Studies on structural and electrical properties of barium strontium titanate thin films developed by metallo-organic decomposition

Thin films of barium strontium titanate (BST) including BaTiO3 and SrTiO3 end members were deposited using the metallo-organic decomposition (MOD) technique. Processing parameters such as nonstoichiometry, annealing temperature and time, film thickness and doping concentration were correlated with the structural and electrical properties of the films. A random polycrystalline structure was observed for all MOD films under the processing conditions in this study. The microstructures of the films showed multi-grains structure through the film thickness. A dielectric constant of 563 was observed for (Ba0.7Sr0.3)TiO3 films rapid thermal annealed at 750 degrees C for 60 s. The dielectric constant increased with annealing temperature and film thickness, while the dielectric constant could reach the bulk values for thicknesses as thin as similar to 0.3 mu m. Nonstoichiometry and doping in the films resulted in a lowering of the dielectric constant. For near-stoichiometric films, a small dielectric dispersion obeying the Curie-von Schweidler type dielectric response was observed. This behavior may be attributed to the presence of the high density of disordered grain boundaries. All MOD processed films showed trap-distributed space-charge limited conduction (SCLC) behavior with slope of similar to 7.5-10 regardless of the chemistry and processing parameter due to the presence of main boundaries through the film thickness. The grain boundaries masked the effect of donor-doping, so that all films showed distributed-trap SCLC behavior without discrete-traps. Donor-doping could significantly improve the time-dependent dielectric breakdown behavior of BST thin films, mostly likely due to the lower oxygen vacancy concentration resulted from donor-doping. From the results of charge storage density, leakage current and time-dependent dielectric breakdown behavior, BST thin films are found to be promising candidates for 64 and 256Mb ULSI DRAM applications. (C) 1997 Elsevier Science S.A.

The role of the interface in foam-bearing glass-epoxy composites subjected to impact loading

foam, either stacked together as three layers (MC) or inserted at three different positions (3L) while arranging the stacking sequence during the fabrication of glass fiber-epoxy composites, form the subject of investigation. This stacking variation resulted in a different interfacial area between these foam materials and the glass-epoxy regions in the laminates. This area in designed to be maximum for the 3L variety. The energy of impact being high enough to cause development of the crack in the samples, how the change in interfacial area affects the traverse of the crack front and the failure feature of the laminated composite are reported in the form of photomacrographs in this work. The results point to significant changes for the impact data, like for instance the peak load attained by the different samples, through thickness crack propagation and tensile fracture features on the non-impacted end for the plain variety, separation about the mid-zone for the MC laminates and two or more layer separations for the 3L variety. The separation for the foam-bearing systems occur invariably at the interface and here again one of the (two identical) interfaces only is chosen for the separation.

Influence of foam/glass-epoxy interface on the macroscopic compressive failure features

The effect on the macroscopic compressive failure features of introduction of two flexible foam layers, either together at mid-region or separately at two locations that are away from the midregion, into a glass-epoxy (G-E) system is studied in this work. In this experimental approach an attempt to look at the possible influence the foam/G-E interface region has on the way the materials respond to compressive loading is made by involving an analyses of macrofractographic features. While foam-free samples fail by extensive ear formation and separation nearer to the mid-region, the foam bearing ones display pronounced interface separation. The positioning of the foam sheet(s) has a bearing on the failure features.

Local environmental structures of Mo in the superionic conducting glass (AgI)(0.6)(Ag2MoO4)(0.4) by anomalous X-ray scattering

The anomalous X-ray scattering (AXS) method using Mo K absorption edges has been employed for obtaining the local structural information of superionic conducting glass having the composition (AgI)(0.6)(Ag2MoO4)(0.4). The possible atomic arrangements in the near-neighbor region of this glass were estimated by coupling the results with the least-squares variational analysis so as to reproduce the differential intensity profile for Mo as well as the ordinary scattering profile. The coordination number of oxygen around Mo is found to be about 4 at the distance of 0.180 mn. This implies that the most probable structural entity in the glass is the MoO4 tetrahedral unit which has been proposed based on infrared spectroscopy. The value of the coordination number of I- around Ag+ is estimated as 4.4 at 0.287 nm, suggesting an arrangement similar to that of crystalline or molten AgI.

Structural, dielectric and optical properties of lithium borate-bismuth tungstate glass-ceramicsc

Glasses in the system (1 - x)Li2B4O7-xBi(2)WO(6) (0.1 less than or equal to x less than or equal to 0.35) were prepared by splat quenching technique. Powder X-ray diffraction (XRD) and differential thermal analysis (DTA) were employed to characterize the as-quenched glasses. High-resolution transmission electron microscopy (HR TEM) revealed the presence of fine, nearly spherical crystallites of Bi2WO6 varying from 1.5 to 20 nm in size, depending on x in the as-quenched glasses. The glasses (corresponding to x = 0.3) heat-treated at 723 K for 6 h gave rise to a clear crystalline phase of Bi2WO6 embedded in the Li2B4O7 glass matrix, as observed by X-ray studies. The dielectric constants of the as-quenched glasses as well as the glass-ceramics decreased with increase in frequency (40Hz-100 kHz) at 300 K, and the value obtained for the glass-ceramic (x = 0.2) is in agreement with the values predicted using Maxwell's model and the logarithmic mixture rule. The dielectric constants for both the as-quenched glass and the glass-ceramic increased with increase in temperature (300 - 873 K) and exhibited anomalies close to the onset of the crystallization temperature of the host glass matrix. The optical transmission properties:of these glass-ceramics were found to be compositional dependant. (C) 2000 Elsevier Science Ltd.

Studies on the variations in compression strengths of graphite powders bearing glass-epoxy composites exposed to humid conditions

The moisture absorption and changes in compression strengths in glass-epoxy (G-E composites without and with discrete quantities of graphite powders introduced into the resin mix prior to its spreading on specific glass fabric (layers) during the lay-up (stacking) sequence forms the subject matter of this report. The results point to higher moisture absorption for graphite bearing specimens. The strengths of graphite-free coupons show a continuous decrease, while the filler bearing ones show an initial rise followed by a drop for larger exposure times. Scanning Fractographic features were examined for an understanding of the process. The observations were explained invoking the effect of matrix plasticizing and the role of interfacial regions.

Embrittlement of a bulk metallic glass due to low-temperature annealing

Embrittlement of a bulk La-based metallic glass due to isothermal and isochronal annealing below the T-g was investigated. Results show that the impact toughness decreases with increasing annealing time or temperature, accompanied by a change in fracture morphology. Reasons for this are discussed in terms of structural relaxation. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Evolution of ferroelectric SrBi2Nb2O9 phase in the Li2B4O7-SrO-Bi2O3-Nb2O5 glass system

Glasses of various compositions in the system (100 - x)Li-2 B-4 O-7 - x (SrO-Bi2O3-Nb2O5) (10 less than or equal to x less than or equal to 60) (in molar ratio) were prepared via a conventional melt-quenching technique. The glassy nature of the as-quenched samples was established by Differential Thermal Analyses (DTA). X-ray powder diffraction (XRD) and Transmission Electron Microscopic (TEM) studies confirmed the amorphous nature of the as quenched and crystallinity in the heat-treated samples. The formation of nanocrystalline layered perovskite SrBi2Nb2O9 (SBN) phase, in the samples heat-treated at temperatures higher than 550degreesC, through an intermediate fluorite phase in the LBO glass matrix was confirmed by both the XRD and High Resolution Transmission Electron Microscopy (HRTEM). The samples that were heat-treated at two different temperatures, 550 and 625degreesC, (containing 0.35 and 0.47 mum sized SBN crystallites) exhibited broad dielectric anomalies in the vicinity of ferroelectric to paraelectric transition temperature of the parent SBN ceramics. A downward shift in the phase transition temperature was observed with decreasing crystallite size of SBN. The observation of pyroelectric and ferroelectric properties for the present samples confirmed their polar nature.

Lithium borate-strontium bismuth tantalate glass nanocomposite: a novel material for nonlinear optic and ferroelectric applications

Glass nanocomposites in the system (100 - x)Li2B4O7-xSrBi(2)Ta(2)O(9) (0 less than or equal to x less than or equal to 22.5, in molar ratio) were fabricated via a melt quenching technique followed by controlled heat-treatment. The as-quenched samples were confirmed to be glassy and amorphous by differential thermal analysis (DTA) and X-ray powder diffraction (XRD) techniques, respectively. The phase formation and crystallite size of the heat-treated samples (glass nanocomposites) were monitored by XRD and transmission electron microscopy (TEM). The relative permittivities (epsilon(tau)') of the glass nanocomposites for different compositions were found to lie in between that of the parent host glass (Li2B4O7) and strontium bismuth tantalate (SBT) ceramic in the frequency range 100 Hz-40 MHz at 300 K, whereas the dielectric loss (D) of the glass nanocomposite was less than that of both the parent phases. Among the various dielectric models employed to predict the effective relative permittivity of the glass nanocomposite, the one obtained using the Maxwell's model was in good agreement with the experimentally observed value. Impedance analysis was employed to rationalize the electrical behavior of the glasses and glass nanocomposites. The pyroelectric response of the glasses and glass nanocomposites was monitored as a function of temperature and the pyroelectric coefficient for glass and glass nanocomposite (x = 20) at 300 K were 27 muC m(-2) K-1 and 53 muC m(-2) K-1, respectively. The ferroelectric behavior of these glass nanocomposites was established by P vs. E hysteresis loop studies. The remnant polarization (P-r) of the glass nanocomposite increases with increase in SBT content. The coercive field (E-c) and P-r for the glass nanocomposite (x = 20) were 727 V cm(-1) and 0.527 muC cm(-2), respectively. The optical transmission properties of these glass nanocomposites were found to be composition dependent. The refractive index (n = 1.722), optical polarizability (am = 1.266 6 10 23 cm 3) and third-order nonlinear optical susceptibility (x(3) = 3.046 6 10(-21) cm(3)) of the glass nanocomposite (x = 15) were larger than those of the as-quenched glass. Second harmonic generation (SHG) was observed in transparent glass nanocomposites and the d(eff) for the glass nanocomposite (x = 20) was found to be 0.373 pm V-1.

A chemical approach to understand fragilities of glass-forming liquids

Fragility is viewed as a measure of the loss of rigidity of a glass structure above its glass transition temperature. It is attributed to the weakness of directional bonding and to the presence of a high density of low-energy configurational states. An a priori fragility function of electronegativities and bond distances is proposed which quite remarkably reproduces the entire range of reported fragilities and demonstrates that the fragility of a melt is indeed encrypted in the chemistry of the parent material. It has also been shown that the use of fragility-modified activation barriers in the Arrhenius function account for the whole gamut of viscosity behavior of liquids. It is shown that fragility can be a universal scaling parameter to collapse all viscosity curves on to a master plot.

Pyroelectric, ferroelectric and optical properties of glass nanocomposite: Lithium borate-bismuth tungstate

Glass nanocomposites in the system (1-x)Li2B4O7-xBi(2)WO(6) (0 less than or equal to x less than or equal to 0.35, in molar ratio) were fabricated by splat quenching technique. The as-quenched samples were X-ray amorphous. Differential Thermal Analyses (DTA) confirmed their glassy nature. The composites on heat-treatment at 720 K yielded monophasic crystalline bismuth tungstate in lithium borate glass matrix. The average size and the spherical nature of the dispersed crystallites were assessed via High Resolution Transmission Electron Microscopy (HRTEM). The dielectric constants (epsilon(r)) of both the as-quenched and post heat-treated composites were found to increase with increase in x (bismuth tungstate content) at all the frequencies (100 Hz-40 MHz) in the temperature range 300 K-870 K. While the dielectric loss (D) decreased with increasing x. The pyroelectric coefficients of the as-quenched (consisting 20 nm sized crystallites) and 720 K heat-treated sample (x = 0.3) were determined as a function of temperature (300 K-873 K) and the values obtained at room temperature were 20 and 60 muC/m(2) K respectively. The as-quenched and heat-treated (720 K) glass nanocomposites exhibited ferroelectric (P Vs E) hysteresis loops. The remnant polarization and coercive field of the heat-treated glass nanocomposite at 300 K were respectively 2.597 muC/cm(2) and 543 V/cm. These glass nanocomposites were birefringent in the 300-873 K temperature range.

Characterization of lithium borate–bismuth tungstate glasses and glass-ceramics by impedance spectroscopy

Transparent glasses in the system (1−x)Li2B4O7–xBi2WO6 (0≤x≤0.35) were prepared via melt quenching technique. Differential thermal analysis was employed to characterize the as-quenched glasses. Glass-ceramics with high optical transparency were obtained by controlled heat-treatment of the glasses at 720 K for 6 h. The amorphous nature of the as-quenched glass and crystallinity of glass-ceramics were confirmed by X-ray powder diffraction studies. High resolution transmission electron microscopy (HRTEM) shows the presence of nearly spherical nanocrystallites of Bi2WO6 in Li2B4O7 glass matrix. Capacitance and dielectric loss measurements were carried out as a function of temperature (300–870 K) in the frequency range 100 Hz–40 MHz. Impedance spectroscopy employed to rationalize the electrical behavior of glasses and glass-ceramics suggest the coexistence of electronic and ionic conduction in these materials. The thermal activation energies for the electronic conduction and ionic conduction were also estimated based on the Arrhenius plots.