Pyroelectric Properties of PZT/PMNZTU Composite Thick Films

Lead zirconate titanate/uranium doped lead magnesium niobate—lead zirconate titanate (PZT/PMNZTU) composite thick films have been fabricated on silicon substrates at 710°C using a composite sol gel technique. A slurry, made up of PMNZTU powder and PZT sol, was spun onto a silicon substrate and fired to yield a porous skeletal ceramic structure. Subsequent sol infiltration and pyrolysis was used to modify the density of the films prior to final sintering at 710°C.Pyroelectric and dielectric properties have been measured as a function of sol infiltration. The pyroelectric coefficients (p _max = 2.41 × 10^–4 Cm^–2 K^–1) of the composite thick films were found to be comparable to the tape cast and monolithic ceramics of similar composition (2.8 and 3.0 × 10^–4 Cm^–2 K^–1 respectively).Maximum figures of merit (F _V = 2.23 × 10^–2 m² C^–1, F _D = 0.89 × 10^–5 Pa^–1/2), calculated using the electrical properties of the thick films, can be compared with those of screen printed thick films (F _V = 2.7–3.9 × 10^–2 m² C^–1, F _D = 0.8–1.1 × 10^–5 Pa^–1/2) processed at temperatures of ca 1100°C. The ability to directly integrate thick pyroelectric films onto substrates at temperatures as low as 710°C, while maintaining competitive figures-of-merit is of considerable interest for future device applications.     

Influence of Heat Treatment at 550–810°C on Inter- and Intragrain Critical Currents of Bi,Pb-2223 Ceramics

The degradation of the superconducting phase Bi_1.65Pb_0.35Sr_1.8Ca_2.2Cu₃O_y and its characteristics: electrical resistivity R, critical temperature T_c (R=0), critical current density j_c, and volume fraction of Meissner phase Cm, have been investigated at 550–810°C and P_O2=2·10¹·1·10⁵ Pa by X-ray diffraction, resistance, inductance and magnetic methods. It was established that at P_O210³ Pa degradation of the ceramic samples runs through the solid solutions decomposition. As a result, the intergrain critical current is decreased 10–20 times while the intragrain one is increased 1.5–2 times. At P_O2=10¹–10² Pa the inter- and intragrain critical currents are decreased by 5–10 times and by 1.5–2 times respectively as a result of oxygen extraction without visible destruction of the Bi,Pb-2223 structure.     

Short-echo-time magnetic resonance phase-shift velocity mapping for assessment of heart valve and great vessel stenoses: three years' experience

Shortening the echo times of magnetic resonance (MR) sequences used for phase-shift velocity mapping to 3.6 ms has extended use of the technique to measurement of velocities in turbulent, poststenotic jet flows. We used a 0.5-T MR machine and field even-echo rephasing (FEER) sequences with 3.6 ms echo times for jet velocity mapping.In vitro trials used continuous flow through a phantom with a 6-mm stenosis. Fifteen patients with mitral and/or aortic valve stenosis and 20 patients with repaired aortic coarctation were studied prospectively, with Doppler ultrasonic measurement of peak jet velocity performed independently on the same day. The clinical contribution of MR jet velocity mapping, used during a 3-year period in 306 patients with congenital and acquired disease of heart valves, great vessels, and conduits, was assessed retrospectively. The 3.6-ms sequence allowed accurate measurement of jet velocities up to 6 m s^–1 in vitro (r=0.996). Prospective studies in patients showed good agreement between MR and Doppler measurements of peak velocity:n=38; range, 1.2–6.1 m s^–1; mean, 2.7 m s^–1; mean of differences (Doppler-MR), 0.22 ms^–1; standard deviation of differences, ±0.38 m s^–1 (±14%). MR jet velocity mapping proved particularly valuable for assessment and localization of stenoses at sites where ultrasonic access was limited. The technique represents a diagnostic advance which can obviate the need for catheterization in selected cases.     

The Enhanced Dielectric and Insulating Properties of Al2O3 Modified Ta2O5 Thin Films

Future-generation memory devices will require materials with higher dielectric constants compared to conventional dielectric materials such as silicon oxide and silicon nitride. Tantalum oxide (Ta₂O₅) is one of the most promising high dielectric constant materials because of its ease of integration into conventional VLSI processes compared to other complex oxide dielectrics. The dielectric constant and thermal stability characteristics of bulk Ta₂O₅ samples were previously reported to enhance significantly through small substitutions of Al₂O₃. However, this improvement in the dielectric constant of (1 – x)Ta₂O₅-xAl₂O₃ was not clearly understood. The present research attempts to explain the higher dielectric constant of (1 – x)Ta₂O₅-xAl₂O₃ by fabricating thin films with enhanced dielectric properties. A higher dielectric constant of 42.8 was obtained for 0.9Ta₂O₅-0.1Al₂O₃ thin films compared to that reported for pure Ta₂O₅ (25–30). This increase was shown to be closely related to a-axis orientation. Pure Ta₂O₅ thin films with similar a-axis orientation also exhibited a high dielectric constant of 51.7, thus confirming the orientation effect. Systematic study of dielectric and insulating properties of (1 – x)Ta₂O₅-xAl₂O₃ thin films indicate improved leakage current properties and reliability characteristics such as temperature coefficient of capacitance and bias stability with increase in Al₂O₃ concentrations.     

Synthesis, Physicochemical Characterization and Ionic Conductivity of LaGa0.4Mg0.2M0.4O3−δ (M = Cr, Mn, Fe, Co)

Partial electronic and ionic conductivities, crystal structure, thermal expansion and infrared absorption spectra of the perovskite-type series, LaGa_0.40Mg_0.20M_0.40O_3– (M = Cr, Mn, Fe, and Co), have been studied. The rhombohedral distortion of the perovskite lattice decreases and the unit cell volume increases in the sequence Co < Cr < Mn < Fe. The p-type electronic conduction increases with atomic number of the transition metal cation; the activation energy varies in air from 15.9 to 32.1 kJ/mol. The oxygen ionic conductivity of the M-doped phases at temperatures below 1200 K is significantly lower than that of LaGa(Mg)O₃. The highest ionic conductivity was found for the Fe- and Co-containing phases. The ion transference numbers of La(Ga,Mg,M)O_3– at 970–1270 K were determined to vary in a wide range, from 2 × 10^–5 to 3 × 10^–2. Thermal expansion coefficients, calculated from the dilatometric data collected in the temperature range 300–1100 K, lie in the range (7.2–15.5) × 10^–6 K^–1.     

Mechanically-Activated Synthesis and Mixed Conductivity of TbMO4−δ (M = Zr, Hf) Ceramics

Terbium hafnate and zirconate ceramics with submicron grain sizes were prepared via mechanically-activated synthesis. X-ray and electron diffraction and infrared (IR) absorption spectroscopy showed that TbZrO_4– has a disordered fluorite-type structure, while TbHfO_4– is partially ordered, containing pyrochlore microdomains. The oxygen ion transference numbers determined by the modified e.m.f. technique under oxygen/air gradient, vary in the range 0.08–0.26 at 873–1123 K, increasing with temperature. The activation energies for ionic and p-type electronic transport are 82–83 and 29–40 kJ/mol, respectively. The ionic conduction becomes dominant in reducing atmospheres, but tends to decrease at low p(O₂). Oxygen partial pressure dependencies of Seebeck coefficient can be described by a model common for oxide phases with mixed ionic and electron-hole conductivity. Due to partial cation ordering, terbium hafnate exhibits lower ionic and hole transport as compared to TbZrO_4–. The average thermal expansion coefficients of TbMO_4– (M = Zr, Hf) ceramics in air, calculated from dilatometric data, are (11.5–12.4) × 10^–6 K^–1 at 600–1200 K and (18.4–20.3) × 10^–6 K^–1 at 1200–1420 K.     

Co-Modification of Ba5NdTi3Ta7O30 Dielectric Ceramics by Substitution and Introducing Secondary Phase

Co-modification of Ba₅NdTi₃Ta₇O₃₀ dielectrics ceramics was investigated through Pb substitution for Ba and introducing Bi₄Ti₃O₁₂ secondary phase. The dielectric constant increased from 150 to 283, the temperature coefficient of the dielectric constant decreased from –2500 ppm/°C to –1279 ppm/°C, and the dielectric loss decreased to 0.0007 at 1 MHz. Meanwhile, the bi-phase ceramics were investigated to achieve temperature stable ceramics with high dielectric constant and low dielectric loss. As the composition x varied from 0.4 to 0.7 for (1 – x)(Ba_0.8Pb_0.2)₅NdTi₃Ta₇O₃₀/xBi₄Ti₃O₁₂, the temperature coefficient of the dielectric constant changed from negative to zero to positive.     

Microstructural Features of RF-sputtered SOFC Anode and Electrolyte Materials

Thin-film samples of yttria-stabilized zirconia (YSZ), nickel oxide (NiO)-YSZ, and YSZ/nickel (Ni)-YSZ bilayer were fabricated by RF-sputtering. The single YSZ layer and YSZ/Ni-YSZ bilayer samples were annealed while the NiO-YSZ layer remained as-deposited. Cross-section transmission electron microscopy (TEM) samples of these thin-films were then prepared, which allowed detailed chemical and structural characterization of these thin-films on the nanometer-scale. Both YSZ and NiO-YSZ layers were fully dense and exhibiting equiaxed grain morphologies. Selected area electron diffraction (SAED) showed the YSZ crystal structure to be predominantly cubic in the annealed samples and amorphous in the as-deposited NiO-YSZ sample. It was found that YSZ film was 70 nm thick and dense, with equiaxed grains ranging from 12–20 nm. Surface roughness of the YSZ in the bilayer fell in the range of 5–20 nm. The Ni-YSZ film in the bilayer was 230 nm thick and porous, which consisted of columnar grains 13–75 nm in length and 9–22 nm in width. The bilayer sample showed no delamination or cracking along the YSZ/Ni-YSZ interface. It is believed that the nano-sized grains, minimal surface roughness and thin layers found in these films are desirable microstructural features for the anode and electrolyte in micro-solid oxide fuel cells (SOFCs). Correlation between microstructural features and electrochemical performance will be reported in a separate study.     

Age-dependent MRI-detected lesions at early stages of transient global ischemia in rat brain

Although ischemic stroke has higher incidence and severity in aged than in young humans, the age factor is generally neglected in ischemia animal models. This study was aimed at comparing age-dependent effects at early stages of transient global cerebral ischemia (TGCI) in rats. TGCI was induced in two groups of rats (3–6 and 20–24 months old, respectively) by exposure to 15% oxygen and 15 min occlusion of the two common carotid arteries. Brains were analysed in vivo by MRI–apparent diffusion coefficient (ADC) and T₂ maps–at 1–3 h post-TGCI and in vitro by histochemical examination of triphenyltetrazolium chloride (TTC)-stained slices. At 1–3 h post-TGCI, a higher incidence of lesions was found in aged than in young rats especially in the hippocampus and cortex (occipital plus parietal) but not in the thalamus. The lesioned regions showed lower ADC values in aged than in younger rats. The most substantial ADC decreases were associated with enhanced spin-spin relaxation and lower TTC staining. The different responses of the two age groups support the use of aged animals for investigations on different ischemia models. Our model of brain ischemia appears appropriate for further studies including drug effects.     

Poling Conditions of Pre-Stressed Piezoelectric Actuators and Their Displacement

The poling procedure has always been the key issue in producing piezoelectric actuators with optimised performance. This is also true with the relatively new category of pre-stressed bender actuators, where mechanical bias achieved with a passive layer is introduced in the actuators during manufacturing. Due to these factors, the behaviour of the actuator under poling is different compared to its bulk counterparts. In this paper, two different thicknesses of commercial PZT 5A and PZT 5H materials were used in bulk actuators and pre-stressed benders realised by new method. Pre-stress was introduced by using a post-fired biasing layer utilising sintering shrinkage and difference in thermal expansion. The hysteresis loop of the actuators was measured under 0.5–7.0 MV/m electric fields at 25–125^∘C temperatures, providing information about their remnant polarisation and coercive field before poling. The results showed that high electric field and 25^∘C temperatures in poling provided higher remnant polarisation and coercive electric field than using 125^∘C temperature at poling. Difference was especially significant in coercive electric field values where up to 114.8% difference was obtained for PZT 5H bulk actuator and 65.9% for pre-stressed actuators. Higher coercive fields can be utilized as increased operating voltage range of piezoelectric devices. The differences in results obtained here and by others can be explained by the different pre-stress level, stronger clamping of the thicker passive layers of the RAINBOW and THUNDER actuators and passive ring area introducing high tensile stresses. The same conditions were used to pole the actuators, after which the displacement and dielectric constant of the actuators were measured. The displacement measurements showed that remnant polarisation has good correlation with displacement. This fact can be used in estimating pre-stressed actuator performance before actual poling. The dielectric constant measurements with a small signal after poling gave even better correlation than the remnant polarisation.     

Measurement of Microwave Dielectric Properties of Pb(Zr1−x Tr x )O3 Thin Films

Ferroelectric Pb(Zr_1–xTi_x)O₃ (PZT) thin films were prepared by sol-gel deposition method. The structural and surface morphologies were investigated using X-ray diffractometer and atomic force microscope. Microwave dielectric properties were obtained using co-circle electrode patterns, which were made by photolithography and etching process. The dielectric constant of PZT films was about 450 from 0.05–1 GHz range.     

Ceramics Materials Based on (Ba, Sr)TiO3 Solid Solutions for Tunable Microwave Devices

Structure and electrical properties at radio frequencies as well as within the 3.5–35 GHz frequency range have been investigated for ceramic samples of the (1–y)(Ba_xSr_{1 – x})TiO₃ · yMgO (BSM) system where x = 0.4–0.6; y = 0.15–0.30. For the compositions studied the bulk ferroelectrics were synthesized with the dielectric constant of 400–600 and high tunability coefficient. We indicated that the quality factor of the samples was in the range of 100–1000 within the frequency band of 3.5–35 GHz. The phase correlations and unit cell constants of the perovskite phase of the BSM samples were studied. The low loss factor and high tunability of the bulk material allowed us using the BSM ferroelectric ceramic layer for tunable accelerating structures of the Argonne Dielectric Wakefield Accelerator and for high power switches design and development for the future linear colliders.     

PMN-PFN Relaxor Ferroelectric Ceramics by a Reaction-Sintering Process

Pb((Mg_1/3Nb_2/3)_0.6(Fe_1/2Nb_1/2)_0.4)O₃ (PMFN) perovskite relaxor ferroelectric ceramics produced by reaction-sintering process were investigated. Without any calcination, the mixture of PbO, Mg(NO₃)₂, Fe(NO₃)₃ and Nb₂O₅ was pressed into pellets and sintered directly. PMFN ceramics of 100% perovskite phase were obtained. Density of 7.84 g/cm³ (95% of theoretical value) was obtained after sintered at 1250°C for 2 h. Grain sizes of 3–6 m were formed after 2 h sintering at 1150–1250°C. Dielectric constant at room temperature under 10 kHz reaches 22400 after sintered at 1250°C for 2 h.     

P-Type Electronic Transport in Ce0.8Gd0.2O2 − δ: The Effect of Transition Metal Oxide Sintering Aids

Small (2 mol%) additions of cobalt, iron and copper oxides into Ce_0.8Gd_0.2O_{2 –} considerably improve sinterability of ceria-gadolinia (CGO) solid electrolyte, making it possible to obtain ceramics with 95–99% density and sub-micron grain sizes at 1170–1370 K. The minor dopant additions have no essential effect on the total and ionic conductivity, whilst the p-type conduction in the transition metal-containing materials at 900–1200 K is 8–30 times higher than that in pure CGO. The oxygen ion transference numbers of the Co-, Fe- and Cu-doped ceramics, determined by the modified e.m.f. technique under oxygen/air gradient, are in the range 0.89–0.99. The electron-hole contribution to the total conductivity increases with temperature, as the activation energy for ionic conduction, 78 to 82 kJ/mol, is significantly lower than that for the p-type electronic transport (139–146 kJ/mol). The results show that CGO sintered with such additions can still be used as solid electrolytes for intermediate-temperature electrochemical applications, including solid oxide fuel cells (SOFCs) operating at 770–970 K, but that increasing operation temperature is undesirable due to performance loss.     

Theoretical Study of Molecular Quantum-Dot Cellular Automata

We present an ab initio quantum chemistry study of {(η⁵-C₅H₅)Fe(η⁵-C₅H₄)}₄(η⁴-C₄)Co(η⁵-C₅H₅)²⁺, a molecular candidate for quantum-dot cellular automata (QCA) which has been synthesized and characterized recently. Our studies support the experimental interpretation that this molecule has a bistable electronic state, and that binary information can be encoded in the molecular charge configuration. We show that when molecules are properly arranged, one molecule can be switched by the Coulomb interaction produced by a neighboring molecule. Our studies also show this molecule is error-tolerant when the four-fold symmetry is broken by a C–C single-bond distortion.     

Defect Chemical Modeling of (La, Sr)(Co, Fe)O3 − δ

Different defect chemical models for calculation of ionic and electronic defect concentrations are discussed regarding their applicability to transition metal perovskite-type oxides (ABO_{3 –} ) with large ranges of oxygen non-stoichiometry. A point defect model, which allows simultaneous consideration of three different B-site species concentrations as a function of the oxygen partial pressure is compared to a simple point defect model, considering only two different B-site species. Additionally, a model assuming electrons/holes as negative resp. positive electronic charge carriers is presented. Further, models involving association of point defects in different complexes are discussed. Examples are given for fits of experimental data of La_{1 – x}Sr_xBO_{3 –} (x = 0.6, B = Fe, Co) to selected models in the temperature range 700–900_C and oxygen partial pressures 10^{– 5} < pO₂/atm < 1.     

Frequency and Voltage Dependent Dielectric Properties of Ni-doped Ba0.6Sr0.4TiO3 Thin Films

Highly (100) preferred undoped and 1–5% Ni-doped Ba_1–xSr_xTiO₃ (BST) thin films were deposited onto MgO (100) single crystal substrate at 750°C using pulsed laser deposition. BST thin film-based interdigital capacitors (IDC) were prepared by standard photolithography process. The microwave properties of BST films were measured at 10 GHz. Ni-doped BST films showed better dielectric properties by exhibiting improved dielectric Q while retaining an appropriate capacitance tuning compared to undoped BST films. 1% Ni-doped BST film showed the maximum figure of merit of 2896.1. It is suggested that 1 mol% Ni doped BST film is an effective candidate for high performance tunable device applications.     

Comparison of Three Quantum Correction Models for the Charge Density in MOS Inversion Layers

In order to obtain high density integration for MOS devices, it is necessary to reduce the gate oxide thickness and increase the substrate doping concentration. This results in a narrow and deep potential well in which electrons are confined at the semiconductor-insulator interface and it becomes necessary to take quantum mechanical (QM) effects into consideration. In this study, we compare three well established quantum correction models, i.e., the Hänsch model (Hänsch W. et al. 1989. Solid State Electronics 32(10): 839–849), the modified local density approximation (MLDA) model (Paasch G. and Ubensee H. 1982. Phys. Stat. Sol. (b) 113: 165–178), and the density-gradient (D-G) model (Ancona M.G. and Tiersten H.F. 1987. Physical Review B 35(15): 7959–7965; Ancona M.G. 1997. JTCAD 97–100) in terms of accuracy for predicting the inversion layer charge distribution.     

Early changes in apparent diffusion coefficient of rat brain following total circulatory arrest

The apparent diffusion coefficient (ADC) of rat brain was determined for the cortex [(771±23)×10^–6 m²/s] and caudate-putamen (CP) [(601±25)×10^–6 m²/s]. Using the ultrafast imaging technique U-FLARE changes in ADC were followed with a 2.4-min temporal resolution after the induction of total circulatory arrest by intravenous KC1 injection. For both tissue types, a biphasic decrease of ADC was observed. The initial fast phase led to an ADC decrease by (27±4)% (cortex) and (29±3)% (CP) within 5 min, whereas the slow continuous decrease of the second phase resulted in (68±3)% (cortex) and (66±3)% (CP) of control after 18 min. The similar relative reduction in ADC for the cortex and the CP meant that an effective distinction between both tissue types persisted after the cessation of systemic and cerebral blood flow.Address for correspondence: Max-Planck Institut für neurologische Forschung, Gleuelerstrasse 50, D-50931 Köln, Germany. Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Venn Plaza, New York, NY 10119.     

The Effects of Lithium and Oxygen Contents Inducing Capacity Loss of the LiMn2O4 Obtained at High Synthetic Temperature

Well-crystallized LiMn₂O₄ has been synthesized at different calcination temperatures using the melt-impregnation method. The lattice constant of LiMn₂O₄ increased with increasing calcination temperatures. Li/LiMn₂O₄cells calcined at lower temperatures (700–800°C) showed excellent cycling performances at room temperature. However, those cells calcined at higher temperature (850–900°C) exhibited abrupt capacity loss in the early stage and very poor cycle retention rate (>65%) after 50 cycles. It was considered that poor cycle performance of the spinels obtained at high temperature resulted from the lithium sublimation and oxygen deficiency during synthetic process. We found that above two factors, lithium sublimation and oxygen deficiency, were the commonly important factors to induce capacity loss in the Li/LiMn₂O₄ system, especially obtained at high synthetic temperature.