JPH0518775B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a ceramic heating element, and more particularly to a method for manufacturing a high-temperature zirconia refractory heating element reinforced with zirconia fibers. [Prior Art] Conventionally, heating elements used in an oxidizing atmosphere are used depending on the operating temperature; SiC heating elements are generally used up to about 1400°C, and MoSi ₂ is used up to about 1700°C. There is generally no heating element that can be used up to temperatures higher than that. La ₂ O ₃ −Cr ₂ O ₃ type heating elements are used in some cases, but La ₂ O ₃ is expensive, it is difficult to manufacture large sintered bodies, and it is difficult to use it as a heating element. However, Cr ₂ O ₃ evaporates and contaminates the inside of the furnace, so it is not widely used. On the other hand, electricity can be generated at around 1000℃ by heating a porcelain material made of zirconia (ZrO ₂ ) or thoria (ThO ₂ ) with calcia (CaO) or yttoria (Y ₂ O ₃ ) or other additives. A resistance heating element with a negative characteristic that exhibits electrical conductivity has been developed (Japanese Patent Publication No. 38-12330, etc.), and some ultra-high temperature furnaces using this resistance heating element have already been put into practical use. It is expected to have a wide range of applications in the production of special high-temperature materials and other research on physical properties at high temperatures. Among these, zirconia heating elements are made by adding small amounts of other oxides to zirconium oxide (ZrO ₂ ) and sintering them at high temperatures to create a heat generating material, and the melting point of zirconia (ZrO ₂ ) is 2690°C. Therefore,
This zirconia heating element can obtain high temperatures up to 2400°C. [Problems to be Solved by the Invention] However, conventional zirconia heating elements are often damaged due to local current flow, and are susceptible to thermal shock due to their low thermal conductivity and large coefficient of expansion. There was a risk of damage due to impact. Therefore, the current situation is that almost no one is manufactured. This invention has been made based on the above-mentioned background, and its purpose is to have excellent electrical current heating characteristics or induction heating characteristics, resistance to thermal shock, and to provide a structure that is resistant to damage in the event of damage. Another object of the present invention is to provide a method for manufacturing a zirconia heating element that is easy to manufacture and has little change in these properties. [Means for Solving the Problems] As a result of our attempts to improve the characteristics of refractory heating elements, the present inventors have developed a flexible fireproof sheet reinforced with zirconia fibers that is rolled or stepped. The inventors have discovered that manufacturing a zirconia heating element by laminating the layers is effective in achieving the object of the invention, and have completed the invention. That is, in the method for producing a zirconia refractory heating element of the present invention, as much as 100 parts by weight of an aggregate consisting of 5 to 80 parts by weight of zirconia fibers and 20 to 95 parts by weight of zirconia powder to which a zirconia stabilizer is added is used. It involves adding and blending 20 to 50 parts by weight of a flexible binder to form a fireproof sheet that is flexible at room temperature, winding the fireproof sheet into a roll, stacking it in a roll or stacking it in steps, and firing. . In a preferred embodiment of the manufacturing method according to the present invention, when the fireproof sheets are laminated, the attachment side of the current-carrying lead wire can be fixed to the fireproof sheet, and then the sheets can be laminated. In another preferred embodiment of the production method according to the present invention, the flexible binder is an emulsion of vinyl acetate resin or acrylic ester, and at least one plasticizer selected from ethylene glycol, glycerin, and dibutylphthalein. can do. [Function] In the method for producing a heating element according to claim 1, a raw material prepared by adding and blending a flexible binder to an aggregate consisting of zirconia fibers and zirconia powder to which a zirconia stabilizer is added is used, and this raw material is stretched. By forming it into a sheet shape, a high-strength fireproof sheet that is flexible at room temperature can be obtained. A preferred fireproof sheet can be obtained by using as a flexible binder an emulsion of vinyl acetate resin or acrylic ester, and at least one plasticizer selected from ethylene glycol, glycerin, and dibutylphthalein. Next, this fireproof sheet is wound into a roll to form a laminate having a substantially cylindrical outer shape.
Furthermore, a stepped laminate is formed by folding the fireproof sheet once or multiple times in a zigzag manner. Furthermore, even if fireproof sheets cut in advance to a predetermined size are piled up, a stepped laminate is similarly formed. By firing this laminate, it is cured in this shape, and the zirconia refractory heating element of the present invention is obtained. In this heating element, the zirconia fiber has excellent mechanical strength and thermal shock resistance, so it significantly improves the mechanical properties of the heating element and also acts as a reinforcing material for the zirconia powder. Furthermore, since zirconia fiber has a high melting point of about 2600° C. or higher, phenomena such as decomposition and melting that often occur with fire-resistant fibers other than zirconia fiber do not occur. Because the heating element sheet is wound in a roll and laminated or layered in steps, even if the heating element is damaged, it will only be in one layer of the heating element sheet, and the heating element as a whole will be damaged. Characteristics such as changes in electrical resistance are suppressed to a small range. The temperature of this heating element is increased by, for example, placing it in a magnetic field generated by a high-frequency current flowing through an induction heating coil, or by directly energizing it. In the manufacturing method according to claim 2, when forming the fireproof sheets in a laminated manner, the mounting side of the electrical lead wire is fixed to the fireproof sheet, and then the fireproof sheets are laminated as described above and fired. A heating element having a current-carrying lead wire is obtained. In this heating element, when a current is passed through the current-carrying lead wire, the heating element is directly energized and generates heat due to Joule heat due to the resistance of the heating element. [Detailed Description of the Invention] In the method for producing a zirconia refractory heating element according to the present invention, first, a refractory sheet having flexibility at room temperature is produced. The fireproof sheet is made by adding 20 to 50 parts by weight of a flexible binder to 100 parts by weight of aggregate consisting of 5 to 80 parts by weight of zirconia fibers and 20 to 95 parts by weight of zirconia powder added with a zirconia stabilizer. It has the following formulation. Zirconia fibers are essentially made of zirconium oxide represented by the chemical formula ZrO ₂ , and may also contain other zirconium compounds such as zirconium carbonate and zirconium hydroxide, as well as Y ₂ O ₃ , MgO, CaO, etc. This stabilizer is added and consists of at least one of these stabilizers. Examples of the zirconia fibers that can be used in the present invention include pure zirconia fibers, lime-stabilized zirconia fibers, magnesia-doped zirconia fibers, ittria-stabilized zirconia fibers, ceria-stabilized zirconia fibers, gadolia-stabilized zirconia fibers, and mixtures thereof. , preferably ittria-stabilized zirconia fibers. This zirconia fiber can be manufactured by various methods. For example, an aqueous solution of a zirconium compound is used as a starting material (spinning solution) to form a fiber, a fiber precursor is formed, and the fiber precursor is fired. The zirconia refractory can be manufactured as appropriate and can be selected depending on the use, shape, etc. of the zirconia refractory. The fiber length and fiber diameter of the zirconia fiber used in this invention are, for example, 0.1 to 50 mm and 0.1 to 20 μm, respectively. The amount of zirconia fiber added is 5 to 80 parts by weight, more preferably 10 to 50 parts by weight, based on 20 to 95 parts by weight of zirconia powder. If the amount of fiber added is less than 5 parts by weight, the auxiliary effect will be small, and if it exceeds 80 parts by weight, it will be difficult to form it into a fireproof sheet. Zirconia powder consists essentially of zirconium oxide, and may also include zirconium compounds such as zirconium carbonate and zirconium hydroxide, or stabilizers such as Y ₂ O ₃ , MgO, and CaO added thereto, depending on the purpose. and consists of at least one of these. This powder can be obtained, for example, by a method for producing fine powder by spray drying. The particle size of this zirconia powder is not particularly limited in the present invention, and can be appropriately selected depending on the shape and size of the heating element according to the present invention.
For example, the particle size is 0.1 to 1000μm, preferably 0.5 to 1000μm.
Can be set to 500μm. The zirconia stabilizer is a crystal stabilizer for zirconia or a precursor that is converted into the crystal stabilizer by heating. That is, the crystal stabilizer is a metal oxide that exhibits a crystal stabilizing effect on zirconia and/or a metal salt that forms a metal oxide upon heating. Examples of such stabilizers include magnesium, yttrium, calcium,
Oxides and carbonates of gadolinium, cerium, samarium, gadmium, lanthanum, neodymium, etc.
Basic carbonates, acetates, oxalates, nitrates, chlorides, sulfates, etc., and oxides, chlorides, carbonates of magnesium, calcium, and ythtrium.
Basic carbonate powders are preferred from an economical point of view. The particle size of metal oxide and metal salt stabilizers is
It is 0.01 μm to 1 mm, preferably 0.1 μm to 0.3 mm.
This is because if the thickness is less than 0.01 μm, a migration phenomenon may occur during drying, and if it exceeds 1 mm, the function as a stabilizer will decrease and the strength will deteriorate. The amount of crystal stabilizer added is zirconia 100
The amount is 0.5 to 30 parts by weight, preferably 1 to 20 parts by weight in terms of oxide. If the amount is less than 1 part by weight, the crystallization and stabilizing effect of the zirconia molded body will be small, and the strength will not be developed.If it exceeds 20 parts by weight, no further effect will be obtained, and the vapor pressure will decrease. If the content is too high, it may contaminate the atmosphere of the electric furnace, thereby impairing the properties of the zirconia refractory heating element. The flexible binder is composed of an emulsion of a suitable synthetic resin such as vinyl acetate resin or acrylic ester, and one or more plasticizers such as ethylene glycol, glycerin, and dibutylphthalein. The amount of plasticizer added is based on the above synthetic resin emulsion.
It is preferably 5 to 25 parts by weight per 100 parts by weight. The blending amount of such a flexible binder is preferably 20 to 50 parts by weight based on 100 parts by weight of the aggregate.
If this amount is less than 20 parts by weight, the fireproof sheet will not have sufficient flexibility, and cracks will easily occur when the fireproof sheet is laminated. Moreover, if it exceeds 50 parts by weight, there is a drawback that the structure of the refractory heating element becomes brittle after the flexible binder disappears by heating. A reinforcing binder can be added to the fireproof sheet if necessary. The reinforcing binder is preferably a sol or an aqueous solution that becomes zirconia or a similar compound thereof upon firing. Examples of reinforcing binders include aqueous solutions of water-soluble salts such as zirconyl acetate, zirconium acetate, yttrium acetate, zirconium chloride, and yttrium chloride, and zirconia sol, which are used in a blending range of about 0 to 30 parts by weight. By using this reinforcing binder, the fireproof sheet becomes self-hardening and has high strength, so that the fireproof sheet can be easily manipulated during drying and firing. A fireproof sheet having flexibility at room temperature can be obtained by forming a raw material for a fireproof sheet having the above-mentioned composition into a sheet by rolling it with a roll or extruding it by extrusion molding. The thickness of this fireproof sheet is preferably about 0.1 to 1 mm. Next, this fireproof sheet is wrapped around a thin rod-shaped core material such as a bamboo strip, or by winding one end of the sheet as a base without using a core material, a substantially cylindrical shape is formed. A laminate can be formed. A metal pipe of the required diameter as the core material,
If a plastic pipe or the like is used, a cylindrical laminate can be formed. The above-mentioned core material is removed by pulling it out after drying, or if it is made of wood or plastic, it is burned away during firing, but it is useful for preventing deformation of the fireproof sheet during drying, and it also helps to prevent through holes in the fireproof sheet after it is removed. It also plays the role of preventing overheating in the center of the heating element. Furthermore, a stepped laminate can be formed by folding the fireproof sheet once or multiple times in a zigzag manner. Alternatively, fireproof sheets cut in advance to a predetermined size may be stacked. The wrapped ends of the above-mentioned laminate may be fixed to the lower layer by pressing, or may be fixed by applying mortar (zirconia hardening composition) as described below. When forming a laminate as described above using a fireproof sheet, a lead wire for conducting electricity can be attached. For columnar or cylindrical laminates,
Bend the end of the lead wire short and insert it into the lead wire attachment part of the fireproof sheet, then crimp and fix it, then wrap and laminate the fireproof sheet as it is, or cut the fireproof sheet to attach a narrower lead wire. Create a sheet, insert the end of the lead wire into the lead wire attachment part of this installation sheet as described above and secure it by crimping, or wrap the lead wire around the lead wire attachment part and secure it, and then stack this installation sheet. There are ways to attach lead wires by winding and stacking them at both ends of the main body. In the case of a tiered laminate, the end of the lead wire bent short as described above is inserted and crimped into the lead wire attachment part of the fireproof sheet, and these fireproof sheets are folded back into a tiered shape and stacked. good. A zirconia-based curing composition may be applied in advance to the above-mentioned lead wire attachment site or attachment sheet to further strengthen the adhesion. Examples of the zirconia hardening composition include a zirconia hardening composition that exhibits self-hardening properties at room temperature by using a water-soluble zirconium salt whose aqueous solution is acidic as a binder and using yttria or a water-insoluble yttrium compound as a hardening agent. Can be mentioned. The water-soluble zirconium salt whose aqueous solution is acidic to be used in this cured zirconia composition includes:
Examples include zirconium acetate, zirconyl acetate, zirconium acid chloride, zirconium nitrate, zirconium sulfate, etc. However, considering the fact that toxic gas is generated due to decomposition of the binder component when the molded product obtained by normal curing is fired, acetic acid is used. Preference is given to using zirconium or zirconyl acetate. Note that the concentration of the aqueous solution must be 5% or more; if the concentration is less than 5%, the strength after curing will be insufficient and the cured product will be difficult to handle. Another essential component of the zirconium-based hardening composition is yttrium or a water-insoluble yttrium compound. Ittria needs to have a purity of 90% by weight or more. Note that it is desirable that the particle size of ittria is 1 mm or less. If the purity of ittria is less than 90% by weight or if the particle size exceeds 1 mm, it is not preferable because it becomes difficult for ittria to function as a curing agent. Examples of water-insoluble yttrium compounds include yttrium carbonate and yttrium hydroxide.
Typically, these water-insoluble yttrium compounds are
It is manufactured and sold in the form of a fine powder of 0.1 mm or less.
In particular, there is no need to consider particle size, but the purity of ittria formed by heating is 90% by weight.
It is necessary that it is above. The ratio of addition of yttria or water-insoluble yttria compound to the water-soluble zirconium salt is 1 part by weight of zirconium oxide formed from the water-soluble zirconium salt, and 1 part by weight of yttria (formed by heating when using a water-insoluble yttria compound). The ratio is preferably from 0.5 to 5. By adjusting the ratio within this range, the curing time can be adjusted from 10 minutes to 10 hours. If the amount of ittria added is less than 0.5, the curing time will be too long, which is not practical.If the amount added is more than 5, the curing time will be too fast, and it may harden during kneading. I don't like it because of this. A composition consisting of the above-mentioned components includes refractory raw material powder,
The zirconia cured composition used in the present invention can be prepared by appropriately adding and mixing organic substances, fiber raw materials, and the like. In addition to applying the cured composition to the wrapped ends of the laminate as described above, the cured composition can also be applied to the entire surface of the fireproof sheet and then formed into a laminate so that the layers are polymerized. It can also be used to strongly adhere materials. The zirconia refractory heating element of the present invention can be obtained by drying and then firing the laminate constructed in this manner. Firing temperature is e.g. 800℃
The temperature above is possible, but the temperature is preferably 1400°C or higher, more preferably 1500 to 2400°C. Below this lower temperature limit, the sintering strength is weak. Moreover, baking can also be performed by induction heating the laminate after drying treatment. In order to prevent the laminate from deforming during firing, a hole is drilled in one end of the laminate and the laminate is hung through an alumina pipe, or the outside of the laminate is protected with a holder to maintain its shape. However, a method such as performing a firing treatment in this state can be used. In this case, the core material may be one that is burnt out. When the firing temperature exceeds 1700° C., it is difficult to attach a platinum lead wire in advance or to fire it by suspending it from the viewpoint of strength. Therefore, in this case, the laminate is first pre-fired at 1,400 to 1,700°C to make it strong enough to be set in the furnace as a heating element, and then installed in the furnace and energized or induced. A method of firing itself by heating is preferred. Note that the lead wire for energization may be attached after firing. Next, the present invention will be explained in more detail by Examples and Comparative Examples with reference to FIGS. 1 to 3. Example 1 50 parts by weight of Ittria stabilized zirconia fiber (Y7Z fiber manufactured by Shinagawa Shirorenga Co., Ltd.), Ittria stabilized zirconia powder (Y ₂ O ₃ 7%, ZrO ₂ 92
%), add polyvinyl acetate emulsion as a flexible binder and 33 parts by weight of a plasticizer (dibutyl phthalein), mix thoroughly, transfer this onto a flat plate, and roll it out using a roller. 0.5
A flexible sheet 1 with a thickness of mm was produced (see FIG. 1). Next, this flexible sheet 1 is wound around a 2φ stainless steel rod 3 as a core material to form a laminate body 1', and the flexible sheet is cut into dimensions that take into account the rate of change due to firing. Then, a lead wire attachment sheet 1a was prepared, and the lead wire 2 with the tip bent as shown in FIG. It was dried for 10 hours. After that, pull out the stainless steel rod 3,
After preliminary firing at 1500°C, it was set in an electric furnace and fired at 2000°C by applying electricity. Table 1 shows the results of using this fired product. Example 2 Example 1 was added to 50 parts by weight of the ittria-stabilized zirconia fiber used in Example 1, 45 parts by weight of the ittria-stabilized zirconia powder, and 5 parts by weight of the magnesia-stabilized zirconia powder (MgO 4%, ZrO ₂ 96%). A flexible sheet was prepared in the same manner by adding the flexible binder used in . This sheet is the second sheet.
As shown in Figure a, cutting is performed so that a tapered recess is formed on one end side, and sheet parts 1a for attaching lead wires are formed on both sides of the cut, and the flexible sheet 1 is After winding and laminating in a cylindrical shape, the above mounting sheet part 1a
was wound inwardly tapered layers around both ends of the laminate main body 1' while attaching lead wires 2 in the same manner as in Example 1, and this was preliminarily fired at 1600°C. After that, this pre-fired product is immersed in an aqueous solution containing zirconium acetate and yttrium acetate, dried, and then installed in an electric furnace.It is preheated in the electric furnace at a heating rate of 3℃/min up to 1500℃, and then the main body It was fired by applying electricity until it reached 2000℃. Table 1 shows the results of using this fired product. Example 3 20 parts by weight of ittria-stabilized zirconia fiber, 65 parts by weight of ittria-stabilized zirconia powder, and 15 parts by weight of unstabilized zirconia powder used in Example 1 were blended with 3% ittria, and a plasticizer ( 28 parts by weight of a polyvinyl acetate emulsion containing 18% (ethylene glycol) and 8 parts by weight of an aqueous zirconyl acetate solution (20% concentration as ZrO ₂ ) were added and kneaded to prepare a 1 mm thick flexible sheet. This sheet is wound around a 100φ paper pipe for 3 turns, and a self-hardening composition containing zirconia fibers, that is, the above-mentioned zirconia hardening composition, is applied to the end and the beginning of the winding in a width of about 2 mm. pasted in layers. This laminate was allowed to stand as it was for 12 hours, then dried at 150°C for 6 hours to reduce its flexibility, and the laminate 1'' was placed in a zirconia brick holder as shown in Figure 3.
The outer periphery was held in a container and preliminarily fired at 1600°C. This pre-fired product was set in an induction furnace, and after preheating, induction heating was performed at a temperature increase rate of 2.5°C/min until the temperature reached 2200°C. Table 1 shows the results of using this fired product. Comparative Example 1 95% Ittria stabilized zirconia powder and 5 hemp threads
%, a flexible sheet was produced using the binder and molding method of Example 1, and a laminate having a substantially cylindrical outer shape was formed. Several cracks occurred in this laminate during pre-firing at 1500°C. Table 1 shows the results of using this fired product. Comparative Example 2 An attempt was made to mold only the Ittria-stabilized zirconia powder into a laminate from a sheet using the same binder and molding method as in Example 1, but a slight crack occurred at the stage of winding the sheet. Therefore, the subsequent steps were discontinued.

〔Effect of the invention〕

Since the present invention is configured as described above, it has the following effects. (b) The zirconia refractory heating element obtained by the production method of the present invention has excellent heat generation properties, and due to reinforcement with zirconia fibers, it has excellent mechanical strength and resistance to thermal shock. In addition, because it has a laminated structure, even if a crack occurs, it will only occur in one layer, and changes in the characteristics of the entire heating element, such as electrical resistance, can be suppressed to a small amount, allowing it to last for a longer period of time. Can withstand use. (b) By the manufacturing method of the present invention, it becomes possible to easily manufacture a zirconia refractory heating element having the effects as in (a). (c) With the manufacturing method of the present invention, there is a large degree of freedom in the shape of the heating element, and if it is cut into a shape that takes into account the rate of change after firing, there is no need for machining for finishing, reducing the number of manufacturing man-hours and production. The number of days can be reduced. (d) According to the manufacturing method of the present invention, air bubbles trapped in the material during the operation of stretching the material into a flexible sheet shape are removed, and a more homogeneous heating element can be easily obtained.

[Brief explanation of the drawing]

1 and 2 are explanatory diagrams of the process of manufacturing a heat generating element from a fireproof sheet, and FIG. 3 is a perspective view showing a state in which the heat generating element is set in a holder. 1... Flexible sheet, 1'... Laminate main body, 1a...
Sheet for attaching lead wires, 2...Lead wires, 3...Core material.

Claims (1)

[Claims] 1. Zirconia powder containing 5 to 80 parts by weight of zirconia fibers and 20 to 95 parts by weight of zirconia stabilizer.
20 to 50 parts by weight of a flexible binder is added to 100 parts by weight of aggregate to form a fire-resistant sheet that is flexible at room temperature, and the fire-resistant sheet is rolled and laminated into a roll. A method for producing a zirconia refractory heating element that generates heat by applying electricity or induction heating, the method comprising laminating layers in a stepwise manner and firing. 2. The manufacturing method according to claim 1, wherein when laminating the fireproof sheets, the attachment side of the current-carrying lead wire is fixed to the fireproof sheet, and then the sheets are laminated. 3. As the flexible binder, an emulsion of vinyl acetate resin or acrylic ester, and at least one plasticizer selected from ethylene glycol, glycerin, and dibutylphthalein are used. 2
Manufacturing method described.