Kristina Gelin

Ionic motion in polypyrrole-cellulose composites: trap release mechanism during potentiostatic reduction.

This work investigates the movement of anions during potentiostatic controlled reduction of novel composite materials consisting of high surface area cellulose substrates, extracted from the Cladophora sp. algae, coated with thin ( approximately 50 nm) layers of the intrinsically conducting polymer (ICP) polypyrrole. The coating was achieved by chemical polymerization of pyrrole on the cellulose fibers with iron(III) chloride and phosphomolybdic acid, respectively. The composites are in the form of paper sheets and can be directly immersed into an electrolyte solution for ion absorption/desorption. The motion of glutamate and aspartate anions during cathodic polarization was investigated as a function of preceding anodic polarization at various potentials. The composite was found to exhibit memory effect as the response to a cathodic polarization of constant magnitude produced different responses depending on the magnitude of the preceding anodic potential. After the application of a cathodic potential to the composite, the reduction current curvesgenerated by anions leaving the compositewere found to initially increase in magnitude followed by a monotonic decay. A similar response has not been described and analyzed for electrochemical reduction of anion containing ICP materials earlier. A theoretical model was developed to aid the analysis of the experimental data. The model accounts for both freely mobile anions and anions that may be temporarily trapped in a contracting PPy network during cathodic polarization. By fitting the recorded reduction current curves to this model, detailed information about the ionic movement in the composite could be obtained, which may be used to further optimize the materials properties of conducting polymer systems aimed for specific electrochemical ion exchange processes.

Infrared reflectance of direct current magnetron sputter deposited films of Ni93V7, Cu89Ni10Fe1(Mn) and Cu

Abstract Direct current magnetron sputter deposition of thin metal films from nickel- and copper-based alloys have been used to prepare infrared reflectors in spectrally selective solar absorbers. The sputtering process was optimized regarding sputter rate, argon pressure and base pressure in order to achieve maximum infrared reflectance for each target material. The structural properties of the films were investigated by scanning electron microscopy and X-ray diffraction. It was found that the near normal thermal emittance at a surface temperature of approximately 100 °C can be reduced from 0.12 to 0.06 if nickel–chromium, which is commonly used as infrared reflector in tandem solar absorbers, is substituted by copper–nickel.

Infrared emittance of Cux–Ni1−x alloys

The aim of this work was to study Cu–Ni alloys and establish a relation between alloy concentration and infrared emittance for wavelengths of the order 10 µm, which is of interest in room temperature applications. The resistivity was measured at room temperature for the same alloy compositions as the emittance in order to investigate the validity of the Hagen–Rubens relation in the infrared wavelength range for Cu–Ni. The Hagen–Rubens relation is verified for both the copper-rich and nickel-rich samples. We therefore assume strong electron scattering from impurities so that intraband transitions dominate over interband transitions in the infrared wavelength range. The validity of the Hagen–Rubens relation can, as a good approximation, also be used for the integrated thermal emittance.

Infrared emittance of Nix–Cr1−x alloys

The optical efficiency of a commercially available sputter deposited spectrally selective solar absorber was improved. The main purposes were to decrease the thermal emittance, increase the solar absorbtance of the absorber and to protect the substrate from degradation due to environmental influence. The adhesion properties between the corrosion-protecting barrier and the substrate were also studied. This project was focused on process improvements that are realistic to implement in industrial production.The thermal emittance of the absorber was decreased from 0.12 to 0.06 by changing the material of the corrosion-protecting layer from nickel-chromium to copper-nickel. Copper-nickel was less sensitive to variations in the sputter parameters than nickel-chromium. A novel method that could simplify the search for alternative corrosion resistant materials with a low thermal emittance has been purposed. Since resistivity data usually exist or can easily be measured and infrared measurements require more sophisticated measurements, the Hagen-Rubens relation was investigated for copper-nickel and nickel-chromium alloys. The dc-resistivity was found to be related to the infrared emittance or the integrated thermal emittance for alloys in their solid soluble fcc phase.The solar absorbtance was increased when a graded index absorbing coating was tailored for a crossover of the reflectance from low to high reflectance at about 2.5 µm. The solar absorber graded index coating was optimized for nickel metal content in nickel oxide and a solar absorptance of 0.89-0.91 was achieved. The solar absorptance was further increased to 0.97 when an antireflection coating was added on top of the absorbing layer.Finally, extrapolation algorithms were developed to assure correct determination of the thermal emittance for coatings on glass since modern spectrometers that do not cover the complete wavelength interval required to calculate the thermal emittance of surfaces at room temperatures accurately. The error arising from the extrapolation algorithms were smaller than the noise from the optical measurements. Similar strategies can be used for other surfaces.