Electrical and thermal properties of lead titanate glass ceramics

Glass samples with composition of (50−X)PbO-(25+X)TiO₂-25B₂O₃ (where X=0, 5, 10 and 12.5 mol%) were prepared using conventional quenching technique. The glass transition temperature, T_g and crystallization temperature T_c were determined from the DTA. These glass samples were converted to glass ceramics by following two stage heat treatment schedule. The glass ceramic samples were characterized by XRD, SEM and dielectric constant measurements. The XRD results revealed the formation of ferroelectric lead titanate (PT) as a major crystalline phase in the glass ceramics. The density increases and the CTE decreases for all glass ceramics with increase in X (mol%). This may be attributed to increase in PT phase. The SEM results which show rounded crystallites of lead titanate, also supports other results. Hysteresis loops observed at room temperature confirms the ferroelectric nature of glass ceramics. The optimized glass ceramic sample exhibits high dielectric constant which is of technical importance.     

Phase change in CoTi2 induced by MeV electron irradiation

The phase change induced by MeV electron irradiation in the intermetallic compound E9₃–CoTi₂ was investigated using high-voltage electron microscopy. Under MeV electron irradiation, CoTi₂ was first transformed into an amorphous phase and, with continued irradiation, crystallite formation in the amorphous phase (i.e. formation of crystallites of a solid-solution phase within the amorphous phase) was induced. The critical temperature for amorphisation was around 250 K. The total dose (dpa) required for crystallite formation (i.e. that required for partial crystallisation) was high (i.e. 27–80 dpa) and, even after prolonged irradiation, the amorphous phase was retained in the irradiated sample. Such partial crystallisation behaviour of amorphous Co₃₃Ti₆₇ was clearly different from the crystallisation behaviour (i.e. amorphous-to-solid solution, polymorphous transformation) of amorphous Cr₆₇Ti₃₃ reported in the literature. A possible cause of the difference is discussed.     

Room-temperature pulsed laser deposition and dielectric properties of amorphous Bi3.95Er0.05Ti3O12 thin films on conductive substrates

Bi_3.95Er_0.05Ti₃O₁₂ (BErT) thin films were prepared on Pt/Ti/SiO₂/Si and indium-tin-oxide (ITO)-coated glass substrates at room temperature by pulsed laser deposition. These thin films were amorphous with uniform thickness. Excellent dielectric characteristics have been confirmed. The amorphous BErT thin films deposited on the Pt/Ti/SiO₂/Si and ITO-coated glass substrates exhibited almost the same dielectric constant of 52 with a low dielectric loss of less than 0.02 at 1 kHz. Meanwhile, the dielectric properties of the thin films had an excellent bias voltage stability and thermal stability. The amorphous BErT thin films might have potential applications in microelectronic and optoelectronic devices.     

Why, when, and how fast innovations are adopted

Structural and dielectric properties of Potassium Lithium Niobate polycrystalline ceramic K₃Li₂Nb₅O₁₅ (KLN), having the tetragonal tungsten bronze (TTB) ?C type structure are studied in the temperature interval 50?550?°C. Special emphasis is given to the diffuse phase transition occurring around 440?°C. Space charge polarization, relaxation phenomena and free charge conductivity have been elucidated using impedance spectroscopy technique. Argand plots have revealed a non Debye and polydispersive type relaxation. In paraelectric phase the Arrhenius activation energy E _?? ?=?0.533?eV was determined. The structural and dielectric results are compared with two others TTB compounds derived from KLN family: Pb_1.85K_1.15Li_0.15Nb₅O₁₅ (PKLN) and GdK₂Nb₅O₁₅ (GKN).     

Nonlinear dielectric properties of planar structures based on ferroelectric betaine phosphite films

Ferroelectric films of partly deuterated betaine phosphite are grown on NdGaO₃(001) substrates with an interdigitated system of electrodes on their surfaces by evaporation at room temperature. These films have a high capacitance in the ferroelectric phase transition range. The dielectric nonlinearity of the grown structures is studied in small-signal and strong-signal response modes and in the intermediate region between these two modes by measuring the capacitance in a dc bias field, dielectric hysteresis loops, and the Fourier spectra of an output signal in the Sawyer-Tower circuit. In the phase transition range, the capacitance control ratio at a bias voltage U _bias = 40 V is K ? 7. The dielectric nonlinearity of the structures in the paraelectric phase is described by the Landau theory of second-order phase transitions. The additional contribution to the nonlinearity in the ferroelectric phase is related to the motion of domain walls and manifests itself when the input signal amplitude is higher than U _st ～ 0.7–1.0 V. The relaxation times of domain walls are determined from an analysis of the frequency dependences of the dielectric hysteresis.     

Revisiting the Dielectric Relaxation of Ethylene-Vinylacetate Copolymers: Influence of Microstructure

The dielectric relaxation behavior of a series of ethylene-vinylacetate (EVA) copolymers was investigated by measuring the complex dielectric permittivity in a broad frequency and temperature range. Crystallinity of EVA copolymers was estimated by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). The shape of the higher temperature relaxation, appearing above the glass transition temperature T _g depends on the VA content. It was found that this relaxation was asymmetric for VA concentrations higher than 40 wt% and changed to a symmetric shape at lower VA values. Concurrently, as the VA content decreased, a major broadening of the relaxation over a wide frequency range was observed. It is found that the dielectric relaxation was preserved on going through the melting range of the semicrystalline samples, although it exhibited changes of its characteristic parameters that are typical for segmental relaxation appearing at T _g. This finding allows one to associate this relaxation to the segmental motions at T _g in the amorphous phase and not to the existence of interfacial regions.     

A. C. behaviour and dielectric relaxation in indium oxide films

Vacuum deposited blackish indium oxide films (In-O) as well as the oxidised films (In₂O₃) were studied for their a.c. behaviour at different temperatures and at various film thicknesses in the audio frequency region. ε of In-O films was thickness dependent and also showed dielectric relaxation at lower frequencies due to the dipolar orientation arising from their non-stoichiometric nature. However at liquid nitrogen temperature region ε was thickness independent similar to the oxidised films which neither showed any relaxation effect nor any thickness dependent ε. The results have been discussed from the classical theory of dielectric polarisation.     

dielectric relaxation in VDF<Subscript>60</Subscript>/Tr<Subscript>40</Subscript> and VDF<Subscript>88</Subscript>/Te<Subscript>12</Subscript> polar copolymers embedded in porous glass matrices

The dielectric properties of composite materials prepared by embedding P(VDF₆₀/Tr₄₀) and P(VDF₈₈/Te₁₂) polar copolymers in porous glass matrices with a mean flow-through pore diameter of around 320 nm were investigated in the temperature range 200–450 K. Strong dielectric relaxation, the characteristic time of which conformed to the Williams-Landel-Ferry law, was observed in the vicinity of glass transition point T _g of an amorphous fraction of polymer inclusions. An increase (≈10 K) in the T _g temperature of the amorphous fraction of incorporated polymeric materials was detected.     

Structural characterisation and thermal stability of SnSe\GaSb stacked films

We have investigated the effect of thermal annealing on the structure of single and stacked phase change memory films based on SnSe and GaSb. Samples were prepared by pulsed laser deposition and investigated by X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) methods. Electrical resistance versus temperature investigations showed crystallisation temperatures of 292°C and 198°C for SnSe and GaSb single films, respectively. Above the transition temperature, GaSb crystallises into a face-centered cubic structure, whereas SnSe has an orthorhombic arrangement. Annealing at three temperatures (160°C, 250°C and 350°C) of the SnSe\GaSb stacked films promotes bond breaking, atom diffusion between the two layers and formation of new phases. At 160°C, GaSb films crystallise partially and no effect is observed on the crystallinity of SnSe films. After 250°C, rhombohedral SnSb emerges in addition to GaSb complete crystallisation. A major, completely new, body-centered orthorhombic unindexed quaternary Ga-Sn-Sb-Se phase formation was observed in the samples annealed at 350°C. The GaSb crystallites are fully dissolved and we have observed the formation of a minor hexagonal SnSe₂ phase. The analysis of EXAFS data, measured at Se and Ga K-edges, revealed changes in the local atomic environment as a function of the annealing temperature. A tetrahedral configuration is obtained for the Ga atoms in both as-deposited and annealed samples, whereas Se is mostly bivalent in the amorphous samples and has an octahedral arrangement in crystalline SnSe. Our results show that inter-layer diffusion should always be considered and evaluated when designing memory cells composed of stacked phase change chalcogenide films.     

Coexistence of glass and ferroelectric order in deuterated betaine phosphate0.05 betaine phosphite0.95 crystals

The dielectric spectra of ferroelectric hydrogen bonded betaine phosphate_0.05 betaine phosphite_0.95 (DBP_0.05DBPI_0.95) was investigated in the very wide temperature (300–20 K) and frequency (20–35 GHz) regions. The dielectric dispersion was analyzed in terms of distribution of relaxation times, using Tichonov regularization method. Strongly asymmetric and broad distribution of relaxation times below ferroelectric phase transition temperature T _c?≈?253 K clearly differs from the one that is usually observed in ferroelectrics. The observed disorder in deuterons system close to ferroelectric phase transition temperature is an embryo of coexistence ferroelectric order and dipolar glass disorder observed at low temperatures.     

Structural relaxation of Zr60Al15Ni25 amorphous ribbon: experimental evidence of the electrical resistivity

In order to study the structural relaxation of the Zr₆₀Al₁₅Ni₂₅ amorphous ribbon, the electrical resistivity was experimentally investigated. The changes in the resistivity before glass transition temperature were observed. Two temperature points (423?K and 573?K) were chosen for cyclic heating experiments. The results showed that both irreversibility and reversibility of structural changes existed in amorphous alloys, which were related to the selected temperature and cycle times. Based on the scattering mechanism of electron conduction in metal, the structural defects model was used to explain the changes of electrical resistivity. The sample was in a highly disorder state after experienced thermal cycling of high temperature (573?K). The number and kind of atoms may be changed to increase the crystallisation range.     

AC conductivity and relaxation behavior in ion conducting polymer nanocomposite

We report the ac conductivity and relaxation behavior analysis for a heterogeneous polymer–clay nanocomposite (PNC) having composition (polyacrylonitrile)₈LiCF₃SO₃ + x wt.% dodecylamine modified montmorillonite. Charge transport behavior in an ionically conducting PNC has been analyzed systematically and correlated with the macroscopic parameters like polymer glass transition temperature and available free mobile charge carriers. Intercalation of cation coordinated polymer into the nanometric clay channels has been confirmed by high-resolution transmission electron microscopy. The electrical properties of the intercalated PNC films have been studied using complex impedance/admittance spectroscopy. Excellent correlation of relaxation behavior with polymer glass transition temperature (T _g) confirmed the objectives of the work. An analysis of dielectric relaxation indicates that PNC films are lossy when compared with polymer–salt film. This result is a direct outcome of faster ion dynamics leading to strong electrode polarization effect due to the accumulation of charge carriers at the interface.     

Ti-doping induced antiferroelectric to ferroelectric phase transition and electrical properties in Sm-PbZrO3 thin films

The antiferroelectric (Pb_0.985Sm_0.01) (Zr_1-xTi_x)O₃ (Ti-PSZO) thin films were synthesized on Pt(111)/Ti/SiO₂/Si substrates using a chemical solution deposition method. The films were crystallized in the perovskite phase with a preferential orientation along (111) direction. With Ti doping in PSZO, a gradual transformation from antiferroelectric to ferroelectric phase transition was noticed at room temperature owing to the Ti doping induced lattice distortion. The phase transition has been confirmed through the P - E hysteresis loops, X-ray diffraction (peak shifting), capacitance-voltage measurements, and Raman scattering analysis. The thin film with Ti = 0.15 doping displayed a ferroelectric behavior with high dielectric constant and large dielectric tunability of about 62%. Also, Ti doping altered the Curie temperature (T_c) and enhanced the order of dielectric diffuseness. It is believed that Ti-doping in PSZO is an effective way to induce an antiferroelectric - ferroelectric phase transition and to tailor the electrical characteristics of PSZO thin films.     

Mechanism and modelling of aluminium nanoparticle oxidation coupled with crystallisation of amorphous Al2O3 shell

The oxidation of aluminium nanoparticles coupled with crystallisation of amorphous alumina shell is investigated through the thermogravimetric analyser and differential scanning calorimetry (TGA-DSC) and the transmission electron microscope (TEM). The thermogravimetric (TG) curves show stepwise shapes with temperature increase and could be divided into four stages. The reaction at the second stage is complex, including the simultaneous crystallisation of amorphous alumina (am-Al₂O₃) and Al oxidation. The crystallisation of am-Al₂O₃ promotes the reaction through generating fast diffusion channels, like micro-cracks and grain boundaries in the oxide shell to accelerate the ionic diffusion. An enhancement factor (f_react), which follows a power-law formula with the crystallisation rate, is introduced to quantify the impact of crystallisation on reaction. With heating rate increase, the second stage of TG curves shifts to the high temperature regime and the total weight gain at the second stage decreases slowly. A crystallisation-reaction model is constructed to fit and predict the weight gain after derivation of diffusivities and crystallisation kinetics. Modelling indicates that with heating rate rise, the mass increment at the second stage of TG curves decreases owing to the reduced reaction time, although the reaction is accelerated. The shift of TG curve to higher temperature is due to the polymorphic phase transition. Actually the derived kinetics of the crystallisation of amorphous alumina indicates that the polymorphic phase transformation mechanism works mainly below the heating rate of 3 K s^–1. At higher heating rate, the melting of Al takes place firstly and the crystallisation of am-Al₂O₃ follows to enhance the ionic diffusion. Therefore, when the heating rate is fast during ignition or combustion, the Al nanoparticles undergo both the melting of Al and the polymorphic phase transition of am-Al₂O₃ to accelerate the reaction.     

Molecular dynamics in thin films of isotactic poly(methyl methacrylate)

The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a shift of the glass transition temperature T _g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T _g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of T _g with decreasing film thickness. The findings concerning the different thickness dependences of T _g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T _g shifts incorporates recently developed models to describe the glass transition in thin polymer films. Received 12 August 2001 and Received in final form 16 November 2001     

Structural-phase state of chromium-fullerite-chromium films subjected to heat treatment in vacuum

The patterns of change in the structure and the phase composition of chromium-fullerite-chromium films subjected to heat treatment in vacuum at different temperatures (470, 570, and 620 K) are studied via scanning electron and atomic force microscopies, X-ray diffraction, reflection-electron diffraction, and X-ray spectral microanalysis. It is found that sample annealing at 470 and 570 K leads to recrystallization of the fullerite and the emergence and growth of a new Cr _x C₆₀ phase, accompanied by volumetric changes that generate internal mechanical stresses. During the process of stress relaxation, the film starts to crack, and lamellar crystallites of the fullerite phase start to grow. Film destruction and the growth of C₆₀ crystallites on the surface are not observed in chromium-fullerite-chromium films annealed at 620 K.     

Leakage current, ferroelectric and structural properties in Pb1−xBaxTiO3 thin films prepared by chemical route

Single-phase perovskite structure Pb_1−xBa_xTiO₃ thin films (x=0.30, 0.50 and 0.70) were deposited on Pt/Ti/SiO₂/Si substrates by the spin-coating technique. The dielectric study reveals that the thin films undergo a diffuse type ferroelectric phase transition, which shows a broad peak. An increase of the diffusivity degree with the increasing Barium contents was observed, and it was associated to a grain decrease in the studied composition range. The temperature dependence of the phonon frequencies was used to characterize the phase transition temperatures. Raman modes persist above tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of breakdown of the local cubic symmetry by chemical disorder. The absence of a well-defined transition temperature and the presence of broad bands in some interval temperature above FE-PE phase transition temperature suggested a diffuse type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. The leakage current density of the PBT thin films was studied at different temperatures and the data follow the Schottky emission model. Through this analysis the Schottky barrier height values 0.75, 0.53 and 0.34 eV were obtained to the PBT70, PBT50 and PBT30 thin films, respectively.     

Glass transition and dynamics of single and stacked thin films of poly(2-chlorostyrene)

The glass transition temperature and the dynamics of the α-process have been investigated using dielectric relaxation spectroscopy for single and stacked thin films of poly(2-chlorostyrene) (P2CS). The stacked film consists of 10 layers of single thin films with thickness of 12 nm or 18 nm. The glass transition temperature T _g of the single thin films of P2CS is found to decrease with decreasing film thickness in a similar way as observed for polystyrene thin films. The magnitude of the depression of T _g for the stacked thin films is larger than that of the single thin films with corresponding thickness. The depression of the temperature at which the dielectric loss shows a peak due to the α-process at a given frequency, T _α, is larger than that of the single thin films, although the magnitude is smaller than that of T _g . Annealing at a high temperature could cause the T _g and T _α of the stacked thin films to approach the values of the bulk system.     

Anomalous properties of crystals with ferroelectric phase transition induced by off-center ions

It is given the theoretical study of some properties of strongly polarizable dielectric crystals in which off-center impurity ions induce ferroelectric phase transition. The spontaneous polarization, transition temperature, soft mode frequency, dielectric susceptibility, ultrasonic attenuation, nuclear spin-lattice relaxation are analyzed. The theory explains observed in K_1?xLi_xTaO₃ saturation of remanent polarization with off-center Li⁺ concentration increasing, close to $\text{x}$ dependence of phase transition temperature, the anisotropy of ultrasonic attenuation, the absence of anomalies of Li nuclear spin-lattice relaxation rate near T_c.     

Dielectric relaxation of poly(ethylenglycol)- b-poly(propylenglycol)-b-poly(ethylenglycol) copolymers above the glass transition temperature

The complex dielectric permittivity has been measured for three poly(ethylenglycol)-b-poly(propylenglycol)-b-poly(ethylenglycol) copolymers with different content of poly(ethylenglycol) (15%, 33% and 80%), and increasing degree of crystallinity (0%, 10% and 20%, respectively). Only the non-crystalline sample shows the normal mode relaxation together with the segmental (α-relaxation) and the Johari-Goldstein (β-relaxation) modes. The crystalline samples show also polarization contributions due to the existence of interfaces between the crystallites and the amorphous phase. The relaxation times of the (α and normal modes can be described by a VFT equation with the same value of T₀. There is a slowing-down of the segmental mode due to the presence of crystallites. The temperature dependence of the α and β relaxations in the copolymers is very similar to that found in pure PPG, while there are significant differences in the case of the normal mode of the non-crystalline sample. The size of the cooperatively rearranging regions CRR, and the width of the glass transition region increase slightly with the degree of crystallinity. The temperature dependence of the size of CRRs is compatible with the prediction of fluctuation theory. No systematic effect of the degree of crystallinity on the β-relaxation has been found. Near T _g the β-relaxation time is close to the primitive time of the coupling model. Received: 31 May 2000