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- 8.2 Zerstörungsfreie Prüfmethoden für das Bauwesen (16)
- 1 Analytische Chemie; Referenzmaterialien (15)
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- 7.6 Korrosion und Korrosionsschutz (2)
Quantitative laser-induced breakdown spectroscopy (LIBS) analysis operates on the assumption that the sample is completely dissociated and diffused within the highly energetic plasma on time-scales of analyte analysis, resulting in analyte emission ideally at the bulk plasma temperature and a signal that is linear with analyte mass concentration. However, recent studies focusing on aerosol analysis have found the heat and mass diffusion rates within laser-induced plasmas to be finite, resulting in particle-rich, locally perturbed areas within the hot bulk plasma. The goal of this study is to observe any related plasma differences, by calculating the bulk and local (i.e. analyte rich regions) plasma temperatures and electron density, to better understand the time frame of equilibrium between the local and bulk plasma properties. This study also seeks to determine whether the presence of large quantities of a matrix element can significantly alter the local plasma conditions, thereby generating matrix effects. We report the temporal profiles of particle-derived species, adding additional insight into the effect of local perturbation of plasma properties, with the conclusion that significant plasma residence (tens of microseconds) is necessary to minimize such effects.
Der Chloridgehalt, ab dem bei der Bewehrung im Beton mit Korrosion gerechnet werden muss, wird als kritischer Chloridgehalt bezeichnet. Die Höhe dieses kritischen Chloridgehaltes bestimmt in hohem Maße den Instandsetzungsumfang und damit die Instandsetzungskosten. Zahlreiche Labor- und Bauwerksuntersuchungen belegen, dass dieser kritische Chloridgehalt von einer Vielzahl von Parametern abhängt und deswegen kein fester Wert sein kann. Aus diesem Grund ist es sinnvoll, über Betrachtung von Korrosions- bzw. Depassivierungswahrscheinlichkeiten einen unteren kritischen Chloridgehalt zu bestimmen, bei dem bei gegebenen Randbedingungen nur mit einer geringen Wahrscheinlichkeit (z. B. 5%-Fraktilwert) Korrosion von Stahl in Beton zu erwarten ist. Die Auswertung von zahlreichen Untersuchungen zeigt, dass der Ansatz eines kritischen Chloridgehaltes von 0, 5 M.-% bezogen auf den Zementgehalt bei Einhaltung bestimmter Randbedingungen mit einer hinreichend geringen Korrosionswahrscheinlichkeit korreliert und somit auch in Übereinstimmung mit der Richtlinie 'Schutz und Instandsetzung von Betonbauteilen' des DAfStb als unterer kritischer Chloridgehalt unter Praxisbedingungen anzusetzen ist.
Der maßgebliche Korrosionsschutz von nicht rostendem Stahl in Beton basiert, anders als bei unlegiertem Betonstahl, auf der Ausbildung einer Chromoxidschicht. Die Chromoxidschicht ist in alkalischem und in carbonatisiertem Beton beständig, sodass nicht rostende Stähle in einem passiven Zustand vorliegen.
Die Initiierung von Lochkorrosionserscheinungen ist dagegen auch an nicht rostenden Stählen in Beton möglich. Der korrosionsauslösende Chloridgehalt ist neben dem Gehalt an Legierungselementen (insbesondere dem Chromgehalt) und der Oberflächenbeschaffenheit auch vom Konzentrationsverhältnis von OH“ zu CI“ im Elektrolyten abhängig. Daher ist prinzipiell zu erwarten, dass in karbonatisierten Betonen geringere Chloridgehalte Korrosion initiieren können als in Betonen mit höheren pH-Werten. Dies gilt in besonderem Maße für nicht rostende Betonstähle mit geringen Chromgehalten, wie z. B. Produkte aus dem Werkstoff 1.4003 (X2CrNi12) mit seiner im Vergleich zu den Standardausteniten geringen Beständigkeit. Um den kritischen korrosionsauslösenden Chloridgehalt am Beispiel eines am Markt verfügbaren ferritischen Chromstahls zu bestimmen, wurden an der Bundesanstalt für Materialforschung und -prüfung (BAM) umfangreiche Untersuchungen zu beschleunigten Korrosionsversuchen in alkalischen und karbonatisierten Mörteln durchgeführt. Mittels Laser-induced Breakdown Spectroscopy (LIBS) konnten die korrosionsauslösenden Chloridgehalte im Phasengrenzbereich Betonstahl-Mörtel ermittelt werden.
In precision agriculture, the estimation of soil parameters via sensors and the creation of nutrient maps are a prerequisite for farmers to take targeted measures such as spatially resolved fertilization. In this work, 68 soil samples uniformly distributed over a field near Bonn are investigated using laser-induced breakdown spectroscopy (LIBS). These investigations include the determination of the total contents of macro- and micronutrients as well as further soil parameters such as soil pH, soil organic matter (SOM) content, and soil texture. The applied LIBS instruments are a handheld and a platform spectrometer, which potentially allows for the single-point measurement and scanning of whole fields, respectively. Their results are compared with a high-resolution lab spectrometer.
The prediction of soil parameters was based on multivariate methods. Different feature selection methods and regression methods like PLS, PCR, SVM, Lasso, and Gaussian processes were tested and compared. While good predictions were obtained for Ca, Mg, P, Mn, Cu, and silt content, excellent predictions were obtained for K, Fe, and clay content. The comparison of the three different spectrometers showed that although the lab spectrometer gives the best results, measurements with both field spectrometers also yield good results. This allows for a method transfer to the in-field measurements
Laser-induced breakdown spectroscopy for on-line sulfur analyses of minerals in ambient conditions
(2009)
The review mainly deals with two topics that became important in applications of laser-induced breakdown spectroscopy (LIBS) in recent years: the emission of halogen- and rare-earth-containing molecules and selective excitation of molecules by molecular laser-induced fluorescence (MLIF). The first topic is related to the emission of alkaline-earth diatomic halides MX, M = Ca, Mg, Ba, Sr and X = F, Cl, Br, and I and rare-earth element (REE) oxides LaO, YO, and ScO. These molecules form in laser-induced plasma (LIP) soon after its ignition and persist for a long time, emitting broad bands in a visible part of the spectrum. They are best detected after relatively long delay times when emission from interfering plasma species (atoms and ions) has already been quenched. Such behavior of molecular spectra allows of using, for their detection, inexpensive CCD detectors equipped with simple electronic or mechanical shutters and low-resolution spectrometers. A main target for analysis by molecular spectroscopy is halogens; these elements are difficult to detect by atomic spectroscopy because their most intense atomic lines lie in the vacuum UV. Therefore, in many situations, emission from CaF and CaCl may provide a substantially more sensitive detection of F and Cl than emission from elemental F and Cl and their ions. This proved to be important in mining and concrete industries and even Mars exploration. A similar situation is observed for REEs; their detection by atomic spectroscopy sometimes fails even despite the abundance of atomic and ionic REEs' lines in the UV-VIS. For example, in minerals and rocks with low concentrations of REEs, emission from major and minor mineral elements hinders the weak emission from REEs. Many REEs do not form molecules that show strong emission bands in LIP but can still be detected with the aid of LIP. All REEs except La, Y, and Sc exhibit long-lived luminescence in solid matrices that is easily excited by LIP. The luminescence can be detected simultaneously with molecular emission of species in LIP within the same time and spectral window. The second topic is related to the combination of MLIF and LIBS, which is a technique that was proved to be efficient for analysis of isotopic molecules in LIP. For example, the characteristic spectral signals from isotopic molecules containing 10B and 11B are easier to detect with MLIF-LIBS than with laser ablation molecular isotopic spectrometry (LAMIS) because MLIF provides strong resonance excitation of only targeted isotopes. The technique is also very efficient in detection of halogen molecules although it requires an additional tunable laser that makes the experimental setup bulky and more expensive.
A significant parameter to monitor the status of concrete buildings like bridges or parking garages is the determination of the depth profile of the chlorine concentration below the exposed concrete surface. This information is required to define the needed volume of restoration for a construction. Conventional methods like wet chemical analysis are time- and cost-intensive so an alternative method is developed using laser-induced breakdown spectroscopy (LIBS). The idea is to deploy LIBS to analyze drill cores by scanning the sample surface with laser pulses. Chlorine spectral lines in the infrared (IR) and ultraviolet (UV)-range were studied for chlorine detection in hydrated cement samples. The excitation energies of these spectral lines are above 9.2 eV. Hence high plasma temperatures and pulse energies in the range of some hundred millijoules are needed to induce sufficient line intensity levels at the required working distance. To further increase the line intensity and to lower the detection limit (LOD) of chlorine a measuring chamber is used where different ambient pressures and gases can be chosen for the measurements. The influences on the line intensity for pressures between 5 mbar and 400 mbar using helium as process gas and the influence of different laser burst modi like single and collinear double pulses are investigated. For the first time a LOD according to DIN 32 645 of 0.1 mass% was achieved for chlorine in hydrated cement using the UV line 134.72 nm.
In this paper experimental temperature and density maps of the laser induced plasma in water during Pulsed Laser ablation in Liquid (PLAL) for the production of metallic nanoparticles (NPs) has been determined. A detection system based on the simultaneous acquisition of two emission images at 515 and 410 nm has been constructed and the obtained images have been processed simultaneously by imaging software. The results of the data analysis show a variation of the temperature between 4000 and 7000 K over the plasma volume. Moreover, by the study of the temperature distribution and of the number densities along the plasma expansion axis it is possible to observe the condensation zone of the plasma where NPs can be formed.
Finally, the time associated to the electron processes is estimated and the plasma charging effect on NPs is demonstrated. The set of observations retrieved from these experiments suggests the importance of the plasma phase for the growth of NPs and the necessity of considering the spatial distribution of plasma parameters for the understanding of one of the most important issues of the PLAL process, that is the source of solid material in the plasma phase.