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- Elfwing, Mattias, 1972-
(författare)
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Nanoscale Characterisation of Barriers to Electron Conduction in ZnO Varistor Materials
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis is concerned with the microstructure of zinc oxide varistor materials used in surge protecting devices. This class of material has been characterised with special emphasis on the functional microstructure and the development of the microstructure during sintering. Several different techniques have been used for the analysis, especially scanning electron microscopy (SEM) in combination with electron beam-induced current (EBIC) analysis and in-situ studies of heat-treatment experiments and transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectrometry (EDS) and electron holography. Detailed TEM analyses using primarily centred dark-field imaging of grain boundaries, especially triple and multiple grain junctions, were used to reveal the morphological differences between the various Bi2O3 phases. The triple and multiple grain junctions were found to exhibit distinct differences in morphology, which could be attributed the difference in structure of the crystalline Bi2O3 polymorphs present in the junctions. Electrical measurements were performed on individual ZnO/ZnO grain boundaries using EBIC in the SEM. The EBIC signal was found to depend strongly on the geometric properties of the interface and also on the symmetry of the depletion region at the interface. A symmetric double Schottky barrier was never observed in the experiments, but instead barriers with clear asymmetry in the depletion region. Experimental results together with computer simulations show that reasonably small differences in the deep donor concentrations between grains could be responsible for this effect.Electron holography in the TEM was used to image the electrostatic potential variation across individual ZnO/ZnO interfaces. The sign of the interface charge, the barrier height (about 0.8 eV) and the depletion region width (100 to 150 nm) were determined from holography data. Asymmetries of the depletion region were also found with this technique. The full sintering process of doped ZnO powder granules was studied in-situ in the environmental SEM. The densification and grain growth processes were studied through the sintering cycle. The formation of a functional microstructure in ZnO varistor materials was found to depend strongly on the total pressure.
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- Mannesson, Karin, et al.
(författare)
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Analysis of WC grain growth during sintering using electron backscatter diffraction and image analysis
- 2008
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Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 26:5, s. 449-455
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Tidskriftsartikel (refereegranskat)abstract
- The WC carbide grain size is important for the technological properties of cemented carbide cutting tools. in the present work the WC carbide grain size distribution is determined after milling and sintering for 0.25, 1 and 8 h at 1430 degrees C. The WC grain size distribution, both for the powder after milling and the sintered specimens, is determined by two different methods, i.e. image analysis on scanning electron microscopy (SEM) images and electron backscatter diffraction (EBSD). It should be noted that in this work the 2D grain size distribution is considered. The EBSD analysis clearly shows that the special 12 boundaries are present in the powder and that their fraction decreases during sintering and particularly during the early stages. When the 12 boundaries are omitted in the EBSD analysis the results of the grain size measurements for the two methods agree quite well.
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