A TEM Investigation of Contact between Nanoparticles
Abstract: Interaction between nanoparticles (1~100nm) provides us with answers to fundamental questions in many important applications e.g. adhesion, friction, and cold welding. The attractive force arises from the reduction of the total free surface and thus the total surface energy due to the formation of a boundary between the contacting particles. Consequently, the squeezed-in excess volume builds up a strain field near the contacting zone. The strained lattice planes generate strain contrast in the micrographs from the transmission electron microscope (TEM).This thesis concerns TEM investigation for quantitatively interpreting adhesion between particles combining elastic contact theory and dynamic electron diffraction theory. The investigation is concentrated on the application of advanced TEM techniques on characterising the properties related to contacting between particles. The main techniques used are diffraction contrast, high resolution electron microscopy (HREM), convergent beam electron diffraction (CBED), microdiffraction, large angle convergent beam electron diffraction (LACBED), electron energy-loss spectroscopy (EELS), and energy filtering image> and diffraction. Three kinds of nanoparticles including CoO, Fe, and MgO are used in this work. They are produced by using precipitation-electrolytic oxidation, spark plasma evaporation and oxidation methods respectively. Bright field and dark field images are recorded with a CCD camera. The corresponding diffraction conditions are determined by means of CBED. The absorption coefficients and the deviation parameters off the Bragg condition are obtained by image refinement of the intensity profiles of thickness fringes. The simulation results from the bright field images give fairly good agreement with the theoretical calculation. In addition to adhesion, different deformation models including the magnetic force, dislocation loops and coherent particles are used to simulate the experimental images. The conclusions from the simulations confirm the dominant effect of an adhesive force. The simulation mapping shows the effects of different parameters on strain contrast, and thus is of great value in guidance to experimental work. The misorientation and boundary orientation between nanoparticles are quantitatively characterised using microdiffraction and LACBED techniques and interpreted with the CSL model. LACBED and defocused CBED show the potential application in analysing the displacement in the contacting area between particles. The contacting boundary areas were also examined using HREM. The reconstruction of the image by Fourier transformation shows the distorted and bent lattice and phase analysis illustrates a quantitative displacement map near the contacting zone. The thickness of particles is measured and the results obtained with different methods (CBED and EELS) are compared. A statistical measurement of the particle thickness can be realised by combining CBED, EELS and thickness mapping with energy filtering. Keywords: nanoparticles, adhesion, contacting, interface, misorientation, strain field TEM, CBED, LACBED, image simulation, energy filtering.
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