Publications

The performance of APCs with relatively low compressive strength and poor stability under hydrothermal conditions make them less than desirable for the DPC use case. Meanwhile, grossite as a primary filler material or as a modifier has resulted in marked improvements in the properties of several DPC cement filler candidates. Grossite CAPCs have substantial mechanical strength even after irradiation. However, the significant decrease in strength observed post-irradiation requires further investigation before it is advanced as a material for the use case. As a modifier, grossite improves strength and set times of the APC and WAPC cements. Hibonite CAPCs also show considerable promise although their degradation under hydrothermal conditions is a potentially significant liability. Finally, with recent improvements in working time and compressive strength, the WAPCs remain in contention as viable candidates for the DPC use case.

Zeolite films are sought as components of molecular sieve membranes. Different routes used to prepare zeolite composite membranes include growing zeolite layers from gels on porous supports, depositing oriented zeolites on supports, and dispersing zeolites in polymeric membranes. In most cases, it is very difficult to control and avoid the formation of cracks and/or pinholes. The approach to membrane synthesis is based on hydrothermally converting films of layered aluminosilicates into zeolite films. The authors have demonstrated this concept by preparing zeolite A membranes on alumina supports from kaolin films. The authors have optimized the process parameters not only for desired bulk properties, but also for preparing thin (ca. 5 {micro}m), continuous zeolite A films. Scanning electron microscopy shows highly intergrown zeolite A crystals over most of the surface area of the membrane, but gas permeation experiments indicate existence of mesoporous defects and/or intercrystalline gaps. It has been demonstrated that the thickness of the final zeolite A membrane can be controlled by limiting the amount of precursor kaolin present in the membrane.

Garnet phosphors have potential for use in field emission displays (FEDs). Green-emitting Gd{sub 3}Ga{sub 5}O{sub 12}:Tb (GGG:Tb) and Y{sub 3}Al{sub 5}O{sub 12}:Tb (YAG:Tb) are possible alternatives to ZnO:Zn, because of their excellent resistance to burn, low-voltage efficiency, (3.5 lm/W from GGG:Tb at 800 V), and saturation resistance at high power densities. Hydrothermal and combustion synthesis techniques were employed to improve the low-voltage efficiency of YAG:Tb, and Y{sub 3}Ga{sub 5}O{sub 12}:Tb (YGG:Tb). Synthetic technique did not affect low-voltage (100--1,000 V) efficiency, but affected the particle size, morphology, and burn resistance. The small particle size phosphors obtained via hydrothermal (<1 {micro}m) and combustion reactions (<1 {micro}m) would benefit projection TV, high-definition TV (HDTV), and heads-up displays (HUDs), where smaller pixel sizes are required for high resolution.

This project studied hydrothermal synthesis as a route to producing green-emitting cathodoluminescent phosphorus isostructural with yttrium aluminum garnet (Y{sub 3}Al{sub 5}O{sub 12}, or YAG). Aqueous precipitation of Y, Gd, Al, Ga, and Tb salts produced amorphous gels, which were heated with water at 600 C and 3,200 bar to produce crystalline YAG:Tb, Y{sub 3}Ga{sub 5}O{sub 12}:Tb, Y{sub 3}Al{sub 3}Ga{sub 2}O{sub 12}:Tb, and Gd{sub 3}Ga{sub 5}O{sub 12}:Tb powders. Process parameters were identified that yielded submicron YAG:Tb and Y{sub 3}Ga{sub 5}O{sub 12}:Tb powders without grinding. Cathodoluminescent efficiencies were measured as functions of power density at 600 V, using both the hydrothermal garnets and identical phosphor compositions synthesized at high temperatures. Saturation behavior was independent of synthetic technique, however, the hydrothermal phosphorus were less susceptible to damage (irreversible efficiency loss) at very high power densities (up to 0.1 W/cm{sup 2}). The fine grain sizes available with hydrothermal synthesis make it an attractive method for preparing garnet phosphorus for field emission, projection, and head-up displays.

Cathodoluminescent (CL) phosphors with improved low-voltage characteristics are needed for use in emissive flat panel displays. Conventional high-temperature methods for phosphor synthesis yield large polycrystalline grains that must be pulverized prior to screen deposition. Grinding has been implicated in reducing phosphor efficiency by causing surface contamination and defects. Hydrothermal synthesis has been used to improve the quality of ceramic powders by producing fine, well-formed crystallites without grinding. Two green-emitting phosphors, Y{sub 3}Al{sub 5}O{sub 12}:Tb (YAG:Tb) and NaY(WO{sub 4}){sub 2}:Tb, were used to test the effects of hydrothermal. synthesis on grain size and morphology, and on low-voltage CL properties. YAG:Th prepared hydrothermally consisted of submicron crystallites with a typical garnet habit. The CL efficiency of hydrothermally synthesized YAG:Tb (3 lm/W at 800 V) was comparable to that of equivalent YAG:Tb compositions prepared via high-temperature solid state reaction. In comparison, CL intensities of Gd{sub 3}Ga{sub 5}O{sub l2}:Tb were slightly better (3.5 lm/W at 800 V), while those of NaY(WO{sub 4}){sub 2}:Tb were approximately 1/100th that of YAG:Tb. Both CL and photoluminescence data show that the difference in the cathodoluminescence of YAG and NaY(WO{sub 4}){sub 2} can be understood in terms of differences in the mechanism of activation.

Field emission flat panel displays place new demands on the performance of cathodoluminescent phosphors. In particular, such phosphors must be efficient at lower voltages (ca. 100-1000 V), and must withstand higher current densities than are present on cathode ray tube screens. ZnO:Zn has been studied extensively as a low-voltage phosphor, but problems such as poor chromatic saturation and temperature sensitivity of emission remain. In this work the use of terbium-doped garnet phases such as yttrium aluminum garnet (YAG) and gadolinium gallium garnet (GGG) as low voltage green-emitting phosphors is evaluated. Hydrothermal synthesis yields well-faceted YAG grains with particle diameters of less than 1 {mu}m. Cathodoluminescent efficiency at a particular voltage was not affected by synthetic route, though the hydrothermally synthesized material was less susceptible to damage at high power densities. An efficiency of 3.5 lm/W was observed for GGG:Tb at 800 V. Deposition of the phosphors onto conducting screens increased their efficiencies at very low voltages (< 200 V). These materials may be considered alternatives to reduced zinc oxide as green-emitting phosphors.

Emissive flat panel display systems operating in full color demand higher performance at low voltages (ca. 50 - 1000 V) from cathodoluminescent (CL) phosphors than cathode ray tubes require. Hydrothermal synthesis has been suggested as a route to phosphors with improved efficiencies, lower voltage thresholds, and increased saturation power. This hypothesis was tested in europium-doped yttrium orthovanadate (YVO4:Eu), an efficient, red emitting CL phosphor. The CL efficiency of YVO4:Eu crystallized from aqueous solution at 200°C is relatively low until it is annealed. The distribution of particle sizes in the low-temperature phosphor is similar to that in material made via a solid-state route, but crystallites remain much smaller (ca. 400 angstrom) until they are annealed. These observations, along with the anomalously strong dependence of CL intensity on europium concentration, support a model in which efficiency principally depends on crystallite size. CL efficiency of both solid state and hydrothermal YVO4:Eu increases with voltage at constant power. Surface-bound electrons are likely the dominant influence on efficiency at voltages near threshold. Saturation power is independent of synthetic route. It is apparent that the CL properties of hydrothermally synthesized YVO4:Eu are essentially the same as those of YVO4:Eu produced via conventional, high-temperature routes.

This document defines the technical requirements for a test program designed to measure time-dependent concentrations of actinide elements from contact-handled transuranic (CH TRU) waste immersed in brines similar to those found in the underground workings of the Waste Isolation Pilot Plant (WIPP). This test program wig determine the influences of TRU waste constituents on the concentrations of dissolved and suspended actinides relevant to the performance of the WIPP. These influences (which include pH, Eh, complexing agents, sorbent phases, and colloidal particles) can affect solubilities and colloidal mobilization of actinides. The test concept involves fully inundating several TRU waste types with simulated WIPP brines in sealed containers and monitoring the concentrations of actinide species in the leachate as a function of time. The results from this program will be used to test numeric models of actinide concentrations derived from laboratory studies. The model is required for WIPP performance assessment with respect to the Environmental Protection Agency`s 40 CFR Part 191B.