Terry T. Xu

Contact thermal resistance between individual multiwall carbon nanotubes

We report on experimental measurements of contact thermal resistance between individual carbon nanotubes. Results indicate that the contact thermal conductance can increase by nearly two orders of magnitude (from 10−8 to 10−6u2002W/K) as the contact area increases from a cross contact to an aligned contact. Normalization with respect to the contact area leads to normalized contact thermal resistance on the order of 10−9u2002m2u2009K/W at room temperature, one order of magnitude lower than that from a molecular dynamics simulation in literature. These results should have important implications in the design of carbon nanotube-polymer composites for tunable thermal properties.

Enhanced and switchable nanoscale thermal conduction due to van der Waals interfaces

Understanding thermal transport in nanostructured materials is important for the development of energy conversion applications and the thermal management of microelectronic and optoelectronic devices. Most nanostructures interact through van der Waals interactions, and these interactions typically lead to a reduction in thermal transport. Here, we show that the thermal conductivity of a bundle of boron nanoribbons can be significantly higher than that of a single free-standing nanoribbon. Moreover, the thermal conductivity of the bundle can be switched between the enhanced values and that of a single nanoribbon by wetting the van der Waals interface between the nanoribbons with various solutions.

Measurement of the Intrinsic Thermal Conductivity of a Multiwalled Carbon Nanotube and Its Contact Thermal Resistance with the Substrate

The intrinsic thermal conductivity of an individual carbon nanotube and its contact thermal resistance with the heat source/sink can be extracted simultaneously through multiple measurements with different lengths of the tube between the heat source and the heat sink. Experimental results on a 66-nm-diameter multiwalled carbon nanotube show that above 100 K, contact thermal resistance can contribute up to 50% of the total measured thermal resistance; therefore, the intrinsic thermal conductivity of the nanotube can be significantly higher than the effective thermal conductivity derived from a single measurement without eliminating the contact thermal resistance. At 300 K, the contact thermal resistance between the tube and the substrate for a unit area is 2.2 × 10(-8) m(2) K W(-1) , which is on the lower end among several published data. Results also indicate that for nanotubes of relatively high thermal conductance, electron-beam-induced gold deposition at the tube-substrate contacts may not reduce the contact thermal resistance to a negligible level. These results provide insights into the long-lasting issue of the contact thermal resistance in nanotube/nanowire thermal conductity measurements and have important implications for further understanding thermal transport through carbon nanotubes and using carbon nanotube arrays as thermal interface materials.

A facile approach to synthesize single-crystalline rutile TiO2 one-dimensional nanostructures

TiO2 one-dimensional (1D) nanostructures such as nanowires and nanoribbons were synthesized by direct heating of Ni-coated TiO powders in argon at 850–920u2009°C and ~760xa0Torr in a tube furnace. The nanostructures were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy and Raman spectroscopy. The 1D nanostructures were found to be single-crystalline rutile TiO2. The preferred growth direction is along the [110] direction. The nanowires are roughly ~10–50xa0nm in diameter, and 5–20xa0µm in length. The nanoribbons are 0.4–2xa0µm in width and 5–20xa0µm in length. The growth of nanostructures under different conditions of substrate, catalytic material, reaction gas environment and chemical composition of solid TixOy precursor was studied. The possible growth mechanisms are discussed.

Boron carbide nanowires: low temperature synthesis and structural and thermal conductivity characterization

Boron carbide nanowires, a promising class of high temperature thermoelectric nanomaterials, are synthesized by co-pyrolysis of diborane and methane in a low pressure chemical vapor deposition system via the vapor–liquid–solid growth mechanism. Nickel and iron are effective catalytic materials. The synthesis is realized at relatively lower temperatures, with 879 °C as the lowest one. Electron microscopy analysis shows that the as-synthesized nanowires have diameters between 15 and 90 nm and lengths up to 10 μm. The nanowires have single crystalline boron carbide cores and thin amorphous oxide sheaths. Both transverse faults and axial faults with fault planes as {101}h-type are observed, which could provide additional measures to tune the nanowire transport properties for better thermoelectric performance. Measurement of individual boron carbide nanowires reveals that the thermal conductivity is diameter-dependent, which indicates that boundary scattering still provides an effective approach to reduce the wire thermal conductivity for enhanced thermoelectric performance.

Selective growth of tungsten oxide nanowires via a vapor-solid process

Selective growth of tungsten oxide nanowires has been achieved using a vapor-solid (VS) process without the assistance of any catalysts. To achieve selective growth, low vapor supersaturation was employed to suppress the spontaneous nucleation of tungsten oxide during the VS process, and patterned tungsten coating was introduced to provide seed nuclei, which promotes the growth of tungsten oxide nanowires and control their growth sites. Patterned tungsten oxide nanowire arrays have been fabricated using simple patterning methods, such as shadow mask and laser-induced nanograting growth. The effects of the source heating temperature and the growth temperature on the nanowire growth have been investigated, showing the morphology of tungsten oxide deposition was sensitive to the vapor supersaturation controlled by both temperatures.

BORON NANOWIRES AND NOVEL TUBE-CATALYTIC PARTICLE-WIRE HYBRID BORON NANOSTRUCTURES

Catalyst-assisted growth of boron nanowires and novel tube–catalytic particle–wire hybrid boron nanostructures were achieved by pyrolysis of diborane at 820–890°C and ~ 200 mTorr in a quartz tube furnace. Electron microscopy imaging and diffraction analysis reveal that most of the nano-structures are amorphous. Elemental analysis by EELS and EDX shows that the nanostructures consist of boron with a small amount of oxygen and carbon. Possible growth mechanisms for the tube–catalytic particle–wire hybrid boron nanostructures are discussed.

Thermal conductivity of individual silicon nanoribbons

The thermal conductivities of two groups of silicon nanoribbons of ∼20 and ∼30 nm thickness and various widths have been measured and analyzed through combining the Callaway model and the Fuchs-Sondheimer (FS) reduction function. The results show that while the data for the ∼30 nm thick ribbons can be well-explained by the classical size effect, the measured thermal conductivities for the ∼20 nm thick ribbons deviate from the prediction remarkably, and size effects beyond phonon-boundary scattering must be considered. The measurements of the Youngs modulus of the thin nanoribbons yield significantly lower values than the corresponding bulk value, which could lead to a reduced phonon group velocity and subsequently thermal conductivity. This study helps to build a regime map for thermal conductivity versus nanostructures surface-area-to-volume ratio that clearly delineates two regions where size effects beyond the Casimir limit are important or not important.

Facile Synthesis and Tensile Behavior of TiO2 One-Dimensional Nanostructures

High-yield synthesis of TiO2 one-dimensional (1D) nanostructures was realized by a simple annealing of Ni-coated Ti grids in an argon atmosphere at 950 °C and 760 torr. The as-synthesized 1D nanostructures were single crystalline rutile TiO2 with the preferred growth direction close to [210]. The growth of these nanostructures was enhanced by using catalytic materials, higher reaction temperature, and longer reaction time. Nanoscale tensile testing performed on individual 1D nanostructures showed that the nanostructures appeared to fracture in a brittle manner. The measured Young’s modulus and fracture strength are ~56.3 and 1.4 GPa, respectively.

Observation of 'hidden' planar defects in boron carbide nanowires and identification of their orientations

The physical properties of nanostructures strongly depend on their structures, and planar defects in particular could significantly affect the behavior of the nanowires. In this work, planar defects (twins or stacking faults) in boron carbide nanowires are extensively studied by transmission electron microscopy (TEM). Results show that these defects can easily be invisible, i.e., no presence of characteristic defect features like modulated contrast in high-resolution TEM images and streaks in diffraction patterns. The simplified reason of this invisibility is that the viewing direction during TEM examination is not parallel to the (001)-type planar defects. Due to the unique rhombohedral structure of boron carbide, planar defects are only distinctive when the viewing direction is along the axial or short diagonal directions ([100], [010], or 1¯10) within the (001) plane (in-zone condition). However, in most cases, these three characteristic directions are not parallel to the viewing direction when boron carbide nanowires are randomly dispersed on TEM grids. To identify fault orientations (transverse faults or axial faults) of those nanowires whose planar defects are not revealed by TEM, a new approach is developed based on the geometrical analysis between the projected preferred growth direction of a nanowire and specific diffraction spots from diffraction patterns recorded along the axial or short diagonal directions out of the (001) plane (off-zone condition). The approach greatly alleviates tedious TEM examination of the nanowire and helps to establish the reliable structure–property relations. Our study calls attention to researchers to be extremely careful when studying nanowires with potential planar defects by TEM. Understanding the true nature of planar defects is essential in tuning the properties of these nanostructures through manipulating their structures.