Field emission of electrons from individually mounted carbon nanotubes has been found to be dramatically enhanced when the nanotube tips are opened by laser evaporation or oxidative etching. Emission currents of 0.1 to 1 microampere were readily obtained at room temperature with bias voltages of less than 80 volts. The emitting structures are concluded to be linear chains of carbon atoms, Cn, (n = 10 to 100), pulled out from the open edges of the graphene wall layers of the nanotube by the force of the electric field, in a process that resembles unraveling the sleeve of a sweater.

The electronic structure of small Ga x Asy clusters (x + y< 10) are calculated using the local density method. The calculation shows that even-numbered clusters tend to be singlets, as opposed to odd-numbered clusters which are open shell systems. This is in agreement with the experimental observations of even/odd alternations of the electron affinity and ionization potential. In the larger clusters, the atoms prefer an alternating bond arrangement; charge transfers are observed from Ga sites to As sites. This observation is also in agreement with recent chemisorption studies of ammonia on GaAs clusters. The close agreement between theoretical calculations and experimental results, together with the rich variation of electronic properties of GaAs clusters with composition makes GaAs clusters an ideal prototype system for the study of how electronic structure influences chemical reactivity.

Articles you may be interested inSimulation and modeling of the electronic structure of GaAs damage clustersThe electronic and geometrical structures of GaxAsyCx + Y = 3-10) clusters in nonstoichiometric (#Y) compositions have been calculated using the local-spin-density method. The results show that all even-numbered GaxAsyCx + y = even) clusters have closed shell electronic structures. The calculated ionization potential and electron affinity for the even-numbered clusters are distinctively different from the neighboring odd-numbered clusters, forming an even/odd alternating pattern with increasing cluster size. This calculation, combined with the results from an earlier calculation for stoichiometric compositions (x=y), shows that the electronic ground states of even-numbered gallium arsenide clusters in the size range 4-10 atoms are closed-shell singlet states with a substantial highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap.1858

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Cyclic peptides containing even numbers of alternating D-and L-amino acids adopt a symmetric ring structure with NH and CO amide functions approximately perpendicular to the average plane of the ring. This results in hydrogen-bonded, -sheet like tubular ensembles by coaxial stacking of rings in which adjacent strands are oriented antiparallel to one another. Starting from a molecular mechanics minimum energy configuration having the C 4 symmetry reported for similar structures, an optimized monomer geometry which retained that symmetry was obtained using density functional theory (DFT). The geometry of the monomer is quite similar to that of stable polypeptides and proteins and the calculated gas-phase harmonic vibrational frequencies are quantitatively reasonable. Two monomers were then brought together coaxially, with adjacent strands arranged antiparallel, creating a dimer of D 4 symmetry. Dimer energy was then minimized with respect to ring separation along, and relative rotation of one ring with respect to the other about, the common C 4 axis. The interstrand separation in this dimer model is reasonable in comparison with experimental values reported for stable -sheet polypeptides and proteins as are the harmonic vibrational frequencies. An approximation to the harmonic N-H‚‚‚O stretching frequency calculated for this dimer model was also physically reasonable by comparison to the interchain motion calculated for -sheet polypeptides from analysis of experimental vibrational frequencies. Though synthesis of the molecules in this study has not been reported, they serve as excellent models for the analysis of ring structure and for isolating the role of backbone-backbone hydrogen bonds in the stacking process for similar assemblies. This study provides theoretical support for experimental work on more complex systems, and the DFT calculations are among the largest ever for the study of molecular systems using double valence single polarization basis sets.

The effect of recrystallization (RX) on the high-temperature tensile and creep properties of a directionally solidified (DS) Ni-base superalloy was studied. The tensile properties were not apparently influenced by RX, the depth of which is up to 18 pct of the overall gage thickness. However, an almost linear reduction of the creep rupture life was observed with the increase of the local RX depth. The failure mechanisms of the specimens were discussed based on the microstructural observations.Recrystallization (RX) induced by residual strains in directionally solidified (DS) Ni-base superalloys is a well-known issue in the investment casting industry. [1] It is generally accepted that the newly formed RX grain boundaries, particularly the transverse grain boundaries, are weak sites during service and may degrade the mechanical properties of the DS alloys. Therefore, strict specifications of RX grains have been proved in foundry manufacturing of the DS components. On the other hand, however, the data concerning the effect of RX on the mechanical properties of DS alloys is surprisingly limited in the open literature. Studies on different alloy systems under different conditions have not yet produced conclusive results. [2][3][4] The purpose of the present investigation was to study the effect of local RX on the high-temperature tensile and creep rupture properties of an experimental DS Nibase superalloy and to explore the role of the RX based on the microstructural observation.The nominal composition of the DS alloy studied is 9Cr, 10Co, 7W, 2Mo, 5Al, 3.5Ti, 4Ta, 0.1C, 0.01B, and balance Ni, in wt pct. The alloy was directionally solidified into a plate with the size of 220 · 70 · 16 mm using a Bridgman furnace. Details of the DS process were reported elsewhere. [5] The resulted DS slab has a primary dendrite arm spacing around 300 lm.Plates were cut by electrodischarge machining and indented on one side using a Brinell hardness tester. After full heat treatment (1220°C/2 h + 1080°C/ 4 h + 900°C/16 h) during which the RX occurred, the indentation was carefully removed by grinding. Based on the relationship between the depth of the local RX and the applied load obtained from our previous work, [6,7] specimens with different depths of local RX were obtained by applying different indentation loads and controlling the subsequent grinding process. (The depth of RX was also cross-checked by cutting and measuring some of the machined specimens after full heat treatment.) The plates were subsequently machined on the opposite side to the final dimension (35 · 8 · 5 mm gage section for tensile tests, and 15 · 5 · 2 mm gage section for creep rupture tests). To check the effect of sample thickness, two specimens with 15 · 5 · 0.8 mm gage section (with and without local RX) were also prepared for creep tests. After macroetching the machined specimens, six to eight grains were found for the 2-mm specimens and three to four grains for the 0.8-mm specimens within the gage section. As for specimens with a different number of local R...

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Scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD) were employed to study the phase in a directionally solidified experimental nickel base alloy. It was observed that the phase distribute at interdendritic area and precipitate mainly from the 0 phase in this experimental alloy. Orientation relationship between the 0 and the phase was found as ð111Þ 0 k ð0001Þ and h " 1 110i 0 k h11 " 2 20i . Detrimental effect of the phase on oxidation resistance of the alloys was also found.