Hidekazu Kodama

Sound isolation by piezoelectric polymer films connected to negative capacitance circuits

Sound isolation has been achieved using a piezoelectric polymer film connected to a negative capacitance feedback circuit. A curved PVDF film was located in the middle of an acoustic tube and the transmission loss of sound through the film was determined in the audio frequency range. At any chosen frequency, the complete isolation of sound was achieved by adjusting the feedback, i.e. the complex capacitance of the circuit was matched precisely to that of the film.

Effects of annealing on the structure and switching characteristics of VDF/TrFE copolymers

Abstract Structure-property relationship has been investigated in concern with the ferroelectric switching characteristics of a VDF(78)/TrFE(22) copolymer. Unoriented and oriented film samples prepared by melt-quenching and melt-extrusion, respectively, were subjected to annealing and resulting structural changes were characterized by SEM, X-ray, density and DSC measurements. It was found that annealing just above the Curie point (120–130°C) caused a marked increase in crystallinity, whereas annealing in the paraelectric state (130–145°C) induced a crystal growth into thick lamellae. Such structural changes resulted in the ferroelectric switching characteristics with larger remanent polarization and shorter switching time. The unoriented sample yielded a smaller remanent polarization than the oriented sample by a factor of π/4, which substantiated that polarization reversal occurred as a result of rotation of molecules rotation about their chain axis. The melt-extruded and well-annealed sample consisted o...

Nonlinear Dielectric Investigation of Trifluoroethylene-Rich Copolymers of Vinylidene Fluoride

Phase transition behavior in trifluoroethylene (TrFE)-rich copolymers of vinylidene fluoride (VDF) has been investigated by means of nonlinear dielectric spectroscopy. Unlike VDF-rich copolymers showing a single ferroelectric-to-paraelectric transition, TrFE-rich copolymers undergo two-stage transitions, loss of ferroelectric polarization at a temperature TA and transition into a paraelectric motional phase at a temperature TC. Between TA and TC, they exhibit the D–E double hysteresis loop characteristic of an antiferroelectric. Analysis of such a double loop allowed the estimation of antiferroelectric polarization PA as well as ferroelectric polarization PF. A comparison of the temperature dependences of reciprocal polarizabilities βn(n = 1, 2, 3) with those of PF and PA revealed that β2 is proportional to PF whereas β1 and β3 are proportional to PF2+PA2. These results are consistent with phenomenological prediction for an antiferroelectric. It was concluded that TrFE-rich copolymers of VDF have an antiferroelectric-like phase consisting of all-trans molecules packed in a disordered manner.

Soft X-ray Charging Method for a Silicon Electret Condenser Microphone

A novel charging method using soft X-ray irradiation for forming an electret of a silicon condenser microphone was developed. Soft X-rays can penetrate through the microphone diaphragm and are suitable for ionizing gas inside the microphone element. The surface potential of a positively biased SiO2/Si3N4 dielectric film negatively increased with irradiation time, since induced anions and electrons are dragged toward the film by an applied electric field. The charging rate is inversely proportional to the bias voltage and electrode-gap distance. Charged electrets have excellent retention properties; therefore, the method is promising for the realization of silicon-electret condenser microphones.

Electret charging method based on soft X-ray photoionization for MEMS transducers

An electret charging method employing soft X-ray irradiation for microelectromechanical systems (MEMS) electret transducers is investigated. Soft X-rays can penetrate obstructions/narrow gaps and ionize gas molecules inside air gaps. This allows embedded electrets even after fabrication processes and vertical electrets on the sidewalls of high-aspect-ratio structures to be charged. The charging properties of polymer electrets subjected to soft X-ray irradiation were systematically examined. The surface potential of embedded electrets increased linearly with increasing irradiation time. The potential could be precisely controlled by the applied bias voltage. The surface potential of vertical electrets after charging was uniform to a depth of 20-30 times the gap opening. Since the photoionization rate depends on the collision diameter of the gas molecules with X-rays, a high charging rate was realized by using Xe gas. In the present charging method, photoionization was found to play a dominant role in generating electrons from gas molecules. Experiments performed at different gas pressures revealed that the effects of photoelectric phenomena and carrier displacement could be neglected. The present charging method was found to have a charge stability as high as that of corona charging. These results indicate that the present charging method has the potential for designing various new MEMS electret transducers.

Elasticity Control of Curved Piezoelectric Polymer Films

Abstract The control of sound transmission through a curved piezoelectric PVDF film was achieved using a negative capacitance circuit as a feedback system. A laser Doppler vibrometer was used to measure the vibration amplitude of the film. The vibration amplitude decreased to near zero and then increased with the reversed phase with increasing feedback voltage. The transmission of sound through the film was significantly suppressed when the vibration amplitude of the film reached near zero. The effect of the thickness and the radius of curvature of the film were tested. The efficiency of feedback depended on the electromechanical coupling factor of the film and the difference of the loss factor between the capacitances of the film and the circuit.

Sound isolation by piezoelectric polymer films connected with negative capacitance circuits

Sound isolation has been achieved using a piezoelectric polymer film connected to a negative capacitance feedback circuit. A curved PVDF film was located in the middle of an acoustic tube and the transmission loss of sound through the film was determined in the audio frequency range. At any chosen frequency, the complete isolation of sound was achieved by adjusting the feedback, i.e. the complex capacitance of the circuit was matched precisely to that of the film.Sound pressure produces strain and polarization in piezoelectric polymer films. The electric charge induced by the polarization in the electrode of the films is introduced to a negative capacitance circuit and the resulting voltage is fed back to the electrode. The field-induced strain cancels the stress-induced strain, leading to the increase of the elastic coefficient of the films. If the capacitance of the circuit is controlled to be equal to that of the film, the elastic coefficient approaches infinity. Using this principle, the sound isolation by piezoelectric polymer films connected with a negative capacitance circuit was undertaken. A PVDF film with a curved plane was located in the middle of an acoustic tube and the transmission loss through the film was determined in the audio frequency range. At any single frequency, the complete isolation of sound was achieved by adjusting the feedback. The precise matching of complex capacitance of the circuit with that of the film was required to obtain the increase of the transmission loss over a broad frequency range.

Vibration Control of Curved Piezoelectric Sheets Using Negative Capacitance Circuits

Sound-induced vibrations of a curved PZT fiber composite sheet have been controlled by a negative capacitance circuit. A laser Doppler vibrometer was used to measure the vibration amplitude. The estimated value of the sound transmission loss was increased up to 85 dB by operating this circuit from 60 dB. The controllable frequency range was increased by using another composite sheet as a capacitor. When compared with the case of a curved PVDF film, the required voltage amplification was reduced to 1/20 because the electromechanical coupling factor was five times greater.

A study of time stability of piezoelectricity in porous polypropylene electrets

Time stability of piezoelectricity in porous polypropylene (PP) electrets was studied. Piezoelectric e<inf>33</inf> and d<inf>33</inf> constants and the electromechanical coupling factor k<inf>t</inf> were evaluated by dielectric resonance method. The amount of storage charge and its temperature stability were measured by short circuit TSD measurement. The experimental results showed e<inf>33</inf>, d<inf>33</inf> and k<inf>t</inf> decreased after a day passed from charging and became stable after that. The TSD spectra showed three peaks at 65 °C, 90 °C and 120 °C. The decrease of piezoelectricity was in correspond with a decrease of TSD peak at 65 °C. Temperature spectra of Youngs modulus and permittivity showed β-relaxation around 10 °C and α-relaxation around 100 °C. The peak temperatures of TSD were covered with the temperature range of α-relaxation in a crystal region of PP. Airborne ultrasonic transducer using the porous PP electrets showed the maximum SPL of 95 dB at 150 kHz.

Analysis of Ferroelectric Switching Process in VDF/Trfe Copolymers

The ferroelectric switching curve of vinylidene fluoride / trifluoroethylene copolymer exhibits a characteristic time evolution consisting of two processes; an initial gradual increase in proportion to t 0 5 followed by a rapid increase according to an exponential function with particularly large exponent 6. Such a switching curve was analyzed by means of computer simulation based on a modified nucleation-growth mechanism. It was found that the initial gradual increase is attributed to generation of considerably large nuclei that grow according to a random-walk scheme. Once such nuclei gain a critical size, they start to grow automatically either one or two dimensionally. The time required to generate critical nuclei serves as an incubation time for the later growth process to result in the large exponent.