Optical characterization of novel materials for photocathodes
Abstract
Cubic-Boron Nitride (c-BN) and hexagonal Boron-Nitride (h-BN), like diamond, carbon nanotubes, graphene, Tungsten Tri-Oxide (WO3), have extraordinary material properties that make them ideal for surviving and improving high power environments like linear accelerators (LINACs), high powered lasers and radio frequency (RF) devices, hypersonic aeronautical regimes, etc. As an example, c-BN is the second hardest known material (tensile strength = 100 GPa) next to diamond. c-BN is chemically inert, has great thermal conductivity (740 -1300 W m-1 K-1 > 4x Copper), tremendous insulation (breakdown voltage Giga-Volts/meter), sublimation temperature of 2973 C, wide optical Band-gap (Eg=6.3 eV), which makes it transparent at mid-wave infrared (3 – 11 micron) wavelengths, and it has a negative electron affinity. The negative electron affinity means the conduction band is above the vacuum band, meaning that any electron that enters c-BN will be emitted easily, which is very useful property for cathodes LINACs and electron multipliers LINACs/High Powered Microwave (HPM) devices. However, due to the high sublimation temperature and hardness of these c-BN and WO3 materials, they are extremely difficult to form single crystalline, macroscopic materials. Existing c-BN, h-BN, MoO3, WO3, CNTs, etc. are polycrystalline & nano-crystalline in grain structure, thus these breaks in the crystalline structure form the weak parts of the macroscopic thin layer material mechanically. In addition, these grain structures in c-BN, MoO3, WO3 alter electromagnetic (EM) propagation with multiple reflections. These reflections combined with current flow can cause surface plasmon-polariton spoofs (SPPS), which are coherent, EM wave and current oscillations at the interface between the thin film and metal/CNT film. In addition, the high binding energies (rigidity) of electronic bonds between atoms in a highly organized, dense lattice (like CNT’s and h-BN SP2 and c-BN SP3 bond) result in high EM/current resonance plasmons and phonons. The CNT’s length and diameter were shown to be consistent with a plasmon resonance explanation of the peak in the conductivity measured in CNT films. Similar ?0 expressions exist for graphene and h-BN, which are dependent on the grain size of the macroscopic graphene and h-BN materials. The plasmon resonance frequency of these nano-structured materials are typically in the 1 to 30 Terahertz (THz) regime, thus requiring unique equipment to interrogate and perform diagnostics on this plasmonic response. The Department of Physics of Sapienza University however, has developed multiple diagnostic tools that are well suited for this task. Understanding, the effect of nano-structure, interfaces, thin films and grain boundaries on the SPPS response of these material is critical to understanding the total EM response of the material under high power, i.e. extreme non-equilibrium conditions. Highly aligned and highly densified CNT fibers and films which come in 100s meter of length were developed by Rice University. These CNT fibers and films tested by Air Force Research Laboratory (AFRL) and have been shown to be exception field emission cathodes and also exhibit highly asymmetric thermal conductivity. Thus, providing the potential to rapidly spread the heat from the hot spot to the bulk heat sink material away from the hotspot. Understanding the SPPS response of c-BN and/or h-BN thin films on these CNT fibers could result in plasmon-enhanced electron emission for improved current/charge per pulse photo-cathodes for LINACs and x-ray beam applications. The phonon and thus energy/heat transfer across the c-BN thin film to CNT film interface is highly important for potentially allowing c-BN coated CNT films to rapidly transport heat away from a localized heating points to the CNT film/fiber. Thus, highly useful for passively mitigating heating problems in applications such as a photo-cathode or hypersonic vehicles’ leading edge.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 20, 2023
- Source ID
- FA86552217232
Entities
People
- S. Lupi
Organizations
- Air Force Office of Scientific Research
- Sapienza University of Rome
- United States Air Force