Additive manufacturing (AM) of alkali-activated materials is a promising method for producing ceramic precursors, construction elements and other parts. A recently introduced AM process is laser-induced slip casting of lithium aluminate/microsilica slurries, which yields parts with excellent mechanical strengths. To clarify the underlying mechanisms, μ-Raman spectroscopy was applied to parts produced by the process, and the dissolution and hydration of lithium aluminate was studied inter alia using conventional and in-situ X-ray diffraction. The results show that significant dissolution of lithium aluminate occurs, particularly at increased temperatures during laser interaction, which leads to an increase of pH and precipitation of an akopovaite-like Li-Al-CO3 layered double hydroxide. The increase of the pH is likely to induce dissolution of the microsilica and possibly formation of a hydrous lithium aluminosilicate gel. These observations explain the strength evolution of the studied parts and can also aid the development and improvement of related AM methods.
Several studies show that thermal and hydrothermal treatment can further improve the excellent properties of UHPC in terms of mechanical strength and durability. While for the thermal treatment the increase in strength is attributed to an intensified pozzolanic and hydraulic reaction, for the hydrothermal treatment previous studies accredited it mostly to the formation of tobermorite. In the presented study thermal and hydrothermal treatment of UHPC samples was systematically varied and the phase formation analysed related to the strength development of a reference sample cured for 28 days in water. For the thermal treatment the results show that the strength increase depends on the protection against desiccation and can be ascribed to an improved pozzolanic reaction of the siliceous fillers. To achieve a significant enhancement of strength, a pre-storage time of few days and a long dwell time at elevated temperature/pressure are required. For the hydrothermal treatment already heating the specimens up to 185 C in saturated steam followed by an immediate cooling leads to a substantial increase in compressive strength. Pre-storage time did not affect the result as far as a minimum of several hours is guaranteed. The improved performance is due to an increase in the pozzolanic and hydraulic reaction. Surprisingly, tobermorite was only found within a very thin layer at the surface of the sample, but not in the bulk. Sulphate and aluminium stemming from the decomposition of the ettringite are bound in the newly formed phases hydroxylellestadite and hydrogarnet.
Polycarboxylate ethers (PCEs) are widely used in construction, but the exact nature of their interaction with cement is still debated. Aiming at a better understanding of the role of tricalcium aluminate (C3A) in cement hydration, we assessed the potential of optical spectroscopy in combination with a water-soluble fluorescent organic reporter dye (S0586) to monitor the early hydration of C3A in the presence of 26 wt% CaSO4·2H2O (C3A26G-S) with and without PCE. As optical methods, steady-state fluorescence and diffuse reflectance (UV–VisDR) spectroscopy were employed. Phase characterization and particle size distribution were performed with in-situ X-ray diffraction (in-situ XRD) and dynamic light scattering (DLS). Our results show that fluorescence and UV–VisDR spectroscopy can be used to monitor the formation of metastable phases by the disaggregation of the dye S0586 in a cement paste as well as changes in ettringite formation. Addition of PCE slowed down the disaggregation of the dye as reflected by the corresponding changes of the dyes absorption and fluorescence. This prolonged induction period is a well-known side effect of PCEs and agrees with previous reported calorimetric studies and the inhibition of gypsum dissolution observed by in-situ XRD. This demonstrates that fluorescence and UV–VisDR spectroscopy together with a suitable optical probe can provide deeper insights into the influence of PCE on C3A-gypsum hydration which could be e.g., utilized as screening method for comparing the influences of different types of PCEs.
Slags from the nonferrous metals industry have great potential to be used as feedstocks for the production of alkali‐activated materials. Until now, however, only very limited information has been available about the structural characteristics of these materials. In the work presented herein, synthetic slags in the CaO–FeOx–SiO2 system, representing typical compositions of Fe‐rich slags, and inorganic polymers (IPs) produced from the synthetic slags by activation with alkali silicate solutions have been studied by means of X‐ray absorption near‐edge structure (XANES) spectroscopy at the Fe K‐edge. The iron in the slags was largely Fe2+, with an average coordination number of approximately 5 for the iron in the amorphous fraction. The increase in average oxidation number after alkali‐activation was conceptualized as the consequence of slag dissolution and IP precipitation, and employed to calculate the degrees of reaction of the slags. The degree of reaction of the slags increased with increasing amorphous fraction. The iron in the IPs had an average coordination number of approximately 5; thus, IPs produced from the Fe‐rich slags studied here are not Fe‐analogs of aluminosilicate geopolymers, but differ significantly in terms of structure from the latter.
Powder x-ray diffraction is a time consuming and challenging task, especially for sensitive phases like ettringite and C-S-H phases. Fine-grained ultra-high performance concrete (UHPC) with an average grain size < 100 μm could be investigated straightly without time-consuming milling. As a proof of concept, small UHPC cylinders with plain surface were investigated with a newly designed sample holder. The comparison with conventionally prepared powder shows the reliability of this approach. As a great benefit, a depth dependent analyses, as well as a comparison of surface layers and core material, were carried out.
This contribution presents the results of structural and compressive strength investigations on cured and high-temperature treated silica-based one-part geopolymer-zeolite composites. The specimens were synthesized from two different silica sources, sodium aluminate and water. The phase content as well as the compressive strength of the cured composites varied depending on the starting mix-design and the silica feedstock. Besides geopolymeric gel, A-type zeolites and hydrosodalites were the major reaction products. One of the silica feedstocks yielded significantly higher compressive strength (19 MPa), while the other one appears to cause less variation in phase content. Strength testing indicated an improvement on heating up to 200–400◦C (28 MPa) followed by a moderate decrease up to 700◦C. Above 700◦C the sys- tems underwent new phase formation and shrinkage (volume decrease) deformations. After exposure at 1000◦C the different mixes consisted of a mix of several stuffed silica phases, almost pure hexago- nal nepheline or amorphous phase. Depending on the mix-design, the onset temperature of the high temperature phase transformations varied.
The structural environment around Y in silicate and aluminosilicate glasses containing 5000 ppm Y was investigated as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The used glass compositions were taken from Prowatke and Klemme (2005) varying in the aluminum saturation index (ASI, molar ratio of Al2O3/(Na2O+K2O+CaO)) from 0.115 to 0.768. Furthermore, a set of glass compositions from the system CaO–Al2O3–SiO2 (CAS) was used, for which structural data from computer simulations are available (Haigis et al., 2013–this issue). Structural parameters of the Y–O pair correlation of the first coordination shell were determined from the EXAFS based on a gamma-like distribution function that accounts for the large static disorder and non-Gaussian pair distribu- tions. The analysis shows an increase in the coordination number from 6 to 8, along with an increase of the average Y–O distance by 0.13 Å for the composition of Prowatke and Klemme (2005). For the CAS-composition an increase of the coordination number from 6 to 7, along with an increase of the average Y–O distance by 0.06 Å is obtained. The change of these parameters is associated with a considerable increase in the asymmetry and width of the Y–O pair distribution. Due to its size and charge, 6-fold coordinated Y will preferentially bond to non-bridging oxygens of the polymeric melt network to form a stable configuration, as is the case for the less polymerized melts with low ASI. In highly polymerized melts with ASI values close to one, 6-fold coordination of Y is not possible because almost only bridging oxygens are available. Consequently, over-bonding of bridging oxygens around Y is counterbalanced by an increase of coordination number and Y–O distance to satisfy local charge balance requirements.
Element partitioning depends strongly on composition and structure of the involved phases. In this study, we use molecular dynamics simulations to investigate the local environment of Y as an exemplary trace element in four silicate melts with different compositions and thus varying degrees of polymerization. Based on these structural results, we propose a mechanism which explains the observed partitioning trends of Y and other rare-earth elements between crystals and melts or between two melts. With our computational approach, we found a systematic correlation between melt composition and Y coordination as well as Y―O bond lengths, a result which was corroborated by EXAFS spectroscopy on glasses with the same compositions as the simulated melts. Our simulations revealed, moreover, the affinity of Y for network modifiers as second-nearest neighbors (Ca in this study) and the tendency to avoid network formers (Si and Al). This is consistent with the observation that Y (and other rare-earth elements) in general prefer depolymerized to polymerized melts in partitioning experiments (see, e.g., Schmidt et al. (2006)). Furthermore, we used the method of thermodynamic integration to calculate the Gibbs free energy which governs Y partitioning between two exemplary melts. These more quantitative results, too, are in line with the observed partitioning trends.
Powder X-ray diffraction (XRD) is a time consuming and challenging task, in particular if a comprehensive series of experiments have to be investigated. The quality and significance of the resulting diffractogram is strongly dependent on the kind of preparation, e.g. type of mill, grain size, packing or ordering of the grains, besides the technical issues of XRD. In principle, prepar- ing a fine grained powder is required to achieve good statistics of every crystal orientation, if the starting material is a coarse aggregate concrete. Fine grained concrete like ultra-high performance concrete (UHPC) with average grain size smaler than 50 µm could be investigated straightly without milling. In this study, small UHPC cylinders were investi- gated with different preparations methods (milling and cutting or grinding with water or petroleum). Powders were measured in the conventional man- ner, while solid cylinders were investigated with the new designed sample holder. The results show a significant influence of the preparation on the susceptible phases like portlandite and ettringite. Besides difference in phase content, good count- ing statistics and a low signal to noise ratio were achieved. Finally, the new designed sample holder provides a reliable and fast method of investiga- tion fine grained concrete without time consuming preparation.
The alkali-silica reaction (ASR) is a harmful process, which can occur in concrete. As product of this chemical reaction a swellable alkali-silica gel can exert pressure leading to cracking of the material. The damages induced by ASR can differ in their magnitude, and to assign certainly that ASR is the main damage mechanism a microscopic investigation is necessary. Therefore this article presents a study on concrete affected by ASR with the purpose of assessing the degree of damage and evaluating various alkali-sensitive aggregates. For texture analysis thin section petrography is the major tool. Optical microscopy offers the best way to analyse ASR-affected concrete samples for damage classification by visualising the gel and the crack formation. The chemical investigation of the gel enables a precise determination of the involved amounts of the major oxides SiO2, CaO, Na2O and K2O. The proportions of these components of the gel are decisive with regard to its ability to swell. For this analysis scanning electron microscopy coupled with energy dispersive X-ray spectroscopy was utilised. Furthermore micro X-ray fluorescence analysis showed elemental distributions, especially the depletion or enrichment of elements involved in ASR.
Knowledge of the local structure around rare earth elements (REE) in aluminosilicate melts is of great interest for the geochemistry of magmatic processes, particularly for understanding the partitioning of REE between melt and the coexisting crystals in a more comprehensive way. Several studies already proposed a significant influence of the melt composition on REE fractionation. In a fundamental study Ponader & Brown (1989) showed that the local environment around La, Gd and Yb changes with polymerization of the melt, this was used to explain differences in element partitioning. However, no direct correlation between partitioning data and structural informations was provided. In this study, melt compositions taken from Prowatke & Klemme (2005) and various halpogranitic, -basaltic compositions were doped with La, Gd, Yb or Y and synthesized as glass. EXAFS was used to gather quantitative information on the local environment of Gd, Yb and Y and XANES for qualitative information on the local environment of La in the glasses. Additional high temperature (HT) in situ Y-EXAFS was performed to prove, if the local structure of Y above TG corresponds to the local structure in the quenched melts. Analysis of the EXAFS data shows that the average bond length, the width and skewness of the REE-O pair distribution function increase with increasing polymerization of the melt for Gd, Yb and Y. XANES spectra confirm a similar trend for La. The HT Y-EXAFS shows no significant change of the local structure above TG . Finally, correlations of structural parameters and partitioning data from Prowatke & Klemme (2005) were obtained.
Core level spectroscopy has gained interest in earth science in the last decade because it provides deep insight into the electronic structure of elements in mineral phases and thus insights to their coordination environment. The electronic structure of rare earth elements (REE) is mainly determined by the interactions between electrons in the localised 4f and in the broad 5d bands. This is probed by L-edge XANES spectroscopy where the main absorption edge arises from 2p to 5d dipole transitions, whereas the fine structure at energies well above the edge is related to scattering of the created photoelectron by neighbouring atoms. In resonant inelastic x-ray scattering (RIXS) a core-electron is promoted to an excited state just as in XANES but also the energy dependence of the scattered or emitted photon is measured, which gives additional information about the intermediate and the final state. The final state, results in a M-core-hole whose lifetime broadening defines the energy resolution.
Consequently, one may acquire the pre-edge fine structure, which corresponds to 2p to 4f quadrupolar transitions, with strongly enhanced resolution. 2p3d-RIXS spectra have been collected for the model compounds of La, Gd, Yb and for silicate glasses doped with REE (2wt% of La, Gd or Yb). Glass compositions are those of Prowatke and Klemme (2005) as well as simplified natural compositions like haplogranite and haplobasalt. The RIXS spectra for both, model compounds and glasses show slightly different pre-edge features. For the model compounds the main spectral features are reproduced employing the FEFF 9 code in combination with the Missing Code. For the glasses, the extracted RIXS spectra show an increase of the pre-edge intensity and a slight shift of the maximum to higher energies with increase of ASI (aluminium saturation index) or polymerisation. For Yb, these changes correlate well with changes in the coordination as observed by our EXAFS study. This implies, that changes in 2p3d-RIXS may be used to trace changes in coordination of REE in silicate melts.
It is generally accepted that the partitioning of trace elements is controlled by temperature, pressure and crystal chemistry. However, among others, the results of Prowatke & Klemme (2005) on trace element partitioning between melt and titanite, which varied over several orders of magnitude, suggest strong effects of the melt composition on partitioning. Ponader & Brown (1989) reported that the coordination of rare earth elements (REE) in quenched melts changes with the degree of polymerization of the melts. However, their data do not allow for a direct correlation between element coordination and partitioning of trace elements.
Here we report melt compositions taken from Prowatke & Klemme (2005) that were doped with selected REE (0.5 wt%) and synthesized as glasses. EXAFS was used to constrain the pair distribution function (PDF) of the neighbouring atoms of the REE. Due to the large static disorder around the absorber, the REE – O correlation was described using an asymmetric PDF based on a gamma-like function. The obtained results for the glasses show a increase of the average REE – O distance (e.g. Y – O: 2.27 Å to 2.39 Å ± 0.01 Å; Yb – O: 2.23 Å to 2.37 Å ± 0.01 Å), an increase of coordination number (Y: 6 to 8; Yb: 6 to 7) and a significant increase in the asymmetry of the PDF with increasing alumina saturation index (ASI) of the glass.
Furthermore, in-situ high temperature EXAFS on melts were performed to check for possible changes in local structure during quenching. In conclusion, our data show that Y and Yb preferentially bond to non-bridging oxygens in the melt structure, which decrease with increasing ASI. Consequently, over-bonding of bridging oxygens around Y is counterbalanced by an increase of coordination number and Y-O distance to satisfy local charge balance requirements. The adaptations of the trace element coordination to the change in melt composition and structure leads to an increase of the incompatibility of REE in melts.
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Element partitioning depends strongly on composition and structure of the involved phases. In this study, we use molecular dynamics simulations to investigate the local environment of Y as an exemplary trace element in four silicate melts with different compositions and thus varying degrees of polymerization. Based on these structural results, we propose a mechanism which explains the observed partitioning trends of Y and other rare-earth elements between crystals and melts or between two melts. With our computational approach, we found a systematic correlation between melt composition and Y coordination as well as Y―O bond lengths, a result which was corroborated by EXAFS spectroscopy on glasses with the same compositions as the simulated melts. Our simulations revealed, moreover, the affinity of Y for network modifiers as second-nearest neighbors (Ca in this study) and the tendency to avoid network formers (Si and Al). This is consistent with the observation that Y (and other rare-earth elements) in general prefer depolymerized to polymerized melts in partitioning experiments (see, e.g., Schmidt et al. (2006)). Furthermore, we used the method of thermodynamic integration to calculate the Gibbs free energy which governs Y partitioning between two exemplary melts. These more quantitative results, too, are in line with the observed partitioning trends.
The structural environment around Y in silicate and aluminosilicate glasses containing 5000 ppm Y was investigated as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The used glass compositions were taken from Prowatke and Klemme (2005) varying in the aluminum saturation index (ASI, molar ratio of Al2O3/(Na2O+K2O+CaO)) from 0.115 to 0.768. Furthermore, a set of glass compositions from the system CaO–Al2O3–SiO2 (CAS) was used, for which structural data from computer simulations are available (Haigis et al., 2013–this issue). Structural parameters of the Y–O pair correlation of the first coordination shell were determined from the EXAFS based on a gamma-like distribution function that accounts for the large static disorder and non-Gaussian pair distribu- tions. The analysis shows an increase in the coordination number from 6 to 8, along with an increase of the average Y–O distance by 0.13 Å for the composition of Prowatke and Klemme (2005). For the CAS-composition an increase of the coordination number from 6 to 7, along with an increase of the average Y–O distance by 0.06 Å is obtained. The change of these parameters is associated with a considerable increase in the asymmetry and width of the Y–O pair distribution. Due to its size and charge, 6-fold coordinated Y will preferentially bond to non-bridging oxygens of the polymeric melt network to form a stable configuration, as is the case for the less polymerized melts with low ASI. In highly polymerized melts with ASI values close to one, 6-fold coordination of Y is not possible because almost only bridging oxygens are available. Consequently, over-bonding of bridging oxygens around Y is counterbalanced by an increase of coordination number and Y–O distance to satisfy local charge balance requirements.
Trace element partitioning strongly depends on major element composition of the involved phases. Whereas the influence of crystal chemistry is well described by the lattice strain model, the role of melt composition and structure is still poorly understood. In experiments with immiscible silicate melts, Schmidt et al. (2006) observed partitioning of Y and rare earth elements into the more depolymerized (silicapoor) melt, where the tetrahedral network is partially destroyed. An explanation of these findings in terms of atomic-scale structure is still missing. We performed molecular dynamics (MD) simulations to investigate the local environment of Y as a trace element in different silicate melts of the system CaO-Al2O3-SiO2. The interactions between atoms were described by a new polarizable ion model which also captures many-body effects and was parametrized non-empirically, using density functional theory data as a reference. As a result, we found a systematic dependence of the atomic-scale structure around Y on melt polymerization, which can be quantified by means of the ratio of non-bridging oxygen to the network forming cations Si and Al (NBO/(Si+Al)): Upon increasing NBO/(Si+Al) from 0.0 to 1.9, the coordination of Y by O drops from 7.7 to 6.2, and the average Y-O distance decreases from 2.56Å to 2.46Å. Moreover, using the method of thermodynamic integration, we obtained first results for the exchange coefficient (ratio of partition coefficients) DYmelt1/melt2 / DAl melt1/melt2 of Y and Al between two melts.
To validate our structural findings, we performed a joint theoretical and experimental study on extended x-ray absorption fine structure (EXAFS) spectra of Y in silicate melts. These spectra contain information about the coordination of Y and distances to neighbouring atoms. Traditionally, a structural model is refined until the measured spectrum is satisfactorily reproduced. However, for amorphous systems it is often difficult to make an appropriate guess for the structure, and hence we propose a converse approach. We modelled EXAFS spectra by averaging over many MD snapshots, and the agreement with experimental spectra confirms that our model faithfully reproduces the atomic-scale environment of Y.
Knowledge on the local structure around rare earth elements (REE) in aluminosilicate melts is of great interest for the geochemistry of magmatic processes, particularly for understanding the partitioning of REE between melt and coexisting crystals in a more comprehensive way. However, the high-temperature (HT) local structure around REE in melts is still not well constrained because in situ characterization at high temperature is a technically challenging task. Therefore, the local structure of REE in melts is usually studied in glasses, with the assumption that the local structure in glasses corresponds to the structure above the glass transition temperature (TG) in the melt. Several authors reported that this is not invariably the case depending on the element of interest. The scope of this work is to investigate in situ the local structure of yttrium in silicate and aluminosilcate melts and thus to verify the results Simon et.al (2013) on the local structure of yttrium in aluminosilicate glasses, particularly the dependence of the local environment on melt composition.
The studied melt compositions ASI200 and ASI60 were taken from Prowatke & Klemme (2005), synthesized from oxides and carbonates and doped with 2wt% Y. To collect high temperature EXAFS spectra the glass samples was fixed in the crunch of a resistance heater and heated stepwise to temperatures above TG (max. 1200 K). The spectra at the Y K-edge (17.038 eV) were collected at beamline C. A Si(111) fixed-exit two-crystal monochromator was used. The spectra were analyzed using the software Athena and Artemis. Amplitudes and phase shift were calculated using Feff6 and checked on crystalline Y2O3. For Y2O3sub>, HT-EXAFS spectra were collected to test the fitting model at HT and to verify backscattering phase shifts and amplitudes obtained by FEFF. Due to large static disorder and non-Gaussian pair distributions in melts and glasses an asymmetric gamma-like distribution function was used to model the Y-O pair correlation of the first coordination shell in the glasses/melts.
The k3-weighted EXAFS spectra of every temperature step of ASI200 and ASI260 are shown in Fig. 1 together with the resulting fit. The EXAFS amplitude decreases considerably from room temperature up to 1173 K for both compositions. ASI200 shows a stronger decrease of the amplitude with increasing temperature. The average bond length is plotted versus temperature in Fig. 2. The obtained average bond length shows a linear increase to 968 K for ASI200 and to 1024 K for ASI260, the change of the slope at this temperature indicates TG. The analysis of the EXAFS data shows that there are no major changes in the local structure during the transition from glass to melt. The average bond length, the skewness and the asymmetry of the pair distribution function of Y-O increase according to the thermal expansion in the glassy state and to increase of disorder above TG. We were able to show that the local structure above TG correspond well to the one of yttrium in quenched melts.
Insights to the local structure of atoms in aluminosilicate melts is of great interest for geochemistry, petrology, ceramic and glass technology. In particular for understanding chemical fractionation in crystal-melt equilibria, the local structure of rare earth elements (REE) in dry and hydrous aluminosilicate melts may provide fundamental information and thus a more detailed view on the formation processes of igneous rocks on earth, moon and terrestrial planets. Previous investigations by Simon et. al. (2012) by EXAFS on the local structure around Y in aluminosilicate melt showed that the coordination of Y (representative for heavy REE) is a function of the melt composition and depends on melt polymerisation. They were able to show that the coordination number, the width of the Y-O pair distribution and the average bond length increases with increasing polymerisation.
Investigation of the electronic structure of REE in these melts may provide additional information on the local structure. A useful tool to probe the electronic structure of 4f-elements is resonant inelastic x-ray scattering (RIXS). In RIXS a core-electron is promoted to an excited state just as in XANES but also the energy dependent intensity of the scattered and emitted photons are measured, which gives additional information about the intermediate state, the final state, especially at energies of the pre-edge fine structure, which corresponds to 2p to 4f quadrupolar transitions in the case of lanthanides. The scope of this study is to investigate the electronic structure of Gd in aluminosilicate glasses in order to gain insight into changes in the Gd coordination as a function of glass composition.
The studied ASI glass compositions were taken from Prowatke and Klemme (2005) varying in the aluminium saturation index (ASI, molar ratio of Al2O3/(Na2O+K2O+CaO)) from 0.115 to 0.768, dry haplogranitic (HPG) compositions (ASI = 0.6 to 1.2) and hydrous HPG compositions (4 wt% H2O). The glasses were synthesized from oxides and carbonates and doped with 2 wt% Gd. The hydrous HPG compositions were synthesized at 2 kbar in an internally heated pressure vessel (IHPV). 2p3d-RIXS and XANES have been collected for Gd2O3 and several Gd-doped aluminosilicate glasses at beamline W1 employing a Johann Spectrometer with a Rowland circle of 1 m and using a spherically bent Si(620) analyzer crystal.
The collected 2p3d RIXS of the pre-edges for ASI glasses, the dry and hydrous HPG glasses show weak but distinct pre-edge features, which are probably related to quadrupolar transitions. The maximum of the pre-edge shows a slight shift to higher energy transfer (ET) with increase of the polymerisation for the ASI, the dry and the hydrous HPG glass compositions, respectively. In contrast, the maximum of the pre-edge for the hydrous HPG is shifted to lower ET compared to the dry HPG for a given composition. The preliminary results indicate that differences in the electronic structure are related to slight differences in intra-atomic multiplet splitting and thus to differences in chemical bonding and Gd site symmetry. The spectra extracted at constant excitation energy (over 2 eV) along the energy transfer axis for the different glass compositions (Fig. 3) show a more quantitative view of the pre-edge shift. The results obtained by a Gaussian fit of the pre-edge maximum show that the position of the maximum for ASI glasses shift by 0.3±0.03 eV from ASI 200 to ASI280, by 0.3±0.03 eV from the dry HPG06 to HPG12 and by 0.2±0.03 eV from the hydrous HPG06 to HPG12. Furthermore, the position shifts by 0.1±0.03 eV between dry HPG06 and hydrous HPG06, for hydrous HPG10 and HPG12 no difference was noticeable. The observed observed changes go along with structural differences obtained by EXAFS.
High-field-strength elements (HFSE), like Zr and Hf, are important trace elements that provide important information about magmatic processes and mass transfer in the Earth’s interior. The formation of silicate melts in the Earth’s crust and the upper mantle and their interaction with the surrounding host rocks are important processes for the evolution of the lithosphere. The geochemical distribution of trace elements between minerals, silicate melts and aqueous solution is controlled by their structural incorporation in the phases at equilibrium. While the effect of the structure and composition of crystalline phases on the chemical partitioning is understood quite well, the effect of the melt composition and structure is far less well known. Here, we investigated the effect of silicate melt composition on the local environment of Hf. XAFS measurements at the L3-edge of Hf in quenched melts of compositions that differ strongly in composition, and structure. In addition, a couple of Hf- bearing minerals with known crystal structure were measured in order to test the simulation of XANES spectra by FEFF9.0.
The EXAFS spectra were collected at beamline C using a Si (311) double-crystal monochromator. Spectra were recorded in fluorescence mode. Glass compositions reported on here are NS3 (Na2Si3O7), and albite (NaAlSi3O8). Glasses were doped with 8000 ppm and 2 wt% of Hf. K3-weighted EXAFS spectra and their Fourier transforms are plotted in figure 1 for the two glasses. The spectra indicate already differences for the local structural environment between the studied compositions. The amplitude of the EXAFS and consequently, the first maximum of the Fourier transform (FT) is much higher for NS3 (partially depolymerised glass). Fitting of the EXAFS using the harmonic approximation yields a Hf-O distance of 2.046 ± 0.006 Å and 6 neighbors for the NS3 glass and only 1.999 ± 0.007 Å and 4 neighbors for albite. For NS3, significant contribution by multiple scattering indicates a regular octahedral coordination. For albite, the unrealistic low number of neighbors and short Hf-O indicate significant anharmonic effects due to a high degree of static disorder for Hf. First attempts to fit asymmetric pair-distribution functions show considerable improvement for the structural parameters. The observed difference between the two samples is consistent with their difference in structural properties. Large cations such as Hf prefer bonding to non-bridging oxygens, which are (almost) not present in the fully polymerized albite glass.
XANES spectra of model compounds are shown. Compounds shown are armstrongite CaZrSi6O15*3(H2O), eudialyte Na4Ca15Ce0.5Fe0.6Mn0.3Y0.1ZrSi3O22(OH)Cl0.5 and wadeite K2ZrSi3O9 that contain Hf as a minor component. Hf replaces Zr in these minerals and is 6 fold coordinated. Spectra of all three compounds show a very strong white line with split peak at the maximum. At higher energies, the spectra of the minerals differ slightly probably owing to the difference in composition and structure. Spectra simulated based on structural data from the literature using FEFF9 describe the spectral features reasonably well, especially for wadeite. Discrepancies may be related to the simplifications implicit to FEFF or due to the fact that the structural data taken from the literature do not match the measured sample.
The scope of this study is the local structure around Rare Earth Elements (REE) in quenched alumino- silicate melts. XANES, EXAFS and RIXS measurements at room temperature were performed at the La, and Gd L3 edge for model compounds ([6]La2O3, [9]LaN3O9*6H2O, [6]LaCl3, [6]Gd2O3, [9]GdN3O9*6H2O, [6]GdCl3) and a set of glass compositions taken from Prowatke and Klemme (2005). The studied melt compositions vary in the aluminium saturation index (ASI, molar ratio of Al2O3/(Na2O+K2O+CaO)) from 0.115 to 0.755.
The spectra were collected at the ID 26 using a Si(111) monochromator crystals for high flux. XANES and EXAFS spectra on model compounds were recorded in transmission mode. RIXS, XANES and EXAFS spectra on the glasses were recorded using the high-resolution wave-length dispersive spectrometer with one spherically bent Si(440) analyzer crystal for La and four spherically bended Ge (422) analyzer crystals for Gd.
pHere only the Gd data are shown. RIXS spectra for both, model compounds (Fig.1) and glasses (Fig.2) show slightly different pre-edge features, which are probably related to quadrupolar transitions. The preliminary results indicate that differences in the pre-edge are related to slight differences in intra-atomic multiplet splitting and thus to differences in chemical bonding and Gd site symmetry.
Furthermore, the extracted high resolution XANES (Fig. 3) for the model compounds, show significant differences in the main edge structure reflecting the different coordination of the nearest neighbors, which is also present in differences of the EXAFS.
For the quenched glasses, high resolution XANES (Fig. 3, right) show that the maximum of the pre-edge shifts to lower energies and increases slightly in intensity with increasing ASI. These changes might be related to an increase of the Gd-O distance as indicated by the shift of the maximum at 7285-7295 eV in the XANES to lower energies (Fig. 3, right) and a slight shift of first maximum of the EXAFS Fourier-transform (Fig. 4).
Overall all spectra indicate that the Gd coordination changes with melt composition.
Trace elements are important indicators in magmatic and metamorphic rock on Earth, Moon and the terrestrial planets. It is generally accepted that the partitioning of trace elements (TE) is controlled by T, P, crystal chemistry and melt composition. Particularly, the trace element partition coefficients of Prowatke and Klemme (2005) between melt and titanite, which varied over several orders of magnitude, suggest a strong control of the melt composition. Ponader and Brown (1989) already reported that the coordination of rare earth elements (REE) in quenched melts changes with the degree of polymerization of the melts. However a direct correlation between element coordination and partitioning of TE was not attempted. The scope of this study is to investigate the local structure of Y in aluminosilicate glasses.
The studied melt compositions were taken from Prowatke and Klemme (2005) and vary in the aluminum saturation index (ASI, molar ratio of Al2O3/(Na2O+K2O+CaO)) from 0.115 to 0.768. The glasses were synthesized from oxides and carbonates and doped with 5000 ppm Y. EXAFS spectra at the Y K-edge (17038 eV) were collected at beamline C in fluorescence mode. A Si(111) fixed-exit two-crystal monochromator was used. The fluorescence signal was detected using a Stern-Heald type detector filled with Kr or a 7-element SDD detector. The spectra were analyzed using the software Athena and Artemis. Amplitudes and phase shift were calculated using Feff6 and checked on crystalline compounds with known structural parameters (Y2O3, Y3Al5O12 and YN3O9 *6H2O). Due to large static disorder and non-Gaussian pair distributions in melts and glasses a histogram fit based on an asymmetric gamma-like distribution function was used to model the Y-O pair correlation of the first coordination shell in the glasses.
The k3-weighted EXAFS spectra of all glasses and the corresponding Fourier-transforms (FT) are shown in Fig. 1 together with the resulting fit. The EXAFS amplitude decreases considerably from ASI200 to ASI280. The position of the first maximum of the FT, which corresponds to the Y-O correlations of the first coordination shell, plots at nearly the same position for all samples (2.26 Å). The first maximum, however, shows a considerable decrease in the magnitude and significant broadening from ASI200 to ASI280. The second maximum of the FT corresponds to pair correlations of the second coordination shell. The analysis of the EXAFS data shows an increase in the coordination number for Y from 6 to 8 with increasing ASI and thus polymerization of the melt. Along with this an increase of the average Y-O distance by 0.12 Å is observed, which is also associated with a considerable increase in the asymmetry and width of the Y-O pair distribution, as displayed in the pair distribution function determined by the fit (Fig. 2). We were able to show that the local structure of Y in aluminosilicate melts changes with the ASI of the melt composition. Changes in ASI will result in a change of the polymerization of the tetrahedral network of silicate melts. The lower amount of non-bridging oxygens in highly polymerized silicate melts forces Y into a higher coordination in order to meet local charge-balance requirements.
Studying trace elements (TE) partitioning data provides useful information about the formation processes of igneous rocks on earth, moon and terrestrial planets, and thus knowledge on their partitioning behaviour during melt crystallisation is essential. Temperature, pressure, crystal chemistry and melt compositions are well known parameters that control the partitioning of TE between coexisting phases. In particular, the trace element partition coefficients of Prowatke and Klemme (2005) between melt and titanite suggest a strong control of the melt composition, because they vary over several orders of magnitude at constant crystal composition. Only little is known about structural changes in the local environment of these trace element that accompany these huge variations, especially for rare earth elements (REE). EXAFS data on Y ([3], [4]) and preliminary EXAFS of Gd and Yb in a suite of melt compositions similar to those of Prowatke and Klemme (2005) show a considerable change in the structural environment of these elements. Investigation of the electronic structure of the REE in these melts provides additional information for understanding the influence of the melt composition on the local structure of these elements. A useful tool to probe the electronic structure of 4f-elements is resonant inelastic x-ray scattering (RIXS). In RIXS a core- electron is promoted to an excited state just as in XANES but also the energy dependent intensity of the scattered and emitted photons are measured, which gives additional information about the intermediate state, the final state, and the pre-edge fine structure., The pre-edge corresponds to 2p to 4f quadrupolar transitions in the case of lanthanides. The scope of this study is to investigate the electronic structure of Yb in aluminosilicate glasses in order to gain insight into changes in the Yb coordination as a function of glass composition.
The studied melt compositions were taken from Prowatke and Klemme (2005) varying in the aluminium saturation index (ASI, molar ratio of Al2O3/(Na2O+K2O+CaO)) from 0.115 to 0.755. The glasses were synthesised from oxides and carbonates and doped with 2wt% Yb. 2p3d-RIXS and XANES have been collected for Yb2O3, Yb3Al5O12 and several Yb-doped aluminosilicate glasses at beamline W1 employing a Johann Spectrometer with a Rowland circle of 1 m and using a spherically bent Si(620) analyzer crystal.
The collected RIXS of the pre-edges for the model compounds and glasses show weak but different pre-edge features, which are probably related to quadrupolar transitions. The preliminary results indicate that differences in the electronic structure are related to slight differences in intra-atomic multiplet splitting and thus to differences in chemical bonding and Yb site symmetry. Similar to the RIXS, The extracted high-resolution XANES spectra for the model compounds reflect the difference in the structural environment of Yb, which is regular [6]Yb for Yb2O3 and regular [8]Yb for Yb3Al5O12. For the glasses the pre-edge shifts to slightly higher energies and increases slightly in intensity with increasing ASI. These changes may be assigned to an increase of the Yb-O distance as indicated by the shift of the maximum at 8980-8990 eV together with a potential change in coordination. For Y, an element chemically quite similar to Yb, an increase in the distance to the nearest oxygen neighbours and a change of the coordination from 6 to 8 was found for the same suite of glass compositions. We were able to show that the electronic structure of Yb changes with melt composition. To quantify the results calculation of spectra is needed to interpret the data.
The aim of the experiment is to study Zr complexation in aqueous fluids containing dissolved silicate components at conditions of the deep Earth. XANES and RIXS measurements at high temperature and pressure are used to provide further insight to the nearest and next-nearest neighbor elements surrounding Zr in the fluid. Temperature and pressure conditions are achieved by using hydrotheral diamond anvil cells.
RIXS and XANES spectra in fluorescence mode were recorded in fluorescence mode, due to the low concentrations of Zr in the fluids, using a high-resolution wave-length dispersive spectrometer. Several spectra on model compounds were also recorded in transmission mode. The measurements were done using different monochromator crystals, Si311 to collect high resolution spectra, and Si111 for higher intensitites especially for samples with low Zr concentrations.
Fig.1 shows XANES spectra collected on zirconosilicates, oxide and sodium aluminosilicate glasses model compounds. Fig.2 shows the comparison of XANES spectra of Zr in various fluids at high P & T. While the spectra of the silicate bearing fluids are very similar to those of zirconosilicates models in which Zr is 6-coordinated, those of HCl and NaOH solutions distinctly differs. This observation clearly points to the formation of different complexes as function of the melt composition.
The results clearly demonstrate that it is possible to obtain information on the complexation of Zr in aqueous fluids at high temperature and pressure using XAFS techniques. The spectra already indicate that Zr-complexation is very sensitive to the chemical composition of the system.
Knowledge of the local structure around rare earth elements (REE) in silicate and aluminosilicate melts is of fundamental interest for the geochemistry of magmatic processes, particularly for comprehensive understanding of the partitioning processes of REE in magmatic systems. It is generally accepted that mineral-melt partitioning of REE’s is controlled by temperature, pressure, oxygen fugacity (in case of polyvalent cations) and crystal chemistry but less is known about the influence of the melt composition. The aim of this thesis is to establish a relationship between the variation of the REE distribution with the melt composition and the coordination chemistry of this REE in the melt.
For this purpose, melt compositions used by Prowatke und Klemme (2005) which show a significant change in the partitioning coefficients between titanite and melt exclusively as a function of melt composition as well as haplogranitic and haplobasaltic melt compositions as a representative of the magmatic systems were doped with La , Gd , Yb and Y and synthesized as glass. The melt compositions systematically vary in aluminum saturationindex ( ASI ), from 0.115 to 0.768 for the Prowatke und Klemme (2005) compositions, from 0.935 to 1.785 for the the haplogranitic composition and from 0.368 to 1.010 for the haplobasaltic composition. Moreover, haplogranitic compositions were synthesized with 4 wt% H2O to study the influence of water on the local structure of REE. To gather information about the local structure of Gd, Yb and Y x-ray absorption spectroscopy was used. While extended x-ray absorption fine structure spectroscopy was used to gather quantitative information of the locale structure around the REE, resonate inelastic x-ray scattering (RIXS) and the extracted high resolution x-ray absorption near edge structure (XANES) was used to gather additional qualitative information on the local environment of La, Gd and Yb in the glasses. Additional high temperature in situ Y-EXAFS was performed to prove, if the local structure of Y above transition region (TG) corresponds to the local structure in the quenched melts.
For the analysis of the EXAFS data a new histogram fit was used, which could describe a non-symmetric respectively non-Gauss-shape pair distribution function, as they may occur with a high degree of polymerization or at high temperatures. The results for Y in the Prowatke und Klemme (2005) compositions show an increase of the width and skewness of the Y-O pair distribution function with increasing polymerization, which goes along with an increase of the coordination number from 6 to 8 while average bond length increases by 0.13 Å. A similar trend is also observed for Gd- and Yb-EXAFS spectra. Furthermore, the high resolution XANES for La, Gd and Yb show that structural difference could be revealed, at least half qualitative, in particular for changes of the average bond length to the oxygen atoms. However, compared to the EXAFS method, this method does not provide information about the shape and width of pair distribution functions. The high temperature EXAFS investigation of Y reveal no significant changes in the local structure above TG except for the thermally induced increase in the average Y-O distance. A comparison of the Y-O distances for compositions with an ASI of 0.115 and 0.755 determined at room temperature and TG indicated that the structural changes in the glass along one composition series could be even stronger in the melts.
The direct correlation of the partitioning coefficient from Prowatke und Klemme (2005) with the structural changes in the glass reveals for Y a linear correlation, whereas Yb and Gd show a nonlinear relationship. Because of its ionic radius and charge, the REE is preferably 6-fold coordinated by nonbridging oxygen in low polymerized melts to form stable configurations. In highly polymerized melts with an ASI close to 1, 6-fold coordination is not possible because almost only bridging oxygens are available. The over bonding of bridging oxygen atoms around the REE will be compensated via increasing coordination number and the average REE-O distance. This means that the configuration in the more depolymerized compositions is energetically more favorable, so that the observed variation of the partitioning coefficient results from these differences, which is eventually different for each element. For the haplogranitic and haplobasaltic compositions an increase of the skewness and the asymmetry of the pair distribution function with increase of polymerization of the melt was observed which result in an increase of the coordination number and average distance. This implies, that the respective REE is also getting more incompatible with the increase of the asymmetry in this compositions. Furthermore, the addition of water shows that the melts depolymerize, which resulted in a more symmetrical pair distribution function by which the compatibility increases again.
Finally, the changes in melt composition result in a change of the polymerization of the melt, which has a significant impact on the local environment of the REE. The structural changes can be directly correlated with distribution data, but the trends differ significantly between light, medium and heavy REE. However, this study was able to show what structural change is required to have a significant impact on the partition coefficient. Furthermore, the influence of melt composition on the distribution of trace elements increase with increase of polymerization and should therefore not be neglected.
Gegenstand der vorliegenden Diplomarbeit ist die Untersuchung des Phasenbestands und der Mikrostruktur von Reibfilmen auf Bremsscheiben in Abhängigkeit von der ther- mischen Beanspruchung. Dazu wurde der unbeanspruchte Bremsbelag zuerst mittels Licht- und Elektronenmikroskopie dargestellt und chemisch charakterisiert. Die Brems- versuche führte Honeywell-Jurid mit einem Dynamometer durch. Aus den dabei erzeugten Beanspruchungen resultieren verschiedene maximale Temperaturen (320 ◦C, 650 ◦C). Mit Hilfe einer neu entwickelten Präparationmethode war es möglich Reib- filmfragmente in einer Ebene parallel zur Oberfläche zu untersuchen und somit einen größeren Bereich, als bei bisher durchgeführten Querschnittsuntersuchungen, im TEM qualitativ zu beschreiben. Es zeigt sich, dass die Reibfilmzusammensetzung von der ther- mischen Beanspruchung abhängt. Bei niedrigen Temperaturen dominieren zerkleinerte Belagsbestandteile den Reibfilm und bei hohen Temperaturen hauptsächlich Magnetit, als Oxidationsprodukt der Scheibe.