Ferrite Toroid Core: Copper-Zinc Ferrite Sintered at 1250 °C, Presintered Three Times at 900 °C (Snoek 1948, Ex. I)

A soft ferrite ring core for coils and transformers: copper, zinc and iron oxides with a little brownstone, presintered three times at 900 °C, pressed into a ring and sintered at 1250 °C in oxygen. Snoek's ring had an initial permeability of 1,580. Its Curie point was about 80 °C. Source: J. L. Snoek, US 2,452,530 (Philips, 1948), Example I and its table. The violet iron oxide has its own blueprint (precipitated hematite calcined at 1000-1100 °C, after Nurdini et al. 2022); Snoek does not say how his was made. Picture: the patent's Fig. 4, the ring core.