【発明の詳細な説明】
本発明は六方晶BN(以下HBNという)を立方
晶BN(以下CBNという)の安定域下に加圧、加
熱してCBNを合成する方法に関する。
一般にCBNはアルカリ金属、アルカリ土類金
属、これらの窒化物を触媒としてHBNより高
温、高圧下で合成されている。
CBNは主として研削、切削剤に使用されるも
のであるが、その性能を向上させるにはCBNは
良質の結晶、即ち、自形の形状を有し、気泡等に
よる結晶欠陥がないことが必要となる。また
CBN合成においては当然転換率が高いこと及び
粒度が揃つていることが望ましい。
従来、これらの目的のため種々の方策がとられ
ている。例えば原料HBNを精製して高純度化す
ることや触媒の改良である。この触媒としては予
じめCa3B2N4を合成して用いる方法(特開昭55
−60008)、Sr3B2N4又はBa3B2N4を用いる方法
(特開昭57−156399)がある。
また本出願人は先にLiCaBN2化合物を触媒と
する方法(特願昭56−181391)(特開昭58−84106
号公報)、LiBaBN2化合物を触媒とする方法(特
願昭57−122973)(特開昭59−18105号公報)を提
案した。
これらの方法において、特に本出願人の提案に
よる方法では優れた結晶形のCBNが得られる。
この場合触媒とHBNが均一に混合されているこ
とが望ましく、不均一だと結晶を悪くする原因と
なつたり、合成試料内でCBNが偏つて生成し、
粒度も不揃いとなる。そして触媒とHBNは夫々
粉砕し、細粉を混合して用いるが、均一な混合に
は困難が多い。特にこの機械的粉砕、混合の方法
では各粒子内に他の成分を含有させることはでき
ない。
本発明はこれらの事情のもとに開発したもの
で、触媒とHBNを均一に混合するために、特定
の触媒調整時に過剰のHBNを混合しておき、焼
成後、粉砕して原料とする方法である。触媒調整
時に過剰のHBNを加えることは前記公報にも記
載されているが、その作用効果は明らかにされて
いない。
本発明者の研究によるとHBNの過剰の効果は
本発明の触媒であるLiXBN2(Xはアルカリ土類
金属、即ち、Be、Mg、Ca、Sr、Baである。)の
場合に大きいことを見出した。LiXBN2の調整は
三元系であること、融点がCa3B2N4等より低い
こと等より、過剰のHBNの分散がよくなるもの
と考えられる。
LiXBN2の生成は次の反応式による。
Li3N+X3N2+3BN→3LiXBN2
この反応はN2等の不活性雰囲気下、700〜1100
℃程度で進行する。X線回折によりこの生成物は
化合物であつて、各成分が混合したものでないこ
とは確認され、また化学分析により上記の組成で
あることが確かめられている。
上記の反応において、生成物は溶融しているが
HBNを過剰に加えると、その量によつては固体
のまま反応が進む。
従つて、触媒の各成分原料は40μm以下が好ま
しい。そして未反応のHBNは生成物の周囲に残
る。このようにして得られた塊状物を粉砕すれ
ば、触媒成分とHBNが結合ないし夾雑したよう
な粒子が得られる。従つて、単に触媒とHBNを
混合した場合に較べ、両粒子の接触が均一にな
る。
HBNの過剰量は上記反応式における化学量論
量のモル比で5〜50倍が適する。この上限値は
CBN合成時に触媒に対し望ましいHBNの量から
定めたものであり、下限値は特に効果を大きくす
るにはこの程度の過剰量が必要である。そして
CBN合成に当り、HBNが不足の場合は、触媒の
反応生成物にHBNを混合して調整する。なお、
Li3NとX3N2はほぼ化学量論量に混合することが
好ましい。
調整された原料は予備成形し、超高圧装置に装
填してCBN合成を行なう。
CBNの合成条件は通常のものと特に変るとこ
ろはなく、温度は1350〜1600℃程度、圧力は40〜
60Kb程度である。この条件に保持する時間は約
20〜40分が適当である。
本発明によれば粒度の揃つたCBN粒が多く得
られ、かつ収率も高い。また各粒形が整つたいわ
ゆる自形粒が多く、結晶欠陥も少ない。
実施例
Li3N、Ca3N2及びHBNの各粉末をモル比で
1:1:20の割合に混合し、N2雰囲気中で約900
℃に60分間加熱した。生成物は塊状の焼結体であ
つた。このものはX線回折、化学分析により
LiCaBN2とHBNの混合物であることが確認され
た。この生成物を100μm以下に粉砕し、その100
重量部にHBN粉末50重量部加え、よく混合して
直径2.6cm、長さ3.1cmに成形した。この成形体を
超高圧装置に装填し、約1450℃、50Kbに30分間
保持してCBNを合成した(実施例1)。
比較のためLi3N、Ca3N2、HBNを前記した化
料量論量に配合し、LiCaBN2を調整した。これ
を100μm以下に粉砕し、これにHBNを全体とし
て触媒とHBNの割合が上記実施例と同じになる
ように配合した。そして実施例と同様にCBNを
合成した。(比較例1)
同様にして、Ca3N2の代りにBa3N2を用い、但
し、Li3N、Ba3N2、HBNをモル比で1:1:30
にして、追加のHBNを用いなかつた外は上記例
と同じにしてCBNを合成した(実施例2)。この
比較のため比較例1と同様にHBNと各触媒を化
学量論量に配合してCBNを合成した(比較例
2)。これらの結果を第1表に示す。
【表】DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for synthesizing CBN by pressurizing and heating hexagonal BN (hereinafter referred to as HBN) to a stable region of cubic BN (hereinafter referred to as CBN). Generally, CBN is synthesized at higher temperatures and pressures than HBN using alkali metals, alkaline earth metals, and their nitrides as catalysts. CBN is mainly used as a grinding and cutting agent, but in order to improve its performance, CBN must have good quality crystals, that is, have an euhedral shape and be free of crystal defects such as air bubbles. Become. Also
In CBN synthesis, it is naturally desirable to have a high conversion rate and uniform particle size. Conventionally, various measures have been taken for these purposes. Examples include refining the raw material HBN to make it highly purified and improving catalysts. As this catalyst, Ca 3 B 2 N 4 is synthesized in advance and used (Japanese Patent Laid-Open No. 55
-60008), a method using Sr 3 B 2 N 4 or Ba 3 B 2 N 4 (Japanese Unexamined Patent Publication No. 156399/1983). In addition, the present applicant has previously proposed a method using LiCaBN 2 compound as a catalyst (Japanese Patent Application No. 56-181391) (Japanese Patent Application No. 58-84106).
(Japanese Patent Application No. 59-18105) using a LiBaBN 2 compound as a catalyst (Japanese Patent Application No. 122973/1982). In these methods, CBN in an excellent crystalline form can be obtained, particularly in the method proposed by the present applicant.
In this case, it is desirable that the catalyst and HBN are mixed uniformly; if they are not uniform, it may cause poor crystallization, or CBN may be unevenly formed in the synthesized sample.
The particle size also becomes irregular. The catalyst and HBN are each ground and used by mixing fine powders, but there are many difficulties in uniformly mixing them. In particular, this method of mechanical grinding and mixing does not allow other components to be contained within each particle. The present invention was developed under these circumstances. In order to uniformly mix the catalyst and HBN, excess HBN is mixed at the time of specific catalyst preparation, and after calcination, it is pulverized and used as a raw material. It is. The above publication also describes the addition of excess HBN during catalyst preparation, but its effects have not been clarified. According to the research of the present inventors, the effect of excess HBN is large in the case of LiXBN 2 (X is an alkaline earth metal, i.e., Be, Mg, Ca, Sr, Ba), which is the catalyst of the present invention. I found it. Since LiXBN 2 is prepared as a ternary system and has a melting point lower than that of Ca 3 B 2 N 4 etc., it is thought that the excess HBN is better dispersed. The production of LiXBN 2 is according to the following reaction formula. Li 3 N+X 3 N 2 +3BN→3LiXBN 2This reaction is carried out under an inert atmosphere such as N
It progresses at about ℃. X-ray diffraction confirmed that this product was a compound and not a mixture of components, and chemical analysis confirmed that it had the above composition. In the above reaction, the product is molten but
If HBN is added in excess, depending on the amount, the reaction will proceed while it remains solid. Therefore, the raw materials for each component of the catalyst preferably have a thickness of 40 μm or less. And unreacted HBN remains around the product. When the thus obtained lumps are pulverized, particles in which the catalyst component and HBN are combined or contaminated are obtained. Therefore, compared to the case where the catalyst and HBN are simply mixed, the contact between the two particles becomes more uniform. The excess amount of HBN is suitably 5 to 50 times the stoichiometric molar ratio in the above reaction formula. This upper limit is
It is determined based on the desired amount of HBN for the catalyst during CBN synthesis, and the lower limit is such an excess amount that this level of excess is required to particularly increase the effect. and
When synthesizing CBN, if HBN is insufficient, it can be adjusted by mixing HBN with the reaction product of the catalyst. In addition,
Li 3 N and X 3 N 2 are preferably mixed in approximately stoichiometric amounts. The prepared raw materials are preformed and loaded into an ultra-high pressure device to perform CBN synthesis. The synthesis conditions for CBN are not particularly different from normal ones; the temperature is about 1350 to 1600℃, and the pressure is about 40 to 40℃.
It is about 60Kb. The time to hold in this condition is approx.
20 to 40 minutes is appropriate. According to the present invention, many CBN grains with uniform particle size can be obtained and the yield is also high. In addition, there are many so-called euhedral grains with regular grain shapes, and there are few crystal defects. Example Li 3 N, Ca 3 N 2 and HBN powders were mixed at a molar ratio of 1:1:20, and about 900 ml of powder was mixed in a N 2 atmosphere.
℃ for 60 minutes. The product was a massive sintered body. This was determined by X-ray diffraction and chemical analysis.
It was confirmed that it is a mixture of LiCaBN 2 and HBN. This product is ground to 100 μm or less, and the 100
50 parts by weight of HBN powder was added to the parts by weight, mixed well, and molded into a diameter of 2.6 cm and a length of 3.1 cm. This compact was loaded into an ultra-high pressure device and held at about 1450° C. and 50 Kb for 30 minutes to synthesize CBN (Example 1). For comparison, Li 3 N, Ca 3 N 2 and HBN were mixed in the stoichiometric amounts of the chemical agents described above to prepare LiCaBN 2 . This was pulverized to 100 μm or less, and HBN was added thereto so that the overall ratio of catalyst to HBN was the same as in the above example. Then, CBN was synthesized in the same manner as in the example. (Comparative Example 1) In the same way, Ba 3 N 2 was used instead of Ca 3 N 2 , but the molar ratio of Li 3 N, Ba 3 N 2 , and HBN was 1:1:30.
CBN was synthesized in the same manner as in the above example except that no additional HBN was used (Example 2). For this comparison, CBN was synthesized by blending HBN and each catalyst in stoichiometric amounts in the same manner as in Comparative Example 1 (Comparative Example 2). These results are shown in Table 1. 【table】