共查询到20条相似文献，搜索用时 562 毫秒
1.
Chu B Zhou X Ren K Neese B Lin M Wang Q Bauer F Zhang QM 《Science (New York, N.Y.)》2006,313(5785):334-336
Dielectric polymers with high dipole density have the potential to achieve very high energy density, which is required in many modern electronics and electric systems. We demonstrate that a very high energy density with fast discharge speed and low loss can be obtained in defect-modified poly(vinylidene fluoride) polymers. This is achieved by combining nonpolar and polar molecular structural changes of the polymer with the proper dielectric constants, to avoid the electric displacement saturation at electric fields well below the breakdown field. The results indicate that a very high dielectric constant may not be desirable to reach a very high energy density. 相似文献
2.
KM Blackwood 《Science (New York, N.Y.)》1996,273(5277):909-912
Side-chain ferroelectric liquid crystalline polymers are currently used in a number of applications, including displays and electrical sensors. Comparisons with existing technologies and materials indicate that relative to ceramics, such polymers have lower figures of merit but offer greater durability in sensor applications. 相似文献
3.
Giant electrostriction and relaxor ferroelectric behavior in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer 总被引：2，自引：0，他引：2
An exceptionally high electrostrictive response ( approximately 4 percent) was observed in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer. The material exhibits typical relaxor ferroelectric behavior, suggesting that the electron irradiation breaks up the coherent polarization domain (all-trans chains) in normal ferroelectric P(VDF-TrFE) copolymer into nanopolar regions (nanometer-size, all-trans chains interrupted by trans and gauche bonds) that transform the material into a relaxor ferroelectric. The expanding and contracting of these polar regions under external fields, coupled with a large difference in the lattice strain between the polar and nonpolar phases, generate an ultrahigh strain response. 相似文献
4.
5.
Applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the electrocaloric effect (ECE), it was determined that a large ECE can be realized in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer at temperatures above the ferroelectric-paraelectric transition (above 70 degrees C), where an isothermal entropy change of more than 55 joules per kilogram per kelvin degree and adiabatic temperature change of more than 12 degrees C were observed. We further showed that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene). 相似文献
6.
利用高氯酸作为氟离子选择性电极法的新介质测定食品中的微量氟 总被引：2，自引：0，他引：2
对HCLO4作为新介质在氟离子选择性电极法中测定食品中的微量氟做了探索和研究。在0.35 MOL/L HCLO4介质中,氟离子浓度在1.0×10-2~1.0×10-6MOL/L范围内呈良好的线性关系,线性相关系数R=0.999 7。利用本法测定了牛奶、卷心菜中氟离子的含量,其结果与现有其他文献方法相当,但本法试验操作大为简化。测定的相对标准偏差在1.54%~2.30%之间。在该介质中测定微量氟操作简便、快速,具有较高的精密度和准确度。所以,HCLO4溶液可作为氟离子选择性电极的测定介质。 相似文献
7.
Hierarchical structure in polymeric materials 总被引：4，自引：0，他引：4
The diversity of monomers available for synthesis of high polymers makes it possible to prepare a wide variety of long-chain macromolecular compounds. It is instructive to consider a hierarchical organization of structure in polymers at four successive levels--the molecular, nano-, micro-, and macrolevels--and to examine how interactions at and between these various levels of structure have important and often quite specific influences. Examples are drawn from semicrystalline polymers with flexible chains, liquid-crystalline polymers composed of rigid macromolecules, and amorphous polymers. Structural hierarchies in biocomposite systems are also discussed, particularly in soft connective tissues such as tendon and intervertebral disk. 相似文献
8.
Biodegradable polymers for the environment 总被引：1，自引：0，他引：1
Biodegradable polymers are designed to degrade upon disposal by the action of living organisms. Extraordinary progress has been made in the development of practical processes and products from polymers such as starch, cellulose, and lactic acid. The need to create alternative biodegradable water-soluble polymers for down-the-drain products such as detergents and cosmetics has taken on increasing importance. Consumers have, however, thus far attached little or no added value to the property of biodegradability, forcing industry to compete head-to-head on a cost-performance basis with existing familiar products. In addition, no suitable infrastructure for the disposal of biodegradable materials exists as yet. 相似文献
9.
Dental caries: prospects for prevention 总被引：44，自引：0，他引：44
H W Scherp 《Science (New York, N.Y.)》1971,173(3):1199-1205
Combined utilization of measures now available or imminent could reduce caries of the crowns of the teeth to the point of negligibility as a public health problem, if public desire were great enough to motivate changes in some of our habits (64). Universal optimum application of fluoride and substitution of starchy foods for sugary ones (or even simply judicious consumption of sugar) would alone do most of the job. Sealing of susceptible occlusal areas with adhesive polymers promises to protect the sites where fluoride evidently cannot be maximally effective. It seems unlikely that any single measure will be found sufficient to control this multifactorial disease. Consequently, we must continue the search for new means to increase the caries resistance of teeth, to reduce the cariogenicity of foodstuffs, and to check the deleterious activities of cariogenic bacteria. Anticaries food additives and antibacterial agents for intraoral use seem to be approaching practicability. Past performance warrants expectation that ongoing fundamental investigations will produce leads for future development and application. 相似文献
10.
Ferroelectric memories 总被引：1，自引：0，他引：1
In the past year it has become possible to fabricate ferroelectric thin-film memories onto standard silicon integrated circuits that combine very high speed (30-nanosecond read/erase/rewrite operation), 5-volt standard silicon logic levels, very high density (2 by 2 micrometer cell size), complete nonvolatility (no standby power required), and extreme radiation hardness. These ferroelectric random-access memories are expected to replace magnetic core memory, magnetic bubble memory systems, and electrically erasable read-only memory for many applications. The switching kinetics of these films, 100 to 300 nanometers thick, are now well understood, with switching times that fit an activation field dependence that scales applied field and temperature. Earlier problems of fatigue and retention failure are also now understood and have been improved to acceptable levels. 相似文献
11.
Low-dimensional ferroelectric structures are a promising basis for the next generation of ultrahigh-density nonvolatile memory devices. Depolarization fields, created by incompletely compensated charges at the surfaces and interfaces, depress the polarization of such structures. Theory suggests that under conditions of uncompensated surface charges, local dipoles can organize in flux-closure structures in thin films and vortex structures in nano-sized ferroelectrics, reducing depolarization fields. However, the continuous rotation of the dipoles required in vortex structures and the behavior of unit cell dipoles in flux-closure structures have never been experimentally established. By aberration-corrected transmission electron microscopy, we obtained experimental evidence for continuous rotation of the dipoles closing the flux of 180° domains in a ferroelectric perovskite thin film. 相似文献
12.
氟离子对多种细菌如变形链球菌、氧化硫硫杆菌和中度嗜热嗜酸菌等细菌具有抑制和杀伤作用。氟离子杀菌机制包括：破坏细胞膜的结构、影响细胞的ATPase活性、抑制多种代谢酶如烯醇酶等。细菌对氟离子的抗性机制包括：将无机氟离子转化为有机氟离子,由氟化物核糖开关控制相关蛋白表达和逆向运输蛋白EriCF对氟离子的运输等。目前关于微生物对氟离子的抗性方面的研究很少,因此,分离并确定与细菌耐氟相关基因的功能,研究其耐氟的机制,对深入理解细菌对氟的抗性以及细菌对阴离子的抗性机制有着重大的理论意义,同时也为解决一些需要在高氟环境下发挥功能的细菌的生长问题以及更好的利用氟离子来杀死有害菌等应用问题奠定良好的基础。 相似文献
13.
Dougherty TP Wiederrecht GP Nelson KA Garrett MH Jensen HP Warde C 《Science (New York, N.Y.)》1992,258(5083):770-774
The microscopic pathway along which ions or molecules in a crystal move during a structural phase transition can often be described in terms of a collective vibrational mode of the lattice. In many cases, this mode, called a "soft" phonon mode because of its characteristically low frequency near the phase transition temperature, is difficult to characterize through conventional frequency-domain spectroscopies such as light or neutron scattering. A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals. The low-frequency lattice dynamics of KNbO(3) and BaTiO(3) are clarified in a manner that permits critical evaluation of microscopic models for their ferroelectric transitions. The results illustrate the advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes. 相似文献
14.
Ferroelectric oxide materials have offered a tantalizing potential for applications since the discovery of ferroelectric perovskites more than 50 years ago. Their switchable electric polarization is ideal for use in devices for memory storage and integrated microelectronics, but progress has long been hampered by difficulties in materials processing. Recent breakthroughs in the synthesis of complex oxides have brought the field to an entirely new level, in which complex artificial oxide structures can be realized with an atomic-level precision comparable to that well known for semiconductor heterostructures. Not only can the necessary high-quality ferroelectric films now be grown for new device capabilities, but ferroelectrics can be combined with other functional oxides, such as high-temperature superconductors and magnetic oxides, to create multifunctional materials and devices. Moreover, the shrinking of the relevant lengths to the nanoscale produces new physical phenomena. Real-space characterization and manipulation of the structure and properties at atomic scales involves new kinds of local probes and a key role for first-principles theory. 相似文献
15.
氟中毒和甲状腺肿是世界范围内的两种严重的地方性疾病。研究表明,人和动物甲状腺肿大的发生与氟中毒有显著的相关性。氟化物是诱发甲状腺肿大的因素之一,在世界上某些地区这两种疾病经常重叠发生。本文综述了环境中氟过量与甲状腺肿的关系以及甲状腺肿发生的机理研究进展,以期为氟化物的毒理学,甲状腺肿的病因学以及甲状腺肿的防治提供有意义的参考。 相似文献
16.
Zheng H Wang J Lofland SE Ma Z Mohaddes-Ardabili L Zhao T Salamanca-Riba L Shinde SR Ogale SB Bai F Viehland D Jia Y Schlom DG Wuttig M Roytburd A Ramesh R 《Science (New York, N.Y.)》2004,303(5658):661-663
We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3-CoFe2O4 ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO3 (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe2O4 nanopillars embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases. 相似文献
17.
Thermal polymers of arginine, glutamic acid, glycine, histidine, phenylalanine, and tryptophan have melanocyte-stimulating activity. The fact that similar polymers lack such activity indicates that the effect is related to the specific amino acid residues. The active polymers are discussed as a model of an evolutionary precursor of contemporary melanocyte-stimulating hormone. 相似文献
18.
Tenne DA Bruchhausen A Lanzillotti-Kimura ND Fainstein A Katiyar RS Cantarero A Soukiassian A Vaithyanathan V Haeni JH Tian W Schlom DG Choi KJ Kim DM Eom CB Sun HP Pan XQ Li YL Chen LQ Jia QX Nakhmanson SM Rabe KM Xi XX 《Science (New York, N.Y.)》2006,313(5793):1614-1616
We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by approximately 500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity. 相似文献
19.
Regulatory role for GTP-binding proteins in endocytosis 总被引：25，自引：0，他引：25
Guanosine 5'-triphosphate (GTP)-binding proteins have been implicated in the transport of newly synthesized proteins along the secretory pathway of yeast and mammalian cells. Early vesicle fusion events that follow receptor-mediated endocytosis as measured by three in vitro assays were blocked by guanosine 5'-O-(3-thiotriphosphate) and aluminum fluoride. The effect was specific for guanosine nucleotides and depended on the presence of cytosolic factors. Thus, GTP-binding proteins may also have a role in the transport of molecules along the endocytic pathway. 相似文献
20.
There has been a phenomenal growth in the field of biosensor development in recent years with emerging applications in a wide range of disciplines, including medical analysis, food and the environment. The increase in the number of analytes requiring monitoring and others that require control, and the need for high sensitivity, speed, and accuracy of analytical measurements have stimulated considerable interest in developing sensors as diagnostics tools. A range of molecules with biorecognition powers are available naturally such as antibodies, enzymes, cell receptors and nucleic acids and are used as the sensing receptors in biosensors. The combined approaches of computer (molecular) modelling and combinatorial synthesis or molecularly imprinted polymers are undertaken today as a new method to produce synthetic receptors that can be used in sensor development. A wide range of transducers is also feasible to fulfil the rapid monitoring needs of the diagnostic market. Biosensors can also be incorporated into simple-to-use instruments as an on-line monitoring device or a one-shot sensor. This paper deals with recent developments in biosensors and their potential use in the agricultural diagnostic market. 相似文献