SYNTHESIS AND PROPERTIES OF NOVEL TUNABLE FERROIC AND MAGNETO-ELECTRIC META-MATERIALS COMPOSITES

Open Access
Author:
AGRAWAL, SHASHNK
Graduate Program:
Materials
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 26, 2006
Committee Members:
  • Amar S Bhalla, Committee Chair
  • Ruyan Guo, Committee Chair
  • Dinesh Kumar Agrawal, Committee Member
  • Michael T Lanagan, Committee Member
  • Jiping Cheng, Committee Member
Keywords:
  • magneto-electric properties
  • dielectric properties
  • meta-materials
  • electroceramics
  • Composite materials
  • micron/nano particles
  • tunable materials
  • microwave sintering
Abstract:
The present demand in the miniaturization of devices has led researchers to use composite material approach to explore new materials with improved properties. Ferroelectric / ferromagnetic materials have been increasingly used for various radio frequency and microwave frequency applications including tunable phase shifter materials, high frequency capacitors and magnetic sensors. High dielectric constants associated with high dielectric / magnetic losses in these materials narrows down the application range of these materials. In order to use ferroelectric materials in their full range of potential, the high frequency properties must be well understood. One way to exploit ferroelectric / ferromagnetic materials is to tailor the dielectric and magnetic properties with the addition of certain oxide materials. The resultant composites prepared with the addition of ferrous and oxide materials have intermediate dielectric and magnetic properties, thereby making these materials more suitable for device implementation and expanding the application range. The focus of this research has been the development of novel composite materials with (a) tunable dielectric and (b) of superior magneto-electric properties. In this research work, ferroelectric components were ceramic materials (Ba1-xSrx)TiO3 (x = 0.4, 0.5, 0.6) → BST and (Pb(Mg1/3Nb2/3)O3)1-x:(PbTiO3)x (x = 0.31) → PMN-PT. The composites for tunable applications were prepared with MgO as non-ferroelectric phase, and for magneto-electric applications magnetic/ferrite materials CoFe2O4, NiMn0.1Fe1.9O4 and Mn0.1Zn0.9Fe2O4 were used. The composites were prepared with the combination of both micron and nano size particles to investigate the grain size effect and distribution of grains on the dielectric and magneto-electric properties of the resultant composites. The composite samples were processed with both conventional and microwave sintering techniques to study the grain growth and connectivity pattern during sintering process. Characterization studies were performed to determine the grain size and phase distribution and electrical properties of composites. Microwave dielectric properties were carried out using modified cavity perturbation technique at ~ 3 GHz to examine the dielectric loss studies in the composite materials. The aim of the research was to develop composite materials for low temperature and room temperature applications in the high frequency areas. For tunable dielectric materials, (Ba1-x,Srx)TiO3 (BST) with various weight ratios with cubic-tetragonal phase transition peak below or around room temperature were used. Micron sized Ba0.4Sr0.6TiO3 and Ba0.6Sr0.4TiO3 and nano size Ba0.5Sr0.5TiO3 were used for ferroelectric phase and both micron and nano sized MgO was used as non-ferroelectric phase. The magneto-electric composites were prepared with both micron sized Ba0.6Sr0.4TiO3 and PMN-PT as ferroelectric/piezoelectric phase and CoFe2O4 (nano), NiMn0.1Fe1.9O4 (micron) and Mn0.1Zn0.9Fe2O4 (nano) as magnetic components. The ferroelectric composite materials were processed using multi mode microwave sintering technique. The microwave sintering resulted in lower sintering time compared to the conventional sintering and also the higher density composites with good connectivity of grains. The connectivity pattern of 0-3 was obtained in the resulted composites with good connectivity of ferroic grains and isolating the non-ferroelectric grains giving very low dielectric constant and appreciable tunability. Using nano particle size of both BST and MgO, a maximum tunability of 48 % at 80 KV/cm was achieved with dielectric constant of 89 at 200 K at 100 KHz. The microwave sintering resulted in very dense composites and low dielectric losses providing composites with high break down strength. The maximum DC electric field applied to the samples was 80 KV/cm. The addition of MgO resulted in the shift of maximum dielectric peak to lower phase transition temperature without shifting the ferroelectric nature of BST. Additional experiments were carried out for pyroelectric measurements to confirm the phase transitions temperatures in the composites. High frequency measurements were done at room temperature with modified cavity perturbation technique where a special cavity was designed to measure samples of very small sizes. At 3 GHz, the dielectric loss observed in the BST:MgO composites were ~ 0.007. The magneto-electric composites were processed using both conventional and single mode microwave heating technique. In a single mode microwave system, the samples were sintered in either electric field (E-field) or magnetic field (H-field). The respective sintering in E-field and H-field resulted in different magneto-electric properties as different connectivity patterns can be achieved with single mode microwave processing. H-field processing resulted in lower sintering temperature by 125 0C than used in conventional sintering. The magnetic component in magneto-electric composites couples faster in H-field thus lowering the sintering temperature and providing better sintered composite materials. The composites processed with microwave sintering showed minimum or no chemical reactivity and lower PbO loss compared to conventional sintering process. PMN-PT (31%):NiMn0.1Fe1.9O4 (60:40) composite showed the maximum magneto-electric coefficient of 520 mV/cm Oe for single mode H-field sintering compared to 430 mV/cm Oe for the composite sintered in conventional sintering. Finally, the microwave sintering process has been studied to examine the absorption rate of microwaves with different materials depending on their properties. The different sintering profiles in E-field and H-field have been studied for composite materials and an attempt has been made in the thesis to describe the reasons of different sintering patterns of electronic composite materials with different constituent materials. This research work clearly showed the potential of microwave sintering in the processing of ferroic composite materials with better properties. Also, the initial grain size influences the connectivity pattern and the resulted dielectric/magneto-electric properties of ceramic composite materials, have been demonstrated.