Fluorochlorozirconate Glass Ceramics Research Articles

The effect of trivalent iron on the properties of fluorochlorozirconate glass ceramics

Heat treating fluorochlorozirconate (FCZ) glasses precipitates nanocrystals in the glass matrix, resulting in glass ceramics that have storage phosphor properties suitable for use as a medical imaging plate. As the temperature of heat treatment rises, the resulting FCZ glass ceramic becomes more turbid as the size of the orthorhombic phase BaCl2 nanocrystals grow. This cloudiness results in scattering of the stimulating laser light, negatively affecting spatial resolution in computed radiography applications. The effect of Fe3+ on the valence state of zirconium and overall glass quality was investigated. The addition of small amounts of FeCl3 (1–2% on a molecular weight basis) to a ZBLAN glass composition allows for the precipitation of orthorhombic phase BaCl2 crystals while maintaining transparency of the glass ceramic.

Nanocrystallization in Fluorochlorozirconate Glass‐Ceramics

Heat treating fluorochlorozirconate (FCZ) glasses nucleates nanocrystals in the glass matrix, resulting in a nanocomposite glass-ceramic that has optical properties suitable for use as a medical imaging plate. Understanding the way in which the nanocrystal nucleation proceeds is critical to controlling the optical behavior. The nucleation and growth of nanocrystals in FCZ glass-ceramics was investigated with in situ transmission electron microscopy heating experiments. The experiments showed the nucleation and growth of previously unreported BaF2 nanocrystals in addition to the expected BaCl2 nanocrystals. Chemical analysis of the BaF2 nanocrystals shows an association with the optically active dopant previously thought only to interact with BaCl2 nanocrystals. The association of the dopant with BaF2 crystals suggests that it plays a role in the photoluminescent (PL) properties of FCZ glass-ceramics.

Rare earth doped downshifting glass ceramics for photovoltaic applications

The ability of fluorochlorozirconate (FCZ) glass ceramics containing hexagonal barium chloride nanocrystals doped with the rare earth elements, holmium and europium, to downshift ultraviolet light to wavelengths more usable by polycrystalline silicon photovoltaic cells was investigated. Six rare-earth-doped and one undoped FCZ glass samples were synthesized and subsequently heat treated to produce glass ceramics containing barium chloride nanocrystals in the hexagonal phase. The glasses were characterized by differential scanning calorimetry to determine crystallization temperatures for the heat treatment process. The resulting glass ceramics were characterized by X-ray diffraction, phosphorimetry, and spectrophotometry. All samples produce light centered at 470nm when excited by ultraviolet radiation. The excitation spectra of FCZ glass ceramics containing hexagonal barium chloride nanocrystals doped or co-doped with divalent europium more closely matches the solar spectrum at the earth's surface than the excitation spectrum of an undoped sample. This feature of europium doped glass ceramics makes them suitable for the process of downshifting to improve the efficiency of polycrystalline silicon photovoltaic cells. The addition of holmium to the glass ceramics gives rise to additional emission at higher wavelengths and nearer to the band gap energy of polycrystalline silicon photovoltaic cells, reducing heating of the cells and thereby increasing cell efficiency. Additionally, the strong absorption by europium in the ultraviolet region of the spectrum may allow these glass ceramics to serve as a protective layer for ultraviolet sensitive materials.

Structural properties of fluorozirconate-based glass ceramics doped with multivalent europium

The structure/property relationships of fluorochlorozirconate glass ceramics as a function of divalent and trivalent europium (Eu) co-doping and thermal processing have been investigated; the influence of doping ratio on the formation of barium chloride (BaCl(2)) nanocrystals therein was elucidated. X-ray absorption near-edge structure spectroscopy shows that the post-thermal annealing changes the Eu valence of the as-poured glass slightly, but during the melting process Eu(3+) is more strongly reduced to Eu(2+), in particular, when doped as a chloride instead of fluoride compound. The Eu(2+)-to-Eu(3+) doping ratio also plays a significant role in chemical equilibrium in the melt. X-ray diffraction measurements indicate that a higher Eu(2+) fraction leads to a BaCl(2) phase transition from hexagonal to orthorhombic structure at a lower temperature.

Optical and selected thermal properties of samarium-doped fluorochlorozirconate (FCZ) glass-ceramics: Formation and growth of BaCl 2 nanocrystals in FCZ glass-ceramics

Photoluminescence (PL) and conventional and modulated temperature differential scanning calorimetry (DSC and MDSC) experiments have been carried out on a typical fluorochlorozirconate glass and glass-ceramics doped with Sm 3+ and Sm 2+ introduced by the addition of SmF 3 and a reducing agent, NaBH 4, into the initial mixture of constituents. The nominal FCZ composition was 53% ZrF 4, 20% NaF, 3% AlF 3, 3% LaF 3, 1% SmF 3, 1% BaF 2, 19% BaCl 2 (molar percentages). Prior to DSC and PL measurements, some of the glasses have been heat treated (annealed at an elevated temperature) under different conditions, which has resulted in glass-ceramics containing BaCl 2 nanocrystals with a hexagonal and/or orthorhombic crystal structure, depending on the heat treatment conditions. Hexagonal nanocrystals may be obtained by a simple one step annealing process while the formation of orthorhombic crystallites requires sequential multistep annealing treatments. Long duration, low temperature annealing, required for the formation of orthorhombic BaCl 2 nanocrystals, leads to the appearance of an endothermic enthalpy peak at around 250 °C on the conventional DSC thermogram. Temperature modulated DSC experiments identify a clear glass transformation in this regions; and the endothermic peak has been attributed to the structural relaxation enthalpy in the host glass which is usually obscured by the thermal effects associated with the formation of BaCl 2 nanocrystals. The observed thermodynamic effects correlate with the suppression of the broad PL band around 900 nm, which is most likely due to Sm 2+ ions near or in the “shell-region” of the glass surrounding the nanocrystals.

Samarium-Doped Fluorochlorozirconate Glass-Ceramics as Red-Emitting X-Ray Phosphors

We have examined the photoluminescence (PL) and X-ray luminescence (XL) of fluorochlorozirconate (FCZ) glass–ceramics (FCZ-GC) doped with Sm3+ and Sm2+ ions. Sm-doped FCZ-GCs have been prepared by annealing a ZBLAN glass of nominal composition 53% ZrF4, 20% NaF, 3% AlF3, 3% LaF3, 1% SmF3, 1% BaF2, 19% BaCl2 (mol%) above the glass transition of the host glass matrix. SmF3 was added to the initial glass mix for trivalent samarium doping. Sm2+ doping was achieved by reducing Sm3+ to Sm2+ during the melting of the initial mix by the addition of NaBH4. FCZ-GC contains BaCl2 nanocrystals formed during the annealing with embedded Sm2+ ions, which dominate PL and XL. The PL of Sm2+ ions embedded in BaCl2 nanocrystals strongly depends on their crystal structure (hexagonal or orthorhombic) and may be used to identify that structure. The correlation of the PL spectra with X-ray diffraction data is used to estimate the size of nanocrystals. The nanocrystals with average dimensions <∼30 nm are mainly hexagonal while larger crystals are mostly orthorhombic. XL is more efficient in glass–ceramics containing relatively large BaCl2 nanocrystals with hexagonal crystal structure.

Luminescence property of Eu-doped fluorochlorozirconate glass-ceramics

A series of Eu 2+-doped fluorochlorozirconate glass-ceramics were prepared by solid state reaction method. X-ray diffraction, photoluminescence, photo-stimulated luminescence (PSL) and the turbidity of fluorozirconate glass containing BaCl 2 nano- and micro-crystals were measured for the samples annealed at 290 °C for 10 min. The PSL was attributed to the characteristic emission of Eu 2+ in nano-crystallites of BaCl 2, which formed in the glass upon annealing. The PSL efficiency of the glass ceramic was increased by increasing the concentration of BaCl 2, which, however, resulted in the decreasing in the transparency of the sample. The sample turned to a semi-transparent glass ceramic or even an opaque and milky white one from a near-transparent glass. The trade-off between optical transparency and PSL intensity over different concentrations of BaCl 2 for X-ray imaging plate applications was briefly discussed.

Photostimulated luminescence from fluorochlorozirconate glass ceramics and the effect of crystallite size

We report the results from a study of europium-doped fluorochlorozirconate glasses that have been thermally processed to induce nucleation and crystallization of BaCl2 crystallites. The resulting glass ceramics show a photostimulated luminescence (PSL) effect with a conversion efficiency which is up to 80% of that found in the commercial crystalline x-ray storage phosphor material BaFBr:Eu2+. Thermal processing for 20 min in the range 240–260 °C produces small (∼6–11nm) hexagonal BaCl2 crystallites, while temperatures in the range 270–290 °C lead to the formation and growth of larger (∼15–100nm) orthorhombic BaCl2 crystallites, as well as additional unidentified phases. We observe only weak PSL from glass ceramics containing hexagonal BaCl2 (∼0.1% conversion efficiency compared to BaFBr:Eu2+ at room temperature), but orthorhombic phase crystallites give rise to a much larger relative efficiency that increases with particle size, and reaches ∼80% for 100 nm diameter particles. The PSL is attributed to the excitation of electrons trapped as F centers in the BaCl2 crystallites, while the dependence on crystallite size is consistent with a PSL-inert surface shell of thickness ∼7nm surrounding the BaCl2 crystallites.

Photostimulable defects in nano-crystallites in fluorozirconate glasses

The photostimulated luminescence (PSL) from Eu-doped fluorobromo- and fluorochlorozirconate glass-ceramics is reviewed. The PSL is attributed to the characteristic emission of Eu2+ present in BaX2 (X = Br, Cl) crystallites which form in the glass upon thermal processing. Both the photoluminescence intensity and the PSL efficiency increase with the size of the crystallites in a way that is consistent with a PSL-inert surface layer surrounding the crystallites. The increasing crystallite sizes also result in decreased optical transparency at the stimulation wavelength due to scattering. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photostimulated luminescence in Eu-doped fluorochlorozirconate glass ceramics

We report the synthesis of Eu2+- and chlorine-doped fluorozirconate glass-ceramics that show an intense photostimulated luminescence (PSL) after x-ray irradiation at room temperature. The PSL efficiency is up to 80% of that found in the well-known crystalline x-ray storage phosphor BaFBr:Eu2+, and it is the largest thus far reported for a glass-ceramic. We attribute the PSL to crystallites of orthorhombic BaCl2 that are formed after annealing above the glass temperature. Hexagonal BaCl2 crystallites are also observed after short annealing times, but they do not provide a measurable PSL signal. The photoluminescence peak from glass-ceramics containing orthorhombic BaCl2 crystallites occurs at 402 nm, and the stimulation band is centered at about 560 nm.