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Organisationseinheit der BAM
- 5.6 Glas (151) (entfernen)
Although the slow crack growth in glass is dominated by stress-corrosion phenomena, it also should reflect the underlaying intrinsic fracture behavior controlled by glass chemistry and structure. To investigate such underlaying phenomena, crack growth velocity in alkali silicate glasses was measured in vacuum across 10 orders of magnitude with double cantilever beam technique. Measured and literature crack growth data were compared with calculated intrinsic fracture toughness data obtained from Young’s moduli and theoretical fracture surface energy. Data analysis reveals slight correlation with the packing density and significant deviations from the intrinsic brittle fracture behavior. These deviations do not follow simple compositional trends. Two opposing processes may explain this finding: a decrease in the apparent fracture surface energy due to stress-induced chemical changes at the crack tip and its increase due to energy dissipation during fracture.
Die technischen Möglichkeiten der robotischen Glasschmelzanlage der BAM werden vorgestellt.
Up to now, oriented surface crystallization phenomena are discussed controversially, and related studies are restricted to few glasses. For silicate glasses we found a good correlation between the calculated surface energy of crystal faces and oriented surface nucleation. Surface energies were estimated assuming that crystal surfaces resemble minimum energy crack paths along the given crystal plane. This concept was successfully applied by Rouxel in calculating fracture surface energies of glasses. Several oriented nucleation phenomena can be herby explained assuming that high energy crystal surfaces tend to be wetted by the melt. This would minimize the total interfacial energy of the nucleus. Furthermore, we will discuss the evolution of the microstructure and its effect on the preferred crystal orientation.
Up to now, oriented surface crystallization phenomena are discussed controversially, and related studies are restricted to few glasses. For silicate glasses we found a good correlation between the calculated surface energy of crystal faces and oriented surface nucleation. Surface energies were estimated assuming that crystal surfaces resemble minimum energy crack paths along the given crystal plane. This concept was successfully applied by Rouxel in calculating fracture surface energies of glasses. Several oriented nucleation phenomena can be herby explained assuming that high energy crystal surfaces tend to be wetted by the melt. This would minimize the total interfacial energy of the nucleus. Furthermore, we will discuss the evolution of the microstructure and its effect on the preferred crystal orientation.
Thermally stimulated interactions between silver and glass, that is, silver dissolution as Ag+ and precipitation as Ag0 were studied in two glass series of molar target composition xAg2O–(19 − x)Na2O–28ZnO–53B2O3 with x = 0, 0.1, 0.5, 5 and (19Na2O–28ZnO–53B2O3)+yAg2O with y = 0.01, 0.05. These act as model for low-melting borate glasses being part of metallization pastes. The occurrence of metallic silver precipitates in melt-quenched glass ingots demonstrated that silver dissolved only in traces (< 0.01 mol%) in the glasses. The dissolved silver was detected by means of Raman spectroscopy and energy-dispersive X-ray spectroscopy. Increasing x in the batch could not lead to a significant increase of the silver ion fraction in the glass as possible in binary silver borate glasses. In situ observation of heated AgNO3 mixed with the base glass frit in a hot stage microscope showed that Ag0 precipitation occurs already at the solid state. At higher temperatures, small droplets of liquid silver were found to move freely within the melt, whereas coalescence caused a stepwise increase of their size.
These results contribute to the understanding of formation of silver precipitates in metallization pastes described in the literature.
Substitution of lead oxide in low-melting glasses, e.g., for application in silver metallization pastes, is a current research goal. This work is focused on the investigation of silver diffusion in alkali zinc borate glasses (X2O-ZnO-B2O3, X = Li, Na, K, Rb). In addition, the redox state of silver (Ag) and thus the type of diffusing silver species were studied. For this purpose, a metallic silver coating was applied on glass cubes by means of sputtering. Heat treatment of the samples was performed close to the glass transition temperatures at 470 °C for 2 h under air and nitrogen atmosphere. Coating thickness was 1.8 µm after heat treatment, measured by a white light interferometer.
Silver diffusion profiles were measured by means of secondary neutral mass spectrometry. The silver diffusion coefficients are in the range of ~10-14 cm2/s and indicate no significant differences depending on the type of alkali ions in the glass. Dissolved Ag+-ions and [Agm]n+-clusters in the glasses were differentiated using fluorescence spectroscopy.
Precipitated metallic silver particles in the sodium containing glass were observed by means of transmission electron microcopy. Their mean particle diameter was ~6 nm.
Silber-Glas Wechselwirkungen spielen eine wichtige Rolle zum Beispiel in Glas-haltigen Silberpasten, die in der Photovoltaik oder Mikroelektronik verwendet werden. Um ein besseres Verständnis der Vorgänge zu erhalten, werden niedrig-schmelzende, bleifreie X2O ZnO B2O3 Gläser (X = Li, Na, K und Rb, Abk.: LZB, NZB, KZB and RZB) zur Untersuchung der Pasten verwendet.
Raman und Infrarot Spektroskopie zeigen, dass die Glasstruktur weitgehend vergleichbar ist, aber Unterschiede in der Bildung von Pentaboratgruppen in LZB und NZB oder Diboratgruppen in KZB und RZB auftreten. Die Anzahl an vierfach koordinierten Bor-Ionen (N4) sinkt mit zunehmendem Alkaliionenradius. Die Verringerung von N4 führt zu einer niedrigeren Glasübergangstemperatur in der Reihe LZB (472 °C) < NZB (465 °C) < KZB (460 °C) < RZB (449 °C).
Zur Herstellung der Pasten wurde kommerzielles Silber-Pulver mit 30 Vol.-% Glas und Organik gemischt. Das Sinterverhalten der gepressten Pulver wurde mittels Erhitzungsmikrospie untersucht. Der Sinterbeginn der Pasten ist vergleichbar und korrespondiert mit den geringen Unterschieden der Viskosität bei vergleichbarer Partikelgröße der Gläser. In den Ag-KZB- und Ag-RZB-Pasten ist die Schwindung durch die stärker einsetzende Kristallisation im Vergleich zu LZB und NZB verzögert.
Melting of Na2CO3-ZnO-B2O3 batches containing up to 16.8 wt% AgNO3 (5 mol% Ag2O in the target glass composition) was observed in situ by means of hot stage microscopy. In all batches metallic silver precipitation took place as most of the silver nitrate was reduced to metallic silver before Ag+ ions could be dissolved in the evolving borate melts. In turn, only traces of Ag+ (<300 ppmw) were dissolved in the sodium zinc borate glass melts under study. It is assumed that the oxidation to Ag+ was limited due to poor availability of reducible oxygen in the glass melts and presence of Na2O being a stronger base than Ag2O. Thus, the precipitated metallic silver formed droplets of different sizes. The larger droplets (d > 20 µm) were already settled at the bottom of the container and remained constant in size upon dwelling for 1 h at 1050 °C of about one hour and the subsequent cooling (45 K/min) to room temperature, whereas the smaller droplets (d < 20 µm) were mobile in the borate melt due to Marangoni and Stokes motion. For the latter droplets, coalescence was observed in situ. A growth of larger droplets at the expense of smaller ones, i.e., Ostwald ripening was also expected but could not be studied with the used experimental equipment.
Liquid phase sintering of glass bearing silver pastes used in photovoltaics and microelectronics is poorly understood. In particular, the role of different network modifiers acting in the glass component and the question of the most suitable atmospheric oxygen level during sintering are still under considerable debate. To tackle these issues, low-melting and lead-free X2O-ZnO-B2O3 glasses with X = Li, Na, K and Rb (LZB, NZB, KZB and RZB) were prepared. Infrared spectroscopy showed that the glass structure was similar to each other, while an increase of tetrahedrally coordinated boron with increasing field strength of the alkali was evident. In turn, the glass transition temperature (from differential thermal analysis) increased in the order: RZB (449 °C) < KZB (460 °C) < NZB (465 °C) < LZB (472 °C). Powders of each glass were mixed with organics to receive silver-glass-pastes containing 30 vol% glass and these were subjected to heating microscopy in air. Although similar onset temperatures of sintering were recorded for Ag-NZB and Ag-LZB glass pastes, and for Ag-RZB paste and KZB-Ag-paste, respectively, differences in the crystallization behavior and final densification were observed between the former and the latter group. These were translated in terms of differences in silver dissolution and reprecipitation among the two groups. Further, running the experiments in nitrogen gas showed that sintering of all pastes was considerably hampered. The slow-down of the sinter kinetics was found to be in line with the assumed lower oxidation and dissolution of silver ions into the glass-forming liquid.
Glasses in the systems Me2O-ZnO-B2O3 with Me = Li, Na, K, Rb (MeZB), Na2O-ZnO-CuO-B2O3 (NZCuB), CaO-ZnO-B2O3 (CaZB), and Li2O-PbO-B2O3 (LPbB) as a reference, were studied by differential thermal analysis, dilatometry, rotational viscometry, and heating microscopy. A decrease of viscosity and sintering range was found with decreasing number of fourfold coordinated boron. The viscosity of the alkali zinc borate glasses varies only slightly. LPbB and CaZB stand out by their reduced and increased viscosities, respectively. Sodium, potassium, and calcium zinc borate glasses possess a fragility above 76. All glasses were sintered to full density before crystallization. Mostly binary zinc borate phases govern crystallization. A ternary crystalline phase was detected only in the potassium containing sample. The Weinberg glass stability parameter ranges between 0.07 and 0.12. This is caused by the presence of several crystalline phases and varying melting points of even the same crystalline phase in different glass matrices.
Low melting Li2O-PbO-B2O3, Me2O-ZnO-B2O3, Me = Li, Na, K, Rb and CaO-ZnO-B2O3 glasses were studied with Raman and infrared spectroscopies to advance the structural understanding of zinc borate glasses as potential candidates for substitution of lead containing glasses. Although the effect of type of alkali ions on the number (N4) of fourfold coordinated boron (B4) in the glasses is small, the alkali ions direct the type of borate groups, i.e., pentaborate in lithium, sodium, and calcium zinc borate glasses, as well as diborate in potassium and rubidium containing ones. Both groups were simultaneously found in Li2O-PbO-B2O3. Alkali ions are mainly responsible for the formation of B4-units and metaborate. Zinc ions favorably compensate non-bridging oxygen and partially form ZnO4. With decreasing N4 and field strength of the alkali ions the atomic packing density, glass transition temper ature and Young’s Modulus also decrease. The coefficient of thermal expansion increases with decreasing N4.
Artificial bone replacement by individual customized three-dimensional resorbable bioactive glass has not yet been widely established in the clinical use. This is mainly due to the antagonism of sintering ability and suitable bioactivity. Competitive crystallization often prevents the generation of dense sintered bodies, especially for additive manufactured 3D structures. Previous studies of the fluoride-containing glass F3 have shown its potential to combine both sintering ability and suitable bioactivity. Furthermore, the occurring sintering blockade by surface crystallization of Na2CaSi2O6 was tunable by glass particle size.
In this study the glasses F3, F3-Cu with 1 mol% CuO added at the expense of CaO and the well-known 13-93 were chosen to determine the influence of surface crystallization on 3D printed sinter bodies. For this purpose, grain size fractions in range of smaller 32 µm to 315 µm in fraction size of 6-20 µm were sieved from jaw crushed glass frit as well as glass cubes were cut from casted blocks for all glasses. Sintering behavior of both pressed and printed powder compacts was observed via heating microscopy. Crystallization was determined by DTA and crystallization progress was monitored on fractured sinter bodies and polished cubes via electron and laser scanning microscopy as well as with diffractometry.
Depending on grain size the formation of crystalline support framework along former grain boundaries shows the capability to stabilize fully densified sinter bodies before softening. Beside of this, the generation of complex hierarchic porosity was possible as well.
Sintering, crystallization, and foaming of 44.8SiO2–2.5P2O3–36.5CaO–6.6Na2O–6.6K2O–3.0CaF2 (F3) and 54.6SiO2–1.7P2O3–22.1CaO–6.0Na2O–7.9K2O–7.7MgO (13–93) bioactive glass powders milled in isopropanol and CO2 were studied via heating microscopy, differential thermal analysis, vacuum hot extraction (VHE), Infrared spectroscopy, and time-of-flight secondary ion mass spectrometry. Full densification was reached in any case and followed by significant foaming.
VHE studies show that foaming is driven by carbon gases and carbonates were detected by Infrared spectroscopy to provide the major foaming source. Carbonates could be detected even after heating to 750◦C, which hints on a thermally very stable species or mechanical trapping. Otherwise, dark gray compact colors for milling in isopropanol indicate the presence of residual carbon as well. Its significant contribution to foaming, however, could not be proved and might be
limited by the diffusivity of oxygen needed for carbon oxidation to carbon gas.
We present an easy-to-apply method to predict structural trends in the internal nucleation tendency of oxide glasses. The approach is based on calculated crystal fracture surface energies derived from easily accessible diatomic bond energy and crystal lattice data. The applicability of the method is demonstrated on literature nucleation data for isochemically crystallizing oxide glasses.
Um die Langzeitbeständigkeit von Hochtemperatur-Festoxidbrennstoffzellen (SOFC) sicherzustellen, ist das grundlegende Verständnis der viskosen Schließung oder Heilung von Rissen, als Folge wechselnder thermischer oder mechanischer Belastung, in Gläsern und teilkristallinen Materialien ein entscheidender Faktor. In Glas ist die Rissheilung hauptsächlich durch viskoses Fließen bestimmt. In teilkristallinen Schmelzen bewirkt der kristalline Volumenanteil die Erhöhung der effektiven Viskosität. Um die Auswirkungen des kristallinen Volumenanteils auf die Rissheilung zu ergründen, wurden Glasmatrixkomposite mit variierten inerten kristallinen Fülleranteilen, die während der Wärme-behandlung konstant blieben, hergestellt. Verwendet wurde ein kristallisationsträges Natrium-Calcium-Silicatglas und ZrO2 als inerter Füller. Komplexe, reproduzierbare Rissstrukturen wurden durch Vickers-Eindrücke erzeugt und die viskose Rissschließung während isothermer Wärmebehandlungs-schritte mittels Laser-Scanning-Mikroskopie verfolgt. Die Untersuchungen zeigen, dass, verglichen zum füllerfreien Glas, der kristalline Phasenanteil die effektive Viskosität erhöht und dadurch großräumiges Fließen verlangsamt. Dies verzögert das Aufweiten der Risse. Dieser Effekt erschwert die Rissverkürzung und führt oftmals zu großen gerundeten Kavitäten und dadurch zu einer verzögerten Rissschließung. Wird dieses Aufweiten verringert, ist zunächst ein lokales viskoses Fließen der Restglasphase weiterhin gegeben, sodass sich die Risse sogar schneller schließen. Für kristalline Anteile > 27 Vol% bildet sich dann ein stabiles Perkolationsgerüst aus, das die weitere Rissschließung auch lokal unterbindet. Nur innerhalb größerer glasiger Bereiche ist hierbei noch eine Rissschließung zu beobachten. Ein Optimum der Risslängenverkürzung konnte bei 17 Vol% beobachtet werden.
Die Analyse von Bruchflächen wird in der Schadensanalyse meist auf der Basis von Erfahrungswissen vorgenommen, welches aus vorliegenden Untersuchungen, eigenen Vergleichsversuchen und aus der Literatur stammt. Durch Vergleiche mit bereits vorliegenden Bildern werden qualitativ Bruchmechanismen ermittelt. Grundlage sind zumeist zweidimensionale Bilder aus licht- und elekt-ronenoptischen Verfahren. Quantitative Aussagen beziehen sich bislang beispielsweise auf makroskopische Anteile von Bruchmechanismen oder die Ausmessung von Schwingstreifen. In jüngerer Zeit gibt es vermehrt Ansätze, Computer-Algorithmen einzusetzen, die in der Lage sind, unterschiedlich strukturierte Bruchmerkmale zu finden und zu klassifizieren. Im hier vorgestellten IGF-Vorhaben „iFrakto“, IGF Vorhaben Nr.: 21477 N, werden licht- und elektronenoptisch Topographie-Bilder erzeugt und die gewonnenen 3D-Informationen zusammen mit den klassischen 2D-Bildern ausgewertet.
The lecture focuses on the mechanisms of non-desired gas bubble formation and foaming during the sintering of glass powder compacts. It is shown that foaming is driven by carbon gases and that carbonates, encapsulated in micropores or mechaniacally dissolved beneath the glass surface, provide the major foaming source.
Crack growth velocity in alkali silicate glasses was measured in vacuum across 10 orders of magnitude with double cantilever beam technique. Measured and literature crack growth data were compared with calculated intrinsic fracture toughness data obtained from Young´s moduli and the theoretical fracture surface energy estimated from chemical bond energies. Data analysis reveals significant deviations from this intrinsic brittle fracture behavior. These deviations do not follow simple compositional trends. Two opposing processes may explain this finding: a decrease in the apparent fracture surface energy due to stress-induced chemical changes at the crack tip and its increase due to energy dissipation during fracture.
Barium silicate glass powders 4 h milled in CO2 and Ar and sintered in air are studied with microscopy, total carbon analysis, differential thermal Analysis (DTA), vacuum hot extraction mass spectroscopy (VHE-MS), Fourier-transformed infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). Intensive foaming of powder compacts is evident, and VHE studies prove that foaming is predominantly caused by carbonaceous species for both milling gases. DTA Shows that the decomposition of BaCO3 particles mix-milled with glass powders occurs at similar temperatures as foaming of compacts. However, no carbonate at the glass surface could be detected by FTIR spectroscopy, XPS, and TOF-SIMS after heating to the temperature of sintering. Instead, CO2 molecules unable to rotate identified by FTIR spectroscopy after milling, probably trapped by mechanical dissolution into the glass bulk. Such a mechanism or microencapsulation in cracks and particle aggregates can explain the contribution of Ar to foaming after intense milling in Ar atmosphere. The amount of CO2 molecules and Ar, however, cannot fully explain the extent of foaming. Carbonates mechanically dissolved beneath the surface or encapsulated in cracks and micropores of particle aggregates are therefore probably the major foaming source.
Sample preparation for analytical scanning electron microscopy using initial notch sectioning
(2021)
A novel method for broad ion beam based sample sectioning using the concept of initial notches is presented. An adapted sample geometry is utilized in order to create terraces with a well-define d step in erosion depth from the surface. The method consists of milling a notch into the surface, followed by glancing-angle ion beam erosion, which leads to preferential erosion at the notch due to increased local surface elevation. The process of terrace formation can be utilized in sample preparation for analytical scanning electron microscopy in order to get efficient access to the depth-dependent microstructure of a material. It is demonstrated that the method can be applied to both conducting and non-conducting specimens. Furthermore, experimental parameters influencing the preparation success are determined. Finally, as a proof-of-concept, an electron backscatter diffraction study on a surface crystallized diopside glass ceramic is performed, where the method is used to analyze orientation dependent crystal growth phenomena occurring during growth of surface crystals into the bulk.
Oriented surface crystallization on polished diopside glass surfaces has been studied with scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy and laser scanning microscopy.
An orientation preference of [001] parallel to the glass surface was detected for separately growing diopside crystals even as small as 700 nm in size. This finding shows that crystal orientation occurs in the outermost surface layer without crystal-crystal interaction and indicates that the crystal orientation is a result of oriented nucleation. Depending on surface preparation, monomodal crystal orientation distributions with [100] perpendicular to the surface or bimodal distributions with [100] and [010] perpendicular to the glass Surface were detected. It was also shown that the degree of crystal orientation increases with decreasing Surface roughness. The observed orientation of diopside crystals could be explained in terms of the interfacial energies of different crystal faces.
The suitability of an Al2O3 coating for corrosion protection on X20Cr13 was evaluated in various artificial geothermal brines, focusing on the influence of different pH (4, 6 and 8) and their chemical compositions on the coating properties. All experiments were performed in the laboratory using autoclaves at 150 ◦C and 1 MPa in deaerated condition for 1 and 7 days. Results showed that the pH of geothermal waters is the most detrimental factor in the transformation of ɣ-Al2O3 and its protective abilities. Delaminations were found in the Coating exposed to geothermal brines with pH 4. FTIR spectra indicated a transformation of ɣ-Al2O3 to boehmite AlOOH after exposure to pH 4 and 6, and bayerite Al(OH)3 was formed after exposure to pH 8. Different Crystal structures of the hydrated Al2O3 also contribute to the stability of the coatings, observed by the SEM- EDX of the surface and cross-section of coatings. This study indicated that ɣ-Al2O3 sol-gel coating presents a promising aspect of corrosion protection in geothermal environment with a neutral pH.
Since decades electric contacts based on silver metallization pastes are key components of photovoltaics and advanced microelectronics. For the metallization of commercial Si solar cells, high conductive silver glass pastes are cost effectively applicated by screen printing. Nevertheless, silver pastes are still one of the most crucial and expensive none Si materials in solar cells. Ever shorter time to market as well as increasing demands on reduced Ag consumption and line width require the targeted development of silver-glass-pastes with increased sinter ability and electrical conductivity. As a main difficulty, however, the liquid phase sintering of silver glass pastes is poorly understood so far.
In the present study, the influence of different network modifier in alkali-zinc-borate paste glasses on liquid phase sintering of silver-glass-pastes was investigated. Low melting X2O-ZnO-B2O3 glasses with X = Na, Li and Rb (abbr. LZB, NZB, and RZB) were utilized to prepare silver-glass-composites containing 30 %Vol glass. Shrinkage behavior of the silver-glass-composites compared with that of pure silver and pure glass powder compacts was studied with heating microscopy. The powder compacts were uniaxially pressed and heated at 5 K/min to the glass softening temperature. Glass transformation temperature and viscosity of the glasses were respectively measured with dilatometry and rotational viscometry. The thermal behavior of the pure glasses was analyzed with thermal analysis. Additionally, the contact angle of glass on pure silver foil was determined by means of heating microscopy between room temperature and 830 °C.
Thermal analysis of the alkali-zinc-borate-glasses under study has shown transformation temperatures between 450 °C (RZB), 460 °C (LZB) and 465 °C (NZB). For all glasses crystallization was found to start approximately at about 550 °C. However, different peak areas hint on a different degree of crystallization. Conformingly, the sintering behavior, measured in terms of area shrinkage, significantly differed for the silver-pastes under study. For silver-pastes with NZB or LZB-glass, sintering starts at 464 °C for NZB Ag pastes and at 451 °C for LZB Ag pastes and ends at 597 °C for NZB Ag paste and at 594 °C for LZB Ag paste. The sintering of the RZB Ag paste proceeds between 426 °C and 703 °C. The final densification was retarded possibly due to crystallization or swelling. The low sinter onset at 426 °C seems to correlate with the good wetting behavior of the RZB glass. Thus, the lowest apparent contact angle between the just densified powder compact sintered at a silver substrate was found for this glass. Moreover, microstructure analyses of the various composites indicate differences in silver dissolution and reprecipitation.
Network modifier ions can decisively influence properties and structure of low melting alkali-zinc-borate glasses and thus cause complex effects on the liquid phase sintering of silver-glass metallization pastes. This effect was studied for X2O-ZnO-B2O3 (X = Li, Na, Rb) glasses for silver-glass metallization pastes. Viscosity and the glass transition temperature, Tg, were measured with rotational viscometry and dilatometry. Dried model pastes with 30 vol% LZB, NZB or RZB glass were prepared for sintering studies by means of heating microscopy measuring the silhouette area shrinkage of uniaxially pressed powder compacts during heating at 5 K/min. For comparison, the silhouette area shrinkage of pure glass and silver powder compacts were determined. Glass-silver wetting was investigated during heating of bulk glass cylinders placed on silver substrates. Glass RZB turned out to have the lowest viscosity among the glasses under study. Its glass transformation temperature, Tg, was found at 444 °C and it caused the lowest sintering onset for its glass and paste powder compacts. Slightly increased values of Tg were found for NZB and LZB (468 °C and 466 °C, respectively) and a slightly retarded sintering was found for both paste powder compacts. These results indicate that liquid phase sintering of silver-glass pastes under air atmosphere is mainly influenced by glass viscosity.
Low melting zinc borate glasses awake interest to replace lead borate glasses in the silver metallization pastes for solar cells or microelectronics. In the current study, characteristic properties of alkali zinc borate glasses (X2O-ZnO-B2O3, X = Li, Na, K, Rb) were compared to an earth alkali zinc borate glass (CaO-ZnO-B2O3). Additionally, zinc oxide is partially substituted by lead oxide or cooper oxide in the borate glasses (Li2O-PbO-B2O3, Na2O ZnO CuO-B2O3). The alkali zinc borate glasses indicate less differences in Raman spectra, and thus in structural properties, in comparison to the Ca and Pb ions influence. LPbB (Tg = 401 °C) has a lower viscosity than LZB (Tg = 468 °C) and CaZB has the highest glass transition temperature (Tg = 580 °C). The Angell plot for the alkali zinc borate glasses shows a high fragility m = 80. Besides Tg, the density measured by means of the Archimedean principle, molar volume, and coefficient of thermal expansion (CTE) of the glasses were investigated. Trends could be found according to alkali ions or intermediate oxides. The density increases with decreasing alkali ion size from KZB (2.632 g/cm3) to LZB (2.829 g/cm3) and increases from LZB to LPbB (3.764 g/cm3). CTE ranges between 7.09 10-6 K-1 for CaZB and 11.5 10 6 K 1 for KZB and RZB. The differential thermal analysis (DTA) and X ray diffraction (XRD) indicate crystallization of various crystalline phases during heating with 5 K/min in most cases.
In many late-breaking research fields as in photovoltaics, microelectronics, nuclear waste glasses or at least mirror glasses silver diffusion in glasses is relevant to the issues of high-level functionality and recycling. The present study is focused on silver diffusion in innovative, low-melting alkali zinc borate glasses (X2O-ZnO-B2O3, X = Li, Na, K, Rb) potentially usable for silver metallization-pastes in solar cells. The glasses were coated with a thin metallic silver layer and heat treatments in air and nitrogen close to Tg at 470 °C for 2 h were performed. After heat treatment under air and nitrogen atmospheres the coating thickness, measured by a white light interferometer, was about 1.8 µm thick. Silver depth profiles determined by means of secondary neutral mass spectrometry (SNMS) indicate the fastest silver diffusion to a depth of 3.5 µm for Li2O-ZnO-B2O3 (LZB) glass. Nevertheless, the influence of the different alkali ions on the silver diffusion is small. The oxygen availability determines the silver diffusion into the glasses. The oxygen promotes the oxidation of the silver layer enabling Ag+ to diffuse into the glass and to precipitate as Ag0. Both species were detected by x-ray absorption spectroscopy (XAS). The precipitated metallic silver particles in Na2O-ZnO-B2O3 (NZB) glass have a mean size of 5.9 nm ± 1.2 nm diameter, which was determined using transmission electron microscopy (TEM). Phase separation in zinc-rich and zinc-poor phases with a mean diameter of 75 nm ± 20 nm occurred in NZB glass after heat treatment. Ion diffusion of the glasses into the silver layer was suggested by EDX-line scans.
Al2O3 has been widely used as a coating in industrial applications due to its excellent chemical and thermal resistance. Considering high temperatures and aggressive mediums exist in geothermal systems, Al2O3 can be a potential coating candidate to protect steels in geothermal applications. In this study, γ-Al2O3 was used as a coating on martensitic steels by applying AlOOH sol followed by a heat treatment at 600 °C. To evaluate the coating application process, one-, two-, and three-layer coatings were tested in the artificial North German Basin (NGB), containing 166 g/L Cl−, at 150 °C and 1 MPa for 168 h. To reveal the stability of the Al2O3 coating in NGB solution, three-layer coatings were used in exposure tests for 24, 168, 672, and 1296 h, followed by surface and cross-section characterization. SEM images show that the Al2O3 coating was stable up to 1296 h of exposure, where the outer layer mostly transformed into boehmite AlOOH with needle-like crystals dominating the surface. Closer analysis of cross-sections showed that the interface between each layer was affected in long-term exposure tests, which caused local delamination after 168 h of exposure. In separate experiments, electrochemical impedance spectroscopy (EIS) was performed at 150 °C to evaluate the changes of coatings within the first 24 h. Results showed that the most significant decrease in the impedance is within 6 h, which can be associated with the electrolyte penetration through the coating, followed by the formation of AlOOH. Here, results of both short-term EIS measurements (up to 24 h) and long-term exposure tests (up to 1296 h) are discussed.
Al2O3 coatings are often used as protective layers on steels against electrochemical and high-temperature corrosion because they are chemically inert and stable at elevated temperatures. This study presents preliminary work on the possibilities of using Al2O3 sol-gel coatings for corrosion protection of martensitic stainless steels in geothermal environments. Al2O3 sol-gel coatings were applied on UNS S42000, which is known to be susceptible to uniform and localized corrosion. The coated steel specimens were then tested in two types of artificial geothermal water, which simulate the geothermal fluids found in Sibayak (SBY), Indonesia, and North German Basin (NGB), Germany, respectively. SBY has pH 4 and 1.5 g/L of chloride ions, whereas NGB has a pH of 6 and 166 g/L of chloride ions. All experiments were carried out in autoclaves at 150 °C and 1 MPa under the deaerated condition. Evaluations were performed by investigating the surface profiles of both uncoated and coated steels before and after the corrosion test using a Laser Scanning Microscope (LSM) and Scanning Electron Microscope (SEM). Finally, Electrochemical Impedance Spectroscopy (EIS) was performed to compare the corrosion resistance of Al2O3 coated steels in SBY and NGB solutions. It was observed from the corrosion test that Al2O3 coatings are more suitable for use in the geothermal water with a higher pH.
A double cantilever beam technique in air equipped with ultrasound modulation was used to measure the crack velocity v in borate and silicate glasses. In all glasses v and the stress intensity KI followed the empirical correlation v ~ KIn. Indicated by its smallest KI at v = 1 μm s − 1, KI* = 0.27 MPa m0.5, the silicoborate glass containing 70 mol% B2O3 was found most susceptible to stress-corrosion enhanced crack growth. Contrarily, the sodium calcium magnesium silicate glass appeared least susceptible with KI* = 0.57 MPa m0.5. No clear correlation is evident between KI*, reflecting the stress-corrosion susceptibility, and the hydrolytic resistance for all glasses under study, but values of n obtained from the present study and taken from previous literature for 35 glasses tend to decrease with increasing network modifier ion fraction. Energy dissipation during stress-corrosion enhanced crack propagation is assumed to cause this trend.
Physical storage of gaseous hydrogen under high-pressure in glassy micro-containers such as spheres and capillaries is a promising concept for enhancing safety and the volumetric capacity of mobile hydrogen storage systems. As very low permeation through the container wall is required for storage of compressed hydrogen, development of glasses of minimal hydrogen permeability is needed. For this purpose, one has to understand better the dependence of hydrogen permeability on glass structure. The paper points out that minimizing the accessible free volume is as one strategy to minimize hydrogen permeability. Based on previously measured and comprehensive literature data, it is shown that permeation is independently controlled by ionic porosity and network modifier content. Thus, ionic porosity in modified and fully polymerized networks can be decreased equally to the lowest hydrogen permeability among the glasses under study. Applying this concept, a drop of up to 30,000 with respect to the permeation of hydrogen molecules through silica glass is attainable.
A heat treated glass that formed crystals under controlled conditions can be a glass ceramic. A proper understanding of crystallization in glassy melts is needed yet still lacking. Involved processes include the mobility of Elements within the glass during heat treatment and the reorganization of atomic bonds during crystallization. This change in coordination number of certain elements is easily observable in bulk glass samples during heat treatment with XAS. Our plan was to monitor these reorganization processes in situ at the immediate surface during heat treatment with PEEM to get time and spatial resolved data.
A summery of previous research regarding surface crystallization is given as well as results regarding thecurrent DFG project are presented.
To explore the impact of ambient and structural water on static fatigue, the initiation and growth of 3279 Vickers induced median radial cracks were automatically recorded and analyzed. We find that humidity is more efficient in initiating cracks and promoting their growth than water, which is dissolved in the glass structure. In particular for slow crack growth (< 3x10-6 m s-1), tests in dry nitrogen showed a considerable decrease in the crack growth exponent with increasing water content of the glasses. On the other hand, if tests were performed in humid air, the crack growth exponent was independent of the water content of the hydrous glasses, while stress intensity decreased slightly. These observations indicate that water promotes the processes at the crack-tip regardless of its origin. However, ambient water is more efficient.
High conductive silver-glass-metallization-pastes are key components in photovoltaics and advanced microelectronics. However, the underlying mechanisms of liquid phase sintering as silver dissolution, diffusion and reprecipitation are poorly understood so far.
In the current work, the influence of different network modifier in alkali-zinc-borate paste-glasses on liquid phase sintering of silver-glass-composites was studied. Therefore, silver-glass-composites containing 30 vol% glass were prepared, using low melting X2O-ZnO-B2O3 glasses with X = Na, Li, and Rb (NZB, LZB, and RZB). Glass transition temperature, viscosity, glass-silver wetting, crystallization and sintering behavior was studied by means of thermal analysis, dilatometry, heating microscopy and microscopy.
Similar glass transition temperatures of 450 °C (RZB), 460 °C (LZB) and 465 °C (NZB) were found by means of thermal analysis for glasses under study. Also, all glasses have a similar crystallization onset at about 550 °C, even though exhibiting with a different degree of crystallization.
Despite these similarities, however, the sintering behavior, measured in terms of area shrinkage, significantly differs for the composites. This finding indicates a different degree of silver dissolution. Assuming that dissolved silver reduces the viscosity, this effect could explain why glass crystallization starts at lower temperature in the composites. For example, the crystallization peak of LZB at 629 °C measured for pure glass powder compacts was decreased to 586 °C for the composite. Confirmatively, microstructure analyses indicate different degrees of silver dissolution, as e.g. revealed by different amount of silver precipitates within the residual glass phase, and reprecipitation. Best silver dissolution appeared for the RZB glass. Nevertheless, the final densification of RZB was retarded probably due to swelling and crystallization.
To understand the influence of water and alkalis on aluminosilicate glasses, three polymerized glasses with varying ratios of Na/K were synthesized [(22. 5-x)Na2O-xK2O-22.5 Al2O3-55 SiO2 with x = 0, 7.5, and 11.25]. Subsequently, these glasses were hydrated (up to 8 wt% H2O) in an internally heated gas pressure vessel. The density of hydrous glasses linearly decreased with water content above 1 wt%, consistent with the partial molar volume of H2O of 12 cm3/mol. Near-infrared spectroscopy revealed that hydroxyl groups are the dominant species at water content of <4 wt%, and molecular water becomes dominating at water content of >5 wt%. The fraction of OH is particularly high in the pure Na-bearing glass compared to the mixed alkali glasses. 27Al magic angle spinning-NMR spectroscopy shows that aluminum is exclusively fourfold coordinated with some variations in the local geometry. It appears that the local structure around Al becomes more ordered with increasing K/Na ratio. The incorporation of H2O reinforces this effect. The differential thermal analysis of hydrous glasses shows a significant mass loss in the range of glass transition already during the first upscan, implying the high mobility of water in the glasses. This observation can be explained by the open structure of the aluminosilicate network and by the low dissociation enthalpy of H2O in the glasses (≈ 8 kJ/mol). The effect of the dissolved H2O on the glass transition temperature is less pronounced than for other aluminosilicate glasses, probably because of the large fraction of Al in the glasses.
In the past few years, a new phase, Ba0.5Sr0.5Zn2Si2O7 with negative thermal expansion has been described in the literature. Some excess of SiO2 is necessary to produce glasses from which the Ba0.5Sr0.5Zn2Si2O7 phase can be crystallized.
Unfortunately, in such glasses usually surface crystallization occurs; however, the addition of nucleating agents such as trace quantities of platinum or relatively high quantities of ZrO2 is necessary to achieve bulk nucleation. These additional components also affect the crystal growth velocity, which furthermore is different for crystal growth from the surface and in the bulk. In this paper, three different chemical compositions containing different ZrO2 concentrations, where one composition additionally contains 100 ppm platinum, are studied with respect to their crystallization behaviour. Although the compositions do not differ much, the crystallization behaviour and also the Crystal growth velocities are surprisingly different.
Stable crack growth was measured for nominal dry and water-bearing (6 wt%) soda-lime silicate glasses in double cantilever beam geometry and combined with DMA studies on the effects of dissolved water on internal friction and glass transition, respectively. In vacuum, a decreased slope of logarithmic crack growth velocity versus stress intensity factor is evident for the hydrous glass in line with an increase of b-relaxation intensity indicating more energy Dissipation during fracture. Further, inert crack growth in hydrous glass is found to be divided into sections of different slope, which indicates different water related crack propagation mechanism. In ambient air, a largely extended region II is observed for the hydrous glass, which indicates that crack growth is more sensitive to ambient water.
Accelerator magnets that can reach magnetic fields well beyond the Nb-Ti performance limits are presently being built and developed, using Nb3Sn superconductors. This technology requires reaction heat treatment (RHT) of the magnet coils, during which Nb3Sn is formed from its ductile precursor materials (a “wind and react” approach). The Nb3Sn microstructure and microchemistry are strongly influenced by the conductor fabrication route, and by the Phase changes during RHT. By combining in situ differential scanning calorimetry, high Energy synchrotron x-ray diffraction, and micro-tomography experiments, we have acquired a unique data set that describes in great detail the phase and microstructure changes that take place during the processing of restacked rod process (RRP), powder-in-tube (PIT), and internal tin (IT) Nb3Sn wires. At temperatures below 450 ° the phase evolutions in the three wire types are similar, with respectively solid state interdiffusion of Cu and Sn, Cu6Sn5 formation, and Cu6Sn5 peritectic transformation. Distinct differences in phase evolutions in the wires are found when temperatures exceed 450 °C. The volume changes of the conductor during RHT are a difficulty in the production of Nb3Sn accelerator magnets. We compare the wire diameter changes measured in situ by dilatometry with the phase and void volume evolution of the three types of Nb3Sn wire. Unlike the Nb3Sn wire length changes, the wire diameter evolution is characteristic for each Nb3Sn wire type. The strongest volume increase, of about 5%, is observed in the RRP wire, where the main diameter increase occurs above 600 °C upon Nb3Sn formation.
Glass
(2019)
Overview about the key activities of division 5.6 glass and the diversity of the material glass. For the vacuum hot extraction method (VHE-MS) some examples are presented e. g. for the measurement of water content or the determination of diffusion coefficient of hydrogen in glasses. Finally, an outlook is shown on the robot controlled glass screening device which will be available next year.
Morphologies of Fresnoite surface crystals of BT0.75S glass under different atmospheric conditions
(2019)
Fresnoite glass-ceramics are characterized by piezoelectric, pyroelectric and non-linear optical properties. These properties can be adjusted by orienting the fresnoite crystals during crystallization. Crystallization begins at the surface. The first surface crystals are not oriented perpendicular and, therfore, produce a surface layer that has not the intended properties. To overcome this issue, the formation of fresnoite surface crystals at different atmospheric conditions was studied.
Glass powders are promising candidates for manufacturing a broad diversity of sintered materials like sintered ¬glass-ceramics, glass matrix composites or glass bonded ceramics with tailored mechanical, thermal, electrical and optical properties and complex shape. Its wide and precise adjustability makes this class of materials a key component for advanced technologies. Processing of glass or composite powders often allow even more flexibility in materials design. At the same time, however, processing can have substantial effects on the glass powder surface and sinterability. Thus, mechanical damage and surface contamination can strongly enhance surface crystallization, which may retard or even fully prevent densification. Whereas sintering and concurrent crystallization have been widely studied, partially as cooperative effort of the TC7 of the ICG, and although glass powder sintering is predominantly applied for glasses of low crystallization tendency, sintering is also limited by gas bubble formation or foaming. The latter phenomenon is much less understood and can occur even for slow crystallizing glass powders. The lecture illustrates possible consequences of glass powder processing on glass sintering, crystallization and foaming.
Due to the stochastic nature of crack nucleation by Vickers indentation, a statistical analysis of propagation rates of 185 radial cracks was performed. Crack growth was observed directly using a video camera with high Image acquisition rate. It is found that propagation rates are controlled by the environmental reactions at the crack-tip shortly after their initiation (< 1 s). Calibration of the stress intensity KI showed that the residual stress factor χ and the exponent n of the equation KI==χPc−n (with P==load and c==crack length) are broadly distributed among the 185 analyzed cracks, ranging from 10−16 to 104 and from 0.1 to 5, respectively. For the most frequent crack, the equation KI==0.052Pc−1.47 holds. The results show that correlations of indentation-induced crack length to stress intensity necessitate the use of statistical significant data that are calibrated by the environmental reactions at the crack-tip.
Hydrogen gas diffusivity of fourteen glasses of the Na2O-Al2O3-SiO2 system are studied along the joins quartzalbite-jadeite-nepheline (Qz-Ab-Jd-Np, fully polymerized) and albite-sodium disilicate (Ab-Ds, depolymerized).
Density measurements show that ionic porosity decreases from 54.4% (Qz) to 51.5% (Np) and from 52.4% (Ab) to 50.2% (Ds). Hydrogen diffusivity D follows similar trends but at another scale. D at 523 K decreases from 4×10−12 to 3×10−14m2 s−1 (Qz-Np) and from 4×10−13 to 3×10−15m2 s−1 (Ab-Ds). Charge compensating Na+ acting as a filling agent in fully polymerized network structures leads to up to one order of Magnitude higher diffusivities as depolymerized glass structures of the same SiO2 content where Na+ takes the role of a
modifier ion. Temperature dependence of the diffusivity indicates that both the activation energy involved with the moving H2 molecule as well as the accessible volume in the structure contribute to this compositional trend.
The crystallization behavior of sodium ion conductive Na2MnP2O7 glass was examined to clarify the crystallization mechanism. The formation of thermodynamically metastable phase, layered Na2MnP2O7, at the surface of the glass occurred. Heat treatment at 430 °C for 3 h lead to surface crystals of Na2MnP2O7 oriented with the (101) direction perpendicular to the sample surface. As the heat treatment temperature increased, the glass-ceramic samples deformed, and the presence numerous micro bubbles due to dissolved water was detected.
Glass strength and fatigue is limited by surface cracks. As subcritical crack growth (SCCG) is governed by ambient humidity, stress corrosion at the crack tip is widely accepted to be the underlying mechanism. However, as water is known to have decisive effect on glass properties and can rapidly enter the crack tip near glass region, SCCG could be affected by such water related phenomena. We tried to mimic these effects studying water dissolution and speciation, mechanical properties, and SCCG in water-bearing glasses. For this purpose, glasses up to 8 wt% water have been prepared by means of high-pressure melting of glass powder - water mixtures.
As part of this effort, SCCG in dry and hydrous commercial micros¬cope slide glass (CW = 6 wt%) was studied in double cantilever beam (DCB) geometry and sub-Tg relaxation was measured by Dynamic Mechanical Analysis (DMA).
For SCCG in ambient air (24% r.h.), SCCG was promoted by the presence of 6wt% bulk water with respect to the dry glass. On the other hand, stress intensity values, KI, required to cause slow crack growth (v < 10-6 ms-1) resemble literature findings for float glass of similar composition in liquid water, which might represent the maximum possible promoting effect of ambient water on SCCG.
For SCCG in vacuum (10-3 mbar), dissolved bulk water causes even more pronounced effects. Most strikingly, it strongly decreases the slope of the log v(KI)-curve, which is a measure of dissipated energy during fracture. A strong increase of sub-Tg relaxation with increasing water content was confirmed by DMA. As a consequence, slow crack growth occurs at KI values as measured in the dry glass whereas fast crack growth occurs at much larger KI than that of the dry glass. Kinks and shoulders shown by the inert log v(KI)-curve indicate that bulk water does not simply affect bulk mechanical properties.