W. Jud Ready

Towards Ultrathick Battery Electrodes: Aligned Carbon Nanotube – Enabled Architecture

Vapor deposition techniques were utilized to synthesize very thick (∼1 mm) Li-ion battery anodes consisting of vertically aligned carbon nanotubes coated with silicon and carbon. The produced anode demonstrated ultrahigh thermal (>400 W·m(-1) ·K(-1)) and high electrical (>20 S·m(-1)) conductivities, high cycle stability, and high average capacity (>3000 mAh·g(Si) (-1)). The processes utilized allow for the conformal deposition of other materials, thus making it a promising architecture for the development of Li-ion anodes and cathodes with greatly enhanced electrical and thermal conductivities.

Ultra Strong Silicon-Coated Carbon Nanotube Nonwoven Fabric as a Multifunctional Lithium-Ion Battery Anode

Materials that can perform simultaneous functions allow for reductions in the total system mass and volume. Developing technologies to produce flexible batteries with good performance in combination with high specific strength is strongly desired for weight- and power-sensitive applications such as unmanned or aerospace vehicles, high-performance ground vehicles, robotics, and smart textiles. State of the art battery electrode fabrication techniques are not conducive to the development of multifunctional materials due to their inherently low strength and conductivities. Here, we present a scalable method utilizing carbon nanotube (CNT) nonwoven fabric-based technology to develop flexible, electrochemically stable (∼494 mAh·g(-1) for 150 cycles) battery anodes that can be produced on an industrial scale and demonstrate specific strength higher than that of titanium, copper, and even a structural steel. Similar methods can be utilized for the formation of various cathode and anode composites with tunable strength and energy and power densities.

Lifetime and Failure Mechanisms of an Arrayed Carbon Nanotube Field Emission Cathode

There is interest in the use of carbon nanotubes (CNTs) to create a field emission (FE) cathode for the neutralization of exhaust plumes of low-power (<; 500 W) electric propulsion devices since FE cathodes do not require a gas flow to operate. To incorporate CNT emitters into propulsion systems, the current emission output over the lifetime of the cathode must be understood. Multiple FE cathodes that consist of multiwalled CNT arrays have been fabricated. Seven cathodes are characterized at pressures below 10 5 T at constant voltage between the CNTs, and the gate until failure occurs. The maximum current density observed is 9.08 mA/cm2 with average current densities up to 2.52 mA/cm2, and the maximum life span is 368 h. The behavior of the cathode current emission is highly unstable and consists of oscillations and sudden shifts. Resistive heating is believed to be the primary cause for failure in two thermally assisted modes: 1) oxidative ablation at the root of the nanotube and 2) field evaporation at the tip.

Band structure effects on resonant tunneling in III-V quantum wells versus two-dimensional vertical heterostructures

Since the invention of the Esaki diode, resonant tunneling devices have been of interest for applications including multi-valued logic and communication systems. These devices are characterized by the presence of negative differential resistance in the current-voltage characteristic, resulting from lateral momentum conservation during the tunneling process. While a large amount of research has focused on III-V material systems, such as the GaAs/AlGaAs system, for resonant tunneling devices, poor device performance and device-to-device variability have limited widespread adoption. Recently, the symmetric field-effect transistor (symFET) was proposed as a resonant tunneling device incorporating symmetric 2-D materials, such as transition metal dichalcogenides (TMDs), separated by an interlayer barrier, such as hexagonal boron-nitride. The achievable peak-to-valley ratio for TMD symFETs has been predicted to be higher than has been observed for III-V resonant tunneling devices. This work examines the effect ...

Material Constraints and Scaling of 2-D Vertical Heterostructure Interlayer Tunnel Field-Effect Transistors

Aggressive scaling of logic devices is quickly approaching the physical limitations of conventional CMOS devices, resulting in the need for novel device architectures. One proposed device is the 2-D interlayer tunnel field-effect transistor (ITFET), which relies on tunneling within a vertical heterostructure of 2-D materials. Steep-slope operation of the ITFET relies on proper band alignment for tunneling between the conduction band of one 2-D electrode and the valence band of the other 2-D electrode. Because of the step-like nature of the density of states of transition metal dichalcogenides (TMDs), the subthreshold slope is infinite for ideal materials. Previous theoretical predictions suggested the possibility for steep-slope operation in TMD-based ITFETs, but did not consider the complete electronic and physical structure of the TMD electrodes. This paper explores the implications of the physical structure of materials, such as lattice constant, on ITFET performance. Further, several design parameters are explored within the MoS2–WSe2 system to develop general design rules for ITFETs based on 2-D materials. Benchmarking is performed of the MoS2–WSe2 ITFET to suggest the potential for both lower power and higher performance than conventional CMOS devices.