William Mickelson

Synthesis and characterization of highly crystalline graphene aerogels.

Aerogels are used in a broad range of scientific and industrial applications due to their large surface areas, ultrafine pore sizes, and extremely low densities. Recently, a large number of reports have described graphene aerogels based on the reduction of graphene oxide (GO). Though these GO-based aerogels represent a considerable advance relative to traditional carbon aerogels, they remain significantly inferior to individual graphene sheets due to their poor crystallinity. Here, we report a straightforward method to synthesize highly crystalline GO-based graphene aerogels via high-temperature processing common in commercial graphite production. The crystallization of the graphene aerogels versus annealing temperature is characterized using Raman and X-ray absorption spectroscopy, X-ray diffraction, and electron microscopy. Nitrogen porosimetry shows that the highly crystalline graphene macrostructure maintains a high surface area and ultrafine pore size. Because of their enhanced crystallinity, these graphene aerogels exhibit a ∼ 200 °C improvement in oxidation temperature and an order of magnitude increase in electrical conductivity.

Scaled Synthesis of Boron Nitride Nanotubes, Nanoribbons, and Nanococoons Using Direct Feedstock Injection into an Extended- Pressure, Inductively-Coupled Thermal Plasma

A variable pressure (up to 10 atm) powder/gas/liquid injection inductively coupled plasma system has been developed and used to produce high-quality boron nitride nanotubes (BNNTs) at continuous production rates of 35 g/h. Under suitable conditions, collapsed BN nanotubes (i.e., nanoribbons), and closed shell BN capsules (i.e., nanococoons) are also obtained. The process is adaptable to a large variety of feedstock materials.

Imaging the life story of nanotube devices

Live imaging of operating multiwall carbon nanotube (MWCNT-) based electronic devices is performed by high resolution transmission electron microscopy. Our measurements allow us to correlate electronic transport with changes in device structure. Surface contamination, contact annealing, and sequential wall removal are observed. Temperature profiles confirm diffusive conduction in MWCNTs in the high bias limit. This technique provides a general platform for studying nanoscale systems, where geometric configuration and electronic transport are intimately connected.

Current-controlled nanotube growth and zone refinement

We describe methods by which the growth of a single carbon nanotube (CNT) can be precisely controlled by an electrical current. In one method a CNT is grown to a predetermined geometry inside another nanotube, which serves as a reaction chamber. Another method allows a preexisting marginal-quality multiwall CNT to be zone refined into a higher-quality multiwall CNT by driving a catalytic bead down the length of the nanotube. In both methods, the speed of nanotube formation is adjustable, and the growth can be stopped and restarted at will.

Simplified synthesis of double-wall carbon nanotubes

We demonstrate a simplified synthesis technique for double-wall carbon nanotubes that is an adaptation of chemical vapor deposition (CVD) techniques used previously for the production of single-wall nanotubes. Double-wall nanotubes (DWNTs) provide ideal geometries for numerous fundamental structural, electronic, thermal and vibrational studies, as well as providing a unique new platform for practical applications. The diameter distribution of DWNTs is broad, and it is possible that in previous studies using CVD-grown small-diameter nanotubes, presumed to be single-wall, there were significant numbers of DWNTs present.

Boron Nitride Nanotube Peapods

We demonstrate that boron nitride (BN) nanotubes can be filled “peapod” fashion with C60 molecules. Filling small‐diameter BN nanotubes results in a linear chain of C60 molecules in the interior, while filling large diameter BN nanotubes leads to nanorods of crystalline C60 in the interior. Electron beam damage can be used to fuse the encapsulated C60 molecules into carbon nanotubes, leading to carbon nanotubes encased within insulating BN nanotubes. BN nanotubes can also be synthesized with mobile magnetic nanocrystals within their core.

Specific Heats of Mg(B1−xCx)2: Two‐Gap Superconductors

Specific heats (C) of two polycrystalline samples of Mg(B1−xCx)2 (nominal x = 0.1 and 0.2) were measured in magnetic fields (B) to 9 T and temperatures (T) from 1 to 40 K. Neither sample shows evidence of magnetic impurities nor do they have large non‐superconducting fractions. The transition temperature (Tc) decrease monotonically with x, and the superconducting specific heats can be well fitted with two energy gaps (α) using the phenomenological α model.