Ezekiel Walker

Dielectric and Piezoelectric Properties of 10% KF-Doped BaTiO3 Ceramics

10% KF-doped barium titanate powders, Ba0.9K0.1TiO2.9F0.1, were synthesized through a sol–gel process. The powders, calcined at 650 °C, consist of cubic crystalline particles of ~70 nm in length; the particle size increases to ~200 nm as the firing temperature increases to ca. 800 °C, above which F2 begins to evaporate. Dense ceramics were fabricated by the spark plasma sintering (SPS) method; the average grain size is ~2 µm in lengths. The ceramics, well annealed at 1,000 °C in an O2 gas flow, have good dielectric and piezoelectric properties; the piezoelectric d33 value is 230 pC/N at room temperature. At the ferroelectric Curie temperature TC = 47 °C, the dielectric constant and loss tan δ are 10,000 and <5% at 10 kHz, respectively. The Curie–Weiss relation holds in the fully disordered cubic and ordered rhombohedral phases, showing the second order 1:2 relation. Below 10 kHz, large dielectric dispersion caused by a domain-wall motion appears at the temperature range of -50 to 107 °C. Some discussions are made for these dielectric properties of the ceramics.

Tunable ultrasonic phononic crystal controlled by infrared radiation

A tunable phononic crystal based ultrasonic filter was designed by stimulating the phase of the polymeric material embedded in a periodic structure using infrared radiation. The acoustic filter can be tuned remotely using thermal stimulation induced by the infrared radiation. The filter is composed of steel cylinder scatterers arranged periodically in a background of bulk poly (N-isopropylacrylamide) polymer hydrogel. The lattice structure creates forbidden bands for certain sets of mechanical waves that cause it to behave as an ultrasonic filter. Since the bandstructure is determined by not only the arrangement of the scatterers but also the physical properties of the materials composing the scatterers and background, modulating either the arrangement or physical properties will alter the effect of the crystal on propagating mechanical waves. Here, the physical properties of the filter are varied by inducing changes in the polymer hydrogel using an electromagnetic thermal stimulus. With particular focus on the k00 -wave, the transmission of ultrasonic wave changes by as much as 20 dBm, and band widths by 22% for select bands.

Anomalous temperature dependence of speed of sound of bulk poly(N-isopropylacrylamide) hydrogels near the phase transition

Bulk Poly(N-isopropylacrylamide) (PNIPAm) hydrogels are thermally responsive polymers that undergo a sharp volumetric phase transition around its lower critical solution temperature of 33 °C. The physical characteristics of bulk, micro-, and nano-form PNIPAm hydrogel have been well-studied, and have applications ranging from biomedical devices to mechanical actuators. An important physical characteristics which reveals lack of available information is speed of sound. Prior studies have utilized Brillouin scattering, multi-echo reflection ultrasound spectroscopy, the sing-around method, and others in measuring the speed of sound. We use a planar resonant cavity with bulk PNIPAm hydrogel in aqueous solution to determine the temperature dependent speed of sound around the lower critical solution temperature. The results show sharp nonmonotonic behavior of the sound velocity in vicinity of the phase transition.

Linear nonreciprocal transmission of sound in a viscous fluid with asymmetric scattteres

Reciprocity theorem is hold for wave equation due to time-reversal symmetry. Dissipative losses introduced by adding imaginary part to elastic modulus make sound propagation irreversible but the reciprocity theorem remains valid. In viscous fluid acoustic oscillations of local velocity follow the Navier-Stokes equation, which is not time-reversible. However, broken T-symmetry does not necessarily lead to nonreciprocity. Here we demonstrate that the necessary condition for nonreciprocal propagation in a viscous fluid is broken P-symmetry, which can be achieved by introducing asymmetric scatterers in a viscous environment. The experiment was done with a phononic crystal of Al rods in water. The rods have asymmetric cross-section in a form of a circular sector with an arc of 120 degrees. The measured transmission spectrum exhibits signatures of nonreciprocity within a range of frequencies from 300 to 450 kHz. The experimental results are in agreement with numerical simulations based on the linearized Navier-Stokes equation. The nonrecipocity is due to different viscous losses accumulated along sound propagation in two opposite directions with broken P-symmetry. Nonreciprocity increases for the rods with more rough surfaces since dissipation occurs mainly in a viscous boundary layer of thickness of few microns. Unlike previously proposed nonreciprocal devices based on nonlinearity or local spinning of fluid, our passive device is small, cheap, and does not require energy source. [Work supported by the NSF under Grant No. 1741677.]Reciprocity theorem is hold for wave equation due to time-reversal symmetry. Dissipative losses introduced by adding imaginary part to elastic modulus make sound propagation irreversible but the reciprocity theorem remains valid. In viscous fluid acoustic oscillations of local velocity follow the Navier-Stokes equation, which is not time-reversible. However, broken T-symmetry does not necessarily lead to nonreciprocity. Here we demonstrate that the necessary condition for nonreciprocal propagation in a viscous fluid is broken P-symmetry, which can be achieved by introducing asymmetric scatterers in a viscous environment. The experiment was done with a phononic crystal of Al rods in water. The rods have asymmetric cross-section in a form of a circular sector with an arc of 120 degrees. The measured transmission spectrum exhibits signatures of nonreciprocity within a range of frequencies from 300 to 450 kHz. The experimental results are in agreement with numerical simulations based on the linearized Navier-...

Funneled focusing of planar acoustic waves utilizing the metamaterial properties of an acoustic lens

Metamaterial acoustic lenses are acoustic devices based on phononic crystal structures that take advantage of negative or near-zero indices of refraction. These unique properties arise due to either the antiparallel direction of the phase and group velocity or strongly anisotropic dispersion characteristics, usually above the first transmission band. In this study, we utilize an FDTD program to examine two phononic lenses that utilize anisotropic effects available in their second band to collimate and focus acoustic waves from a plane-wave source with a k00 wavevector. The phononic crystals consist of stainless steel rods arranged in a square lattice with water as the ambient material. Results show collimation and focusing in the second band for select frequencies, fc ± 0.005𝑓𝑐.