Frances Ann Hill

High-Throughput Ionic Liquid Ion Sources Using Arrays of Microfabricated Electrospray Emitters With Integrated Extractor Grid and Carbon Nanotube Flow Control Structures

We report the design, fabrication, and experimental characterization of dense, monolithic, and planar arrays of externally-fed electrospray emitters with an integrated extractor grid and carbon nanotube flow control structures for low-voltage and high-throughput electrospray of the ionic liquid EMI-BF4 in vacuum. Microfabricated arrays with as many as 1900 emitters in 1 cm2 were fabricated and tested. Per-emitter currents as high as 5 μA in both polarities were measured, with start-up bias voltages as low as 470 V and extractor grid transmission as high as 80%. Maximum array emission currents of 1.35 mA (1.35 mA/cm2) were measured using arrays of 1900 emitters in 1 cm2. A conformal carbon nanotube forest grown on the surface of the emitters acts as a wicking structure that transports liquid to the emitter tips, providing hydraulic impedance to regulate and uniformize the emission across the array. Mass spectrometry of the electrospray beam confirms that emission in both polarities is composed of solvated ions, and etching of the silicon collector electrode is observed. Collector imprints and per-emitter current-voltage characteristics for different emitter array sizes spanning three orders of magnitude show excellent emission uniformity across the array. Performance estimates of the devices as nanosatellite thrusters are provided.

Low-Bremsstrahlung X-Ray Source Using a Low-Voltage High-Current-Density Nanostructured Field Emission Cathode and a Transmission Anode for Markerless Soft Tissue Imaging

We report the design, fabrication, and proof-of-concept characterization of an X-ray generator for improved X-ray absorption imaging that uses a nanostructured field emission cathode as the electron source and a microstructured transmission anode as the X-ray generating structure. Field emission cathodes consume less power, respond faster, and tolerate lower vacuum than the thermionic cathodes used in conventional X-ray generators. The use of a transmission anode, instead of a conventional reflection anode, allows filtering of the background radiation (bremsstrahlung) while allowing efficient generation of X-ray at lower voltages by exciting atomic shell transitions, resulting in emission of X-ray with narrow spectral linewidth for sharper imaging of biological tissue. The fabricated field emission cathode contains arrays of self-aligned and gated silicon field emitters. The field emission cathodes turn on at bias voltages as low as 25 V, and their gates transmit almost 100% of the electrons to the anode. The cathodes produce per-emitter electron currents in excess of 2 μA (current density >2 A/cm2) at a bias voltage of 80 V. A desktop rig is built to generate X-ray with a field emission cathode and transmission anode. Using the facility, we obtained X-ray absorption images of several objects. The images clearly show details under 500 μm in size, as well as soft tissue and fine bone structures without using contrast agents.

Characterizing the failure processes that limit the storage of energy in carbon nanotube springs under tension

We report measurements of the mechanical properties and energy storage capabilities of carbon nanotube (CNT) springs under tensile loading, including correlated measurements of their cyclic loading and electrical resistance behavior. Tests are conducted on fibers of multi-walled CNTs fabricated from 6 mm tall forests. The highest measured strength and stiffness of the fibers are 2 N tex−1 and 70 N tex−1 respectively. The highest recorded energy density is approximately 7 kJ kg−1 or 500 kJ m−3, more than an order of magnitude higher than the gravimetric energy density of steel springs, and half the volumetric energy density of steel springs. The resistance and stress responses of the fibers during loading to failure and cyclic loading demonstrate that disorder at the nanoscale affects the bulk response. CNT springs show limited effects of fatigue under 75 tensile cyclic loading cycles. Improving the structural quality of the CNTs and the organization of the fibers offers potential to significantly increase the energy storage capacity of the springs.

Parallel nanomanufacturing via electrohydrodynamic jetting from microfabricated externally-fed emitter arrays

We report the design, fabrication, and characterization of planar arrays of externally-fed silicon electrospinning emitters for high-throughput generation of polymer nanofibers. Arrays with as many as 225 emitters and with emitter density as large as 100 emitters cm(-2) were characterized using a solution of dissolved PEO in water and ethanol. Devices with emitter density as high as 25 emitters cm(-2) deposit uniform imprints comprising fibers with diameters on the order of a few hundred nanometers. Mass flux rates as high as 417 g hr(-1) m(-2) were measured, i.e., four times the reported production rate of the leading commercial free-surface electrospinning sources. Throughput increases with increasing array size at constant emitter density, suggesting the design can be scaled up with no loss of productivity. Devices with emitter density equal to 100 emitters cm(-2) fail to generate fibers but uniformly generate electrosprayed droplets. For the arrays tested, the largest measured mass flux resulted from arrays with larger emitter separation operating at larger bias voltages, indicating the strong influence of electrical field enhancement on the performance of the devices. Incorporation of a ground electrode surrounding the array tips helps equalize the emitter field enhancement across the array as well as control the spread of the imprints over larger distances.

A Portable Power Source Based on MEMS and Carbon Nanotubes

There is a growing need for small, lightweight, reliable, highly efficient and fully rechargeable portable power sources. The focus of this project is the design and modeling of a system in which energy is stored in the elastic deformation of carbon nanotube (CNT)-based springs. The CNTs are coupled to a MEMS electric generator. When the CNT deformation is released, the stored energy actuates the generator, which then converts the energy into electricity. The MEMS generator may be operated in reverse, as a motor, in order to wind the CNT springs and recharge the system. Alternatively, the stored elastic energy may be used to supply a mechanical load directly. This project is motivated by recent research into the mechanical properties of CNTs. The CNTs have a high stiffness, low defect density, and a consequently high yield strain that enables them to store elastic energy with significantly greater energy density than typical spring materials such as high-carbon steel. Models suggest that CNTs can be reversibly stretched by up to 15% [1]; lower strains of up to 6% have been demonstrated experimen-tally to date [2-3].This type of system offers several important potential advantages. First, due to CNTs’ high strength, high flexibility, and low defect density, they can store energy at very high energy density. Con-sidering just the CNT-based spring itself, the energy density of an array of CNTs stretched to a reversible 15% strain is about 1500 W-hr/kg, about ten times the energy density of Li-ion batteries. The energy density of the final system will be lower because of the finite conversion efficiency of the generator and the weight of both the supporting structure and the generator hardware. In addition, because energy storage in the CNT system is based on stretching chemical bonds rather than breaking and reforming chemical bonds as in batteries, the CNT-MEMS generator sys-tem has the potential to operate at higher power densities, un-der harsher conditions, to deeper discharge levels, and through a greater number of charge-discharge cycles than a chemical bat-tery.The system architecture consists of a CNT-based energy storage element, an energy release rate mechanism, and a MEMS gen-erator. This project is examining and modeling different varia-tions on this system architecture that incorporate different modes of deformation of the CNT-based energy storage element, vari-ous types of generators, different types of coupling between the storage element and the generator, and different size scales for the various components. One conceptual example is illustrated below, in which the axial relaxation of an axially-stretched CNT-based storage element is converted to rotational motion of a wheel. The wheel is coupled to a piezoelectric generator through a mechanism that regulates the rate of energy release, much as in a mechanical watch.

High-throughput manufacturing of polymer nanofibers via electrohydrodynamic jetting from planar arrays of microfabricated externally-fed emitters

We report the design, fabrication, and characterization of novel microfabricated, multiplexed sources for highthroughput production of polymer nanofibers. The devices are planar arrays of high-aspect-ratio silicon emitters with surfaces covered by an array of micropillars that enable surface tension-driven feed of liquid to the emitter tips. The sources are assembled from monolithic linear arrays of emitters etched out of a silicon wafer using deep reactiveion etching. Experimental data show high array utilization and uniform generation of nanofibers with average diameter equal to ~250 nm. Increase of the operational bias voltage results in increasing the emission current and nanofiber flux.

High-throughput electrospray for nanomanufacturing and nanosatellite propulsion

Electrospray is the ionization of electrically conductive liquids using high electric fields. Under the influence of a strong enough field (150 million volts per meter, for example), the free surface of the liquid becomes a cone that ejects particles from its tip (see Figure 1).1 The electrospray process generates nearmonodisperse particles (those of uniform size) and—depending on the working liquid and the process conditions—these can be droplets, ions, fibers, or a mix.2 However, electrospray emitter throughput is very small (<1 A, <1 N, or <10 l/h, for example), which restricts its application. In fact, the only major role for electrospray so far is as an ion source for mass spectrometry of large biomolecules, for which John Fenn won the Nobel Prize in Chemistry in 2002.3 An attractive approach to greatly increase the throughput of electrospray sources is to operate many emitters in parallel. Several research groups have reported that miniaturized electrospray multiplexed sources, which demonstrate a throughput more than two orders of magnitude larger than for single-emitter sources,4, 5 achieve visibly less power consumption per emitter than their macroscaled counterparts (the start-up voltage of an electrospray emitter is proportional to the square root of the tip diameter). Ion emission using electrospray is possible at a sufficiently low flow rate using ionic liquids.6 These are ionic salts that are liquid at room temperature, and can have high electrical conductivity (>1S/m) and virtually no vapor pressure, which makes them ideal for vacuum applications. Ionic liquids have historically been expensive (

Advances In Vertical Solid-State Current Limiters For Individual Field Emitter Regulation In High-Density Arrays

1000/cm3), but are now available for tens of dollars. The ionic liquid ion source has applications such as large-area nanomanufacturing (operating many electrospray ion emitters in parallel to deposit or etch material) and Figure 1. A Taylor cone, formed during ionization, and from which uniform particles are ejected. The outer diameter of the capillary is about 300 m. (Reproduced from http://multiplexed.mit.edu.)

Modeling mechanical energy storage in springs based on carbon nanotubes

We report the design, fabrication, and characterization of improved solid-state elements intended for individual regulation of field emitters part of high-density arrays. We demonstrate a high-yield, CMOS compatible fabrication process of single-crystal, vertical, ungated, n-type silicon field-effect transistors (FETs); each device behaves as a current source when is biased at a voltage larger than its drain-source saturation voltage. An ungated FET in saturation connected in series to a field emitter can compensate for the wide variation in current-voltage characteristics of the field emitters due to the tip radii spread present in any field emitter array, which should result in emitter burn-out protection, larger array utilization, and smaller array emission non-uniformity. Using 1-2 Ωcm single-crystal n-Si wafers, we fabricated arrays of 25 μm tall vertical ungated FETs with 0.5 μm diameter that span two orders of magnitude of array size. Experimental characterization of the arrays demonstrates that the current is limited with > 3.5 V bias voltage to the same ~6 μA (6 A.cm-2) per-FET value. Finite element simulations of the device predict a saturation voltage close to the experimental value and a saturation current within a factor of two of the experimental value.