Complete Band Gaps in Nano-Piezoelectric Phononic Crystals

We study the band structure of elastic waves propagating in a nano-piezoelectric phononic crystal consisting of a polymeric matrix reinforced by BaTiO3 inclusions in square, rectangular, triangular, honeycomb and Kagomé lattices. We also investigate the influence of inclusion cross section geometry circular, hollow circular, square and rotated square with a 45o angle of rotation with respect to x and y axes. Plane wave expansion method is used to solve the governing equations of motion of a piezoelectric solid based on classical elasticity theory, ignoring nanoscopic size effects, considering two-dimensional periodicity and wave propagation in the xy plane. Complete band gaps between XY and Z modes are observed for all inclusions and the best performance is for circular inclusion in a triangular lattice. Piezoelectricity influences significantly the band gaps for hollow circular inclusion in lower frequencies. We suggest that nano-piezoelectric phononic crystals are feasible for elastic vibration management in GHz.


Introduction
Phononic crystals (PCs) are artificial periodic composites designed to exhibit phononic band gaps and they have been quite studied 1-14 . There are no mechanical (elastic or acoustic) propagating waves in phononic band gaps, only evanescent waves. These band gaps are created by the periodically mismatch between the constituent materials. This mismatch can be considered to arise either from difference of material properties or geometry (continuum-scale theory), or from interatomic force constants and masses (atomic-scale theory).
The ability of creating phononic band gaps is similar to the electronic and photonic band gaps in semiconductors and photonic crystals [15][16] , respectively. The physical origin of phononic and photonic band gaps can be understood at micro-scale using the classical wave theory to describe the Bragg and Mie resonances, respectively, based on the scattering of mechanical and electromagnetic waves propagating within the crystal 17 .
The main purpose of this study is to investigate the elastic band structure, also known as dispersion relation, of a nano-piezoelectric phononic crystal with square, rectangular, triangular, honeycomb and Kagomé lattices, composed by a polymeric matrix and BaTiO 3 inclusions, considering wave propagation in the xy plane and two-dimensional periodicity. We consider the following inclusions geometries -circular, hollow circular, square and rotated square with a 45º angle of rotation with respect to the x and y axes. To obtain the elastic band structure we use the semi-analytical plane wave expansion 1,2 (PWE) method.
To the best of our knowledge, it is the first time that the band structure of nano-piezoelectric PCs with different inclusion geometries and different lattices is investigated.

2D Nano-Piezoelectric Phononic Crystal Model by PWE Method
PWE formulation of a 2D PC is presented in this section. PWE method 1,2 , also known as ω(k) method, is one of the most used methods to calculate the elastic band structure of PCs and it has been applied in micro 10 and nanophononic crystals 71,75 (NPCs). PWE method is used to solve the constitutive equations of a piezoelectric material based on classical elasticity theory, ignoring nanoscopic size effects, similar to other studies 10,71,75 .
We consider two-dimensional periodicity, transversely isotropic elastic solid and wave propagation in the xy plane. Complete band gaps between XY (longitudinal-transverse vibration) and Z (transverse vibration) modes are observed for all inclusion geometry -circular, hollow circular, square and rotated square, considering square, rectangular, triangular, honeycomb and Kagomé lattices. Piezoelectric effect on the Z modes is also investigated.

2D Phononic Crystal Model
Figure 1 (a-e) sketches the cross section of nanopiezoelectric PC unit cells taking in to account square, rectangular, triangular, honeycomb and Kagomé lattices, respectively, with arbitrary inclusion geometry. Figure 1 (f-h) represents the first Brillouin zone 80 (FBZ) for square, rectangular, triangular, honeycomb and Kagomé lattices. Note that triangular, honeycomb and Kagomé lattices present the same FBZ shape (Figure 1 (h)). We consider four types of BaTiO 3 inclusions: circular, hollow circular, square and rotated square with a 45º angle of rotation with respect to the x, y axes. We underline that only BaTiO 3 inclusions are rotated for rotated square configuration, not the entire lattice.
In addition, we highlight that we consider 1/4 of the FBZ as the actual first irreducible Brillouin zone 80 (FIBZ) for square and rotated square inclusions in triangular, honeycomb and Kagomé lattices because the symmetries of phononic crystals are reduced since the phononic crystal basis has different symmetry from the lattice 81 . Thus, for these configurations we scan M'-Γ-X-M-X'-M'. It is very important to make this adjustment since it changes the phononic crystal band structures 81 . For circular and hollow circular inclusions in triangular, honeycomb and Kagomé lattices, phononic crystal symmetry is not reduced thus we scan only M-Γ-X-M.
The constitutive equations of an elastic piezoelectric material are 41 : (1) (2) where i,j,k,l = 1,2,3, σ ij is the elastic stress tensor, D i is the electric displacement vector, u i is the elastic displacement vector, E l is the electric field vector, c ijkl is the elastic stiffness tensor, e lij is the piezoelectric tensor and ϵ il is the dielectric tensor. The standard tensor notation is used with Latin indices running from 1 to 3. They obey Einstein's summation convention when repeated.
We restricted the treatment to linear media, thus the elastic strain tensor ε kl is simplified: Furthermore, based on the quasi-static approximation, there is no electro source (i.e. since there is no voltage source, this configuration is also known as an open circuit) and the curl is zero, thus the electric field is taken as gradient of scalar potential and one can write: The differential equations of motion in absence of body forces are given by: where ρ is the mass density and dot denotes differentiation with respect to time. Substituting Eqs. (1-2) in Eqs. (5)(6), applying the simplifications of Eqs. (3)(4), considering a transversely isotropic elastic solid and for a two-dimensional problem, ∂/∂x 3 =0, results: where c 66 = ** (c 11 -c 12 ). Note that Eqs. (7-10) are written using Voigt notation and from now on this notation is adopted.
In addition, considering a piezoelectric PC, one can note that c 11 = c 11 (r), c 12 = c 12 (r), c 66 = c 66 (r), c 44 = c 44 (r), e 15 = e 15 (r), ϵ 11 = ϵ 11 (r), ρ = ρ(r), because we consider two different materials -BaTiO 3 inclusions and a polymeric matrix, and u i = u i (r,t). For a two-dimensional periodicity (the system has translational symmetry in z direction and material parameters depend only on the x and y coordinates), then r = xe 1 + ye 2 (x,y ∈ ℝ) is the two-dimensional spatial vector and e i (i = 1,2) are the basis vectors in real space.
Furthermore, we may expand c 11 ,c 12 ,c 66 ,c 44 ,e 15 ,ϵ 11 , ρ in Fourier series on the reciprocal space as: , , where P is one of the c 11 ,c 12 ,c 66 ,c 44 ,e 15 ,ϵ 11 , ρ and g has the same expressions of g with m, n ∈ ℤ. We use g instead of g to highlight the difference between the Fourier series expansion of material properties and displacements.
The Fourier coefficients are defined as: where the indexes A and B in Eq. (25) are related to inclusion (BaTiO 3 ) and polymeric matrix, respectively, F(g) is the structure function and ƒ is the filling fraction of each type of inclusion, considering circular section of radius r, square section of width 2l, rotated square section of width 2l with a 45º angle of rotation with respect to x and y axes and hollow circular section with external radius R̃ and internal radius r, R̃ > r. The hollow cylinder inclusion has an internal radius r̃ of the same polymer at matrix and a thickness R̃ -r̃ of BaTiO 3 , similar to Anjos & Arantes 10 .
The filling fraction is defined as ƒ = S A /S C for square, rectangular and triangular lattices, whereas for honeycomb and Kagomé lattices it is defined as ƒ = 2S A /S C and ƒ = 3S A /S C , respectively, where S A is the cross section area of the BaTiO 3 inclusions and S C = ‖a 1 × a 2 ‖ is the cross section area of the unit cell. Note that differently from other lattices honeycomb and Kagomé lattices have two and three inclusions per unit cell, respectively.
The filling fractions for square, rectangular, triangular, honeycomb and Kagomé lattices are: (26) , r g P P e .
respectively. The structure function for square, rectangular and triangular lattices F(g) is given by: (31) The integral in Eq. (31) is performed over the unit cell cross section. The structure functions for square, rectangular and triangular lattices are calculated by: (32) where g = ‖g‖ and g 1,2 = ‖g 1,2 ‖. The structure function for honeycomb lattice F H (g) is defined as 84 : where ū 1 = -ū 2 = a(0,1/2) are vectors that define central position of the two BaTiO 3 inclusions within the honeycomb unit cell. We choose these vectors similar to Gao et al. 85 . Thus, the structure functions of BaTiO 3 inclusions for honeycomb lattice are the same of Eq. (31) multiplied by cos(g • u 1 ), considering ƒ from Eq. (29). 11.2 x 10 -9 C 2 /Nm 2 , 0.0398 x 10 -9 C 2 /Nm 2 Note that a is the lattice parameter for square, triangular, honeycomb and Kagomé lattices, and a 1 , a 2 are the lattice parameters for rectangular lattice.

Results and Discussion
The physical parameters of BaTiO 3 inclusion (A) and polymeric matrix (B) are listed in Table 1. We calculate the elastic band structure considering initially a fixed filling fraction, 0.335, for the four inclusion cross section geometries considered -circular, hollow circular, square and rotated square with a 45º angle of rotation with respect to the x, y axes, in a square, rectangular, triangular, honeycomb and    Kagomé lattices. In the course of numerical calculations, we consider 441 plane waves for Fourier series expansion (i.e. M = 10), which results in a good convergence. Band structure plots are limited until a maximum frequency of 0.12 THz. Figure 2 (a-d) shows the band structures of the nanopiezoelectric PC for square lattice, considering the four types of inclusions and XY (red) and Z (blue) modes. We plot the band structures in the principal symmetry directions of FIBZ (see Figure 1 (f-h)). Plots are given in terms of frequency in Hz versus the reduced Bloch wave vector, k = ka/2π. The reduced Bloch wave vector for rectangular lattice is calculated taking into account ************. In Figure 2 (a), two complete band gaps are obtained for circular inclusion.
From now on the relation between parameters R̃ and r̃ for hollow circular inclusion is r̃ = 0.2R̃ and we do not investigate the influence of BaTiO 3 thickness, i.e.R̃ -r, in the band structure. The thickness influence of a carbon microstructure inclusion for square lattice was investigated by Anjos & Arantes 10 . Figure 2 (b) presents one complete band gap for hollow circular inclusion and we can observe that first branches occur in higher frequencies compared to the other inclusions. Figure 2 (c) shows two complete band gaps for square inclusion and its first complete band gap is broader than the complete band gaps for circular and hollow circular inclusions. When these square inclusions are rotated 45º with respect to x and y axes (Figure 2 (d)) three complete gaps are opened up. The broadest complete band gap for square lattice is for rotated square inclusion with a bandwidth of 11.97 GHz (43.5 GHz is the center frequency). Figure 3 (a-d) shows the band structures for rectangular lattice. For circular and hollow circular inclusions, the band gap behavior of rectangular lattice is similar to square lattice, however they are narrower than the complete band gaps in a square lattice. For square and rotated square inclusions, only one and two complete band gaps are opened up, respectively. Similar to square lattice, the broadest complete band gap for rectangular lattice is opened up for rotated square inclusion with a bandwidth of 8.51 GHz (44.59 GHz is the center frequency). Figure 4 (a-d) illustrates the band structures for triangular lattice. For circular, square and rotated square inclusions, a wide complete band gap is created. The broadest complete band gap for triangular lattice is for circular inclusion with a bandwidth of 12.98 GHz (36.31 GHz is the center frequency). A narrow complete band gap is also opened up for circular inclusion in high bands. For hollow inclusion, one complete band gap is created between 58.69 GHz-49.94 GHz. Comparing the band structures of triangular lattice to square and rectangular lattices, we observe that triangular lattice with circular inclusion presents the broadest complete band gap.
In Figures 5 (a-d) and 6 (a-d), honeycomb and Kagomé lattices, respectively, the band structures of square and rotated square inclusions present similar band gap behavior. The band structure of circular inclusion in honeycomb and Kagomé lattices presents seven and five complete band gaps, respectively. For Kagomé lattice, all the complete band gaps are narrower than the broadest complete band gap from square, rectangular, triangular and honeycomb lattices. The broadest complete band gap for honeycomb lattice is for circular inclusion with a bandwidth of 9.77 GHz (31.19 GHz is the center frequency). For hollow inclusion in honeycomb and Kagomé lattices, we can observe two and one complete band gaps, respectively, and the first bands appear in higher frequencies compared to the other inclusions, similar as observed to other lattices -square, rectangular and triangular.
Comparing the band structures for honeycomb and Kagomé lattices, we observe that honeycomb lattice presents the best performance (more complete band gaps and broader complete band gaps) for all inclusions.
In Figures 7-11, we show the comparison between Z modes of the nano-piezoelectric PC with (blue asterisks) and without (e 15A = 0) (black circles) piezoelectricity for square, rectangular, triangular, honeycomb and Kagomé lattices, respectively. The influence of piezoelectricity is significant on the band gaps for all lattices and inclusions. This influence is more evident in high frequencies 60 for circular, square and rotated square inclusions. However, for hollow circular inclusion in all lattices (Figures 7-11 (b)), the piezoelectricity is also significant in lower frequencies.
We illustrate in Figures 12-16 the complete band gap widths between XY and Z modes as a function of filling fraction for all inclusions and lattices. Piezoelectricity is included in this analysis.
In a square lattice (Figure 12), circular and square inclusions provide the best behavior with high complete band gap widths in a broad range of filling fraction. Considering the first band gap, square inclusion shows the highest one in a broad range of filling fraction. Furthermore, square inclusion also presents four complete band gaps. Hollow circular inclusion presents only one complete band gap, whereas rotated square inclusion does not present better performance than square inclusion.
For rectangular lattice, Figure 13, hollow circular inclusion opens up just one narrow complete band gap, whereas rotated square inclusion creates three complete band gaps, however, only the second one appear in a broad range of filling fraction. Similar as square lattice (Figure 12), circular and square inclusions present the broadest complete band gaps for rectangular lattice. Complete band gap widths in rectangular lattice are lower than in square lattice for all inclusions.          Figure 14 shows complete band gap widths for triangular lattice. Circular inclusion presents the best performance. Comparing triangular lattice ( Figure 14) to square lattice (Figure 12), triangular lattice presents the best performance for circular (first complete band gap), hollow circular and rotated square inclusions. However, for square inclusion, square lattice shows better performance than triangular lattice. There are two important variables to conclude which is the best band gap performance, that is to say the band gap width (the higher, the better) and the filling fraction range in which the band gaps are opened (the broader, the better).
Comparing honeycomb ( Figure 15) and Kagomé ( Figure  16) lattices, the best performance is found for honeycomb lattice considering circular, hollow circular and rotated square inclusions. However, the difference among these lattices is subtle. The highest band gap width for honeycomb and Kagomé lattices is for circular inclusion. Comparing honeycomb and Kagomé lattices to the other lattices, we can observe that for all inclusions, the best behavior is found for square ( Figure 12) and triangular (Figure 14) lattices. The worst performance is found for rectangular lattice with hollow circular inclusion (Figure 13 (b)).

Conclusion
Broad complete band gaps are obtained for a nanopiezoelectric PC, consisting of BaTiO 3 inclusions embedded in a polymeric matrix, for different inclusion geometries and different lattices. We firstly study the elastic band structure for a fixed filling fraction of 0.335 considering XY and Z modes. For square lattice, the broadest complete band gap is found for rotated square inclusion with 11.97 GHz of bandwidth. This inclusion geometry is also the best one for rectangular lattice. Triangular lattice presents the broadest complete band gap than the other lattices for circular inclusion. The elastic band structure for honeycomb and Kagomé lattices presents several complete band gaps. However, for Kagomé lattice, all of them are narrower than the complete band gaps from square, rectangular, triangular and honeycomb lattices.
The Z mode with and without piezoelectric effect is also investigated for a fixed filling fraction of 0.335. This effect is significant on the band gaps for all lattices and inclusions, and it is more evident in higher frequencies for circular, square and rotated square inclusions. However, piezoelectricity is significant in lower frequencies for hollow circular inclusion.
We also analyze the complete band gap bandwidths as a function of filling fraction for all inclusions and lattices with piezoelectric effect. Among square, rectangular and triangular lattices, triangular lattice presents the best performance for circular inclusion (first complete band gap), hollow circular and rotated square inclusions. For square inclusion, square lattice shows the best performance. Among honeycomb and Kagomé lattices, the best performance is found for honeycomb lattice considering circular, hollow circular and rotated square inclusions.
The best band gap performance (i.e. highest band gap width and band gap opened up in a broad range of filling fraction) of the nano-piezoelectric PC investigated is for circular inclusion in a triangular lattice. However, it also depends on the application, for instance the frequency range of interest. Finally, we suggest that elastic complete band gaps in nano-piezoelectric PCs enlarge the potential applications for elastic vibration control in GHz.

Acknowledgments
The authors gratefully acknowledge the financial support of this investigation by the Brazilian research funding agency FAPEMA (State Funding Agency of Maranhão) and by IFMA (Federal Institute of Education, Science and Technology of Maranhão).