4 resultados para Ptoject 2005-001-C : Sydney Opera House – FM Exemplar project

em CaltechTHESIS


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The organometallic chemistry of the hexagonally close-packed Ru(001) surface has been studied using electron energy loss spectroscopy and thermal desorption mass spectrometry. The molecules that have been studied are acetylene, formamide and ammonia. The chemistry of acetylene and formamide has also been investigated in the presence of coadsorbed hydrogen and oxygen adatoms.

Acetylene is adsorbed molecularly on Ru(001) below approximately 230 K, with rehybridization of the molecule to nearly sp^3 occurring. The principal decomposition products at higher temperatures are ethylidyne (CCH_3) and acetylide (CCH) between 230 and 350 K, and methylidyne (CH) and surface carbon at higher temperatures. Some methylidyne is stable to approximately 700 K. The preadsorption of hydrogen does not alter the decomposition products of acetylene, but reduces the saturation coverage and also leads to the formation of a small amount of ethylene (via an η^2-CHCH_2 species) which desorbs molecularly near 175 K. Preadsorbed oxygen also reduces the saturation coverage of acetylene but has virtually no effect on the nature of the molecularly chemisorbed acetylene. It does, however, lead to the formation of an sp^2-hybridized vinylidene (CCH_2) species in the decomposition of acetylene, in addition to the decomposition products that are formed on the clean surface. There is no molecular desorption of chemisorbed acetylene from clean Ru(001), hydrogen-presaturated Ru(001), or oxygen-presaturated Ru(001).

The adsorption and decomposition of formamide has been studied on clean Ru(001), hydrogen-presaturated Ru(001), and Ru(001)-p(1x2)-O (oxygen adatom coverage = 0.5). On clean Ru(001), the adsorption of low coverages of formamide at 80 K results in CH bond cleavage and rehybridization of the carbonyl double bond to produce an η^2 (C,O)-NH_2CO species. This species is stable to approximately 250 K at which point it decomposes to yield a mixture of coadsorbed carbon monoxide, ammonia, an NH species and hydrogen adatoms. The decomposition of NH to hydrogen and nitrogen adatoms occurs between 350 and 400 K, and the thermal desorption products are NH_3 (-315 K), H_2 (-420 K), CO (-480 K) and N_2 (-770 K). At higher formamide coverages, some formamide is adsorbed molecularly at 80 K, leading both to molecular desorption and to the formation of a new surface intermediate between 300 and 375 K that is identified tentatively as η^1(N)-NCHO. On Ru(001)- p(1x2)-O and hydrogen-presaturated Ru(001), formamide adsorbs molecularly at 80 K in an η^1(O)- NH_2CHO configuration. On the oxygen-precovered surface, the molecularly adsorbed formamide undergoes competing desorption and decomposition, resulting in the formation of an η^2(N,O)-NHCHO species (analogous to a bidentate formate) at approximately 265 K. This species decomposes near 420 K with the evolution of CO and H_2 into the gas phase. On the hydrogen precovered surface, the Η^1(O)-NH_2CHO converts below 200 K to η^2(C,O)-NH_2CHO and η^2(C,O)-NH^2CO, with some molecular desorption occurring also at high coverage. The η^2(C,O)-bonded species decompose in a manner similar to the decomposition of η^2(C,O)-NH_2CO on the clean surface, although the formation of ammonia is not detected.

Ammonia adsorbs reversibly on Ru(001) at 80 K, with negligible dissociation occurring as the surface is annealed The EEL spectra of ammonia on Ru(001) are very similar to those of ammonia on other metal surfaces. Off-specular EEL spectra of chemisorbed ammonia allow the v(Ru-NH_3) and ρ(NH_3) vibrational loss features to be resolved near 340 and 625 cm^(-1), respectively. The intense δ_g (NH_3) loss feature shifts downward in frequency with increasing ammonia coverage, from approximately 1160 cm^(-1) in the low coverage limit to 1070 cm^(-1) at saturation. In coordination compounds of ammonia, the frequency of this mode shifts downward with decreasing charge on the metal atom, and its downshift on Ru(001) can be correlated with the large work function decrease that the surface has previously been shown to undergo when ammonia is adsorbed. The EELS data are consistent with ammonia adsorption in on-top sites. Second-layer and multilayer ammonia on Ru(001) have also been characterized vibrationally, and the results are similar to those obtained for other metal surfaces.

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The goal of this thesis is to develop a proper microelectromechanical systems (MEMS) process to manufacture piezoelectric Parylene-C (PA-C), which is famous for its chemical inertness, mechanical and thermal properties and electrical insulation. Furthermore, piezoelectric PA-C is used to build miniature, inexpensive, non-biased piezoelectric microphones.

These piezoelectric PA-C MEMS microphones are to be used in any application where a conventional piezoelectric and electret microphone can be used, such as in cell phones and hearing aids. However, they have the advantage of a simplified fabrication process compared with existing technology. In addition, as a piezoelectric polymer, PA-C has varieties of applications due to its low dielectric constant, low elastic stiffness, low density, high voltage sensitivity, high temperature stability and low acoustic and mechanical impedance. Furthermore, PA-C is an FDA approved biocompatible material and is able to maintain operate at a high temperature.

To accomplish piezoelectric PA-C, a MEMS-compatible poling technology has been developed. The PA-C film is poled by applying electrical field during heating. The piezoelectric coefficient, -3.75pC/N, is obtained without film stretching.

The millimeter-scale piezoelectric PA-C microphone is fabricated with an in-plane spiral arrangement of two electrodes. The dynamic range is from less than 30 dB to above 110 dB SPL (referenced 20 µPa) and the open-circuit sensitivities are from 0.001 0.11 mV/Pa over a frequency range of 1 - 10 kHz. The total harmonic distortion of the device is less than 20% at 110 dB SPL and 1 kHz.

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The initial probabilities of activated, dissociative chemisorption of methane and ethane on Pt(110)-(1 x 2) have been measured. The surface temperature was varied from 450 to 900 K with the reactant gas temperature constant at 300 K. Under these conditions, we probe the kinetics of dissociation via trapping-mediated (as opposed to 'direct') mechanism. It was found that the probabilities of dissociation of both methane and ethane were strong functions of the surface temperature with an apparent activation energies of 14.4 kcal/mol for methane and 2.8 kcal/mol for ethane, which implys that the methane and ethane molecules have fully accommodated to the surface temperature. Kinetic isotope effects were observed for both reactions, indicating that the C-H bond cleavage was involved in the rate-limiting step. A mechanistic model based on the trapping-mediated mechanism is used to explain the observed kinetic behavior. The activation energies for C-H bond dissociation of the thermally accommodated methane and ethane on the surface extracted from the model are 18.4 and 10.3 kcal/mol, respectively.

The studies of the catalytic decomposition of formic acid on the Ru(001) surface with thermal desorption mass spectrometry following the adsorption of DCOOH and HCOOH on the surface at 130 and 310 K are described. Formic acid (DCOOH) chemisorbs dissociatively on the surface via both the cleavage of its O-H bond to form a formate and a hydrogen adatom, and the cleavage of its C-O bond to form a carbon monoxide, a deuterium adatom and an hydroxyl (OH). The former is the predominant reaction. The rate of desorption of carbon dioxide is a direct measure of the kinetics of decomposition of the surface formate. It is characterized by a kinetic isotope effect, an increasingly narrow FWHM, and an upward shift in peak temperature with Ɵ_T, the coverage of the dissociatively adsorbed formic acid. The FWHM and the peak temperature change from 18 K and 326 K at Ɵ_T = 0.04 to 8 K and 395 K at Ɵ_T = 0.89. The increase in the apparent activation energy of the C-D bond cleavage is largely a result of self-poisoning by the formate, the presence of which on the surface alters the electronic properties of the surface such that the activation energy of the decomposition of formate is increased. The variation of the activation energy for carbon dioxide formation with Ɵ_T accounts for the observed sharp carbon dioxide peak. The coverage of surface formate can be adjusted over a relatively wide range so that the activation energy for C-D bond cleavage in the case of DCOOH can be adjusted to be below, approximately equal to, or well above the activation energy for the recombinative desorption of the deuterium adatoms. Accordingly, the desorption of deuterium was observed to be governed completely by the desorption kinetics of the deuterium adatoms at low Ɵ_T, jointly by the kinetics of deuterium desorption and C-D bond cleavage at intermediate Ɵ_T, and solely by the kinetics of C-D bond cleavage at high Ɵ_T. The overall branching ratio of the formate to carbon dioxide and carbon monoxide is approximately unity, regardless the initial coverage Ɵ_T, even though the activation energy for the production of carbon dioxide varies with Ɵ_T. The desorption of water, which implies C-O bond cleavage of the formate, appears at approximately the same temperature as that of carbon dioxide. These observations suggest that the cleavage of the C-D bond and that of the C-O bond of two surface formates are coupled, possibly via the formation of a short-lived surface complex that is the precursor to to the decomposition.

The measurement of steady-state rate is demonstrated here to be valuable in determining kinetics associated with short-lived, molecularly adsorbed precursor to further reactions on the surface, by determining the kinetic parameters of the molecular precursor of formaldehyde to its dissociation on the Pt(110)-(1 x 2) surface.

Overlayers of nitrogen adatoms on Ru(001) have been characterized both by thermal desorption mass spectrometry and low-energy electron diffraction, as well as chemically via the postadsorption and desorption of ammonia and carbon monoxide.

The nitrogen-adatom overlayer was prepared by decomposing ammonia thermally on the surface at a pressure of 2.8 x 10^(-6) Torr and a temperature of 480 K. The saturated overlayer prepared under these conditions has associated with it a (√247/10 x √247/10)R22.7° LEED pattern, has two peaks in its thermal desorption spectrum, and has a fractional surface coverage of 0.40. Annealing the overlayer to approximately 535 K results in a rather sharp (√3 x √3)R30° LEED pattern with an associated fractional surface coverage of one-third. Annealing the overlayer further to 620 K results in the disappearance of the low-temperature thermal desorption peak and the appearance of a rather fuzzy p(2x2) LEED pattern with an associated fractional surface coverage of approximately one-fourth. In the low coverage limit, the presence of the (√3 x √3)R30° N overlayer alters the surface in such a way that the binding energy of ammonia is increased by 20% relative to the clean surface, whereas that of carbon monoxide is reduced by 15%.

A general methodology for the indirect relative determination of the absolute fractional surface coverages has been developed and was utilized to determine the saturation fractional coverage of hydrogen on Ru(001). Formaldehyde was employed as a bridge to lead us from the known reference point of the saturation fractional coverage of carbon monoxide to unknown reference point of the fractional coverage of hydrogen on Ru(001), which is then used to determine accurately the saturation fractional coverage of hydrogen. We find that ƟSAT/H = 1.02 (±0.05), i.e., the surface stoichiometry is Ru : H = 1 : 1. The relative nature of the method, which cancels systematic errors, together with the utilization of a glass envelope around the mass spectrometer, which reduces spurious contributions in the thermal desorption spectra, results in high accuracy in the determination of absolute fractional coverages.

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PART I

The total cross-section for the reaction 21Ne(α, n)24Mg has been measured in the energy range 1.49 Mev ≤ Ecm ≤ 2.6 Mev. The cross-section factor, S(O), for this reaction has been determined, by means of an optical model calculation, to be in the range 1.52 x 1012 mb-Mev to 2.67 x 1012 mb-Mev, for interaction radii in the range 5.0 fm to 6.6 fm. With S(O) ≈ 2 x 1012 mb-Mev, the reaction 21Ne(α, n)24Mg can produce a large enough neutron flux to be a significant astrophysical source of neutrons.

PART II

The reaction12C(3He, p)14N has been studied over the energy range 12 Mev ≤ Elab ≤ 18 Mev. Angular distributions of the proton groups leading to the lowest seven levels in 14N were obtained.

Distorted wave calculations, based on two-nucleon transfer theory, were performed, and were found to be reliable for obtaining the value of the orbital angular momentum transferred. The present work shows that such calculations do not yield unambiguous values for the spectroscopic factors.