Fluorochlorozirconate Glasses Research Articles

Luminescence of Fluorochlorozirconate Glasses Doped with Manganese Ions

A series of fluorochlorozirconate glasses doped with manganese ions is synthesized and investigated as possible red‐emitting phosphors in comparison with conventional green‐emitting fluorozirconate glasses doped with Mn2+. It is found that the replacement of fluorine by chlorine ions results in broadening and a long‐wavelength shift of the manganese luminescence band into the red region (peaked at 615 nm) although the emission spectrum still has some contribution of the green sub‐band. It turns out also that decay kinetics of luminescence from fluorochlorozirconate glasses becomes nonexponential in contrast to single‐exponential decay observed for green luminescence from Mn2+‐doped fluorozirconate glasses. The observation of two emission components in the luminescence spectrum of fluorochlorozirconate glasses is interpreted as being due to the presence of two Mn2+ emission centers in different local environments. The presence of 2+ valence states of manganese ions in both types of glasses is confirmed by recording X‐ray absorption near edge structure (XANES) spectra at Mn L2,3 edge. Substitution of F− by Cl− ions in anion sites neighboring to Mn2+, leads to a significant rearrangement of the local environment of Mn2+ ions, which results in the long‐wavelength shift of Mn2+ emission spectrum.

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.

Nd3+ doped fluorochlorozirconate glass: 3.9 μm MIR emission properties and energy transfer

The Nd3+ doped fluorochlorozirconate (FCZ) glass was prepared by melt-quenching method. The 3.9 μm emission from Nd3+ ions is attributed to the two-photon absorption process. The strong emission transition at 3.9 μm fluorescence peak intensity, corresponding to the 4G11/2→2K13/2 transition, is directly proportional to the NaCl concentration. With the increase of the Cl- ions amount, the mid-infrared (MIR) luminescent intensity is significantly enhanced. Additionally, the Judd-Ofelt (J-O) parameter Ω 2 is larger than that of the fluorozirconate (FZ) glass, which indicates the covalency of the bond between RE ions and ligand is stronger as Cl- ions substitution of F- ions in chloride FZ glass. The X-ray diffraction (XRD) patterns show that the amorphous glassy state keeps the FZ glass network structure. In brief, the advantageous spectroscopic characteristics make the Nd3+-doped FCZ glass be a promising candidate for application of 3.9 μm emission.

Radioluminescence properties of Sm-doped fluorochlorozirconate glasses and glass-ceramics

We have investigated X-ray induced radioluminescence (XL) properties of Sm-doped fluorochlorozirconate (FCZ) glasses and glass-ceramics. The FCZ glass is a modified ZBLAN glass which shows a very high optical transmission over a wide spectral range. The glass matrix includes Sm3+-doped nanocrystals of BaCl2 after heat-treatment at temperatures above 250 °C. The glass-ceramic emits red light under UV and X-ray exposure. Since conventional Si-based photodetectors, e.g., CCDs, have the highest quantum efficiency to red light in general, the Sm-doped FCZ glass-ceramic plate can be a good candidate as a scintillator material for indirect radiation detection. Moreover, a very broad emission is present in the glass-ceramic around 300–500 nm, which is attributed to a self-trapped exciton (STE) emissions. The temperature dependence of X-ray induced luminescence and photoluminescence are very similar. The XL light yield is linearly proportional to the X-ray exposure rate for rates higher than 20 mR/s. For low exposure rates, emissions by Sm2+ are more sensitive than others, leading to a nonlinear response.

Evaluation of a Fluorochlorozirconate Glass‐Ceramic Storage Phosphor Plate for Gamma‐Ray Computed Radiography

A fluorochlorozirconate (FCZ) glass‐ceramic containing orthorhombic barium chloride crystals doped with divalent europium was evaluated for use as a storage phosphor in gamma‐ray imaging. X‐ray diffraction and phosphorimetry of the glass‐ceramic sample showed the presence of a significant amount of orthorhombic barium chloride crystals in the glass matrix. Transmission electron microscopy and scanning electron microscopy were used to identify crystal size, structure, and morphology. The size of the orthorhombic barium chloride crystals in the FCZ glass matrix was very large, ~0.5–0.7 μm, which can limit image resolution. The FCZ glass‐ceramic sample was exposed to 1 MeV gamma rays to determine its photostimulated emission characteristics at high energies, which were found to be suitable for imaging applications. Test images were made at 2 MeV energies using gap and step wedge phantoms. Gaps as small as 101.6 μm in a 440 stainless steel phantom were imaged using the sample imaging plate. Analysis of an image created using a depleted uranium step wedge phantom showed that emission is proportional to incident energy at the sample and the estimated absorbed dose. The results showed that the sample imaging plate has potential for gamma‐ray‐computed radiography and dosimetry applications.

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.

Influence of BaCl2 Nanocrystal Size on the Optical Properties of Nd3+ in Fluorozirconate Glass

The optical properties of Nd3+ ions embedded in fluorochlorozirconate glasses were studied, addressing in particular the effect of BaCl2 nanocrystal growth in the matrix upon annealing. A Judd–Ofelt analysis for these glasses and for a Nd3+-doped fluorozirconate reference glass without nanocrystals yielded various radiative decay rates as a function of nanocrystal size. For the 4F3/2 to 4I9/2 transition, where multiphonon relaxation is negligible, the rates agree very well with experimental results. Further support for the validity of the Judd–Ofelt results was obtained using an alternative approach for the determination of radiative decay rates involving emission and absorption spectra and their connection through the McCumber theory and the Füchtbauer–Ladenburg equation. The obtained variations of the Judd–Ofelt intensity parameters Ωλ reflect systematic changes in the local environment of the Nd3+ ions as an effect of the BaCl2 nanocrystal growth, indicating partial inclusion of Nd3+ into the nanocrystals.

Mössbauer spectroscopy of europium-doped fluorochlorozirconate glasses and glass ceramics: optimization of storage phosphors in computed radiography

Eu2+-doped fluorochlorozirconate (FCZ) glasses and glass ceramics, which are being developed for medical and photovoltaic applications, have been analysed by Mössbauer spectroscopy. The oxidation state and chemical environment of the europium ions, which are important for the performance of these materials, were investigated. Routes for maximizing the divalent europium content were also investigated. By using EuCl2 instead of EuF2 in the starting material a fraction of about 90% of the europium was maintained in the Eu2+ state as opposed to about 70% when using EuF2. The glass ceramics produced by subsequent thermal processing contain BaCl2 nanocrystals in which Eu2+ is incorporated, as shown by the narrower linewidth in the Mössbauer spectrum. Debye temperatures of 147 K and 186 K for Eu2+ and Eu3+, respectively, were determined from temperature dependent Mössbauer measurements. The f-factors were used to obtain the Eu2+/Eu3+ ratio from the area ratio of the corresponding absorption lines.

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.

Particle size monitoring of BaCl2 nanocrystals in fluorozirconate glasses

Thermal processing of fluorochlorozirconate glasses leads to the formation of BaCl2 nanocrystals within the glass matrix. An alternative method to determine the size of the nanocrystals is provided by a Rayleigh scattering model based analysis of optical transmittance data. For comparison, the size is extracted from X-ray diffraction data with the Scherrer equation. Additional unwanted inclusions of non-crystalline scattering centers lead to significant differences between the results obtained from both of these techniques. Using these two methods allows for a controlled monitoring of nanocrystal growth in optical 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.

Trivalent Er and Sm ions in fluorochlorozirconate glasses: optical properties and X‐ray luminescence

AbstractThe optical properties and X‐ray luminescence of fluorochlorozirconate glasses and glass‐ceramics with composition 53% ZrF4, 20% NaF, 3% AlF3, 3% LaF3, 1% SmF3, (20‐ x)% BaF2 and x% BaCl2 with x varying from 0% to 19% and doped by the addition of ErCl3 or SmF3 have been investigated. The glasses show absorption and emission bands typical for trivalent Er3+ and Sm3+ ions. The samples doped with Sm showed X‐ray stimulated luminescence while those doped with Er gave no response. The annealing of samples at temperatures 250‐290 °C increases the X‐ray stimulated luminescence while there were no changes in optical absorption and photoluminescence. The observed effects are explained in terms of changes in the host glass and the enhancement of excitation transfer from the glass to embedded Sm3+ ions (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of erbium-doped fluorochlorozirconate glasses

We report the optical properties of a fluorochlorozirconate (FCZ) glass with the composition 53% ZrF 4, 20% NaF, 3.5% AlF 3, 3% LaF 3, 0.5% InF 3, (20 − x)% BaCl 2, x% BaF 2 with x varying from 0% to 2%, and doped with various amounts of trivalent erbium by the addition of ErCl 3. Annealing of the as-prepared glass in inert (N 2) or reducing (5%H 2 + 95%Ar) atmospheres at temperatures that ensure the conversion of the glass into a glass-ceramic by the nucleation of BaCl 2 nanocrystals, does not significantly change any of Er 3+ related absorption and photoluminescence (PL) characteristics. We have carried out a Judd–Ofelt analysis of the absorption spectra and obtained Ω 2 = (1.92 ± 0.3) × 10 − 20 cm 2, Ω 4 = (0.88 ± 0.16) × 10 − 20 cm 2 and Ω 6 = (0.59 ± 0.08) × 10 − 20 cm 2, and also the radiative lifetimes of the 4I 13/2→ 4I 15/2, 4I 11/2→ 4I 15/2 and 4S 3/2→ 4I 15/2 bands. The radiative lifetime from the Judd–Ofelt analysis for the 4I 13/2→ 4I 15/2 band is in good agreement with the experimentally measured PL decay time. The examination of the optical properties of powdered samples with different average particle size does not show any photon trapping effects. We have determined the spectral absorption and emission cross-sections and then estimated the possible spectral optical gain for varying degrees of relative populations of the 4I 13/2 and 4I 15/2 manifolds.

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.

Crystallization behavior of rare-earth doped fluorochlorozirconate glasses

A series of fluorochlorozirconate (FCZ) glasses, each doped with a different rare-earth, was prepared and examined to determine thermal stability and activation energy, E a , of the dopant dependent BaCl 2 crystallization. Non-isothermal differential scanning calorimetry (DSC) measurements were done to investigate the endothermic and exothermic reactions upon heat treatment of the glass samples. In comparison to the rare-earth free FCZ glass, significant changes in the Hruby constant, H r , and E a were found due to the addition of a rare-earth and also between the individual dopants.

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.

Differential scanning calorimetry investigations on Eu-doped fluorozirconate-based glass ceramics

The properties of Eu-doped fluorochlorozirconate (FCZ) glass ceramics upon thermal processing and the influence of Eu-doping on the formation of BaCl 2 nanocrystals therein have been investigated. Differential scanning calorimetry indicates that higher Eu-doping shifts the crystallization peak of the nanocrystals in the glass to lower temperatures, while the glass transition temperature remains constant. The activation energy and the thermal stability parameters for the BaCl 2 crystallization are determined.

Crystallization in heat-treated fluorochlorozirconate glasses

Crystallization phenomena of fluorochlorozirconate glasses were investigated bymeans of differential scanning calorimetry and inelastic neutron scattering. Theprecipitation of barium chloride nanoparticles from the glass matrix upon heattreatment was found to be suppressed when re-melting the glass with a reducingagent but not if the agent was present in the initial synthesis. Addition of smallamounts of oxide to the predominantly fluoride melt was found to maintain thepresence of nanoparticles but not to induce the predicted phase transition ofthe barium chloride nanoparticles from hexagonal to orthorhombic structure.Inelastic neutron scattering performed on an ‘as-made’ glass and a heat-treatedglass showed an increase in ‘hardness’, consistent with a more ordered structure.

Erbium- and chlorine-doped fluorozirconate-based glasses for up-converted fluorescence

A series of glass ceramics was prepared and the structure–property relationships evaluated. The base composition of the series was comprised of fluorozirconate (FZ) and fluorochlorozirconate (FCZ) glass ceramics. Thermal processing of as-made FCZ-based glass ceramics produced sharp peaks in X-ray diffraction patterns, which was attributed to BaCl 2 nanocrystals formed in the material; this was not observed in the FZ glass. Up-conversion fluorescence experiments were carried out for both the FZ and FCZ samples doped with erbium. The fact that an increase in the up-conversion efficiency was not observed indicates that erbium ions were not embedded in the BaCl 2 nanocrystals. A comparison is made between this series and a similar series that was doped with neodymium.

Optical properties of divalent samarium-doped fluorochlorozirconate glasses and glass ceramics

Glass ceramics comprising Sm 2+-doped barium chloride in both hexagonal and orthorhombic phases embedded in a fluorochlorozirconate glass matrix have been prepared. Divalent samarium doping was effected with the aid of NaBH 4 to partly reduce Sm 3+ in the starting materials to Sm 2+. The optical absorption spectrum in unannealed samples shows a strong, broad 4f → 5d band centred at around 340 nm and attributed to Sm 2+, but no photoluminescence is observed. Following annealing to induce crystallisation of Sm 2+-doped barium chloride crystals in the glass, photoluminescence is observed in the form of sharp line 4f → 4f transitions, and two broad 5d → 4f bands, the latter centred at around 680 and 900 nm. The 680 nm band disappears below about 250 K whilst the 900 nm band intensity increases with decreasing temperature. The 4f → 4f transitions for Sm 2+ ions in the orthorhombic phase of barium chloride show an abrupt change in the relative intensity of transitions originating from the 5D 1 and 5D 0 levels at around 90 K. The effect is attributed to the role of the 5d level in thermally assisted indirect transitions between the 5D 1 and 5D 0 levels. In contrast, transitions from only the 5D 0 level are observed for the hexagonal phase. The temperature dependence of the photoluminescence intensity, and the observation of a different crystal field splitting pattern for the 7F 1 level, can be used to distinguish between the two phases. The splitting pattern for the 7F 1 level for the hexagonal phase of barium chloride suggests that both possible crystallographic barium sites are occupied by Sm 2+ ions.