71 Matching Results

Search Results

Advanced search parameters have been applied.

A Method to Improve Activation of Implanted Dopants in SiC

Description: Implantation of dopant ions in SiC has evolved according to the assumption that the best electrical results (i.e., carrier concentrations and mobility) is achieved by using the highest possible processing temperature. This includes implantation at > 600 C followed by furnace annealing at temperatures as high as 1,750 C. Despite such aggressive and extreme processing, implantation suffers because of poor dopant activation, typically ranging between < 2%--50% with p-type dopants represented in the lower portion of this range and n-types in the upper. Additionally, high-temperature processing can led to several problems including changes in the stoichiometry and topography of the surface, as well as degradation of the electrical properties of devices. A novel approach for increasing activation of implanted dopants in SiC and lowering the activation temperature will be discussed. This approach utilizes the manipulation of the ion-induced damage to enhance activation of implanted dopants. It will be shown that nearly amorphous layers containing a small amount of residual crystallinity can be recrystallized at temperatures below 900 C with little residual damage. It will be shown that recrystallization traps a high fraction of the implanted dopant residing within the amorphous phase (prior to annealing) onto substitutional sites within the SiC lattice.
Date: January 16, 2001
Creator: Holland, O.W.
Partner: UNT Libraries Government Documents Department

On the origin of laser-induced surface activation of ceramics

Description: Pulsed-laser irradiation of Al{sub 2}O{sub 3} and AlN surfaces promotes Cu deposition when the irradiated substgrates are immersed in an electroless bath. In this paper, the nature of the surface modification is analyzed using Auger emission spectroscopy (AES) and cross sectional transmission electron microscopy. During irradiation, AlN thermaly decomposes, leaving a discontinuous metallic film on the surface. A film of Al{sub 2}O{sub 3} is detected at the surface of the irradiated AlN substrate, much thicker when the irradiation is done in an oxidizing atmosphere than in a reducing one. Nanoparticles of metallic Al are generated during laser irradiation of Al{sub 2}O{sub 3} in a reducing atmosphere. When the Al{sub 2}O{sub 3} irradiation is done in an oxidizing atmosphere, regions containing Al or substoichiometric alumina are detected by AES. It is concluded that the presence of metallic Al is the main reason why electroless deposition can occur in both AlN and Al{sub 2}O{sub 3}. Deposition kinetics are consistent with this conclusion. It is likely that also substoichiometric alumina helps to catalyze the electroless deposition.
Date: February 1996
Creator: Pedraza, A. J.; Park, J. W.; Cao, S.; Allen, W. R.; Lowndes, D. H. & Allen, W. R.
Partner: UNT Libraries Government Documents Department

Studies Relevent to Catalytic Activation Co & other small Molecules

Description: Detailed annual and triannual reports describing the progress accomplished during the tenure of this grant were filed with the Program Manager for Catalysis at the Office of Basic Energy Sciences. To avoid unnecessary duplication, the present report will provide a brief overview of the research areas that were sponsored by this grant and list the resulting publications and theses based on this DOE supported research. The scientific personnel participating in (and trained by) this grant's research are also listed. Research carried out under this DOE grant was largely concerned with the mechanisms of the homogeneous catalytic and photocatalytic activation of small molecules such as carbon monoxide, dihydrogen and various hydrocarbons. Much of the more recent effort has focused on the dynamics and mechanisms of reactions relevant to substrate carbonylations by homogeneous organometallic catalysts. A wide range of modern investigative techniques were employed, including quantitative fast reaction methodologies such as time-resolved optical (TRO) and time-resolved infrared (TRIR) spectroscopy and stopped flow kinetics. Although somewhat diverse, this research falls within the scope of the long-term objective of applying quantitative techniques to elucidate the dynamics and understand the principles of mechanisms relevant to the selective and efficient catalytic conversions of fundamental feedstocks to higher value materials.
Date: February 22, 2005
Creator: Ford, Peter C.
Partner: UNT Libraries Government Documents Department

Role of C and P Sites on the Chemical Activity of Metal Carbide and Phosphides: From Clusters to Single-Crystal Surfaces

Description: Transition metal carbides and phosphides have shown tremendous potential as highly active catalysts. At a microscopic level, it is not well understood how these new catalysts work. Their high activity is usually attributed to ligand or/and ensemble effects. Here, we review recent studies that examine the chemical activity of metal carbide and phosphides as a function of size, from clusters to extended surfaces, and metal/carbon or metal/phosphorous ratio. These studies reveal that the C and P sites in these compounds cannot be considered as simple spectators. They moderate the reactivity of the metal centers and provide bonding sites for adsorbates.
Date: July 1, 2007
Creator: Rodriguez, J. A.; Vines, F.; Liu, P. & Illas, F.
Partner: UNT Libraries Government Documents Department

Novel catalysts for methane activation. Final progress report, September 30, 1992--April 30, 1996

Description: This final report summarizes the results of our research under Contract No. DE-AC22-92PC92112, Novel Catalysts for Methane Activation. In this research we prepared and tested fullerene soots for converting methane into higher hydrocarbons. We conducted the methane conversions using dehydrocoupling conditions, primarily in the temperature regimes of 600{degrees}-1000{degrees}C and atmospheric pressures. The research was divided into three sections. The first section focused on comparing fullerene soots with other forms of carbon such as acetylene black and Norit-A. We found that the fullerene soot was indeed more reactive than the other forms of carbon. However, due to its high reactivity, it was not selective. The second section focused on the effect of metals on the reactivity of the soots, including both transition metals and alkali metals. We found that potassium could enhance the selectivities of fullerene soot to higher hydrocarbons, but the effect was unique to fullerene soot and did not improve the performance of other forms of carbon. The third part focused on the use of co-feeds for methane activation to enhance the selectivities and lower the temperature threshold of methane activation.
Date: June 11, 1996
Creator: Hirschon, A.S.; Du, Y.; Wu, H.J.; Malhotra, R. & Wilson, R.B.
Partner: UNT Libraries Government Documents Department

[Towards computer-aided catalyst design: Three effective core potential studies of C-H activation]. Final report

Description: Research in the initial grant period focused on computational studies relevant to the selective activation of methane, the prime component of natural gas. Reaction coordinates for methane activation by experimental models were delineated, as well as the bonding and structure of complexes that effect this important reaction. This research, highlighted in the following sections, also provided the impetus for further development, and application of methods for modeling metal-containing catalysts. Sections of the report describe the following: methane activation by multiple-bonded transition metal complexes; computational lanthanide chemistry; and methane activation by non-imido, multiple-bonded ligands.
Date: December 31, 1998
Partner: UNT Libraries Government Documents Department

Catalytic conversion of light alkanes - phase V. Topical report, February 1993--October 1994

Description: We have made excellent progress toward a practical route from field butanes to MTBE, the oxygenate of choice for high-octane, clean-burning, environmentally acceptable reformulated gasoline. We have evaluated two proprietary process possibilities with a potential commercial partner and have conducted a joint catalyst evaluation program. The first of the two potential processes considered during the past quarter utilizes a two-step route from isobutane to tert-butyl alcohol, TBA. Not only is TBA an intermediate for MTBE production but is equally applicable for ETBE-an oxygenate which utilizes renewable ethanol in its` manufacture. In the two-step process, isobutane is oxidized in a non-catalytic reaction to a roughly equal mixture of TBA and tert-butyl hydroperoxide. TBHP, eq. 1. We have developed an inexpensive new catalyst system based on an electron-deficient macrocyclic metal complex that selectively converts TBHP to TBA, eq. 2, and meets or exceeds all of the process criteria that we have set.
Date: December 31, 1998
Partner: UNT Libraries Government Documents Department

Maleimide Functionalized Siloxane Resins

Description: In-situ filling through hydrolysis and condensation of silicon alkoxides has been utilized to generate nanocomposites in which the filler phase can be intimately associated with the polymer on relatively small length scales. One problem of the method has been achieving useful fill volumes without bulk phase separation of the reacting silicon monomer from the polymer. In this paper, we describe the preparation of a new class of nanocomposite materials in which the inorganic filler phase is pre-assembled before copolymerization with an organic species. Maleimide monomers, prepared from alkoxysilylpropyl amines and maleic anhydride, were protected against side reactions by forming the oxonorbornene Diels-Alder adduct with furan. The monomers were then reacted under sol-gel conditions to form oligomers or polymers-the filler phase. The material was activated by thermal deprotection of the maleimide and reacted with organic monomers or polymers to form the filled nanocomposite.
Date: April 21, 1999
Creator: Shaltout, R.M.; Loy, D.A. & Wheeler, D.R.
Partner: UNT Libraries Government Documents Department

Task 5.10 -- Value-added coproducts. Semi-annual report, January 1--June 30, 1995

Description: Research focused on end-use products is needed to integrate value-added coproducts with power generation and to reduce escalating waste disposal costs by developing profit centers from recycling. Substantial public benefits can be derived from by-products utilization through conservation of energy and resources, reduction in gaseous emissions, and prevention of solid waste pollution. Continued research on carbon products under the base Cooperative Agreement addresses the central question of whether uniquely cost-effective activated carbons can be produced by taking advantage of the inherently porous and reactive structure of low-rank coals (LRCs). Carbon products will be evaluated in reference to their potential use in air toxics control and other applications. The Energy and Environmental Research Center (EERC) designated value-added carbon as a product line, with the goal of developing superior carbon products at competitive prices for a range of applications, including SOx/NOx/air toxics adsorption, wastewater treatment, molecular sieves, catalyst supports, metallurgical and foundry carbons, and clean-burning solid fuels using EERC processes previously developed under the US Department of Energy (DOE) Mild Gasification program. The approach to be used in meeting specifications for marketable activated carbon products is to develop a fundamental understanding of the extent and quality of the porous surface needed to achieve rapid, high-capacity adsorption of pollutant gases or toxic metals and the processing conditions needed to transform coal and other raw materials into products possessing these superior properties.
Date: August 1, 1997
Creator: Olson, E.S. & Aulich, T.R.
Partner: UNT Libraries Government Documents Department


Description: Accurate simulation of in-situ combustion processes is computationally very challenging because the spatial and temporal scales over which the combustion process takes place are very small. In this sixth quarter of our DoE funded research, we continued the development of our new simulation tool which is based on an efficient Cartesian Adaptive Mesh Refinement technique. This methodology allows much higher grid densities to be used near typical fronts than current simulators. We improved the upscaling strategy on these grids, and derived an effective way to generate upscaled permeabilities that preserve local fluxes. We have started more in-depth research into splitting methods for stiff PDEs such as those found in in-situ combustion simulation. We will report on these new developments extensively in the next quarterly report. This quarterly report, we focus on experimental work. On the experimental side, we have fleshed out a mechanism of improved in-situ combustion with aqueous metallic salts using scanning electron microscopy (SEM) and the transport phenomenon of such additives through porous media. Based on the observations from SEM analysis, we propose cation exchange of metallic salts with clay as a mechanism to create activated sites that enhance combustion reactions between oil and oxygen. Moreover, the empirical ranking of the success of metallic ions as catalytic additives for in-situ combustion is interpreted as originating from three factors: cation replacing power, distribution of metallic additive adsorption sites, and cation catalytic power for oxidation and cracking of hydrocarbon.
Date: April 1, 2005
Creator: Gerritsen, Margot & Kovscek, Anthony R.
Partner: UNT Libraries Government Documents Department

Time resolved studies of bond activation by organometallic complexes

Description: In 1971, Jetz and Graham discovered that the silicon-hydrogen bond in silanes could be broken under mild photochemical conditions in the presence of certain transition metal carbonyls. Such reactions fall within the class of oxidative addition. A decade later, similar reactivity was discovered in alkanes. In these cases a C-H bond in non-functionalized alkanes was broken through the oxidative addition of Cp*Ir(H){sub 2}L (Cp* = (CH{sub 3}){sub 5}C{sub 5}, L = PPh{sub 3}, Ph = C{sub 6}H{sub 5}) to form Cp*ML(R)(H) or of Cp*Ir(CO){sub 2} to form Cp*Ir(CO)(R)(H). These discoveries opened an entirely new field of research, one which naturally included mechanistic studies aimed at elucidating the various paths involved in these and related reactions. Much was learned from these experiments but they shared the disadvantage of studying under highly non-standard conditions a system which is of interest largely because of its characteristics under standard conditions. Ultrafast time-resolved IR spectroscopy provides an ideal solution to this problem; because it allows the resolution of chemical events taking place on the femto-through picosecond time scale, it is possible to study this important class of reactions under the ambient conditions which are most of interest to the practicing synthetic chemist. Certain of the molecules in question are particularly well-suited to study using the ultrafast IR spectrophotometer described in the experimental section because they contain one or more carbonyl ligands.
Date: May 1, 1998
Creator: Wilkens, M. J.
Partner: UNT Libraries Government Documents Department

Hydrothermal Transport and Deposition of Silica

Description: The strong Si-O-bond under dry conditions and in absence of impurities needs high activation energy for mobilization. Chemical activation with water based on the reaction: SiO{sub 2(s)} + nH{sub 2}O{sub (g,f)} = SiO{sub 2} {center_dot} nH{sub 2}O{sub (g,f)} lowers the energy barriers remarkably. Under normal conditions the equilibrium is extremely shifted to the left, however at increased temperatures and water-pressures (hydrothermal conditions) the reaction may cause strong chemical transport. The efficiency (rate) of transport depends on the physico-chemical and physical parameters (temperature, pressure, viscosity e.g.) as well as on the mechanism of flow. The phase composition and micro-structure of the silica-deposit formed by the reverse process, strongly depends on the rate of transport and deposition and on temperature of the substrate.
Date: January 1, 1976
Creator: Florke, Otto W.
Partner: UNT Libraries Government Documents Department

Activation Layer Stabilization of High Polarization Photocathodes in Sub-Optimal RF Gun Environments

Description: We have developed an activation procedure by which the reactivity to CO{sub 2}, a principal cause of yield decay for GaAs photocathodes, is greatly reduced. The use of a second alkali in the activation process is responsible for the increased immunity of the activated surface. The best immunity was obtained by using a combination of Cs and Li without any loss in near bandgap yield. Optimally activated photocathodes have nearly equal quantities of both alkalis.
Date: August 25, 2010
Creator: Mulhollan, Gregory & /SLAC /Saxed Surface Science, Austin, TX
Partner: UNT Libraries Government Documents Department

Development of White-Light Emitting Active Layers in Nitride Based Heterostructures for Phosphorless Solid State Lighting

Description: This report provides a summary of research activities carried out at the University of California, San Diego and Central Research of OSRAM SYLVANIA in Beverly, MA partially supported by a research contract from US Department of Energy, DE-FC26-04NT422274. The main objective of this project was to develop III-V nitrides activated by rare earth ions, RE{sup 3+}, which could eliminate the need for phosphors in nitride-based solid state light sources. The main idea was to convert electron-hole pairs injected into the active layer in a LED die to white light directly through transitions within the energy levels of the 4f{sup n}-manifold of RE{sup 3+}. We focused on the following materials: Eu{sup 3+}(red), Tb{sup 3+}(green), Er{sup 3+}(green), Dy{sup 3+}(yellow) and Tm{sup 3+}(blue) in AlN, GaN and alloys of AlN and GaN. Our strategy was to explore candidate materials in powder form first, and then study their behavior in thin films. Thin films of these materials were to be deposited on sapphire substrates using pulsed laser deposition (PLD) and metal organic vapor phase epitaxy (MOVPE). The photo- and cathode-luminescence measurements of these materials were used to investigate their suitability for white light generation. The project proceeded along this route with minor modifications needed to produce better materials and to expedite our progress towards the final goal. The project made the following accomplishments: (1) red emission from Eu{sup 3+}, green from Tb{sup 3+}, yellow from Dy{sup 3+} and blue from Tm{sup 3+} in AlN powders; (2) red emission from Eu{sup 3+} and green emission from Tb{sup 3+} in GaN powder; (3) red emission from Eu{sup 3+} in alloys of GaN and AlN; (4) green emission from Tb{sup 3+} in GaN thin films by PLD; (5) red emission from Eu{sup 3+} and Tb{sup 3+} in GaN thin films deposited by MOVPE; (6) energy transfer from ...
Date: December 31, 2007
Creator: Talbot, Jan & Mishra, Kailash
Partner: UNT Libraries Government Documents Department


Description: The increasing role of coal as a source of energy in the 21st century will demand environmental and cost-effective strategies for the use of coal combustion by-products (CCBPs), mainly unburned carbon in fly ash. Unburned carbon is nowadays regarded as a waste product and its fate is mainly disposal, due to the present lack of efficient routes for its utilization. However, unburned carbon is a potential precursor for the production of adsorbent carbons, since it has gone through a devolatilization process while in the combustor, and therefore, only requires to be activated. Accordingly, this report evaluates and compares several routes for the production of activated carbons from unburned carbon in fly ash, including physical and chemical activation methods. During the present reporting period (June 30, 2001-June 29, 2002), additional characterization work was conducted under Task 1 ''Procurement and characterization of CCBPs''. The suite collected includes samples from pulverized utility boilers, a utility cyclone unit equipped with a beneficiation technology, a stoker, and a fluidized bed combustor. Proximate, ultimate, and petrographic analyses of the fly ash samples previously collected were measured. Furthermore, the surface areas of the samples assembled were characterized by N{sub 2} adsorption isotherms at 77 K. The proximate analyses showed that all the samples had very low moisture contents (0.17 to 3.39 wt%), while volatile matter contents of the samples varied between 0.45 to 24.8 wt%. The ultimate analyses of all the fly ash samples showed that they contained primarily carbon, while the hydrogen contents of all the samples were very low. In addition, during the current reporting period, also Task 2 ''Development of activated carbons'' and Task 3 ''Characterization of activated carbons'' were continued.
Date: September 27, 2002
Creator: Schobert, Prof. Harold H.; Maroto-Valer, Dr. M. Mercedes & Lu, Ms. Zhe
Partner: UNT Libraries Government Documents Department

Synthesis and properties of lanthanide-exchanged Preyssler`s heteropolyanions

Description: Na{sup +} in the Preyssler heteropolytungstate anion [NaP{sub 5}W{sub 30}O{sub 110}]{sup 14{minus}} can be exchanged for a trivalent lanthanide ion. The potential significance of this new class of lanthanide heteropolyanions relates to their applications in catalysis science. This view follows from the fact that Keggin heteropolyanions and their free acids are used as heterogeneous solid catalysts and homogeneous solution catalysts. The authors describe synthetic conditions that lead to the incorporation of Ce{sup 3+} and Pr{sup 3+} within the Preyssler anion, and the coprecipitation of Ce{sup 3+} and the Preyssler anion. Initial studies indicate that the latter, coprecipitated, material deserves study for bifunctional catalytic activity.
Date: December 1994
Creator: Antonio, M. R.; Malinsky, J. & Soderholm, L.
Partner: UNT Libraries Government Documents Department

Alternative fuels and chemicals from synthesis gas. Quarterly status report number 2, 1 January--31 March 1995

Description: The overall objectives of this program are to investigate potential technologies for the conversion of synthesis gas to oxygenated and hydrocarbon fuels and industrial chemicals, and to demonstrate the most promising technologies at DOE`s LaPorte, Texas, Slurry Phase Alternative Fuels Development Unit. Results are discussed for the following tasks: liquid phase hydrodynamic run; catalyst activation with CO; new processes for DME (dehydration catalyst screening runs, and experiments using Robinson-Mahoney basket internal and pelletized catalysts); new fuels from DME; and new processes for alcohols and oxygenated fuel additives.
Date: December 31, 1995
Partner: UNT Libraries Government Documents Department

Mechanism of hydrogen incorporation in coal liquefaction: Fifteenth report, progress as of March 31, 1995

Description: The authors continued work on the reaction of deuterium with coal model materials. Their recent efforts have emphasized models in which organic compounds are immobilized on the surface of silica. They have also carried out some studies on the effect of silica itself on the reaction with deuterium. Work in progress and results are described for the subtask entitled, Silica-catalyzed hydrogenations of alkenes.
Date: May 1, 1995
Partner: UNT Libraries Government Documents Department

Assessment of Neptunium, Americium, and Curium in the Savannah River Site Environment

Description: A series of documents has been published in which the impact of various radionuclides released to the environment by Savannah River Site (SRS) operations has been assessed. The quantity released, the disposition of the radionuclides in the environment, and the dose to offsite individuals has been presented for activation products, carbon cesium, iodine, plutonium, selected fission products, strontium, technetium, tritium, uranium, and the noble gases. An assessment of the impact of nonradioactive mercury also has been published.This document assesses the impact of radioactive transuranics released from SRS facilities since the first reactor became operational late in 1953. The isotopes reported here are 239Np, 241Am, and 244Cm.
Date: December 17, 1997
Creator: Carlton, W.H.
Partner: UNT Libraries Government Documents Department

Time resolved infrared studies of C-H bond activation by organometallics

Description: This work describes how step-scan Fourier Transform Infrared spectroscopy and visible and near infrared ultrafast lasers have been applied to the study of the photochemical activation of C-H bonds in organometallic systems, which allow for the selective breaking of C-H bonds in alkanes. The author has established the photochemical mechanism of C-H activation by Tp{sup *}Rh(CO){sub 2}(Tp{sup *} = HB-Pz{sup *}{sub 3}, Pz = 3,5-dimethylpyrazolyl) in alkane solution. The initially formed monocarbonyl forms a weak solvent complex, which undergoes a change in Tp{sup *} ligand connectivity. The final C-H bond breaking step occurs at different time scales depending on the structure of the alkane. In linear solvents, the time scale is <50 ns and cyclic alkanes is {approximately}200 ps. The reactivity of the Tp{sup *}Rh(CO){sub 2} system has also been studied in aromatic solvents. Here the reaction proceeds through two different pathways, with very different time scales. The first proceeds in a manner analogous to alkanes and takes <50 ns. The second proceeds through a Rh-C-C complex, and takes place on a time scale of 1.8 {micro}s.
Date: June 1, 1998
Creator: Asplund, M.C.
Partner: UNT Libraries Government Documents Department

Ultrafast studies of organometallic photochemistry: The mechanism of carbon-hydrogen bond activation in solution

Description: When certain organometallic compounds are photoexcited in room temperature alkane solution, they are able to break or activate the C-H bonds of the solvent. Understanding this potentially practical reaction requires a detailed knowledge of the entire reaction mechanism. Because of the dynamic nature of chemical reactions, time-resolved spectroscopy is commonly employed to follow the important events that take place as reactants are converted to products. For the organometallic reactions examined here, the electronic/structural characteristics of the chemical systems along with the time scales for the key steps in the reaction make ultrafast UV/Vis and IR spectroscopy along with nanosecond Step-Scan FTIR spectroscopy the ideal techniques to use for this study. An initial study of the photophysics of (non-activating) model metal carbonyls centering on the photodissociation of M(CO){sub 6} (M = Cr, W, Mo) was carried out in alkane solutions using ultrafast IR spectroscopy. Next, picosecond UV/vis studies of the C-H bond activation reaction of Cp{sup *}M(CO){sub 2} (M = Rh, Ir), conducted in room temperature alkane solution, are described in an effort to investigate the origin of the low quantum yield for bond cleavage ({approximately}1%). To monitor the chemistry that takes place in the reaction after CO is lost, a system with higher quantum yield is required. The reaction of Tp{sup *}Rh(CO){sub 2} (Tp{sup *} = HB-Pz{sub 3}{sup *}, Pz{sup *} = 3,5-dimethylpyrazolyl) in alkanes has a quantum yield of {approximately}30%, making time resolved spectroscopic measurements possible. From ultrafast IR experiments, two subsequently formed intermediates were observed. The nature of these intermediates are discussed and the first comprehensive reaction mechanism for a photochemical C-H activating organometallic complex is presented.
Date: May 1, 1998
Creator: Bromberg, S.E.
Partner: UNT Libraries Government Documents Department

New Carbon Activation Process for Increased Surface Accessibility in Electrochemical Capacitors

Description: A process for making carbon film or powder suitable for double capacitor electrodes having a capacitance of up to about 300 F/cm{sup 3} is disclosed. This is accomplished by treating in aqueous nitric acid for a period of about 5 to 15 minutes thin carbon films obtained by carbonizing carbon-containing polymeric material having a high degree of molecular directionality, such as polyimide film, then heating the treated carbon film in a non-oxidizing atmosphere at a non-graphitizing temperature of at least 350 C for about 20 minutes, and repeating alternately the nitric acid step and the heating step from 7 to 10 times. Capacitors made with this carbon may find uses ranging from electronic devices to electric vehicle applications.
Date: March 16, 1999
Creator: Doughty, Daniel H. & Eisenmann, Erhard T.
Partner: UNT Libraries Government Documents Department