High Efficient,high Accurate Image Simulation And The Study Of The Structure Model Of AlCuCo 10-fold Quasicrystal

Materials are developing rapidly,the requirements for material's performance become more and more high,the structure of material determines it's performance,while the microstructure is one of the most important parts of the material's structure.At present the electron crystallography method has advantages on micro structure analysis contrast with X-ray and it has been an efficient method to research nano-materials or many other microstructures.Although electron crystallography method play an important role in material's structure research,but electron has great dynamic effect with crystal which makes structure analysis complicated.Some unreal structure information may get because of the imperfection of transmission electron microscope in imaging system.So it is necessary to develop some high efficient and high accurate electron crystallography methods,i.e.,image simulation method,and structure reconstruction method.The main works of the thesis have five parts listed as following:1.Quantitative comparison between real space(RS)and Bloch wave(BW)methods as far as simulation accuracy and speed of these two methods are concerned.According to the definition of the scattering matrix,the dimension of the matrix for the BW method is proportional to the square of area of a two dimensional unit cell,while that for the RS method is proportional to the area of a unit cell.If the lattice constants of the crystals are small,both methods works well,for example,Cu and MgAl2O4.When the area of a two dimensional unit cell is larger than(2,0 nm)2,the BW method does not work any more;however,the RS method still works well,for example,Mg44Rh7 and KNbO3.2.Improvement of the direct product method of scattering matrix(DPSM).The original DPSM method is based on the full NxN scattering matrix and the calculation time is too long to be implemented.Therefore the matrix should be reduced.According to Bethe potential method,the dimension of the scattering matrix can be reduced dramatically.The reduced scattering matrix can be both implemented by the DPSM and BW methods.The calculated results showed that the calculating speed for the DPSM method is almost about 3 times faster than that for the BW method and the accuracy is exactly the same.We call the improved DPSM method as modified DPSM method.3.Accurate determination of lattice parameters based on Niggli reduced cell theory.Traditional methods to determine lattice parameters by using a double tilt holder to take a series electron diffraction patterns in a TEM and records the rotation angles at the same time.While the method has some difficulties in two aspects;first,it is hard to operate a double holder to obtain a series diffraction patterns;second,the angles are not precision enough,in general,the angle error is about 1°-2°,sometimes may reach 5°.All of these prevent from the accurate determination of lattice parameters,particularly for the crystal system determination.In this article,we develop a method based on Niggli reduced cell which avoid using the inaccuracy angles of a double tilt holder,and which only need electron patterns of three different zone axes The angle precision between zone axes can be only 0.5° by measuring the reciprocal lattice directions from the diffraction patterns.In addition,a least square method are taken to make the Niggli reduced cell method more accurately.4.Real space method to determine the space group of a crystal.The space group should be determined first if you want to determine an unknown crystal structure.Traditional methods are convergent-beam electron diffraction and precession electron diffraction methods,they both operate in reciprocal space.In this thesis we develop a new method called real space method.The method is based on high angle annular dark field(HAADF)image which has no phase losing.The principle of the method is to reduce three dimension space to two dimension space,then the complicated three dimension space group can be simplified by determination of the two dimension space group.One more advantage of the method is that it can make full usage of the HAADF image which used to determine crystal structure,it is a win-win scheme.So HAADF image as an incoherent image is a wonderful method to measure both space group and crystal's structure because its contrast can reflect the atomic potential structure directly.5.The study of the structure model of 10-fold quasicrystal Al65Cu20Co15.Quasicrystal is a special material with forbidden system in crystal and does not have translational periodicity which both makes it is hard to analysis its structure.It has been 29 years since the first discovering of two dimension 10-fold quasicrystal,the former's quasicrystal structure models are all the model about a local atomic arrangements,the whole structures are still not clearly.In this thesis,the whole structure model of 10-fold quasi crystal Al65Cu20Co15 has been presented in two aspects of theory and practice.Firstly,solved the quasi-unit-cell,the 2 nm cluster is always recorded as a quasi-unit-cell previously,however,we found that the 2 nm cluster is not able to fill the whole space and it has to be expanded to 3.2 nm.Secondly,how to fill the whole space with the 3.2 nm cluster with the inflation rules.In order to solve the problem,we project a cubic cell in five dimension space to two dimension space to build a two dimension quasi-lattice.Then the inflation rules are built according to the quasi-lattice which can make the quasi-unit-cell lattice extending infinitely.At last,we can use the 3.2 nm cluster to fill the whole space.The quasi-unit-cell and the inflation rules are all based on the experiments,we are confident that the results are right.