Materials Innovation Amplifies Terahertz Power and Performance

My note: Thanks
to numbr, on the
IV board for bringing this story to my attention.

by Heyward Burnett
Materials and Manufacturing

12/13/2010 - WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- Air Force Research Laboratory materials experts' discovery that single-walled [carbon] nanotubes boost terahertz imaging capabilities resolves a fundamental limitation in the lab's Hazardous Material Identification System. That directed energy system has heretofore generated its terahertz beam via traveling wave tube amplifier; however, the limited current density of the carbon-fiber-based cathodes used for device emissions has restricted the system's power and, thus, its range and resolution. In pursuing the desired enhancements, the scientists conceived of using SWNT fiber cathodes in the system's electron gun. The outcome of this exploratory effort, a collaborative venture with Rice University, is a material solution not only more durable than carbon fiber counterparts, but more powerful (10x greater current) and less power-hungry (4x lower turn-on voltage). Though originally devised for military purposes, this technology could also benefit terahertz systems used for security surveillance, medical diagnostics, nondestructive evaluation, production quality control, and similarly diverse applications.

Imaging via "terahertz" entails the use of electromagnetic waves sent at frequencies in the terahertz range. Waves in this range can pass through clothing, paper, cardboard, wood, masonry, plastic, and ceramics, making terahertz imaging a technology of choice for security surveillance and other noninvasive (but detection-oriented) applications. Further, the nonionizing properties of waves in this range pose no risk to human tissue, which heightens the technology's appeal for medical use. SWNTs, meanwhile, are unparalleled in their combination of strength, stiffness, thermal and electrical conductivity, and field emission. Consequently, the AFRL/academic team set about investigating the properties--and potential applications--of SWNTs spun as a polymer into 100 µm diameter continuous fibers via a process similar to that used for Kevlar ™ production.

Long-term testing indicates that SWNT fibers demonstrate substantially improved emission current and exhibit minimal damage compared to carbon fibers, which produce limited current and suffer catastrophic failure due to Joule heating. The team's groundbreaking work has established an Air Force-unique cathode material, prompted a patent application addressing SWNT fibers as a cathode material, and shifted the research focus on carbon fiber cathodes towards implementing SWNT-based technology instead.