GB2142494A - Methods of recording information in and reading information from a ferroelectric polymer material - Google Patents

Abstract

Information can be stored in a ferroelectric polymer memory in a polarized-and-non-polarized or depolarized-and-polarized pattern and the information thus stored can be retrieved by sweeping the memory, for example by using a light or heat source, and detecting discrete polarizations or depolarizations one after another. <IMAGE>

Description

SPECIFICATION Methods of recording pieces of information in and reading pieces of information from a ferroelectric polymer material This invention relates to methods of recording pieces of information in and reading pieces of information from a ferroelectric polymer material.

Japanese Patent Application Laid-Open No. 55 126905 discloses that pieces of information can be stored by subjecting selected areas of ferroelectric polymer sheet to an electric field for local polarizations, and that the so-stored pieces of information are read by using the pyroelectric effect, which is caused by application of heat to the ferroelectric polymer sheet or by exposure of the ferroelectric polymer sheet to light. Also, it describes a vinylidene fluoride-and-trifluoroethylene copolymer (viny lidene of 80 to 30 mole % and fluoroethylene of 20 to 70 mole %) as a ferroelectric polymer material appropriate for the purpose of providing a memory.

According to a first aspect of the present invention, there is provided a method of recording pieces of information in a ferroelectric polymer material comprising the steps of laying a photoconductive layer on the ferroelectric polymer material, applying an electric voltage across the lamination of the photoconductive layer and the ferroelectric polymer, and projecting a beam of light to a selected part of the lamination for polarization to produce recorded pieces of information in a polarized-andnonpolarized pattern.

According to a second aspect of the present invention, there is provided a method of recording pieces of information in a ferroelectric material comprising the steps of polarizing the ferroelectric polymer material and controlling the temperature of a selected part of the material to remove polarization from said selected part of the material, thereby recording pieces of information in a depolarizedand-polarized pattern.

According to a third aspect of the present invention, there is provided a ferroelectric polymer material bearing pieces of information which have been recorded substantially by the method as defined in either of the two preceding paragraphs.

According to a fourth aspect of the present invention, there is provided a method of reading pieces of information from a ferroelectric polymer material bearing said pieces of information in a polarized-and-nonpolarized or depolarized-andpolarized pattern comprising the steps of raising the temperature of selected parts of the material one after another and determining the spatial distribution of polarizations in the material.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 represents the hysteresis loop of a ferroelectric polymer material, showing how the polarization of the material varies with the strength of an electric field; Figure 2 shows the polarization versus tempera ture characteristics of the ferroelectric polymer material.

Figure 3 illustrates in a graph how quickly the ferroelectric polymer can be polarized in response to application of an electric field; Figures 4 and 5 show a memory unit including a ferroelectric polymer sheet and one mode of storing pieces of information in the memory unit in a polarized-and-nonpolarized pattern; Figure 6 shows another memory unit including a ferroelectric polymer sheet and another mode of storing pieces of information in the memory unit in a depolarized-and-polarized pattern; and Figures 7, 8 and 9 show different systems for retrieving pieces of information from the memory units.

Figure 1 shows how the degree of polarization of vinylidene fluoride-and-trifluoroethylene copolymer varies with the strength of an electric field applied to the material and Figure 2 shows how the degree of polarization varies with the surrounding temperature.

It was previously not known that a ferroelectric polymer material is quick to polarize when subjected to a varying electric field, and therefore it was not thought practical to use such material for the purpose of storing and reading pieces of information. Furthermore, the use of such material as a memory was supposed to require numerous discrete electrodes on the surface of the single piece of ferroelectric polymer for storing pieces of information, thus causing inconvenience in handling.

This invention is based on the discovery of the quick polarization of a vinylidene fluoride-based polymer, particularly a vinylidene fluoride-andtrifluoroethylene copolymer in response to application of an electric field as illustrated in Figure 3.

The term 'vinylidene fluoride-based polymer' should be understood as including a homopolymer of vinylidene fluoride and a copolymer containing vinylidene fluoride of 50 or more mole %. Some examples of such copolymer are: vinylidene fluoride-and-trifluoroethylene copolymer; vinylidene fluoride-and-hexafluoropropylene copolymer; vinylidene fluoride-and-trifluoroethylene-andmonochlorotrifluoroethylene copolymer; and vinylidene fluoride-and-trifluoroethylene-and hexafluoropropylene copolymer.

Among these copolymers a vinylidenefluorideand-trifluoroethylene copolymer is the most preferable for use with the present method.

Referring to Figures 4 and 5, there is shown how pieces of information can be recorded in a memory unit by polarization. A transparent electrode 4, a memory element 1, a photo-conductive or photoelectric film 2 whose electric resistance will reduce when exposed to light, and another transparent electrode 4' are laid on each other on a transparent substrate 5, for instance made of glass, in the order named. The transparent electrodes 4 and 4' are adapted to have an electric voltage applied therebetween. In storing pieces of information, first, a flash lamp 6 is lit to heat the lamination, thereby instantly raising the temperature of the lamination to the temperature "T1" as shown in Figure 2. Then, the whole upper surface of the lamination is cleared off, leaving no polarization thereon.

An electric voltage is applied between the transparent electrodes 4 and 4', and a beam of light 3, for instance a laser beam, is projected to a selected part "S" of the upper surface of the lamination so that the part of the photoconductive film 2 just below the selected part "S" of the transparent electrode 4' turns electrically conductive. Thus, the selected part S' of the memory element 1 sandwiched between the transparent electrode 4 and the selected and now electrically conductive part S of the photoconductive film 2 is subjected to an electric field as strong as E1 in Figure 1. Then, the selected part S' of the memory element 1 is polarized, remaining in the state of polarization even after removing the beam of light and the electric field, and retaining the piece of information in the form of discrete polarization.

As an alternative to the recording in the form of polarization just described the recording in the form of depolarization is possible and is described below with reference to Figure 6.

A transparent electrode 4, a memory element 1 and another transparent electrode 4' are laid on each other on a transparent substrate Sin the order named, thus making up a lamination 9. In erasing or clearing off the lamination 9, first, an electric voltage is applied between the transparent electrodes 4 and 4', thereby subjecting the memory element 1 to the electric field E1 to polarize the whole body of the memory element 1. As an alternative a corona discharge may be used to polarize the memory element 1, in which case no upper electrode would be required.In recording pieces of information an impulse beam of light 3 is projected to a selected part of the memory element 1 to bring instantly the temperature of the selected part of the memory element up to the temperature T1, thereby depolanz- ing the selected part of the memory element 1. Thus, pieces of information are stored in the form of discrete depolarizations.

The pieces of information thus stored can be read in different ways as follows (a) A memory element bearing pieces of informa tion in the form of a polarized-and-nonpolarized (or depolarized-and-polarized) pattern is exposed to light, and the phase difference a between the light passing through the polarizations (or depolarizations) of the memory element and the light passing through the nonpolarizations (or polarizations) of the memory element is determined from the interference of these lights, and finally the information bearing pattern is determined; (b) A beam of light is projected to the memory element, and the polarized-and-nonpolarized (or depolarized-and polarized) pattern is determined in terms of refraction of light at the boundaries between discrete polarizations (or depolarizations) and nonpolarizations (or polarizations);; (c) A beam of polarized light is projected to a memory element which is inclined with respect to the beam of polarized light, and elliptically polarized light so produced at each discrete polarization is detected with the aid of a photodetector, thereby determining the distribution of polarizations; or (d) A memory is swept across with a moving thermal spot, and then the pyroelectric polarization caused thereby is detected in terms of electric current, which is measured by a detector connected between opposite electrodes sandwiching the mem ory element. Thus, spatial distribution of polariza tions representing pieces of information is converted into the time-sequential distribution in the form of electric current.

The present recording methods permit the use of a light spot for quick and well defined recording, thus requiring no electrodes. Also, the present recording methods permit the practiical use of large-capacity memory disks of a ferroelectric polymer material.

Figure 7 shows how pieces of information can be stored in and retrieved from a large-capacity mem ory disk 8, which is composed of a lamination such as that shown in Figures 4 and 5 or 6. As shown, a beam of light 3 is projected to write pieces of information in the lamination disk 8 whereas another beam of light 13 is projected, and is detected by a detector 14 after passing through the lamination thereby reading the pieces of information.

Figure 8 shows another mode in which pieces of information can be stored in and retrived from a memory lamination 8. As shown, an image 23 to be stored is exposed to an incoherent light 22, and the reflection of light is projected to the memory lamination 8 after passing through a convex lens 24, thus storing the image 23 on the memory lamination 8. In reading out the pieces of image information thus stored, a beam of coherent light 21 is converted to a polarized light by passing through a polarizer 11, and the polarized light 25 is projected to the memory lamination 8 so that the elliptical polarization pas sing through the polarized bits of the memory lamination is detected by a photodetector 12, there by providing an image of coherent light.

Figure 9 shows still another mode of reading pieces of information from a memory lamination 18, which is used, for instance as a cash-withdrawal card. The card 18 is displaced in the direction as indicated by the arrow while the card 18 is heated by a beam of light 3, and then a pyroelectric current representing the degree of each local polarization flows between a pair of opposite rolls 16 and 17, and is amplified by an amplifier 15 to appear at an output terminal OUT. In place of the beam of light for heating the lamination the rolls 16 and 17 may be heated to constitute a heat source.

As is apparent from the above, the present methods use a beam of light for writing and reading pieces of information, this obviating the necessity of electrodes which otherwise would be required.

Because of the use of photoconductive layers, recording is possible even with a faint light. Furthermore, the practical use of a ferroelectric polymer material is feasible as a memory element. This material is very cheap, and is insensitive to the surrounding magnetic field. Therefore, the lamination may be safely used or retained in the vicinity of magnets, which are used in many articles for daily use such as television sets.

Claims (8)

1. A method of recording pieces of information in a ferroelectric polymer material comprising the steps of laying a photoconductive layer on the ferroelectric polymer material, applying an electric voltage across the lamination of the photoconductive layer and the ferroelectric polymer, and project ing a beam of light to a selected part of the lamination for polarization to produce recorded pieces of information in a polarized-and nonpolarized pattern.

2. A method of recording pieces of information in a ferroelectric polymer material comprising the steps of polarizing the ferroelectric polymer material and controlling the temperature of a selected part of the material to remove polarization from said selected part of the material, thereby recording pieces of information in a depolarized-and-polarized pattern.

3. A method according to claim 1 or 2, wherein said material is a vinylidene fluoride homopolymer or a vinylidene fluoride containing a copolymer.

4. A method according to claim 2 or claim 3 as appendant to claim 2, wherein the temperature controlling step includes projecting a beam of light to said selected part of the material to raise the temperature thereof thereby to depolarize said selected part of the material.

5. A method of recording pieces of information in a ferroelectric polymer material, substantially as hereinbefore described with reference to Figures 4 and 5, or Figure 6, or Figure 7, or Figure 8 of the accompanying drawings.

6. Aferroelectric polymer material bearing pieces of information which have been recorded by the method according to any preceding claim.

7, A method of reading pieces of information from a ferroelectric polymer material bearing said pieces of information in a polarized-and nonpolarized or depolarized-and-polarized pattern comprising the steps of raising the temperature of selected parts of the material one after another and determining the spatial distribution of polarizations in the material.

8. A method of reading pieces of information from a ferroelectric polymer material, substantially as hereinbefore described with reference to Figure 7, Figure 8 or Figure 9 of the accompanying drawings.