JP4701426B2

JP4701426B2 - Copper powder and copper powder manufacturing method

Info

Publication number: JP4701426B2
Application number: JP2000358802A
Authority: JP
Inventors: 和司佐野; 美洋岡田; 宏昌三好; 賢臣高田
Original assignee: Dowa Electronics Materials Co Ltd
Current assignee: Dowa Electronics Materials Co Ltd
Priority date: 1999-12-01
Filing date: 2000-11-27
Publication date: 2011-06-15
Anticipated expiration: 2020-11-27
Also published as: JP2001220607A

Description

【０００１】
【発明の属する技術分野】
本発明は，粒径が小さくても凝集の少ない銅粉の製法に係り，低い焼成温度でもポアのない疑融体焼成品が得られる銅粉に関する。
【０００２】
【従来の技術】
従来より，絶縁基板の意図する位置に導電回路を接合または配属したりチップ部品に外部電極を付与したりするのに導電ペーストが使用されている。導電ペーストの導電材料としては，銅，ニッケル，銀などの粉体が適用されているが，銅粉は廉価でありながら抵抗値が低く且つ銀のようにマイグレーションが起き難いという特徴があるので，銅ペーストが多く使用されている。
【０００３】
最近では，積層セラミックスコンデンサーの外部電極として，金属粉をフイラーとした導電性ペーストの使用が提案され使用されている。この場合には，高温で焼き固めた誘電体であるセラミックスに，外部電極として金属粉を焼き付けることになる。例えば，セラミックス素体を導電性ペーストにディップ後加熱処理する。加熱中にペースト中のビヒクル分が蒸発または分解除去すると共に金属粉体が焼結して外部電極が形成される。この金属粉体としても銅粉が多く場合使用される。
【０００４】
銅粉の製造法としては，機械的粉砕法，溶融銅を噴霧するアトマイズ法，陰極への電解析出法，蒸発蒸着法，湿式還元法等が知られているが，湿式還元法は他の方法に比べると小さい粒径の均一な粒子を比較的容易に得ることができるので，導電ペースト用銅粉を製造する場合の主流となっており，例えば特開平４−１１６１０９号公報，特開平２−１９７０１２号公報および特開昭６２−９９４０６号公報には湿式還元法による銅粉の製造法が記載されている。
【０００５】
【発明が解決しようとする課題】
前記の積層セラミックスコンデンサーの外部電極を銅ペーストの焼成によって形成する場合，従来の銅粉を使用したものでは，緻密な導体とするには一般に８００℃を超える焼成温度を必要としている。８００℃以下の温度では，焼結が起きないか，起きても粒子同士の一体的な接合が十分ではなく空隙の多い焼結体となり，良好な導電体を形成できないからである。したがって，８００℃より高い焼結温度（不活性雰囲気中１気圧のもとで）を一般に必要とする。
【０００６】
この場合，高温に昇温されると，積層セラミックスコンデンサーの材質によっては熱的影響により素体の一部が還元されるなどのダメージを受けたり，導電体が過度の収縮を起こして内部電極との接合不備を生じたり焼結体自身にクラックや膨れが発生させるなどして，これが品質低下や容量低下をもたらす要因ともなりかねない。
【０００７】
このような品質低下のおそれのみならず，高温での焼成では，加熱エネルギー，加熱時間，加熱装置等のエネルギー的および設備的な負荷が増大し，製造コストを押し上げる要因となり，また歩留りを低下させる要因ともなる。
【０００８】
したがって，本発明は，焼成温度を下げても空隙のない一体的な焼成品が得られるような銅粉を得ることを目的としたものである。
【０００９】
【課題を解決するための手段】
前記の課題を解決すべく本発明者らは鋭意研究を重ねたところ，粒径を小さくしても凝集が発生しがたく，したがって，８００℃以下の焼成温度でも，空隙のない焼成品（一見したところ，一たん融けてメルトダウンしたような形態の焼成品，本明細書ではこれを「疑融体焼成品」と呼ぶ）が得られる銅粉を得ることができた。具体的には，湿式還元法における二次還元の前または途中において，懸濁液をアンモニアまたはアンモニウム塩と接触させると，粒径が小さくても，粒度分布の狭く且つ表面が滑らかな（ＢＥＴ比表面積がその粒径にしては相対的に小さい）銅粉が得られ，このものは，粒子同士の凝集が起こり難くて，低温焼成に適することがわかった。
【００１０】
したがって，本発明によれば，銅塩水溶液とアルカリ剤を反応させて水酸化銅を析出させ，得られた水酸化銅を亜酸化銅に液中で一次還元し，得られた亜酸化銅を金属銅に液中で二次還元し，得られた金属銅を液から分離する銅粉の製造法において，該二次還元の前または還元途中の懸濁液をアンモニアまたはアンモニウム基と接触させることを特徴とする銅粉の製法を提供する。
【００１１】
このようにして本発明によれば，平均粒径が０.１μｍ以上で１.５μｍ未満の範囲にあり，下記に定義するＸ２５，Ｘ５０およびＸ７５の値の間で下式(1)に従うＡ値が１.２以下を示す粒度分布幅の狭い銅粉であって，不活性ガス１気圧雰囲気下８００℃の温度に維持したときに疑融体焼成品となる銅粉を提供する。
Ａ値＝（Ｘ７５−Ｘ２５）／Ｘ５０・・・(1)
ただし，Ｘ２５，Ｘ５０およびＸ７５は，横軸に粒径Ｘ（μｍ），縦軸にＱ％（その粒径以下の粒子が存在する割合・単位は粒子の容積％）をとった累積粒度曲線において，Ｑ％＝２５％，５０％および７５％に対応するそれぞれの粒径Ｘの値を言う。
【００１２】
【発明の実施の形態】
湿式還元法による銅粉の製造法は，銅塩水溶液とアルカリ剤を反応させて水酸化銅を析出させる工程，得られた水酸化銅を亜酸化銅に水中で一次還元する工程，得られた亜酸化銅を金属銅に水中で二次還元する工程からなり，得られた金属銅は液から分離したあと，耐酸化性付与のための表面処理を施し或いは施すことなく，乾燥することによって金属銅粉を得るものであるが，本発明者らは，二次還元する工程において，アンモニアまたはアンモニウム基の存在下で還元を進行させると，平均粒径が例えば１.５μｍ以下，好ましくは１.２μｍ以下，さらに好ましくは１.０μｍ以下の微細粉であっても，その微細な粒径にしてはＢＥＴ比表面積が小さく（すなわち，表面に凹凸が少なく）且つ粒度分布の狭い（すなわち，ほぼ同じ粒径のものが揃った）銅粉が得られ，このものは，微粒子であっても，凝集し難い性質を有することがわかった。そして，この銅粉は焼成温度が低くても，疑融体焼成品となることがわかった。
【００１３】
一般に，前述の積層基板の外部電極を形成するための焼成処理は非酸化性常圧雰囲気（実際には不活性ガス常圧雰囲気）中で行われるが，銅ペースト中の銅粉の粒径が小さいほど，低い温度でも焼成が進行する。しかし，従来の湿式還元法によって例えば平均粒径が１μｍ以下のような微粉（サブミクロン粉）が得られたとしても，実際には，数個ないし数１０個の粒子同士が互いに付着（接着）または絡み合った粗大粒子（数μｍないし数１０μｍの径をもつ複合粒子）を形成しやすく，このような複合粒子とサブミクロン粒子とが混在した粉体（凝集した粉体）となり，これを，同様に低温で焼成処理した場合には，部分的には焼成が進行するとしても空隙の多い焼成品となる。したがって，焼成温度を下げるには，単に粒径を小さくすればよい，というわけのものではない。
【００１４】
ところが，後記の実施例にも示すように，湿式還元法の二次還元をアンモニアまたはアンモニウム基の存在下で進行させて得た銅粉は，例えば粒径が１μｍ以下のような微粉でも，前記のような粗大粒子を形成しがたく（凝集しがたく），８００℃以下の焼成温度でも空隙のない又は少ない疑融体焼成品を得ることができる。その理由は必ずしも明らかではないが，液中にアンモニアまたはアンモニウム塩が存在すると，これらが錯化剤の役割を果たし，Ｃｕがいったん錯体となって液側に移行し，これから還元が進行する結果，表面が滑らかで粒径の揃った金属銅が形成されるのではないかと考えられる。添加するアンモニアまたはアンモニウム基は，アンモニア気体，アンモニア水，水酸化アンモニウム，各種アンモニウム塩が適用できるが，アンモニア水が取扱いに便利である。その添加量は，系内の銅１モルに対してアンモニア換算で０.０１〜０.１モル，好ましくは０.０２〜０.０８であればよい。実際には，金属銅への還元が終了した時点でアンモニアまたはアンモニウム基が液中に残存しているのが望ましい。
【００１５】
他方，金属銅粉の平均粒径を小さくするには，二次還元に使用する還元剤の量を当量以上として一挙に添加するのがよい。具体的には，抱水ヒドラジンを還元剤とする場合に，亜酸化銅を金属銅に還元するに必要な化学量論量の１.１倍以上の抱水ヒドラジンを一挙に添加するのがよい。これによって，平均粒径が０.１〜１.５μｍ，好ましくは０.３〜１.２μｍの範囲の微細な金属銅粉を得ることができる。また，一次還元された亜酸化銅の懸濁液に酸素含有ガスを吹き込むとその吹込量に応じて粒径制御ができ，また，粒度分布の幅を小さくすることもできる。酸素含有ガスの吹込量が多くなるほど粒径は大きくなるが，粒径は小さくしながら粒度分布の幅を小さくする効果を期待する場合には，全体として少量の酸素含有ガスを時間をかけて吹き込むのがよい。
【００１６】
そのほかの処理工程は公知の方法を採用することができる。例えば水酸化銅の析出工程では，銅塩水溶液としては硫酸銅水溶液が普通に使用できるが，塩化銅，炭酸銅，硝酸銅などの水溶液であってもよい。アルカリ剤としてはＮａＯＨ水溶液が最も普通に使用できるが，これ以外にも，他に影響を与えないアルカリ剤であれば使用可能である。水酸化銅の析出反応は，所定濃度の銅塩水溶液と所定濃度のアルカリ水溶液を別途に作製し，アルカリ過剰となるように両液を混合し直ちに強攪拌する方法，或いは該銅塩水溶液にアルカリ水溶液を攪拌下に添加し続けるという方法で進行させればよい。得られた水酸化銅懸濁液に対して，還元剤を添加して水酸化銅を亜酸化銅に還元するには，還元剤としてグルコース（ブドウ糖）を普通に使用できる。この一次還元工程は不活性ガス雰囲気下で昇温しながら（例えば５０〜９０℃）で行うのがよい。前述のように酸素含有ガスを吹き込む場合には，空気を液中にバブリングさせればよい。
【００１７】
アンモニアまたはアンモニウム基の存在下で抱水ヒドラジンを添加して金属銅にまで最終還元したあとは，液中の金属銅を液から分離し，これを耐酸化性付与のための表面処理を施し或いは施すことなく，乾燥することによって，平均粒径が小さく且つ凝集の少ない金属銅粉を得ることができる。
【００１８】
この銅粉は，平均粒径が０.１μｍ以上で１.５μｍ未満，好ましくは０.３〜１.２μｍの範囲にある。そして，その平均粒径に近い粒径の粒子の数が多く，平均粒径から遠い粒径の粒子が少ないものである。具体的には，例えばヘロス粒度分布測定装置によって粒度分布を計測したときに，粒径Ｘ（μｍ）を横軸とし，縦軸にＱ％をとって表わされた累積粒度曲線（後記実施例の図１参照）において（Ｑ％はその粒径以下の粒子が存在する割合・単位は粒子の容積％），Ｑ％＝２５％，５０％および７５％に対応するそれぞれの粒径Ｘの値，Ｘ２５，Ｘ５０およびＸ７５の値の間で式(1) に従うＡ値，すなわち，
Ａ値＝（Ｘ７５−Ｘ２５）／Ｘ５０・・・(1)
がいずれも１.２以下，好ましくは１.０以下を示す粒度分布幅の狭い銅粉である。加えて，この銅粉は，平均粒径が小さいにも拘わらず，ＢＥＴ比表面積が相対的に低いものである。すなわち表面に凹凸が少なく，滑らかである（後記実施例の図２参照）。
【００１９】
このような平均粒径，Ａ値および表面平滑性（小さいＢＥＴ比表面積，例えば平均粒径が０.８μｍ程度でもＢＥＴ値が２.０ｍ²/ｇ以下）を満たした本発明に従う銅粉は，不活性ガス１気圧の雰囲気下８００℃の温度に維持したときに疑融体焼成品となる（後記実施例の図３参照）。これに対して平均粒径が本発明で規定する範囲にあっても，Ａ値が本発明で規定する範囲外の銅粉は同じ８００℃の温度で焼成しても，空隙のあるポーラスな焼成品となり（例えば図６参照），また，Ａ値が本発明で規定する範囲であっても，平均粒径が本発明で規定する範囲より大きい場合には，焼成が進行しない（例えば図８参照）。
【００２０】
したがって，本発明に従う銅粉をフイラーとした導電ペーストを例えば積層セラミックコンデンサーの外部電極形成用に使用すれば，低い焼成温度で空隙のない外部電極を形成することができる。
【００２１】
実際のところ，本発明に従う銅粉は図９に見られるように昇温中の焼結挙動が従来粉とは異なっている。図９は後記の実施例１と比較例１の銅粉を一定荷重下で定速昇温したときの収縮挙動を調べたものであるが（試験条件は後述），比較例１のものは６２０〜６３０℃付近から収縮が開始し，その収縮は１０００℃に至ってもまだ続いているのに対し，実施例１のものは５９０〜６００℃付近から主たる収縮が始まって８００℃に至ると収縮はほぼ完了し，それ以上高温に達しても収縮は進行しない。すなわち本発明に従う銅粉は８００℃で焼結が終了し，それ以上加熱してももはや収縮が生じないことから，その焼結終了時点で空隙のないメルトダウンしたような疑焼成品が得られていると見ることができる。
【００２２】
【実施例】
〔実施例１〕
１.０４ＫｇのＣｕＳＯ₄・５Ｈ₂Ｏを２.５４Ｋｇの純水に溶かした硫酸銅水溶液Ａと，濃度４９％のＮａＯＨ水溶液８５０ｇを３.２Ｋｇの純水に加えたアルカリ水溶液Ｂとを準備し，温度２９℃に保持した溶液Ａと温度２７℃に保持した溶液Ｂとを全量反応容器内で攪拌混合した。発熱により液温度は３６℃まで昇温し，水酸化銅が析出した懸濁液を得た。
【００２３】
得られた水酸化銅懸濁液の全量に対し，純水１.５９Ｋｇに１.１２Ｋｇのぶどう糖を溶かしたぶどう糖溶液を添加し，添加後３０分間で液の温度を７０℃まで昇温したあと，３０分間保持した。ここまでの処理操作（水酸化銅の析出および亜酸化銅への還元操作）は全て窒素雰囲気下で行った。
【００２４】
ついで，この液中に１リットル／分の流量で２００分間にわたって空気をバブリングさせたあと，その懸濁液を窒素雰囲気中で２日間静置したうえ，上澄液（ｐＨ５.５）を除去して，沈澱をほぼ全量採取し，この沈澱物に純水２.２５Ｋｇを追加して懸濁液とした。
【００２５】
この懸濁液に，２０wt％のアンモニア水を，懸濁液重量に対して２wt％添加した。このアンモニア添加量は，系内の銅１モルに対してアンモニア０.０４モルに相当する。これにより液のｐＨは１０となった。そして，液温を５０℃に調整し，抱水ヒドラジン１３０ｇを一挙に添加した。発熱により液温は８０℃まで上昇し，反応が終了した。反応終了後の懸濁液を固液分離して銅粉を採取し，これを１１０℃の不活性ガス雰囲気中で乾燥して銅粉を得た。
【００２６】
得られた銅粉をサブシーブサイザー（ＳＳＳ）で平均粒径を測定したところ，平均粒径は０.８μｍであった。ＢＥＴ比表面積を測定したところ１.６ｍ²/ｇであった。さらに酸素含有量と炭素含有量を分析したところ，Ｏ＝０.１６wt％，Ｃ＝０.０９wt％であった。
【００２７】
図１は，この銅粉をＳＹＭＰＡＴＥＣ社製のヘロス粒度分布測定装置（ＨＥＬＯＳ＆ＲＯＤＯＳ）で粒度分布を測定した結果を示したものである。図１において「曲線１」は横軸に粒径Ｘ（μｍ）を対数目盛でとり，縦軸に分布密度（右側スケール）をとったときの粒度分布曲線を示しており，「曲線２」は横軸は同じく粒径Ｘ（μｍ），縦軸にＱ％（左側スケール）をとったときの累積粒度曲線を示している。Ｑ％は，粒径Ｘμｍ以下の粒子が存在する容積（％）である。この曲線２からＱ％が２５％，５０％および７５％のときの粒径Ｘは，それぞれＸ２５＝０.４７，Ｘ５０＝０.７７，Ｘ７５＝１.０８μｍであることがわかる。したがって，Ａ値＝０.７９である。これらの測定結果を表１に総括して示した。
【００２８】
図２は本例の銅粉のＳＥＭ像である。図２から，この銅粉は，粒径がほぼ０.８μｍのものが揃った表面平滑な粒子からなることがわかる。
【００２９】
次に，本例の銅粉３０ｇと樹脂６ｇ（エチルセルロース９５％＋ターピネオール５％）を脱泡式混練機で３分間混練し，その混練物をアルミナ基板上に厚み３０μｍで塗布して窒素雰囲気中で１００℃で３時間乾燥した。次いでこの乾燥品を窒素雰囲気中（１気圧）で８００℃で３０分間の焼成処理を行った。得られた焼結体をＳＥＭ観察し，そのＳＥＭ像を図３に示した。図３から，本例の銅粉は８００℃で疑融体焼成品となることがわかる。すなわち，８００℃でメルトダウンしたような状態の殆ど空隙のない一体品となることがわかる。
【００３０】
〔実施例２〕
２０wt％アンモニア水の添加量を懸濁液重量に対して１.５wt％に変更した以外は，実施例１を繰り返した。このアンモニア添加量は，系内の銅１モルに対してアンモニア０.０３モルに相当する。得られた銅粉の諸性質を実施例１と同様にして測定した結果を表１に併記した。この銅粉を用いて実施例１と同じ条件で焼成したところ，実施例１と同様に空隙の殆どない疑融体焼成品が得られた。
【００３１】
〔実施例３〕
２０wt％アンモニア水の添加量を懸濁液重量に対して１.０wt％に変更した以外は，実施例１を繰り返した。このアンモニア添加量は，系内の銅１モルに対してアンモニア０.０２モルに相当する。得られた銅粉の諸性質を実施例１と同様にして測定した結果を表１に併記した。この銅粉を用いて実施例１と同じ条件で焼成したところ，実施例１と同様に空隙の殆どない疑融体焼成品が得られた。
【００３２】
〔比較例１〕
アンモニア水の添加処理を実施しなかった以外は，実施例１を繰り返した。得られた銅粉の諸性質を実施例１と同様にして測定した結果を表１に併記した。
【００３３】
また図４に，実施例１の場合と同様にヘロス粒度分布測定装置で測定して得た本例の銅粉の粒度分布曲線１と累積粒度曲線２を示した。さらに，図５に本例の銅粉のＳＥＭ像を，また図６に本例の銅粉を実施例１と同じ条件（８００℃）で焼成して得た焼成品のＳＥＭ像を示した。図５から，本例の銅粉では，数個若しくは数１０個の粒子が付着接合または絡みあった粗大粒子が多く見られ，凝集が進行している状況がわかる。そして，図６から，本例のような凝集した粉体では，たとえ平均粒径が小さくても，８００℃の温度では，図１のような空隙のない疑融体焼成品は得られず，粒子同士が部分的に接合した空隙の多い焼成品となることがわかる。このものは，実施例１のものよりも導電率が低くなることは明らかである。
【００３４】
〔比較例２〕
７リットル／分の流量で２００分間にわたって空気をバブリングした以外は比較例１を繰り返した。得られた銅粉の諸性質を実施例１と同様にして測定した結果を表１に併記した。
【００３５】
また図７に本例の銅粉のＳＥＭ像を，また図８に本例の銅粉を実施例１と同じ条件（８００℃）で焼成して得た焼成品のＳＥＭ像を示した。図７から，本例の銅粉は粒径が大きく（図７は図２と図５よりも倍率が半分である），各粒子は凝集していないことがわかる。そして，図８から，本例のような粒径の大きな粉体では，８００℃の温度では焼成は進行しないことがわかる。
【００３６】
図９は，実施例１と比較例１の銅粉について，昇温中の収縮率測定結果を対比して示したものである。収縮率測定試験は，銅粉0.97±0.001gをバインダー (ターピネオール＋4.5wt.% のエチルセルロース)0.03 〜0.05g と混合し，成形冶具を用いて直径５mmの円柱形状に成形 (165kgf加圧, 10秒保持) し，この成形品を熱収縮測定装置 (真空理工株式会社製のTM 7000)にセットし, 10.0 gの一定荷重をかけながら, 窒素雰囲気中で昇温速度10℃/minで定速昇温し，成形品に接触させた熱電対の温度を計測しながら成形品の収縮率 (円柱の高さ減少率) を自動計測する。図９の結果にみられように，比較例１の銅粉は６２０〜６３０℃付近から収縮を開始し１０００℃に至るまで徐々に収縮を続けるのに対し，実施例１の銅粉は５９０〜６００℃付近から主たる収縮を開始し，８００℃では収縮をほぼ完了し，それ以上昇温を続けても，もはや収縮は生じないことがわかる。このことは８００℃において実施例１のものは空隙のない焼結体（メルトダウンに似た疑融体焼成品）が得られていると見てよい。
【００３７】
【表１】

【００３８】
【発明の効果】
以上説明したように，本発明によると，８００℃で空隙が無いか少ない疑融体焼結品となる金属銅粉が提供される。この金属銅粉は低温で一体品に焼成する性質を有するので，例えば積層セラミックコンデンサーの外部電極材を形成するのに好適である。
【図面の簡単な説明】
【図１】本発明に従う銅粉のヘロス粒度分布測定チャートである。
【図２】本発明に従う銅粉のＳＥＭ像である。
【図３】本発明に従う銅粉の８００℃焼成品のＳＥＭ像である。
【図４】比較例の銅粉のヘロス粒度分布測定チャートである。
【図５】比較例の銅粉（粒径は小さいが凝集が起きている）のＳＥＭ像である。
【図６】比較例の銅粉（粒径は小さいが凝集が起きている）の８００℃焼成品のＳＥＭ像である。
【図７】比較例の銅粉（凝集は起きていないが粒径が大きい）のＳＥＭ像である。
【図８】比較例の銅粉（凝集は起きていないが粒径が大きい）の８００℃焼成品のＳＥＭ像である。
【図９】本発明に従う銅粉と比較例の銅粉の成形品について一定荷重下で定速昇温した場合の収縮率の変化を示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a copper powder with little agglomeration even when the particle size is small, and relates to a copper powder from which a pseudo-melt fired product having no pore can be obtained even at a low firing temperature.
[0002]
[Prior art]
Conventionally, a conductive paste has been used to join or assign a conductive circuit to an intended position of an insulating substrate or to apply an external electrode to a chip component. As the conductive material of the conductive paste, powders such as copper, nickel, silver, etc. are applied. However, copper powder has a low resistance value and is unlikely to cause migration like silver. A lot of copper paste is used.
[0003]
Recently, it has been proposed and used as an external electrode for multilayer ceramic capacitors. In this case, metal powder is baked as an external electrode on the ceramic, which is a dielectric material baked at high temperature. For example, the ceramic body is dipped into a conductive paste and then heated. During heating, the vehicle component in the paste is evaporated or decomposed and the metal powder is sintered to form an external electrode. As this metal powder, copper powder is often used.
[0004]
Known methods for producing copper powder include mechanical pulverization, atomization by spraying molten copper, electrolytic deposition on the cathode, evaporation deposition, and wet reduction. Since uniform particles having a small particle size can be obtained relatively easily as compared with the method, it has become a mainstream in the production of copper powder for conductive pastes. For example, JP-A-4-116109 and JP-A-2 -197012 and JP-A-62-99406 describe a method for producing copper powder by a wet reduction method.
[0005]
[Problems to be solved by the invention]
When the external electrode of the multilayer ceramic capacitor is formed by firing a copper paste, a conventional one using copper powder generally requires a firing temperature exceeding 800 ° C. to make a dense conductor. This is because sintering does not occur at a temperature of 800 ° C. or lower, or even if the sintering occurs, the particles are not sufficiently joined together to form a sintered body with many voids, and a good conductor cannot be formed. Therefore, sintering temperatures higher than 800 ° C. (under 1 atmosphere in an inert atmosphere) are generally required.
[0006]
In this case, if the temperature is raised to a high temperature, depending on the material of the multilayer ceramic capacitor, the element body may be damaged due to thermal effects, or the conductor may be excessively contracted and This may cause defects in quality and capacity, such as incomplete bonding or cracks and swelling in the sintered body itself.
[0007]
In addition to the risk of quality degradation, high-temperature firing increases the energy and equipment loads such as heating energy, heating time, and heating equipment, which increases manufacturing costs and decreases yield. It becomes a factor.
[0008]
Therefore, an object of the present invention is to obtain a copper powder that can obtain an integrally fired product without voids even when the firing temperature is lowered.
[0009]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors have conducted extensive research and found that aggregation does not easily occur even when the particle size is reduced. As a result, it was possible to obtain a copper powder from which a baked product that melted down and melted down, which is referred to as “suspected melt baked product” in this specification. Specifically, when the suspension is brought into contact with ammonia or an ammonium salt before or during the secondary reduction in the wet reduction method, the particle size distribution is narrow and the surface is smooth (BET ratio) even if the particle size is small. Copper powder was obtained (surface area is relatively small in terms of particle size), and it was found that the particles are less likely to aggregate and suitable for low-temperature firing.
[0010]
Therefore, according to the present invention, a copper salt aqueous solution and an alkali agent are reacted to precipitate copper hydroxide, and the obtained copper hydroxide is primarily reduced to cuprous oxide in the liquid. In the method for producing copper powder, in which secondary reduction to metallic copper is performed in a liquid and the resulting metallic copper is separated from the liquid, the suspension before or during the secondary reduction is brought into contact with ammonia or an ammonium group. The manufacturing method of the copper powder characterized by these is provided.
[0011]
Thus, according to the present invention, the average particle size is in the range of 0.1 μm or more and less than 1.5 μm, and the A value according to the following formula (1) between the values of X25, X50 and X75 defined below. Is a copper powder having a narrow particle size distribution range of 1.2 or less, and a copper powder that becomes a suspected melt-baked product when maintained at a temperature of 800 ° C. in an atmosphere of 1 atmosphere of inert gas.
A value = (X75−X25) / X50 (1)
However, X25, X50, and X75 are cumulative particle size curves in which the horizontal axis is the particle size X (μm) and the vertical axis is Q% (the ratio of the particles having the particle size below the unit / unit is the volume percent of the particles). , Q% = 25%, 50% and 75% respectively corresponding to the value of particle size X.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The method for producing copper powder by the wet reduction method was a step of causing copper hydroxide aqueous solution and an alkali agent to react to precipitate copper hydroxide, a step of primary reduction of the obtained copper hydroxide to cuprous oxide in water, It consists of a step of secondary reduction of cuprous oxide to metallic copper in water, and the obtained metallic copper is separated from the liquid and then dried by applying or drying with or without surface treatment for imparting oxidation resistance. Although copper powder is obtained, the present inventors proceeded with reduction in the presence of ammonia or ammonium groups in the secondary reduction step, and the average particle size is, for example, 1.5 μm or less, preferably 1. Even fine powder of 2 μm or less, more preferably 1.0 μm or less, has a small BET specific surface area (that is, less irregularities on the surface) and a narrow particle size distribution (that is, almost the same). Particle size Uniform) copper powder was obtained, this thing, even fine particles were found to have agglomerated hard nature. And it turned out that this copper powder turns into a suspicious melt baked product, even if a calcination temperature is low.
[0013]
In general, the firing process for forming the external electrode of the above-mentioned multilayer substrate is performed in a non-oxidizing atmospheric pressure atmosphere (actually an inert gas atmospheric pressure atmosphere). The smaller the temperature, the lower the firing temperature. However, even if a fine powder (submicron powder) having an average particle diameter of 1 μm or less is obtained by a conventional wet reduction method, in reality, several to several tens of particles are adhered (adhered) to each other. Or, it is easy to form entangled coarse particles (composite particles having a diameter of several μm to several tens of μm), and such composite particles and submicron particles are mixed (powdered powder). When the baking treatment is performed at a low temperature, a burned product with many voids is obtained even if the baking proceeds partially. Therefore, reducing the firing temperature does not simply mean reducing the particle size.
[0014]
However, as shown in the examples described later, the copper powder obtained by allowing the secondary reduction of the wet reduction method to proceed in the presence of ammonia or ammonium groups is, for example, a fine powder having a particle size of 1 μm or less. Such a coarse-grained product is difficult to form (it is difficult to aggregate), and even a firing temperature of 800 ° C. or less can provide a pseudo-melt fired product with little or no voids. The reason for this is not necessarily clear, but when ammonia or ammonium salt is present in the liquid, these serve as a complexing agent, Cu once forms a complex and moves to the liquid side, and the reduction proceeds from this, It is thought that metallic copper having a smooth surface and a uniform particle size may be formed. As the ammonia or ammonium group to be added, ammonia gas, ammonia water, ammonium hydroxide, and various ammonium salts can be applied, but ammonia water is convenient for handling. The amount of addition may be 0.01 to 0.1 mol, preferably 0.02 to 0.08 in terms of ammonia with respect to 1 mol of copper in the system. In practice, it is desirable that ammonia or ammonium groups remain in the liquid when the reduction to metallic copper is completed.
[0015]
On the other hand, in order to reduce the average particle size of the metallic copper powder, it is preferable to add the reducing agent used for the secondary reduction at an equivalent amount or more. Specifically, when hydrazine hydrate is used as the reducing agent, it is better to add hydrazine hydrate at least 1.1 times the stoichiometric amount required to reduce cuprous oxide to metallic copper. . As a result, it is possible to obtain fine metallic copper powder having an average particle diameter of 0.1 to 1.5 μm, preferably 0.3 to 1.2 μm. In addition, when an oxygen-containing gas is blown into the primarily reduced cuprous oxide suspension, the particle size can be controlled according to the blown amount, and the width of the particle size distribution can be reduced. The larger the amount of oxygen-containing gas blown, the larger the particle size. However, if you expect the effect of reducing the particle size distribution width while reducing the particle size, blow a small amount of oxygen-containing gas over time. It is good.
[0016]
Other processing steps can employ known methods. For example, in the precipitation process of copper hydroxide, a copper sulfate aqueous solution can be normally used as the copper salt aqueous solution, but an aqueous solution of copper chloride, copper carbonate, copper nitrate or the like may be used. As the alkali agent, an NaOH aqueous solution can be most commonly used, but besides this, any alkali agent that does not affect others can be used. The copper hydroxide precipitation reaction may be carried out by separately preparing a copper salt aqueous solution having a predetermined concentration and an alkali aqueous solution having a predetermined concentration, mixing both solutions so that the alkali is excessive, and immediately stirring vigorously, or adding an alkali solution to the copper salt aqueous solution. What is necessary is just to advance by the method of continuing adding aqueous solution under stirring. In order to reduce the copper hydroxide to cuprous oxide by adding a reducing agent to the obtained copper hydroxide suspension, glucose (glucose) can be commonly used as the reducing agent. This primary reduction step is preferably performed while raising the temperature in an inert gas atmosphere (for example, 50 to 90 ° C.). As described above, when the oxygen-containing gas is blown, air may be bubbled into the liquid.
[0017]
After adding hydrazine hydrate in the presence of ammonia or ammonium groups to final reduction to metallic copper, the metallic copper in the liquid is separated from the liquid and subjected to a surface treatment for imparting oxidation resistance or By drying without applying, a metallic copper powder having a small average particle diameter and little aggregation can be obtained.
[0018]
The copper powder has an average particle size of 0.1 μm or more and less than 1.5 μm, preferably in the range of 0.3 to 1.2 μm. The number of particles having a particle size close to the average particle size is large, and the number of particles having a particle size far from the average particle size is small. Specifically, for example, when the particle size distribution is measured by a heros particle size distribution measuring apparatus, the particle size X (μm) is taken as the horizontal axis, and the cumulative particle size curve is expressed by taking Q% on the vertical axis (Examples described later). (See Fig. 1) (Q% is the ratio of particles having a particle size equal to or less than the volume% of the particles), Q% = values of particle diameter X corresponding to 25%, 50% and 75%, respectively. , X25, X50 and X75, the value A according to equation (1), ie
A value = (X75−X25) / X50 (1)
Are copper powders having a narrow particle size distribution width of 1.2 or less, preferably 1.0 or less. In addition, this copper powder has a relatively low BET specific surface area despite its small average particle size. That is, the surface is smooth with few irregularities (see FIG. 2 in the examples described later).
[0019]
The copper powder according to the present invention satisfying such an average particle size, A value and surface smoothness (small BET specific surface area, for example, even if the average particle size is about 0.8 μm, the BET value is 2.0 m ² / g or less) When it is maintained at a temperature of 800 ° C. under an atmosphere of 1 atm of inert gas, it becomes a suspicious melt baked product (see FIG. 3 in Examples described later). On the other hand, even if the average particle diameter is in the range specified by the present invention, the copper powder having an A value outside the range specified by the present invention is porous baked with voids even if baked at the same temperature of 800 ° C. Even if the A value is in the range specified by the present invention, if the average particle size is larger than the range specified by the present invention, firing does not proceed (see, for example, FIG. 8). ).
[0020]
Therefore, if a conductive paste using copper powder as a filler according to the present invention is used, for example, for forming an external electrode of a multilayer ceramic capacitor, an external electrode without voids can be formed at a low firing temperature.
[0021]
As a matter of fact, the copper powder according to the present invention is different from the conventional powder in the sintering behavior during temperature elevation as seen in FIG. FIG. 9 shows the shrinkage behavior when the copper powders of Example 1 and Comparative Example 1 described later were heated at a constant speed under a constant load (the test conditions are described later). Shrinkage started from around 630 ° C., and the shrinkage still continued at 1000 ° C., whereas in Example 1, the main shrinkage started from around 590 to 600 ° C. and reached 800 ° C. It is almost complete, and shrinkage does not proceed even when the temperature reaches a higher temperature. That is, since the copper powder according to the present invention is sintered at 800 ° C. and is no longer contracted even when heated further, a suspicious sintered product having no voids at the end of the sintering can be obtained. You can see that.
[0022]
【Example】
[Example 1]
An aqueous copper sulfate solution A prepared by dissolving 1.04 kg CuSO ₄ .5H ₂ O in 2.54 kg pure water and an alkaline aqueous solution B obtained by adding 850 g of 49% NaOH aqueous solution to 3.2 kg pure water were prepared. The solution A maintained at a temperature of 29 ° C. and the solution B maintained at a temperature of 27 ° C. were all stirred and mixed in the reaction vessel. Due to heat generation, the liquid temperature was raised to 36 ° C. to obtain a suspension in which copper hydroxide was precipitated.
[0023]
After adding a glucose solution in which 1.12 Kg of glucose is dissolved in 1.59 Kg of pure water to the total amount of the obtained copper hydroxide suspension, the temperature of the solution is raised to 70 ° C. in 30 minutes after the addition. , Held for 30 minutes. All of the treatment operations so far (precipitation of copper hydroxide and reduction to cuprous oxide) were performed in a nitrogen atmosphere.
[0024]
Next, after bubbling air through the solution at a flow rate of 1 liter / min for 200 minutes, the suspension was allowed to stand in a nitrogen atmosphere for 2 days, and the supernatant (pH 5.5) was removed. Then, almost all of the precipitate was collected, and 2.25 kg of pure water was added to the precipitate to form a suspension.
[0025]
To this suspension, 2 wt% of 20 wt% aqueous ammonia was added based on the weight of the suspension. This added amount of ammonia corresponds to 0.04 mol of ammonia with respect to 1 mol of copper in the system. As a result, the pH of the solution became 10. Then, the liquid temperature was adjusted to 50 ° C., and 130 g of hydrazine hydrate was added all at once. The liquid temperature rose to 80 ° C. due to exotherm, and the reaction was completed. The suspension after completion of the reaction was subjected to solid-liquid separation to collect copper powder, which was dried in an inert gas atmosphere at 110 ° C. to obtain copper powder.
[0026]
When the average particle diameter of the obtained copper powder was measured with a sub-sieve sizer (SSS), the average particle diameter was 0.8 μm. The BET specific surface area was measured and found to be 1.6 m ² / g. Further, when the oxygen content and the carbon content were analyzed, O = 0.16 wt% and C = 0.09 wt%.
[0027]
FIG. 1 shows the results of measuring the particle size distribution of this copper powder with a HELOS particle size distribution measuring device (HELOS & RODOS) manufactured by SYMPATEC. In FIG. 1, “Curve 1” is a particle size distribution curve when the horizontal axis represents the particle size X (μm) on a logarithmic scale, and the vertical axis represents the distribution density (right scale). The horizontal axis shows the particle size X (μm), and the vertical axis shows the cumulative particle size curve when Q% (left scale) is taken. Q% is a volume (%) in which particles having a particle diameter of X μm or less exist. From the curve 2, it can be seen that the particle diameters X when the Q% is 25%, 50% and 75% are X25 = 0.47, X50 = 0.77 and X75 = 1.08 μm, respectively. Therefore, A value = 0.79. These measurement results are summarized in Table 1.
[0028]
FIG. 2 is an SEM image of the copper powder of this example. From FIG. 2, it can be seen that the copper powder is composed of particles having a smooth surface with a particle diameter of approximately 0.8 μm.
[0029]
Next, 30 g of the copper powder of this example and 6 g of resin (ethyl cellulose 95% + terpineol 5%) were kneaded for 3 minutes with a defoaming kneader, and the kneaded material was applied to an alumina substrate with a thickness of 30 μm in a nitrogen atmosphere. And dried at 100 ° C. for 3 hours. Next, this dried product was baked for 30 minutes at 800 ° C. in a nitrogen atmosphere (1 atm). The obtained sintered body was observed by SEM, and the SEM image is shown in FIG. From FIG. 3, it can be seen that the copper powder of this example becomes a pseudo-melt fired product at 800 ° C. That is, it turns out that it becomes an integrated product with almost no void in a state melted down at 800 ° C.
[0030]
[Example 2]
Example 1 was repeated except that the addition amount of 20 wt% ammonia water was changed to 1.5 wt% with respect to the suspension weight. This amount of ammonia added corresponds to 0.03 mol of ammonia per 1 mol of copper in the system. The results of measuring various properties of the obtained copper powder in the same manner as in Example 1 are also shown in Table 1. When this copper powder was used for firing under the same conditions as in Example 1, a pseudo melt fired product having almost no voids was obtained as in Example 1.
[0031]
Example 3
Example 1 was repeated except that the addition amount of 20 wt% ammonia water was changed to 1.0 wt% with respect to the suspension weight. This ammonia addition amount corresponds to 0.02 mol of ammonia with respect to 1 mol of copper in the system. The results of measuring various properties of the obtained copper powder in the same manner as in Example 1 are also shown in Table 1. When this copper powder was used for firing under the same conditions as in Example 1, a pseudo melt fired product having almost no voids was obtained as in Example 1.
[0032]
[Comparative Example 1]
Example 1 was repeated except that the ammonia water addition treatment was not performed. The results of measuring various properties of the obtained copper powder in the same manner as in Example 1 are also shown in Table 1.
[0033]
FIG. 4 shows a particle size distribution curve 1 and a cumulative particle size curve 2 of the copper powder of this example obtained by measuring with a heros particle size distribution measuring apparatus in the same manner as in Example 1. Further, FIG. 5 shows an SEM image of the copper powder of this example, and FIG. 6 shows an SEM image of a fired product obtained by firing the copper powder of this example under the same conditions (800 ° C.) as in Example 1. From FIG. 5, in the copper powder of this example, a large number of coarse particles in which several or several tens of particles are adhered or entangled are seen, and it can be seen that aggregation is progressing. And from FIG. 6, even if the average particle size is small, the aggregated powder as in this example cannot obtain the no-melt-sintered sintered product as shown in FIG. 1 at a temperature of 800 ° C. It turns out that it becomes a baked product with many space | gap which particle | grains joined partially. Obviously, this has a lower conductivity than that of Example 1.
[0034]
[Comparative Example 2]
Comparative Example 1 was repeated except that air was bubbled for 200 minutes at a flow rate of 7 liters / minute. The results of measuring various properties of the obtained copper powder in the same manner as in Example 1 are also shown in Table 1.
[0035]
FIG. 7 shows an SEM image of the copper powder of this example, and FIG. 8 shows an SEM image of a fired product obtained by firing the copper powder of this example under the same conditions (800 ° C.) as in Example 1. From FIG. 7, it can be seen that the copper powder of this example has a large particle size (FIG. 7 is half the magnification of FIGS. 2 and 5), and each particle is not agglomerated. From FIG. 8, it can be seen that the powder having a large particle size as in this example does not proceed at a temperature of 800 ° C.
[0036]
FIG. 9 shows a comparison of the shrinkage measurement results during temperature rise for the copper powder of Example 1 and Comparative Example 1. In the shrinkage measurement test, 0.97 ± 0.001 g of copper powder was mixed with 0.03 to 0.05 g of binder (terpineol + 4.5 wt.% Ethylcellulose) and formed into a cylindrical shape with a diameter of 5 mm using a forming jig (165 kgf pressure, 10 Set the molded product in a heat shrinkage measuring device (TM 7000 manufactured by Vacuum Riko Co., Ltd.) and apply a constant load of 10.0 g at a constant rate of 10 ° C / min in a nitrogen atmosphere. The shrinkage rate (cylinder height reduction rate) of the molded product is automatically measured while measuring the temperature of the thermocouple that is heated and brought into contact with the molded product. As seen in the results of FIG. 9, the copper powder of Comparative Example 1 starts shrinking from around 620 to 630 ° C. and continues to shrink gradually until reaching 1000 ° C., whereas the copper powder of Example 1 has 590 to 590 ° C. It can be seen that the main shrinkage starts from around 600 ° C., the shrinkage is almost completed at 800 ° C., and no further shrinkage occurs even if the temperature is further increased. This can be considered that a sintered body having no voids (a sintered product similar to meltdown) was obtained at 800 ° C. in Example 1.
[0037]
[Table 1]

[0038]
【The invention's effect】
As described above, according to the present invention, metallic copper powder is provided that becomes a suspicious melt sintered product having no or few voids at 800 ° C. Since this metallic copper powder has the property of firing into an integral product at a low temperature, it is suitable for forming an external electrode material of a multilayer ceramic capacitor, for example.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a measurement chart of heros particle size distribution of copper powder according to the present invention.
FIG. 2 is an SEM image of copper powder according to the present invention.
FIG. 3 is an SEM image of an 800 ° C. sintered product of copper powder according to the present invention.
FIG. 4 is a chart showing a measurement of a heros particle size distribution of a copper powder of a comparative example.
FIG. 5 is an SEM image of a copper powder of comparative example (particle size is small but aggregation occurs).
FIG. 6 is an SEM image of a sintered product of 800 ° C. of copper powder of comparative example (particle size is small but aggregation occurs).
FIG. 7 is an SEM image of a copper powder of a comparative example (no aggregation occurs but the particle size is large).
FIG. 8 is an SEM image of an 800 ° C. fired product of a copper powder of a comparative example (no agglomeration but large particle size).
FIG. 9 is a diagram showing a change in shrinkage rate when a constant temperature rise is performed under a constant load for a molded product of copper powder according to the present invention and a copper powder of a comparative example.

Claims

A copper salt aqueous solution and an alkali agent are reacted to precipitate copper hydroxide. The obtained copper hydroxide is primarily reduced to cuprous oxide in liquid, and the obtained cuprous oxide is secondary to metallic copper in liquid. In the method for producing copper powder for reducing and separating the obtained metallic copper from the liquid, 1 mol of copper in the liquid is added to the suspension after the primary reduction and before the secondary reduction or during the secondary reduction. A method for producing a copper powder, characterized by adding 0.01 to 0.1 mol of ammonia or ammonium group in terms of ammonia.

The method for producing a copper powder according to claim 1, wherein a non-aggregating copper powder having an average particle size of 1.2 μm or less is obtained.