US3924324A - Method of making electret - Google Patents

Method of making electret Download PDF

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Publication number
US3924324A
US3924324A US485112A US48511274A US3924324A US 3924324 A US3924324 A US 3924324A US 485112 A US485112 A US 485112A US 48511274 A US48511274 A US 48511274A US 3924324 A US3924324 A US 3924324A
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United States
Prior art keywords
film
charge
electret
charged
making
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Expired - Lifetime
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US485112A
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English (en)
Inventor
Yoichi Kodera
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Sony Corp
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Sony Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G7/00—Capacitors in which the capacitance is varied by non-mechanical means; Processes of their manufacture
    • H01G7/02—Electrets, i.e. having a permanently-polarised dielectric
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G7/00—Capacitors in which the capacitance is varied by non-mechanical means; Processes of their manufacture
    • H01G7/02—Electrets, i.e. having a permanently-polarised dielectric
    • H01G7/021—Electrets, i.e. having a permanently-polarised dielectric having an organic dielectric
    • H01G7/023—Electrets, i.e. having a permanently-polarised dielectric having an organic dielectric of macromolecular compounds
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T29/00—Metal working
    • Y10T29/49—Method of mechanical manufacture
    • Y10T29/49002—Electrical device making

Definitions

  • ABSTRACT [51] Int. Cl. H01S 4/00 A method of making an electret by annealing a corona [58] Field of Search 29/592, 592 E; 317/262 A; charged dielectric film on a metal plate is disclosed.
  • This invention relates generally to a method of making an electret, and more particularly is directed to a method of making an electret with a single electric charge or a monocharge electret.
  • a thin film electret is used as an oscillating plate or vibrating diaphragm of a microphone, speaker or the like.
  • a high polymer film such as Teflon FEP (Trade name) consisting of a copolymer of tetrafluoro ethylene-hexafluoro propyrene or Teflon TEF (Trade name) is used.
  • thermo-electret method a method for producing an electric charge on a dielectric material.
  • a dielectric material is gripped between two electrodes and subjected to heating process to make it as an electret by a heterocharge, which is known as a socalled thermo-electret method.
  • a heterocharge which is known as a socalled thermo-electret method.
  • an electret which is charged with positive and negative charges on its both surfaces, is used as an electrode layer is attached to one surface of the electret and the electric charge produced on the other surface thereof is utilized.
  • an electret produced by the above prior art method is employed in an ear-speaker, two of such electrets are used in such a manner that they are mounted on fixed electrodes or back electrodes with apertures to face the charges of the electrets of the same polarity or sign, and an electric conductive vibrating diaphragm is located between the two electrets at their central portions from a spacial point of view to vibrate'the vibrating diaphragm with an input signal applied to an input termi-,
  • a method of making an electret in which a high polymer film with no electrode is placed on a metal table and is charged with a negative corona, and then the high polymer film is subjected to an annealing treatment to be an electret with an electric charge of a single sign or a monocharge electret.
  • the film is peeled off from the metal table once, and the film is mounted on the table ,with its surface, which is charged with a positive charge, being faced to the surface of the table, and then the film is subjected to the annealing treatment to carry out a charging process with high efficiency for a short time period.
  • a negative corona discharge is applied to a non-polar high polymer film disposed on a metal table, and thereafter the film is annealed to obtain a mono-charge electret film.
  • a stylus electrode is disposed apart from the surface of the film by 15mm and supplied with a voltage of 4 to 8KV (kilovolts).
  • a voltage of 4 to 8KV kilovolts
  • the electret is, at it is, annealed at a temperature of about C to have a negative surface charge density of l09 C/cm and then peeled off from the metal table to be a monocharge electret film with a net charge as maintained.
  • a monocharge electret film can be obtained for a short time period.
  • FIGS. 1A to IE, inclusive are diagrams showing a process of one of the methods according to this invention.
  • FIGS. 2A to 2C, inclusive, are diagrams showing a process of another of the methods of this invention.
  • FIGS. 3 and 4 are respectively schematic theoretical diagrams showing head-phones used for the explanation of the invention.
  • FIG. 5 is a graph showing the surface charge densitytime lapse characteristics used for the explanation of the invention.
  • the two electrets 1 and 2 are used in a single transducer, and upon the manufacture thereof two dielectric films to be made as the electrets 1 and 2 are attached to the back electrodes 4 and 5, respectively, which is thereafter subjected to an electret treatment.
  • an electret treatment due to the type of material used in the back electrodes. That is, there are problems of the distortion of the back electrodes and the deterioration of the adhesive used to attach the dielectric films to the back electrodes which may be caused by the heating during the electret treatment.
  • a vibrating diaphragm is made as an electret.
  • the vibrating diaphragm must have a monocharge that is, an electric charge of a single polarity so as to be operative as a vibrating diaphragm.
  • a monocharge that is, an electric charge of a single polarity so as to be operative as a vibrating diaphragm.
  • the vibrating diaphragm 7 which is made to be an electret to have, for example, only a negative charge, is centrally disposed and the back electrodes 4 and 5, each of which has formed therethrough a plurality of apertures 3, are disposed at the both sides of the diaphragm 7 in opposed relation with each other.
  • the headphone becomes simple in construction.
  • a dielectric film or a high polymer film 10 to be made as an electret is mounted on a metal table or plate 1 1 and a stylus electrode 12 is positioned above the high polymer film 10 apart from its upper surface 10A by a predetermined distance d.
  • a predetermined high voltage is applied across the stylus electrode 12 and the metal plate 11 from a high voltage source 13 with the metal plate 11 being connected to a positive electrode while the stylus electrode 12 being connected to a negative electrode to carry out a corona discharge between the stylus electrode 12 and the high polymer film 10 and to charge the high polymer film 10 with negative charge on its upper surface 10A.
  • the high polymer film 10 is peeled off from the metal plate 11. At this time, a discharge occurs between a peeled off surface 10B of the polymer film 10 and an upper surface 11A of the metal plate 11, and hence the surface 10B of the high polymer film 10 is charged with a positive charge.
  • the high polymer film 10 is charged with negative and positive charges on its surfaces 10A and 10B, respectively.
  • the high polymer film 10 is again placed on another metal plate 14, with the surface 10B having the positive charge being in contact with the plate, and is heated in a furnace 15 for a constant time period and then cooled.
  • the metal plate 14 is grounded, as shown in FIG-1D.
  • reference numeral 16 indicates a heater.
  • the heating temperature is selected higher than the glass transition temperature of the high polymer film 10, for example, more than C for the case that the high polymer film 10 is made of Teflon FEP. If the heating temperature is too high, there may be a fear that the film is distorted or molten. Accordingly, it is necessary that the heating temperature is selected lower than the melting point of the film.
  • an electret film 17 which is charged with the negative charge on its both surfaces 10A and 10B is produced as shown in FIG. 1E.
  • the surface 10B which is charged with the positive charge before heating, is charged with the negative charge by suitably selecting the time interval of the heating, while the amount of the negative electric charge stored in the surface 10A before the heating is decreased somewhat, but the surface 10A has still the negative charge, that is, both the surfaces 10A and 10B carry the negative charge.
  • the amount of initial charge by the corona discharge is much greater than can be at- .tained by the thermo electret method, and the charge where 0 represents the charge density and s the dielectric constant in vacuum.
  • FIGS 2A to 2C' show another'niethod of the invention.
  • the high "polymer film to be made as an electretis mounted on the metal plate 11 and the stylus electrode 1.2 is locate'dabove.
  • the film 10 apart from its upper surface 10A by a predetermined distance in opposed relation-Apredetermined high voltage is applied across the .stylus electrode 12 and the metal plate 11 from the high voltage source 13 with the metal plate 11 being positivenand the stylus I electrode 12 being'negativezto produce a 'negativeelec; tric charge on the upper surface 10A of thezfilm 10 by the corona discharge (refer to,F-IG.. 2A-).
  • EXAMP A Teflon FEP ioof ao After the corona discharge is finished; thefilm' is peeled off from the metal plate 1 l'a nd' placed onthe electrode of an electric charge measuringapparatusTAfter measuring the charge amount of the film, it is8 X 10 C/cni at its portion opposing the stylus electrode 12. After turning over the film'ahdnieas'urin'g the charge ambunt on the surfaceopposite to the discharged surface, it is+7;8 X' 1 0 Clem. 'Aftertlie measurement, 45
  • the film is mounted on the metal plate with its positive charged s'urfaceffacin'g the plategand heated at 155C for'a'timeintefval of te'r'i minutes/When the charge amount of the film after heating is measured, it is l .9
  • EXAMPLE II of 30 seconds. After the corona discharge is finishedf the film is peeled off from the metal plate 11. Then, the film is placed on the second metal plate 14 with its corona-discharged surface facing upward and heated at traflu'oroe- 160C fora-time interval of 20 minutes. It is ascertained by the aforedescribed measurement that the charged amount after heating is 4.7 X 10 C/cm on the corona-discharged:surface and 4.0 X 10' C/cm 5 on the surface that was-in-contact with the metal plate, that-is, both the surfaces are charged negatively. The change in the amount of the charge of the film with respect to the time lapse is shown in FIG.
  • the corona-discharge is finished, the film is heated at 150C for a time interval of 1.5 hours without :the film being-peelef offfrom the metal plate 11. It is ascertained that the charge amount of the film after i-lieatingi's --11 .3. X 10" Clcrn on its corona-discharged surface and .2.4 X 10 Clcm on the surface which was in contact with the metal plate, both surfaces thus *negatively'charged.
  • the amount of the charge is 5.5 X 10" to 7 X 10 its reproducibility is deteriorated.
  • a Teflon FEP film is used as a high polymer film, but even if a Teflon TFE film, a Tefzel film and the like are used as the high polymer film, an electret film with a single sign of an electric charge can be also obtained.
  • an electret having a monocharge that is, a single sign of an electric charge such as a negative electric charge
  • an apparatus for performing the method of this invention requires no complete redesign and thus can be the apparatus used for making the prior art electret.
  • a method of making a mono-charge electret comprising the steps of charging a dielectric film which is mounted on a metal electrode by subjecting said film to a corona discharge to thereby charge one surface of said film with the polarity of said corona discharge; peeling off said charged dielectric film from said electrode; placing said dielectric film on a metal backing with the other surface of said film in contact with said metal backing; and, vwhile said film is on said metal backing, heating said dielectric film.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Treatments Of Macromolecular Shaped Articles (AREA)
  • Electrostatic, Electromagnetic, Magneto- Strictive, And Variable-Resistance Transducers (AREA)
US485112A 1973-07-05 1974-07-02 Method of making electret Expired - Lifetime US3924324A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
JP7602773A JPS5650408B2 (ja) 1973-07-05 1973-07-05

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US3924324A true US3924324A (en) 1975-12-09

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US (1) US3924324A (ja)
JP (1) JPS5650408B2 (ja)
CA (1) CA1008187A (ja)
DE (1) DE2432377C2 (ja)
FR (1) FR2236259B1 (ja)
GB (1) GB1445943A (ja)
NL (1) NL181062C (ja)

Cited By (30)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4127898A (en) * 1976-06-18 1978-11-28 Battelle-Institute E.V. Storage element for an erasable, digital permanent storage
US4127681A (en) * 1976-09-24 1978-11-28 Pennwalt Corporation Single electrode poling of dielectric films
US4248808A (en) * 1978-12-29 1981-02-03 Bell Telephone Laboratories, Incorporated Technique for removing surface and volume charges from thin high polymer films
US4308223A (en) * 1980-03-24 1981-12-29 Albany International Corp. Method for producing electret fibers for enhancement of submicron aerosol filtration
US4346505A (en) * 1979-01-04 1982-08-31 Thomson-Csf Method of fabricating piezoelectric transducer with polymer element
WO1982004346A1 (en) * 1981-06-08 1982-12-09 Western Electric Co Stable positively charged electrets
US4365283A (en) * 1980-10-16 1982-12-21 Pennwalt Corporation Corona discharge poling process
US4397702A (en) * 1980-01-09 1983-08-09 Johnson Controls, Inc. Fabrication of non-conductive charged sensing probe unit
US4407852A (en) * 1979-10-19 1983-10-04 Sapieha Slawomir W Electrets from plasma polymerized material
US4451736A (en) * 1982-04-16 1984-05-29 Wisconsin Alumni Research Foundation Method and apparatus for measuring air ion concentrations
US4513049A (en) * 1983-04-26 1985-04-23 Mitsui Petrochemical Industries, Ltd. Electret article
US5045747A (en) * 1990-02-28 1991-09-03 Industrial Technology Research Institute Apparatus for poling a piezoelectric ceramic
EP0447166A2 (en) * 1990-03-12 1991-09-18 Mitsui Petrochemical Industries, Ltd. Process for producing an electret, a film electret, and an electret filter
US5652609A (en) * 1993-06-09 1997-07-29 J. David Scholler Recording device using an electret transducer
WO1997039464A1 (en) * 1996-04-18 1997-10-23 California Institute Of Technology Thin film electret microphone
US5952645A (en) * 1996-08-27 1999-09-14 California Institute Of Technology Light-sensing array with wedge-like reflective optical concentrators
WO1999065277A1 (en) * 1998-06-11 1999-12-16 Microtronic A/S A method of manufacturing a transducer having a diaphragm with a predetermined tension
US20020185003A1 (en) * 2001-06-11 2002-12-12 Rochester Institute Of Technology Electrostatic filter and a method thereof
US20050254673A1 (en) * 1999-05-19 2005-11-17 California Institute Of Technology High performance MEMS thin-film teflon electret microphone
US7195393B2 (en) 2001-05-31 2007-03-27 Rochester Institute Of Technology Micro fluidic valves, agitators, and pumps and methods thereof
US7211923B2 (en) 2001-10-26 2007-05-01 Nth Tech Corporation Rotational motion based, electrostatic power source and methods thereof
US7217582B2 (en) 2003-08-29 2007-05-15 Rochester Institute Of Technology Method for non-damaging charge injection and a system thereof
US7280014B2 (en) 2001-03-13 2007-10-09 Rochester Institute Of Technology Micro-electro-mechanical switch and a method of using and making thereof
US7287328B2 (en) 2003-08-29 2007-10-30 Rochester Institute Of Technology Methods for distributed electrode injection
US7378775B2 (en) 2001-10-26 2008-05-27 Nth Tech Corporation Motion based, electrostatic power source and methods thereof
DE102011013258A1 (de) 2010-03-08 2011-12-15 Sabo Industria E Comercio De Autopecas Ltda. Bausatz Melderad
US20120043142A1 (en) * 2007-05-09 2012-02-23 Grivna Edward L Electret stylus for touch-sensor device
US8581308B2 (en) 2004-02-19 2013-11-12 Rochester Institute Of Technology High temperature embedded charge devices and methods thereof
EP2019298A4 (en) * 2006-04-27 2016-03-02 Univ Saitama Nat Univ Corp DEVICE FOR CONVERTING MECHANICAL IN ELECTRICAL AND MANUFACTURING METHOD THEREFOR
US11081285B2 (en) * 2019-05-08 2021-08-03 Deborah Duen Ling Chung Electrically conductive electret and associated electret-based power source and self-powered structure

Families Citing this family (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2643260C2 (de) * 1976-09-25 1979-02-15 Elektro-Thermit Gmbh, 4300 Essen Ortsbewegliche, tragbare Vorrichtung zum Abarbeiten von Schweißgutüberständen bei der Schienen-Verbindungsschweißung
NL7907539A (nl) * 1979-10-11 1981-04-14 Tno Werkwijze voor het vervaardigen van elektreten.
JPS62160919U (ja) * 1986-03-31 1987-10-13
ATA80098A (de) * 1998-05-12 1999-06-15 Johannes Dr Heitz Dünne elektretschichten und ein verfahren zu ihrer herstellung
JP2001122996A (ja) * 1999-10-28 2001-05-08 Bridgestone Corp フッ素樹脂の表面処理方法及び積層体の製造方法
JP2011181748A (ja) * 2010-03-02 2011-09-15 Daikin Industries Ltd 分極化樹脂フィルムの製造方法
DE102017101950A1 (de) 2017-02-01 2018-08-02 Achim Limbeck Blutstillendes Material sowie Verfahren und Vorrichtung zu seiner Herstellung

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3660736A (en) * 1970-03-10 1972-05-02 Kureha Chemical Ind Co Ltd Process for the production of high-efficient electrets
US3794986A (en) * 1971-04-08 1974-02-26 Kureha Chemical Ind Co Ltd Pyroelectric element of polymer film

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3644605A (en) * 1969-02-11 1972-02-22 Bell Telephone Labor Inc Method for producing permanent electret charges in dielectric materials
JPS5024440B1 (ja) * 1970-07-10 1975-08-15
FR2144933A5 (ja) * 1971-07-02 1973-02-16 Anvar

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3660736A (en) * 1970-03-10 1972-05-02 Kureha Chemical Ind Co Ltd Process for the production of high-efficient electrets
US3794986A (en) * 1971-04-08 1974-02-26 Kureha Chemical Ind Co Ltd Pyroelectric element of polymer film

Cited By (39)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4127898A (en) * 1976-06-18 1978-11-28 Battelle-Institute E.V. Storage element for an erasable, digital permanent storage
US4127681A (en) * 1976-09-24 1978-11-28 Pennwalt Corporation Single electrode poling of dielectric films
US4248808A (en) * 1978-12-29 1981-02-03 Bell Telephone Laboratories, Incorporated Technique for removing surface and volume charges from thin high polymer films
US4346505A (en) * 1979-01-04 1982-08-31 Thomson-Csf Method of fabricating piezoelectric transducer with polymer element
US4407852A (en) * 1979-10-19 1983-10-04 Sapieha Slawomir W Electrets from plasma polymerized material
US4397702A (en) * 1980-01-09 1983-08-09 Johnson Controls, Inc. Fabrication of non-conductive charged sensing probe unit
US4308223A (en) * 1980-03-24 1981-12-29 Albany International Corp. Method for producing electret fibers for enhancement of submicron aerosol filtration
US4365283A (en) * 1980-10-16 1982-12-21 Pennwalt Corporation Corona discharge poling process
US4527218A (en) * 1981-06-08 1985-07-02 At&T Bell Laboratories Stable positively charged Teflon electrets
WO1982004346A1 (en) * 1981-06-08 1982-12-09 Western Electric Co Stable positively charged electrets
US4451736A (en) * 1982-04-16 1984-05-29 Wisconsin Alumni Research Foundation Method and apparatus for measuring air ion concentrations
US4513049A (en) * 1983-04-26 1985-04-23 Mitsui Petrochemical Industries, Ltd. Electret article
US5045747A (en) * 1990-02-28 1991-09-03 Industrial Technology Research Institute Apparatus for poling a piezoelectric ceramic
EP0447166A2 (en) * 1990-03-12 1991-09-18 Mitsui Petrochemical Industries, Ltd. Process for producing an electret, a film electret, and an electret filter
EP0447166A3 (en) * 1990-03-12 1992-10-21 Mitsui Petrochemical Industries, Ltd. Process for producing an electret, a film electret, and an electret filter
US5256176A (en) * 1990-03-12 1993-10-26 Mitsui Petrochemical Industries, Ltd. Film electret and an electret filter
US5652609A (en) * 1993-06-09 1997-07-29 J. David Scholler Recording device using an electret transducer
WO1997039464A1 (en) * 1996-04-18 1997-10-23 California Institute Of Technology Thin film electret microphone
US20010033670A1 (en) * 1996-04-18 2001-10-25 California Institute Of Technology A California Institute Of Technology Thin film electret microphone
US6806593B2 (en) * 1996-04-18 2004-10-19 California Institute Of Technology Thin film electret microphone
US6243474B1 (en) 1996-04-18 2001-06-05 California Institute Of Technology Thin film electret microphone
US5952645A (en) * 1996-08-27 1999-09-14 California Institute Of Technology Light-sensing array with wedge-like reflective optical concentrators
WO1999065277A1 (en) * 1998-06-11 1999-12-16 Microtronic A/S A method of manufacturing a transducer having a diaphragm with a predetermined tension
US20050254673A1 (en) * 1999-05-19 2005-11-17 California Institute Of Technology High performance MEMS thin-film teflon electret microphone
US7280014B2 (en) 2001-03-13 2007-10-09 Rochester Institute Of Technology Micro-electro-mechanical switch and a method of using and making thereof
US7195393B2 (en) 2001-05-31 2007-03-27 Rochester Institute Of Technology Micro fluidic valves, agitators, and pumps and methods thereof
US20020185003A1 (en) * 2001-06-11 2002-12-12 Rochester Institute Of Technology Electrostatic filter and a method thereof
US6773488B2 (en) * 2001-06-11 2004-08-10 Rochester Institute Of Technology Electrostatic filter and a method thereof
US7211923B2 (en) 2001-10-26 2007-05-01 Nth Tech Corporation Rotational motion based, electrostatic power source and methods thereof
US7378775B2 (en) 2001-10-26 2008-05-27 Nth Tech Corporation Motion based, electrostatic power source and methods thereof
US7287328B2 (en) 2003-08-29 2007-10-30 Rochester Institute Of Technology Methods for distributed electrode injection
US7217582B2 (en) 2003-08-29 2007-05-15 Rochester Institute Of Technology Method for non-damaging charge injection and a system thereof
US7408236B2 (en) 2003-08-29 2008-08-05 Nth Tech Method for non-damaging charge injection and system thereof
US8581308B2 (en) 2004-02-19 2013-11-12 Rochester Institute Of Technology High temperature embedded charge devices and methods thereof
EP2019298A4 (en) * 2006-04-27 2016-03-02 Univ Saitama Nat Univ Corp DEVICE FOR CONVERTING MECHANICAL IN ELECTRICAL AND MANUFACTURING METHOD THEREFOR
US20120043142A1 (en) * 2007-05-09 2012-02-23 Grivna Edward L Electret stylus for touch-sensor device
US9285930B2 (en) * 2007-05-09 2016-03-15 Wacom Co., Ltd. Electret stylus for touch-sensor device
DE102011013258A1 (de) 2010-03-08 2011-12-15 Sabo Industria E Comercio De Autopecas Ltda. Bausatz Melderad
US11081285B2 (en) * 2019-05-08 2021-08-03 Deborah Duen Ling Chung Electrically conductive electret and associated electret-based power source and self-powered structure

Also Published As

Publication number Publication date
JPS5025677A (ja) 1975-03-18
GB1445943A (en) 1976-08-11
NL7409136A (nl) 1975-01-07
DE2432377A1 (de) 1975-01-23
FR2236259B1 (ja) 1977-10-14
DE2432377C2 (de) 1987-10-01
NL181062B (nl) 1987-01-02
CA1008187A (en) 1977-04-05
NL181062C (nl) 1987-06-01
FR2236259A1 (ja) 1975-01-31
JPS5650408B2 (ja) 1981-11-28

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