3D IMAGING OF INVERTED DOMAIN STRUCTURES BY CONFOCAL SHG INTERFERENCE MICROSCOPE

ABSTRACT

A novel nonlinear optical interference microscope which enables us to take 3D images of ferroelectric domain structures is developed. This microscope is of a transmission type equipped with the confocal optical system, and exploits, as a probe, second harmonic waves generated in a sample by an incident laser beam. We apply the microscope to the observation of periodically poled domain structure in a LiTaO₃ quasi-phase matching device.     

SELF-ASSEMBLY OF BISMUTH OXIDE NANOSTRUCTURE USING THE MOD METHOD

ABSTRACT

δ-Bi₂O₃ has attracted extensive attention in recent years for its high ionic conductivity especially at high temperatures. The bismuth sesquioxide can serve as the potential top electrodes of the Bi-containing ferroelectric thin films. In order to investigate the property of self-patterning Bi₂O₃, the self-assembled bismuth oxide was fabricated on Pt/Ti/SiO₂/Si substrates and the Bi_3.25La_0.75Ti₃O₁₂ thin films. The method we used here was spin coating the Bi-containing precursors on the substrates and then annealing in the oxygen atmosphere to get the self-patterning Bi₂O₃ particles. To avoid the affection of the substrate to the largest extent, the substrates were annealed first for different time in oxygen atmosphere in order to select appropriate conditions. Scanning probe microscope (SPM), scanning electron microscope (SEM) and the x-ray diffraction (XRD) were used to detect the morphology and the structure of the particles mentioned above. It showed that the change from separated particles to continuous thin films occurred at the precursor's concentration of 0.008 mol/l.     

Electrical and optical characterisation of aluminium nitride piezoelectric films on silicon nitride membranes

Aluminium nitride (AlN) is a thin film piezoelectric material having excellent potential for integration with microelectronic systems. We have investigated flexural modes of Si₃N₄ membrane structures with and without an AlN active layer. AlN films typically 3 μm thick were deposited by RF sputtering. Mechanical excitation was provided acoustically by sweeping the excitation frequency of a 1 MHz air-coupled ultrasonic transducer. Mode shapes were verified by scanning laser vibrometry up to the [3,3] mode, in the frequency range 100 kHz to 1 MHz. Resonant frequencies were identified at the predicted values provided the tension in the layers could be estimated. For a membrane structure incorporating an AlN layer, acoustic and electrical excitation of flexural modes was confirmed by displacement measurements using laser vibrometry and resonant frequencies were compared with analytical calculations.     

Investigation of ferroelectric domains and structural twins in barium titanate crystal

Abstract

The Ferroelectric domain patterns and the {111} twin boundaries are frequently observed in the BaTiO₃ ceramics. The ferroelectric domain structures in BaTiO₃ single crystal were investigated by a conventional and a synchrotron white beam X-ray topographies. The domain structure of {111} twinned crystal sample was also observed under a polarizing microscope and under a TEM. The relation between the {111} twin and the ferroelectric domain structure was verified by insitu observation of the behaviors of 90° domains during the phase transitions. It is observed that the 90° ferroelectric domain boundaries around the {111} twin boundary which was produced during the crystal growth obey the symmetry relation of the {111} twinning and that they meet each other coherently at the same position of the {111} twin boundary.     

EFFECT OF SILICON CARBIDE PASSIVATION FILM ON THE RESONANCE CHARACTERISTICS OF CANTILEVER

ABSTRACT

We have used silicon carbide (SiC) thin films as an insulating material of the PZT micro cantilevers for electrical and biological passivation. The use of SiC thin films as a passivation layer of the PZT microcantilevers is also seemingly viable to insure the high mass sensitivity as well as the stable passivation. In this study, we report the effect of SiC passivation layer on the performance of the PZT microcantilevers. The micromachined PZT microcantilevers having a structure of SiN_x/Ta/Pt/PZT/Pt were fabricated through MEMS processes. In order to improve the mass sensitivity and the passivation, SiC thin films of the high elasticity material were deposited on the cantilever using plasma enhanced chemical vapor deposition (PECVD) at the temperature of 400°C. Plane-strain modulus of SiC thin film was measured by nanoindentation. We observed that SiC thin films showed higher Young's modulus than Si and SiO₂. Before and after the deposition of SiC thin films, the end-tip deflection and the resonant frequency change of microcantilevers were measured by a confocal microscope and an impedance analyzer. It was confirmed that end-tip deflection of microcantilever was reduced by 13～18% through the deposition of SiC thin films, indicating the stress relaxation of the microcantilevers.     

GHZ polarization dynamics in ferroelectric thin films

Abstract

We present results from experiments which measure the local dielectric response of ferroelectric thin films driven by microwave-frequency electric fields. The repetition rate of a mode-locked Ti:Sapphire laser is used to generate a microwave drive signal that is phase-locked to an optical probe pulse and applied to the ferroelectric thin film. The induced polarization change in the ferroelectric film is measured stroboscopically via the electro-optic effect. Polarization images are acquired by scanning the laser beam across the sample in a confocal geometry. Time resolution is achieved by changing the delay between the electrical pump and the optical probe. Initial results show large local phase shifts in the ferroelectric response of closely separated regions of a Ba_0.5Sr_0.5TiO₃ thin film. This new experimental technique may help to understand the physical mechanisms of dielectric loss in these materials.     

Electrically tunable Ag(Ta,Nb)O3 thin film structures on oxide substrates

Abstract

We report on design, fabrication, and comparative test of three different types of voltage-variable interdigital capacitors made on ferroelectric Ag(Ta,Nb)O₃ films deposited on MgO and A1₂O₃ substrates. X-ray diffraction patterns show that ATN films pulsed laser deposited on MgO(001) and Al₂O₃(0112) single crystals have preferential (00/) and (0kk) orientation. Capacitance and loss tangent in interdigital capacitors were measured as the functions of frequency and applied dc voltage bias. Loss tangent was as low as 0.0025 and 0.0034 and AT-factor (tunability/tanδ) was around 26.2 and 20.0 for MgO and A1₂O₃, respectively, @ ± 40 V (maximum electric field 200 kV/cm), 300 K, and 1 MHz. Both of polarization and steady leakage currents were observed in the current-time domain measurements. 0.1 pF interdigital capacitors have pA leakage current level @ ± 40 V.     

Synthesis and Luminescence Properties of Eu3+ Doped Sr2SiO4 Phosphor

Abstract

The europium doped Sr₂SiO₄ phosphors were prepared by the combustion synthesis technique. The prepared samples of europium doped Sr₂SiO₄ phosphors were characterized by the X-Ray Diffraction (XRD), Ultraviolet Visible spectroscopy (UV), Fourier Transform Infrared Spectroscopy (FT-IR), Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive Spectra (EDS) and Photoluminescence Technique (PL). The orthorhombic crystal structure of the prepared sample was confirmed by using XRD. The formation of fiber like nano structured nature was confirmed by the images captured using the FE-SEM technique. The band gap energies were calculated using the UV-Visible spectra of the samples and these band gap energies were observed as 4.5826?eV for Sr₂SiO₄ and 4.1748?eV for Eu (5?m%) doped Sr₂SiO₄. The two different PL emission peaks were observed for two different excitation wavelengths. One peak was observed at the 590?nm under 393?nm excitation and another peak was observed at the 615?nm under 408?nm excitation. The CIE color coordinates of the Eu³⁺ doped Sr₂SiO₄ phosphors are x?≈?0.6615, y?≈?0.3382 (red color) observed for 408?nm excitation and x?≈?0.5636, y?≈?0.4356 (orange) observed for 393?nm excitation calculated using the color calculator program radiant imaging.     

Design and analysis of a novel SAW sensor for civil structural health monitoring

This paper presents a novel surface acoustic wave sensor for civil structural health monitoring. The sensing chip packaged in a shell is composed of two SAW delay line dice. One is utilized to sense the pressure induced by a gauge, and the other is used to sense the surrounding temperature for temperature compensation. Each SAW delay line die consists of an inter-digital transducer and two open grating reflectors. The SAW devices are analyzed by using coupling of modes model. Then, the devices in different parameters are fabricated on a 128°Y-X cut LiNbO₃ substrate by lift-off technique. Finally, a network analyzer is utilized to test the devices. Large signal-to-noise ratio, low loss and clear sharp peaks of the reflection coefficients S₁₁ in frequency and time domain can be observed. The experimental results matched well with the theoretical analysis by the finite element software COMSOL. The IDT finger pairs number, electrodes number of per reflector and acoustic aperture effect the reflection coefficient S₁₁ is discussed in detail.     

High frequency response of Bragg reflector type film bulk acoustic wave resonator

Abstract

We fabricated a Bragg reflector type FBAR using AlN piezoelectric with quarter wavelength thickness, where the Bragg reflector was composed of W-SiO₂ pairs. By numerical simulation considering actual acoustic losses of each layer, we analyzed the frequency response of the resonator and could explain it using an equivalent circuit with parasitic elements. The Effective electromechanical coupling constant (K ² _eff ) and the Quality factor (Q_s ), figures of merit of the resonator, were about 1.1% and 307, respectively.</ab>     

Microwave frequency conversion in coplanar waveguide on bilayered substrate with (Ba,Sr)TiO3 film

Abstract

Experimental and simulated spectra of microwave (30GHz) signal at the output of the coplanar waveguide (CPW) based on Ba_xSr_1-xTiO₃ film are presented and discussed. The modulation of Ba_xSr_1-xTiO₃ film dielectric constant by the application of high frequency (f ≈ 20MHz) control signal results in the generation of side frequency components in the output signal spectrum. The analysis of experimental data is performed on the basis of considering the CPW section with tunable effective dielectric constant as a phase modulator     

Silent BOLD imaging

Pulsed magnetic field gradients in magnetic resonance imaging produce high levels of acoustic noise. In functional magnetic resonance imaging, acoustic scanner noise causes unwanted masking effects. Recently, we proposed a method to perform magnetic resonance imaging experiments undisturbed by acoustic scanner noise by utilizing the property of standard gradient coils to poorly submit acoustic noise in the low frequency range. The silent gradient scheme is now incorporated into a standard T ₂ ^* -weighted sequence. Additionally, simultaneous multi-slice excitation (SIMEX) pulses were implemented to improve the intrinsic low volume coverage of the silent sequence. The proposed silent SI M EX technique was tested and compared with a standard noisy technique using auditory and visual stimulation paradigms. The scanner noise during the silent experiments could be reduced below the range of the ambient noise of the magnet room. This feasibility study shows a trend of decreased activated areas in the noisy experiment for both, the visual and auditory paradigm.     

DEVELOPMENT OF SELF-SENSITIVE PIEZOELECTRIC CANTILEVER UTILIZING PZT THIN FILM DEPOSITED ON SOI WAFER

ABSTRACT

Piezoelectric micro cantilevers were applied to a resonant type mass sensor, biosensor, vapor sensor and probe for scanning force microscopes. These devices utilize impedance, phase, and charge as output signals. However, the measurement system could be greatly simplified if an output voltage were utilized. Therefore, a self-sensitive piezoelectric cantilever with separated electrode for actuation and sensing was developed. The self-sensitive piezoelectric cantilever was fabricated from a multilayer of Pt/Ti/PZT/Pt/Ti/SiO₂/SOI through MEMS bulk micromachining. The cantilever was oscillated by a sweep sine wave and the output voltage was measured as a function of the input frequency. The resonant frequency determined from the output voltage spectrum agrees well with that measured by a laser Doppler vibrometer.     

Study of abnormal transient current of PMN-32%PT ferroelectric single crystal in time domain and frequency domain

A special test circuit was designed to measure the transient current in time domain. The measurement was performed on 0.68Pb(Mg_1/3Nb_2/3)O₃–0.32PbTiO₃ (PMN-32%PT) single crystal. Abnormal transient current was observed in time domain. Two oscillations were observed in transient current. By means of discrete Fourier transform (DFT), two peaks were found in frequency domain, which were corresponded to the resonance frequencies measured by frequency domain method. This phenomenon can be attributed to piezoelectric resonance. The transient current was calculated and fitted well based on a piezoelectric resonance equivalent electrical circuit model. Experimental results confirmed that no oscillation can be observed in PMN-32%PT single crystal samples after thermal annealing, which also verified that the piezoelectric resonance was the origin of the abnormal transient current.     

ELECTRICAL PROPERTIES OF LEAD FERROTANTALATE CERAMICS

ABSTRACT

Perovskite structures with high dielectric constants and magnetic properties play an important role in micro- and optoelectronics and have numerous practical applications. Relaxor type ferroelectric ceramics PbFe_1/2Ta_1/2O₃ with perovskite structure was subject of present studies. Pyrochlore free lead ferrotantalate ceramics were produced by solid state technology from oxides. Low- and infra-low frequency studies of dielectric properties of PbFe_1/2Ta_1/2O₃ ceramics have revealed considerable infra-low frequency dispersion at temperatures above the temperature of maximum dielectric permittivity. The observed dispersion described by lemniscates is due to a considerable Maxwell-Wagner relaxation.     

White-Light-Emitting Single Component Metal-Organic Frameworks Based on 2,3-Pyridinedicarboxylic Acid

Abstract

Two white-light-emitting MOFs, [Dy(pydc)·(H₂O)₂]·DMF_0.66 (1) and [Gd_1.9Dy_0.1(pydc)_1.5(H₂O)₃]·DMF_1.5?C460 (2), were synthesized by the reaction of a rigid ligands H₂pydc (H₂pydc = 2,3-pyridinedicarboxylic acid) and Dy(III) ions under solvothermal conditions. The emission color of compound 1 is tunable by variation of excitation light to get yellow-to-white photoluminescence. When proportion of Dy³⁺ and H₂pydc is 1:3 could obtain the single component compound 1 with CIE coordinates of (0.2935, 0.3335), which is close to (0.3333, 0.3333) of the pure white light. To investigate the matrixes effect and luminescent dye C460@MOF, Gd(III) ions and C460 were introduced to form MOFs 2 with the mixed metal centers. When concentration of C460 is 10^?3 mol/L, MOFs 2 shows CIE coordinates of (0.3255, 0.3452). This work provides a practical strategy for the development of white-light emitting phosphor based on single component MOFs, which opens a new region for WLEDs.     

Design and Verification of VFTO Differentiating-Integrating Measurement Equivalent Circuit

Abstract

Operation of isolating switches and circuit breakers in fully enclosed gas insulated substations (GIS) produces very fast transient overvoltage (VFTO). According to IEC61321-1, VFTO contains four frequency components with frequencies ranging from 0.1?MHz to 100?MHz. Therefore, the VFTO measurement system needs to have good transient response and steady state response and puts high requirements on the measurement frequency band. As a measurement method that does not need to modified GIS, the differentiating-integrating measurement system has the characteristics of high frequency bandwidth and high voltage divider ratio. In this paper, the theoretical derivation and numerical calculation of the equivalent circuit of differentiating-integrating measurement are carried out, and the simulation of time domain, frequency domain and synthetic VFTO waveform as excitation source was constructed, and its performance is fully verified, which proves that this system can meet the needs of VFTO measurement.     

A comparison of phase imaging and quantitative susceptibility mapping in the imaging of multiple sclerosis lesions at ultrahigh field

Objective

The aim of this study was to compare the use of high-resolution phase and QSM images acquired at ultra-high field in the investigation of multiple sclerosis (MS) lesions with peripheral rings, and to discuss their usefulness for drawing inferences about underlying tissue composition.

Materials and methods

Thirty-nine Subjects were scanned at 7 T, using 3D T ₂*-weighted and T ₁-weighted sequences. Phase images were then unwrapped and filtered, and quantitative susceptibility maps were generated using a thresholded k-space division method. Lesions were compared visually and using a 1D profiling algorithm.

Results

Lesions displaying peripheral rings in the phase images were identified in 10 of the 39 subjects. Dipolar projections were apparent in the phase images outside of the extent of several of these lesions; however, QSM images showed peripheral rings without such projections. These projections appeared ring-like in a small number of phase images where no ring was observed in QSM. 1D profiles of six well-isolated example lesions showed that QSM contrast corresponds more closely to the magnitude images than phase contrast.

Conclusions

Phase images contain dipolar projections, which confounds their use in the investigation of tissue composition in MS lesions. Quantitative susceptibility maps correct these projections, providing insight into the composition of MS lesions showing peripheral rings.

     

DIELECTRIC BEHAVIOUR OF A NANOGRAIN PMN POWDERS

ABSTRACT

Dielectric permittivity of 150 nm mean grain size Pb(Mn_1/3Nb_2/3)O₃ (PMN) nanopowder has been investigated by dielectric spectroscopy in the 20 Hz–1 MHz frequency range and 100 K–400 K temperature range. The broad and diffused dielectric dispersion has been observed, but the dispersion region and the maximum of the real part of dielectric permittivity is shifted to lower temperatures compare to PMN single crystal and ceramics. The mean relaxation time, obtained from fits of the frequency dependences of the dielectric permittivity with Cole-Cole formula, changes according to the Vogel-Fulcher law with the freezing temperature T ₀ = 88 K which is much lower than in bulk PMN materials.     

Microwave Dielectric Mechanism Studied by Microwave Near-Field Microscopy and Raman Spectroscopy

Microwave dielectric ceramics such as Ba(Mg_1/3Ta_2/3)O₃ and Ba₂Ti₉O₂₀ possess high dielectric constant and low dielectric loss in microwave frequency regime and have tremendous potential for device applications. In these materials, the presence of extrinsic defects, such as secondary phases, usually altered the microwave dielectric properties of the materials markedly, but the correlation of the microwave dielectric response of the materials with their microstructure has not been fully understood due to the lack of dielectric response in the local area. In this article, microwave near-field microscopy and Raman spectroscopy were used to investigate the microwave dielectric mechanism, viz. we measured the microwave dielectric properties of the materials in micron region by using a evanescent microwave probe (EMP) and, at the same time, examined the lattice vibration characteristics of the region by using a micro-Raman spectrum. How the presence of the secondary phase affects the microwave dielectric properties of the materials is thus systematically investigated. The causes of intrinsic or extrinsic dielectric loss were explored by comparing the dielectric images in SEMP at microwave frequencies and the corresponding Raman Spectra.