Dielectric relaxation and ac conductivity of sodium tungsten phosphate glasses

Studies of dielectric relaxation and ac conductivity have been made on three samples of sodium tungsten phosphate glasses over a temperature range of 77–420 K. Complex relative permitivity data have been analyzed using dielectric modulus approach. Conductivity relaxation frequency increases with the increase of temperature. Activation energy for conductivity relaxation has also been evaluated. Measured ac conductivity (σ_m(ω)) has been found to be higher than σ_dc at low temperatures whereas at high temperature σ_m(ω) becomes equal to σ_dc at all frequencies. The ac conductivity obeys the relation σ_ac(ω)=Aω ^S over a considerable range of low temperatures. Values of exponent S are nearly equal to unity at about 78 K and the values decrease non-linearly with the increase of temperature. Values of the number density of states at Fermi level (N(E _F)) have been evaluated at 80 K assuming values of electron wave function decay constant α to be 0.5 (Å)^?1. Values of N(E _F) have the order 10²⁰ which are well within the range suggested for localized states. Present values of N(E _F) are smaller than those for tungsten phosphate glasses.     

Ac electrical conductivity and dielectric relaxation behavior of amorphous Selinum36 Antimony31Cubber33 thin films

ABSTRACT

The alternating-current (Ac) conductivity measurements and dielectric behaviors were observed in the range of temperature (from 303 to 393?K) and in the frequency range from 10² to10⁶?Hz for amorphous films of Selinum₃₆ Antimony₃₁ Cubber₃₃ chalcogenide glass. The ac conductivity has temperature dependency and the frequency dependency. The reduction of the exponent S values with raising temperature was introduced with the correlated barrier hopping model. The maximum height of the barrier W_M for Sellinum₃₆ Antimony₃₁Cubber₃₃ films is reliable with carrier hopping over a potential barrier. The number of localized states per unit volume at the Fermi level enhances with the elevation of ambient temperature of the film sample. Both dielectric constant ε₁ and loss ε₂ increase with the rise of temperature rising and decrease with frequency. The computation of the dielectric modulus M^/ and M^// revealed that the interfacial is the most suitable polarization type.     

Stretched exponential relaxation and dispersive conductivity behavior in lithium bismuth silicate glasses

Glasses having compositions xLi₂O∙(85 − x)Bi₂O₃∙15SiO₂ (x = 35, 40, and 45 mol%) were prepared by normal melt quenching technique. Electrical relaxation and conductivity in these glasses were studied using impedance spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature range from 453 to 603 K. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency were extracted from the impedance spectra. The dc conductivity increases with increase in Li₂O content providing modified glass structure and large number of mobile lithium ions. Similar values of activation energy for dc conduction and for conductivity relaxation time indicate that the ions overcome the same energy barrier while conducting and relaxing. The non-exponential character of relaxation processes increases with decrease in stretched exponential parameter ‘β’ as the composition parameter ‘x’ increases. The observed conductivity spectra follow a power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. Nearly constant losses (NCL) characterize this linearly dependent region of the conductivity spectra. A deviation from the ‘master curve’ for various isotherms of conductivity spectra was also observed in the high-frequency region and at low temperatures, which supports the existence of different dynamic processes like NCL in addition to the ion hopping processes in the investigated glass system.     

Insulated electrodes for eliminating conductivity in dielectric relaxation experiments

In a variety of samples, conductivity is a limiting factor regarding the resolution of dielectric loss peaks of interest. One approach to eliminating the signal that originates from conductivity is the use of insulating layers on one or both electrodes. For the typical case, it is shown that insulation layers add errors rather than improving the situation.     

Ac conductivity and dielectric behavior of hydrated homoionic alkali-cations-exchanged montmorillonite

The dielectric properties of a series of homoionic alkali-exchanged montmorillonites were studied at different treatment temperatures and various water loadings by means of complex impedance spectroscopy. To date, however, this method has been underutilized in clay minerals studies. The main objective of the present work is to understand the relaxation mechanisms of water molecules interacting with different hydration centers in clay minerals, with a view to eventually control their interactions with the alkali extra-framework cations. The other part of our study is to study the dielectric properties such as real and imaginary parts of dielectric permittivity, loss tangent, and ac conductivity in the frequency range 10^?2–10⁶?Hz and temperature range 173–333?K of these clay minerals. The obtained results have been discussed in terms of the Jonscher model.     

AC conductivity and dielectric properties of sulphate glasses

AC conductivity and dielectric properties of sulphate glasses have been studied as a function of temperature, frequency and variation in interalkali concentration. AC conductivity at frequencies well beyond the dielectric loss peaks seems to arise from local motion of alkali ions within the neighbouring potential wells. Activation energies for AC conductivity were found to be very much lower than those for DC conductivity. Further, AC conductivity seems to be independent of interalkali variation, whereas ε′ and tan δ show a mild degree of mixed alkali effect. The observations made here have been explained on the basis of a structural model earlier proposed by us for these glasses. Communication No. 167 from Solid State and Structural Chemistry Unit.     

Investigation on crystalline structure and dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) film

The dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) (PVDF) film have been studied using dielectric spectroscopy in the frequency domain from 20 Hz to 5 MHz at temperatures between 20 °C and 200 °C. Crystalline/amorphous interphase is suggested with methods of FTIR, XRD, and DSC. Frequency and temperature dependence of dielectric spectroscopy reveals the relaxation behavior and structural dynamics of the samples, and three types of relaxation processes are suggested, α_Ac relaxation process contributed by the hopping transport process near the periphery of conduction band or valence zones at Fermi energy, α_c relaxation process related to the structure change of crystal lattice trapped dipoles in crystalline regions, and α_a relaxation process arising from segmental dipole rearrangement of interphases in amorphous regions. Cole-Cole and Havriliak-Negami experimental equations were utilized to analyze these relaxation processes, and differences of Arrhenius parameters for α_Ac and α_c relaxation processes obtained from Cole-Cole and Havriliak-Negami equations were discussed in detail. Activity energy of different relaxation processes obtained from Arrhenius equation and VFT equation indicates non-single thermal activation mechanism for hot pressed PVDF film.     

AC conductivity and dielectric behavior of polyaniline/sodium metavenadate (PANI/NaVO3) composites

The conducting polyaniline/sodium metavenadate (PANI/NaVO₃) composites were synthesized by single step in situ polymerization technique by placing finely grinded powder of NaVO₃ during the polymerization of aniline. The formation of mixed phases of the polymer together with the conducting emeraldine salt phase was confirmed by spectroscopic techniques like FTIR. SEM images indicated a systematic morphological variation of particles aggregated in the composite matrix as compared to the pristine PANI. AC conductivity and dielectric behavior of these composites were investigated in the frequency range 50 Hz to 5 MHz. It is found that AC conductivity obeyed the power law index and the variation of conductivity with wt% of NaVO₃ could be related to conductivity relaxation phenomenon. These composites have shown high dielectric constant, which is related to polarization. It is seen that both dielectric constant and dielectric loss decrease with increase in frequency. Variations in measured parameters of AC response with increasing frequency of these composites are found to follow systematic trends that are similar to those observed with temperature and doping.     

Effects of plasticizer and nanofiller on the dielectric dispersion and relaxation behaviour of polymer blend based solid polymer electrolytes

Solid polymer electrolytes consisted of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) blend (50:50 wt/wt%) with lithium triflate (LiCF₃SO₃) as a dopant ionic salt at stoichiometric ratio [EO + (CO)]:Li⁺ = 9:1, poly(ethylene glycol) (PEG) as plasticizer (10 wt%) and montmorillonite (MMT) clay as nanofiller (3 wt%) have been prepared by solution cast followed by melt–pressing method. The X–ray diffraction study infers that the (PEO–PMMA)–LiCF₃SO₃ electrolyte is predominantly amorphous, but (PEO–PMMA)–LiCF₃SO₃–10 wt% PEG electrolyte has some PEO crystalline cluster, whereas (PEO–PMMA)–LiCF₃SO₃–10 wt% PEG–3 wt% MMT electrolyte is an amorphous with intercalated and exfoliated MMT structures. The complex dielectric function, ac electrical conductivity, electric modulus and impedance spectra of these electrolytes have been investigated over the frequency range 20 Hz to 1 MHz. These spectra have been analysed in terms of the contribution of electrode polarization phenomenon in the low frequency region and the dynamics of cations coordinated polymer chain segments in the high frequency region, and also their variation on the addition of PEG and MMT in the electrolytes. The temperature dependent dc ionic conductivity, dielectric relaxation time and dielectric strength of the plasticized nanocomposite electrolyte obey the Arrhenius behaviour. The mechanism of ions transportation and the dependence of ionic conductivity on the segmental motion of polymer chain, dielectric strength, and amorphicity of these electrolytes have been explored. The room temperature ionic conductivity values of the electrolytes are found ∼10⁻⁵ S cm⁻¹, confirming their use in preparation of all-solid-state ion conducting devices.     

Conduction mechanism and the dielectric relaxation process of a-Se75Te25−xGax (x=0, 5, 10 and 15 at wt%) chalcogenide glasses

Se₇₅Te_25−xGa_x (x=0, 5, 10 and 15 at wt%) chalcogenide compositions were prepared by the well known melt quenching technique. Thin films with different thicknesses in the range (185–630 nm) of the obtained compositions were deposited by thermal evaporation technique. X-ray diffraction patterns indicate that the amorphous nature of the obtained films. The ac conductivity and the dielectric properties of the studied films have been investigated in the frequency range (10²–10⁵ Hz) and in the temperature range (293–333 K). The ac conductivity was found to obey the power low ω^s where s≤1 independent of film thickness. The temperature dependence of both ac conductivity and the exponent s can be well interpreted by the correlated barrier hopping (CBH) model. The experimental results of the dielectric constant ε₁ and dielectric loss ε₂ are frequency and temperature dependent. The maximum barrier height W_m calculated from the results of the dielectric loss according to the Guintini equation, and agrees with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott for chalcogenide glasses. The density of localized state was estimated for the studied film compositions. The variation of the studied properties with Ga content was also investigated. The correlation between the ac conduction and the dielectric properties were verified.     

Dielectric and conductivity relaxation in mixtures of glycerol with LiCl

We report a thorough dielectric characterization of the α relaxation of glass-forming glycerol with varying additions of LiCl. Nine salt concentrations from 0.1 to 20mol% are investigated in a frequency range of 20Hz-3GHz and analyzed in the dielectric loss and modulus representation. Information on the dc conductivity, the dielectric relaxation time (from the loss) and the conductivity relaxation time (from the modulus) is provided. Overall, with increasing ion concentration, a transition from reorientationally to translationally dominated behavior is observed and the translational ion dynamics and the dipolar reorientational dynamics become successively coupled. This gives rise to the prospect that, by adding ions to dipolar glass formers, dielectric spectroscopy may directly couple to the translational degrees of freedom determining the glass transition, even in frequency regimes where usually strong decoupling is observed.     

Crystallization evolution and relaxation behavior of high entropy bulk metallic glasses using microalloying process

The role of the microalloying process in relaxation behavior and crystallization evolution of Zr_(20) Cu_(20) Ni_(20) Ti_(20) Hf_(20) high entropy bulk metallic glass(HEBMG) was investigated. We selected Al and Nb elements as minor elements, which led to the negative and positive effects on the heat of mixing in the master HEBMG composition, respectively. According to the results, both elements intensified β relaxation in the structure; however, α relaxation remained stable. By using different frequencies in dynamic mechanical analysis, it was revealed that the activation energy of β relaxation for the Nb-added sample was much higher, which was due to the creation of significant structural heterogeneity under the microalloying process. Moreover, it was found that Nb addition led to a diversity in crystallization stages at the supercooled liquid region.It was suggested that the severe structural heterogeneity in the Nb-added sample provided multiple energy-level sites in the structure for enhancing the crystallization stages.     

Ionic conduction and dielectric relaxation in Ag/Na ion-exchanged aluminosilicate glasses: mixed mobile ion effect and KWW relaxation

Ag⁺/Na⁺ ion-exchanged aluminosilicate glasses with uniform concentration profiles were prepared, and their electrical conductivities were investigated as functions of the ion-exchange ratio and the initial glass compositions. In the case of the ion-exchanged glasses of x20Ag₂O–(1−x)20Na₂O–10Al₂O₃–70SiO₂ in mol%, the conductivity, σ, and its activation energy, E_σ, showed a minimum and a maximum at the same ion-exchange ratio x=0.3, respectively, and the mixed mobile ion effect (MMIE) was observed. The fully ion-exchanged sample attained σ=3.5×10⁻⁵ S/cm at 200 °C, which was 1.5 orders of magnitude larger than that of initial glass. In the case of x25Ag₂O–(1−x)25Na₂O–25Al₂O₃–50SiO₂, the mixed mobile ion effect was also observed at x=0.5. The maximum conductivity of 2×10⁻⁴ S/cm at 200 °C was obtained in the fully ion-exchanged glass sample.

The electric relaxation analysis was also conducted on both systems, and Kohlrausch–Williams–Watts (KWW) fractional exponent β was obtained as a function of x. The decrease of β was observed near x≈0.3 in the former system, while that of the later system was independent of the ion-exchange ratio. Based on the structural analysis results, the observed behaviors were investigated from the point of view of the occupation of Ag⁺ ions on the non-bridging oxygen-site (NBO-site) and the charge compensation-site (CC-site) of AlO₄ tetrahedral unit.     

Ag concentration dependent transport properties of LiF-MoO3-P2O5 glasses

LiF-MoO₃-P₂O₅ glasses mixed with different concentrations of Ag₂O (ranging from 0 to 1.0 mol%) was prepared. D.C. conductivity and dielectric properties over a range of temperature have been investigated. The analysis of the results of d.c. conductivity has indicated that T>θ_D/2, the small polaron hoping model seems to be fit and the conduction is adiabatic in nature. These results further indicated that there is a change over of conduction mechanism from electronic to ionic at about 0.4 mol% of Ag₂O. The low temperature part of a.c. conductivity is explained based on quantum mechanical tunneling model. The quantitative analysis of these results is further extended with the aid of the data on optical absorption, ESR and IR spectral studies.     

Ion implantation effects in glasses

Abstract

Ion implantation can be used to introduce network damage and to alter the chemical composition in glasses. Structural changes can be inferred from IR measurements near 1000 cm^?1 and by optical absorption near 2150 Å. Implantation-induced damage decreases the implanted volume in fused silica with consequent changes in the refractive index, the near-surface hardness, and the tensile surface stress. Prior work in these areas is reviewed. Implantation into alkali silicate glasses depletes the alkali content in the implanted region. These changes allow preferential surface crystallization in Li₂O-2SiO₂ glasses. Crystallization of amorphous SiO2 can be induced by implantation of Li. Insight into the crystallization process is obtained by following the associated ion movement by elastic recoil detection (ERD) and optical techniques. Implantation of 20keV H shows that saturation of implanted H-sites in fused silica occurs at about 2.2 × 10²¹ H/cm³ in agreement with free volume estimates of the maximum number of available interstitial sites. Details of H and D interactions in fused silica were studied as a function of fluence and temperature. Results are of interest in studies of corrosion in glasses considered for nuclear waste encapsulation and for components in fusion reactors.

IV Summary     

Ce3+对Er3+/Yb3+共掺TeO2-WO3-ZnO玻璃发光性能的影响

在Er³⁺/Yb³⁺共掺TeO₂-WO₃-ZnO玻璃中引入Ce³⁺，研究了Ce³⁺对Er³⁺1.5μm发射性能及其上转换发光性能的影响。结果表明，随Ce³⁺浓度的增加Er³⁺1.5μm波段的荧光强度先增强后降低，优化的Ce³⁺掺杂浓度在2.07×10²⁰/cm³左右；1.5μm波段的荧光寿命则随Ce³⁺浓度的增加有轻微降低，从3.4ms降到3.0ms，但Ce³⁺浓度的增加对1.5μm波段的荧光半高宽基本无影响；Er³⁺/Ce³⁺间的交叉弛豫Er³⁺(⁴I_11/2)+Ce³⁺(²F_5/2)→Er³⁺(⁴I_13/2)+Ce³⁺(²F_7/2)使玻璃的上转换发光强度大大降低，但在过高的Ce³⁺浓度下，Er³⁺/Ce³⁺间的另一交叉弛豫Er³⁺(⁴I_13/2)+Ce³⁺(²F_5/2)→Er³⁺(⁴I_15/2)+Ce³⁺(²F_7/2)则使Er³⁺⁴I_13/2能级粒子数减少，导致1.5μm波段荧光强度和荧光寿命降低. 关键词： 碲钨酸盐玻璃 发光性能 3+离子')" href="#">Er³⁺离子 3+离子')" href="#">Ce³⁺离子 交叉弛豫     

Improved techniques for the calculation of ab initio ion-neutral interaction potentials: application to coinage metal ions interacting with rare gas atoms

Ab initio values for the potential energy functions for ion–neutral interactions can be tested by comparison with gaseous ion transport coefficients, but only if special care is taken to compute the interaction potentials accurately over wide ranges of internuclear separation. This is illustrated here by a reanalysis of the ab initio values for the coinage metal ions interacting with rare gas atoms, precise calculations of the transport cross sections over extremely wide ranges of energy, and similarly precise calculations of the zero-field ion mobilities as functions of gas temperature and the field-dependent ion mobilities at various fixed temperatures. The calculations indicate that the mobilities for Ag⁺(¹S) moving in Ne or Ar can distinguish between the existing, very similar ab initio potentials. They also show that substantial differences exist among the mobilities of the coinage metal anions and the ground and excited states of the cations. The techniques implemented are recommended for future ab initio calculations.     

Thermal neutron damage in CdS and CdTe

Photoluminescence and electrical resistivity changes in CdS and CdTe produced by thermal neutrons are discussed. The damage is produced principally by the neutron capture reaction ¹¹³Cd (n,y) ¹¹⁴Cd. Since the reaction product ¹¹⁴Cd is stable, complications arising from impurity introduction is minimal. The cumulative recoil nuclear recoil energy is about 143 eV, but is not the recoil energy at the time atomic displacement occurs. Thermal and fast neutrons enhance the CdS luminescence band at 7200A in the ratio of 28:1, but the resistivity changes are in the ratio of 40:1 Cd interstitial is suggested as the luminescence center. Hall measurements on n-type CdTe suggest that only Cd defects are produced for low thermal neutron doses. The acceptor introduction rate is about 1.0 to 0.6, compared to 0.098 for CdS. These are in good agreement with the values reported by R. O. Chester. The fast neutron effects in high resistivity CdS reported by Johnson indicate the need for further measurement.     

Properties of (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 analysed within dielectric permittivity,conductivity, electric modulus and derivative techniques approach

The (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ ceramics with perovskite structure were sintered. The XRD test proved that the samples are cubic (a?=?3.920?±?0.001?Å). Microstructure and atomic composition were determined with a SEM (JSM-5410) equipped with energy dispersion X-ray analyser (ISIS-300). The fluctuation in the chemical composition was found indicating on local disorder. Broadband dielectric spectroscopy in the range 10^?1–3?·?10⁷?Hz was applied within the range of 100–650?K. The real, ?′(f,?T), and imaginary, ?″(f,?T), parts of complex dielectric permitivity characteristics, both in the temperature and frequency domain, show relaxation processes partially covered by electric conductivity. At high temperatures the electric conductivity exhibits a thermally activated behaviour σ(f,?T)?∝?exp(?E _a/kT) but the variable range hopping (VRH) dependence σ?∝?exp[?(T ₀/T)^1/4] is manifested at low temperatures. The derivatives technique in the frequency (??log??/??log?ω) and temperature (??log??/?T) domain enabled various relaxation processes to be distinguished. The data converted to electric modulus representation, M*(f,?T)?=?1/?*, exhibited clearly resolved relaxation peaks. The relaxation times obtained from the peaks position show a slightly non-Arrhenius temperature behaviour with the activation energy varying in 0.4–0.6?eV range and characteristic time of the electric conductivity relaxation of the order of 10^?12?s. The relaxation times can be fitted at better accuracy with the VRH dependence where T ₀ are of the order of 10⁸?K. It is shown that the low frequency ac-conductivity converges to dc-conductivity and the relation σ(0)?～?ω_m?～?τ_m ^?1 typical for the disordered solids applies. The conduction current relaxation relationship behaves in accord with the VRH system: σ_dc?∝?(T/T ₀)^q (e ²/kT) ω_c, where ω_c?=?ν_ph exp[?(T ₀/T)^1/4] is valid for the locally disordered (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ compound.