George E. Overturf

Fabrication of low-density foam shells from resorcinol-formaldehyde aerogel

Resorcinol-formaldehyde (RF) aerogel chemistry has been used with encapsulation techniques to fabricate low-density, transparent, foam shells. To accomplish this, the gelation time was reduced from several hours to several minutes by the addition of acid following base-catalyzed RF particle growth. However, additional “annealing” of the gel for at least 20 h was needed to maximize crosslinking and minimize swelling in exchange solvents. Increasing the molar ratio of formaldehyde to resorcinol from 2 to 3 also helped to increase crosslinking. Densification of the foam shells due to dehydration during curing was greatly reduced by judicious choice of immiscible oil phases and by saturating the exterior oil phase during the annealing stage. Shells have been produced with diameters of about 2 mm, wall thicknesses ranging from 100 to 200 μm and foam densities approaching 50 mg/cc.

Gamma radiation effects on corrosion—I. Electrochemical mechanisms for the aqueous corrosion processes of austenitic stainless steels relevant to nuclear waste disposal in tuff☆

Abstract The effects of gamma irradiation on the corrosion mechanisms of 304L and 316L stainless steels in groundwater from a proposed nuclear waste repository site in tuffaceous rock are presented. When gamma irradiation is initiated, corrosion potential shifts in the positive direction are observed. These potential shifts are associated with the radiation-induced production of hydrogen peroxide. The electrochemical mechanisms involved in the corrosion potential shifts, as well as the subsequent effect on pitting resistance, are considered.

Production and characterization of doped mandrels for inertial‐confinement fusion experiments

A key feature of current inertial‐confinement fusion (ICF) experiments is the incorporation of dopant atoms into the thin polymer microshell which, in a finished ICF capsule, forms its inner wall. These dopants provide a spectroscopic signal during the implosion that can be used to diagnose the degree of mix at the capsule–fuel interface. The high‐Z dopants can also be used to directly image the fuel–pusher interface. The current status of doped mandrel development is reviewed, with a focus on the mandrel surface smoothness. With the development of unique surface mapping characterization tools which will be described, it has been discovered that mandrel surface smoothness is a function of the polymers used to form the mandrels. In this report it will be shown that Cl‐doped mandrels produced from a blend of polystyrene and poly(p‐chlorostyrene) are rough on a length scale of 10’s of microns with amplitudes of as much as a 100 nm. The origin of this roughness will be discussed, and it will be shown that thi...

Resorcinol/Formaldehyde Foam Shell Targets for ICF

Resorcinol/formaldehyde (R/F) low-density foam making processes have been adapted to microencapsulation techniques. This has been done in an effort to make low density, low Z, transparent foam shells for use as cryogenic ICF targets. It was necessary to modify the normal R/F formulation and processing to accelerate the gelation time from tens of hours to less than one hour. Proper selection of the inner and outer oil phase solvents was critical for density matching and prevention of the dehydration of the gelling preform, respectively. 12 refs., 5 figs., 1 tab.

Towards unraveling the photochemistry of TATB

A combined theoretical and experimental chemical analysis has been conducted to unravel the mechanism, underlying the color change of yellow 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) to green upon UV irradiation. There is a strong evidence to show, for the first time, that the process is photochemical in nature and due to the formation of the mono nitroso derivative. We have identified a chemical synthesis by which this derivative compound can be produced in the laboratory, thus allowing for direct testing and determination of its chemical and physical properties. Calculations also show only a slight decrease in the sensitivity and performance of the irradiated materials, attributed to the formation of this previously unidentified species.

Hollow foam microshells for liquid‐layered cryogenic inertial confinement fusion targets

Future U.S. inertial confinement fusion (ICF) targets will use capsules, 1–2 mm in diameter, with uniform 100 μm thick, cryogenic fuel layers. Research is currently underway to identify optimal methods for producing these thick, uniform layers. One method is to use a spherical polymer foam layer within a full density polymer overcoat to support the fuel. Targets of this type, 0.4–0.8 mm in diameter, with 10–30 μm walls, have been developed by the Institute of Laser Engineering at Osaka University, Japan. Reported here are the results obtained from work to extend the method to the future ICF target design. Overcoated foam shells of the proper dimensions were produced, but their optical properties precluded the use of current diagnostics to determine the amount and uniformity of the fuel fill. Briefly outlined are options for improving the optical properties.

Reactions of Titanocene Derivatives with Molecular Carboxylic Acids and Copolymers Bearing Carboxylic Acid Groups

Linear, soluble copolymers containing titanium are of interest for use in targets for inertial-confinement fusion (ICF) experiments because the titanium is a useful spectroscopic probe for studying the nuclear fusion process. We have studied the reactions of dichloro- titanocene and diphenyltitanocene with carboxylic acids to determine if these can be used to prepare such polymers. Model reactions of dichlorotitanocene with p-toluic acid show that both mono- and di-carboxylates are formed. We have prepared a soluble titanium-containing copolymer via the reaction of poly(styrene-co-methacrylic acid) with an excess of dichlorotitanocene but attempts to remove the excess molecular titanocene from the copolymer result in crosslinking of the copolymer. Model reactions of benzoic and pivalic acids with diphenyltitanocene demonstrate that monosubstituted carboxylato(phenyl)titanocenes are formed but that these products are stable only at low temperatures and decompose readily at ambient temperature. Proton NMR studies of the reaction of diphenyltitanocene and benzoic acid at −20 °C indicate that this reaction is second order, suggesting that the reaction does not proceed through a benzyne intermediate.

Moisture desorption rates from TATB formulations: experiments and kinetic models.

The rate of water desorption from PBX-9502, a formulation containing 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), is measured using temperature-programmed desorption and modeled using conventional kinetic modeling methods. The results of these studies show two stages of moisture release. At lower temperatures, the release is likely assisted by thermal expansion of the TATB and melting of the Kel-F binder. At higher temperatures, a considerable amount of water is released and is attributed to sublimation of the TATB, which exposes new surfaces for water desorption.