Effect of pH value on electromagnetic loss properties of Co–Zn ferrite prepared via coprecipitation method

Journal of Magnetism and Magnetic Materials 405 (2016) 36–41

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Effect of pH value on electromagnetic loss properties of Co–Zn ferrite prepared via coprecipitation method Xiaogu Huang a,b,n, Jing Zhang c, Wei Wang a, Tianyi Sang d, Bo Song b, Hongli Zhu e, Weifeng Rao a, Chingping Wong b a

School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China School of Material Science and Engineering, Georgia Institute of Technology, Atlanta 30332, United States c Nanjing Center, China Geological Survey, Nanjing 210016, China d Department of Electrical and Computer Engineering, University of California Davis, Davis 95616, United States e Institute 53 of China North Industries Group Corporation, Jinan 250031, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 1 October 2015 Received in revised form 4 December 2015 Accepted 16 December 2015 Available online 18 December 2015

In this paper, the cobalt zinc ferrite was prepared by coprecipitation method at different pH conditions. The influence of pH values on the coprecipitation reaction was theoretically analyzed at first. The calculated results showed that the pH values should be controlled in the range of 9–11 to form the stable precipitation. The XRD investigation was used to further confirm the formation of the composite on specific pH values. In addition, the morphological study revealed that the average particle size of the composite decreased from 40 nm to 30 nm when the pH value increased from 9–11. The variation of microstructure plays a critical role in controlling the electromagnetic properties. From the electromagnetic analysis, the dielectric loss factor was 0.02–0.07 and magnetic loss factor was 0.2–0.5 for the composite synthesized at pH of 9, which presents dramatically improved dielectric loss and magnetic loss properties than the samples prepared at pH of 10 and 11. The as-prepared cobalt zinc ferrite are highly promising to be used as microwave absorption materials. & 2015 Elsevier B.V. All rights reserved.

Keywords: Coprecipitation method Microstructure Magnetic properties Microwave absorption Electromagnetic loss properties

1. Introduction Recently, cobalt zinc ferrite has attracted worldwide interests due to its excellent electromagnetic properties and physical/chemical stability [1]. It can be generally used as active material in catalysis [2,3], supercapacitive energy storage [4,5], magnetic record medium [6] and microwave absorption [7,8]. In the mixed spinel structure of cobalt 2þ 3þ 2þ ion in the B-site can zinc ferrite ((Zn2x þ Fe3þ 1 x)A[Co1 xFe1þ x]BO4), Co induce the occupation of the equal number of Fe3þ ions in the A-site. Cations occupation can significantly affect electrons hopping mechanism and magnetic exchange interaction [9]. Moreover, shape control is also an efficient strategy to modify magnetic properties. For example, the paramagnetic and ferromagnetic properties of spinel ferrite can be controlled by varying its particle size and distribution [10]. Therefore, a general approach to synthesis cobalt zinc ferrite is of great technical importance for various applications. Herein, the cobalt zinc ferrite was prepared by coprecipitation method. The theoretical pH values were firstly calculated to obtain n Corresponding author at: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China. E-mail addresses: [email protected], [email protected] (X. Huang).

http://dx.doi.org/10.1016/j.jmmm.2015.12.051 0304-8853/& 2015 Elsevier B.V. All rights reserved.

the stable precipitation state. Then, the influences of pH values on phase composition, particle size and electromagnetic properties were discussed in detail. The as-prepared cobalt zinc ferrite can be potentially applied as microwave absorption materials.

2. Experimental procedure 2.1. Powder synthesis The cobalt zinc ferrite was prepared by coprecipitation method. Iron (III) nitrate nonahydrate (Fe(NO3)3  9H2O, AR), cobalt (II) nitrate hexahydrate (Co(NO3)2  6H2O, AR), zinc nitrate hexahydrate (Zn(NO3)2  6H2O, AR) and sodium hydroxide (NaOH, AR) were used as the raw materials. Firstly, Zn(NO3)2  6H2O, Co(NO3)2  6H2O and Fe(NO3)3  9H2O were dissolved into DI water with the mole ratio of 1:1:4 The as-prepared solution was magnetic stirred for 30 minutes and then moved into 70 °C water bath. Then, the precipitant NaOH was added into the solution to adjust the pH value. The coprecipitation reaction was kept for 4 h under magnetic stirring and the precipitate was further aged for about 6 h. Then, the as-prepared precipitate was washed repeatedly by deionized water and ethanol. Finally, the precursor was calcined at 700 °C for

X. Huang et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 36–41

2.2. Measurements The crystallographic structure was identified by X-ray diffraction (XRD, Rigaka, D/MAX2500). The morphology was measured by transmission electron microscopy (TEM, Tecnai G2 F20). The electromagnetic parameters (ε′, ε″, μ′, μ″) were measured by vector network analyzer (Agilent PNA 8363B) in the frequency range from 2 to 18 GHz. The samples used for electromagnetic parameters measurement were prepared by uniformly mixing 30 wt% of wax and 70 wt% of ferrite. Then, the wax/ferrite composites were pressed in a toroidal-shaped mould (3.0 mm inner diameter, 7.0 mm outer diameter and 2.0 mm thickness).

3. Results and discussion 3.1. Theoretical calculation of pH values In this study, sodium hydroxide NaOH was adopted as the precipitant and reacted with the metal ions. During this process, the precipitate reaction can be expressed as:

Me + 2OH − = Me (OH )2 (s )

Table 1 Equilibrium constants in co-precipitation system. Ligand

Metal ions

Coordination number (n)

lgβn

OH-

Co2 þ Fe3 þ Zn2 þ

1, 2, 3, 4 1, 2, 3 1, 2, 3, 4

4.3, 8.4, 9.7, 10.2 11.87, 21.17, 29.67 4.4, 11.3, 14.14, 17.66

14

3+ Fe 2+ Zn 2+ Co

12 10

log([Me]T/(mol/L))

2 h at 3 °C/min and the final product was obtained after cooling to room temperature.

37

8 6 4 2 0 -2 -4 -6 -8 0

2

4

6

8

10

12

14

pH

(1)

Fig. 1. Relationships between total concentration of the residual metal ions and pH value in Me–NaOH–H2O system.

Ksp = [Me][OH −]2

[Me] =

Ksp [OH −]2

=

(2) [L ]T = [L ] + [MeL ] + 2 ⎡⎣ MeL2 ⎤⎦ + ⋯

Ksp 2 102pH ) (K w

(3)

where Ksp is solubility product constant, Kw is ion product constant of water. In the solution, the KspFe(OH)3 ¼10  37.4, KspCo(OH)2 ¼10  14.7, KspZn(OH)2 ¼10  16.92. The relationships between the metal ion concentration and the pH values can be expressed as:

[Fe 3 +] = [Co2 +] = [Zn2 +] =

Ksp Fe (OH)3 − 3

[OH ] Ksp Co (OH)2 − 2

[OH ] Ksp Zn (OH)2 [OH− ]2

=

10−37.4 = 104.6 − 3pH 103pH − 27.86

=

10−14.7 = 1011.08 − 2pH 102pH − 28

=

10−16.92 102pH − 28

⎪

(7)

where L is ligands, n is coordination number, and βn (n ¼1, 2, …) represents the cumulative equilibrium constant of ligands. The equilibrium constants of different metal ions (25 °C, I ¼0) are displayed in Table 1 [11]. The metal ion concentration can be calculated and the results are listed as-following:

⎡ ⎤ = ⎣ Fe 3 +⎦ 1 + 10pH − 2.13 + 10 2pH − 6.83 + 10 3pH − 12.33

{

}

⎡ 2 +⎤ ⎡ 2 +⎤ 4.3 pH − 14 + 10 8.410 2pH − 28 + 109.710 3pH − 42 + 1010.210 4pH − 56 ⎣ Co ⎦T = ⎣ Co ⎦ 1 + 10 10

{

}

⎡ ⎤ = ⎣ Co 2 +⎦ 1 + 10pH − 9.7 + 10 2pH − 19.6 + 10 3pH − 32.3 + 10 4pH − 45.8

{

(4)

Considering the ligand in the solution, the coordination reaction can be expressed as:

Me + L = MeLβ1 Me + 2L = MeL2 β2 … … Me + nL = MeL n βn

(5)

Then, the total concentration of residual metal ions [Me]T and the total ligands concentration [L]T in Me–NaOH–H2O system can be calculated according to the conservation law: ⎪

⎪

2 3⎫ ⎡ Fe 3 +⎤ = ⎡ Fe 3 +⎤ ⎧ 1 + β1⎡⎣ OH−⎤⎦ + β2 ⎡⎣ OH−⎤⎦ + β3 ⎡⎣ OH−⎤⎦ ⎬ ⎣ ⎦T ⎣ ⎦⎨ ⎩ ⎭

= 1013.3 − 2pH

⎧ ⎨1 + [Me]T = [Me] + [MeL ] + ⋯ + ⎡⎣ MeL n ⎦⎤ = [Me] ⎪ ⎩

n ⎧ ⎫ ⎨ 1 + [Me] ∑ iβi [L ]i − 1⎪ ⎬ + n ⎡⎣ MeL n ⎤⎦ = [L ] ⎪ ⎩ ⎭ i=1

⎫ ∑ βi [L]i − 1⎪⎬ ⎭ i=1 n

⎪

(6)

}

⎡ 2 +⎤ ⎡ 2 +⎤ 4.4 pH − 14 + 1011.310 2pH − 28 + 1014.410 3pH − 42 + 1017.6610 4pH − 56 ⎣ Zn ⎦T = ⎣ Zn ⎦ 1 + 10 10

{

⎡ ⎤ = ⎣ Zn2 +⎦ 1 + 10pH − 9.6 + 10 2pH − 16.7 + 10 3pH − 27.86 + 10 4pH − 38.34

{

}

}

(8)

Finally, the total concentration of the residual metal ions [Me]T in Me–NaOH–H2O system with different pH values can be calculated according to formula (4) and (8). The results were expressed as formula (9). Moreover, the relationships between total concentration of the residual metal ions [Me]T and pH value in Me–NaOH–H2O system are shown in Fig. 2 (Fig. 1). ⎡ Fe3 +⎤ = 104.6 − 3pH + 102.47 − 2pH + 10−2.23 − pH + 10−7.73 ⎣ ⎦T ⎡ Zn2 +⎤ = 1011.08 − 2pH + 101.48 − pH + 10−5.62 + 10pH − 16.78 ⎣ ⎦T + 102pH − 27.26 ⎡ Co2 +⎤ = 1013.3 − 2pH + 103.6 − pH + 10−6.3 + 10pH − 19 + 102pH − 32.5 ⎣ ⎦T

(9)

From Fig. 2, it can be seen that the concentration of residual iron ion is less than 10  5 mol/L when pH value is more than 4. The

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X. Huang et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 36–41

pH=11

Intensity (a.u)

pH=10

pH=9 pH=8 pH=5 JCPDS NO. 22-1086 JCPDS NO. 65-3111

20

30

40

50

60

70

2θ/(°) Fig. 2. XRD patterns of samples prepared with different pH conditions.

complete precipitation of Fe3 þ Zn2 þ and Co2 þ were pH 4 4, 8o pH o11 and 9 opH o 14, respectively. Therefore, the theoretical pH values should be controlled in the range of 9–11 in this coprecipitation preparation. The higher pH values can induce the solution of Zn(OH)2. 3.2. Microstructure analysis

Fe2O3 Co0.5Zn0.5Fe2O4 (533)

(440)

(333)

(422)

(400)

(311) (222)

(220)

concentration of residual zinc ion is less than 10  5 mol/L when pH value is in the range of 8–11. In addition, the concentration of residual cobalt ion is less than 10  5 mol/L when pH value is in the range of 9–14. These results indicated that the pH range for the

80

The XRD patterns of as-prepared cobalt zinc ferrite with different pH conditions are shown in Fig. 2. It can be seen that the sample prepared exhibits the Fe2O3 phase when the pH value is 5. Co2 þ and Zn2 þ can not be completely precipitated at this pH value. When the pH value increases to 8, some Fe2O3 impurity also exists in the product. When the pH value reaches 9, the Fe2O3 phase disappears, and all the diffraction peaks can be ascribed to cobalt zinc ferrite phase. The XRD patterns confirm that the Co2 þ , Zn2 þ and Fe3 þ are all precipitated and then the pure Co0.5Zn0.5Fe2O4 is obtained after calcination. Moreover, the phase composition of the samples prepared at pH 10 and 11 also corresponds to pure phase of Co0.5Zn0.5Fe2O4. These XRD results are in accordance with theoretical calculation of pH values, in which pure Co0.5Zn0.5Fe2O4 ferrite can be obtained when the pH values are in the range of 9–11. The TEM images of as-prepared Co0.5Zn0.5Fe2O4 ferrites under different pH conditions are presented in Fig. 3. It can be seen that the particle size increases with the increasing pH values. The particle size is about 30 nm when the pH value is 9, and the

Fig. 3. TEM images of Co–Zn ferrites prepared with different pH conditions.

X. Huang et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 36–41

4.8 4.6

1.6

(a)

pH=9 pH=10 pH=11

39

pH=9 pH=10 pH=11

1.5

(a)

1.4

4.4

1.3

ε′

μ′

4.2 4.0

1.2 1.1

3.8

1.0

3.6

0.9 0.8

3.4 2

4

6

8

10

12

14

16

2

18

4

6

0.4

(b)

pH=9 pH=10 pH=11

0.3

8

10

12

14

16

18

Frequency/GHz

Frequency/GHz pH=9 pH=10 pH=11

0.8

(b)

0.2

μ″

ε″

0.6

0.1

0.0

0.4

0.2

2

4

6

8

10

12

14

16

18

Frequency/GHz

0.0 2

4

6

8

10

12

14

16

18

Frequency/GHz

Fig. 4. Complex permittivity of cobalt zinc ferrite prepared with different pH conditions, (a) the real part of complex permittivity ε′ and (b) the imaginary part of complex permittivity ε″.

Fig. 5. Complex permeability of cobalt zinc ferrite prepared with different pH conditions, (a) the real part of complex permeability m′ and (b) the imaginary part of complex permeability m″.

particle size increases to about 40 nm when the pH value is 11. It is known that nucleation and aggregation are two important processes during the particle formation: numerous small crystallites are firstly formed at the beginning of precipitation (nucleation), and then these precipitation (nucleation) aggregates to produce larger particles. In this sense, the large-sized particles can be obtained when aggregate rate is faster than nucleation rate. The nucleation of grains is highly affected by the types, composition, concentration of cations and anions in solution, the interactions among these ions, and the nature of precipitated grains [12]. Therefore, high concentration of OH  ions in alkaline medium leads to the formation of a supersaturated network that slowed down the nucleation rate, resulting in larger particle size.

pH value in the frequency range of 5–18 GHz. According to the Koops model, the high dielectric constant can be attributed to the high resistive medium of grain boundaries [13]. From the TEM analysis, it is concluded that the grain size increased with the increase of pH value. As a result, the low dielectric constant of the composite at high pH value is attributed from the weakened grain boundaries. Based on Fig. 5, it can be seen that the samples synthesized at different pH values show similar trends of the real part (μ′) and the imaginary part (μ″) of the complex permittivity, both of which decrease with the increase of pH value. Quantitatively, the values of μ′ are in the range of 0.97–1.55 and the values of μ″ are in the range of 0.02–0.78. It can be also observed that the values are high at lower frequencies and they gradually decreased with the increase of frequency. The reason is that the natural resonant frequency of the as-prepared spinel ferrite is lower than 2 GHz and the dispersion relaxation phenomenon occurs in the frequency range of 2–18 GHz [14]. In addition, the dielectric loss properties and the magnetic loss properties can be represented by the dielectric loss factor (tan δε ¼ ε″/ε′) and the magnetic loss factor (tan δu ¼ μ″/μ′) [15,16]. The dielectric loss factor tanδε and the magnetic loss factor tanδu are calculated and the results are displayed in Fig. 6. It can be seen from Fig. 6(a) that dielectric loss factor decreases with the increase of pH value in the frequency range of 5–18 GHz, which might be

3.3. Electromagnetic properties The electromagnetic parameters of as-prepared Co0.5Zn0.5Fe2O4 ferrites with different pH conditions are measured and illustrate in Figs. 4 and 5. As shown in Fig. 4(a), the real part of the complex permittivity (ε′) decreases with the increase of pH value. The values of ε′ are nearly 4.3 when the pH is 9, but the values of ε′ decrease to about 3.7 when the pH is 11. Moreover, it can be seen from Fig. 4(b) that the imaginary part of complex permittivity (ε″) increases with the increase of pH value in the frequency range of 2–5 GHz. However, the values of ε″ decrease with the increase of

40

X. Huang et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 36–41

7.0

(a)

pH=9 pH=10 pH=11

0.07

-10.00

6.5

-9.000 -8.000

0.06

Thickness / mm

Dielectric loss factor

6.0

0.05 0.04

-7.000

5.5

-6.000 5.0

-5.000 -4.000

4.5

-3.000 4.0

0.03

-2.000 -1.000

3.5

0.02

Reflection loss / dB

0.08

0 3.0 2 2

4

6

8

10

12

14

16

4

6

18

8

Frequency/GHz

12

14

16

18

(a) pH=9 7.0

(b)

0.5

10

Frequency / GHz

pH=9 pH=10 pH=11

-10.00

6.5

-9.000 6.0

-8.000

Thickness / mm

Magnetic loss factor

0.4

0.3

0.2

-7.000

5.5

-6.000 5.0

-5.000

4.5

-4.000 -3.000

4.0

-2.000 -1.000

3.5

0.1

Reflection loss / dB

0.01

0 3.0 2

0.0 2

4

6

8

10

12

14

16

4

6

8

10

12

14

16

18

Frequency / GHz

18

(b) pH=10

Frequency/Ghz

7.0

Fig. 6. Frequency dependent (a) dielectric loss factor and (b) magnetic loss factor of cobalt zinc ferrite prepared with different pH conditions.

-10.00

6.5

-8.000

5.5

-7.000 -6.000

5.0

-5.000

4.5

-4.000 -3.000

4.0

Reflection loss / dB

caused by the enhanced interfacial polarization with smaller particle size. However, the values of tan δε are as low as 0.02–0.07, which indicated that the dielectric loss is not the dominant loss factor of the as-prepared cobalt zinc ferrite. The magnetic loss factor is shown in Fig. 6(b). It can be found that the values of tan δu are in the range of 0.2–0.5, indicating the strong magnetic loss properties. In addition, the magnetic loss factor decreases with the increase of the pH value. Generally, the magnetic properties in the spinel ferrite originates from the magnetic exchange between the magnetic ions in A-site and magnetic ions B-site. In this study, the theoretical ratio between Zn2 þ and Co2 þ is 1:1 according to the chemical composition of Co0.5Zn0.5Fe2O4. However, the concentration of residual cobalt ion decreases and the concentration of residual zinc ion increases when the pH value increases from 9 to 11 from the coprecipitation analysis in Fig. 1. This phenomenon suggests that more cobalt ion is doped in the ferrite. As a result, more Fe3 þ ions move from Bsite to A-site. The final product might be expressed as Co0.5 þ δZn0.5  δFe2O4, where δ is a positive number. The change in composition can induce inversing magnetic moments between Asite and B-site and causes the declining saturation magnetization, which further verifies the point that the magnetic loss properties are weakened with the increase of pH value.

Thickness / mm

-9.000 6.0

-2.000 -1.000

3.5

0

3.0 2

4

6

8

10

12

14

16

18

Frequency / GHz

(c) pH=11 Fig. 7. Contour maps of calculated RL values of the cobalt zinc ferrite prepared with different pH conditions.

3.4. Microwave absorption properties According to the transmission line theory, the reflection loss can be calculated as follows: [17,18]

RL = 20 log

Zin − Z 0 Zin + Z 0

(10)

X. Huang et al. / Journal of Magnetism and Magnetic Materials 405 (2016) 36–41

Zin = Z 0 μ r /εr tanh ⎡⎣ j (2πfd/c ) εr μ r ⎤⎦

(11)

where Zo is the intrinsic impedance of free space (with the value of approximate 376.7 Ω), Zin is the input impedance at free space/ material interface, εr is the complex permittivity (εr ¼ ε′  jε″), μr is the complex permeability (μr ¼ μ′  jμ″), f is the frequency, d is the coating thickness and c is the light velocity. For the Co–Zn ferrite prepared under different pH values, the contour maps of their reflection loss are illustrated in Fig. 7. It can be seen that the Co–Zn ferrite prepared under pH ¼9.0 shows the maximum area with reflection loss less than  5 dB, indicating the excellent and enhanced microwave absorption properties. When the coating thickness is 5 mm, the reflection loss can reach to  5 dB in the frequency range of 3.3–9.5 GHz and the peak value is about 11.2 dB. However, the reflection loss reduces with the increase of pH value. In addition, with reflection loss less than  5 dB, the optimal efficient bandwidth decreases to 3 GHz as the pH value increases to 11. The results are in accordance with the analysis of electromagnetic loss properties. From the above discussion, the condition of pH value around 9 should be considered to synthesize the Co0.5 þ δZn0.5  δFe2O4 ferrite with well electromagnetic loss performance. Moreover, the magnetic loss is the dominating loss mechanism for the as-prepared sample.

4. Conclusion Cobalt zinc ferrite was prepared by coprecipitation method with different pH conditions. The influences of pH values on the microstructure and electromagnetic properties were investigated. The theoretical analysis and the XRD measurement both confirmed that the pH value should be controlled in the range of 9–11 to obtain the pure cobalt zinc ferrite. TEM images revealed that the particle size decreased with the increase of pH value. The electromagnetic properties analysis also demonstrated that complex permittivity and complex permittivity were significantly modified by varying the pH values. The final dielectric loss and magnetic loss properties enhanced when the pH value was 9. The reasons can be ascribed to the well-performed interfacial polarization loss and magnetic hysteresis loss. Due to these unique characteristics,

41

the as-prepared cobalt zinc ferrite can be potentially used as microwave absorption materials. The effects of pH values on the microstructure of cobalt zinc ferrite are beneficial to improve its dielectric and magnetic properties.

Acknowledgments This work was financially supported by the Project Funded by the National Natural Science Foundation of China (51402154), Natural Science Foundation of Jiangsu Province (BK20141000) and Natural Science Foundation of Jiangsu Provincial Universities (14KJB430019).

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