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Crystallinity and magnetic properties dependence on sintering temperature and soaking time of mechanically alloyed nanometer-grain Ni0.5Zn0.5Fe2O4
Journal of Magnetism and Magnetic Materials 333 (2013) 100–107
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Crystallinity and magnetic properties dependence on sintering temperature and soaking time of mechanically alloyed nanometer-grain Ni0.5Zn0.5Fe2O4 Ismayadi Ismail a,n, Mansor Hashim a,b, Idza Riati Ibrahim b, Rodziah Nazlan b, Fadzidah Mohd Idris a, Shamsul Ezzad Shafie a, Masni Manap a, Ghazaleh Bahmanrokh b, Nor Hapishah Abdullah b, Wan Norailiana Wan Rahman b a b
Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology (ITMA), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Physics Department, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2012 Received in revised form 17 December 2012 Available online 9 January 2013
The separate influences of sintering temperature and soaking time on mechanically alloyed ferrites’ crystallity degree and properties are reported. To understand the two influences in the case of a ferrite with the composition Ni0.5Zn0.5Fe2O4, toroids of this composition were separated into three groups for investigation of three respective and different sintering parameters. The first parameter was the sintering temperature of 500 1C to 1400 1C with a 100 1C increment. The second parameter was the sintering temperature from 800 1C to 1000 1C with a 25 1C increment which were believed the temperature range of magnetic phase transition. All toroidal samples were sintered for 10 hours in an ambient air atmosphere. The soaking time was the third parameter employed with prolonged soaking from 1 to 96 hours and a fixed sintering temperature of 800 1C. The X-ray diffraction (XRD) results of the three parameters show that the first appearance of a single phase occurred at as low as 600 1C and the intensity peaks increased as the sintering temperature and soaking time increased, yielding for the entire sintering sequence three distinct families of magnetic hysteresis loops. The density results against temperatures show almost a linear increase until 1200 1C and remain relatively unchanged for 1300 1C and 1400 1C. The increase of XRD intensity with the sintering temperature indicates higher crystallinity and possible higher values of magnetization. From the differential scanning calorimetry (DSC) result, maximum exothermic peak appear at 960 1C and the nanometer starting powders could be speculated to give high reactivity due to a high surface area. The Curie temperatures were all found to be the same i.e. 232 1C. & 2012 Elsevier B.V. All rights reserved.
Keywords: Mechanical alloying Sintering temperature Soaking time M–H loop
1. Introduction Ferrites are usually non-conductive ceramic compounds derived from an oxide of iron i.e. hematite (Fe2O3) or magnetite (Fe3O4) as well as oxides of other metals. Unlike most materials, they possess both high permeability and moderate permittivity at frequencies from dc to millimeter [1]. One of the most commonly known ferrites is nickel zinc ferrite which was developed for a wide range of applications due to their high permeability and low loss characteristics. Today, NiZn ferrite is still one of the most important ferrites for such application and constitutes a substantial portion of the ferrite market. NiZn ferrites have been extensively used as core materials for a large number of devices and electrical components.
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0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.12.047
One of the factors affecting the distribution of the ions between the two types of site for spinel ferrites is temperature [2]. The effect of increasing the temperature would also change the microstructure of the materials which, in turn, would, affect magnetic properties of the materials [3–10]. It is already widely known that magnetic properties of ferrite materials are sensitive to their microstructures. Rezlescu et al. [11] investigated the effect of adding Me bivalent cations (Me¼Cu2 þ , Cd2 þ , Co2 þ , Ca2 þ , Mn2 þ and Mg2 þ ) on the electric and magnetic properties of Ni–Zn ferrites obtained by the conventional method of mixing oxides with a Ni0.25Me0.25 Zn0.5Fe2O4 nominal composition. They found that the partial substitution of Ni2 þ for bivalent cations of Mg2 þ , Cu2 þ , Co2 þ , Mn2 þ , Cd2 þ and Ca2 þ caused the Ni–Zn ferrite lattice parameter to increase, because Ni2 þ has a smaller ionic radius than does the Me2 þ ion. Substituting nickel for Cu2 þ ions favored greater densification of Ni–Zn ferrites at low temperature. Ref. [12] has investigated the effects of quencher after calcination on the microstructure and magnetic properties of manganese–zinc ferrites by
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measuring the magnetic properties, electrical resistivity and density. The powder of Mn0.68Zn0.25Fe2.07O4 was prepared by adopting the conventional ceramic technique. Their toroidal cores were sintered at 1350 1C for 4 h in an atmosphere controlled by using the equation for equilibrium oxygen partial pressure. They reported the inner stress of calcined powder increases, abnormal grains of ferrite grow up and initial permeability decreases and power losses of ferrite rise with the increase in quenching temperature. From their results the microstructure and magnetic properties of manganese–zinc ferrites can be improved with the gradual cooling of calcined powder to room temperature (25 1C). Costa et al. [13] however used the combustion synthesis technique to prepare Ni– Zn ferrite powders with a nominal composition of Ni0.5Zn0.5Fe2O4, using urea as fuel. The large quantity of gas that developed inhibited particle aggregation and yielded soft powders suitable for dispersion and use. They reported the sintering temperature of 1200 1C producing a homogeneous microstructure with an average grain size of 2.0 mm and magnetic parameter values comparable to the values obtained for samples sintered at 1300 1C and 1400 1C. The sintering temperature Z1300 1C contributed to abnormal grain growth and greater trapped porosity. Although their samples presented poor microstructural characteristics, the greater average grain size contributed strongly to the better magnetic properties (lower coercivity and higher permeability), considering the conditions under which the magnetic measurements were taken. Hu et al. [14] studied the effect of heat treatment on the structural and magnetic properties of auto-combusted Mn–Zn ferrite samples with temperatures from 400 1C to 1200 1C. They reported poor magnetic properties after sintering at 550 1C in air. However, above 1100 1C they had fine crystallites with uniform size and the samples showed larger saturation magnetization. In this paper, the physical evolution and phase of the samples subjected to various sintering temperatures and soaking times are reported. Magnetic properties such as saturation magnetization and Curie temperature, characteristic of the prepared material as a function of sintering temperature are presented and discussed. This is necessary because knowledge of the experimental conditions which produced the properties is a prerequisite to the materials’ applications.
2. Methodology A powder composition, towards obtaining Ni0.5Zn0.5Fe2O4, was prepared by mechanical alloying of a mixture of metallic oxides. The materials used were Fe2O3 (Alfa Aesar, 99.95%), NiO (Alfa Aesar, 99.99%) and ZnO (Alfa Aesar, 99.99%) weighed according to the composition formula. The chemicals, in the above order, were mixed with a chosen molar ratio of 1:0.5:0.5. This composition was later mechanically alloyed using a Spex8000D milling machine with a ball to powder ratio (BPR) of 10:1 for 24 h of milling. The alloyed powder was then divided and uniaxially pressed into a few toroidal shapes of 20 mm outer diameter and 9 mm inner diameter with 2 wt% of PVA as binder under a pressure of 686.4 MPa. The toroidal samples produced were separated into 3 groups which were respectively subjected to 3 different sintering parameters. The first parameter was the sintering temperature covering 500 to 1400 1C with each toroidal sample in this first group sintered only once at a temperature different from those of the other samples in the group within the range 500 1C to 1400 1C with 100 1C increments. The second sintering temperature parameter ranged from 800 to 1000 1C with 25 1C increments. Each sample in this second group was sintered only once at a temperature different from those of the other samples in this group. All the toroidal samples were sintered for 10 hours in an ambient air atmosphere. The third,
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soaking-time parameter was employed with prolonged soaking time from 1 to 96 h, keeping the sintering temperature fixed at 800 1C. They were examined with X-ray diffraction (Phillips Expert Pro PW3040) using CuKa radiation. Field emission scanning electron microscopy (FESEM) micrographs were taken using an FEI NOVA NanoSEM 230 machine to reveal the microstructure of the samples. The average grain size of a sintered body was measured over 200 grains by the linear intercept method. The Archimedes principle with water as the fluid medium was used to measure the density of the samples. The percentage theoretical density (%Dth) was calculated using this formula: %Dth g=cm3 ¼ measured density=theoretical density 100% ð1Þ The theoretical density of Ni0.5Zn0.5Fe2O4 was calculated by taking the molecular weight of Ni0.5Zn0.5Fe2O4 to be 237.73 g. The weight of 8 molecules in 1 unit cell is ð8Þ 237:73g=A, where A is Avogadro’s number. The volume of a cube of side, length a is a3. ˚ of Ni0.5Zn0.5Fe2O4 ¼ 8.3827 A˚ therefore The unit cell edge ao (A) a3 ¼589.0495 A˚ 3. As 1 A˚ 3 ¼10 24 cm3, theoretical density is mass/ volume equal to ½ð8Þ ð237:73=6:022 1023 Þ=589:0495 1024 , which is equal to 5.3614 g/cm3. The theoretical density, rx was calculated as above. The percentage of porosity, P, of the sample was calculated using the relation below: r P ¼ 1 100% ð2Þ
rx
where r is the measured density of the sample. The magnetic properties of the samples were investigated using a Vibrating Sample Magnetometer (VSM) (VSM model LDJ 9600). The temperature range for crystallization was measured using a differential scanning calorimetry apparatus (Mettler Toledo). The Curie temperature was measured using an Agilent Precision Impedance Analyzer model 4294A.
3. Results and discussion Fig. 1 shows the XRD spectra of Ni0.5Zn0.5Fe2O4 milled samples sintered at different temperatures starting from 500 1C to 1400 1C. At 500 1C a small trace of Fe2O3 still existed suggesting that the thermal energy supplied to the materials during the sintering process was not enough to complete the reaction. The occurrence of (121) peak shows the existence of a-Fe2O3 at 331. A further increase of sintering temperature at 600 1C yielded a single phase of crystallization of Ni0.5Zn0.5Fe2O4, which indicated the starting sintering temperature for crystallization of Ni0.5Zn0.5Fe2O4. Zn2 þ ions started to diffuse into the tetrahedral sites and Ni2 þ ions moved to octahedral sites. The full crystallization achieved at as low as 600 1C shows the advantage of mechanical alloying. Upon sintering the sample at 600 1C, the major peaks began to form and the intensity of the peaks increased with increasing sintering temperature (Table 1). It shows that the amount of crystalline phase of Ni0.5Zn0.5Fe2O4 is increasing. The theoretical density (%Dth) for each sintered sample is given in Table 1. At lower sintering temperature from 500 1C to 900 1C, we could see almost a linear increase of density against sintering temperature. The increasing trend continued until 1200 1C and remained nearly constant for 1300 1C and 1400 1C. The effect of sintering temperature could be seen in the density and also the porosity of the samples (Table 1). As the sintering temperature increased, the average grain size (Table 1) increased causing the porosity to segregate to the grain boundary and the samples became denser due to the process as found by Ref. [15].
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Fig. 1. XRD spectra of pure Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C. Table 1 Average grain size, Densities, Theoretical Density, Porosities, Lattice Parameter, Cell Volume and peak intensity of the Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C. Sintering temperature (1C) 500 600 700 800 900 1000 1100 1200 1300 1400
Average grain size (lm)
Density (g/cm3)
Theoretical density (%)
Porosity (%) A (A) ˚
Vo (A˚ 3)
2h Obs.
XRD Peak intensity (counts)
0.10(6) 0.22(6) 0.16(6) 0.17(1) 0.41(2) 0.97(3) 3.00(9) 10.09(3) 10.10(5) 10.99(6)
4.37(3) 4.56(3) 4.69(2) 4.75(5) 4.80(6) 5.01(3) 5.19(0) 5.19(0) 5.16(2) 5.12(6)
82 85 88 89 90 94 95 97 96 96
18 15 12 11 10 6 5 3 4 4
590.88 592.94 591.97 591.27 590.77 591.46 590.92 590.78 589.77 587.73
35.33 35.50 35.56 35.51 35.43 35.52 35.46 35.46 35.48 35.67
1759.51 3479.72 6168.39 6571.80 6723.35 6812.34 6768.76 6605.57 6536.59 6715.19
Fig. 2 shows the spectra of XRD patterns for the samples sintered at 800 1C to 1000 1C each for 10 h in air with 25 1C increments. This temperature range believed to be the first appearance of the ordered magnetic moment as reported before [16–19]. All the sintered samples could be indexed to a single spinel phase. As we look closely at these spectra, we could observe the peaks of nickel zinc ferrite which increase with the sintering temperature. The increased intensity of the major peaks of nickel zinc ferrite demonstrated an improvement in the degree of crystallinity of the sintered samples. As the sintering temperature increased, the grain size would also be increased, yielding more crystallization. At the same time, the magnetic phase would be increased due to the increasing crystalline mass having the cubic spinel structure. The increase of magnetic phase in the samples causes the A–B interaction contribution in the spinel to be increased. Therefore the higher sintering temperature leads to higher values of magnetization. The XRD spectra of Ni0.5Zn0.5Fe2O4 sintered at 800 1C each with different soaking times are shown in Fig. 3. These samples were sintered with different soaking times in order to study what
8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.38
would happen if we could increase the grain growth with the same starting crystal phase. The diffraction spectra of the materials revealed the presence of the signature peaks of Ni0.5Zn0.5Fe2O4 at 2y ¼30.131, 35.541, 37.141, 43.321, 53.411, 57.011, 62.591 which can be indexed to (220), (311), (222), (400), (422), (333), (440) of a cubic unit cell respectively. Upon sintering the samples at 800 1C, the major peaks appeared and the intensity peaks increased with the soaking time. This suggests the increase in amount of crystallinity phase of the samples and the increasing of peak intensity could be observed in Table 3. The nanometer particles of the starting powders greatly helped in accelerating the phase formation that even when the sample was sintered within only one hour, all the seven major peaks of the Ni0.5Zn0.5Fe2O4 were formed. The XRD spectra (Fig. 3) at various soaking times show that the overall XRD spectra peaks on the 2y position remain the same, but with an increasing intensity. Besides this, no change in the crystal lattice structure is observed which affirms its stability over a wide range of soaking times. Similar trends of data as mentioned above for the case of sintering at 500 to
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Fig. 2. XRD spectra of sintered Ni0.5Zn0.5Fe2O4 samples from 800 1C to 1000 1C.
Fig. 3. XRD spectra of sintered Ni0.5Zn0.5Fe2O4 samples at 800 1C with various soaking time.
Table 2 Average grain size, Densities, Theoretical Density, Porosities, Lattice Parameter, Cell Volume and Peak Intensity of the Ni0.5Zn0.5Fe2O4 sintered at 800 to 1000 1C. Sintering temperature (1C) 800 825 850 875 900 925 950 975 1000
Average grain size (lm)
Density (g/cm3)
Theoretical density (%)
Porosity (%) A (A) ˚
Vo (A˚ 3)
2h Obs.
XRD Peak intensity (counts)
0.11(3) 0.12(6) 0.12(6) 0.17(1) 0.19(7) 0.29(9) 0.31(2) 0.38(8) 0.49(9)
4.37(3) 4.56(3) 4.69(2) 4.75(5) 4.80(6) 5.01(3) 5.11(6) 5.19(0) 5.16(2)
82 85 88 89 90 94 95 97 96
18 15 12 11 10 6 5 3 4
591.36 591.19 591.15 591.21 591.03 591.16 591.11 590.99 591.41
35.52 35.52 35.52 35.52 35.53 35.55 35.54 35.54 35.56
3143.27 3104.83 3808.33 4085.23 4447.31 4175.14 4707.71 4940.95 5057.22
8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.39
Table 3 Average grain size, Densities, Theoretical Density, Porosities, Lattice Parameter, Cell Volume and Peak Intensity of the Ni0.5Zn0.5Fe2O4 sintered at 800 1C with various soaking time. Soaking time (h) 1 5 10 20 30 40 96
Average grain size (lm)
Density (g/cm3)
Theoretical density (%)
Porosity (%)
˚ A (A)
Vo (A˚ 3)
2h Obs.
XRD Peak intensity (counts)
0.14(0) 0.15(8) 0.19(7) 0.21(0) 0.23(2) 0.25(0) 0.25(6)
4.51(1) 4.61(6) 4.66(0) 4.69(8) 4.70(1) 4.72(2) 4.92(0)
84 86 87 88 88 88 92
16 14 13 12 12 12 8
8.39 8.39 8.39 8.39 8.39 8.39 8.40
592.24 591.45 591.47 591.14 591.53 591.53 591.97
35.60 35.51 35.51 35.49 35.53 35.52 35.53
1688.52 1765.71 1987.82 1933.04 2577.72 2606.21 2957.25
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1400 1C could be observed for the average grain size, density and porosity of the samples sintered at 800 1C. In the 800–1000 1C sintering case the porosity was reduced significantly after being sintered at 900 1C which correlates with denser and bigger grain size (Table 2). All the peaks of the sintered samples can be clearly indexed to the seven major peaks of the spinel ferrites, which are (220), (311), (222), (400), (422), (333) and (440) planes of a cubic unit cell for the 500 to 1400 1C (Fig. 1), 800 to 1000 1C (Fig. 2) and 1 to 96 h sintered samples (Fig. 3), except for the sample sintered at 500 1C where a trace of hematite occurred with (121) planes. A closer look on the XRD spectra at various temperatures show that, the overall XRD spectra peaks on the 2y position remain the same, but increase in intensity with sintering temperatures and soaking times. Besides this, no significant change in the crystal lattice structure is observed. Since the corresponding peaks of the diffraction patterns of the compound were bearably the same, it was found that unit cell parameters were equally the same with increasing temperature, except for some slight discrepancy in the unit cell volume (Tables 1–3). This reaffirms the spinel phase stability over a wide temperature range. Upon sintering at 500 1C, the X-ray diffraction peaks appeared, indicating an improvement in the crystallinity of the material. The degree of crystallinity of the sample increased with increasing sintering temperature from 500 to 1400 1C (Table 1) and from 800 to 1000 1C (Table 2) and increasing soaking time from 1 to 96 h (Table 3). A fresh sample of the 24-h milled powder was examined using DSC to ascertain the temperature range for crystallization (Fig. 4). The sample was heated under an oxygen environment starting from room temperature up to 1400 1C. The maximum exothermic peak appeared at 960 1C. The nanometer starting powders could be speculated to give high reactivity due to their high surface area. Particles with a higher surface area also have a higher number of particle–particle contact points leading to a higher number of necking contacts between particles, which stimulate the diffusion and evaporation–condensation of the matter on surfaces with consequent bulk densification [20]. From the graph shown, we could see that the temperature range for crystallization started from 500 1C and increasing until 960 1C. The XRD graph (Fig. 1) shows that at 600 1C the milled samples were already forming Ni0.5Zn0.5Fe2O4. By contrast, conventional ceramic methods involving the mixing, grinding and firing of the constituents oxides and/ or carbonates invariably yield large particles in the micrometer size range because the diffusion controlled solid state reaction requires temperatures of 1000 1C or above [21]. The VSM results in Fig. 5 shows the M–H loops of the samples sintered at 500 to 1400 1C with 100 1C increments. From these loops, we could observe the evolution of increasing saturation magnetization of the M–H loop with increasing sintering temperatures. For the range 500 to 600 1C, Fig. 5 shows the first group
Fig. 4. DSC of milled Ni0.5Zn0.5Fe2O4 samples.
Fig. 5. M–H loops of Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C.
Fig. 6. Variation of saturation magnetization (Ms) of Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C.
of loops with low Ms indicating the degree of crystallinity of the samples was also low and can also be seen in Fig. 6. They show the signs of the initial stage of sintering where the presence of grain growth was not noticeable and the magnetization detected was probably contributed entirely by the rotation of the spin magnetic moment. A weak ordered magnetization was detected for the sample sintered at 500 1C. The sample, we believe, is dominated by paramagnetism with a small superparamagnetism. The lower Ms values which relate to the particles with the smaller size could be attributed to a surface distortion due to the interaction of transition metal ions in the spinel lattice with oxygen atoms, which can reduce the net magnetic moment in the particle. This effect is particularly prominent for the ultrafine particles due to their large surface to volume ratio [22]. This caused the magnetic phase transformation from paramagnetism to ferromagnetism to be not so drastic. This behavior could be related to the increasing trend of XRD spectra (Fig. 1) and XRD peak intensity (Table 1). The intermediate phase of hysteresis loop was seen for 700–800 1C and another gap appeared before the samples sintered at 900 1C. This shows that the range of 700– 800 1C is critical for the development of the strong bulk magnetic properties. The transition from 800 1C to 900 1C could be speculated to evolve parallel to the domain wall formation. The changes in the magnetic properties of the samples can also be recognized by the changes of the crystallite sizes dependant on the sintering temperature. Generally the permeability of polycrystalline ferrite ceramics are due to two different magnetizing mechanisms, which are spin rotation and domain wall movement. This relationship can
I. Ismail et al. / Journal of Magnetism and Magnetic Materials 333 (2013) 100–107
be described as
mi ¼ 1 þ X w þ X spin ,
ð3Þ
where Xw is the domain wall susceptibility; Xspin is the intrinsic rotational susceptibility. Meanwhile the Xw and Xspin may be written [23] as follows: X w ¼ 3pM2s D=4g
ð4Þ
and X spin ¼ 2pM2s =K,
ð5Þ
where Ms is the saturation magnetization, K is the total anisotropy, D is the average grain diameter, and g is the domain wall energy. From the formulae given, we know that the domain wall motion is affected by the grain size and enhanced with the increase of grain size. The initial permeability is therefore a function of grain size and magnetization [23]. Fig. 6 shows increasing Ms from lower sintering temperature up to about 800 1C. The second group of samples sintered at 900 1C and above showed a flat trend with small fluctuation values of Ms. This region showed a very strong ferromagnetism-related behavior and probably existed due to movement of domain wall. Looking back at the DSC spectrum in Fig. 4, this is where the crystallization area peaked and started to decrease suggesting that the process of crystallization was nearing completion Fig. 7 shows the saturation magnetization (Ms) dependence of the average grain size of Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C. Comparing Figs. 1, 4 and 7 and the peak intensity (Table 1), we could observe that the degree of crystallinity of the samples was developing in stages. The early stage of sintering (500 to 800 1C)
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was the region where crystallization rate was very high. However, at 960 1C, crystallization reaction peaked and the rate of reaction subsequently reduced. Increase in crystallization reaction caused the material to possess a greater degree of magnetocrystalline anisotropy and exchange interaction that determine the direction in which magnetization prefers to be oriented. This effect can be seen in Fig. 7, where the sample with average grain size of 0.41 mm and above showed a flat trend of saturation magnetization of the sample showing a sign of complete crystallization process. Samples with grains of 0.41 mm and below would contain some paramagnetic and superparamagnetic states as mention in Ref. [17]. Fig. 8 shows the M–H loops of samples sintered at 800 to 1000 1C with 25 1C increments. The 800 1C and 825 1C samples (Figs. 8 and 9) show a slightly lower magnetization compared to others. This shows that there was a certain critical temperature or degree of crystallinity phase for the sample to have a complete
Fig. 9. Variation of saturation magnetization (Ms) of Ni0.5Zn0.5Fe2O4 sintered at 800 to 1000 1C.
Fig. 7. Ms as a function of average grain size of Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C.
Fig. 10. M–H loops of Ni0.5Zn0.5Fe2O4 sintered at 800 with various soaking time.
Fig. 8. M–H loops of Ni0.5Zn0.5Fe2O4 sintered at 800 to 1000 1C.
Fig. 11. Variation of saturation magnetization (Ms) of Ni0.5Zn0.5Fe2O4 sintered at 800 1C with various soaking time.
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Fig. 12. The variation of initial permeability with temperature for the sample Ni0.5Zn0.5Fe2O4 sintered at different temperatures.
single phase which resulted in higher magnetization. Fig. 9 shows a small rise of Ms with increasing sintering temperature. Slight increases of Ms after 850 1C indicate that there was not much change in terms of ordered magnetism. Fig. 10 shows the effect of soaking time on the evolution of ordered magnetism. Only two groups of M–H loops were observed. The first, sintered at 800 1C with 1 h soaking time, indicated that the degree of crystallinity was low. The second group (5 h and more of soaking time) whose Ms increased with soaking time showed a high degree of crystallinity. Fig. 11 showed a sharp increase of Ms from 1 h to 5 h of soaking time. Within this range of soaking time (1 to 5 h), it shows that very high reactivity of transformation of Fe2 þ into Fe3 þ [24]. Looking at the grain growth and density (Table 3), the increases are moderately small, therefore we believe the increase of ordered magnetism was due to the slow increase in the degree of crystallinity of the sample. Prolonged soaking time after 40 h, up to 96 h, did not show significant rise of Ms. This is because all the Fe2 þ has turned into Fe3 þ , and a complete single phase transformation achieved [24]. Fig. 12 shows the permeability variation as the sample temperature approached the Curie temperature of the samples sintered at 500 to 1400 1C for 10 h. The initial permeability increases slowly with the temperature until 190 1C before reaching the peak value at a certain temperature. The permeability was measured at 10 kHz, i.e. below resonance, and was therefore dependant on the microstructure i.e. on grain size and porosity. After 190 1C, the permeability reached peak values, because the thermal energy can drastically define the domain wall from the strain in the grain. It shows almost a sudden and greater release of pinning. The maximum of initial permeability indicates the point of zero anisotropy field [25]. The samples become paramagnetic at about 232 1C which destroyed the ordered magnetism, indicating the point of curie temperature. The Curie temperature behavior of the samples confirms the existence of superexchange interaction of in spinel ferrites. It also indicates that the samples produced are pure single magnetic phase.
of Ni0.5Zn0.5Fe2O4 at a temperature as low as 600 1C. The density against sintering temperature also increased for the samples sintered from 500 1C giving maximum density at 1200 1C and remained the same for both the sintering temperature of 1300 1C and 1400 1C. The average grain size increased causing the porosity to segregate to the grain boundary and the samples became denser due to the process. The sintered samples showed increases in the degree of crystallinity with the increase of sintering temperature and soaking time which could be observed by the increase of grain size and peak intensity. The mechanical alloying method with nanometer particles of the starting powders helps in accelerating the phase formation even when the sample was sintered within one hour. From the DSC measurement, the maximum exothermic peak appeared to be at 960 1C and, relating this to the degree of crystallinity, we could see there is a transition in the magnetic properties measured. The samples with the lower sintering temperatures indicated the M–H loops associated with negligible grain growth and the magnetization detected was contributed by the spin magnetic moment. The weak ordered magnetization was dominated by the paramagnetic phase with a small amount of the superparamagnetic phase. The magnetic transition from 800 1C to 900 1C was due to the domain wall formation. The strong ferromagnetism was revealed through movement of domain wall. From the effect of soaking time on the M–H loop, the sharp increase of Ms shows the chemical reactivity was high within the 1–5 h soaking time due to transformation of Fe2 þ into Fe3 þ . Whereas the soaking time of 40 h up to 96 h did not show a significant rise of Ms and can be attributed to the complete transition of Fe2 þ to Fe3 þ . The Curie temperatures of all the samples sintered at 500 to 1400 1C showed no significant changes. They had the same value of 232 1C, indicating that the Curie temperature is not microstructure-sensitive, dependent only on crystal-structure details and above which the samples become paramagnetic.
3.1. Conclusion
The authors are thankful to Ministry of Higher Education (MOHE) for providing the Prototype Research Grant Scheme (PRGS) with the Vot no. 5528000 and Fundamental Research Grant Scheme (FRGS) with the Vot no. 5524164.
In summary, both sintering temperature and soaking time directly influence phase crystallinity. The degree of crystallinity of the sample increased with increasing sintering temperature from 500 1C to 1400 1C, from 800 1C to 1000 1C and also with 1 to 96 h of soaking time for a single sintering temperature of 800 1C. The XRD spectra peaks on the 2y position remained the same. The XRD results proved that the powders subjected to mechanical alloying for nanometer starting powders and later sintered according to the planned parameters result in full crystallization
Acknowledgment
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[3] L. Tung, V. Kolesnichenko, G. Caruntu, D. Caruntu, Y. Remond, V.O. Golub, C. O’Connor, L. Spinu, Physica B: Condensed Matter 319 (2002) 116–121. [4] M.M. Hessien, M.M. Rashad, K. El-Barawy, I.A. Ibrahim, Journal of Magnetism and Magnetic Materials 320 (2008) 1615–1621. [5] S. Dasgupta, J. Das, J. Eckert, I. Manna, Journal of Magnetism and Magnetic Materials 306 (2006) 9–15. [6] V. Sepelak, D. Baabe, D. Mienert, D. Schultze, F. Krumeich, F.J. Litterst, K.D. Becker, Journal of Magnetism and Magnetic Materials 257 (2003) 377–386. [7] A. Azizi, H. Yoozbashizadeh, a. Yourdkhani, M. Mohammadi, Journal of Magnetism and Magnetic Materials 322 (2010) 56–59. [8] A. Azizi, S.K. Sadrnezhaad, Ceramics International 36 (2010) 2241–2245. [9] O.M. Hemeda, M.I.A. El-Ati, Materials Letters 51 (2001) 42–47. [10] N. Tashtoush, O.M. Hemeda, a. Tawfik, Journal of Magnetism and Magnetic Materials 256 (2003) 168–173. [11] E. Rezlescu, L. Sachelarie, P.D. Popa, N. Rezlescu, IEEE Transactions on Magnetics 36 (2000) 3962–3967. [12] L. Lezhong, L. Zhongwen, Y. Zhong, S. Ke, J. Haining, Journal of Magnetism and Magnetic Materials 318 (2007) 39–43. [13] A.C.F.M. Costa, E. Tortella, M.R. Morelli, R.H.G.A. Kiminami, Journal of Magnetism and Magnetic Materials 256 (2003) 174–182. [14] P. Hu, H.-bo Yang, D.-an Pan, H. Wang, J.-jun Tian, S.-gen Zhang, X.-feng Wang, A.A. Volinsky, Journal of Magnetism and Magnetic Materials 322 (2010) 173–177.
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[15] S.L. Kang, Sintering: Densification, Grain Growth and Microstructure, Elsevier Butterworth-Heinemann, Burlington, 2005. [16] N. Rodziah, M. Hashim, I.R. Idza, I. Ismail, A.N. Hapishah, M.A. Khamirul, Applied Surface Science 258 (2012) 2679–2685. [17] I. Ismail, M. Hashim, K.A. Matori, R. Alias, J. Hassan, Journal of Superconductivity and Novel Magnetism 25 (2011) 71–77. [18] I.R. Idza, M. Hashim, N. Rodziah, I. Ismail, A.R. Norailiana, Materials Research Bulletin 47 (2012) 1345–1352. [19] I. Ismail, M. Hashim, K.A. Matori, R. Alias, J. Hassan, Journal of Magnetism and Magnetic Materials 324 (2012) 2463–2470. [20] C. Santos, M.H. Koizumi, J.K.M.F. Daguano, F.A. Santos, C.N. Elias, A.S. Ramos, Materials Science and Engineering: A 502 (2009) 6–12. [21] V.K. Sankaranarayanan, Q.A. Pankhurst, D.P.E. Dickson, C.E. Johnson, Journal of Magnetism and Magnetic Materials 125 (1993) 199–208. [22] M. Gharagozlou, Journal of Alloys and Compounds 495 (2010) 217–223. [23] A.K.M.A. Hossain, S.T. Mahmud, M. Seki, T. Kawai, H. Tabata, Journal of Magnetism and Magnetic Materials 312 (2007) 210–219. [24] P.I. Slick, in: E.P. Wohlforth (Ed.), In Ferromagnetic Materials, Vol. 25, NorthHolland, New York, 1980pp. 189–241. [25] K.J. Standley, Oxide Magnetic Materials, Clarendon Press, Oxford, Northern Ireland, 1972.
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Crystallinity and magnetic properties dependence on sintering temperature and soaking time of mechanically alloyed nanometer-grain Ni0.5Zn0.5Fe2O4 Ismayadi Ismail a,n, Mansor Hashim a,b, Idza Riati Ibrahim b, Rodziah Nazlan b, Fadzidah Mohd Idris a, Shamsul Ezzad Shafie a, Masni Manap a, Ghazaleh Bahmanrokh b, Nor Hapishah Abdullah b, Wan Norailiana Wan Rahman b a b
Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology (ITMA), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Physics Department, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2012 Received in revised form 17 December 2012 Available online 9 January 2013
The separate influences of sintering temperature and soaking time on mechanically alloyed ferrites’ crystallity degree and properties are reported. To understand the two influences in the case of a ferrite with the composition Ni0.5Zn0.5Fe2O4, toroids of this composition were separated into three groups for investigation of three respective and different sintering parameters. The first parameter was the sintering temperature of 500 1C to 1400 1C with a 100 1C increment. The second parameter was the sintering temperature from 800 1C to 1000 1C with a 25 1C increment which were believed the temperature range of magnetic phase transition. All toroidal samples were sintered for 10 hours in an ambient air atmosphere. The soaking time was the third parameter employed with prolonged soaking from 1 to 96 hours and a fixed sintering temperature of 800 1C. The X-ray diffraction (XRD) results of the three parameters show that the first appearance of a single phase occurred at as low as 600 1C and the intensity peaks increased as the sintering temperature and soaking time increased, yielding for the entire sintering sequence three distinct families of magnetic hysteresis loops. The density results against temperatures show almost a linear increase until 1200 1C and remain relatively unchanged for 1300 1C and 1400 1C. The increase of XRD intensity with the sintering temperature indicates higher crystallinity and possible higher values of magnetization. From the differential scanning calorimetry (DSC) result, maximum exothermic peak appear at 960 1C and the nanometer starting powders could be speculated to give high reactivity due to a high surface area. The Curie temperatures were all found to be the same i.e. 232 1C. & 2012 Elsevier B.V. All rights reserved.
Keywords: Mechanical alloying Sintering temperature Soaking time M–H loop
1. Introduction Ferrites are usually non-conductive ceramic compounds derived from an oxide of iron i.e. hematite (Fe2O3) or magnetite (Fe3O4) as well as oxides of other metals. Unlike most materials, they possess both high permeability and moderate permittivity at frequencies from dc to millimeter [1]. One of the most commonly known ferrites is nickel zinc ferrite which was developed for a wide range of applications due to their high permeability and low loss characteristics. Today, NiZn ferrite is still one of the most important ferrites for such application and constitutes a substantial portion of the ferrite market. NiZn ferrites have been extensively used as core materials for a large number of devices and electrical components.
n
Corresponding author. Tel.: þ603 89467546; fax: þ 603 86566061. E-mail address: [email protected] (I. Ismail).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.12.047
One of the factors affecting the distribution of the ions between the two types of site for spinel ferrites is temperature [2]. The effect of increasing the temperature would also change the microstructure of the materials which, in turn, would, affect magnetic properties of the materials [3–10]. It is already widely known that magnetic properties of ferrite materials are sensitive to their microstructures. Rezlescu et al. [11] investigated the effect of adding Me bivalent cations (Me¼Cu2 þ , Cd2 þ , Co2 þ , Ca2 þ , Mn2 þ and Mg2 þ ) on the electric and magnetic properties of Ni–Zn ferrites obtained by the conventional method of mixing oxides with a Ni0.25Me0.25 Zn0.5Fe2O4 nominal composition. They found that the partial substitution of Ni2 þ for bivalent cations of Mg2 þ , Cu2 þ , Co2 þ , Mn2 þ , Cd2 þ and Ca2 þ caused the Ni–Zn ferrite lattice parameter to increase, because Ni2 þ has a smaller ionic radius than does the Me2 þ ion. Substituting nickel for Cu2 þ ions favored greater densification of Ni–Zn ferrites at low temperature. Ref. [12] has investigated the effects of quencher after calcination on the microstructure and magnetic properties of manganese–zinc ferrites by
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measuring the magnetic properties, electrical resistivity and density. The powder of Mn0.68Zn0.25Fe2.07O4 was prepared by adopting the conventional ceramic technique. Their toroidal cores were sintered at 1350 1C for 4 h in an atmosphere controlled by using the equation for equilibrium oxygen partial pressure. They reported the inner stress of calcined powder increases, abnormal grains of ferrite grow up and initial permeability decreases and power losses of ferrite rise with the increase in quenching temperature. From their results the microstructure and magnetic properties of manganese–zinc ferrites can be improved with the gradual cooling of calcined powder to room temperature (25 1C). Costa et al. [13] however used the combustion synthesis technique to prepare Ni– Zn ferrite powders with a nominal composition of Ni0.5Zn0.5Fe2O4, using urea as fuel. The large quantity of gas that developed inhibited particle aggregation and yielded soft powders suitable for dispersion and use. They reported the sintering temperature of 1200 1C producing a homogeneous microstructure with an average grain size of 2.0 mm and magnetic parameter values comparable to the values obtained for samples sintered at 1300 1C and 1400 1C. The sintering temperature Z1300 1C contributed to abnormal grain growth and greater trapped porosity. Although their samples presented poor microstructural characteristics, the greater average grain size contributed strongly to the better magnetic properties (lower coercivity and higher permeability), considering the conditions under which the magnetic measurements were taken. Hu et al. [14] studied the effect of heat treatment on the structural and magnetic properties of auto-combusted Mn–Zn ferrite samples with temperatures from 400 1C to 1200 1C. They reported poor magnetic properties after sintering at 550 1C in air. However, above 1100 1C they had fine crystallites with uniform size and the samples showed larger saturation magnetization. In this paper, the physical evolution and phase of the samples subjected to various sintering temperatures and soaking times are reported. Magnetic properties such as saturation magnetization and Curie temperature, characteristic of the prepared material as a function of sintering temperature are presented and discussed. This is necessary because knowledge of the experimental conditions which produced the properties is a prerequisite to the materials’ applications.
2. Methodology A powder composition, towards obtaining Ni0.5Zn0.5Fe2O4, was prepared by mechanical alloying of a mixture of metallic oxides. The materials used were Fe2O3 (Alfa Aesar, 99.95%), NiO (Alfa Aesar, 99.99%) and ZnO (Alfa Aesar, 99.99%) weighed according to the composition formula. The chemicals, in the above order, were mixed with a chosen molar ratio of 1:0.5:0.5. This composition was later mechanically alloyed using a Spex8000D milling machine with a ball to powder ratio (BPR) of 10:1 for 24 h of milling. The alloyed powder was then divided and uniaxially pressed into a few toroidal shapes of 20 mm outer diameter and 9 mm inner diameter with 2 wt% of PVA as binder under a pressure of 686.4 MPa. The toroidal samples produced were separated into 3 groups which were respectively subjected to 3 different sintering parameters. The first parameter was the sintering temperature covering 500 to 1400 1C with each toroidal sample in this first group sintered only once at a temperature different from those of the other samples in the group within the range 500 1C to 1400 1C with 100 1C increments. The second sintering temperature parameter ranged from 800 to 1000 1C with 25 1C increments. Each sample in this second group was sintered only once at a temperature different from those of the other samples in this group. All the toroidal samples were sintered for 10 hours in an ambient air atmosphere. The third,
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soaking-time parameter was employed with prolonged soaking time from 1 to 96 h, keeping the sintering temperature fixed at 800 1C. They were examined with X-ray diffraction (Phillips Expert Pro PW3040) using CuKa radiation. Field emission scanning electron microscopy (FESEM) micrographs were taken using an FEI NOVA NanoSEM 230 machine to reveal the microstructure of the samples. The average grain size of a sintered body was measured over 200 grains by the linear intercept method. The Archimedes principle with water as the fluid medium was used to measure the density of the samples. The percentage theoretical density (%Dth) was calculated using this formula: %Dth g=cm3 ¼ measured density=theoretical density 100% ð1Þ The theoretical density of Ni0.5Zn0.5Fe2O4 was calculated by taking the molecular weight of Ni0.5Zn0.5Fe2O4 to be 237.73 g. The weight of 8 molecules in 1 unit cell is ð8Þ 237:73g=A, where A is Avogadro’s number. The volume of a cube of side, length a is a3. ˚ of Ni0.5Zn0.5Fe2O4 ¼ 8.3827 A˚ therefore The unit cell edge ao (A) a3 ¼589.0495 A˚ 3. As 1 A˚ 3 ¼10 24 cm3, theoretical density is mass/ volume equal to ½ð8Þ ð237:73=6:022 1023 Þ=589:0495 1024 , which is equal to 5.3614 g/cm3. The theoretical density, rx was calculated as above. The percentage of porosity, P, of the sample was calculated using the relation below: r P ¼ 1 100% ð2Þ
rx
where r is the measured density of the sample. The magnetic properties of the samples were investigated using a Vibrating Sample Magnetometer (VSM) (VSM model LDJ 9600). The temperature range for crystallization was measured using a differential scanning calorimetry apparatus (Mettler Toledo). The Curie temperature was measured using an Agilent Precision Impedance Analyzer model 4294A.
3. Results and discussion Fig. 1 shows the XRD spectra of Ni0.5Zn0.5Fe2O4 milled samples sintered at different temperatures starting from 500 1C to 1400 1C. At 500 1C a small trace of Fe2O3 still existed suggesting that the thermal energy supplied to the materials during the sintering process was not enough to complete the reaction. The occurrence of (121) peak shows the existence of a-Fe2O3 at 331. A further increase of sintering temperature at 600 1C yielded a single phase of crystallization of Ni0.5Zn0.5Fe2O4, which indicated the starting sintering temperature for crystallization of Ni0.5Zn0.5Fe2O4. Zn2 þ ions started to diffuse into the tetrahedral sites and Ni2 þ ions moved to octahedral sites. The full crystallization achieved at as low as 600 1C shows the advantage of mechanical alloying. Upon sintering the sample at 600 1C, the major peaks began to form and the intensity of the peaks increased with increasing sintering temperature (Table 1). It shows that the amount of crystalline phase of Ni0.5Zn0.5Fe2O4 is increasing. The theoretical density (%Dth) for each sintered sample is given in Table 1. At lower sintering temperature from 500 1C to 900 1C, we could see almost a linear increase of density against sintering temperature. The increasing trend continued until 1200 1C and remained nearly constant for 1300 1C and 1400 1C. The effect of sintering temperature could be seen in the density and also the porosity of the samples (Table 1). As the sintering temperature increased, the average grain size (Table 1) increased causing the porosity to segregate to the grain boundary and the samples became denser due to the process as found by Ref. [15].
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Fig. 1. XRD spectra of pure Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C. Table 1 Average grain size, Densities, Theoretical Density, Porosities, Lattice Parameter, Cell Volume and peak intensity of the Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C. Sintering temperature (1C) 500 600 700 800 900 1000 1100 1200 1300 1400
Average grain size (lm)
Density (g/cm3)
Theoretical density (%)
Porosity (%) A (A) ˚
Vo (A˚ 3)
2h Obs.
XRD Peak intensity (counts)
0.10(6) 0.22(6) 0.16(6) 0.17(1) 0.41(2) 0.97(3) 3.00(9) 10.09(3) 10.10(5) 10.99(6)
4.37(3) 4.56(3) 4.69(2) 4.75(5) 4.80(6) 5.01(3) 5.19(0) 5.19(0) 5.16(2) 5.12(6)
82 85 88 89 90 94 95 97 96 96
18 15 12 11 10 6 5 3 4 4
590.88 592.94 591.97 591.27 590.77 591.46 590.92 590.78 589.77 587.73
35.33 35.50 35.56 35.51 35.43 35.52 35.46 35.46 35.48 35.67
1759.51 3479.72 6168.39 6571.80 6723.35 6812.34 6768.76 6605.57 6536.59 6715.19
Fig. 2 shows the spectra of XRD patterns for the samples sintered at 800 1C to 1000 1C each for 10 h in air with 25 1C increments. This temperature range believed to be the first appearance of the ordered magnetic moment as reported before [16–19]. All the sintered samples could be indexed to a single spinel phase. As we look closely at these spectra, we could observe the peaks of nickel zinc ferrite which increase with the sintering temperature. The increased intensity of the major peaks of nickel zinc ferrite demonstrated an improvement in the degree of crystallinity of the sintered samples. As the sintering temperature increased, the grain size would also be increased, yielding more crystallization. At the same time, the magnetic phase would be increased due to the increasing crystalline mass having the cubic spinel structure. The increase of magnetic phase in the samples causes the A–B interaction contribution in the spinel to be increased. Therefore the higher sintering temperature leads to higher values of magnetization. The XRD spectra of Ni0.5Zn0.5Fe2O4 sintered at 800 1C each with different soaking times are shown in Fig. 3. These samples were sintered with different soaking times in order to study what
8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.38
would happen if we could increase the grain growth with the same starting crystal phase. The diffraction spectra of the materials revealed the presence of the signature peaks of Ni0.5Zn0.5Fe2O4 at 2y ¼30.131, 35.541, 37.141, 43.321, 53.411, 57.011, 62.591 which can be indexed to (220), (311), (222), (400), (422), (333), (440) of a cubic unit cell respectively. Upon sintering the samples at 800 1C, the major peaks appeared and the intensity peaks increased with the soaking time. This suggests the increase in amount of crystallinity phase of the samples and the increasing of peak intensity could be observed in Table 3. The nanometer particles of the starting powders greatly helped in accelerating the phase formation that even when the sample was sintered within only one hour, all the seven major peaks of the Ni0.5Zn0.5Fe2O4 were formed. The XRD spectra (Fig. 3) at various soaking times show that the overall XRD spectra peaks on the 2y position remain the same, but with an increasing intensity. Besides this, no change in the crystal lattice structure is observed which affirms its stability over a wide range of soaking times. Similar trends of data as mentioned above for the case of sintering at 500 to
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Fig. 2. XRD spectra of sintered Ni0.5Zn0.5Fe2O4 samples from 800 1C to 1000 1C.
Fig. 3. XRD spectra of sintered Ni0.5Zn0.5Fe2O4 samples at 800 1C with various soaking time.
Table 2 Average grain size, Densities, Theoretical Density, Porosities, Lattice Parameter, Cell Volume and Peak Intensity of the Ni0.5Zn0.5Fe2O4 sintered at 800 to 1000 1C. Sintering temperature (1C) 800 825 850 875 900 925 950 975 1000
Average grain size (lm)
Density (g/cm3)
Theoretical density (%)
Porosity (%) A (A) ˚
Vo (A˚ 3)
2h Obs.
XRD Peak intensity (counts)
0.11(3) 0.12(6) 0.12(6) 0.17(1) 0.19(7) 0.29(9) 0.31(2) 0.38(8) 0.49(9)
4.37(3) 4.56(3) 4.69(2) 4.75(5) 4.80(6) 5.01(3) 5.11(6) 5.19(0) 5.16(2)
82 85 88 89 90 94 95 97 96
18 15 12 11 10 6 5 3 4
591.36 591.19 591.15 591.21 591.03 591.16 591.11 590.99 591.41
35.52 35.52 35.52 35.52 35.53 35.55 35.54 35.54 35.56
3143.27 3104.83 3808.33 4085.23 4447.31 4175.14 4707.71 4940.95 5057.22
8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.39 8.39
Table 3 Average grain size, Densities, Theoretical Density, Porosities, Lattice Parameter, Cell Volume and Peak Intensity of the Ni0.5Zn0.5Fe2O4 sintered at 800 1C with various soaking time. Soaking time (h) 1 5 10 20 30 40 96
Average grain size (lm)
Density (g/cm3)
Theoretical density (%)
Porosity (%)
˚ A (A)
Vo (A˚ 3)
2h Obs.
XRD Peak intensity (counts)
0.14(0) 0.15(8) 0.19(7) 0.21(0) 0.23(2) 0.25(0) 0.25(6)
4.51(1) 4.61(6) 4.66(0) 4.69(8) 4.70(1) 4.72(2) 4.92(0)
84 86 87 88 88 88 92
16 14 13 12 12 12 8
8.39 8.39 8.39 8.39 8.39 8.39 8.40
592.24 591.45 591.47 591.14 591.53 591.53 591.97
35.60 35.51 35.51 35.49 35.53 35.52 35.53
1688.52 1765.71 1987.82 1933.04 2577.72 2606.21 2957.25
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1400 1C could be observed for the average grain size, density and porosity of the samples sintered at 800 1C. In the 800–1000 1C sintering case the porosity was reduced significantly after being sintered at 900 1C which correlates with denser and bigger grain size (Table 2). All the peaks of the sintered samples can be clearly indexed to the seven major peaks of the spinel ferrites, which are (220), (311), (222), (400), (422), (333) and (440) planes of a cubic unit cell for the 500 to 1400 1C (Fig. 1), 800 to 1000 1C (Fig. 2) and 1 to 96 h sintered samples (Fig. 3), except for the sample sintered at 500 1C where a trace of hematite occurred with (121) planes. A closer look on the XRD spectra at various temperatures show that, the overall XRD spectra peaks on the 2y position remain the same, but increase in intensity with sintering temperatures and soaking times. Besides this, no significant change in the crystal lattice structure is observed. Since the corresponding peaks of the diffraction patterns of the compound were bearably the same, it was found that unit cell parameters were equally the same with increasing temperature, except for some slight discrepancy in the unit cell volume (Tables 1–3). This reaffirms the spinel phase stability over a wide temperature range. Upon sintering at 500 1C, the X-ray diffraction peaks appeared, indicating an improvement in the crystallinity of the material. The degree of crystallinity of the sample increased with increasing sintering temperature from 500 to 1400 1C (Table 1) and from 800 to 1000 1C (Table 2) and increasing soaking time from 1 to 96 h (Table 3). A fresh sample of the 24-h milled powder was examined using DSC to ascertain the temperature range for crystallization (Fig. 4). The sample was heated under an oxygen environment starting from room temperature up to 1400 1C. The maximum exothermic peak appeared at 960 1C. The nanometer starting powders could be speculated to give high reactivity due to their high surface area. Particles with a higher surface area also have a higher number of particle–particle contact points leading to a higher number of necking contacts between particles, which stimulate the diffusion and evaporation–condensation of the matter on surfaces with consequent bulk densification [20]. From the graph shown, we could see that the temperature range for crystallization started from 500 1C and increasing until 960 1C. The XRD graph (Fig. 1) shows that at 600 1C the milled samples were already forming Ni0.5Zn0.5Fe2O4. By contrast, conventional ceramic methods involving the mixing, grinding and firing of the constituents oxides and/ or carbonates invariably yield large particles in the micrometer size range because the diffusion controlled solid state reaction requires temperatures of 1000 1C or above [21]. The VSM results in Fig. 5 shows the M–H loops of the samples sintered at 500 to 1400 1C with 100 1C increments. From these loops, we could observe the evolution of increasing saturation magnetization of the M–H loop with increasing sintering temperatures. For the range 500 to 600 1C, Fig. 5 shows the first group
Fig. 4. DSC of milled Ni0.5Zn0.5Fe2O4 samples.
Fig. 5. M–H loops of Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C.
Fig. 6. Variation of saturation magnetization (Ms) of Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C.
of loops with low Ms indicating the degree of crystallinity of the samples was also low and can also be seen in Fig. 6. They show the signs of the initial stage of sintering where the presence of grain growth was not noticeable and the magnetization detected was probably contributed entirely by the rotation of the spin magnetic moment. A weak ordered magnetization was detected for the sample sintered at 500 1C. The sample, we believe, is dominated by paramagnetism with a small superparamagnetism. The lower Ms values which relate to the particles with the smaller size could be attributed to a surface distortion due to the interaction of transition metal ions in the spinel lattice with oxygen atoms, which can reduce the net magnetic moment in the particle. This effect is particularly prominent for the ultrafine particles due to their large surface to volume ratio [22]. This caused the magnetic phase transformation from paramagnetism to ferromagnetism to be not so drastic. This behavior could be related to the increasing trend of XRD spectra (Fig. 1) and XRD peak intensity (Table 1). The intermediate phase of hysteresis loop was seen for 700–800 1C and another gap appeared before the samples sintered at 900 1C. This shows that the range of 700– 800 1C is critical for the development of the strong bulk magnetic properties. The transition from 800 1C to 900 1C could be speculated to evolve parallel to the domain wall formation. The changes in the magnetic properties of the samples can also be recognized by the changes of the crystallite sizes dependant on the sintering temperature. Generally the permeability of polycrystalline ferrite ceramics are due to two different magnetizing mechanisms, which are spin rotation and domain wall movement. This relationship can
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be described as
mi ¼ 1 þ X w þ X spin ,
ð3Þ
where Xw is the domain wall susceptibility; Xspin is the intrinsic rotational susceptibility. Meanwhile the Xw and Xspin may be written [23] as follows: X w ¼ 3pM2s D=4g
ð4Þ
and X spin ¼ 2pM2s =K,
ð5Þ
where Ms is the saturation magnetization, K is the total anisotropy, D is the average grain diameter, and g is the domain wall energy. From the formulae given, we know that the domain wall motion is affected by the grain size and enhanced with the increase of grain size. The initial permeability is therefore a function of grain size and magnetization [23]. Fig. 6 shows increasing Ms from lower sintering temperature up to about 800 1C. The second group of samples sintered at 900 1C and above showed a flat trend with small fluctuation values of Ms. This region showed a very strong ferromagnetism-related behavior and probably existed due to movement of domain wall. Looking back at the DSC spectrum in Fig. 4, this is where the crystallization area peaked and started to decrease suggesting that the process of crystallization was nearing completion Fig. 7 shows the saturation magnetization (Ms) dependence of the average grain size of Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C. Comparing Figs. 1, 4 and 7 and the peak intensity (Table 1), we could observe that the degree of crystallinity of the samples was developing in stages. The early stage of sintering (500 to 800 1C)
105
was the region where crystallization rate was very high. However, at 960 1C, crystallization reaction peaked and the rate of reaction subsequently reduced. Increase in crystallization reaction caused the material to possess a greater degree of magnetocrystalline anisotropy and exchange interaction that determine the direction in which magnetization prefers to be oriented. This effect can be seen in Fig. 7, where the sample with average grain size of 0.41 mm and above showed a flat trend of saturation magnetization of the sample showing a sign of complete crystallization process. Samples with grains of 0.41 mm and below would contain some paramagnetic and superparamagnetic states as mention in Ref. [17]. Fig. 8 shows the M–H loops of samples sintered at 800 to 1000 1C with 25 1C increments. The 800 1C and 825 1C samples (Figs. 8 and 9) show a slightly lower magnetization compared to others. This shows that there was a certain critical temperature or degree of crystallinity phase for the sample to have a complete
Fig. 9. Variation of saturation magnetization (Ms) of Ni0.5Zn0.5Fe2O4 sintered at 800 to 1000 1C.
Fig. 7. Ms as a function of average grain size of Ni0.5Zn0.5Fe2O4 sintered at 500 to 1400 1C.
Fig. 10. M–H loops of Ni0.5Zn0.5Fe2O4 sintered at 800 with various soaking time.
Fig. 8. M–H loops of Ni0.5Zn0.5Fe2O4 sintered at 800 to 1000 1C.
Fig. 11. Variation of saturation magnetization (Ms) of Ni0.5Zn0.5Fe2O4 sintered at 800 1C with various soaking time.
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Fig. 12. The variation of initial permeability with temperature for the sample Ni0.5Zn0.5Fe2O4 sintered at different temperatures.
single phase which resulted in higher magnetization. Fig. 9 shows a small rise of Ms with increasing sintering temperature. Slight increases of Ms after 850 1C indicate that there was not much change in terms of ordered magnetism. Fig. 10 shows the effect of soaking time on the evolution of ordered magnetism. Only two groups of M–H loops were observed. The first, sintered at 800 1C with 1 h soaking time, indicated that the degree of crystallinity was low. The second group (5 h and more of soaking time) whose Ms increased with soaking time showed a high degree of crystallinity. Fig. 11 showed a sharp increase of Ms from 1 h to 5 h of soaking time. Within this range of soaking time (1 to 5 h), it shows that very high reactivity of transformation of Fe2 þ into Fe3 þ [24]. Looking at the grain growth and density (Table 3), the increases are moderately small, therefore we believe the increase of ordered magnetism was due to the slow increase in the degree of crystallinity of the sample. Prolonged soaking time after 40 h, up to 96 h, did not show significant rise of Ms. This is because all the Fe2 þ has turned into Fe3 þ , and a complete single phase transformation achieved [24]. Fig. 12 shows the permeability variation as the sample temperature approached the Curie temperature of the samples sintered at 500 to 1400 1C for 10 h. The initial permeability increases slowly with the temperature until 190 1C before reaching the peak value at a certain temperature. The permeability was measured at 10 kHz, i.e. below resonance, and was therefore dependant on the microstructure i.e. on grain size and porosity. After 190 1C, the permeability reached peak values, because the thermal energy can drastically define the domain wall from the strain in the grain. It shows almost a sudden and greater release of pinning. The maximum of initial permeability indicates the point of zero anisotropy field [25]. The samples become paramagnetic at about 232 1C which destroyed the ordered magnetism, indicating the point of curie temperature. The Curie temperature behavior of the samples confirms the existence of superexchange interaction of in spinel ferrites. It also indicates that the samples produced are pure single magnetic phase.
of Ni0.5Zn0.5Fe2O4 at a temperature as low as 600 1C. The density against sintering temperature also increased for the samples sintered from 500 1C giving maximum density at 1200 1C and remained the same for both the sintering temperature of 1300 1C and 1400 1C. The average grain size increased causing the porosity to segregate to the grain boundary and the samples became denser due to the process. The sintered samples showed increases in the degree of crystallinity with the increase of sintering temperature and soaking time which could be observed by the increase of grain size and peak intensity. The mechanical alloying method with nanometer particles of the starting powders helps in accelerating the phase formation even when the sample was sintered within one hour. From the DSC measurement, the maximum exothermic peak appeared to be at 960 1C and, relating this to the degree of crystallinity, we could see there is a transition in the magnetic properties measured. The samples with the lower sintering temperatures indicated the M–H loops associated with negligible grain growth and the magnetization detected was contributed by the spin magnetic moment. The weak ordered magnetization was dominated by the paramagnetic phase with a small amount of the superparamagnetic phase. The magnetic transition from 800 1C to 900 1C was due to the domain wall formation. The strong ferromagnetism was revealed through movement of domain wall. From the effect of soaking time on the M–H loop, the sharp increase of Ms shows the chemical reactivity was high within the 1–5 h soaking time due to transformation of Fe2 þ into Fe3 þ . Whereas the soaking time of 40 h up to 96 h did not show a significant rise of Ms and can be attributed to the complete transition of Fe2 þ to Fe3 þ . The Curie temperatures of all the samples sintered at 500 to 1400 1C showed no significant changes. They had the same value of 232 1C, indicating that the Curie temperature is not microstructure-sensitive, dependent only on crystal-structure details and above which the samples become paramagnetic.
3.1. Conclusion
The authors are thankful to Ministry of Higher Education (MOHE) for providing the Prototype Research Grant Scheme (PRGS) with the Vot no. 5528000 and Fundamental Research Grant Scheme (FRGS) with the Vot no. 5524164.
In summary, both sintering temperature and soaking time directly influence phase crystallinity. The degree of crystallinity of the sample increased with increasing sintering temperature from 500 1C to 1400 1C, from 800 1C to 1000 1C and also with 1 to 96 h of soaking time for a single sintering temperature of 800 1C. The XRD spectra peaks on the 2y position remained the same. The XRD results proved that the powders subjected to mechanical alloying for nanometer starting powders and later sintered according to the planned parameters result in full crystallization
Acknowledgment
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