Лако-красочные материалы — производство

Hexagonal ferrites have a wide range of structures distinguished by different stacking arrangements of three basic elements known as M, S, and Y blocks [5.25]. For mag­netic pigments, the M-type structure (barium hexaferrite [12047-11-9] BaFe12O19) is the most important. The magnetic properties of M-ferrite can be controlled over a fairly wide range by partial substitution of the Fe3+ ions, usually with combinations of di — and tetravalent ions such as Co and Ti. Barium ferrite crystallizes in the form of small hexagonal platelets. The preferred direction of magnetization is parallel to the c-axis and is therefore perpendicular to the surface of the platelet. The spe­cific saturation magnetization of the undoped material is ca. 72 A m2 kg-1 and is

therefore somewhat lower than that of other magnetic oxide pigments. In barium ferrite, the coercive field strength is primarily determined by the magneto-crystalline anisotropy and only to a limited extent by particle morphology. This is the reason why barium ferrite can be obtained with extremely uniform magnetic properties. Barium ferrite pigments have a brown color and chemical properties similar to those of the iron oxides. Typical values of physical properties of barium ferrite pigments used in magnetic recording are given in Table 5.2. Figure 5.3 shows the TEM photograph of barium ferrite pigments listed in Table 5.2.

Fig. 5.3 TEM-photograph showing typical morphology ofbarium ferrite pigments (Table 5.2).