Sealing green sheet

Abstract

To provide a sealing green sheet capable of forming a sealing portion excellent in bondability and compactness.SOLUTION: A sealing green sheet disclosed herein contains at least glass powder, a binder, and a solvent. Furthermore, this sealing green sheet has a glass softening point Y of the glass powder of 670°C or lower. With such a composition, it is possible to realize a gas-sealing portion with high adhesion, even with low-temperature firing at 800°C or below. In addition, in this sealing green sheet, the difference (Y-X) between the binder 97% combustion temperature X and the glass softening point Y of the glass powder is 200°C or more. This allows the binder to be sufficiently burned before the softening of the glass powder begins, thereby suppressing the formation of voids in the gas-sealing portion after firing and achieving high density.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The technology disclosed herein relates to a sealing green sheet that forms a gas sealing portion of an electrochemical cell. [Background technology]

[0002] Solid oxide electrolysis cells (SOECs) generate oxygen and hydrogen gas through the electrolysis of water. This allows them to convert electrical energy, which is difficult to store, into chemical substances that are easy to store, and they have attracted attention in fields such as solar power generation. Meanwhile, some SOEC cells can also be used as solid oxide fuel cells (SOFCs). SOFCs generate electricity by reacting hydrogen and oxygen gases. In this specification, cells that can be used as both SOECs and SOFCs are referred to as "electrochemical cells." In these electrochemical cells, porous layers, namely, hydrogen and oxygen electrodes, are stacked on top of each other, sandwiching a dense solid electrolyte layer.

[0003] Pipes (gas pipes) for supplying desired gases to the electrodes (hydrogen electrodes, oxygen electrodes, etc.) of electrochemical cells are sometimes connected. A gas sealing portion, which is a dense glass body, is formed at the connection between the electrode and the gas pipe. This prevents performance degradation due to gas leakage from the connection. For example, the gas sealing portion of an electrochemical cell is formed by attaching a sealing green sheet containing glass powder to the connection and then firing it. An example of this sealing green sheet is disclosed in JP 2020-167093 A. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2020-167093 Summary of the Invention [Problem to be solved by the invention]

[0005] In order to properly prevent gas leakage from connections, the gas sealing portions of electrochemical cells require high density. For example, the sealing green sheet described in JP 2020-167093 A allows the formation of gas sealing portions with a relative density of approximately 70% to 85%. However, in recent years, there has been an increasing demand for extremely dense gas sealing portions with a relative density of approximately 95% due to demands for improved performance of electrochemical cells. Meanwhile, in consideration of the impact on other components of the electrochemical cell (such as electrodes), it has recently become necessary to sinter sealing green sheets at low temperatures (e.g., 800°C or lower). Such low-temperature firing tends to further reduce the density of the gas sealing portions after firing. [Means for solving the problem]

[0006] The present inventors conducted various studies to solve the above-mentioned problems and have come to the following conclusions. First, a binder is added to a typical sealing green sheet to bind the components and maintain the sheet shape. This binder usually burns during low-temperature heating (degreasing process) in the early stages of firing. However, to meet the demand for low-temperature firing, recent sealing green sheets have used low-temperature softening glass with a low glass softening point. This low-temperature softening glass sufficiently softens even in low-temperature environments, preventing a decrease in bondability. However, this low-temperature softening glass may begin to soften during the binder combustion stage in the early stages of firing. In this case, unburned binder is embedded within the softened glass. Then, as the firing process progresses and the binder in the glass burns, voids are formed in the gas sealing portion after firing, significantly reducing the density. To solve these problems, the present inventors conducted extensive experiments and studies and have come up with a sealing green sheet having the following configuration.

[0007] The sealing green sheet disclosed herein is used for forming a gas sealing portion of an electrochemical cell. This sealing green sheet contains at least a glass powder, a binder, and a solvent. The sealing green sheet disclosed herein satisfies the following formulas (1) and (2), where X is the binder 97% combustion temperature, which is the temperature at which the weight loss rate of the binder reaches 97%, and Y is the glass softening point of the glass powder. Y≦670℃ (1) 200℃≦YX≦400℃ (2)

[0008] First, the sealing green sheet disclosed herein uses low-temperature softening glass with a glass softening point Y of 670°C or less. This allows the glass powder to be sufficiently softened even during low-temperature firing at 800°C or less, thereby maintaining sufficient bonding strength of the glass sealing portion after firing. Next, in the sealing green sheet disclosed herein, the difference (YX) between the glass softening point Y of the glass powder and the binder 97% combustion temperature X is set to a range of 200°C or more and 400°C or less. This ensures that most of the binder (typically 97% or more) is burned before the glass powder starts to soften. As a result, it is possible to prevent unburned binder from being embedded in the glass during the initial firing stage, thereby preventing voids from forming inside the gas sealing portion after firing. DETAILED DESCRIPTION OF THE INVENTION

[0009] Preferred embodiments of the technology disclosed herein are described below. It should be noted that matters other than those specifically mentioned in this specification that are necessary for implementing the technology disclosed herein (e.g., the structure of an electrochemical cell) can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. In the following description, "A to B (where A and B are any values)" includes the values ​​of A and B (upper and lower limits).

[0010] <Sealing green sheet> The sealing green sheet disclosed herein is used to form a gas sealing portion of an electrochemical cell. Specifically, the sealing green sheet disclosed herein is attached between two or more sealed members (e.g., a gas pipe and an electrode) in an electrochemical cell, and then fired. This allows a gas sealing portion made mainly of glass to be formed at the connecting portion of the sealed members.

[0011] The encapsulating green sheet disclosed herein contains at least a glass powder, a binder, and a solvent. Each component will be described below.

[0012] 1. Glass powder Glass powder is the main component of the sealing green sheet. This glass powder softens during firing. The softened glass then solidifies to form the gas sealing portion. Here, in this sealing green sheet, low-temperature softened glass having a glass softening point Y of 670°C or less is used, as shown in the following formula (1). This allows the glass powder to be sufficiently softened even during low-temperature firing at 800°C or less, so that the bondability of the gas sealing portion after firing can be sufficiently maintained. From the viewpoint of further improving the bondability of the gas sealing portion, the glass softening point Y of the glass powder is preferably 665°C or less, more preferably 663°C or less, and particularly preferably 660°C or less. From the viewpoint of the bondability and denseness of the gas sealing portion, the lower limit of the glass softening point Y of the glass powder is not particularly limited and may be 500°C or more or 550°C or more. However, in consideration of the heat resistance of the gas sealing portion after firing, the glass softening point Y of the glass powder is preferably 600°C or higher, more preferably 610°C or higher, even more preferably 620°C or higher, and particularly preferably 630°C or higher. Y≦670℃ (1)

[0013] The type of glass powder is not particularly limited as long as it satisfies the above-mentioned glass softening point Y, and conventionally known glasses can be used without any restrictions. For example, the glass powder may be a typical amorphous glass or a crystallized glass containing crystals. The components of the glass powder are also not particularly limited. Examples include SiO2-RO (R represents, for example, Mg, Ca, Zn, Ba, or Sr; the same applies hereinafter)-based glass, SiO2-R'2O (R' represents, for example, Li, K, or Na; the same applies hereinafter)-based glass, SiO2-RO-Al2O3-based glass, SiO2-RO-Bi2O3-based glass, SiO2-RO-Y2O3-based glass, SiO2-RO-B2O3-based glass, SiO2-Al2O3-based glass, SiO2-ZnO-based glass, SiO2-ZrO2-based glass, RO-R'2O-based glass, and RO-based glass. In addition to the above-mentioned components, lead-based glass, lead-lithium-based glass, borosilicate-based glass, and the like can also be used. The glass powder may contain one or more components in addition to the above-mentioned glass components. Furthermore, one of the above-mentioned glass components may be used alone, or two or more may be mixed.

[0014] Among the above-mentioned glasses, SiO2-RO-B2O3-based glass is particularly preferred for use in the sealing green sheet disclosed herein. This SiO2-RO-B2O3-based glass contains B2O3 as one of its main components. This glass powder containing a large amount of B2O3 has the characteristic of a low glass softening point. As a result, a gas seal with excellent bonding strength can be formed even when fired at a low temperature. Furthermore, the technology disclosed herein can prevent the formation of voids in the gas seal after firing, even when using such glass with excellent low-temperature softening properties. Furthermore, glass containing a large amount of B2O3 also has excellent fluidity after softening. This allows the softened glass to easily penetrate voids in the sheet, thereby contributing to improving the density of the gas seal after firing. Specifically, the B2O3 content in the glass powder is preferably 13 mol% or more, more preferably 13.5 mol% or more, even more preferably 14 mol% or more, and particularly preferably 14.5 mol% or more. On the other hand, glass with too high a B2O3 content tends to soften significantly in the early stages of firing, which can easily cause voids to form due to the embedded binder. From this perspective, the B2O3 content is preferably 25 mol% or less, more preferably 24 mol% or less, even more preferably 23 mol% or less, and particularly preferably 22 mol% or less. In this specification, the "content" is expressed as mol% in terms of oxide when the total moles of glass is 100 mol%.

[0015] Furthermore, it is preferable that the components other than B2O3 in the SiO2-RO-B2O3-based glass are also appropriately adjusted in consideration of the performance of the gas seal after firing. For example, SiO2 is a component that constitutes the glass skeleton (glass matrix) and can also contribute to improving the durability (water resistance, chemical resistance, thermal shock resistance, etc.) of the gas seal. For this reason, the SiO2 content in the glass powder is preferably 7 mol% or more, more preferably 8 mol% or more, even more preferably 9 mol% or more, and particularly preferably 10 mol% or more. On the other hand, glass containing a large amount of SiO2 has a high softening point, making it difficult to use in low-temperature firing. From this perspective, the SiO2 content is preferably 20 mol% or less, more preferably 18 mol% or less, even more preferably 16 mol% or less, and particularly preferably 15 mol% or less.

[0016] As mentioned above, RO is an oxide of a Group 2 element (e.g., Mg, Ca, Ba, Sr) or zinc (Zn). These components modify the glass skeleton and improve the fluidity of the glass after softening. Therefore, glass containing a large amount of RO can contribute to improving the density of the gas sealing part after firing. For example, the total content of RO in the glass powder is preferably 50 mol% or more, more preferably 55 mol% or more, even more preferably 57 mol% or more, and particularly preferably 60 mol% or more. On the other hand, if the components that form the glass skeleton (e.g., SiO2, BO3) are insufficient, vitrification becomes difficult. Therefore, the total content of RO is preferably 75 mol% or less, more preferably 74 mol% or less, even more preferably 72 mol% or less, and particularly preferably 70 mol% or less.

[0017] Among the above-mentioned Group 2 elements, Ba has a larger atomic radius than the other Group 2 elements (Mg, Ca), which makes the glass skeleton brittle. Therefore, BaO has the effect of increasing the thermal expansion coefficient of the glass powder. When the sealing target is a component of an electrochemical cell, the content of BaO in the glass powder is preferably 30 mol% or more, more preferably 32 mol% or more, even more preferably 34 mol% or more, and particularly preferably 35 mol% or more. This allows the thermal expansion coefficient of the component of the electrochemical cell to approximate that of the gas sealing portion. Furthermore, in consideration of the strength of the gas sealing portion, the content of BaO is preferably 50 mol% or less, more preferably 48 mol% or less, even more preferably 46 mol% or less, and particularly preferably 45 mol% or less. Furthermore, the content of CaO is preferably 10 mol% to 20 mol%, and more preferably 12 mol% to 16 mol%. The MgO content is preferably 1 mol% to 10 mol%, more preferably 4 mol% to 6 mol%, the ZnO content is preferably 1 mol% to 10 mol%, more preferably 4 mol% to 6 mol%, and the SrO content is preferably 0 mol% to 10 mol%, more preferably 0 mol% to 4 mol%.

[0018] The SiO2-RO-B2O3-based glass may also contain Al2O3. Al2O3 has the function of stabilizing the glass skeleton, which can contribute to improving the durability of the gas sealing portion after firing. For example, the content of Al2O3 in the glass particles is preferably 0.5 mol% or more, more preferably 0.75 mol% or more, and particularly preferably 1 mol% or more. From the viewpoint of fully exhibiting the functions of the SiO2-RO-B2O3-based glass described above, the upper limit of the Al2O3 content is preferably 10 mol% or less, more preferably 7.5 mol% or less, and particularly preferably 5 mol% or less.

[0019] The SiO2-RO-B2O3-based glass may contain other optional components such as Fe2O3, ZrO2, Y2O2, and TiO2. However, from the viewpoint of fully exhibiting the functions of the SiO2-RO-B2O3-based glass, the upper limit of the content of these optional components is preferably 5 mol% or less, more preferably 2.5 mol% or less, even more preferably 1 mol% or less, and particularly preferably 0.5 mol% or less. The glass powder of the sealing green sheet disclosed herein is preferably alkali-free glass that is substantially free of alkali metal elements (such as Li, Na, and K). This can suppress reactions between the electrochemical cell to be joined and the sealing green sheet. In this specification, "substantially free" means that the content is 0.1 mol% or less (preferably 0.05 mol% or less, more preferably 0.01 mol% or less, even more preferably 0.005 mol% or less, and particularly preferably 0.001 mol% or less).

[0020] As described above, SiO2-RO-B2O3-based glass can form a gas sealing portion that is excellent in bondability, density, and durability, and is therefore particularly suitable for use in the sealing green sheet disclosed herein. A suitable example of the composition of this SiO2-RO-B2O3-based glass is shown below. SiO2 7-20mol% Al2O3 1-10mol% CaO 10-20mol% B2O3 13~25mol% BaO 30~45mol% MgO 1 to 10 mol% ZnO 1 to 10 mol%

[0021] In addition, the D of glass powder 50 The particle size is preferably 45 μm or less, more preferably 40 μm or less, even more preferably 35 μm or less, and particularly preferably 31.7 μm or less. 50As the particle size becomes smaller, the glass powder becomes more likely to soften during firing, which tends to improve the bonding strength of the gas sealing portion. 50 The lower limit of the particle size is preferably 10 μm or more, more preferably 15 μm or more, even more preferably 20 μm or more, and particularly preferably 21.3 μm or more. 50 As the particle size increases, the glass powder is less likely to soften in the initial stage of firing, and therefore, it becomes easier to prevent the binder from being embedded in the softened glass. 50 "Particle size" refers to the particle size corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by laser diffraction / light scattering method.

[0022] In addition, the D of glass powder 90 The particle size is preferably 100 μm or less, more preferably 90 μm or less, even more preferably 85 μm or less, and particularly preferably 82.2 μm or less. 90 Glass powder with a small particle size prevents the inclusion of coarse glass particles, which can prevent a decrease in bonding strength due to insufficient softening of some particles. 90 The lower limit of the particle size is preferably 30 μm or more, more preferably 35 μm or more, even more preferably 40 μm or more, and particularly preferably 41.1 μm or more. 50 particle size and relatively large D 90 The glass powder having a particle size is in a state where fine particles fill the gaps between the coarse particles. This makes it easier to improve the density of the gas sealing part after firing. 90 "Particle size" refers to the particle size corresponding to the cumulative 90% from the fine particle side in the volume-based particle size distribution based on the laser diffraction / light scattering method.

[0023] 2. Binder The binder is a component that binds the various components together and maintains the shape of the sheet. Typically, the binder burns during low-temperature heating (degreasing treatment) in the early stages of firing. This prevents impurities from being mixed into the gas seal portion after firing. In the technology disclosed herein, the binder used in the sealing green sheet is determined so as to satisfy the following formula (2). Specifically, X in the formula (2) is the temperature at which the weight loss rate of the binder reaches 97% during firing (binder 97% combustion temperature). When this binder 97% combustion temperature X is close to the glass softening point Y of the glass powder, softening of the glass powder begins before the binder is fully burned. As a result, the binder is embedded in the softened glass, resulting in the formation of numerous voids in the gas seal portion after firing. On the other hand, when the binder 97% combustion temperature X is significantly lower than the glass softening point Y of the glass powder, most of the binder has been burned by the time the glass powder begins to soften, thereby preventing the formation of voids and reducing the density of the gas seal portion. From this viewpoint, in the sealing green sheet disclosed herein, the difference (YX) between the 97% combustion temperature X of the binder and the glass softening point Y of the glass powder is set to 200°C or more. From the viewpoint of further improving the denseness of the gas sealing portion after firing, the lower limit of the difference (YX) between the 97% combustion temperature X of the binder and the glass softening point Y of the glass powder is preferably 210°C or more, more preferably 220°C or more, and particularly preferably 230°C or more. From the viewpoint of improving the denseness of the gas sealing portion after firing, the upper limit of the difference (YX) is not particularly limited. However, to set the difference (YX) to more than 400°C, it is necessary to use high-temperature softened glass (glass with a glass softening point exceeding 670°C) that is not suitable for low-temperature firing. From this viewpoint, in the following formula (2), the difference (YX) between the 97% combustion temperature X of the binder and the glass softening point Y of the glass powder is set to 400°C or less. The upper limit of the difference (YX) may be 395°C or less, or 390°C or less. 200℃≦YX≦400℃ (2)

[0024] Furthermore, the specific value of the binder 97% combustion temperature X is preferably 450°C or less, more preferably 400°C or less, and particularly preferably 350°C or less. This further promotes binder combustion in the early stages of firing, thereby more effectively suppressing a decrease in density due to the formation of voids. On the other hand, from the viewpoint of the density of the gas-sealed portion, the lower limit of the binder 97% combustion temperature X is not particularly limited. However, if a binder with a 97% combustion temperature X that is too low is used, there is a risk that the binder will volatilize before the firing process begins. From this viewpoint, the lower limit of the binder 97% combustion temperature X is preferably 70°C or more, more preferably 120°C or more, even more preferably 170°C or more, and particularly preferably 200°C or more. The "binder 97% combustion temperature X" is based on thermogravimetry-differential thermal analysis (TG-DTA) measurements. Specifically, a predetermined weight (100%) of binder is heated while gradually increasing the temperature, and the temperature when the weight loss rate reaches 97% is measured, thereby obtaining the binder 97% combustion temperature X.

[0025] The type of binder is not particularly limited as long as it satisfies the above formula (2), and can be selected without particular limitation from conventionally known binders that can be used in sealing green sheets. However, it is preferable to select the binder in relation to the solvent described below. For example, when the solvent is an aqueous solvent, an aqueous binder dispersible in the aqueous solvent is used. On the other hand, when the solvent is a non-aqueous solvent, a non-aqueous binder dispersible in the non-aqueous solvent is used.

[0026] Suitable examples of the aqueous binder include resin materials (acrylic resins, polyether resins, etc.) having carboxyl groups at least on the side chains and terminals. Resin materials having carboxyl groups are easily dispersed uniformly in aqueous solvents, so the shape of the sealing green sheet can be favorably maintained. In addition, many acrylic resins and polyether resins have a low binder 97% combustion temperature X, so sealing green sheets that satisfy the above formula (2) can be easily manufactured.

[0027] Furthermore, in the sealing green sheet disclosed herein, it is preferable to adjust the amount of binder added in relation to the amount of glass powder added. For example, if the amount of binder is too much relative to the amount of glass powder, the density is likely to decrease due to poor combustion of the binder. From this viewpoint, when the mass of the glass powder is taken as 100 wt%, the mass of the binder is preferably 20 wt% or less, more preferably 18 wt% or less, even more preferably 17 wt% or less, and particularly preferably 16.1 wt% or less. On the other hand, if the amount of binder is too little relative to the glass powder, it becomes difficult to maintain the shape of the sealing green sheet. From this viewpoint, when the mass of the glass powder is taken as 100 wt%, the mass of the binder is preferably 5 wt% or more, more preferably 7 wt% or more, even more preferably 8 wt% or more, and particularly preferably 9.5 wt% or more.

[0028] 3. Solvent The solvent used is a liquid that can suitably disperse the glass powder and binder. The type of solvent can be appropriately selected from conventionally known solvents depending on the types of glass powder and binder. As described above, examples of the solvent include aqueous solvents and non-aqueous solvents. Among these, aqueous solvents are particularly suitable for use in terms of safety, operating costs, and environmental impact. Examples of aqueous solvents include ion-exchanged water (deionized water), pure water, ultrapure water, and distilled water. The aqueous solvent may also contain a small amount of a non-aqueous solvent (such as a lower alcohol or a lower ketone) that can be uniformly mixed with water. In this specification, a mixed solvent containing an aqueous solvent of 95% by volume or more (preferably 99% by volume or more) is also considered to be an aqueous solvent.

[0029] The amount of solvent added to the sealing green sheet is not particularly limited and can be adjusted as appropriate. For example, when the mass of the glass powder is 100 wt%, the amount of solvent added is preferably 100 wt% or more, more preferably 150 wt% or more, more preferably 175 wt% or more, and particularly preferably 200 wt% or more. This makes it possible to obtain a sealing green sheet in which the glass powder is suitably dispersed. The upper limit of the amount of solvent added is preferably 400 wt% or less, more preferably 350 wt% or less, more preferably 325 wt% or less, and particularly preferably 300 wt% or less. This makes it easier to maintain the shape of the sealing green sheet.

[0030] 4. Other additives The sealing green sheet may contain conventionally known additives as long as they do not significantly impair the effects of the technology disclosed herein (achieving both good bondability and denseness). Specific examples of such additives include plasticizers, sintering aids, antifoaming agents, antioxidants, preservatives, pH adjusters, colorants (pigments, dyes, etc.), etc.

[0031] Among the additives described above, plasticizers are particularly preferably used in the sealing green sheet disclosed herein. Plasticizers can impart strength, flexibility, and moisture retention to the molded sheet. Examples of plasticizers include trihydric or higher alcohols and polyethers. These resin materials can particularly effectively improve the strength and flexibility of the sheet when dispersed in an aqueous solvent. Suitable examples of trihydric or higher alcohols include polyglycerols such as diglycerol, triglycerol, and tetraglycerol, as well as glycerol, trimethylolmethane, trimethylolpropane, pentaerythritol, arbitol, sorbitol, xylose, arabinose, glucose, galactose, sorbose, fructose, palatinose, maltotriose, and maleitose. Suitable examples of polyethers include polyoxyethylene polyglyceryl ether and polyoxypropylene polyglyceryl ether.

[0032] When the mass of the glass powder is taken as 100 wt%, the amount of plasticizer added is preferably 0.1 wt% or more, more preferably 0.5 wt% or more, even more preferably 1 wt% or more, and particularly preferably 1.5 wt% or more. This allows the effects of the plasticizer to be fully exerted. On the other hand, if the amount of plasticizer added is too large, the viscosity of the sheet may increase significantly, causing it to adhere to the molding device and making it impossible to peel off. From this perspective, the amount of plasticizer added is preferably 5 wt% or less, more preferably 4.5 wt% or less, even more preferably 4 wt% or less, and particularly preferably 3.5 wt% or less.

[0033] <Applications of encapsulation green sheets> As described above, the sealing green sheet disclosed herein is suitable for use in sealing joints between electrodes of electrochemical cells and gas pipes. That is, the sealing green sheet disclosed herein is used for sealing joints between metals, ceramics, ceramics, and metals.

[0034] In the technology disclosed herein, a firing process is performed with a sealing green sheet attached to the connection portion of the joining object. In this firing process, the joining object is placed in a firing furnace, and the temperature inside the furnace is gradually increased. Then, a first heating temperature for burning the binder is maintained for a certain period of time, and then a second heating temperature for softening the glass is maintained for a certain period of time. As a result, after the binder is burned, the glass powder softens and adheres to the joining object. Then, the temperature inside the furnace is lowered, and the softened glass solidifies, forming a gas sealing portion. Note that this firing process is preferably performed as a low-temperature firing in which the second heating temperature is set to 800°C or less (preferably 700°C to 800°C). This prevents thermal damage to components other than the gas sealing portion (such as electrodes and a solid electrolyte layer). Furthermore, the first heating temperature is set to a lower temperature (approximately 500°C to 600°C) than the second heating temperature.

[0035] Here, the sealing green sheet having the above configuration uses glass powder with a glass softening point Y of 670°C or less. This allows the glass powder to be sufficiently softened even during low-temperature firing at 800°C or less, making it possible to form a gas sealing portion with excellent bondability. In addition, in the sealing green sheet disclosed herein, the glass powder and binder are selected so that the difference (YX) between the binder 97% combustion temperature X and the glass softening point Y of the glass powder is 200°C or more. This allows the binder to be sufficiently burned before the glass powder starts to soften, preventing a decrease in density due to the formation of voids. As described above, the sealing green sheet disclosed herein makes it possible to form a gas sealing portion that achieves both high levels of bondability and density.

[0036] In this specification, the phrase "a dense gas sealing portion can be formed even by low-temperature firing" means that when the firing temperature (second heating temperature) is set to 800°C or lower, a gas sealing portion having a relative density of 90% or higher based on Archimedes' law can be formed. The sealing green sheet disclosed herein can prevent the binder from being embedded by softened glass in the early stages of firing, so that a dense gas sealing portion having a relative density of 90% or higher can be achieved even when firing at a low temperature of 800°C or lower. The relative density of the gas sealing portion after firing is preferably 91% or higher, more preferably 92% or higher, even more preferably 93% or higher, and particularly preferably 94% or higher. This more preferably prevents performance degradation due to gas leakage. The upper limit of the relative density is not particularly limited and may be 99.9% or lower, 99.8% or lower, 99.7% or lower, or 99.6% or lower.

[0037] The encapsulating green sheet disclosed herein is not limited to use in low-temperature firing at 800°C or less. For example, it has been difficult for conventional encapsulating green sheets to achieve a relative density of 90% or more even when subjected to high-temperature firing, which tends to improve the relative density. That is, the encapsulating green sheet disclosed herein can achieve a relative density of 90% or more even when fired at a temperature above 800°C, demonstrating an advantageous effect over conventional techniques.

[0038] [Test example] Below, several test examples relating to the technology disclosed herein will be described, but it is not intended that the technology disclosed herein be limited to those shown in these test examples.

[0039] 1. Preparation of each sample In this test, 11 types of encapsulating green sheets (samples 1 to 11) with different compositions were prepared, and the performance of each sample was evaluated. Each sample will be described below.

[0040] (1) Sample 1 To prepare Sample 1, a slurry was prepared by adding 15.9 wt% of an acrylic binder, 2.0 wt% of a plasticizer, and 156 wt% of water to 100 wt% of glass powder. For Sample 1, SiO2-RO-B2O3-based glass was used as the glass powder. The detailed composition of this glass is shown in Table 1. To prepare the slurry, a kneading process was performed for 30 minutes using a tornado. Next, a granulated powder with a particle size of approximately 30 μm was prepared from the slurry using a spray-drying method. The granulated powder was then rolled using a roll molding machine to form a 0.7 mm-thick green sheet for sealing (Sample 1).

[0041] (2) Samples 2 to 11 For Samples 2 to 11, sealing green sheets were formed under the same conditions as for Sample 1, except that the components of the slurry (such as the type and amount of glass powder and acrylic binder) were different. The detailed composition of the slurry for each sample is shown in Table 1.

[0042] 2.Evaluation Test (1) Firing of encapsulation green sheets The green sheets of Samples 1 to 11 were punched with a die to prepare test pieces with a diameter of 10 mm. Next, two sheets of metal foil made of SUS430 were prepared, and the test pieces were placed between the metal foils. Then, a weight of 120 g / cm was placed on this laminate. 2The firing process was carried out while applying a load of 100°C / h. In this firing process, the temperature was increased at a rate of 100°C / h, and a degreasing process was carried out by maintaining the temperature at 600°C for 4 hours, followed by a main firing process by maintaining the temperature at 800°C for 2 hours. This resulted in a laminate in which two metal foils were bonded together via the fired green sheet (gas sealing portion).

[0043] (2) Density evaluation The metal foil was peeled off from both sides of the fired laminate, and the relative density (density) was calculated by Archimedes measurement of the fired gas seal portion. The relative density was calculated by determining the ratio of the apparent volume (closed pores and solid itself) to the total volume of the sample (open pores, closed pores, and solid itself). The results are shown in Table 1.

[0044] (3) Bondability evaluation In this test, the bondability of the laminate after firing was evaluated. In this bondability evaluation, the metal foil was first peeled off from only one side of the laminate after firing. Then, the metal foil on the other side that had not been peeled off was grasped, and the fired green sheet (gas sealing portion) was held facing downward in the direction of gravity for one minute. If the gas sealing portion peeled off from the metal foil and fell freely, it was evaluated as "×", and if it did not fall freely, it was evaluated as "◯". The results are shown in Table 1.

[0045] [Table 1]

[0046] As shown in Table 1, samples 1 to 8 and 10 to 11 were confirmed to have sufficient bondability in the gas seals after firing. In contrast, the gas seal of sample 9 had poor bondability and easily peeled off from the metal foil. This indicates that when firing at a low temperature of 800°C, it is necessary to use glass powder with a glass softening point Y of 670°C or less. Next, samples 1 to 7 were confirmed to have a very high density of around 95% while maintaining sufficient bondability. In contrast, the density of samples 8, 10, and 11 was only around 70%. This indicates that if the glass softening point Y is kept below 670°C, the difference (YX) between the 97% combustion temperature X of the binder and the glass softening point Y of the glass powder must be 200°C or more. This is thought to be because most of the binder is burned before the glass powder begins to soften, preventing the formation of voids due to the binder being embedded in the glass.

[0047] While specific examples of the technology disclosed herein have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0048] The technology disclosed herein includes the following items 1 to 8. The following items 1 to 8 are not limited to the above-described embodiment.

[0049] [Item 1] A sealing green sheet used to form a gas sealing portion of an electrochemical cell, The glass paste contains at least a glass powder, a binder, and a solvent, A green sheet for sealing, which satisfies the following formulas (1) and (2), where X is a binder 97% combustion temperature, which is a temperature at which the weight loss rate of the binder reaches 97%, and Y is a glass softening point of the glass powder. Y≦670℃ (1) 200℃≦YX≦400℃ (2)

[0050] [Item 2] 2. The encapsulating green sheet according to item 1, wherein the binder 97% combustion temperature X is 250°C or higher and 450°C or lower.

[0051] [Item 3] 3. The encapsulating green sheet according to item 1 or 2, wherein the binder is an acrylic resin having a carboxyl group at least on one of a side chain and an end.

[0052] [Item 4] 4. The sealing green sheet according to any one of items 1 to 3, wherein the mass of the binder is 5 wt % or more and 20 wt % or less when the mass of the glass powder is taken as 100 wt %.

[0053] [Item 5] D of the glass powder 50 5. The encapsulating green sheet according to any one of items 1 to 4, wherein the particle size is 20 μm or more and 35 μm or less.

[0054] [Item 6] D of the glass powder 90 6. The encapsulating green sheet according to any one of items 1 to 5, wherein the particle size is 40 μm or more and 85 μm or less.

[0055] [Item 7] The glass powder has the following composition in terms of oxide mass ratio: SiO2 7~20mol%; Al2O3 1~10mol%; CaO 10-20mol%; B2O3 13~25mol%; BaO 30-45 mol%; MgO 1-10 mol%; ZnO 1 to 10 mol% 7. The encapsulating green sheet according to any one of items 1 to 6, which is essentially composed of:

[0056] [Item 8] 8. The encapsulating green sheet according to any one of items 1 to 7, which is used for low-temperature firing at a firing temperature of 700°C to 800°C.

Claims

1. A sealing green sheet used to form a gas sealing portion of an electrochemical cell, The glass paste contains at least a glass powder, a binder, and a solvent, A sealing green sheet which satisfies the following formulas (1) and (2), where X is a binder 97% combustion temperature, which is a temperature at which a weight reduction rate of the binder reaches 97%, and Y is a glass softening point of the glass powder. Y≦670° C. (1) 200°C≦Y−X≦400°C (2)

2. 2. The encapsulating green sheet according to claim 1, wherein the binder 97% combustion temperature X is 250°C or higher and 450°C or lower.

3. 2. The sealing green sheet according to claim 1, wherein the binder is an acrylic resin having a carboxyl group at least on one of a side chain and an end.

4. 2. The sealing green sheet according to claim 1, wherein a mass of the binder is 5 wt % or more and 20 wt % or less when a mass of the glass powder is taken as 100 wt %.

5. D of the glass powder ５０ The encapsulating green sheet according to claim 1 , wherein the particle size is 20 μm or more and 35 μm or less.

6. D of the glass powder ９０ The encapsulating green sheet according to claim 1 , wherein the particle size is 40 μm or more and 85 μm or less.

7. The glass powder has the following composition in terms of oxide mass ratio: Yes ２ 7~20 mol%; Al ２ 9 ３ 1~100mlle; CaO 10~20mol%; B ２ O ３ 13~25mmol%; BaO 30~45mol%; MgO 1~10mol%; ZnO 1~10mol% The encapsulating green sheet according to claim 1 , which consists essentially of

8. The encapsulating green sheet according to any one of claims 1 to 7, which is used for low-temperature firing at a firing temperature of 800°C or less.

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