Yingqi Jiang

Uniformly Embedded Metal Oxide Nanoparticles in Vertically Aligned Carbon Nanotube Forests as Pseudocapacitor Electrodes for Enhanced Energy Storage

Carbon nanotube (CNT) forests were grown directly on a silicon substrate using a Fe/Al/Mo stacking layer which functioned as both the catalyst material and subsequently a conductive current collecting layer in pseudocapacitor applications. A vacuum-assisted, in situ electrodeposition process has been used to achieve the three-dimensional functionalization of CNT forests with inserted nickel nanoparticles as pseudocapacitor electrodes. Experimental results have shown the measured specific capacitance of 1.26 F/cm(3), which is 5.7 times higher than pure CNT forest samples, and the oxidized nickel nanoparticle/CNT supercapacitor retained 94.2% of its initial capacitance after 10,000 cyclic voltammetry tests.

Planar MEMS Supercapacitor using Carbon Nanotube Forests

Planar micro supercapacitors utilizing vertically aligned carbon nanotube (CNT) forests with the design of interdigital electrodes have been demonstrated. Conductive substrates using Mo/Al/Fe metal stack shows best CNT forest synthesis results with high electrical conductivity and dense CNT structures. Prototype devices show measured specific capacitances about 1000 times higher than those with plain metal electrodes without CNT forests. Furthermore, charging/discharging experiments show over 92% efficiency and very robust cycling stability. As such, we believe these planar MEMS supercapacitors could be applicable in various systems including energy harvesters, pulse-power supplies and advanced microelectronics as on-chip capacitors.

Characterizations of contact and sheet resistances of vertically aligned carbon nanotube forests with intrinsic bottom contacts

Comprehensive studies on the sheet and contact resistances of vertically aligned carbon nanotube (CNT) forests with as-grown bottom contacts to the metal layer have been conducted. Using microfabrication and four distinct methods: (1) the transfer length method (TLM), (2) the contact chain method, (3) the Kelvin method, and (4) the four point probe method, we have designed multiple testing devices to characterize the resistances of CNT-forest-based devices. Experimental results show that devices based on stripe-shaped CNT forests 100 µm in height and 100 µm in width have a sheet resistance of approximately [Formula: see text]. The corresponding specific contact resistance to the molybdenum layer is roughly 5 × 10(4) Ω µm(2). Consistency of the results from the four different methods validates the study. After two months of storage of the CNT forest samples in open air, less than 0.9% deviations in the resistance values were observed. We further demonstrated one application of CNT forests as an NH(3) gas sensor and measured 0.5 ppm of sensing resolution with a detection response time of 1 min.

3D supercapacitor using nickel electroplated vertical aligned carbon nanotube array electrode

Three-dimensional (3D) supercapacitors have been demonstrated using vertically aligned carbon nanotubes (CNT) arrays and electroplated nickel nanoparticles. Two significant achievements have been demonstrated in this work: (1) a unique and reliable vacuum-assisted electroplating methodology for the uniform deposition of nickel nanoparticles on CNTs up to 150µm in length, and (2) usage of pseudo capacitance to significantly enhance the energy density of CNT supercapacitors up to one order of magnitude higher than those without nickel nanoparticles coating. Experimental results show robust operation of more than 100 times charge/discharge tests.

Micromachined W-band plastic slot array antenna with self-aligned and integrated flange

A W-band slot array antenna using a plastic substrate with micromachining techniques and injection molding processes has been demonstrated. A unique design feature by integrating the input flange directly with the waveguide structure using a one-shot molding process is presented which greatly improves the robustness of the fabrication process. Measurement results show that a prototype 8-slot antenna in good agreement with simulation prediction with reflection loss (s11) of −26.3dB at 76.8GHz, 10dB bandwidth of 9.3GHz, gain and side lobe level (SLL) of 9.6dB and 13.5dB, respectively. As such, this waveguide-based plastic slot array antenna might find potential applications in low-cost mm-wave systems, including automobile radars.

Flexible energy storage devices based on lift-off of CNT films

We report a transfer process for CNT films with intrinsic bottom metal contacts to construct mechanically bendable, densely aligned carbon nanotube (CNT) forests for energy storage devices. The flexible electrode created and investigated as a supercapacitor has the following salient features: (1) excellent transfer of charge from the aligned CNTs to the substrate, (2) simple fabrication, and (3) easy integration with a variety of surfaces and topographies. Preliminary testing results with a CNT forest of 5 × 10 mm2 in area on Au/Kapton® film show a specific capacitance of 7.0 mF/cm2.

Amorphous silicon-coated CNT forest for energy storage applications

This work is based on developing an amorphous silicon-coated, vertically aligned carbon nanotube (CNT) forest for energy storage applications. The architecture of the electrode has three valuable features: (1) enhanced charge transfer via aligned CNTs to the substrate, (2) excellent charge storage capability via amorphous silicon, and (3) the preservation of high surface area and porosity with enhanced energy storage capacity. As such, this work could offer a new type of nanomaterial for energy storage applications. A supercapacitor electrode is used in this work for the energy storage demonstration.

Densely packed carbon nanotube forest on silicon substrate for MEMS supercapacitor applications

Densely packed carbon nanotube (CNT) forest on silicon wafer has been fabricated by combining the process of mechanical press and liquid densification. Advantages from the densely packed CNT forest prototype devices include: (1) 16-fold volume reduction with the same amount of CNT forest as compared to the as-grown CNT forest; (2) preservations of aligned CNT organization and natural contacts to the substrate; (3) improved surface flatness of the CNT films. Possible applications for densely packed CNT forests are in energy storage, sensors and other field and here we demonstrate and characterize CNT forest as electrodes in supercapacitors. Experimental results show this volume densification process has successfully increased the energy density of a prototype supercapacitor by 16 times. The resistance of the CNT electrode has also reduced 25% due to the improved material conductivity. Stable performance has been observed during cyclic charge/discharge tests. As such, the presented method of organized densification in aqueous solution for CNT films provides a unique path for improved performance in MEMS/energy applications.

Electrostatic oscillation of CNT bundles

Electrostatic oscillations of conductive CNT bundles have been successfully demonstrated in air environment. Three unique characteristics have been observed and characterized. First, CNT bundles have been constructed by a simple “pick, stick and break” process using a regular probe tip from vertically aligned CNT forest on a silicon substrate. Second, CNT bundles have been actuated statically under applied DC voltages. Third, CNT bundles oscillated following the applied AC input frequencies. As such, this research present electrostatic oscillation of CNT bundles and could potentially open up a new class of actuator/oscillator research for applications in the general field of MEMS and NEMS.

Polymeric Microelectromechanical Millimeter Wave Systems

Polymeric millimeter-wave components and systems based on micro molding technologies have been demonstrated, including waveguides, iris filters, tunable filters, phase shifters, waveguide-fed horn antennas and waveguide-based feeding networks. Fundamental issues in polymer metallization process such as conformal and uniform deposition as well as mass transfer and current density effects on the novel in-channel electroplating encapsulation, surface morphology and roughness on mm-wave attenuation will be discussed in detail. We believe this new class of polymeric millimeter-wave systems has potential applications in replacing the expensive metallic counterparts (a few thousand dollars for each waveguide) in current millimeter-wave systems.