Photo-thermal enhanced carbon nanotube rectenna arrays for solar energy conversion

Issued as final reportThis project was aimed at fabricating dense ordered arrays of vertical carbon nanotube (CNT) rectenna with diode components exhibiting high asymmetry and nonlinearity. Its broad objectives were to explore and demonstrate the devices with solar energy conversion at high solar frequencies and with efficient power conversion that can be used in a variety of defense applications.Space and Naval Warfare Systems Center San Diego (U.S.

Buckling-driven delamination of carbon nanotube forests

We report buckling-driven delamination of carbon nanotube (CNT) forests from their growth substrates when subjected to compression. Macroscale compression experiments reveal local delamination at the CNT forest-substrate interface. Results of microscale flat punch indentations indicate that enhanced CNT interlocking at the top surface of the forest accomplished by application of a metal coating causes delamination of the forest from the growth substrate, a phenomenon not observed in indentation of as-grown CNT forests. We postulate that the post-buckling tensile stresses that develop at the base of the CNT forests serve as the driving force for delamination

Compressive response of vertically aligned carbon nanotube films gleaned from in situ flat-punch indentations

We report the mechanical behavior of vertically aligned carbon nanotube films, grown on Si substrates using atmospheric pressure chemical vapor deposition, subjected to in situ large displacement (up to 70 μm) flat-punch indentations. We observed three distinct regimes in their indentation stress–strain curves: (i) a short elastic regime, followed by (ii) a sudden instability, which resulted in a substantial rapid displacement burst manifested by an instantaneous vertical shearing of the material directly underneath the indenter tip by as much as 30 μm, and (iii) a positively sloped plateau for displacements between 10 and 70 μm. In situ nanomechanical indentation experiments revealed that the shear strain was accommodated by an array of coiled carbon nanotube “microrollers,” providing a low-friction path for the vertical displacement. Mechanical response and concurrent deformation morphologies are discussed in the foam-like deformation framework with a particular emphasis on boundary conditions

Effects of morphology on the micro-compression response of carbon nanotube forests

This study reports the mechanical response of distinct carbon nanotube (CNT) morphologies as revealed by flat punch in situ nanoindentation in a scanning electron microscope. We find that the location of incipient deformation varies significantly by changing the CNT growth parameters. The initial buckles formed close to the growth substrate in 70 and 190 µm tall CNT forests grown with low pressure chemical vapor deposition (LPCVD) and moved to ~100 µm above the growth substrate when the height increased to 280 µm. Change of the recipe from LPCVD to CVD at pressures near atmospheric changed the location of the initial buckling event from the bottom half to the top half of the CNT forest. Plasma pretreatment of the catalyst also resulted in a unique CNT forest morphology in which deformation started by bending and buckling of the CNT tips. We find that the vertical gradients in CNT morphology dictate the location of incipient buckling. These new insights are critical in the design of CNT forests for a variety of applications where mechanical contact is important

A pulsed source-sink fluid mixing device,

Abstract-Efficient fluid mixing can be achieved in a high-aspect-ratio volume by periodically pulsing an arrangement of source-sink pairs. In order to conserve fluid and promote mixing, the fluid extracted through a sink is subsequently injected through a source. We present an implementation of this approach that consists of a disposable chip with embedded microchannels and external fluidic control. When both the mixing chamber geometry and the source-sink arrangement are fixed, mixing is controlled by choosing , the fraction of the mixing chamber volume that is exchanged with each pulse. Experimental results in a rectangular chamber show that the value of has a significant effect on mixing efficiency. This device shows promise for enhancing the performance of massively parallel sensing systems such as DNA microarrays. [1592

Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An in-Situ Study

We report mechanical behavior and strain rate dependence of recoverability and energy dissipation in vertically aligned carbon nanotube (VACNT) bundles subjected to quasi-static uniaxial compression. We observe three distinct regimes in their stress–strain curves for all explored strain rates from 4 × 10^(–2) down to 4 × 10^(–4) /sec: (1) a short initial elastic section followed by (2) a sloped plateau with characteristic wavy features corresponding to buckle formation and (3) densification characterized by rapid stress increase. Load–unload cycles reveal a stiffer response and virtually 100% recoverability at faster strain rates of 0.04/sec, while the response is more compliant at slower rates, characterized by permanent localized buckling and significantly reduced recoverability. We propose that it is the kinetics of attractive adhesive interactions between the individual carbon nanotubes within the VACNT matrix that governs morphology evolution and ensuing recoverability. In addition, we report a 6-fold increase in elastic modulus and gradual decrease in recoverability (down to 50%) when VACNT bundles are unloaded from postdensification stage as compared with predensification. Finally, we demonstrate energy dissipation capability, as revealed by hysteresis in load–unload cycles. These findings, together with high thermal and electrical conductivities, position VACNTs in the “unattained-as-of-to-date-space” in the material property landscape