Short Notes K69

phys. stat. sol. (a) 139, K69 (1993)

Subject classification: 77.20; 72.80; S11.2

Department of Physics, Osmania University, Hyderabad

Effect of Sintering Temperature on the Dielectric Behaviour of Lithium Ferrite

BY D. RAVINDER )

The polycrystalline ferrites, which have many applications ranging from microwave frequencies to radio frequencies, are very good dielectric materials. The dielectric properties of these materials are very sensitive to the method of preparation [l], sintering temperature [3], and sintering atmosphere [3]. Lithium ferrite is an important material for applications at microwave frequencies. Lithium ferrite is an interesting ferrimagnetic material. The undoped lithium ferrite has a high Curie temperature of about 650 "C. Lithium ferrite has attracted considerable attention because of the squareness of the hysteresis loop coupled with a superior temperature performance. Recently considerable attention has been focused on the compounds of the Li spinel family for possible applications as cathode materials in Li batteries [4]. In view of the wide-ranging applications lithium ferrite has been chosen for the present study with special reference to the dependence of its dielectric behaviour on sintering temperature. Four samples of lithium ferrite have been prepared under identical conditions except for the final sintering temperature. The final sintering was carried out at four different temperatures viz., 950, 1000, 1050, and 1100 "C. The results thus obtained are presented in this note.

The dielectric constant ( E ' ) and the dielectric loss tangent (tan 6) of the lithium ferrite sintered at different temperatures were measured using a capacitance bridge (General Radio type 1615-A) with a three-terminal network and a crystal holder assembly. Lithium ferrite was prepared by a double sintering ceramic method. The final sintering was done at 950, 1000, 1050, and 1100°C. The details of the preparation method have been given in an earlier publication [5].

The room temperature values of dielectric constant (c'), and dielectric loss tangent (tan 6) and the complex dielectric constant ( E " ) for lithium ferrite sintered at different temperatures are given in Table 1. The values of electrical conductivity (o), bulk densities, and percentage porosity are also included in the table to facilitate discussion. It can be seen from Table 1 that the dielectric constant increases continuously with increasing sintering temperature, ranging from 47 for lithium ferrite sintered at 950 "C to 997 for lithium ferrite sintered at 1100 "C. This behaviour is similar to that observed for Li-Cd ferrites [6]. The increase in E' with increasing sintering temperature of lithium ferrite can be explained on the basis of a model used earlier [7, 81 to explain the dependence of the dielectric constant of Li-Zn and Li-Cd ferrites on composition. In this model the electron exchange between Fe2+ and Fe3+ in an n-type ferrite results in local displacements of electrons in the direction

I ) Present address: Department of Physics, Post-Graduate College of Science, Saifabad, Hyderabad 500004, India.

23*

K70 physica status solidi (a) 139

Table 1 Room temperature dielectric data for lithium ferrite at 5 kHz

No. sintering dielectric dielectric complex electrical bulk porosity tempera- constant, loss tan- dielectric conduc- density (YO) ture c8 gent, tan 6 constant, tivity (g/cm 3, ("C) &I' (n- cm- ' )

1 950 47 0.3 1 15 2.83 x lo-' 4.38 8.0 2 1000 I1 0.51 44 1.62 x 4.52 5.1 3 1050 410 0.86 352 8.35 x 4.62 3.0 4 1100 997 0.96 957 4.69 x lo-" 4.73 0.7

of the applied electric field and these displacements determine the polarization of the ferrites.

Fig. 1 shows the variation of dielectric constant (c ' ) with frequency at room temperature for lithium ferrite sintered at 950, 1000, 1050, and 1100 "C. It can be seen from the figure that the value of the dielectric constant decreases continuously with increasing frequency in lithium ferrite sintered at different temperatures. In the case of lithium ferrite sintered at 1100 "C, the dispersion of the dielectric constant with frequency is maximum and the magnitude of the dispersion decreases continuously with the decrease of sintering tem- perature. It is minimum in the case of lithium ferrite sintered at 950 "C. The decrease of the dielectric constant with increasing frequency as observed in the case of lithium ferrite sintered at different temperatures is the normal dielectric behaviour of spinel ferrites. This normal dielectric behaviour was also observed by Pran Kishan et al. [9] in the case of lithium-zinc ferrites. Reddy and Reddy [lo] observed similar normal dielectric behaviour in the case of Li-Ti ferrites.

The plot of dielectric loss tangent (tan 6) against frequency is shown in Fig. 2. It may be noted from the figure that the loss tangent decreases continuously with increasing frequency without showing any peaks, unlike the behaviour of a number of other ferrites. This behaviour

u3

500

0 20 60 80 700 frequency (kHz1 -

Fig. 1. Plot of dielectric constant ( E ' ) versus frequency for lithium ferrite (Lio,5Fe2,504) sintered at 0 950, A 1000, 0 1050, 0 1100 "c

Short Notes K71

I I I I I I 1 I I I

0 20 GO 60 80 100 frequency (kHz) -

Fig. 2. Plot of dielectric loss tangent (tan 6) versus frequency for lithium ferrite (Li,,,Fe2,50,) sintered a t 950, A 1000, 0 1050, 0 1100 "C

can be related to the previous observation that samples fired to high relative densities do not show a peak in the curves of tan 6 versus frequency [l 11. The samples used in the present in- vestigation had relatively high densities with porosities (Table 1) ranging from 0.7 to 8.0%. Fig. 2 also shows that the dielectric loss tangent is almost independent of the sintering temper- ature at low frequencies, whereas it varies with the sintering temperature at higher frequencies.

The values of the dc resistivity of lithium ferrite sintered at different temperatures were obtained by the well-known two-probe method [12] and are given together with the values of dielectric constant in Table2 It can be seen from the table that c' is approximately inversely proportional to the square root of the resistivity (e). A similar relationship between E' and eli' has been found [12] for data obtained by Koops [3] for Ni-Zn ferrite samples fired in various atmospheres. This relationship between the conductivity and the dielectric constant confirms the assumption that the mechanisms of the conductivity and of the dielectric constant are the same.

T a b l e 2 Relation between the dielectric constant (6') and the resistivity (e ) of lithium ferrite sintered at different temperatures

No. sintering dielectric resistivity, Q C ' l Q ' '2

temperature constant, ( W m ) ("C) El

1 950 47 3.53 x lo6 1880 8.84 104 2 1000 7 1 6.17 105 786 6.05 x 104 3 1050 410 1.20 105 346 1.42 105 4 1100 997 2.13 8 103 56 4.61 x 104

The author is grateful to Prof. K. Rama Reddy, Head of the Department of Physics, Osmania University, Hyderabad, for his interest in this work. The author also thanks Prof. T. Seshagiri Rao, Prof. K. Shankaraiah, Principal and Prof. U. V. Subba Rao, Head of the Department of Physics, Post-Graduate College of Science, Osmania Unversity, Saifabad, Hyderabad for their encouragement.

K72 physica status solidi (a) 139

References

[I] A. VASILIU, E. CUCIUREANU, N . REZLESCU, and N. LUCA, phys. stat. sol. (a) 14, K105 (1973). [3] A. S. HUDSON, Marconi Rev. 37, 43 (1968). [3] C. G. Koops, Phys. Rev. 83, 121 ( I 95 1 ). [4] A. DEPIEEIOTTOL and M. M. DETHACKERAY, Mater. Rcs. Bull. (USA) 21, 583 (1986). [5] D. RAVINDER, Crystal Res. Technol. 26, 457 (1991). [6] D. RAVINDER, Thesis, Osmania University, 1988. [7] D. RAVINDER, S. RAMANA MURTHY, V. N. MULAY, and K. B. REDDY, phys. stat. sol. (a) 134, 273

[8] D. RAVINDER. phys. stat. sol. (a ) 129, 549 (1993). [9] PRAN KISHAN, D. R. SAGAR, and PREM SWARUP, J. less-common Metals 108, 345 (1985). 101 M. BIIAGAVANTIIA REDDY and P. VLNUCOPAL REDDY, J. Phys. D 24, 975 (1991). 1 I] L. G. VANUITERT, Proc. IRE 44, 1294 (1956). 171 D. RAVINDER and T. SESHAGIRI RAO, Crystal Res. Technol. 25, 963 (1990).

(1993).

( Recriwd October 30, 1992; in revised fo rm hfuy II, 1993)