Mark L. F. Phillips

Predicting and Modeling the Low‐Voltage Cathodoluminescent Efficiency of Oxide Phosphors

A novel ceramic synthesis technique, combustion synthesis, was used to produce submicron-sized Y{sub 2}O{sub 3}:Eu{sup 3+} phosphors. This technique exploits the exothermic redox reaction of yttrium and europium nitrates (oxidizers) with urea (CH{sub 4}N{sub 2}O) fuel (reducing agent). Resulting powders were luminescent in the as-synthesized state. However, their luminous intensity increased with increasing postreaction annealing treatments (1,000--1,600 C for 2 h). The low-voltage (200--1,000 V) cathodoluminescence efficiency of Y{sub 2}O{sub 3}:Eu{sup 3+} was found to increase with increasing crystallite size, independent of the particle size. A model of low-voltage cathodoluminescence that includes the effects of the crystallite size, the probability of radiative recombination, and the effect of surface-bound electrons was developed to predict phosphor efficiency at low voltages. The efficiencies predicted by the model are in very good agreement with experimental results.

Zeolite membranes from kaolin

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.

Phosphor synthesis routes and their effect on the performance of garnet phosphorus at low-voltages

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.

Low-Voltage Cathodoluminescent Properties of Europium-Activated Anion-Deficient Fluorites

The authors examined the optical bandgap, particle size distribution, photoluminescence spectra, and cathodoluminescent response of fifteen phosphors that contain transition-metal and main-group sensitizers. They determined luminance versus applied voltage curves for electron energies from 5 to 1000 eV. Seven phosphors exhibit threshold voltages less than 110V. Y{sub 1.96} Eu{sub 0.04} O{sub 3} exhibits a threshold voltage of 13 V and, at 300 V, displays a luminance of 25 fL and a luminous efficiency of 0.02 lm/W.

Inorganic materials for anomalous-dispersion phase-matched second-harmonic generation: rubidium titanyl arsenate isomorphs, Rb[Ti1−2xLnxNbx]OAsO4

Abstract The defect chemistry and optical properties of Rb[Ti 1−2 x Ln x Nb x ]OAsO 4 are reported. The solid solubility of lanthanide ions in these materials decreases exponentially as the size of the lanthanide ion increases. The high-temperature chemistry is complex: Rb[Ti 1−2 x Ln x Nb x ]OAsO 4 is formed at low dopant concentrations and Rb 1− z □ z [Ti 1−2 x − z Ln x Nb x + z ]OAsO 4 and additional phases are formed at higher concentrations. All the materials exhibit absorption spectra that are characteristic of the particular lanthanide ion in the structure. The charge transfer band is red-shifted 0–27 nm relative to RbTiOAsO 4 (midpoint 331 nm). The magnitude of the shift depends on the method of sample preparation. Second-harmonic intensities measured at 532 nm decrease exponentially as the lanthanide ion concentration is increased.

Nonlinear optical properties of new KTiOPO4 isostructures

The atomic structures of the nonlinear optical materials potassium titanyl phosphate (KTiOPO4, or KTP) and potassium titanyl arsenate (KTiOAsO4) feature one- dimensional channels through which the potassium ions are relatively free to migrate. Ion exchange results when these materials are immersed in molten salts containing alkali metal ions. Sodium, lithium, and silver all exchange readily for K+ in single crystals of both KTP and KTA to yield the exchanged derivatives Na.95K.05TiOPO4 (NaTP), Na.83K.17TiOAsO4 (NaTA), Ag.85K.15TiOPO4 (AgTP), Ag.98K.02TiOAsO4 (AgTA), Li.45K.55TiOPO4 (KLTP), and Li.46K.54TiOAsO4 (KLTA), which are all KTP isostructures. The optical nonlinearities (measured as SHG intensities) of the limiting compositions in the NaTA, KLTP, and KLTA systems are similar to that of KTP, but are much smaller in NaTP, AgTP, and AgTA. Single crystal x-ray data have revealed differences in coordination of the mobile cations to oxygen atoms linking the TiO6 groups in these compounds, and these differences correlate with changes in optical nonlinearity. The observed nonlinearities can be rationalized if they are viewed as being dependent on the degree to which delocalized charge- transfer excited state character can be mixed into ground state bonding and nonbonding orbitals in the TiO6 chains. The relative lack of association of Na+ and Li+ ions with these chains in NaTA, KLTP, and KLTA allows extensive excited state delocalization, and thus significant electronic hyperpolarizability.

Comment on: ‘‘Modified KTiOPO4 crystals for noncritical phase matching applications’’ [Appl. Phys. Lett. 64, 16 (1994)]

The nonlinear optical properties of sodium doped potassium titanate phosphate crystal are discussed. (AIP)

Desalination of brackish ground waters and produced waters using in-situ precipitation.

The need for fresh water has increased exponentially during the last several decades due to the continuous growth of human population and industrial and agricultural activities. Yet existing resources are limited often because of their high salinity. This unfavorable situation requires the development of new, long-term strategies and alternative technologies for desalination of saline waters presently not being used to supply the population growth occurring in arid regions. We have developed a novel environmentally friendly method for desalinating inland brackish waters. This process can be applied to either brackish ground water or produced waters (i.e., coal-bed methane or oil and gas produced waters). Using a set of ion exchange and sorption materials, our process effectively removes anions and cations in separate steps. The ion exchange materials were chosen because of their specific selectivity for ions of interest, and for their ability to work in the temperature and pH regions necessary for cost and energy effectiveness. For anion exchange, we have focused on hydrotalcite (HTC), a layered hydroxide similar to clay in structure. For cation exchange, we have developed an amorphous silica material that has enhanced cation (in particular Na{sup +}) selectivity. In the case of produced waters with high concentrations of Ca{sup 2+}, a lime softening step is included.

Synthesis, structure and properties of (CN{sub 3}H{sub 6}){sub 4}Zn{sub 3}(SeO{sub 3}){sub 5}, the first organically-templated selenite

An astonishing variety of inorganic networks templated by organic species have been reported over the last 10 years. A great deal of attention has been paid to the structure-directing role of the organic species, and the structural effect of variously coordinated cations, for example distorted octahedral vanadium and pyramidal tin. Less exploratory work has been carried out on the anionic part of the inorganic network, and most groups reported so far (phosphate, germanate, etc.) invariably adopt tetrahedral coordination. The possibilities of incorporating the pyramidal [HP0{sub 3}]{sup 2{minus}} hydrogen phosphite group into extended structures templated by inorganic, alkaline earth cations was explored a few years ago. In this paper the authors report the synthesis, crystal structure, and some properties of (CN{sub 3}H{sub 6}){sub 4}{center_dot}Zn{sub 3}(SeO{sub 3}){sub 5}, the first organically-templated phase to contain the pyramidal selenite [SeO{sub 3}]{sup 2{minus}} anion.

Low-voltage cathodeluminescence of europium-activated yttrium orthovanadate

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 degree(s)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.