Influence of melting and annealing conditions on the optical spectra of a borosilicate glass doped with CoO and NiO

In the high-viscosity borosilicate glass (NBS2) doped with 0.3 mol% CoO or NiO quenching resulted in a freeze-in snap-shot of the glass structure within the dopants' transformation process from their high-temperature tetrahedral coordination to the octahedral form normally present in this glass at room temperature. In this transitional state the octahedral, tetrahedral and a third pseudotetrahedral transitional coordination are simultaneously present. The optical spectra of the doped glasses are discussed in relation to the different melting and cooling conditions applied. Quenched glasses were also tempered on a heating table, which permitted to take the optical spectra at each temperature step. In contrast to Co2+, Ni2+ has a strong octahedral preference. Thus for NiO-doped NBS2 glass tempering or annealing always results in relaxation into the octahedral coordination. For Co2+, which is also octahedrally coordinated in the annealed NBS2, tempering of the quenched glass leads to a relaxation into octahedrally and tetrahedrally coordinated Co2+. These structural changes are especially strong when the applied temperatures lie 150 to 200 °C above Tg of the NBS2 glass where also the viscosity-temperature curve implies structural changes within the glass matrix

Irradiation-induced defects in different glasses demonstrated on a metaphosphate glass

The influence of the two polyvalent ions, cobalt and nickel, on the formation of irradiation-induced defects was studied in several different model glasses (silicate, borosilicate, fluoride- and phosphate glasses). In this article the defects are demonstrated on the example of the (SrPO3)2-metaphosphate glass P100. Sample plates of high-purity glasses, undoped and doped with 0.3 mol% CoO and NiO, were irradiated with a UV lamp and with X-rays. The subsequent defect centers, formed at ppm levels, were characterized by EPR as well as optical UV-VIS spectroscopy. Defect recovery experiments were also studied in these glasses. The newly found optical bands and EPR signals evolving in the irradiated glass are in part characteristic for intrinsic defects. These are different types of electron centers (EC) and hole centers (HC) connected with phosphate groups. Other signals arise from extrinsic defects, which are caused by the two dopant ions. The predominant extrinsic defect stems from the photooxidation of Co2+ to (Co2+)2. As an HC the latter replaces some of the intrinsie phosphate-bonded HC and dominates the optical spectra with two bands at 300 and 400 nm. In the glass P100 lamp irradiation photoionizes only Co2+ but not Ni2+. Α new optical band at 330 nm, as well as a new EPR signal at g = 2.08 can be seen only after X-ray irradiadon. Both can be attributed to a nickel-related EC created via the photoreduction of Ni2+ to (Ni2+)-. At the same time the band of the intrinsic oxygen-related HC is intensified. Generally X-ray irradiation causes stronger irradiation-induced defects (excitation of inner electrons) than UV-lamp irradiation (selective excitation of valence electrons)

UV light induced photoreduction in phosphate and fluoridephosphate glasses doped with Ni2+, Ta5+, Pb2+, and Ag+ compounds

The photoreduction of polyvalent ions was studied in high purity fluoride-phosphate and metaphosphate glasses doped with Ni2+ (3d8), Ta5+ (5d0), Pb2+ (6d2), and Ag+ (3d10). Compared to the undoped base glasses all doped samples display different electronic transitions in the UV at the irradiation wavelength. Glass samples containing 50 to 5000 ppm dopants were irradiated with excimer lasers at 193 and 248 nm, respectively. The subsequent defect centers, formed at ppm levels, were characterized by EPR and optical UV-VIS spectroscopy. The observed laser induced transmission losses in the UV and visible range increased in the order Ni, Ta, Pb to Ag. Extrinsic electron centers are formed by photoreduction of the dopants. (Ni2+)- is characterized by an optical transition with a maximum at 355 nm and an EPR signal around g ≈ 2.07. The maxima of the optical transitions of the (Pb2+)- -EC are positioned at 395 and 500 nm, of the (Ta5+)- -EC at 465 nm. The photoionization products of silver depend strongly on the silver concentration. At a silver content of 50 ppm only the (Ag+)- -EC is formed, visible in the optical spectra with a maximum around 450 nm. Α second silver species, (Ag+)2 -, which absorbs at 305 nm, is additionally observed in the sample doped with a silver concentration of 500 ppm. In the sample doped with 5000 ppm silver a third defect, the photooxidized (Ag+)+ -HC, with an optical band maximum at 405 nm and an EPR signal around g ≈ 2.3 is observed as well. The formation of extrinsic electron centers causes in all glasses an increase in the formation of intrinsic hole centers and often a decrease in the formation of intrinsic electron centers. Defect generation curves show that a very rapid darkening in the glasses is initiated by the addition of any of these dopants. The recovery rates of the defeets formed depend strongly on the dopant, not on the glass matrix

Microcompression experiments on glasses ‐ strain rate sensitive cracking behavior

Figure 11 – microcompression experiments on glasses showing stable crack growth (a) and reversible deformation (b) It is well known that the mechanical properties of glasses are closely related to their atomic structure. The exact structure-property-relationship, however, is only poorly understood even for fundamental mechanisms like shear and densification. Nanomechanical test methods like micropillar compression and nano indentation can help fill this gap. In this study a sodium-boro-silicate glass is quenched from different temperatures to induce changes in the atomic structure. Micropillar compression was used to introduce plastic deformation into these glasses at room temperature under a uniaxial stress state. By changing the strain rate it is shown that deformation shifts from completely reversible deformation, to stable crack growth, and finally brittle failure. It is shown that by changing the glass structure, the strain rates corresponding to these deformation regimes are shifted. Finally, the occurrence of shear and densification is discussed. These findings are analysed against the background of the glass structure. Please click Additional Files below to see the full abstract

Using impact‐nanoindentation to test glasses at high strain rates and room temperature

In many daily applications glasses are indispensable, and novel applications demanding improved strength and crack resistance are appearing continuously. Up to now, the fundamental mechanical processes in glasses subjected to high strain rates at room temperature are largely unknown and thus guidelines for one of the major failure conditions of glass components are non-existent. Here, we elucidate this important regime for the first time using glasses ranging from a dense metallic glass to open fused silica by impact as well as quasi-static nano-indentation. We show that towards high strain rates, shear deformation becomes the dominant mechanism in all glasses accompanied by Non-Newtonian behavior evident in a drop of viscosity with increasing rate covering eight orders of magnitude. All glasses converge to the same limit stress determined by the theoretical hardness, thus giving the first experimental and quantitative evidence that Non-Newtonian shear flow occurs at the theoretical strength at room temperature

Radiation-induced defects in CoO- and NiO-doped fluoridephosphate glasses

Irradiation-induced defect formation is a common phenomenon in glasses. The influence of the two polyvalent ions cobalt and nickel was studied in several model glasses, two of those were fluoride-phosphate glasses. These studies were done in order to contribute to the ongoing research on solarization. Dopants and impurities may influence the intensity of intrinsic defects and may cause the evolution of additional extrinsic defects. Sample plates of high-purity glasses, undoped and doped with CoO and NiO, were irradiated by UV lamps and X-rays. The formed defect centers displayed absorption bands in the UV-VIS range, which were recorded by absorption spectroscopy. As many defect centers are paramagnetic, EPR spectra of the irradiated samples were taken. The newly found optical bands and EPR signals evolving in the irradiated glasses are in part characteristic of intrinsic defects, which are different types of electron and hole centers connected with phosphate groups. The other signals arise from extrinsic defects, which are caused by the two dopant ions. Co2+ is photooxidized to (Co2+)+, and replaces some of the intrinsic hole centers (POHC), while Ni2+ is photoreduced to (Ni2+)-

Radiation-induced defects in CoO- and NiO-doped fluoride, phosphate, silicate and borosilicate glasses

The influence of cobalt and nickel on the formation of irradiation-induced defects was studied in fluoride, phosphate, silicate and borosilicate glasses. Sample plates of high-purity glasses, undoped and doped with 0.3 mol% CoO and NiO, respectively, were irradiated with UV lamps and with X-rays. The subsequent defect centers, formed at ppm levels, were characterized by EPR and optical UV-VIS spectroscopy. X-ray irradiation caused stronger solarization (excitation of inner electrons) than UV lamp irradiation (selective excitation of valence electrons). More defects were formed in doped than in undoped glasses, generally stronger for Co2+ - than for Ni2+ -doped glasses and especially strong in glasses of high optical basicity where Co2+ and Ni2+ were tetrahedrally coordinated. Co2+ was photooxidized to (Co2+)+ in all glasses, replacing some of the intrinsic hole centers (HC), with (Co2+)+ in tetrahedral coordination: charge transfer band < 400 nm, and (Co2+)+ in octahedral coordination: two bands between 300 and 450 nm. Ni2+ was photooxidized in the (boro-)silicate glasses, which all had a higher basicity, but was photoreduced in the fluoride-phosphate glasses of low basicity. Photoreduced (Ni2+)- was found in the phosphate glass of medium basicity only after X-ray irradiation. The photoionized nickel species also displayed distinct EPR signals, with (Ni2+)+: several bands from 700 to 200 nm, g=2.10; and (Ni2+)-: 330 nm, g1=2.08 and g2=2.26

Metal ions and their interactions in covalent to ionic glass systems: a spectroscopic study

The cumulative thesis is based on 21 publications and describes different aspects of the glass structure in very different glass systems. The role of conventional glass formers, such as SiO2, B2O3, and P2O5 is discussed for changes in the coordination, changes in the fraction of bridging and non-bridging oxygen ions, and in regard to the interconnectivity of multicomponent systems. The impact of various modifier cations, which by themselves do not form glasses, on the basic glass network is discussed; as is the contribution these cations have on the network stability through cross-linking. The thesis compares the full concentration range of modifier ions in glasses; starting from very low dopant concentrations, where dopants act as probe ions or as active coloring or luminescing agents, to higher concentrations where preferential bonding and clustering of these ions impacts the glass structure, all the way to even higher concentrations where cation oxides are an integral part of the glass structure and thus fundamental constituents of the glass itself. Optical (absorption or photo-luminescence) and ESR spectroscopy can be used in the identification of the oxidation state and coordination of transition metal ions in glasses. Preferential bonding of dopants or of certain glass modifier cations is studied, as is preferential bonding between conventional network formers. Raman, IR and NMR spectroscopies in the low alkali borosilicate system show for example preferential bonding between tetrahedral borate units to borate entities (tetrahedral or trigonal) rather than to silicate tetrahedra. Modifications of the glass structure depending on the thermal history of a sample, after external impact such as irradiation, laser modification or indentation are discussed, as are surface modifications of archaeological glass samples from the late bronze age.Die auf 21 Veröffentlichungen beruhende kumulative Habilitationsschrift beschreibt die Glasstruktur sehr unterschiedlicher Glassysteme. Die Rolle konventioneller Glasbildner, wie SiO2, B2O3 und P2O5, wird in Bezug auf die Verknüpfung sowie in Hinblick auf die Wirkung verschiedener Netzwerkwandler diskutiert. Neben Veränderungen, die durch die Zugabe von Wandleroxiden im Netzwerk hervorgerufen werden, wurde auch der Beitrag untersucht, den diese Ionen zur gesamten Netzwerkstabilität beisteuern, indem sie die Glasbildner miteinander vernetzen. Der gesamte Konzentrationsbereich verschiedener zugegebener Metalloxide wird erörtert. Beginnend mit niedrigen Dotierungen im Spurenbereich, wie Indikatorionen oder aktive Dotanden (Chromophore oder Luminophore), zu immer höhere Konzentrationen werden schließlich auch Gläser behandelt, in denen die zugegebenen Metalloxide ein integraler Bestandteil der Gläser sind und damit nun selber als Glasbildner fungieren. Für die Bestimmung der Oxidationsstufen und Koordinationszahlen verschiedener Übergangsmetallionen werden optische (UV-Vis und Photolumineszenz) sowie ESR Spektroskopie eingesetzt. Vorzugsbindungen in Gläsern werden für bestimmte Netzwerkwandler beobachtet, wie z.B. für Alkali- und Erdalkali-Ionen, die sich in einigen Gläsern spezifische Liganden für die Koordination und den Ladungsausgleich auswählen. Weiterhin sind Vorzugsbindungen auch zwischen bestimmten Netzwerkbildnern bekannt. In alkaliarmen Borosilicatgläsern zeigen IR, Raman und NMR Spektroskopie, dass tetraedrische Boratgruppen bevorzugt an anderen Boratgruppen (tertaedrische oder trigonale) anstatt an Silicatgruppen gebunden vorliegen. Modifikationen der Glasstruktur in Abhängigkeit der thermischen Vorgeschichte, nach Bestrahlung oder Indentierung, werden ebenso behandelt, wie Oberflächenveränderungen an archäologischen Glasproben der späten Bronzezeit