J. S. Brooks

Enhancement of thermal and electrical properties of carbon nanotube polymer composites by magnetic field processing

We show that the thermal and electrical properties of single wall carbon nanotube (CNT)-polymer composites are significantly enhanced by magnetic alignment during processing. The electrical transport properties of the composites are mainly governed by the hopping conduction with localization lengths comparable to bundle diameters. The bundling of nanotubes during the composite processing is an important factor for electrical, and in particular, for thermal transport properties. Better CNT isolation will be needed to reach the theoretical thermal conductivity limit for CNT composites.

The calculated thermodynamic properties of superfluid helium‐4

Comprehensive tables of the primary thermodynamic properties of superfluid helium‐4, such as the specific heat and entropy, are presented as computed from the Landau quasiparticle model, with the aid of inelastic neutron scattering data. The neutron data are presented by continuous functions of temperature pressure, and wave number and certain excitation properties such as number density, normal and superfluid densities are calculated directly from it. A discussion of the methods used in our computations is included, and comparisons of computed and experimental results are made where applicable. Certain inadequacies of present theoretical methods to describe the thermodynamic properties are reported, and the use of an effective spectrum is introduced to offset some of these difficulties. Considerable experimental effort is also needed to improve the present situation.

Electromagnetic interference shielding properties of carbon nanotube buckypaper composites

Preformed carbon nanotube thin films (10-20 microm), or buckypapers (BPs), consist of dense and entangled nanotube networks, which demonstrate high electrical conductivity and provide potential lightweight electromagnetic interference (EMI) solutions for composite structures. Nanocomposite laminates consisting of various proportions of single-walled and multi-walled carbon nanotubes, having different conductivity, and with different stacking structures, were studied. Single-layer BP composites showed shielding effectiveness (SE) of 20-60 dB, depending on the BP conductivity within a 2-18 GHz frequency range. The effects on EMI SE performance of composite laminate structures made with BPs of different conductivity values and epoxy or polyethylene insulating layer stacking sequences were studied. The results were also compared against the predictions from a modified EMI SE model. The predicted trends of SE value and frequency dependence were consistent with the experimental results, revealing that adjusting the number of BP layers and appropriate arrangement of the BP conducting layers and insulators can increase the EMI SE from 45 dB to close to 100 dB owing to the utilization of the double-shielding effect.

Superconductivity in an Organic Insulator at Very High Magnetic Fields

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor lambda-(BETS)2FeCl4. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe3+ ions. We further argue that the Fe3+ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state.

New opportunities in science, materials, and biological systems in the low-gravity (magnetic levitation) environment (invited)

We discuss new opportunities that present themselves with the advent of very high magnetic field resistive magnets with appreciable central bore access. A detailed description of the parameters of the magnetic force environment for the case of diamagnetic materials in a water-cooled Bitter-type resistive magnet is provided for the reader who may have an interest in low-gravity experiments. We discuss emerging research activities involving novel uses of magnetic forces in high field resistive magnets at the National High Magnetic Field Laboratory. Particular attention is given to the area of diamagnetic materials that allow a low or “zero” gravity state, i.e., magnetic levitation. These include studies involving plant growth, protein crystallization, and dynamics of single particles and granular materials. In the latter case, unique aspects of the magnetic force environment allow low gravity experiments on particulates that cannot be performed on the Space Shuttle due to the lack of a weak confining potent...

Carbon nanotubes on a spider silk scaffold

Understanding the compatibility between spider silk and conducting materials is essential to advance the use of spider silk in electronic applications. Spider silk is tough, but becomes soft when exposed to water. Here we report a strong affinity of amine-functionalised multi-walled carbon nanotubes for spider silk, with coating assisted by a water and mechanical shear method. The nanotubes adhere uniformly and bond to the silk fibre surface to produce tough, custom-shaped, flexible and electrically conducting fibres after drying and contraction. The conductivity of coated silk fibres is reversibly sensitive to strain and humidity, leading to proof-of-concept sensor and actuator demonstrations.

Effects of surfactants and alignment on the physical properties of single-walled carbon nanotube buckypaper

Single-walled carbon nanotubes were dispersed in an aqueous medium using surfactants and filtered to make entangled networks, called buckypaper (BP), and the Raman spectra of BP samples revealed the degree of entanglement and residual surfactant content. The temperature dependence of the G-band peak shift in the BP was found to depend on the reduction in residual surfactant and nanotube oxidation. The electrical conductivity was improved after removing the surfactant and increasing the nanotube alignment, although the temperature dependence of electrical resistivity still followed a variable range hopping conduction behavior. The mechanical properties were affected by the degree of entanglement, alignment, and residual surfactant content, and tensile properties were found to improve with the reduction in surfactant and enhancement of alignment.

Tunneling characteristics of amorphous Si barriers

Tunnel junctions using a barrier of amorphous silicon (a‐Si) between normal and superconducting metals were studied at temperatures from 300 to 0.45 K. These junctions were reliably made by depositing on a 77 K glass substrate Ni or Au, a‐Si 60 to 100 A thick, and then Al. It was demonstrated that the dominant conduction process was elastic tunneling by the presence of structure caused by the superconducting energy gap of Al, and by comparing measurements of the voltage, temperature, and barrier thickness dependence of the conductance with theory. The effective barrier heights were grouped close to 2×10−2 eV. A semiquantitative argument suggests that the barriers controlling the elastic tunneling are much the same as those controlling the phonon‐activated variable range tunneling at higher temperatures. Although a‐Si barriers can be formed reliably and have low leakage, the low barrier height leads to large nonlinearity even at low voltages.

Anomalous Fermi surface in FeSe seen by Shubnikov–de Haas oscillation measurements

We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order of 0.01 carriers/ Fe, an order-of-magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering.

Small sample magnetometers for simultaneous magnetic and resistive measurements at low temperatures and high magnetic fields

We describe a very simple method for making measurements of the isotropic and anisotropic static magnetization of small samples which is especially useful in the limit of very low temperatures and high magnetic fields. The sensitivity of this technique can surpass that of commercial superconducting magnetometers in the high magnetic field limit. Methods for calibration are presented and magnetization measurements on several materials are shown to demonstrate the technique.