IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 5 , SEPTEMBER 1989

THE EFFECT OF POST-HEAT TREATMENC ON STRUCTURE, MAGXFTIC AND MECHANICAL PROPERTIES OF HOT ISOSTATIC PRESSED (HIP) NiZn FERRITE.

Zheng Yang and Bian Xiao-ping Research Institute of Magnetic Materials

Lanzhou University, Lanzhou, Gansu, China.

f(MHz)

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1 3 5 7 9

Abstract: The effects of hot isostatic pressing and post-heat treatment on the structure, magnetic and me- chanical properties of NiZn ferrites were studied. It was found that magnetic and mechanical properties of NiZn ferrites were damaged in the HIP process. The X- ray diffraction analysis shows that the equilibrium oxygen content of the ferrite cannot be kept during HIP process in argon gas, and a part of the spinel phase should be decomposed into some oxides because of loss oxygen in the surface layer of the sample. This leads to decrease both soft magnetic properties and bending strength of the NiZn ferrite. The larger the ratio of surface to volume of sample, the more striking the abo- ve-mentioned damaging effect. Experiments show that the magnetic and other physical properties of HIP NiZn ferrite could be improved remarkably by appropriate post-annealing.

After post-annealing Tp=9 0 0 "C

looooc

IPiTIIODUCTION

560 390 220 210 220 140 190 200 110 140 880 610 410 420 320 150 350 320 270 210

~ - - - -

- - - - ~

The HIP NiZn ferrites have been widely used for digital recording heads (e.g. the erasing part of flop- py disk heads and the substrate of thin film heads for perpendicular recording), because their high-frequency magnetic properties, bending strength and machinability are good. Properties of NiZn ferrite depend on the chemical composition, crystal structure (grain size, density, defect size and distribution) and the unifor- mity of oxygen content throughout the sample [l] 121.

In this paper high density NiZn ferrites with uni- form fine grain structure were prepared by proper com- bination of presintering and HIP conditions. We inves- tigated the effect of HIP and post-heat treatment on the phase composition, magnetic permeability spectrum, B-H hysteresis loop and mechanical properties of NiZn ferri tes.

EXF'ERIMENML

The process of fabricating HIP ferrite consists of the following stages [31: preparing ferrite powder by chemical coprecipitation in the form oxalates [41; cal- cining, ball milling, cold isostatic pressing; first sintering, non-encapsulating hot isostatic pressing and post-annealing. HIP was realized at 116O0C-120OoC at pressure 1400kg/cmz. The chemical composition of ferrite is Ni0.32Zn0.68Fe204.

X-ray diffraction patterns and the crystal lattice constant were measured by the Rigaku X-ray diffracto- meter system Geigerflex DIMAX-3A. The morphology of ferrite powder and microstructure of HIP ferrites were observed by Hitachi S-550 SEM. The microhardness, Hv, was determined by a semi-automatic optical microhard- ness meter, type HAUS 249-A. The toroidal samples (@lOxQ6x2m) for measurements were prepared from fer- rite ingots. The B-H loops at different frequencies and magnetic fields were measured by a YEW-3262 AC hys- teresis loop tracer. The flux density, Bm, and coer cive force, Hc, at h=2.0 Oe, 5.0 Oe and 10 Oe were obtained from respective B-H loops. The high-frequency magnetic permeability spectrum and the temperature re- lation of low frequency permeability IJo(T) were measu- red by a CD-5 impedance bridge and a LCCG-1 inductance- capacitance meter, respectively. The bending strength 'Jf was determined by a three-point bending method on small strips of NiZn ferrites with dimensions of 28x4~ x 4 m and 28x6x2m. Theoretical calculation gives [51:

f =3PL/2bd (1)

where P=fracture load, L=span, b=sample width and d=sarnple thickness. The average value of Of was calcu- lated from measurements on 10-20 strips cut from the same ferrite ingot, with L=18m.

RESULTS AND DISCUSSION

1. The effect of HIP and post-annealing on the perme- ability spectrum and B-H loops. It was found that magnetic properties of NiZn fer-

rites were damaged by HIP, especially for samples which have high values of specific surface arealvolume ratios during the HIP process. There were two presintered samples of different dimensions: @5Ox2Om (called a bulk sample) and @10x@6x2m (called a toroidal sample). Table 1 sumnarizes the dependence of the permeability spectrum for toroidal sample of HIP and post-annealing. The post-annealing was carried out at 90O0C-12O0"C (6h) in air. It was found the magnetic properties of NiZn toroidal samples were deteriorated after the HIP pro- cess. The real permeability p'(lMHz), for example, de- creased to 78% of its initial value by HIP. By means of the post-annealing at 90O0C-120O0C, the permeability IJ'(1MHz) partially recovered, but did not achieve its initial value before HIP. The p'(lMHz) only recovered to 85% of its initial value even at optimal annealing condition. Fig.1 shows the B-H loops of toroidal sam- ples both before and after post-annealing at 1100°C.

of sample Before HIP

After HIP

1320 860 600 540 470 390 470 340 360 330 290 250 200 190 140 30 86 105 130 108

- - - - -

- - - - -

1100°C 1120 630 440 390 1280 350 320 320 210 1040 560 360 330 270 380 400 290 260 240

_ _ - - - - 1200°C

Fig.1 The effect of post-annealing on 8-H loop of toroidal ferrite (A-As HIP; B-After post-annealing at 1100°C)

0018-9464/89/0900-4239$01 .WO 1989 IEEE

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BioIGl

3 0 0 0 -

2000

1000-

In Table 2 are given relations between the permeability spectrum and both HIP and post-annealing of bulk sam- ples. In this case the samples for magnetic measure- ments were still toroids of @10x@6x2mn, but were cut from the center part of the HIP ferrite ingot of dimn- sions @5Ox20mn. In comparison to the toroidal samples (Table 11 the high-frequency permeability of bulk sam- ples decreased slightly due to the HIP process, and its magnetic properties could be increased by post-anneal- ing. The (L'(lMHz), for example, increased over 20% of its initial value after annealing at llOO"C(6h) in air.

-

Table 2 The dependence of ratio of the bulk sample on HIP and post-annealing temperatures.

1 3 5 7 9

- e: 2

-0.50

-0.40

-0.30

-0.20

- 0 . 1 0

940 530 380 350 280 - - - - -

?p=9OO0C 790 490 400 410 1 % 440 370 280 270 1340 840 620 500 420 360 500 370 330 270 1450 800 550 450 380 410 570 440 360 260

- - - - - looooc

1100°C - - - - - 1030 550 390 320 287 340 420 330 200 200 - ~ - - - 1200°C

Similar regularities were observed for parameters rela- ted to B-H loops: B2, B10, Hc(2 Oe) and Hc(l0 Oe) etc.. Fig.2 shows the effect of post-annealing temperature, Tp, on magnetic properties of both toroidal and bulk samples with the same chemical compositions. From Fig.2 we can see that the effect of the atmosphere during the HIP process on properties of small samples is striking, and it is important to carry out the post-heat treat- ment even for bulk samples.

1 1 1 1 1 l I I 0 "as HIP 900 1000 1100 Tp("C)

Fig.2 Effect of the post-annealing temperature (Tp) on magnetic properties of toroidal(0)

and bulk(.) ferrites.

2. Relation between the low-frequency permeability and temperature. It was sham experimentally that the HIP process

and post-annealing act upon not only the high-frequency permeability spectrum, but also on the Po(T) curve of NiZn ferrites (Fig.3). sharply, and Curie temperature changes from 113°C to 186°C due to the HIP process. The Curie temperature is a structure-insensitive characteristic which is determined by the chemical composition and phase struc- ture of the ferrite. This means that the phase struc- ture of toroidal samples have changed during HIP pro- cess and post-annealing. X-ray structure analysis and crystal lattice constant measurements, which will be discussed in detail in the next section. Clearly, in order to improve the soft

It was found that the po drops

This has been verified by

Fig.3 The CIO(T) curves of toroidal ferrite (0-Before HIP, .-As HIP,

' 4 ti -After post-annealing)

magnetic properties and remove the strains of as HIPPed ferrite, a suitable post-annealing is indispensable.

3. The effect of the HIP process and post-annealing on

Fig.4 shows X-ray diffraction patterns of toroidal ferrite samples. It was found that the sample consists of a single spinel phase before HIP, but after HIP other phases (such as Fe2O3, ZnO etc.) appear. This is the reason why soft magnetic properties of these samp- les deteriorated remarkably by the HIP process. post-annealing the oxides disappear gradually, and mag- netic properties recover partly with increasing anneal- ing temperature. Although a sample annealed at 1100°C also has a spinel structure (Fig.Q-C), the amplitude of the diffraction peaks declines clearly and the position of peaks shifts slightly in comparison with that of the same sample before HIP. It is difficult to identify

the structure of NiZn ferrite.

In

Fig.4 X-ray diffraction patterns of toroidal samples (A-Before HIP, B-As HIP and

C-Post-annealing at 1100°C).

the changes of phase structure by X-ray diffraction patterns because related phases are all spinels. We can see directly the changes of phases of samples from results in Fig.3. It was known that the Curie tempera- ture of NiZn ferrite is related to its stoichiometry, so the changes of phases shoud effect the Po(T) curves.

X-ray diffraction patterns show that there is not change of the phase structure inside the bulk sample, and no extra oxides appear after HIP. Fig.5 shows the relation between lattice constant of both toroidal and bulk samples via HIP and annealing temperature. In the case of the bulk samples, the lattice constant doesn't change until Tp=llOO°C. For toroidal samples the lat- tice constant decreases after HIP and then increases with rising of the temperature up to Tp=1100"C. The change of phase structure could be analysed by means of the change of lattice constant. In Nii-,ZnxFe2O4 fer- rites the relationship between lattice $onstant,a, and parameter,X, is nearly linear. a=8.343A for Ni-fer- rite (X=O) and a=8.444A for Zn-ferrite (X=l) [6]. From Fig.4 and Fig.5 i t was found that the part of original spinel phase had been decomposed into Fe2O3, ZnO etc. and a new spinel phase was formed in which the Zn con- tent is lower. This caused the decreasing of lattice constant and the increasing of the Curie temperature (Fig.3). The phase structure can be gradually recover- ed towards the original one by rising of Tp(but incomp- letely). Therefore, i t is necessary to control the atmosphere in HIP process because the recovery of the stoichiometry is incomplete in the post-annealing.

a(W)

8.415-

8.410-

.-Bulk sample 0-Toroidal sample

.--e -e.- -0-0-

0 , ,o-o,o-o~ "\. ' 0

' -0 - -

I I l l I I I I ,

Fig.5 The dependence of the lattice constant on the HIP and post-annealing temp.(Tp).

properties \ Bending strength

uf(kg/mn2) Vicker Hardness

4. The mechanical properties of NiZn HIP ferrite

bending strength of of HIP ferrites decreases with in- creasing grain size and experimental data could be ex- plained qualitatively by the empirical equation: Of= =Oo+olD-h, where D is average grain diameter [51. The bending strength of ferrites with uniform grain size is bigger than that of ferrites with exaggerated grains. It was found the of and Vicker hardness(Hv) are rela- ted to the HIP process and post-annealing temperature (Tp) as given in the Table 3. The Uf increases at first with the increasing of Tp and then decreases when Tp71100"C. On the contrary, the Hv remaines constant with increasing Tp. Experimental results show that the

It was shown in our previous paper 141 that the

I 18.0 18.4 19.2 20.3 21.0 19.2 8.9

900 -- 820 820 820 820 --

Table 3 The effect of HIP and post-annealing on of and Hv (bulk sample).

TP( "C) M e c h a > c d 1 as HIP1800 1900 ~1000~1100~1200~1300

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of and Hv of NiZn ferrite are dependent of the density. For comparison in Table 4 were given mechanical proper- ties of the sintered, hot pressed and HIP ferrites. A SEM photograph of fracture surface of the HIP-ferrite is shown in the Fig.6. It was evident that NiZn HIP ferrites have an improved magnetic and mechanical pro- perties and are very good head materials for digital magnetic recording.

Table 4 The mechanical properties of ferrites Type of ferrite of ( kg/m2 ) Hv Sintered 13.0 680 Hot -pressed 18.5 830 Hot-isostatic pressed 21.0 900

Fig.6 SEM photograph of fracture surface of HIP NiZn ferrite

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The X-ray diffraction analysis and magnetic mea- surements show that equilibrium oxygen content of the NiZn ferrites cannot be kept during HIP process, and a part of the spinel phase in the surface layer of the sample should be decomposed into some oxides (such as Fe203,ZnO etc.). This leads to decrease both soft mag- netic and mechanical properties of the NiZn ferrites. Therefore, in order to obtain uniform oxygen content and remove the strains of as HIP ferrites, a suitable post-annealing is indispensable. The HIP Ni0.32Zn0.68 Fe 04 ferrite with B10=3100 G, Hc=0.19 Oe, P'(lMHz)= =l$OO, Uf=21kg/m2 and Hv=850 was obtained.

REFERENCES

T. Shinohar et al., "High B Polycrystalline Mn-Zn Ferrite", Proceedings of ICF-3, 1980, pp.321-323. E.B. Rinbv et al., "Preparation of Coprecipitated - . Ferrite", IEEE Trans. Magn., Vol. MAG-20, No.5, 1984, pp.1506-1508. Z. Yang et al., "The Study on HIP Ferrites", J- Magnetic Materials and Devices, No4, 1986, pp.1-5. 2 . Yang et al., "The Mechanical Propertics of Fer- rites", Acta Electronics Sinica, Vol. 15, No4, 1987, pp.29-32. S.C. Carniglia, "Reexamination of Experimental Strength-vs-Grain-size Data for Ceramics", J.Am. Ceram. Soc., Vo1.55, Nos, 1972, pp.243-249. H.H. Shortz at al., Ferrite for Radio-frequency, Moscow, "Energy" Press, 1966, Ch. 3.