Michael B. Jakubinek

Hydrogen-Catalyzed, Pilot-Scale Production of Small-Diameter Boron Nitride Nanotubes and Their Macroscopic Assemblies

Boron nitride nanotubes (BNNTs) exhibit a range of properties that are as compelling as those of carbon nanotubes (CNTs); however, very low production volumes have prevented the science and technology of BNNTs from evolving at even a fraction of the pace of CNTs. Here we report the high-yield production of small-diameter BNNTs from pure hexagonal boron nitride powder in an induction thermal plasma process. Few-walled, highly crystalline small-diameter BNNTs (∼5 nm) are produced exclusively and at an unprecedentedly high rate approaching 20 g/h, without the need for metal catalysts. An exceptionally high cooling rate (∼10(5) K/s) in the induction plasma provides a strong driving force for the abundant nucleation of small-sized B droplets, which are known as effective precursors for small-diameter BNNTs. It is also found that the addition of hydrogen to the reactant gases is crucial for achieving such high-quality, high-yield growth of BNNTs. In the plasma process, hydrogen inhibits the formation of N2 from N radicals and promotes the creation of B-N-H intermediate species, which provide faster chemical pathways to the re-formation of a h-BN-like phase in comparison to nitridation from N2. We also demonstrate the fabrication of macroscopic BNNT assemblies such as yarns, sheets, buckypapers, and transparent thin films at large scales. These findings represent a seminal milestone toward the exploitation of BNNTs in real-world applications.

Temperature Dependence of Thermal Conductivity Enhancement in Single-walled Carbon Nanotube/polystyrene Composites

The thermal conductivity of single-walled carbon nanotube (SWCNT)/polystyrene composites, prepared by a method known to produce a uniform distribution of SWCNT bundles on the micrometer length scale, was measured in the temperature range from approximately 140 to 360 K. The thermal conductivity enhancement (50% for 1 mass % at 300 K) is reasonably constant above room temperature but is reduced at the lower temperatures. This result is consistent with the expected, large contribution of interfacial thermal resistance in SWCNT/polymer composites. Enhancements in electrical conductivity show that 1 mass % loading is in the region of the electrical percolation threshold.

Temperature excursions at the pulp-dentin junction during the curing of light-activated dental restorations

OBJECTIVES Excessive heat produced during the curing of light-activated dental restorations may injure the dental pulp. The maximum temperature excursion at the pulp-dentin junction provides a means to assess the risk of thermal injury. In this investigation we develop and evaluate a model to simulate temperature increases during light-curing of dental restorations and use it to investigate the influence of several factors on the maximum temperature excursion along the pulp-dentin junction. METHODS Finite element method modeling, using COMSOL 3.3a, was employed to simulate temperature distributions in a 2D, axisymmetric model tooth. The necessary parameters were determined from a combination of literature reports and our measurements of enthalpy of polymerization, heat capacity, density, thermal conductivity and reflectance for several dental composites. Results of the model were validated using in vitro experiments. RESULTS Comparisons with in vitro experiments indicate that the model provides a good approximation of the actual temperature increases. The intensity of the curing light, the curing time and the enthalpy of polymerization of the resin composite were the most important factors. The composite is a good insulator and the greatest risk occurs when using the light to cure the thin layer of bonding resin or in deep restorations that do not have a liner to act as a thermal barrier. SIGNIFICANCE The results show the importance of considering temperature increases when developing curing protocols. Furthermore, we suggest methods to minimize the temperature increase and hence the risk of thermal injury. The physical properties measured for several commercial composites may be useful in other studies.

3D chemically cross-linked single-walled carbon nanotube buckypapers

Single-walled carbon nanotubes (SWCNTs) covalently modified with OH functional groups were assembled into buckypapers through solvent dispersion and vacuum filtration. These SWCNT-OH buckypaper sheets were subsequently crosslinked by wetting with bifunctional linkers followed by hot compression causing reaction between the functional groups of the reagent and OH functional groups on the side-walls of SWCNTs to create three-dimensional (3D) covalently cross-linked buckypapers. Cross-linking also was performed using SWCNTs encapsulated with a functionalized polymer wrapping in a core–shell structure, where OH or/and NH2 groups are available on the surface of the polymeric shell for reaction. The 3D cross-linked SWCNT buckypapers retain the porous character typical of buckypaper, and were characterized for their tensile properties and thermal and electrical conductivities. Several cross-linking approaches dramatically improved the mechanical properties. The strongest and stiffest papers (32 MPa, E = 3.1 GPa), which approach 10× stronger and stiffer than the pristine non-crosslinked buckypaper, were obtained at the expense of a loss of electrical conductivity. In other cases, such as cross-linking using a high-performance epoxy resin monomer, improvements in strength and stiffness of ∼5× were obtained while retaining electrical and thermal conductivity. Therefore, the optimal cross-linking approach would be determined by the desired, multifunctional properties. Additionally, the approach can be used in the preparation buckypaper composites and it is demonstrated that cross-linking using a multifunctional epoxy resin prior to impregnation with the same epoxy resin results in substantially better mechanical properties in comparison to just epoxy-impregnation of pristine buckypaper.

Linear and nonlinear optical properties of carbon nanotube-coated single-mode optical fiber gratings

Single-wall carbon nanotube deposition on the cladding of optical fibers has been carried out to fabricate an all-fiber nonlinear device. Two different nanotube deposition techniques were studied. The first consisted of repeatedly immersing the optical fiber into a nanotube supension, increasing the thickness of the coating in each step. The second deposition involved wrapping a thin film of nanotubes around the optical fiber. For both cases, interaction of transmitted light through the fiber core with the external coating was assisted by the cladding mode resonances of a tilted fiber Bragg grating. Ultrafast nonlinear effects of the nanotube-coated fiber were measured by means of a pump-probe pulses experiment.

Polymer nanocomposites from free-standing, macroscopic boron nitride nanotube assemblies†

Here we report the fabrication of free-standing boron nitride nanotube (BNNT) sheets by direct deposition and by vacuum filtration methods, including novel hybrid assemblies with BNNT and carbon nanotubes. Such sheets have enabled production of polymer nanocomposites with high nanotube content. Two example cases, BNNT–epoxy nanocomposites (>30 wt% BNNTs) produced by impregnation of dry sheets and BNNT sheets modified by integration of a thermoplastic polyurethane are described. Related methods have proven advantageous for carbon nanotube composites and, enabled by new technology for large scale BNNT production, such composites have now been realized with BNNTs. This represents an important milestone towards the development of BNNT-based multifunctional composites.

Fabrication of High Content Carbon Nanotube–Polyurethane Sheets with Tailorable Properties

We have fabricated carbon nanotube (CNT)-polyurethane (TPU) sheets via a one-step filtration method that uses a TPU solvent/nonsolvent combination. This solution method allows for control of the composition and processing conditions, significantly reducing both the filtration time and the need for large volumes of solvent to debundle the CNTs. Through an appropriate selection of the solvents and tuning the solvent/nonsolvent ratio, it is possible to enhance the interaction between the CNTs and the polymer chains in solution and improve the CNT exfoliation in the nanocomposites. The composition of the nanocomposites, which defines the characteristics of the material and its mechanical properties, can be precisely controlled. The highest improvements in tensile properties were achieved at a CNT:TPU weight ratio around 35:65 with a Youngs modulus of 1270 MPa, stress at 50% strain of 35 MPa, and strength of 41 MPa, corresponding to ∼10-fold improvement in modulus and ∼7-fold improvement in stress at 50% strain, while maintaining a high failure strain. At the same composition, CNTs with higher aspect ratio produce nanocomposites with greater improvements (e.g., strength of 99 MPa). Electrical conductivity also shows a maximum near the same composition, where it can exceed the values achieved for the pristine nanotube buckypaper. The trend in mechanical and electrical properties was understood in terms of the CNT-TPU interfacial interactions and morphological changes occurring in the nanocomposite sheets as a function of increasing the TPU content. The availability of such a simple method and the understanding of the structure-property relationships are expected to be broadly applicable in the nanocomposites field.

Thermal Conductivity of Nanotube Assemblies and Superfiber Materials

Individual carbon nanotubes (CNTs) have been reported to have the highest thermal conductivities of any known material. However, significant variability exists both for the reported thermal conductivities of individual CNTs and the thermal conductivities measured for macroscopic CNT assemblies (e.g. CNT films, buckypapers, arrays, and fibers), which range from comparable to metals to aerogel-like. This chapter reviews the current status of the field, summarizing a wide selection of experimental results and drawing conclusions regarding present limitations of the thermal conductivity of CNT assemblies and opportunities for improvement of the performance of nanotube superfiber materials.

Four-wave mixing in carbon nanotube-coated optical fiber gratings

The observation of four-wave mixing (FWM) in single-walled carbon nanotubes (SWCNTs) deposited around a tilted fiber Bragg grating (TFBG) has been demonstrated. A thin, floating SWCNT film is manually wrapped around the outer cladding of the fiber and FWM occurs between two core-guided laser signals by TFBG-induced interaction of the core mode and cladding modes. The effective nonlinear coefficient is calculated to be 1.8 × 103 W−1 Km−1. The wavelength of generated idlers is tunable with a range of 7.8 nm.

Epoxy resin nanocomposites with hydroxyl (OH) and amino (NH2) functionalized boron nitride nanotubes

Abstract Hydroxyl (OH) and amino (NH2) functionalized boron nitride nanotubes (f-BNNTs) were integrated into an epoxy resin (Epon828) to achieve improved mechanical properties. While raw BNNT-composites yielded the largest values for Young’s modulus and appeared to be well mixed, f-BNNTs were found to provide a superior combination of mechanical properties yielding improvements in strain at failure, tensile strength and toughness that were not observed using raw BNNTs. In particular, an increase of 21% in Young’s modulus is observed with 5 wt% of f-BNNT, and increases of 12, 21, and 49% are observed in tensile strength, failure strain, and toughness, respectively, with 2 wt% f-BNNT while a 34% increase in fracture toughness is observed with 3 wt% f-BNNT.