【発明の詳細な説明】[Detailed description of the invention]
本発明はコネクター、リレー、スイツチ等に用
いられる電気接触部材に関するもので、特に加工
性を損なうことなく長期間に亘り安定した電気接
触特性を示す経済的な接触部材を提供するもので
ある。
従来電気接触部材には、卑金属からなる部材の
電気接触面に、Au、Ag、Pd等の貴金属又はこれ
等の合金を被着したものが用いられている。これ
等貴金属は耐食性に富み、電気接触抵抗の小さい
良導体で、アーク損耗にも耐え、適度の硬さと可
撓性を有し、機械的摩耗特性も満足できる優れた
電気接触特性を示す。しかしながらこれ等金属は
何れも高価な貴金属であるため、電気接触面のみ
に可及的に薄く被着している。
Auは最も高価な貴金属で、電気接触抵抗は最
も低いが、軟質のため高度の耐摩耗性が要求され
る用途には適さない。特に卑金属からなる部材の
電気接触面にのみ被着したものは、周辺卑金属の
硫化腐食物がAu層上に進展する、所謂クリープ
現象が極めて顕著に現われる欠点がある。Agは
比較的安価な貴金属であるが、硫化し易い致命的
な欠点がある。Pdはコスト的にAuとAgの中間に
ある貴金属で、硬質、高融点であるが、触媒性が
強く、有機蒸気や大気汚染物質を吸着して表面に
絶縁性の化合物を生じ易い欠点がある。特にPd
は貴金属中最も活性で、所謂褐色粉末（Brown
Powder）、動的、静的重合（Friction Polymer、
Static Polymer）等を起す危険性が大きいとこ
ろから、Pd層上に更にAuメツキなどの高価なメ
ツキを施す必要があり、コネクターなどの弱電電
子部品には利用されていない。
本発明はこれに鑑み種々研究の結果、少量の
Pdを有効に活用し、加工性を損なうことなく長
期間に亘り安定した電気接触特性を示す経済的な
電気接触部材を開発したもので、電気接触面が
Ni、Co又はこれ等の合金からなる部材の接触面
に、Pdの拡散層を形成し、該拡散層の表面Pd濃
度を40〜90wt％としたことを特徴とするもので
ある。
即ち本発明はCu又はCu合金等からなる電気接
触部材の接触面にNi、Co又はこれ等の合金層を
形成するか、或いは電気接触部材をNi、Co又は
これ等の合金で構成し、該部材のNi、Co又はこ
れ等の合金からなる電気接触面にPdの拡散層を
形成し、該拡散層の表面Pd濃度を40〜90wt％と
したものである。Pd拡散層は熱拡散による場合
電気接触面から内部に向つて漸次Pd濃度を低下
するもので、Ni、Co又はこれ等合金に拡散させ
ることにより、少量のPdの使用で優れた電気接
触特性を得たものであるが又後述する方法によれ
ば拡散層中の濃度分布を大巾に平均化することが
可能である。
Ni、Coは強固な酸化皮膜を生成し、電気接触
抵抗を著しく高める欠点があり、またPdは前記
の如き欠点を有しているが、Ni、Co又はこれ等
の合金の表面にPdを拡散させることにより、両
者の欠点を軽減緩和する。即ち表面のPd濃度を
90wt％以下とすることにより、前記絶縁性化合
物の発生を大巾に低下抑止し、他方Pd濃度を
40wt％以上とすることにより、Ni、Co又はこれ
等の合金の酸化皮膜の生成を抑止したものであ
る。Pd拡散層の厚さは使用条件により特定する
ことはできないが、高荷重で多数回の挿板を行な
うコネクターやアーク損耗を伴うリレー接点など
ではμオーダを要するが、通常の用途では0.01〜
1μ程度で十分である。
本発明部材は上記の如くNi、Co又はこれ等の
合金からなる電気接触面に、表面のPd濃度が40
〜90wt％のPd拡散層を形成したもので、PdはNi
やCoと全率固溶の安定した合金を生成するため、
表面から連続してPdの濃度を勾配を有する拡散
層となる。従つて部材と拡散層の境界には欠陥が
起り得ない。電気接触部材の接触面にPdとNi又
はCo均一な合金層を機械的クラツド、蒸着又は
メツキにより被着してもある程度の電気接触特性
を得ることができるが、機械的クラツドでは薄い
合金層を能率的、経済的に被着することが困難で
あり、蒸着やメツキでは所望組成の合金を安定し
て析出させることが困難である。更にこのような
合金層は上記拡散層と異なり、部材との境界の密
着力が劣り、加工中又は使用中に合金層の剥離や
破壊が起り易いものである。
本発明部材はNi、Co又はそれ等の合金からな
る電気接触面に、Pdの拡散層を形成してNi、Co
又はこれ等合金とPdの双方の欠陥を軽減緩和し、
優れた電気接触特性を得たもので、Cu又はCu合
金等からなる電気接触部材の接触面にNi、Co又
はこれ等の合金層を形成するか、或いはNi、Co
又はそれ等の合金からなる電気接触部材を用い、
該部材の接触面にPd又はPd合金を被着し、これ
を加熱拡散することにより容易に得ることができ
る。加熱拡散はPd又はPd合金の被着厚さ、Pdの
表面濃度によつて異なるが、不活性雰囲気中、
400〜1200℃の温度で加熱処理し、加熱温度、加
熱時間は、Pd又はPd合金の被着厚さ、Pdの表面
濃度により、経験的に決めればよい。尚接触面は
Pdの表面濃度が40〜90wt％で、残部はNi又はCo
を主体とするものであるが、必要に応じてAg、
Au、白金族などの貴金属、Cuなどの卑金属を含
有させることもできる。
拡散方法としては、他にレーザー光を用い極短
時間（ｍsecオーダー）に溶融金属化し、急冷凝
固させる方法も開発されている。この方法によれ
ば合金属内のPd濃度の分布はより均一化される。
次に本発明を実施例について詳細に説明する。
実施例 １
厚さ0.5mmのリン青銅条に、下記スルフアミン
酸浴を用いて厚さ10μのNiメツキを行ない、その
上に下記Pd（NH3）2（NO2）2浴を用いて厚さ1μの
Pdメツキを行なつた。これをH2−Arガス中で
900℃の温度に２時間加熱した後、厚さ0.25mmま
で圧延して電気接触部材を得た。
スルフアミン酸浴
Ni（SO3NH2）2 500ｇ／
NiCl2・6H2O 30ｇ／
H3BO3 30ｇ／
浴 温 50℃
電流密度 10A／ｄm2
Pd（NH3）2（NO2）2浴
Pd（NH3）2（NO2）2 ６ｇ（Pd）／
NaNO2 40ｇ／
NaCl 40ｇ／
H3BO3 10ｇ／
浴 温 50℃
電流密度 2.5A／ｄm2
実施例 ２
実施例１においてスルフアミン酸浴を代えて下
記ワイズベルク浴を用い、厚さ5μのNi−18wt％
Co合金メツキを行ないPd（NH3）2（NO2）2浴に代
えて市販のメツキ浴（田中貴金属社製パラテツク
ス）を用い、浴温45℃、電流密度1A／ｄm2で
厚さ0.5μのPdをメツキした。これをH2−Arガス
中で950℃の温度に１時間加熱した後、厚さ0.25
mmまで圧延して電気接触部材を得た。
ワイズベルク浴
NiSO4・6H2O 240ｇ／
CoSO4・7H2O 15ｇ／
H3BO3 30ｇ／
NiCl2・6H2O 45ｇ／
（NH4）2SO4 ２ｇ／
PH 3.0
浴 温 40℃
電流密度 2.5A／ｄm2
実施例 ３
実施例１において、Pd（NH3）2NO2浴に代えて
市販のメツキ浴（日進化成製PNP−80）を用い、
浴温30℃、電流密度0.5A／ｄm2で厚さ0.7μのPd
−23wt％Ni合金をメツキした。これをH2気流中
で750℃の温度に４時間加熱した後、厚さ0.25mm
まで圧延して電気接触部材を得た。
比較例 １〜３
実施例１〜３において、加熱処理することなく
圧延した。
比較例 ４
実施例１において、Pdメツキと加熱処理を省
略して圧延した。
比較例 ５
実施例１において、Pdメツキに代えて、厚さ
2.5μのAu−25wt％Ag−５％Cu合金をNi上に冷
間圧接した。
これ等電気接触部材を巾5.0mmにスリツトした
ものについて、表面Pd濃度、拡散層の厚さ、接
触抵抗、及び加工性を測定した、これ等の結果を
第１表に示す。
表面Pd濃度及び拡散層の厚さは、オージユー
分光法により表面Pd濃度を分析すると共に、Ar
イオンスパツターで表面をエツチングしながら同
様にPd濃度を分析し、Pd濃度が40％以上の部分
を拡散層とした。接触抵抗は部材を加湿劣化（温
度60℃、湿度95％、100時間）、硫化劣化
（3ppmH2S、湿度100％、室温、500時間）、有機
蒸気劣化（トルエンと水の混合物、室温、500時
間）を施した後、部材表面にAg棒を荷重50grで
接触させ、0.1Aの電流で抵抗を測定した。
また加工性は直径2.0mmの棒に巻き付け、その
外表面を1000倍の電子顕微鏡で観察した。
The present invention relates to electrical contact members used in connectors, relays, switches, etc., and in particular provides an economical contact member that exhibits stable electrical contact characteristics over a long period of time without impairing workability. BACKGROUND ART Conventionally, electrical contact members are made of a member made of a base metal and coated with a noble metal such as Au, Ag, or Pd or an alloy thereof on the electrical contact surface. These precious metals are highly corrosion resistant, are good conductors with low electrical contact resistance, are resistant to arc wear, have appropriate hardness and flexibility, and exhibit excellent electrical contact characteristics with satisfactory mechanical wear characteristics. However, since these metals are all expensive noble metals, they are deposited as thinly as possible only on the electrical contact surfaces. Au is the most expensive precious metal and has the lowest electrical contact resistance, but its softness makes it unsuitable for applications that require a high degree of wear resistance. Particularly, those deposited only on the electrical contact surfaces of members made of base metals have the disadvantage that the so-called creep phenomenon, in which sulfide corrodes of the surrounding base metals develop onto the Au layer, appears very clearly. Although Ag is a relatively inexpensive precious metal, it has the fatal drawback of being easily sulfided. Pd is a noble metal that is between Au and Ag in terms of cost, and is hard and has a high melting point, but it has strong catalytic properties and has the disadvantage of adsorbing organic vapors and air pollutants and easily forming insulating compounds on the surface. . Especially Pd
is the most active of all precious metals, and is the so-called brown powder (Brown powder).
Powder), dynamic, static polymerization (Friction Polymer,
Since there is a high risk of causing static polymers, etc., it is necessary to further apply an expensive plating such as Au plating on the Pd layer, so it is not used for low-power electronic parts such as connectors. In view of this, the present invention was developed as a result of various studies.
By effectively utilizing Pd, we have developed an economical electrical contact member that exhibits stable electrical contact characteristics over a long period of time without compromising workability.
A Pd diffusion layer is formed on the contact surface of a member made of Ni, Co, or an alloy thereof, and the surface Pd concentration of the diffusion layer is set to 40 to 90 wt%. That is, the present invention forms a layer of Ni, Co, or an alloy of these on the contact surface of an electrical contact member made of Cu or a Cu alloy, or forms an electrical contact member of Ni, Co, or an alloy of these, and A Pd diffusion layer is formed on the electrical contact surface of the member made of Ni, Co, or an alloy thereof, and the surface Pd concentration of the diffusion layer is set to 40 to 90 wt%. When the Pd diffusion layer is used by thermal diffusion, the Pd concentration gradually decreases from the electrical contact surface toward the inside. By diffusing into Ni, Co, or their alloys, excellent electrical contact characteristics can be achieved with a small amount of Pd. However, according to the method described later, it is possible to average the concentration distribution in the diffusion layer over a wide range. Ni and Co have the disadvantage of forming a strong oxide film and significantly increasing electrical contact resistance, and Pd has the disadvantages mentioned above, but Pd is diffused onto the surface of Ni, Co or their alloys. By doing so, the drawbacks of both can be alleviated. In other words, the Pd concentration on the surface
By setting the Pd concentration to 90wt% or less, the generation of the insulating compound can be significantly reduced, while the Pd concentration can be reduced.
By setting the content to 40 wt% or more, the formation of an oxide film of Ni, Co, or an alloy thereof is suppressed. The thickness of the Pd diffusion layer cannot be specified depending on the usage conditions, but it is on the order of μ for connectors that are inserted many times under high loads, relay contacts that are subject to arc wear, etc., but for normal applications it is 0.01~
About 1μ is sufficient. As mentioned above, the member of the present invention has an electrical contact surface made of Ni, Co, or an alloy thereof, with a surface Pd concentration of 40%.
~90wt% Pd diffusion layer is formed, Pd is Ni
In order to produce a stable alloy with complete solid solution with Co and Co,
This becomes a diffusion layer with a continuous Pd concentration gradient from the surface. Therefore, no defects can occur at the interface between the component and the diffusion layer. A certain degree of electrical contact properties can be obtained by depositing a uniform alloy layer of Pd and Ni or Co on the contact surface of an electrical contact member by mechanical cladding, vapor deposition or plating, but mechanical cladding requires a thin alloy layer. It is difficult to deposit efficiently and economically, and it is difficult to stably deposit an alloy with a desired composition by vapor deposition or plating. Furthermore, unlike the above-mentioned diffusion layer, such an alloy layer has poor adhesion at the boundary with the member, and is likely to peel off or break during processing or use. The member of the present invention has a Pd diffusion layer formed on the electrical contact surface made of Ni, Co, or an alloy thereof.
Or reduce the defects of both these alloys and Pd,
With excellent electrical contact properties, a layer of Ni, Co or an alloy of these is formed on the contact surface of an electrical contact member made of Cu or Cu alloy, or a layer of Ni, Co or an alloy of these is formed.
or using an electrical contact member made of an alloy thereof,
It can be easily obtained by depositing Pd or a Pd alloy on the contact surface of the member and heating and diffusing it. Heating diffusion varies depending on the deposited thickness of Pd or Pd alloy and the surface concentration of Pd, but in an inert atmosphere,
Heat treatment is performed at a temperature of 400 to 1200° C., and the heating temperature and heating time may be determined empirically depending on the deposited thickness of Pd or Pd alloy and the surface concentration of Pd. The contact surface is
The surface concentration of Pd is 40 to 90wt%, and the balance is Ni or Co.
The main focus is Ag, but if necessary,
It is also possible to contain noble metals such as Au and platinum group metals, and base metals such as Cu. As another diffusion method, a method has also been developed in which laser light is used to melt the metal in an extremely short period of time (on the order of milliseconds) and rapidly solidify it. According to this method, the distribution of Pd concentration within the alloy metal is made more uniform. Next, the present invention will be explained in detail with reference to examples. Example 1 A phosphor bronze strip with a thickness of 0.5 mm was plated with Ni to a thickness of 10 μm using the sulfamic acid bath shown below, and then plated with Ni to a thickness of 10 μm using the Pd(NH 3 ) 2 (NO 2 ) 2 bath shown below. 1μ
I did Pd Metsuki. This was carried out in H 2 −Ar gas.
After heating to a temperature of 900° C. for 2 hours, the material was rolled to a thickness of 0.25 mm to obtain an electrical contact member. Sulfamic acid bath Ni (SO 3 NH 2 ) 2 500g / NiCl 2・6H 2 O 30g / H 3 BO 3 30g / Bath temperature 50℃ Current density 10A/dm 2 Pd (NH 3 ) 2 (NO 2 ) 2 bath Pd (NH 3 ) 2 (NO 2 ) 2 6g (Pd) / NaNO 2 40g / NaCl 40g / H 3 BO 3 10g / Bath temperature 50°C Current density 2.5A/dm 2Example 2 In Example 1, the sulfamic acid bath was Instead, use the following Weisberg bath and use 5μ thick Ni-18wt%
Co alloy plating was performed using a commercially available plating bath (Paratex, manufactured by Tanaka Kikinzoku Co., Ltd.) instead of the Pd(NH 3 ) 2 (NO 2 ) 2 bath, at a bath temperature of 45°C and a current density of 1 A/dm 2 to a thickness of 0.5 μm. Pd was marked. After heating this in H 2 -Ar gas to a temperature of 950℃ for 1 hour, a thickness of 0.25
An electrical contact member was obtained by rolling to mm. Weisberg bath NiSO 4・6H 2 O 240g / CoSO 4・7H 2 O 15g / H 3 BO 3 30g / NiCl 2・6H 2 O 45g / (NH 4 ) 2 SO 4 2g / PH 3.0 Bath temperature 40℃ Current density 2.5A/dm 2Example 3 In Example 1, a commercially available plating bath (PNP-80 manufactured by Nichika Seisaku Co., Ltd.) was used instead of the Pd(NH 3 ) 2 NO 2 bath,
Pd with a thickness of 0.7μ at a bath temperature of 30℃ and a current density of 0.5A/ dm2.
-23wt%Ni alloy was plated. After heating it to a temperature of 750℃ in H2 stream for 4 hours, the thickness of 0.25mm
An electrical contact member was obtained by rolling the product to a temperature of 100%. Comparative Examples 1 to 3 In Examples 1 to 3, rolling was performed without heat treatment. Comparative Example 4 In Example 1, rolling was performed without Pd plating and heat treatment. Comparative Example 5 In Example 1, instead of Pd plating, the thickness
A 2.5μ Au-25wt%Ag-5%Cu alloy was cold welded onto Ni. Table 1 shows the results of measurements of surface Pd concentration, diffusion layer thickness, contact resistance, and workability of these electrical contact members slit to a width of 5.0 mm. The surface Pd concentration and the thickness of the diffusion layer were determined by analyzing the surface Pd concentration by Aussieur spectroscopy and by using Ar
While etching the surface with an ion sputter, the Pd concentration was similarly analyzed, and the portion with a Pd concentration of 40% or more was defined as a diffusion layer. Contact resistance is measured by humidification deterioration (temperature 60℃, humidity 95%, 100 hours), sulfurization deterioration (3ppm H 2 S, humidity 100%, room temperature, 500 hours), and organic vapor deterioration (toluene and water mixture, room temperature, 500 hours). time), an Ag rod was brought into contact with the surface of the member under a load of 50gr, and the resistance was measured with a current of 0.1A. Workability was also determined by wrapping the material around a rod with a diameter of 2.0 mm and observing its outer surface with an electron microscope at 1000x magnification.
【表】
第１表から明らかな如く、本発明部材は何れの
劣化条件にも耐え、安定した接触抵抗を示し、か
つ加工性も優れていることが判る。これに対し加
熱拡散処理を省略したものでは有機蒸気劣化が認
められ、加工性も劣り、特にPd−Ni合金メツキ
について加熱拡散処理を省略したものでは加熱、
硫化、有機による劣化が認められた。またPdメ
ツキと加熱拡散処理を省略したものでは、加湿及
び硫化劣化が著しく、高価なAu−Ag合金メツキ
したものではH2S中で切断面のリン青銅の硫化物
がクリープし、黒褐色に変色して接触抵抗が著し
く大きくなることが判る。
実施例 ４
実施例１において、Pdメツキ厚さを、それぞ
れ0.5μ、0.25μ、0.1μとして同様の加熱拡散処理
し、表面Pd濃度、拡散層の厚さ、加湿、硫化、
有機劣化の測定を行なつた。その結果を第２表に
示す。[Table] As is clear from Table 1, the members of the present invention can withstand any deterioration conditions, exhibit stable contact resistance, and have excellent workability. On the other hand, when heating and diffusion treatment was omitted, organic vapor deterioration was observed, and workability was poor, especially when heating and diffusion treatment was omitted for Pd-Ni alloy plating.
Sulfurization and organic deterioration were observed. In addition, in the case where Pd plating and heat diffusion treatment are omitted, deterioration due to humidification and sulfurization is significant, and in the case of the plated with expensive Au-Ag alloy, the phosphor bronze sulfide on the cut surface creeps in H 2 S, turning the color blackish brown. It can be seen that the contact resistance increases significantly. Example 4 In Example 1, the Pd plating thickness was changed to 0.5μ, 0.25μ, and 0.1μ, respectively, and the same heat diffusion treatment was performed, and the surface Pd concentration, diffusion layer thickness, humidification, sulfurization,
Organic deterioration was measured. The results are shown in Table 2.
【表】
第２表から明らかなように表面Pd濃度が40％
に満たないものは加湿による劣化が著しく大きい
ことが判る。
実施例 ５
実施例１〜３により得た本発明部材よりコネク
ターソケツトを成型し、これにAuメツキピンを
104回挿抜し、その前後における接触抵抗を測定
した。またこれ等について、加湿劣化を行つた
後、同様にして接触抵抗を測定した。これ等の結
果を第３表に示す。[Table] As is clear from Table 2, the surface Pd concentration is 40%.
It can be seen that the deterioration due to humidification is significantly large for those that are less than . Example 5 A connector socket was molded from the members of the present invention obtained in Examples 1 to 3, and Au plated pins were attached to it.
10 The contact resistance was measured before and after insertion and removal four times. Moreover, after humidification deterioration was performed on these, the contact resistance was measured in the same manner. These results are shown in Table 3.
【表】
第３表から明らかなように本発明部材は挿抜後
加湿劣化を行なつても接触抵抗の増加は小さく、
本発明部材は耐摩耗性も実用上満足し得ることが
判る。尚実施例３からなる部材では、加湿劣化に
より接触抵抗に若干の増加が認められた。
実施例 ６
厚さ0.5mmのNi条に、常法により下記メツキ浴
を用いてNiストライクメツキを施してからPd
（NH3）2（NO2）2浴を用いて厚さ2μのPdメツキを
行ない、これを水素ガス中900℃の温度で0.5時間
加熱拡散処理した後、厚さ0.3mmまで圧延して本
発明部材を得た。
この部材をリードレール用Fe系接点基体に溶
接により取付けてリードリレーの固定側接点を形
成し、可動側接点基体に厚さ3.5μのAu−0.6％Co
合金をメツキしたものを用い、電圧30V、電流
1A、振動数１Hzの条件で溶着するまでの接点寿
命を測定した。その結果溶着までの振動数は平均
55×104回であつた。尚本発明部材の表面Pd濃度
は85％拡散層の厚さは3.6μであつた。
比較のため固定側接点基体に、可動側接点基体
と同様厚さ3.5μのAg−0.6％Co合金をメツキした
従来のリードリレーでは、溶着までの寿命は30〜
45×104回であり、本発明部材によれば性能及び
経済性が著しく改善されることが判る。
以上何れもPd又はPd合金をメツキし、これを
加熱拡散してPdの拡散層を形成した例について
説明したが、これに限るものではなく、Pd又は
Pd合金を蒸着、スパツタリングその他任意の方
法で被着してもよい。
このように本発明部材によれば、Ni、Co又は
これ等の合金からなる接触面にPdを拡散させる
ことにより、両者の欠点を軽減緩和し、かつ加工
性を損なうことなく長期間に亘り安定した電気接
触特性を得ることができるもので、工業上顕著な
効果を奏するものである。[Table] As is clear from Table 3, even when the members of the present invention undergo humidification deterioration after insertion and removal, the increase in contact resistance is small;
It can be seen that the members of the present invention have practically satisfactory wear resistance. In the member made in Example 3, a slight increase in contact resistance was observed due to deterioration due to humidification. Example 6 A Ni strip with a thickness of 0.5 mm was plated with Ni strike plating using the following plating bath in a conventional manner, and then plated with Pd.
Pd plating with a thickness of 2 μm was performed using a (NH 3 ) 2 (NO 2 ) 2 bath, which was heat-diffused in hydrogen gas at a temperature of 900°C for 0.5 hours, and then rolled to a thickness of 0.3 mm. An invented member was obtained. This member is attached to the Fe-based contact base for the lead rail by welding to form the fixed side contact of the reed relay, and the movable side contact base is attached to a 3.5μ thick Au-0.6% Co
Uses plated alloy, voltage 30V, current
The contact life until welding was measured under the conditions of 1A and 1Hz frequency. As a result, the frequency of vibration until welding is the average
It was 55×10 4 times. The surface Pd concentration of the member of the present invention was 85%, and the thickness of the diffusion layer was 3.6μ. For comparison, a conventional reed relay in which the fixed side contact base is plated with 3.5 μ thick Ag-0.6% Co alloy like the movable side contact base has a lifespan of 30 to 30 minutes until welding.
45×10 4 times, indicating that the member of the present invention significantly improves performance and economical efficiency. In each of the above examples, Pd or Pd alloy is plated and heated and diffused to form a Pd diffusion layer. However, the present invention is not limited to this.
The Pd alloy may be deposited by vapor deposition, sputtering, or any other method. In this way, according to the member of the present invention, by diffusing Pd into the contact surface made of Ni, Co, or an alloy of these, the drawbacks of both are alleviated and the material is stable for a long period of time without impairing workability. This makes it possible to obtain electrical contact characteristics that are industrially significant.