Kilho Lee

Domain structures and piezoelectric properties in epitaxial Pb(Zr0.35,Ti0.65)O3 thin films

The ferroelastic twin-domain structures and piezoelectric properties were investigated in epitaxial Pb(Zr0.35,Ti0.65)O3 thin films with various c-domain volume fractions by metal organic chemical vapor deposition on (100)cSrRuO3∕(100)SrTiO3 substrates. The domain structures could be characterized into three regions: a-domain dominant, c-domain dominant, and their mixed domain regions. The mixed domain structure is quite unusual but formed a bilayer with two regions of different a∕c domain proportions: the bottom layer is the a1-a2 domain structure and top layer is the a-c domain structure. The films of the mixed region have attained largest piezoelectric coefficients, d33, presumably due to enhanced a-to-c domain switching.

Two-dimensional planar size effects in epitaxial PbTiO3 thin films

In order to test a critical lateral dimension in two-dimensional (2D) planar ferroelectrics, epitaxial PbTiO3 thin films are patterned into discrete islands lithographically with different lateral sizes. As the pattern size decreases, the substrate clamping effect is significantly reduced and thus the misfit strain in the films could be relaxed further. Evolution of 90° domain structures as a function of lateral dimensions was characterized extensively by reciprocal space mapping using synchrotron x ray. As the lateral 2D planar size decreases in the PbTiO3 patterns on MgO(001), some of the a domains turned into c domains due to the relaxed tensile strain. In the PbTiO3 patterns on Pt(001)∕MgO(001), on the other hand, the formation of 90° domains is enhanced by the reduction in compressive misfit strain. Equilibrium domain structures in the PbTiO3 thin film islands are also analyzed by the finite element simulation and found to be consistent with the experimental observation.

Morphotropic phase boundary in epitaxial Pb(Zr,Ti)O3 thin films: Two-dimensional planar size effect

The epitaxial Pb(Zr,Ti)O3 (PZT) thin films were patterned into small islands by a photolithographic process, and the morphotropic phase boundary (MPB) in which the tetragonal and rhombohedral phases coexist was realized by changing the two-dimensional planar sizes. The MPB moved toward the Zr-rich composition due to two-dimensional film stress in the continuous epitaxial PZT thin films. The 600‐nm-thick epitaxial Pb(Zr0.55Ti0.45)O3 and Pb(Zr0.60Ti0.40)O3 films were found highly rich in tetragonal phase. As the films were patterned progressively in smaller sizes and the misfit strains were relaxed further accordingly, the MPB was restored to the original bulk composition, showing higher fractions of rhombohedral phase. In the epitaxial Pb(Zr0.55Ti0.45)O3 film, two-dimensional patterns of 100×100, 10×10, and 5×5μm2 were found within the MPB region, whereas the smallest 2×2μm2 patterns were fully rhombohedral. On the other hand, in the epitaxial Pb(Zr0.60Ti0.40)O3 film, only the 100×100μm2 patterns belonged ...

Study of single-phase bidirectional battery charger for high power application

This paper presents a thorough scope of a wide-used single-phase bidirectional battery charger which composes of a pulse-width modulation (PWM) converter in the front-end and a dual active bridge (DAB) converter for high power applications. The computer simulations of PWM converter and DAB converter are separately shown to support theoretical analyses. A prototype converter was built and tested at different operating conditions. Experiment results at 3.3 kW are given to verify theory and simulation results as well.

Finite element analysis of domain structures in epitaxial PbTiO3 thin films

Equilibrium domain structures commonly observed in epitaxial Pb-based ferroelectric thin films are analyzed by the finite element method (FEM) using a commercial package, ABAQUS (Hibbit, Karlsson & Sorensen, Inc., 1080 Main Street, Pawtucket, RI 02860-4847). Structures of periodic 90° domains in epitaxial PbTiO3 thin films on cubic single crystalline substrates are analyzed as a function of decreasing temperature in order to simulate cooling process after the film deposition at elevated temperature (TG). The degree of c-axis orientation (α) is determined as a function of temperature below the Curie temperature and compared to the experimental results. It is then possible to calculate the magnitude of misfit strain during film growth and its relaxation due to dislocation generation. The effect of PZT composition on c-domain abundance is also simulated. FEM simulation is performed with the assumption that the major driving force for such domain formation is thermoelastic strains arising from the film–substr...

Fluctuating Current Control Method for a PMSM Along Constant Torque Contours

The dc-link voltage fluctuation problem is more serious for an inverter system which utilizes a single phase ac source. As an effort to reduce the dc-link capacitance, there is a tendency to make torque oscillate in synchronism with the input power. Then, the fluctuating motor power matches the converter power. The key idea is to enforce current oscillation along a constant torque line which only affects the reactive power of the motor. In other words, the proposed method uses the motor as inductive energy storage by controlling the motor flux by the reactive current component. Analytic method for obtaining the current bounds was derived. Also, a practical recursive algorithm was proposed. Simulation and experimental results support the effectiveness of the proposed method.

Direct observation of Cr magnetic order in CoCrTa and CoCrPt thin films

Magnetic circular dichroism measurements of room temperature, sputter deposited Co86Cr12Ta2 and Co86Cr12Pt2 films were performed to investigate the local magnetic ordering of the Co and Cr atoms. The results demonstrate that the Cr has a net magnetic moment and that a small fraction of the Cr is magnetically oriented opposite to the Co moment.

Ferroelectric Domain Structures in Epitaxial PZT Thin Films: A Review

Heteroepitaxial growth of thin films of non-cubic ferroelectric materials on cubic single crystal substrates inevitably accompanies twin domain formation. Since the presence of twin domains strongly affects on the ferroelectric properties, studies on the twinning phenomena are crucial for controlling their properties. Ferroelectric twin-domain formation in epitaxial Pb(Zr,Ti)O 3 thin films has been investigated by changing several processing parameters such as film composition, substrate selection, and film thickness in order to determine the crucial stain factors controlling final domain structures. In order to investigate the effect of transformation strains involved in the phase transition at and below the Curie temperature, epitaxial (Pb 1 m x La x )(Zr y Ti 1 m y )O 3 thin films grown on MgO(001) have been examined as a function of composition, x and y, and temperature. The effect of misfit strain and its relaxation during film fabrication on the final domain structures of epitaxial PbTiO 3 was studied by changing cubic substrates including MgO(001), SrTiO 3 (001), and KTaO 3 (001). In particular, considering thermodynamic equilibrium relief of coherency strain at the initial growth stage based on the Mattews-Blakeslee criteria, effect of film thickness on domain formation was analyzed for the epitaxial PbTiO 3 thin films grown on MgO(001) substrates, and the correlation of final domain structures with the degree of misfit strain relaxation by dislocation generation was established. Finally, evolution of equilibrium domain structures in epitaxial PZT thin films was simulated successfully by the finite element method employing a commercial package, ABAQUS, and the strains un-relaxed during film fabrication could be reasonably estimated.

Domain Engineering of Epitaxial PbTiO3 Thin Films by the Control of Misfit Strain

Ferroelectric domain structures and their evolution in the epitaxial PbTiO3 thin films are greatly affected by the misfit strain and its relaxation during epitaxial film deposition on single crystalline MgO(001) substrates with epitaxial Pt electrode interlayer. The misfit strain could be controlled by varying the underlying interlayer thickness. As the thickness of Pt electrode decreases from 120 nm to 12 nm, the effective lattice constant of Pt electrode increases due to the suppressed generation of misfit dislocations at the Pt(001)/MgO(001) interface. Consequently, the compressive misfit strain in PbTiO3 thin films decreases due to the decreased lattice mismatch with Pt electrode, which enhances the formation of 90° domains. Equilibrium domain structures in the epitaxial thin films are also analyzed by the finite element simulation and found to be consistent with the experimental observation. The results manifest that the domain structure and evolution of the epitaxial PbTiO3 thin films could be engineered by the control of misfit strain.

Fluctuating current control for a PMSM under reduced DC link capacitance

The DC link voltage fluctuation problem is more serious for an inverter system which utilizes a single phase AC source. As an effort to reduce the DC link capacitance, the fluctuating motor power control is proposed to match the converter power. Specifically, the key idea is to enforce current oscillation along a constant torque line which only affects the reactive power of the motor. Analytic method for obtaining the current bounds was derived. Also, a practical recursive algorithm was proposed. Simulation and experimental results support the effectiveness of the proposed method.