JP4452810B2 - Manufacturing method of transparent magnesia sintered body - Google Patents

Description

The present invention relates to a method for producing a transparent sintered body made of magnesia .

Magnesia is one of the important materials for refractories because it has the advantages of high fire resistance, excellent electrical insulation at high temperatures, large heat conduction, and low cost. Magnesia is cubic, and when the pores are completely removed, the sintered body becomes transparent. Recently, research and development of optical ceramics using heat resistance and transparency with more emphasis on function has been underway.

Since magnesia has a high melting point (2800 ° C.), it was necessary to fire at 1800 ° C. or higher to produce a high-density sintered body. However, magnesia has a relatively high dissociation pressure at high temperatures among heat-resistant materials, and pores grow by evaporation-condensation and surface diffusion mechanisms when fired at temperatures above 1800 ° C.
For this reason, in order to completely remove the pores under normal pressure sintering, the porosity is 1% before reaching 1800 ° C.
It had to be densified until the following.

As a method for producing transparent magnesia, a method of hot pressing after adding LiF and further vacuum sintering was developed. A hot isostatic press (HIP) has also been developed that uses gas pressure to isotropically pressurize the sample at the firing temperature to promote densification.

On the other hand, as a method using a sintering aid, magnesium oxide that produces magnesia by magnesia powder or calcining, metal oxides such as 2CaO and Ga ₂ O ₃ , Al ₂ O ₃ , SiO ₂ and B ₂ O ₃ A method has been developed in which a substance and fluoride ions are added and calcined, followed by cold pressure molding and firing in a vacuum atmosphere of 1500 ° C. or higher (Non-Patent Document 1).
The inventors have filed a patent application for an invention relating to a method for producing a magnesia sintered body that is made transparent by promoting densification with SiO ₂ and B ₂ O ₃ (Patent Document 1).

Mat.Res.Bull. Vol.2 1075-1086 (1967) JP 2000-281428 A

In recent years, the usage environment of various arc tubes, heat shields for projectors, and high-temperature windows has become severe.
The development of transparent ceramic materials with high thermal conductivity has become desirable. Al ₂ O ₃ ceramics with a small amount of MgO added have been developed as translucent ceramics, but since Al ₂ O ₃ is hexagonal, the light is birefringent at the grain boundaries, and even if the pores are completely removed, the rubbed glass There was a drawback that it became invisible and could not be made transparent.

Y ₂ O ₃ and Y ₃ Al ₅ O ₁₂ are cubic crystals that can be made transparent if the pores are completely removed, but Y ₂ O ₃ is a resource-poor and expensive material, so they are an inexpensive material that cannot be substituted. There was a drawback that it could only be used. Magnesia is cubic and transparent, has a very high melting point of 2800 ° C, has high thermal conductivity at high temperatures, is abundant in resources, and is inexpensive, making it ideal as a transparent ceramic material. For this reason, various manufacturing techniques related to the magnesia transparent sintered body have been developed.

Among them, the hot press method has the advantage that a transparent sintered body can be manufactured relatively easily regardless of the sinterability of the powder, but has the disadvantage that the workability is poor and only simple shapes can be manufactured. there were. The HIP method has relatively small restrictions on the shape, but the sample is covered with a thin metal plate,
There was a disadvantage that the open pores had to be removed in advance by normal pressure sintering, and the cost was increased accordingly.

Although the method using a sintering aid is highly productive and economical, the metal oxide additive remains in the material even after sintering, and has the disadvantage of reducing the original properties of the material. On the other hand, when fluoride ions are used as a sintering aid, most of the added fluoride ions evaporate during the sintering and leave the sintered body, but the remaining trace amount of fluoride ions reduces the high-temperature strength. There was a drawback of making it.

The present invention can be synthesized by (1) a safe and environmentally friendly method of aqueous precursor reaction, (2)
An existing apparatus can be used for calcining the precursor and calcining the calcined powder, and a method for producing a transparent magnesia sintered body at a very low cost is provided.

That is, the method for producing a transparent magnesia sintered body according to the first aspect of the present invention includes a solvent dispersion of an aggregate in which flaky basic magnesium carbonate particles having a purity of 99 % by mass or more are aggregated at three-dimensional intervals, and a chloride. A solid obtained by mixing a chloride solvent containing ions and drying the mixed solution is calcined at 600 ° C. to 1200 ° C. to obtain a magnesia powder, and the magnesia powder is molded to obtain a green compact. forming a, and firing the green compact at 1200 ° C. to 1900 ° C..

Further, in the present invention 2 is the preparation of transparent magnesia sintered body of the invention 1, a solvent dispersion of the resultant magnesia powder particles the agglomerates calcined at below 1100 ° C. or higher thermal decomposition temperature, the chloride It is characterized by mixing with a solvent .

Furthermore, the present invention 3 is characterized in that, in the method for producing a transparent magnesia sintered body of the invention 1 or 2, the thickness of the flaky basic magnesium carbonate particles is less than 200 nm .
Invention 4 is the method for producing a transparent magnesia sintered body according to any one of Inventions 1 to 3, wherein the chloride solvent contains 0.1 to 15 mol% of chloride ions, and metals and fluoride ions are contained. It is characterized by not being mixed.

The present invention uses (1) an aqueous solution reaction that is easy to handle, and a magnesia precursor is prepared using a resource-rich and inexpensive compound. Since the transparent sintered body can be produced not only in a vacuum atmosphere but also in an atmosphere containing oxygen, the sintering cost is very low, and the transparent sintered body can be produced economically.

<A> Precursor suitable for the method of the present invention The easily sinterable magnesia powder exhibiting the characteristics of the present invention is a bulky basic magnesium carbonate in which flaky particles having a thickness of less than 200 nm are three-dimensionally aggregated. Chloride ions are added and calcined. When the thickness of the precursor flake is less than 200 nm, magnesia powder particles having a diameter of 200 nm or less and individually separated by calcination can be obtained. This powder is preferable because it has good sinterability and can produce a transparent sintered body. On the other hand, when the thickness of the precursor is 200 nm or more, the calcined powder particles are agglomerated and the sinterability is lowered, which is not preferable. In the present invention, the basic magnesium carbonate powder needs to be produced so that the powdery flaky particles are three-dimensionally arranged so that large voids are formed between each other and are bulky and agglomerated. On the other hand, a powder aggregated in a state in which flaky particles are two-dimensionally stacked is not preferable because it becomes a magnesia powder in which the particles are hard and aggregated after calcination.

<B> Precursor production conditions The concentrations of the magnesium salt and the basic precipitating agent are each preferably 0.1 mol / L to 0.7 mol / L.
If it becomes thinner than this, the precipitate becomes a gel, and if dried, hard aggregated particles are formed, which is not preferable. On the other hand, when the concentration is 0.7 mol / L or more, the suspension becomes viscous and the change from the normal carbonate to the basic carbonate does not proceed sufficiently, and the normal carbonate remains after aging, which is not preferable.

From the viewpoint of yield, the mixing ratio of the magnesium salt and the basic precipitating agent (hereinafter referred to as MB ratio) is preferably 1 or more when the basic precipitating agent is a carbonate, and 2 or more when it is a bicarbonate. However,
When the MB ratio is 1.5 or more in the case of carbonate and 3 or more in the case of bicarbonate, impurities such as Na, Ca, and Si are taken into the precursor, which is not preferable.

Even if the MB ratio is slightly smaller than 1, a precursor that becomes highly pure and easily sinterable magnesia can be synthesized.
Since magnesia is an inexpensive material, it is industrially valuable if a sintered body with high added value can be produced even if the yield is somewhat reduced. Considering all of the above, the MB ratio is
A range of 0.7 to 1.5 is preferred, and in the case of bicarbonate, a range of 1.5 to 3 is preferred. 0.7 if the MB ratio is carbonate
If it is less than 1.5 or less than 1.5, the yield is very small.

In the method of the present invention, the ripening of the precipitate is preferably performed at 15 ° C to 60 ° C. Aging temperature is 15 ℃
The following is not preferable because the change from normal magnesium carbonate to basic magnesium carbonate does not proceed sufficiently. On the other hand, when the temperature exceeds 60 ° C., the shape of the precipitated particles changes from flaky to granular,
Since sinterability falls, it is not preferable.

In the method of the present invention, the preferable aging time varies depending on the aging temperature. When the aging temperature is 15 ° C or higher and lower than 30 ° C, it is 6 hours or longer. When the aging temperature is 30 ° C or higher and lower than 40 ° C, the aging temperature is 3 hours or longer.
At 40 ° C or higher and 60 ° C or lower, aging is required for 1 hour or longer.

Na ⁺ and K in the above basic by-products resulting in precipitation reaction of magnesium carbonate ^- like metal ions reduces the purity of the sintered body finally obtained. On the other hand, non-metallic ions such as SO ₄ ²⁻ and NO ₃ ⁻ aggregate the calcined powder to lower the sinterability. For this reason, after ripening the precipitated particles, it is necessary to wash with pure water to remove by-products that reduce the purity of the sintered body and aggregate the calcined powder particles.

In the method of the present invention, when a basic precipitating agent that does not contain Na ⁺ or K ⁻ or the like, which is an impurity, such as magnesium chloride and ammonium carbonate or basic ammonium carbonate, is used, it is dried without washing after aging and filtration. Even so, since the aqueous solution existing between the filtered precipitated particles contains chloride ions, a transparent magnesia sintered body can be produced without further adding chloride ions. In this case as well, the chloride ions in the precursor are removed by washing,
It is necessary to newly add chloride ions. Even commercially available basic magnesium carbonate, if it is a bulky powder in which flaky particles are three-dimensionally aggregated, the characteristics of the precursor of the present invention are exhibited.

If the flaky particles are three-dimensionally aggregated precursors, preferable results can be obtained even with magnesia prepared by calcining the precursors at a thermal decomposition temperature of 1100 ° C. or lower. The lower the calcination temperature, the more preferable the magnesia crystallites become finer. The magnesia crystallite immediately after pyrolysis is very fine, and the form of the mother salt composed of the particles can be easily destroyed by chloride ions, which is particularly preferable. When the calcining temperature exceeds 1100 ° C., the magnesia particles grow and the shape of the mother salt becomes strong, and even if calcined with the addition of chloride ions, the shape cannot be sufficiently destroyed.

<B> Addition of Chloride Ion In the method of the present invention, chloride ion is added to break up the magnesia mother salt and separate the particles individually. Further, a small amount of chloride ions remaining after calcination also has an action of directly promoting densification of the calcination powder.

In the method of the present invention, the synthesized precursor is dispersed in pure water, an organic solvent, or a mixed solvent before drying after the last filtration, and pure water, an organic solvent, or them are mixed in this dispersion. Addition of magnesium chloride, ammonium chloride or the like dissolved in a solvent is particularly preferable because chloride ions can be uniformly added to the precursor.

The chloride that supplies chloride ions in the present invention is not particularly limited as long as it is a chemical substance that contains chloride ions and deteriorates the properties of magnesia and does not contain fluoride ions. For example, hydrochloric acid, magnesium chloride, ammonium chloride and the like can be exemplified.

The amount of chloride ion used in the present invention is 0.1 to 15 with respect to the precursor or magnesia.
Mole% is preferred. If the addition amount is less than 0.1 mol%, the effect of adding chloride ions is small,
The destruction of the mother salt form is insufficient, and the pores cannot be removed so as to become transparent by the atmospheric pressure sintering method. On the other hand, when 15 mol% or more of chloride ions are added, necks between particles grow due to calcination, and aggregated particles are formed, so that sinterability is lowered, and pores can be removed as it becomes transparent by atmospheric pressure sintering. Can not.

<D> Calcination In the method of the present invention, the precursor or magnesia added with chloride ions is 600 ° C. to 120 ° C.
Must be calcined at 0 ° C. If it is less than 600 ° C., the calcined powder is fine and has poor filling properties, and the packing density distribution in the green compact is wide, and large pores are generated in the sparsely packed region, making it impossible to produce a dense sintered body. On the other hand, when the temperature exceeds 1200 ° C., strong secondary particles are formed and the characteristics of the present invention are lost.

<E> Sintering temperature In the present invention, in order to produce a transparent sintered body from which chloride ions have been completely removed,
It is necessary to calcinate at a temperature of 1900C. When the temperature is 1200 ° C. or lower, the pores cannot be completely removed by sintering. From the viewpoint of obtaining a good transparent body and a sintered body excellent in corrosion resistance, a larger particle size is generally preferable, and thus a higher sintering temperature is more preferable.

However, when the firing temperature is 1900 ° C. or higher, the energy for heating increases rapidly, and the firing furnace also requires a special refractory material that can withstand ultra-high temperatures. When a sintered body having high mechanical strength at the same time as transparency is required, it is necessary to lower the firing temperature to slow grain growth. The grain growth rate differs depending on the manufacturing process because it varies depending on a small amount of impurities mixed in the manufacturing process of the sintered body. Except for a specific example, it is preferable to fire at 1700 ° C. or lower in order to produce a transparent sintered body having fine particles.

<F> Firing atmosphere A gas containing an element that is difficult to diffuse in magnesia such as nitrogen or argon cannot be removed from the pores by sintering if the pores are isolated. For this reason, there is no particular limitation on the means, but it is necessary to perform firing in an atmosphere in which the total partial pressure is adjusted to 0.03 MPa or less.

It is preferable that the atmosphere is a vacuum because even if the densification progresses and the pores are isolated, there is no gas that suppresses the disappearance of the pores in the pores. Since oxygen ions and hydrogen ions diffuse easily in magnesia oxide, a good transparent body can be produced even if a large amount of oxygen gas or hydrogen gas is present in the atmosphere.

Commercially available sodium carbonate was dissolved in pure water, and 500 mL of an aqueous solution having a concentration of 0.4 mol / L was kept at 25 ° C. in a circulating thermostat while stirring with a magnetic stirrer. This solution was also kept at 25 ° C.
0.4 mL / L of commercially available magnesium chloride hexahydrate (500 mL) was added dropwise at a rate of 100 mL / min, and the mixture was stirred and held for 30 minutes. Thereafter, the mixture was aged at 35 ° C. for 18 hours to obtain a slurry containing basic magnesium carbonate.

The slurry was suction filtered with an aspirator. Disperse the filtered precipitate in 500 mL of pure water.
The operation of stirring for 5 minutes and filtering again was repeated 6 times for washing. A part of the precipitate filtered after washing was dried and subjected to chemical analysis. The purity of this precursor was 99.9% by weight or more.
The precursor flaky particles were bulky basic magnesium carbonate aggregated three-dimensionally while forming large voids between the particles.

Before the remaining precipitate is dried, it is dispersed in ethanol in a wet state and stirred with a magnetic stirrer with a heater, and 50 mL of an aqueous solution in which ammonium chloride corresponding to 5 mol% is dissolved is added dropwise to this dispersion. Compound ions were mixed. After dripping, the plate part of the stirrer was heated to 50 ° C. to evaporate the alcohol and dried. After sufficiently drying, it was thoroughly loosened in a mortar, heated to 900 ° C. at a constant temperature increase of 5 ° C./min in an oxygen stream in a tubular electric furnace, held for 4 hours, and calcined to obtain magnesia powder.

The magnesia powder obtained by calcination was loosened in a menor mortar and primary molded at 10 MPa using a mold and then hydrostatically pressed at 200 MPa to produce a molded body. The molded green compact was heated to 1600 ° C. at a constant temperature increase of 5 ° C./min in an oxygen atmosphere using a tubular electric furnace, held at this temperature for 3 hours, and sintered. The transparency of the obtained sintered body was very excellent.

Using a thermomechanical analyzer, measure the shrinkage ratio of the green compact due to sintering at a constant temperature rise of 2.5 ° C / min, 5 ° C / min, 10 ° C / min, 15 ° C / min. The sintered density at each temperature, ρ, was calculated from the bulk density of the green compact, ρ _o . The calcined powder contained 0.03 wt% of chloride ions. However, when the powder compact was fired at 1200 ° C. or higher, chloride ions could not be detected by chemical analysis, and the added chloride ions were not burned. It was found that it disappeared completely.

In FIG. 1, the SEM photograph of the precursor prepared in Example 1 was shown. From the SEM photograph of FIG. 1, it was found that the precursor formed secondary particles in which flaky particles having a diameter of 1000 nm and a thickness of 50 nm were three-dimensionally aggregated. FIG. 2A shows an SEM photograph of the magnesia powder calcined by the method of Example 1. From the SEM picture, it was found that the mother salt was completely destroyed. Fig. 3 (A) shows the relationship between the sintered density calculated from the green bulk density and the shrinkage ratio of the green compact measured using a thermomechanical analyzer, and the sintering temperature ρ (1 -ρ _o ) / ρ _o. It is shown by the relationship between (1 -ρ) and absolute temperature, the reciprocal of T, 1 / T.

50 mL of bulky commercially available basic magnesium carbonate in which flaky particles with a purity of 99% by weight or more are three-dimensionally aggregated are dispersed in ethanol, and ammonium chloride corresponding to 5 mol% is dissolved.
Was added dropwise and mixed with chloride ions. After the dropping, the sintered body was produced by the method of Example 1, and the sintered body was transparent.

The basic magnesium carbonate prepared by the method of Example 1 was calcined at 800 ° C. for 4 hours. The magnesia powder obtained by calcination was dispersed in ethanol, and 50 mL of an aqueous solution in which ammonium chloride corresponding to 5 mol% was dissolved was added dropwise and mixed with chloride ions. After the dropping, the sintered body was transparent when fired by the method of Example 1.

The basic magnesium carbonate prepared by the method of Example 1 was dried. This dry powder was lightly pulverized in an agate mortar and then dispersed in ethanol, and 50 mL of a solution in which ammonium chloride corresponding to 5 mol% was dissolved was added dropwise and mixed with chloride ions. After the dropwise addition, calcination was carried out by the method of Example 1, and when baked, a transparent sintered body was obtained.
(Comparative Example 1)

The basic magnesium carbonate prepared by the method of Example 1 was calcined, molded and sintered under the conditions of Example 1 without adding chloride ions. The sintered body was opaque. FIG. 2 (B) shows a SEM photograph of magnesia powder calcined by the method of Comparative Example 1. As shown in the SEM picture, a form of mother salt was observed.

FIG. 3B shows the relationship between 1 / T and ρ (1 −ρ _o ) / ρ _o (1 −ρ) of magnesia prepared by the method of Comparative Example 1. As can be seen by comparing Fig. 3 (A) and (B), there is no value of ρ (1-ρ _o ) / ρ _o (1 -ρ) for magnesia added with chloride ions at a sintering temperature of 1300 ° C or lower. Greater than that of the addition. This indicates that magnesia densification was promoted by chloride ions.
(Comparative Example 2)

Except for aging the precursor at 65 ° C., the sintered density of magnesia produced by the method of Example 1 was 99% of the theoretical density and was opaque.
(Comparative Example 3)

Except that the concentration of magnesium salt was 1 mol / L, the sintered density of magnesia produced by the method of Example 1 was 96% of the theoretical density and was opaque.

The production method of the present invention is useful for the production of a transparent magnesia sintered body that can be used for various arc tubes, laser materials, thermal shielding plates for projectors, high-temperature windows, and the like.

2 is a SEM photograph substituting for a drawing of the basic carbonate precursor prepared in Example 1. FIG. 2 is a drawing-substitute SEM photograph of the magnesia powder (A) produced by the method of Example 1 and the magnesia powder (B) produced by the method of Comparative Example 1. Graph showing the relationship between ρ (1-ρ _o ) / ρ _o (1-ρ) and 1 / T of magnesia (A) prepared by the method of Example 1 and magnesia (B) prepared by the method of Comparative Example 1 It is.

Claims (4)

A method for producing a transparent sintered body made of magnesia, comprising a solvent dispersion of an aggregate in which flaky basic magnesium carbonate particles having a purity of 99 % by mass or more are aggregated at three-dimensional intervals, and chloride ions mixing the chloride solvent containing a solid obtained by drying this mixture was calcined at 600 ° C. to 1200 ° C. to obtain a magnesia powder, a green compact by molding the magnesia powder A method for producing a transparent magnesia sintered body, comprising forming and firing the green compact at 1200 to 1900 ° C. In the production method of the transparent magnesia sintered body according to claim 1, a solvent dispersion of the resultant magnesia powder particles the agglomerates calcined at below 1100 ° C. or higher thermal decomposition temperature, and said chloride solvent It is characterized by mixing. The method for producing a transparent magnesia sintered body according to claim 1 or 2, wherein the thickness of the flaky basic magnesium carbonate particles is less than 200 nm . In the manufacturing method of the transparent magnesia sintered compact in any one of Claim 1 to 3, the said chloride solvent contains 0.1-15 mol% chloride ion, and metals and fluoride ion are mixed. It is characterized by not.

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