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Composition - Microstructure - Property Relationships In Dual Phase Bulk Magnetoelectric Composite

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Composition - Microstructure - Property Relationships In Dual Phase Bulk Magnetoelectric Composite

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Title: Composition - Microstructure - Property Relationships In Dual Phase Bulk Magnetoelectric Composite
Author: Islam, Rashed Adnan
Abstract: The coexistence of coupled electrical and magnetic properties in the “magnetoelectric” material has led to the possibility of developing smarter and smaller electronic components. In order to make this possibility a reality, significant efforts are required to understand the science of magnetoelectric (ME) behavior and apply this understanding to develop higher sensitivity material. The primary aims of this thesis are to identify the role of composition, microstructural variables, composite geometry, texturing, post sintering heat treatment, and nanoscale assembly on ME coefficient. The overall objective is to synthesize, characterize and utilize a high ME coefficient composite. The desired range of ME coefficient in the sintered composite is more than 1.5 V/cm.Oe. At first, a piezoelectric composition in the system of Pb(Zr,Ti)O3 - Pb[(Zn,Ni)1/3Nb2/3]O3 was designed and synthesized which has high energy density (d.g) parameter of 18456.2 x 10- 15 m2/N and high g constant of 83.1 V-m/N in order to use it as the matrix in piezoelectric – magnetostrictive composite. Secondly it was found that soft piezoelectric phase shows much better magnetoelectric response. The magnetoelectric coefficient for Pb(Zr0.52Ti0.48)O3 - 15% Pb(Zn1/3Nb2/3)O3 [PZT – 15 PZN] - 20% Ni0.8Zn0.2Fe2O4 was found to be around 186 mV/cm.Oe. Thridly, soft magnetic phase with lower coercivity and higher magnetization was found to be suitable for high ME coefficient. Zinc doped Ni-ferrite has higher resistivity, permeability, magnetization and it was found that with increasing Zn concentration the ME coefficient increases exhibiting maxima near 30 at% Zn (138 mV/cm.Oe). Fourthly, if the connectivity was changed from (0-3) to (2-2) which is a bilayer geometry, improved piezoelectric (d33 ~ 80 pC/N), ferroelectric (polarization = 60 μC/cm2), magnetization (25 emu/gm) and lower coercive field (2.8 Oe) were measured. The bilayer shows an enhancement of 67% increase in ME coefficient compared to bulk composite. Finally it was found that the electrical, magnetic and magnetoelectric properties of (1-x) Pb(Zr0.52Ti0.48)O3 – xNiFe1.9Mn0.1O4 (PZT-NFM) composites were enhanced after post-sinter annealing and aging. The thermal treatment relaxed the strain in the matrix as observed by change in PZT lattice constant from (a = 3.87Å, c = 4.07 Å) to (a = 4.07Å, c = 4.09 Å). This signifies that strain relaxation helps to enhance the ME coefficient by ~ 50%. A trilayer composite was synthesized using pressure assisted sintering with soft phase [0.9PZT – 0.1 PZN] having grain size larger than 1μm and soft ferromagnetic phase of composition Ni0.8Cu0.2Zn0.2Fe2O4 [NCZF]. The composite showed a high ME coefficient of 412 and 494mV/cm.Oe after sintering and annealing respectively. Optimized ferrite to PZT thickness ratio was found to be 5.33, providing ME coefficient of 525mV/cm.Oe. The ME coefficient exhibited orientation dependence with respect to applied magnetic field. Multilayering the PZT layer increased the magnitude of ME coefficient to 782mV/cm.Oe. Piezoelectric grain texturing and nano-particulate assembly techniques were incorporated with the layered geometry. It was found that with moderate texturing, d33 and ME coefficient reached up to 325 pC/N and 878 mV/cm.Oe respectively. Nano-particulate core shell assembly shows the promise for achieving large ME coefficient in the sintered composites. A systematic relationship between composition, microstructure, geometry and properties is presented which will lead towards development of high performance magnetoelectric materials. Using the particulate type ME composite developed in thesis, a high sensitivity magnetic field sensor based on piezoelectric transformer principle was fabricated and characterized. This application demonstrates the advantages of ME composites for magnetic field sensing.
URI: http://hdl.handle.net/10106/957
Date: 2008-08-08

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