EP0974984A1 - Production de varistors - Google Patents

Production de varistors Download PDF

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Publication number
EP0974984A1
EP0974984A1 EP98650042A EP98650042A EP0974984A1 EP 0974984 A1 EP0974984 A1 EP 0974984A1 EP 98650042 A EP98650042 A EP 98650042A EP 98650042 A EP98650042 A EP 98650042A EP 0974984 A1 EP0974984 A1 EP 0974984A1
Authority
EP
European Patent Office
Prior art keywords
powder
binder
temperature
glass transition
transition temperature
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP98650042A
Other languages
German (de)
English (en)
Other versions
EP0974984B1 (fr
Inventor
Francis O'sullivan
Brendan Harmon
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Littelfuse Ireland Development Co Ltd
Original Assignee
Littelfuse Ireland Development Co Ltd
Harris Ireland Development Co Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Littelfuse Ireland Development Co Ltd, Harris Ireland Development Co Ltd filed Critical Littelfuse Ireland Development Co Ltd
Priority to EP98650042A priority Critical patent/EP0974984B1/fr
Priority to DE69836074T priority patent/DE69836074D1/de
Priority to IE980588 priority patent/IES80693B2/en
Priority to AT98650042T priority patent/ATE341820T1/de
Publication of EP0974984A1 publication Critical patent/EP0974984A1/fr
Application granted granted Critical
Publication of EP0974984B1 publication Critical patent/EP0974984B1/fr
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01C—RESISTORS
    • H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
    • H01C7/10—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material voltage responsive, i.e. varistors
    • H01C7/105—Varistor cores
    • H01C7/108—Metal oxide
    • H01C7/112—ZnO type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01C—RESISTORS
    • H01C17/00—Apparatus or processes specially adapted for manufacturing resistors
    • H01C17/30—Apparatus or processes specially adapted for manufacturing resistors adapted for baking

Definitions

  • the invention relates to production of varistors having a multiphase composition, such as, ZnO (zinc oxide) varistors.
  • the electrical characteristics of such varistors are determined by their detailed microstructure.
  • the three main micro-structural features that are of importance for the performance of ZnO varistors containing bismuth oxide as one of the additives are for example, ZnO grains, grain boundaries and intergranular network of bismuth rich phases. These features constitute the functional microstructure which develops during fabrication and their detailed structure varies with changes in fabrication parameters. There is a correlation between the microstructure and the electrical (current-voltage) characteristics and this acts as an important tool to adjust the electrical property of the grain boundary as well as that of the grain to fit the requirements of a given application.
  • Such varistors are typically produced in one of two types of process, namely hot pressing and cold pressing.
  • the hot pressing technique is described, for example, in US4180483. This involves simultaneously pressing and heating the powder at a high temperature to provide a consolidated body.
  • JP56101714 describes a variation of hot pressing, in which the powder is pre-sintered at 1000°C for 2 hours, is then compressed to a disc, and is then sintered again at 1000°C for 2 hours.
  • the hot pressing technique is effective in some circumstances, however, the equipment required is expensive and the range of sizes which may be effectively produced in this manner is limited.
  • the cold pressing technique involves pressing the powder to provide what is referred to as a "green disc” of compressed powder. This is then sintered in a separate operation to consolidate the structure.
  • An example of this technique is described in US 5004573, in which oxides are mixed, ground in a ball mill, pressed into discs using PVA as a binder, and are then sintered at 1200 - 1350°C. This process is relatively simple and is widely used for high-volume varistor production. However, it suffers from the problem that there is a high defect rate caused by discs becoming chipped between the pressing and sintering stations.
  • the invention is directed towards providing a cold pressing process of the type set out in the preamble of claim 1 to provide a reduced green disc defect rate. Another object is to achieve improved green disc consistency.
  • the invention is characterised in that the powder is heated before pressing to a temperature above the glass transition temperature of the binder. Accordingly, the binder provides much reduced resistance to relative motion of the particles as they are compressed, to provide a more uniform and less brittle compressed body structure.
  • the powder is heated to a temperature in the range of 1°C to 70°C above the binder glass transition temperature. This range ensures that the glass transition temperature of the binder is exceeded, and also that organics in the powder are not damaged.
  • the temperature is in the range of 20°C to 70°C above the glass transition temperature. Most preferably, the temperature is in the range of 20°C to 40°C above the glass transition temperature. These temperature ranges have been found to be particularly suitable to achieve a reduced green disc defect rate without adversely affecting the varistor performance.
  • the powder is heated in a conduit leading to a press head. This is a particularly simple and effective way of heating the powder.
  • the powder is heated in a series of baffles as it falls to the press head.
  • the baffles are mounted to provide a cascading action as the powder falls in the conduit. This arrangement causes turbulence in the powder as it falls to ensure that the heat is well distributed across the whole range of powder particles as they fall to the press head. As there are no moving parts, the arrangement is very reliable and effective.
  • the baffles are electrically heated.
  • the powder transit time between heating and pressing is less than 5 seconds.
  • the powder particle size is in the range 130 to 150 ⁇ m, and most preferably is approximately 140 ⁇ m.
  • the step of preparing the powder includes the sub-step of adding a plasticizer component to reduce the glass transition temperature of the binder.
  • the ratio of binder to plasticizer is preferably 60:40 by weight.
  • step 2 of the method there is slurry preparation using an agitator 3 in a container 4.
  • zinc oxide and various oxide additives are weighed and milled and are mixed with a PVA binder solution in the container 4 to provide a slurry 5.
  • the particle size of the zinc oxide is approximately 2 ⁇ m.
  • the PVA binder has a glass transition temperature Tg (at which it becomes soft and pliable) in the range of 70°C to 90 °C.
  • a PEG plasticizer is also added in step 2. This lowers the glass transition temperature Tg of the PVA.
  • the ratio of binder to plasticizer is 60:40 by weight.
  • step 10 the slurry is convened to powder 11 by spray drying in a spray drying chamber 12 feeding a batch container 13.
  • the spray drying pump pressure is approximately 20 kg/cm 2
  • the outlet temperature is approximately 145°C.
  • the powder 11 is pre-heated in step 20 and is pressed in step 40.
  • the powder is drawn from a batch container 13 through a vacuum tube 22 which is perforated along the length immersed in the powder. These perforations ensure that an even distribution of powder is drawn at any one time because they extend through the depth of the powder in the container 13.
  • the pre-heating step 20 involves ejection of the powder from a nozzle 24 at the end of the vacuum tube 22 into a pre-heating chamber 23, shown in more detail in Fig. 2.
  • the chamber 23 has insulated walls 25 which support a series of baffles 26 extending downwardly and inwardly.
  • the baffles 26 are heated by application of electrical potential at silicon mat heaters 27.
  • An outlet chute 28 is mounted at the base of the pre-heating chamber 23.
  • the baffles 26 and the chute 28 are of aluminium material.
  • the powder falls through the chamber 23, it is gradually heated by the baffles 26 until the glass transition temperature of the binder is exceeded.
  • the glass transition temperature of the binder For example, for a glass transition temperature of 70°C, it has been found that a powder temperature of 100°C is suitable.
  • the silicon mat heaters 27 have an upper temperature limit of 180°C. They are mounted on the sides of the chamber 23, the heat transferring by conduction to the baffles 26. This ensures that the powder is gently heated over a large surface area from ambient to approximately 100°C as it falls under gravity to the outlet chute 28 in a cascading action.
  • the powder is pressed in step 40 by press heads 41 mounted on a carousel press over a powder bed 42.
  • the powder is then in the form of compressed "green" discs.
  • the discs 46 are delivered by a chute 43 onto a conveyor 44.
  • Pick-and-place heads 45 pick the discs by suction and automatically pack them into racks.
  • step 50 the discs are sintered in an oven 51 through which they are conveyed on a conveyor 52 in racks 53.
  • the discs 46 are mounted vertically in the racks 53.
  • step 60 leads 61 are applied to the discs 46. This is performed by an assembly unit having wire reels and soldier guns.
  • the discs are coated in epoxy using an epoxy shell. The epoxy shell is mixed, pre-heated, cured, and assembled.
  • the pre-heating step 2 of the invention achieves greater density and strength in the green disc. There is also improved consistency in the green discs. In one test, in room temperature pressing the green disc density was 2.937g/cc and this improved to 3.005g/c for 70°C pre-heating and to 3.028g/cc for 120°C preheating. The strength improved from 46.57N for room temperature to 74.44N for 70°C and to 103.19 N for 120°C. This improved strength significantly reduces the defect rate caused by discs being chipped en route to the sintering station. The defect rate was found to reduce by approximately 20%.

Landscapes

  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Manufacturing & Machinery (AREA)
  • Thermistors And Varistors (AREA)
  • Compositions Of Oxide Ceramics (AREA)
EP98650042A 1998-07-20 1998-07-20 Production de varistors Expired - Lifetime EP0974984B1 (fr)

Priority Applications (4)

Application Number Priority Date Filing Date Title
EP98650042A EP0974984B1 (fr) 1998-07-20 1998-07-20 Production de varistors
DE69836074T DE69836074D1 (de) 1998-07-20 1998-07-20 Herstellung von Varistoren
IE980588 IES80693B2 (en) 1998-07-20 1998-07-20 Manufacture of varistors
AT98650042T ATE341820T1 (de) 1998-07-20 1998-07-20 Herstellung von varistoren

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP98650042A EP0974984B1 (fr) 1998-07-20 1998-07-20 Production de varistors
IE980588 IES80693B2 (en) 1998-07-20 1998-07-20 Manufacture of varistors

Publications (2)

Publication Number Publication Date
EP0974984A1 true EP0974984A1 (fr) 2000-01-26
EP0974984B1 EP0974984B1 (fr) 2006-10-04

Family

ID=26151843

Family Applications (1)

Application Number Title Priority Date Filing Date
EP98650042A Expired - Lifetime EP0974984B1 (fr) 1998-07-20 1998-07-20 Production de varistors

Country Status (2)

Country Link
EP (1) EP0974984B1 (fr)
IE (1) IES80693B2 (fr)

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS52152907A (en) * 1976-06-16 1977-12-19 Matsushita Electric Works Ltd Manufacture of inorganic mold materials
EP0196370A1 (fr) * 1985-02-07 1986-10-08 BBC Brown Boveri AG Méthode de fabrication d'un dériveur de surtension utilisant un varistor à base de ZnO et dériveur de surtension ainsi fabriqué
US5004573A (en) * 1989-11-02 1991-04-02 Korea Institute Of Science And Technology Fabrication method for high voltage zinc oxide varistor
JPH07291743A (ja) * 1994-04-22 1995-11-07 Matsushita Electric Ind Co Ltd セラミック成形体の焼成方法

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS52152907A (en) * 1976-06-16 1977-12-19 Matsushita Electric Works Ltd Manufacture of inorganic mold materials
EP0196370A1 (fr) * 1985-02-07 1986-10-08 BBC Brown Boveri AG Méthode de fabrication d'un dériveur de surtension utilisant un varistor à base de ZnO et dériveur de surtension ainsi fabriqué
US5004573A (en) * 1989-11-02 1991-04-02 Korea Institute Of Science And Technology Fabrication method for high voltage zinc oxide varistor
JPH07291743A (ja) * 1994-04-22 1995-11-07 Matsushita Electric Ind Co Ltd セラミック成形体の焼成方法

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
DATABASE WPI Section Ch Week 7805, Derwent World Patents Index; Class L02, AN 78-09562A, XP002086531 *
PATENT ABSTRACTS OF JAPAN vol. 096, no. 003 29 March 1996 (1996-03-29) *
YODOGAWA M: "Zinc oxide varistors provide urgent surge control", JEE (JOURNAL OF ELECTRONIC ENGINEERING), AUG. 1987, JAPAN, VOL. 24, NR. 248, PAGE(S) 76 - 81, ISSN 0385-4507, XP002086530 *

Also Published As

Publication number Publication date
IES980588A2 (en) 1998-12-02
EP0974984B1 (fr) 2006-10-04
IES80693B2 (en) 1998-12-02

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