Evidence for electret polarization in polyoxymethylene

It has been found that under certain experimental conditions, polyoxymethylene films undergo a microscopic as well as macroscopic polarization resulting in electret formation. Thermally stimulated discharge (TSD) currents have been measured for polyoxymethylene film electrets of 100 μM prepared at various polarizing fields and polarization temperatures. Experimental evidence like the shift of the discharge current peak with polarization temperature, and a nonlinear dependence of the accumulated charge on field strength suggests the formation of a space charge and interfacial polarization. The observed TSD peak at 130°C coincides with the α -relaxation temperature in low-frequency dielectric measurements. Activation energies and relaxation times have been calculated on the basis of the experimental data.     

退火与极化温度对尼龙11薄膜驻极体内陷阱能级分布的影响

用热释电技术研究了尼龙11薄膜驻极体制备过程中热处理与极化温度对驻极体陷阱能级分布的影响.结果显示，淬火驻极体的热释电流谱上存在四个空间电荷退陷阱电流峰，而在退火处理后则显示两个退陷阱电流峰.采用多点法对热释电流谱进行理论拟合可以将各个退陷阱电流峰分离并得到它们的陷阱深度参数.这些参数进一步表明，淬火尼龙11薄膜驻极体内存在四个空间电荷的陷阱能级，极化温度升高对它们的分布情况影响不大；退火处理后，陷阱能级减少为两个，且随着极化温度的升高，较浅的陷阱能级有明显向较深陷阱能级接近的趋势. 关键词： 尼龙11 薄膜驻极体 热释电 热处理     

介电极化新概念--边界屏蔽理论

熟知的介电极化只是一种快效应,还有一种慢效应。慢极化效应使电介质的极化偏离了麦克斯韦方程组描述的规律,引入边界屏蔽电荷的概述２来描述介质的极化过程,可以固避电介质内部复杂的电位移运动,从而能用屏蔽电荷激发的量子统计理论来说明驻极体和铁电体的许多性质。     

Scope for simulating ionic conducting lattices as spherical electrets

Spherical electrets are proposed to simulate an ionic crystal lattice containing polarizable ions, whose electric fields persist for long periods. When there are conducting paths between the charged elements, polarization can exist in thin layers.Moscow State Institute for Electronics and Mathematics, Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 27–30, April, 1994.     

Costa ribeiro effect in some cholesteryl esters

In this paper we confirm the existence of the thermodielectric effect at phase transitions of some cholesteryl esters (cholesteryl chloride, cholesteryl nonanoate and cholesteryl laurate). Significant sudden variations of the potential difference across the samples were detected at solid crystal → isotropic, isotropic → mesophase phase transitions of these liquid crystals. The variations are due to the formation of thermodielectric electrets by spontaneous polarization. The phase transitions are identified and their temperatures are in agreement with the literature.     

Electrical properties of polylactides

A PLA film with low crystalline-phase content was investigated in a DC field within the temperature range of 25–70 °C. The polymer exhibits low electrical conduction with increasing activation energy within the area of the glass transition temperature. Electrets were made from the PLA film precursor using an isothermal polarization method. The charge on the electrets was assessed by measuring the field intensity. The surface charge of an electret has the character of a hetero-charge. After 4 months from termining the isothermal charging and cooling, the PLA film polarized at E_p > 16.7 MV/m was affirmed the applicable electret properties.     

Influence of the presence of humidity,ambient aerosols and thoron on the detection responses of electret radon monitors

This paper describes how the presence of humidity, ambient aerosols and thoron influences the detection responses of an electret passive environmental radon monitor (commercially E-PERM^®) using radon and thoron chambers of National Institute of Radiological Sciences (NIRS). From the initial experiments, the linear correlation between the conversion factor for radon and the midpoint voltage for the short-term and long-term electrets was determined. As a result, it was observed that the conversion factors were approximately 10% lower for the short-term electrets and 3% higher for the long-term electrets than those reported by the manufacturer. The evaluation of different magnitudes of radon concentration showed that the presence of humidity and ambient aerosols had little influence on the detection response for radon, with less than 10% relative standard deviation. In the case of thoron detection response, net voltage drop values increased linearly with time-integrated concentrations for both short-term and long-term electrets, as was the case with radon. The thoron responses to radon were less than 5% for both the electrets from the results of experiments and calculations.     

Piezoelectricity of porous polytetrafluoroethylene single- and multiple-film electrets containing high charge densities of both polarities

Single-film bipolar electrets of porous polytetrafluoroethylene are generated by means of a two-step corona-charging process at elevated temperatures. Quasi-static direct piezoelectric coefficients of up to 0.15 nC/N have been observed on these films. In addition, multiple-layer stacks of porous and non-porous polytetrafluoroethylene films with monopolar charge were also investigated. While the piezoelectric responses of the stacks were not as high as those of the single films, the multiple-film arrangements may have other advantages such as better electrical shielding or tunable mechanical properties and adjustable acoustical impedance. Our new results are discussed in the context of the emerging field of porous polymer electrets with many potential device applications.     

The strong piezoelectricity in polyvinylidene fluroide (PVDF)

Polymers have been playing an important role already for a long time in electronic engineering as dielectrics. Recently a search has been made for polymers with unusual properties: organic semiconductors or photoconductors, electrets, and polymeric piezo — and pyroelectrics have been developed. And now, polyvinylidene fluroide (PVDF), a new piezoelectric film material is being produced on an industrial scale.^1,2 In this article, recent developments and applications of piezoelectric polymers are surveyed.     

Das Strom-Spannungs-Widerstands-Ersatzschaltbild von Elektreten

For one-dimensional electrets there is set up a current-voltage-resistance equivalent circuit, the aim of which is to change the boundary value problem of electrostatics, that is the calculation of the electric fields in electrets from the electric and geometric data and the applied voltage into a problem of network analysis of a circuit consisting of ohmic resistances, current sources and voltage sources.      

充电参数对聚丙烯蜂窝膜驻极体压电性的影响

通过接触法充电、恒压电晕充电，以及开路热刺激放电技术，对充电参数(例如充电电压，充电时间和充电温度等)对聚丙烯蜂窝膜(PP cellular)驻极体的压电性及其热稳定性的影响的研究，并从孔洞型薄膜压电体的材料结构和理论模型出发，讨论了这些参数对其影响的原因，分析了与压电活性密切相关的空间电荷的动态特性.结果表明，合理地优化充电参数，能使这类呈闭合孔洞的聚合物蜂窝膜驻极体的压电性及其稳定性得以改善，为拓宽这类新型压电功能膜应用领域，推动其产业化进程提供了一定的理论和技术依据. 关键词： 聚丙烯蜂窝膜 压电性 充电参数 电荷输运     

Transient photocurrents accompanying irradiation of charged films of polytetrafluoroethylene

It was established experimentally that the transient photocurrents arising when PTFE films charged in a corona discharge are illuminated are reversible. It is shown that the transient current is of a thermal origin and is formed by the superposition of two oppositely directed components. The characteristics of the photocurrents are explained by the existence of internal polarization (heterocharge), arising in a strong field generated by the homocharge trapped on the surface, in the films. The existence of heterocharge in nonpolar films indicates that it is in principle possible to make stable electrets, in which the potential is maintained constant owing to slow self-consistent relaxation of the homo- and heterocharge.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 16–19, September, 1987.     

不连续Co/SiO2多层膜的结构及其输运性质的研究

用射频磁控溅射方法制备了系列Co/SiO2不连续磁性金属绝缘体多层膜（DMIM） .经研究发现：对［SiO2(2.4 nm)/Co(t)］20体系，在Co层厚度小于2.5 nm时，Co层由连续变为不连续；Co层不连续时，其导电机理为热激发的电子隧穿导电，lnR与T-1/2接近正比关系; 隧道磁电阻（TMR）在Co层厚度为1.4 nm时出现极大值-3%.DMIM 的性质 不仅与磁性金属层厚度密切相关，而且与绝缘层厚度有密切的关系.在固定Co层厚度为 1.9 nm的情况下，研究了TMR随SiO2层厚度的变化 关键词： 不连续磁性金属/绝缘体多层膜 隧道磁电阻效应     

正充电聚四氟乙烯薄膜驻极体的电荷储存及其动态特性

研究了栅控恒压正电晕充电的聚四氟乙烯（PTFE）薄膜驻极体的电荷储存与 输运特性.结果显示在100℃以下的较低温区和高于150℃，尤其是高于180℃的较高 温区内慢再俘获效应控制着脱阱电荷的输运；而在约100—150℃的温区内快再俘获效应占主导地位.初始表面电位的增加将导致电荷密度衰减加剧，通过合理控制充电参数和组合 热处理工艺，可使同样储存有足够高的电荷密度的正负充电的PTFE薄膜驻极体既显示出相近 的电荷储存寿命，又具有突出的电荷稳定性. 关键词： 聚四氟乙烯 正充电驻极体 交越现象 电荷稳定性 电荷 输运     

Review of ferroelectric hydroxyapatite and its application to biomedicine*

ABSTRACT

Hydroxyapatite (HA) is a major component of bone in humans and animals. Until about 10 years ago, it was considered to have a centrosymmetric crystal structure and could not contribute to the well-known piezoelectric effect in bone. This review describes the theoretical and experimental studies that showed that HA does have a non-centrosymmetric structure. Recent experiments have shown that HA exhibits piezoelectricity, pyroelectricity, and ferroelectricity. It has been made in the form of thick films and as space-charge electrets. It has an important biomedical application as an implant for bone cell attachment and growth.     

Higher stabilities of positive and negative charge on tetrafluoroethylene–hexafluoropropylene copolymer (FEP) electrets treated with titanium-tetrachloride vapor

Tetrafluoroethylene–hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor in a molecular-layer deposition process. As a result of the surface treatment, significant improvements of the thermal and temporal charge stability were observed. Charge-decay measurements revealed enhancements of the half-value temperatures and the relaxation times of positively charged FEP electrets by at least 120 °C and two orders of magnitude, respectively. Beyond previous publications on fluoropolymer electrets with surface modification, we here report enhanced charge stabilities of the FEP films charged in negative as well as in positive corona discharges. Even though the improvement for negatively charged FEP films is moderate (half-value temperature about 20 °C higher), our experiments show that the asymmetry in positive and negative charge stability that is typical for FEP electrets can be overcome by means of chemical surface treatments. The results are discussed in the context of the formation of modified surface layers with enhanced charge-trapping properties.     

Influence of gamma irradiation on surface potential decay of corona charged polyethylene

In the present paper the surface potential decay after dc corona charging of gamma-irradiated low-density polyethylene (LDPE) has been studied. The irradiation was carried out in air, nitrogen gas and vacuum to investigate the effect of irradiation environment on the surface potential of LDPE. Samples with different absorbed doses were obtained by exposing samples to ⁶⁰Co for various times at room temperature. It has been found that there is a significant change in the surface potential decay characteristics after different irradiation dose and the surface potential decay pattern depends also on irradiation environment. The possible surface potential decay mechanisms responsible for the above were discussed. The results are useful in the context of long-term performance of electrets.     

A review of developments in thermal techniques for charge profile measurements in polymer electrets

A number of methods were evolved during last three decades to understand the internal charge profile of polymer electrets. These methods essentially are based on the propagation of heat or pressure waves inside the charged samples. In both cases, electrical signals are generated due to mechanical or dielectric changes caused by heat diffusion or propagation of a pressure discontinuity in the sample. The charge distribution can be obtained from the electrical response. This paper presents detailed information on the thermal techniques to probe the charge distributions in the thickness direction of the polymer electrets as well as a comprehensive review of thermal data analysis.     

充电栅压对聚四氟乙烯多孔膜驻极体储电能力的影响

利用室温下栅控恒压电晕充电、热脉冲技术、开路热刺激放电电流谱以及对在充电过程中通过样品电流的监测等方法，系统地研究了充电栅压对具有开放性孔洞结构的聚四氟乙烯（PTFE）多孔膜储电能力的影响，并讨论了导致这类影响的电荷动力学特性和材料的微结构根源 .结果显示，过高的充电栅压会导致沉积电荷密度下降和电荷衰减加剧，不利于这类新结构 功能材料压电活性的提高及其热稳定性的改善.合理的优化充电条件能使负极性充电PTFE多 孔膜驻极体在有机聚合物材料中显示优异的储电能力及电荷稳定性，并改善其作为双极性空 间电荷型压电传 关键词： 聚四氟乙烯 驻极体 储电能力 多孔膜     

ChargingE-perm for radon measurements

Recharging of commercial E-perm electret detectors used for radon measurement has been investigated. The E-perm was recharged using the poling charging method where a high voltage is applied on a metal nozzle to polarize the electret material. The optimum parameters to recharge the electrets were 5 kV charging voltage and 7 mm height of the nozzle from the electret surface and using an insulator while charging. Under these conditions, charging of E-perm carried out for 15 s attained required voltage and retained the charge for long durations. The response of recharged E-perm for radon activity measurements in a radon calibration chamber and in an open environment was compared with that of CR-39 detectors. Results showed close proximity in radon values.