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- Tengstrand, Olof, 1983-
(författare)
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Transition metal carbide nanocomposite and amorphous thin films
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis explores thin films of binary and ternary transition metal carbides, in the Nb-C, Ti-Si-C, Nb-Si-C, Zr-Si-C, and Nb-Ge-C systems. The electrical and mechanical properties of these systems are affected by their structure and here both nanocomposite and amorphous thin films are thus investigated. By appropriate choice of transition metal and composition the films can be designed to be multifunctional with a combination of properties, such as low electric resistivity, low contact resistance and high mechanical strength. Electrical contacts are one example of application that has been of special interest in this thesis. Since some industrially important substrates used in electrical contacts soften at higher temperature, all films were deposited with dc magnetron sputtering at a low substrate temperature (200-350 °C).I show that the electrical resistivity and mechanical properties of composites consisting of nanocrystalline NbC grains (nc-NbC) in a matrix of amorphous C (a-C) depend strongly on the amount of amorphous C. The best combination of hardness (23 GPa) and electrical resistivity (260 μΩ*cm) are found in films with ~15 at.% a-C phase. This is a higher hardness and lower resistivity than measured for the more well studied Ti-C system if deposited under similar conditions. The better results can be explained by a thinner matrix of amorphous C phase in the case of NbC. The nc-NbC/a-C is therefore interesting as a material in electrical contacts.Si can be added to further control the structure and thereby the properties of binary Me-C systems. There are however, different opinions in the literature of whether Si is incorporated on the Ti or C site in the cubic NaCl (B1) structure of TiC. In order to understand how Si is incorporated in a Me-Si-C material I use a model system of epitaxial TiCx (x ~0.7). In this model system a few atomic percent of Si can be incorporated in the cubic TiC structure. The experimental results together with theoretical stability calculations suggest that the Si is positioned at the C sites forming Ti(Si,C)x. The calculation further shows a strong tendency for Si segregation, which is seen at higher Si contents in the experiments, where Si starts segregate out from the TiCx to the grain boundaries causing a loss of epitaxy.If Si is added to an Nb-C nanocomposite, it hinders the grain growth and thus a reduced size of the NbC grains is observed. The Si segregates to the amorphous matrix forming a-SiC. At the same time the resistivity increases and the hardness is reduced. With even higher amounts of Si (>25 at.%) into the Nb-Si-C material, grain growth is no longer possible and the material becomes amorphous. In order to separate between effects from the addition of Si and the choice of transition metal I compare the Nb-Si-C system to already published results for the Zr-Si-C system. I find that the hardness of the material depends on the amount of strong Si-C bonds rather than the type of transition metal. The reduced elastic modulus is, however, dependent on the choice of transition metal. I therefore suggest that it is possible to make Me-Si-C films with high wear resistance by an appropriate choice of transition metal and composition.Electron microscopy was of importance for determining amorphous structures of Nb-Si-C and Zr-Si-C at high Si contents. However, the investigations were obstructed by electron beam induced crystallization. Further investigations show that the energy transferred from the beam electrons to C and Si atoms in the material is enough to cause atomic displacements. The displacements cause volume fluctuations and thereby enhance the mobility of all the atoms in the material. The result is formation of MeC grains, which are stable to further irradiation.Finally, I have studied substitution of Ge for Si in a ternary system looking at Nb-Ge-C thin films. I show that the films consist of nc-NbC/a-C/a-Ge and that Ge in a similar way to Si decreases the size of the crystalline NbC grains. However, a transition to a completely amorphous material is not seen even at high Ge contents (~30 at.%). Another dissimilarity is that while Si bonds to C and forms a matrix of a-SiC, Ge tends to bond to Ge.
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- Lilja, Carl
(författare)
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Tephrochronological Studies in Scotland and Sweden
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The study of palaeoclimates, especially the rapid climate changes during the Last Glacial-Interglacial Transition (LGIT ca. 15-8 ka BP), requires precise dating as well as the ability to link climate records over large regional areas. As has recently been demonstrated by the Eyjafjallajökull eruption, volcanic ash (tephra) has the possibility of a widespread geographical distribution which for geological purposes is instantaneous. Tephrochronology, the study of volcanic ash as a geochronological method, has over the past few decades developed methods to identify distal cryptotephras, tephras invisible to the naked eye. This has expanded the number of maker horizons as well as the areas in which they can be found. The studies presented here expand the geographical extent of one marker horizon, the Hässeldalen Tephra, in Southern Sweden. At the same time, some of the problems of the method are examined. The method builds on horizons being distinct and geochemically fingerprinted. The Borrobol Tephra has been found in Scotland and Sweden but with widely differing dates leading to the possibility of there actually being two geochemically identical horizons in close stratigraphic proximity. The present studies provide new results from the original type-site as well as reanalysis of the geochemistry of shards from the first Swedish findings. The major element analysis used here reflects improvements in Electron Probe Microanalysis (EPMA) settings which provides more robust data with slightly altered values from those previously published. These altered values must be taken into account when making inter-site comparisons. Trace element data for the Borrobol type-site and for the two original Swedish sites are also published. Results remain inconclusive as no differences were found between the Swedish and Scottish horizons. Even some problems with confusing the Hässeldalen Tephra with another horizon, the Askja-S Tephra, in Swedish sites are considered.
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