JP3677812B2 - Silicon nitride sintered body and method for producing the same - Google Patents

Description

【００４６】
得られた各Ｓｉ原料粉末に、窒素空孔生成剤としてＳｍ₂Ｏ₃粉末を８モル％添加し、更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た。各成形体は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、Ｓｍは価数＋３であり、その共有結合半径Ｒ_MとＳｉの共有結合半径Ｒ_Siとの関係は（Ｒ_M−Ｒ_Si）／Ｒ_Si＝０.５である。
【００４７】
次に、上記の各成形体を窒素気流中において表２に示すように１３５０℃で２時間の窒化処理を行った後、同じく窒素気流中にて下記表２に示す１７５０〜１８００℃で３時間の焼結を行った。かくして得られた各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。
【００４８】
【表２】

【００４９】
上記のごとく、市販Ｓｉ粉末を予めｐＨ４〜１１の水で洗浄した後、窒素以外の雰囲気中にて３００〜８００℃で２〜５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の範囲に制御することができ、且つこのＳｉ原料粉末に共有結合半径Ｒ_MがＳｉの共有結合半径Ｒ_Siとの間で０.５≦（Ｒ_M−Ｒ_Si）／Ｒ_Si＜０.８の関係を満たすＳｍのような価数＋１〜＋３の元素を含む窒素空孔生成剤と混合して、反応焼結することにより、相対密度９６％以上で３点曲げ強度８００ＭＰａ以上のＳｉ₃Ｎ₄焼結体を得ることができる。
【００５０】
参考例
上記実施例１と同じ市販のＳｉ粉末（Ａ）をｐＨ３の水で洗浄した。得られたＳｉ粉末（Ａ₁）の不対電子濃度は６×１０²⁰／ｃｍ³であった。又、同じく市販のＳｉ粉末（Ｂ）をｐＨ１２の水で洗浄し、得られたＳｉ粉末（Ｂ₁）の不対電子濃度は９×１０²⁰／ｃｍ³であった。
【００５１】
上記各Ｓｉ粉末（Ａ₁）及び（Ｂ₁）を、実施例３の試料１と同様にアルゴン雰囲気中にて３００℃で５時間熱処理した。得られた各Ｓｉ原料粉末の不対電子濃度を表３に示した。この各Ｓｉ原料粉末に、実施例３の試料１と同様に窒素空孔生成剤としてＳｍ₂Ｏ₃を８モル％添加して混合、成形、脱バインダー処理し、更に実施例３の試料１と同様に窒素雰囲気中で１３５０℃×２時間の窒化処理と、同雰囲気中で１８００℃×３時間の焼結を行い、それぞれＳｉ₃Ｎ₄焼結体を製造した。得られた各焼結体の特性を表３に併せて示した。
【００５２】
【表３】

実施例４
窒素空孔生成剤の種類がＳｉ₃Ｎ₄焼結体の特性に及ぼす影響を調べるため、下記の試験を行った。
【００５３】
前記実施例２で得られた洗浄後のＳｉ粉末（Ａ₁）を、酸素雰囲気中にて５００℃で２時間の熱処理を行った。得られたＳｉ原料粉末の不対電子濃度は８×１０¹⁵／ｃｍ³であった。比較のために、熱処理を行わないＳｉ原料粉末も用意した。これらの各Ｓｉ原料粉末に、下記表４に示す窒素空孔生成剤をそれぞれ添加し、実施例３と同様に混合、成形、及び脱バインダー処理を行った。
【００５４】
【表４】

【００５５】
上記の窒素空孔生成剤を添加し、混合た各Ｓｉ原料粉末を、成形、及び脱バインダー処理を行った後、実施例３と同様に窒素気流中において下記表４に示す条件での窒化処理をと、それに続けての焼結を行った。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５に示した。
【００５６】
【表５】

【００５７】
上記のごとく、不対電子濃度が１０¹⁵〜１０²⁰／ｃｍ³の範囲にあるＳｉ原料粉末を使用し、＋１〜＋３価の価数を有し且つその共有結合半径Ｒ_MとＳｉの共有結合半径Ｒ_Siとが０.５≦（Ｒ_M−Ｒ_Si）／Ｒ_Si≦０.８の関係にある元素を添加して焼結することにより、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。
【００５８】
実施例５
この実施例では、特に窒素空孔生成剤の添加量が及ぼす影響について調べるため以下の試験を行った。まず、前記実施例２で得たＳｉ粉末（Ａ₁）及び（Ｂ₁）を下記表６に示す条件で熱処理した。得られた各Ｓｉ原料粉末の不対電子濃度を表６に併せて示した。
【００５９】
【表６】

【００６０】
得られた各Ｓｉ原料粉末に、下記表７に示す窒素空孔生成剤を添加し、実施例３と同様の方法で混合、成形、脱バインダー処理を行った。次に、各成形体を窒素気流中において下記表８に示す条件で窒化処理、及び焼結を行った。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に併せて示した。
【００６１】
【表７】

【００６２】
【表８】

【００６３】
上記の結果から、窒素空孔生成剤が合計で１〜１０モル％の範囲にあれば、緻密且つ高強度のＳｉ₃Ｎ₄焼結体が得られることが分かる。
【００６４】
【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】Ｓｉよりも共有結合半径の小さいＦ原子が介在し歪を生じたＳｉ格子を示す模式図である。
【図６】図５のＦ原子による歪を緩和するため格子間Ｓｉを生じた状態を示す模式図である。
【図７】不対電子濃度７×１０¹²／ｃｍ³の市販Ｓｉ粉末（Ａ）と同濃度２×１０¹³／ｃｍ³の市販Ｓｉ粉末（Ｂ）を洗浄する水のｐＨと、洗浄後の各Ｓｉ粉末の不対電子濃度との関係を示すグラフである。
【図８】市販Ｓｉ粉末（Ａ）をｐＨ７の水で洗浄した後の熱処理温度と、得られるＳｉ原料粉末の不対電子濃度の関係を示すグラフである。
【図９】市販Ｓｉ粉末（Ｂ）をｐＨ８の水で洗浄した後の熱処理温度と、得られるＳｉ原料粉末の不対電子濃度の関係を示すグラフである。
【図１０】窒化促進剤とアモルファスＳｉＯ₂層との反応により表面に液相が生成されたＳｉ粉末の模式図である。
【図１１】図１０の液相の内部に窒化によりＳｉ₃Ｎ₄層が生成された状態を示す模式図である。
【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
５ 液相
６ Ｓｉ₃Ｎ₄層[0001]
[Industrial application fields]
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. This is because, as shown in FIG. 1, presumably because the amorphous SiO ₂ layer 1 ₀ that is generating the surface of the Si powder is inhibited from diffusing into the Si layer 3 of nitrogen.
[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. Why nitride is promoted, amorphous SiO ₂ layer and the nitriding agent is reacted in Si powder surface, as shown in FIG. 10, since the amorphous SiO ₂ layer surface becomes a liquid phase 5, Nitrogen into Si layer 3 This is thought to be because the diffusion rate of the liquid crystal increases.
[0004]
[Problems to be solved by the invention]
However, even in the method using these nitriding accelerators, as shown in FIG. 11, the diffusion coefficient of nitrogen in the Si ₃ N ₄ layer 6 formed inside the liquid phase 5 on the surface of the Si powder is small, and further inside Nitrogen diffusion into silicon and generation of Si ₃ N ₄ are hindered, so that further reduction in nitriding time cannot be achieved. 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. In addition, the added nitriding accelerator coarsens the Si ₃ N ₄ crystal particles, which may be a starting point of destruction, and thus is not necessarily a preferable method.
[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]
To achieve the above object, silicon nitride sintered body provided by the present invention, concentration of unpaired electrons obtained by the reaction sintering of Si powder ^{10 15 / cm 3 ~10 21 /} cm 3 of silicon nitride sintered a body, in valence +1 to +3 valence element, covalent radii covalent radius _{R Si} and the 0.5 ≦ a _{R M} and _{Si (R M -R Si) /} R Si <0.8 for the It is characterized by containing related elements.
[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 . In the method of the present invention, the Si powder having an unpaired electron concentration within this range is obtained by washing a commercially available Si powder with water having a pH of 4 to 11, and then 3 to 5 at a temperature of 300 to 800 ° C. in an atmosphere other than nitrogen. It is obtained by a method of time annealing.
[0009]
More specifically, in the method for producing a silicon nitride sintered body of the present invention, an unpaired electron concentration Si powder obtained by the above method is composed of an element having a valence of +1 to +3 and its covalent bond radius R _M. And at least one compound powder containing elements in which the covalent bond radius R _{Si of Si} is 0.5 ≦ (R _M −R _Si ) / R _Si <0.8 is mixed, molded, and reacted Sinter.
[0010]
The element contained in the compound powder added to and mixed with the Si powder is preferably a rare earth element or a Group 2A element of the periodic table, and among the rare earth elements, La, Sm, or Yb, and within the Group 2A element, Ca or Sr. Is preferred. The form of the compound powder to be mixed is preferably an oxide, nitride, or oxynitride, and the mixing amount is 1 to 10 mol% in terms of the rare earth or 2A group element contained in the compound. Is preferable.
[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 as described above and the Si ₃ N formed near the surface of the Si powder by nitriding. _This is because there are two layers of two nitrogen diffusion inhibiting factors.
[0012]
In the present invention, in order to overcome the above two factors for inhibiting nitrogen diffusion, the present inventors succeeded in improving the nitrogen diffusion rate and greatly shortening the nitriding time by mainly incorporating two characteristic steps. 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 the Si ₃ N ₄ layer.
[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 innermost Si layer 3. . Amorphous SiO ₂ layer 1 ₀ surface inhibits diffusion into the interior of the nitrogen as described above, the oxygen solid-solved in Si (O) layer 2 _0, which is an intermediate layer is present between lattices, interstitial oxygen Increases the lattice constant of Si.
[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. When +1 to +3 valent elements are dissolved in the Si ₃ N ₄ lattice, the positively charged elements occupy the lattice points 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. However, 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.
[0018]
In the present invention, the Si (O) layer 2 described above is used in order to solidify +1 to +3 elements into Si ₃ N ₄ produced by nitriding. That is, the 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, as shown in FIG. 3, the Si powder in this state of the present invention mainly includes Si (O) _(1-y) M _z Vsi _(yz) layer (hereinafter abbreviated as SiM layer) 2 ₂ , Of the Si layer 3. When the diffusion further proceeds, M is diffused by u from the SiM layer 2 ₂ to the Si layer 3 to generate a Si _{(1-u) Mu} layer. At this time, when the covalent bond radius of the element M is larger than that of Si, as shown in FIG. 4, the generation of silicon vacancies is promoted to suppress the increase of the lattice constant of Si, and Si _{(1- uw)} abbreviated as M _u Vsi _w layer (hereinafter SiMV layer) 4 is formed.
[0020]
Then, SiM layer 2 ₂ and SiMV layer 4 of the powder will react as the each reaction formula by nitriding:
[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, nitrogen vacancies are generated in any Si ₃ N ₄ , and by using an element that can have a valence of +1 to +3 as a nitrogen vacancy generator, Si ₃ N _It can be expected that the diffusion rate of nitrogen in the _four layers will increase.
[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 solid solution of the nitriding accelerator 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]
On the other hand, components having a smaller covalent bond radius than Si, such as fluorine (F), carbon (C), and sulfur (S), are present on the surface of commercially available Si powder. Since these components have a small covalent bond radius, they are dissolved in the Si crystal of the Si powder by the annealing process of the commercial Si powder described above, and as a result, interstitial Si is generated, and coexisting elements other than Si (additives and impurities) The phenomenon of negative effects is likely to occur in nitriding to suppress the diffusion of).
[0024]
That is, when, for example, F atoms enter the Si crystal lattice, the Si crystal lattice contracts and is distorted as shown in FIG. In order to alleviate this distortion, interstitial Si is generated as shown in FIG. This interstitial Si has a function of preventing solid solution and diffusion of other elements, and this tendency is strong particularly for an element having a large covalent bond radius.
[0025]
Therefore, for example, when the Si powder is annealed without removing elemental components having a smaller covalent bond radius than Si, such as F, C, S, etc., the elements of the valences +1 to +3 added as a nitrogen vacancy generator covalent radius R _M of the have a relation of _{(R M -R Si) / R} Si <0.5 between the covalent bond radius R _Si of Si is required.
[0026]
For this reason, in the method of the present invention, prior to the annealing of the Si powder, the Si powder is washed with water having a pH of 4 to 11, so that the covalent bond radius is larger than that of Si such as F, C, and S existing on the surface of the Si powder. A step of removing small elemental components in advance; As a result, in the present invention, an element having a large covalent bond radius R _M that satisfies the relationship of 0.5 ≦ (R _M −R _Si ) / R _Si <0.8 is added as an element added as a nitrogen vacancy generator. It can be used.
[0027]
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 washed with water having a pH of 4 to 11. Here, the pH of water is set to 4 to 11 when the pH is outside this range, the thickness of the oxide layer on the surface of the Si powder becomes large, and in any case, the unpaired electron concentration becomes larger than 10 ²⁰ / cm ^3. Because it will end up.
[0028]
Next, the cleaned Si powder is annealed at 300 to 800 ° C. for 3 to 5 hours. As a result, the oxygen in the amorphous SiO ₂ layer generated on the powder surface is diffused into Si to form a Si (O) layer in which many Si vacancies exist. However, the processing atmosphere must be other than a nitrogen atmosphere, such as oxygen, 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 generating agent is hardly dissolved.
[0029]
In addition, the formation method of the nitrogen vacancy Si (O) layer is not only the above-mentioned annealing, but also a method of ion-implanting oxygen ions into the Si powder, a method of forcibly mixing oxygen when producing bulk Si Etc. can also be used.
[0030]
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.
[0031]
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 generating 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.
[0032]
Next, Si powder having a Si (O) layer produced by the above method and having a high concentration of unpaired electrons is mixed with one or more nitrogen vacancy generating agents and molded. Here, the nitrogen vacancy generating agent of the present invention is a compound containing an element having a valence of +1 to +3, and the covalent bond radius R _{M of the} element and the covalent bond radius R _Si of the element are 0. It is a compound of an element having a relationship of 0.5 ≦ (R _M −R _Si ) / R _Si <0.8. Elements in the above range tend to generate nitrogen vacancies. Of these elements, it is preferable to use Ca, Sr, La, Sm, and Yb compounds that act as nitrogen vacancy generating agents and are also effective as sintering agents.
[0033]
The nitrogen vacancy generating agent may be added in powder form, but in particular when producing a large Si ₃ N ₄ sintered body, in order to disperse uniformly on the Si powder surface, alkoxides of these elements, stearates, It can also be added in the form of laurate. The addition amount of the nitrogen vacancy generating agent is preferably controlled in the range of 1 to 10 mol% in terms of elements in each sintered body. That is, when the nitrogen vacancy generating agent is less than 1 mol%, the amount of solid solution in Si or Si ₃ N ₄ is small, so the effect as a nitrogen vacancy generating agent is not achieved. This is because it precipitates at the boundary and becomes the starting point of fracture, and a high strength sintered body having a three-point bending strength of 800 MPa or more cannot be obtained.
[0034]
Finally, the molded body is nitrided and sintered. The temperature pattern of nitriding and sintering may be very simple control, for example, holding at 1100 to 1400 ° C. for 2 to 4 hours and then holding at 1500 to 1900 ° C. for 1 to 3 hours. By this treatment, a Si ₃ N ₄ sintered body having a relative density of 96% 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.
[0035]
Unpaired electrons of the Si ₃ N ₄ sintered body is considered to be due to oxygen contained in the _{Si 3 N 4, Si 3 N} 4 of the present invention in which a solid solution of oxygen by heat treatment to the Si Then, it is 10 ¹⁵ / cm ³ or more and 10 ²¹ / cm ³ or less. On the other hand, when a sintered body is produced without applying heat treatment to Si, only an Si ₃ N ₄ sintered body having an unpaired electron of less than 10 ¹⁵ / cm ³ can be obtained due to a small amount of oxygen solid solution. .
[0036]
【Example】
Example 1
Two types of Si powders (A) and (B) were prepared as starting materials. The unpaired electron concentration of these Si powders was 7 × 10 ¹² / cm ³ for Si powder (A) and 2 × 10 ¹³ / cm ³ for Si powder (B). These commercially available Si powders (A) and (B) were washed with water having a pH of 1 to 14 and dried, and the results of measuring the unpaired electron concentration were shown in FIG.
[0037]
As is clear from FIG. 7, the Si powder washed with water having a pH of 1 to 3 and 12 to 14 has an unpaired electron concentration exceeding 10 ²⁰ / cm ³ and is suitable as the raw material powder of the present invention as described above. I understand that it is not.
[0038]
Example 2
The same raw Si powder (A) with an unpaired electron concentration of 7 × 10 ¹² / cm ^{3 and} a commercially available Si powder (B) with an unpaired electron concentration of 2 × 10 ¹³ / cm ³ were used as starting materials. The Si powder (A) was washed with water at pH 7 and the Si powder (B) at pH 8 to obtain Si powders (A ₁ ) and (B ₁ ), respectively. Next, the relationship between the heat treatment conditions of these powders and the unpaired electron concentration was examined.
[0039]
That is, the Si powder (A ₁ ) and the Si powder (B ₁ ) were respectively placed in an oxygen atmosphere (a), a hydrogen atmosphere (b), an argon atmosphere (c), and a vacuum of 10 Torr (d). At 100 ° C. intervals from 100 ° C. to 900 ° C., the Si powder (A ₁ ) was held for 5 hours and the Si powder (B ₁ ) was held for 2 hours, respectively, and then the unpaired electron concentration was measured by the ESR method.
[0040]
As a comparative example, the same Si powder (A ₁ ) and Si powder (B ₁ ) 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.
[0041]
The results are shown in FIG. 8 for the Si powder (A ₁ ) and in FIG. 9 for the Si powder (B ₁ ). In addition, the amount of F, C, and S in each Si powder (A ₁ ) and (B ₁ ) was 50 ppm or less, which was about a trace. From these results, the commercially available Si powder was washed with water having a pH of 4 to 11, and further heat-treated at 300 to 800 ° C. for 2 to 5 hours in an atmosphere other than nitrogen such as oxygen, hydrogen, argon, vacuum of 10 Torr or less. It can be seen that the unpaired electron concentration of the Si powder can be controlled in the range of 10 ¹⁵ to 10 ²⁰ / cm ³ and that F, C, and S having a smaller covalent bond radius than Si can be removed.
[0042]
Incidentally, the Si powder washed with water of less than pH 4 and more than pH 11 in Example 1 has an unpaired electron concentration of 10 ²⁰ / cm ³ or less even if heat treatment is performed under the conditions in the range confirmed in this example. (The following reference example). Moreover, even if it is Si powder wash | cleaned with the water of pH 4-11, when heat processing was performed at 100 degreeC, 200 degreeC, and 900 degreeC other than the range confirmed in the present Example, it is a comparative example of following Example 3 As shown in the sample, the unpaired electron concentration could not be controlled between 10 ¹⁵ to 10 ²⁰ / cm ³ .
[0043]
Example 3
For the Si powders (A ₁ ) and (B ₁ ) obtained by cleaning in Example 2 above, the subsequent heat treatment conditions, the unpaired electron concentration of the Si raw material powder after the heat treatment, and the Si ₃ obtained from each Si raw material powder The following tests were conducted to investigate the correlation between the density, strength, and unpaired electron concentration of the N ₄ sintered body.
[0044]
Were heat-treated under the conditions of the above Si powder (A ₁₎ or 300 to 800 ° C. × 2 to 5 hours at (B ₁₎ the in each atmosphere other than nitrogen shown in the following Table 1. The unpaired electron concentration of each obtained Si raw material powder is shown together in Table 1. As comparative examples, the unpaired electron concentration was measured and shown in Table 1 for those not heat-treated and those heat-treated under conditions other than the above conditions.
[0045]
[Table 1]

[0046]
8 mol% of Sm ₂ O ₃ powder as a nitrogen vacancy generator was added to each obtained Si raw material powder, and 4 mol% of a thermoplastic resin binder was further added and mixed, and then molded by a dry press. Each molded body was debindered in a nitrogen stream at 600 ° C. for 2 hours. Sm has a valence of +3, and the relationship between the covalent bond radius R _M and the covalent bond radius R _{Si of Si} is (R _M −R _Si ) / R _Si = 0.5.
[0047]
Next, after performing nitriding treatment at 1350 ° C. for 2 hours as shown in Table 2 in the nitrogen stream, each of the above molded bodies was also subjected to 3 hours at 1750 to 1800 ° C. shown in Table 2 below in the same nitrogen stream. Was sintered. Table 2 shows the unpaired electron concentration, the relative density, and the three-point bending strength of each Si ₃ N ₄ sintered body thus obtained.
[0048]
[Table 2]

[0049]
As described above, after the commercially available Si powder is previously washed with water having a pH of 4 to 11, the unpaired electron concentration in the Si powder is set to 10 by holding at 300 to 800 ° C. in an atmosphere other than nitrogen for 2 to 5 hours. ^{15 ~10 20} / cm ³ of can be controlled within the range, and 0.5 ≦ between covalent radius R _M in the Si raw material powder and the covalent radius R _Si of Si (R _M -R _Si) / R _Si <0.8 satisfying the relationship of valence +1 to +3 such as Sm satisfying the relationship, and by reactive sintering, three-point bending at a relative density of 96% or more A Si ₃ N ₄ sintered body having a strength of 800 MPa or more can be obtained.
[0050]
Reference Example The same commercially available Si powder (A) as in Example 1 was washed with water at pH 3. The unpaired electron concentration of the obtained Si powder (A ₁ ) was 6 × 10 ²⁰ / cm ³ . Similarly, commercially available Si powder (B) was washed with water at pH 12, and the unpaired electron concentration of the obtained Si powder (B ₁ ) was 9 × 10 ²⁰ / cm ³ .
[0051]
Each of the Si powders (A ₁ ) and (B ₁ ) was heat-treated at 300 ° C. for 5 hours in an argon atmosphere in the same manner as Sample 1 of Example 3. Table 3 shows unpaired electron concentrations of the obtained Si raw material powders. To each of these Si raw material powders, 8 mol% of Sm ₂ O ₃ as a nitrogen vacancy generating agent was added, mixed, molded, and debindered in the same manner as in Sample 1 of Example 3, and further processed with Sample 1 of Example 3. Similarly, nitriding treatment at 1350 ° C. × 2 hours in a nitrogen atmosphere and sintering at 1800 ° C. × 3 hours in the same atmosphere were performed to produce Si ₃ N ₄ sintered bodies, respectively. Table 3 shows the characteristics of the obtained sintered bodies.
[0052]
[Table 3]

Example 4
In order to investigate the influence of the type of nitrogen vacancy generating agent on the characteristics of the Si ₃ N ₄ sintered body, the following test was performed.
[0053]
The cleaned Si powder (A ₁ ) obtained in Example 2 was heat-treated at 500 ° C. for 2 hours in an oxygen atmosphere. The unpaired electron concentration of the obtained Si raw material powder was 8 × 10 ¹⁵ / cm ³ . For comparison, a Si raw material powder not subjected to heat treatment was also prepared. To each of these Si raw material powders, a nitrogen vacancy generating agent shown in Table 4 below was added, and mixing, molding, and debinding were performed in the same manner as in Example 3.
[0054]
[Table 4]

[0055]
After adding said nitrogen void | hole production | generation agent and performing each mixing Si raw material powder and performing a binder removal process, it nitrided on the conditions shown in following Table 4 in nitrogen stream like Example 3. Then, the subsequent sintering was performed. Table 5 shows the unpaired electron concentration, relative density, and three-point bending strength of each obtained Si ₃ N ₄ sintered body.
[0056]
[Table 5]

[0057]
As described above, Si raw material powder having an unpaired electron concentration in the range of 10 ¹⁵ to 10 ²⁰ / cm ³ is used, has a valence of +1 to +3 and has a covalent bond radius R _M and a covalent bond of Si. An unpaired Si ₃ N ₄ sintered body formed by adding an element having a radius R _Si of 0.5 ≦ (R _M −R _Si ) / R _Si ≦ 0.8 and sintering. The electron concentration can be controlled in the range of 10 ¹⁵ to 10 ²¹ / cm ³ , and a dense and high-strength Si ₃ N ₄ sintered body can be obtained.
[0058]
Example 5
In this example, the following test was conducted in order to examine the influence of the addition amount of the nitrogen vacancy generating agent. First, the Si powders (A ₁ ) and (B ₁ ) obtained in Example 2 were heat-treated under the conditions shown in Table 6 below. Table 6 shows the unpaired electron concentration of each obtained Si raw material powder.
[0059]
[Table 6]

[0060]
Nitrogen vacancy generators shown in Table 7 below were added to the obtained Si raw material powders, and mixing, molding, and debinding were performed in the same manner as in Example 3. Next, each molded body was subjected to nitriding treatment and sintering under the conditions shown in Table 8 below in a nitrogen stream. Table 8 shows the unpaired electron concentration, relative density, and three-point bending strength of each obtained Si ₃ N ₄ sintered body.
[0061]
[Table 7]

[0062]
[Table 8]

[0063]
From the above results, it can be seen that a dense and high-strength Si ₃ N ₄ sintered body can be obtained if the total amount of nitrogen vacancy generators is in the range of 1 to 10 mol%.
[0064]
【The invention's effect】
According to the present invention, instead of expensive Si ₃ N ₄ powder as raw material powder, silicon powder having a price of about 1/10 is used, and by reaction sintering in a much shorter time than in the past, the cost is low. 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 schematic diagram showing a Si lattice in which F atoms having a covalent bond radius smaller than Si are present and strain is generated.
6 is a schematic diagram showing a state in which interstitial Si is generated in order to relax strain caused by F atoms in FIG. 5. FIG.
FIG. 7 shows the pH of water for washing 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 the same concentration of 2 × 10 ¹³ / cm ³ , and It is a graph which shows the relationship with the unpaired electron density | concentration of each Si powder.
FIG. 8 is a graph showing the relationship between the heat treatment temperature after washing commercially available Si powder (A) with water at pH 7 and the unpaired electron concentration of the obtained Si raw material powder.
FIG. 9 is a graph showing the relationship between the heat treatment temperature after washing commercially available Si powder (B) with water at pH 8 and the unpaired electron concentration of the obtained Si raw material powder.
FIG. 10 is a schematic view of a Si powder in which a liquid phase is generated on the surface by a reaction between a nitriding accelerator and an amorphous SiO ₂ layer.
11 is a schematic view showing a state in which a Si ₃ N ₄ layer is generated by nitriding inside the liquid phase of FIG.
[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 formed with Si vacancies by heat treatment 2 ₂ SiM layer 3 Si layer 4 SiMV layer 5 Liquid Phase 6 Si ₃ N ₄ layer

Claims (15)

A reaction sintering more unpaired electron density obtained is ^{10 15 / cm 3 ~10 21 /} cm silicon nitride sintered body of the ^third Si powder, in valence +1 to +3 valence element, the covalent bond radius _{R M} and the covalent radius _{R Si} and the 0.5 ≦ a _{Si (R M -R Si) /} R Si < silicon nitride sintered body, characterized in that it comprises an element in the 0.8 relationship. The silicon nitride sintered body according to claim 1, wherein the element is a rare earth element or a group 2A element of the periodic table. The silicon nitride sintered body according to claim 2, wherein the rare earth element is La, Sm, or Yb. The silicon nitride sintered body according to claim 2, wherein the group 2A element is Ca or Sr. The silicon nitride sintered body according to any one of claims 1 to 4, wherein the element is contained as an oxide, a nitride, or an oxynitride. The silicon nitride sintered body according to any one of claims 1 to 5, wherein the element is contained in an amount of 1 to 10 mol%. The silicon nitride sintered body according to claim 1, wherein the relative density is 96% or more and the three-point bending strength is 800 MPa or more. A method for producing a silicon nitride sintered body by reactive sintering of Si powder, after washing commercially available Si powder with water having a pH of 4 to 11, and annealing at 300 to 800 ° C. in an atmosphere other than nitrogen for 3 to 5 hours Si powder having an unpaired electron concentration of 10 ¹⁵ / cm ³ to 10 ²⁰ / cm ³ obtained by
In this Si powder, elements having a valence of +1 to +3 and the covalent bond radius R _M and the Si covalent bond radius R _Si are 0.5 ≦ (R _M −R _Si ) / R _Si <0.8. Mixing at least one compound powder containing the elements concerned,
A method for producing a silicon nitride sintered body, characterized by molding and reaction sintering. The method for producing a silicon nitride sintered body according to claim 8, wherein the atmosphere other than nitrogen is oxygen, hydrogen, argon, or a vacuum of 10 Torr or less. The method for producing a silicon nitride sintered body according to claim 8, wherein the element contained in the compound powder is a rare earth element or a group 2A element of the periodic table. The method for producing a silicon nitride sintered body according to claim 10, wherein the rare earth element is La, Sm, or Yb. The method for producing a silicon nitride sintered body according to claim 10, wherein the group 2A element is Ca or Sr. The method for producing a silicon nitride sintered body according to any one of claims 10 to 12, wherein the compound of the element is contained as an oxide, a nitride, or an oxynitride. The method for producing a silicon nitride sintered body according to claim 8, wherein the mixing amount of the compound powder is 1 to 10 mol% in terms of the element contained in the compound. 9. The reaction sintering according to claim 8, wherein the reaction sintering is performed by nitriding the molded body while maintaining the molded body at 1100 to 1400 ° C. in a nitrogen atmosphere, and further sintering at 1500 to 1900 ° C. in the same atmosphere. A method for producing a silicon nitride sintered body.

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