JP7172209B2 - sealing material - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sealing material for forming a thin sealing layer, and more particularly to a sealing material that is suitable for small and thin piezoelectric vibrator packages and the like.

In general, piezoelectric vibrators represented by semiconductor devices, crystal vibrators, and surface acoustic wave devices have a plurality of metallized wiring layers composed of a high-melting-point metal such as tungsten or molybdenum. It is housed in a package composed of a base made of an alumina insulator having a recess for accommodating the piezoelectric vibrator and a cover made of an alumina insulator (see, for example, Patent Document 1).

In the package, one end of the piezoelectric vibrator is fixed to the substrate with conductive resin such as conductive epoxy resin, and each electrode of the piezoelectric vibrator is electrically connected to the metallized wiring layer. The base and the lid are sealed with a sealing material containing glass powder and refractory filler powder in order to airtightly house the piezoelectric vibrator in the package.

JP 2006-261684 A

2. Description of the Related Art In recent years, with the spread of portable electronic devices, there is an increasing demand for smaller and thinner piezoelectric vibrator packages and the like. In order to reduce the size and thickness of piezoelectric vibrator packages and the like, it is necessary to reduce the size and thickness of the base body and the lid, and to reduce the thickness of the sealing layer formed of the sealing material.

When a conventional sealing material is used to form a thin sealing layer on a substrate, part of the refractory filler powder is exposed on the surface of the sealing layer, forming surface projections on the sealing layer. When surface protrusions are formed on the sealing layer, undue stress remains in the vicinity of the surface protrusions of the sealing layer, and undue stress remains in the lid that abuts on the surface protrusions. Due to the physical impact, cracks tend to occur in the sealing layer and the lid, and there is a risk that the airtightness inside the package will be impaired.

In addition, when a sealing material that does not contain refractory filler powder and is composed only of glass powder is used, surface protrusions are less likely to occur in the sealing layer. However, since the sealing material composed only of glass powder has a high thermal expansion coefficient, it is difficult to match the thermal expansion coefficients of the base and lid. Undue stress remains in the deposited layers, and mechanical shock can easily cause cracks in the substrate, lid, and sealing layer, ultimately compromising the hermeticity of the package.

Therefore, the present invention provides a sealing material containing a glass powder and a refractory filler powder, in which even if a sealing layer having a small thickness is formed, it is difficult for undue stress to remain in the sealing layer or the object to be sealed. Intended to provide material.

As a result of diligent efforts, the present inventors have solved the above technical problem by restricting the contents of the glass powder and the refractory filler powder within a predetermined range and restricting the particle size of the refractory filler powder within a predetermined range. We have found that the problem can be solved, and propose it as the present invention. That is, the sealing material of the present invention is (1) a sealing material for forming a sealing layer having a thickness of 50 μm or less (sealing thickness of 50 μm or less), and (2) the sealing material has a volume %, containing 50 to 99% glass powder and 1 to 50% refractory filler powder, (3) the refractory filler powder is substantially spherical, and (4) the ₉₀ % particle diameter D90 of the refractory filler powder is It is characterized by being 1 to 20 μm. Here, the "thickness of the sealing layer" refers to the thickness after firing, that is, after the sealing step. Here, "90% particle size _D90 " refers to a value measured by a laser diffraction method, and refers to the particle size (volume) at which the cumulative particle size is 90%.

The sealing material of the present invention is a sealing material for forming a sealing layer having a thickness of 50 µm or less. By reducing the thickness of the sealing layer, it becomes easier to achieve miniaturization and thinning of the piezoelectric resonator package. Moreover, if the thickness of the sealing layer is set to 50 μm or less, the stress remaining in the sealing layer and the object to be sealed can be relaxed, and the reliability of the piezoelectric vibrator package and the like can be improved.

The sealing material of the present invention regulates the content of the refractory filler powder to 1 to 50% by volume. In this way, it is possible to reduce the coefficient of thermal expansion of the sealing material so as to match the coefficient of thermal expansion of the object to be sealed.

The sealing material of the present invention regulates the refractory filler powder into a substantially spherical shape. By doing so, it becomes easier to suppress deterioration in fluidity of the sealing material due to the refractory filler.

In the sealing material of the present invention, the ₉₀ % particle diameter D90 of the refractory filler powder is regulated to 1 to 20 μm. If the ₉₀ % particle diameter D90 of the refractory filler powder is regulated to 20 μm or less, the probability of surface protrusions occurring in the sealing layer can be reduced, and as a result, the airtightness inside the package is impaired by mechanical impact. It is possible to prevent the situation where Further, when the coefficient of thermal expansion of the refractory filler powder is low, if the ₉₀ % particle diameter D90 of the refractory filler powder is regulated to 20 μm or less, microcracks are less likely to occur on the surface of the sealing layer, and mechanical impact Accordingly, it is possible to further prevent a situation in which the airtightness inside the package is impaired. On the other hand, if the ₉₀ % particle diameter D90 of the refractory filler powder is regulated to 1 μm or more, the effects of the refractory filler powder, such as the effect of lowering the coefficient of thermal expansion and the effect of improving the mechanical strength of the sealing layer. etc. will be easier to enjoy.

The sealing material of the present invention preferably contains 10 to 60% TeO ₂ and 10 to 60% MoO ₃ in terms of mol % of the glass powder, and does not substantially contain PbO.

The sealing material of the present invention preferably contains 5 to 50% Ag ₂ O+CuO+WO ₃ in terms of mol % of the glass powder. Here, " _Ag2O + _CuO +WO3" means the total amount of _Ag2O , _CuO and WO3.

The sealing layer of the present invention is (1) a sealing layer having a thickness of 50 μm or less, and (2) the sealing layer contains 50 to 99% by volume of glass powder and 1 to 50% by volume of refractory filler powder. (3) the refractory filler powder is substantially spherical, (4) the ₉₀ % particle diameter D90 of the refractory filler powder is 1 to 20 μm, and (5) the 90% particle diameter of the refractory filler powder Characterized by _D90 being less than the thickness of the sealing layer.

According to the present invention, in the sealing material containing the glass powder and the refractory filler powder, even if a sealing layer having a small thickness is formed, the sealing layer and the object to be sealed are unlikely to have unreasonable stress. materials can be provided.

It is a schematic diagram which shows the measurement curve obtained by a macro-type differential thermal analysis apparatus.

First, the sealing material of the present invention will be explained.

The sealing material of the present invention is for forming a sealing layer having a thickness of 50 μm or less, and the thickness of the sealing layer is 40 μm or less, 30 μm or less, 25 μm or less, 24 μm or less, particularly 23 μm or less. is preferred. If the thickness of the sealing layer is too large, it becomes difficult to reduce the size and thickness of the piezoelectric vibrator package. Moreover, the stress remaining in the sealing layer and the object to be sealed tends to increase, and the reliability of the piezoelectric vibrator package and the like tends to decrease. Although the lower limit of the thickness of the sealing layer is not particularly limited, it is more than 1 μm in reality.

In the sealing material of the present invention, the mixing ratio of the glass powder and the refractory filler powder is 50 to 99% by volume, 1 to 50% by volume of the refractory filler powder, and 50 to 85% by volume of the glass powder and 50% to 85% by volume of the refractory filler powder. 15 to 50% glass powder, and 25 to 45% refractory filler powder. If the content of the refractory filler powder is too small, the sealing layer and the object to be sealed are likely to have undue stress remaining. There is a risk that poor airtightness, etc., may occur. On the other hand, if the content of the refractory filler powder is too high, the content of the glass powder is relatively low, which makes it difficult to form a dense sealing layer, and the fluidity of the sealing material tends to decrease. As a result, the sealing strength between the members tends to decrease.

The sealing material of the present invention has a thermal expansion coefficient of 20×10 ⁻⁷ /° C. to 180×10 ⁻⁷ /° C., 30×10 ⁻⁷ /° C. to 160×10 ⁻⁷ /° C., particularly 40×10 ⁻⁷ /°C to 140×10 ^-7 /°C. If the coefficient of thermal expansion of the sealing material is too low or too high, undue stress will remain in the sealing layer and the object to be sealed, which may cause airtightness failure due to mechanical impact. Cracks may occur in the adhered layer or the material to be sealed, and airtightness failure may occur in the piezoelectric vibrator package or the like. Here, "thermal expansion coefficient" refers to a value measured with a push rod type thermal expansion coefficient measuring (TMA) device, and the measurement temperature range is 30 to 150°C.

The sealing material of the present invention preferably has a softening point of 400° C. or lower, 390° C. or lower, 380° C. or lower, particularly 370° C. or lower. If the softening point is too high, the viscosity of the glass increases, which raises the sealing temperature and may deteriorate the element during sealing. Although the lower limit of the softening point is not particularly limited, it is practically 180° C. or higher. Here, the “softening point” refers to a value measured with a macro-type differential thermal analysis apparatus using a glass composition and a sealing material having an average particle diameter _D50 of 0.5 to 20 μm as measurement samples. As for the measurement conditions, the measurement is started at room temperature, and the temperature rise rate is 10° C./min. The softening point measured by the macro-type differential thermal analyzer refers to the temperature (Ts) at the fourth inflection point in the measurement curve shown in FIG.

The sealing material of the present invention preferably has a bending strength of 40 MPa or more, 45 MPa or more, particularly 50 MPa or more. Here, the "flexural strength" is obtained by a three-point load measurement method in accordance with JIS R1601, using a measurement sample that is processed into a prism of 3 × 4 × 40 mm after densely sintering the sealing material. Each measurement is performed 20 times, and the average value is calculated. If the bending strength is too small, the sealing layer is likely to break due to cracks or the like, and the reliability of the piezoelectric vibrator package, etc., especially the airtightness, is likely to deteriorate. Although the upper limit of the bending strength is not particularly limited, it is practically 200 MPa or less.

Next, the refractory filler used in the present invention will be explained.

The refractory filler is approximately spherical. By doing so, it is easy to suppress deterioration in fluidity of the sealing material due to the refractory filler, and as a result, it is easy to increase the sealing strength between the members. It should be noted that the closer to a true sphere, the easier it is to obtain the above effects.

In the refractory filler powder, the 90% particle size D ₉₀ is 1-20 μm, preferably 1-15 μm, 1-13 μm, 2-12 μm, especially 3-11 μm. If the ₉₀ % particle size D90 of the refractory filler powder is too small, the effect of lowering the coefficient of thermal expansion becomes poor, and the refractory filler powder easily melts into the glass in the heat treatment process. Fluidity and devitrification resistance tend to decrease. On the other hand, if the ₉₀ % particle diameter D90 of the refractory filler powder is too large, surface protrusions tend to occur in the sealing layer, and undue stress tends to remain in the vicinity of the surface protrusions, and the surface protrusions are contacted. Cracks are more likely to occur in the material to be sealed.

In the refractory filler powder, the ₉₀ % particle size D90 is smaller than the thickness of the sealing layer, preferably at least 5 μm smaller than the thickness of the sealing layer, particularly preferably at least 7 μm smaller than the thickness of the sealing layer. When the ₉₀ % particle diameter D90 of the refractory filler powder is equal to or greater than the thickness of the sealing layer, surface protrusions are likely to occur in the sealing layer, and undue stress tends to remain in the vicinity of the surface protrusions of the sealing layer. , cracks are likely to occur in the object to be sealed that abuts on the surface projections.

The refractory filler powder is not particularly limited, and various materials can be selected, but those that hardly react with the glass powder are preferred.

Specifically, _{NbZr (} PO4) ₃ , _{Zr2WO4 (} PO4) ₂ , _{Zr2MoO4 ( PO4} ) ₂ , _{Hf2WO4 ( PO4} ) ₂ , _Hf2MoO as the refractory filler ₄ (PO ₄ ) ₂ , zirconium phosphate, zircon, zirconia, tin oxide, aluminum titanate, quartz, β-spodumene, mullite, titania, quartz glass, β-eucryptite, β-quartz, willemite, cordierite _, Sr _0.5 Zr _{2 (} PO ₄ ) ₃ , etc. can be used alone or in combination of _two or more.

Next, the glass powder used in the present invention will be explained.

The glass powder is not particularly limited as long as it has low softening properties. For example, the glass powder preferably contains 10-60% TeO ₂ and 10-60% MoO ₃ in mole percent. The reason why the glass composition range is limited in this way will be described below.

TeO ₂ is a component that forms a glass network and improves weather resistance. The content of TeO ₂ is 10-60%, preferably 15-57%, especially 25-55%. If the TeO ₂ content is too low, the glass becomes thermally unstable, and the glass tends to devitrify during melting or firing, and the weather resistance tends to decrease. _On the other hand, if the TeO2 content is too high, the viscosity (softening point, etc.) of the glass becomes high, making low-temperature sealing difficult, and the glass becomes thermally unstable. Devitrification easily occurs. Also, the coefficient of thermal expansion of glass tends to be too high.

_MoO3 is a component that forms a glass network and improves weather resistance. The content of MoO ₃ is 10-60%, preferably 15-55%, especially 20-50%. If the _MoO3 content is too low, the glass becomes thermally unstable, and the glass tends to devitrify during melting or firing. become difficult. On the other hand, if the content of MoO ₃ is too large, the glass becomes thermally unstable, the glass tends to devitrify during melting or firing, and the coefficient of thermal expansion of the glass tends to become too high.

The glass powder may contain the following components in the glass composition in addition to the above components.

Ag ₂ O, CuO, and WO ₃ are components that lower the viscosity (softening point, etc.) of glass. Ag ₂ O+CuO+WO ₃ (total amount of Ag ₂ O, CuO and WO ₃ ) is preferably 5-50%, 6-48%, especially 7-46%. If the total amount of Ag ₂ O, CuO, and WO ₃ is too small, the viscosity (softening point, etc.) of the glass increases, and low-temperature sealing tends to become difficult. On the other hand, if the total amount of Ag ₂ O, CuO and WO ₃ is too large, the glass becomes thermally unstable and tends to devitrify during melting or firing.

In addition, preferable ranges of contents of Ag ₂ O, CuO, and WO ₃ are as follows.

The content of Ag ₂ O is preferably 0-40%, 0-35%, particularly 0.1-30%.

The CuO content is preferably 0 to 40%, 0 to 35%, particularly 0.1 to 30%.

The content of WO ₃ is preferably 0-30%, 0-25%, especially 0-20%.

Bi ₂ O ₃ is a component that lowers the viscosity (softening point, etc.) of the glass and lowers the coefficient of thermal expansion of the glass. The content of Bi ₂ O ₃ is preferably 0-10%, 0-6%, especially 0-2%. If the content of Bi ₂ O ₃ is too high, the glass becomes thermally unstable and tends to devitrify during melting or firing.

TiO ₂ is a component that thermally stabilizes the glass and lowers the thermal expansion coefficient of the glass. The content of TiO ₂ is preferably 0-10%, 0-6%, especially 0-2%. If the content of TiO ₂ is too high, the viscosity (softening point, etc.) of the glass increases, and low-temperature sealing tends to become difficult.

AgI is a component that lowers the viscosity (softening point, etc.) of glass. The AgI content is preferably 0 to 10%, 0 to 5%, particularly 0 to 2%. If the AgI content is too high, the coefficient of thermal expansion of the glass tends to be too high.

P ₂ O ₅ is a component that forms a glass network and thermally stabilizes the glass. The content of P ₂ O ₅ is preferably 0-5%, 0-2%, especially 0-1%. If the content of P ₂ O ₅ is too large, the viscosity (softening point, etc.) of the glass increases, making low-temperature sealing difficult and weather resistance more likely to decrease.

Li ₂ O, Na ₂ O, and K ₂ O have the effect of lowering the viscosity (softening point, etc.) of the glass. 2% is preferred. If the total amount of Li ₂ O, Na ₂ O, and K ₂ O is too large, the glass becomes thermally unstable, the glass tends to devitrify during melting or firing, and the weather resistance tends to decrease. Become. The contents of Li ₂ O, Na ₂ O and K ₂ O are each preferably 0 to 10%, particularly preferably 0 to 5%.

MgO, CaO, SrO, and BaO have the effect of thermally stabilizing glass and improving weather resistance, and their total content is 0 to 20%, particularly 0 to 10%. is preferred. If the total amount of MgO, CaO, SrO and BaO is too large, the glass becomes thermally unstable and tends to devitrify during melting or firing. The contents of MgO, CaO, SrO and BaO are each preferably 0 to 10%, particularly preferably 0 to 5%.

ZnO is a component that lowers the viscosity (softening point, etc.) of glass and improves weather resistance. The content of ZnO is preferably 0-10%, more preferably 0-5%. If the ZnO content is too high, the glass becomes thermally unstable and tends to devitrify during melting or firing.

Nb ₂ O ₅ is a component that thermally stabilizes glass and improves weather resistance. The content of Nb ₂ O ₅ is preferably 0-10%, especially 0-5%. If the content of Nb ₂ O ₅ is too large, the viscosity (softening point, etc.) of the glass increases, and low-temperature sealing tends to become difficult.

V ₂ O ₅ is a component that forms a glass network and lowers the viscosity (softening point, etc.) of the glass. The content of V ₂ O ₅ is preferably 0-10%, especially 0-5%. If the V ₂ O ₅ content is too high, the glass becomes thermally unstable, and the glass tends to devitrify during melting or firing, and the weather resistance tends to decrease.

Ga ₂ O ₃ is a component that thermally stabilizes glass and improves weather resistance, but is very expensive, so its content is less than 0.01%, and it is particularly preferable not to contain it. .

SiO ₂ , Al ₂ O ₃ , GeO ₂ , Fe ₂ O ₃ , NiO, CeO ₂ , B ₂ O ₃ , Sb ₂ O ₃ and ZrO ₂ are components that thermally stabilize the glass and suppress devitrification. and each can be added up to less than 2%. If the content of these elements is too high, the glass becomes thermally unstable and tends to devitrify during melting or firing.

The glass powder is preferably substantially free of PbO for environmental reasons. Here, "substantially free of PbO" as used in the present invention refers to the case where the content of PbO in the glass composition is 1000 ppm or less.

Next, an example of a method for producing the sealing material of the present invention and a method of using the sealing material of the present invention will be described.

First, raw material powder prepared to have the above composition is melted at 800 to 1000° C. for 1 to 2 hours until a homogeneous glass is obtained. Next, the molten glass is shaped into a film or the like, pulverized, and classified to produce glass powder. Incidentally, the average particle diameter _D50 of the glass powder is preferably about 2 to 20 μm.

Next, various refractory filler powders are added to the glass powder to obtain a sealing material.

Next, a sealing material paste is prepared by adding a vehicle to the sealing material and kneading the mixture. The vehicle mainly consists of an organic solvent and a resin, and the resin is added for the purpose of adjusting the viscosity of the paste. Moreover, a surfactant, a thickening agent, etc. can also be added as needed.

The organic solvent preferably has a low boiling point (for example, a boiling point of 300° C. or lower), leaves little residue after firing, and does not degrade the glass, and its content is preferably 10 to 40% by mass. preferable. Organic solvents include propylene carbonate, toluene, N,N'-dimethylformamide (DMF), 1,3-dimethyl-2-imidazolidinone (DMI), dimethyl carbonate, butyl carbitol acetate (BCA), isoamyl acetate, It is preferred to use dimethyl sulfoxide, acetone, methyl ethyl ketone and the like. Further, it is more preferable to use a higher alcohol as the organic solvent. Since the higher alcohol itself has viscosity, it can be made into a paste without adding a resin to the vehicle. Pentanediol and its derivatives, specifically diethylpentanediol (C ₉ H ₂₀ O ₂ ), can also be used as the solvent because of their excellent viscosity.

The resin preferably has a low decomposition temperature, leaves little residue after firing, and does not easily degrade the glass, and its content is preferably 0.1 to 20% by mass. As the resin, it is preferable to use nitrocellulose, polyethylene glycol derivatives, polyethylene carbonate, acrylic acid ester (acrylic resin), and the like.

Next, the paste is applied to the sealing portion between the first member made of metal, ceramic, or glass and the second member made of metal, ceramic, or glass using a dispenser or an applicator such as a screen printer. It is applied, dried and heat treated at 300-400°C. This heat treatment causes the sealing material to soften and flow to seal the first and second members.

The sealing layer thus formed between the two members has a thickness of 50 μm or less, contains 50 to 99% by volume of glass powder and 1 to 50% by volume of refractory filler powder, and the refractory filler powder is substantially spherical. and the 90% particle size D90 of the refractory filler powder is 1 to 20 μm, and the ₉₀ % particle size _D90 of the refractory filler powder is smaller than the thickness of the sealing layer.

The present invention will be described in detail based on examples. Tables 1 to 3 show examples of the present invention (Sample Nos. 1 to 12) and Comparative Examples (Sample Nos. 13 to 15).

First, glass raw materials such as various oxides and carbonates were mixed so as to obtain the glass composition shown in the table, and a glass batch was prepared. Melted for 2 hours. Next, it was molded into a film with a water-cooled roller. Finally, the film-like glass was pulverized with a ball mill and passed through a sieve with an opening of 75 μm to obtain a glass powder having an average particle diameter _D50 of about 10 μm.

As the refractory filler powder, the refractory filler powder shown in the table was used. Each refractory filler powder was prepared to have the particle size and shape shown in the table. ZWP is Zr ₂ WO ₄ (PO ₄ ) ₂ and NZP is NbZr(PO ₄ ) ₃ . Incidentally, the particle sizes of the glass powder and the refractory filler powder were measured by a laser diffraction method.

As shown in the table, glass powder and refractory filler powder were mixed to obtain a sealing material. No. Samples 1 to 15 were evaluated for coefficient of thermal expansion, softening point, bending strength, and fluidity.

The coefficient of thermal expansion was determined with a TMA apparatus. The measurement temperature range was 30 to 150°C.

Softening points were measured with a DTA apparatus. The measurement was performed in the atmosphere at a temperature elevation rate of 10° C./min, and the measurement was started from room temperature.

The bending strength was obtained by a three-point load measurement method based on JIS R1601, using a measurement sample that was processed into a prism of 3×4×40 mm after densely sintering each sample. Each measurement was performed 20 times, and the average value was calculated.

Liquidity was evaluated as follows. 5 g of the powder sample was placed in a mold with a diameter of 20 mm, press-molded, and then baked on a glass substrate at 450° C. for 30 minutes. A flow diameter of 19 mm or more of the sintered body was evaluated as "○", and a flow diameter of less than 19 mm was evaluated as "X".

Subsequently, a sealing layer was produced as follows. First, an alumina substrate having a size of 25 mm square and a thickness of 5 mm was prepared, each sample was mixed with a vehicle (α-terpineol containing an acrylic resin), and the resulting paste was applied to the entire surface (only one side) of the substrate. The coating conditions and vehicle composition were adjusted so that a sealing layer having the thickness shown in the table could be obtained after the heat treatment. Next, the coating film was dried at 130° C. for 10 minutes to evaporate and remove the solvent in the vehicle, followed by heat treatment at 450° C. for 30 minutes to obtain the sealing layer shown in the table.

The surface projections of the sealing layer are measured by a surface roughness meter on the surface of the sealing layer obtained by the above method. was evaluated as "x".

As is clear from the table, No. 1, which is an embodiment of the present invention. Samples 1 to 12 were capable of forming a sealing layer having a thickness of 25 μm or less after heat treatment, and no surface projections were observed on the sealing layer.

On the other hand, no. Sample No. 13 had poor fluidity due to the excessive content of the refractory filler powder. No. Sample No. 14 had a large 90% particle size D90 of the refractory filler powder, so the evaluation of surface projections was unsatisfactory. No. Sample No. 15 had poor fluidity because the refractory filler powder was crushed.

The sealing material of the present invention is suitable for sealing semiconductor integrated circuits, crystal oscillators, flat display devices, and glass terminals for LDs.

Claims (4)

(1) A sealing material for forming a sealing layer having a thickness of 50 μm or less,
(2) The sealing material contains 50 to 99% by volume of glass powder and 1 to 50% of refractory filler powder,
(3) the refractory filler powder is substantially spherical,
(4) the ₉₀ % particle size D90 of the refractory filler powder is 1 to 20 μm ;
A sealing material characterized in that the glass powder contains 10-60% MoO ₃ and 0-5 % V ₂ O ₅ in mol %. 2. The sealing material according to claim 1, wherein the glass powder contains, in mol %, TeO ₂ 10-60 % and substantially no PbO. 3. The sealing material according to claim 1, wherein the glass powder contains 5 to 50% Ag ₂ O+CuO+WO ₃ in mol %. (1) A sealing layer having a thickness of 50 μm or less,
(2) The sealing layer contains 50 to 99% by volume of glass powder and 1 to 50% by volume of refractory filler powder,
(3) the refractory filler powder is substantially spherical,
(4) the ₉₀ % particle size D90 of the refractory filler powder is 1 to 20 μm;
(5) the ₉₀ % particle size D90 of the refractory filler powder is smaller than the thickness of the sealing layer;
A sealing layer characterized in that the glass powder contains 10-60% MoO ₃ and 0-5 % V ₂ O ₅ in mol %.

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