Synthesis of copper–magnesium–zinc ferrites and correlation of magnetic properties with microstructure

Materials Science and Engineering B65 (1999) 79 – 89 www.elsevier.com/locate/mseb

Synthesis of copper–magnesium–zinc ferrites and correlation of magnetic properties with microstructure D.N. Bhosale a,*, S.R. Sawant a, S.A. Gangal b, R.R. Mahajan c, P.P. Bakare d a

Department of Electronics, Shi6aji Uni6ersity, Kolhapur-416 004, India Department of Electronic Science, Pune Uni6ersity, Pune-411 007, India c High Energy Material Research Laboratory, Pashan, Pune-411 042, India d Physical Chemistry Di6ision (SIL), National Chemical Laboratory, Pune-411 008, India b

Received 19 February 1999; received in revised form 22 June 1999

Abstract A novel route for the preparation of high density, high permeability Cu – Mg – Zn ferrites using oxalate precursors is reported, with various Mg2 + contents represented as Cu(0.5 − x)Mgx Zn0.5Fe2O4 wherein x= 0.00, 0.20, 0.25, 0.40. To investigate the ferritization temperature of this system, TG/DTG/DTA studies have been carried out varying from 599 to 743 K with increasing x. X-Ray diffraction (XRD) study revealed that all the samples had a single spinel phase and the cubic lattice parameter ‘a’ decreased with an increase in x. Particle size distribution (PSD) and scanning electron microscopy (SEM) techniques were employed to determine the average particle size and to study the microstructure respectively. Initial permeability of all the samples increased with increase in temperature and Mg2 + content (x5 0.20). It also exhibited a thermal hysteresis effect. The magnetic properties are correlated with high density and grain structure. The magnetic properties of this ferrite system prepared by novel route are superior to those of commercial ETKMG ferrites. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cu – Mg – Zn ferrites; Magnetic properties; Microstructure

1. Introduction The quest for ceramic materials with high density, high purity, high permeability has led to the investigation and evaluation of various unconventional preparative methods. Precursors are advantageous because they ensure excellent stoichiometry, low trace impurity content and maximum homogeneity [1 – 6]. For chemical synthesis, a precursor compound with intended stoichiometry is prepared first which is decomposed at temperatures uB 873 K in a subsequent calcination reaction to obtain the required metal oxides [7]. The oxalate precursors are usually preferred due to their low solubility, low decomposition temperature and very fine particle nature [8]. Ideally, in order to achieve a complete reaction within the shortest time and at the lowest possible temperatures, mixing of component cations on an atomic scale is necessary. Compound-precursors achieve this goal, but the stoi* Corresponding author.

chiometry of the precursor does not often strictly coincide with the stoichiometry of the desired product [9]. Various authors [10,11] have investigated the coprecipitation of metal oxalates from appropriately composed solutions in order to produce precursor compounds for spinel ferrite MFe2O4. According to Schuele [10] when oxalate complex of general composition MFe2(C2O4)3·nH2O (where M=Ni2 + , Cu2 + ) is decomposed at 673 K it yields the required ferrite. Wickham [12] has synthesized spinel ferrites MFe2O4 (where M= Mg2 + , Ni2 + , Zn2 + , Mn2 + , Co2 + ) by thermal decomposition of MFe2(C2O4)3·nH2O (where M= Mg2 + , Ni2 + , Zn2 + , Mn2 + , Co2 + ) in air at 873 K. Schroder [13] concluded that decomposition of mixed oxalates is an apparently low temperature phenomenon. Koh and colleagues [14,15] have synthesized Cu– Mg–Zn ferrites by a ceramic method, which involves solid state reactions between intimately mixed fine powders at high temperatures. They have reported the effects of powder preparation conditions on electrical

0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 1 8 2 - 8

80

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

properties, magnetic properties and microstructure, as well as the effect on physical properties by changing raw material compositions and sintering temperatures of ferrites. Park et al. [16] have synthesized Mg –Cu–Zn ferrites by a ceramic technique and have investigated the effect of additives such as NiO and Cr2O3 on the magnetic properties of the Mg – Cu – Zn ferrites. They have further reported that the Mg – Cu – Zn ferrites have magnetic properties similar to those of Ni –Cu–Zn ferrites, and the former are economical. The motivation to investigate Cu – Mg – Zn ferrites also lies in its low cost and wide range of applications. However, attempts to synthesize Cu– Mg – Zn ferrites by other non-conventional methods and their subsequent characterization appear to be lacking. In the present paper, we report the synthesis of high density and high permeability Cu – Mg – Zn ferrites by a novel route, i.e. a non-conventional method using oxalate precursors. The method is advantageous in obtaining a homogeneous solid solution of oxalates which on further decomposition yield the required ferrite compositions. Chemical analysis of ferrite compositions has been carried out by thermogravimetry. Thermal analysis has been carried out for determination of ferritization temperatures by simultaneous recording of thermogravimetry (TG)/differential thermogravimetry (DTG)/differential thermal analysis (DTA) plots (simultaneous thermal analysis — STA) and also to confirm the experimental results of the thermogravimetry. X-Ray diffraction (XRD) studies have been carried out to characterize the single spinel phase. Density measurements of these samples have been carried out by pycknometric method using xylene medium. Particle size distribution (PSD) data have been used to determine the average particle size. The scanning electron microscopy (SEM) technique has been used to study the grain structure of the samples which has been correlated with the magnetic properties. The experimental results of PSD and SEM were further used to correlate the magnetic properties in the light of density data. Initial permeability studies have been carried out to investigate the magnetic properties and thermal hysteresis effect. A comparative study of magnetic properties is also included.

required metal acetates [17]. The method of preparation was further modified and used for preparation of Cu– Mg–Zn–Fe oxalate complexes, the details of which are given below. In the present case, Fe(II) acetate was prepared by adding glacial acetic acid (slight excess) to the required quantity of AR grade Fe metal powder. To avoid oxidation of Fe(II) to Fe(III), the reaction was carried out in a CO2 atmosphere instead of N2 atmosphere employed in the above method [7]. To maintain the desired stoichiometry, the required quantities of copper acetate, zinc acetate, magnesium acetate were dissolved in doubly distilled water, warmed at 333 K. The aboveprepared Fe(II) acetate solution and Cu-, Mg-, Zn-acetate solutions (total metal ion concentration=0.45 M) were drop-wise added to 0.60 M oxalic acid solution to precipitate the required oxalate complex. The oxalic acid was continuously stirred during addition of acetate solutions for 1 h for homogenous mixing until its completion. The solution with yellow crystalline precipitate was stirred for 1 h at its boiling point, digested for 10 min and allowed to cool to room temperature. The supernatant liquid/solution was filtered off. The precipitate was washed with doubly distilled water and dried in an oven at 373 K. Thus oxalate complexes having general composition Cu(0.5 − x)Mgx Zn0.5Fe2(C2O4)3·nH2O were synthesized, which yielded the desired ferrite compositions on subsequent decomposition. The flow chart (Fig. 1) summarises the synthesis of Cu– Mg–Zn ferrites and pellet, toroid formation.

2.2. Physicochemical characterization

2.1. Synthesis

2.2.1. Quantitati6e chemical analysis: thermogra6imetry Quantitative chemical analysis of ferrite compositions were carried out by thermogravimetry. For this purpose, initially 10× 10 − 3 kg coprecipitated oxalate complex was weighed out in two separate silica crucibles for each composition. For this purpose, a single pan microbalance having least count 10 − 8 kg (10 − 5 g). was used. The oxalate complex was decomposed at 873 K for 1 h in air. The rates of heating and cooling were 5 and 3°C/min, respectively. After satisfactory completion of the heating procedure, the mass loss was found for each sample and therefrom mass loss (%) was computed. It was further compared with theoretically calculated mass loss (%). The experimental results obtained are presented in Table 1.

Bremer et al. [7] have prepared manganese –zinc ferrites using a modified method of Wickham [12] for preparation of individual oxalates. The method has an advantage over the conventional metal oxide process since acetic acid has been obtained as an important by-product after the synthesis of oxalates using the

2.2.2. Study of ferritization temperature TG/DTG/DTA curves were simultaneously recorded in the temperature range from room temperature to 973 K using a Netz’sch 409 Differential Thermal Analyser in a static air atmosphere. The ferritization (i.e. formation of respective ferrite) temperatures were determined

2. Experimental

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

81

Table 2 Effect of Mg2+ content on lattice parameter x

0.00 0.20 0.25 0.40

Lattice parameter (‘a’ 90.002)×10−10 m Oxalate complex calcined at 873 K

End product sintered at 1273 K

8.398 8.391 8.386 8.395

8.425 8.387 8.380 8.365

tometer Model PW 1710 using Cu Ka radiation (l= 1.5405 A, ) in the 2u range of 20–80° and in the intensity range of 250–1000 counts (Fig. 3). Using a standard conversion table, ‘d’ values for corresponding 2u were derived. The lattice parameters were computed using these ‘d’ values and respective (h,k,l) parameters, which identified that samples were single spinel ferrites. To characterize samples, the experimental results of XRD have been compared with reported lattice parameter values [19,20] for Mg0.5Zn0.5Fe2O4 —0.840×10 − 9 m (0.840 nm) and Cu0.5Zn0.5Fe2O4 —0.841×10 − 9 m (0.841 nm), respectively.

from experimental data (Table 2). It was also used to confirm the thermogravimetric experimental results. A detailed study of ferritization temperature, mass loss (%) during dehydration and decomposition, onset and termination temperatures of dehydration and decomposition processes are reported separately [18].

2.2.4. Density Density measurements were carried out by the pycknometric method using a xylene medium. The weights of sample in air and on immersion in xylene were recorded using a single pan digital balance (Adlair Dutt, Calcutta) having least count 0.1× 10 − 4 kg (0.1 mg). The density of samples was calculated using the formula, r= Wr%/(W − W %) where W is the weight of sample in air, W% is the weight of sample in xylene and r% is the density of xylene.

2.2.3. X-Ray diffraction (XRD) studies The solid solutions of co-precipitated oxalate complexes were decomposed at their respective ferritization temperatures obtained from TG’s for 3 h and at 873 K for 1 h in air. XRD patterns of all the earlier mentioned samples were recorded using a Philips X-ray Diffrac-

2.2.5. Particle size distribution (PSD) The particle size distribution (PSD) study of oxalate complexes decomposed at their respective ferritization temperatures, 873 and 1273 K was carried out using a Galai Type CIS-1 Laser Particle Size Analyser. The samples processed at different temperatures were dis-

Fig. 1. A flow chart explaining a novel route of synthesis for the Cu– Mg – Zn ferrite system.

Table 1 Experimental results obtained by thermogravimetry and TG/DTG/DTA studies and ferritization temperatures (temperature of ferrite formation) as a function of Mg2+ content x

0.00 0.20 0.25 0.40

Proposed formulae for oxalate complexes

Cu0.5Zn0.5Fe2(C2O4)·6.5H2O Cu0.30Mg0.20Zn0.50Fe2(C2O4)·7.5H2O Cu0.25Mg0.25Zn0.50Fe2(C2O4)·6.5H2O Cu0.10Mg0.40Zn0.50Fe2(C2O4)·7.5H2O

Mass loss (%)

Ferritization temperatures (K)

Theoretical

Thermogravime- Netz’sch STA 409 try

56.90 58.62 56.49 59.67

56.87 58.16 56.60 59.40

57.63 59.02 56.52 59.33

599 610 610 643

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

82

persed in a glycerol/water (60:40) suspension. Sonication was carried out for 30 s before analysis.

2.2.6. Scanning electron microscopy (SEM) studies The sintered toroids were polished optically and then broken into small pieces. These small pieces of toroid were gold-coated using a Sputter coating unit (ES 5200 Auto Sputter Coater Bai-rad, UK) operating at a voltage of 1 kV. The micrographs of the Cu(0.5 − x)Mgx Zn0.5Fe2O4 (x = 0.00, 0.20, 0.25, 0.40) were taken with the help of a scanning electron microscope (Model 2097 OM Cambridge Stereoscan 120 TL). The average grain size was calculated [21] and reported in Table 3. 2.2.7. Initial permeability (mi) Initial permeability measurements were carried out using an LCR-Q meter (APLAB, India). The inductance values of the toroidal core of samples having outer diameter (o.d.) of 2×10 − 2 m, inner diameter (i.d.) of 1× 10 − 2 m and thickness of 0.3×10 − 2 m and having 100 turns of winding were measured at 1 kHz at room temperature and in the temperature range 298– 773 K. The initial permeability (mi) was determined by using the relation, mi =

L 0.0046N 2h log10(d2/d1)

where L is the inductance (mH), N is the number of turns, d2 is the outer diameter, d1 is the inner diameter, and h is the height of core in inches.

3. Results and discussion

3.1. Quantitati6e chemical analysis: thermogra6imetry The temperature ranges for the different steps observed during dehydration and decomposition processes for each separate required metal oxalate system represented by MFe2(C2O4)3·nH2O (M= Mg2 + , Zn2 + , Cu2 + , Fe2 + ) have been reported by Duval [22] and presented in Table 4. In the case of decomposition of zinc oxalate to zinc oxide, the required maximum tem-

perature is 863 K. Therefore, to ensure the completion of dehydration and decomposition of oxalate complexes, the calcination temperature was maintained at 873 K for 1 h. The experimentally observed total mass loss (%) (Table 1) agrees very well with the theoretically calculated mass loss (%) indicating that the requisite stoichiometry is maintained.

3.2. Study of ferritization temperature TG/DTG/DTA studies (Fig. 2) reveal that the decomposition takes place in two steps for oxalate complex with composition x= 0.00 and 0.20. In the case of composition with x= 0.25 and 0.40, the first step indicates the presence of two substeps. Step I is of dehydration and step II is decomposition of anhydrous oxalate complex (Fig. 2). The formation of respective MOs (where M= Fe2 + , Cu2 + , Mg2 + , Zn2 + ) can be explained by the reaction [23] MC2O4 “ MO+ CO  + CO2  The ferritization (formation of ferrite) and decomposition processes occur simultaneously in the temperature range 482–643 K (maximum) as nascent MOs are highly reactive and therefore ferritization occurs at such a low temperature in the range of 599–643 K. The variation of ferritization temperature with increasing Mg2 + content is shown in Table 1. The details of TG/DTG/DTA (STA) studies have been reported separately [18]. Although ferritization takes place at very low temperatures, to ensure the complete decomposition of oxalate complexes to the requisite ferrite composition, we have calcined the oxalate complex at 873 K during the present synthesis. The observed total mass loss (%) by thermogravimetry and by TG studies (Netz’sch 409 Differential Thermal Analyser) are in close agreement with theoretically calculated values. This has been further used to determine the total number of water molecules present in the respective oxalate complex system and to predict their proposed formulae.

Table 4 Thermoanalytical data for individual oxalates Oxalate

Stable up to (K)

Dehydration (K)

Stable as anhydrous oxalate (K)

Decomposition

MgC2O4·2H2O CuC2O4·0.5H2O

453 –

453–523 –

523–673 373–543

ZnC2O4·2H2O

348

348–438

Unstable

FeC2O4

–

–

–

773 K“MgO 561 K (abrupt) 767 K“CuO (black) 553–863 K 863 K“ZnO 468 K (two stages)

x

0.00 0.20 0.25 0.40

Density×103 (kg/m3) Theoretical

Mass/volume

Xylene

5.340 5.174 5.132 5.008

4.850 5.132 5.087 5.080

4.913 4.714 4.971 4.770

Porosity (%)

Average grain size ‘D’×10−6 m (SEM)

Average particle size×10−6 m (PSD)

mih

mic

Dmi

%Dmi/mic

9.20 0.81 0.88 –

15 6 5 4

6.76 6.69 7.52 5.85

3728 5034 4997 411

3398 4969 4950 410

330 65 47 1

9.71 1.31 0.95 0.24

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

Table 3 Effect of Mg2+ content on density, porosity percentage, particle size distribution, grain size in relation to initial permeability

83

84

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

Fig. 2. Thermograms of solid solution of the oxalate complex system. Cu(0.5 − x)Mgx Zn0.5Fe2(C2O4)·nH2O.

3.3. X-Ray diffraction (XRD) studies The XRD patterns of oxalate complexes decomposed at 610 K (ferritization temperature) for 3 h, calcined at 873 K for 1 h and finally sintered at 1273 K for 4 h are shown in Fig. 3 for representative ferrite composition with x=0.25 in the Cu(0.5 − x)Mgx Zn0.5Fe2O4 ferrite system. The analysis of XRD pattern reveals the following points: 1. Using experimental data of ferritization temperatures (Table 1), the oxalate complexes were decomposed at their respective ferritization temperatures. The end-products after decomposition were identified as single spinel phase Cu – Mg – Zn ferrite, from the analysis of their recorded XRD patterns (Fig. 3(I)). This proves the simultaneous completion of decomposition process of oxalate complex and ferritization. The XRD patterns exhibit peaks corresponding to typical Cu – Mg – Zn ferrites and the absence of any other impurity phase. It has also been confirmed by energy dispersive analysis by X-ray (EDAX). 2. Recorded XRD patterns of oxalate complexes calcined at 873 K for 1 h and sintered at 1273 K for 4 h with x =0.00, 0.20, 0.25 and 0.40 were identified as a single spinel phase of the Cu – Mg – Zn ferrites. The computed lattice parameters ‘a’ for these compositions are comparable with those of Cu0.5Zn0.5Fe2O4 and Mg0.5Zn0.5Fe2O4 having a single spinel phase with reported ‘a’=0.841 ×10 − 9 m (0.841 nm) and 0.840× 10 − 9 m (0.840 nm) respectively [19,20]. The lattice parameter values for oxalate complexes decomposed at 873 K for 1 h and end ferrite sintered at

1273 K for 4 h are given in Table 2. It is seen from Table 2 that for the present ferrite system the lattice parameter ‘a’ gradually decreases with increasing Mg2 + content. This is attributed to the ionic radii [24,25] of Cu2 + (0.70× 10 − 10 m) and Mg2 + (0.65× 10 − 10 m).

3.4. Density It is clear from Table 3 that the theoretically calculated density decreased with increasing Mg2 + content. Such variation in the experimental density data is not closely observed for the present Cu–Mg–Zn ferrite system (Table 3). The best mi value for ferrite compositions with x= 0.20 can be attributed to the lowest porosity percentage of this sample, which has been derived from theoretical and experimental density data (Table 3).

3.5. Particle size distribution (PSD) Particle size distribution data of oxalate complexes decomposed at ferritization temperatures, calcined at 873 K for 1 h and finally sintered at 1273 K for 4 h are recorded and the average particle size observed for these samples are given in Table 5. Fig. 4 shows typical particle size distribution data observed for the samples with x= 0.40 representing the data obtained for Cu(0.5 − x)Mgx Zn0.5Fe2O4. Fig. 4 indicates that the particle size distribution is narrow and average particle size is 5.85 × 10 − 6 m (5.85 mm). It is clearly seen from Table 5 that the average particle size increases with an increase in processing temperature and Mg2 + content. The average particle size determined using PSD data closely matches the average grain size seen from

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

85

Fig. 4. Particle size distribution of Cu(0.5 − x)Mgx Zn0.5Fe2O4 system with x= 0.40. The average particle size is 5.85 mm.

3.6. Scanning electron microscopy (SEM) studies

Fig. 3. XRD pattern for typical ferrite composition with x= 0.25. (I) Oxalate complex decomposed at 610 K for 3 h. (II) Oxalate complex decomposed at 873 K for 1 h. (III) End ferrite sintered at 1273 K for 4 h.

recorded micrographs of these samples by SEM technique.

Table 3 gives the values of average grain size obtained from micrographs. For measuring grain size/average particle size, the procedure was that adopted by Ishikawa et al. [21]. Average grain size is determined as follows [21]: 1. Draw a diagonal on the micrograph; 2. Measure the maximum unidirectional particle size in the vertical direction against diagonal; 3. Note the average of the maximum unidirectional particle size. The micrograph (Fig. 5(a)) for composition with x= 0.00 shows a discontinuous grain structure with a large number of voids. The very fine striations associated with fatigue are readily observed. Although the grain size is large for this composition, the permeability value is comparatively low. This is attributable to the porosity percentage and the presence of a number of voids indicating comparatively large grain separation ($ 7×10 − 6 m) causing a decrease in bulk density (Table 3). On the other hand, the micrograph of ferrite composition with x= 0.20 (Fig. 5(b)) shows a well-packed, crack-free, continuous grain structure without any voids with distinct grain boundaries. The porosity percentage value being lowest, the initial permeability value of this sample is highest.

Table 5 Effect of Mg2+ content and processing temperatures on average particle size of Cu(0.5−x)Mgx Zn0.5Fe2O4 system x

0.00 0.20 0.25 0.40

Average particle size×10−6 m (PSD) Oxalate complex decomposed at ferritization temperature

Calcined at 873 K

Sintered at 1273 K

1.05 2.78 2.66 4.10

0.64 3.77 4.29 5.57

6.76 6.69 7.52 5.85

86

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

The micrograph of ferrite composition with x= 0.25 (Fig. 5(c)) also shows a well packed, crack free, continuous grain structure without any voids, having clear grain boundaries. It also indicates a microstructure having high density/low porosity exhibiting high initial permeability (mi) comparable with that of x =0.20. The micrograph of ferrite composition with x=0.40 (Fig. 5(d)) shows a discontinuous grain growth with a large number of voids indicating very poor sintering and initiation of grain growth process. This is obvious for the ferrite composition having the highest Mg2 + content and thereby results in the lowest mi. To study microstructure control through the sintering and densification process for ferrite compositions prepared by the present novel route, micrographs of the sample prepared at 610 K (ferritization temperature), 873 K (calcination temperature) and powder obtained by crushing and grinding the toroid of x= 0.25 sintered at 1273 K are recorded in Fig. 6. Fig. 6(a) indicates the grain structure of the samples obtained from decomposition of the oxalate complex decomposed at 610 K with x =0.25. It indicates discontinuous grain growth with a large number of voids and agglomerated grains. The grain size ranges from 2 to 4×10 − 6 m (2 – 4 mm). Fig. 6(b) indicates the initiation of grain growth process having average grain size of 4×10 − 6 m, with relatively fewer voids. Fig. 6(c) shows the completion of the sintering process having larger grains (average grain size, 5×10 − 6 m). The presence of a large number of voids is caused by grinding and crushing the grains/samples. The microstructure differs very much from the microstructure observed for a broken polished gold-coated toroid (Fig. 5(c)). The experimental

observations of PSD and SEM techniques prove that a very good microstructural control is achieved using the novel route adopted for the synthesis of Cu–Mg–Zn ferrites.

3.7. Initial permeability (mi) The initial permeability (mi) of samples sintered at 1273 K at room temperature and its variation with temperature are shown in Figs. 7–9. The initial permeability increased with an increase in temperature. This phenomenon was considered to be due to a faster decrease in anisotropy field rather than a decrease in saturation moment [15]. It has also been reported that initial permeability increases with an increase in temperature showing a sharp fall in mi value just before reaching the Curie temperature [26]. The plot of mi vs T for the composition with x=0.00 (Fig. 7) shows the presence of two slopes before reaching the Curie temperature Tc, i.e. first slope in the temperature range 300–350 K and second slope in the temperature range 350–370 K. The presence of two slopes can be explained on the basis of the contributions of mrk and mw to mi, i.e. (1) resulting from spin rotation (mrot); (2) resulting from domain wall motion/ displacement (mw). The values of rotational permeability and wall permeability have been calculated using the formula [27,28], mrk = 1+2pM 2s / K1 and mw = mi − (mrk −1). Now the contribution to mi by mrot is a slow and temperature-independent process, whereas the contribution due to mw is a relatively quicker and temperature-dependent process, which are again characteristics of the materials.

Fig. 5. Micrographs of ferrite system Cu(0.5 − x)Mgx Zn0.5Fe2O4. (a) x =0.00; (b) x =0.20; (c) x =0.25; (d) x = 0.40.

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

87

Loaec has reported the thermal spectrum of the first category in the Ni–Zn ferrite sample directly sintered from a mixture of oxides. All the ferrites in this category are characterised by a cubic structure and a magnetocrystalline anisotropy constant K1, increasing smoothly from TC to lower temperature.

Fig. 7. Thermal hysteresis of initial permeability for composition with x= 0.00.

Fig. 6. Micrographs of the ferrite composition with x= 0.25 prepared at (a) 610 K; (b) 873 K; (c) Crushed toroid powder sintered at 1273 K.

With the addition of Mg2 + there is a distinct change in mi vs T behaviour (Figs. 7 – 9). As Mg2 + content increases the slope due to mrot has an insignificant contribution to mi, as a result the lower slope disappears and only one slope remains corresponding to the contribution of mw to mi. Thus the addition of Mg2 + suppresses the contribution due to spin rotation. Loaec [29] has classified the thermal spectra of initial permeability into two categories: 1. Ferrites exhibiting one peak in permeability named the Hopkinson peak near TC, followed by a steady decrease with decreasing temperature. 2. Ferrites exhibiting an additional peak at lower temperature and named a secondary peak.

Fig. 8. Thermal hysteresis of initial permeability for composition with x= 0.20.

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

88

Fig. 9. Thermal hysteresis of initial permeability for composition with x= 0.25. Table 6 Comparison of magnetic properties Parameter

ETKMG-3

Initial permeability 350 (mi) Saturation flux den- 2400 sity (Bs) Coercive force (Hc) 0.4 Curie temperature 155 (Tc)

ETKMG-5 550 2500 0.45 160

Our sample 4997 7091 Gauss (4pMs) – 145

pinning and competition between intrinsic parameters such as saturation magnetization (Ms) and anisotropy constant (K1) around transition points. The explanation seems to support the fundamental role of the domain wall mechanism in magnetization process and the idea of domain wall topography specific to toroidal soft ferrite samples. The ferrite compositions under investigation exhibited similar thermal hysteresis, with a Hopkinson peak of category (1) and type (1) and this is attributable to irreversible wall motion. A similar effect has been reported by other research workers [30,31]. For the present system, similar observation, i.e. mih B mic is observed as in part (1) (Table 3). Additionally, Table 3 gives the values of Dmi, where Dmi =mic −mih. It is very clear from Figs. 7–9 of mi vs T plot, that peak heights indicate a difference in the magnitude of mi recorded during cooling and heating cycles. This difference occurs due to the decrease in thermal disturbance on cooling and thereby due to the resultant magnetisation on effective ordering of the domains present in the grains/grain structure. The variation in the percentage of Dmi to mic decreases with increasing Mg2 + content (Table 3, column 11) and is attributable to the increase in K1. (For CuFe2O4 [32], K1 = − 6×10 − 4 J/m3 while for MgFe2O4 [33], K1 = − 2.5× 10 − 4 J/m3.) The maximum mi is observed for the sample with x= 0.20 which decreases with further increase in x due to the dilution effect. Density, PSD and SEM studies show that addition of Mg2 + significantly affects the densification behaviour and microstructure resulting in changes in initial permeability values as is clear from Table 3. The comparative data of magnetic properties is given in Table 6. It shows that the magnetic properties of the ferrite system prepared by the novel route are superior to those of commercial ETKMG ferrites.

4. Conclusions Loaec [29] has reported that thermal hysteresis of mi consists of two types: 1. One which occurs below the Curie temperature (Tc), the permeability obtained during heating cycles is always lower than that obtained during cooling cycle. mih B mic 2. Another type which occurs between the Hopkinson peak and Curie temperature (Tc) when the heating curve is above the cooling one. Loaec [29] has investigated thermal hysteresis of initial permeability in soft ferrites (such as Mn – Zn, Ni– Zn) near the transition temperature. This phenomenon has been explained by taking into account domain wall

1. Cu–Mg–Zn ferrites synthesized by a novel route/ non-conventional method using oxalate precursors exhibit high density and high permeability. The substitution of Cu2 + by Mg2 + in Cu–Zn ferrite leads to low cost production/better economy. 2. For the present ferrite system, ferritization takes place at a very low temperature, ranging from 599 to 743 K varying with Mg2 + content. 3. The thermal hysteresis effect around the transition point depends upon the domain wall pinning and competition between intrinsic parameters such as saturation magnetization (Ms) and anisotropy constant (K1). 4. The most sintered sample with x= 0.20 has the highest permeability, i.e. mi = 5034, which is at-

D.N. Bhosale et al. / Materials Science and Engineering B65 (1999) 79–89

tributable to continuous grain structure, having clearly defined grain boundaries and without any voids, i.e. having the lowest porosity. 5. The Cu–Mg– Zn ferrites synthesized by the novel route exhibit superior magnetic properties compared with commercial ETKMG ferrites prepared by conventional ceramic techniques. 6. PSD and SEM studies indicate that a very good control on the microstructure is achieved using the novel route for the synthesis of Cu – Mg – Zn ferrites and having superior magnetic properties.

Acknowledgements The authors express their sincere thanks to Dr D.R. Ketkar and R.D. Kale of the Department of Metallurgical Engineering and Material Science, Indian Institute of Technology, Mumbai (India) for recording particle size distribution data. Thanks are extended to the Director, Regional Sophisticated Instrumentation Centre, Nagpur University Campus, Nagpur for SEM studies.

References [1] K.C. Patil, S. Sundar Manoharan, D. Gajpathy, in: N.P. Cheremisinoff (Ed.), Handbook of Ceramics and Composites, Part I, Marcel Dekker, New York, 1990, p. 469. [2] M.N. Sankarshana Murthy, C.E. Deshpande, P.P. Bakare, J.J. Shrotri, Bull. Chem. Soc. Jpn. 52 (2) (1979) 571–573. [3] P.P. Bakare, J.J. Shrotri, C.E. Deshpande, M.P. Gupta, S.K. Date, Ind. J. Chem. 26A (1987) 1–6. [4] P.P. Bakare, V.G. Gunjikar, C.E. Deshpande, S.K. Date, J. Mater. Sci. 26 (1991) 484–490. [5] P.P. Bakare, C.E. Deshpande, V.G. Gunjikar, P. Singh, A.B. Mandale, S.K. Date, Bull. Mater. Sci. 17 (6) (1994) 1015 – 1028. [6] P.P. Bakare, C.E. Deshpande, J.J. Shrotri, M.G. Gupta, S.K. Date, Ceram. Int. 13 (1987) 247–252. [7] M. Bremer, S. Fisher, H. Topelmann, H. Scheler, Thermochim. Acta 209 (1992) 323–330.

.

89

[8] M. Paulus, in: P. Hagenmuller (Ed.), Preparative Methods in Solid State Chemistry, Academic Press, New York, 1972, p. 487. [9] J.M. Longo, H.S. Horowitz, L.R. Clevenna, in: S.L. Holt, J.L. Milstein, M. Robbins (Eds), Solid State Chemistry —A Contemporary Overview, Advances in Chemistry Series, 1986, p. 139. [10] W.J. Schuele, J. Phys. Chem. 63 (1959) 83 – 86. [11] P. Klienert, A. Funke, Z. Chem. 1 (5) (1961) 155 –157. [12] D.G. Wickham Jr, in: S. Young Tyree Jr (Ed.), Inorganic Synthesis, vol. IX, McGraw-Hill, New York, 1967, p. 152. [13] V.H. Schroder, Z. Phys. Chem. 236 (1967) 200 – 212. [14] J.G. Koh, C.I. Yu, New Phys. (Korean Phys. Soc.) 24 (4) (1984) 359 – 371. [15] J.G. Koh, K.U. Kim, New Phys. (Korean Phys. Soc.) 26 (6) (1986) 540 – 549. [16] J. Park, J. Kim, S. Cho, J. Phys IV France, 7, 1997, Colloq. C1, Suppl. (1997) C1-193 – C1-194. [17] F.K. Lotgering, Phil. Res. Rep. 11 (1956) 337 – 350. [18] D.N. Bhosale, V.Y. Patil, K.S. Rane, R.R. Mahajan, P.P. Bakare, S.R. Sawant, Thermochim. Acta 316 (2) (1998) 159– 166. [19] K. Suresh, K.C. Patil, J. Mater. Sci. 13 (1994) 1712–1714. [20] S.R. Sawant, R.N. Patil, J. Mater. Sci. 16 (1981) 3496–3499. [21] S. Ishikawa, T. Mochizuki, I. Sasaki, in: C.M. Shrivastava, M.J. Patni (Eds.), Advances in Ferrites, vol. 1, IBH Publishing, New Delhi, 1990, p. 651. [22] C. Duval, Inorganic Thermogravimetric Analysis, 2nd edn, Elsevier, Amsterdam, 1963, p. 223, 330, 387, 411. [23] Dollimore, D.L. Griffiths, D. Nicholson, J. Chem. Soc. Part III (1963) 2617 – 2623. [24] J. Smit, H.P.J. Wijn, in: Ferrites: Physical Properties of Ferromagnetic Oxides in Relation to their Technical Properties, Philips Technical Library, 1959, p. 143, 163. [25] C.D. Hodgman, Handbook of Chemistry and Physics, 36th edn, Chemical Rubber Co, Cleveland, OH, 1954-55, pp. 3095–3097. [26] J. Smit, Magnetic Properties of Materials, McGraw-Hill, New York, 1971. [27] A. Globus, P. Duplex, M. Guyot, IEEE Trans. Magn. mag-7 (3) (1971) 617 – 622. [28] A. Globus, P. Duplex, J. Appl. Phys. 39 (2) (1988) 723–730. [29] J.D. Loaec, J. Phys. D Appl. Phys. 26 (1993) 963 –966. [30] E. Roess, I. Hanke, E. Moser, Z. Angew. Phys. 17 (7) (1964) 504 – 508. [31] R.K. Gallagher, E.M. Gyorgy, D.W. Johnson, Am. Ceram. Soc. Bull. 57 (1978) 872 – 877. [32] T. Okamura, Y. Kojima, Phys. Rev. 86/67 (1952) 1040–1041. [33] G.T. Rado, V.J. Folen, E.W. Emerson, Proc. I.E.E., 104B, (1956) 118 – 205, 213 – 215.