RU2523337C1 - Method of film electret production - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method involves application of fluoropolymer layer onto metal electrode, application of discrete layer consisting of isolated nano-sized aggregates out of titan-containing nanostructures onto fluoropolymer surface, and further electreting in positive corona discharge. Before application of titan-containing nanostructures, fluoropolymer surface is triboelectrified by a dielectric to communicate negative charge to the surface.

EFFECT: enhanced magnitude and stability of positive charge surface density in film fluoropolymer electret.

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Description

The invention relates to the field of manufacturing technologies for film electrets. In recent years, there has been a dynamic growth in the use of polymer film electrets in modern high-tech devices and technologies. The field of application of polymer film electrets is constantly expanding - from well-known technical applications (such as electret microphones, piezoelectric and pyroelectric transducers and sensors) to innovative developments in the field of microsystem engineering, nonlinear optics and molecular biology.

The most important factors determining the effectiveness of the practical use of polymer film electrets are the magnitude and stability of the electret charge formed in them. First of all, we mean temporary stability, the criterion of which is sometimes used by the parameter τ (electret lifetime) - the time during which the effective surface charge density of the electret σ decreases by a factor of e. For polymers with pronounced electret properties, typical values of the parameter τ under normal conditions range from several days to several years. However, the lifetime of electrets decreases sharply as the effective surface density of the charge accumulated in them increases. Therefore, the criterion for the quality of the electret is also the value of the stable residual surface charge density σ _st . Typical values of stable residual charge density σ _st , which actually can be obtained in practice, are, as a rule, values of the order of 10 ^-6 -10 ^-5 C / m ² , rarely reach values of 10 ^-4 -10 ^-3 C / m ² .

Along with the stability of the surface charge density over time, the most important integrated characteristic of electrets is the thermal stability of the charge, which, firstly, determines the nominal temperature operating conditions of electret materials, and secondly, indicates the maximum temperatures to which short-term heating of such materials is allowed without significant decline electret charge.

The formation of a stable electret charge in polymer film materials usually involves their processing in an electric field (electretization). The most effective and technologically advanced method of electretization at present is the method of charging polymer films in a corona discharge. In some cases, substances capable of increasing the polarization effects in the polymer are introduced (deposited) into the polymer matrix or onto the film surface.

A known method of manufacturing electrets by condensing vapors on a dielectric product, followed by drying [1]. A method for electrifying a non-woven dielectric fabric by impacts of jets of water under pressure with subsequent drying [2]. The disadvantages of these methods:

- they are varieties of contact electrification methods for dielectric films [3], which are characterized by a low value and stability of the surface charge density of the obtained electrets. In addition, these methods do not provide uniformity and reproducibility of the distribution of surface charge density over the surface of manufactured electrets and are generally not technological.

- they lack direct information about the magnitude and stability of the electret charge, and the electret properties of the resulting objects are judged indirectly (by increasing the filtration efficiency of filtering systems based on them), which does not allow to predict the magnitude and stability of the electret charge.

- these methods do not allow to produce electrets from fluoropolymers with a positive charge, since fluoropolymers are the most electronegative dielectrics and are charged only with a negative charge in contact electrization [3].

For the manufacture of film electrets with a positive charge from fluoropolymers, corona charging is usually used. However, electrets obtained in this way are characterized by low stability of a positive charge [4]. In order to increase the stability of the positive charge in fluoropolymers, a number of methods have been proposed [5, 6].

So, in the patent [5], in order to stabilize the positive charge, they charge in the positive corona discharge at elevated temperatures. The specified method can significantly increase the thermal stability of a positive charge, however, it has the following disadvantages:

- low surface density of the electret charge (only 0.2 mC / m ² );

- the impossibility of reliable control of the initial value of the electret charge;

- there are no experimental data on the long-term stability of the charge (only the calculated values of the electret lifetime τ are indicated).

The prototype of the invention is a method of manufacturing a film electret [6], comprising applying a fluoropolymer layer to a metal electrode, applying a discrete layer to the fluoropolymer surface consisting of nanoscale aggregates of titanium-containing nanostructures isolated from each other, and subsequent electrification in a positive corona discharge. The essence of this technical solution lies in the fact that nanoscale aggregates of titanium-containing nanostructures on the surface of a fluoropolymer film are energetically deep traps for a positive charge. In addition, titanium-containing nanostructures significantly reduce the mobility of surface macromolecules, which together leads to an increase in the stability of the surface density of the positive charge imparted to the fluoropolymer during electretization. As a result, it is possible to obtain electrets with a stable surface charge density σ _st up to 1.44 mC / m ² . This creates the conditions for the use of such electrets in devices where, in addition to negative charges, positive charges must also have acceptable stability.

The disadvantages of the prototype:

- insufficient amount of stable surface charge density of the electret;

- insufficient thermal stability and stability over time of the electret charge.

The purpose of the invention is to increase the magnitude and stability of the surface density of a positive charge in film fluoropolymers.

The choice of fluoropolymers as an object for the implementation of the proposed method is due to the following considerations. To date, fluoropolymers have the highest electret characteristics. It is fluoropolymers that are actually used in the mass production of electrets. And finally, it is on their basis that there are real prospects for creating the latest technical devices. For example, bipolar electret microphones and a new class of piezoelectric sensors with a giant piezoelectric module (up to 1000 pC / N) - “ferroelectrets”. The constraining factor for creating such devices is the insufficient size and stability of the positive charge in fluoropolymers (recall that negative charges in fluoropolymers are very stable). Therefore, the development of methods for the manufacture of film electrets from fluoropolymers bearing a stable positive charge is an urgent task.

The desired technical result is achieved due to the fact that in the known method of manufacturing a film electret before applying a discrete layer to the surface of a fluoropolymer, consisting of nanoscale aggregates of titanium-containing nanostructures isolated from each other, the surface of the fluoropolymer is triboelectrified with a dielectric counterbody, reporting a negative charge.

The essence of the invention lies in the fact that the triboelectrification of the surface of the fluoropolymer dielectric counterbody leads to:

1) mechanical destruction and partial removal of physically sorbed contaminants on the surface of the polymer film;

2) the formation of a uniform distribution of microroughnesses on the surface of the polymer film;

3) the accumulation of negative tribo charges on the surface of the polymer film.

All three of these factors increase the reactivity of the surface of the fluoropolymer. As a result, upon subsequent deposition of a discrete layer of isolated nanoscale aggregates of titanium-containing nanostructures onto the surface of a triboelectrified fluoropolymer by molecular layering [7], a larger number of nanostructures are grafted onto the surface macromolecules. So, according to x-ray photoelectron spectroscopy, the total titanium content in the discrete layer deposited by the prototype method is 2 atomic percent, then in the layer deposited by the present method, 2.5-3 atomic percent. Thus, it is possible to increase the surface concentration of energetically deep traps, and consequently the magnitude and stability of the positive charge imparted to the fluoropolymer during subsequent corona discharge electrification.

The sequence of operations when implementing the proposed method is as follows. A fluoropolymer film is applied to the metal electrode. The triboelectrification of the free surface of the fluoropolymer is carried out in the process of sliding friction, in which the dielectric counterbody makes reciprocating movements with an average speed of 8 cm / s for 10 seconds. (At a normal load in the friction pair of not more than 500 grams, the maximum negative tribo charge acquired by the fluoropolymer does not exceed ((8.2-8.8) · 10 ^-4 C / m ² ). After that, the triboelectrified surface of the fluoropolymer is treated with titanium tetrachloride vapors (TiCl ₄ ) in a flow-type reactor at a temperature of 130 ° C for 10 minutes, Then the reactor is blown with a stream of dried carrier gas (air) without supplying a reagent (TiCl ₄ ) and cooled for 5 minutes After removing samples from the reactor, they are electrified positive charge house.

List of figures

Figure 1 The results of climatic tests (at 40 ° C and 98% relative humidity) of film electrets on the stability of the surface charge density:

2 - electrets made by the prototype method;

1 - electrets made according to the claimed method.

Figure 2 The test results (with linear heating of the samples at a rate of 5 ° C / min) of film electrets for thermal stability of the surface charge density:

2 - electrets made by the prototype method;

1 - electrets made according to the claimed method.

Here are examples of the implementation of the method.

Example 1. A batch (15 pcs.) Of polymer film electrets according to the invention is made. A 13 μm thick polytetrafluoroethylene film is applied to the metal electrode, after which its free surface is triboelectrified with a dielectric counterbody, reporting a negative charge ((4-5) · 10 ^-4 C / m ² ). For this, the dielectric counterbody (polyethylene fiber), during sliding friction, made reciprocating movements with an average speed of 8 cm / s for 10 seconds at normal load in a friction pair of 500 grams. After that, the triboelectrified surface of polytetrafluoroethylene is treated with vapors of titanium tetrachloride (TiCl ₄ ) in a flow-type reactor at a temperature of 130 ° C for 10 min. Then the reactor is purged with a stream of dried carrier gas (air) without supplying a reagent (TiCl ₄ ) and cooled for 5 minutes. After removing the samples from the reactor, they are electrified with a positive charge. Electretization is carried out in a positive corona discharge in air to an initial surface charge density of 20.4 · 10 ^-4 C / m.

The results of climatic tests (40 ° C, 98% relative humidity) of such electrets for stability are shown in figure 1 - curve 1.

Example 2. A batch is made (15 pcs.) Of polymer film electrets according to the prototype method. A 13 μm thick polytetrafluoroethylene film is deposited on a metal electrode, after which a discrete layer consisting of nanoscale aggregates of titanium-containing nanostructures isolated from one another is deposited on its free surface. For this, the free surface of the film is treated with vapors of titanium tetrachloride (TiCl ₄ ) in a flow-type reactor at a temperature of 130 ° C for 10 min. Then the reactor is purged with a stream of dried carrier gas (air) without supplying a reagent (TiCl ₄ ) and cooled for 5 minutes. After removing the samples from the reactor, they are electrified with a positive charge. Electretization is carried out in a positive corona discharge in air to an initial surface charge density of 20.4 · 10 ^-4 C / m ² .

The results of climatic tests (40 ° C, 98% relative humidity) of such electrets for stability are shown in figure 1 - curve 2.

Example 3. A batch is produced (15 pcs.) Of polymer film electrets according to the invention (as in example 1).

The test results of the obtained film electrets for thermal stability (in the mode of linear heating of samples at a rate of 5 ° C / min) are presented in figure 2 - curve 1.

Example 4. A batch is made (15 pcs.) Of polymer film electrets according to the prototype method (as in example 2).

The test results of the obtained film electrets for thermal stability (in the mode of linear heating of samples at a rate of 5 ° C / min) are presented in figure 2 - curve 2.

Analysis of the results presented in figure 1 and figure 2, indicates the following.

1. The temporary stability of the surface density of a positive charge in electrets made from a fluoropolymer film according to the invention (Fig. 1, curve 1) is significantly higher than that of electrets obtained in a known manner (Fig. 1, curve 2).

2. The value of the residual (stable) surface density of a positive charge in electrets made from a fluoropolymer film according to the invention (Fig. 1, curve 1) is 1.25 times higher than that of electrets obtained in a known manner (Fig. 1, curve 2 )

3. The thermal stability of the surface density of the positive charge of electrets made from a fluoropolymer film according to the invention (figure 2, curve 1) is higher than that of electrets obtained in a known manner (figure 2, curve 2). This, in particular, can be judged by the characteristic points (indicated by arrows in FIG. 2) on the temperature dependences of the relative surface charge density of the electrets.

Thus, the aim of the invention, which is to increase the magnitude and stability of the surface density of a positive charge in electrets based on fluoropolymer films, is achieved.

Information sources

1. Patent 2260866.

2. Patent 2130521.

3. Polymer electrets //. G.A. Luschaykin. - M .: Chemistry. - 1984. - 184 p.

4. Electrets // G.M. Sessler (Ed.). - 3rd ed., Vol. 1. - Laplacian press, Morgan Hill, Ca, 1999, pp. 41-42.

5. US Patent 4,527,218.

6. A positive decision (from 10.17.12) on the grant of a patent for an application No. 20111114060 for an invention (prototype).

7. Technology of molecular layering and some areas of its application // A.A. Malygin. - Journal of Applied Chemistry. - 1996. - T. 69, No. 10. - p. 1585-1593.

Claims (1)

A method of manufacturing a film electret, including applying a fluoropolymer layer to a metal electrode, applying a discrete layer to a fluoropolymer surface consisting of nanoscale aggregates of titanium-containing nanostructures isolated from each other, and subsequent electrification in a positive corona discharge, characterized in that the fluoropolymer fluoropolymer surface is tribo-polymer before applying the titanium-containing nanostructures dielectric counterbody, reporting a negative charge.

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