Debye type dielectric relaxation in carbon nano-balls’ and 4-DMAABCA acid doped E7 coded nematic liquid crystal

The electrical properties of carbon nano-balls’ and 4-dimethylaminoazobenzene-2′-carboxylic acid doped dispersed nematic liquid crystal composite were investigated by impedance spectroscopy technique. The conductivity and capacitance were measured in the frequency range 100 kHz-1 MHz and temperature range 300-380 K. The loss peak was observed in the dielectric loss spectra and was identified as nearly-Debye type relaxation. Cole-Cole plots have been used to describe the characteristic changes of electrical properties in mentioned temperature interval. The sample presents monodispersive relaxation behavior with a relaxation time of ∼10⁻⁷ s. The relaxation process is attributed to the dipolar rotation of the long molecular axis and the activation energy is found to be 0.097 eV.     

Investigation of near constant loss contribution to conductivity in lithium bismo-silicate glasses

Glasses having compositions 20Li₂O · (80 − x)Bi₂O₃ · xSiO₂ (x = 55, 60, 65, 70 mol%) were investigated using impedance spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature range from 543 to 663 K. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency are extracted from the impedance spectra. The increase in conductivity with increase in SiO₂ content is attributed to the change in the structural units of bismuth. Both electric modulus and the conductivity formalism have been employed to study the relaxation dynamics of charge carriers in these glasses. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates the temperature independence of the dynamic processes for ions in these glasses. Similar values of activation energy for dc conduction and for conductivity relaxation time indicates that the ions overcome same energy barrier while conducting and relaxing. The observed conductivity spectra follows power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. Near constant losses (NCL) characterize this linearly dependent region of conductivity spectra. A deviation from ‘super curve’ for various isotherms of conductivity spectra was also observed in 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.     

Molecular dynamics simulation for cooling rate dependence of solidification microstructures of silver

A molecular dynamics simulation study has been performed to investigate the solid microstructures of Ag at room temperature resulted from rapid solidification with six cooling rates by using Quantum Sutton-Chen (QSC) many body potential. Several structural analysis methods have been employed to characterize and analyze the atomic configurations in the system. The results reveal that as the cooling rate γ ? 1.0 × 10¹³ K/s, an amorphous structure can be obtained; as the cooling rates in the range of γ = 5.0 × 10¹² ∼ 1.38 × 10¹¹ K/s, the coexistence structures of the meta-stable hcp with stable fcc configurations can be formed with phase separating or layering structures. Moreover, by using the 3D graphics technology, the arrangement of atoms in the system reveals that the phase separating occurs before the layering structures can be formed following the cooling rate decreasing. And an interesting layering structures have been found, in which the central atoms of fcc and hcp basic clusters are arranged by alternate close-packed-layers with a new repeating sequence ABCB.     

Study of sodium tellurite glass using the thermally stimulated depolarization current technique (TSDC)

In order to have a better understanding of the role of the structure and the defects involved in the polarization processes in an 85TeO₂-15Na₂O mol% glass, we used the thermally stimulated depolarization currents (TSDC technique). The TSDC of the non-irradiated sample presented a strong negative peak of current at the temperature of 340 K, preceded by a relatively weak positive peak at about 300 K, after different d.c. voltages of 1200, 1500 and 2000 V were applied. No response was obtained with 1000 V, but the peak intensity increased considerably for voltages above 1200 V. After γ-irradiation of 25 and 50 KGy doses, a depolarization of the negative peak was observed in the sample submitted to25 KGy, whereas for the sample irradiated with 50 KGy, six TSDC peaks appeared at regular intervals of 5 KGy, in the temperature range of 100 and 300 K.     

Johari-Goldstein relaxation as the origin of the excess wing observed in metallic glasses

Measurements of the shear loss modulus G″ of amorphous Zr₆₅Al_7.5Cu_27.5 at 5.4 kHz were reported by Rösner, Samwer and Lunkenheimer. They observed that the measured G″ are in excess of the contribution from the α-relaxation, indicating the existence of an ‘excess wing’ in amorphous metals like that found in molecular glass-formers. They speculated that the excess wing found in amorphous metal is due to the presence of an unresolved Johari-Goldstein (JG) secondary relaxation. In this work, the coupling model is used to calculate the temperature T_JG at which the JG relaxation frequency coincides with the experimental frequency of the isochronal measurement. The location of T_JG is well within the temperature range where the excess wing of Zr₆₅Al_7.5Cu_27.5 appears at 5.4 kHz. Hence, the result supports the assertion of Rösner et al. that the excess wing found in the metallic glass originates from the intrinsic JG relaxation.     

The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623 K. Two weak relaxations have been observed around 337 K and around 402 K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W = (0.84 ± 0.03) eV ) and above ( W = (0.82 ± 0.03) eV) the temperature range where the β relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the β relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the β₁ sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the β₂ sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573 K, is high than 8895 s.     

The ionic transition and the glass transition in ion-conducting glasses

In an ion-conducting glass, there may be the ionic transition which characterizes ‘melting’ of the modifying ions in a similar way as the glass transition characterizes ‘melting’ of the glass network former. The ionic transition can be observed, electrically, by the thermally stimulated depolarization current technique. It is demonstrated that the relaxation time for the ionic transition is the same (near 100 s) with that for the glass transition in so far as the heating/cooling rate lies within, say, 10⁻¹-10⁻⁵ K/s as adopted in our routine works.     

Disorder in LixFePO4: From glasses to nanocrystallites

Li_xFePO₄ glasses have been prepared by fast-quenching method in the whole range of composition 0 ? x ? 1. The amorphous state of glassy materials is confirmed by X-ray diffraction. Information concerning the local environment of Li and Fe cations and the configuration of (PO₄)³⁻ oxo-anions is obtained by Fourier transform infrared (FTIR) spectroscopy. While the LiFePO₄ crystalline materials undergo a transition from the paramagnetic to the antiferromagnetic ordering at 52 K, no magnetic ordering is observed in the vitreous samples that realize random field systems, so that a spin glass-like freezing is observed at low temperature. The paramagnetic Curie temperature of Li_xFePO₄ is independent of x and shifted to θ = −60 K in the glassy state, due to a significant distortion of the FeO₆ octahedra that alters the superexchange path inside the atomic FeO₄ layers of the crystallized structure. On another hand, the PO₄ tetrahedra are not significantly distorted in the glassy phase. The results are compared with highly disordered, but nanocrystallized LiFePO₄ recently obtained at the early stage of synthesis by solid state reaction at 300 °C. In this latter case, the lack of long-range antiferromagnetic ordering is due to substitutional disorder among the cationic sublattice.     

The dielectric behavior of a thermoelectric treated B2O3-Li2O-Nb2O5 glass

A transparent glass with the composition 60B₂O₃-30Li₂O-10Nb₂O₅ (mol%) was prepared by the melt-quenching technique. Glass-ceramics, containing LiNbO₃ ferroelectric crystallites, were obtained by heat-treatment (HT) above 500 °C, with and without the presence of an external electric field. The dielectric properties of the glass and glass-ceramic were investigated, as a function of temperature (270-315 K), in the 10 mHz-32 MHz frequency range. The presence of an external electric field, during the heating process, improves the formation of LiNbO₃ crystallites. The rise of the treatment temperature and the applied field, during the heat-treatment, leads to a decrease in the dc electric conductivity (σ_dc), indicating a decrease of the charge carriers number. The dielectric permittivity (ε′) values (300 K;1 kHz) are between 16.25 and 18.83, with the exception of the 550 °C HT sample that presents a ε′ value of 11.25. An electric equivalent circuit composed by an R in parallel with a CPE element was used to adjust the dielectric data. The results reflect the important role carried out by the heat-treatment and the electric field during the HT in the electric properties of glass-ceramics.     

Dielectric relaxation study of chalcogenide glasses

The paper reports dielectric measurements carried out for a variety of threshold and memory alloys of glassy AsGeTe and SeGeTe at different temperatures (83 to 373 K) and various frequencies (0.2, 0.5, 1.0, 2.6 and 5.0 MHz). It is found that the glassy system of chalcogenides exists in the form of molecular dipoles which remain frozen at low temperatures and, as the temperature is increased, the molecules attain freedom of rotation at temperatures which are sometimes as low as 253 K. All the materials displayed dielectric dispersion in the radio frequency range. Gevers' formula has been used to calculate the dielectric loss (?′') and loss-angle (tan δ) from the measured values of the real part of dielectric constant (?′). The curves: log ?′ versus temperature, ?′ versus log ω, ?″ versus log ω, tan δ versus log ω and tan δ versus temperature, gave a direct evidence of the existence of a Debye-type relaxation having a wide distribution of relaxation times.Cole-Cole diagrams have been used to determine the distribution parameter (α) and the molecular relaxation time (τ). The temperature dependence of α and τ for all the alloys is consistent with the “molecular relaxation mechanism”. The paper also reports accurate values of the static and optical dielectric constants for all the alloys.Eyring's relaxation rate equations have been used to determine the free energy of activation (ΔF), and enthalpy of activation (ΔH) for all the alloys. These results indicate the existence of a stronger intermolecular interaction for SeGeTe alloys. mott's concept of “dangling bonds” has also been used to explain the existence of a stronger intermolecular interaction, and hence a greater density of defect states in case of SeGeTe as compared with AsGeTe alloys.It has been finally concluded that the dielectric behaviour of chalcogenide glasses, in general, can be successfully explained by using the theory of molecular relaxation.     

Growth and characterization of near-band-edge transitions in β-In2S3 single crystals

Single crystals of β-In₂S₃ were grown by chemical vapor transport method using ICl₃ as a transport agent. The below and above band-edge transitions in β-In₂S₃ have been characterized using optical absorption and photoluminescence (PL) measurements in the temperature range 20–300 K. Thermoreflectance (TR) and photoconductivity (PC) measurements were carried out to verify the band-edge nature of the diindium trisulfide tetragonal crystals. Experimental analyses of the transmittance, PL, PC, and TR spectra of β-In₂S₃ confirmed that the chalcogenide compound is a direct semiconductor with a band gap of about 1.935 eV at 300 K. β-In₂S₃ is familiar with its defect nature. For the β-In₂S₃ crystals, two defect emissions and two above band-edge luminescences were simultaneously detected in the PL spectra at low temperatures. The energy variations of the defect emissions showed temperature-insensitive behavior with respect to the temperature change from 20 to 300 K. Temperature dependences of transition energies of the near-band-edge (NBE) transitions below and above band gap are analyzed. The origins for the NBE transitions in the β-In₂S₃ defect crystals are discussed.     

Crystallization kinetics of (Ni0.75Fe0.25)78Si10B12 amorphous alloy

Amorphous ribbon specimen of (Ni_0.75Fe_0.25)₇₈Si₁₀B₁₂ has been prepared by a single roller melt-spinning technique in the air atmosphere. The crystallization kinetics of the alloy has been investigated using different thermal analysis by means of continuous heating and isothermal heating. The activation energy of the alloy has been calculated by using Kissinger plot method and Ozawa plot method based on differential thermal analysis data, respectively. The products of crystallization have been analyzed by X-ray diffraction. A single phase of γ-(Fe, Ni) solid solution with grain size of about 10.3 and 18.5 nm precipitates in the amorphous matrix after annealing at temperatures 715 and 745 K, respectively. The crystallized phases are γ-(Fe, Ni) solid solution, Fe₂Si, Ni₂Si, Fe₃B and unidentified phase after annealing at 765 K. The details of nucleation and growth during the isothermal crystallization are expatiated in terms of local Avrami exponent and local activation energy.     

On the Urbach rule in non-crystalline solids

A comparative analysis of temperature behavior of optical absorption edge is performed for non-crystalline materials where the Urbach behavior is observed (SiO₂), with deviations from the Urbach behavior (20PbO · 80SiO₂) and with a mixed behavior (As₂S₃). For glassy As₂S₃, the Urbach behavior of the absorption edge in a limited temperature range is explained from the point of view of two components of structural disordering - static and dynamic. A parallel red shift of the absorption edge in As₂S₃ and, consequently, the temperature-independent Urbach energy at T < 300 K, are related to the lack of medium-range order in the atomic arrangement in the presence of short-range order.     

Spin probe reorientation and its connections with free volume and relaxation dynamics: Diglycidyl-ether of bis-phenol A

We report the dynamical behavior of a series of four spin probes of different geometries in amorphous van der Waals-type glass-former diglycidyl-ether of bis-phenol A (DGEBA) as measured by electron spin resonance spectroscopy (ESR). To reveal the relative influence of physical factors influencing spin probe dynamics, the reorientation properties were related to the free volume data of DGEBA from positron annihilation lifetime spectroscopy (PALS) and to the relaxation results of DGEBA from broadband dielectric spectroscopy (BDS). The characteristic ESR temperatures () determined from the temperature dependences of the spectral parameter of mobility, 2A_zz^′, for all spin probes and those ( and ) from the rotational correlation time, τ_c, as a function of temperatures for the smallest spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo) were found to coincide with the characteristic PALS temperatures T_b1, T_b2, and T_σ3 from phenomenological analyses of the temperature dependencies of the mean o-Ps lifetime, τ₃, and the width of the o-Ps lifetime distribution, σ₃. Moreover, for the smallest spin probe Tempo, the favorable mutual relationship between the spin probe size and hole size distribution g(V_h) and the closeness of T_50 G with T_b1 point to the role of free volume in the slow to rapid regime crossover. On the other hand, for the three larger spin probes T_50 G lies in the vicinity of T_b2∼T_σ3 and is independent of their size, mass, and shape. The origin of these ESR and PALS coincidences can be traced out to the secondary γ-process for the smallest spin probe Tempo and to the primary α-process for all larger spin probes.     

Formation, mechanical properties and corrosion resistance of Ti-Pd base glassy alloys

No biocompatible Ti-based glassy alloys without a harmful element have been reported. We have examined the mechanical and chemical properties of Ti-Pd-Zr-Si glassy alloy in comparison with pure Ti metal and Ti-6Al-4V alloy which have been used so far for biomaterials. The present Ti-Pd base glassy alloys do not contain Al and Ni elements which are considered to be rather toxic. Melt-spun Ti₄₅Zr_50−xPd_xSi₅ glassy alloy ribbons (x = 35, 40, 45) exhibited good bend ductility and had higher Vickers’s hardness and lower Young’s modulus as compared to pure titanium and Ti-6Al-4V alloy. In addition, the Ti₄₅Zr_50−xPd_xSi₅ glassy alloys had higher corrosion resistance and were passivated over a wide range and at the lower passive current density of approximately 10⁻² Am⁻² than at of pure titanium and Ti-6Al-4V alloy in 1 mass% lactic acid and PBS(−) solutions at 310 K.     

Far-infrared transmission and bonding arrangement in Ge10Se90−xTex semiconducting glassy alloys

The far-infrared spectra of Ge₁₀Se_90−xTe_x where x = 0, 10, 20, 30, 40, 50 glassy alloys were measured in the wavenumber region 50-650 cm⁻¹ at room temperature. The results were explained in terms of the vibrations of the isolated molecular units. The addition of Te in Ge₁₀Se₉₀ has shown the appearance of GeTe₂ and GeTe₄ molecular units and vibrations of Se-Te bond as Se_8−xTe_x mixed rings. The assignment of various absorption bands has been made on the basis of absorption spectra of pure Se, binary Ge-Se, Ge-Te, Se-Te and ternary Ge-Se-Te glassy alloys. The far-infrared transmission spectrum has been found to shift a little towards lower wavenumber side with the addition of Te content to Ge₁₀Se₉₀. The addition of Te to Ge-Se system replacing Se has found to reduce the Se-Se bonds and Ge-Se bonds and leads to the formation of Se-Te, Ge-Te and Te-Te bonds.     

Ac conductivity and dielectric relaxation in ionically conducting soda-lime-silicate glasses

The frequency dependent conductivity and dielectric relaxation of alkali ions in some soda-lime-silicate (Na₂O-CaO-SiO₂) glasses are investigated over a frequency range from 50 Hz to 1 MHz and in a temperature range from room temperature to 603 K by using alternating current impedance spectroscopy. The conductivity isotherms show a transition from frequency independent dc region to dispersive region where the conductivity continuously increases with increasing frequency. The electric modulus representation has been used to provide comparative analysis of the ion transport properties in these glasses. The scaling behavior of imaginary part of electric modulus indicates that all dynamical processes occurring at different frequencies give the same activation energy.     

Characterization and identification of the nature of two different kinds of secondary relaxation in one glass-former

Dielectric measurements were carried out in the van der Waals liquid di-n-octyl phthalate at ambient pressure. Two secondary relaxation processes were found in the dielectric spectra, namely, an excess wing and a γ-process at higher frequencies. It has been established that the excess wing is the universal JG relaxation. Moreover we pointed out that γ-relaxation times show a minimum in temperature dependence. A Minimal Model proposed by Dyre and Olsen was successfully applied to explain this unusual behaviour of the γ-relaxation in di-n-octyl phthalate.     

Spectroscopic parameters related to non-bridging oxygen hole centers in amorphous-SiO2

The relationship between the luminescence at 1.9 eV and the absorption bands at 2.0 eV and at 4.8 eV were investigated in a wide variety of synthetic silica samples exposed to different γ- and β-ray irradiation doses. We found that the intensities of these optical bands are linearly correlated in agreement with a model in which they are assigned to a single defect. This finding allows the determination of spectroscopic parameters related to the optical transitions efficiencies. In this case the absorption oscillator strength at 4.8 eV is ∼200 times higher than that at 2.0 eV; while the 1.9 eV luminescence quantum yield under 4.8 eV excitation is lower (by a factor ∼3) than that under 2.0 eV excitation. These results are consistent with the energetic level scheme proposed in the literature for the non-bridging oxygen hole center. Moreover, they account for the excitation → luminescence pathways occurring after UV and visible absorption.     

Effect of frit size on sintering, crystallization and electrical properties of wollastonite glass-ceramics

Wollastonite glass-ceramics were prepared through pressureless sintering. The sinterability of the prepared samples of the glassy powder in the system (SiO₂-CaO-Na₂O-Fe₂O₃-WO₃) was investigated in the temperature range 720-900 °C and soaking time of 180 min. The influence of the increase in the glass powder particle size on the sinterability and dielectric properties of the glass-ceramic samples was studied.The sintered specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods. XRD analysis revealed that wollastonite was the main crystalline phase in the sintered glass-ceramics. Dielectric properties such as dielectric constant and dielectric loss were measured via a network analyzer at 10 GHz.It was observed that the increase of the glassy powder particle size improved the sinterability and dielectric properties of the glass-ceramic specimens. Wollastonite glass-ceramics with 16 μm particle size had maximum constant and minimum loss (ε_r = 10.10 and tan δ = 0.005) compared with the other glass-ceramics.