JP2012046365A - Alumina ceramics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide low-cost alumina ceramics with high purity and low dielectric loss.SOLUTION: The alumina ceramics contain, in terms of chemical compositions, SiOtwice as much as NaO or more in molar ratio and of 0.05-0.5 mass%, and YOof 0.05-0.5 mass%, the balance being AlOand inevitable impurities. Dielectric loss at 1 MHz-1 GHz is tanδ=10×10or less. Volume intrinsic resistivity is 1×10Ωcm or more, and density is 3.8 g/cmor more.

Description

  The present invention relates to a low dielectric loss and high resistance alumina ceramic used for an insulating member of a device using high frequency or microwave.

Alumina ceramics are used as structural members, wear-resistant members, and insulating members as relatively inexpensive ceramics. In particular, general alumina ceramics are widely used for insulating members and the like because they have a volume resistivity of 10 ¹⁴ Ωcm or more and are high resistance materials among ceramics. However, when used as an insulating member of a high-frequency power supply unit or a packaging of microwave parts, particularly low-purity alumina ceramics have disadvantages such as a large dielectric loss and a significant loss of performance of the apparatus and parts.

  In response to such problems, attempts have been made to reduce dielectric loss by using high-purity alumina with less impurities. However, in recent years, there has been an increasing need for higher efficiency of devices and parts, and lower dielectric loss. There is a need for materials.

In order to meet such needs, various studies have been made on impurity concentration, additive components, and the like. Patent Documents 1 and 2 disclose a technique for reducing dielectric loss by setting Na to 100 ppm or less in terms of Na ₂ O because alkali metals cause increase in dielectric loss. However, the relatively inexpensive alumina raw material on the market contains Na ₂ O of more than 100 ppm even if the purity of Al ₂ O ₃ is 99.9%, and the raw material of 100 ppm or less is made from refined metallic aluminum. Therefore, the cost is 10 times higher than that of a general low-soda alumina raw material, which is a major factor in increasing the cost of the product.

In Patent Document 3, since SiO ₂ increases the ionic vibration in the alumina sintered body, the SiO ₂ content is reduced to 0.2% by mass or less, and further YAG is contained in an amount of 3 to 50% by mass. A technique for reducing dielectric loss is disclosed. However, this technique has a drawback in that high-purity alumina ceramics of 98% by mass or more cannot be obtained because the dielectric loss increases when the YAG content is less than 3% by mass.

Patent Documents 4 and 5 disclose a method for reducing dielectric loss by precipitating TiO ₂ . However, in order to reduce the dielectric loss, it is necessary to deposit TiO ₂ alone, and for that purpose, CuO or Nb ₂ O ₅ needs to be added in an amount of 4% by mass or more together with TiO _2. % High-purity alumina ceramics cannot be obtained. Moreover, when the amount of TiO ₂ increases, the volume resistivity decreases and the insulating property is impaired.

Patent Document 6 discloses a method of generating and removing a metal salt with Na ₂ O using an acid compound molding binder. However, in order to produce a metal salt, it is necessary to add a large amount of a binder for molding, and the volatile matter at the time of degreasing and firing increases, so that there is a drawback that cracks are likely to occur.

JP-A-8-143358 JP-A-8-235933 JP 2003-112964 A JP-A-9-52761 JP-A-9-52762 JP-A-9-263440

  As described above, in the prior art, it was necessary to use an expensive high-purity raw material or to add many additives for reducing dielectric loss in order to obtain low dielectric loss alumina ceramics. Therefore, an object of the present invention is to obtain low-cost and high-purity low dielectric loss alumina ceramics.

The gist of the present invention is as follows.
(1) In the chemical component, SiO ₂ is contained in a molar ratio of not less than twice that of Na ₂ O and not less than 0.05% by mass and not more than 0.5% by mass, and further Y ₂ O ₃ is not less than 0.05% by mass. 0.5% by mass or less, the balance is Al ₂ O ₃ and inevitable impurities, the dielectric loss at 1 MHz to 1 GHz is tan δ = 10 × 10 ⁻⁴ or less, and the volume resistivity is 1 × 10 ¹⁴ Ωcm. As described above, alumina ceramics having a density of 3.8 g / cm ³ or more.
(2) The alumina ceramic according to (1), which contains TiO ₂ at 0.5% by mass or less.
(3) Grain boundary phase composed of an oxide containing at least 33.3 mol% to 90 mol% of Si, 5 mol% to 33.3 mol% of Al, and 5 mol% to 33.3 mol% of Na and unavoidable impurities (1) or alumina ceramics as described in (2).
(4) The alumina ceramic according to any one of (1) to (3), wherein the average crystal grain size is 10 μm or more.
(5) It contains 100 ppm or more and 1000 ppm or less Na in terms of Na ₂ O, and Al ₂ O ₃
The alumina ceramic according to any one of (1) to (4), wherein an alumina powder having a content of 99.9% by mass or more is used as a raw material powder.

According to the present invention, an inexpensive alumina ceramic having a dielectric loss from 1 MHz to 1 GHz of 10 × 10 ⁻⁴ or less and a volume resistivity of 1 × 10 ¹⁴ Ωcm or more is obtained, and is used for high frequency or microwave insulation. It is possible to obtain a material useful for a member or the like.

  The present invention is described in detail below.

The object of the present invention is to obtain an inexpensive low dielectric loss alumina ceramic, and the main factors that increase the dielectric loss of alumina include the effects of loss at the grain boundary, impurities, and pores. As a result of earnestly examining the method of reducing the influence of Na unavoidable on an inexpensive alumina raw material, the inventors have made a major cause of increasing dielectric loss due to Na contained as an impurity in the raw material. It has been found that dielectric loss can be reduced by containing an appropriate amount of SiO ₂ with respect to the amount of Na.

This is presumably because an effect of reducing the influence of Na appears by incorporating Na into an amorphous glass having a SiO ₂ crystal or SiO ₂ as a skeleton. As for the incorporation of Na into SiO ₂ , when trivalent Al substitution occurs at the tetravalent Si site of SiO ₂ , the positive charge is insufficient, so that monovalent Na is easily incorporated in a form that compensates for this. . SiO ₂ incorporating Na is present in the sintered body as a grain boundary phase. NaAlSi ₃ O ₈ is known as a stable compound as a compound formed by incorporating Na together with substitution of SiO ₂ with Al. The grain boundary phase exists in the form of crystals represented by NaAlSi ₃ O ₈ or Na _x Al _x Si _4−x O ₈ (0 <x ≦ 2), or in an amorphous state close to these compositions.

In Na _x Al _x Si _4-x O ₈ , when x = 2, the molar ratio of SiO _{2 to} Na is 1: 1, and if SiO ₂ increases from this ratio, the effect of reducing dielectric loss can be obtained. Although the ratio of SiO ₂ is too large, the ratio of the grain boundary phase is increased, which causes a decrease in strength and a decrease in sinterability. For this reason, it is desirable that the ratio of SiO _{2 to} Na is equal to or greater than 18 times. Further, the amount of substitution by trivalent Al generated at the tetravalent Si site of SiO ₂ is almost the same as the amount of Na taken up thereby. From the above, the presence ratio of at least Si, Al, and Na (in the case of oxide, mol% not including oxygen in the oxide) is as follows: Si 33.3 to 90 mol%, Al 5 to 33.3 mol%, It is important to produce an oxide with Na 5-33.3 mol%. In this grain boundary phase, in addition to Si, Al, and Na, other impurities such as K, Mg, and Ca in the Al ₂ O ₃ raw material may exist in a crystalline or amorphous state.

On the other hand, as a ratio of SiO _{2 to} Na in alumina ceramics, dielectric loss at 1 MHz to 1 GHz is greatly reduced by containing SiO ₂ twice or more in a molar ratio of Na converted to Na ₂ O. Can do. However, if the content of SiO ₂ is less than 0.05% by mass, the effect of reducing the dielectric loss is small, and the effect of improving the sinterability cannot be obtained. Moreover, since the liquid phase which exists in a grain boundary at the time of sintering will increase when it exceeds 0.5 mass%, since grain growth is hindered and sinterability is reduced, the content of SiO _{2 in} the alumina ceramics is It is necessary that the molar ratio is not less than twice Na ₂ O and not less than 0.05% by mass and not more than 0.5% by mass.

Furthermore, by including 0.05 mass% or more and 0.5 mass% or less of Y ₂ O ₃ , the dielectric loss at 1 MHz to 1 GHz targeted by the present invention is tan δ = 10 × 10 ⁻⁴ or less, and the volume resistivity is An alumina ceramic having excellent characteristics such as a rate of 1 × 10 ¹⁴ Ωcm or more and a density of 3.8 g / cm ³ or more can be obtained. Y ₂ O ₃ has a great effect of improving the sinterability of alumina, and since the resulting sintered body becomes dense, pores are reduced and dielectric loss at the pores is reduced. In particular, when SiO ₂ is added, the sinterability deteriorates as the amount added increases only by adding SiO ₂ alone. However, by adding Y ₂ O ₃ simultaneously with SiO ₂ , a dense sintered body can be obtained. Can be obtained. When Y ₂ O ₃ is less than 0.05% by mass, the effect of improving the sinterability cannot be obtained. On the other hand, when the amount is more than 0.5% by mass, a large amount of Y ₂ O ₃ containing phase such as YAG phase is generated at the grain boundary, which causes the inhibition of grain growth and lowers the dielectric characteristics. Therefore, the content of Y ₂ O ₃ needs to be 0.05% by mass or more and 0.5% by mass or less.

In addition to the effect of reducing dielectric loss due to Na incorporation, the effect of adding SiO ₂ is to improve the sinterability of alumina by generating a liquid phase with Al ₂ O ₃ and Y ₂ O _{3 during} sintering. This has the effect of increasing the crystal grain size of the alumina sintered body. When the particle size is increased, the number of grain boundaries is reduced, and the loss at the grain boundaries that causes an increase in the dielectric loss is reduced, so that the dielectric loss is reduced.

TiO ₂ has the effect of promoting the diffusion of Al ₂ O ₃ by being dissolved in alumina crystals. For this reason, the effect of increasing the crystal grain size of the obtained sintered body is particularly great due to the diffusion promoting effect during the sintering of alumina. For this reason, it is possible to reduce the dielectric loss due to the effect of increasing the particle size as well as the reduction of the dielectric loss due to Na incorporation by SiO ₂ . When TiO ₂ is less than 0.05% by mass, grain growth during sintering does not occur sufficiently, and the effect of increasing the particle size due to the addition of TiO ₂ is reduced. When added, 0.05% by mass or more is preferable. . On the other hand, if the amount is more than 0.5% by mass, the grain growth becomes remarkable, and a large amount of pores are taken into the crystal grains and pores tend to remain at the grain boundaries, making it difficult to obtain a dense sintered body. It is necessary to be not more than mass%.

As for the particle size of the alumina powder used as a raw material for producing the alumina ceramic of the present invention, it is desirable to use a powder having an average particle size of 0.3 μm or more and 2.0 μm or less. Although it is possible to use a raw material having an average particle size smaller than 0.3 μm, a raw material smaller than 0.3 μm is generally expensive, so it is desirable to use a low-cost raw material of 0.3 μm or more. In addition, when a raw material having an average particle size larger than 2 μm is used, since the sinterability is inferior and it becomes difficult to obtain a target dense sintered body, a raw material having an average particle size of 2.0 μm or less should be used. Is desirable. As for the amount of Na of impurities contained in the alumina powder, it is desirable to use 100 ppm or more and 1000 ppm or less in terms of Na ₂ O. Since raw materials with Na less than 100 ppm in terms of Na ₂ O generally use a special production method, the price is one or more orders of magnitude higher than general low soda alumina raw materials, and inexpensive alumina ceramics cannot be obtained. Further, when a raw material with Na more than 1000 ppm in terms of Na ₂ O is used, the dielectric loss increases due to the influence of Na, and the low dielectric loss material of tan δ = 10 × 10 ⁻⁴ or less which is the object of the present invention is obtained. It becomes difficult. In a commercially available low-soda alumina raw material, since Na is 100 ppm or more and 500 ppm or less in terms of Na ₂ O, the raw material is relatively inexpensive. By applying the present invention using these raw materials, low cost and low dielectric loss can be achieved. Since alumina ceramics can be obtained, it is particularly preferable to use these raw materials.

  As the yttria, silica, and titania raw materials used as additives, it is desirable to use a powder having an average particle size of 0.1 μm to 2 μm. A raw material having an average particle size of less than 0.1 μm is generally expensive and easily agglomerates, so that it is difficult to uniformly disperse it. In addition, when the average particle size is larger than 2 μm, it is difficult to obtain a sufficient effect of the additive and it is difficult to obtain a sintered body having the desired characteristics. desirable.

The amount of Na in the alumina raw material to be used is not less than twice the molar ratio converted to Na ₂ O, and the addition amount is 0.05 mass% or more and 0.5 mass% or less of silica, and an appropriate amount for yttria and titania in a predetermined amount. Although the raw material is composed of a binder, it is common for the alumina raw material to contain about several hundred ppm of SiO ₂ as an impurity, and depending on the production method, Na in the alumina raw material may volatilize during the production. Therefore, it is necessary to appropriately adjust the addition amount of silica so as to be within the scope of the present invention after sintering in consideration of the impurity amount of SiO ₂ and the volatilization amount of Na.

  In order to obtain a uniform mixed powder, it is desirable to use wet mixing for mixing the raw material powder. An organic solvent, water, or the like is used as the solvent for wet mixing, but more uniform mixing is possible by using a dispersant. Further, it is desirable to use additives such as a binder and a plasticizer in order to enhance the moldability of the mixed powder and the processability of the molded body as necessary. A rotary ball mill, an attritor, etc. can be used for these mixing.

  After mixing, drying and molding are performed, and in particular, by using spray drying, it is possible to produce a large amount of powder with good fluidity at a time. The dried powder is molded into a desired shape by uniaxial molding or isostatic pressing (CIP). In order to obtain a sintered body having a uniform density distribution, the CIP molding method may be used. desirable. Further, a method of directly molding from a slurry mixed by a method such as a mud casting method or an injection molding method without passing through a dry powder can be used.

  A dense sintered body can be obtained by firing the obtained molded body. The firing is preferably performed in the air. Although it is possible to carry out in a nitrogen gas atmosphere or in an inert gas atmosphere such as Ar, it is desirable to carry out in a general atmosphere for firing alumina.

Firing is preferably performed at a temperature of 1300 ° C. or higher and 1700 ° C. or lower. At a temperature lower than 1300 ° C., the diffusion of alumina hardly occurs, and the liquid phase formed by the additive components Y ₂ O ₃ , SiO ₂ and Al ₂ O _3, etc. hardly forms, and the sintering via the liquid phase Therefore, it is difficult to obtain a dense sintered body. In order to reduce the dielectric loss, it is effective to reduce the number of grain boundaries, which is a factor that increases the dielectric loss, by increasing the crystal grain size of the sintered body. In particular, when the average grain size is 10 μm or more, the influence on the dielectric loss due to the grain boundary can be remarkably reduced. Therefore, the firing temperature should be a temperature at which densification occurs sufficiently and grain growth is likely to occur. is important. In particular, the firing is more preferably performed at 1450 ° C. or higher and 1700 ° C. or lower in that liquid phase generation occurs due to Y ₂ O ₃ , SiO ₂ , or Al ₂ O ₃ . Although it is possible to obtain a dense sintered body even if fired at a temperature higher than 1700 ° C., the additive may start to volatilize, and the grain growth becomes remarkable, and the crystal grains of alumina and the grain boundaries Since it becomes easy to produce a big pore, the density of the obtained sintered compact will fall. Also, from the viewpoint that a special furnace that can be used at a temperature higher than 1700 ° C. needs to be used for firing, it is desirable to fire at a temperature of 1700 ° C. or less at which a sufficiently dense sintered body can be obtained.

  The average size (average crystal grain size) of the crystal grains of the obtained sintered body is preferably 10 μm or more and 100 μm or less. When the average crystal grain size is less than 10 μm, the number of grain boundaries increases and the dielectric loss increases. However, by setting the average crystal grain size to 10 μm or more, the dielectric loss at the grain boundaries can be reduced. Also, when the average crystal grain size exceeds 100 μm, large pores are likely to remain as defects, and this increase in dielectric loss at the pores causes an increase in the dielectric loss of the sintered body. Furthermore, when the average crystal grain size exceeds 100 μm, the strength of the resulting alumina is significantly reduced. Therefore, in order to obtain a material with low dielectric loss and high strength and high reliability, the average crystal grain size Is preferably 10 μm or more and 100 μm or less.

Among the alumina ceramics produced by the above method, the dielectric loss from 1 MHz to 1 GHz is tan δ = 10 × 10 ⁻⁴ or less, the volume resistivity is 1 × 10 ¹⁴ Ωcm or more, and the density is 3.8 g / cm ³ or more. The present invention becomes the present invention, and it becomes possible to obtain alumina ceramics having a low dielectric loss and a high resistance used for an insulating member of an apparatus using a high frequency or microwave.

Example 1
As three kinds of alumina raw material powders, alumina powder A (purity 99.9 mass%, average particle diameter 0.5 μm, impurity Na content; 170 ppm in terms of Na ₂ O), alumina powder B (purity 99.9 mass%, Average particle size 0.5 μm, impurity Na content: 250 ppm in terms of Na ₂ O), alumina powder C (purity 99.9% by mass, average particle size 0.5 μm, impurity Na content: 430 ppm in terms of Na ₂ O) Further, after silica powder (purity 99.5%, particle size 0.5 μm) and yttria powder (purity 99.9%, average particle size 0.3 μm) were added with distilled water and wet mixed using an alumina ball mill. The mixed powder was obtained by spray drying. The obtained mixed powder was uniaxially pressure-molded with a mold and then CIP-molded at a pressure of 1.4 ton / cm ² . This molded body was fired at 1600 ° C. in the air for 8 hours to obtain a sintered body. The results of chemical analysis of the Na ₂ O, SiO ₂ and Y ₂ O ₃ contents of the obtained sintered body are shown in the column of chemical components in Table 1. Moreover, the density measured according to JIS-R1634 and the result of dielectric loss measured according to JIS-C2138 are shown in the characteristic column of Table 1. In addition, as a result of measuring the volume resistivity in accordance with JIS-C2139, it was 1 × 10 ¹⁴ Ωcm or more in all the sintered bodies, so description in the table is omitted.

As shown in the table, the sintered body according to the present invention has a dielectric loss at 1 MHz to 1 GHz of tan δ = 10 × 10 ⁻⁴ or less, a volume resistivity of 1 × 10 ¹⁴ Ωcm or more, and a density of 3. An alumina ceramic having excellent characteristics of 8 g / cm ³ or more was obtained.

On the other hand, Comparative Examples 1 to 3 having a molar ratio of SiO ₂ to Na ₂ O of 2 or less, and Comparative Example 4 having a SiO ₂ or Y ₂ O ₃ content outside the range of 0.05 mass% to 0.5 mass%. 10 cannot satisfy the required characteristic of the present invention that the dielectric loss at 1 MHz to 1 GHz is tan δ = 10 × 10 ⁻⁴ or less.

  The sintered bodies produced in Example 5, Example 13, and Comparative Example 1 were mirror-polished and subjected to grain boundary etching, and then the chemical composition of the grain boundary part was investigated by EPMA. The chemical composition was obtained by measuring the grain boundary portion at five points and calculating the mol% of each component with the total amount of Si, Al and Na as 100% from the average value. In Example 5, Si = 74 mol%, Al = 13 mol%, and Na = 13 mol%. In Example 13, Si = 86 mol%, Al = 7 mol%, and Na = 7 mol% were obtained. In Comparative Example 1, Si = 30 mol%, Al = 35 mol%, and Na = 35 mol%. From these, a grain boundary phase composed of an oxide containing Si in a ratio of 33.3 mol% to 90 mol%, Al in a ratio of 5 mol% to 33.3 mol%, and Na in a ratio of 5 mol% to 33.3 mol% is formed. It can be seen that the better the dielectric properties.

(Example 2)
In addition to the alumina powder B, silica powder, and yttria powder used in Example 1, titania powder (purity 99.9%, average particle size 0.5 μm) was used to produce a sintered body in the same manner as in Example 1. did. The results of chemical analysis of the content of Na ₂ O, SiO ₂ , Y ₂ O ₃ and TiO _{2 of the} obtained sintered body are shown in the column of chemical components in Table 2. Further, the density measured according to JIS-R1634 and the result of dielectric loss measured according to JIS-C2138 are shown in the characteristic column of Table 2. In addition, as a result of measuring the volume resistivity based on JIS-C2139, all were 1 * 10 < ¹⁴ > ohm-cm or more, Therefore The description in a table | surface is abbreviate | omitted.

As shown in Examples 17 to 20 in the same table, it is clear that the inclusion of TiO ₂ in an amount of 0.05% by mass to 0.5% by mass has an effect of reducing dielectric loss. Comparative Example 11 containing 0.7% by mass of TiO ₂ is not suitable because the dielectric loss tends to increase remarkably as the density decreases.

(Example 3)
To obtain the chemical composition shown in Table 3, the sintered body produced by the same manufacturing method as in the above-described examples was mirror-polished and subjected to grain boundary etching treatment, and then the average particle diameter measured by SEM observation and others. The characteristics are shown in Table 3.

As shown in the table, the material having a dielectric loss of tan δ = 10 × 10 ⁻⁴ or less according to the present invention has an average particle size of 10 μm or more, whereas the material according to the comparative example has an average particle size of less than 10 μm. Tan δ = 10 × 10 ⁻⁴ .

Claims (5)

In the chemical component, SiO ₂ is contained in a molar ratio of not less than twice that of Na ₂ O and not less than 0.05% by mass and not more than 0.5% by mass, and further, Y ₂ O ₃ is not less than 0.05% by mass and not more than 0.5%. It is contained by mass% or less, the balance consists of Al ₂ O ₃ and inevitable impurities, the dielectric loss at 1 MHz to 1 GHz is tan δ = 10 × 10 ⁻⁴ or less, the volume resistivity is 1 × 10 ¹⁴ Ωcm or more, and the density Is an alumina ceramic having a 3.8 g / cm ³ or more. The alumina ceramic according to claim 1, containing TiO ₂ at 0.5 mass% or less.   There is a grain boundary phase composed of an oxide and inevitable impurities containing at least Si of 33.3 mol% to 90 mol%, Al of 5 mol% to 33.3 mol%, and Na of 5 mol% to 33.3 mol%. The alumina ceramic according to claim 1 or 2.   The alumina ceramic according to any one of claims 1 to 3, wherein the average crystal grain size is 10 µm or more. It contains Na of 100 ppm or more and 1000 ppm or less in terms of Na ₂ O, Al ₂ O ₃
The alumina ceramic according to any one of claims 1 to 4, wherein an alumina powder having a content of 99.9% by mass or more is used as a raw material powder.