Influence of γ irradiation on the electrical conductivity and dielectric properties of β TlInS2 in the temperature range of the incommensurate phase

The influence of γ irradiation on the electrical conductivity σ, the dielectric permittivity ɛ, and the dielectric loss tangent tanδ of β-TlInS₂ crystals is investigated in the temperature range of the incommensurate phase. It is established that γ irradiation lowers the values of ɛ and tanδ over the entire investigated temperature range and decreases the maxima of the σ(T) and tanδ(T) curves in the vicinity of the transition temperature T _c, but does not alter the transition temperature. Fiz. Tverd. Tela (St. Petersburg) 39, 1088–1090 (May 1997)     

Dielectric response of makrofol-KG polycarbonate irradiated with 145 MeV Ne6+ and 100 MeV Si8+ ions

The passage of heavy ions in a track detector polymeric material produces lattice deformations. These deformations may be in the form of latent tracks or may vanish by self annealing in time. Heavy ion irradiation produces modifications in polymers in their relevant electrical, chemical and optical properties in the form of rearrangement of bonding, cross-linking, chain scission, formation of carbon rich clusters and changes in dielectric properties etc. Modification depends on the ion, its energy and fluence and the polymeric material. In the present work, a study of the dielectric response of pristine and heavy ion irradiated Makrofol-KG polycarbonate is carried out. 40 μm thick Makrofol-KG polycarbonate films were irradiated to various fluences with Si⁸⁺ ions of 100 MeV energy from Pelletron at Inter University Accelerator Centre (IUAC), New Delhi and Ne⁶⁺ ions of 145 MeV from Variable Energy Cyclotron Centre, Kolkata. On irradiation with heavy ions dielectric constant (ɛ′) decreases with frequency where ɛ′ increases with fluence for both the ions. Variation of loss factor (tan δ) with frequency for pristine and irradiated with Si ions reveals that tan δ increases as the frequency increases. Tan δ also increases with fluence. While Ne irradiated samples tan δ shows slight variation with frequency as well as with fluence. Tan δ has positive values indicating the dominance of inductive behavior.      

Dielectric properties of perovskite ceramics of (1–x)BiFeO3 ? x(KBi)1/2TiO3 (0.40 < x < 0.85) solid solutions from impedance spectroscopy data

The dielectric properties of the ceramics of (1 − x)BiFeO₃ − x(KBi)_1/2TiO₃ (0.40 < x < 0.85) solid solutions with an orthorhombic structure have been studied using impedance spectroscopy in the frequency range 25–10⁶ Hz at different temperatures. It has been shown that these solid solutions undergo a diffuse ferroelectric phase transition. The Curie temperature is found to be in the range 620–640 K. The activation energies of dielectric polarization relaxation (δE _M ) and dc charge carriers (ΔE _dc) are determined. It has been established that, in the vicinity of 460 K, ΔE _dc increases jumpwise as the temperature increases.     

Transport and dielectric properties of thin polycrystalline CoO films

Transport and dielectric properties of polycrystalline CoO films were studied as functions of the applied field, frequency and temperature. TheI–V plots showed that the Poole-Frenkel field emission mechanism is responsible for conduction at fields>10⁵ V/cm. The ac conductivity σ(ω), the imaginary part of the dielectric constantε ₂, and tan δ plots as functions of frequency revealed three dispersion regions. The σ(ω) andε ₂ frequency dependence indicates a non-adiabatic hopping of charge carriers at low frequencies and adiabatic hopping at high frequencies. The activation energy of a dielectric oscillator is 0.15 eV. Work supported by the Office of Naval Research.     

Structural and dielectric properties of Pb(Li1/4Dy1/4W1/2)O3

The polycrystalline samples of Pb(Li_1/4Dy_1/4W_1/2)O₃ have been synthesized by high-temperature solid-state reaction techniques. Room temperature X-ray diffraction (XRD) studies of the compound provided preliminary structural data, and hence formation of a single phase desired material was confirmed. Detailed studies of dielectric constant (ε) and loss (tanδ) as a function of frequency (100 Hz to 10 kHz) at room temperature (298 K) and also as a function of temperature (liquid nitrogen to 403 K) at 10 kHz suggest that the compound undergoes a ferroelectric phase transition of diffuse type.     

Preparation and characterisation of compositionally graded BaxSr1-xTiO3 thin films

Ba_xSr_1-xTiO₃ thin films with a compositional gradient of x=0.3 to 1 (in 0.1 mole fraction increments) were fabricated on Pt/Ti/SiO₂/Si substrates using a modified sol–gel technique. The graded film crystallised in a perovskite structure and consists of a uniform microstructure with comparatively larger grains. The room-temperature relative dielectric constant (ε_r) and dielectric loss (cosδ) at 100 kHz were found to be 305 and 0.03 respectively. Dielectric peaks were not observed in the temperature range from -20 °C to 120 °C. The dielectric constant and dielectric loss were almost independent of temperature. Polarisation–electric field measurements at room temperature revealed a saturated but slim hysteresis loop with a remanent polarisation (P_r) of 0.6 μC/cm² and a coercive field (E_c) of 2.4 kV/mm. The graded film behaves as a stack of Ba_xSr_1-xTiO₃ capacitors connected in series and hence the dielectric Curie peaks are removed. Received: 4 October 2001 / Accepted: 17 October 2001 / Published online: 23 January 2002     

Dielectric, ferroelectric and piezoelectric properties of 0-3 barium titanate–Portland cement composites

In this work, barium titanate (BT) and cement composites of 0-3 connectivity were produced with BT concentrations of 30%, 50% and 70% by volume using the mixing and pressing method. The dielectric constant (ε _r ) and the dielectric loss (tan δ) at room temperature and at various frequencies (0.1–20 kHz) of the ferroelectric BT-Portland cement composites with different BT concentrations were investigated. The results show that the dielectric constant of BT-PC composites was found to increase as BT concentration increases, and that the highest value for ε _r —of 436—was obtained for a BT concentration of 70%. In addition, the dielectric loss tangent decreased with increasing BT concentration. Moreover, several mathematical models were used; the experimental values of the dielectric constants are closest to those calculated from the cube model. The 0-3 cement-based piezoelectric composites show typical ferroelectric hysteresis loops at room temperature. The instantaneous remnant polarization (P _ir ), at an applied external electrical field (E ₀) of 20 kV/cm (90 Hz) of 70% barium titanate composite, was found to have a value ≈3.42 μC/cm². Furthermore, the piezoelectric coefficient (d ₃₃) was also found to increase as BT concentration increases, as expected. The highest value for d ₃₃ was 16 pC/N for 70% BT composite.     

Permittivity and conductivity of triglycine sulfate films on Al/SiO2 and α-Al2O3 substrates

Triglycine sulfate (TGS) films have been prepared by evaporation from a saturated aqueous solution on substrates of fused quartz coated by a layer of thermally deposited aluminum (Al/SiO₂) and white sapphire (α-Al₂O₃) on whose surface interdigital electrodes have been deposited by photolithography. The TGS films have a polycrystalline structure made up of blocks measuring 0.1–0.3 mm (Al/SiO₂) and 0.1 × 1.0 mm (α-Al₂O₃). The polar axis in the blocks is mostly confined to the substrate plane. The temperature dependences of the capacitance and dielectric losses normal to and in the film plane have maxima at the temperature coinciding with that of the ferroelectric phase transition in a bulk crystal, T _c . The low-frequency conductivity G in TGS/Al/SiO₂ structures displays a frequency dispersion described by the relation G ∼ ω ^s (s ≈ 0.82). The conduction can be tentatively ascribed to the hopping mechanism involving localized carriers with a ground state energy of 0.8–0.9 eV. At temperatures above and below T _c , the low-frequency conductivity in TGS/α-Al₂O₃ films operates through a thermally-activated mechanism with an activation energy of 0.9–1.0 eV. At the phase transition, an additional contribution to conductivity appears in TGS/α-Al₂O₃ films with a dispersion G ∼ ω^0.5, which can be associated with domain-wall relaxation.     

Effect of γ irradiation on the hysteresis phenomena at the incommensurate-commensurate phase transition in Rb2ZnBr4

The effect of γ irradiation on the temperature hysteresis in dielectric permittivity ɛ and loss tangent tan δ of crystalline Rb₂ZnBr₄ has been studied in the vicinity of the incommensurate-commensurate phase transition. The ɛ(T) and tan δ(T) curves were found to exhibit anomalies in the form of maxima. Hysteresis was observed in the measured properties, including the transition temperature T _c (ΔT=T _c ^h −T _c ^c ), in both unirradiated and irradiated samples. It is shown that, as the radiation dose increases the extent of the hysteresis ΔT increases, the values of ɛ _max and tan δ _max at the transition point decrease, and the anomalies wash out. Fiz. Tverd. Tela (St. Petersburg) 40, 1911–1914 (October 1998)     

Dielectric properties of natural,modified, and irradiated zeolites

The complex permittivity (ɛ = ɛ′ − iɛ″) of natural, modified, and irradiated zeolites as a function of the frequency of a variable electric field is measured. For Ba²⁺-modified (doped) zeolite, the permittivity and dielectric loss tangent (tanδ) as functions of the electric field frequency are found to decrease monotonically. When the irradiation dose rises to 10⁵ cm⁻², the dielectric loss tangent reaches a maximum and then, starting from a dose of 3 × 10¹⁶ cm⁻², sharply drops. Such behavior of tanδ is assumed to be associated with Ba²⁺- and irradiation-induced structural modifications in the unit cell of zeolite.     

Effect of gamma irradiation on the dielectric properties and electrical conductivity of the TlInS<Subscript>2</Subscript> single crystal

The effect of gamma irradiation on the dielectric properties and ac conductivity of a TlInS₂ single crystal with a layered structure has been investigated in the frequency range from 5 × 10⁴ to 3.5 × 10⁷Hz. It has been shown that gamma irradiation of the TlInS₂ single crystal with a dose of 10⁴–2.25 × 10⁶ rad leads to a considerable increase in the dielectric loss tangent tanδ, the real part ɛ′ and imaginary part ɛ″ of the complex permittivity, and the ac conductivity σ _ac across the layers. It has been established that, for all gamma irradiation doses, the TlInS₂ single crystal is characterized by the dielectric loss due to electrical conduction up to a frequency of 10⁷ Hz and by the relaxation loss at a higher frequency. Irradiation of the TlInS₂ single crystal results in an increase in the dispersion of tan δ, ɛ′, and ɛ″. It has been demonstrated that, as the gamma irradiation dose is accumulated in the TlInS₂ single crystal, the density of localized states near the Fermi level N _F increases (from 5.2 × 10¹⁸ to 1.9 × 10¹⁹ eV⁻¹ cm⁻³).     

Equivalent circuit of silicon diodes subjected to high-fluence electron irradiation

Silicon diodes with a p ⁺-n junction made in a 48-μm-thick phosphorus-doped silicon epilayer (resistivity ρ = 30 Ω cm) grown on antimony-doped Si(111) wafers (ρ = 0.01 Ω cm) are studied. The diodes are irradiated by high-energy (3.5 MeV) electrons with fluences from 5 × 10¹⁵ to 2 × 10¹⁶ cm⁻². It is shown that the conventional equivalent circuit of the diode that consists of a parallel RC network and a series-connected resistor inadequately describes the dependence of the dielectric loss tanδ on variable current frequency f in the range 1 × 10²–3 × 10⁷ Hz. Another equivalent circuit is suggested that includes not only the capacitance and resistance of the n-base (the latter increases because radiation-induced defects are compensated for by shallow donors) but also the f dependence of the capacitance of the space-charge region, which is due to retarded charge exchange between deep-level radiation-induced defects.     

Magnetoelectric effect in cobalt ferrite-barium titanate composites and their electrical properties

CoFe₂O₄-BaTiO₃ composites were prepared using conventional ceramic double sintering process with various compositions. Presence of two phases in the composites was confirmed using X-ray diffraction. The dc resistivity and thermoemf as a function of temperature in the temperature range 300 K to 600 K were measured. Variation of dielectric constant (ɛ′) with frequency in the range 100 Hz to 1 MHz and also with temperature at a fixed frequency of 1 kHz was studied. The ac conductivity was derived from dielectric constant (ɛ′) and loss tangent (tan δ). The nature of conduction is discussed on the basis of small polaron hopping model. The static value of magnetoelectric conversion factor has been studied as a function of magnetic field.     

Theory of the oscillatory dependence of the conductivity of two-component dielectric composites at room temperatures

It is predicted that at room temperatures a hopping mechanism of charge transfer plays a very important role and leads to temperature oscillations of the conductivity σ(T) of a dielectric composite. The dependence of the conductivity σ(ω) on the frequency of an alternating electric field is calculated. The relation obtained can be used to determine, first, the electron relaxation times and, second, and more importantly, the frequency of electron tunneling through the dielectric matrix from measurements of the conductivity in various frequency ranges. Zh. Tekh. Fiz. 69, 31–34 (March 1999)     

Dielectric properties of CdS nanoparticles synthesized by soft chemical route

CdS nanoparticles have been synthesized by a chemical reaction route using thiophenol as a capping agent. The frequency-dependent dielectric dispersion of cadmium sulphide (CdS) is investigated in the temperature range of 303-413 K and in a frequency range of 50 Hz-1 MHz by impedance spectroscopy. An analysis of the complex permittivity (ɛ′ and ɛ″) and loss tangent (tan δ) with frequency is performed by assuming a distribution of relaxation times. The scaling behaviour of dielectric loss spectra suggests that the relaxation describes the same mechanism at various temperatures. The frequency-dependent electrical data are analysed in the framework of conductivity and modulus formalisms. The frequency-dependent conductivity spectra obey the power law.     

Detecting a true quantum pump effect

Reflectance measurements from p-type GaSb:Zn epitaxial films with different hole concentrations (10¹⁷–10¹⁸ cm^-3) have been investigated over the frequency region of 100–1000 cm^-1. A minimum broadening feature corresponding to the hole plasmon was observed in the reflectance spectra. The experimental infrared spectra were well fitted using a Lorentz-Drude dispersion model. The real part ε₁ of the dielectric function decreases with increasing hole concentration. However, the imaginary part ε₂ increases with hole concentration in the far-infrared region. This indicates that the acoustic- and optic-phonons mainly participate in the free carrier absorption processes. The hole mobility obtained from Hall-effect measurements is slightly larger than that derived from optical measurements and the average ratio of mobilities is estimated to be 1.33. Owing to overdamping effects, the upper branch of longitudinal-optical phonon plasmon (LPP) coupled modes was observed. The upper LPP⁺ frequency increases with hole concentration and it shows a transition from phonon-like to plasmon-like behavior. A theoretical analysis with solutions in the complex frequency plane describes these experimental results.     

Microwave dielectric tangent losses in KDP and DKDP crystals

By adding cubic and quartic phonon anharmonic interactions in the pseudospin lattice coupled mode (PLCM) model for KDP-type crystals and using double-time temperature dependent Green's function method, expressions for soft mode frequency, dielectric constant and dielectric tangent loss are obtained. Using model parameters given by Ganguliet al [9] the dielectric losses are calculated for KDP and DKDP crystals. In the microwave frequency range an increase in frequency (1–35 GHz) is followed by an increase in dielectric tangent loss (1–35) at 98 K and (1–15) × 10⁻² at 333 K for KDP and DKDP crystals respectively. The dielectric tangent loss decreases from 0.052 to 0.042 for KDP crystals with increase in temperature from 130 to 170 K and for DKDP crystals it decreases from 0.0166 to 0.0074 with an increase in temperature from 230–343 K in their paraelectric phases at 10 GHz. This shows Curie-Weiss behavior of the dielectric tangent loss     

Electrical study and dielectric relaxation behavior in?nanocrystalline Ce0.85Gd0.15O2?δ material at intermediate temperatures

The nanocrystalline material of 15 mol% Gd-doped ceria (Ce_0.85Gd_0.15O_2−δ ) was prepared by citrate auto ignition method. The electrical study and dielectric relaxation technique were applied to investigate the ionic transport process in this nanocrystalline material with an average grain size of 13 nm and the dynamic relaxation parameters are deduced in the temperature range of 300–600°C. The ionic transference number in the material is found to be 0.85 at 500°C at ambient conditions. The oxygen ionic conduction in the nanocrystalline Ce_0.85Gd_0.15O_2−δ material follows the hopping mechanism. The grain boundary relaxation is found to be associated with migration of charge carriers. The frequency spectra of modulus M″ exhibited a dielectric relaxation peak corresponding to defect associates (Gd-V_{o^{\blacksquare \blacksquare}})^{_\blacksquare}(\mathrm{Gd}\mbox{-}\mathrm{V}_{\mathrm{o}}^{_{_{{\blacksquare\,\blacksquare}}}})^{_{_{{\blacksquare}}}}. The material exhibits very low values of migration energy and association energy of the oxygen vacancies in the long-range motion, i.e., 0.84 and 0.07 eV, respectively.     

Ferroelectric relaxor behaviour in Pb(Fe0.5Ta0.5)O3

The relaxor ferroelectric lead iron tantalate, Pb(Fe_0.5Ta_0.5)O₃ (PFT) is synthesized by Coulombite precursor method. The X-ray diffraction pattern of the sample at room temperature shows a cubic phase. The field dependence of dielectric response is measured in a frequency range 0.1 kHz — 1 MHz and in a temperature range from 173–373 K. The temperature dependence of permittivity (ɛ′) shows broad maxima at various frequencies. The frequency dependence of the permittivity maximum temperature (T _m ) has been modelled using Vogel-Fulcher relation.      

Structural, optical and electrical properties of cubic AlN films deposited by laser molecular beam epitaxy

Cubic AlN films were successfully deposited on TiN buffered Si (100) substrates by a laser molecular beam epitaxy (LMBE) technique, and their crystal structure and optical and electrical properties were studied. The results indicate that cubic AlN films show the NaCl-type structure with a (200) preferred orientation, and the lattice parameter is determined to be 0.4027 nm. The Fourier transform infrared (FTIR) pattern of the cubic AlN film displays sharp absorption peaks at 668 cm⁻¹ and 951 cm⁻¹, corresponding to the transverse and longitudinal optical vibration modes. Ellipsometric measurements evidence a refractive index of 1.66–1.71 and an extinction coefficient of about zero for the cubic AlN film in the visible range. Capacitance–voltage (C–V) traces of the metal–insulator–semiconductor (MIS) device exhibit that the cubic AlN film has a dielectric constant of 8.1, and hysteresis in the C–V traces indicates a significant number of charge traps in the film.