Dielectric behavior and transport properties of ZnO nanorods

Highly optical, good crystalline and randomly aligned ZnO nanorods were synthesized by the hydrothermal method. The dielectric properties of ZnO nanorods were attributed to the interfacial polarization at low frequencies (below 10 kHz) and orientational polarization at higher frequencies. The observed ω^(n?1) dependence of dielectric loss was discussed on the basis of the Universal model of dielectric response. Dielectric loss peak was composed of the Debye like loss peak at higher frequencies and interfacial loss peak at lower frequencies. Charge transport through the grain and grain boundary region was investigated by impedance spectroscopy. At higher temperatures the conductivity of the nanorod was mainly through the grain interior and the overall impedance was contributed by the grain boundary region. The activation energy of nanorod was calculated as 0.078 eV, which is slightly higher than the reported bulk value.     

An electrochemical study of copper diffusion in non-stoichiometric copper sulphide

The diffusion of copper in hexagonal chalcocite (Cu₂S) is not very well-known. In this work electrochemical cells with natural polycrystalline chalcocite working electrodes have been studied by electrochemical impedance spectroscopy (EIS) over a large frequency range at equilibrium potential. To study this phenomenon between 120 and 160°C a solid electrolyte RbCu₄Cl₅ has been used. The impedance spectra present two distinct regions. At high frequencies the general shape of the diagram in the Nyquist plane is a depleted are of a circle. Changes in electrolyte resistance, interfacial capacitance and transfer resistance have been studied as a function of temperature. At low frequencies, a diffusion impedance is observed attributed to the mobility of copper vacancies. Diffusion coefficients, with an activation energy of 1.7 eV, have been deduced from the impedance diagrams (4.7·10⁻⁵cm² s⁻¹ at 130°C). These results are compared with those obtained with orthorhombic chalcocite between 30 and 60 °C by EIS and using an electrochemical cell with a cupric liquid electrolyte. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Studies of dielectric and impedance properties of KCa2V5O15 ceramics

A polycrystalline sample, KCa₂V₅O₁₅, with tungsten bronze structure was prepared by a mixed-oxide method at low temperature (i.e., at 630 °C). A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound was studied by scanning electron microscopy (SEM). Two dielectric anomalies at 131 and 275 °C were observed in the temperature dependency of dielectric response at various frequencies, which may be attributed to the ferroelastic-ferroelectric and ferroelectric-paraelectric transitions, respectively. The nature of variation of the electrical conductivity, and value of activation energy of different temperature regions, suggest that the conduction process is of mixed-type (i.e., ionic-polaronic and space charge generated from the oxygen ion vacancies). The impedance plots showed only bulk contributions, and non-Debye type of relaxation process occurs in the material. A hopping mechanism of electrical transport processes in the system is evident from the modulus analysis. The activation energy of the compound (calculated both from loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers.     

Structural and electrical properties of KCa<Subscript>2</Subscript>Nb<Subscript>5</Subscript>O<Subscript>15</Subscript> ceramics

A polycrystalline sample of KCa₂Nb₅O₁₅ with tungsten bronze structure was prepared by a mixed oxide method at high temperature. A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound shows a uniform grain distribution throughout the surface of the sample. Studies of temperature variation on dielectric response at various frequencies show that the compound has a transition temperature well above the room temperature (i.e., 105°C), which was confirmed by the polarization measurement. Electrical properties of the material have been studied using a complex impedance spectroscopy (CIS) technique in a wide temperature (31–500°C) and frequency (10²–10⁶ Hz) range that showed only bulk contribution and non-Debye type relaxation processes in the material. The activation energy of the compound (calculated from both the loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers. A possible ‘hopping’ mechanism for electrical transport processes in the system is evident from the modulus analysis. A plot of dc conductivity (bulk) with temperature variation demonstrates that the compound exhibits Arrhenius type of electrical conductivity.      

The Electrochemical Stability of Starch Carbon as an Important Property in the Construction of a Lithium-Ion Cell

This paper shows use of starch-based carbon (CSC) and graphene as the anode electrode for lithium-ion cell. To describe electrochemical stability of the half-cell system and kinetic parameters of charging process in different temperatures, electrochemical impedance spectroscopy (EIS) measurement was adopted. It has been shown that smaller resistances are observed for CSC. Additionally, Bode plots show high electrochemical stability at higher temperatures. The activation energy for the SEI (solid–electrolyte interface) layer, charge transfer, and electrolyte were in the ranges of 24.06–25.33, 68.18–118.55, and 13.84–15.22 kJ mol⁻¹, respectively. Moreover, the activation energy of most processes is smaller for CSC, which means that this electrode could serve as an eco-friendly biodegradable lithium-ion cell element.     

Relaxation studies of bulk samarium doped ceria electrolyte

The nanocrystalline samarium doped ceria powder of phase Ce_0.8Sm_0.2O_1.9 was pelletized and sintered at about 1,623 K for the study of relaxation process. The impedance measurements were done within the frequencies from 1 Hz to 1 MHz for wide temperature range of 529–673 K. The temperature-dependent relaxation frequency found to be shifted towards higher frequency region from 17.26 to 707.96 KHz and the corresponding relaxation time for oxygen ions transport was varied from 9.22 to 0.22 μS. The temperature dependent conductivity and relaxation frequency revealed Arrhenius nature, showing activation energies 1.18 and 1.01 eV, respectively. This difference was attributed to pronounced interference due to the grain boundaries. The effect of sintering on microstructure of the sample was studied by using SEM and X-ray diffractometer (XRD). The structural and phase stability study was done by high-temperature (HT)-XRD. The thermal expansion coefficient of the samarium doped ceria powder determined from HT-XRD was found to be 13.9 × 10⁻⁶ K⁻¹.     

Effect of pulse frequency on the microstructure, phase composition and corrosion performance of a phosphate-based plasma electrolytic oxidation coated AM50 magnesium alloy

An AM50 magnesium alloy was plasma electrolytic oxidation treated using a pulsed DC power supply at three different pulse frequencies viz., 10 Hz, 100 Hz and 1000 Hz with a constant pulse ratio for 15 min in an alkaline phosphate electrolyte. The resultant coatings were characterized by X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy for their phase composition and microstructural features. The 10 Hz condition yielded relatively thick and rough coatings, which was attributed to the higher energy input per individual pulse during the PEO processing. The phase composition was also found to be influenced by the processing frequency. Electrochemical impedance spectroscopy studies performed in 0.1 M NaCl solutions revealed that the coatings produced at 10 Hz condition had a better corrosion resistance, which was attributed to the higher thickness, more compact microstructural features and a relatively stable phase composition.     

Crystal structure,NMR spectroscopy,electrical properties of catena-poly[(bis-glycinium-k 2 O:O)-di-μ-thiocyanate- k 2 N:S; k 2 S:N -cadmium (II)]

The crystal structure, the ¹¹¹Cd and ¹³C NMR spectroscopy, and the complex impedance have been carried out on a new polymeric hybrid compound: [Cd(NH₃CH₂COO)₂(SCN)₂] _n . Crystal structure shows that in the title compound the cadmium atoms have a 2N2S2O-hexa-coordination sphere, exhibiting pseudo-octahedral geometry. The cadmium atoms are bridged by two thiocyanate ions generating 1D polymeric chains. ¹¹¹Cd and ¹³C MAS NMR spectroscopy show multiplets that result from ¹¹¹Cd, ¹⁴N and ¹³C, ¹⁴N spin–spin coupling, respectively. The AC impedance measurements were performed as a function of both frequency and temperature. The AC and DC electrical conduction have been studied. The activation energy associated with the bulk resistance determined from equivalent circuit was found close to that of the activation energy obtained from DC conductivity. The conduction mechanisms are attributed to the correlated barrier hopping model.     

Effect of growth pressure on the morphology evolution and doping characteristics in nonpolar a-plane GaN

Nonpolar a-plane GaN layers grown on r-plane sapphire substrates were examined by using a two-step growth process. The higher initial growth pressure for the nucleation layer resulted in the improved crystalline quality with lower density of both threading dislocations and basal stacking faults. This was attributed to the higher degree of initial roughening and recovery time via a growth mode transition from three-dimensional (3D) to quasi two-dimensional (2D) lateral growth. Using Hall-effect measurements, the overgrown Si doped GaN layers grown with higher initial growth pressure were found to have higher mobility. The scattering mechanism due to the dislocations was dominant especially at low temperature (<200 K) for the lower initial growth pressure, which was insignificant for the higher initial growth pressure. The temperature-dependent Hall-effect measurements for the Mg doped GaN with a higher initial growth pressure yielded the activation energy and the acceptor concentration to be 128 meV and 1.2 × 10¹⁹ cm⁻³, respectively, corresponding to about 3.6% of activation at room temperature. Two-step growth scheme with a higher initial growth pressure is suggested as a potential method to improve the performance of nonpolar a-plane GaN based devices.     

高温高压下黑云斜长片麻岩的电性研究

 在压力1 GPa、温度250~1 100 ℃、频率0.1 Hz~1 MHz的条件下,采用阻抗谱方法,对来自阴山造山带的太古代黑云斜长片麻岩进行了电导率实验研究。实验发现：（1） 所有样品的阻抗谱都是由一个大的高频半圆弧Ⅰ和一个小的被压缩了的低频弧Ⅱ组成,阻抗弧Ⅰ主要代表矿物颗粒内部传导机制,阻抗弧Ⅱ更有可能代表的是样品和电极之间的传导机制;（2） 电导率随温度的变化遵循Arrhenian定律,但在700~750 ℃间有近一个量级的跳跃,通过对比实验前后样品的显微照片和探针分析数据,认为这一电导率的大幅跳跃可能是样品中黑云母发生大量脱水熔融的缘故;（3） 在高温段750~1 100 ℃,部分熔融样品的电导率主要由钠离子传导控制;（4） 在低温段250~700 ℃,样品的激活能为0.53 eV;在高温段750~1 100 ℃,样品的激活能为1.41 eV。激活能的改变可能与样品的结构变化有关,还与样品和熔体中钠的含量以及扩散有关。     

Thermal quenching of luminescence in erbium doped semiconductors

The nature of the temperature dependence of luminescence intensity from Er⁺ ions in GaInAsP, Si, InP, GaAs, AlGaAs, ZnTe, as observed by Favennecet al [1] has been examined in terms of a double exponential model. The smaller activation energy is found to be 58–100 meV, characteristic of a localized energy barrier at the Er⁺ centre while the higher activation energy is approximately 0.8E _g attributed to an Auger non-radiative process of carrier excitation into bands. This model has been found to describe the observed temperature dependences with reasonably good agreement.     

An acoustic study of the mixed-alkali-mixed phase β-aluminas. Part 1: Naβ″/βAl2O3, Kβ″/βAl2O3 and (Na0.6K0.4)β″/βAl2O3

The relative attenuation of compressional sound waves of frequencies 10–60 MHz in mixed alkali (Na/K) mixed phase (β″/β)-aluminas is reported for temperatures 80–550 K. The internal friction peaks shift to higher frequencies at higher temperatures and are attributed to Na⁺ interactions in Naβ″/β alumina and Na⁺ and K⁺ in NaK β″/β alumina. The broad attenuation peaks occuring at low temperatures (< 300 K) and at higher temperatures (> 400 K) suggest multi-relaxation processes giving a distribution of activation energies. The estimated average activation energy for Na⁺ diffusion in Naβ″/βAl₂ O₃ at low temperatures and high temperatures is 0.183 eV and 0.387 eV respectively. In the NaK β″/βAl₂o₃ samples, the Na ⁺ values were 0.239 eV and 0.386 eV, respectively. The estimated average activation energies for K⁺ diffusion at low and high temperatures in the Kβ″/β-alumina samples were 0.269 eV and 0.371 eV and for K⁺ in the NaK β″/β samples, 0252 eV and 0.339 eV, respectively. The low temperature attenuation peaks were interpreted in terms of ionic interaction in the bulk and the high temperature peaks were related to interactions in the grain boundaries. The measured activation energies confirmed these interpretations. A reversal of the temperature appearance of the Na⁺ and K⁺ high temperature peaks in the NaKβ″/βAl₂ O₃ is explained by the disorder at the grain boundaries.     

Optical probes of interlamellar redox chemistry: Intercalation of the Creutz-Taube complex into hydrogen uranyl phosphate

Hydrogen uranyl phosphate (HUP) can be intercalated by the Creutz-Taube (C-T) complex, {[Ru(NH₃)₅]₂(μ-pyrazine)}⁵⁺, to yield hydrated, partially-substituted lamellar solids, H(C-T)UP, of approximate composition H_1−5x(C-T)_xUO₂PO₄ (0.001<x<0.01). The more crystalline samples exhibited an X-ray powder pattern similar to that of HUP: the structure can be indexed in tetragonal symmetry and possesses an interlamellar spacing of ∼8.69 Å. The absorption spectrum of H(C-T) UP appears to be a simple superposition of bands due to the UO²⁺₂ moiety and the C-T complex. Photoluminescence (PL) of H(C-T) UP is quenched relative to HUP; PL decay curves are nonexponential and span a shorter time domain relative to HUP. Exposure of H(C-T) UP to Br₂ vapor causes the purple solid to become yellow. The spectral changes are consistent with redox chemistry wherein the intercalated C-T complex undergoes oxidation. For most samples examined, the PL decay times were enhanced upon oxidation.     

High-temperature impedance and modulus spectroscopy characterization of La/Mn modified PbTiO3 nanoceramics

Nanocrystalline samples of Pb_1−yLa_y(Ti_1−xMn_x)_(1−y/4)O₃ (PLMT) (y=0.06, x=0, 0.04, 0.07 and 0.10) were prepared by mechanical activation process (i.e., ball milling) followed by some annealing. The formation of single phase tetragonal crystal structure is confirmed by high-resolution X-ray diffraction study and by High resolution transmission electron micrographs (HRTEM), nano-scale compounds. The electrical behavior (i.e., impedance (Z) and electrical modulus (M)) of PLMT ceramics was studied by impedance spectroscopy technique in high temperature range. This study was carried out by means of the simultaneous analysis of the complex impedance (Z^?) and electrical modulus (M^*) functions in a wide frequency range (1 kHz-1 MHz). Impedance analysis has shown the grain and grain boundary contributions by an equivalent circuit model. Modulus analysis has provided vast information on charge transport processes. The simultaneous representation of the imaginary part of impedance and electric modulus (Z″, M″) vs. frequency revealed the localization of relaxation. The activation energy obtained from relaxation data may be attributed to oxygen ion vacancies.     

Effect of grain boundaries on the conductivity of high-purity ZrO2Y2O3 ceramics

Grain boundary conductivities are determined by complex impedance measurements (1–10⁶ Hz) on high-purity ceramics prepared by the alkoxide synthesis and on less pure ceramics obtained from a commercial powder. The grain size was varied systematically in the region 0.36–55 μm. The grain boundary conductivity is strongly influenced by the grain size, impurities and cooling procedure. The grain boundary conductivity increases linearly with the grain size for small grain sizes (0.3 to 2–4 μm) and is constant for larger grain sizes. The calculated specific conductivity of the grain boundary for pure materials is about 100 times smaller than that of the bulk. The grain boundary thickness was estimated to be 5.4 nm. The activation energy of the grain boundary conductivity is 7 kJ mole^?1 higher than of the bulk.     

Impedance analysis of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric ceramic

AC impedance spectroscopy technique has been used to study electrical properties of Bi_3.25La_0.75Ti₃O₁₂ (BLT) ceramic. Complex impedance plots were fitted with three depressed semicircles, which are attributed to crystalline layer, plate boundary and grain boundary and all three were found to comprise of universal capacitance nature [C = C0w ⁿ⁻¹]. Grain boundary resistance and capacitance evaluated from complex impedance plots have larger values than that of plate boundary and crystalline layer. The activation energies (E _a) for DC-conductance in grain boundary, plate boundary and crystalline layer are 0.68 eV, 0.89 eV and 0.89 eV, respectively. Relaxation activation energies calculated from impedance plots showed similar values, 0.81 eV and 0.80 eV for crystalline layer and plate boundary, respectively. These activation energy values are found to be consistent with the E _a value of oxygen vacancies in perovskite materials. A mechanism is offered to explain the generation of oxygen vacancies in BLT ceramic and its role in temperature dependence of DC-conductance study.      

On the physical and electrical properties of poly propenal prepared by electropolymerization technique

Electropolymerization of acrolein in an aprotic solution leads to the formation of a highly crosslinked homopolymer structure. The formed polymer was identified by elemental analysis and IR spectrum. Based on the proposed model, the chemical structure of the formed polymer is assigned. The investigated polymer is stable up to 114 °C with a thermal activation energy 0.72 eV. AC conductivity measurements, at different temperatures and frequencies, showed that the prepared polymer is a highly polarized material. The relaxation process is an Arrhenus type with activation energy 6.1 eV. The conduction behaviour of the formed polymer could be attributed to the quantum tunneling mechanism.     

Origin of dielectric relaxations in KHSeO4 above room temperature

The dispersion curves of the dielectric response for KHSeO₄ were obtained in the radio frequency range at several isotherms below the fast proton conducting phase (T?<?415 K). The results reveal a distinct dielectric relaxation at low frequency, which is about 682 Hz at 320 K, and then, it shifts to higher frequencies (～10 kHz) as the temperature increases. The f _max vs. reciprocal T shows an activated relaxation process with an activation energy of 0.5 eV, which is in close agreement with that associated with transport of charge carriers. We suggest that the observed dielectric relaxation could be attributed to polarization induced by the proton jump and selenate tetrahedral reorientations. The displacement of mobile H⁺ proton accompanied by SeO ₄ ^??2 tetrahedra reorientations creates structural distortion in both sublattices which induce localized dipoles like HSeO ₄ ^? .     

Impedance spectroscopy and dielectric analysis in KH2PO4 single crystal

Optical observation, differential scanning calorimetry, thermogravimetric analysis, and differential thermogravimetric measurements have been carried out on KH₂PO₄ single crystals. As compared with the optical observation of crystal under polarizing microscope, the dehydration process occurred gradually over the crystal surface at temperatures above 195 °C and then the interior of the sample. The ac impedance measurements were performed as a function of both frequency and temperature. The electrical conduction and dielectric relaxation have been studied. The activation energy of migration is 1.02 eV in the temperature range between 150 and 179 °C. The conduction mechanism in this temperature range is attributed to the hopping of proton among hydrogen vacancies. At temperatures above 186 °C, a higher conductivity activation energy with 2.94 eV is obtained. In addition to the proton conduction, the migration of the heavier ions (such as potassium ion) is also suggested.     

Pyroelectric Spectroscopy of the Hydrogen Uranyl Phosphate

Absorption, transmission and reflection properties of hydrogen uranyl phosphate (HUP) are determined, by using pyroelectric (PE) sensors in thermally-coupled (PPE) and thermally-decoupled (conventional) spectroscopy configurations. Absorption bands at 1.25, 1.5 and 2.0μm, stronger for HUP than for the corresponding ammonium salt, have been observed. This can be correlated with the much higher protonic conductivity of HUP and suggests polaronic effects on the conduction mechanism in this solid electrolyte.