【発明の詳細な説明】[Detailed description of the invention]
産業上の利用分野
本発明は電気絶縁性基板、電気抵抗体などに使
用されるSiC焼結体に関し、さらに詳しくは高密
度にして電気比抵抗の大きなSiC焼結体に関す
る。
従来の技術
SiCは一般に導電性であり、その通常の焼結体
も導電性があつて電気絶縁基板には適さない。特
にSiCは焼結しにくいため、密度が小さい焼結体
は強度が弱く、実用的でないので高密度にする必
要がある。
電気絶縁性を高めるためにSiCに少量のBe又は
BeOを添加して焼結したものがある（特開昭57
−166365、同59−69473、同57−156373、同57−
166365、同59−69473号）。
発明が解決しようとする問題点
Be又はBeOを添加したSiC焼結体は確かに電気
抵抗は上るが、Beが有毒であるため扱いにくい。
またSiCにBeやBeOを加えただけでは密度が上ら
ないのでホツトプレスする必要があり、そのため
単純な形状のものしかできず、高価になるなどの
欠点を有する。
SiCは焼結しにくいので常圧下では高密度の焼
結体を得ることは容易でないが、SiCに少量の
Ｃ、Ｂ、Al等の焼結助剤を添加すれば常圧下で
も高密度になることが知られている（特公昭58−
9785、同59−34147）。SiCにはα、βと呼ばれる
結晶形の異なるものがあるが、上記の添加物によ
り両者とも高密度化が図れる。
一般にこれらの焼結体はSiCを粉砕して微粉末
にし、これに上記の添加物を加え、成形後、加圧
もしくは非加圧下で高温に加熱して製造されてい
る。この焼結体の電気比抵抗を計ると通常103Ω
cm以下とかなり低い。
SiCにBe又はBeOを添加すれば電気抵抗は上る
ことはわかつているが、上記したような欠点があ
るため、本発明者はBe又はBeOを添加せずに電
気抵抗を上げることを種々研究し、本発明に到達
した。
即ち、本発明の目的はBe又はBeOを添加しな
くとも高密度にして電気抵抗の高いSiC焼結体を
得ることにある。
問題点を解決するための手段
本発明者の研究によればSiC焼結体の電気抵抗
はその中に含有するＮ成分に大きく影響され、さ
らにＢ、Alに遊離のＣの含有量にも影響される
ことがわかつた。そこで本発明は焼結体中のＮ成
分を300ppm以下に抑えて電気抵抗を上げ、かつ
遊離のＣ、Ｂ、Alを特定量含有させてSiCの高密
度化を図つたものである。
即ち、本発明は遊離のC0.6〜3.0重量％、B0.05
〜0.13重量％、N300ppm以下、残部が実質的に
SiCからなり、嵩密度が3.0g／cm3以上、電気比抵
抗が105Ωcm以上である電気比抵抗の高いSiC焼結
体である。SiCはα、β、あるいはその混合のい
ずれでもよい。上記において、Ｂは遊離のもの又
はB4C等のように結合したものである。Alは
Al4C3等のような結合形で大部分存在している。
Ｎは固溶等の形で存在していると思われるが、
300ppm以下にしないと電気抵抗が上らない。Ｎ
が多いとなぜ電気抵抗が下るかは、ＮがSiCに固
溶し、ドナーとして働き、フリーキヤリヤー（電
子）を増加させると考えられる。
遊離のＣが上記範囲としたのはSiC焼結体の密
度を上げるため及びＣがこの範囲外では電気抵抗
体が下るからである。上記Ｃの量はB4Cのように
結合した炭素は含まない。Ｂについては本発明の
焼結体はAlが含まれているので、Alを含まない
焼結体に較べて少なくてよく、0.05〜0.13重量％
の範囲がSiCの密度を上げる上、また電気抵抗を
上げる点からも特に適する。AlはＢ同様、SiCの
密度を上げ、かつ電気抵抗を上げるため必要なも
のでその範囲は0.1〜1.0重量％である。前記特公
昭58−9785は電気抵抗を上げることを目的として
いないので、遊離Ｃは少なく、また特公昭59−
34147はＢが多く、そしてAlは含まれていない。
SiC焼結体は電気絶縁基板にする場合は高熱伝
導性が要求され、それには密度を上げることが必
要であり、また結晶粒界や格子内不純物の欠陥の
原因となるＢ、AlはSiCの焼結密度に支障のない
範囲で少なくする必要がある。この点本発明の
Ｂ、Alの含有量は少ないので十分この要求に適
合している。SiC焼結体の密度は電気絶縁基板に
限らず一般に高い方が望ましいので本発明では密
度を3.0g／cm3以上とした。
次に本発明の焼結体の製法について述べる。
SiCはシリカの炭素還元法、いわゆるアチソン
法でつくつたものが工業的には有利である。この
SiCを微粉砕する。通常のSiC焼結体ではＮの含
有量は問題にしていないのでボールミル等で空気
中で粉砕されている。しかし空気中での粉砕では
SiCにかなりのＮが吸着されることがわかつた。
そしてこのＮは焼結中には殆んど抜けない。従つ
てＮの低いSiC焼結体にするには粉末SiCのＮの
含有量を下げなければならない。従来のSiC粉末
中のＮは殆んどが粉砕中に入るので、本発明にお
いてはSiCの粉砕を非窒素雰囲気下、例えばアル
ゴン雰囲気下で行なう。粉砕はできるだけ細か
く、望ましくは5μ以下とする。粉砕中に入つた
不純物、例えば鉄等は、酸で除去する。従来は一
般に空気中での粉砕（例えば前記特公昭58−9785
の実施例１）であり、N2の吸着が避けられない
ので、Ｎは300ppmを越えている。
こうして得られたSiC粉末は純度が95重量％以
上、Ｎ含有量300ppm以下である。その結晶形は
α，βのいずれも用いることができる。
SiC粉末に加えるＣはカーボンブラツク等の炭
素粉末を分散媒により分散して用いることもでき
るが、液状のフエノール樹脂等を加え、SiCの焼
結中に炭化してＣにすることが望ましい。この場
合は、樹脂の炭化率等を考慮し、またＣがSiC中
の微量のSiO2等の還元によりガス化して逸出す
る場合はその量も考慮し、焼結体中に遊離の状態
で残るＣの量が前記の範囲になるように樹脂の添
加量を定める必要がある。
Ｂは単体ホウ素、B4C、等の粉末が使用され
る。単体ホウ素の形で添加してもSiC焼結体中で
はB4Cの形になつているものが多いと考えられ
る。
Alは金属アルミニウム粉末またはAl2O3などの
化合物でも良いがAlNは使用できない。
上記のようにＮを極力少なくしたSiC粉末にＣ
源、Ｂ源、Al源を加え、PVA等の一次結合剤を
加え所望の形状に成形する。成形体は加熱焼結す
る。焼結は加圧でも非加圧でもよいが、成形体が
複雑な形状の場合は非加圧が適する。焼結温度は
1900〜2200℃の範囲が適当である。焼結はAr等
の不活性雰囲気下で行なうが、N2雰囲気は避け
る。
実施例
アチソン法でつくつた純度96％のα−SiC粒を
粉砕して2.5μm以下の粉末を次のようにして得
た。粉砕はＮが入らないようにするため以下のよ
うにした。ボールミル内に数mmの大きさのSiC粉
を入れ、真空ポンプで脱気し、10-1TorrにしAr
を注入した。これを２回くり返し、その後１日間
粉砕した。このものを取出し、塩酸でFeを除去
し、沈降法で分級した。
その結果得られたSiC粉末はＮ含有量180ppm
であつた。このSiC粉末100重量部にフエノール
樹脂（固形分）２重量部、B0.06重量部、Al0.18
重量部を加えさらに水190c.c.とPVA2重量部を加
え20時間混合した後スプレードライヤーで顆粒化
した。この顆粒を直径50mm、厚さ６mmに加圧成形
し、Ar雰囲気中無加圧で2100℃、60分加熱し焼
結した。その特性を第１表のNo.１に示す。
以下同様にし、但し、Ｂ、Al、Ｃの量を変え
て焼結した結果を第１表のNo.２以下に示す。表中
比較例のNo.１はSiC粉砕を空気中で行なつたもの
である。
INDUSTRIAL APPLICATION FIELD The present invention relates to a SiC sintered body used for electrically insulating substrates, electrical resistors, etc., and more specifically to a SiC sintered body that is highly dense and has a large electrical specific resistance. BACKGROUND ART SiC is generally conductive, and its ordinary sintered body is also conductive, making it unsuitable for use as an electrically insulating substrate. In particular, SiC is difficult to sinter, and a sintered body with a low density has low strength and is not practical, so it is necessary to have a high density. A small amount of Be or Be is added to SiC to improve electrical insulation.
Some products are sintered with the addition of BeO (Japanese Unexamined Patent Publication No. 1983
-166365, 59-69473, 57-156373, 57-
166365, No. 59-69473). Problems to be Solved by the Invention Although SiC sintered bodies doped with Be or BeO do have increased electrical resistance, they are difficult to handle because Be is toxic.
Furthermore, simply adding Be or BeO to SiC does not increase the density, so hot pressing is required, which results in the disadvantages that only simple shapes can be produced and they are expensive. SiC is difficult to sinter, so it is not easy to obtain a high-density sintered body under normal pressure, but if a small amount of sintering aids such as C, B, and Al are added to SiC, high density can be obtained even under normal pressure. It is known that (Tokuko Sho 58-
9785, 59-34147). SiC has different crystal forms called α and β, and the above additives can increase the density of both. Generally, these sintered bodies are produced by pulverizing SiC into a fine powder, adding the above-mentioned additives thereto, shaping the powder, and then heating it to a high temperature with or without pressure. The electrical resistivity of this sintered body is usually 10 3 Ω.
It is quite low, less than cm. It is known that adding Be or BeO to SiC increases the electrical resistance, but because of the drawbacks mentioned above, the inventor conducted various studies to increase the electrical resistance without adding Be or BeO. , arrived at the present invention. That is, an object of the present invention is to obtain a SiC sintered body with high density and high electrical resistance without adding Be or BeO. Means for Solving the Problems According to the research of the present inventor, the electrical resistance of a SiC sintered body is greatly influenced by the N component contained therein, and is also influenced by the content of free C in B and Al. I found out that it would happen. Therefore, the present invention aims to increase the electrical resistance by suppressing the N component in the sintered body to 300 ppm or less, and to increase the density of SiC by containing specific amounts of free C, B, and Al. That is, the present invention has free C0.6-3.0% by weight, B0.05
~0.13% by weight, less than 300ppm of N, the remainder being substantially
This SiC sintered body is made of SiC and has a bulk density of 3.0 g/cm 3 or more and an electrical resistivity of 10 5 Ωcm or more. SiC may be α, β, or a mixture thereof. In the above, B is free or bound, such as B 4 C. Al is
It mostly exists in bonded forms such as Al 4 C 3 .
It is thought that N exists in the form of solid solution, etc.
Electrical resistance will not increase unless it is below 300ppm. N
The reason why electrical resistance decreases when there is a large amount of N is thought to be that N dissolves in SiC, acts as a donor, and increases free carriers (electrons). The reason why free C is set to the above range is to increase the density of the SiC sintered body, and because if C is outside this range, the electric resistance decreases. The above amount of C does not include bonded carbon such as B 4 C. Regarding B, since the sintered body of the present invention contains Al, it may be less than a sintered body that does not contain Al, and is 0.05 to 0.13% by weight.
This range is particularly suitable from the viewpoint of increasing the density of SiC and increasing the electrical resistance. Like B, Al is necessary to increase the density and electrical resistance of SiC, and its content ranges from 0.1 to 1.0% by weight. The above-mentioned Special Publication No. 58-9785 was not intended to increase electrical resistance, so there was little free C, and the Special Publication No. 59-9785
34147 contains a lot of B and does not contain Al. If SiC sintered bodies are to be used as electrically insulating substrates, high thermal conductivity is required, which requires increasing the density, and B and Al, which cause defects at grain boundaries and intralattice impurities, cannot be used in SiC. It is necessary to reduce the amount within a range that does not affect the sintered density. In this respect, since the content of B and Al in the present invention is small, it satisfies this requirement. Since it is generally desirable for the density of the SiC sintered body to be high, not only for electrically insulating substrates, the density is set to 3.0 g/cm 3 or higher in the present invention. Next, a method for manufacturing the sintered body of the present invention will be described. SiC produced by the silica carbon reduction method, the so-called Acheson method, is industrially advantageous. this
Pulverize SiC. In ordinary SiC sintered bodies, the N content is not an issue, so they are pulverized in air using a ball mill or the like. However, in air crushing
It was found that a considerable amount of N was adsorbed on SiC.
This N hardly escapes during sintering. Therefore, in order to obtain a SiC sintered body with a low N content, the N content of powdered SiC must be lowered. Since most of the N in conventional SiC powder enters the pulverization process, in the present invention the SiC is pulverized in a non-nitrogen atmosphere, for example, in an argon atmosphere. Grinding should be as fine as possible, preferably less than 5 microns. Impurities introduced during grinding, such as iron, are removed with acid. Conventionally, pulverization was generally carried out in air (for example, the
Example 1), and since adsorption of N 2 is unavoidable, N exceeds 300 ppm. The SiC powder thus obtained has a purity of 95% by weight or more and an N content of 300 ppm or less. Both α and β crystal forms can be used. The C added to the SiC powder can be used by dispersing carbon powder such as carbon black with a dispersion medium, but it is preferable to add a liquid phenol resin or the like and carbonize it to C during sintering of the SiC. In this case, take into consideration the carbonization rate of the resin, and if carbon gasifies and escapes due to the reduction of a small amount of SiO 2 in SiC, also take into account the amount of carbon that will be released in the sintered body in a free state. It is necessary to determine the amount of resin added so that the amount of remaining C falls within the above range. B is a powder of elemental boron, B 4 C, or the like. Even if elemental boron is added, it is thought that most of it is in the form of B 4 C in the SiC sintered body. Al may be metallic aluminum powder or a compound such as Al 2 O 3 , but AlN cannot be used. As mentioned above, C is added to SiC powder with as little N as possible.
Source, B source, and Al source are added, and a primary binder such as PVA is added and molded into the desired shape. The molded body is heated and sintered. Sintering may be performed with or without pressure, but if the molded object has a complicated shape, non-pressure is suitable. The sintering temperature is
A range of 1900 to 2200°C is suitable. Sintering is performed under an inert atmosphere such as Ar, but an N 2 atmosphere is avoided. Example α-SiC grains with a purity of 96% produced by the Acheson method were crushed to obtain powder of 2.5 μm or less in the following manner. The pulverization was carried out as follows to prevent N from entering. SiC powder several mm in size was placed in a ball mill, degassed with a vacuum pump, and heated to 10 -1 Torr with Ar.
was injected. This was repeated twice, and then crushed for one day. This material was taken out, Fe was removed with hydrochloric acid, and it was classified using a sedimentation method. The resulting SiC powder has a N content of 180ppm.
It was hot. 100 parts by weight of this SiC powder, 2 parts by weight of phenol resin (solid content), 0.06 parts by weight of B, 0.18 parts by weight of Al.
After adding 190 c.c. of water and 2 parts of PVA by weight, the mixture was mixed for 20 hours and then granulated using a spray dryer. The granules were pressure-molded to a diameter of 50 mm and a thickness of 6 mm, and sintered by heating at 2100° C. for 60 minutes in an Ar atmosphere without pressure. Its characteristics are shown in No. 1 of Table 1. The results of sintering were performed in the same manner, but with different amounts of B, Al, and C, as shown in No. 2 and below in Table 1. Comparative Example No. 1 in the table is one in which SiC pulverization was performed in air.
【表】【table】
【表】
発明の効果
上記の結果が示すように本発明の焼結体は密度
が高く、かつ電気比抵抗が大きいので電気絶縁基
板等に用いることができる。[Table] Effects of the Invention As shown by the above results, the sintered body of the present invention has a high density and a large electrical resistivity, so it can be used for electrically insulating substrates and the like.