Method for producing precursor of continuous zirconia fiber and method for producing continuous zirconia fiber

Abstract

To provide a manufacturing method of a zirconia continuous fiber having excellent heat resistance.SOLUTION: A disclosed manufacturing method includes: a first step of forming a precursor having excellent continuous spinnability by partial hydrolytic condensation of a zirconium alkoxide in coexistence of a first metal alkoxide for generating an oxide component for forming a partially stabilized zirconia or a stabilized zirconia and a second metal alkoxide for generating an oxide component for suppressing crystal growth; a second step of high speed continuous spinning this by dry spinning method; a third step of infusibilization under controlled steam atmosphere; a forth step of converting to a fiber having high intensity by primary firing in an atmospheric air or a non-oxidation atmosphere containing steam as required; and a fifth step of high speed temperature rising to convert to a crystal grown fiber.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a precursor of a continuous fiber. The present invention also relates to raw materials for zirconia continuous fibers and a manufacturing method for suppressing crystal growth and softening within the fibers during the infusible and sintering steps. [Background technology]

[0002] Zirconia is monoclinic at room temperature, and as the temperature increases, its crystal structure undergoes a phase transition to tetragonal and then cubic. This phase transition is accompanied by a change in volume, so repeated temperature increases and decreases can lead to destruction of the sintered body. In particular, the phase transition from monoclinic to tetragonal causes a volumetric shrinkage of approximately 4%.

[0003] When zirconia is dissolved with rare earth oxides such as calcium oxide, magnesium oxide, or yttrium oxide, oxygen vacancies are formed in the structure, making the cubic and tetragonal crystals stable or metastable at room temperature, and preventing destruction due to temperature changes. Zirconia with the addition of such oxides known as stabilizers is called stabilized zirconia or partially stabilized zirconia.

[0004] Stabilized zirconia has superior mechanical properties, such as strength and toughness, compared to zirconia without added oxides. This is because the phase transformation from tetragonal to monoclinic inhibits the propagation of cracks that cause fracture, alleviating stress concentration at the crack tip. This unique mechanism is called the "stress-induced phase transformation strengthening mechanism," in which up to approximately 40% of the tetragonal crystals transform into monoclinic crystals. It is also known that partially stabilized zirconia, in which the amount of additives is reduced to allow a slight transformation, has better mechanical properties than fully stabilized zirconia, in which the transformation is completely suppressed. Furthermore, ceramic fibers are useful materials used in various fields such as electrical insulation, heat insulation, fillers, and filters, taking advantage of their properties such as electrical insulation, low thermal conductivity, and high elasticity, and the same is true for zirconia.

[0005] Zirconia fibers can be produced by various methods. For example, they can be produced by forming an aqueous solution of zirconium salt into fibers as a starting material and calcining the fibers at a high temperature. Specifically, there are the inorganic salt method (Patent Document 1), in which a water-soluble organic polymer such as polyethylene oxide or polyvinyl alcohol is added to an aqueous solution of zirconium salt to form a viscous liquid, which is then sprayed through a nozzle and calcined; the slurry method (Patent Document 2), in which ZrOCl2 is added as a binder to ZrO2 fine powder and a crystal stabilizer to form a viscous slurry, which is dry-spun and calcined; an industrial method for producing continuous zirconia fibers with excellent heat resistance and mechanical strength, in which an aqueous solution prepared from a zirconium-containing compound, a partial stabilizer, a sintering aid, and a spinning aid is used as a spinning dope that is easy to spin (Patent Document 3); and a method in which an organic acid salt of zirconium is melted, spun, and calcined (Non-Patent Document 1). It has also been well known that fibers can be produced by the sol-gel method using metal alkoxides as raw materials.

[0006] As described above, it is possible to make zirconia into fibers. However, because these fibers are polycrystalline, they are prone to degradation due to grain growth at high temperatures. In addition, at temperatures above 1300°C, the fibers fuse together, resulting in problems such as inferior mechanical strength to single-crystal fibers. Therefore, many methods for producing single-crystal fibers of ceramic materials have been proposed. Regarding zirconia, single-crystal fibers can be produced by hydrothermal synthesis, laser heating, and vapor phase synthesis. Furthermore, electrospinning is known as a method for producing fibers thinner than conventional fibers and increasing their strength. In electrospinning, a high voltage is applied to a fiber-forming solution, causing the solution to be ejected toward an electrode, and the solvent evaporates as it is ejected, easily producing ultrafine fiber structures. Zirconia fibers with an average fiber diameter of 50 to 1,000 nm and a fiber length of 100 μm or more have been obtained (Patent Document 4).

[0007] However, these fibers are extremely short and cannot be used as reinforcing fibers for continuous fiber reinforced ceramics. Conventional zirconia continuous fibers, particularly stabilized zirconia, can be produced by a melting method, i.e., by melting and drawing the zirconia into a fibrous form, but the melting point is as high as 2700° C., which results in extremely high production costs. Furthermore, with the above-mentioned methods for producing stabilized zirconia fibers using the inorganic salt method and the sol-gel method, the resulting fibers are polycrystalline, and therefore prone to degradation due to grain growth at high temperatures, and also have the problem of insufficient mechanical strength.

[0008] Among these conventional methods, the method disclosed in Patent Document 3 is superior as a method for obtaining continuous fibers that exhibit relatively high strength up to about 1300°C. However, at temperatures higher than this, the fibers fuse together and crystal growth causes a significant decrease in strength, which is a drawback and makes it unsuitable for use as reinforcing fibers for highly heat-resistant oxide-based composite materials. Therefore, the present inventors aimed to develop a fiber for long-fiber-reinforced composite materials that prevents the deterioration caused by grain growth at high temperatures of conventional polycrystalline zirconia fibers, improves properties such as mechanical strength, and is easy to handle. They provided a stabilized zirconia continuous fiber or a partially stabilized zirconia continuous fiber that can be used in zirconia fiber-reinforced zirconia composite materials, and a method for producing the same. As a result, by incorporating a small amount of carbon into the zirconia continuous fiber, it is possible to suppress porosity in the fiber due to crystal growth within the fiber, thereby obtaining a fiber with excellent mechanical strength. Furthermore, if necessary, a relatively dense, highly crystallized zirconia fiber with a low carbon content can be obtained by subsequent rapid heating and firing at a higher temperature (Patent Document 5). However, these fibers have a fiber diameter of 10 to 50 μm, and a diameter of around 10 μm is preferable for weaving into the fabric required for composite materials. Therefore, it is necessary to be able to spin fibers at a diameter of around 5 to 20 μm. Furthermore, there is a drawback in that when calcined at temperatures above 1000°C, the strength decreases significantly, and in particular the elastic modulus increases, making them difficult to handle. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Special Publication No. 55-36726 [Patent Document 2] Japanese Patent Application Publication No. 60-246817 [Patent Document 3] Japanese Patent Application Publication No. 4-91227 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-336121 [Patent Document 5] Japanese Patent Application Laid-Open No. 2015-94055 [Non-patent literature]

[0010] [Non-Patent Document 1] J.Am.Ceram.Soc.,70(1987)C187 Summary of the Invention [Problem to be solved by the invention]

[0011] Conventional zirconia continuous fibers for composite materials are polycrystalline, so they are prone to degradation due to grain growth at high temperatures. Furthermore, when fired at high temperatures exceeding 1,300°C to make them heat-resistant, fusion occurs between the fibers, resulting in a decrease in properties such as mechanical strength, making them unsuitable for use in long-fiber-reinforced composite materials. Therefore, an object of the present invention is to provide a method for producing a stabilized zirconia continuous fiber or a partially stabilized zirconia continuous fiber that can be used for a zirconia fiber-reinforced zirconia composite material, that suppresses crystal growth even at high temperatures, that has high strength even at high temperatures, and that is easy to handle, thereby solving the above-mentioned problems. Another object of the present invention is to provide a method for producing a precursor of a continuous zirconia fiber, which is suitable for use in producing a stabilized continuous zirconia fiber or a partially stabilized continuous zirconia fiber that is easy to handle. [Means for solving the problem]

[0012] The present inventors have conducted extensive research into a method for producing continuous zirconia fibers that use alkoxides as the main raw material, have extremely low levels of impurities such as alkali, and have a controlled crystallite size of approximately 20 nm or less by controlling the amount of carbon and the amount of added elements such as Y, Al, and Si. As a result, they have arrived at the present invention.

[0013] [1] The method for producing a precursor of a continuous zirconia fiber of the present invention involves carrying out partial hydrolysis and condensation of a zirconium alkoxide in the presence of a first metal alkoxide that produces partially stabilized zirconia or an oxide component that forms stabilized zirconia, and a second metal alkoxide that produces an oxide component that inhibits crystal growth, thereby forming a precursor with excellent continuous spinnability. [2] In the method [1] for producing a precursor of a continuous zirconia fiber of the present invention, preferably, a partial co-hydrolysis condensation with a zirconium alkoxide is carried out in the presence of a first metal alkoxide that produces an oxide component that forms the partially stabilized zirconia or stabilized zirconia, and a second metal alkoxide that produces an oxide component that inhibits crystal growth, to form a precursor with excellent continuous spinnability, and polycarbosilane that produces SiC is further added to the precursor to form a precursor with excellent continuous spinnability.

[0014] [3] In the method [1] or [2] for producing a precursor of a continuous zirconia fiber of the present invention, the oxide component that inhibits crystal growth is preferably at least one selected from the group consisting of alumina (Al2O3), silica (SiO2), titania (TiO2), and boron oxide (B2O3). [4] In the method [3] for producing a precursor of a zirconia continuous fiber of the present invention, the amount of oxide component that inhibits crystal growth to be mixed is preferably 0.1 mol % or more and 20 mol % or less relative to ZrO2 for one or more selected from the group consisting of alumina (Al2O3), silica (SiO2), titania (TiO2), and boron oxide (B2O3). [5] In the methods [2] to [4] of the present invention for producing a precursor of a continuous zirconia fiber, the amount of SiC that inhibits crystal growth to be mixed is preferably 0.1 mol % or more and 20 mol % or less relative to ZrO2. [6] In the methods [2] to [5] of the present invention for producing a zirconia continuous fiber precursor, the ratio of the polycarbosilane added to the zirconia fiber precursor obtained by partial hydrolysis in the coexistence of the first metal alkoxide and the second metal alkoxide is preferably such that the ratio of the number of Si atoms contained in the polycarbosilane to the number of Zr atoms contained therein is 0.001 or more and 0.1 or less. [7] In the method [6] for producing a zirconia continuous fiber precursor of the present invention, preferably, the ratio of the polycarbosilane added to the zirconia fiber precursor obtained by partially hydrolyzing the first metal alkoxide is such that the ratio of the number of Si atoms contained in the polycarbosilane to the number of Zr atoms contained therein is 0.001 or more and 0.1 or less.

[0015] [8] The method for producing a precursor of a continuous zirconia fiber of the present invention comprises carrying out partial co-hydrolysis condensation with a zirconium alkoxide in the presence of a first metal alkoxide that produces the partially stabilized zirconia or an oxide component that forms stabilized zirconia, thereby forming a precursor with excellent continuous spinnability, and further adding polycarbosilane that produces SiC to the precursor to form a precursor with excellent continuous spinnability. [9] In the method [8] for producing a precursor of a continuous zirconia fiber of the present invention, the amount of SiC that inhibits the crystal growth to be mixed is preferably 0.1 mol % or more and 20 mol % or less relative to ZrO2.

[0016]

[10] In the methods [1] to [9] for producing a precursor of a continuous zirconia fiber of the present invention, the oxide component forming the partially stabilized zirconia or stabilized zirconia is preferably at least one selected from the group consisting of yttria (Y2O3), ceria (Ce2O3), and erbia (Er2O3).

[11] In the method

[10] for producing a precursor of a continuous zirconia fiber of the present invention, the amount of oxide components forming the partially stabilized zirconia or stabilized zirconia mixed is preferably 2 mol % or more and 12 mol % or less relative to ZrO2.

[12] In the methods [1] to

[11] of the present invention for producing a precursor of a zirconia continuous fiber, the precursor preferably has a spinnability value, defined by the following formula, that is at least twice the value of the polycarbosilane of the SiC fiber precursor. Spinnability value = Winding drum rotation speed (rpm) × Continuous winding time (min) ÷ Extrusion pressure (MPa)

[0017]

[13] The method for producing a continuous zirconia fiber of the present invention comprises the steps of: a first step of preparing a precursor produced by any one of the methods [1] to

[12] for producing a continuous zirconia fiber precursor; a second step of continuously spinning the precursor by dry spinning; a third step of infusibility in a controlled water vapor atmosphere; a fourth step of low-temperature calcination in a controlled atmosphere to convert the precursor into a high-strength fiber; and a fifth step of high-temperature calcination to convert the precursor into a zirconia-based fiber with a small rate of loss in strength at high temperatures. Here, calcination at a high temperature refers to calcination in the temperature range of 1000 to 1600°C.

[0018]

[14] In the method for producing continuous zirconia fiber

[13] of the present invention, preferably, in the second step, the precursor obtained in the first step is continuously spun at high speed by a dry spinning method. The solvent for the spinning solution is preferably xylene, and the concentration is generally 75 to 85 wt %. The spinning temperature may be room temperature, but is preferably about 5 to 50°C depending on the viscosity of the spinning solution.

[15] In the method

[13] for producing a continuous zirconia fiber of the present invention, the infusibility of the precursor fiber in the third step can be preferably achieved by holding the precursor fiber in a water vapor-containing atmosphere at room temperature to 200°C for a predetermined period of time. It is more preferable to hold the precursor fiber in a saturated water vapor atmosphere at 95°C to 200°C for 0.5 hours or more.

[16] In the method for producing a zirconia continuous fiber of the present invention

[13] , preferably, in the fourth step, the infusible precursor fiber is heated to 1,000°C at a temperature increase rate of 10 to 200°C / hour in air, an inert atmosphere, or an inert gas containing water at saturated vapor pressure at room temperature, and then maintained at that temperature for a predetermined period of time to be converted into a high-strength fiber. When the low-temperature firing is performed in an inert gas containing water at saturated vapor pressure at room temperature, an inert gas atmosphere is preferably used at temperatures above 500°C.

[17] In the method

[16] for producing continuous zirconia fibers of the present invention, the fiber diameter obtained in the fourth step is preferably 6 μm or more and 20 μm or less.

[18] In the method

[13] for producing a continuous zirconia fiber of the present invention, preferably, in the fifth step, the gel fiber obtained in the fourth step is subjected to secondary firing by heating in air, nitrogen gas, argon gas, or the like to 1000 to 1600°C at a heating rate of 500 to 20,000°C / hour to promote crystallization and advance sintering, thereby obtaining a stabilized zirconia fiber or a partially stabilized zirconia fiber.

[19] In the method

[18] for producing continuous zirconia fibers of the present invention, the fiber diameter obtained in the fifth step is preferably 6 μm or more and 20 μm or less. [Effects of the Invention]

[0019] According to the method for producing zirconium oxide continuous fibers of the present invention, it is possible to obtain zirconia continuous fibers that have high strength even when fired at temperatures exceeding 1000°C, i.e., at high temperatures. Furthermore, according to the method for producing zirconia continuous fiber of the present invention, by incorporating carbon and a small amount of a metal that inhibits crystal growth into the zirconia continuous fiber, the generation of voids at the grain boundaries within the fiber due to crystallization can be suppressed, and fibers with excellent mechanical strength can be obtained. Furthermore, by adding the fifth step of the present production method, dense, highly crystallized zirconia fiber can be obtained through a firing step at a higher temperature by rapid heating. [Brief explanation of the drawings]

[0020] [Figure 1]FIG. 1 shows the crystallite size of a 5YSZ (Y / Zr=0.10) based precursor after pyrolysis at 1500 and 1600° C. [Figure 2] This figure shows the relationship between the infusibilization temperature of dry-spun Zr-Y(0.1)·Al(0.1)·Si(0.01)+PCS(0.1) fibers and the tensile strength (◯) and crystallite size (●) of zirconia fibers obtained by firing at 1000°C in nitrogen gas. [Figure 3] This figure shows the dependence of the tensile strength of fibers made from Zr-Y(0.1)·Al(0.1)·Si(0.1)-061 precursor on the firing end temperature (TH2O) in an Ar atmosphere containing water vapor. The fibers were heated to 1000°C at a heating rate of 100 to 200°C / hour and held for 1 hour for primary firing. [Figure 4] FIG. 1 is a graph showing the secondary-baking temperature dependence of the tensile strength of fibers that have been primarily baked in Ar at 1000° C. and then secondary-baked in air to a predetermined temperature. [Figure 5A] This is an SEM photograph of the cross section of fiber No. 7 obtained from Zr-Y(0.1)-077+PCS(0.05) after the first calcination (1000°C). [Figure 5B] This is an SEM photograph of the cross section of fiber obtained by secondary calcination (1300°C) of fiber No. 7 obtained from Zr-Y(0.1)-077+PCS(0.05). DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be described in detail below. The upper and lower limits of a range indicated by "~" are inclusive of the boundary value unless otherwise specified. For example, "XX~△△" indicates a range between XX and △△. The method for producing the zirconia continuous fiber of the present invention is as follows. In the first step of the method for producing continuous zirconia fibers of the present invention, partial hydrolysis and condensation of zirconium alkoxide is carried out in the presence of a metal alkoxide to synthesize a precursor with excellent continuous spinnability. Here, the metal alkoxide used as the stabilizer generates oxide components that form partially stabilized zirconia or stabilized zirconia. Furthermore, the crystal growth inhibitor may be any alkoxide that forms alumina (Al2O3), silica (SiO2), titania (TiO2), or boron oxide (B2O3), which has a confirmed inhibitory effect. Furthermore, the Si component may be polycarbosilane, which forms SiC and is a precursor of SiC fibers.

[0022] When zirconium alkoxide is used as the metal alkoxide, for example, zirconium tetra-n-butoxide Zr(OC4H9)4 is suitable, but zirconium tetra-i-propoxide, zirconium tetra-sec-butoxide, zirconium tetra-t-butoxide, etc. may also be used. Stabilizers can be compounds containing rare earth elements such as yttrium, cerium, and erbium, or alkaline earth metals such as calcium and magnesium. However, alkoxides are preferred for uniform mixing with zirconium alkoxide, such as yttrium tetraisopropoxide, yttrium tri-n-butoxide, and erbium acetylacetonate. The alkoxide is dissolved in isopropanol or similar and mixed with the zirconium alkoxide in a predetermined amount. The amount of stabilizer, calculated as oxide, mixed with ZrO2 is preferably 2–12 mol%. Less than 2 mol% is ineffective, while more than 12 mol% is undesirable due to oxide precipitation and other factors that degrade fiber properties. A particularly suitable oxide is yttria (YO). Addition of 5 mol% yttria, even in the absence of other crystal growth inhibitors, results in zirconia crystallite sizes of approximately 10 nm after firing at 1600°C (Figure 1).

[0023] Figure 1 shows the crystallite sizes of a 5YSZ (Y / Zr = 0.10) precursor after pyrolysis at 1500 and 1600 °C. The horizontal axis represents the Al / Zr ratio from 0.0 to 0.3 in 0.1 increments, the depth axis represents the Si / Zr ratio from 0.0 to 0.3 in 0.1 increments, and the vertical axis represents the crystallite size. In the figure, for an Al / Zr ratio of 0.1 and a Si / Zr ratio from 0.0 to 0.3 in 0.1 increments, the crystallite sizes after pyrolysis at 1500 °C are 16.8, 9.4, 6.3, and 18.9 nm. In contrast, after pyrolysis at 1600 °C, the crystallite sizes are 10.0, 7.5, 3.9, and 10.8 nm. Note that for lattice coordinates without a bar graph showing the crystallite size, no pyrolysis experiments were performed.

[0024] As raw alkoxides for the crystal growth inhibitor, (ethylacetoacetato)aluminum diisopropoxide is suitable for alumina (Al2O3), tetraethoxysilane for silica (SiO2), tetra-i-propoxytitanium for titania (TiO2), and triethyl borate for boron oxide (BO3). When adding SiC as the Si component, polycarbosilane, a precursor to SiC fibers, is suitable. The amount of Si atom contained in the polycarbosilane to the Zr contained in the zirconia fiber precursor is preferably 0.001 to 0.2, more preferably 0.001 to 0.1. Adding an amount of 0.2 or more is undesirable because it impairs the properties of zirconia.

[0025] A mixture of zirconium alkoxide, the stabilizer, and the crystal growth inhibitor is partially hydrolyzed by chelation with β-diketone. The β-diketone is preferably 3-oxobutanoic acid ethyl. The amount of 3-oxobutanoic acid ethyl is preferably 0.1 to 2 times the molar amount of zirconium alkoxide and metal alkoxide. To hydrolyze zirconium tetra-n-butoxide, for example, a mixed solution of water and isopropanol is added and stirred. There are no particular restrictions on the amount of each component used relative to the metal alkoxide, but isopropanol can be used in an amount of about 3 times by mole. Water is preferably used in an amount of 0.3 to 3 times by mole. If the amount of water relative to the alkoxide is less than 0.3 times by mole, the reaction does not proceed sufficiently and the polymer does not become high-molecular. If the amount of water is more than 3 times by mole, the polymer becomes too high-molecular, making spinning difficult or causing gelation, which is not desirable. The stabilizer and the crystal growth inhibitor may be mixed after the hydrolysis and condensation reaction of the zirconium alkoxide.

[0026] Next, the mixture is concentrated under reduced pressure while the temperature is gradually increased from room temperature to obtain a spinnable precursor. The concentration temperature can be up to approximately 90°C. In the second step of the present invention, the precursor obtained in the first step is continuously spun at high speed using a dry spinning method. The solvent for the spinning solution is preferably toluene or xylene, with xylene being preferred for fibers with a circular cross section. The concentration is generally 75-85%. The spinning temperature can be room temperature, but may be around 5-50°C depending on the viscosity of the spinning solution. The precursor obtained in the first step has excellent spinnability in dry spinning. Under the above spinning solution preparation conditions and spinning conditions, the spinnability value is several times (>680) higher than the value (168) of polycarbosilane, a SiC fiber precursor whose industrialization is being considered for dry spinning, when using a 100 μm nozzle. This indicates spinning using a multi-hole nozzle, i.e., industrial mass production, and enables spinning of fiber diameters of 5-20 μm. Here, the spinnability value is expressed by the following formula and is an index of the ease of spinning. Spinnability value = Winding drum rotation speed (rpm) × Continuous winding time (min) ÷ Extrusion pressure (MPa)

[0027] In the third step of the present invention, the resulting precursor fiber is infusible in a controlled water vapor atmosphere. The purpose of infusibility is to hydrolyze and condense the alkoxy groups or chelate moieties remaining in the precursor, thereby crosslinking the molecules and preventing the fiber from melting and fusing in the third step. To efficiently infusibly perform infusibility in a short time, the temperature is raised from room temperature to 200°C at a rate of 5 to 100°C / hour in a saturated water vapor atmosphere for a predetermined period of time, and then maintained for the predetermined period of time. However, maintaining the temperature in a saturated water vapor atmosphere between 95°C and 200°C for 0.5 hours or more increases the strength of the resulting zirconia continuous fiber (Figure 2). Water vapor treatment for longer periods than this is not effective and is therefore not efficient, and is therefore not preferred.

[0028] In the fourth step of the present invention, the infusible precursor fiber is heated to 1000°C at a rate of 10 to 200°C / hour in air, an inert atmosphere, or an inert gas containing water at saturated vapor pressure at room temperature, and then maintained for a predetermined period of time for low-temperature calcination, i.e., primary calcination, to convert it into a high-strength fiber. Low-temperature calcination in an inert gas containing water at saturated vapor pressure at room temperature is preferably performed in an inert gas atmosphere at temperatures above 500°C. Here, room temperature refers to a range of 20 to 30°C. The purpose of the primary firing is to adequately remove residual organic matter, such as solvents and alkoxy groups, from the infusible fiber, allowing for the production of heat-resistant zirconia fiber in the fifth firing step. Therefore, depending on the precursor composition after the infusible firing step (step 3), dry air may be used if no carbon is required. Alternatively, a non-oxidizing atmosphere such as dry nitrogen gas may be used if a trace amount of carbon is desired. Furthermore, to control the crystal growth mechanism during the primary firing, firing in an inert gas atmosphere containing water at saturated vapor pressure at room temperature is effective for achieving high strength. The primary firing temperature may be up to 1000°C, the temperature at which ceramicization is completed, i.e., the temperature at which mass loss due to firing is almost eliminated. A heating rate of 10 to 200°C / hour is preferred, since too fast a rate of heating can prevent the release of residual organic matter from the fiber, resulting in the generation of bubbles or the formation of an inhomogeneous structure. Furthermore, in the case of precursors containing Al and Si components, whiskers are generated at 1300° C. or higher in a nitrogen gas atmosphere, resulting in a decrease in strength, so that an Ar gas atmosphere is preferred as the non-oxidizing atmosphere.

[0029] In the fifth step of the present invention, crystallization is further promoted to obtain a heat-resistant, high-strength fiber. The fiber obtained in the fourth step is heated to 1000 to 1600°C at a temperature increase rate of 500 to 20,000°C / hour in an atmosphere of air, nitrogen gas, argon gas, or the like to promote crystallization and proceed with sintering, thereby obtaining a stabilized zirconia fiber or a partially stabilized zirconia fiber. The fiber obtained in the fourth step may contain carbon. If a lower carbon content is desirable for the application, the fiber can be heated in air to 1000-1600°C and sintered while oxidizing and removing the carbon, yielding a sinterable stabilized zirconia fiber or partially stabilized zirconia fiber with a low carbon content. A heating rate of less than 500°C / hour is not preferable, as excessive crystallization will occur, resulting in a low-strength fiber. The oxidizing firing atmosphere may be oxygen or dry air.

[0030] The above manufacturing method can produce dense stabilized zirconia fibers or partially stabilized zirconia fibers with a fiber diameter of 5 to 20 μm and small crystallite size. Furthermore, fibers with added carbon or SiC are flexible and easy to process into woven fabrics. After processing or after making into a composite material, these can be oxidized and removed or converted to SiO2 to produce fibers that are stable in an oxidizing atmosphere. The reason for this is presumed to be as follows, but is not necessarily limited to this presumption.

[0031] As shown in the examples, the fibers obtained in the fifth step of the present invention differ from conventional zirconia fibers in that they contain approximately 1 to 6 mass% carbon and metal oxides or SiC. This effect is evident from the powder X-ray diffraction results, which show that even after firing at 1600°C, the crystallite size is very small, at 10 nm or less, as shown in Figure 1. Therefore, the addition of metal oxides inhibits zirconia crystal growth. Furthermore, the presence of residual carbon or carbides such as SiC is presumably responsible for the uneven distribution of carbon and SiC at zirconia grain boundaries, as with metal oxides, which inhibits zirconia crystallization and, as a result, reduces defects that cause strength loss associated with high-temperature crystal growth. Furthermore, the presence of carbon and SiC is presumably responsible for somewhat lowering the elastic modulus of the fibers, making them easier to weave. The strength derived from this structure is presumably maintained even during secondary firing in an oxidizing atmosphere, i.e., firing in air. It is therefore presumed that strength loss can be minimized even when further firing at high temperatures in an oxidizing atmosphere promotes crystallization somewhat.

[0032] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Example 1 In the first step, a predetermined amount of ethyl 3-oxobutanoate was mixed with a predetermined amount of zirconium tetra-n-butoxide. After the exothermic reaction was complete, a predetermined amount of H2O was diluted with 2-propanol and mixed. After the exothermic reaction was complete, a 2-propanol solution of yttrium tri-n-butoxide or erbium acetylacetonate was added in a calculated amount to achieve a predetermined mole percentage of Y2O3 or Er2O3 relative to ZrO2, and the mixture was stirred for 15 minutes. Next, predetermined amounts of aluminum ethyl acetoacetate diisopropylate, tetraethoxysilane, and tetra-i-propoxytitanium, which are raw alkoxides of metal oxides used as crystallization inhibitors, were added, and a predetermined amount of H2O was diluted with 2-propanol and mixed. After the exothermic reaction was completed, the reaction was continued by gradually heating from room temperature in a water bath using a rotary evaporator to remove low-boiling components such as the solvent and the hydrolysis product 2-propanol, and the mixture was concentrated to 95°C to obtain yttria- or erbia-stabilized zirconia fiber precursor.

[0033] In the second step, the precursor obtained in the first step was put into a spinning tube as a xylene solution of a predetermined concentration, optionally with the addition of polycarbosilane (PCS). The spinning tube was pressurized to approximately 2.5 MPa or less with nitrogen gas at a temperature at which the precursor solution could be extruded with an appropriate viscosity, and the precursor was extruded from a nozzle with a diameter of 100 μm and continuously wound up at a speed of approximately 1,800 m / min. In the third step, the resulting raw fiber was heated in saturated steam from 45 to 200°C at a rate of 10°C / hour and held for 1 hour to make it infusible. The infusibility was confirmed by the fact that it did not melt on a hot plate at 150°C.

[0034] In the fourth step, the infusible fibers were subjected to low-temperature calcination (i.e., primary calcination) by heating to 1000°C at a rate of 100–200°C / hour in air, an inert atmosphere, or an inert gas containing water at saturated vapor pressure at room temperature, and then holding for 1 hour. The inert gas containing water at saturated vapor pressure was prepared by bubbling Ar gas into water at room temperature and then introducing it into a calcination furnace. Fibers calcined in air yielded white fibers, while fibers calcined in an inert atmosphere or an inert gas containing water at saturated vapor pressure at room temperature yielded black fibers, and carbon analysis revealed that they contained less than 6% carbon. The fiber diameters were approximately 6–20 μm.

[0035] Table 1 shows the synthesis conditions (numbers in parentheses indicate the atomic ratio of the added element to Zr) and spinnability values ​​of the precursors obtained in this example. The three-digit number following the "-" of the precursor is the synthesis lot number. There are precursors with the same composition, but the synthesis conditions are different as shown in Table 1, so the different synthesis lots are noted to indicate that they were synthesized under different lots. The notation for synthesis lot numbers is the same in Tables 2 and 3. Table 1 also shows the spinnability value of the SiC fiber precursor polycarbosilane (PCS-UUH) for comparison, which shows that it exhibits an extremely high spinnability value and is therefore industrially viable for spinning.

[0036] [Table 1]

[0037] Table 2 shows the conditions for infusibilizing the spun fibers, the conditions for the primary calcination, and examples of the mechanical properties of the resulting fibers. Infusibilization was performed in a saturated steam atmosphere by heating to a predetermined temperature at a rate of 10°C / hour and holding for one hour. The infusible yarn obtained from the precursor Zr-Y(0.1)·Al(0.1)·Si(0.1)-061 had an average fiber diameter of 18 μm, a tensile strength of 0.32 MPa, and an elongation at break of 0.7%, and was easy to handle.

[0038] The primary firing was then performed in air, an inert atmosphere, or an inert gas containing water at saturated vapor pressure at room temperature, by heating to 1000°C at a temperature increase rate of 100-200°C / hour and holding for 1 hour. The firing conditions were as follows: specified temperature / heating rate (°C / hour) - holding time (HO) + primary firing temperature / heating rate (°C / hour) - holding time (atmosphere), since firing in an atmosphere containing water vapor was switched to an atmosphere without water vapor below the primary firing temperature.

[0039] [Table 2]

[0040] Figure 2 shows the relationship between the curing temperature of dry-spun Zr-Y(0.1)·Al(0.1)·Si(0.01)+PCS(0.1) fibers and the tensile strength (◯) and crystallite size (●) of zirconia fibers obtained by firing at 1000°C in nitrogen gas. This corresponds to precursor No. 5. At curing temperatures above 95°C, the crystallite size is 12-15 nm and the tensile strength is stable at 1.0-1.3 GPa.

[0041] Figure 3 shows the tensile strength of the fibers with Zr-Y(0.1)·Al(0.1)·Si(0.1)-061 precursor, which were heated to 1000°C at a heating rate of 100-200°C / hour and held for 1 hour for the primary firing, as a function of the firing completion temperature (T H2O The figure shows the dependence of the tensile strength of the fibers made from Zr-Y(0.1)·Al(0.1)·Si(0.1)-061 precursor, which were heated to 1000°C at a rate of 100-200°C / hour in an inert gas containing water at saturated vapor pressure at room temperature and held for 1 hour for primary firing. H2O ), and high strength was obtained at 400 to 600°C (Figure 3).

[0042] <Example 2> The fibers obtained in Example 1 were heated to 1200-1400°C at a rate of 500°C / hour in various atmospheres and held at that temperature for 1 hour to obtain secondary sintered fibers. The carbon contents of these fibers after sintering in an inert gas atmosphere or an inert gas atmosphere containing water vapor were 6% by mass or less and 1% by mass or less, respectively, and it was found that the carbon content had almost completely disappeared after sintering in air. XRD of these fibers showed that they were tetragonal or a mixture of tetragonal and cubic crystals, with a crystallite size of 20 nm or less.

[0043] Table 3 shows the calcination conditions and properties of the secondary calcined fibers. [Table 3] Fig. 4 shows the dependence of tensile strength on secondary firing temperature for a fiber (fiber No. 7) that was primarily fired at 1000°C in Ar and then secondary fired in air to a predetermined temperature. When the secondary firing temperatures were 1000°C, 1200°C, 1300°C, and 1400°C, the tensile strength gradually decreased from 1.7 GPa to 1.0 GPa.

[0044] The cross sections of these fibers showed slight grain growth from the fibers obtained by the first firing at 1000°C, but showed a dense structure with almost no defects such as voids. SEM photographs of the cross sections of fiber No. 7 obtained from Zr-Y(0.1)-077+PCS(0.05) after the first firing (1000°C) and the second firing (1300°C) are shown (Figures 5A and 5B). [Industrial Applicability]

[0045] According to the method for producing zirconia continuous fibers of the present invention, by incorporating small amounts of components that stabilize the crystalline phase of zirconia and oxides, SiC, or carbon that inhibit crystal growth into the zirconia continuous fibers, it is possible to suppress the deterioration of the mechanical properties of the fibers due to crystallization at high temperatures, and to obtain fibers that are resistant to a decrease in mechanical strength even at high temperatures. These zirconia continuous fibers can maintain their strength even when the carbon is burned off during the production of zirconia fiber-reinforced oxide ceramic composite materials with an oxide matrix, and are therefore suitable as reinforcing fibers for oxide-based composite materials. According to the method for producing a precursor of a continuous zirconia fiber of the present invention, a precursor suitable for producing the above-mentioned continuous zirconia fiber can be easily obtained.

Claims

1. a partial co-hydrolysis condensation with a zirconium alkoxide in the presence of a first metal alkoxide that produces an oxide component that forms partially stabilized zirconia or stabilized zirconia, and a second metal alkoxide that produces an oxide component that inhibits crystal growth, to form a precursor that is excellent in continuous spinnability; A method for producing a precursor to continuous zirconia fibers.

2. a first metal alkoxide that produces an oxide component that forms the partially stabilized zirconia or stabilized zirconia, and a second metal alkoxide that produces an oxide component that inhibits crystal growth, and the first metal alkoxide and the second metal alkoxide are subjected to partial co-hydrolysis and condensation to form a precursor that is excellent in continuous spinnability; Further, polycarbosilane that generates SiC is added to the precursor to form a precursor that is excellent in continuous spinnability. The method for producing a precursor of the zirconia continuous fiber according to claim 1.

3. In the precursor formation step, the oxide component that inhibits crystal growth is alumina (Al ２ O ３ ), silica (SiO ２ ), titania (TiO ２ ), boron oxide (B ２ O ３ 2. The method for producing a precursor of a zirconia continuous fiber according to claim 1, wherein the precursor is one or more selected from the group consisting of:

4. In the precursor formation step, the amount of oxide component that inhibits the crystal growth is mixed in the amount of alumina (Al ２ O ３ ), silica (SiO ２ ), titania (TiO ２ ), boron oxide (B ２ O ３ 4. The method for producing a precursor of a continuous zirconia fiber according to claim 3, wherein the content of at least one selected from the group consisting of:

5. In the precursor formation step, the amount of polycarbosilane to be mixed to generate SiC is determined based on the amount of ZrO contained in the precursor in terms of SiC. ２ 3. The method for producing a precursor of zirconia continuous fiber according to claim 2, wherein the content of the zirconia continuous fiber is 0.1 mol % or more and 20 mol % or less.

6. 3. The method for producing a zirconia continuous fiber precursor according to claim 2, wherein the ratio of the number of Si atoms contained in the polycarbosilane to the number of Zr atoms contained in the zirconia fiber precursor obtained by partial hydrolysis in the coexistence of the first metal alkoxide and the second metal alkoxide is 0.001 or more and 0.1 or less.

7. 7. The method for producing a zirconia continuous fiber precursor according to claim 6, wherein the ratio of the number of Si atoms contained in the polycarbosilane to the number of Zr atoms contained in the zirconia fiber precursor obtained by partially hydrolyzing the first metal alkoxide is 0.001 or more and 0.1 or less.

8. In the precursor formation step, the oxide component forming the partially stabilized zirconia or stabilized zirconia is yttria (Y ２ O ３ ), ceria (Ce ２ O ３ ), Ervia (Er ２ O ３ 2. The method for producing a precursor of a zirconia continuous fiber according to claim 1, wherein the precursor is one or more selected from the group consisting of:

9. In the precursor formation step, the amount of oxide components to be mixed to form the partially stabilized zirconia or stabilized zirconia is set to be ZrO ２ 9. The method for producing a precursor of a zirconia continuous fiber according to claim 8, wherein the content of the zirconia continuous fiber is 2 mol % or more and 12 mol % or less.

10. The precursor formed in the precursor formation step has a spinnability value defined by the following formula:

2. The method for producing a zirconia continuous fiber precursor according to claim 1, wherein the value is at least twice that of the polycarbosilane of the SiC fiber precursor. Spinnability value = winding drum rotation speed (rpm) × continuous winding time (min) ÷ extrusion pressure (MPa)

11. carrying out partial co-hydrolysis and condensation with a zirconium alkoxide in the presence of a first metal alkoxide that produces partially stabilized zirconia or an oxide component that forms stabilized zirconia, to form a precursor that is excellent in continuous spinnability; Further, polycarbosilane that generates SiC is added to the precursor to form a precursor that is excellent in continuous spinnability. A method for producing a precursor to continuous zirconia fibers.

12. In the precursor formation step, the amount of polycarbosilane to be mixed to generate SiC is determined based on the amount of ZrO contained in the precursor in terms of SiC. ２ 12. The method for producing a precursor of a zirconia continuous fiber according to claim 11, wherein the content of the zirconia continuous fiber is 0.1 mol % or more and 20 mol % or less.

13. In the precursor formation step, the oxide component forming the partially stabilized zirconia or stabilized zirconia is yttria (Y ２ O ３ ), ceria (Ce ２ O ３ ), Ervia (Er ２ O ３ 12. The method for producing a precursor of a zirconia continuous fiber according to claim 11, wherein the precursor is one or more selected from the group consisting of:

14. In the precursor formation step, the amount of oxide components to be mixed to form the partially stabilized zirconia or stabilized zirconia is set to be ZrO ２ 14. The method for producing a precursor of a zirconia continuous fiber according to claim 13, wherein the content of the zirconia continuous fiber is 2 mol % or more and 12 mol % or less.

15. The precursor formed in the precursor formation step has a spinnability value defined by the following formula: The method for producing a zirconia continuous fiber precursor according to claim 11, wherein the value is at least twice that of the polycarbosilane of the SiC fiber precursor. Spinnability value = winding drum rotation speed (rpm) × continuous winding time (min) ÷ extrusion pressure (MPa)

16. A first step of preparing a precursor formed by the method for producing a zirconia continuous fiber precursor according to any one of claims 1 to 15; a second step of continuously spinning the precursor at high speed by a dry spinning method; a third step of infusibilizing the mixture in a controlled water vapor atmosphere; a fourth step of primary firing in air, an inert atmosphere, or an inert gas containing water at saturated vapor pressure at room temperature; a fifth step of converting the zirconia-based fiber into a highly crystallized fiber by secondary firing at a higher temperature in the air, in an inert atmosphere, or in an inert gas containing water at saturated vapor pressure at room temperature; A method for producing zirconia continuous fibers comprising:

17. 17. The method for producing a zirconia continuous fiber according to claim 16, wherein in the second step, the solvent of the solution for dry spinning of the precursor is xylene, and the concentration is 75 to 85 mass %.

18. The method for producing a zirconia continuous fiber according to claim 16, characterized in that in the third step, the precursor fiber is infusible in a saturated steam atmosphere by raising the temperature from 95°C to 200°C at a rate of 5 to 100°C / hour over a predetermined period of time and maintaining the temperature at that temperature for 0.5 hours or more.

19. 17. The method for producing a zirconia continuous fiber according to claim 16, wherein in the fourth step, the infusible precursor fiber is heated to 1,000°C at a temperature increase rate of 10 to 200°C / hour in the air, an inert atmosphere, or an inert gas containing water at saturated vapor pressure at room temperature, to convert it into a high-strength, low-elasticity fiber.

20. 17. The method for producing a zirconia-based continuous fiber according to claim 16, wherein the fiber diameter obtained in the fourth step is 6 μm or more and 20 μm or less.

21. 17. The method for producing a zirconia continuous fiber according to claim 16, wherein in the fifth step, the fiber obtained in the fourth step is heated to 1000 to 1600°C at a temperature increase rate of 500 to 1000°C / hour in the air, an inert atmosphere, or an inert gas containing water at saturated vapor pressure at room temperature, to convert it into a highly crystallized zirconia fiber with high strength and high elastic modulus.

22. 22. The method for producing a zirconia-based continuous fiber according to claim 21, wherein the fiber diameter obtained in the fifth step is 6 μm or more and 20 μm or less.

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