Effect of bead milling on heat generation ability in AC magnetic field of FeFe2O4 powder

Materials Chemistry and Physics 129 (2011) 1081–1088

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Effect of bead milling on heat generation ability in AC magnetic field of FeFe2 O4 powder Hiromichi Aono a,∗ , Yusuke Watanabe a , Takashi Naohara a , Tsunehiro Maehara a , Hideyuki Hirazawa b , Yuji Watanabe c a

Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan Department of Environmental Materials Engineering, Niihama National College of Technology, Niihama 792-8580, Japan c Department of Surgery, Graduate school of Medicine, Ehime University, Toon 791-0295, Japan b

a r t i c l e

i n f o

Article history: Received 14 February 2011 Received in revised form 17 May 2011 Accepted 24 May 2011 Keywords: Magnetic materials Nanostructures Oxides Magnetic properties

a b s t r a c t Nano-sized FeFe2 O4 ferrite powder having a heat generation ability in an AC magnetic field was prepared by bead milling for a thermal coagulation therapy application. A commercial powder sample (non-milled sample) of ca. 2.0 ␮m in particle size showed a temperature enhancement (T) of 3 ◦ C in an AC magnetic field (powder weight 1.0 g, 370 kHz, 1.77 kA m−1 ) in ambient air. The heat generation ability in the AC magnetic field improved with the milling time, i.e., due to a decrease in the average crystallite size for all the examined ferrites. The highest heat ability (T = 26 ◦ C) in the AC magnetic field in ambient air was for the fine FeFe2 O4 powder with a 4.7 nm crystallite size (the samples were milled for 6 h using 0.1 mm␾ beads). However, the heat generation ability decreased for the excessively milled FeFe2 O4 samples having average crystallite sizes of less than ca. 4.0 nm. The heat generation of the samples showed some dependence on the hysteresis loss for the B–H magnetic property. The reasons for the high heat generation properties of the milled samples would be ascribed to an increase in the Néel relaxation of the superparamagnetic material. The hysteresis loss in the B–H magnetic curve would be generated as the magnetic moment rotates (Néel relaxation) within the crystal. The heat generation ability (W g−1 ) can be estimated using a 1.07 × 10−4 fH2 frequency (f, kHz) and the magnetic field (H, kA m−1 ) for the samples milled for 6 h using 0.1 mm␾ beads. Moreover, an improvement in the heating ability was obtained by calcination of the bead-milled sample at low temperature. The maximum heat generation (T = 59 ◦ C) ability in the AC magnetic field in ambient air was obtained at ca. 5.6 nm for the sample calcined at 500 ◦ C. The heat generation ability (W g−1 ) for this heat treated sample was 2.54 × 10−4 fH2 . © 2011 Elsevier B.V. All rights reserved.

1. Introduction Thermal coagulation therapy can be realized by application of an AC magnetic field from external coils to cancer tumors using magnetic ferrite materials [1–7]. Up to now, FeFe2 O4 (or Fe3 O4 ) (magnetite) nanoparticles have been mainly investigated as the candidate material for this type of therapy. A drug delivery system (DDS) using nano-sized magnetic particles encapsulated in a liposome (<100 nm) can be used for this therapy (Fig. 1). These capsules selectively concentrate at the cancer tumors because of the liposome with an antibody of its cancer. Heat from the magnetic particles in the AC magnetic field locally damages the cancer tumor, and there is an effect by the medicine which appeared from a capsule broken

∗ Corresponding author. Tel.: +81 89 927 9856; fax: +81 89 927 9856. E-mail address: [email protected] (H. Aono). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.05.061

by heat for treatment. Furthermore, a few days later, it can expect that the immune system begin to act for spontaneous cure. This therapy might be the ultimate method for treating cancer tumor. We have studied new magnetic materials having higher heat generation abilities when compared to that of the FeFe2 O4 [8–11]. For the commercial ferrite powder (particle size: several ␮m) of MFe2 O4 (M = Mg, Mn, Fe, Co, Ni, Cu, and Sr), MgFe2 O4 has the highest heating ability of the various commercial ferrites [8,9]. Furthermore, we reported that the Mg1−X CaX Fe2 O4 system has a higher heat generation compared to the X = 0 (MgFe2 O4 ) material [10,11]. The heat generation in an AC magnetic field for these ferrite materials depends on the hysteresis loss value of the B–H magnetic property. Recently, the needle-type materials obtained by the sintering of MgFe2 O4 powder were studied for breast cancer therapy [12–14]. These ferrite materials have been synthesized by a chemical method. In particular, a nano-sized FeFe2 O4 is very easy

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H. Aono et al. / Materials Chemistry and Physics 129 (2011) 1081–1088 2. Experimental 2.1. Preparation of samples The fine ferrite powders of various particle sizes were prepared using a beadmill (DMS65, Ashizawa Finetech, Ltd.). The apparatus consisted of a zirconia (0.14 L) vessel and beads. A commercial FeFe2 O4 (99.9%, Kojyundo Chemical Lab.) powder was used as the milling samples. Ethanol was used as the solvent during milling. The milling time and bead size (0.3, 0.1, 0.05 mm␾) were varied in order to obtain different particle sizes. For the samples using the 0.1 mm␾ beads, the final milling was carried out after milling using 0.3 mm␾ beads for 2 h. For the samples with the 0.05 mm␾ beads, the final milling was carried out after milling using the 0.3 mm␾ beads for 2 h and then the 0.1 mm␾beads for 2 h. After the bead milling, the solvent was evaporated at 100 ◦ C. To study the dependence of the crystallite growth and heating ability, the milling sample using the 0.1 mm␾ beads for 6 h was calcined (5 ◦ C min−1 heating rate) and kept at an elevated temperature for 5 min in an Ar atmosphere. 2.2. Characterization

Fig. 1. DDS application of thermal coagulation therapy for cancer tumors.

to obtain using chemical methods, such as the coprecipitation method and thermal decomposition of organometallic compounds. The mechanism for the heat generation of magnetite nanoparticles has been postulated to be due to the hysteresis loss of ferromagnetic and ferrimagnetic materials [15,16], Néel relaxation, and the Brownian relaxation of superparamagnetic materials [17–22]. The effect of the crystallite size on the heat generation was not clear for these materials, because the particle size and the crystallite size of the samples prepared by chemical processes might not be small enough to study these properties. We studied the heat generation ability in an AC magnetic field of fine MgFe2 O4 powders physically prepared using a bead-milling method in order to understand the effect of the particle size [23]. The highest heating ability in an AC magnetic field (370 kHz, 1.77 kA m−1 ) was obtained for the fine MgFe2 O4 powder having an ca. 6 nm crystal size (the samples were milled for 6–8 h using 0.1 mm␾ beads). The heat generation of the samples was closely related to the hysteresis loss for the B–H magnetic property. However, we have not qualitatively discussed the heat generation ability of these ferrite samples. In this study, we investigated the heat generation ability in an AC magnetic field of fine FeFe2 O4 powders physically prepared using a bead-milling method and quantitatively discussed the heat generation ability.

The crystallite size of the FeFe2 O4 was estimated by the FWHM (full-width at half maximum) of the X-ray diffraction peaks (2◦ min−1 , XRD, Model Rint 2000, Rigaku Co., using Cu-K␣ radiation). The specific surface area was measured by the onepoint BET method, and then the particle size was calculated by assuming a spherical particle size. The hysteresis loss and the magnetic permeability in the AC magnetic field (150–370 kHz, 0.1–0.9 kA m−1 ) were obtained using a B–H analyzer (Iwatsu Electric Co., Ltd., SY-8258). For this measurement, ring-type samples (about 20 mm outside diameter, about 13 mm inside diameter, about 5 mm height) were prepared using a mixture of the ferrite powder and epoxy resin adhesive (4:1 weight ratio). 2.3. Measurement of heat generation ability Fig. 2 shows the apparatus for measuring of the heat generation ability of the ferrite powder in the AC magnetic field. The powder sample (1.0 g) was placed in a glass case (Pyrex: 20 mm␾, 45 mm), and an AC magnetic field (370 kHz, 1.77 kA m−1 ) was applied to the sample using an external coil. The coil consisted of eight loops of copper pipe (6␾) wound around a polypropylene (PP) bobbin (48 mm␾ × 40 mm). The copper pipe was cooled by flowing water to maintain its temperature and impedance. The coil was connected to a power supply (T162-5712B, Thamway Co., Ltd.) through an impedance tuner. The temperature of the sample was measured using a radiation thermometer (505s, Minolta Co., Ltd.). The temperature measurement was started after maintaining the temperature at 25 ◦ C in ambient air for several hours. For estimating the heat generation ability (W g−1 = J s−1 g−1 ) using this apparatus (Fig. 2), the sample (2.0 g) was placed in a glass case with 10 ml of water (a). Air was bubbled into the glass case for stirring the water. The heat generation ability (W g−1 ) was calculated using the temperature enhance ratio (dT/dt = K s−1 ) for the initial 5 min of the T measurement using the following:



heat generation ability = C

dT dt



M

(1)

where M and C are the sample weight (g), and the estimated total heat capacity (J K−1 ) of 10 ml of water and glass case, respectively. For comparison, the powder sample (2.0 g) with 10 ml of water was hardened using agar (b) and its heat generation ability was measured in order to eliminate the heat generation from the Brownian relaxation. In addition, the ring-type mass samples for the B–H measurement was cut as a ferrite (2.0 g) containing an epoxy resin adhesive mass, and its heat generation ability was measured with 10 ml of water (c). The average value for the measurement of three or more samples was used for this calculation.

Fig. 2. The apparatus for measuring of the heat generation ability.

H. Aono et al. / Materials Chemistry and Physics 129 (2011) 1081–1088

(440)

(422) (511)

(311)

▼

(400)

(111)

(220)

▼

(a)

Fe O 2

1083

3

(b)

(c)

10h

10h

10h

8h

8h

4h

6h 4h

2h

30

40

50

60

20

70

6h 4h 2h

2h

0h 20

Intensity/a.u.

6h

Intensity/a.u.

Intensity/a.u.

8h

30

2θ /degree

40

50

2θ /degree

60

70

20

30

40

50

60

70

2θ /degree

Fig. 3. XRD results for the fine FeFe2 O4 powder prepared by bead milling at various milling times using (a) 0.3 mm␾ beads, (b) 0.1 mm␾ beads after rough milling (0.3 mm␾ beads for 2 h), and (c) 0.05 mm␾ beads after rough milling (0.1 mm␾ beads for 2 h).

3. Results and discussion 3.1. Size of milled samples Fig. 3 shows the XRD results for the dried FeFe2 O4 powder after bead milling using the (a) 0.3 mm␾, (b) 0.1 mm␾, and (c) 0.05 mm␾ beads. The XRD peaks of a commercial sample (no milling time) were very sharp due to the large crystallite size. They became broader with an increase in the milling time. This means that the crystallite diameter was significantly reduced by this physical milling method even for 1 h using the 0.3 mm␾ beads (Fig. 3(a)). Furthermore, broader peaks were obtained using smaller beads (Fig. 3(b and c)). A broad peak for Fe2 O3 was detected and its intensity increased with the milling time. The divalent Fe2+ ion in the magnetite would be partially oxidized to trivalent Fe3+ ion to form the Fe2 O3 . Fig. 4 shows the crystallite and particle diameters for the FeFe2 O4 samples prepared at various milling times using the 0.3 mm␾, 0.1 mm␾ and 0.05 mm␾ beads. The crystallite and particle diameters were calculated using the Scherrer equation from the XRD peak at 2 = 62.5◦ (440) and the BET surface area, respectively. The average particle diameter of the commercial sample was 2.0 ␮m, which was also estimated using the BET method. The crys-

tallite diameter could not be calculated due to the sharp XRD peak for the commercial sample. The crystallite diameter and particle diameter decreased with the increased milling time. The use of the smaller 0.1 mm␾ and 0.05 mm␾ beads was effective for obtaining finer crystallite powders. The particle size was 2–20 times greater than the crystallite size for all the samples. This means that cohesion of the small crystallites resulted in larger particles as shown in the TEM photo of Fig. 5. The particle size approached the crystallite size by a step-up in the milling level (time and bead size). 3.2. Heat generation ability in AC magnetic field Fig. 6 shows a comparison of the heat generation ability (T) in ambient air for a commercial FeFe2 O4 sample and a typical milled sample. This measurement in ambient air was carried out to qualitatively compare the heat generation ability. The temperature of these samples significantly increased with time in the AC magnetic field. The temperature enhancement of the sample after 20 min reached a saturated value in the AC magnetic field. The heat ability and the cooling rate in ambient air were equilibrated at elevated temperature. The temperature enhancement (T) after 20 min was 3 ◦ C for the commercial sample (non-milled sample). However, the milled sample showed a T = 26 ◦ C temperature enhancement. The heat generation ability was significantly improved by this milling.

100

Particle size

Diameter / nm

0.3 mmφ 0.1 mmφ 0.05mmφ

10

Crystallite size 0.3 mmφ 0.1 mmφ 0.05mmφ

1 0

2

4

6

8

10

Milling time / h Fig. 4. Crystallite diameter and particle diameter for bead-milled FeFe2 O4 samples calculated using the Scherrer equation from the XRD peak and BET method.

Fig. 5. TEM photo of FeFe2 O4 powder prepared by bead milling (using 0.1 mm␾ beads for 6 h).

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Particle size /nm

30

0.1mmφ

0

6 h milling

20

40

60

80

100

30

25

o

Δ T/ C

▲ Particle size ● Crystallite size

25

20

20

Δ T/ oC

15

10

15

Commercial sample

5

10

0 5

0

5

10

15

20

Time/min 0 Fig. 6. Comparison of temperature enhancement (T) from room temperature for FeFe2 O4 powder (1.0 g) prepared by bead milling (using 0.1 mm␾ beads for 6 h) and commercial sample in an AC magnetic field (370 kHz, 1.77 kA m−1 ) in ambient air.

Fig. 7 plots the enhanced temperature in the AC magnetic field after 20 min versus the bead-milling time for the FeFe2 O4 powder. The heat generation ability increased with the milling time for the 0.3 mm␾ beads. The T value of the milled sample using the 0.1 mm␾ beads was higher than that using the 0.3 mm␾ beads. The maximum heat generation of T = 26 ◦ C was obtained for the samples milled for 6 h using the 0.1 mm␾ beads. However, the T value decreased with the milling time when the milling time was longer than 6 h. Moreover, the T value for FeFe2 O4 using the 0.05 mm␾ beads decreased with an increase in the milling time. Fig. 8 plots the temperature enhancement (T) versus the crystallite and particle diameters for the milled FeFe2 O4 samples. The T value increased with the decreasing crystallite size. The maximum heat generation ability was obtained for the sample having an

0

2

4

6

8

10

Crystal size /nm Fig. 8. Effect onT of crystallite diameter and particle diameter for bead-milled FeFe2 O4 samples.

ca. 4 nm crystallite size. The highest heat ability (T = 26 ◦ C) in the AC magnetic field was determined for the fine FeFe2 O4 powder with a 4.7 nm crystallite size (the samples milled for 6 h using 0.1 mm␾ beads). The T value decreased with a decrease in the crystallite diameter below ca. 4 nm. For the particle size, the maximum temperature was obtained at around 10–25 nm.

30

25

0.1 mmφ

o

ΔT/ C

20

15

0.05mmφ 10

0.3 mmφ

5

0 0

2

4

6

8

10

Milling time / h Fig. 7. Relationship between the milling time and temperature enhancement (T) for the FeFe2 O4 powders in an AC magnetic field in ambient air. The T value for the commercial FeFe2 O4 sample is plotted as the 0 h milling time in the figure.

Fig. 9. The hysteresis loops in B–H relationship (370 kHz) (0.1, 0.5, 0.9 kA m−1 ) for FeFe2 O4 powder prepared by bead milling for 6 h using 0.1 mm␾ beads.

H. Aono et al. / Materials Chemistry and Physics 129 (2011) 1081–1088

1085

30

25

o

Δ T/ C

20

15

10

5

0 0

5

10

15

20

Hysteresis loss/(mW/g) Fig. 10. Relationship between the hysteresis loss of the B–H magnetic property (370 kHz, ±0.5 kA m−1 ) and temperature enhancement (T) for all the examined samples.

Fig. 11. Relationship between the square of the magnetic field (370 kHz) and heat generation ability for FeFe2 O4 powder prepared by bead milling (using 0.1 mm␾ beads for 6 h). Three types of samples, (a) FeFe2 O4 powder with 10 ml of water, (b) hardened FeFe2 O4 powder with 10 ml of water using agar, (c) solidified FeFe2 O4 powder using epoxy resin, with hysteresis loss for B–H analyzer (370 kHz, 0.1–0.9 kA m−1 ) are plotted in this figure.

3.3. Mechanism of the heat generation ples and extrapolated hysteresis value. The energy losses for all the samples were proportional to the frequency of the AC magnetic field. It was reported that the energy losses (heat generation ability) of the magnetite nanoparticles due to the hysteresis loss, Néel relaxation, and Brownian relaxation are proportional to the square of the magnetic field (H2 ) and its frequency (f). The equation is expressed as follows: heat generation ability (W g−1 ) = kfH 2

(2)

0.14

(a) 0.12 -1

(c) Heat generation value/W•g

We have been reported that the heat generation ability of the ferrites was related to the hysteresis loss for the B–H magnetic property [9–11,16,23]. Fig. 9 shows typical results of the hysteresis loops in the B–H relationship of an AC magnetic field (370 kHz) (0.1, 0.5, 0.9 kA m−1 ) for a FeFe2 O4 powder prepared by bead milling for 6 h using 0.1 mm beads. The magnetic permeability (B/H slope) was not influenced by the strength of the AC magnetic field. Although this milled sample having a several nm crystallite size would be considered superparamagnetic nanoparticles, the coercivity value of 1.1, 5.2, 9.2 A m−1 was obtained for 0.1, 0.5, 0.9 kA m−1 , respectively. The coercivity value was proportional to the strength of the AC magnetic field. While not shown in figure, the magnetic permeability (B/H slope) was hardly influenced by the frequency of the AC magnetic field. The relationship between the temperature enhancement in Fig. 7 and the calculated hysteresis loss value for the B–H curve of these milled samples is plotted in Fig. 10. For the bead-milled FeFe2 O4 , the heat generation of the milled samples showed some dependence on the hysteresis loss value. To study the heat generation mechanism, the heat generation was quantitatively measured for three types of samples, (a) FeFe2 O4 powder (2.0 g) with 10 ml water, (b) the hardened FeFe2 O4 powder sample (2.0 g) with 10 ml of water using agar, and (c) the solidified samples containing the FeFe2 O4 powder (2.0 g) using an epoxy resin adhesive with 10 ml of water. The FeFe2 O4 powder milled for 6 h using 0.1 mm␾ beads having the maximum heat ability (Fig. 7) was selected for this measurement. The heat generation ability was calculated by Eq. (1) using the slope (dT/dt) of the initial 5 min in the AC magnetic field. Fig. 11 shows the relationship between the square of the magnetic field (H2 ) and heat generation ability for the three types of samples. The hysteresis value is also plotted in the same figure in order to compare the energy losses. The energy losses for all samples were proportional to the square of the magnetic field. For the hysteresis loss, the heat generation was extrapolated to the high magnetic field region, because the magnetic field of the B–H analyzer was limited to under 1.0 kA m−1 . Fig. 12 plots the relationship between the frequency of the AC magnetic field (150–370 kHz, 1.77 kA m−1 ) and heat generation ability for the three types of sam-

0.1

(b)

0.08

0.06

Hysteresis loss 0.04

0.02

0

0

50

100

150

200

250

300

350

Frequency/ kHz Fig. 12. Relationship between frequency of the AC magnetic field (150–370 kHz, 1.77 kA m−1 ) and heat generation ability for FeFe2 O4 powder prepared by bead milling for 6 h using 0.1 mm␾ beads. Three types of samples, (a) FeFe2 O4 powder with 10 ml of water, (b) hardened FeFe2 O4 powder with 10 ml of water using agar, and (c) solidified FeFe2 O4 powder using epoxy resin, with hysteresis loss for B–H analyzer are plotted in this figure.

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H. Aono et al. / Materials Chemistry and Physics 129 (2011) 1081–1088

Sample

Heat generation loss (W g−1 )

(a) (b) (c) Hysteresis loss

0.123 0.114 0.108 0.092

where k, f, and H are a constant value, frequency (kHz), and the magnetic field (kA m−1 ), respectively [15–22]. Table 1 lists the heat generation values for the three samples in an AC magnetic field (370 kHz, 1.77 kA m−1 ) along with the hysteresis loss value. The result of the sample (a) in water is the total energy loss value. The estimated k value in Eq. (2) using the result in Table 1 was 1.07 × 10−4 for the total loss of this sample having the best heat generation ability. When the heat generation of sample (a) was compared to that of sample (b), this difference (ca. 8%) would be due to the Brownian relaxation, because the particle rotation could not occur in the hardened sample containing the agar. The energy loss of the solidified sample (c) using epoxy resin was almost same as that of sample (b). Sample (c) was the same material as the ring-type sample used for the measurement of the hysteresis loss by the B–H analyzer. This means that the concentration of the ferrite in the sample does not affect the heat generation ability in an AC magnetic field. The calculated hysteresis loss was ca. 75% of the total energy loss for sample (a). In general, the hysteresis loss is mainly due to the domain wall motion in an AC magnetic field for ferromagnetic and ferrimagnetic materials. For the Néel relaxation, the energy loss is due to the magnetic moment rotation within the crystal for the superparamagnetic material. In our previous paper, the FeFe2 O4 was prepared by the reverse coprecipitation method, and it showed the maximum heating ability in an AC magnetic field when the crystallite size was ca. 12 nm [16]. The heat generation in an AC magnetic field for the samples synthesized using a chemical method also depended on the hysteresis loss of the B–H magnetic property. However, this crystallite size of ca. 12 nm was very close to the calculated size having the maximum loss of the Néel relaxation value for the superparamagnetic FeFe2 O4 sample [19,22]. This 70

60

Δ T/ oC

50

40

Intensity/a.u.

Table 1 Comparison of heat generation ability in AC magnetic field (370 kHz, 1.77 kA) for FeFe2 O4 powder prepared by bead milling (using 0.1 mm␾ beads for 6 h).

10

20

30

40

50

60

70

80

90

2θ /degree Fig. 14. XRD results for the calcined FeFe2 O4 powder milled using 0.1 mm␾ beads for 6 h.

magnetic moment rotates, i.e., the Néel relaxation would cause the hysteresis loss in the B–H curve. Based on this combination, the heat generation value would depend on the hysteresis loss values. 3.4. Calcined samples In this study, the maximum heat ability was obtained for the milled sample having a 4.7 nm crystal diameter. This size was smaller than ca. 12 nm for the sample prepared by the chemical method [16]. The crystals of the FeFe2 O4 sample having the maximum T value (milling time of 6 h using 0.1 mm␾ beads) grew again during calcination at low temperature in an argon atmosphere. Fig. 13 shows the relationship between the calcination temperature and temperature enhancement (T) from room temperature in an AC magnetic field in ambient air. The T value was significantly improved by the calcination. However, the heating ability decreased for the calcined samples over 500 ◦ C. Fig. 14 shows the XRD results for the calcined FeFe2 O4 powder milled using 0.1 mm␾ beads for 6 h. The broad peaks due to the bead milling slightly changed into sharp peaks with an increase in the calcination temperature. However, the Fe2 O3 phase was formed by the oxidation of the divalent Fe2+ ions in the FeFe2 O4 to trivalent Fe3+ ions. The decrease in T value was ascribed to the oxidation of magnetite to form the Fe2 O3 phase. Table 2 lists the crystallite size, particle size, and enhanced temperature (T) for the calcined samples.

30 ●

Table 2 The crystallite size, particle size, and enhanced temperature (T) in ambient air for calcined samples.

20

0.1 mmφ 6h milled sample

Calcining temperature (◦ C)

Crystallite size (nm)

Particle size (nm)

T (◦ C)

Noncalcined sample 350 400 450 500 550 600

4.7 4.8 4.8 5.6 5.6 6.3 9.8

16.5 17.4 18.3 19.5 20.9 22.1 24.2

26 47 49 52 59 56 36

10

0 200

300

400

500

600

o

Calcining temperature/ C Fig. 13. Relationship between the calcining temperature and temperature enhancement (T) for FeFe2 O4 powders in an AC magnetic field.

H. Aono et al. / Materials Chemistry and Physics 129 (2011) 1081–1088

1087

70

(a) (b) (c) Hysteresis loss

0.3

FeFe2O4(calcined sample) (this study)

60

50

o

Temperature ( C)

-1

Heat Generation ability (W•g )

0.4

0.2

40

FeFe2O4(reverse coprecipitation)

30

20

0.1

FeFe O 2

(milled sample)(this

4

10

Hysteresis loss

FeFe O 2

0 0

1

2

3

4

The crystallite diameter increased with an increase in the calcining temperature. The maximum heat generation (T = 59 ◦ C) ability was obtained at ca. 5.6 nm for the sample calcined at 500 ◦ C. The T value might improve with particle growth if the oxidation did not occur at elevated temperature. Fig. 15 shows the relationship between the square of the magnetic field (H2 ) and heat generation ability for the sample calcined at 500 ◦ C. The hysteresis value is also plotted in the same figure in order to compare the energy losses. The energy losses for all the samples were also proportional to the square of the magnetic field. The result for sample (a) in water is the total value of the energy loss. However, the heat generation abil-

(a) (b) (c) Hysteresis loss

-1

(Commercial sample)

5

10

15

20

Time (min)

Fig. 15. Relationship between the square of the magnetic field (370 kHz) and heat generation ability for FeFe2 O4 powder prepared by calcination at 500 ◦ C after bead milling (using 0.1 mm␾ beads for 6 h). Three types of samples (a) FeFe2 O4 powder with 10 ml of water, (b) hardened FeFe2 O4 powder with 10 ml of water using agar, and (c) solidified FeFe2 O4 powder using epoxy resin, with hysteresis loss for B–H analyzer (370 kHz, 0.1–0.9 kA m−1 ) are plotted in this figure.

Heat generation value/W•g

4

0 0

-1 2

(H/kA•m )

0.3

study)

Fig. 17. Comparison of temperature enhancement (T) in an AC magnetic field (1.0 g, 370 kHz, 1.77 kA m−1 ) for samples prepared by the bead milling method and chemical method [16].

ity was almost the same value for all the examined samples. This means that the heat generation ability was clearly dependent on the Néel relaxation for the calcined samples, because the Brownian relaxation hardly occurred due to the sintering of the particles. For this sample, the hysteresis value for the B–H magnetic property showed a value similar to the heat generation ability in an AC magnetic field. Fig. 16 shows the relationship between the frequency of the AC magnetic field (150–370 kHz, 1.77 kA m−1 ) and the heat generation ability for the three types of samples and the extrapolated hysteresis value. For this calcined sample, the energy losses for all samples were also proportional to the frequency of the AC magnetic field. Based on these results, the energy losses for this sample are proportional to the square of the magnetic field (H2 ) and its frequency (f) as expressed by Eq. (2). The estimated k value in Eq. (2) was 2.54 × 10−4 for the total loss of this sample having the best heat generation ability. The heat generation ability of this heated sample was about 2.5 times greater than that of the bead milled sample. 3.5. Comparison to chemical method In our previous study, we prepared FeFe2 O4 by a chemical method (reverse coprecipitation method), and the maximum T was obtained when the crystallite size was around 12 nm [16]. Fig. 17 plots a comparison of the heat generation ability in ambient air for the bead milling method and for the best sample prepared by the chemical method. Table 3 lists the crystallite size, particle size, and enhanced temperature (T) in ambient air. The temperature enhancement in the AC magnetic field was very similar for both samples prepared by the milling breakdown method and crystallite

0.2

0.1

0 0

50

100

150

200

250

300

350

Frequency/ kHz Fig. 16. Relationship between frequency of AC magnetic field (150–370 kHz, 1.77 kA m−1 ) and heat generation ability for FeFe2 O4 powder prepared by calcination at 500 ◦ C after bead milling (using 0.1 mm␾ beads for 6 h). Three types of samples, (a) FeFe2 O4 powder with 10 ml water, (b) hardened FeFe2 O4 powder with 10 ml of water using agar, and (c) solidified FeFe2 O4 powder using epoxy resin, with hysteresis loss for B–H analyzer are plotted in this figure.

Table 3 Comparison of bead milling method and chemical method related to crystallite size, particle size, and enhanced temperature (T) in ambient air. Calcining temperature (◦ C)

Crystallite size (nm)

Particle size (nm)

T (◦ C)

Noncalcined sample 500 Chemical method

4.7 5.6 10.9

16.5 20.9 13.1

26 59 29

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growth. The heat generation ability was significantly improved by the calcination at low temperature. The calcination at low temperature might allow the growth of excessively small particles (<4 nm) to form a suitable crystal size for the high heat generation. The distorted crystal by physical milling might be restored by the calcination. 4. Conclusions A nano-sized FeFe2 O4 powder was prepared by the bead milling method. The heat generation was influenced by the crystallite diameter. The highest heat generation (T = 26 ◦ C) in an AC magnetic field was obtained for the 6 h milled samples using 0.1 mm␾ beads. The heat generation ability of this milled sample was additionally improved by calcination at low temperature. The crystallite size having the maximum T value for the FeFe2 O4 prepared by bead milling was ca. 4 nm, and the highest heat generation was obtained using the ca. 6 nm crystallites prepared by calcination after bead milling. This study suggests that control of the FeFe2 O4 crystallite diameter using the bead milling was a very effective method to improve the heat generation ability. One of the most important results is that the heat generation ability (W g−1 ) of these ferrite materials in an AC magnetic field knowing the arbitrary frequency (f, kHz) and the magnetic field (H, kA m−1 ) can be easily estimated using the kfH2 equation. Acknowledgement The present work was supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 21360341) (H. Aono). References [1] A. Jordan, R. Scholz, K. Maier-Hauff, M. Johannsen, P. Wust, J. Nadobny, H. Schirra, H. Schmidt, S. Deger, S. Loening, W. Lanksch, R. Felix, J. Magn. Magn. Mater. 225 (2001) 118.

[2] A. Jordan, R. Scholz, P. Wust, H. Fähling, R. Felix, J. Magn. Magn. Mater. 201 (1999) 413. [3] P. Moroz, S.K. Jones, B.N. Gray, Int. J. Hyperthermia 18 (2002) 267. [4] M. Johannsen, U. Gneveckow, B. Thiesen, K. Taymoorian, C.H. Cho, N. Waldöfner, R. Scholz, A. Jordan, S.A. Loening, P. Wust, Eur. Urol. 52 (2007) 1653. [5] A. Jordan, R. Scholz, K. Maier-Hauff, F.K.H. Landeghem, N. Waldoefner, U. Teichgraeber, J. Pinkernelle, H. Bruhn, F. Neumann, B. Thiesen, A. Deimling, R. Felix, J. Neurooncol. 78 (2006) 7. [6] K. Maier-Hauff, R. Rothe, R. Scholz, U. Gneveckow, P. Wust, B. Thiesen, A. Feussner, A. Deimling, N. Waldoefner, R. Felix, A. Jordan, J. Neurooncol. 81 (2007) 53. [7] M. Yoshida, Y. Watanabe, M. Sato, T. Maehara, H. Aono, T. Naohara, H. Hirazawa, A. Horiuchi, S. Yukumi, K. Sato, H. Nakagawa, Y. Yamamoto, H. Sugishita, K. Kawachi, Int. J. Cancer 126 (2010) 1955. [8] T. Maehara, K. Konishi, T. Kamimori, H. Aono, T. Naohara, H. Kikkawa, Y. Watanabe, K. Kawach, Jpn. J. Appl. Phys. 41 (2002) 1620. [9] T. Maehara, K. Konishi, T. Kamimori, H. Hirazawa, H. Aono, T. Naohara, S. Nomura, H. Kikkawa, Y. Watanabe, K. Kawachi, J. Mater. Sci. 40 (2005) 135. [10] H. Aono, H. Hirazawa, T. Ochi, T. Naohara, K. Mori, Y. Hattori, T. Maehara, H. Kikkawa, Y. Watanabe, Chem. Lett. 34 (2005) 482. [11] H. Hirazawa, S. Kusamoto, H. Aono, T. Naohara, K. Mori, Y. Hattori, T. Maehara, Y. Watanabe, J. Alloy Compd. 461 (2008) 467. [12] S. Yukumi, Y. Watanabe, A. Horiuchi, T. Doi, K. Sato, M. Yoshida, T. Maehara, H. Aono, T. Naohara, K. Kawachi, Anticancer Res. 28 (2008) 69. [13] S. Yukumi, Y. Watanabe, A. Horiuch, T. Doi, K. Sato, M. Yoshida, Y. Yamamoto, T. Maehara, H. Aono, T. Naohara, K. Kawachi, Int. J. Hyperthermia 25 (2009) 416. [14] Y. Watanabe, K. Sato, S. Yukumi, M. Yoshida, Y. Yamamoto, T. Doi, H. Sugishita, T. Naohara, T. Maehara, H. Aono, K. Kawachi, Bio-Med. Mater. Eng. 19 (2–3) (2009) 101. [15] M. Ma, Y. Wu, J. Zhou, Y. Sun, Y. Zhang, N. Gu, J. Magn. Magn. Mater. 268 (2004) 33. [16] H. Aono, H. Hirazawa, T. Naohara, T. Maehara, H. Kikkawa, Y. Watanabe, Mater. Res. Bull. 40 (2005) 1126. [17] A. Jordan, P. Wust, H. Fähling, W. John, A. Hinz, R. Felix, Int. J. Hyperthermia 9 (1993) 51. [18] R. Hergt, W. Andrä, C.G. D’Ambly, I. Hilger, W.A. Kaiser, U. Richter, H. Schmidt, IEEE Trans. Magn. 34 (1998) 3745. [19] R.E. Rosensweig, J. Magn. Magn. Mater. 252 (2002) 370. [20] J. Fortin, C. Wilhelm, J. Servais, C. Ménager, J. Bacri, F. Gazeau, J. Am. Chem. Soc. 129 (2007) 2628. [21] T. Atsumi, B. Jeyadevan, Y. Sato, K. Tohji, J. Magn. Magn. Mater. 310 (2007) 2841. [22] B. Jeyadevan, J. Ceram. Soc. Jpn. 118 (6) (2010) 391. [23] H. Hirazawa, H. Aono, T. Naohara, T. Maehara, Y. Watanabe, J. Magn. Magn. Mater. 323 (2011) 675.