The Metallography of Ferrites

Abstract

I. lntroductionAn accurate interpretation of the microstructures, however, cam be made only if suitable specimen preparation has been achieved.EASUREMENTS of grain size, grain orientation, and porosity and the identification of the phases present can be important "tools" for coordinating the fabrication techniques with the electrical characteristics of ferrite materials. II. Preparation of Ferrite Specimens M ( I ) Grinding and PolishingThe method of grinding and polishing a ferrite specimen may vary slightly with the particular ferrite. However, the following procedure, or a modification of it, worked successfully in all instances. Rough grind on a silicon carbide cloth (grit 400) and then silicon carbide cloth (grit 600) and Microcut paper. All the grinding operations were done on a horizontal wheel at 550 r.p.m. using a 25% alcohol-water solution.For the more friable samples, an intermediate step between the fine grinding and the usual polishing may be necessary. This consists of polishing on a silk wheel lightly charged with 1200-grit lapping compound.Most surfaces obtained with wet-grinding techniques were easily polished with 1 p Diamet-Hyprez paste on kitten-ear broadcloth or with aluminum oxide (1 p ) or chromic oxide ( l p ) on a heavily loaded silk wheel at 550 r.p.m. For the final polishing 0.5p chromic oxide was used on a silk wheel a t 160 r.p.m. (2) EtchingSince ferrite materials are prepared in a wide range of compositions, their etching properties may vary considerably. Grain boundaries were clearly delineated with the following etches: etch No. 1, 50% aqueous HCl; etch No. 2, 50% concentrated H2S04, 50% saturated oxalic acid.The time necessary to bring out the grain boundaries varies from 4 to 15 minutes in Fig. 1. Typical microstructure of a nickel ferrite (bright field, 550 mp).Etch No, 1 is used on manganese ferrites. Fig. 3. Inter-and trans-crystalline cracking in a nickel ferrite (bright fleld, 550 mpl. Fig. 2. Etch flgures in a zinc-nickel ferrite (bright field, 550 mp). the temperature range 85" to 90°C. Etch No. 2 is used on nickel and zinc ferrites (Fig. 1). The etching time is approximately 1 hour a t 110°C. The condition of the etched surface, as might be expected, is a function of time and temperature. At higher temperatures the grains are more likely to be attacked, whereas a t lower temperatures only the grain boundaries are attacked. An optimum combination of temperature, timp, and etchants can be used to produce specific results in any given ferrite. 111. ( I ) Cryrfolline OrientofionOn etching, once an atom is removed from the crystal surface, dissolution will start quite readily near this void; an etch pit will then be formed. The development of etch figures depends on lattice imperfections as well as on mechanical stresses and ruptures on the surface of a crystal plane. In the study and analysis of mature etch figures, the most useful information is derived from the geometry of the etch patterns produced on planes of high atomic density. Each crystallographic plane has its own ...

Sintering of Ni-Zn ferrite nanopowders by the constant heating rate (CHR) method

“…This knowledge is important to allow changes in the composition and processing of ferrites to be effected, with the purpose of obtaining microstructures that confer on the sintered material properties and characteristics appropriate for their application. Some of these studies are the following: Levesque 10 and Paulus 11,12 studied the grain growth of Mn-Zn ferrites in the early stage of sintering; Jain et al 13 reported on grain growth kinetics in the intermediate stage of Mn-Zn-Fe ferrite sintering, which was obtained by the conventional oxide blending method. The activation energy determined for the intermediate stage was 130 Kcal/mol; Dias et al 3 investigated the sintering kinetics of the intermediate and final stages of Ni-Zn ferrites prepared by hidrothermal synthesis; Parvatheeswara Rao et al 14 studied the evolution of Ni-Zn ferrite microstructures at sintering temperatures in the range of 1150 ° to 1300 °C for 1 to 4 h. Densification and grain growth was reported as Arrhenius rate controlled processes with activation energies of 63.9 and 64.4 Kcal/mol, respectively.…”

Study of NiZn Ferrite Complex Permeability ‐ Effect of Relative Density and Microstructure

“…(5), Fig. 4 plots the evolution of Љ r with average grain size (G a ) for three values of s (80%, 90%, and 100%) and for three grain size distribution widths (3,15, and 28 m). Equation (5) and Fig.…”

“…They were then subjected to thermal etching by heating at 100°-120°C below sintering temperature for 30 min, using a heating rate of 15°C/min. 15 The etched surfaces were observed in a scanning electron microscope (SEM).…”

In situ “artificial plasma” calibration of tokamak magnetic sensors

“…The geometry of the new conducting wall and magnetic sensors were measured with ROMER and FARO coordi- nate measuring machines (CMMs) 17 . First, a ROMER INFINITE portable measuring arm was used to measure sensors and conducting wall segments as assembled on the bench.…”