This thesis describes an investigation into the causal mechanisms of anomalous non-linearity in quartz crystal resonators. In general terms anomalous non-linearity is taken to be any non-linearity which is greater than that produced by the higher order elastic constants of quartz. More specifically, anomalous non-linear behaviour is that which does not obey the conventional non-linear law of a fixed coefficient polynomial in crystal current or strain. Both types of anomalous behaviour are studied. Five aspects of the AT-type crystal unit are considered for potential non-linear mechanisms: i) surface contamination ii) bulk impurities and defects iii) internally coupled unwanted modes iv) electrode and mounting defects v) externally applied stress. The details of experiments performed to determine the influence of these factors on the non-linear behaviour of quartz crystal resonators axe presented and the interpretation of results discussed. Seven physical mechanisms are proposed as possible causes of anomalous non-linearity in quartz crystal resonators. The most significant of these is surface contamination. This is found to be temperature and frequency dependent. The effectiveness of current industrial surface cleaning techniques is assessed. It is shown that good correlation exists between surface contamination related non-linearity and anomalous activity and frequency variations. A simple model of the surface contamination mechanism is presented and it is demonstrated that microscopic particles of either quartz or electrode material, in conjunction with a surface fluid film, can produce activity and frequency variations of the magnitude and form of those measured in quartz crystal resonators. The non-linearity of the mechanism is associated with the complex non-linear rheological properties of the fluid film. It is suspected that this film is composed of a phenylated silicone oil which is commonly used in diffusion pumps.
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