Jun-Hong Lin

Active Piezoelectric Energy Harvesting: General Principle and Experimental Demonstration

In piezoelectric energy harvesting systems, the energy harvesting circuit is the interface between a piezoelectric device and an electrical load. A conventional view of this interface is based on impedance matching concepts. In fact, an energy harvesting circuit can also apply electrical boundary conditions, such as voltage and charge, to the piezoelectric device for each energy conversion cycle. An optimized electrical boundary condition can therefore increase the mechanical energy flow into the device and the energy conversion efficiency of the device. We present a study of active energy harvesting, a type of energy harvesting approach which uses switch-mode power electronics to control the voltage and/or charge on a piezoelectric device relative to the mechanical input for optimized energy conversion. Under quasi-static assumptions, a model based on the electromechanical boundary conditions is established. Some practical limiting factors of active energy harvesting, due to device limitations and the efficiency of the power electronic circuitry, are discussed. In the experimental part of the article, active energy harvesting is demonstrated with a multilayer PVDF polymer device. In these experiments, the active energy harvesting approach increased the harvested energy by a factor of five for the same mechanical displacement compared to an optimized diode rectifier-based circuit.

Ion transport and storage of ionic liquids in ionic polymer conductor network composites

We investigate ion transport and storage of ionic liquids in ionic polymer conductor network composite electroactive devices. Specifically, we show that by combining the time domain electric and electromechanical responses, one can gain quantitative information on transport behavior of the two mobile ions in ionic liquids (i.e., cation and anion) in these electroactive devices. By employing a two carrier model, the total excess ions stored and strains generated by the cations and anions, and their transport times in the nanocomposites can be determined, which all depend critically on the morphologies of the conductor network nanocomposites.

Morphological study of the Al–Ti ohmic contact to p-type SiC

Abstract The composition 70 wt.% Al was recently reported to provide low resistance Al–Ti ohmic contacts with excellent electrical uniformity on p-type SiC. Using scanning electron microscopy and atomic force microscopy, an investigation of the surface morphology and edge definition of the annealed contacts was conducted, and the morphology of the buried metal/semiconductor interface was examined by etching away the contact metallization and imaging the freshly exposed SiC surface. This information provides guidance on the suitability of the contact for devices with small feature sizes and shallow p-type epilayers. Patterned contacts exhibited good edge definition, a root-mean-square surface roughness of 11 nm, and a root-mean-square interfacial roughness of 12 nm. The deepest observed penetration of the metallization into the SiC was 65 nm, and the lateral length scale of the morphological features at the buried metal/semiconductor interface was sufficiently small compared to the active area of the contact to allow good contact-to-contact reproducibility. The interfacial reactions and ohmic contact formation mechanism are considered from the point of view of the materials characterization study presented here and the binary Al–Ti and quaternary Al–C–Si–Ti phase diagrams.

Direct Observation of Ion Distributions near Electrodes in Ionic Polymer Actuators Containing Ionic Liquids

The recent boom of energy storage and conversion devices, exploiting ionic liquids (ILs) to enhance the performance, requires an in-depth understanding of this new class of electrolytes in device operation conditions. One central question critical to device performance is how the mobile ions accumulate near charged electrodes. Here, we present the excess ion depth profiles of ILs in ionomer membrane actuators (Aquivion/1-butyl-2,3-dimethylimidazolium chloride (BMMI-Cl), 27 μm thick), characterized directly by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) at liquid nitrogen temperature. Experimental results reveal that for the IL studied, cations and anions are accumulated at both electrodes. The large difference in the total volume occupied by the excess ions between the two electrodes cause the observed large bending actuation of the actuator. Hence we demonstrate that ToF-SIMS experiment provides great insights on the physics nature of ionic devices.

Nonlinear conduction in aromatic polyurea thin films and its influence on dielectric applications over a broad temperature range

We investigate nonlinear conduction in aromatic polyurea thin films and its influence on the dielectric applications over a broad temperature range. The experimental data indicate that several high field conduction mechanisms coexist in the polymer films and one conduction mechanism may dominate over the others depending on different temperatures. The results also reveal that owing to the non-linear increase of the conduction loss with field, the dielectric loss at high field can be many orders of magnitude larger than that at low field. Therefore, besides the electrical breakdown strength, the conduction loss at high field and their temperature dependence behavior are other key factors in developing polymer dielectric materials for high energy density dielectric applications over a broad temperature range.

Evolution of relaxor ferroelectric behavior of poly(vinylidene fluoride trifluoroethylene chlorofluoroethylene) terpolymer nanorods

The evolution of relaxor ferroelectric behavior of poly(vinylidene fluoride trifluoroethylene chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer nanorods with diameters reduced from 200 to 25 nm was investigated. It was observed that all the nanorods studied exhibited relaxor ferroelectric behavior, as characterized by the dielectric peak shifting toward high temperatures with increasing frequency. The frequency-permittivity peak temperature characteristics fit well with the Vogel–Fulcher (VF) relation. Moreover, the freezing temperature in the VF relation decreased with the reduction of nanorod diameter, indicating that the scaling down of the lateral size of nanorods influenced the relaxor ferroelectric behavior of the terpolymer.

Ion distribution in ionic electroactive polymer actuators

Ionic electroactive polymer (i-EAP) actuators with large strain and low operation voltage are extremely attractive for applications such as MEMS and smart materials and systems. In-depth understanding of the ion transport and storage under electrical stimulus is crucial for optimizing the actuator performance. In this study, we show the dominances of ion diffusion charge and we perform direct measurements of the steady state ion distribution in charged and frozen actuators by using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). High temperature actuators that consist Aquivion ionomer membrane and high melting temperature ionic liquid 1-butyl-2,3-dimethylimidazolium chloride (BMMI-Cl]) served in this study. Electrical impedance, I-V characteristics, and potential step charging of the actuator are characterized at 25°C and 100°C. The conductivity of the actuator is 0.3mS/cm at 100°C and 2.9μS/cm at 25°C, respectively. The electrochemical window of the device is 3V and a 2mm tip displacement is observed under 2.5V 0.2Hz at 100°C. A semi-quantitative depth profile of the relative ion concentration in charged and frozen actuators is measured by ToF-SIMS. The result shows that, unlike semiconductors, ions do not deplete from the electrodes with same signs. Due to a strong cluster effect between the ions, Cl- and BMMI+ accumulate near both cathode and anode. Furthermore, the profile indicates that the ion size difference causes the BMMI+ space charge layers (~6um) much thicker than those of Cl- (~0.5um).

Aromatic Polyurea for High Temperature High Energy Density Capacitors

We investigate aromatic polyureas which can be fabricated in the form of thin films through CVD. It was found that the polymer possesses a flat dielectric response (k∼ 4.2 and loss 800MV/m), high energy density (>12J/cm3) and high efficiency suggest this polymer can be a good candidate material for high temperature energy storage capacitors. Breakdown strength was analyzed with Weibull model over a broad temperature range (25°C ∼180°C). Experimental results indicate that aromatic polyurea is more like a nonpolar linear dielectric material because of its highly cross-linked structures. The experiment results further show that this polymer maintains its high performance even at high temperatures.

Transports of ionic liquids in ionic polymer conductor network composite actuators

We investigate the influence of ionic liquids on the electromechanical performance of Ionic Polymer Conductor Network Composite (IPCNC) bending actuators. Two imidazolium ionic liquids (ILs) with one cation, which is 1-ethyl-3- methylimidazolium ([EMI+]), and two different anions, which are tetrafluoroborate ([BF4-]) and trifluoromethanesulfonate ([Tf-]), are chosen for the study. By combining the time domain electric and electromechanical responses, we developed a new model that describes the ion transports in IPCNC actuators. The time constant of excess cation and anion migration in various composite electrodes are deduced: 6s and 25s in RuO2/Nafion; 7.9s and 36.3s in RuO2/Aquivion; 4.8s and 53s in Au/PAH, respectively. NMR is also applied to provide quantitative measures of self-diffusion coefficients independently for IL anions and cations both in pure ILs and in ILs absorved into ionomers. All the results indicate that the motion of cation, in the studied pure ionic liquids, polymer matrix and conductor network composites, is faster than that of anion. Moreover, the CNC morphology is playing a crucial role in determining the ion transport in the porous electrodes.

Conductive filler morphology effect on performance of ionic polymer conductive network composite actuators

Several generations of ionic polymer metal composite (IPMC) actuators have been developed since 1992. It has been discovered that the composite electrodes which are composed of electronic and ionic conductors, have great impact on performance of ionic polymer actuators by affecting strain level, efficiency and speed. One of important factors in composite electrodes is the shape and morphology of electronic conductor fillers. In this paper, RuO2 nanoparticles and vertically aligned carbon nanotube (Va-CNT) are used as conductor fillers. Making use of unique properties of Va-CNT forests with ultrahigh volume fraction in Nafion nanocomposite, an ionic polymer actuator is developed. Ion transport speed is greatly increased along CNT alignment direction. The high elastic anisotropy, arising from the high modulus and volume fraction of Va-CNTs, enhances actuation strain while reducing the undesirable direction strain. More than 8% actuation strain under 4 volts with less than one second response time has been achieved.