Hiroki Matsuo

Structural and piezoelectric properties of high-density (Bi0.5K0.5)TiO3-BiFeO3 ceramics

The crystal structures and dielectric, polarization, and piezoelectric properties of high-density x(Bi0.5K0.5)TiO3–(1−x)BiFeO3 ceramics were investigated. The results obtained using x-ray and neutron powder diffractions and transmission electron microscopy showed that a morphotropic phase boundary between the rhombohedral (ferroelectric) and pseudocubic (ferroelectric) phases is present in 0.4<x<0.43. Ceramics with x=0.4 exhibited a large remanent polarization of 52 μC/cm2 at 25 °C and their piezoelectric properties were maintained up to 300 °C. It is suggested that the presence of nanosized domains with a polar rhombohedral structure observed for x=0.4 is the origin of relaxor-like dielectric properties.

Japanese first double lunar swingby mission “Hiten”

Abstract The paper discusses the Japanese first lunar mission named “Hiten”. The spacecraft system, its major mission objectives, namely, the double lunar swingby experiments and the insertion of a miniaturized sub-satellite into an orbit around the moon, are presented. The results of these engineering missions, and other experiments such as optical navigation, packet telemetry by use of a fault tolerant computer and detection of micro-meteorite particles in space of the Earth-moon system, are also described. A couple of follow-on missions, which may be conducted if the Hiten spacecraft will survive the aerobraking experiments at the end of its original 1 year mission life, are discussed. These follow-on missions include an excursion to the Lagrange points of the Earth-moon system and orbiting the Hiten spacecraft itself around the moon.

Dynamics and control of flexible multibody systems: Part II: simulation code and parametric studies with nonlinear control

Abstract This paper describes a rather general numerical simulation code newly developed for studying dynamics and control of flexible multibody systems. It accounts for an arbitrary level of topological branching, character of the structural members, slewing, deployment, orbital perturbations, etc. The program is based on a recursive order n formulation whose details are given in Part I. Versatility and validity of the code are illustrated through parametric studies of several distinctively different systems. The dynamical analysis indicates that critical situations can lead to an undesirable system response. To establish specified performance, feasibility of a nonlinear control strategy based on the feedback linearization technique (FLT) in conjunction with the order n algorithm is explored. The FLT can control both the rigid and flexible degrees of freedom. The effectiveness of the control is assessed through numerical simulation.

On making use of lunar and solar gravity assists in LUNAR-A, PLANET-B missions

Abstract ISAS (the Institute of Space and Astronautical Science, Japan) is currently planning to launch the LUNAR-A spacecraft to the Moon in 1997 and the PLANET-B spacecraft toward Mars in 1998. Since these two spacecraft have been facing mass budget hurdles, ISAS have been studying how to make good use of lunar and solar gravity effects in order to increase the scientific payload as much as possible. In the LUNAR-A mission, the current orbital sequence uses one lunar swingby via which the spacecraft can be thrown toward the SOI (sphere of influence) boundary for the purpose of acquiring solar gravity assist. This sequence enables the approach velocity to the Moon to be diminished drastically. In the PLANET-B mission, use of lunar and solar gravity assist can help in boosting the increase in velocity and saving the amount of fuel. The sequence discussed here involves two lunar swingbys to accelerate spacecraft enough to exceed the escape velocity. This paper focuses its attention on how such gravity assist trajectories are designed and stresses the significance of such utilization in both missions.

DYNAMICS AND CONTROL OF FLEXIBLE MULTIBODY SYSTEMS: PART I: GENERAL FORMULATION WITH AN ORDER N FORWARD DYNAMICS

Abstract This paper presents a rather general recursive formulation of flexible multibody systems. It accounts for an arbitrary level of topological branching, character of the structural members, slewing, deployment, orbital perturbations, etc. By introducing the concept of equivalent disconnected system, it leads to an efficient order n algorithm. Validity and efficiency of the formulation are assessed through several numerical examples of flexible multibody systems, which show high accuracy in terms of system energy and angular momentum conservation. A simple and unified methodology for extending the formulation to systems in general topology is also given. A companion paper describes a numerical simulation code which has been developed based upon the formulation, and illustrates versatility of the approach through parametric studies of several particular systems.

Switchable diode-effect mechanism in ferroelectric BiFeO3 thin film capacitors

We investigate the mechanism of a switchable diode behavior observed in ferroelectric SrRuO3/BiFeO3 (BFO)/SrRuO3 capacitors. We experimentally demonstrate that the switchable diode effect observed in the capacitors is induced by the polarization reversal in the BFO film. The conductivity in an Ohmic region in different oxidation states provides direct evidence that electron hole acts as the majority carrier, delivering p-type conduction. Density functional theory (DFT) calculations show that the p-type conduction arises from an unoccupied gap state of Fe4+ in an FeO5 pyramid which is derived from Bi vacancy. Our experimental and DFT study leads to the conclusion that the switchable diode effect originates from an asymmetric band bending in the top and bottom depletion layers modulated by ferroelectric polarization and oxygen vacancies.

Microstructures Related to Ferroelectric Properties in (Bi0.5K0.5)TiO3–BiFeO3

Crystal structures and microstructures associated with ferroelectric properties in a lead-free piezoelectric compound, x(Bi0.5K0.5)TiO3–(1-x)BiFeO3, have been investigated mainly by transmission electron microscopy (TEM). Complicated ferroelectric domain structures with a width of approximately ~200 nm were found in the polar rhombohedral structure of the x = 0.30 compound. On the other hand, a tweed-like contrast, which stems from local lattice distortion, was observed in the pseudocubic structure of the x = 0.40, 0.46, and 0.50 compounds. The present results suggest that the pseudocubic phase is characterized by nanodomain structures consisting of non-polar cubic and polar rhombohedral structures.

An approach to dynamics and control of flexible systems

This paper presents a rather general r~cursive formulation of flexible niultibody sys ten~s It accormts for an arbitrary level of branching, character of the structural members, orbital perturbations, etc , and introducing a concept of equivalent disconnected system, it easily leads to an efficient order 11 algorithm A numerical simulation code based upon the formulation has been developed, which showed high accln-acy in terms of sys ten~ energy and ang~dar moment~un conse rva t io~~ Versatility of tllc approach is illr~stratcd t h o u g h paran~etric studies of several particular sysvectors ( column mahices ) matrix which is equivalent to the vector product operatio11 E x ; Eq.( l4) a E matrix of elements where Si, b are vectors. a b value of n a t b 0 f o r c ~ w.r t 1,lw i~tertial frame inboard body nunlhw of body 1 set of body nu~r~bcr s o~~t,boart l to botly T set of level I body n u ~ r ~ b e r s mass generalized coordinate vect,or ~lonholono~nic velocity vect,or velocity vector after constraint relative velocity vector of body i w.r.t. the body ii, fixed frame; Eq.(12) velocit,y vrctor for tlte rq~~ iva . I~ ,~ l l discotnlected sys te~n; 15q.(34) position vector to a Itlass cdcmr~lt w.r.L. tllc illertial framc position vector to the ~lorniml ccnt.c\r of rnass w.r.t. the inertial frame Graduate student, University of Tokyo t Professor t Piofessor, Fellow AIAA Copyright 0 1 9 9 4 by the Amet-ican lnstitrttr of nlat,ricf,s transpose unit ~na t r ix forces other than the generalized force; Eq ( 7 ~ ) total force; Eq.(7d) total force after cor~straint; Eq.(lOb) n ~ a x i n ~ u ~ n bra ch level mass matrix; Eq. (7a) mass matrix aft,er constraint,; Eq (IOa) mass matrix of the equivalent discon~ t ( ~ t , c t i sys t e~n ; Eqs.(36) arid (37) nrmthrr of bodies ge~leralizcd force; Eq (7b)

Cooperative effect of oxygen-vacancy-rich layer and ferroelectric polarization on photovoltaic properties in BiFeO3 thin film capacitors

Photovoltaic (PV) properties of ferroelectric SrRuO3/BiFeO3/SrRuO3 (SRO/BFO/SRO) epitaxial thin-film capacitors are investigated. The experimental results of the markedly reduced PV response caused by the doping of Mn provide evidence that the PV properties originate not from the bulk-derived PV effect but from an interfacial band bending. We show that the capacitors having a defective layer composed of oxygen vacancies at the SRO/BFO interface exhibit a markedly large photocurrent. Our study demonstrates that a cooperative effect between the interface charges arising from ferroelectric polarization and the oxygen-vacancy-rich layer enhances the PV response in capacitor form in the BFO system.

Gap-state engineering of visible-light-active ferroelectrics for photovoltaic applications

Photoferroelectrics offer unique opportunities to explore light energy conversion based on their polarization-driven carrier separation and above-bandgap voltages. The problem associated with the wide bandgap of ferroelectric oxides, i.e., the vanishingly small photoresponse under visible light, has been overcome partly by bandgap tuning, but the narrowing of the bandgap is, in principle, accompanied by a substantial loss of ferroelectric polarization. In this article, we report an approach, ‘gap-state’ engineering, to produce photoferroelectrics, in which defect states within the bandgap act as a scaffold for photogeneration. Our first-principles calculations and single-domain thin-film experiments of BiFeO3 demonstrate that gap states half-filled with electrons can enhance not only photocurrents but also photovoltages over a broad photon-energy range that is different from intermediate bands in present semiconductor-based solar cells. Our approach opens a promising route to the material design of visible-light-active ferroelectrics without sacrificing spontaneous polarization.Overcoming the optical transparency of wide bandgap of ferroelectric oxides by narrowing its bandgap tends to result in a loss of polarization. By utilizing defect states within the bandgap, Matsuo et al. report visible-light-active ferroelectrics without sacrificing polarization.