Hydroxide co-precipitation route for synthesis of La0.7Sr0.3MnO3-BaTiO3 nanocomposites

The paper deals with synthesis of nanocrystalline La0.7 Sr0.3 Mn O3 (LSMO) and BaTiO3 (BT) powder via hydroxide co-precipitation route. The particle size of the powder is determined using Williamson Hall method and is observed to be nearly 35 nm for LSMO and 120 nm for BT. To avoid occurrence of impurity phases the composites of LSMO and BT are formed at 1273 K using Bi2O3 as a sintering aid. The paper focuses mainly on synthesis and dielectric properties of LSMO and BT composites.


INTRODUCTION
The Ferroic materials have regained a renewed interest in the recent years; owing to the useful magnetoelectric (ME) susceptibilityX ME exhibited by the ferrite-Pb (TiZr) O 3 (PZT), ferrite-BaTiO 3 (BT) composites and multilayer laminates 1-3. The manganites e.g. La 0.7 Sr 0.3 Mn O 3 (LSMO), are potential candidates to form ME composites for various reasons as elaborated in the literature 1,2 . The ME composites are expected to be mechanically sturdy as compared to the laminated composites 4 . Therefore we have opted to investigate the ME properties on particulate composites of LSMO and BT. To have well dispersed particles of LSMO and BT nanopowders of LSMO and BT are used as starting materials. Here, the hydroxide coprecipitation route is used for synthesis of LSMO and BT nanopowders [5][6][7] .
The paper reports synthesis of LSMO and BT nanopowders and formation of following series of composites.
It has been reported that the use of sintering aid allows formation of dense ceramic bodies at relatively low sintering temperature 8 . The need of low sintering temperature is needed to avoid formation of impurity phases in case of LSMO based composites (2). Here, the paper additionally reports structural investigations, dielectric and ME properties of nanopowders LSMO and BT and their composites.

EXPERIMENTAL
Hydroxide co-precipitation route has been used for synthesis of LSMO and BT nanopowders.  2 , the molar ratio of KOH to (BaTi) of 1.6 has been used, based on the earlier reports (9). It has been observed that the Ba (OH) 2 is fractionally soluble in water but insoluble in alkaline medium 10 . Therefore the precipitates are washed in dilute NH 4 OH solution with p H ~ 8. The remaining procedure of coprecipitation is similar to that of LSMO. The precipitates are subjected to TG analysis to examine the validity of proposed precipitation reactions and to determine the minimum required sintering temperature. Considering TG analysis the precipitates of Ba(OH) 2 and TiO(OH) 2 are subjected to the pre sintering process at 1473 K and the final sintering has been carried out at 1553 K for 4 hrs.

Synthesis of LSMO
The nano powders of LSMO and BT are used to form composites using formulae of series 1 and series 2 as above. The Bi 2 O 3 is used as sintering aid to lower the temperature of sintering and achieve higher levels of densification 8 . The composites are pressed in the form of pellets for further process of investigations. The composites are sintered at 1273 K for 8 hours. The HP4284A LCR-Q meter is used for the purpose of the measurement of dielectric constant and a custom built setup is used for the measurement of magneto-electric properties.

XRD and particle size
The X-ray spectra of LSMO are observed to be in accordance with the standard data (ICDD card # 89-4461). The observed peak widths of Xray spectra are used to determine the particle size using Williamson Hall method. The analysis shows that LSMO posses particle size of nearly 35 nm. The XRD spectrum of BT is observed to reproduce the JCPD data and earlier reports¹¹. Here the particle size estimated using Williamson Hall method is observed to be nearly 120 nm. Therefore it appears that the hydroxide co-precipitation route could be used to produce nano scale ceramics. The particle size is also confirmed by using SEM pictures. Figure 1 shows the SEM pictures of LSMO,

BT and composite respectively
The XRD spectra on pellets of the composites are observed to show the reflections corresponding to both the LSMO and BT phases and confirm formation of the desired composite form.   2 shows the variation of the dielectric constant ε r , at excitation frequency f = 1 KHz, as a function of T for series 1. The overall behavior for series 2 also is similar to these observations except for the magnitude of ε r . It is observed that the ε r , ε rmax (the maximum value of ε r at T=T c ) and Q for series 2 are lower as compared that of series 1. Now, the features common for series 1 and series 2 could be summarized as (i) the ε r passes through a diffused phase transistion (DPT) at T ~ 387 K, (ii) for frequencies less than 100 KHz, the ε r shows possibility of another DPT for T ~ 300 K or less, but for f = 100 KHz and 1 MHz, ε r do not show any significant DPT behavior in this temperature range as shown in fig. 1 and (iii) The ε rmax is observed to increase with increasing x, while the Q reduces with increasing x.
The frequency variation of å r shows a characteristic behavior indicating a presence of interfacial/space charge polarization. Fig. 3 shows variation of ε r as a function of T for varying f for x = 0.15 of series 2. From fig. 3 it could be seen that the These observations of ε r could be understood by assuming two contributions to the ε r, one due to the parent BT particles and other due to interfacial/space charge polarization at grain boundaries of LSMO and BT, occurring because of very large difference in the conductivities of these two sub-systems. A DPT that is seen near room temperature is significant only at low frequencies (f <10 KHz) and for x > 0.15. This feature could be attributed to a small percentage of impurity phases at grain surface of BT (2).
For the measurements of longitudinal magnetoelectric coefficient α 33 , the samples are polled at an electric field of 1.5 KV/cm for 8 hours, which is sufficient to poll the BT particles. The α 33 is  It is observed that α 33 is higher for composites of series 2 as compared to corresponding composites of series 1 except for x=0.2. The observed magnitudes of α 33 are within the range of values reported earlier for composites of ferrite-BT and LSMO-PZT (1, 2).

Conclusions
Hydroxide co-precipitation route has been successfully used for synthesis of nanocrystalline LSMO and BT powders. To form dense composites of LSMO-BT at lower temperatures (1273 K), Bi 2 O 3 could be used as a sintering aid. The preliminary investigations on ε r and α 33 indicate that LSMO-BT composites will also posses a useful figure of merit for the device applications of ME materials.