Journal of Magnetism and Magnetic Materials 7 (1978) 26-30

0 North-Holland Publishing Company

GAS-PHASE FLUORINATED FERRIMAGNETIC MATERIALS

R. HEINDL, R. DESCHAMPS, M. DOMINE-BERGES and J. LORIERS

Centre National de la Recherche Scientifique, Laboratoire de Bellevue, 92190 _ Meudon, France

Mn-Zn and Ni-ferrites have been treated by diffusion-fluorination techniques in order to increase their sheet resistivity

and to preserve their bulk magnetic properties. Improvements have been obtained: an increase of the resistivity and of the

quality factor Q: the permeability, however, decreases. The penetration process of fluorine in the porous materials is dis-

cussed.

1. Introduction

The usefulness of ferrites in electronics industry is not determined by their magnetic properties alone,

but also by their insulating properties which keep the eddy currents at a tolerable level at high frequencies. It is well known that the substitution of oxygen by fluorine induces an increase of the resistivity of mag-

netic oxides; this substitution can be easily achieved because the monovalent F- ion has almost the same ionic size as that of the divalent 02- ion. Charge

compensation in the fluorine-substituted compound is obtained by partial reduction of some of the com-

ponent cations [l] [2] or by replacing part of these by other cations of lower valence [3-51. A typical

formula of such substituted compounds is for instance for spine1 ferrites:MtL, Mi+Fe204_XFX.

Fluorinated magnetic oxides have been prepared

till now only by solid state reaction, the fluorine being introduced in the starting ingredients in the form.of fluoride. The substituted compounds do indeed have a higher resistivity, but their permeability is generally more or less lowered, the magnetization and anisotropy of the materials being modified by the fluorine substitution [3-S].

The purpose of the present study was to investigate a gradual gas-phase fluorination process of already sintered ferrites, allowing an increase of the sheet resistivity without affecting the magnetic bulk prop- erties of the materials too much.

Both laboratory basic compounds and industrial ferrites have been treated by this method. The results

given here concern two typical classes of industrial materials: Mn-Zn ferrites and Ni-Zn ferrites.

2. Experimental

Three methods of fluorination have been investigated

in preliminary studies:

(1) solid state reaction with ammonium fluoride, (2) reaction in a flow-system of gaseous fluorhydric

acid, (3) treatment by molecular fluorine under pressure.

Fluorination of ferrites by NH,F was obtained by placing the samples in a platinum crucible filled with

hand-pressed NH4F, covering the crucible and heating it in an electrical furnace in air at about 300°C.

Attack by gaseous HF was carried out in a monel- tube with “teflon” connections; through it, a gas-flow of HF diluted in 90% of argon was passed. The sam- ples were heated at temperatures ranging from 200°C

to 6OO”C, during 30 min to 4 h. A more sophisticated monel-apparatus * was used

for the fluorination by molecular fluorine under pres- sure. The samples are placed in a portable cylindrical reactor vessel, that can be connected to a fluorine- bottle for filling and a CaO-containing furnace for fluorine destruction (fig. 1). After evacuation of the whole apparatus by high-vacuum pumping, the cal- culated amount of fluorine is introduced in the reac-

* Built at the Atomic Energy Center, Saclay.

26

R. Heindl et al. / Gas phase fluorinated ferrimagnetic materials 21

roughing pump

Fig. 1. “Monel” alloy apparatus for fluorination of solid ma- terials under pressure.

tor vessel by cooling it with liquid nitrogen; then the

reactor is closed, separated from the assembly, and put into an electrical furnace. Experiments have been performed with heating temperatures ranging from

180°C to 400°C fluorine-pressures from 1 bar to 4 bars, and reaction times from 1 h to 9 h.

The effectiveness of the fluorination processes was

checked in all cases by weighing the samples before and after treatment, taking into account the difference of molecular weight between 0, and F, (with the

assumption that each substituted F- ion replaces one

oxygen). The concentration of the cations were deter-

mined before and after fluorination by X-ray-fluor-

escence spectrometry. Cross-section examinations of

some samples have been also performed in the scan- ning electron microscope.

Table 1 Experimental conditions of the fluorination of the Mn-Zn ferrite

The variations of the typical properties of the fer-

rites (permeability and losses) were determined before and after fluorination in the l-5 MHz frequency

range by the usual methods. The losses, which are lowered by increase of resistivity, were evaluated by

winding coils on the initial and reacted cores, and measuring the effective resistance I&.

3. Results and discussion

Among the three fluorination methods used, only

the F, gas process gave convincing results. In HF gas flow, no fluorination took place under 3OO”C, and above this temperature, the samples were severely

attacked. Diffusion-treatment in NHbF also corroded the samples. Measurements could still be carried out on specimens of Mn-Zn ferrite treated in NH4F at

300°C after removal of a dusty brownish fluorinated layer; both permeability and resistivity increased to a

slight extent, but it is difficult to ascertain whether this gain is due to the fluorination or more probably to the elimination of some surface-stresses.

The experimental conditions of treatment by

molecular fluorine under pressure are listed in tables 1

and 2, for Mn-Zn and Ni-Zn ferrites respectively. Very

different rates of fluorination are observed, the more

rapid reaction tacking place in Ni-Zn ferrite. It seems

that the fluorination rate is not determined only by

the porosity of the materials (the more porous spec- imen studied being the Mn-Zn ferrite with about 70%

Time Pressure Temperature Weight A weight

(h) (bars) eo before (g) (mg)

1 1 200 3.3255 - 2.5 -

1 2 200 3.3251 - 1.0 1 4 200 3.3425 - 0.1 -

2 1 200 3.3423 - 0.4 -

2 2 200 3.3047 + 0.5 0.015 2 4 200 3.2945 + 1.1 0.033 3 4 200 2.9458 + 3.2 0.11 6 4 200 2.9244 + 3.6 0.12 6 4 200 2.9136 + 4.4 0.15 9 4 200 2.9185 + 7.4 0.25 9 4 200 2.9796 + 10.5 0.35

A wt.% Fluorination (%/mole)

- - - - 0.07 0.15 0.51 0.58 0.70 1.20 1.65

28 R. Heindl et al. / Gas phase fluorinated ferrimagnetic materials

Table 2

Experimental conditions of the fluorination of N-Zn ferrites

(A. porous cores - B. compact cores)

Time Pressure

(h) (bars)

A. 1 1

1 2

1 4

2 1

2 2

2 4

B 6 4

6 4

18 3.9

18 3.8 __ _~. _

Temperature Weight A weight

(“C) before (6) (md

200 1.4801 + 1.9

200 1.4659 + 4.9

200 1.4919 + 6.3

200 1.4461 + 3.8

200 1.4465 + 8.9

200 1.4816 t18.7

200 1.4849 + 4.0

200 1.4516 + 3.1

200 1.4831 + 9.6

300 1.4718 +61.0

ID), but also by chemical factors (composition and

reactivity). This is well illustrated by the electron-microscope

photographs made on samples before and after fluorina-

tion. The starting materials are prepared by sintering

powders of a few microns sized particles, obtained by

spray-drying. The as-sintered samples of Mn-Zn ferrite

A wt.q’

0.12

0.33

0.42

0.26

0.615

1.26

0.27

0.21

0.64

4.14

Fluorination

(%/mole) _~

0.57

1.57

1.99

1.23

2.92

6.0

1.28

1.0

3.04

19.6

have almost an open hole porosity (see fig. 2a), leaving the fluorine free to enter the bulk material and to

react at the grains surface. After treatment moreover,

the pores disappear (fig. 2b): an internal expansion of

the material occurs first by fluorination and fills up the pores; the penetration of fluorine into the bulk

material can then only proceed by diffusion, and is

Fig. 2. Electron microscope photographs of Mn-Zn ferritesamples (a) as-sintered (b) fluorinated.

After ret

R. Heindl et al. / Gas phase fluorinated ferrimagnetic materials 29

Fig. 3. Variation of properties (/J and R,ff) of Mn-Zn ferrite versus frequency before and after fluorination.

slowed down. In the denser Ni-Zn ferrite on the other hand, the penetration of fluorine is surprisingly not hindered, and the reaction goes on continuously (table 2) accompanied by a large external expansion (20% increase in volume after 18 h at 200°C); the dif- fusion process is therefore different and faster in this

case. The X-ray spectrometric analysis of the samples

before and after fluorination showed no change in the concentration of the cations (Zn, Mn, Ni, Fe, Ti). The weight increase is due therefore only to the sub-

stitution of oxygen by fluorine. The weight losses found on short-time fluorinated

Zn-Mn ferrites (table 1) originates from evaporation of volatile impurities when the vacuum in the reactor vessel is established. This has been checked by “blank”

experiments.

The variations of the permeability p and Reff value

versus frequency concerning the Mn-Zn ferrite are

represented in fig. 3 for different reaction times. The curves show that the Reff term (proportional to the

losses) decreases, i.e. the resistivity appreciably

increases, but that the permeability strongly decreases for long fluorination times. Fig. 4 gives a more per- tinent representation of the relative variation with fluorine-diffusion time of some typical parameters: permeability, losses, quality factor Q, and figure of

merit ~1 Q = p*/R, at 2 MHz and 3 MHz, respectively; the PQ product is somewhat lowered by fluorination for these ferrites.

In the case of nickel-ferrites, however, the merit factor should be improved, because the losses are dras- tically decreased and the permeability only slightly reduced. More precise determinations have to be per-

R. Heindl et al. / Gas phase fluorinated ferrimagnetic materials

RF Ro

. . . . .

Fig. 4. Relative variations of permeability p, losses, surtension Q and merit factor PQ of Mn-Zn ferrite with fluorination time.

formed on these ferrites owing the inadequate detec- laboratory on more compact specimens to solve this

tion threshold of our equipment for Reff measurements. difficulty.

4. Conclusion References

The partial fluorination of ferrites by molecular

fluorine appears to be an interesting technique for the obtention of highe! resistivities and lower losses; but the simultaneous reduction in magnetic permeability remains a striking problem. It is evident that the porosity of the materials we studied prevented us from to limit the penetration of fluorine into the bulk of the samples. Further experiments are in progress in our

111

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