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Domain Engineering in Textured (Na1/2Bi1/2)TiO3 – BaTiO3 Bulk Ceramics and Thin Films

By Deepam Murya

1. Design and fabrication of new materials with high dielectric constant for multilayer ceramic capacitors (MLCCs)
We designed new modified BaTiO3 material for Y5V type capacitor application. In order to get volume efficiency of MLCCs, the grain size of the material should be in submicron regime (Fig-1) and at the same time material should have high dielectric constant (Fig. 2) at room temperature with low temperature coefficient of capacitance (TCC).  Various properties of these materials are listed in Table -1.


   

SEM micrographs (a) BT (b) BTBCN (c) SBTBCN-1 (d) SBTBCN-2 (e) SBTBCN-3 (f) SBTBCN-4, Temperature dependence of dielectric constant at 1 kHz for various compositions Fig.-1 SEM micrographs (a) BT (b) BTBCN (c) SBTBCN-1 (d) SBTBCN-2 (e) SBTBCN-3 (f) SBTBCN-4 Fig.-2 Temperature dependence of dielectric constant at 1 kHz for various compositions

 

   

table1

 

2.  Compositionally graded composites

We prepared compositionally graded composite (Fig.3) and studied the effect of interface on the ferroelectric response.  Fig. 4(a-c) represents SEM image of sharp interface and Fig. 4(d) represents the domain structure across the interface of these compositionally graded composites with different domain width.

   

Schematic of various graded structure prepared, (a-c) SEM micrographs of interface of bilayer composite (d) TEM image representing domain structure across the interface Fig.-3 Schematic of various graded structure prepared Fig.-4 (a-c) SEM micrographs of interface of bilayer composite (d) TEM image representing domain structure across the interface

  The PE –hysteresis loop measured on various laminate composites are provided in Fig. 5.  The effect of composite structure is clearly observed in PE hysteresis on various specimens.  Trilayer specimen was found to exhibit double loop behavior due finite coupling across the interface. 

   

PE-hysteresis loop of various graded composite Fig.-5 PE-hysteresis loop of various graded composite

3.  Synthesis of Na0.5Bi0.5TiO3-BaTiO3 (NBTBT) ceramics using Na2 Ti6O13 (NaTiO) whiskers

 NBTBT is an important lead free piezoelectric material. We synthesized NBTBT materials with good piezoelectric properties using NaTiO whiskers as starting materials and thoroughly studied microstructural evolution mechanism and sintering behavior.   Fig.6 represents the HR-TEM microstructure of the NaTiO whiskers and schematic representation of octahedra connectivity. 

   

HR-TEM image of the NaTiO whiskers, HR-TEM image of specimen processed at 1175C (2 h) Fig.-6 HR-TEM image of the NaTiO whiskers Fig.-7 HR-TEM image of specimen processed at 1175C (2 h)

After aligning NaTiO whiskers in base matrix powder of NBTBT using tape casting method and processing at high temperature, the specimen transformed to NBTBT phase after liquid phase sintering. A HRTEM image of the specimen processed at 1175C (2 h) have been depicted in Fig.7, which represents localized phases due to chemical fluctuation during liquid phase sintering.  These kind of localized also phases contributes to diffuse nature of phase transitions along with the polar nano regions (PNRs).  
The growth of grains was found to be in <111>c direction resulting smaller (111)c plane. Also, the assumed equilibrium shape of grains was found to exhibit bigger faces with (100)c plane indicating the surface energy in <100>c direction is lower than that of <111>c direction (Fig. 8). Fig. 9 represents the AFM image of the growth steps confined preferably on the corners of grains. The step height of these growth steps was measured using AFM images.

   

Growth steps observed on NBTBT grains / AFM image of growth steps observed on the NBTBT grains Fig.-8 Growth steps observed on NBTBT grains / AFM image of growth steps observed on the NBTBT grains

4.  Synthesis of Na0.5Bi0.5TiO3-BaTiO3 (NBTBT) textured ceramics and single crystal
We used TGG and RTGG method to synthesize textured NBTBT ceramics. However, NBTBT single crystals were prepared using flux growth method.  EBSD orientation maps of textured and singe crystal NBTBT materials are given in Fig. 10.

   

(a) EBSD orientation map of NBTBT textured ceramics (b) EBSD orientation map of NBTBT single crystal and (c) corresponding inverse pole figure Fig. 10 (a) EBSD orientation map of NBTBT textured ceramics (b) EBSD orientation map of NBTBT single crystal and (c) corresponding inverse pole figure