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We report measurements of temperature- and length-dependent 1/f noise in vertical single-walled carbon nanotube (SWCNT) arrays. Carbon nanotubes are synthesized in a porous anodic alumina template with sub-micrometer channel lengths ranging from 100 to 700 nm. A significant difference is observed in the 1/f noise magnitude of quasi-ballistic and diffusive SWCNT devices, with quasi-ballistic devices exhibiting 1/f noise levels that are one to two orders of magnitude less than diffusively conducting devices. Furthermore, 1/f noise was measured from 90 to 400 K, and the noise prefactor decreased significantly at temperatures below 250 K. [ABSTRACT FROM AUTHOR]

This work describes an experimental study of thermal conductance across multiwalled carbon nanotube (CNT) array interfaces, one sided (Si-CNT-Ag) and two sided (Si-CNT-CNT-Cu), using a photoacoustic technique (PA). Well-anchored, dense, and vertically oriented multiwalled CNT arrays have been directly synthesized on Si wafers and pure Cu sheets using plasma-enhanced chemical vapor deposition. With the PA technique, the small interface resistances of the highly conductive CNT interfaces can be measured with accuracy and precision. In addition, the PA technique can resolve the one-sided CNT interface component resistances (Si-CNT and CNT-Ag) and the two-sided CNT interface component resistances (Si-CNT, CNT-CNT, and CNT-Cu) and can estimate the thermal diffusivity of the CNT layers. The thermal contact resistances of the one- and two-sided CNT interfaces measured using the PA technique are 15.8±0.9 and 4.0±0.4 mm2 K/W, respectively, at moderate pressure. These results compare favorably with those obtained using a steady state, one-dimensional reference bar method; however, the uncertainty range is much narrower. The one-sided CNT thermal interface resistance is dominated by the resistance between the free CNT array tips and their opposing substrate (CNT-Ag), which is measured to be 14.0±0.9 mm2 K/W. The two-sided CNT thermal interface resistance is dominated by the resistance between the free tips of the mating CNT arrays (CNT-CNT), which is estimated to be 2.1±0.4 mm2 K/W. [ABSTRACT FROM AUTHOR]

The one-dimensional, cylindrical nature of single-walled carbon nanotubes (SWCNTs) suggests that the ideal gating geometry for nanotube field-effect transistors (FETs) is a surround gate (SG). Using vertical SWCNTs templated in porous anodic alumina, SGs are formed using top-down processes for the dielectric/metal depositions and definition of the channel length. Surround gates allow aggressive scaling of the channel to 25% of the length attainable with a bottom-gate geometry without incurring short-channel effects. The process demonstrated here for forming SGs on vertical SWCNTs is amenable for large-scale fabrication of multinanotube FETs. [ABSTRACT FROM AUTHOR]

Heat-conduction interfaces that employ carbon nanotube (CNT) arrays have been fabricated and studied experimentally using a reference calorimeter testing rig in a vacuum environment with infrared temperature measurements. Arrays of multiwalled CNTs are grown directly on silicon substrates with microwave plasma-enhanced chemical vapor deposition. Iron and nickel were used as CNT catalysts. CNT arrays grown under different synthesis conditions exhibit different pressure-contact conductance characteristics. The thermal contact resistance of CNTs with a copper interface exhibits promising results with a minimum value of 19.8 mm² K/W at a pressure of 0.445 MPa. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes an experimental study of thermal contact conductance enhancement enabled by carbon nanotube (CNT) arrays synthesized directly on silicon wafers using plasma-enhanced chemical vapor deposition. Testing based on the one-dimensional reference bar method occurred in a high-vacuum environment with radiation shielding, and temperature measurements were made with an infrared camera. Results from other thermal interface materials are presented, as well as combinations of these materials with CNT arrays. Dry CNT arrays produce a minimum thermal interface resistance of 19.8mm2 K/W, while the combination of a CNT array and a phase change material produces a minimum resistance of 5.2mm2 K/W. [Copyright &y& Elsevier]

A correlation between growth temperature and electrical resistance of multiwalled carbon nanotubes (MWNTs) has been established by measuring the resistance of individual MWNTs grown by microwave plasma-enhanced chemical vapor deposition (PECVD) at 800, 900, and 950 °C. The lowest resistances were obtained mainly from MWNTs grown at 900 °C. The MWNT resistance is larger on average at lower (800 °C) and higher (950 °C) growth temperatures. The resistance of MWNTs correlated well with other MWNT quality indices obtained from Raman spectra. This study identifies a temperature window for growing higher-quality MWNTs with fewer defects and lower resistance by PECVD. [ABSTRACT FROM AUTHOR]

The thermal performance of an interface material comprised of a metal foil with dense, vertically oriented carbon nanotube (CNT) arrays synthesized on both of its surfaces is characterized for rough and smooth interfaces. The CNT/foil deforms in the interfaces by two mechanisms, CNT deformation and foil deformation, that may significantly increase the number of CNT contact spots on both sides of the foil. As a result, thermal interface resistances less than 10 mm2 K/W are achieved at moderate pressures and compare very favorably to alternative interface materials and structures. [ABSTRACT FROM AUTHOR]

Abstract: We report the thermal behavior of gadolinium foils to be used in magneto thermoelectric generator cells. Magneto thermoelectric generator cell technology exploits the ferromagnetic phase transition of gadolinium to drive the movement of a diaphragm ‘shuttle’ whose mechanical energy is converted to electrical form and which enhances heat transfer through both conduction and convection. Efficient heat transfer at mechanical interfaces is critical to increase shuttle speed and the commensurate rate of heat transfer. The synthesis and characterization of carbon nanotube thermal interfaces for the gadolinium foils are described. The total thermal interface resistance of the carbon nanotube coated gadolinium was measured using a one-dimensional reference calorimeter technique. Improvement of carbon nanotube growth based on parametric process variations is described, and the effect of hydrogen embrittlement on the magnetic properties of the gadolinium foils during carbon nanotube growth is quantified. The samples generated in this study were consistently measured with total thermal interface resistances in the range of 65–105mm2 K/W, a reduction of 55–70% compared to bare gadolinium (Rint ≈230mm2 K/W). The addition of carbon nanotube arrays did not alter the magnetic properties of the gadolinium foils and only a slight decrease in the magnetic moment of the gadolinium samples (8–13%) was measured after growth. [Copyright &y& Elsevier]

Abstract: A parametric study of carbon nanotube (CNT) synthesis from catalytically active porous anodic Al–Fe–Al multilayer templates was conducted with respect to pore aspect ratio, Fe layer thickness, CNT synthesis temperature, and pre-anodization thermal annealing. Performance metrics included CNT catalytic activity and the pore wall integrity at the Al–Fe–Al interface. The observed CNT density was a strong function of pore diameter, synthesis temperature and pre-anodization annealing of the catalyst film. Vertical pore wall integrity at the Al–Fe–Al interface was optimized by selection of pre-anodization annealing conditions, with interfacial void formation observed in the absence of this technique. Based on CNT growth rates, an activation energy of 0.52eV was observed for CNT synthesis for all film structures, regardless of pore aspect ratio. The optimization of templated CNT synthesis is expected to assist in the development of high-density vertically oriented CNT-based devices. [Copyright &y& Elsevier]

Abstract: Freestanding single-walled carbon nanotubes (SWCNTs) have been synthesized in a vertical direction, perpendicular to the growth substrate, using applied DC substrate bias in a microwave plasma-enhanced chemical vapor deposition (PECVD) synthesis process. The degree of alignment and spatial density of SWCNTs demonstrate a strong dependence on the magnitude of applied bias, with increased alignment and decreased density with increased bias. The unique synthesis environment created by the application of a negative substrate bias in PECVD aligns SWCNTs along electric field lines and decreases SWCNT density due to bombardment by positively charged hydrogen ions. Multi-excitation wavelength Raman spectroscopy reveals shifts in dominant RBM peaks with the application of dc bias. Use of this technique to orient SWCNTs in the vertical direction may allow for three-dimensional SWCNT-based device architectures. [Copyright &y& Elsevier]

Abstract: High-quality single-walled carbon nanotubes (SWCNTs) have been synthesized from H2–CH4 mixtures on a MgO-supported bimetallic Mo/Co catalyst using microwave plasma-enhanced chemical vapor deposition (PECVD). Reaction parameters including temperature, H2:CH4 ratio, plasma power, and synthesis time have been examined to assess their influence on SWCNT synthesis. Raman spectroscopy and high-resolution field emission scanning electron microscopy reveal that the quality, selectivity, density and predominant diameter of SWCNTs depend on the varied synthesis parameters. Results of this study can be used to optimize SWCNT synthesis conditions and products and to improve understanding of the growth of SWCNTs by PECVD. [Copyright &y& Elsevier]

An exploratory thermal interface structure, made of vertically oriented carbon nanotubes directly grown on a silicon substrate, has been thermally characterized using a 3-omega method. The effective thermal conductivities of the carbon nanotubes (CNT) sample, including the effects of voids, are found to be 74 W/m K to 83 W/m K in the temperature range of 295 K to 323 K, one order higher than that of the best thermal greases or phase change materials. This result suggests that the vertically oriented CNTs potentially can be a promising next-generation thermal interface solution. However, fairly large thermal resistances were observed at the interfaces between the CNT samples and the experimental contact. Minimizing these contact resistances is critical for the application of these materials. [ABSTRACT FROM AUTHOR]