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Title: TEM investigations of NdFeB based permanent magnetic materials
Author: Young, Stuart
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1994
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This thesis documents work carried out on NdFeB based permanent magnetic materials using electron microscopy and related techniques. The first chapter introduces the basic theory of magnetism, the various energy terms which determine the overall state of a magnetic system and dicusses magnetic domains and domain walls. The development of permanent magnets and their uses are outlined. Chapter 2 is a discussion of transmission electron microscopy (TEM) and related techniques. The principles of image formation in an electron microscope are given. The general technique of Lorentz microscopy is described and the various modes available are reviewed. The advantages and disadvantages are highlighted. The specific instruments in the Department of Physics and Astronomy at Glasgow University and their relative merits are described. Chapter 3 describes the properties which are used to characterise a permanent magnet and gives a brief history of permanent magnets including the improvements recently realised to these properties. The material upon which most of the research was carried out is then discussed including how the bulk material is processed and how products are made from this material. The advantages and disadvantages of the material are described along with its properties and the various approaches used to enhance these properties. The preparation of samples from the bulk material suitable for the TEM is discussed in some detail. Chapter 4 presents the results of TEM studies of thinned sections of sintered NdFeB. The domain structures observed are explained in terms of the inclination of the c-axis with respect to the sample normal. Data is shown for three different samples of the variation of domain period with the c-axis inclination. It is found that for a given inclination, the domain period is different for each material. The behaviour of domain walls at the boundary between two grains is studied. Various types of behaviour have been observed and these have been explained in terms of the inclination of the c-axes, the orientation of the boundary with respect to the domains and the possible existence of a Nd rich grain boundary phase. Chapter 5 takes the data from the previous chapter and, using a modified version of a model derived by Kooy and Enz, calculates the domain wall energy of each of the three samples. It is found that despite sample B and C having additions of Dy, the domain wall energies are very similar. This is explained on the basis that although Dy increases the anisotropy of the material, the exchange coupling is decreased. This results in a smaller domain wall width which theoretical models predict should result in a higher coercivity. Assuming that the increase in anisotropy field and the decrease in exchange coupling are linearly proportional to the Dy content, a factor of two increase in the coercivity is expected. Magnetising experiments have been undertaken which show that the domain width of favourably oriented domains increases as expected under the influence of an increasing field which supports the validity of the model as applied to these materials. Chapter 6 gives the theory behind x-ray production and its subsequent extension to x-ray analysis and shows how this technique can be used to derive compositional information. The hardware is briefly described and the method of ratios for obtaining compositional information is explained. Finally, the accuracy of this approach is discussed. Chapter 7 shows the results of x-ray analysis of sintered NdFeB. The basic nature of the materials has been studied and it has been observed to consist of Nd2Fe14B grains separated by a predominantly Nd phase. The distribution of Nb within the materials takes various forms i.e. small precipitate-like features of varying composition or much larger Fe/Nb containing features. The smaller features, whilst similar in appearance can in fact be distinguished by their composition i.e. some of them are regions where Nb has substituted into the matrix for Fe while others appear to be genuine precipitates of what is presumed to be FeNbB. The remaining ones are rich in Nd but have not yet been identified. The larger features are also FeNbB which are crystals left over from the primary melt; these were found in Nd rich areas suggesting that the FeNbB has prevented formation of the Nd2Fe14B phase. Chapter 8 shows the results of investigations into melt-spun NdFeB. Foucault images have been obtained of three melt-spun materials with Nd contents ranging from 9.5% to 14%. The mean crystallite size and domain width are given and it has been found that the domain width increases with crystallite size. It has also been observed that the mean domain width decreases with increasing Nd content suggesting that, as in the sintered case, the Nd leads to increased decoupling of the grains.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available