JP3876453B2 - Silicon nitride sintered body and manufacturing method thereof - Google Patents
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Description
【0001】
【発明の属する技術分野】
本発明は、反応焼結により製造される窒化ケイ素(Si3N4)焼結体、及びその製造方法に関する。
【0002】
【従来の技術】
窒化ケイ素焼結体の製造方法の一つに、シリコン(Si)粉末の成形体を窒素雰囲気中で窒化すると同時に焼結する反応焼結法がある。しかし、純粋なSi粉末からなる成形体を窒素と反応させて窒化ケイ素焼結体を得るには、例えばJ.Mater.Sci.22(1987)P.3041〜3086、特にそのP.3074〜3075に記載されるように、100時間以上にもおよぶ長時間の窒化時間を必要とするため、生産性に劣るという大きな欠点があった。
【0003】
これに対して、特公昭61−38149号公報、特開平5−330921号公報、特許出願公表平5−508612号公報には、Si粉末にNi、Co、Ti、Zr等の窒化促進剤を添加して、窒化を速める方法が提案されている。これらの方法によれば、Si粉末表面のアモルファスSiO2層が窒化促進剤と反応して液相化し、窒素の拡散速度が大きくなるため、窒化が促進されるものと考えられる。特に特開平5−330921号公報によれば、窒化時間を約8時間にまで短縮できるとしている。
【0004】
【発明が解決しようとする課題】
しかしながら、これらの窒化促進剤を用いる方法においても、Si粉末表面の液相の内側に生成したSi3N4内での窒素の拡散係数は小さく、更に内側への窒素の拡散とSi3N4の生成が阻害されるため、これ以上の窒化時間の短縮は達成できない。又、特開平5−330921号公報に記載のごとく、1200〜1450℃の高温において50℃刻みで何段階にも窒化処理を行う複雑な温度制御を必要とするなど、生産性において優れているとはいえなかった。
【0005】
更に、これらの窒化促進剤を用いる反応焼結法で得られたSi3N4焼結体は、気孔率が4〜11体積%と緻密化しておらず、3点曲げ強度も460MPaと低強度であるため、構造用材料として使用するには不適切であった。
【0006】
本発明は、かかる従来の事情に鑑み、窒化ケイ素焼結体の反応焼結における窒化時間の短縮を図り、生産性を向上させると共に、反応焼結により得られる緻密で高強度の窒化ケイ素焼結体を提供することを目的とする。
【0007】
【課題を解決するための手段】
上記目的を達成するため、本発明が提供する窒化ケイ素焼結体は、Si粉末の反応焼結により得られた窒化ケイ素焼結体であって、不対電子濃度が1015/cm3〜1021/cm3であることを特徴とする。
【0008】
本発明の窒化ケイ素焼結体を製造するためには、不対電子濃度が1015/cm3〜1020/cm3のSi粉末を用いて反応焼結させる。不対電子濃度がこの範囲にあるSi粉末は、市販のSi粉末を窒素以外の雰囲気中において300〜800℃の温度で1〜5時間焼鈍する方法等により得られる。この場合の雰囲気としては、空気、水素、アルゴン、それらの混合物又は10torr以下の真空とするのが好ましい。
また、本発明の窒化ケイ素焼結体では、焼結助剤および窒素空孔形成剤の粉末を上記不対電子濃度を有するSi粉末に加えて反応焼結させる。
【0009】
更に具体的には、本発明の窒化ケイ素焼結体の製造方法では、前記不対電子濃度のSi粉末に、焼結助剤として希土類元素、Al、Mg、Caの少なくとも1種の元素の化合物を元素換算で0.1〜15モル%、好ましくは0.5〜10モル%、及び窒素空孔形成剤としてAl、Mg、Ca以外の価数+1〜+4価、好ましくは+1〜+3価の元素で、その共有結合半径RMとSiの共有結合半径RSiとが(RM−RSi)/RSi<0.5の関係にある元素又はその化合物を元素換算で0.5〜15モル%、好ましくは1〜10モル%添加し、反応焼結させる。
【0010】
窒化空孔生成剤の具体例としては、Sc、Ni、V、Cr、Mn、Fe、Co、Cu、Ti、Zn、Ga、Geの少なくとも1種の元素、又はその化合物、好ましくはそのアルコキシド、ステアリン酸塩、ラウリン酸塩がある。
【0011】
【作用】
従来の窒化珪素の反応焼結において、窒化処理が長時間化するのは、Si粉末表面に形成されているアモルファスSiO2層と、窒化によりSi粉末表面に生成したSi3N4層の、2つの窒素拡散阻害要因が存在するからである。
【0012】
本発明では、上記2つの窒素拡散阻害要因を克服するため2つの特徴ある工程を取り入れることにより、窒素の拡散速度を向上させ、窒化時間を大幅に短縮化することに成功した。この2つの特徴ある工程とは、後述するように、原料粉末として使用するSi粉末にシリコン空孔を導入する工程と、Si3N4内で窒素空孔を生成させる工程である。
【0013】
市販のSi粉末は、図1に示すように3層構造になっていると考えられる。即ち、表面層はSiが自然酸化されることで生成したアモルファスSiO2層10、その内側にあるSiに酸素が固溶したSi(O)層20、そして最も内側のSi層3である。表面のアモルファスSiO2層10は前記のごとく窒素の内部への拡散を抑制し、中間層であるSi(O)層20では固溶した酸素は格子間に存在しており、格子間酸素はSiの格子定数を増加させる。
【0014】
従って、本発明では、Si(O)層20の格子定数の増加を抑制するため、シリコン空孔を生成させるのである。具体的には、市販のSi粉末に適切な熱処理等を行い、図2に示すように、表面のアモルファスSiO2層11の酸素を内部へ拡散させ、アモルファスSiO2層11の厚さを低減すると共にシリコン空孔が多数存在するSi(O)層21を増加させる。このシリコン空孔はSi中に固溶した元素の拡散速度を増加させると考えられる。
【0015】
次に、窒化により生成したSi3N4内での窒素の拡散速度を大きくするため、Si3N4格子内に生成させる窒素空孔について説明する。Si3N4格子はSiの+4価とNの−3価が電気的中性を保つように構成されている。そのSi3N4格子内に例えば+1〜+3価の元素が固溶するとプラスの電荷を持った元素はSiの格子点を占有し、且つ電気的中性を保つため窒素の空孔が生成する。
【0016】
例えば、+3価のMイオンがxだけSi3N4結晶中に固溶した場合、以下の反応式で示すことができる:
【数1】
Si3N4+xM→Si(3-x)N(4-x/3)MxVN(x/3)
ここで、VNは窒素空孔を表す。
【0017】
このような窒素空孔を多数持つSi3N4は、空孔を介しての窒素の拡散が促進されるため、Si3N4層の内側のSi層への窒素の供給量が多くなる、即ち窒化速度が劇的に増大するものと考えられる。なお、+4価以上のイオンが固溶した場合はプラスの電荷が過剰となるため、窒素空孔ではなくシリコン空孔が生成され、窒化速度が劇的に増大することはなくなる。しかしながらSi4+と同じ4価のMイオンを添加した場合でも、例えばRMがRSiより大きいものを用い、Si粉末の不対電子濃度を1015/cm3〜1020/cm3の範囲にしさえすれば、後述のようにSiの格子定数が大きくなるのを抑制する効果があるため、+1価〜+3価の範囲のものの添加ほどではないが、その結果、窒素の拡散が助長されて短時間で窒化させることが可能となる。
【0018】
本発明では特に+1〜+3価の元素を用いることが望ましいが、この場合窒化により生成したSi3N4にそれらの元素を固溶させるため、先に述べたSi(O)層2を利用する。即ち、Si(O)層2に多数存在するシリコン空孔に上記元素を固溶させるのである。かかる元素Mをzだけシリコン空孔に固溶させたときの反応式は以下の通りである:
【数2】
Si(O)(1-y)Vsi(y)+zM→Si(O)(1-y)MzVsi(y-z)
ここでVsiはシリコン空孔を表す。
【0019】
従って、本発明のこの状態におけるSi粉末は、図3に示すように、主にSi(O)(1-y)MzVsi(y-z)層(以下SiM層と略記する)22と、内部のSi層3とからなる。更に拡散が進行すると、SiM層22からSi層3にMがuだけ拡散され、Si(1-u)Mu層が生成される。この時、元素Mの共有結合半径がSiのそれよりも大きいと、図4に示すように、Siの格子定数が大きくなるのを抑制するためシリコン空孔の生成が促進され、Si(1-u-w)MuVsiw層(以下SiMV層と略記する)4が形成される。
【0020】
そして、この粉末のSiM層22とSiMV層4は、窒化処理によってそれぞれ下記の反応式のごとく反応する:
【数3】
3Si(1-y)(O)MzVsi(y-z)+2N2
→Si3(1-y)(O)N(4-3y+9z/4)M3zVsi3(y-z)VN(4y-3z)
【数4】
3Si(1-u-w)MuVsiw+2N2
→Si3(1-u-w)N(4-u-4w)M3uVsi3wVN(u+4w)
ここでVNは窒素空孔を表す。尚、Mは+3価とした。
【0021】
上記の反応式から分かるように、いずれのSi3N4においても窒素空孔が生成しており、Si3N4内での窒素の拡散速度が大きくなることが期待できる。
【0022】
以上の説明から分かるように、特公昭61−38149号公報、特開平5−330921号公報、特許出願公表平5−508612号公報に記載されている、Si粉末に単に窒化促進剤を添加する従来の方法では、Si(O)層が薄いために窒化促進剤の固溶量が少なく、Si粉末内にSiM層並びにSiMV層が生成しない。その結果、窒化により生成したSi3N4内で窒素空孔が生成しないので、窒素の拡散速度が小さく、窒化時間が長期化するのである。
【0023】
次に、本発明の反応焼結によるSi3N4焼結体の製造を具体的に説明する。まず、市販のSi粉末を300〜800℃で1〜5時間焼鈍することにより、粉末表面に生成しているアモルファスSiO2層の酸素をSi中に拡散させ、Si空孔が多数存在するSi(O)層を形成させる。ただし、処理雰囲気は空気、水素、アルゴン、10torr以下の真空など、窒素雰囲気以外の雰囲気とする。窒素雰囲気で処理すると、Si粉末表面にSi3N4膜が形成され、窒素空孔形成剤が固溶し難くなるためである。
【0024】
尚、窒素空孔Si(O)層の形成方法は上記の焼鈍だけでなく、他にもSi粉末に酸素イオンをイオン注入する方法、バルクSiを作製する際に酸素を強制的に混入させる方法等を用いることもできる。
【0025】
かくして得られたSi粉末中のSi(O)層中のSi空孔量の定量的な計測は、Si(O)層内のSi空孔内にトラップされた不対電子数として、電子スピン共鳴法(ESR法)を用いて測定することができる。その結果、市販のSi粉末の不対電子濃度が1012〜1013/cm3であるのに対し、上記方法によりSi粉末の不対電子濃度を1015〜1020/cm3の範囲に制御したとき、特に窒化反応が促進されることが判明した。
【0026】
即ち、不対電子数、即ちSi空孔量が1015/cm3より少ないと、Si空孔量が不足して窒素空孔形成剤の固溶が促進されないからである。又、逆に不対電子数が1020/cm3を越えると、窒化は促進されるが、Si結晶中の酸素量が多いためSi3N4結晶中に残存した酸素あるいは空孔が強度低下の原因となり、3点曲げ強度で800MPa以下の低強度のSi3N4焼結体しか得られない。
【0027】
次に、上記方法により得られた不対電子濃度の高いSi粉末を、希土類元素、Al、Mg、Caの少なくとも1種の元素の化合物からなる焼結助剤、及び窒素空孔形成剤と混合し、成形する。窒素空孔形成剤とは、Al、Mg、Ca以外の価数+1〜+4価の元素、望ましくは+1〜+3価の元素で、その共有結合半径RMとSiの共有結合半径RSiとが(RM−RSi)/RSi<0.5の関係にある元素、又はその化合物である。
【0028】
上記範囲の元素はSi結晶中に固溶しやすく、窒素空孔を生成させやすい。これらの元素として、具体的にはSc、Ni、V、Cr、Mn、Fe、Co、Cu、Ti、Zn、Ga、Geを挙げることができる。尚、窒素空孔形成剤及び焼結助剤は粉末で添加しても良いが、特に大型のSi3N4焼結体を作製する場合には、これらをSi粉末表面に均一に分散させるため、アルコキシド、ステアリン酸塩、ラウリン酸塩の形で添加することが好ましい。
【0029】
焼結助剤の添加量は元素換算で0.1〜15モル%の範囲、及び窒素空孔形成剤の添加量は元素換算で0.5〜15モル%の範囲が好ましい。焼結助剤が0.1モル%未満又は窒素空孔形成剤が0.5モル%未満では共にその効果が得られず、これらが15モル%を越えると粒界に析出して破壊の起点となるため、3点曲げ強度で800MPa以上の焼結体を得ることができないからである。
【0030】
最後に、上記成形体を窒化及び焼結する。窒化及び焼結の温度パターンは、例えば1300〜1400℃で2〜4時間保持した後、1500〜1800℃で1〜3時間保持するという、極めて簡単な制御で良い。この処理により、相対密度が99%以上のSi3N4焼結体を得ることができる。特に、得られたSi3N4焼結体の不対電子濃度が1015〜1021/cm3の範囲にある場合、3点曲げ強度で800MPa以上の高い強度が達成される。
【0031】
【実施例】
実施例1
出発原料として、不対電子濃度7×1012/cm3の市販Si粉末(A)と、不対電子濃度2×1013/cm3の市販Si粉末(B)とを使用し、粉末の加熱処理条件と不対電子濃度との関係を調べた。
【0032】
即ち、上記市販Si粉末(A)と市販Si粉末(B)を、それぞれ空気中(a)、水素雰囲気中(b)、アルゴン雰囲気中(c)、及び10torrの真空中(d)において、100℃から900℃まで100℃間隔で、Si粉末(A)についてはそれぞれ5時間及びSi粉末(B)についてはそれぞれ2時間保持した後、ESR法により不対電子濃度を測定した。
【0033】
比較例として、同じ市販Si粉末(A)と市販Si粉末(B)を、それぞれ窒素雰囲気中(e)及び100torrの真空中(f)において、100℃から900℃まで100℃間隔で共にそれぞれ5時間保持した後、ESR法により不対電子濃度を測定した。
【0034】
その結果を、市販Si粉末(A)については図5に、及び市販Si粉末(B)については図6に示した。これらの結果から分かるように、市販Si粉末を空気、水素、アルゴン、10torr以下の真空等の窒素以外の雰囲気中で、300〜800℃で熱処理することで、Si粉末の不対電子濃度を1015〜1020/cm3の範囲に制御できることが分かる。
【0035】
実施例2
上記市販Si粉末(A)を、下記表1に示す各雰囲気と条件で熱処理した。得られた各Si粉末の不対電子濃度を表1に併せて示した。
尚、Si粉末(A)、(B)を各々表1のNo.2〜No.5と同じ温度と雰囲気中で1時間に満たない時間でバッチ処理したものについては、不対電子濃度が(A)(B)ともにバッチ全体では1015/cm3以上となったものの、バッチ塊の中心では1015/cm3に満たない部分も生じた。以上の結果から、市販Si粉末バッチ全体にわたり不対電子濃度を1015〜1020/cm3の範囲に収めるように確実にコントロールするためには300〜800℃の温度で最低1時間の処理をする必要のあることがわかる。
【0036】
【表1】
【0037】
得られた各Si粉末に、焼結助剤として3モル%のY2O3粉末と5モル%のAl2O3粉末を添加し、更に窒素空孔形成剤としてCu粉末を2モル%添加した。これに熱可塑性樹脂バインダーを4モル%加えて混合した後、乾式プレスにより成形し、600℃の窒素気流中において2時間脱バインダー処理を行った。尚、Cuは、その共有結合半径RMとSiの共有結合半径RSiとが(RM−RSi)/RSi=0.5である。
【0038】
次に、各成形体を窒素気流中において1350℃で2時間窒化処理した後、下記表2に示す1700〜1800℃で1〜3時間の焼結を行った。得られた各Si3N4焼結体の不対電子濃度、相対密度、及び3点曲げ強度を表2に示した。
【0039】
【表2】
【0040】
上記のごとく、市販のSi粉末を窒素以外の雰囲気中で300〜800℃で1〜5時間保持することにより、Si粉末中の不対電子濃度を1015〜1020/cm3の範囲に処理粉末中に同濃度のムラのない状態で制御することができ、このSi粉末を焼結助剤及び窒素空孔形成剤と共に用いて反応焼結することにより、相対密度99%以上で3点曲げ強度1000MPa以上のSi3N4焼結体が得られることが分かる。
【0041】
実施例3
前記市販Si粉末(A)を空気中500℃で2時間熱処理し、不対電子濃度8×1015/cm3のSi粉末を得た。又、市販Si粉末(B)をアルゴン雰囲気中500℃で4時間熱処理し、不対電子濃度2×1019/cm3のSi粉末を得た。
【0042】
又、さらに、市販Si粉末(A)を空気中500℃で3時間熱処理し、不対電子濃度1×1016/cm3のSi粉末を得た。又、市販Si粉末(B)をアルゴン雰囲気中500℃で4時間熱処理し、不対電子濃度2×1019/cm3のSi粉末を得た。
【0043】
得られた各Si粉末に、焼結助剤として3モル%のY2O3粉末と5モル%のAl2O3粉末を添加し、更に下記表3に示す窒素空孔形成剤を添加した。使用したSi粉末とその不対電子濃度、並びに窒素空孔形成剤の種類(全て粉末)、価数、及びその共有結合半径RMとSiの共有結合半径RSiとの(RM−RSi)/RSiの値、並びにその添加量を下記表3に併せて示した。
【0044】
【表3】
【0045】
上記のSi粉末に焼結助剤及び窒素空孔形成剤を添加した各粉末を、実施例2と同様に混合し、成形した。次に、各成形体を窒素気流中において下記表4に示す1300〜1350℃で2〜4時間の窒化処理を行った後、下記表4に示す1600〜1800℃で1〜3時間の条件にて焼結を行った。得られた各Si3N4焼結体の不対電子濃度、相対密度、及び3点曲げ強度を表4に示した。
【0046】
【表4】
【0047】
上記のごとく、不対電子濃度が1015〜1020/cm3の範囲にあるSi粉末を使用することによって、生成するSi3N4焼結体の不対電子濃度を1015〜1021/cm3の範囲に制御でき、緻密且つ高強度のSi3N4焼結体を得ることができる。特に窒素空孔形成剤として+1〜+3価の価数を有し且つその共有結合半径RMとSiの共有結合半径RSiとの比率(RM−RSi)/RSiが0.5未満である元素を添加して焼結することにより、さらに高強度の焼結体を得ることができる。
【0048】
実施例4
前記市販Si粉末(A)を空気中500℃で3時間熱処理し、不対電子濃度1×1016/cm3のSi粉末を得た。又、市販Si粉末(B)をアルゴン雰囲気中500℃で4時間熱処理し、不対電子濃度2×1019/cm3のSi粉末を得た。
【0049】
得られた各Si粉末に、下記表5に示す焼結助剤と窒素空孔形成剤(共に全て粉末)を添加し、実施例1と同様の方法で混合、成形、脱バインダー処理を行った。尚、使用した窒素空孔形成剤は、いずれも価数が+1〜+3価の元素であって、その共有結合半径RMとSiの共有結合半径RSiとの比率(RM−RSi)/RSiが0.5未満の元素又はその化合物である。
【0050】
【表5】
【0051】
次に、上記の各成形体を窒素気流中において下記表6に示す1300〜1350℃で2〜4時間の窒化処理を行った後、1600〜1800℃で3時間の焼結を行った。得られた各Si3N4焼結体の不対電子濃度、相対密度、及び3点曲げ強度を表6に示した。
【0052】
【表6】
【0053】
上記の結果から、焼結助剤が合計で0.1〜15モル%及び窒素空孔形成剤が0.5〜15モル%の範囲にあれば、緻密且つより高強度のSi3N4焼結体が得られる。
【0054】
実施例5
前記市販Si粉末(A)を空気中500℃で3時間熱処理し、不対電子濃度1×1016/cm3のSi粉末を得た。又、市販Si粉末(B)をアルゴン雰囲気中500℃で4時間熱処理し、不対電子濃度2×1019/cm3のSi粉末を得た。
【0055】
得られた各Si粉末に、下記表7に示す形態の各焼結助剤と窒素空孔形成剤を添加し、実施例1と同様の方法で混合、成形、脱バインダー処理を行った。尚、使用した窒素空孔形成剤は、いずれも価数が+1〜+3価の元素であって、その共有結合半径RMとSiの共有結合半径RSiとの比率(RM−RSi)/RSiが0.5未満の元素又はその化合物である。
【0056】
【表7】
【0057】
次に、上記の各成形体を窒素気流中において1350℃で2時間窒化処理した後、下記表8に示す1600〜1800℃で3時間の条件で焼結を行った。得られた各Si3N4焼結体の不対電子濃度、相対密度、及び3点曲げ強度を表8に示した。
【0058】
【表8】
【0059】
上記の結果から、焼結助剤及び窒素空孔形成剤を粉末で添加するよりも、ステアリン酸、ラウリン酸、アルコキシドとして添加した方が、少量の添加量でより高強度のSi3N4焼結体が得られることが分かる。
【0060】
【発明の効果】
本発明によれば、原料粉末として高価なSi3N4粉末の代わりに約1/10の価格のシリコン粉末を使用して、従来よりも遥かに短時間の反応焼結により、低価格でしかも緻密且つ高強度の窒化ケイ素焼結体を提供することができる。
【図面の簡単な説明】
【図1】市販のSi粉末の模式図である。
【図2】熱処理によりSi(O)層を増大させたSi粉末の模式図である。
【図3】Si(O)層に不純物元素Mが固溶した状態のSiM層を有するSi粉末の模式図である。
【図4】Si層に不純物元素Mが拡散した状態のSiMV層を有するSi粉末の模式図である。
【図5】不対電子濃度7×1012/cm3の市販Si粉末の処理温度と得られるSi粉末の不対電子濃度の関係を示すグラフである。
【図6】不対電子濃度2×1013/cm3のSi粉末の処理温度と得られるSi粉末の不対電子濃度の関係を示すグラフである。
【符号の説明】
10 アモルファスSiO2層
11 厚さの減少したSiO2層
20 Si(O)層
21 熱処理によりSi空孔を形成したSi(O)層
22 SiM層
3 Si層
4 SiMV層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a silicon nitride (Si 3 N 4 ) sintered body produced by reactive sintering and a method for producing the same.
[0002]
[Prior art]
One method for producing a silicon nitride sintered body is a reactive sintering method in which a silicon (Si) powder compact is nitrided and sintered in a nitrogen atmosphere. However, in order to obtain a silicon nitride sintered body by reacting a molded body made of pure Si powder with nitrogen, for example, J. Org. Mater. Sci. 22 (1987) P.A. 3041-3086, especially its P.P. As described in 3074-3075, since a long nitriding time of 100 hours or more is required, there is a great disadvantage that productivity is inferior.
[0003]
On the other hand, in Japanese Patent Publication No. 61-38149, Japanese Patent Laid-Open No. 5-330921, and Japanese Patent Application Publication No. 5-508612, a nitriding accelerator such as Ni, Co, Ti, Zr or the like is added to Si powder. Thus, a method for accelerating nitriding has been proposed. According to these methods, the amorphous SiO 2 layer on the surface of the Si powder reacts with the nitriding accelerator to form a liquid phase, and the diffusion rate of nitrogen increases, so that nitriding is promoted. In particular, according to Japanese Patent Laid-Open No. 5-330921, the nitriding time can be reduced to about 8 hours.
[0004]
[Problems to be solved by the invention]
However, even in the method using these nitriding accelerators, the diffusion coefficient of nitrogen in Si 3 N 4 formed inside the liquid phase on the surface of the Si powder is small, and further, the diffusion of nitrogen into the Si 3 N 4 and Si 3 N 4 Therefore, the nitriding time cannot be further shortened. Moreover, as described in JP-A-5-330921, it is excellent in productivity, such as requiring complicated temperature control in which nitriding is performed in steps of 50 ° C. at a high temperature of 1200 to 1450 ° C. I could not say.
[0005]
Furthermore, the Si 3 N 4 sintered body obtained by the reactive sintering method using these nitriding accelerators is not densified with a porosity of 4 to 11% by volume, and the three-point bending strength is also low at 460 MPa. Therefore, it was inappropriate for use as a structural material.
[0006]
In view of such conventional circumstances, the present invention aims at shortening the nitriding time in the reactive sintering of the silicon nitride sintered body, improving productivity, and dense and high-strength silicon nitride sintering obtained by reactive sintering. The purpose is to provide a body.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the silicon nitride sintered body provided by the present invention is a silicon nitride sintered body obtained by reactive sintering of Si powder, and has an unpaired electron concentration of 10 15 / cm 3 to 10 It is characterized by 21 / cm 3 .
[0008]
In order to produce the silicon nitride sintered body of the present invention, reaction sintering is performed using Si powder having an unpaired electron concentration of 10 15 / cm 3 to 10 20 / cm 3 . The Si powder having an unpaired electron concentration in this range can be obtained by a method of annealing a commercially available Si powder at a temperature of 300 to 800 ° C. for 1 to 5 hours in an atmosphere other than nitrogen. The atmosphere in this case is preferably air, hydrogen, argon, a mixture thereof, or a vacuum of 10 torr or less.
In the silicon nitride sintered body of the present invention, the sintering aid and nitrogen vacancy forming agent powder are added to the Si powder having the unpaired electron concentration and subjected to reaction sintering.
[0009]
More specifically, in the method for producing a silicon nitride sintered body of the present invention, a compound of at least one element selected from the group consisting of rare earth elements, Al, Mg, and Ca as a sintering aid in the unpaired electron concentration Si powder. 0.1 to 15 mol% in terms of element, preferably 0.5 to 10 mol%, and valences other than Al, Mg, and Ca as nitrogen vacancy forming agents are +1 to +4, preferably +1 to +3. An element or a compound thereof, in which the covalent bond radius RM and the covalent bond radius RSi of Si have a relationship of (RM-RSi) / RSi <0.5, or 0.5 to 15 mol% in terms of element, preferably 1-10 mol% is added and reaction-sintered.
[0010]
Specific examples of the nitriding vacancy generator include at least one element of Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga, and Ge, or a compound thereof, preferably an alkoxide thereof, There are stearates and laurates.
[0011]
[Action]
In the conventional reactive sintering of silicon nitride, the nitriding treatment takes a long time because the amorphous SiO 2 layer formed on the surface of the Si powder and the Si 3 N 4 layer formed on the surface of the Si powder by nitriding are 2 This is because there are two factors that inhibit nitrogen diffusion.
[0012]
In the present invention, by incorporating two characteristic steps in order to overcome the above two factors for inhibiting nitrogen diffusion, the diffusion rate of nitrogen was improved and the nitriding time was successfully shortened. As will be described later, these two characteristic steps are a step of introducing silicon vacancies into Si powder used as a raw material powder and a step of generating nitrogen vacancies in Si 3 N 4 .
[0013]
Commercially available Si powder is considered to have a three-layer structure as shown in FIG. That is, the surface layer is an amorphous SiO 2 layer 1 0 generated by natural oxidation of Si, an Si (O) layer 2 0 in which oxygen is dissolved in Si inside, and an
[0014]
Accordingly, in the present invention, in order to suppress an increase in the lattice constant of Si (O) layer 2 0, it is to produce an silicon vacancies. Specifically, performs the appropriate heat treatment or the like in commercial Si powder, as shown in FIG. 2, by diffusing the oxygen of the amorphous SiO 2 layer 1 1 of the surface to the inside, the thickness of the amorphous SiO 2 layer 1 1 silicon vacancies increases the Si (O) layer 2 1 present a number while reducing. These silicon vacancies are thought to increase the diffusion rate of elements dissolved in Si.
[0015]
Next, nitrogen vacancies generated in the Si 3 N 4 lattice in order to increase the diffusion rate of nitrogen in the Si 3 N 4 generated by nitriding will be described. The Si 3 N 4 lattice is configured such that +4 valence of Si and -3 valence of N are kept electrically neutral. For example, when a +1 to +3 valent element is dissolved in the Si 3 N 4 lattice, a positively charged element occupies a lattice point of Si, and nitrogen vacancies are generated to maintain electrical neutrality. .
[0016]
For example, when only x trivalent M ions are solid-solved in the Si 3 N 4 crystal by x, it can be represented by the following reaction formula:
[Expression 1]
Si 3 N 4 + xM → Si (3-x) N (4-x / 3) M x VN (x / 3)
Here, VN represents a nitrogen vacancy.
[0017]
Si 3 N 4 having a large number of such nitrogen vacancies promotes the diffusion of nitrogen through the vacancies, so that the amount of nitrogen supplied to the Si layer inside the Si 3 N 4 layer increases. That is, the nitriding rate is considered to increase dramatically. When ions having a valence of +4 or more are dissolved, positive charges are excessive, so that silicon vacancies are generated instead of nitrogen vacancies, and the nitriding rate does not increase dramatically. However, even when the same tetravalent M ion as Si 4+ is added, for example, a material having an RM larger than RSi is used, and the concentration of unpaired electrons in the Si powder is even in the range of 10 15 / cm 3 to 10 20 / cm 3. Then, since it has an effect of suppressing an increase in the lattice constant of Si as will be described later, it is not as much as the addition of one in the range of +1 valence to +3 valence, but as a result, the diffusion of nitrogen is promoted for a short time. It becomes possible to nitride.
[0018]
In the present invention, it is particularly desirable to use +1 to +3 valent elements. In this case, the Si (O) layer 2 described above is used to dissolve these elements in Si 3 N 4 produced by nitriding. . That is, the above element is dissolved in a large number of silicon vacancies existing in the Si (O) layer 2. The reaction formula when this element M is dissolved in silicon vacancies by z is as follows:
[Expression 2]
Si (O) (1-y) Vsi (y) + zM → Si (O) (1-y) Mz Vsi (yz)
Here, Vsi represents a silicon hole.
[0019]
Therefore, the Si powder in this state of the present invention is mainly composed of a Si (O) (1-y) M z Vsi (yz) layer (hereinafter abbreviated as SiM layer) 2 2 , as shown in FIG. Of the
[0020]
Then, SiM layer 2 2 and
[Equation 3]
3Si (1-y) (O) M z Vsi (yz) + 2N 2
→ Si 3 (1-y) (O) N (4-3y + 9z / 4) M 3z Vsi 3 (yz) VN (4y-3z)
[Expression 4]
3Si (1-uw) M u Vsi w + 2N 2
→ Si 3 (1-uw) N (4-u-4w) M 3u Vsi 3w VN (u + 4w)
Here, VN represents a nitrogen vacancy. Note that M was +3.
[0021]
As can be seen from the above reaction formula, and generates nitrogen vacancies in any of Si 3 N 4, it can be expected that the diffusion rate of nitrogen in the Si 3 N 4 is increased.
[0022]
As can be seen from the above description, the conventional nitriding accelerator is simply added to the Si powder described in Japanese Patent Publication No. 61-38149, Japanese Patent Laid-Open No. 5-330921, and Japanese Patent Application Publication No. 5-508612. In this method, since the Si (O) layer is thin, the amount of the nitriding accelerator dissolved is small, and the SiM layer and the SiMV layer are not generated in the Si powder. As a result, nitrogen vacancies are not generated in Si 3 N 4 generated by nitriding, so that the diffusion rate of nitrogen is small and the nitriding time is prolonged.
[0023]
Next, the production of the Si 3 N 4 sintered body by the reactive sintering of the present invention will be specifically described. First, commercially available Si powder is annealed at 300 to 800 ° C. for 1 to 5 hours to diffuse oxygen in the amorphous SiO 2 layer formed on the powder surface into Si, and Si ( O) A layer is formed. However, the treatment atmosphere is an atmosphere other than the nitrogen atmosphere, such as air, hydrogen, argon, or a vacuum of 10 torr or less. This is because when the treatment is performed in a nitrogen atmosphere, a Si 3 N 4 film is formed on the surface of the Si powder, and the nitrogen vacancy forming agent is hardly dissolved.
[0024]
In addition, the method of forming the nitrogen vacancy Si (O) layer is not only the above annealing, but also a method of ion-implanting oxygen ions into Si powder, a method of forcibly mixing oxygen when producing bulk Si Etc. can also be used.
[0025]
The quantitative measurement of the amount of Si vacancies in the Si (O) layer in the Si powder thus obtained is based on the electron spin resonance as the number of unpaired electrons trapped in the Si vacancies in the Si (O) layer. It can be measured using the method (ESR method). As a result, while the unpaired electron concentration of the commercially available Si powder is 10 12 to 10 13 / cm 3 , the unpaired electron concentration of the Si powder is controlled in the range of 10 15 to 10 20 / cm 3 by the above method. In particular, it was found that the nitriding reaction was promoted.
[0026]
That is, if the number of unpaired electrons, that is, the amount of Si vacancies is less than 10 15 / cm 3 , the amount of Si vacancies is insufficient and solid solution of the nitrogen vacancy forming agent is not promoted. On the other hand, if the number of unpaired electrons exceeds 10 20 / cm 3 , nitriding is promoted, but oxygen or vacancies remaining in the Si 3 N 4 crystal decrease in strength due to the large amount of oxygen in the Si crystal. As a result, only a low-strength Si 3 N 4 sintered body having a three-point bending strength of 800 MPa or less can be obtained.
[0027]
Next, the high unpaired electron concentration Si powder obtained by the above method is mixed with a sintering aid composed of a compound of at least one element of rare earth elements, Al, Mg, and Ca, and a nitrogen vacancy forming agent. And molding. The nitrogen vacancy forming agent is an element having a valence of +1 to +4 other than Al, Mg and Ca, preferably +1 to +3, and the covalent bond radius RM and the Si covalent bond radius RSi are (RM) -RSi) / RSi <0.5, or an element thereof.
[0028]
Elements in the above range are easily dissolved in the Si crystal and easily generate nitrogen vacancies. Specific examples of these elements include Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga, and Ge. The nitrogen vacancy forming agent and the sintering aid may be added in powder form, but particularly when a large Si 3 N 4 sintered body is produced, in order to disperse them uniformly on the Si powder surface. , Alkoxide, stearate and laurate are preferably added.
[0029]
The addition amount of the sintering aid is preferably in the range of 0.1 to 15 mol% in terms of element, and the addition amount of the nitrogen vacancy forming agent is preferably in the range of 0.5 to 15 mol% in terms of element. If the sintering aid is less than 0.1 mol% or the nitrogen vacancy forming agent is less than 0.5 mol%, the effect cannot be obtained. Therefore, a sintered body having a three-point bending strength of 800 MPa or more cannot be obtained.
[0030]
Finally, the molded body is nitrided and sintered. The temperature pattern of nitriding and sintering may be very simple control, for example, holding at 1300 to 1400 ° C. for 2 to 4 hours and then holding at 1500 to 1800 ° C. for 1 to 3 hours. By this treatment, a Si 3 N 4 sintered body having a relative density of 99% or more can be obtained. In particular, when the unpaired electron concentration of the obtained Si 3 N 4 sintered body is in the range of 10 15 to 10 21 / cm 3 , a high strength of 800 MPa or more is achieved with a three-point bending strength.
[0031]
【Example】
Example 1
As starting materials, a commercially available Si powder (A) having an unpaired electron concentration of 7 × 10 12 / cm 3 and a commercially available Si powder (B) having an unpaired electron concentration of 2 × 10 13 / cm 3 were used to heat the powder. The relationship between processing conditions and unpaired electron concentration was investigated.
[0032]
That is, the commercially available Si powder (A) and the commercially available Si powder (B) were respectively mixed in air (a), in a hydrogen atmosphere (b), in an argon atmosphere (c), and in a vacuum of 10 torr (d). From 100 ° C. to 900 ° C. at intervals of 100 ° C., each of the Si powder (A) was held for 5 hours and each of the Si powder (B) was held for 2 hours, and then the unpaired electron concentration was measured by the ESR method.
[0033]
As a comparative example, the same commercially available Si powder (A) and commercially available Si powder (B) were each 5% from 100 ° C. to 900 ° C. at 100 ° C. intervals in a nitrogen atmosphere (e) and 100 torr vacuum (f), respectively. After holding for a time, the unpaired electron concentration was measured by the ESR method.
[0034]
The results are shown in FIG. 5 for the commercially available Si powder (A) and in FIG. 6 for the commercially available Si powder (B). As can be seen from these results, the unpaired electron concentration of the Si powder is set to 10 by heat-treating the commercially available Si powder at 300 to 800 ° C. in an atmosphere other than nitrogen such as air, hydrogen, argon, vacuum of 10 torr or less. it can be seen that controlled in the range of 15 ~10 20 / cm 3.
[0035]
Example 2
The commercially available Si powder (A) was heat-treated in each atmosphere and conditions shown in Table 1 below. Table 1 shows the unpaired electron concentration of each of the obtained Si powders.
In addition, the unpaired electron concentration of the Si powders (A) and (B) that were batch-treated at the same temperature and the same atmosphere as No. 2 to No. 5 in Table 1 for less than 1 hour, respectively ( Although both A) and (B) were 10 15 / cm 3 or more in the whole batch, a portion less than 10 15 / cm 3 was also generated at the center of the batch lump. From the above results, in order to reliably control the unpaired electron concentration in the range of 10 15 to 10 20 / cm 3 throughout the commercially available Si powder batch, treatment at a temperature of 300 to 800 ° C. for at least 1 hour is required. I know you need to do that.
[0036]
[Table 1]
[0037]
To each of the obtained Si powders, 3 mol% Y 2 O 3 powder and 5 mol% Al 2 O 3 powder were added as sintering aids, and 2 mol% of Cu powder was added as a nitrogen vacancy forming agent. did. 4 mol% of a thermoplastic resin binder was added thereto and mixed, and then molded by a dry press, and debinding was performed in a nitrogen stream at 600 ° C. for 2 hours. In addition, as for Cu, the covalent bond radius RM and the covalent bond radius RSi of Si are (RM-RSi) /RSi=0.5.
[0038]
Next, each molded body was nitrided at 1350 ° C. for 2 hours in a nitrogen stream, and then sintered at 1700-1800 ° C. shown in Table 2 for 1 to 3 hours. Table 2 shows the unpaired electron concentration, the relative density, and the three-point bending strength of each of the obtained Si 3 N 4 sintered bodies.
[0039]
[Table 2]
[0040]
As indicated above, the process commercially available Si powder by holding for 1 to 5 hours at 300 to 800 ° C. in an atmosphere other than nitrogen, the unpaired electron concentration in the Si powder to the range of 10 15 ~10 20 / cm 3 It can be controlled in the powder without unevenness of the same concentration, and by reacting and sintering this Si powder together with a sintering aid and a nitrogen vacancy forming agent, a three-point bending is performed at a relative density of 99% or more. It can be seen that a Si 3 N 4 sintered body having a strength of 1000 MPa or more can be obtained.
[0041]
Example 3
The commercially available Si powder (A) was heat-treated in air at 500 ° C. for 2 hours to obtain a Si powder having an unpaired electron concentration of 8 × 10 15 / cm 3 . Further, the commercially available Si powder (B) was heat-treated at 500 ° C. for 4 hours in an argon atmosphere to obtain a Si powder having an unpaired electron concentration of 2 × 10 19 / cm 3 .
[0042]
Furthermore, the commercially available Si powder (A) was heat-treated in air at 500 ° C. for 3 hours to obtain a Si powder having an unpaired electron concentration of 1 × 10 16 / cm 3 . Further, the commercially available Si powder (B) was heat-treated at 500 ° C. for 4 hours in an argon atmosphere to obtain a Si powder having an unpaired electron concentration of 2 × 10 19 / cm 3 .
[0043]
To each of the obtained Si powders, 3 mol% Y 2 O 3 powder and 5 mol% Al 2 O 3 powder were added as sintering aids, and further nitrogen vacancy forming agents shown in Table 3 below were added. . Si powder used and its unpaired electron concentration, as well as the type of nitrogen vacancy forming agent (all powders), valence, and the covalent bond radius RM and the covalent bond radius RSi of (RM-RSi) / RSi The values and addition amounts thereof are shown in Table 3 below.
[0044]
[Table 3]
[0045]
Each powder obtained by adding a sintering aid and a nitrogen pore forming agent to the above Si powder was mixed and molded in the same manner as in Example 2. Next, each molded body was subjected to nitriding treatment for 2 to 4 hours at 1300 to 1350 ° C. shown in Table 4 below in a nitrogen stream, and then for 1 to 3 hours at 1600 to 1800 ° C. shown in Table 4 below. Sintering was performed. Table 4 shows the unpaired electron concentration, relative density, and three-point bending strength of each of the obtained Si 3 N 4 sintered bodies.
[0046]
[Table 4]
[0047]
As described above, by using Si powder having an unpaired electron concentration in the range of 10 15 to 10 20 / cm 3 , the unpaired electron concentration of the resulting Si 3 N 4 sintered body is set to 10 15 to 10 21 / It can be controlled within the range of cm 3 , and a dense and high-strength Si 3 N 4 sintered body can be obtained. In particular, an element having a valence of +1 to +3 and a ratio of the covalent bond radius RM to the covalent bond radius RSi (RM-RSi) / RSi of less than 0.5 is added as a nitrogen vacancy forming agent Thus, a sintered body with higher strength can be obtained by sintering.
[0048]
Example 4
The commercially available Si powder (A) was heat-treated in air at 500 ° C. for 3 hours to obtain an Si powder having an unpaired electron concentration of 1 × 10 16 / cm 3 . Further, the commercially available Si powder (B) was heat-treated at 500 ° C. for 4 hours in an argon atmosphere to obtain a Si powder having an unpaired electron concentration of 2 × 10 19 / cm 3 .
[0049]
To each of the obtained Si powders, a sintering aid and a nitrogen pore forming agent (both powders) shown in Table 5 below were added, and mixing, molding, and debinding were performed in the same manner as in Example 1. . The nitrogen vacancy forming agents used are all elements having a valence of +1 to +3, and the ratio of the covalent bond radius RM to the covalent bond radius RSi of Si (RM-RSi) / RSi is 0. Less than 5 elements or compounds thereof.
[0050]
[Table 5]
[0051]
Next, each of the above molded bodies was subjected to nitriding treatment at 1300 to 1350 ° C. for 2 to 4 hours shown in Table 6 below in a nitrogen stream, and then sintered at 1600 to 1800 ° C. for 3 hours. Table 6 shows the unpaired electron concentration, relative density, and three-point bending strength of each of the obtained Si 3 N 4 sintered bodies.
[0052]
[Table 6]
[0053]
From the above results, if the sintering aids are in the range of 0.1 to 15 mol% in total and the nitrogen vacancy forming agent is in the range of 0.5 to 15 mol%, dense and higher-strength Si 3 N 4 firing is achieved. A knot is obtained.
[0054]
Example 5
The commercially available Si powder (A) was heat-treated in air at 500 ° C. for 3 hours to obtain an Si powder having an unpaired electron concentration of 1 × 10 16 / cm 3 . Further, the commercially available Si powder (B) was heat-treated at 500 ° C. for 4 hours in an argon atmosphere to obtain a Si powder having an unpaired electron concentration of 2 × 10 19 / cm 3 .
[0055]
Each of the obtained Si powders was added with each sintering aid and nitrogen vacancy forming agent in the form shown in Table 7 below, and mixed, molded and debindered in the same manner as in Example 1. The nitrogen vacancy forming agents used are all elements having a valence of +1 to +3, and the ratio of the covalent bond radius RM to the covalent bond radius RSi of Si (RM-RSi) / RSi is 0. Less than 5 elements or compounds thereof.
[0056]
[Table 7]
[0057]
Next, each molded body was subjected to nitriding treatment at 1350 ° C. for 2 hours in a nitrogen stream, and then sintered at 1600 to 1800 ° C. for 3 hours as shown in Table 8 below. Table 8 shows the unpaired electron concentration, relative density, and three-point bending strength of each obtained Si 3 N 4 sintered body.
[0058]
[Table 8]
[0059]
From the above results, rather than adding a sintering aid and a nitrogen pore forming agent in powder, stearic acid, lauric acid, who was added as alkoxide, a small amount of the addition amount of higher strength the Si 3 N 4 sintered It turns out that a ligation is obtained.
[0060]
【The invention's effect】
According to the present invention, a silicon powder having a price of about 1/10 is used instead of an expensive Si 3 N 4 powder as a raw material powder. A dense and high-strength silicon nitride sintered body can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic view of a commercially available Si powder.
FIG. 2 is a schematic view of Si powder having an increased Si (O) layer by heat treatment.
FIG. 3 is a schematic view of a Si powder having a SiM layer in which an impurity element M is dissolved in a Si (O) layer.
FIG. 4 is a schematic view of a Si powder having a SiMV layer in which an impurity element M is diffused in the Si layer.
FIG. 5 is a graph showing the relationship between the processing temperature of a commercially available Si powder having an unpaired electron concentration of 7 × 10 12 / cm 3 and the unpaired electron concentration of the obtained Si powder.
FIG. 6 is a graph showing the relationship between the treatment temperature of Si powder having an unpaired electron concentration of 2 × 10 13 / cm 3 and the unpaired electron concentration of the obtained Si powder.
[Explanation of symbols]
1 0 Amorphous SiO 2 layer 1 1 Reduced thickness SiO 2 layer 2 0 Si (O) layer 2 1 Si (O) layer 2 SiM layer formed by heat treatment 2 2
Claims (12)
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