZnF2-P2O5-TeO2 glass system by the study of dielectric dispersion and some other physical properties

ZnF₂-P₂O₅-TeO₂ glasses containing different concentrations of MnO (ranging from 0 to 0.6%) were prepared. A number of studies, viz. differential thermal analysis, optical absorption, thermo luminescence, infrared spectra, magnetic susceptibility, elastic properties (Young's modulus Y, shear modulus n and micro hardness H) and dielectric properties (constant ε, loss tan δ, a.c. conductivity σ_ac over a range of frequency and temperature and dielectric breakdown strength), of these glasses were carried out as a function of manganese ion concentration. The analysis of the results indicate manganese ions mostly exist in Mn²⁺ state in these glasses when the concentration of MnO≤0.4% and above this concentration manganese ions predominantly exist in Mn³⁺ state; from this analysis an attempt is made to identify the role of these ions on the stability of glass network.     

An atypical coordination in hexacyanometallates: Structure and properties of hexagonal zinc phases

In hexacyanometallates, the involved transition metals are usually found with octahedral coordination. The exception corresponds to the hexagonal zinc phases where this metal appears tetrahedrally coordinated to N ends from the CN ligands. Those zinc hexacyanometallates where such atypical coordination appears were identified and for four of them the crystal structure was refined from X-ray diffraction powder patterns using the Rietveld method. Zinc hexacyanoferrates (III), hexacyanocobaltate (III), hexacyanoiridate (III) and the mixed zinc-cesium hexacyanoferrate (II) were found to be dimorphic, cubic (Fm-3m) and hexagonal (R-3c), related to the zinc atom in octahedral or tetrahedral coordination, respectively. In the absence of an exchangeable cation, the hexagonal phases result anhydrous. This last feature was attributed to a low polar character for the pores surface. The Mössbauer spectrum of hexagonal zinc hexacyanoferrate (III) is an unresolved quadrupole splitting doublet (Δ=0.18 mm/s). The iron nucleus is sensing a weak electric field gradient related to a relatively high symmetry for its ligands and charge environment. The IR spectrum appears to be an excellent sensor to identify the coordination for the zinc atom in a given sample. For the tetrahedral coordination, the CN stretching absorption was found at least 8 cm⁻¹ above the frequency observed for this vibration in the octahedral one. For hydrated phases, the crystal water evolves on heating preserving the material porous framework. The temperature at which the material becomes anhydrous parallels the polarizing power of the charge balancing cation sited within the channels. Hexagonal Zn-Cs ferrocyanide becomes anhydrous at 100 °C, while for the Zn-Na analogue a heating close to 200 °C is required. The stability temperature range for the anhydrous phases depends on the nature of the engaged hexacyanometallate anion; the higher stability was observed for hexacyanoferrates (II). Zinc ferricyanide shows the weaker magnetic interaction for the hexagonal modification due to an unfavourable geometry for the overlapping path between the unpaired electrons on the iron(III) atoms. The open 3D porous network is formed by relatively large ellipsoidal cavities, three per cell, communicated through elliptical openings (windows), six per cavity. For dimorphic zinc hexacyanometallates (III), the most compact structure (higher density) corresponds to the hexagonal modification, however, it has the largest cavity windows and cavity (pore) size, and also the higher thermal stability.     

Thermal, optical, mechanical and dielectric properties of nonlinear optical crystal 4-methoxy benzaldehyde-N-methyl 4-stilbazolium tosylate

We have grown organic nonlinear optical 4-methoxy benzaldehyde-N-methyl-4-stilbazolium tosylate (MBST) crystals by slow evaporation technique. The grown crystals were analyzed by powder XRD, FTIR, NMR, UV, Thermal, mechanical and dielectric measurements. Lattice parameters and crystallinity of the crystal is confirmed by the XRD studies. The FTIR and NMR studies give the details about the various functional groups present. In order to study the optical quality of the crystal, the UV-vis absorption spectrum was recorded and cut-off wavelength was determined. Melting point of the crystal is found from the differential scanning calorimetry. The powder SHG studies were done with the Kurtz powder technique and the NLO efficiency is 17.2 times greater than that of urea. The surface of the grown crystal was analyzed with etching. Laser damage threshold studies have been carried out for the crystal using a Q-switched Nd:YAG laser of 10 ns pulses.     

Densification, microstructural evolution, and dielectric properties of CaTiO3-LaGaO3 microwave ceramics

The relations among the densification, microstructural evolution, and microwave dielectric properties of the (1−x)CaTiO₃-xLaGaO₃ ceramics with x=0.34 and 0.36 were investigated in this study. The results indicated that (1−x)CaTiO₃−xLaGaO₃ ceramics can be densified at 1300 °C with at least 97% of the theoretical value. The ceramics reported an orthorhombic perovskite structure, and no other detectable phases were found. Both ε_r and Q×f values can be improved by slowing the cooling rate during sintering. The ε_r and Q×f values of the 0.64CaTiO₃-0.36LaGaO₃ ceramics at cooling rates of >10 °C/min and 0.1 °C/min are 48.1 and 27,500 and 48.7 and 38,000, respectively. The higher densification obtained at a slower cooling rate plays an important role in improving the microwave dielectric properties.     

Preparation and characterization of carbamoylphosphonate(CMPO) silane grafted on various mesoporous silicas

The carbamoylphosphosphonate silane (CMPO analogue; 2-(diphenylphosphoryl)-N-(3-(triethoxysilyl)propyl) acetamide) modified mesoporous silica was prepared via a post-synthesis grafting method for the effective purification of rare earth elements. The guest CMPO analogue was synthesized by direct coupling reaction of 2-(diphenylphosphoryl) acetic acid and 3-(triethoxysilyl)propan-1-amine. Various mesoporous silicates such as MCM-41, SBA-15, or amorphous silica nanoparticles were adopted as host materials. The resulting surface-modified mesoporous materials were characterized with respect to their structural integrity, surface area, and pore size and the concentration of the CMPO silane species. These CMPO functionalized periodic mesostructured silicates offer the potential of applications as catalysts, sensors, or environmental sorbents.     

Characterization, sintering and dielectric properties of nanocrystalline zinc oxide prepared by a citric acid-based combustion route

Nanosized zinc oxide has been synthesized through a novel single step solution combustion route using citric acid as fuel. The X-ray diffraction (XRD) analysis revealed that the synthesized ZnO nanopowder has the pure wurtzite structure. The phase purity of the nanopowder has been confirmed using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The morphology and crystalline size of the as-prepared nanopowder characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the powder consisted of a mixture of nanoparticles and nanorods. The nanocrystalline ZnO could be sintered to ∼97% of the theoretical density at 1200 °C in 4 h. The dielectric constant (ε_r) and dielectric loss (ε_i) of sintered ZnO pellets at 5 MHz were 1.38 and 9×10⁻², respectively, at room temperature.     

A systematic study on magnetic, dielectric and magnetocapacitance properties of Ni doped bismuth ferrite

In this paper we report the structural, magnetic, magnetocapacitance and dielectric properties of BiFe_1−xNi_xO₃ nanoceramics (with x=0, 0.1) prepared by the sol-gel method. XRD analysis showed formation of single phase nanoceramics (particle size ∼50 nm by TEM). Samples of BiFe_1−xNi_xO₃ were divided into two parts—one of them quenched in liquid nitrogen and another sintered in the normal way. We observed the enhancement in magnetic and dielectric properties of quenched sample. The splitting of zero field cool (ZFC) and field cool (FC) magnetization curves at low temperature reveals spin- glass behavior. Quenched sample showed the enhancement of blocking temperature.     

Phase evolution and microwave dielectric properties of Bi3SbO7 ceramic

The Bi₃SbO₇ ceramic was prepared by the solid state reaction method and its phase evolution at different temperatures was studied. Low temperature phase α-Bi₃SbO₇ was formed at about 890 °C and it started to transform to high temperature phase β-Bi₃SbO₇ at about 960 °C. Microwave dielectric constants of α-Bi₃SbO₇ ceramic and β-Bi₃SbO₇ ceramic were 43.2 and 37.6, Qf value were 2080 and 5080 GHz, respectively. TCF of α-Bi₃SbO₇ ceramic was near zero and TCF of β-Bi₃SbO₇ ceramic was about −120 ppm/°C. The Bi₃SbO₇ ceramic is a promising candidate for low temperature co-fired ceramic (LTCC) technology due to its large dielectric constant, low dielectric loss at microwave region, low sintering temperature and simple composition.     

Structural characterization of dense reduced BaTiO3 and Ba0.95La0.05TiO3 nanoceramics showing colossal dielectric values

BaTiO_3−x and Ba_0.95La_0.05TiO_3−x nanoceramics showing colossal permittivity values have been characterized. While starting powders are of cubic symmetry, X-ray and Neutron Diffraction techniques and Raman Spectroscopy measurements show that the one-step processed ceramics obtained by Spark Plasma Sintering (SPS) contain cubic and tetragonal phases. Rather large oxygen deficiency determined in such ceramics by Electron Micro Probe analysis and Electron Energy Loss Spectroscopy analyzes is explained by the presence of Ti³⁺, as evidenced by X-ray Photoelectron Spectroscopy measurements. Transmission Electron Microscopy and High Resolution Transmission Electron Microscopy show that these ceramics contain 50-300 nm grains, which have single-domains, while grain boundaries are of nanometer scale. Colossal permittivity values measured in our dense nanoceramics are explained by a charge hopping mechanism and an interfacial polarization of a large number of polarons generated after sample reduction in SPS apparatus.     

Electronic structure,optical dielectric constant and born effective charge of EuAlO3

EuAlO₃ (EAO) is synthesized by the sol–gel process. The Rietveld refinement of the X-ray diffraction data shows that the material has orthorhombic structure with Pbnm space group. The density functional theory calculations are initiated with the experimental lattice parameters. The full potential linearized augmented plane wave method and projector augmented wave method are used to investigate the ground state properties of EAO. An indirect band gap of 1.8 eV is observed with the valence band maximum at the Γ point and the conduction band minimum at the R point. The X-ray photoemission spectroscopy (XPS) spectra of EAO are obtained in the energy window of 0–1000 eV. Using the electronic density of states, the valence band (VB) spectrum of EAO is generated and compared with the observed VB-XPS spectrum. The optical dielectric constant and the refractive index of the material are calculated for the photon energy radiation. The optical properties show a considerable anisotropy in the material. The Born effective charge of various elements and the dielectric tensor of EAO have been calculated.     

Thermoelectric properties and electronic structure of Al-doped ZnO

Impure ZnO materials are of great interest for high temperature thermoelectric application. In this work, we present the effects of Al-doping on the thermoelectric properties and electronic structures of a ZnO system. We find that, with increasing Al concentrations, the electrical conductivity increases and the thermal conductivity decreases significantly, whereas, the Seebeck coefficient decreases slightly. Nevertheless, the figure of merit (ZT) increases owing to high electrical conductivity and low thermal conductivity. On the other hand, the electronic band structures show that the position of the Fermi level is moved upwards and the bands split near the valence-band top and conduction-band bottom. This is due to the interaction between the Al3p and Zn4s orbitals, which drive the system towards semimetal. Besides, the Density Of States (DOS) analysis shows that the introduction of Al atom obviously reduces the slope d(DOS)/dE near the Fermi level. Based on the calculated band structures, we are able to explain qualitatively the measured transport properties of the Al-doped ZnO system.     

Influence of fluorination and chain length on the static dielectric properties of nematogenic bi-cyclohexyl phenyl derivatives

Effects of fluorination and chain length of six bi-cyclohexyl phenyl compounds (3ccp-f, 3ccp-ff, 3ccp-fff, 5ccp-f, 5ccp-ff, 5ccp-fff) on their physical properties have been investigated. All the compounds exhibit nematic phase over a wide range of temperatures. Molecular mechanics calculation reveals that dipole moments of the molecules increases (1.93D to 3.37D) with fluorination in both propyl- and pentyl-based systems; increment in f to ff derivatives is more than in ff to fff derivatives. However, no change in dipole moment is observed with increased chain length. In the nematic phase, average value of dielectric constant (?_avg) is less than the extrapolated values of ?_iso in isotropic phase. Increased fluorination also decreases splay elastic constant, and thus faster response is expected in triply fluorinated compounds.     

Orientation-dependent dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films prepared by sol-gel method

The orientation-dependent dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃, BTS) thin films grown on (1 0 0) LaAlO₃ single-crystal substrates through sol-gel process were investigated. The nonlinear dielectric properties of the BTS films were measured using an inter-digital capacitor (IDC). The results show that the in-plane dielectric properties of BTS films exhibited a strong sensitivity to orientation. The upward shift of Curie temperature (T_c) of the highly (1 0 0)-oriented BTS thin films is believed to be attributing to a tensile stress along the in-plane direction inside the film. A high tunability of 47.03% was obtained for the highly (1 0 0)-oriented BTS films, which is about three times larger than that of the BTS films with random orientation, measured at a frequency of 1 MHz and an applied electric field of 80 kV/cm. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

Low-temperature sintering and microwave dielectric properties of Ca2Zn4Ti15O36 ceramics

The sintering behavior, microstructures, and microwave dielectric properties of Ca₂Zn₄Ti₁₅O₃₆ ceramics with B₂O₃ addition were investigated. The crystalline phases and microstructures of Ca₂Zn₄Ti₁₅O₃₆ ceramics with 0-10 wt% B₂O₃ addition were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The sintering temperature of Ca₂Zn₄Ti₁₅O₃₆ ceramic was lowered from 1170 to 930 °C by 10 wt% B₂O₃ addition. Ca₂Zn₄Ti₁₅O₃₆ ceramics with 8 wt% B₂O₃ addition sintered at 990 °C for 2 h exhibited good microwave dielectric properties, i.e., a quality factor (Qf) 11,400 GHz, a relative dielectric constant (ε_r) 41.5, and a temperature coefficient of resonant frequency (τ_f) 94.4 ppm/°C.     

液体中静态介电常数随温度渡越行为及其关联特征研究

为了通过液体静态介电常数随温度的变化行为,归纳总结液体中分子间取向关联极化的一般规律,在现有的文献中,筛选了分子的电子极化远小于分子间取向关联极化的18种液体材料,并对其静态介电常数随温度的变化行为进行了分析研究.结果表明,液体中静态介电常数随温度变化存在一个普遍的渡越行为,并且对应于该种渡越,至少存在两种分子之间的取向关联序,一种随温度降低而增强,而另一种则减弱.