Co-reporter:W. C. Wang;Q. H. Le;Q. Y. Zhang;L. Wondraczek
Journal of Materials Chemistry C 2017 vol. 5(Issue 31) pp:7969-7976
Publication Date(Web):2017/08/10
DOI:10.1039/C7TC01853C
Unusually stable multi-anion glasses of the fluoride-sulfophosphate type (FPS) are introduced as a new host material for optically active cation species. Despite a notoriously low polymerization grade, anion mixing in this glass system enables facile manufacture of bulk or fiber devices which combine several advantages of fluoride and phosphate glasses while using the stabilizing effect of sulfate additions. Using the example of chromium doping, we demonstrate broad red photoluminescence at 734 nm and inhomogeneous broadening of the R-line at 694 nm, originating from the 4T2 → 4A2 and 2E → 4A2 transitions of Cr3+, respectively. The luminescence mechanism is further analyzed on the basis of the corresponding Tanabe–Sugano diagram. Tailored through chemical composition, internally nucleated precipitation of a nanocrystalline fluoride phase enables switching between high-field and low-field configurations of the Cr3+ ion, resulting in the specific emission properties and setting the path towards FPS-based optical devices.
Co-reporter:Enhai Song;Jinliang Wang;Shi Ye;Xiao-Bao Yang;Mingying Peng;Qinyuan Zhang
Advanced Optical Materials 2017 Volume 5(Issue 13) pp:
Publication Date(Web):2017/07/01
DOI:10.1002/adom.201700070
In modern phosphor materials, in contrast to single-band emission, simultaneous photoluminescence through two or more emission bands enables broad and tailorable emission color, but requires fine control of activator precipitation on specific crystallographic sites. This presents a major obstacle in the design of tunable emitters, since dedicated control of site occupancy is rarely possible. Here, a new paradigm of tunability is considered, which overcomes this limitation. It is demonstrated in CaO:Mn2+ that single- and dual-band emission can be generated by involving only a single type of crystallographic sites. This enables tailoring of the emission color alone through dopant concentration. The two emission bands exhibit different spectral, dynamic, and thermal characteristics, but very similar excitation spectra. While the variations in thermal quenching provide a tool for tunability, the variations in decay behavior suggest interesting potential for lifetime-multiplexing and coding. Energy transfer is observed between the two emission bands, using time-resolved emission spectra. From the analysis of crystal structure, spectral data, electron paramagnetic resonance analyses, and density functional theory calculations analyses, it is found that the emission bands at ≈600 and 660 nm originate from isolated Mn2+ ions and superexchange reactions in Mn2+ Mn2+ dimers, respectively.
Co-reporter:Yuko Nakatsuka;Kilian Pollok;Torsten Wieduwilt;Falko Langenhorst;Markus A. Schmidt;Koji Fujita;Shunsuke Murai;Katsuhisa Tanaka
Advanced Science 2017 Volume 4(Issue 4) pp:
Publication Date(Web):2017/04/01
DOI:10.1002/advs.201600299
Magnetooptical (MO) glasses and, in particular, Faraday rotators are becoming key components in lasers and optical information processing, light switching, coding, filtering, and sensing. The common design of such Faraday rotator materials follows a simple path: high Faraday rotation is achieved by maximizing the concentration of paramagnetic ion species in a given matrix material. However, this approach has reached its limits in terms of MO performance; hence, glass-based materials can presently not be used efficiently in thin film MO applications. Here, a novel strategy which overcomes this limitation is demonstrated. Using vitreous films of xFeO·(100 − x)SiO2, unusually large Faraday rotation has been obtained, beating the performance of any other glassy material by up to two orders of magnitude. It is shown that this is due to the incorporation of small, ferromagnetic clusters of atomic iron which are generated in line during laser deposition and rapid condensation of the thin film, generating superparamagnetism. The size of these clusters underbids the present record of metallic Fe incorporation and experimental verification in glass matrices.
Co-reporter:Quyen Huyen Le, Theresia Palenta, Omar Benzine, Kristin Griebenow, Rene Limbach, Efstratios I. Kamitsos, Lothar Wondraczek
Journal of Non-Crystalline Solids 2017 Volume 477(Volume 477) pp:
Publication Date(Web):1 December 2017
DOI:10.1016/j.jnoncrysol.2017.09.043
•the structure of fluoro-sulfo-phosphate glasses is reported•intermediate-range structural heterogeneity is related to anion mixingPhosphate glasses exhibit high solubility for secondary anion species. This includes sulfate, fluoride or nitride ions, and enables the preparation of poly-anionic glasses. Here, we consider fluoro-sulfo-phosphates. We show that in this system, even equimolar anion mixing can be achieved without notably compromising the glass forming ability. The derived glasses exhibit surprising stability, what enables the fabrication of bulk samples mostly without the need to employ rapid quenching techniques. In terms of molecular structure, they rely on primarily ionic bonding among the different constituents, whereby the fluoride and sulfate anions seem to act as stabilizing ionic cross-linkers between highly-depolymerized phosphate entities. Investigation of the low-frequency Raman scattering indicates characteristic variations on intermediate-range structure, where a shift in the Boson peak appears to correlate inversely with increasing sulfate content. We discuss this observation in terms of the low level of bond localization and low field strength of the sulfate anion relative to phosphate Q0 groups. Adjusting the degree of anion competition enables to tailor super-structural heterogeneity. Beyond this, the glass system provides an interesting variety of ligand situations for the potential incorporation of optically active cation species.
Co-reporter:Elham Moayedi, Lothar Wondraczek
Journal of Non-Crystalline Solids 2017 Volume 470(Volume 470) pp:
Publication Date(Web):15 August 2017
DOI:10.1016/j.jnoncrysol.2017.05.003
We employ instrumented nanoindentation for obtaining quantitative information on the onset of scratch-induced microabrasion on silica glass. For this, in situ evaluation of lateral force and friction coefficient is compared to post mortem optical inspection, following edge-forward scratching with a Berkovich indenter at velocities of 10–500 μm/s under continuously increasing normal load of up to 300 mN. In the two approaches, the onset of microabrasion is identified from the occurrence of pop-ins in the load-displacement curve and phenomenologically determined from the scratch pattern, respectively. Obtained data are analyzed in terms of a Weibull distribution, assuming that microabrasion sets-on as a result of acting stress as well as surface state. Aside of the occurrence of occasional outliers at low load (probably induced through individual surface defects), data indicate two underlying probability functions, i.e., the probability for the propagating scratch to hit a surface flaw and the probability that such an event causes an observable micro-crack. Dominance of the former leads to an exponential function with Weibull modulus ~ 1, reflecting a purely random distribution with load-independent probability of failure. This is observed in particular at high scratching velocity after passing a certain normal load. For the latter, the Weibull modulus increases with increasing scratching velocity, that is, from ~ 1.6 to 4.4, at intermediate load. Here, low Weibull modulus at low load is attributed to the increasing time of local strain, which leads to a reduction of the load-dependence of micro-cracking relative to a faster-moving scratch. In the present case, the critical lateral load for microabrasion of silica (50th percentile) is around 30–40 mN. Within the employed experimental conditions, this value is practically independent of scratching velocity.
Co-reporter:Kristin Griebenow, Efstratios I. Kamitsos, Lothar Wondraczek
Journal of Non-Crystalline Solids 2017 Volume 468(Volume 468) pp:
Publication Date(Web):15 July 2017
DOI:10.1016/j.jnoncrysol.2017.04.036
The effect of modifier mixing on structure and properties of (Ca, Mg) metaphosphate glasses is considered across the binary join of xMgO-(1-x)CaO-P2O5. Pronounced extrema in hardness, Young's modulus, local creep and glass transition temperature are related to variations in modifier precipitation. Both alkaline earth species compete for octahedral lattice sites, what leads to an increase in the fraction of [MgO4] tetrahedral units at equimolar mixing. At the same time, the structural backbone of phosphate Q2 species is affected through favorable formation of ring- and chain-like superstructures in the presence of Ca2 + and Mg2 +, respectively.
Co-reporter:Z.Y. Yao, D. Möncke, E.I. Kamitsos, P. Houizot, F. Célarié, T. Rouxel, L. Wondraczek
Journal of Non-Crystalline Solids 2016 Volume 435() pp:55-68
Publication Date(Web):1 March 2016
DOI:10.1016/j.jnoncrysol.2015.12.005
•Copper–lead and copper–zinc borate glasses are studied for relations between structure and mechanical properties.•Cations are used to tailor the intermediate-range speciation of borate groups.•Zinc metaborates show a high disproportionation tendency of the metaborate group.•In both glass types, the addition of copper oxide results in a more homogenous network of [BØ2O]- and [BØ4]-metaborate units.Copper–lead and copper–zinc borate glasses with different copper contents were prepared and studied for correlations between structure and mechanical properties, where the cations are used to tailor the intermediate-range speciation of borate groups. Structural characterization was done by optical absorption, electron spin resonance (ESR), Raman and infrared (IR) spectroscopy. The mechanical properties were investigated through in-depth instrumented indentation and mechanical resonance analyses. The zinc metaborate glass series shows a high disproportionation of metaborate into mainly trigonal pyroborate [B2O5]4 − and polyborate [BØ3]0 units, while the lead borate glass series exhibits a network based on trigonal [BØ2O]− and tetrahedral [BØ4]− metaborate units and a minor PbO-pseudophase (Ø = bridging oxygen and O = non-bridging oxygen). For both glass types, the addition of copper oxide results in a more homogenous network containing [BØ2O]− and [BØ4]− metaborate units. This induces an enhancement of the elastic moduli and hardness in the lead borate glass series, but a decrease of these properties in the copper–zinc borate glasses, whereby copper–zinc borate glasses are stiffer and harder than copper–lead borate glasses.
Co-reporter:Enhai Song;Shi Ye;Tianhui Liu;Peipei Du;Rui Si;Xiping Jing;Sha Ding;Mingying Peng;Qinyuan Zhang
Advanced Science 2015 Volume 2( Issue 7) pp:
Publication Date(Web):
DOI:10.1002/advs.201500089
Biomedical imaging and labeling through luminescence microscopy requires materials that are active in the near-infrared spectral range, i.e., within the transparency window of biological tissue. For this purpose, tailoring of Mn2+–Mn2+ activator aggregation is demonstrated within the ABF3 fluoride perovskites. Such tailoring promotes distinct near-infrared photoluminescence through antiferromagnetic super-exchange across effective dimers. The crossover dopant concentrations for the occurrence of Mn2+ interaction within the first and second coordination shells comply well with experimental observations of concentration quenching of photoluminescence from isolated Mn2+ and from Mn2+–Mn2+ effective dimers, respectively. Tailoring of this procedure is achieved via adjusting the Mn–F–Mn angle and the Mn–F distance through substitution of the A+ and/or the B2+ species in the ABF3 compound. Computational simulation and X-ray absorption spectroscopy are employed to confirm this. The principle is applied to produce pure anti-Stokes near-infrared emission within the spectral range of ≈760–830 nm from codoped ABF3:Yb3+,Mn2+ upon excitation with a 976 nm laser diode, challenging the classical viewpoint where Mn2+ is used only for visible photoluminescence: in the present case, intense and tunable near-infrared emission is generated. This approach is highly promising for future applications in biomedical imaging and labeling.
Co-reporter:Lothar Wondraczek;Esa Tyystjärvi;Jorge Méndez-Ramos;Frank A. Müller;Qinyuan Zhang
Advanced Science 2015 Volume 2( Issue 12) pp:
Publication Date(Web):
DOI:10.1002/advs.201500218
Solar energy harvesting is largely limited by the spectral sensitivity of the employed energy conversion system, where usually large parts of the solar spectrum do not contribute to the harvesting scheme, and where, of the contributing fraction, the full potential of each photon is not efficiently used in the generation of electrical or chemical energy. Extrinsic sensitization through photoluminescent spectral conversion has been proposed as a route to at least partially overcome this problem. Here, we discuss this approach in the emerging context of photochemical energy harvesting and storage through natural or artificial photosynthesis. Clearly contrary to application in photovoltaic energy conversion, implementation of solar spectral conversion for extrinsic sensitization of a photosynthetic machinery is very straightforward, and—when compared to intrinsic sensitization—less-strict limitations with regard to quantum coherence are seen. We now argue the ways in which extrinsic sensitization through photoluminescent spectral converters will—and will not—play its role in the area of ultra-efficient photosynthesis, and also illustrate how such extrinsic sensitization requires dedicated selection of specific conversion schemes and design strategies on system scale.
Co-reporter:R. Limbach, A. Winterstein-Beckmann, J. Dellith, D. Möncke, L. Wondraczek
Journal of Non-Crystalline Solids 2015 Volumes 417–418() pp:15-27
Publication Date(Web):1–15 June 2015
DOI:10.1016/j.jnoncrysol.2015.02.019
•Elasticity, plasticity and defect-resistance of alkali borosilicate glasses are reported.•Mechanical properties are related to glass structure.•A complete picture is obtained for the Na2O–B2O3–SiO2 ternary.We provide a comprehensive description of the defect tolerance of sodium-borosilicate glasses upon sharp contact loading. This is motivated by the key role which is taken by this particular glass system in a wide variety of applications, ranging from electronic substrates, display covers and substrates for biomedical imaging and sensing to, e.g., radioactive waste vitrification. The present report covers the mechanical properties of glasses in the Na2O–B2O3–SiO2 ternary over the broad range of compositions from pure SiO2 to binary sodium-borates, and crossing the regions of various commercially relevant specialty borosilicate glasses, such as the multi-component Duran-, Pyrex- and BK7-type compositions and typical soda-lime silicate glasses, which are also included in this study. In terms of structure, the considered glasses may be separated into two groups, that is, one series which contains only bridging oxygen atoms, and another series which is designed with an increasing number of non-bridging oxygen ions. Elastic moduli, Poisson ratio, hardness as well as creep and crack resistance were evaluated, as well as the contribution of densification to the overall amount of indentation deformation. Correlations between the mechanical properties and structural characteristics of near- and mid-range order are discussed, from which we obtain a mechanistic view at the molecular reactions which govern the overall deformation reaction and, ultimately, contact cracking.
Co-reporter:Arnulf Thieme, Doris Möncke, René Limbach, Sindy Fuhrmann, Efstratios I. Kamitsos, Lothar Wondraczek
Journal of Non-Crystalline Solids 2015 410() pp: 142-150
Publication Date(Web):
DOI:10.1016/j.jnoncrysol.2014.11.029
Co-reporter:Anja Winterstein-Beckmann, Doris Möncke, Dimitrios Palles, Efstratios I. Kamitsos, and Lothar Wondraczek
The Journal of Physical Chemistry B 2015 Volume 119(Issue 7) pp:3259-3272
Publication Date(Web):January 20, 2015
DOI:10.1021/jp5120465
The structure and properties of melt-quenched glasses and partially crystallized samples from the borate series (1–2x)Eu2O3–x((Eu,Sr)O–B2O3) were investigated in the supermodified regime of x < 0.5, using Raman, infrared (IR), electron spin resonance (ESR), and UV–vis absorption and fluorescence spectroscopic techniques. ESR and optical spectroscopy showed that, despite the strongly reducing synthesis conditions, the Eu2+/Eu3+ equilibrium remained shifted to the side of trivalent Eu3+. Stable and transparent overmodified borate glasses were produced for compositions with x ≥ 0.36. Higher europium oxide concentrations resulted in precipitation of crystalline Eu2Sr3(BO3)4 and EuBO3 phases, as traced by X-ray diffraction. Raman and IR spectroscopy showed that the metaborate configuration which is present at x = 0.46 transforms gradually, with increasing Eu2O3 levels, into orthoborate [BO3]3– triangular units. At higher europium oxide content (x ≤ 0.36), the presence of Eu3+ supports the formation of orthoborate [BØ2O2]3– tetrahedral species. These units organize into [B3O9]9– rings, which exist in equilibrium with [BO3]3– triangles. As a consequence, distinct variations can be observed also in the macroscopic properties such as density, glass transition temperature, refractive index, optical basicity, and oxygen polarizability. This observation confirms previous findings on manganese–strontium borates with high modification levels.
Co-reporter:Guojun Gao, Jingxue Wei, Yang Shen, Mingying Peng and Lothar Wondraczek
Journal of Materials Chemistry A 2014 vol. 2(Issue 41) pp:8678-8682
Publication Date(Web):21 Aug 2014
DOI:10.1039/C4TC01447B
We report on the photoluminescence (PL) properties of heavily Eu2O3 doped Y2O3–Al2O3–B2O3 glasses as a high-gain red-emitting material for photoconversion. A UV-to-red conversion efficiency of up to 60% is achieved in this material. Concentration quenching is related to the formation of –{Eu–O–Me–O–Eu}– (Me = B, Y, Al) and –{Eu–O–Eu}– linkages in the first and second coordination shell, respectively, of Eu3+. For a quantitative consideration, we employ a statistical approach to estimate the number of those species as a function of europium concentration. In this way, a crossover composition is obtained at a dopant concentration of ∼3 mol% of Eu2O3 where further addition of europium directly increases the number of –{Eu–O–Eu}–. In the latter species, the critical distance for concentration quenching of PL is achieved so that for concentrations above the crossover, PL quantum efficiency decreases. This observation is confirmed experimentally through lifetime analyses as well as direct measurements of internal quantum efficiency. The proposed tool can now be used to predict and optimize cluster formation and concentration quenching in rare-earth doped glasses.
Co-reporter:Guojun Gao, Mingying Peng and Lothar Wondraczek
Journal of Materials Chemistry A 2014 vol. 2(Issue 38) pp:8083-8088
Publication Date(Web):14 Aug 2014
DOI:10.1039/C4TC01242A
We report on spectral modification through NIR down-conversion (DC) photoluminescence (PL) in Yb3+-Bi3+-co-doped Zn2GeO4. Energetic downshifting (DS) of UV-A irradiation occurs via intrinsic luminescence of the high-bandgap semiconductor Zn2GeO4 as well as via active Bi3+ centres. In parallel, both species act as sensitizers for Yb3+, strongly extending its excitation region to ∼500 nm. In the absence of Bi3+, band-to-band absorption of Zn2GeO4 in the UV region results in PL at ∼475–625 nm. Doping with Yb3+ initiates energy transfer from trapped defect states to two neighbouring Yb3+ ions in a cooperative DC process, resulting in Yb3+-related photoemission at ∼1000 nm. The introduction of Bi3+ into Zn2GeO4:Yb3+ greatly extends the absorption band to the visible blue region. Then, energy transfer also occurs through cooperative DC from Bi3+ to Yb3+. As a result, a strong increase in the absolute Yb3+-related PL intensity is observed. This enables ultra-efficient harvesting of UV-A to visible radiation for energy conversion processes.
Co-reporter:Guojun Gao and Lothar Wondraczek
Journal of Materials Chemistry A 2014 vol. 2(Issue 4) pp:691-695
Publication Date(Web):30 Oct 2013
DOI:10.1039/C3TC31660B
We report on the photoluminescence of heavily Eu3+-doped SrO–La2O3–Al2O3–B2O3–SiO2 glasses. The specific glass system enables highly efficient photoemission for a dopant concentration of up to 8 mol% of Eu2O3 before concentration quenching is observed. This allows for the realization of high absorption cross section combined with an internal quantum efficiency of greater than 60% without further optimization of batch materials and processing conditions. In this way, the problem of luminescence quantum yield from active glass substrates in smart windows, greenhouses or solar energy harvesting may be overcome. In the present case, more than 50% of the photons coming in at the peak excitation wavelengths of 394 or 464 nm are converted to red.
Co-reporter:Karsten H. Nielsen, Dominik K. Orzol, Svetoslav Koynov, Steve Carney, Eric Hultstein, Lothar Wondraczek
Solar Energy Materials and Solar Cells 2014 Volume 128() pp:283-288
Publication Date(Web):September 2014
DOI:10.1016/j.solmat.2014.05.034
•A low-cost, large-area AR coating has been developed for c-Si and thin-film solar modules.•An absolute transmission increase of up to 3.6 % is reported for single-side coated glass.•The coating relies on wet-deposition of aqueous silicate solutions and atmospheric treatment.•The process of coating deposition and consolidation is elucidated.We present on line formation of a sol from wet-deposited aqueous potassium silicate solutions as a novel route for the generation of highly performing large-area anti-reflective (AR) surfaces on glasses for solar energy conversion. Compared to alternative technologies, the present approach enables processing at very low cost. The mechanism of coating formation and consolidation was evaluated. Following deposition, the aqueous potassium silicate solution dries into a gel of interconnected SiO2 colloids and dispersed potassium hydroxides and carbonates. A typical size of ~20–40 nm of the colloids is established already at this stage without significant growth in the later process. Potassium species are removed in a subsequent washing procedure, leaving only a minor amount of residual potassium at the interface between coating and substrate in an otherwise nanoporous silica layer. Physically and chemically bound water species are driven-out of the coating in a final annealing step. In this way, an AR layer of nanoporous silica with a thickness of ~100–150 nm is easily created. The AR effect is caused by two-beam interference at this layer, enabling an absolute transmission increase of 3.6% at a wavelength of 550 nm and a relative transmission increase of 3.1% over the spectral range of 400–1100 nm for a single-side coating on solar glass for c-Si as well as thin-film modules.
Co-reporter:Bruno P. Rodrigues, John C. Mauro, Yuanzheng Yue, Lothar Wondraczek
Journal of Non-Crystalline Solids 2014 Volume 405() pp:12-15
Publication Date(Web):1 December 2014
DOI:10.1016/j.jnoncrysol.2014.08.035
•The concept of cationic constraint strength is applied for Tg prediction in ultraphosphate glasses.•Network modifiers are treated as coexisting in isolated and in crosslinking sites.•A significant improvement over previously published work is achieved.In applying the recently introduced concept of cationic constraint strength [J. Chem. Phys. 140, 214501 (2014)] to bond constraint theory (BCT) of binary phosphate glasses in the ultraphosphate region of xR2O-(1 − x)P2O5 (with x ≤ 0.5 and R = {Li, Na, Cs}), we demonstrate that a fundamental limitation of BCT can be overcome. The modifiers are considered to exist in either “isolated” or “crosslinking” sites, in line with the so-called modifier sub-network [J. Chem. Phys. 140, 154501 (2014)] and each site is associated with a certain number of constraints. We estimate the compositional dependence of the modifier sites and then use this to calculate the glass transition temperature as a function of chemical composition. A statistical distribution of sites achieves a remarkable agreement with experimental data for all tested glasses and greatly improves upon previously published work.
Co-reporter:Doris Möncke, Sindy Reibstein, Daniel Schumacher, Lothar Wondraczek
Journal of Non-Crystalline Solids 2014 Volume 383() pp:33-37
Publication Date(Web):1 January 2014
DOI:10.1016/j.jnoncrysol.2013.04.029
•Defect formation in X-ray irradiated sulfophosphate glasses was studied.•A variety of positively and negatively charged defects was identified.•Defect relaxation was studied by EPR and UV–VIS spectroscopy.•Silver-doping enables stabilization of defects at room temperature.X-ray-induced defects in undoped and Ag-doped sulfophosphate glasses were studied by optical and electron paramagnetic resonance (EPR) spectroscopy. Besides intrinsic phosphate-related positively charged hole centers (HC) and negatively charged electron centers (EC), also oxygen hole centers (OHC), sulfate-related electron centers (SEC) and zinc centers (Zn*-HC) were identified. The formation of three different Ag-related defects was evident in the Ag+-doped glasses: (Ag+)− and (Ag+)2−-EC, as well as (Ag+)+-HC. Undoped glasses recover at room temperature within two weeks. In contrast, other defects are stabilized when Ag+ is present in the glasses, as some intrinsic defects transform over time at room temperature into more stable Ag-related defects.
Co-reporter:A. Winterstein-Beckmann, D. Möncke, D. Palles, E.I. Kamitsos, L. Wondraczek
Journal of Non-Crystalline Solids 2014 Volume 401() pp:110-114
Publication Date(Web):1 October 2014
DOI:10.1016/j.jnoncrysol.2013.12.038
•The effect of micro-indentation on local glass structure is studied.•A scheme for indentation-induced structural changes is established.•Indentation-induced structural changes are correlated to different deformation mechanisms.•Deformation changes from density-driven (silica) to shear-driven (alkali-borate).We report on the effect of micro-indentation on the local structure of glasses in the ternary borosilicate glass system Na2O–B2O3–SiO2. Indentation experiments were performed with loads ranging from 0.49 to 4.91 N and for loading times of 10–30 s in order to analyze the impact on the silicate and borate sub-networks in glasses ranging from pure SiO2 to binary Na2O–4B2O3, with a ratio of R = Na2O/B2O3 < 0.5. The Raman spectra reveal distinct changes in the glass structure, which can be correlated to different densification mechanisms occurring in the silicate and borate sub-networks, respectively. A clear trend from anomalous (densification-driven) deformation in silica-rich to normal (shear-driven) deformation behavior in alkali borate-rich glasses is observed.
Co-reporter:Sindy Fuhrmann, Daniel Schumacher, Jonas Herbst, Lothar Wondraczek
Journal of Non-Crystalline Solids 2014 Volume 401() pp:82-86
Publication Date(Web):1 October 2014
DOI:10.1016/j.jnoncrysol.2014.01.024
•Irreversible formation of silver clusters occurs in Ag-doped sulfophosphate glasses upon X-ray irradiation.•Reversible cluster formation occurs upon UV-irradiation in the presence of CeO2.•Decay kinetics of defects and clusters are reported.Silver-doped sulfophosphate glasses show irreversible Agmx + silver cluster formation upon X-ray irradiation. Reversible cluster formation occurs upon UV irradiation in the presence of Ce3 + as electron reservoir. Additional intrinsic irradiation induced defects such as POHC, PEC, OHC, SEC and Zn*-HC recover at room temperature. The presence of silver clusters results in the occurrence of an optical absorption band at λ ~ 330 nm. The formation and decay kinetics of irradiation induced defects and clusters are followed by photoluminescence and UV–VIS spectroscopy as well as by small-angle X-ray scattering.
Co-reporter:Rene Limbach, Bruno P. Rodrigues, Lothar Wondraczek
Journal of Non-Crystalline Solids 2014 Volume 404() pp:124-134
Publication Date(Web):15 November 2014
DOI:10.1016/j.jnoncrysol.2014.08.023
•The strain-rate sensitivity of local deformation in covalent, metallic and ionic glasses is studied.•A strain-rate jump test through instrumented nanoindentation is adopted for brittle glasses.•Correlations between strain-rate sensitivity and Poisson ratio, packing density, bond strength and temperature are discussed.We report on the loading-rate dependence of localized plastic deformation in inorganic covalent, metallic, ionic and superionic glasses. For this, the strain-rate sensitivity is determined through instrumented nanoindentation in a load-controlled strain-rate jump test. Through relating the strain-rate sensitivity to the reduced temperature, the packing density, the network dimensionality and the average single bond strength of the system, a qualitative mechanistic description of the strain-mediating process is possible. A strong variability of strain-rate sensitivity is obtained only at intermediate values of packing density, network connectivity or bond strength, when other parameters such as chemical composition and specific structural arrangement are dominating the deformation process. On the other side, for high bond strength and connectivity or for high packing density, the strain-rate sensitivity of the considered glasses is always low, which is also confirmed through the dependence of strain-rate sensitivity on Poisson ratio. Here, only for glasses with a Poisson ratio of ~ 0.3–0.4 we observe a wide variability of the loading-rate dependence of local deformation. For higher or lower Poisson ratio, the observed dependence is always low: when the limiting factor in deformation is primarily network connectivity and bond strength or packing density, respectively, once an activation barrier is overcome, deformation is only weakly loading-rate-dependent. This is regardless of the height of the activation barrier. When approaching the glass transition temperature, high strain-rate sensitivity is observed only in glasses where non-Newtonian flow is expected also in the corresponding liquid.
Co-reporter:Lothar Wondraczek, Sebastian Krolikowski and Peter Nass
Journal of Materials Chemistry A 2013 vol. 1(Issue 26) pp:4078-4086
Publication Date(Web):03 May 2013
DOI:10.1039/C3TC30609G
We report on partitioning of europium in bi-phasic åkermanite–feldspar glass ceramics with broadly tunable photoluminescence. Conditions for precipitating Eu-doped microcrystalline (Sr,Ca)-åkermanite from supercooled liquids of the type MO–B2O3–SiO2–Al2O3 (M = Mg, Ca and Sr) were evaluated. Formation of secondary anorthite and slawsonite feldspar is controlled by chemical composition and crystallization kinetics. Eu2+ precipitates on Sr2+ and, to a lesser extent, on Ca2+ sites. Static and dynamic Eu2+ luminescence spectroscopy and spatially resolved cathodoluminescence spectroscopy are used to explore the distribution of europium between crystallite species and the residual glass phase. It was found that åkermanite precipitation occurs congruently with respect to Sr and Ca partitioning for liquids with Sr/(Sr + Ca) ≥ 0.4. Due to kinetic selection, the Ca-rich side of the (Ca,Sr)2MgSi2O7 solid solution is favored and less strontium is incorporated into the crystal phase relative to the precursor material for liquids with lower strontium content, meaning that strontium enriches in the residual glass phase. Using a selective etching process, the crystalline feldspar phase was separated from the residual glass and åkermanite phases, respectively. In this way, the spectroscopic contributions of each phase could be distinguished from each other, clarifying the problem of external quantum efficiency of photoluminescence and re-absorption in glass ceramics relative to the spectroscopic properties of pure crystals.
Co-reporter:A. Winterstein-Beckmann, D. Möncke, D. Palles, E.I. Kamitsos, L. Wondraczek
Journal of Non-Crystalline Solids 2013 Volume 376() pp:165-174
Publication Date(Web):15 September 2013
DOI:10.1016/j.jnoncrysol.2013.05.029
•The structure and properties of highly modified borate glasses are studied.•Glass formation occurs in SrO–MnO–B2O3 down to 20 mol% of B2O3.•Boron-speciation into [BØ2O]−, [BØ4]−, [B2O5]4 −, [BO3]3 −, [BØ2O2]3 − and [B3O9]9 − is explored.•Structural inhomogeneities occur due to chemical isomerization processes.•MnO-clustering was observed for all MnO contents.Highly modified borate glasses with the composition (1 − 2x)MnO–x(SrO–B2O3) (x = 0.46, 0.42, 0.36, 0.25, and 0.20) were prepared and investigated by Raman, infrared (IR), and electron paramagnetic resonance (EPR) spectroscopy. Optical properties were studied in regard to photoluminescence, optical absorption, and refractive index. The Mn2 +/Mn3 + equilibrium was shifted towards the divalent manganese ion as a result of the strongly reducing melting conditions employed in this work, which facilitate the preparation of transparent glasses with up to 80 mol% total SrO and MnO content. Changes in the optical and physical properties within this glass series were related to structural variations. The structure of glasses with relatively low MnO content was found to involve mainly trigonal [BØ2O]− and tetrahedral [BØ4]− metaborate groups, which are replaced progressively by pyroborate [B2O5]4 − and orthoborate [BO3]3 − triangular units upon increasing MnO content. At the highest modification level (x = 0.20) the structure is built of orthoborate isomeric species in triangular [BO3]3 − and tetrahedral [BØ2O2]3 − configuration. The latter species form [B3O9]9 − rings, which reestablish some degree of network connectivity, as they involve three bridging and six non-bridging oxygen atoms, and this is reflected by the increase of the glass transition temperature for x = 0.25 over x = 0.20. Micro-Raman measurements showed structural inhomogeneities in these glasses due to chemical isomerization processes involving short- and medium-range order structures. Also, increasing MnO content was shown to cause MnO-clustering in the glasses as revealed by luminescence and EPR measurements.
Co-reporter:W.J. Zhang, G. Gao, A. Winterstein-Beckmann, D. Möncke, Q.Y. Zhang, M. Peng, L. Wondraczek
Journal of Non-Crystalline Solids 2013 380() pp: 60-64
Publication Date(Web):
DOI:10.1016/j.jnoncrysol.2013.08.004
Co-reporter:Lothar Wondraczek, Guojun Gao, Doris Möncke, Thangaraj Selvam, Andreas Kuhnt, Wilhelm Schwieger, Dimitrios Palles, Efstratios I. Kamitsos
Journal of Non-Crystalline Solids 2013 360() pp: 36-40
Publication Date(Web):
DOI:10.1016/j.jnoncrysol.2012.10.001
Co-reporter:Karsten H. Nielsen, Morten M. Smedskjaer, Mingyeng Peng, Yuanzheng Yue, Lothar Wondraczek
Journal of Non-Crystalline Solids 2012 Volume 358(Issue 23) pp:3193-3199
Publication Date(Web):1 December 2012
DOI:10.1016/j.jnoncrysol.2012.09.021
We report on the effect of hydrogen annealing on the optical properties of bismuth-doped sodium aluminosilicate glasses. The redox state of bismuth in the as-melted glasses is governed by the composition, viz., NIR luminescence is observed only in the glasses with low optical basicity. Upon thermal reduction, visible emission from Bi3+ and, eventually, minor amounts of Bi2+ is significantly lowered, depending on heat-treatment time and temperature, and glass composition. Hydrogen treatment was also found to result in a decrease of the NIR emission intensity and, at the same time, formation of metallic bismuth particles in the surface region. Surface-tinting as well as the decrease of visible luminescence follow Arrhenian kinetics, suggesting that hydrogen permeation is the rate-governing process. Upon re-annealing in air, the effects of thermal reduction on the optical properties are reversible only to a limited extent.Highlights► The effect of thermal reduction on luminescence of Bi-doped aluminosilicate glasses was studied. ► Variations in luminescence are attributed to variations in the redox state of bismuth species. ► Metallic Bi-particles form near the surface by thermal annealing in reducing atmosphere. ► Thermal reduction was found to be only partially reversible.
Co-reporter:Kristin Griebenow, Uwe Hoppe, Doris Möncke, Efstratios I. Kamitsos, Lothar Wondraczek
Journal of Non-Crystalline Solids (15 March 2017) Volume 460() pp:136-145
Publication Date(Web):15 March 2017
DOI:10.1016/j.jnoncrysol.2017.01.022
Co-reporter:F. Lind, D. Palles, D. Möncke, E.I. Kamitsos, L. Wondraczek
Journal of Non-Crystalline Solids (15 April 2017) Volume 462() pp:47-50
Publication Date(Web):15 April 2017
DOI:10.1016/j.jnoncrysol.2017.02.006
Co-reporter:R. Limbach, K. Kosiba, S. Pauly, U. Kühn, L. Wondraczek
Journal of Non-Crystalline Solids (1 March 2017) Volume 459() pp:130-141
Publication Date(Web):1 March 2017
DOI:10.1016/j.jnoncrysol.2017.01.015
Co-reporter:Guojun Gao and Lothar Wondraczek
Journal of Materials Chemistry A 2014 - vol. 2(Issue 4) pp:NaN695-695
Publication Date(Web):2013/10/30
DOI:10.1039/C3TC31660B
We report on the photoluminescence of heavily Eu3+-doped SrO–La2O3–Al2O3–B2O3–SiO2 glasses. The specific glass system enables highly efficient photoemission for a dopant concentration of up to 8 mol% of Eu2O3 before concentration quenching is observed. This allows for the realization of high absorption cross section combined with an internal quantum efficiency of greater than 60% without further optimization of batch materials and processing conditions. In this way, the problem of luminescence quantum yield from active glass substrates in smart windows, greenhouses or solar energy harvesting may be overcome. In the present case, more than 50% of the photons coming in at the peak excitation wavelengths of 394 or 464 nm are converted to red.
Co-reporter:Lothar Wondraczek, Sebastian Krolikowski and Peter Nass
Journal of Materials Chemistry A 2013 - vol. 1(Issue 26) pp:NaN4086-4086
Publication Date(Web):2013/05/03
DOI:10.1039/C3TC30609G
We report on partitioning of europium in bi-phasic åkermanite–feldspar glass ceramics with broadly tunable photoluminescence. Conditions for precipitating Eu-doped microcrystalline (Sr,Ca)-åkermanite from supercooled liquids of the type MO–B2O3–SiO2–Al2O3 (M = Mg, Ca and Sr) were evaluated. Formation of secondary anorthite and slawsonite feldspar is controlled by chemical composition and crystallization kinetics. Eu2+ precipitates on Sr2+ and, to a lesser extent, on Ca2+ sites. Static and dynamic Eu2+ luminescence spectroscopy and spatially resolved cathodoluminescence spectroscopy are used to explore the distribution of europium between crystallite species and the residual glass phase. It was found that åkermanite precipitation occurs congruently with respect to Sr and Ca partitioning for liquids with Sr/(Sr + Ca) ≥ 0.4. Due to kinetic selection, the Ca-rich side of the (Ca,Sr)2MgSi2O7 solid solution is favored and less strontium is incorporated into the crystal phase relative to the precursor material for liquids with lower strontium content, meaning that strontium enriches in the residual glass phase. Using a selective etching process, the crystalline feldspar phase was separated from the residual glass and åkermanite phases, respectively. In this way, the spectroscopic contributions of each phase could be distinguished from each other, clarifying the problem of external quantum efficiency of photoluminescence and re-absorption in glass ceramics relative to the spectroscopic properties of pure crystals.
Co-reporter:Guojun Gao, Jingxue Wei, Yang Shen, Mingying Peng and Lothar Wondraczek
Journal of Materials Chemistry A 2014 - vol. 2(Issue 41) pp:NaN8682-8682
Publication Date(Web):2014/08/21
DOI:10.1039/C4TC01447B
We report on the photoluminescence (PL) properties of heavily Eu2O3 doped Y2O3–Al2O3–B2O3 glasses as a high-gain red-emitting material for photoconversion. A UV-to-red conversion efficiency of up to 60% is achieved in this material. Concentration quenching is related to the formation of –{Eu–O–Me–O–Eu}– (Me = B, Y, Al) and –{Eu–O–Eu}– linkages in the first and second coordination shell, respectively, of Eu3+. For a quantitative consideration, we employ a statistical approach to estimate the number of those species as a function of europium concentration. In this way, a crossover composition is obtained at a dopant concentration of ∼3 mol% of Eu2O3 where further addition of europium directly increases the number of –{Eu–O–Eu}–. In the latter species, the critical distance for concentration quenching of PL is achieved so that for concentrations above the crossover, PL quantum efficiency decreases. This observation is confirmed experimentally through lifetime analyses as well as direct measurements of internal quantum efficiency. The proposed tool can now be used to predict and optimize cluster formation and concentration quenching in rare-earth doped glasses.
Co-reporter:Guojun Gao, Mingying Peng and Lothar Wondraczek
Journal of Materials Chemistry A 2014 - vol. 2(Issue 38) pp:NaN8088-8088
Publication Date(Web):2014/08/14
DOI:10.1039/C4TC01242A
We report on spectral modification through NIR down-conversion (DC) photoluminescence (PL) in Yb3+-Bi3+-co-doped Zn2GeO4. Energetic downshifting (DS) of UV-A irradiation occurs via intrinsic luminescence of the high-bandgap semiconductor Zn2GeO4 as well as via active Bi3+ centres. In parallel, both species act as sensitizers for Yb3+, strongly extending its excitation region to ∼500 nm. In the absence of Bi3+, band-to-band absorption of Zn2GeO4 in the UV region results in PL at ∼475–625 nm. Doping with Yb3+ initiates energy transfer from trapped defect states to two neighbouring Yb3+ ions in a cooperative DC process, resulting in Yb3+-related photoemission at ∼1000 nm. The introduction of Bi3+ into Zn2GeO4:Yb3+ greatly extends the absorption band to the visible blue region. Then, energy transfer also occurs through cooperative DC from Bi3+ to Yb3+. As a result, a strong increase in the absolute Yb3+-related PL intensity is observed. This enables ultra-efficient harvesting of UV-A to visible radiation for energy conversion processes.
