BI CONCENTRATION DEPENDENCE OF STRUCTURAL,FERROELECTRIC AND MAGNETIC PROPERTIES OF BiFeO3FILMS

ABSTRACT

Bi-rich BiFeO₃ films were fabricated by chemical solution deposition followed by a post-deposition annealing at 823 K in air. Not only the polycrystalline BiFeO₃ phase but also the bismuth oxide phases were formed at high excess Bi contents. This suggested that the Bi atoms were not significantly evaporated. The remanent polarization decreased as the excess Bi contents increased at 90 K, though the remanent polarization of 33 μ C/cm² was still obtained at the excess Bi contents of 30 at.%. The magnetization monotonically decreased as the excess Bi content increased. It could be considered that the optimal Bi content is the stoichiometric value of BiFeO₃ in the preparing way of the CSD followed by the annealing at 823 K.     

Discrete variable stochastic approximation procedures and recursive autoregressive model identification

Two stochastic approximation procedures are proposed for finding a point attaining the maximum of a regression function defined and observable only at points on a set of discrete variables. The asymptotic convergence property of the procedures is discussed using the theorem of almost supermartingales. The procedures are applied to the recursive identification of autoregressive time series models. The identification procedure consists of a recursive order estimation stage and a recursive autoregressive parameter updating stage, and gives the true autoregressive model or the best autoregressive approximation model.     

Improvement of Data Readout Disturbance Effect in 1T2C-Type Ferroelectric Memory

The data disturbance effect in readout operation was investigated in a 1T2C-type FeRAM cell array. Based on the analysis, a method for reducing the data disturbance effect is proposed and verified by SPICE simulation as well as experimental results. Current on/off ratio of about two orders of magnitude which is sufficient to distinguish stored binary data was maintained even under the maximum disturbance condition.     

Electrical Conductivity of TiO2-V2O5-P205 Glasses

The dc conductivities (σ) of V₂O₅-P₂O₅ glasses containing up to 30 mol% TiO₂ were measured at T=100° to ∼10°C below the glass-transition temperature. Dielectric constants from 30 to 10⁶ Hz, densities, and the fraction of reduced V ion were measured at room temperature. The conduction mechanism was considered to be small polaron hopping between V ions, as previously reported for V₂O₅-P₂O₅ glass. The temperature dependence of σ was exponential with σ = σ₀ exp(-W/kT ) in the high-temperature range. When part of the P₂O₅ was replaced by TiO_2,σ increased and W decreased. The hopping energy depended on the reciprocal dielectric constant which, in this case, increased with increasing TiO₂ content.     

A dynamic economic model for an x¯-control chart design

Parkhideh and Case (HE Transactions, 21, 313-323 (1989)) developed an economic model for a dynamic jc-control chart. In developing the model they considered six decision variables in their design methodology. It therefore became very complicated to obtain the optimal combination that resulted in the minimum loss-cost. This note proposes an alternative and simplified design methodology that reduces the number of design variables from six to three. The optimal values are obtained by imposing the following constraints. The optimal sampling interval h_i, (i = 1,2,⃛) is chosen such that the integrated hazard rate over each sampling interval is constant. The optimal sample size n_i (i = 1,2,⃛) is chosen such that the relative sample size per unit time during each sampling interval is constant. Analogously, the optimal control limit coefficient k_i (i = 1,2,⃛) is chosen so that the power of the control charts remains constant over each sampling interval. The process failure mechanism is assumed to follow a Weibull shock model and the product quality characteristic is considered to be normal. Computational experience indicates that the proposed dynamic nonuniform control chart design is much simpler and provides a lower cost than that of Parkhideh and Case's dynamic model.     

Polymorphic Transformation of PbO by Isothermal Wet Ball-Milling

Mechanochemical polymorphic transformation of massicot to litharge was achieved by isothermal wet ball-milling. A first-order relation was observed between the fractional conversion and the milling time; the apparent activation energies for various milling conditions were obtained by kinetic treatments. The fact that the Arrhenius equation can be applied to the mechanochemical transformation suggests that the reaction cannot be explained by a simple local heating mechanism and that a mechanical effect has a fundamental role. The effects of the number, volume, and density of the milling balls on the rate constant agreed fairly well with those predicted by Matsui's equations for impact comminution in ball-milling, indicating that the action of the milling balls may be similar to that in impact comminution of granular materials.     

Microstructure of SiC Prepared by Chemical Vapor Deposition

Silicon carbide prepared by chemical vapor deposition at a substrate temperature of ∼1400425-429°C was investigated. Optical microscopy showed that the growth characteristic is dendritic; the deposits consist of columnar blocks within each of which the orientation of crystals is approximately the same. Small pores occur along the boundaries of these blocks. Transmission electron microscopy revealed finer details of the growth characteristics. The crystals were mostly 3 C in structure, but crystals with 2H and one-dimensionally-disordered structures were also found. The 3 C crystals are mostly dendritic, but some columnar growth with alternating twin bands occurs. Both 2 H and one-dimensionallydisordered crystals exhibit a columnar habit, reflecting the difference in crystal symmetry from 3 C . In all these crystals, the c axis (or one of the <111> axes in 3 C crystals) lies perpendicular to the substrate.