Primary objectives:

Tentative Topic :- Multifunctional studies on BiFeO₃based ferroelectric alloy Systems

Development of self grown 0-3 magnetoelectric multiferroic composites out of BiFeO₃based ferroelectric alloy Systems:- Working on the merits of additive assisted sintering (AAS) to develop a self grown 0-3 particulate multiferroic composite with a microstructure that is conducive for good ME coupling between the ferroelectric and ferromagnetic phases. We demonstrated the concept on a pseudo ternary multi-cation ferroelectric alloy system BiFeO₃-PbTiO₃-DyFeO₃ which when sintered using MnO₂ as an additive in optimized quantity resulted in the precipitation of dysprosium-iron-garnet (DIG) phase from out of the matrix. Now we are looking forward to get Different type of ferromagnetic precipitates with higher magnetostriction values such as Lead Hexaferrite, Nickel Ferrite, Cobalt Ferrite Etc. Which gives us the possibility to explore higher magnetoelectric coupling values in 0-3 composites(5 digit coupling values) using similar liquid phase sintering technique.

Countering the limitations in Conventional 0-3 magnetoelectric multiferroic Composites: - One does not have direct control over the amount of precipitate we are getting in the additive assisted self-grown multiferroic composites. We would add secondary magnetic phase (highly magnetostrictive) additionally as we do in conventional synthesis of 0-3 composite and would use an additive to get it distributed homogeneously in the matrix.

Development of materials with improved piezoelectric performance and high curie temperature (High d₃₃and T_c Materials): BiFeO₃ as a ferroelectric system have high curie temperature but also have its own demerits of leaky behavior and presence of secondary phases. We can overcome these demerits by alloying BiFeO₃ with other suitable perovskite ferroelectric systems. We have decided to work on one such alloy system BiFeO₃-PbTiO₃(BF-PT), which inherently have high curie temperature. The challenge is to improve on piezoelectric and ferroelectric properties of the matrix. which can be best dealt with the reduction of c/a ratio (1.18) around the morphotropic phase boundary (MPB) compositions which will make the domain switching easy. The reduction of c/a can be achieved by (i) Substitution of large cation to expand a & b axis, (ii)Weakly ferroelectric active cation to shorten c axis. In order to Incorporate other metal ions by modification at A site and B site, we decided to substitute Ti ion with weakly ferroelectric Zr ion in BiFeO₃-PbTiO₃(BF-PT) solid solutions making the system as BiFeO3-PbZr_1-xTi_xO₃ (BF-PZT). Which will further be modified with different cations at its near morphotropic phase boundary compositions to modify Gibbs free energy profile and to induce structural heterogeneity which will help us getting high piezoelectric co-efficient.

Bulk photovoltaic effect in ferroelectrics:- Ferroelectrics may have a bright future for solar-energy generation, the domain walls (replica of low width depletion region) of such materials can be engineered to exhibit a photovoltaic effect with an impressively high above bandgap voltage outputs. BiFeO₃have a bandgap of ~2.1 ev, which will make it and its' alloy systems a promising candidate for ferroelectric photovoltaics.

Development on Objective 1 & 2

Self grown 0-3 magnetoelectric multiferroic composites

The coupling between magnetization and electric polarization in magnetoelectric materials provides opportunity to control magnetization by electric field and polarization by magnetic field and offers avenues for novel sensors and devices. Further, the intrinsic ME coupling α in single phase multiferroics is limited by the fundamental constraint α ≤ (μe)^1/2, where μ and e are permeability and permittivity of the medium. While large μ is characteristic of a ferro/ferrimagnetic material, large e is most often observed in ferroelectrics such as BaTiO₃. Both features are rarely realized in one material, and that too at room temperature. The limitations of the single phase multiferroics no longer holds for composites, formed by combining ferroelectric and ferro/ferrimagnetic phases.The magnetoelectric (ME) effect in a ferroelectric-ferro/ferrimagnetic composite is a product property resulting from a cross interaction between the ferroelectric (FE) and ferro/ferrimagnetic (FM) phases.The large ME effect in multiferroic composites can be used for wireless powering systems, tunable inductors, energy harvesting, magnetic field sensors etc. Among the different schemes (i.e. 3-0, 2-2, 3-1) of connecting the FE and the FM phases, the 0-3 particulate composite comprising of randomly mixed FE and FM grains is most attractive because of the simplicity of fabrication and amenable to mass production.

0-3

3-1

2-2

Schematic illustration of three bulk composites with the three common connectivity schemes: a 0–3 particulate composite, 2–2 laminate composite, and 1–3 fiber/rod composite .(*C.W. Nan et al - 10.1063/1.2836410)

Existence of secondary garnet phase and its effect on ferroelectric polarization

Though the simple to make bulk particulate multiferroic composites are promising, they often fail to show large ME coupling due to serious material challenges like (i) low resistivity of the composite, (ii) non-uniform distribution of the phases, (iii) microcracks formation, and (iv) formation of unwanted parasitic phase(s) & (v) unfavorable microstructural features which cumulatively prevent the realization of large ME coupling such composites.

I am currently working on a new material design strategy to make self-grown 0-3 particulate composites, wherein we exploit some of the merits of additive assisted sintering (AAS) and develop a particulate multiferroic composite with a microstructure that is conducive for good ME coupling between the ferroelectric and ferromagnetic phases. We demonstrated the concept on a pseudo ternary multi-cation ferroelectric alloy system BiFeO₃-PbTiO₃-DyFeO₃ which when sintered using MnO₂ as an additive in optimized quantity resulted in the precipitation of dysprosium-iron-garnet (DIG) phase from out of the matrix. Despite low volume fraction (6 %) of the DIG grains it resulted in a substantially enhanced switching of the ferroelectric domains under magnetic field, causing almost ~ 50 % increase in the ferroelectric polarization under a modest magnetic field of 1 Tesla.

Evidence of secondary garnet phase in multi-cation ferroelectric alloy system BiFeO₃-PbTiO₃-DyFeO₃

Demonstration of magnetic enhancement of Ferroelectric polarization

Amplification of magnetostrictive deformation :

The same multiferroic composite system can be taken as model systems for the purpose of investigating an interesting question of "how local stimuli within the interior of the system would make the ferroelectric system respond on the global scale?" The magnetostriction measurement revealed no measurable magnetostrain in the specimen in unpoled state. Once it is electrically poled, a strain of ~ - 4 ppm was observed at ~1 kOe. Despite the small (6%) volume fraction of DyIG, the macroscopic strain in Dy-modified BF-PT is almost 50% of the strain of pure DyIG The phenomenon confirms that electrically poled ferroelectric matrix acts as an amplifier of the magnetic field-driven insignificant deformations induced in embedded ferrimagnetic (FM) islands. Our experiments validate that the sole contributor to the magnetic field induced macroscopic measured strain is caused by a slight rearrangement of the aligned ferroelectric-ferroelastic domains by localized stress exerted by the isolated FM islands.

Magnetostriction measurements on (a) DyIG(Dysprosium iron garnet) (b) and (c) Poled and unpoled samples of (0.66)BF-PT(0.34)-Dy10

Effect of different additives:

The use of additives as a sintering aid for promoting densification is well-known in processing structural ceramics. In leaky ferroelectric ceramics (e.g., BiFeO₃-based systems), additives are also used to reduce the leakage current, improve the poling efficiency and the piezoelectric response of a material. A less investigated aspect is how additives affect the multiferroic (ferroelectric and magnetic) features of BiFeO₃-based ceramics. Here, we report a systematic study of the effect of different additives viz, MnO₂, CuO, K₂CO₃, and KMnO₄ on the microstructural, electromechanical, and magnetic properties of Dy-modified BiFeO₃–PbTiO₃. We found that all the additives help in the precipitation of the ferrimagnetic dysprosium iron garnet phase. The additives containing potassium (K) tend to suppress the grain size. We found that the best combination of ferroelectric and ferromagnetic behavior was found in a mixture of additives.

Primary objectives:

Development on Objective 1 & 2

Evidence of secondary garnet phase in multi-cation ferroelectric alloy system BiFeO3-PbTiO3-DyFeO3

Demonstration of magnetic enhancement of Ferroelectric polarization

Evidence of secondary garnet phase in multi-cation ferroelectric alloy system BiFeO₃-PbTiO₃-DyFeO₃