Microwave absorbing properties of NdFeCo magnetic powder

JOURNAL OF RARE EARTHS, Vol. 30, No. 6, June 2012, P. 529

Microwave absorbing properties of NdFeCo magnetic powder WANG Lei (⥟⺞), LIN Peihao (ᵫ෍䈾) , PAN Shunkang (┬乎ᒋ), ZHOU huaiying (਼ᗔ㧹) (Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China) Received 17 October 2011; revised 6 February 2012

Abstract: NdFeCo magnetic powder was prepared by the process of smelting, high-energy ball milling and oxidation heat treating. The effects of oxidation heat treatment and Co content on phase composition and microwave absorbing properties of NdFeCo magnetic powder were investigated by an X-ray diffractometer (XRD) and vector network analyzer. The minimum reflectivity of Nd23.25Fe36.75Co40 powder before oxidation heat treatment was –6.2 dB, and that of oxidized powder decreased to –14.0 dB. The microwave absorbing properties of NdFeCo magnetic powder could be improved effectively by oxidation heat treatment. With the increase of Co content, the Fe2O3 reduced and the Nd2O3 increased; Fe3Co7 phase appeared when the content of Co increased to 40% (mass ratio); the absorption peak was found to move towards lower frequency region first, and then it moved towards a higher frequency region. Nd23.25Fe66.75Co10 powder had better comprehensive properties in absorbing microwave in the frequency band of 3–13 GHz. The value of minimum reflectivity and absorption peak frequency, when the coating thickness (d) was 1.8 mm, were –19.7 dB and 4.8 GHz, respectively. Keywords: NdFeCo; absorbing property; oxidation heat treatment; rare earths

With the rapid development of science and technology, the application of electromagnetic technology has brought material civilization to our society. However, the electromagnetic waves pollution has aroused concerns about our human beings. Therefore, microwave absorbing materials for controlling the electromagnetic radiation pollution is a pressing matter of the moment[1,2]. Furthermore, the microwave absorbing materials can be used to reduce radar cross section for military use. So researchers are now intensely interested in microwave absorbers and correlative devices. Currently, the research of absorbing properties has yielded fruitful results in the X-band (8–12 GHz)[3–8], but it is relatively weak in the S-band (2–4 GHz) and C-band (4–8 GHz)[9–11]. According to the relationship between the composition of FeCo alloy[12] and its magnetic properties, and the absorption peak frequency inversely proportional to the square of saturation magnetization (Ms2)[13], the purpose of moving absorption peak could be realized by changing the relative content of Fe, Co. In addition, the reflectivity could be reduced by the oxidation heat treatment[14]. In fact, the magnetic powder, prepared with the above conception, has a good absorption performance in the S-band and C-band.

1 Experimental With the purity of no less than 99.50%, the NdFeCo powder was prepared by Nd, Fe and Co, according to the for-

mula Nd23.25Fe76.75–xCox (x=0, 10, 20, 30, 40) (mass ratio). The powder was smelted in electric arc furnace under the protection of high purity argon. The NdFeCo ingot casting was homogenized for 48 h at 1050 ºC in the vacuum environment, and then crushed into coarse powder of which particle size was smaller than 1 mm. The coarse powder was ground under the protection of petrol for 48 h by using QM-lSP planet ball milling with a speed of 300 r/min. The mass ratio of the balls to the powder was 20:1. Then the ground powder was oxidized in air at 100 ºC for 2 h. XRD (D8 ADVANCE) was used for phase analysis. The prepared NdFeCo powder was mixed with paraffin in the mass ratio of 4:1. The mixture was made into a coaxial ring with a thickness of 3.5 mm, of which the outside and inside diameters were 7 and 3 mm, respectively. The complex permeability and the complex permittivity of the samples were measured with a vector network analyzer (HP8722ES) in the frequency band of 2–18 GHz.

2 Results and discussion 2.1 Effects of oxidation heat treatment on microwave absorbing properties of NdFeCo magnetic powder The XRD pattern of Nd23.25Fe36.75Co40 powders before and after oxidation heat treatment are demonstrated in Fig. 1. It shows that the powder without oxidation heat treatment mainly consists of -(Fe·Co) and Nd2(Fe·Co)17, while the

Foundation item: Project supported by National Natural Science Foundation of China (50961005), Natural Science Foundation of Guangxi (0991002Z) and Innovation Project of Guangxi Graduate Education (2011105950805M38) Corresponding author: PAN Shunkang (E-mail: [email protected]; Tel.: +86-773-2291434) DOI: 10.1016/S1002-0721(12)60085-4

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parameters in Fig. 2, and the reflectivity R of the powders were computed from Formula (1)[15]

R

Fig. 1 XRD patterns of Nd23.25Fe36.75Co40 powders

phase composition includes Nd2O3, Fe2O3 and Fe3Co7 phases which except -(Fe·Co) and Nd2(Fe·Co)17 phases after oxidation heat treatment, and the peak value of -(Fe·Co) decreases slightly. Frequency dependences of complex dielectric constant ' and  complex magnetic permeability ' and  are demonstrated in Fig. 2. It shows that the ' and  decrease relatively after oxidation heat treatment. The ' and  of oxidized powder are greater than those of powder in the frequency band of 2–10 GHz before oxidation heat treatment, while the ' and  of oxidized powder are inferior to those of powder in the frequency band of 10–18 GHz without oxidation heat treatment. The values of relative permeability (r) and relative permittivity (r) can be obtained in terms of the electromagnetic

20 lg

Pr 2Sfd . tanh( j Hr c

P rH r)  1

Pr 2Sfd . tanh( j Hr c

P rH r)  1

(1)

where f is the frequency of electromagnetic waves, c is wave propagation velocity in free space (or the speed of light), j is the imaginary unit, with different contents when the coating thickness d=1.8 mm, in the frequency band ranging from 2 to18 GHz, as we can see in Fig. 3. The minimum reflectivity of powder before oxidation is –6.2 dB, and that of oxidized powder decreases to –14.0 dB. In addition, the absorption peak frequency moves slightly towards higher region, and the absorption band has been broadened. It is mainly because of the formations of Nd2O3, Fe2O3 and Fe3Co7 which increases resistivity () and decreases saturation magnetization (Ms) after oxidation heat treatment. The relaxation domain wall resonant frequency () and line width ()of resonance curve are both proportional to /Ms2 [13], thus the increase of  and the decrease of Ms are beneficial for the enlargement of  and . As a result, the absorption frequency will move towards higher frequency region and the absorption bandwidth will be broadened. We can see from Fig. 2 that  of the oxidized powder is greater in the frequency band of 2 to 10 GHz, the magnetic loss effect could be better after oxidation heat treatment in this frequency band. Hence, the electromagnetic wave absorption property of the oxidized powder is superior to the powder with out

Fig. 2 Electromagnetic properties of Nd23.25Fe36.75Co40 powders (a) Curves of  vs f; (b) Curves of  vs f; (c) Curves of  vs f; (d) Curves of  vs f

WANG Lei et al., Microwave absorbing properties of NdFeCo magnetic powder

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Fig. 3 Reflectivity of Nd23.25Fe36.75Co40 powders (d=1.8 mm)

 oxidation heat treatment in the frequency band ranging from 2 to 10 GHz. The mainly reason is that oxidate was formed on the surface of powder after heat treatment, which increased the surface impedance, and it led to the formation of the similar core-shell composite structure, benefiting to the match of space and electromagnetic parameters[14]. Thus, the incident electromagnetic wave could enter into the material smoothly, and we can reach the purpose of improving performance of absorbing materials. 2.2

Effect of Co content on microwave absorbing properties of NdFeCo magnetic powder

2.2.1 Analysis of X-ray diffraction pattern The X-ray diffraction patterns of Nd23.25Fe76.75–xCox (x=0, 10, 20, 30, 40) powders are demonstrated in Fig. 4, which show that Nd23.25Fe76.75 phases are mainly composed of Nd2Fe17 , -Fe phases, a spot of Nd2O3 and Fe2O3 phases. The phases of Nd23.25Fe66.75Co10, Nd23.25Fe56.75Co20 and Nd23.25Fe46.75Co30 are the same as Nd23.25Fe76.75. Co is mainly dissolved into -Fe and Nd2Fe17, generates -(Fe·Co) and Nd2(Fe·Co)17 phases. The phase compositions of Nd23.25Fe36.75Co40 powders includes Fe3Co7 phase besides Nd2(Fe·Co)17, -(Fe·Co), Nd2O3 and Fe2O3 phases. The existence of Fe3Co7 phase leads to the depression of -(Fe·Co) peak value. It can be seen from the X-ray diffraction patterns that with the increase of Co the relative amounts of Fe2O3 decrease, but that of Nd2O3 increase.

Fig. 4 XRD patterns of Nd23.25Fe76.75–xCox (x=0, 10, 20, 30, 40) powders

2.2.2 Analysis of electromagnetic parameters The relation between the frequency and complex permeability and permittivity with different Co contents in the frequency range of 2–18 GHz are demonstrated in Fig. 5. It shows that the real part of complex permittivity (') of Nd23.25Fe76.75–xCox decreased after Co was added in, compared to that of Nd23.25Fe76.75 except Nd23.25Fe56.75Co20. The ' of Nd23.25Fe66.75Co10 powder reduced more dramatically than Nd23.25Fe46.75Co30 and Nd23.25Fe36.75Co40 powder did in the frequency band of 2 to 13 GHz; The imaginary part of complex permittivity () of Nd23.25Fe66.75Co10 powder in the frequency band of 3–16 GHz increased the fastest. The value of  reached the highest at 2 GHz, however it varied within a small range when the frequency exceeded 3 GHz. The  of Nd23.25Fe46.75Co30 remained steady in the frequency band of 2 to18 GHz. The  of Nd23.25Fe36.75Co40 powder decreased in the frequency band of 2 to 5.5 GHz, and it enlarged when the frequency exceeded 5.5 GHz; The real part of complex permeability (') increased with the addition of Co, in which that of Nd23.25Fe66.75Co10 increased greatest; The imaginary part of complex permeability () of Nd23.25Fe66.75Co10 was greater than that of Nd23.25Fe76.75 in the frequency band of 2 to 18 GHz, and the  of Nd23.25Fe66.75Co10 and Nd23.25Fe56.75Co20 are fairly high. The  of Nd23.25Fe46.75Co30 was higher than that of Nd23.25Fe76.75 in the frequency exceeding 5 GHz. The value of  for Nd23.25Fe36.75Co40 and Nd23.25Fe76.75 are approximately equal. 2.2.3 Analysis of microwave absorbing properties The reflectivities of Nd23.25Fe76.75–xCox (x=0, 10, 20, 30, 40) powders, with the coating thickness d=1.8 mm, are demonstrated in Fig. 6. We can see that the absorption peak frequency rises at the beginning and then declines with the increase of Co content, and that is related to the saturation magnetization (Ms). According to the relationship between magnetic property and FeCo alloy phase: the saturation magnetization (Ms) increases at the beginning and then declines with the increase of Co content, and the maximum appears when atomic ratio of Co is about 34%[12]. The atomic ratios of Co in Nd23.25Fe56.75Co20 and Nd23.25Fe46.75Co30 are 26% and 39%, respectively, so the inflection point of Ms would appear between Nd23.25Fe56.75Co20 and Nd23.25Fe46.75Co30. As we can see in Fig. 5, the complex permeability curve is the relaxation curve of domain wall resonance. The frequency of relaxation domain wall resonance is inversely proportional to the square of Ms[13], thus the resonance frequency moves towards lower frequency region at the beginning, which is resulted from the enhancement of Ms, with the increase of Co. While the resonance frequency moves towards a higher frequency region, with the weakening of Ms at the moment, the Co content increases to 30%. Resonance frequency moves further towards higher frequency region when the content of Co is 40%, because the Nd2O3 content increases and -(Fe·Co) and Fe2O3 content decreases in the powder. The minimum reflectivity values of Nd 23.25 Fe 76.75 , Nd23.25Fe66.75Co10, Nd23.25Fe56.75Co20, Nd23.25Fe46.75Co30 and

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Fig. 5 Electromagnetic parameter of Nd23.25Fe76.75–xCox (x=0, 10, 20, 30, 40) powders (a) Curves of  vs f; (b) Curves of  vs f; (c) Curves of  vs f; (d) Curves of  vs f

Nd23.25Fe36.75Co40 are –11.7, –19.7, –20.0, –23.3 and –14.0 dB respectively. In view of this, the minimum reflectivity can be reduced through the introduction of Co, but it varies inconspicuously while Co content increased. In addition, there is an absorption peak of Nd23.25Fe66.75Co10 which was probably caused by natural resonance[13] at 17 GHz, and the corresponding reflectivity of Nd23.25Fe66.75Co10 is –9.3 dB, as we can see in Fig. 6. This shows that Nd23.25Fe66.75Co10 has an excellent broadband absorption effect. We can see from the analysis of Fig. 5 that the values of ', ',  and  of Nd23.25Fe66.75Co10 powders are greater in the frequency band of 3 to 13 GHz. The energy loss of microwave is proportional to  and , and the energy stored in the powder is proportional to ' and ' [13]. So, Nd23.25Fe66.75Co10 powder

Fig. 6 Reflectivities of Nd23.25Fe76.75–xCox (x=0, 10, 20, 30, 40) powders (d=1.8 mm)

has good microwave absorbing properties in the frequency band of 3 to 13 GHz.

3 Conclusions (1) The minimum reflectivity of Nd23.25Fe36.75Co40 powder before oxidation was –6.2 dB, and that of oxidized powder decreased to –14.0 dB. The absorption peak frequency moved slightly towards a higher region, and the absorption band had been broadened. In conclusion, microwave absorbing properties of the powders could be improved by surface oxidation heat treatment. (2) As the Co content increased in NdFeCo alloy, the content of Fe2O3 would decrease, and while the content of Nd2O3 would increase after high-energy ball milling and oxidation heat treatment. When the content of Co increased to 40%, Fe3Co7 phase appeared. (3) The minimum reflectivity of NdFe powder could be reduced through the introduction of Co, but it varied inconspicuously while Co content increased. The absorption peak frequency moved towards lower frequency region at the beginning and then moved towards higher frequency region. Nd23.25Fe66.75Co10 powder had better properties in absorbing comprehensive microwave in the frequency band of 3 to 13 GHz. The value of minimum reflectivity and absorption peak frequency were –19.7 dB and 4.8 GHz respectively when the coating thickness was 1.8 mm.

WANG Lei et al., Microwave absorbing properties of NdFeCo magnetic powder

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