Shuting Chen

Gate-controlled nonvolatile graphene-ferroelectric memory

In this letter, we demonstrate a nonvolatile memory device in a graphene field-effect-transistor structure using ferroelectric gating. The binary information, i.e., “1” and “0”, is represented by the high and low resistance states of the graphene working channels and is switched by controlling the polarization of the ferroelectric thin film using gate voltage sweep. A nonvolatile resistance change exceeding 200% is achieved in our graphene-ferroelectric hybrid devices. The experimental observations are explained by the electrostatic doping of graphene by electric dipoles at the ferroelectric/graphene interface.

Nonlinear dielectric thin films for high-power electric storage with energy density comparable with electrochemical supercapacitors

Although batteries possess high energy storage density, their output power is limited by the slow movement of charge carriers, and thus capacitors are often required to deliver high power output. Dielectric capacitors have high power density with fast discharge rate, but their energy density is typically much lower than electrochemical supercapacitors. Increasing the energy density of dielectric materials is highly desired to extend their applications in many emerging power system applications. In this paper, we review the mechanisms and major characteristics of electric energy storage with electrochemical supercapacitors and dielectric capacitors. Three types of in-house-produced ferroic nonlinear dielectric thin film materials with high energy density are described, including (Pb_0.97La_0.02)(Zr_0.90Sn_0.05Ti_0.05)O₃ (PLZST) antiferroelectric ceramic thin films, Pb(Zn_1/3Nb_2/3)O_3-Pb(Mg_1/3Nb_2/3) O_3-PbTiO₃ (PZN-PMN-PT) relaxor ferroelectric ceramic thin films, and poly(vinylidene fluoride) (PVDF)-based polymer blend thin films. The results showed that these thin film materials are promising for electric storage with outstandingly high power density and fairly high energy density, comparable with electrochemical supercapacitors.

Ferroelectric poly(vinylidene fluoride) thin films on Si substrate with the β phase promoted by hydrated magnesium nitrate

Solution derived poly(vinylidene fluoride) (PVDF) homopolymer thin films on silicon substrates with the addition of hydrated salt [Mg(NO3)2⋅6H2O] were systematically investigated. β phase dominant ferroelectric PVDF thin films with a remnant polarization of 77 mC∕m2 were achieved by optimizing the concentration of Mg(NO3)2⋅6H2O and the processing condition. Our experimental results and theoretical analysis indicated that the hydrogen bonds between water in the hydrated salt and the PVDF molecules result in the interchain registration of the all-trans conformation, and the hydrated salt acts as the nucleation agent and promotes the crystallization of the β phase. The obtained effective d33 was −14.5 pm∕V, tested with a laser scanning vibrometer, without taking into account the substrate clamping effect. The numerical simulation, after considering the elastic constrain of the substrate, determined that the corresponding actual d33 is −30.8 pm∕V, which is comparable to that of uniaxially stretched piezoelect...

High electric breakdown strength and energy density in vinylidene fluoride oligomer/poly(vinylidene fluoride) blend thin films

Dense α-phase blend films of vinylidene fluoride (VDF) oligomer and poly(vinylidene fluoride) (PVDF) of various compositions were prepared from chemical solution deposition. The dielectric constant of the films was unexpectedly lower, and the mechanical strength was higher than either of the two components, leading to high electromechanical dielectric breakdown strength (>850 MV/m vs. 300∼500 MV/m for typical PVDF-based films). The properties were attributed to the unique blend structure with high crystallinity and densely packed rigid amorphous phase incorporating long and short chains. A maximum polarization of 162 mC/m2 and a large electric energy density up to 27.3 J/cm3 were obtained.

Nanoconfinement induced crystal orientation and large piezoelectric coefficient in vertically aligned P(VDF-TrFE) nanotube array

Vertically aligned piezoelectric P(VDF-TrFE) nanotube array comprising nanotubes embedded in anodized alumina membrane matrix without entanglement has been fabricated. It is found that the crystallographic polar axes of the P(VDF-TrFE) nanotubes are oriented along the nanotubes long axes. Such a desired crystal orientation is due to the kinetic selection mechanism for lamellae growth confined in the nanopores. The preferred crystal orientation in nanotubes leads to huge piezoelectric coefficients of the P(VDF-TrFE). The piezoelectric strain and voltage coefficients of P(VDF-TrFE) nanotube array are observed to be 1.97 and 3.40 times of those for conventional spin coated film. Such a significant performance enhancement is attributed to the well-controlled polarization orientation, the elimination of the substrate constraint, and the low dielectric constant of the nanotube array. The P(VDF-TrFE) nanotube array exhibiting the unique structure and outstanding piezoelectric performance is promising for wide applications, including various electrical devices and electromechanical sensors and transducers.

Direct-Write Piezoelectric Ultrasonic Transducers for Non-Destructive Testing of Metal Plates

Current real time structural health monitoring is implemented by assembling multiple discrete sensors on a structure with each sensor providing only point measurement. The installation of the multiple sensors results in high global cost and low reliability. This paper reports the design, direct-write fabrication, and testing of ultrasonic transducers on a plate structure and a non-destructive testing method for detecting defects using the sensor array comprising the direct-write ultrasonic transducers. The transducers are made of piezoelectric poly(vinylidenefluoride/trifluoroethylene) (P(VDF/TrFE)) polymer coatings that are aerosol-spray deposited and patterned directly on the plate structure to be monitored. The ultrasonic transducers bearing annular array comb electrodes are designed for generating and selectively detecting fundamental antisymmetric Lamb-mode ultrasonic waves in the plate structure. The ultrasonic transducers can serve as both the actuators to generate ultrasonic waves and the sensors to detect ultrasonic waves. The ultrasonic waves propagating through the plate structure contain the information about the structural integrity. With copper bars of different thicknesses introduced at the plate center as mock defects of different severity, the correlation between the transducer response and the defect thickness and hence, the severity is verified. It is also demonstrated that four ultrasonic transducers located at the square plate (100 mm

Lead-Free Piezoelectric Ceramic Coatings Fabricated by Thermal Spray Process

\times 100

P(VDF-HFP) Polymer as Sensing Material for Capacitive Carbon Dioxide Sensors

mm

Hybrid local piezoelectric and conductive functions for high performance airborne sound absorption

\times 1.27

High-frequency ultrasonic methods for determining corrosion layer thickness of hollow metallic components

mm) corners forming a transducer array, which can locate the defect on the plate. A short time Fourier transform algorithm and an imaging algorithm are developed for processing signal of the pitch-catch ultrasonic wave spectra to determine the location of the defects, which is verified by experimental results.