David D. Allred received Alumni Professorship Award

Abstract: The utility of unmanned micro underwater vehicles (MUVs) is paramount for exploring confined spaces, but their spatial agility is often impaired when maneuvers require burst-propulsion. Herein we develop high-aspect ratio (150:1), multiwalled carbon nanotube microarray membranes (CNT-MMs) for propulsive, MUV thrust generation by the decomposition of hydrogen peroxide (H₂O₂). The CNT-MMs are grown via chemical vapor deposition with diamond shaped pores (nominal diagonal dimensions of 4.5 × 9.0 μm) and subsequently decorated with urchin-like, platinum (Pt) nanoparticles via a facile, electroless, chemical deposition process. The Pt-CNT-MMs display robust, high catalytic ability with an effective activation energy of 26.96 kJ mol^–1 capable of producing a thrust of 0.209 ± 0.049 N from 50% [w/w] H₂O₂ decomposition within a compact reaction chamber of eight Pt-CNT-MMs in series.

Abstract: Carbon nanotubes (CNTs) can be grown in dense lithographically patterned forests to form framework structures that can be filled in via chemical vapor deposition to form solid structures. These solid structures can then be used in microelectromechanical systems (MEMS) applications. Initial testing with these structures suggests that when these frameworks are filled with carbon, the resulting material exhibits favorable properties for use in compliant MEMS. To better understand this material's properties, we conducted tests to measure its Young's modulus, failure stress, and stress relaxation in the direction perpendicular to the CNT growth, as well as the modulus and stress in the direction parallel to the CNTs. To determine the properties in the transverse direction, we applied vertical loads to the tips of simple cantilever beam samples, and recorded the force and deflection until failure. The results showed failure strain up to 2.48%. Cantilever samples prepared from the same pattern were also used to measure the stress relaxation of the material. The first test for each sample showed an average force relaxation of 3.72%, while successive tests only produced 1.23% after 24 h. To determine the properties in the direction parallel to the CNTs, we prepared simple rectangular beams and subjected them to 3-point bending tests. The average strain calculated in the parallel direction was 8.17%.

Abstract:

Experimentally and computationally, the structure of Pt–Cu at 1:3 stoichiometry has a convoluted history. The L1₃ structure has been predicted to occur in binary alloy systems, but has not been linked to experimental observations. Using a combination of electron diffraction, synchrotron X-ray powder diffraction, and Monte Carlo simulations, we demonstrate that it is present in the Cu–Pt system at 1:3 stoichiometry. We also find that the 4-atom, fcc superstructure L1₃ is equivalent to the large 32-atom orthorhombic superstructure reported in older literature, resolving much of the confusion surrounding this composition. Quantitative Rietveld analysis of the X-ray data and qualitative trends in the electron-diffraction patterns reveal that the secondary X1+(a,0,0) order parameter of the L1₃ phase is unexpectedly weak relative to the primary L1+(a,a,0,0) order parameter, resulting in a partially-ordered L1₃ ordering, which we conclude to be the result of kinetic limitations. Monte Carlo simulations confirm the formation of a large cubic superstructure at high temperatures, and its eventual transformation to the L1₃ structure at lower temperature, but also provide evidence of other transitional orderings.

Abstract: We describe the direct, conformal, atomic layer deposition (ALD) of silica onto carbon nanotubes (CNTs) in the microfabrication of thinlayer chromatography (TLC) plates. As before, these plates were prepared with zig-zag hedge and channel microstructures, with high aspect ratio, porous hedges. After ALD, scanning electron microscopy (SEM) showed an increase in the radius of the CNTs of 8–40 nm. X-ray photoelectron spectroscopy (XPS) showed that the plates were composed almost entirely of silicon and oxygen, without contamination of metals or other elements that might compromise chromatographic performance, e.g., aluminum. Time-of-flight secondary ion mass spectrometry confirmed the extremely low level of aluminum in the plates. The final TLC layer thickness was ca. 50 μm. Separations of a test mixture of dyes from CAMAG (Muttenz, Switzerland) on an uncoated silica plate under traditional, normal phase conditions gave efficiencies of 40,000–140,000 plates m⁻¹ with migration distances ranging from 2 to 36 mm. A separation of two fluorescent dyes, eosin Y disodium salt and sulforhodamine B, on an amino silane-coated plate gave efficiencies of ca. 170,000 and 200,000 plates m⁻¹, with hR_F values of 76 and 88, respectively. Run times on these new plates were much faster than on conventional TLC plates.

Abstract: Described is a method for ultrasonically spraying thin films of carbon nanotubes that have been suspended in organic solvents. Nanotubes were sonicated in N -methyl-2-pyrrolidone or N -cyclohexyl-2-pyrrolidone (CHP) and then sprayed onto a heated substrate using an ultrasonic spray nozzle. The solvent was quickly evaporated, leaving a thin film of randomly oriented nanotubes. Unlike other methods of spraying nanotube films, this does not require removal of surfactant after spraying and is compatible with creating films of functionalized nanotubes. Film thickness was controlled by the spray time and films were sprayed with thicknesses between 10 and 500 nm. Single-walled, multiwalled, and functionalized multiwalled nanotubes were sprayed. Transparent conducting thin films prepared by spraying single-walled carbon nanotubes dispersed in CHP demonstrated similar sheet resistance (for a given optical transmittance) as those prepared by spraying aqueous polymer-based dispersions that required postdeposition polymer removal.

Abstract: We apply a suite of analytical tools to characterize materials created in the production of microfabricated thin layer chromatography plates. Techniques used include X-ray photoelectron spectroscopy (XPS), valence band spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS) in both positive and negative ion modes, Rutherford backscattering spectroscopy (RBS), and helium ion microscopy. Materials characterized include: the Si(100) substrate with native oxide: Si/SiO2, alumina (35nm) deposited as a diffusion barrier on the Si/SiO2: Si/SiO2/Al2O3, iron (6nm) thermally evaporated on the Al2O3: Si/SiO2/Al2O3/Fe, the iron film annealed in H-2 to make Fe catalyst nanoparticles: Si/SiO2/Al2O3/Fe(NP), and carbon nanotubes (CNTs) grown from the Fe nanoparticles: Si/SiO2/Al2O3/Fe(NP)/CNT. The Fe films and nanoparticles appear in an oxidized state. Some of the analyses of the CNTs/CNT forests appear to be unique: (i) the CNT forest appears to exhibit an interesting channeling' phenomenon by RBS, (ii) we observe an odd-even effect in the SIMS spectra of C-n(-) species for n=1 - 6, with the n6 ions showing a steady decrease in intensity, and (iii) valence band characterization of CNTs using X-radiation is reported. Initial analysis of the CNT forest by XPS shows that it is 100at.% carbon. After one year, only ca. 0.25at.% oxygen is observed. The information obtained from the combination of the different analytical tools provides a more complete understanding of our materials than a single technique, which is analogous to the story of The Blind Men and the Elephant'. The raw XPS and ToF-SIMS spectra from this study will be submitted to Surface Science Spectra for archiving. Copyright (c) 2013 John Wiley & Sons, Ltd.