Tunable Bulk Research Articles

Microwave Response of BiFeO3 Films in Parallel-Plate Capacitors

Dielectric response of the BiFeO3 films in a parallel-plate capacitor configuration is studied in the frequency range 1 MHz–30 GHz and under dc electric field up to 45 V/μm in view of their application in tunable film bulk acoustic wave resonators. The observed relatively high permittivity 130 without remarkable frequency dispersion is explained by contribution of the domain wall vibrations. It is shown that the substrate induced strain and Maxwell-Wagner contributions are negligible. The measured dielectric response allows estimation of the BiFeO3 sound velocity and acoustic impedance as 3100 m/s and 26 · 106 kg/m2s, respectively.

Switchable molecular magnets

Various molecular magnetic compounds whose magnetic properties can be controlled by external stimuli have been developed, including electrochemically, photochemically, and chemically tunable bulk magnets as well as a phototunable antiferromagnetic phase of single chain magnet. In addition, we present tunable paramagnetic mononuclear complexes ranging from spin crossover complexes and valence tautomeric complexes to Co complexes in which orbital angular momentum can be switched. Furthermore, we recently developed several switchable clusters and one-dimensional coordination polymers. The switching of magnetic properties can be achieved by modulating metals, ligands, and molecules/ions in the second sphere of the complexes.

Tunable bulk acoustic wave resonators based on Ba<sub>0.25</sub>Sr<sub>0.75</sub>TiO<sub>3</sub> thin films and a HfO<sub>2</sub>/SiO<sub>2</sub> bragg reflector

A switchable and tunable bulk acoustic wave resonator based on a paraelectric phase Ba(0.25)Sr(0.75)TiO(3) thin film and an all-dielectric HfO(2)/SiO(2) Bragg reflector is presented. The achieved tuning range (3.8%) and effective electromechanical coupling coefficient (7.1%) are the highest reported for solidly mounted tunable bulk acoustic wave resonators. The non-conductive Bragg reflector stack provides excellent integration possibilities.

Surface-Grafted, Covalently Cross-Linked Hydrogel Brushes with Tunable Interfacial and Bulk Properties

Tethering macromolecules to surfaces represents a versatile approach for functionalizing, protecting, and structuring both organic and inorganic materials. In this study, thin films of poly(acrylamide) (PAAm) brushes and covalently cross-linked hydrogel brushes were grown from iniferter-functionalized silicon substrates by UVLED-initiated photopolymerization and their properties subsequently studied by means of a variety of analytical methods. The employed photografting method allowed the controlled fabrication of very thick films (up to 1 μm) in an aqueous environment, over a period of less than 1 h of polymerization and in the absence of side reactions. PAAm covalently cross-linked hydrogel brushes were prepared by feeding trace amounts of the cross-linker bis(acrylamide) (up to 1.0 wt % of monomer solution) into the reaction vessel. Both bulk and interfacial properties of these polymer films were found to be strongly influenced by lateral cross-linking of the grafted polymer chains. In agreement with theoretical expectations, the decrease of polymer-brush conformational freedom with increasing cross-link density resulted in a substantial increase of film wettability with water. The swelling ratio of the hydrogel brushes, as measured by ellipsometry and atomic force microscopy (AFM), also confirmed the formation of grafted networks and was found to be directly related to the amount of cross-linker in the monomer feed. In addition, the Young’s moduli and tribological properties of PAAm brushes and hydrogel brushes were tuned by adjusting the cross-linker concentration. Because of the additional constraint given by the surface tethering of each chain end, intermolecular cross-linking generated very high mechanical stresses within the brush structure. Covalently cross-linked hydrogel brushes thus displayed higher Young’s moduli and coefficients of friction, when compared to the grafted polymer-brush analogues. These hydrogel brushes present an opportunity for readily tailoring physical properties, especially as they allow tuning of the physical characteristics of surfaces while maintaining the interfacial chemical composition nearly constant.

Active acoustic metamaterials.

Extensive efforts are being exerted to develop various types of acoustic metamaterials to effectively control the flow of acoustical energy through these materials. However, all these efforts are focused on passive metamaterials with fixed material properties. The emphasis is placed here on the development of a class of acoustic metamaterials with tunable effective densities and bulk modulus in an attempt to enable the control of wave propagation. More importantly, the active metamaterials can be tailored to have increasing or decreasing variation of the material properties along and across the material volume. With such unique capabilities, physically realizable acoustic cloaks can be achieved and objects treated with these active metamaterials can become acoustically invisible. The theoretical analysis of this class of active acoustic metamaterials will be presented and the theoretical predictions are determined for an array of fluid cavities separated by piezoelectric boundaries. These boundaries control the stiffness of the individual cavity and in turn its dynamical density and bulk modulus. Various configurations will be considered to achieve different spectral and spatial control of the density and bulk modulus of this class of acoustic metamaterials. Extensive efforts are now exerted to build and test modules of these active acoustic metamaterials in order to build practical configurations of acoustic cloaks.

Self-Alignment of Plasmonic Gold Nanorods in Reconfigurable Anisotropic Fluids for Tunable Bulk Metamaterial Applications

We demonstrate the bulk self-alignment of dispersed gold nanorods imposed by the intrinsic cylindrical micelle self-assembly in nematic and hexagonal liquid crystalline phases of anisotropic fluids. External magnetic field and shearing allow for alignment and realignment of the liquid crystal matrix with the ensuing long-range orientational order of well-dispersed plasmonic nanorods. This results in a switchable polarization-sensitive plasmon resonance exhibiting stark differences from that of the same nanorods in isotropic fluids. The device-scale bulk nanoparticle alignment may enable optical metamaterial mass production and control of properties arising from combining the switchable nanoscale structure of anisotropic fluids with the surface plasmon resonance properties of the plasmonic nanorods.

Quasiequilibrium lattice Boltzmann models with tunable bulk viscosity for enhancing stability

Taking advantage of a closed-form generalized Maxwell distribution function [P. Asinari and I. V. Karlin, Phys. Rev. E 79, 036703 (2009)] and splitting the relaxation to the equilibrium in two steps, an entropic quasiequilibrium (EQE) kinetic model is proposed for the simulation of low Mach number flows, which enjoys both the H theorem and a free-tunable parameter for controlling the bulk viscosity in such a way as to enhance numerical stability in the incompressible flow limit. Moreover, the proposed model admits a simplification based on a proper expansion in the low Mach number limit (LQE model). The lattice Boltzmann implementation of both the EQE and LQE is as simple as that of the standard lattice Bhatnagar-Gross-Krook (LBGK) method, and practical details are reported. Extensive numerical testing with the lid driven cavity flow in two dimensions is presented in order to verify the enhancement of the stability region. The proposed models achieve the same accuracy as the LBGK method with much rougher meshes, leading to an effective computational speed-up of almost three times for EQE and of more than four times for the LQE. Three-dimensional extension of EQE and LQE is also discussed.

Tunable optical metamaterial based on liquid crystal-gold nanosphere composite

Effect of the surrounding anisotropic liquid crystal medium on the surface plasmon resonance (SPR) exhibited by concentrated suspensions of gold nanospheres has been investigated experimentally and compared with the Mie scattering theory. The observed polarization-sensitive SPR and the red-shift in the SPR wavelength with increasing concentration of the gold nanospheres in the liquid crystal matrix have been explained using calculations based on the Maxwell Garnet effective medium theory. Agglomeration of the gold nanospheres that could also lead to such a red-shift has been ruled out using Atomic force microscopy study of thin nanoparticle-doped smectic films obtained on solid substrates. Our study demonstrates feasibility of obtaining tunable optical bulk metamaterials based on smectic liquid crystal - nanoparticle composites.

Tunable bulk acoustic wave resonators with the induced piezoelectric effect in a ferroelectric

An electromechanical model of the piezoelectric effect induced in an acoustic resonator based on a ferroelectric film under the action of a dc or weak ac voltage is developed. The basic equation is obtained by expansion of the free energy in a series with respect to the electric induction and the mechanical deformation. The system of electromechanical equations for variable components of the induction and the mechanical deformation involves all linear terms along with the component of the electrostriction nonlinear with respect to the mechanical deformation. These electromechanical equations made it possible to obtain a one-dimensional approximation for the effective parameters of the material: the piezoelectric modulus and the elastic modulus as a function of the strength of the electric field applied to the acoustic layer. Expressions for the controlled electromechanical coupling coefficient and resonance frequencies of the tunable acoustic resonator are found. It is shown that the most significant parameter responsible for the tuning is the nonlinear electros-triction coefficient M, whose magnitude and sign were evaluated from the available experimental data.

A New, Flexible N,N,N‐Tripodal Facially Capping Ligand System: Synthesis and Structural Characterization of β‐Triketimines and Their M(CO)3 Complexes (M = Cr, Mo, W)

AbstractReaction of imidoyl chlorides Ar*N=CClR (Ar* = 2‐iPrC6H4, 2,6‐iPr2C6H3, 2‐MeOC6H4; R = Me, Ph, tBu) with [Li(nacnac)] [nacnac = (Ar*NCMe)2CH] gives β‐triketimines L, most of which exist in solution in equilibrium with their imine/enamine tautomers. The route is highly modular, allowing independent variation of at least five parameters. The solution equilibria are very sensitive to such substituent pattern variation. Single‐crystal X‐ray diffraction analyses of examples of both tautomers and a geometric isomer in the solid state are presented, alongside solution NMR studies of the tautomerism. All examples revert exclusively to the β‐triimine form on complexation with M(CO)3 fragments (M = Cr, Mo, W). Facial isomers result. The ligands are weak σ‐donors, as adjudged by CO IR stretching frequencies in [LM(CO)3]. Crystal‐structure determination on the isostructural pair [HC(2‐iPrC6H4N=CMe)3M(CO)3] (M = Cr, Mo) revealed a hexagonal packing arrangement composed of aryl–aryl and carbonyl (CO)–H–C interactions which generates pseudocylindrical voids accounting for 6–9 % of the crystal volume. In only one case were these occupied by solvent molecules. This family of facially capping N,N,N‐ligands with finely tunable bulk have wide potential in coordination chemistry.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

De Novo Design of Strand-Swapped β-Hairpin Hydrogels

De novo designed peptides, capable of undergoing a thermally triggered beta-strand-swapped self-assembly event leading to hydrogel formation were prepared. Strand-swapping peptide 1 (SSP1) incorporates an exchangeable beta-strand domain composed of eight residues appended to a nonexchangeable beta-hairpin domain. CD shows that, at pH 9 and temperatures less than 35 degrees C, this peptide adopts a random coil conformation, rendering it soluble in aqueous solution. On heating to 37 degrees C or greater, SSP1 adopts a beta-hairpin that displays an exchangeable beta-strand region. The exchangeable strand domain participates in swapping with the exchangeable domain of another peptide, affording a strand-swapped dimer. These dimers further assemble into fibrils that define the hydrogel. A second peptide (SSP2) containing an exchangeable strand composed of only four residues was also studied. Microscopy and scattering data show that the length of the exchangeable domain directly influences the fibril nanostructure and can be used as a design element to construct either twisted (SSP1) or nontwisted (SSP2) fibril morphologies. CD, FTIR, and WAXS confirm that both peptides adopt beta-sheet secondary structure when assembled into fibrils. Fibril dimensions, as measured by TEM, AFM, and SANS indicate a fibril diameter of 6.4 nm, a height of 6.0 nm, and a pitch of 50.4 nm for the twisted SSP1 fibrils. The nontwisted SSP2 fibrils are 6.2 nm in diameter and 2.5 nm in height. Oscillatory rheology, used to measure bulk hydrogel rigidity, showed that the gel composed of the nontwisted fibrils is more mechanically rigid (517 Pa at 6 rad/s) than the gel composed of twisted fibrils (367 Pa at 6 rad/s). This work demonstrates that beta-strand-swapping can be used to fabricate biomaterials with tunable fibril nanostructure and bulk hydrogel rheological properties.

Modeling tunable bulk acoustic resonators based on induced piezoelectric effect in BaTiO3 and Ba0.25Sr0.75TiO3 films

A model for tunable thin film bulk acoustic resonators (TFBARs) based on ferroelectric films is proposed. The model is based on electromechanical equations taking into account piezoelectric effect and electrostriction effect induced by the dc electric field. The dc field induced shift of the resonant frequency is explained by the high-order nonlinear effects in the ferroelectric material. The main contribution to the tunability of the resonant frequency under dc electric field can be attributed to electrostriction, which is nonlinear with respect to the mechanical deformation. It is shown that the upward or downward shift in the resonant frequency is given by the sign of the nonlinear component of the electrostriction tensor M. The model is verified by comparing the results with the measured microwave input impedance of BaTiO3 and Ba0.25Sr0.75TiO3 based TFBARs. For a positive sign of the nonlinear coefficient of electrostriction M, the model predicts an upward shift of the resonant frequencies (resonance and antiresonance) under dc biasing in case of the TFBAR based on BaTiO3, whereas a negative sign of the nonlinear coefficient of electrostriction M predicts downward shift of the resonant frequencies for TFBAR based on Ba0.25Sr0.75TiO3 films.

Electrical Frequency Tuning of Film Bulk Acoustic Resonator

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper describes the design, fabrication, and measurement of an electrically tunable film bulk acoustic resonator (FBAR) that is formed by integrating FBAR with an electrostatic microelectromechanical systems actuator. Around 1.47% tuning of the series resonant frequency (<formula formulatype="inline"> <tex>$\Delta {\rm f} \cong $</tex></formula> 22.5 MHz) at 1.5 GHz is experimentally obtained with an electrostatic actuation voltage of 7 V. This is the highest frequency tuning reported for FBAR operating at above 1 GHz without any extra power consumption. Two integration approaches of FBAR and air-gap capacitor are presented and compared, in terms of fabrication process and Q factor. The approach that minimizes any possible energy loss in the acoustic wave propagation path shows a quality factor (160–304) significantly higher than the one having a capacitor right on top surface of the FBAR's piezoelectric film. Furthermore, we have characterized the electrical tuning of FBAR through piezoelectric stiffening due to an applied dc electric field and report a linear frequency shift of about <formula formulatype="inline"><tex>${-}$</tex> </formula>8 ppm/V at 3.4 GHz.<papernumber>1755</papernumber> </para>

Dc electric field tunable bulk acoustic wave solidly mounted resonator using SrTiO3

A tunable SrTiO3 solidly mounted bulk acoustic wave resonator has been designed, fabricated, and tested. The solidly mounted resonator was implemented using an acoustical Bragg reflector of alternating high and low acoustic impedance layers. The resonator demonstrated a frequency tunability of 1%, from 5.87to5.81GHz with an applied bias of 0–7.5V, respectively. A return loss of −6.5dB was observed at an applied bias of 7.5V. The quality factor at the resonant frequency was 78 and relatively constant with applied bias. The effective electromechanical coupling coefficient was 2% and near linear with applied bias.

Starbons: New Starch‐Derived Mesoporous Carbonaceous Materials with Tunable Properties

A star is born: The complex molecular architecture found in plants is used in the design of new nanostructured materials. Carbon-based mesoporous materials with tunable surfaces and bulk properties can be synthesized by the low-temperature carbonization of expanded natural starches that are in turn prepared by a simple gelatinization–retrogradation process.

A surface micromachined electrostatically tunable film bulk acoustic resonator

The design, modeling, fabrication and characterization of a film bulk acoustic resonator (FBAR) that allows frequency tuning by MEMS actuation are reported. FBARs are micromachined versions of bulk acoustic wave (BAW) resonators working in the microwave regime. A small range of frequency tuning is desired to cope with drifts from different origins. In conventional approaches, this functionality has been realized by introducing a discrete element. In this paper, a different design is proposed, in which the MEMS tuning element is integrated in the FBAR structure and tuning is realized by electrostatic actuation. A first demonstration of frequency tuning is also presented.

Ferroelectric Materials for Microwave Tunable Applications

A review of the properties of ferroelectric materials that are relevant to microwave tunable devices is presented: we discuss the theory of dielectric response of tunable bulk materials and thin films; the experimental results from the literature and from own work are reviewed; the correspondence between the theoretical results and the measured properties of tunable materials is critically analyzed; nominally pure, real (defected), and composite bulk materials and thin films are addressed. In addition, techniques for characterization of tunable ferroelectrics and applications of these materials are briefly presented.