Peter J. Newman

Eu-activated fluorochlorozirconate glass-ceramic scintillators

Rare-earth-doped fluorochlorozirconate (FCZ) glass-ceramic materials have been developed as scintillators and their properties investigated as a function of dopant level. The paper presents the relative scintillation efficiency in comparison to single-crystal cadmium tungstate, the scintillation intensity as a function of x-ray intensity and x-ray energy, and the spatial resolution (modulation transfer function). Images obtained with the FCZ glass-ceramic scintillator and with cadmium tungstate are also presented. Comparison shows that the image quality obtained using the glass ceramic is close to that from cadmium tungstate. Therefore, the glass-ceramic scintillator could be used as an alternative material for image formation resulting from scintillation. Other inorganic scintillators such as single crystals or polycrystalline films have limitations in resolution or size, but the transparent glass-ceramic can be scaled to any shape or size with excellent resolution.

Lithium-ion conducting ceramic/polyether composites

Composites of a lithium ion conducting ceramic with a lithium salt based polymer electrolyte matrix are described. Conductivity measurements as a function of the lithium ion conducting ceramic phase content in the composite show that there is a significant increase in conductivity at approximately 40 vol% of the ceramic. The room temperature conductivity above this ceramic content is enhanced by at least 100% over that of the polymer electrolyte phase alone. It is believed that this additional contribution is substantially lithium ion conduction. The major barrier to ion-motion in these materials appears to be the interface between the polymer and ceramic. This interfacial resistance is strongly moisture-sensitive.

NMR STUDIES OF MODIFIED NASICON-LIKE, LITHIUM CONDUCTING SOLID ELECTROLYTES

27Al, 31P and 7Li NMR measurements have been performed on lithium conducting ceramics based on the LiTi2(PO4)3 structure with Al, V and Nb metal ions substituted for either Ti or P within the framework NASICON structure. The 27Al magic angle spinning NMR measurements have revealed that, although Al is intended to substitute for octahedral Ti sites, additional substitution into tetrahedral environments (presumably phosphorous sites) occurs with increasing amount of Al addition. This tetrahedral substitution appears to occur more readily in the presence of vanadium, in Li1+xAlxTi2−x(PO4)2.9(VO4)0.1, whereas similar niobium additions (in place of vanadium) appear to stifle tetrahedral substitution. 7Li static NMR spectra reveal quadrupolar structure with Cq approximately 42 kHz, largely independent of substitution. Measurement of the 7Li central transition linewidth at room temperature reveals a relatively mobile lithium species (300–900 Hz) with linewidth tending to decrease with Al substitution and increase with increasing V or Nb. This new structural information is discussed in the context of ionic conduction in these ceramics.

Towards elucidating microscopic structural changes in Li-ion conductors Li1+yTi2–yAly[PO4 ]3 and Li1+yTi2–yAly[PO4 ]3–x [MO4 ]x(M=V and Nb): X-ray and27Al and31P NMR studies

A combination of X-ray powder diffraction (XRD) and nuclear magnetic resonance (NMR) studies has demonstrated that attempted substitutions of Al, V and Nb into the framework of LiTi2(PO4)3 yield several impurity phases in addition to direct substitutions of Al into Ti and V, Nb into P sites. Direct substitutions were confirmed by changes in the unit cell dimensions as indicated by the peak shifts observed in the X-ray diffractographs and by analyses of the 27Al and 31P magic angle spinning (MAS) spectra. A major impurity phase was identified as AlPO4 (found in at least two polymorphs) and the amount present increases with increasing Al additions. The formation of AlPO4 appeared to be enhanced by further V but suppressed by Nb substitution. These results suggest that the presence of AlPO4 , together with the non-stoichiometric modified LTP, may be the cause for the observed densification of this material upon sintering and the increased ionic conductivity.

PHOTOSTIMULATED LUMINESCENCE IN A RARE EARTH-DOPED FLUOROBROMOZIRCONATE GLASS CERAMIC

Photostimulated luminescence at room temperature was observed in a fluorozirconate glass which was doped with 1% Eu2+ or 1% Ce3+ and 5% Br− ions. Small crystals of BaBr2 (high-pressure phase) form in the glass on cooling. The Eu2+ and Ce3+ luminescence spectra comprised two emission lines each: for Eu2+ one line at 413 nm and a broader band centered at 485 nm, and for Ce3+ one line at 320 nm and a broader band at 425 nm. The luminescence is attributed to the 5d14fn−1→4fn emissions of the rare earth ions. In both cases only one of the emission bands showed photostimulated luminescence. The stimulation band for both rare earth ions was centered at 570 nm.

Characterisation and impedance spectroscopy of substituted Li1.3Al0.3Ti1.7(PO4)3-x(ZO4)x (Z=V, Nb) ceramics

Lithium ion conducting ceramics based on the lithium aluminium titanium phosphate (LATP) NASICON structure have been prepared with various substitutions of the phosphorous. The effect of the processing method has been shown to be the key factor in determining the conductivity, both bulk and grain boundary, as well as the conductivity trends observed as a function of substitution.

In situ generation of Eu2+ in glass-forming melts

Abstract We report on the efficacy of in situ reduction of Eu 3+ to Eu 2+ in a fluoride melt by a variety of chemical reductants and by bulk electrochemical reduction. The Eu 2+ content of the resultant glasses was determined by a combination of electron paramagnetic resonance and magnetic susceptibility measurements. Addition of LiH, or use of a hydrogen atmosphere, produced near-complete reduction, whilst the electrochemical technique produced approximately 30% conversion. None of the fluorozirconate glasses luminesced from Eu 2+ ions, although a fluoroaluminate glass made using the LiH reduction technique did luminesce.

Fluorescence from highly-doped erbium fluorozirconate glasses pumped at 800 nm

Abstract A study of the fluorescence properties, under 800 nm excitation, of new ZBEAN glasses with Er 3+ concentrations up to 18 mol% is reported. This represents the highest reported doping concentration for this type of glass. The fluorescence intensity at 550 nm, 1.53 μm and 2.7 μm did not exhibit the expected strong quenching at these high concentrations and the corresponding decay times were consistent with this observation. Cooperative energy transfer between the relevant levels is discussed as a possible explanation of this behaviour.

Photoluminescence and crystallization in europium-doped fluorobromozirconate glass-ceramics

Abstract We have investigated barium bromide crystallization in glasses which have an overall composition of 53% ZrF 4 , 20% BaF 2 , 5% NaF, 15% NaBr, 1% LaF 3 , 3% AlF 3 , 2% YF 3 , and 1% EuF 2 . The glasses were prepared by melting at 850°C in an inert atmosphere and quenching into a brass mold. Subsequent thermal treatment of the glass yields a transparent or translucent fluorozirconate glass containing BaBr 2 crystals; this glass-ceramic material has potential applications as an X-ray storage phosphor. The glasses and glass-ceramics were investigated using photoluminescence, optical density, X-ray diffraction, and differential scanning calorimetry methods. Glasses which have been quenched to temperatures below 260°C are transparent and show no photoluminescence from the europium ions. When the glasses are subsequently heated above about 263°C, crystals of BaBr 2 in the hexagonal phase start to precipitate and photoluminescence from Eu 2+ ions is observed. It is possible to prepare the resulting glass-ceramic in transparent form by heating to temperatures just below the temperature of most rapid crystallization, T x =280 C. The photoluminescence is attributed to Eu 2+ ions replacing Ba 2+ ions in the BaBr 2 crystals.

A new fluorozirconate glass-ceramic X-ray storage phosphor

A new fluorozirconate glass-ceramic has been discovered that displays an X-ray storage phosphor effect with an efficiency of up to 8% of that observed in the established crystalline storage phosphor, BaFBr:Eu 2+ . The glass-ceramic is based on the well-known ZBLAN20 formulation, but the replacement of Na by Rb and Li, partial substitution of F by Br, as well as doping with Eu 2+ , results in the formation of RbBa 2 Br 5 :Eu 2+ microcrystallites. X-ray diffraction, photostimulated luminescence and EPR measurements on the as-made glass and the annealed glass-ceramic show that the X-ray storage phosphor effect arises from the RbBa 2 Br 5 :Eu 2+ microcrystallites.