High-resistivity nickel zinc ferrites by the citrate precursor method
A. Verma
a, T.C. Goel
a'*, R.G. Mendiratta
a, R.G. Gupta
b^Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi-110 016, India
^Department of Electronics, Electronics Niketan, New Delhi-110 003, India Received 12 August 1998; received in revised form 28 October 1998
Abstract
Nickel-zinc ferrites of different compositions, N i Z n F e O with x " 0.2, 0.35, 0.5 and 0.6, have been prepared by a precursor method involving citrate precursors of the concerned metals and mixing them in solution state. Resistivity has been studied as a function of composition and sintering temperature. It is observed that NiZn ferrites prepared by this method have resistivity * 108 Q cm which is higher by atleast two orders of magnitude than that reported () 106 Q cm) for ferrites prepared by the conventional ceramic method. This is attributed to better purity as well as better compositional and microstructural control achievable by the citrate method. High resistivity makes these ferrites suitable particularly for high-frequency applications where eddy current losses are required to be low.
Keywords: Nickel-zinc ferrites; Preparation; Citrate method; Microstructure; Resistivity
1. Introduction
Nickel-zinc ferrites are a well-known class of technologically important ferrites. This material enjoys special significance, particularly at high fre- quencies, because of its high resistivity and there- fore low eddy current losses. The usual method of preparing ferrites is the conventional solid-solution method which apart from being cumbersome, has some serious limitations [1,2]. This method re- quires prolonged heating at high temperatures
during their preparation which may result in evap- oration of some of the constituents and thereby changing the desired stoichiometry. Also, in NiZn ferrites, zinc volatilization at high temperatures re- sults in formation of Fe2 + ions thereby increasing the electron hopping and reducing the resistivity.
Also the grinding or milling operations involved in the process lead to loss of some material and to impurity pick-ups which result in nonstoichiomet- ric compositions. All this can result in nonrep- roducible samples having varying characteristics.
Efforts are, therefore, needed to develop a tech- nique which involves heat treatment at relatively lower temperatures and for shorter durations, and which should also result in better stoichiometry and reproducibility of the samples.
Wet chemical methods [3-5] for preparation of ferrites are gaining prominence as these are ex- pected to overcome the above limitations to a great extent. In the present work, nickel-zinc ferrites have been prepared by the citrate precursor method [6-9] which is simple and commercially viable. The technological advantage of this method is that mix- ing of the metal ions takes place on an atomic scale in the solution state during the initial stages of preparation giving rise to homogeneous mixtures.
Also, this method requires the material to be heat treated for relatively shorter durations and at lower temperatures. Although, NiZn ferrites have been prepared by different solution methods, such as coprecipitation [10] and other precursor methods [11-13] their electrical properties have not been investigated in detail. The present investigations are, therefore, directed towards preparation of vari- ous compositions of N i Z n F e O (x " 0.2, 0.35, 0.5 and 0.6) with a view to have better perfor- mance materials with high resistivity and greater reproducibility. Microstructural studies have been undertaken to understand the enhanced resistivity of these ferrites.
2. Experimental procedure
The materials used in the preparation of N i Z n F e O were nickel nitrate (97%, Merck, India), zinc nitrate (96%, Merck, India), iron(III) citrate (Merck, Germany) and citric acid (99.5%, Merck, India). The chemicals were weighed accord- ing to the required stoichiometric proportion.
Iron(III) citrate solution was prepared in distilled water by heating at 40°C with continuous stirring.
Nickel nitrate and zinc nitrate were added to citric acid dissolved in distilled water. The solution was heated at 40°C for about 30 min and added to iron citrate solution under constant stirring. This mix- ture was evaporated to dryness to obtain a homo- geneous uniformly coloured brown transparent glassy material. The dried citrate mixture was cal- cined for 1 h at 1000°C to obtain the spinel ferrite.
The spinel formation was confirmed by X-ray dif- fraction pattern taken on a Rigaku Geiger flex 3 kW X-ray diffractometer. The ferrite powder ob- tained on calcination was pressed into 1.5 mm thick
pellets of 1.3 cm diameter at a pressure of 10 tons.
Poly vinyl alcohol, 2% by weight, was used as bind- er. The pellets were sintered at various temper- atures in the range 1100-1400°C for 1 h in air at a heating rate of 100°C/h in a box-type furnace and were subsequently furnace cooled. Silver paste was coated on polished pellets to provide electrical con- tacts. DC -Resistivity was measured by a two-probe method using Keithley Electrometer (model 610) with current measuring capability down to 10"1 3 A. Scanning electron microphotographs were recorded on a Cambridge Stereo Scan 360 scanning electron microscope (SEM).
3. Results and discussion
Resistivity of ferrites is known to depend upon the purity of the starting compounds and the prep- aration details such as sintering temperature and atmosphere which also influence the microstruc- ture and the composition of the prepared samples.
The grain size, grain boundaries, porosity and stoichiometry are important factors in this regard.
In the present work, the variation of room temper- ature DC resistivity of NiZn ferrites sintered at various temperatures is reported. Resistivity values of the order of * 10s Q. cm have been observed which are much higher than those ( < 1 06Q c m ) reported [14,15] for NiZn ferrites prepared by the conventional ceramic method. Table 1 lists the values of DC resistivity of ferrites of different com- positions sintered at various temperatures. The re- sistivity is observed to increase with sintering temperature upto 1250°C for all the compositions (except the composition withx " 0.6) after which it drops. In case of the composition with x " 0.6, it increases only upto a sintering temperature of 1200°C decreasing thereafter. It is also observed that resistivity decreases with decreasing zinc con- tent except for the composition with x " 0.2 which shows a slight increase in resistivity.
It has been reported [16] that resistivity of a polycrystalline material in general increases with decreasing grain size. Smaller grains imply larger number of insulating grain boundaries which act as barriers to the flow of electrons. Smaller grains also imply smaller grain-to-grain surface contact area
Table 1
Room temperature DC resistivity of nickel-zinc ferrites sintered at different temperatures; A,B,C,D and E refer to sintering temperatures of 1100°, 1200°, 1250°, 1300° and 1400°C, respectively.
Composition Ni1_-tZn:tFe2O4
Ni0.8Zn0.2Fe2O4
DC resistivity (H cm)
A 1.5 x l O8
9.6 xlO7
5.5 xlO7
1.4 x l O8
B 1.3 2.2 1.9 7.3 X X
X X o o o o
c
5.2 x 4.9 x 4.8 x 2.2 x
109
108
108
109
D 3.3xlO9
1.5 x l O8
9.5 x l O7
1.7 x l O9
E 1.1 X 1.1 X 1.2 x 1.0 x
105
106
106
108
I - SEl EHT- 20.0 KV Ut- 30 m 20.0|»l
PHOTO- 8131 1
'A
Fig. 1. SEM photographs of fractured surfaces of Ni Zn F e O sintered at (a) 1100°C, (b) 1200°C, (c) 1300°C and (d) 1400°C.
and therefore a reduced electron flow. Fig. 1 shows typical microstructure of NiZn ferrite with x " 0.6, sintered at different temperatures. As expected [16,17], the increase in grain size with increasing
sintering temperature is clearly evident. The grain size increases monotonically from around 0.6 lm for sample sintered at 1100°C to about 8 lm for the sample sintered at 1400°C. It is also observed that
the grains are more or less spherical in shape and that the grain size and the size distribution over different grains of each sample are smaller as com- pared to those of the samples prepared by the ceramic method [18-21]. For example, Su-il Pyun et al. [18] have obtained a grain size of the order of 7.8 lm for N i Z n F e O sintered at 1200°C as compared to our value of ~2 |am. A gradual in- crease in grain size distribution is observed espe- cially at higher sintering temperatures (*1300°C).
At 1400°C there is a considerable increase in grain size, the shape of the grains is no more spherical and the structure appears to be nonhomogeneous (Fig. 1d).
According to the above arguments, the resistivity of the ferrites is expected to decrease with increas- ing sintering temperature. However, a reverse trend is observed in the present work for sintering tem- peratures upto about 1250°C. This indicates that some factors other than those considered above are also important in determining the resistivity of fer- rite ceramics. The comparatively lower values of resistivity in samples sintered at lower temper- atures, below 1250°C, are possibly due to the pres- ence of localized states in the forbidden energy gap which arise due to lattice imperfections. The pres- ence of these states effectively lowers the energy barrier to the flow of electrons. Increase in resistiv- ity with sintering temperature can be explained in terms of increasing structural improvement. In- creased sintering temperature results in more uni- form crystal structures with reduced imperfections thereby increasing the sample resistivity. The above discussion suggests that the effect of structure has an influence which is more significant than that of the anticipated effect of the grain size. A similar trend in resistivity was reported by Van Uitert [22], who also explained it in terms of increased homo- geneity and structural perfection with increase in sintering temperature.
The decrease in resistivity in samples sintered above 1250°C is attributable to the formation of Fe2 + phase. The formation of Fe2 + gives rise to electron hopping [23] between F e2 + and F e3 + ions which brings about a reduction in resistivity. At higher sintering temperature, loss of zinc takes place due to volatilization [24,25]. This results in excess unsaturated oxygen ions which then
associate with surrounding F e3 + ions reducing them to Fe2 +. It is, therefore, expected that samples sintered at higher temperatures would result in nonstoichiometric compositions with lower resis- tivity. As discussed above, the microphotograph of the sample sintered at 1400°C presents a nonuni- form microstructure with larger grains. This, in addition to the presence of increased Fe2 + , is re- sponsible for reduction in resistivity at this temper- ature, see Table 1. Normally, small and relatively uniform grain sizes are preferred as oxidation ad- vances faster in smaller grains [26] thus reducing the probability of F e2 + formation and conse- quently increasing the resistivity. That is why, samples sintered at lower temperatures which have smaller grain sizes exhibit higher values of resistivity.
In sample with x " 0.6, the zinc content being relatively high, the evaporation of the latter and consequent formation of F e2 + ion probably be- comes important even at temperatures relatively lower than 1250°C. For this sample, therefore, the maximum resistivity is observed at temperatures around 1200°C. The higher value of resistivity ob- served in case of composition with x " 0.2 can also be explained on the same basis. The x-value of this sample being small, the amount of zinc lost and F e2 + formed would also be small. Satyanarayana et al. [27] have also reported higher values of resistivity for NiZn ferrites having lower zinc contents.
The magnetic properties of these samples have been measured and will be reported shortly. The preliminary results indicate that the magnetic prop- erties of the present samples are comparable, in fact slightly better in certain respects, as compared to those reported for the samples prepared by the conventional ceramic method. For instance, the curie temperature values of the present samples is about 5-10% higher than those for conventionally prepared samples. The value of saturation magnet- isation for the samples with x " 0.4 studied in the present work is around 63 emu/g whereas the re- ported value for the similar samples prepared by the ceramic technique is about 60 emu/g [28]. The value of coercive field for the sample corresponding to x " 0.2 studied in the present work is about 30%
less than that reported for similar sample prepared
by the ceramic method. It may also be remarked that all the magnetic properties of all the samples studied in the present work are superior than those of the samples prepared by the hydrazine precursor method [13].
The present citrate precursor technique for the preparation of ferrites, therefore, results in produ- cing ferrites with improved electrical as well as magnetic properties.
4. Conclusion
The significantly higher values of resistivity ob- tained in the present investigation compared to those obtained for NiZn ferrites prepared by the conventional ceramic method are attributed to the reduction in Fe2 + content of the ferrite and obtain- ing more uniform structures and compositions.
This has been possible by using relatively lower preparation temperatures and durations, decreas- ing the grain size and obtaining relatively pure ferrite powders. As no milling is involved in the citrate process, the possibilty of introduction of impurities through milling is avoided. Highest values of resistivity (lC^-lO11 Q. cm) have been ob- tained for ferrites sintered around 1200°C for a short duration of 1 h. This shows the promise of the citrate method in preparing high-performance ferrites suitable for high-frequency applications where eddy current losses are of paramount con- cern. The higher values of resistivity suggest that NiZn ferrites processed by the citrate route could be used at frequencies much higher than at which the conventionally prepared ferrites could be used.
This method, moreover, is simple and inexpensive as it does not require elaborate experimental set-up or costly chemicals. On the industrial front, this method is environment friendly and safe as it does not involve handling of the oxides. In future, the citrate precursors could also be used for preparing ferrites by co-spray roasting.
Acknowledgements
The authors are grateful to Dr. Pran Kishan of Solid-State Physics Laboratory, New Delhi and to
Dr. M.I. Alam of Central Electronics, Sahibabad (U.P.), for their valuable suggestions.
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