Magnetische werkstoffe in elektromechanischen resonatoren und filtern

In the last 15 years a great number of new devices for the frequency selection of electrical signals has been developed. Primarily they use mechanically vibrating resonators, which are driven by electromechanical transducers. Two different classes exist. In Class 1, mechanical properties are transposed by means of electromechanical conversion effects to the electrical terminal of the transducer. These devices are only electrically mounted in an electrical filter. The other class (Class 2) of devices is more complicated. After conversion of electrical energy to mechanical energy, mechanical vibrations are transmitted into a pure mechanical filterstructure. Finally the transmitted frequencies are reconverted into electrical vibrations by means of a second transducer. In these two classes of electromechanical filters, magnetic materials play a very important role. The transducers mainly are made of magnetostrictive materials, preferrable special Ni-Zn ferrites with additives for getting a good temperature behavior. For these transducer materials the electromechanical coupling coefficient must be as high as needed or (Class 2) as stable as possible with respect to temperature and aging. The -factor of mechanical resonances must also overcome a minimal value. All these properties are influenced by the pure magnetic properties in a complicated way. By use of the temperature dependence of the magnetic anisotropy of cobalt-additives it is possible to compensate the temperature dependence of the mechanical resonance frequency of magnetostrictive resonators (bars, rings, tubes, etc.). For getting a linear conversion effect, transducers have to be polarized by permanent magnets. Because of the high stability requirements for mechanical resonators in pure mechanical filter structures of Class 2 filters, metallic resonators and coupling elements are used. The resonators must be made of special Ni-Fe alloys, worked for getting a high mechanical -factor, a low-frequency temperature coefficient for the special vibration mode used in the filter and a low aging rate for resonance frequency. The low temperature coefficient is achieved by an adjustment of the interaction of magnetostriction and internal stresses and their temperature dependenc- e. The internal stresses are created by a cold working process ; by a final precipitation hardening, using additives like beryllium or titanium, the low aging rate is guaranteed, plus a value of the frequency coefficient. Now aging rates for frequency of approximately 10^-8per day or better are possible. The temperature coefficient is /°C or better, the -factor approximately . At the end of this report, some applications of these magnetic materials in filters and resonators are shown.