ROLE OF CHLORINE AND ALKALI PROMOTERS IN ETHYLENE EPOXIDATION OVER SINGLE CRYSTAL SILVER SURFACES.

Experiments with N//2O were carried out with a view to obtaining additional information about the reactivity of oxygen surface species. On clean Ag, N//2O decomposition was found to be an activated process which led exclusively to the deposition of O(a) species. The presence of preadsorbed oxygen or subsurface oxygen served to enhance the deposition rate of O(a). Subsequent dosing with ethylene at 300 K of such an oxygen-populated surface followed by TPR examination showed it to be active for ethylene oxide formation. Control experiments established that adventitious decomposition of N//2O at the reactor walls or specimen supports followed by possible re-absorption of O//2(a) was an entirely negligible process. ) The oxidation activity of N//2O was also investigated at elevated pressures in the batch reactor.

Hydrogen content estimation of hydrogenated amorphous carbon by visible Raman spectroscopy

In the present study, we report the hydrogen content estimation of the hydrogenated amorphous carbon (a-C:H) films using visible Raman spectroscopy in a fast and nondestructive way. Hydrogenated diamondlike carbon films were deposited by the plasma enhanced chemical vapor deposition, plasma beam source, and integrated distributed electron cyclotron resonance techniques. Methane and acetylene were used as source gases resulting in different hydrogen content and sp2/sp3 fraction. Ultraviolet-visible (UV-Vis) spectroscopic ellipsometry (1.5-5 eV) as well as UV-Vis spectroscopy were provided with the optical band gap (Tauc gap). The sp2/sp3 fraction and the hydrogen content were independently estimated by electron energy loss spectroscopy and elastic recoil detection analysis-Rutherford back scattering, respectively. The Raman spectra that were acquired in the visible region using the 488 nm line shows the superposition of Raman features on a photoluminescence (PL) background. The direct relationship of the sp2 content and the optical band gap has been confirmed. The difference in the PL background for samples of the same optical band gap (sp2 content) and different hydrogen content was demonstrated and an empirical relationship between the visible Raman spectra PL background slope and the corresponding hydrogen content was extracted. © 2004 American Institute of Physics.

Pressure and thermal effects on elastic properties of single crystal alpha-Al2O3 from Monte-Carlo simulation

A rectangular structural unit cell of a-Al2O3 is generated from its hexagonal one. For the rectangular structural crystal with a simple interatomic potential [Matsui, Mineral Mag. 58A, 571 (1994)], the relations of lattice constants to homogeneous pressure and temperature are calculated by using Monte-Carlo method at temperature 298K and 0 GPa, respectively. Both numerical results agree with experimental ones fairly well. By comparing pair distribution function, the crystal structure of a-Al2O3 has no phase transition in the range of systematic parameters. Based on the potential model, pressure dependence of isothermal bulk moduli is predicted. Under variation of general strains, which include of external and internal strains, elastic constants of a-Al2O3 in the different homogeneous load are determined. Along with increase of pressure, axial elastic constants increase appreciably, but nonaxial elastic constants are slowly changed.

A bulk acoustic mode single-crystal silicon microresonator with a high-quality factor

This paper details a bulk acoustic mode resonator fabricated in single-crystal silicon with a quality factor of 15 000 in air, and over a million below 10 mTorr at a resonant frequency of 2.18 MHz. The resonator is a square plate that is excited in the square-extensional mode and has been fabricated in a commercial foundry silicon-on-insulator (SOI) MEMS process through MEMSCAP. This paper also presents a simple method of extracting resonator parameters from raw measurements heavily buried in electrical feedthrough. Its accuracy has been demonstrated through a comparison between extracted motional resistance values measured at different voltage biases and those predicted from an analytical model. Finally, a method of substantially cancelling electrical feedthrough through system-level electronic implementation is also introduced. © 2008 IOP Publishing Ltd.

Ultrasensitive mass balance based on a bulk acoustic mode single-crystal silicon resonator

A single-crystal silicon resonant bulk acoustic mass sensor with a measured resolution of 125 pg cm2 is presented. The mass sensor comprises a micromachined silicon plate that is excited in the square-extensional bulk acoustic resonant mode at a frequency of 2.182 MHz, with a quality factor exceeding 106. The mass sensor has a measured mass to frequency shift sensitivity of 132 Hz cm2 μg. The resonator element is embedded in a feedback loop of an electronic amplifier to implement an oscillator with a short term frequency stability of better than 7 ppb at an operating pressure of 3.8 mTorr. © 2007 American Institute of Physics.

5.4-MHz single-crystal silicon wine glass mode disk resonator with quality factor of 2 million

This paper reports the design and electrical characterization of a micromechanical disk resonator fabricated in single crystal silicon using a foundry SOI micromachining process. The microresonator has been selectively excited in the radial extensional and the wine glass modes by reversing the polarity of the DC bias voltage applied on selected drive electrodes around the resonant structure. The quality factor of the resonator vibrating in the radial contour mode was 8000 at a resonant frequency of 6.34 MHz at pressure below 10 mTorr vacuum. The highest measured quality factor of the resonator in the wine glass resonant mode was 1.9 × 106 using a DC bias voltage of 20 V at about the same pressure in vacuum; the resonant frequency was 5.43 MHz and the lowest motional resistance measured was approximately 17 kΩ using a DC bias voltage of 60 V applied across 2.7 μm actuation gaps. This corresponds to a resonant frequency-quality factor (f-Q) product of 1.02 × 1013, among the highest reported for single crystal silicon microresonators, and on par with the best quartz crystal resonators. The quality factor for the wine glass mode in air was approximately 10,000. © 2009 Elsevier B.V. All rights reserved.

Raman spectroscopy of silicon nanowires

We measure the effects of phonon confinement on the Raman spectra of silicon nanowires. We show how previous spectra were inconsistent with phonon confinement, but were due to intense local heating caused by the laser. This is peculiar to nanostructures, and would require orders of magnitude more power in bulk Si. By working at very low laser powers, we identify the contribution of pure confinement typical of quantum wires.

Measurement of Microscopic Deformation in a Cualni Single Crystal Alloy by Nanoindentation with a Heating Stage

Thermally induced recovery of nanoindents in a CUAINi single crystal shape memory alloy was studied by nanoindentation in conjunction with a heating stage. Nanoindents formed by a Berkovich indenter at room temperature were heated to 40, 70 and 100 degrees C. Partial recovery was observed for the nanoindents. The recovery ratio depended on the heating temperature. Indentation of CuAlNi can induce inelastic deformation via dislocation motion and a stress-induced matensitic transformation. The percentages of dislocation-induced plastic strain would affect the thermal deformation of CuAlNi, because the induced dislocations could stabilize stress-induced martensite plates even when the temperature above austenite finish temperature, A(f). When the applied indentation load is low (less than 10,000 mu N), the shape recovery strain is predominant, compared with the dislocation-induced plastic strain. Therefore, the degree of indent recovery in the depth direction, delta(D), is high (about 0.7-0.8 at 100 degrees C).

A Molecular Dynamics Simulation: the Effect of Finite Size on the Thermal Conductivity in a Single Crystal Silicon

Non-equilibrium molecular dynamics (NEMD) simulations are performed to calculate thermal conductivity. The environment-dependent interatomic potential (EDIP) potential on crystal silicon is adopted as a model system. The issues are related to nonlinear response, local thermal equilibrium and statistical averaging. The simulation results by non-equilibrium molecular dynamics show that the calculated thermal conductivity decreases almost linearly as the film thickness reduced at the nanometre scale. The effect of size on the thermal conductivity is also obtained by a theoretic analysis of the kinetic theory and formulas of the heat capacity. The analysis reveals that the contributions of phonon mean free path (MFP) and phonon number in a finite cell to thermal conductivity are very important.

Large Scale Fabrication Of Periodical Bowl-Like Micropatterns Of Single Crystal Zno

Nanostructured ZnO materials are of great significance for their potential applications in photoelectronic devices, light-emitting displays, catalysis and gas sensors. In this paper, we report a new method to produce large area periodical bowl-like micropatterns of single crystal ZnO through aqueous-phase epitaxial growth on a ZnO single crystal substrate. A self-assembled monolayer of polystyrene microspheres was used as a template to confine the epitaxial growth of single crystal ZnO from the substrate, while the growth morphology was well controlled by citrate anions. Moreover, it was found that the self-assembled monolayer of colloidal spheres plays an important role in reduction of the defect density in the epitaxial ZnO layer. Though the mechanism is still open for further investigation, the present result indicates a new route to suppress the dislocations in the fabrication of single crystal ZnO film. A predicable application of this new method is for the fabrication of two-dimensional photonic crystal structures on light emitting diode surfaces.

Preliminary-results of gaas single-crystal growth under high gravity conditions

GaAs single crystals have been grown under high gravity conditions, up to 9g0, by a recrystallization method with decreasing temperature. The impurity striations in GaAs grown under high gravity become weak and indistinct with smaller striation spacings. The dislocation density of surcharge-grown GaAs increases with increase of centrifugal force. The cathodoluminescence results also show worse perfection in the GaAs grown at high gravity than at normal earth gravity.