Benpeng Zhu

Polyimide/nanosized CaCu3Ti4O12 functional hybrid films with high dielectric permittivity

This work reports the high dielectric permittivity of polyimide (PI) embedded with CaCu3Ti4O12 (CCTO) nanoparticles. The dielectric behavior has been investigated over a frequency of 100 Hz-1 MHz. High dielectric permittivity (e = 171) and low dielectric loss (tan δ = 0.45) at 100 Hz have been observed near the percolation threshold. The experimental results fit well with the Percolation theory. We suggest that the high dielectric permittivity originates from the large interface area and the remarkable Maxwell-Wagner-Sillars effect at percolation in which nomadic charge carriers are blocked at internal interfaces between CCTO nanoparticles and the polyimide matrix.

Design and fabrication of PIN-PMN-PT single-crystal high-frequency ultrasound transducers

High-frequency PIN-PMN-PT single crystal ultrasound transducers at center frequencies of 35 MHz and 60 MHz were successfully fabricated using lead indium niobate-lead magnesium niobate-lead titanate (0.23PIN- 0.5PMN-0.27PT) single crystal. The new PIN-PMN-PT single crystal has higher coercivity (6.0 kV/cm) and higher Curie temperature (160°C) than PMN-PT crystal. Experimental results showed that the PIN-PMN-PT transducers have similar performance but better thermal stability compared with the PMN-PT transducers.

Enhanced pinning and proliferation of matching effects in a superconducting film with a Penrose array of magnetic dots

The vortex dynamics in superconducting films deposited on top of a fivefold Penrose array of magnetic dots is studied by means of transport measurements. The authors show that in the low pinning regime (demagnetized dots) a few periodic and aperiodic matching features coexist. In the strong pinning regime (magnetized dots) a richer structure of unforeseen periodic and aperiodic vortex patterns appear, giving rise to a clear enhancement of the critical current in a broader field range. Possible stable vortex configurations are determined by molecular dynamics simulations.

High Frequency PMN-PT 1-3 Composite Transducer for Ultrasonic Imaging Application

Development of PMN-PT single crystal/epoxy 1-3 composites for high-frequency ultrasonic transducers application is presented. The composite was fabricated by using a DRIE dry etching process with a 45% volume fraction of PMN-PT. A 35 MHz ultrasound flat transducer was fabricated with the composite, which was found to have an effective electromechanical coupling coefficient of 0.81, an insertion loss of 18 db, and a −6 dB bandwidth as high as 100%. Tungsten wire phantom image shows that the transducer had an axial resolution of 30 μm, which was in good agreement with the theoretical expectation. The initial results showed that the PMN-PT/epoxy 1-3 composite has many attractive properties over conventional piezoelectric materials for medical imaging applications.

Biologically inspired devices for easily controlling the motion of magnetic flux quanta

Abstract Motor proteins employ non-equilibrium fluctuations in anisotropic media to transport cargo at the cellular level. Here we consider anisotropic pinning to transport magnetic flux quanta inside superconductor. In particular, we consider: (1) composite pins by superimposing two interpenetrating arrays of weak and strong pinning centers; (2) triangular blind antidots; (3) V-shaped pinning sites. Specifically, we study stochastic transport of fluxons by alternating current (AC) rectification. Our simulated systems provide fluxon pumps, or fluxon “rectifiers”, because the applied electrical AC force is transformed into a net DC motion of fluxons. The asymmetry of the ratchet-shaped pinning landscape induce this “diode” effect, which can have important applications in devices, like SQUID magnetometers, and for fluxon optics, including convex and concave fluxon lenses.

Enhanced dielectric performance of three phase percolative composites based on thermoplastic-ceramic composites and surface modified carbon nanotube

Three-phase composites were prepared by embedding CaCu3Ti4O12(CCTO) nanoparticles and Multiwalled Carbon Nanotube (MWNT) into polyimide (PI) matrix via in-situ polymerization. The dependences of electric and dielectric properties of the resultant composites on volume fractions of filler and frequency were investigated. The dielectric permittivity of PI/CCTO-surface modified MWNT (MWNT-S) composite reached as high as 252 at 100 Hz at 0.1 vol. % filler (MWNT-S), which is about 63 times higher than that of pure PI. Also the dielectric loss is only 0.02 at 100 Hz. The results are in good agreement with the percolation theory. It is shown that embedding high aspect ratio MWNT-S in PI/CCTO composites is an effective means to enhance the dielectric permittivity and reduce the percolation threshold. The dielectric properties of the composites will meet the practical requirements for the application in high dielectric constant capacitors and high energy density materials.

New fabrication of high-frequency (100-MHz) ultrasound PZT film kerfless linear array [Correspondence]

The paper describes the design, fabrication, and measurements of a high-frequency ultrasound kerfless linear array prepared from hydrothermal lead zirconate titanate (PZT) thick film. The 15-μm hydrothermal PZT thick film with an area of 1 × 1 cm, obtained through a self-separation process from Ti substrate, was used to fabricate a 32-element 100-MHz kerfless linear array with photolithography. The bandwidth at -6 dB without matching layer, insertion loss around center frequency, and crosstalk between adjacent elements were measured to be 39%, -30 dB, and -15 dB, respectively.

Micro-machined high-frequency (80 MHz) PZT thick film linear arrays

This paper presents the development of a micromachined high-frequency linear array using PZT piezoelectric thick films. The linear array has 32 elements with an element width of 24 μm and an element length of 4 mm. Array elements were fabricated by deep reactive ion etching of PZT thick films, which were prepared from spin-coating of PZT sol-gel composite. Detailed fabrication processes, especially PZT thick film etching conditions and a novel transferring-and-etching method, are presented and discussed. Array designs were evaluated by simulation. Experimental measurements show that the array had a center frequency of 80 MHz and a fractional bandwidth (-6 dB) of 60%. An insertion loss of -41 dB and adjacent element crosstalk of -21 dB were found at the center frequency.

Exchange bias effect and enhanced magnetoresistance in La0.67Sr0.33MnO3∕SrTiO3 superlattices

The magnetization and electrical transport in the superlattices consisting of ferromagnetic La0.67Sr0.33MnO3 and nonmagnetic insulating SrTiO3 layers have been investigated. A significant displacement of the hysteresis loop along the field axis is observed when the sample is field-cooled through the blocking temperature TB. The strength of displacement, termed as exchange field HE, is found to exponentially decay with temperature. The magnetoresistance in field-cooling process is obviously enhanced compared to that in zero-field-cooling process. The existence of the disordered spin state at the interface is suggested to be the origin of such phenomena.

Beating patterns in the oscillatory magnetoresistance originatedfrom zero-field spin splitting in AlxGa1−xN∕GaN heterostructures

Beating patterns in the oscillatory magnetoresistance in Al0.11Ga0.89N∕GaN heterostructures with one subband occupation have been investigated by means of temperature dependent Shubnikov–de Haas measurements at low temperatures and high magnetic fields. The zero-field spin splitting effect is observed by excluding the magnetointersubband scattering effect. The obtained zero-field spin splitting energy is 2.5meV, and the obtained spin-orbit coupling parameter is 2.2×10−12eVm. Despite the strong polarization-induced electric field in the heterostructures, the spin-orbit coupling parameter in AlxGa1−xN∕GaN heterostructures is smaller than that in other heterostructures, such as InxGa1−xAs∕InyAl1−yAs ones. This is due to the large effective mass of the two-dimensional electron gas and the large GaN energy band gap in AlxGa1−xN∕GaN heterostructures. With an increase in magnetic field, the spin splitting energy becomes smaller. The zero-field effect is still the dominant mechanism in AlxGa1−xN∕GaN heterostructures at a magnetic field as high as 4.4T.Beating patterns in the oscillatory magnetoresistance in Al0.11Ga0.89N∕GaN heterostructures with one subband occupation have been investigated by means of temperature dependent Shubnikov–de Haas measurements at low temperatures and high magnetic fields. The zero-field spin splitting effect is observed by excluding the magnetointersubband scattering effect. The obtained zero-field spin splitting energy is 2.5meV, and the obtained spin-orbit coupling parameter is 2.2×10−12eVm. Despite the strong polarization-induced electric field in the heterostructures, the spin-orbit coupling parameter in AlxGa1−xN∕GaN heterostructures is smaller than that in other heterostructures, such as InxGa1−xAs∕InyAl1−yAs ones. This is due to the large effective mass of the two-dimensional electron gas and the large GaN energy band gap in AlxGa1−xN∕GaN heterostructures. With an increase in magnetic field, the spin splitting energy becomes smaller. The zero-field effect is still the dominant mechanism in AlxGa1−xN∕GaN heterostructu...