Fluoroethylenepropylene ferroelectrets with patterned microstructure and high, thermally stable piezoelectricity

Layered fluoroethylenepropylene (FEP) ferroelectret films were prepared from sheets of FEP films by template-patterning followed by a fusion-bonding process and contact charging. The layered ferroelectret films show consistency and regularity in their void structures and good bonding of the layers. For films composed of two 12.5???m thick FEP layers and a typical void of 60???m height, the critical voltage necessary for the built-up of the ??macro-dipoles?? in the inner voids is approximately 800 V. At room temperature, Young??s modulus in the thickness direction, determined from dielectric resonance spectra of the fabricated films with a typical thickness of 85???m, is about 0.21 MPa. Initial quasistatic piezoelectric d ₃₃ coefficients of samples contact charged at a peak voltage of 1500 V are in the range of 1000?C3000 pC/N. From these, ferroelectrets with high quasistatic and dynamic (up to 20 kHz) d ₃₃ coefficients of up to 1000 pC/N and 400 pC/N, respectively, which are thermally stable at 120°C, can be obtained by proper annealing treatment. This constitutes a significant improvement compared to previous results.     

Thermally stable fluorocarbon ferroelectrets with high piezoelectric coefficient

Fused layers of polytetrafluoroethylene (PTFE) and fluoroethylenepropylene (FEP) films with small interfacial cellular gas voids show large quasistatic piezoelectric d₃₃-coefficients of more than 1000 pC/N. After annealing at a temperature of 90 °C for 4 days, d₃₃ amounts to about 400 pC/N and thereafter changes by less than 10 percent over a period of 5 days at this temperature. The piezoelectric coefficient is independent of applied pressure in the range up to 20 kPa. Its frequency response shows a small decrease up to the vicinity of the thickness resonance frequency at 40 to 150 kHz. PACS 77.65.Bn; 77.55.+f; 77.84.Jd     

Polarization and piezoelectricity in polymer films with artificial void structure

Laminated polymer-film systems with well-defined void structures were prepared from fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE) layers. First the PTFE films were patterned and then fusion-bonded with the FEP films. The laminates were subjected to either corona or contact charging in order to obtain the desired piezoelectricity. The build-up of the “macro-dipoles” in the laminated films was studied by recording the electric hysteresis loops. The resulting electro-mechanical properties were investigated by means of dielectric resonance spectroscopy (DRS) and direct measurements of the stress-strain relationship. Moreover, the thermal stability of the piezoelectric d ₃₃ coefficient was investigated at elevated temperatures and via thermally stimulated discharge (TSD) current measurements in short circuit. For 150 μm thick laminated films, consisting of one 25 μm thick PTFE layer, two 12.5 μm thick FEP layers, and a void of 100 μm height, the critical voltage necessary for the build-up of the “macro-dipoles” in the inner voids was approximately 1400 V, which agrees with the value calculated from the Paschen Law. A quasi-static piezoelectric d ₃₃ coefficient up to 300 pC/N was observed after corona charging. The mechanical properties of the film systems are highly anisotropic. At room temperature, the Young’s moduli of the laminated film system are around 0.37 MPa in the thickness direction and 274 MPa in the lateral direction, respectively. Using these values, the theoretical shape anisotropy ratio of the void was calculated, which agrees well with experimental observation. Compared with films that do not exhibit structural regularity, the laminates showed improved thermal stability of the d ₃₃ coefficients. The thermal stability of d ₃₃ can be further improved by pre-aging. E.g., the reduction of the d ₃₃ value in the sample pre-aged at 150°C for 5 h was less than 5% after annealing for 30 h at a temperature of 90°C.     

Pressure dependence of space charge deposition in piezoelectric polymer foams: simulations and experimental verification

The piezoelectric activity of PQ-50 cellular polypropylene (PP) foam (an example of a so-called ferroelectret) is measured after repeated charging in a nitrogen atmosphere at a range of pressures between 61 and 381?kPa. The results are compared against simulations using a multilayer electromechanical model based on Townsend??s model of Paschen breakdown and a realistic distribution of void heights determined from scanning electron micrographs. The modeled piezoelectric coefficients versus pressure are in good agreement with experimental data when adjusted Paschen coefficients are used, indicating that the Paschen curve for electric breakdown in gases needs to be modified for dielectric barrier discharges in microcavities. The highest d ₃₃ coefficients were achieved for pressures above 251?kPa. For previously uncharged PP foam, the model predicts an optimal charging pressure of 186 kPa.     

氟聚合物压电驻极体的压电性及其电荷的动态行为

描述了一种可控微结构的多孔聚合物压电功能膜的制备方法,讨论了采用该工艺制备的聚四氟乙烯(PTFE)和全氟乙丙烯共聚物(FEP)复合膜压电驻极体的压电性能及其热稳定性.通过等温压电系数衰减和短路热刺激放电(TSD)方法,研究了氟聚合物复合膜压电活性热稳定性改善的根源,以及脱阱电荷输运和复合的特性.结果表明,这类氟聚合物压电驻极体膜的准静态压电系数d₃₃可高达2200pC/N;压电系数d₃₃的压强特性在直到20kPa的压强范围内呈现良好的 关键词： 氟聚合物 压电驻极体 热稳定性 电荷动态特性     

聚四氟乙烯和氟化乙丙烯共聚物复合膜的压电性

以多孔PTFE膜为骨架,而以致密（非多孔）FEP膜为储电介质层的孔洞结构复合压电驻极体膜的制备方法.利用正压电效应,测量了复合膜的准静态压电系数d₃₃;研究了压电系数的热稳定性和复合膜中空间电荷的动态特性;并通过介电谐振谱的分析,比较了这类复合膜的准静态和动态压电系数.结果表明：FEP和PTFE复合膜压电驻极体的准静态压电系数d₃₃可以达到300 pC/N.经90℃老化20 h 后的d₃₃仍保持在初 关键词： 压电驻极体 压电性 多孔聚四氟乙烯 致密氟化乙丙烯共聚物     

人工调控微结构压电驻极体的热稳定性和电荷动态特性

利用表面带有周期性结构的硬质模板,通过冷压工艺将周期结构图案复制到多孔聚四氟乙烯(PTFE)薄膜表面,再经过热黏合工艺与致密氟化乙丙烯共聚物(FEP)薄膜复合,制备出了高度有序的微孔结构复合膜,并用电晕充电的方法对复合膜进行极化处理,最终获得氟聚合物复合膜压电驻极体.借助对这类复合膜压电驻极体介电谐振谱的测量,得到了材料的杨氏模量.并利用等温热老化工艺对它们的压电系数d33的热稳定性进行了考察.最后通过短路热刺激放电谱的测量和分析,讨论了该复合膜在热老化处理后的电荷动态 关键词： 有序结构 压电驻极体 压电性 电荷动态特性     

Physical foaming of fluorinated ethylene-propylene (FEP) copolymers in supercritical carbon dioxide: single-film fluoropolymer piezoelectrets

Cellular polymer foams with significant piezoelectric activity were prepared from fluorinated ethylene-propylene (FEP) copolymers. The required void structure is obtained by saturation of FEP films with supercritical carbon dioxide and a subsequent heat treatment for controlled inflation. After bipolar electric charging of the voids and evaporation of electrodes, the FEP films show piezoelectric d₃₃ coefficients up to 50 pC/N. The present physical foaming process generates cellular fluoropolymer piezoelectret films with usually only one single void across the film thickness. PACS 81.05.Rm; 77.65.-j; 77.84.Jd; 61.41.+e     

Piezoelectric properties of new ternary Bi1/2(Na, Li)1/2TiO3–(Bi1/2K1/2)TiO3–Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics

0.852[Bi_1/2(Na_1?x Li _x )_1/2]TiO₃?C0.110(Bi_1/2K_1/2)TiO₃?C0.038Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BNLT?CBKT?CBCTZ-x) new ternary piezoelectric ceramics were fabricated by the conventional solid-state method, and their piezoelectric properties as a function of the Li content were mainly investigated. A?stable solid solution with a single perovskite structure has been formed, and the depolarization temperature (T _d) of these ceramics was identified by using the temperature dependence of the dielectric loss. The T _d value of these ceramics gradually decreases, while the T _m value increases with increasing the Li content. The dielectric constant increases and the dielectric loss decreases with increasing the Li content, and an enhanced piezoelectric behavior of d ₃₃??223 pC/N and k _p??35.2?% has been demonstrated in these ceramics with x=0.06.     

Dielectric and piezoelectric properties of the Ba0.92Ca0.08Ti0.95Zr0.05O3 thin films grown on different substrate

Lead free Ba_0.92Ca_0.08Ti_0.95Zr_0.05O₃ (BCZT) thin films were deposited on Pt/Ti/SiO₂/Si and LaNiO₃(LNO)/Pt/Ti/SiO₂/Si substrates by a sol–gel processing technique, respectively. The effects of substrate on structure, dielectric and piezoelectric properties were investigated in detail. The BCZT thin films deposited on LNO/Pt/Ti/SiO₂/Si substrates exhibit (100) orientation, larger grain size and higher dielectric tunability (64%). The BCZT thin films deposited on Pt/Ti/SiO₂/Si exhibit (110) orientation, higher Curie temperature (75 °C), better piezoelectric property (d₃₃ of 50 pm/V) and lower dielectric loss (0.02). The differences in dielectric and piezoelectric properties in the two kinds of oriented BCZT films should be attributed to the difference of structure, in-plane stress and polarization rotation in orientation engineered BCZT films.     

Piezoelectric properties of non-uniform polymeric structures under static load

ABSTRACT

Piezo-electrical properties of structures containing ‘soft’ and ‘hard’ dielectric layers with charge stored on the interfaces are described in the paper. The piezo-activation process of structures containing layers with gas voids by partial discharges is described. The influence of the mechanical properties of the ‘soft’ layer on the piezoelectric parameter d₃₃ value and its dependence on the static pressure p are also discussed. It was found experimentally, that for the fibrous type of ‘soft’ dielectric layer, the dependence of the piezoelectric parameter d₃₃ (p) can be described by the function d₃₃ ÷ p^?n, where n ≤ 1.     

Polytetrafluoroethylene piezoelectrets prepared by sintering process

Polytetrafluoroethylene (PTFE) films with a void structure are prepared by a sintering process. Such void PTFE films are piezoelectric after proper corona charging. The quasi-static piezoelectric d ₃₃ coefficients up to 250 pC/N are achieved for the samples which were made of compact and biaxial-tension porous PTFE layers. Pre-ageing treatment is an effective method to further improve the thermal stability. For the samples with pre-ageing treatment, the reduction of the d ₃₃ coefficients is around 2% per day when exposed to 120 °C.     

Nonlinear properties of barium titanate in the electric field range 0≤<Emphasis Type="Italic">E</Emphasis>≤5.5×0<Superscript>7</Superscript> V/m

X-ray structural studies and dielectric measurements of BaTiO₃ single crystals were performed in the dc electric field range 0≤E≤5.5×10⁷ V/m. The field dependence of the tetragonal cell parameter obtained was used to calculate the field dependence of the piezoelectric modulus d₃₃. The piezoelectric modulus d₃₃ and the dielectric permittivity vary in magnitude by a factor of two with the field varied in the above range. The observed nonlinear behavior is shown to fit well to a relation connecting the dielectric with the electromechanical characteristics of the crystal.     

The influence of different doping elements on microstructure, piezoelectric coefficient and resistivity of sputtered ZnO film

ZnO film is attractive for high frequency surface acoustic wave device application when it is coupled with diamond. In order to get good performance and reduce insertion loss of the device, it demands the ZnO film possessing high electrical resistivity and piezoelectric coefficient d₃₃. Doping ZnO film with some elements may be a desirable method. In this paper, the ZnO films undoped and doped with Cu, Ni, Co and Fe, respectively (doping concentration is 2.0 at.%) are prepared by magnetron sputtering. The effect of different dopants on the microstructure, piezoelectric coefficient d₃₃, and electrical resistivity of the film are investigated. The results indicate that Cu dopant can enhance the c-axis orientation and piezoelectric coefficient d₃₃, the Cu and Ni dopant can increase electrical resistivity of the ZnO film up to 10⁹ Ω cm. It is promising to fabricate the ZnO films doped with Cu for SAW device applications.     

Improvement of piezoelectric coefficient of cellular polypropylene films by repeated expansions

Pressure expansion can be used to improve the piezoelectric d₃₃-coefficients of customary cellular polypropylene (PP) films. In the present paper, the experimental procedure for a double expansion process is described and experimental results of d₃₃-coefficients that can be achieved by choosing expansion pressures from 0.2 to 2 MPa and expansion temperatures from 20 to 120 °C for the two processes are discussed. For example, expansion pressures of 2 MPa, a temperature of 90 to 100 °C for the first expansion and 60 °C for the second expansion, and exposure times of the order of 1 h result in quasistatic d₃₃-coefficients of more than 1000 pC/N which are relatively stable at room temperature.     

Piezoelectric property of hot pressed electrospun poly(γ-benzyl-α, L-glutamate) fibers

Since the 1960s, the piezoelectricity in biopolymers (e.g. proteins and polynucleotides) has attracted considerable scientific attention. In particular, poly(glutamate)s have been one of the most popular targets for this research due to their well-defined helical structure and permanent polarity along the helical axis. To date, films of poly(glutamate)s have been shown to exhibit piezoelectricity only in shear mode (d₁₄), mainly due to the limitation in fabricating electrically poled polymer samples. This paper describes a combined electrospinning and hot press method that allows production of poled poly(??-benzyl-??,L-glutamate) (PBLG) films with piezoelectricity in all d₃₃, d₃₁ and d₁₄ modes for the first time. It is found that this PBLG film belongs to the matrix structure of C_??v group, which is the same as that of poled PVDF film. The moderately high piezoelectric coefficients in both d₃₃ and d₁₄ modes as well as their thermal stability make the poled PBLG film an excellent candidate for use in flexible transducers and small energy harvesting devices.     

Thickness dependent studies of dielectric breakdown in Langmuir thin molecular films

The usefulness of Langmuir thin molecular films for making devices has made it necessary to study their dielectric breakdown behaviour. Study of thickness dependence of the breakdown field in these films is important from the point of view of device applications as well as the understanding of breakdown phenomenon. In the present paper, thickness dependence of two breakdown events corresponding to ‘onset breakdown’ and ‘maximum breakdown’ voltages have been reported for the ‘build-up’ Langmuir films of barium palmitate, margarate and behenate in the thickness ranges (23–300 Å) and (370–2400 Å), respectively. These two events are of particular interest because the former determines the permissible operating voltage and the latter leads to the ‘practically’ important ultimate dielectric strength of the film. The Langmuir films have been chosen because of their accurately known and controllable uniform thickness, high dielectric strength and high structural perfection. The onset breakdown strength of all the substances is found to vary as d^-1 (d being film thickness) in the ultra-thin range which have been explained qualitatively in terms of boundary effects. The maximum breakdown strength is also found to be almost inversely proportional to the film thickness but the results remain inexplicable because the actual mechanism of destructive breakdown is not yet fully understood. The typical J-V characteristics in the ‘non-destructive’ phase have also been given.     

Relationship between surface potential and d33 constant in cellular piezoelectric polymers

The development of cellular piezoelectric polymers has shown very promising results thanks to their high d₃₃ piezoelectric constants which make them candidates for many applications. Cellular piezoelectric polymers, known as ferroelectrets, are obtained by means of an activation process which consists in generating an internal dipole with electrostatic charges produced by internal electric discharges. The most common system for this activation process is the application of a corona discharge on the surface of the sample in order to produce a high internal electric field. The theoretical electrostatic model of the process which is widely used is the Sessler model which relates the internal surface charge density, the air and polymer layers thickness, the dielectric permittivity of the polymer and the Young's Modulus of the cellular material to the d₃₃ piezoelectric constant. In our work, we relate the internal charges of the material with the d₃₃ piezoelectric constant by means of a surface potential scanning of cellular polypropylene biaxially stretched samples. Samples were charged by a corona discharge controlled with a triode electrode. Surface potentials were high enough to generate internal discharges and obtain measurable d₃₃ piezoelectric constants but low enough to be measured with spatial resolution by means of a 3 kV electrostatic probe. Surface potential profiles showed some deviations from the expected bell-shape profile due to the internal electric field generated by the internal static charge. These deviations can be numerically related to the measured d₃₃ piezoelectric constant with the electrostatic Sessler model.     

Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ thin films were deposited on LaNiO₃(LNO)/SiO₂/Si(1 0 0) and Pt/Ti/SiO₂/Si(1 0 0) substrates by metal-organic decomposition, and the effects of bottom electrodes LNO and Pt on the ferroelectric, dielectric and piezoelectric properties were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy, respectively. For the thin films deposited on LNO and Pt electrodes, the remnant polarization 2P_r are about 22.6 and 8.8 μC/cm² under 375 kV/cm, the dielectric constants 238 and 579 at 10 kHz, the dielectric losses 0.06 and 0.30 at 10 kHz, the statistic d_33eff values 95 and 81 pm/V. The improved piezoelectric properties could make (Na_1−xK_x)_0.5Bi_0.5TiO₃ thin film as a promising candidate for piezoelectric thin film devices.     

Screen printing for producing ferroelectret systems with polymer-electret films and well-defined cavities

We report a process for preparing polymer ferroelectrets by means of screen printing—a technology that is widely used for the two-dimensional patterning of printed layers. In order to produce polymer-film systems with cavities that are suitable for bipolar electric charging, a screen-printing paste is deposited through a screen with a pre-designed pattern onto the surface of a polymer electret film. Another such polymer film is placed on top of the printed pattern, and well-defined cavities are formed in-between. During heating and curing, the polymer films are tightly bonded to the patterned paste layer so that a stable three-layer system is obtained. In the present work, polycarbonate (PC) films have been employed as electret layers. Screen printing, curing and charging led to PC ferroelectret systems with a piezoelectric d ₃₃ coefficient of about 28 pC/N that is stable up to 100 ^°C. Due to the rather soft patterned layer, d ₃₃ strongly decreases already for static pressures of tens of kPa. The results demonstrate the suitability of screen printing for the preparation of ferroelectret systems.