Unusually low thermal conductivity of gallium nitride nanowires

We report measurements of thermal conductivity κ on individual gallium nitride nanowires (GaN NWs) with diameters ranging from 97 to 181 nm grown by thermal chemical vapor deposition. We observed unexpectedly small kappa values, in the range of 13–19 W/m K at 300 K, with very weak diameter dependence. We also observe unusual power law κ~Tn behavior with n=1.8 at low temperature. Electron-energy-loss-spectroscopy measurements indicate Si and O concentrations in the ranges of 0.1–1 and 0.01–0.1 at. %, respectively. Based on extensive numerical calculations, we conclude that both the unexpectedly low κ and the T1.8 dependence are caused by unusually large mass-difference scattering, primarily from Si impurities. Our analysis also suggests that mass-difference scattering rates are significantly enhanced by the reduced phonon group velocity in nanoscale systems. Planar defects running the length of the NW, previously characterized in detail, may also play a role in limiting the phonon mean free path

An infiltration method for preparing single-wall nanotube/epoxy composites with improved thermal conductivity

Recent studies of SWNT/polymer nanocomposites identify the large interfacial thermal resistance at nanotube/nanotube junctions as a primary cause for the only modest increases in thermal conductivity relative to the polymer matrix. To reduce this interfacial thermal resistance, we prepared a freestanding nanotube framework by removing the polymer matrix from a 1 wt % SWNT/PMMA composite by nitrogen gasification and then infiltrated it with epoxy resin and cured. The SWNT/epoxy composite made by this infiltration method has a micron-scale, bicontinuous morphology and much improved thermal conductivity (220% relative to epoxy) due to the more effective heat transfer within the nanotube-rich phase. By applying a linear mixing rule to the bicontinuous composite, we conclude that even at high loadings the nanotube framework more effectively transports phonons than well-dispersed SWNT bundles. Contrary to the widely accepted approaches, these findings suggest that better thermal and electrical conductivities can be accomplished via heterogeneous distributions of SWNT in polymer matrices

Correlation of properties with preferred orientation in coagulated and stretch-aligned single-wall carbon nanotubes

We report structure-property correlations in single wall carbon nanotube (SWNT) fibers, among electrical, thermal and chemical parameters with respect to stretch-induced preferential SWNT alignment along the fiber axis. Purified HiPco tubes are dispersed with the aid of an anionic surfactant and coagulated in the co-flowing stream of an adsorbing polymer. The fibers are then dried, rewetted under tensile load and redried to improve the alignment. Complete removal of the polymer was assured by annealing in hydrogen at 1000oC. The degree of alignment was determined by x-ray scattering from individual fibers using a 2-dimensional detector. The half width at half maximum (HWHM) describing the axially symmetric distribution of SWNT axes decreases linearly from 27.5o in the initial extruded fiber to 14.5o after stretching by 80%. The electrical resistivity ρ at 300 K decreases overall by a factor ~4 with stretching, for both as-spun composite and polymer-free annealed fibers. However, the temperature dependence ρ(T) is markedly different for the two, implying different electron transport mechanisms with and without the polymer. Thermal conductivity also improves with increasing alignment, while the absolute values are limited by the disordered network of finite length tubes and bundles. Comparisons are made with results from similar fibers spun from oleum, and with magnetically aligned buckypapers

Magnetically aligned single wall carbon nanotube films: preferred orientation and anisotropic transport properties

Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 Tesla fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5–9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both x-ray and Raman data are required for a complete texture analysis of oriented SWNT materials

Macroscopic Neat Single-Walled Carbon Nanotubes Fibers

The first-ever well-aligned continuous macroscopic neat single-walled carbon nanotube (SWNT) fibers were produced using conventional spinning techniques. Neat SWNT fibers, containing no surfactant or polymer, were made by spinning dispersions of SWNTs in 102% sulfuric acid into different coagulants. The critical role of sulfuric acid in dispersing and aligning SWNTs during fiber formation has been explored. Characterization shows alignment greater than any other macroscopic neat SWNT material reported to-date while providing insight into the fundamental hierarchy and nature of SWNT fiber formation. Electrical, thermal, and mechanical measurements indicate that neat SWNT fibers hold tremendous potential for future applications

Thermal transport in SWNT-PMMA composites and individual gallium nitride nanowires

Single-wall carbon nanotubes (SWNT) are considered as promising filler materials for improving the thermal conductivity of conventional polymers. We investigated the thermal conductivity of SWNT/PMMA nanocomposites with random SWNT orientations and loadings up to 10 wt% using the comparative technique. The composites were prepared by coagulation [1] and exhibit ∼250% improvement in the thermal conductivity at 10 wt%o. The experimental results were analyzed using the versatile Nielsen model [2], which accounts for many important factors, including filler aspect ratio and maximum packing fraction. In this work the aspect ratio was determined by AFM [3] and used as an input parameter in the Nielsen model. The comparison between our results and the predictions of the Nielsen model indicates that higher aspect ratio fillers are needed to achieve further enhancement. Our analysis also suggests that improved thermal contact between the SWNT network and the matrix material would be beneficial. In the second set of experiments we studied nanoscale thermal transport in individual GaN nanowires grown by thermal CVD method in our group. We measured the thermal conductivity κ, of several GaN nanowires with diameters ranging from 97 nm to 181 nm by the suspended islands method [4]. An unexpectedly large reduction of κ, is observed in these nanowires. They also exhibit an unusual T1.8 low-temperature κ dependence. We analyzed our experimental results within the framework of the Callaway model of heat conduction [5]. A moderate reduction of κ is expected due to the increase of boundary scattering for small cross-sections [6]. TEM analysis [7] of our GaN NWs revealed the presence of stacking faults (SFs). These SFs are expected to further reduce the phonon mean free path. Based on our extensive numerical calculations we concluded that both the unexpected reduction in κ as well as the strange T1.8 low-temperature κ dependence is caused by unusually large mass-difference scattering. From the growth chemistry one expects Si and O to be the main impurities; from EELS measurements [7] we estimate their concentration at 0.1–1 at% and 0.01–0.1 at%, respectively. However, we obtain ∼3 at% Si from the best fits of our experimental data assuming the bulk GaN phonon group velocity vG of 5071 m/s. Our numerical calculations of confined phonon dispersions for a 40 nm GaN NW indicate a 2-fold reduction of vG. A natural explanation for the discrepancy lies in the enhancement of mass-difference scattering rates due to the inverse cubic dependence on vG

Thermal transport in SWNT-PMMA composites and individual gallium nitride nanowires

Single-wall carbon nanotubes (SWNT) are considered as promising filler materials for improving the thermal conductivity of conventional polymers. We investigated the thermal conductivity of SWNT/PMMA nanocomposites with random SWNT orientations and loadings up to 10 wt% using the comparative technique. The composites were prepared by coagulation [1] and exhibit ∼250% improvement in the thermal conductivity at 10 wt%o. The experimental results were analyzed using the versatile Nielsen model [2], which accounts for many important factors, including filler aspect ratio and maximum packing fraction. In this work the aspect ratio was determined by AFM [3] and used as an input parameter in the Nielsen model. The comparison between our results and the predictions of the Nielsen model indicates that higher aspect ratio fillers are needed to achieve further enhancement. Our analysis also suggests that improved thermal contact between the SWNT network and the matrix material would be beneficial. In the second set of experiments we studied nanoscale thermal transport in individual GaN nanowires grown by thermal CVD method in our group. We measured the thermal conductivity κ, of several GaN nanowires with diameters ranging from 97 nm to 181 nm by the suspended islands method [4]. An unexpectedly large reduction of κ, is observed in these nanowires. They also exhibit an unusual T1.8 low-temperature κ dependence. We analyzed our experimental results within the framework of the Callaway model of heat conduction [5]. A moderate reduction of κ is expected due to the increase of boundary scattering for small cross-sections [6]. TEM analysis [7] of our GaN NWs revealed the presence of stacking faults (SFs). These SFs are expected to further reduce the phonon mean free path. Based on our extensive numerical calculations we concluded that both the unexpected reduction in κ as well as the strange T1.8 low-temperature κ dependence is caused by unusually large mass-difference scattering. From the growth chemistry one expects Si and O to be the main impurities; from EELS measurements [7] we estimate their concentration at 0.1–1 at% and 0.01–0.1 at%, respectively. However, we obtain ∼3 at% Si from the best fits of our experimental data assuming the bulk GaN phonon group velocity vG of 5071 m/s. Our numerical calculations of confined phonon dispersions for a 40 nm GaN NW indicate a 2-fold reduction of vG. A natural explanation for the discrepancy lies in the enhancement of mass-difference scattering rates due to the inverse cubic dependence on vG

Thermal conductivity of single-walled carbon nanotube/pmma nanocomposites

Single-walled carbon nanotubes (SWNTs