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Rhodium Nanoparticle Shape Dependence in the Reduction of NO by CO

Description: The shape dependence of the catalytic reduction of NO by CO on Rhodium nanopolyhedra and nanocubes was studied from 230-270 C. The nanocubes are found to exhibit higher turnover frequency and lower activation energy than the nanopolyhedra. These trends are compared to previous studies on Rh single crystals.
Date: July 13, 2009
Creator: Renzas, J.R.; Zhang, Y.; Huang, W. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Near-Monodisperse Ni-Cu Bimetallic Nanocrystals of Variable Composition: Controlled Synthesis and Catalytic Activity for H2 Generation

Description: Near-monodisperse Ni{sub 1-x}Cu{sub x} (x = 0.2-0.8) bimetallic nanocrystals were synthesized by a one-pot thermolysis approach in oleylamine/1-octadecene, using metal acetylacetonates as precursors. The nanocrystals form large-area 2D superlattices, and display a catalytic synergistic effect in the hydrolysis of NaBH{sub 4} to generate H{sub 2} at x = 0.5 in a strongly basic medium. The Ni{sub 0.5}Cu{sub 0.5} nanocrystals show the lowest activation energy, and also exhibit the highest H{sub 2} generation rate at 298 K.
Date: July 22, 2008
Creator: Zhang, Yawen; Huang, Wenyu; Habas, Susan E.; Kuhn, John N.; Grass, Michael E.; Yamada, Yusuke et al.
Partner: UNT Libraries Government Documents Department

Permeation and Diffusion of Hydrogen Through Pd Membranes

Description: Hydrogen diffusion through Pd membranes has been measured under non-UHV conditions, i.e., the membranes are evacuated under non-UHV conditions. Despite this, the results indicate that bulk diffusion is the slow step and the diffusion constants agree with earlier workers results where UHV conditions obtained. The activation energy for H2 permeation in the dilute phase was determined from an Arrhenius plot over a series of temperatures from 423 to 503 K. The solubilities of H2 were determined over the same temperature range and from these data, the diffusion constants were determined.
Date: January 29, 2003
Creator: Shanahan, K.L.
Partner: UNT Libraries Government Documents Department

Localized Pd Overgrowth on Cubic Pt Nanocrystals for Enhanced Electrocatalytic Oxidation of Formic Acid

Description: Binary Pt/Pd nanoparticles were synthesized by localized overgrowth of Pd on cubic Pt seeds for the investigation of electrocatalytic formic acid oxidation. The binary particles exhibited much less self-poisoning and a lower activation energy relative to Pt nanocubes, consistent with the single crystal study.
Date: March 20, 2008
Creator: Lee, H.; Habas, S.E.; Somorjai, G.A. & Yang, P.
Partner: UNT Libraries Government Documents Department

The effect of CO2(aq), Al(aq) and temperature on feldspar dissolution

Description: The authors measured labradorite (Ca{sub 0.6}Na{sub 0.4}Al{sub 1.6}Si{sub 2.4}O{sub 8}) dissolution rates using a mixed flow reactor from 30 to 130 C as a function of CO{sub 2} (3 x 10{sup -3} and 0.6 M), and aluminum (10{sup -6} to 10{sup -3}M) at pH 3.2. Over these conditions, labradorite dissolution can be described with a single rate expression that accounts for observed increases in dissolution rate with temperature and decreases in dissolution rate with dissolved aluminum: Rate{sub Si} (mol Labradorite cm{sup -2} s{sup -1}) = k{double_prime} x 10{sup -Ea/2.303RT} [(a{sub H{sup +}}{sup 3n}/a{sub Al{sup 3+}}{sup n})K{sub T}/(1+K{sub T} (a{sub H{sup +}}{sup 3n}/a{sub Al{sup 3+}}{sup n}))] where the apparent dissolution rate constant, k{double_prime} = 10{sup -5.69} (mol Labradorite cm{sup -2}s{sup -1}); the net activation energy, E{sub a} = 10.06 (kcal mol{sup -1}); H{sup +}-Al{sup 3+} exchange coefficient, n = 0.31; and silica rich surface complex formation constant K{sub T} = 4.5 to 5.6 from 30 to 130 C. The effect of CO{sub 2}(aq) on mineral dissolution is accounted for by changes in solution pH. At temperatures below 60 C, labradorite dissolves incongruently with preferential dissolution of Na, Ca and Al over Si.
Date: October 14, 2003
Creator: Carroll, S & Knauss, K
Partner: UNT Libraries Government Documents Department

The Role of Organic Capping Layers of Platinum Nanoparticles in Catalytic Activity of CO Oxidation

Description: We report the catalytic activity of colloid platinum nanoparticles synthesized with different organic capping layers. On the molecular scale, the porous organic layers have open spaces that permit the reactant and product molecules to reach the metal surface. We carried out CO oxidation on several platinum nanoparticle systems capped with various organic molecules to investigate the role of the capping agent on catalytic activity. Platinum colloid nanoparticles with four types of capping layer have been used: TTAB (Tetradecyltrimethylammonium Bromide), HDA (hexadecylamine), HDT (hexadecylthiol), and PVP (poly(vinylpyrrolidone)). The reactivity of the Pt nanoparticles varied by 30%, with higher activity on TTAB coated nanoparticles and lower activity on HDT, while the activation energy remained between 27-28 kcal/mol. In separate experiments, the organic capping layers were partially removed using ultraviolet light-ozone generation techniques, which resulted in increased catalytic activity due to the removal of some of the organic layers. These results indicate that the nature of chemical bonding between organic capping layers and nanoparticle surfaces plays a role in determining the catalytic activity of platinum colloid nanoparticles for carbon monoxide oxidation.
Date: December 17, 2008
Creator: Park, Jeong Y.; Aliaga, Cesar; Renzas, J. Russell; Lee, Hyunjoo & Somorjai, Gabor A.
Partner: UNT Libraries Government Documents Department

An Evaluation of Power Law Breakdown in Metals, Alloys, Dispersion Hardened Materials and Compounds

Description: Creep at high stresses often produces strain rates that exceed those that would be predicted by a power law relationship. In this paper, we examine available high stress creep data for pure metals, solid solution alloys, dispersion strengthened powder metallurgy materials and compounds for power law breakdown (PLB). The results show that, if PLB is observed, then the onset of PLB is generally observed at about {epsilon}/D{sub eff} = 10{sup 13} m{sup -2}, where D{sub eff} is the effective diffusion coefficient incorporating lattice and dislocation pipe diffusion. The common origins of PLB for the various systems studied can be found in the production of excess vacancies by plastic deformation. Anomalous behavior in two pure metals (nickel and tungsten) and a solid solution alloy (Fe-25Cr and Fe-26Cr-1Mo) has been analyzed and provides insight into this excess vacancy mechanism. In metal systems, the onset of PLB is related to a change in the nature of the subgrain structure developed. In the PLB region, subgrains become imperfect containing dislocation tangles adjacent to the sub-boundary, and dislocation cells are evident. The dislocation tangles and cells are the source of excess vacancies and increase the creep rate above that predicted from power law creep. If subgrains do not form then PLB is not observed. In solid solution alloys, in which the dominant deformation resistance results from the interaction of solute atoms with moving dislocations, excess vacancies influence the diffusion of these solute atoms. PLB is not observed in many systems. This is attributed either to the presence of a high equilibrium vacancy concentration (because of a low activation energy for vacancy formation) or to the inability to form subgrains.
Date: October 20, 1999
Creator: Lesuer, D.R.; Syn, C.K. & Sherby, O.D.
Partner: UNT Libraries Government Documents Department

Diffusion of Ca and Mg in Calcite

Description: The self-diffusion of Ca and the tracer diffusion of Mg in calcite have been experimentally measured using isotopic tracers of {sup 25}Mg and {sup 44}Ca. Natural single crystals of calcite were coated with a thermally-sputtered oxide thin film and then annealed in a CO{sub 2} gas at one atmosphere total pressure and temperatures from 550 to 800 C. Diffusion coefficient values were derived from the depth profiles obtained by ion microprobe analysis. The resultant activation energies for Mg tracer diffusion and Ca self-diffusion are respectively: E{sub a}(Mg) = 284 {+-} 74 kJ/mol and E{sub a}(Ca) = 271 {+-} 80 kJ/mol. For the temperature ranges in these experiments, the diffusion of Mg is faster than Ca. The results are generally consistent in magnitude with divalent cation diffusion rates obtained in previous studies and provide a means of interpreting the thermal histories of carbonate minerals, the mechanism of dolomitization, and other diffusion-controlled processes. The results indicate that cation diffusion in calcite is relatively slow and cations are the rate-limiting diffusing species for the deformation of calcite and carbonate rocks. Application of the calcite-dolomite geothermometer to metamorphic assemblages will be constrained by cation diffusion and cooling rates. The direct measurement of Mg tracer diffusion in calcite indicates that dolomitization is unlikely to be accomplished by Mg diffusion in the solid state but by a recrystallization process.
Date: February 10, 1999
Creator: Cygan, R.T. & Fisler, D.K.
Partner: UNT Libraries Government Documents Department

Single- and multi-photon ionization studies of organosulfur species

Description: Accurate ionization energies (IE`s) for molecular species are used for prediction of chemical reactivity and are of fundamental importance to chemists. The IE of a gaseous molecule can be determined routinely in a photoionization or a photoelectron experiment. IE determinations made in conventional photoionization and photoelectron studies have uncertainties in the range of 3--100 meV (25--250 cm{sup {minus}1}). In the past decade, the most exciting development in the field of photoionization and photoelectron spectroscopy has been the availability of high resolution, tunable ultraviolet (UV) and vacuum ultraviolet (VUV) laser sources. The laser pulsed field ionization photoelectron (PFI-PE) scheme is currently the state-of-the-art photoelectron spectroscopic technique and is capable of providing photoelectron energy resolution close to the optical resolution. The author has focused attention on the photoionization processes of some sulfur-containing species. The studies of the photoionization and photodissociation on sulfur-containing compounds [such as CS{sub 2}, CH{sub 3}SH, CH{sub 3}SSCH{sub 3}, CH{sub 3}CH{sub 2}SCH{sub 2}CH{sub 3}, HSCH{sub 2}CH{sub 2}SH and C{sub 4}H{sub 4}S (thiophene) and sulfur-containing radicals, such as HS, CS, CH{sub 3}S, CH{sub 3}CH{sub 2}S and CH{sub 3}SS], have been the major subjects in the group because sulfur is an important species contributing to air pollution in the atmosphere. The modeling of the combustion and oxidation of sulfur compounds represents important steps for the control of both the production and the elimination of sulfur-containing pollutants. Chapter 1 is a general introduction of the thesis. Chapters 2 and 6 contain five papers published in, or accepted for publication in, academic periodicals. In Chapter 7, the progress of the construction in the laboratory of a new vacuum ultraviolet laser system equipped with a reflectron mass spectrometer is presented. Chapters 2 through 7 have been removed for separate processing. A general conclusion of these studies are given in Chapter 8 followed by an ...
Date: February 12, 1999
Creator: Cheung, Y.S.
Partner: UNT Libraries Government Documents Department

Methods for Predicting More Confident Lifetimes of Seals in Air Environments

Description: We have been working for many years to develop improved methods for predicting the lifetimes of polymers exposed to air environments and have recently turned our attention to seal materials. This paper describes an extensive study on a butyl material using elevated temperature compression stress-relaxation (CSR) techniques in combination with conventional oven aging exposures. The results initially indicated important synergistic effects when mechanical strain is combined with oven aging, as well as complex, non-Arrhenius behavior of the CSR results. By combining modeling and experiments, we show that diffusion-limited oxidation (DLO) anomalies dominate traditional CSR experiments. A new CSR approach allows us to eliminate DLO effects and recover Arrhenius behavior. Furthermore, the resulting CSR activation energy (E{sub a}) from 125 C to 70 C is identical to the activation energies for the tensile elongation and for the oxygen consumption rate of unstrained material over similar temperature ranges. This strongly suggests that the same underlying oxidation reactions determine both the unstrained and strained degradation rates. We therefore utilize our ultrasensitive oxygen consumption rate approach down to 23 C to show that the CSR E{sub a} likely remains unchanged when extrapolated below 70 C, allowing very confident room temperature lifetime predictions for the butyl seal.
Date: March 5, 1999
Creator: Celina, M.; Gillen, K.T. & Keenan, M.R.
Partner: UNT Libraries Government Documents Department

Thermal properties and chemical reactivity. Quarterly report, October 1971--December 1971

Description: A very high boiling impurity was concentrated from a sample of FEFO with a hexane wash. Additional washing of this sample has increased the concentration of this impurity. A mass spectrum was obtained but an identification has not been made. The results of the analysis of the products from the thermal decomposition of FEFO at 120, 135, 150 C are discussed. A chromatogram of FEFO heated for 22 hours at 150 C shows a definite increase in low and high boiling impurities. The evaluation of the condition of the two coupon test assemblies aged at 80 C for 21 and 27 months are discussed. Thermal analysis of the LX-09 from these two coupon tests, a PASS A mechanical test specimen and a control sample are reported. A PDP-12/30 was interfaced with a Perkin Elmer DSC-1 to measure the heat of fusion of PETN. Some of the problems associated with getting reproducible data are discussed. The heat of fusion for six lots of LX-13 grade PETN are given.
Date: December 31, 1998
Creator: Myers, L.C.
Partner: UNT Libraries Government Documents Department

Rate constants for H{sub 2}CO + O{sub 2} {yields} HCO + HO{sub 2} at high temperature

Description: The reaction between H{sub 2}CO and O{sub 2} has been studied in a reflected shock tube apparatus between 1633--2027 K using trioxane, (H{sub 2}CO){sub 3}, as the source of H{sub 2}CO. O-atom atomic resonance absorption spectrometry (ARAS) was used to observe absolute [O]{sub t} under conditions of low [H{sub 2}CO]{sub 0} so that most secondary reactions were negligible. Hence, the observed [O]{sub t} was the direct result of the rate controlling reaction between H{sub 2}CO and O{sub 2}. Ab initio theoretical results indicated that the process, H{sub 2}CO + O{sub 2} {yields} HCO + HO{sub 2}, is the only possible reaction. After rapid HCO and HO{sub 2} dissociations, O-atoms are then instantaneously produced from H + O{sub 2} {yields} O + OH. Using the ab initio result, variational transition state theoretical calculations (CTST) give k{sub 1} = 4.4929 x 10{sup {minus}20} T{sup 2.9116} exp{minus}18692/T cm{sup 3} molecule{sup {minus}1} S{sup {minus}1}. This theoretical result is consistent with the present experimental determinations and those at lower temperatures.
Date: January 5, 2000
Creator: Michael, J. V.; Su, M.-C.; Sutherland, J. W.; Fang, D.-C.; Harding, L. B. & Wagner, A. F.
Partner: UNT Libraries Government Documents Department

Rate of Pu(IV) polymer formation in nitric acid solutions. A parametric study

Description: The kinetics of Pu(IV) polymer formation has been examined with the intent of developing a simple mathematical equation that would predict the appearance of polymer. The fundamental polymerization rate has been found to be dependent on [Pu(IV)]{sup 1} {sup 2} and [HNO{sub 3}]{sup -6}. The activation energy for polymer formation is real temperature dependent, varying from 66.9 kJ/mol (16 kcal/mol) at 25{sup 0}C to 150.5 kJ/mol (36 kcal/mol) at 105{sup 0}C. These relationships have guided the developement of an empirical model that gives time to form 2% polymer in hours, t = [Pu/sub T/]/sup a/[HNO{sub 3}]/sup b/ Ae/sup c/T/, where a = -1.6, b = 4.6, c = 12.300 K, and A = 7.66 x 10{sup -16} h M{sup -3}; [Pu/sub T/] is the total plutonium concentration, mol/L; and [HNO{sub 3}] is the makeup nitric acid concentration, mol/L. 11 references, 26 figures, 1 table.
Date: July 1, 1984
Creator: Toth, L.M. & Osborne, M.M.
Partner: UNT Libraries Government Documents Department

The effects of pressure, temperature and concentration on the reactivity of alkanes; experiments and modeling in a rapid compression machine

Description: Experiments in a rapid compression machine have examined the influences of variations in pressure, temperature, and equivalence ratio on the autoignition of n-pentane. Equivalence ratios included values from 0.5 to � 2.0, compressed gas initial temperatures were varied between 675K and 980K, and compresed gas initial pressures varied from 8 to 20 bar. Numerical simulations of the same experiments were carried out using a detailed chemical kinetic reaction mechanism. The results are interpreted in terms of a low temperature oxidation mechanism involving addition of molecular oxygen to alkyl and hydroperoxyalkyl radicals. Idealized calculations are reported which identify the major reaction paths at each temperature. Results indicate that in most cases, the reactive gases experience a two-stage autoigni tion. The first stage follows a low temperature alkylperoxy radical isomerization pathway that is effectively quenched when the temperature reaches a level where dissociation reactions of alkylperoxy and hydroperoxyalkylperoxy radicals are more rapid than the reverse addition steps. The second stage is controlled by the onset of dissociation of hydrogen peroxide. Results also show that in some cases, the first stage ignition takes place during the compression stroke in the rapid compression machine, making the interpretation of the experiments somewhat more complex than generally assumed. At the highest compression temperatures achieved, little or no first stage ignition is observed.
Date: January 8, 1998
Creator: Curran, H J; Griffiths, J F; Mohamed, C; Pitz, W J; Westbrook, C & Wo, S K
Partner: UNT Libraries Government Documents Department

Diffusion Kinetics in the Pd/Cu(001) Surface Alloy

Description: We use atom-tracking scanning tunneling microscopy to study the diffusion of Pd in the Pd/Cu(001) surface alloy. By following the motion of individual Pd atoms incorporated in the surface, we show that Pd diffuses by a vacancy-exchange, mechanism. We measure an effective activation energy for the diffusion of incorporated Pd atoms of 0.88 eV, which is consistent with an ab initio calculated barrier of 0.94 eV.
Date: December 8, 2000
Partner: UNT Libraries Government Documents Department

Reaction mechanisms and microstructures of ceramic-metal composites made by reactive metal penetration

Description: Ceramic-metal composites can be made by reactive penetration of molten metals into dense ceramic performs. The metal penetration is driven by a large negative Gibbs energy for reaction, which is different from the more common physical infiltration of porous media. Reactions involving Al can be written generally as (x+2)Al + (3/y)MO{sub y} {yields} Al{sub 2}O{sub 3} + M{sub 3/y}Al{sub x}, where MO{sub y} is an oxide that is wet by molten Al. In low Po{sub 2} atmospheres and at temperature above about 900{degrees}c, molten Al reduces mullite to produce Al{sub 2}O{sub 3} + M{sub 3/y}Al{sub x}, where MO is an oxide that is wet by molten Al. In low Po{sub 2} atmospheres and at temperatures above about 900{degrees}C, molten al reduces mullite to produce Al{sub 2}O{sub 3} and Si. The Al/mullite reaction has a {Delta}G{sub r}{degrees} (1200K) of -1014 kJ/mol and, if the mullite is fully dense, the theoretical volume change on reaction is less than 1%. A microstructure of mutually-interpenetrating metal and ceramic phases generally is obtained. Penetration rate increases with increasing reaction temperature from 900 to 1150{degrees}C, and the reaction layer thickness increases linearly with time. Reaction rate is a maximum at 1150{degrees}C; above that temperature the reaction slows and stops after a relatively short period of linear growth. At 1300{degrees}C and above, no reaction layer is detected by optical microscopy. Observations of the reaction front by TEM show only al and Al{sub 2}O{sub 3} after reaction at 900{degrees}C, but Si is present in increasing amounts as the reaction temperature increases to 1100{degrees}C and above. The kinetic and microstructural data suggest that the deviation from linear growth kinetics at higher reaction temperatures and longer times is due to Si build-up and saturation at the reaction front. The activation energy for short reaction times at 900 to 1150{degrees}C varies ...
Date: December 31, 1996
Creator: Fahrenholtz, W.F.; Ewsuk, K.G. & Loehman, R.E.
Partner: UNT Libraries Government Documents Department

Mechanisms and optimization of coal combustion. Semiannual report, November 1, 1998--April 30, 1999

Description: The effects of process conditions on the reactivity of Illinois No.6 coal in the kinetic and diffusion control regime were studied using new sequential combustion procedures. Reactivity patterns in the kinetic controlled regime were not influenced by the pyrolysis heating rates. Results presented in the previous DOE report have shown, however, that combustion rates were significantly affected by the pyrolysis heating rates at reaction temperatures higher than 550 C. These results establish the importance of the macropore structure of chars in determining their combustion rates under conditions leading to significant intraparticle diffusional limitations.
Date: December 1, 1999
Creator: Zygourakis, K.
Partner: UNT Libraries Government Documents Department

Effects of oxygen cover gas and NaOH dilution on gas generation in tank 241-SY-101 waste

Description: Laboratory studies are reported of gas generation in heated waste from tank 241-SY-101. The rates of gas generation and the compositions of product gas were measured. Three types of tests are compared. The tests use: undiluted waste, waste diluted by a 54% addition of 2.5 M NaOH, and undiluted waste with a reactive cover gas of 30% Oxygen in He. The gas generation rate is reduced by dilution, increased by higher temperatures (which determines activation energies), and increased by reactions of Oxygen (these primarily produce H{sub 2}). Gases are generated as reduction products oxidation of organic carbon species by nitrite and oxygen.
Date: May 30, 1996
Creator: Person, J.C.
Partner: UNT Libraries Government Documents Department

Mechanisms of Atmospheric Copper Sulfidation and Evaluation of Parallel Experimentation Techniques

Description: A physics-based understanding of material aging mechanisms helps to increase reliability when predicting the lifetime of mechanical and electrical components. This report examines in detail the mechanisms of atmospheric copper sulfidation and evaluates new methods of parallel experimentation for high-throughput corrosion analysis. Often our knowledge of aging mechanisms is limited because coupled chemical reactions and physical processes are involved that depend on complex interactions with the environment and component functionality. Atmospheric corrosion is one of the most complex aging phenomena and it has profound consequences for the nation's economy and safety. Therefore, copper sulfidation was used as a test-case to examine the utility of parallel experimentation. Through the use of parallel and conventional experimentation, we measured: (1) the sulfidation rate as a function of humidity, light, temperature and O{sub 2} concentration; (2) the primary moving species in solid state transport; (3) the diffusivity of Cu vacancies through Cu{sub 2}S; (4) the sulfidation activation energies as a function of relative humidity (RH); (5) the sulfidation induction times at low humidities; and (6) the effect of light on the sulfidation rate. Also, the importance of various sulfidation mechanisms was determined as a function of RH and sulfide thickness. Different models for sulfidation-reactor geometries and the sulfidation reaction process are presented.
Date: March 1, 2002
Partner: UNT Libraries Government Documents Department

Thermal Decomposition Kinetics of HMX

Description: Nucleation-growth kinetic expressions are derived for thermal decomposition of HMX from a variety of types of data, including mass loss for isothermal and constant rate heating in an open pan, and heat flow for isothermal and constant rate heating in open and closed pans. Conditions are identified in which thermal runaway is small to nonexistent, which typically means temperatures less than 255 C and heating rates less than 1 C/min. Activation energies are typically in the 140 to 165 kJ/mol regime for open pan experiments and about 150-165 kJ/mol for sealed-pan experiments. The reaction clearly displays more than one process, and most likely three processes, which are most clearly evident in open pan experiments. The reaction is accelerated for closed pan experiments, and one global reaction fits the data fairly well. Our A-E values lie in the middle of the values given in a compensation-law plot by Brill et al. (1994). Comparison with additional open and closed low temperature pyrolysis experiments support an activation energy of 165 kJ/mol at 10% conversion.
Date: March 17, 2005
Creator: Burnham, A K & Weese, R K
Partner: UNT Libraries Government Documents Department

Comparison of cracking kinetics for Kern River 650{degrees}F{sup +} residuum and Midway Sunset crude oil

Description: Kern River 650{degrees}F{sup +} residuum and Midway Sunset crude oil were examined by micropyrolysis at several constant-heating rates to determine pyrolysis cracking kinetics. Determined by the discrete distribution method, both feeds exhibited principal activation energies of 50 kcal/mol and frequency factors {approximately} 10{sup 13} sec{sup -1}. Energy distributions were similar ranging from 45 to 57 kcal/mol. Determined by the shift-in-T{sub max} method, E{sub approx}, A{sub approx} for Kern River 650{degrees}F{sup +} and Midway Sunset were 48 kcal/mol, 1.3 X 10{sup 12} sec{sup -1}, and 46 kcal/mol, 4.6 X 10{sup 11} sec{sup -1}, respectively. These results are similar, but not identical to other kinetic parameters for heavy oils from type II source rocks.
Date: May 1, 1995
Creator: Reynolds, J.
Partner: UNT Libraries Government Documents Department