JP2678775B2 - Method for manufacturing high-density silicon nitride sintered body - Google Patents
Method for manufacturing high-density silicon nitride sintered bodyInfo
- Publication number
- JP2678775B2 JP2678775B2 JP63255953A JP25595388A JP2678775B2 JP 2678775 B2 JP2678775 B2 JP 2678775B2 JP 63255953 A JP63255953 A JP 63255953A JP 25595388 A JP25595388 A JP 25595388A JP 2678775 B2 JP2678775 B2 JP 2678775B2
- Authority
- JP
- Japan
- Prior art keywords
- sintered body
- pressure
- silicon nitride
- temperature
- gas
- 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.)
- Expired - Fee Related
Links
- 229910052581 Si3N4 Inorganic materials 0.000 title claims description 12
- HQVNEWCFYHHQES-UHFFFAOYSA-N silicon nitride Chemical compound N12[Si]34N5[Si]62N3[Si]51N64 HQVNEWCFYHHQES-UHFFFAOYSA-N 0.000 title claims description 12
- 238000004519 manufacturing process Methods 0.000 title claims description 9
- 238000000034 method Methods 0.000 title description 19
- 238000005245 sintering Methods 0.000 claims description 24
- 239000007789 gas Substances 0.000 claims description 23
- 238000010304 firing Methods 0.000 claims description 15
- 239000007791 liquid phase Substances 0.000 claims description 14
- 239000000843 powder Substances 0.000 claims description 9
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 7
- 230000015572 biosynthetic process Effects 0.000 claims description 6
- 239000012298 atmosphere Substances 0.000 claims description 5
- 239000012071 phase Substances 0.000 claims description 4
- 238000010438 heat treatment Methods 0.000 claims description 3
- 239000011261 inert gas Substances 0.000 claims description 2
- 239000007788 liquid Substances 0.000 claims 1
- 239000011148 porous material Substances 0.000 description 13
- QSHDDOUJBYECFT-UHFFFAOYSA-N mercury Chemical compound [Hg] QSHDDOUJBYECFT-UHFFFAOYSA-N 0.000 description 6
- 229910052753 mercury Inorganic materials 0.000 description 6
- 230000008602 contraction Effects 0.000 description 3
- 229910001873 dinitrogen Inorganic materials 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 2
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000001513 hot isostatic pressing Methods 0.000 description 2
- 238000007731 hot pressing Methods 0.000 description 2
- 239000012299 nitrogen atmosphere Substances 0.000 description 2
- 238000010008 shearing Methods 0.000 description 2
- 239000000919 ceramic Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000000280 densification Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 239000011521 glass Substances 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 238000002459 porosimetry Methods 0.000 description 1
- 238000003825 pressing Methods 0.000 description 1
- 229910001404 rare earth metal oxide Inorganic materials 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 239000011800 void material Substances 0.000 description 1
- 238000009736 wetting Methods 0.000 description 1
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- Ceramic Products (AREA)
Description
【発明の詳細な説明】 ＜産業上の利用分野＞ 本発明は高密度窒化珪素焼結体の製造方法さらに詳細
には内部にピンホールの残留しない焼結体を得るための
製造方法に関する。DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention relates to a method for producing a high-density silicon nitride sintered body, and more particularly to a method for producing a sintered body in which pinholes do not remain inside.
＜従来の技術＞ 近年熱効率の向上、燃料の節約、低公害、軽量化を目
的として高温ガスタービン、ディーゼルエンジンなど高
温で稼動する機器の開発が盛んである。これら高温で使
用される機器に用いられる材料としてはセラミック中で
も窒化珪素が注目されている。<Prior Art> In recent years, high-temperature gas turbines, diesel engines, and other devices that operate at high temperatures have been actively developed for the purpose of improving thermal efficiency, saving fuel, reducing pollution, and reducing weight. Among ceramics, attention has been paid to silicon nitride as a material used for devices used at these high temperatures.
この窒化珪素（Si3N4）は優れた物理的、化学的特性
を有するがそれ単味では緻密化させるのが困難であっ
た。このようなSi3N4焼結体の製造法としてはガス圧
焼結法、焼結助剤（Al2O3,MgO,Y2O3;希土類酸化物等
の焼結助剤）を添加して大気圧下で焼結する常圧焼結
法、ホットプレス法、熱間静水圧プレス法など４つ
の方法がある。This silicon nitride (Si 3 N 4 ) has excellent physical and chemical properties, but it was difficult to densify it by itself. Gas pressure sintering and sintering aids (Al 2 O 3 , MgO, Y 2 O 3 ; sintering aids such as rare earth oxides) are added as the manufacturing method of such Si 3 N 4 sintered body. Then, there are four methods such as an atmospheric pressure sintering method of performing sintering under atmospheric pressure, a hot pressing method, and a hot isostatic pressing method.
このうち、ホットプレス法は比較的高密度で高強度の
焼結体が得られているが、複雑な形状の焼結体が得難い
ことと、コストアップが避けられない。Among them, the hot pressing method has obtained a sintered body having a relatively high density and high strength, but it is difficult to obtain a sintered body having a complicated shape and an increase in cost is inevitable.
常圧焼結法は予め任意形状に成形したSi3N4粉末を焼
結する方法であるので複雑形状品の製造は容易であるが
比較的多量の焼結助剤が必要であるため、粒界のガラス
成分が増し、高温特性に優れた焼結体が得難い。この点
ガス圧焼結法や、熱間静水圧プレス法は、少量の焼結助
剤で緻密化させることができ、高温構造材料用Si3N4焼
結体の製造方法として有用である。Since the atmospheric pressure sintering method is a method of sintering Si 3 N 4 powder that has been previously molded into an arbitrary shape, it is easy to manufacture a product with a complicated shape, but a comparatively large amount of sintering aid is required, so the grain It is difficult to obtain a sintered body with excellent high temperature characteristics because the glass component in the field increases. The point gas pressure sintering method and the hot isostatic pressing method can be densified with a small amount of a sintering aid and are useful as a method for producing a Si 3 N 4 sintered body for a high temperature structural material.
一例として特公昭61−46430号があるが、その概要はS
i3N4粉末を所定形状に成形するとともに、これを相対密
度92％以上に予備焼結し閉気孔させ後、1500℃以上、窒
素分圧500気圧以上で熱間静水圧プレス（HIP）で２次焼
結する方法であり、得られた焼結体の密度は98％以上で
あるが、完全には内部ピンホールが消滅していない焼結
体を得ており、２次焼結時の温度と圧力のスケジュール
の緻密化との関係については何等言及していない。ま
た、予備焼結体の密度が97％以下の場合、上記の方法で
ピンホールのない焼結体を得ることは困難である。An example is Japanese Examined Patent Publication No. 61-46430, the outline of which is S
i 3 N 4 powder is molded into a predetermined shape, pre-sintered to a relative density of 92% or more and closed pores are then subjected to a hot isostatic press (HIP) at 1500 ° C or more and a nitrogen partial pressure of 500 atm or more. It is a method of secondary sintering, the density of the obtained sintered body is 98% or more, but a sintered body in which the internal pinholes have not completely disappeared is obtained. No mention is made of the relationship between temperature and pressure schedule densification. Further, when the density of the pre-sintered body is 97% or less, it is difficult to obtain a pin-hole-free sintered body by the above method.
＜発明が解決すべき課題＞ 本発明者らは上記の焼結方法に於て、1000atmの窒素
ガス圧力下で２次焼成して相対密度98.4％の焼結体を
得、1atmの窒素ガス雰囲気で1500℃2hrの条件で熱処理
を行なったところ、焼結体中には内部にクラックが発生
したが、その原因は過度の引張応力が発生したことによ
るものと推定される。これは２次焼成時に何等かの経路
で焼結体中に圧力媒体に用いたガス（窒素ガス等）が侵
入し、焼結体中に残留していたピンホールの中に高圧の
まゝ閉じ込められたことによるものと推定される。<Problems to be Solved by the Invention> In the above sintering method, the present inventors performed a secondary firing under a nitrogen gas pressure of 1000 atm to obtain a sintered body having a relative density of 98.4%, and a nitrogen gas atmosphere of 1 atm. When heat treatment was performed at 1,500 ° C. for 2 hours, cracks were generated inside the sintered body, and it is presumed that the cause was excessive tensile stress. This is because the gas used as the pressure medium (nitrogen gas, etc.) penetrates into the sintered body through some route during the secondary firing, and the high pressure is confined in the pinhole remaining in the sintered body. It is presumed that this is due to what was done.
換言すれば、高圧のガス圧下で２次焼成した後に焼結
体内部に残留したピンホールの中に高圧の雰囲気ガスが
封じ込まれる場合、高温構造材として使用するとその特
性の劣化が避けられないということを意味している。In other words, when the high-pressure atmospheric gas is confined in the pinholes remaining inside the sintered body after the secondary firing under the high-pressure gas pressure, the deterioration of the characteristics cannot be avoided if it is used as a high-temperature structural material. It means that.
本発明はこのような状況に鑑み、高圧のガス圧力下で
２次焼成した場合でも、高温での特定の劣化を生じさせ
るピンホールが残留しない焼結体を得るための焼結方法
を目的とするものである。In view of such a situation, the present invention aims at a sintering method for obtaining a sintered body in which pinholes that cause specific deterioration at high temperature do not remain even when secondary sintering is performed under a high gas pressure. To do.
＜課題を解決するための手段＞ 本発明者等は予備成形体を300atm以上のガス圧下で２
次焼成を行う焼成法において前述の問題を生じるような
内部ピンホールが残留しない焼結体を得るための焼結方
法について鋭意検討したところ、２次焼成の温度と圧力
のスケジュールがピンホールの残留の有無に大きな影響
を持つことが判った。<Means for Solving the Problems> The inventors of the present invention used a preformed body under a gas pressure of 300 atm or more.
When the sintering method for obtaining the sintered body in which the internal pinholes that do not cause the above-mentioned problems remain in the sintering method in which the secondary sintering is performed, the schedule of the secondary sintering temperature and pressure shows that the residual pinholes remain. It was found to have a great influence on the presence or absence of.
本発明はこの知見に基づき以下の手段によりこの問題
を解決し得ることができた。The present invention was able to solve this problem by the following means based on this finding.
窒化珪素粉末及び焼結助剤を所定の形状に予備成形
し、N2雰囲気下で１次焼成した後にN2又はAr等の不活性
ガスの加圧雰囲気下で２次焼成するスケジュールを、
（イ）焼結体中における液相生成温度までは300atm以下
で加圧加熱し、（ロ）焼結体中における液相生成温度以
上では300atmを越えて所定の圧力まで昇圧させて各圧加
熱する高密度窒化珪素焼結体の製造方法である。A schedule of preforming silicon nitride powder and a sintering aid into a predetermined shape, performing primary firing in an N 2 atmosphere, and then secondary firing in a pressurized atmosphere of an inert gas such as N 2 or Ar,
(A) Pressurize and heat up to 300 atm up to the liquid phase generation temperature in the sintered body, and (b) pressurize and heat up to a predetermined pressure above 300 atm above the liquid phase generation temperature in the sintered body and heat to each pressure. And a method for producing a high-density silicon nitride sintered body.
本発明においては１次焼結体に開気孔が存在しないこ
とが必要であり、開気孔が存在すれば上記のスケジュー
ルによってもピンホールが残留することになる。In the present invention, it is necessary that there are no open pores in the primary sintered body, and if there are open pores, pinholes will remain even with the above schedule.
１次焼結体の開気孔の有無は水銀圧入法を用いて大気
圧以上の圧入圧力での水銀の侵入の有無によって決定す
ることができ、水銀の侵入が認められなければ開気孔が
存在せず、水銀の侵入が認められれば開気孔が存在する
と判断することができる。The presence or absence of open pores in the primary sintered body can be determined by the presence or absence of mercury intrusion at a press-in pressure of atmospheric pressure or higher using the mercury intrusion method. If no intrusion of mercury is recognized, open pores must be present. However, if mercury intrusion is recognized, it can be determined that there are open pores.
ここで焼結体における液相生成温度は成形体を加熱し
ていたときに急激な焼成収縮が開始する温度として決定
することができる。Here, the liquid phase formation temperature in the sintered body can be determined as the temperature at which abrupt firing contraction starts when the molded body is heated.
ただしこの方法では液相生成温度の決定温度に若干の
誤差を生じるため、昇圧は、この温度より30℃以上高い
温度で行うのがよく、より好ましくは50℃以上高い温度
で行うのがよい。However, this method causes a slight error in the temperature at which the liquid phase formation temperature is determined, and therefore the pressure is preferably raised at a temperature higher by 30 ° C. or more, more preferably 50 ° C. or higher.
＜作 用＞ 高密度焼結体を得るためには、１次焼結体に開気孔の
有無が重要であることは前述のとおりである。すなわ
ち、開気孔が無ければ一般的に２次焼結時の雰囲気ガス
が焼結体に侵入することは考えられない。ただし、ガス
圧焼結に於ては雰囲気ガスの圧力を高めて焼成するため
に、ガス圧力が外力として焼結体に作用し、これにより
影響されることが考えられる。本発明者はこの点につい
て検討したところ、１次焼結体中に液相が生成していな
い温度で300atmを越える圧力に昇圧すると開気孔の存在
しない１次焼結体にあっても２次焼結後にピンホールが
残留し、ピンホール中には圧力媒体として用いたガスが
高圧で存在することが判った。<Operation> As described above, the presence or absence of open pores in the primary sintered body is important for obtaining a high-density sintered body. That is, without open pores, it is generally unlikely that the atmospheric gas at the time of secondary sintering will enter the sintered body. However, in the gas pressure sintering, since the pressure of the atmospheric gas is increased and firing is performed, the gas pressure acts on the sintered body as an external force, and it is considered that this is affected. The present inventor has examined this point and found that when the pressure is increased to more than 300 atm at a temperature at which the liquid phase is not formed in the primary sintered body, the secondary sintered body does not have the open pores even if the primary sintered body has no open pores. It was found that pinholes remained after sintering, and the gas used as a pressure medium was present in the pinholes at high pressure.
ところが液相生成温度では300atm以下とし、300atmを
越える圧力への昇圧は液相生成温度以上で行なうスケジ
ュールでは２次焼結体にピンホールが存在しないことが
判った。However, it was found that there was no pinhole in the secondary sintered body in the schedule in which the liquid phase formation temperature was 300 atm or less, and the pressure rise to a pressure over 300 atm was performed at the liquid phase formation temperature or higher.
その理由は１次焼結体中の粒界相の状態の差が考えら
れる。液相生成温度以上では粒界相は液相状態で、窒化
珪素粒とは充分に濡れているので、ガス圧力が作用し、
両者の間にせん断力が生じても両者の間にはガスが侵入
し得る空隙は生じない。液相が生成していない温度では
窒化珪素粒と粒界相の濡れが不充分で、加圧によるせん
断力が両者間に作用した場合に、両者間にはガスの侵入
し得る微小な空隙が生じ、これによって圧力媒体として
用いたガスが１次焼結体中に侵入し高圧で閉じ込められ
ることになり、熱処理によりクラックが発生する原因と
なる。ただし圧力が300atm以下の場合は、外力としての
作用に乏しくたとえ液相が生成していない温度であって
も焼結体にガスの侵入し得る空隙が生じない。The reason is considered to be the difference in the state of the grain boundary phase in the primary sintered body. At the liquid phase generation temperature or higher, the grain boundary phase is in a liquid phase state and is sufficiently wet with the silicon nitride particles, so gas pressure acts,
Even if a shearing force is generated between the two, there is no gap between the two where gas can enter. At the temperature where the liquid phase is not formed, the wetting of the silicon nitride grains and the grain boundary phase is insufficient, and when the shearing force due to the pressure acts between them, there is a minute void into which gas can enter. As a result, the gas used as the pressure medium enters the primary sintered body and is confined at a high pressure, which causes cracks due to the heat treatment. However, when the pressure is 300 atm or less, the action as an external force is poor, and even at a temperature at which the liquid phase is not generated, no voids into which gas can enter the sintered body do not occur.
そこで１次焼結体中に液相が生成しない温度ではガス
圧を300atm以下に保持し、液相が生成する温度以上では
300atmを越えて所定の圧力まで次第に昇圧するスケジュ
ールを用いることによって、焼結体中に圧力媒体ガスを
侵入させることなく、高圧焼成することができ、その結
果としてピンホールの残留しない高密度焼結体が得られ
る。Therefore, the gas pressure is kept below 300 atm at the temperature at which the liquid phase is not formed in the primary sintered body,
By using a schedule of gradually increasing the pressure to more than 300 atm to a predetermined pressure, it is possible to perform high-pressure firing without injecting pressure medium gas into the sintered body, and as a result, high density sintering without pinholes remaining. The body is obtained.
＜実施例＞ 比表面積12m2/g、α率92％のSi3N4粉末93重量％と比
表面積8m2/gのAl2O3粉末３重量％と、比表面積10m2/gの
Y2O3粉末３重量％と、比表面積20m2/gのMgO粉末１重量
％をエタノール中で混合した後乾燥し造粒した。<Example> Specific surface area of 12 m 2 / g, 93% by weight of Si 3 N 4 powder having an α ratio of 92%, 3 % by weight of Al 2 O 3 powder having a specific surface area of 8 m 2 / g, and specific surface area of 10 m 2 / g
3% by weight of Y 2 O 3 powder and 1% by weight of MgO powder having a specific surface area of 20 m 2 / g were mixed in ethanol, dried and granulated.
この粉末を10×５×30mmに、金型プレスした後、1atm
のN2雰囲気下で1500,1550,1600,1650℃で各々２時間保
持し、表に示した４種の予備焼結体を得た。After pressing this powder into a mold of 10 x 5 x 30 mm, 1 atm
Under N 2 atmosphere at 1500, 1550, 1600 and 1650 ° C. for 2 hours each, and four kinds of pre-sintered bodies shown in the table were obtained.
この予備焼結体の開気孔の有無を水銀圧入法で測定し
た。これによれば密度88.7％のものは水銀の侵入が認め
られ、気孔率として4.99％であった。密度91.1〜97.8％
のものは大気圧から30000psiの圧力においても侵入は認
められず開気孔が存在しなかった。また焼成温度と収縮
の関係を８×８×40mmの成形体の長手方向の線収縮率に
より測定した。その結果は第１図に示すとおりである。
第１図によればこの試料の液相生成温度（収縮が急激に
開始する温度）は約1370℃であることが判った。次にこ
のデータを基に第２図に示す４種のスケジュールで2000
atm−1800℃−2hrの条件で２次焼成を行なった。Ｔは２
次焼成時の温度、Ｐは圧力を示し、横軸は時間を示す。
第２図Ａ〜Ｃは比較例、Ｄは本発明を示す。なお圧力媒
体ガスは窒素（N2）ガスである。The presence or absence of open pores in this pre-sintered body was measured by the mercury porosimetry method. According to this, intrusion of mercury was recognized in the case of the density of 88.7%, and the porosity was 4.99%. Density 91.1-97.8%
No intrusion was observed and no open pores were present even at pressures from atmospheric pressure to 30,000 psi. Further, the relationship between the firing temperature and the shrinkage was measured by the linear shrinkage ratio in the longitudinal direction of the 8 × 8 × 40 mm compact. The result is as shown in FIG.
According to FIG. 1, it was found that the liquid phase formation temperature (temperature at which contraction abruptly starts) of this sample was about 1370 ° C. Next, based on this data, 2000
Secondary firing was performed under the conditions of atm-1800 ° C-2hr. T is 2
The temperature at the time of the subsequent firing, P shows the pressure, and the horizontal axis shows the time.
2A to 2C show comparative examples, and D shows the present invention. The pressure medium gas is nitrogen (N 2 ) gas.
得られた２次焼結体については密度、内部ピンホール
及び1atmN2雰囲気下1500℃−2hrの条件で熱処理をした
後焼結体内部のクラックの発生の有無について調査し
た。The obtained secondary sintered body was heat-treated under the conditions of density, internal pinholes and 1atmN 2 atmosphere at 1500 ° C. for 2 hours, and then examined for occurrence of cracks inside the sintered body.
その結果は表に示すとおりである。 The results are shown in the table.
＜発明の効果＞ 本発明によれば、１次焼結体に開気孔が無ければ１次
焼結体の密度が97％以下であっても２次焼成により完全
に緻密化し、又、高温下においても焼結体中に変化生じ
ない安定な焼結体が得られた。 <Effects of the Invention> According to the present invention, if there are no open pores in the primary sintered body, even if the density of the primary sintered body is 97% or less, it is completely densified by the secondary firing, and under high temperature. Even in the above, a stable sintered body was obtained in which no change occurred in the sintered body.
第１図は、窒化珪素成形体の熱収縮率と焼成温度の関係
を示すグラフ。 第２図は２次焼成時の温度Ｔと圧力Ｐのスケジュールを
表わすグラフである。FIG. 1 is a graph showing the relationship between the thermal contraction rate of a silicon nitride compact and the firing temperature. FIG. 2 is a graph showing a schedule of temperature T and pressure P at the time of secondary firing.
Claims (1)
予備成形し、窒素（N2）ガス雰囲気下で１次焼成し後に
不活性ガスの加圧雰囲気下で２次焼成し、緻密化する窒
化珪素焼結体の製造方法に於て、２次焼成を （イ）焼結体中における液相生成温度までは300atm以下
で加圧加熱し、 （ロ）焼結体中における液相生成温度以上では300atmを
越えて所定の圧力まで次第に昇圧させて加圧加熱する ことを特報とする高密度窒化珪素焼結体の製造方法1. A silicon nitride powder and a sintering aid are preformed into a predetermined shape, are primarily fired in a nitrogen (N 2 ) gas atmosphere, and then are secondarily fired in a pressurized atmosphere of an inert gas, In the method for producing a compacted silicon nitride sintered body, the secondary firing is (a) pressurized and heated to 300 atm or less up to the liquid phase generation temperature in the sintered body, and (b) the liquid in the sintered body. Manufacturing method of high-density silicon nitride sintered body, which is characterized by gradually increasing the pressure to more than 300 atm to a predetermined pressure and heating under pressure above the phase formation temperature.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP63255953A JP2678775B2 (en) | 1988-10-13 | 1988-10-13 | Method for manufacturing high-density silicon nitride sintered body |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP63255953A JP2678775B2 (en) | 1988-10-13 | 1988-10-13 | Method for manufacturing high-density silicon nitride sintered body |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH02107567A JPH02107567A (en) | 1990-04-19 |
| JP2678775B2 true JP2678775B2 (en) | 1997-11-17 |
Family
ID=17285872
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP63255953A Expired - Fee Related JP2678775B2 (en) | 1988-10-13 | 1988-10-13 | Method for manufacturing high-density silicon nitride sintered body |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP2678775B2 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH075390B2 (en) * | 1989-07-18 | 1995-01-25 | 住友電気工業株式会社 | Method for producing high-strength silicon nitride sintered body |
| JP4795588B2 (en) * | 2001-01-12 | 2011-10-19 | 株式会社東芝 | Wear resistant parts made of silicon nitride |
-
1988
- 1988-10-13 JP JP63255953A patent/JP2678775B2/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| JPH02107567A (en) | 1990-04-19 |
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