Heat Treatment Effects on Electrical Resistivity of Spinel Ferrite Layer for PowderMagnetic Core+1

Junghwan Hwang1,+2, Seishi Utsuno1 and Ken Matsubara2

1Toyota Central R&D Inc., Nagakute 480-1192, Japan2Denso Corporation, Kariya 448-8661, Japan

The powder magnetic core having a spinel ferrite insulating layer has enabled high magnetic flux density and high electrical resistivity,because spinel ferrite exhibits magnetic insulating properties. However, its the electrical resistivity of the powder magnetic core was reducedafter annealing at 873K, since FeO was formed in the spinel ferrite insulating layer. The eutectoid transformation caused the decomposition ofFeO, which decompose into Fe3O4 and ¡-Fe at the temperature range below 833K. We verified the possibility of eutectoid transformation of FeOin the insulating layer by the two-step heat treatment at 873K and 773K. The presented results clearly showed that the two-step heat treatmentincreases the electrical resistivity of the insulating layer due to the disappearance of FeO by eutectoid transformation even in the insulating layer.[doi:10.2320/matertrans.Y-M2019813]

(Received November 14, 2018; Accepted March 8, 2019; Published April 26, 2019)

Keywords: spinel ferrite, magnetic insulating layer, heat treatment, electrical resistivity, eutectoid transformation

1. Introduction

A powder magnetic core was fabricated by compactingpowders of approximately 100 µm in diameter with thesurface coated by an insulating layer. This core showed goodisotropic magnetic properties, a high electrical resistivity, anda low eddy current loss at high frequencies. These benefitscan improve its applicability in the electromagnetic valves ofinjectors1,2) and motors.3,4) Typically, the powder for powdermagnetic cores is coated with nonmagnetic insulators,including silicone resins and phosphates, which inducemagnetic resistance between the powder particles becauseof their demagnetizing field effects. Therefore, it is hard tomagnetize as much as the magnetic field applied and achievehigh permeability and saturation magnetization with typicalpowders. In our previous study, we applied a spinel ferrite,a magnetic insulator, to an insulating layer to solve thisproblem, and found that the magnetic resistance between thepowder particles was reduced and that the magnetic fluxdensity of the powder magnetic core was improved.5)

However, the electrical resistivities of the powder magneticcores with spinel ferrite layers were decreased during the heattreatment (HT) performed to remove compaction-relatedprocessing strain. This decrease in the electrical resistivitypresumably occurred because of the production of FeO(wüstite), having a resistivity lower than that of spinel ferrite,6)

was generated in the insulating layer by HT at >873K.7,8)

Accordingly, suppressing the retention of FeO after HT isnecessary to produce a powder magnetic core with a spinelferrite layer that retains high resistivity even after HT at873K. The present study was conducted to verify whetherthe eutectoid transformation of FeO at <843K,9) i.e., thedisappearance of FeO caused by its decomposition into Fe3O4

(magnetite) and ¡-Fe, could be observed within a spinelferrite layer on a powder surface. For this purpose, weutilized a two-step HT with primary heating at 873K using

pure Fe powder coated with a spinel ferrite layer, whichmight remove the processing strain, and subsequentsecondary heating at 773K. We thereby evaluated thepossibility that this two-step HT could improve the electricalresistivity of the powder magnetic core in the insulating layerby focusing on the eutectoid transformation of FeO.

2. Experimental Methods

The pure Fe powder with spherical particles of ³150 µmmean diameter coated with MnZn spinel ferrite was preparedas a precursor. The MnZn spinel ferrite layer was formed byMA (Mechanical Alloying). In brief, 100 g of Fe powdersand 0.3 g of MnZn spinel ferrite powders of the particlediameter <1µm (Kojundo Chemical Lab. Co. Ltd.) werepoured into a 500-mL plastic bottle, this plastic bottle wasrotated with a rotary mixer mill at a speed of 300 rpm for360 ks followed by a two-step HT. The specimens underwentprimary HT at 873K for 3.6 ks in a tubular furnace in anitrogen atmosphere followed by furnace cooling to 300K,and then secondary HT at 773K lasted 3.6 ks. The heatingrate was 0.17K/s for both HTs. Each of the as-coated and theheat-treated powders were each analyzed regarding for theirlayer characteristics.

The electrical resistivities of the surface layers of theindividual powder particles were measured using a microp-robe10) set in a scanning electron microscope (SEM JSM-7000F; JEOL Ltd.). The equipment configuration, aschematic of the electrical resistivity measurement system,and the obtained SEM images are shown in Fig. 1. Thetwo W-electrode probes coated with Au (PT-14-6705-B;Kleindiek Nanotechnik), denoted as probes 1 and 2 in Fig. 1,were used to monitor the electrical resistivity. The powderspecimen for measurement was press-fitted to an In sheet,mounted on a SiO2/Si substrate with Cu tape, and set in theSEM. The current­voltage (I­V ) relationship of the powderspecimen was measured for the electrode voltages ¹0.1­0.1V during the observation at 3 keV. The electricalresistance R was obtained by fitting this I­V correlationusing the least-squares method. Three powder specimens

+1This Paper was Originally Published in Japanese in J. Jpn. Soc. PowderPowder Metallurgy 65 (2018) 171­175.

+2Corresponding author, E-mail: hwang@mosk.tytlabs.co.jp

Materials Transactions, Vol. 60, No. 6 (2019) pp. 1061 to 1065©2019 Japan Society of Powder and Powder Metallurgy

were used for the measurements. The distance between theprobe electrodes was fixed at 5 µm during the probe contactwith the powder. In addition, the maximum and minimumvalues for the probes were determined to measure theelectrical resistivity by adjusting the contact pressure. Theminimum value obtained was 37³ when the probes were inmutual contact. The maximum value was 3.2G³ when theprobes were floating in the absence of any contact.

The phases formed in the insulating layers were identifiedusing Auger electron spectroscopy (AES PHI 700; Ulvac-PhiInc.) and the powder X-ray diffraction (XRD Ultima IV;Rigaku Corp.). Furthermore, after depositing carbon onthe powders in vacuum, the specimens were prepared usingthe micro-sampling technique with focused ion beam (FIBNB5000; Hitachi High-Technologies Corp.). These speci-mens underwent cross-sectional layer observation bytransmission electron microscopy (TEM), phase identifica-tion via selected-area electron diffraction (SAED) byintegrating the diffraction spot intensity, and elementalanalysis via energy dispersive X-ray spectral analysis(EDX) using a scanning transmission electron microscope(STEM JEM-2100F; JEOL Ltd.).

3. Results

3.1 Changes in the electrical resistivity of spinel ferritelayers by HT

Figures 2(a), 2(b), and 2(c) show the I­V curves of thepowder coated with spinel ferrite before the HTs, after the

primary HT at 873K, and after the subsequent secondaryHT at 773K, respectively. Based on these I­V curves, theiraverage electrical resistivities were determined (Fig. 3). Thepowder coated with spinel ferrite initially displays a highelectrical resistivity, whereas its the resistivity is decreased byfive orders after exposure to the primary HT. Notably, thesubsequent exposure to the secondary HT results in anincrease in its electrical resistivity. Collectively, the electricalresistivity of the powder with the spinel ferrite layer isdecreased and increased by the primary and the primary plussecondary HT, respectively.

Embed in Indium sheet

Insulating layer

Probe 2

Pure FePowder

Probe controller

Probe

Holder

Specimen

Enlarged View

100 µm

Probe 1

Fig. 1 Schematic of electrical resistivity measurement system by microprobe in SEM.

Voltage (V)0.10.050- 0.05- 0.1

Curr

ent (

10-9

) (A

)

5

10

15

-5

-10

-15

0

(a)

Voltage (V)0.10.050- 0.05- 0.1

0

2

3

-2

-3

Curr

ent (

10- 4

) (A

) (b)

1

-1

0

4

6

-2

-4Curr

ent (

10-6

) (A

) (c)

-6

2

Voltage (V)0.10.050- 0.05- 0.1

107 Ω

2 107 Ω

6 107 Ω

4 102 Ω

6 102 Ω

103 Ω

2 104 Ω

4 104 Ω

Fig. 2 The I­V curves of the insulating layer after each heat treatment. (a) Before HT, (b) 1st HT (873K), (c) 2nd HT (773K).

BeforeHT

1st HT(873 K)

2nd HT (773 K)

1

104

106

108

Elec

trica

l res

itivi

ty(Ω

)

103

105

107

102

10

Fig. 3 Change in electrical resistivity of powder having the spinel ferriteinsulating layer after each heat treatment.

J. Hwang, S. Utsuno and K. Matsubara1062

3.2 Analysis of layer properties by XRD and TEMobservations

To investigate the phase changes in the electricallyresistive spinel ferrite layer upon HT, the coated powdersbefore and after HT were analyzed using XRD to investigatethe phase changes in the electrically resistive spinel ferritelayer upon HT. As shown in Fig. 4(a), the peaks of the spinelferrite are detected in the pattern from the layer of the as-coated powder, as shown in Fig. 4(a), while the generation ofFeO other than spinel ferrite is observed after the primary HT(Fig. 4(b)). As seen in Fig. 4(c), the FeO peak disappearsfrom the pattern of the layer after the secondary HT and onlythe peaks of spinel ferrite present were detectable. The peakintensities of spinel ferrite are enhanced, probably because ofthe coarsening of ferrite crystal grains during HT.

The phases formed in the layers of the as-coated powderand those after exposure to each HT were analyzed by TEMobservation. The cross-sectional TEM image of the as-coatedpowder particle before the exposure to the primary HTshowed the presence of the thin layer of 10­100 nm inthickness (Fig. 5(a)). The concentric Debye­Scherrer ringsare observed in the SAED pattern of the layer. Figures 5(b)and 5(c) suggest that the peak positions determined by thediffraction intensities of the Debye­Scherrer rings indicatethe lattice spacing of spinel ferrite.

Figure 6 presents the TEM image and the SAED pattern ofthe particle layer (100 nm in diameter) in the selected-areaafter the primary HT. The insulating layer thickness is 20­150 nm. Particulate grains of ³100 nm in diameter areobserved. The SAED pattern of the grain marked with adashed circle shows the presence of the diffraction patternsuggesting a specific orientation (Fig. 6(a)). The resultsobtained by the XRD demonstrated the coexistence of thetwo phases of spinel ferrite (Fd-3m, a = 0.8411 nm) and FeO(Fm-3m, a = 0.4354 nm) in the layer of this specimen11)

(Fig. 4(b)). The plane index and incidence orientation of eachdiffraction spot determined for each crystal structure in theSAED pattern matched those of both phases (Fig. 6(b)). Thisagreement is related to both the phases possessing cubiccrystal structures; in addition, the lattice parameter of spinelferrite is approximately twice that of FeO. This was in goodagreement with the extinction rule of diffraction reflections.Generally, MnZn spinel ferrite compounds are produced bymixing precursor powders of oxides of Fe, Mn, and Zninto prescribed compositions and calcining the mixtures attemperatures above 1273K.12)

Accordingly, spinel ferrite is markedly heat-resistant athigh temperatures. Although the spinel ferrite naturallygenerates no FeO at ³873K, FeO is formed even underHT at the low temperature of 873K in the spinel ferrite

(b)

(c)

α-Fe

α-Fe

α-FeSpinel ferrite

FeO

(a)

Inte

nsity

(arb

.uni

t)

25 30 40 5035 452θ (deg)

Fig. 4 XRD patterns. (a) Before HT, (b) 1st HT (873K), (c) 2nd HT(773K).

Selected-area aperture

Insulating layerProtective layer(C)

Protective layer(W)200 nm

(a)

(b)

(c)

Fig. 5 The Phase identification of Crystal phase in an insulating layer. (Before HT) (a) TEM Image, (b) Debye-Scherrer rings, (c) RadialIntensity.

Selected-area aperture

000

2-22

-22-2

40-8

6-2-6

-626

-408

Insulating layer

Protective layer(C)

Protective layer(W)

BD=[-2-3-1]X=14.6 Y=15.1

200 nm

(a)

(b)

Fig. 6 The Phase identification of Crystal phase in an insulating layer.(1st HT at 873K) (a) TEM Image, (b) Index of spinel ferrite and FeO.

Heat Treatment Effects on Electrical Resistivity of Spinel Ferrite Layer for Powder Magnetic Core 1063

surface layer on Fe powder. The layer compositions of thecoated powder before and after primary HT analyzed by AESare shown in Fig. 7(a) and (b), respectively. These resultsdemonstrate that Fe contents near the surface after theprimary HT. Since the Fe concentration gradient betweenthe powder core and the spinel ferrite layer was observed, thisincrease is attributed to the diffusion of iron ions (Fe2+) fromthe Fe powder to the layer. It is likely that FeO is generatedafter the primary HT at 873K in the layer because of thereaction of diffused Fe2+ ions with oxygen ions (O2¹) alreadypresent in the layer, i.e., the interfacial reaction between theFe powder and the spinel ferrite layer.

As shown in Fig. 8, the thickness of the layer aftersecondary HT is around 100­200 nm. The crystal grains inthe layer have cubic-based crystal structures similar to thatshown in Fig. 6. Together with the results obtained by theXRD (Fig. 4(c)), this observation indicates that the layermainly contains the spinel ferrite crystals. Since precipitationnuclei were too fine to detect by the TEM resolution used for

this experiment, these results cannot yet verify the existenceof ¡-Fe in the layer arising from the eutectoid transformation.Consequently, these results suggest that the change inelectrical resistivity of the spinel ferrite layer depends onFeO generation in the layer.

3.3 Analysis of layer composition by STEM-EDXThe combination of the simple determination method with

the point EDX analysis determined the elemental contentsat the locations marked with circles in Fig. 9 (Table 1). Thelayer compositions after secondary HT in Fig. 9(c) areevaluated at two locations: near the Fe powder surface(1) and the layer surface (2). Relative to the content of theas-coated powder, the Fe content and the total contents ofMn and Zn in the layer after primary HT are increased anddecreased, respectively. As described above, Fe ions in Fepowder are diffused into the layer to form FeO and Fe3O4.Therefore, it is likely that the Fe content in the layer isincreased and the relative contents of Mn and Zn aredecreased. Although these Mn and Zn may exist in the layeras spinel ferrite as shown in Fig. 4(b), further study isrequired to understand the details such as its effect. Theelement contents vary with the layer thickness after thesecondary HT at 773K. The total contents of Mn and Zn

Inte

nsity

(10

4 )

Depth (nm) Depth (nm)

(a) (b)layer powder layer powder

Fig. 7 The elemental contents in the insulating layer analyzed by AES. (a) Before HT, (b) 1st HT (873K).

Table 1 The elemental contents in the insulating layer after each heattreatment.

Selected-area apertureX=11.7º, Y=13.9º

000 -2-4-2242

2-4-2

-242-400

400

Insulating layer

BD=[0-12]

Protective layer(C)Protective layer(W)

(a)

(b)

200 nm

Fig. 8 The Phase identification of Crystal phase in an insulating layer. (2ndHT at 773K) (a) TEM Image, (b) Index of spinel ferrite.

12

500 nm

(a) (b) (c)

Fig. 9 TEM images of coated powder cross section with EDX analysis point. (a) Before HT, (b) 1st HT (873K), (c) 2nd HT (773K).

J. Hwang, S. Utsuno and K. Matsubara1064

approach their contents in the layer of the as-coated powdernear the surface of the Fe powder (1), although thecomposition at the layer surface (2) remains almost identicalto that after the primary HT. The eutectoid transformationof FeO to Fe3O4 was inferred as a major cause of theincreased Mn and Zn contents after the secondary HT in thelayer. The Fe content was decreased and the contents ofMn and Zn are increased because Fe3O4 has a lower Fecomposition ratio than FeO. Hayashi et al.13) investigated thephase transformation behavior of scale on pure Fe. Accordingto their results, Fe3O4 nucleation preferentially occurredpreferentially at FeO/Fe interfaces in the eutectoid trans-formation of FeO on pure Fe. The elemental contentspresented in Table 1 also indicated high concentrations ofMn and Zn in the layer near the Fe powder surface, but lowconcentrations of Mn and Zn at the FeO/Fe interface. Theseresults strongly suggest that the eutectoid transformationfrom FeO to spinel ferrite also occurs near the Fe powdersurface under these experimental conditions. In future, thedetails of this phase transformation and composition changemust be further verified.

We infer that the primary cause of the increase in theelectrical resistivity in the insulating layer after the secondaryHT (Fig. 3) are the disappearance of FeO generated by theprimary HT via the eutectoid transformation during thesecondary HT. However, the electrical resistivity was notfully recovered by the secondary HT to that of the as-coatedpowder, which might be attributed to the presence of the highfraction of Fe3O4 other than MnZn ferrite in the insulatinglayer. The total contents of Mn and Zn were increased toonly ³40% of those of the coated powder after the eutectoidtransformation from FeO to the spinel ferrite by secondaryHT; this suggests that Fe3O4, a spinel ferrite phase containingneither Mn nor Zn, occupies the remainder. Among spinelferrites, Fe3O4 has a low electrical resistivity, with the lowspecific resistance of 40 µ³·m8) as compared to that of 106­107 µ³·m.14,15) Based on our present the present results, tofurther improve the electrical resistivity of the powder afterthe secondary HT, it is necessary to reduce the Fe3O4

contents and increase the fraction of MnZn spinel ferrite inthe insulating layer.

4. Conclusion

In the present study, we have assessed the possible effectsof the two-step HT-induced eutectoid transformation of FeOon the electrical resistivity of the powder magnetic core in a

spinel insulating layer. To this end, the pure Fe powder witha spinel ferrite coat was subjected to the primary HT at 873Kfollowed by the secondary HT at 773K.(1) The electrical resistivity of the powder coated with

spinel ferrite was decreased after primary HT at 873K,however, the subsequent exposure to secondary HT at773K restored electrical resistivity.

(2) Primary HT at 873K produced FeO, whereas FeObecame undetectable in the layer after the subsequentexposure to secondary HT at 773K, indicating that theeutectoid transformation of FeO might affect the changein electrical resistivity of the layer.

(3) The total contents of Mn and Zn in the layer after theexposure to secondary HT varied dependent on thedepth of the layer. Although the composition at thetopmost surface of the layer was almost identical to thatafter primary HT, Mn and Zn contents approachedtheir concentrations in the layer of the as-coated powdernear the Fe powder surface. Secondary HT did not fullyrestore the electrical resistivity of the as-coated powder,which might be due to the higher fraction of Fe3O4

relative to MnZn spinel ferrite in the insulating layer.

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Heat Treatment Effects on Electrical Resistivity of Spinel Ferrite Layer for Powder Magnetic Core 1065