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Transparent plastic scintillators based on polyvinyltoluene (PVT) have been fabricated with high loading of bismuth carboxylates for gamma spectroscopy, and with lithium carboxylates for neutron detection. When activated with a combination of standard fluors, 2,5-diphenyloxazole (PPO) and tetraphenylbutadiene (TPB), gamma light yields with 15 wt% bismuth tripivalate of 5000 Ph/MeV are measured. A PVT plastic formulation including 30 wt% lithium pivalate and 30 wt% PPO offers both pulse shape discrimination, and a neutron capture peak at ~400 keVee. In another configuration, a bismuth-loaded PVT plastic is coated with ZnS(6Li) paint, permitting simultaneous gamma and neutron detection via pulse shape discrimination with a figure-of-merit of 3.8, while offering gamma spectroscopy with energy resolution of R(662 keV)=15%.

New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI2, CaI2, SrBr2, BaI2 and BaBr2-SrI2 is the most promising. SrI2(Eu) emits into the Eu2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.

For rapid, unambiguous isotope identification, scintillator detectors providing high-resolution gamma ray spectra are required. We have fabricated Lutetium Aluminum Garnet (LuAG) using transparent ceramic processing, and report a 2-mm thick ceramic exhibiting 75% transmission and light yield comparable to single-crystal LuAG:Ce. The LuAG:Ce luminescence peaks at 550 nm, providing an excellent match for Silicon Photodiode readout. LuAG is dense (6.67 g/cm3) and impervious to water, exhibits good proportionality and a fast decay (∼40 ns), and we measure light yields in excess of 20,000 photons/MeV.

We have developed a cleaning procedure for aluminum alloys for effective minimization of surface-adsorbed sub-micrometer particles and nonvolatile residue. The procedure consists of a phosphoric acid etch followed by an alkaline detergent wash. To better understand the mechanism whereby this procedure reduces surface contaminants, we characterized the aluminum surface as a function of cleaning step using surface enhanced Raman spectroscopy (SERS). SERS indicates that phosphoric acid etching re-establishes a surface oxide of different characteristics, including deposition of phosphate and increased hydration, while the subsequent alkaline detergent wash appears to remove the phosphate and modify the new surface oxide, possibly leading to a more compact surface oxide. We also studied the zeta potential of5 microm pure aluminum and aluminum alloy 6061-T6 particles to determine how surface electrostatics may be affected during the cleaning process. The particles show a decrease in the magnitude of their zeta potential in the presence of detergent, and this effect is most pronounced for particles that have been etched with phosphoric acid.

Reactions of (CH 3 O) 4 Si and RSi(OCH 3 ) 3 (R = CH 3 , C 2 H 5 , C 3 H 7 , C 4 H 9 , C 16 H 33 ) at a mole ratio of 1 to 1 were performed in methanol using base catalysis. Subsequent supercritical drying produced materials that have a wide range of surface areas and porosities. Spectroscopic characterization, revealed normal Si–O substitution as well as incorporation of the carbon chain into the Si framework. Heating of the stable forms of the materials in air at different temperatures yielded, depending upon oxidation conditions, several materials with much higher surface areas, typical of aerogels. Pore-size distribution measurements revealed mesopore features that differed from the non-oxidized materials for some R groups. Spectroscopic characterization revealed the disappearance of the R–Si substitution for all R groups and the appearance of an oxidized intermediate for R = C 16 H 33 .

The use of the R-substituted organosiloxanes in hydrolysis-condensation reactions with tetraalkoxysilanes is a flexible way to incorporate functional groups into silica gels and aerogels. When R = CH2CH2CF3, chemically and physically stable gels and aerogels that are hydrophobic can be readily made. These types of aerogels have been shown to be adsorbents for organics compounds from aqueous phase. In order to understand the role of the fluorinated group in making these materials hydrophobic, various formulations of CF3CH2CH2-modified aerogels have been synthesized and characterized. These materials were formed by the reaction of CF3CH2CH2Si(OCH3)3 and Si(OCH3)4 in CH3OH with NH4OH as the catalyst followed by supercritical drying in CO2. A 0.7 mole ratio of the reactants CF3CH2CH2Si(OCH3)3/Si(OCH3)4 is necessary to make hydrophobic aerogels that meet the 2.5% water adsorption limit. Lower reactant mole ratios yield materials that adsorb more water, reaching a saturation value in roughly 1 month, where the amount adsorbed is inversely proportional to the amount of CF3CH2CH2 group incorporated.

The hexadecyl substituted siloxane, C16H33Si(OCH3)3, was reacted with (CH3O)4Si at the mole ratio of 1–3 in methanol using HBF4 catalysis. Sol–gel materials were formed that have low surface areas (∼10 m2/g). Subsequent supercritical drying using CO2 at 40 °C produced materials that are very different than traditional aerogels, with surface areas around the same values as the corresponding sol–gels, as well as no detectable meso-pore features. In some cases, the aerogels even melted upon heating. Spectroscopic characterization, using IR, 29Si and 13C NMR revealed normal Si–O substitution as well as incorporation of the carbon substituent into the Si framework. Heating of the stable forms of the materials in air at different temperatures yielded, depending upon oxidation conditions, several materials with much higher surface areas, typical of aerogels. Pore size distribution measurements revealed meso-pore features with a narrow distribution of 37–39 A. Spectroscopic characterization revealed the disappearance of the R-Si substitution and the appearance of an oxidized intermediate. This paper describes the chemistry and characterization of these unusual sol–gels, aerogels, and oxidation products.

We have utilized a sol–gel synthetic approach in preparing nano-sized transition metal oxide components for new energetic nanocomposites. Nanocomposites of Fe 2 O 3 /Al(s), are readily produced from a solution of Fe(III) salt by adding an organic epoxide and a powder of the fuel metal. These materials can be processed to aerogel or xerogel monolithic composite solids. High resolution transmission electron microscopy (HRTEM) of the dried energetic nanocomposites reveal that the metal oxide component consists of small (3–10 nm) clusters of Fe 2 O 3 that are in intimate contact with ultra fine grain (UFG) ∼25 nm diameter Al metal particles. HRTEM results also indicate that the Al particles have an oxide coating ∼5 nm thick. This value agrees well with analysis of pristine UFG Al powder and indicates that the sol–gel synthetic method and processing does not significantly perturb the fuel metal. Both qualitative and quantitative characterization has shown that these materials are indeed energetic. The materials described here are relatively insensitive to standard impact, spark, and friction tests, results of which will be presented. Qualitatively, it does appear that these energetic nanocomposites burn faster and are more sensitive to thermal ignition than their conventional counterparts and that aerogel materials are more sensitive to ignition than xerogels. We believe that the sol–gel method will at the very least provide processing advantages over conventional methods in the areas of cost, purity, homogeneity, and safety and potentially yield energetic materials with interesting and special properties.

We have developed a new sol–gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol–gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its rate was studied by varying the solvent used, the Cr(III) precursor salt, the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe3+, Al3+, In3+, Ga3+, Zr4+, Hf4+, Ta5+, Nb5+, and W6+), provided the formal oxidation state of the metal ion is greater than or equal to +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.

Iron oxide-based porous solids were prepared by a sol−gel process using Fe(III) salts in various solvents. It was observed that the addition of propylene oxide to Fe(III) solutions resulted in the formation of transparent red-brown monolithic gels. The resulting gels were converted to either xerogels by atmospheric drying or aerogels by supercritical extraction with CO2(l). Some of the dried materials were characterized by nitrogen adsorption and desorption analysis and transmission electron microscopy (TEM). The results of those analyses indicate that the materials have high surface areas (∼300−400 m2/g), pore sizes with mesoporic dimensions (2−23 nm), and a microstructure made up of 5−10 nm diameter clusters of iron(III) oxide. The dependence of both gel formation and its rate was studied by varying the epoxide/Fe(III) ratio, the Fe(III) precursor salt, amount of water (H2O/Fe(III)) present, and the solvent employed. All of these variables were shown to affect the rate of gel formation and provide a con...

Traditional manufacturing of energetic materials involves processing of granular solids. One application is the production of detonators where powders of energetic material and a binder are typically mixed and compacted at high pressure to make pellets. Performance properties are strongly dependent on particle size distribution, surface area of its constituents, homogeneity of the mix, and void volume. The goal is to produce detonators with fast energy release rate that are insensitive to unintended initiation. Preparation of detonators from xerogel molding powders and aerogels, and comparison with materials produced by state-of-the-art technology are described.

Aqueous polycondensation of resorcinol with formaldehyde leads to organic gels the structure of which can be controlled by the reaction parameters. The amount of catalyst controls the size of the particles constituting the gel network. We could show that for very low catalyst concentrations in a high dilution of reactants, the particle growth can be further enhanced if the reaction temperature is kept low, and the gel time is prolonged. The resultant structure is largely affected by this treatment, producing particles with sizes of about 2 μm and average pore sizes of up to 7 μm. Due to these coarse structures it is possible to dry these RF gels subcritically with very little shrinkage. The structure has been studied by nitrogen sorption, small angle X-ray scattering and acoustic sound propagation. The change of the elastic modulus caused by pyrolysis at around 1000°C has been investigated. Mechanical properties of carbon aerogels are correlated with their electrical properties. The derived carbon aerogels have large specific surface areas, very little mesopore volume however, micron-sized macropores.

High resolution transmission electron microscopy (HRTEM) was used to study the morphology of ultralow-density transparent condensed-silica (CS) aerogels. Silica aerogels were synthesized by two slightly different two-step polymerization processes and they were supercritically dried in a high temperature autoclave. Aerogel CS1 had a density of 9 kg/m3 and a BET surface area of 574 m2/g; aerogel CS2 had a density of 30 kg/m3 and a specific surface area of 630 m2/g. Both samples were fractured, vertically replicated with 0.95 nm Pt-C and backed with approximately 12 nm of rotary evaporated carbon. The silica aerogel was then removed from the replica with dilute HF acid and the replicas were studied by HRTEM. The stereoscopic HRTEM images reveal that connectors in both CS aerogels are extended filaments which resemble bottlebrushes, having microporosity. This morphology results from side-chain formation on a nearly linear CS stem. The slightly different chemistry leads to different morphologies for the two aerogels. For CS1, the connectors between stems have diameters ranging from 1.7 to 14.2 nm with an average of 6.4 ± 0.5 nm and connector lengths averaged 62 ± 21 nm with some as long as 132 nm. Pore sizes ranged from 13 to 240 nm with an average of 74 ± 43 nm. The pores were slightly larger than those in CS2 which ranged from 12 to 277 nm and averaged 61 ± 56 nm. For CS2, the connectors had diameters ranging from 1.5 to 16.5 nm and averaging 9.7 ± 0.5 nm. The connector lengths in the CS2 aerogel averaged 58 ± 27 nm with some as long as 127 nm. The connector diameters in CS2 (9.7 ± 0.5 nm) were the only important aerogel feature significantly different and greater than those in CS1 (6.4 ± 0.5 nm). The CS1 and CS2 connectors had side-chain diameters of 2 ± 0.7 and 0.95 ± 0.5 nm, respectively, and a similar microporosity of approximately 2.0 ± 1 nm.

Commercialization of aerogels has been slow due to several factors including cost and manufacturability issues. The technology itself is well enough developed as a result of extensive work over the past decade by an international community of researchers. Several substantial markets appear to exist for aerogels as thermal and sound insulators, if production costs can keep prices in line with competing established materials. In the present paper, the elements identified as key cost drivers are discussed, and a prognosis for the evolution of the technology leading to reduced cost aerogel production is given.

Vanadium/silica and copper/silica aerogels have been prepared using the sol-gel method followed by CO{sub 2} exchange and supercritical extraction. Structural properties of samples supercritically dried, oxidized and used in reactions studies conducted with a feed representing the average composition of automobile exhaust from a lean burn engine were investigated using laser Raman spectroscopy and temperature-programmed reduction. No evidence of crystalline V{sub 2}O{sub 5} was found for the vanadium/silica aerogel, freshly extracted, oxidized or following exposure to reaction conditions using these techniques. However, results obtained for the copper/silica sample indicate that changes in the structure of the copper species had occurred as the sample was oxidized and exposed to reaction conditions.

Sol-gel synthesis and processing have the potential to create catalytic sites atomically isolated in the aerogel framework. V or Cu were incorporated during the solution chemistry into silica aerogels. Extended X-ray absorption fine structure and X-ray absorption near edge structure were employed to determine that copper is present as an oxide and probably isolated in the silica matrix, while vanadium is present as V 2 O 4 and V 2 O 5 . High resolution transmission electron microscopy and selected area diffraction were used to elucidate the characteristics of the aerogel structure itself, including the diameter of the chains, as well as to look for phase separation and crystallinity of the active metal oxide. It was found that the metals were highly dispersed in the silica aerogel, even after oxidation and reaction treatments.

Bismuth-loaded plastic scintillators capable of gamma spectroscopy are being scaled up to multiple cubic inch sizes. High light yields of >38,888 Ph/MeV are obtained with Iridium-complex fluors due to efficient harvesting of both singlet and triplet excitons, providing energy resolution of ~10% at 662 keV for 3 in3 scintillators. Although singlet fluors provide a poorer light yield of ~12,888 Ph/MeV and resolution at 662 keV of 14% in the Bismuth-loaded plastics, the fast decay time of

In this paper we report on the preparation of mixed lanthanide-silicate and pure lanthanide aerogels from the chlorides of erbium, praseodymium, and neodymium. A two-step sol-gel method is described for preparing the mixed aerogels, using a sub-stoichiometric amount of water in the first step to prepare a partially condensed silica-lanthanide precursor. The pure lanthanide aerogels are prepared directly from the chlorides using propylene oxide as a scavenger for reaction generated hydrochloric acid. The aerogel microstructures vary from colloidal for the pure lanthanide and high weight percent lanthanide-silicate aerogels to polymeric for the low weight percent lanthanide-silicate aerogels. This change in microstructure is also indicated by the BET analyses, which show that the surface areas decrease with increasing lanthanide concentrations. In general, we measured reductions of lanthanide contents during the supercritical drying step due to insufficient linking and subsequent “washing out” of the lanthanides from the gels. Also, the retention efficiency for the lanthanide improves with higher silica concentrations, making quantitative doping by this method practical only for the lower lanthanide concentrations.

Conventional silica sol-gel chemistry is limited for the production of transparent ultralow-density aerogels because (1) gelation is either slow or unachievable, and (2) even when gelation is achieved, the large pore sizes result in loss of transparency for aerogels

Aerogels are a special class of open-cell foams that have an ultrafine cell/pore size (

Transparent ceramics offer an alternative to single crystals for scintillator applications such as gamma ray spectroscopy and radiography. We have developed a versatile, scaleable fabrication method, using Flame Spray Pyrolysis (FSP) to produce feedstock which is readily converted into phase-pure transparent ceramics. We measure integral light yields in excess of 80,000 Ph/MeV with Cerium-doped Garnets, and excellent optical quality. Avalanche photodiode readout of Garnets provides resolution near 6%. For radiography applications, Lutetium Oxide offers a high performance metric and is formable by ceramics processing. Scatter in transparent ceramics due to secondary phases is the principal limitation to optical quality, and afterglow issues that affect the scintillation performance are presently being addressed.

Ceramic and single crystal Lutetium Aluminum Garnet scintillators exhibit energy resolution with bialkali photomultiplier tube detection as good as 8.6% at 662 keV. Ceramic fabrication allows production of garnets that cannot easily be grown as single crystals, such as Gadolinium Aluminum Garnet and Terbium Aluminum Garnet. Measured scintillation light yields of Cerium-doped ceramic garnets indicate prospects for high energy resolution.

Aerogels are a unique type of ultrafine cell size, low density foam. Traditional aerogels are inorganic, but the synthesis of organic aerogels has also been reported. In all cases, solution chemistry can be used to tailor the structure and properties of the resultant aerogels. This study examines the microstructural dependence of the compressive mechanical properties of silica, resorcinol-formaldehyde, carbon, and melamine-formaldehyde aerogels.

A stable mixture of partially condensed polysilicates is formed from the substoichiometric, acid catalysed hydrolysis of tetramethoxysilane [Si(OCH3)4] and the removal of the reaction- generated methanol by distillation. This mixture has been used as a precursor to make silica aerogels that show microstructural differences from base catalysed TMOS aerogels. In this paper, we report the analysis of this mixture for a determination of the type and distribution of the polysilicate species formed, and we include measurements of some physical properties of this fluid.

Aerogels are highly porous solids derived from the hypercritical drying of covalently crosslinked gels. SEM, TEM and scattering studies reveal that these materials have an open-cell morphology with a solid matrix composed of interconnected colloidal-like particles (30–200 A) Processing variables and synthetic conditions largely determine the microstructure and morphology. In this study, the structure-mechanical property relationships of silica, resorcinolformaldehyde, and carbon aerogels are examined.

Silica aerogels having densities in the range of 0.003 to 0.05 g/cm3, have been prepared from a condensed silica formulation involving a two-step hydrolysis/condensation process with tetramethoxysilane (TMOS). This work describes the preparation of these unique ultralow-density aerogels, and compares several of their properties with those of aerogels made by a single step, base catalysed process.

The classical sol-gel process for synthesizing SiO2 aerogels involves the hydrolysis/condensation of tetraethyoxysilane (TEOS) and/or teramethyoxysilane (TMOS) to produce a gel which can then be super-critically extracted to a low density, highly porous aerogel glass. Controlled hydrolysis of TEOS and TMOS leads to partially hydrolyzed compounds that can be subsequently water processed to form silica aerogels in the density range from .020 to .500 gms/cc. The partially hydrolyzed compounds are stable when sealed from moist air and can be stored for future use.We discuss the controlled conditions used to obtain these compounds and present data that characterize their structure. We detail the procedures for preparing the wide range of aerogel densities. We also report on their use as an adhesive.