Ferrites , crystals

Stainless and heat-resisting steels containing at least 18% by weight chromium and 8% nickel are in widespread use in industry. The structure of these steels is changed from magnetic body centered cubic or ferritic crystal structure to a nonmagnetic, face-centered cubic or austenitic crystal structure. 

This class of steels has an austenitic-ferritic crystal structure, with at least 25 or 30% of the lesser phase with a balance of austenite and ferrite stabilizing alloying elements to... 

Swayze (S8) noted that equilibrium was often difficult to achieve in this system. One effect was the tendency for crystals of the ferrite phase to be zoned. For bulk compositions in the Ca2(Al Fei J2O5 series, the liquid is of higher Al/Fe ratio than the ferrite phase with which it is in equilibrium the crystals that are initially deposited on cooling such a liquid therefore have a lower Al/Fe ratio than the bulk composition of the mix. On further cooling, the Al/Fe ratio of the material deposited progressively increases. Equilibrium within the ferrite crystals is difficult to attain, causing them to remain zoned, with cores richer in Fe and outer regions richer in AF" than the mean composition. 

When electrolytic iron foil is immersed in concentrated solutions of jodium or potassium hydroxide for several weeks, and, after thorough cleaning, allowed to corrode in distilled water, the latter gradually becomes contaminated with traces of sodium or potassium salts, the bresence of which can be detected by the spectroscope or by the usual Bunsen flame test.6 Similar results have been obtained with lithium lydroxide, barium hydroxide, and with ammonia.7 It appears probable hat the alkali penetrates in minute quantities into the metal between he ferrite crystals, possibly in consequence of a certain amount of borosity in the intercrystalline cement. This theory is supported by he fact that iron which has been soaked in alkali invariably pits ... 

The solution pressure of the cement might, conceivably, lie between that of the more positive and the more negative ferrite crystals, in which case the cement would function cathodically towards the one and anodically towards the other. 

Cubic ferrites crystallizing in the spinel (MgAl204) structure with the general chemical composition... 

Fig. 3.30. Furnace for growing ferrite crystals by the Czochralski-Kryopoulos technique.

Fig. 3.32. Growing ferrite crystals by CVT. (Adapted from Wold, 1980.)...

AuCoin, T. R., Savage, R. O. Tauber, A. (1966). Growth of hexagonal ferrite crystals by a modified pulling technique. Journal of Applied Physics, 37,2908-9. 

Broese van Groenou, A., Schulkes, J. A. Annis, D. A. (1967). Magnetic anisotropy of some nickel zinc ferrite crystals. Journal of Applied Physics, 38, 1133-4. 

The aim of this study was to include selenium as an actual impurity in the ferrite crystals under a reaction-ciystallization. We let the ferrite crystals play the role of a scavenger as we wanted to produce selenium-fiee solutions. 

Distribution of Selenium in Ferrite Crystals Some dissolution tests of the resultant ferrite crystals were performed to investigate how selenium was distributed in the crystals. About 10 g of ferrite crystals which were usually obtained from each ferritization, were dissolved. They were gradually dissolved with several acidic solutions from the surface to the center. The dissolution operation was repeated she times in order to thoroughly dissolve the residual ferrite crystals. The distributions of selenium in the crystals formed with Se(IV) and Se(VT) were found as shown in Figures 4-1 and 4-2, respectively. The reduced radius, r/R, on the abscissa was determined by the amount of dissolved iron. The ratio of Se/Fe on the vertical axis was the average value calculated from the amounts of iron and selenium dissolved in each acidic solution. 

In the former crystals, selenite was almost uniformly distributed in the ferrite crystals, but in the latter, selenate was concentrated near the center of the crystals. The reason why Se(VI) was included more deeply or more tightly into the ferrite crystals is not presently understood. However, it is possible to speculate that one of the reasons may be due to its ionic radius and ionic valence. The radius of Se(VI) is smaller than... 

Figure 4-1. Distribution of selenium in the ferrite crystals formed from Se(IV) soln.

A similar phenomenon was observed with the inclusion of arsenic (3) when it is also treated by the ferritization method. As(IU), having a larger radius, was uniformly distributed in the ferrite crystals formed from As(in) solution. When As(V), having a smaller radius, was treated, a large amount of it was concentrated near the center of the crystals with small amounts of reduced As(III) near the surface. 

Particle Size and Crystallinity of Ferrite Crystals The particle size of ferrite crystals was measured because the larger particles are, of course, desirable for their easy separation from a suspension. The size was measured using Laser Diffraction Analysis (LDA). Just before the measurement, the suspension containing a small amount of ferrite crystals was sonicated to disperse the ferrite particles. The particle size distribution of the ferrite crystals formed from 400 mg-Se(IV) solution is shown in Figure 8-1. The distribution has two peaks at nearly 0.2 and 1.8 fim. The median, mode and average diameters were slightly smaller than those obtained from the ferrite crystals formed from only iron(II) sulfate solution. 

The X-ray diffraction (XRD) intensity of the ferrite crystals formed from various solutions of selenium was observed ( Figure 9 ) in order to examine the crystallinity of ferrite particles. It is preferable to make ferrite particles with high crystallinity. The relative intensity at a d-value of 2.530 (2 0 was about 35.5 degrees, the strongest value among three d-values) was shown with respect to the initial concentration of selenium. 

Figure 7. Amounts of selenium and iron leached from the ferrite crystals coated with magnetite, following the ferrite formation with Se(VI) soln.

Figure 8-1. Particle size distribution of the ferrite crystals formed from 400 mg-Se(rV) soln., by SALD-3000 with sonicator. (Median dia. 1.930 m, mode dia. 2.616, avg. dia. 1.505)...

Figure 9. XRD intensity of ferrite crystals formed from the solutions containing Se(IV) or Se(VI).

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