Dmitry Rychkov

Electret properties of polyethylene and polytetrafluoroethylene films with chemically modified surface

This paper investigates the effect of chemical surface modification of polytetrafluoroethylene (PTFE) and low density polyethylene (LDPE) films on their electret properties. PTFE films were subjected to wet treatment with three different chemicals: orthophosphoric acid, tetrabutyl titanate and tetraethoxysilane. The technique based on the principles of molecular layer deposition (MLD) method was used to modify the surface of LDPE films with phosphorus trichloride vapors. The surfaces of the films were then corona charged, and the electret charge stability was studied by means of isothermal and thermally stimulated surface potential decay. Both PTFE and LDPE films, after the surface treatment, displayed a considerable enhancement in the charge stability compared to the virgin samples. It is important to note that the enhancement of the charge stability was achieved in the positively charged PTFE films, a result important to practical applications. We attribute this effect of charge stabilization to the formation of new energetically deep traps on the modified surface. Decrease in molecular mobility, due to attachment of new chemical structures to the surface macro molecules, may also contribute to the overall growth of the charge stability.

Stabilization of positive charge on polytetrafluoroethylene electret films treated with titanium-tetrachloride vapor

The surface of polytetrafluoroethylene films was treated with titanium-tetrachloride vapor. The treatment was carried out in a flow reactor by means of molecular-layer deposition, a method from the arsenal of chemical nanotechnology. X-ray photoelectron spectroscopy reveals that such a treatment results in considerable changes in the chemical composition at and near the surface of the fluoropolymer film. Both, defluorination and oxidation of the surface were observed. At the same time, samples treated with titanium tetrachloride show a significant enhancement in the thermal stability of the positive homocharge. The thermally stimulated surface-potential-decay curves were observed to shift to higher temperatures by more than 100 °C

Treatment with orthophosphoric acid enhances the thermal stability of the piezoelectricity in low-density polyethylene ferroelectrets

Ferroelectrets have been fabricated from low-density polyethylene (LDPE) films by means of a template-based lamination. The temperature dependence of the piezoelectric d33 coefficient has been investigated. It was found that low-density polyethylene ferroelectrets have rather low thermal stability with the piezoelectric coefficient decaying almost to zero already at 100 °C. This behavior is attributed to the poor electret properties of the polyethylene films used for the fabrication of the ferroelectrets. In order to improve the charge trapping and the thermal stability of electret charge and piezoelectricity, LDPE ferroelectrets were treated with orthophosphoric acid. The treatment resulted in considerable improvements of the charge stability in LDPE films and in ferroelectret systems made from them. For example, the charge and piezoelectric-coefficient decay curves shifted to higher temperatures by 60 K and 40 K, respectively. It is shown that the decay of the piezoelectric coefficient in LDPE ferroelec...

Laminated tubular-channel ferroelectret systems from low-density polyethylene films and from fluoroethylene- propylene copolymer films - A comparison

A template-based lamination technique for the manufacture of ferroelectrets from uniform electret films was recently reported. In the present work, this technique is used to prepare similar ferroelectret structures from low-density polyethylene (LDPE) films and from fluoro-ethylene-propylene (FEP) copolymer films. A comparative analysis of the pressure-, temperature-, and frequency-dependent piezoelectric properties has been performed on the two ferroelectret systems. It is observed that the FEP ferroelectrets exhibit better piezoelectric responses and are thermally more stable. The difference between the piezoelectric d33 coefficients of the two ferroelectret systems is partially explained here by their different elastic moduli. The anti-resonance peaks of both structures have been investigated by means of dielectric resonance spectroscopy and electroacoustic sound-pressure measurements. A difference of more than 10 kHz is observed between the anti-resonance frequencies of the two ferroelectret systems.

Enhanced electret charge stability on polyethylene films treated with titanium-tetrachloride vapor

Low-density polyethylene (LDPE) films have been treated with titanium-tetrachloride vapor by means of the molecular-layer-deposition method. It is shown that such a treatment leads to a considerable improvement of the electret properties for both positively and negatively charged films. The temperature stability of the electret homo-charge has been increased by approximately 60°C. At the same time, the temporal stability of charge is also considerably improved. Modified low-density polyethylene films show no “cross-over phenomenon” when charged to higher voltages. Thus, it is now possible to produce electrets from polyethylene films with high initial charge densities, but without a strongly reduced charge stability. The influence of a chemical treatment with titanium-tetrachloride vapor on charge injection from aluminum electrodes into polyethylene films was also investigated. It is found that the interface between an aluminum electrode and a modified LDPE surface layer has different injection properties for positive and negative charges. Electrons can be injected across the modified interface, whereas injection of holes is either very limited or non-existent.

Unipolar ferroelectrets - Following the example of the electret microphone more closely

We report first investigations on a ferroelectret transducer material that is based on the same layer sequence as a simple electret microphone. Only one electrically charged polymer film is employed instead of two as in previous tubular-channel ferroelectrets. The new materials design has major advantages such as a reduced number of polymer films to be processed and a reduced amount of expensive electret material, as well as the possibility to choose the polarity of the charged layer for optimal stability. A drawback seems to be a lower piezoelectric coefficient, since the values obtained in our preliminary experiments amount to roughly half of those found on bipolar ferroelectrets made from two polymer-electret films. Nevertheless, the d33 values observed on our samples are still twice as high as those of polyvinylidene fluoride (PVDF) and can be further improved, e.g. through modifications of the ferroelectret geometry, the layer sequence and the electret properties or via multi-layer stacking as already demonstrated for polypropylene piezoelectrets.

Stabilization of positive charge on FEP electret films modified with titanium-tetrachloride vapor: Formation of a two-dimensional nanodielectric?

It has been shown recently that the stability of positively corona-charged polytetrafluoroethylene (PTFE) electrets is considerably enhanced as a result of a surface treatment with titanium-tetrachloride (TiCl4) vapor. In the present study, we investigate the influence of a gas-phase treatment with TiCl4 on the electret properties of tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films. FEP-film samples were treated with titanium-tetrachloride vapor by means of a nanotechnological chemical process that is based on molecular-layer deposition. Subsequently, the samples were poled in a positive corona discharge at room temperature. A very significant improvement of the surface-charge stability has been observed on the modified FEP films. The phenomenon is not restricted to the thermal stability of the surface charge, but is also found in long-term charge-decay measurements. Typical relaxation times for positive charge under isothermal conditions are at least one order of magnitude higher after the surface modification. Thermally-stimulated-decay data indicate that the half-value temperatures of the vapor-treated samples have been enhanced to almost 200°C. Thus, the thermal stability of positive charge in modified FEP films may reach values that were so far only obtained on negatively charged FEP electrets. The results are discussed in connection with the formation of deep traps on the chemically modified polymer surfaces and in the context of two-dimensional surface-layer nanodielectrics.

Molecular level materials design for improvements of actuation properties of dielectric elastomer actuators

Dielectric elastomer actuators are soft electro-mechanical transducers with possible uses in robotic, orthopaedic and automotive applications. The active material must be soft and have a high ability to store electrical energy. Hence, three properties of the elastic medium in a dielectric elastomer actuator affect the actuation properties directly: dielectric constant, electric breakdown strength, and mechanical stiffness. The dielectric constant of a given elastomer can be improved by mixing it with other components with a higher dielectric constant, which can be classified as insulating or conducting. In this paper, an overview of all approaches proposed so far for dielectric constant improvement in these soft materials will be provided. Insulating particles such as TiO2 nanoparticles can raise the dielectric constant, but may also lead to stiffening of the composite, such that the overall actuation is lowered. It is shown here how a chemical coating of the TiO2 nanoparticles leads to verifiable improvements. Conducting material can also lead to improvements, as has been shown in several cases. Simple percolation, relying on the random distribution of conducting nanoparticles, commonly leads to drastic lowering of the breakdown strength. On the other hand, conducting polymer can also be employed, as has been demonstrated. We show here how an approach based on a specific chemical reaction between the conducting polymer and the elastomer network molecules solves the problem of premature breakdown which is otherwise typically found.

Materials science on the nano-scale for improvements in actuation properties of dielectric elastomer actuators

We discuss various approaches to increasing the dielectric constant of elastomer materials, for use in dielectric elastomer actuators. High permittivity metal-oxide nano-particles can show elevated impact compared to larger size particles, but suffer from water uptake. Composites with conducting particles lead to extremely high permittivity caused by percolation, but they often suffer early breakdown. We present experiments on approaches combining metal-oxides and metal particles, which compensate for the drawbacks, and may lead to useful DEA materials in which all relevant properties are technologically useful. The key seems to be to avoid percolation and achieve a constant nearest-neighbor separation.

Nano-scale materials science for soft dielectrics: Composites for dielectric elastomer actuators

Electro-mechanical transducers based on soft elastomers can be improved by suitable manipulation of dielectric properties, mechanical stiffness and electric breakdown strength. The dielectric constant of an elastomer can be improved by mixing with other components with a higher dielectric constant, and both insulating or conducting filler materials may be employed. We present our results on insulating nanoparticulate TiO2 with various chemical modifications, which may lead to devices with improved properties. Conducting nanoparticles such as carbon black may lead to percolation-related enhancement, though with strongly detrimental side effects. On the other hand, a “molecular composite” approach, in which the conducting nanoparticles are grafted chemically to the backbone, appears valuable.