Hsin-Yi Kuo

Effective magnetoelectric effect in multicoated circular fibrous multiferroic composites

Rayleigh’s formalism is generalized for the evaluation of the effective material properties in multicoated circular fibrous multiferroic composites. The derived solution is applied to the special three-phase composite in which coated fibers are embedded in a matrix. For composites made of piezoelectric (BaTiO3) and piezomagnetic (CoFe2O4 or Terfenol-D) phases, we find that the magnetoelectric effect in the composite made of CoFe2O4 coated BaTiO3 in matrix Terfenol-D is five times larger than that in the composite made of BaTiO3 coated Terfenol-D in matrix CoFe2O4. Furthermore, in each case, with appropriate coating to the circular fiber, the magnetoelectric effect in the coated composites can be enhanced by more than one order of magnitude as compared to the corresponding noncoating composite.

Effective property of multiferroic fibrous composites with imperfect interfaces

This paper studies the effective behavior of piezoelectric and piezomagnetic circular fibrous composites with imperfect interfaces under longitudinal shear with in-plane electromagnetic fields. Two kinds of imperfect contact are investigated: mechanically stiff and dielectrically/magnetically highly conducting interfaces, and mechanically compliant and dielectrically/magnetically weakly conducting interfaces. For the former case, the potential field is continuous, while the normal component of the flux undergoes a discontinuity across the interface. For the latter case, the normal component of the flux is continuous, while there is a jump of potential field at such a contact. The classic work of Rayleigh (1892 Phil. Mag. 34 481?502) in a periodic conductive perfect composite is generalized to the current coupled magnetoelectroelastic composites with imperfect interfaces. It is shown that the expression of the effective property has exactly the same form as that in the ideal coupling composite. Finally, this method is used to study BaTiO3?CoFe2O4 composites and provide insights into enhancing the effective magnetoelectric voltage coefficient by properly choosing the interface.

Optimization of magnetoelectricity in piezoelectric?magnetostrictive bilayers

Magnetoelectric coupling is of interest for a variety of applications, but is weak in monolithic materials. Strain-coupled bilayers or multilayers of piezoelectric and magnetostrictive material are an attractive way of obtaining enhanced effective magnetoelectricity. This paper studies the optimization of magnetoelectricity with respect to the crystallographic orientations and the relative thickness of the two materials. We show that the effective transverse (α_(E, 31)) and longitudinal (α_(E, 33)) coupling constants can be enhanced many-fold at the optimal orientation compared to those at normal orientation. For example, we show that the constants are 17 and 7 times larger for the optimal orientation of a lithium niobate/Terfenol-D bilayer of equal thickness compared to the normal orientation. The coupling also increases as the piezoelectric phase gets thinner.

Domain pattern and piezoelectric response across polymorphic phase transition in strained bismuth ferrite films

A model is developed to investigate the domain pattern and piezoelectric response across the polymorphic phase transition in strained epitaxial bismuth ferrite films. The orientations of stripelike pattern of the mixed rhombohedral and tetragonal phases are predicted as the consequence of competition between elastic and depolarization energies. The abnormally large piezoelectric response is attributed to the strain-driven softening in dielectric stiffness. The results are in good agreement with recent experimental observations and provide a guidance for developing similar strain-driven polymorphic phase transition in other related perovskite material systems.

Transport properties of composites consisting of periodic arrays of exponentially graded cylinders with cylindrically orthotropic materials

The work is concerned with the determination of effective conductivities and field potentials of matrix-based composites consisting of periodic arrays of cylinders which are cylindrically orthotropic and exponentially graded along the radial direction. We generalize Rayleigh’s method to account for the periodic arrangements of these cylinders. The potential field and effective conductivities of composite systems were calculated to a very high order to achieve a sufficient accuracy. We find that the cylindrical orthotropy of the inclusions has a dramatic effect on the potential field of the inclusions. In addition, we discuss the effect of the grading factor on the effective conductivity. Interestingly, we find that when the inclusions are purely cylindrically orthotropic, their effects can be fully described by homogeneous isotropic cylinders. This equivalent isotropic conductivity is simply the geometric mean of the radial and tangential conductivities of cylindrically orthotropic cylinders.

Magnetoelectric effect of three-phase core-shell-matrix particulate multiferroic composites

This paper studies the magnetoelectricity of a core-shell-matrix three-phase particulate composite made of piezoelectric (PE) and piezomagnetic (PM) phases. We propose a micromechanical model, the two-level recursive scheme in conjunction with Mori-Tanaka’s method, to investigate the effective magnetoelectric coupling coefficients of the composite. We compare this micromechanical solution with those predicted by finite element analysis, which provides the benchmark results for a periodic array of inclusions. Both the magnitudes and trends between them are in good agreement. Based on this micromechanical approach, we show that, for the case of PE/PM/PM (core/shell/matrix) multiferroic composite, with a coating appropriate for the inhomogeneity, the effective magnetoelectric coupling can be enhanced many-fold as compared to the noncoated counterpart. Further, useful design principles are proposed for engineering magnetoelectric composites.

Effective transport properties of arrays of multicoated or graded spheres with spherically transversely isotropic constituents

This work is concerned with the determination of the effective conductivity and potential fields of a periodic array of spherically transversely isotropic spheres in an isotropic matrix. We generalize Rayleigh’s method to account for the periodic arrangements of the inclusions. The inclusions considered in the formulation could be multicoated, generally graded, or exponentially graded. For the multicoated spheres, we derive a recurrence procedure valid for any number of coatings. We show that a (2×2) array alone can mathematically represent the effect of the multiple coatings. For a graded inclusion, the method of Frobenius is adopted to obtain series solutions for the potential fields. For an exponentially graded sphere, we show that the admissible potential field in the inclusion admits a closed-form expression in terms of confluent hypergeometric functions. All these types of inclusions can be characterized by simple scalar coefficients Tl in the estimate of effective conductivities. Simple orthorhombi...

Magnetoelectricity in multiferroic particulate composites with arbitrary crystallographic orientation

This paper studies the magnetoelectricity (ME) of a spherical particulate composite made of piezoelectric and piezomagnetic phases. The effects of crystallographic orientations and the volume fraction of inclusion are investigated by a micromechanical approach. The solutions are in good agreement with predictions by finite element analysis. Based on this micromechanical method, we show that, for the CoFe2O4–LiNbO3 particulate composite, the effective ME voltage coefficient can be enhanced at the optimal orientation as compared to those at normal cut orientation. Further, we observe that the ME coupling is sensitive to the piezoelectric constant e15. The optimal orientation of the ME voltage coefficient is near that of the piezoelectric constant e15.