Preparation of ferrites

The choice of composition of a soft ferrite is made to achieve one or more of the following properties  

temperature variation of properties between prescribed limits  

satisfactory behaviour over the required frequency band  

finally but by no means the least important, minimum cost. 

High-purity oxides can be obtained by calcining oxalates in an atmosphere with a controlled oxygen content  

---

For the practical use of ferrite it is necessary to get high-density material. In the firing process, the starting compound mixture has to be ferritized and densified by solid -solid and solid-gas reactions. In this process, the value of (5 and the grain size also have to be controlled. For this purpose, a heat-treatment, which was called atmosphere controlled two step firing by Tanaka, was devised, as shown schematically in Fig. 3.12 and in detail in Table 3.1. For reference, the usual heat-treatment for preparation of ferrites is shown in Fig. 3.12(b). The new heat treatment is composed of the following six processes. In process (1), the starting mixed compounds are heated to... 

The precursor method allows the preparation of ferrites with a precise stoichiometry. It involves the synthesis of a compound, the precursor, in which the reactants are present in the required stoichiometry upon heating, the precursor decomposes yielding the ferrite. 

Fig. 3.26. Schematic sol-gel process for the preparation of ferrite thin films. (Adapted from Tanaka et al, 1989.)...

Fig. 3.29. Preparation of ferrite thin films using arc plasma. (Adapted from Harris et aL, 1970.)...

Takada, T. Kiyama, M. (1971). Preparation of ferrites by wet method. In Ferrites Proceedings of the International Conference, July 1970, Japan. Eds Y. Hoshino, S. lida and M. Sugimoto. University of Tokyo Press, Tokyo, pp. 69-71. 

Spray drying of solutions has been found to be useful for the preparation of ferrite powders (88). For Ni-Zn ferrite, the solution of sulfates was broken up into droplets (10-20 p.m) by a rotary atomizer. The powder obtained by spray drying was in the form of hollow spheres having the same size as the original droplets. Calcination at 800-1000°C produced a fully reacted powder consisting of agglomerates with a primary particle size of 0.2 xm. The ground powder (particle size < 1 p,m) was compacted and sintered to almost theoretical density. 

---