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Im Rahmen dieser Arbeit wurde ein Niedertemperatur Sol-Gel Spin-Coating Verfahren entwickelt, mit dem sich Metalloxide und -fluoride in Form von Nanometer-dicken Schichten abscheiden lassen. Ziel dieses Verfahrens ist die Herstellung von anti- (AR) und hoch reflektierenden (HR) Schichtsystemen, die aus einem alternierenden Aufbau von niedrig brechenden und hoch brechenden Materialien bestehen. Zur Darstellung der Metallfluoride wurde dabei eine neuartige nichtwässrige Sol-Gel Synthese ausgehend von Metallalkoxiden und in Alkoholen einkondensiertem Fluorwasserstoff angewendet. Die abgeschiedenen Schichten, die bei 100 °C getrocknet wurden, sind mit REM, TEM und AFM auf ihre morphologische Struktur, mit EDX und XPS auf ihre chemische Zusammensetzung und mit Ellipsometrie und UV-vis Transmissionsspektroskopie auf ihre optischen Eigenschaften hin untersucht worden. Mit diesem Verfahren gelang die Präparation von homogenen Magnesiumfluorid- und Titandioxidschichten mit geringen Rauheiten (Ra < 2,0 nm) auf Silicium- und Quarzglassubstraten. Die Magnesiumfluoridschichten sind amorph bis teilkristallin, bestehen aus 2 nm bis 10 nm großen Partikeln und haben einen Brechungsindex von n500 = 1,36. Die Titandioxidschichten sind amorph und weisen einen Brechungsindex von n500 = 2,05 auf. Die Schichtdicken der Magnesiumfluorid- und Titandioxidschichten sind zwischen 25 nm und 500 nm einstellbar. Durch einen alternierenden Aufbau von Magnesiumfluorid- und Titandioxidschichten gelang außerdem erstmalig die Herstellung von Metallfluorid- und Metalloxid-basierten Schichtsystemen über einen Niedertemperatur Sol-Gel Prozess. Mit Hilfe der bestimmten optischen Eigenschaften der Magnesiumfluorid- und Titandioxideinzelschichten wurden einfache AR und HR Designs entwickelt und aufgebaut. Die Transmissionsspektren der Designs und der jeweiligen Schichtsysteme zeigten dabei gute Übereinstimmungen, gleiches galt für die mit Hilfe der Spektralellipsometrie aufgenommenen Reflexionsspektren. In dieser Arbeit konnte erstmals gezeigt werden, dass sich das Niedertemperatur Sol-Gel Verfahren prinzipiell zur Herstellung von einfachen AR und HR Schichtsystemen eignet.
In this work, thin iron doped tin oxide layers (3–6 nm) were investigated with regard to gas sensitivity by means of surface plasmon resonance (SPR) effect with ellipsometric readout. The experimental set-up was a Kretschmann configuration with gold as metal layer. The sensor system was exposed to different concentrations of methane in the low ppm range and the sensor response of an uncoated gold layer compared to a gold layer coated with iron doped tin oxide. The additional layer effects stability with regard to drift behavior of the sensor and an increase in sensitivity. Furthermore, the sensor is able to detect the toxic gas carbon monoxide in low concentration range (down to 0.5 ppm). The thin layers were investigated by X-ray photoelectron spectroscopy, secondary electron microscopy and spectroscopic ellipsometry. The possible mechanisms taking place on the surface are discussed.
Preparation and characterisation of MgF2-low-temperature sol-gel-films for optical applications
(2006)
Dielectric, anti-reflective or high reflective systems consist of low and high refractive index layers. Common systems are oxides. The preparation of low refractive index MgF2-films of optical quality by means of an anhydrous low temperature solgel synthesis is presented. The MgF2-sol is prepared by spin-coating on silicon and glass substrates. Various film thicknesses between 20 nm and 435 nm have been deposited. It has been shown that the thickness increase is proportional to the number of coating steps. The deposited MgF2-films consist of 10 nm to 20 nm large nanoparticles and have smooth surfaces with an average roughness (Ra) of (1.7 ± 0.3) nm. The optical constants n and k of the films are in agreement with the literature data of bulk-MgF2.
Thin films of MgF2 have been deposited on silicon wafers in optical quality with a novel sol-gel deposition process at temperatures as low as 100 °C. The properties of the fluoride layers are compared to bulk MgF2 with respect to the optical constants. By employing spectroscopic mapping ellipsometry, the uniformity of the thickness of fluoride layers is confirmed to be within acceptable limits for optical applications. The optical constants are close to the bulk data. High-quality low-index films of suitable and homogeneous thickness have already been produced on a laboratory scale.
A gas sensor system based on the surface plasmon resonance (SPR) effect in a 44 nm Au layer was studied. The usability of this sensor system was investigated for several analyte gases and gas mixtures which are interesting due to their flammability, toxicity or greenhouse effect. The SPR effect of gold was measured in nitrogen and compared with the gaseous alkane series which caused a shift of the position of the SPR effect. For in situ measurements, the phase shift Δ at the SPR angle (ψ minimum) in the analyte gas mixtures was measured for investigated gases in comparison with nitrogen. With this method, detection limits of different gases could be determined, which lie in a range interesting for safety engineering. Furthermore the aging effect of the gold layers which leads to loss in sensitivity of the sensor was investigated.
MgF2 and TiO2 single layers and layer stacks were produced by a spin-coating sol-gel process. The final temperature treatment was carried out at 100 °C. The layers were deposited onto silicon and fused silica substrates and were analysed by means of atomic force microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, ellipsometry, and UV-vis transmission spectroscopy. MgF2 and TiO2 single layers have morphological and optical properties comparable with physical vapour deposited layers. By using spectroscopic mapping ellipsometry, a good inter- and intra-sample homogeneity was confirmed. Multiple deposition steps result in a linear increase of layer thickness. Various films were deposited with thicknesses between 25 nm and 350 nm.
It was shown that the low temperature sol-gel process results in films of optical quality. Anti-reflective and high reflective layer stacks consisting of MgF2 and TiO2 were designed and can be produced now by a sol-gel process, whereas the MgF2 layers in the layer stacks contains also traces of MgF2-2xOx.
An ellipsometric gas sensor based on the surface Plasmon resonance (SPR) effect of ~ 43 nm thick gold layers was investigated. To protect the gold layer from contamination and to improve the detection limits, the gold layers were top-coated with 5–6 nm thick layers of organic a-C:H or with inorganic metal oxides TiO2 or ZrO2. The additional layers increased the long-term stability, whereas the metal oxide layers protect better than a-C:H. Furthermore, the additional layers decreased the detection limits by one order of magnitude in case of a-C:H and two orders of magnitude in case of the oxides. It could be shown that the detection limits also depend on the kind of preparation (sol–gel process or physical vapour deposition) of the additional layers.
Phosphorus (P) is essential for all forms of life and cannot be substituted. It is one of the macro nutrients applied in form of mineral or organic fertilizers in agricultural crop production. Mineral P fertilizers are generally based on phosphate rock, a limited resource. Western Europe completely depends on imports as it has no own relevant phosphate mines. The most abundant phosphate deposits are located only in a few countries such as Morocco, China, South Africa and the United States of America. Furthermore, phosphate rock and as a consequence mineral P fertilizers are often contaminated with the toxic elements cadmium and uranium. In Order to substitute phosphate rock P-bearing waste streams are investigated for their potential to be recyded as fertilizers. Wastewater is one of the important P-bearing waste streams that were in the focus of research in the last 10 years. German wastewater is e.g. a carrier of approx. 70,000 t of phosphorus per year.
Klärschlammaschen weisen hohe Gehalte an Phosphor auf, dieser ist allerdings schlecht für Pflanzen verfügbar. Des Weiteren enthalten sie Schwermetalle die vor dem Einsatz als Düngemittel entfernt werden müssen. Vor diesem Hintergrund wurden in den letzten Jahren nasschemische und thermochemische Verfahren entwickelt und zum Teil bereits in Demonstrationsanlagen getestet. Ein thermochemischer Verfahrensansatz wird in diesem Beitrag näher vorgestellt. Klärschlammaschen werden mit Klärschlamm als Reduktionsmittel und Natriumsulfat als Aufschlussmittel im Drehrohrofen thermisch behandelt. Bei diesem Prozess wird pflanzenverfügbares NaCaPÜ4 gebildet, unerwünschte Elemente wie As, Cd, Hg und Pb werden zum Teil über die Gasphase separiert. Die Ergebnisse eines Demonstrationsversuchs werden vorgestellt.
Phosphorus fertilizer from sewage sludge ashes by thermochemical treatment - Benefits and challenges
(2015)
Mineral phosphorus (P) fertilizers are solely produced from phosphate rock. Europe completely depends on imports since there are no relevant phosphate rock deposits on the continent. Furthermore, phosphate rock is contaminated with heavy metals such as Cd and U that pollute the farmlands and pose environmental risks. Sewage sludge ash (SSA) might be a promising source for recycling fertilizer since it contains large amounts of P (up to 13 %). However, fertilizer from SSA has to comply with the respective ordinances, particularly the heavy metal limit values stated in the fertilizer ordinance, and requires sufficient P bioavailability. Thus, we conducted a complete survey of SSA from German mono-incineration facilities and developed a thermochemical treatment for SSA to reduce toxic elements and increase P bioavailability.
Sewage sludge incineration is extensively practiced in some European countries such as the Netherlands, Switzerland, Austria and Germany. A survey of German sewage sludge ash showed that the recovery potential is high, approx. 19,000 t of phosphorus per year. However, the survey also discovered that the bioavailability of phosphorus in the sewage sludge ash is poor and that more than half of the ashes cannot be used as fertilizers due to high heavy metal content. A new thermochemical process for sewage sludge ash treatment was developed that transforms the ash into marketable fertilizer products. Sewage sludge ash was thermochemically treated with sodium and potassium additives under reducing conditions, whereby the phosphate-bearing mineral phases were transformed into plant available phosphates. High P-bioavailability was achieved with a molar Na/P ratio > 1.75 in the starting materials. Sodium sulfate, carbonate and hydroxide performed comparably as additives for this calcination process. Potassium carbonate and -hydroxide have to be added in a molar K/P ratio > 2.5 to achieve comparable P-solubility. The findings of the laboratory scale investigations were confirmed by an industrial demonstration trial for an ash treatment with sodium sulfate. Simultaneously, the volatile transition metal arsenic (61% removal) as well as volatile heavy metals such as cadmium (80%), mercury (68%), lead (39%) and zinc (9%) were removed via the off-gas treatment system. The product of the demonstration trial is characterized by high bioavailability and a toxic trace element mass fraction below the limit values of the German fertilizer ordinance, thus fulfilling the quality parameters for a P-fertilizer.
Phosphorus-fertilizers from secondary resources such as sewage sludge ash (SSA) will become more important in the future as they could substitute conventional fertilizers based on the nonrenewable resource phosphate rock. Thermochemical approaches were developed which remove heavy metals from SSA prior to its fertilizer application on farmlands. We analyzed the chemical state of mercury and selenium in SSA before and after thermochemical treatment under different conditions for P-fertilizer production by X-ray absorption near edge structure (XANES) spectroscopy. In some incineration plants the mercury loaded carbon adsorber from off-gas cleaning was collected together with the SSA for waste disposal. SSAs from those plants contained mercury mainly bound to carbon/organic material. The other SSAs contained inorganic mercury compounds which are most probably stabilized in the SSA matrix and were thus not evaporated during incineration. During thermochemical treatment, carbon-bound mercury was removed quantitatively. In contrast, a certain immobile fraction of inorganic mercury compounds remained in thermochemically treated SSA, which were not clearly identified. HgSe might be one of the inorganic compounds, which is supported by results of Se K-edge XANES spectroscopy. Furthermore, the chemical state of selenium in the SSAs was very sensitive to the conditions of the thermochemical treatment.
Im Rahmen des EU-Projekts P-REX werden unter anderem die zwei thermischen Verfahren ASH DEC und Mephrec zur technischen Rückgewinnung von Phosphor aus Klärschlamm und Klärschlammaschen für die P-Düngemittelproduktion betrachtet. Die Produkte haben geringe Konzentrationen an Schwermetallen und eine bessere P-Löslichkeit als die nicht behandelten Klärschlammaschen. Die Klärschlammasche wird im ASH DEC-Verfahren mit Natriumverbindungen unter reduzierenden Bedingungen im Drehrohrofen behandelt. Das Verfahren wurde erfolgreich bei einem Demonstrationsversuch getestet. Das Mephrec-Verfahren ist ein metallurgisches Schmelzverfahren und wird unter reduzierenden Bedingungen im Schachtofen durchgeführt. Es ist eine Pilotanlage in Nürnberg geplant.
In Denmark, increasing amounts of woody biomass are being used for the production of renewable energy, resulting in more wood ashes being generated. While these materials have been mainly landfilled, wood ashes may also be utilised for fertilizing and liming purposes on top of soils. Pre-treatments involving hardening or granulation may be carried out prior to soil application. In this study, two Danish wood ash samples were hardened and/or granulated. Lab-hardening induced rapid changes in the shape of the acid neutralisation capacity curve of the ashes. Up-flow column tests, assuming local equilibrium conditions, were employed to investigate the leaching from pre-treated ashes. Granules and loose ashes demonstrated similar leaching behaviours, indicating that similar geochemical processes were governing their leaching. In comparison with untreated fresh ashes, the hardened ashes demonstrated reduced leaching of Ca, Ba, Pb and Zn with concentration levels generally below or close to the analytical limits of quantification; to the contrary, the leaching of As, P, Sb, Si, V and Mg was enhanced in the hardened ashes. The release of alkalinity was reduced by hardening. In general, all granules were barely breakable by finger-pinching and they could withstand one month of continuous leaching, preserving their overall shape. The solubility of phosphorous in neutral ammonium citrate indicated that about 30–51% of the total P content in the ash samples was released, suggesting that the ashes could be potentially valuable as P-fertiliser if applied onto soil.