In recent experiments the compositions and methods of preparation of various ferrites were varied with the aim of achieving optimum mechanical and piezomagnetic performance. Essential improvements were obtained by small cobalt substitutions in the chemical composition and by suitable modifications of the mechanical and thermal treatments.
In CW operation, appropriate ferrites can excite radiation intensities up to at least 10 w/cm2, with an electroacoustic efficiency of more than 75% up to 50 kc/sec. So they are highly promising for underwater transmission and processing, in spite of their brittleness (ultimate tensile strength up to about 6 kg/mm2). Wide‐band hydrophones with linear frequency response and high pressure‐sensitivity can also be built from such ferrites having magnetomechanical coupling coefficients at remanence up to almost 0.25, like Permendur.
The total variation of the mechanical resonant and antiresonant frequencies of ferrite filter elements in the temperature range from +20 to +50°C, being 0.1–0.25% for existing commercial ferrites, has been lowered to 0.03% and less. Since the mechanical quality factors are usually much better than 2000, such ferrites can be profitably applied to the construction of electrical and electromechanical band filters.