ZrO2-reinforced mullite fiber, process for its manufacture, and its use

Abstract

The present invention relates to ZrO2-reinforced mullite fibers having a content of at least 0.1 wt.% crystalline ZrO2, which are distinguished by significantly improved mechanical properties compared to unmodified mullite fibers. The invention further relates to a process for producing such fibers, green fibers produced as an intermediate product in the process, and the use of ZrO2-reinforced mullite fibers in fiber-matrix composites.

Description

[Technical Field]

[0001] The present invention relates to ZrO2-reinforced mullite fibers having a crystalline ZrO2 content of at least 0.1% by weight, wherein the mullite fibers are distinguished by significantly improved mechanical properties compared to unmodified mullite fibers. The present invention further relates to a process for producing such fibers, green fibers produced as intermediate products in the process, and the use of ZrO2-reinforced mullite fibers in fiber-matrix composite materials. [Background technology]

[0002] Mullite is a mineral that rarely occurs naturally and is formed under high temperatures and low pressures. It is the only stable Al2O3·SiO2 binary compound. In technical applications, mullite is one of the most important phases in ceramics due to its advantageous properties, and has a wide range of applications. These properties include, among others, high thermal stability (up to temperatures exceeding 1700°C), low thermal expansion and thermal conductivity, low density, and strength and fracture toughness suitable for many applications. Mullite ceramics also exhibit outstanding low creep rate and good corrosion resistance. Traditionally, mullite ceramics have been used for porcelain and stoneware, and are also used as refractory materials in the iron, cement, and chemical industries.

[0003] In addition to the advantages mentioned above, a major advantage of mullite is the fact that the starting materials from which mullite can be obtained are available in large quantities at a relatively low cost, and for this reason, mullite has become increasingly important in the field of high-performance ceramics in recent years. The increasing use of mullite is particularly related to monolithic ceramics, and its typical applications include, among others, refractory materials and catalyst supports, films and coatings to prevent material degradation in oxidizing atmospheres, and mullite fibers that can be used on their own or in ceramic composite materials.

[0004] Document EP2173683A1 describes, among other things, the use of mullite fibers to form solid, highly porous structures for filtration, thermal insulation, and high-temperature processes, as well as for chemical reactions. DE102008004532A1 relates to mullite fibers having nanoscale particle sizes up to 200 nm for use in ceramic composite materials.

[0005] Mullite fibers, which have been commercially available for some time, also contain a certain proportion of corundum particles dispersed within the mullite and are available under the trade name Nextel 720®. These fibers have very high creep resistance and room temperature tensile strength, but show a significant decrease in Weibull strength at higher temperatures. Pure mullite fibers with a composition close to the stoichiometric 3:2 mullite exhibit lower absolute Weibull strength, but the decrease in Weibull strength at temperatures above 1000°C is far less.

[0006] Zirconium dioxide (ZrO2) is used in the field of refractories and as a functional material. ZrO2 exists in three structural transformations: the monoclinic transformation (m-ZrO2) is converted to the tetragonal transformation (t-ZrO2) at approximately 1170°C, and the cubic transformation (c-ZrO2) is formed from the tetragonal transformation at approximately 2370°C.

[0007] ZrO2 stands out for its low thermal conductivity of 2 W / m·K at 1000°C (Al2O3 has a thermal conductivity of 7 W / m·K at the same temperature). Further advantageous properties of ZrO2 include its high melting point in the range of 2680-2710°C, as well as its extremely high hardness and wear resistance. For this reason, ZrO2 ceramics belong to the oxide ceramics with the highest hardness and have been synthesized to date.

[0008] The problem with using ZrO2 in ceramics is that such ceramics often exhibit very undesirable fracture behavior, which arises from the fact that ZrO2 undergoes a 3-5% volume expansion when transitioning from tetragonal to monoclinic transformation. This transformation usually occurs at temperatures below approximately 950°C.

[0009] To counteract these conversion processes, various additives and stabilizers, such as Y2O3, have been proposed in the literature on the manufacture of mullite ceramics (see S. Prusty et al., Adv. Appl. Ceram. 2013, pp. 110, 360-366). In addition, Li et al., in J. Ceram. Sci Technol. 2016, 7, pp. 417-422, describe a discharge sintering process that was able to incorporate 1-4 wt% ZrO2 into mullite ceramics. However, the most uniform distribution of ZrO2 particles possible in the mullite structure inhibits grain growth and cannot be achieved with the rod-shaped ZrO2 investigated in this study; therefore, such a process would not be suitable for the manufacture of ceramic fibers. [Overview of the project] [Problems that the invention aims to solve]

[0010] Given the background of the prior art presented above, there is a need for ceramic fibers, particularly mullite fibers, that have an improved strength profile compared to known mullite fibers, exhibit higher Weibull strength compared to pure mullite fibers, and do not show a significant decrease in Weibull strength at high temperatures compared to the commercially available fibers described. The present invention addresses this need. [Means for solving the problem]

[0011] This invention is based on the remarkable finding that by including ZrO2 in the fibers, the desired improvements in the breaking load and elongation at break of mullite fibers can be achieved without any degradation of other properties. Such fibers can be produced by mixing the necessary starting materials for the fibers, then spinning them to produce green fibers, firing the fibers, and sintering them, thereby obtaining crystalline ZrO2 in at least a proportional amount.

[0012] According to a first aspect, the present invention relates to a ZrO2-reinforced mullite fiber containing at least 0.1% by weight of crystalline ZrO2. [Brief explanation of the drawing]

[0013] [Figure 1] This is an X-ray diffraction pattern of a fiber. [Modes for carrying out the invention]

[0014] manufacturing After the process, synthetic mullite with a composition of 3Al2O3×2SiO2 is called sintered mullite, and mullite with a composition of 2Al2O3×1SiO2 is called molten mullite. The term "mullite" as defined herein includes both deformities. Regarding the ratios stated, it is clear that there is no free Al2O3 or SiO2 in mullite, but these ratios are specified here to provide a simplified description of the ratio of Al2O3 or SiO2 in mullite. These ratios are described as molar ratios.

[0015] The term "crystalline ZrO2" specifies that at least a certain proportion of ZrO2 is present in a crystalline form within the fiber, but does not exclude the presence of amorphous or non-crystalline proportions of ZrO2 within the mullite fiber. The presence of crystalline ZrO2 can be demonstrated by characteristic reflections in X-ray diffraction patterns.

[0016] Regarding the content rate of crystalline ZrO₂, the present invention is not subject to any relevant limitations. On the one hand, the content rate of crystalline ZrO₂ should be at a preferably high level for the strengthening effect. However, on the other hand, the proportion of ZrO₂ should not be so large as to be harmful to the overall properties of the fibers. Here, a proportion of at least 0.5 wt%, preferably at least 1 wt%, more preferably at least 2 wt%, even more preferably at least 3.1 wt%, and even more preferably at least 3.5 wt% is described as preferable for achieving good strengthening properties. The maximum content rate of ZrO₂ should preferably not exceed 15 wt% here, and in particularly preferred embodiments, it should not exceed 10 wt%. Particularly preferred strengthening properties are achieved by a ZrO₂ content rate within the range of approximately 2 to 15 wt% of crystalline ZrO₂, particularly approximately 3 to 10 wt%, and particularly preferably approximately 3.5 to 10 wt%. Therefore, within the context of the present invention, these are particularly preferred.

[0017] Within the tests forming the basis of the present invention, it has been found that ZrO₂ exists in the fibers mainly as tetragonal ZrO₂ at temperatures below 950 °C, which is related to the conversion to monoclinic ZrO₂. Therefore, within the scope of the present invention, it is preferable for ZrO₂ -reinforced mullite fibers to contain ZrO₂ in the tetragonal transformation.

[0018] The particle size of ZrO₂ in ZrO₂ -reinforced mullite fibers is somewhat important because, with a large particle size, the tetragonal transformation of ZrO₂ formed unstably is not sufficiently stable. A range of approximately 10 to 60 nm, particularly approximately 20 to 35 nm, can be specified here as a particularly suitable diameter length for the crystalline ZrO₂ particle size. The particle size is determined graphically using a microscopic image of the thermal-etched cross-section surface by the line intercept method according to DIN EN ISO 13383-1:2016-11. Based on the determined particle size, it is advantageously clear that there is a uniform distribution of ZrO₂ in the mullite fibers, which is preferable for high-strength fibers. The average particle size of ZrO₂ in ZrO₂ -reinforced mullite fibers is preferably within the range specified above.

[0019] In the mullite fiber described, mullite occupies the largest proportion of the components. In this context, mullite in the mullite fiber, In other words the proportion of crystalline mullite preferably occupies at least 80% by weight, particularly at least 85% by weight, and particularly preferably 90 - 97.5% by weight. At such a proportion, it is ensured that the overall properties of the resulting fiber are as similar as possible to the corresponding properties of pure mullite fiber.

[0020] The ratio of the ceramic-forming Al2O3 precursor to the ceramic-forming SiO2 precursor forming mullite should be appropriately set so that the maximum possible proportion of mullite can be formed in the fiber when the precursors are processed to form the fiber. Here, with respect to the total amount of Al2O3 and SiO2, it is preferable that the mullite fiber contains a theoretical Al2O3 proportion of approximately 71 - 80% by weight, particularly approximately 73 - 78% by weight, and particularly preferably approximately 75 - 78% by weight within the mullite phase. In this case of this proportion, since it is a reference to the mullite phase, additional ZrO2 or further ceramic components present are not considered.

[0021] The particle size of mullite in the ZrO2 - reinforced mullite fiber is suitably in the range of 50 - 200 nm, particularly approximately 80 - 150 nm. The particle size of mullite is determined graphically here, in the same way as the determination of the particle size of ZrO2, using a microscopic image of the thermally etched cross-section surface by the line intercept method according to DIN EN ISO 13383 - 1:2016 - 11. Mullite with a relatively small particle size brings about the high strength found in the ZrO2 - reinforced mullite fiber according to the present invention, and this strength is impaired by larger mullite grains. The average particle size of mullite in the ZrO2 - reinforced mullite fiber is preferably within the range specified above.

[0022] In addition to mullite and ZrO2, the ZrO2-reinforced mullite fibers according to the present invention may contain further inorganic components and residual phases, particularly in the form of crystalline Al2O3, which can be formed from excess ceramic-forming Al2O3 precursors during the production of the fibers. In one embodiment, such residual phases, particularly those formed of corundum, are preferred according to the present invention because they can thus further improve the mechanical properties of the mullite fibers.

[0023] In addition to mullite and crystalline ZrO2, further materials that can be contained in the mullite fibers according to the present invention include, for example, Y2O3, Yb2O3, HfO2, CeO2, and other transition metal oxides or lanthanide oxides.

[0024] The ZrO2-reinforced mullite fibers according to the present invention preferably exist in the form of filaments (i.e., fibers of practically unlimited length, which are used synonymously here), also referred to as endless filaments or endless fibers. In addition, the fibers preferably have a diameter greater than 5 μm so as to provide the desired stability. Fiber diameters in the range of 7 to 13 μm are very particularly preferred.

[0025] The ZrO2-reinforced mullite fibers according to the present invention are distinguished from known mullite fibers by improved breaking load and maximum elongation in bending tests. As a result, the ZrO2-reinforced mullite fibers according to the present invention preferably have a breaking load of at least 12.0 N, particularly at least 14.0 N, particularly preferably at least 15.0 N, determined at a fixed deflection section with a diameter of 2.5 mm at a test speed of 5 mm / min in a fiber bundle of 468 filaments having a thickness of approximately 10 μm. In addition or alternatively, the fibers have a breaking elongation of at least 1.5%, particularly at least 1.6%, particularly preferably 2.0%. For further details regarding the determination of breaking load and breaking elongation, please refer to the information in the Examples section.

[0026] As described above, the ZrO2-reinforced mullite fibers according to the present invention can be manufactured by mixing the necessary starting materials for the fibers, then spinning them to produce green fibers, firing the fibers, and sintering them. As a result, a further aspect of the invention described herein is a process for manufacturing ZrO2-reinforced mullite fibers, the following steps: (i) A step of producing a spinning solution from a ceramic-forming SiO2 precursor, a ceramic-forming Al2O3 precursor, a water-soluble organic polymer, a ceramic-forming ZrO2 precursor, and optionally a stabilizer, wherein the use of a stabilizer preferably results in a solution having a ceramic-forming ZrO2 precursor. (ii) In order to produce a spindle having a zero shear rate viscosity of at least 150 Pa·s, particularly in the range of 180 to 350 Pa·s (determined in each case at 25°C), the step of partially evaporating the water in the spinning solution, if necessary. (iii) Dry spinning of the spindle to obtain green fibers, dry spinning step, (iv) A step of calcining green fibers to form inorganic precursor fibers, wherein volatile components are removed by thermal decomposition, and (v) A step of sintering inorganic precursor fibers while forming a mullite phase in order to obtain ZrO2-reinforced mullite fibers. Regarding processes that include this.

[0027] In the first step (i), a spinning solution is produced that contains all the components of the subsequent mullite fibers, one or more additional water-soluble polymers to temporarily stabilize the green fibers, and optionally a stabilizer to stabilize the ceramic-forming ZrO2 precursor in the spinning solution. Due to the calcination performed in step (iv), the inorganic and organic components initially contained in the green fibers, such as polymers in particular, are removed by thermal decomposition, i.e., converted into gaseous decomposition products, and are no longer contained in the resulting fibers.

[0028] To stabilize the ceramic-forming ZrO2 precursor, it is advantageous to add a stabilizer to the solution of the ceramic-forming ZrO2 precursor, or to mix a stabilizer solution into the solution of the ceramic-forming ZrO2 precursor.

[0029] Step (ii) is optional; for example, if the spinning solution produced in step (i) already has the zero shear rate viscosity described in (ii), this step can be omitted. The purpose of step (ii) is to adjust the viscosity to suit dry spinning. In the context of the present invention as described herein, the zero shear rate viscosity is determined using a plate-plate geometry having a plate diameter of 25 mm and a gap distance of 0.5 mm at 25°C. For this purpose, a Physica MCR 301 rheometer by Anton Paar can be used.

[0030] Step (i) of the process described above can be carried out as a one-pot process, and the spinning solution can be produced from the components described by mixing the components with water. In particular, when a stabilizer for ZrO2 is incorporated into the spinning solution, it is appropriate to prepare a pre-solution of the stabilizer and the ceramic-forming ZrO2 precursor, and then mix this with the other components of the spinning solution.

[0031] Generally, it is appropriate to first prepare a mixture of the inorganic components of the subsequent spinning solution and, if applicable, an optional stabilizer, and then add a water-soluble organic polymer or an aqueous solution thereof to this mixture. In a very particularly preferred embodiment, the spinning solution is prepared in step (i) as a substep: (ia) Converting the ceramic-forming SiO2 precursor and the ceramic-forming Al2O3 precursor into an aqueous mullite precursor solution by adding water, and converting the ceramic-forming ZrO2 precursor into an aqueous ZrO2 precursor solution by adding water and optionally a stabilizer. (ib) Combine the aqueous mullite precursor solution produced according to step (ia) and the ZrO2 precursor solution produced according to step (ia) to form a ZrO2 mullite precursor solution. (ic) Add the water-soluble organic polymer to the ZrO2 mullite precursor solution formed in step (ib), thereby forming an aqueous spinning solution. It is generated by [the following method / system].

[0032] To aid crystallization, mullite seed crystals can be added, for example, in the form of an aqueous suspension, in step (i) or in one of the steps (ia) to (ic) of the process.

[0033] The ceramic-forming ZrO2 precursor is not subject to any relevant limitations, provided that, within the scope of the present invention, it should be possible to convert it to ZrO2 as completely as possible within the scope of processing during the process. Therefore, the ceramic-forming ZrO2 precursor should not contain any components that cannot react to form gaseous products within the scope of this processing (under oxidative conditions). For example, halides can react to form gaseous products by conversion to halogenated hydrogen.

[0034] The ceramic-forming ZrO2 precursor is preferably a Zr(O) O X (4-2o) [In the formula, o = 0 to 2, and X is a halide ion, an organic anion, preferably an alcoholate, or NO3] - The zirconium compound is a zirconium compound that exists as zirconium oxide sol when o=2. Particularly preferred halide ions are Br - and Cl - And among them, Cl - This is very, especially preferable.

[0035] Particularly preferred ceramic-forming ZrO2 precursors are, for example, zirconium dichloride oxide (ZrOCl2), and especially zirconium dichloride oxide octahydrate.

[0036] As described above, a proportion of approximately 2 to 15% by weight of ZrO2, particularly approximately 3 to 10% by weight, and particularly preferably approximately 3.5 to 10% by weight, is suitable for the ZrO2 - reinforced mullite fibers according to the present invention. Correspondingly, in the process described herein, the ZrO2 mullite precursor solution contains the ceramic - forming ZrO2 precursor in an amount corresponding to approximately 2 to 15% by weight, particularly approximately 3 to 10% by weight, and particularly preferably approximately 3.5 to 10% by weight, based on the total amount of the phases of ZrO2, Al2O3, and mullite formed from the precursor solution. Due to different counter - ions or crystallization water contained in the ceramic - forming ZrO2 precursor, the amount of the precursor may, therefore, vary within a certain range for the production of ZrO2 - reinforced mullite fibers. However, after determining for a particular ZrO2 precursor, a person skilled in the art cannot easily adjust this amount so that the desired ZrO2 content is present in the ceramic fibers.

[0037] The water - soluble Al2O3 precursor is also, in the context of the present invention, initially only subject to the limitation of the ability to convert (during firing) into Al2O3 without residue, from which mullite can form in the reaction with SiO2 during sintering. However, here, particularly suitable water - soluble Al2O3 precursors include aluminum salts of the formula Al n (OH) m X (3n-m) [where X is a halide ion, NO3 - or an organic anion, such as an alcoholate, n = 1 or 2, and m = 0 to 5]. Particularly preferred ions X are halide ions, particularly Cl - . A particularly preferred water - soluble Al2O3 precursor is Al2(OH)5Cl, particularly having 2.5 units of H2O.

[0038] In the case of the ZrO2-reinforced mullite fiber according to the present invention, a higher proportion of mullite is preferable. Accordingly, the mullite precursor solution in the process described above preferably contains a water-soluble Al2O3 precursor in an amount corresponding to approximately 71-80% by weight, preferably approximately 73-78% by weight, and particularly approximately 75-78% by weight, relative to the total theoretical proportion of Al2O3 and SiO2 in the mullite phase resulting from the precursor solution. The term "theoretical" here takes into account the fact that Al2O3 and SiO2 are contained in the mullite fiber as mullite, and not as "Al2O3" and "SiO2" themselves.

[0039] Water-soluble SiO2 precursors are, in principle, subject to the same limitations as Al2O3 precursors. Particularly preferred water-soluble SiO2 precursors are colloidal silicon dioxide, or water-soluble or dispersible Si-containing organic compounds, such as silanes, aminosilanes, or orthosilicate esters.

[0040] To produce fibers with a high proportion of mullite, it is preferable that the mullite precursor solution contains a water-soluble SiO2 precursor in an amount corresponding to approximately 20-28% by weight, particularly approximately 22-27% by weight, and especially preferably approximately 23-25% by weight, relative to the total theoretical proportion of Al2O3 and SiO2 in the mullite phase formed from the precursor solution. The same applies to the term "theoretical" as to the corresponding Al2O3 proportion.

[0041] In the process described herein, the water-soluble organic polymer plays, above all, a role as a temporary binder for the inorganic components in the initially formed green fibers, and as a means of increasing viscosity for spinning. Since the polymer is no longer contained in the ceramic fibers, the structure of the polymer itself is not essential to the process. For example, poly(vinylpyrrolidone), poly(vinyl alcohol), and / or poly(ethylene oxide) can be used as particularly suitable water-soluble organic polymers, with poly(vinylpyrrolidone) being particularly preferred. Furthermore, the polymer helps to avoid the formation of gels, which are undesirable for spinning.

[0042] The amount of water-soluble organic polymer incorporated into the spinning solution should preferably be selected so that the desired viscosity is achieved and sufficient stabilization of the green fibers until further processing is ensured. Therefore, excessively high proportions should be avoided for cost reasons and because higher proportions of polymer result in greater fiber shrinkage during firing. The weight ratio of polymer to total oxide contents in the spinning solution can be specified as preferably between 20:80 and 40:60, and particularly between 25:75 and 30:70.

[0043] By selecting the appropriate molecular weight of the water-soluble polymer, advantageous control of the viscosity and stabilization of the green fibers can be achieved. Here, the molecular weight M is less than 200,000 g / mol, preferably in the range of approximately 20,000 to 70,000 g / mol. w It is particularly appropriate to use polymers having the following characteristics. The use of mixtures of water-soluble polymers having different molecular weights is particularly advantageous, especially those with molecular weights less than 200,000 g / mol, particularly in the range of approximately 20,000 to 70,000 g / mol. w A first polymer having and a molecular weight M greater than 1,000,000 g / mol w A second polymer having the following properties is used. Particularly preferred second polymers have a molecular weight M in the range of 1,100,000 to 1,500,000 g / mol. wIt has the following properties. The first polymer and optionally the second polymer are particularly preferred materials, and poly(vinylpyrrolidone). Various molecular weights are determined here by GPC using a preferred standard (e.g., poly(styrene)).

[0044] Stabilizers for the ZrO2 precursor, which are preferably used in spinning solutions and aqueous ZrO2 precursor solutions, are appropriately carboxylic acids, preferably selected from the group including glycine, serine, cysteine, oxalic acid, malonic acid, glutaric acid, adipic acid, acetic acid, glycolic acid, lactic acid, tartaric acid, and / or citric acid. Glycine and serine are particularly preferred stabilizers.

[0045] The calcination in step (iv) of the process described above is carried out at a temperature in which each of the precursors of ZrO2, Al2O3, and SiO2 is converted to its respective oxide, but the mullite that results in the final effect has not yet formed. In addition, volatile components (i.e., polymers and any stabilizers present) are removed by thermal decomposition within the range of calcination. The temperature favorable for conversion is therefore the preferred temperature for step (iv), which is in the range of approximately 700–1000°C, and particularly approximately 850–900°C.

[0046] Since calcination and the removal of the most complete volatile components by thermal decomposition generally take some time, 240–600 minutes, particularly 300–400 minutes, is specified as a suitable period for this step. A continuous heating furnace can be appropriately used for calcination.

[0047] Sintering and the subsequent conversion of inorganic precursor fibers to mullite fibers occur in a tubular furnace at a higher temperature than calcination, preferably in the range of approximately 1200-1600°C, and particularly in the range of approximately 1300-1500°C. Since this conversion generally proceeds very rapidly, exposure of the inorganic precursor fibers to these conditions for a period of preferably 60-400 seconds, and particularly 180-300 seconds, is usually sufficient.

[0048] In tests forming the basis of the invention described herein, the ZrO2-reinforced mullite fibers produced by the process described above were preferably found to exhibit outstanding uniform dispersion of the ZrO2 phase within the fibers, which was also observed at higher proportions of ZrO2 (particularly 3% by weight or more). Simultaneously, these ZrO2-reinforced mullite fibers possessed advantageous mechanical properties, such as particularly high breaking load and breaking elongation. It is clear that both observed values ​​are related. Consequently, further aspects of the invention described herein relate to ZrO2-reinforced mullite fibers that can be produced, or have been produced, according to the process described above.

[0049] In addition, the process described above offers the advantage that mullite is formed only at higher temperatures above 1200°C (compared to production from molecularly dispersed precursor systems). This means that creep-resistant mullite is formed after the material has been compressed more significantly. In contrast, if mullite is already formed at lower temperatures (e.g., around 1000°C), it forms earlier and then resists the compaction of the material, which usually leads to the formation of pores and the resulting undesirable mechanical properties.

[0050] In a further embodiment, the present invention relates to ceramic-forming green fibers in the form of filaments obtained or obtained according to process steps (i) to (iii) of the process described above. The term "ceramic-forming" here indicates that the green fibers are suitable for subsequent conversion into ceramic fibers.

[0051] In further embodiments, the present invention relates to the use of the ZrO2-reinforced mullite fibers described above in fiber matrix composite materials, i.e., materials in which fibers are incorporated into a surrounding matrix material. In particular, polymers, metals, or ceramic materials are the potential matrix materials, but metals and ceramic materials are preferred from the viewpoint of high temperature resistance. The ZrO2-reinforced mullite fibers according to the present invention can provide fiber matrix composite materials having improved strength, stiffness, and / or high temperature stability. This use generally involves the immersion of ZrO2-reinforced mullite fibers or woven fabrics produced therefrom using a liquid or liquefied matrix material or a precursor thereof, and the coagulation of the mixture.

[0052] The present invention will be described in more detail below based on examples, but these are intended for illustrative purposes only and should not be construed as limiting the scope of protection of this application. [Examples]

[0053] The mechanical properties of the fibers produced in the following examples were determined as follows.

[0054] The tensile strength and modulus of the fibers were determined using a Textechno Favimat. For this purpose, individual filaments with a clamp length of 25 mm were placed between two draw-off clamps, and their resonance frequencies were measured after reaching a pre-load of 0.25 cN / tex. For this purpose, a vibration process incorporated into the test apparatus was used, in which the linear density of the fiber was calculated using the resonance frequency, and then the fiber diameter was determined. Each filament was loaded at a test speed of 1 mm / min. The tensile tests were evaluated using software belonging to the measuring apparatus. At least 30 valid measurements were used for evaluation. The tensile strength was determined from the arithmetic mean of the individual tensile strengths, and the Weibull strength and Weibull coefficient were calculated.

[0055] Bending tests were performed on a Zwick / Roell general-purpose testing machine (Z010) using sized fiber bundles containing 468 filaments. The software testXpert® II was used for evaluation. Fiber bundles were bent using fixed flexures with specified diameters of 2.0, 2.25, and 2.5 mm and secured to a holder. The clamp length was 10 cm, and the initial force was 40 cN. The fiber bundles were stretched at a test speed of 5 mm / min, and the maximum breaking load and elongation at break were measured at the flexure. The average values ​​of breaking load and elongation were formed from at least 15 measurements in each case. [Examples]

[0056] Fiber production by winding test Production of spinning solution Locron L®, a 50 wt% aqueous solution of Al2(OH)5Cl·2.5H2O as an Al2O3 precursor, and Levasil®, a 30 wt% aqueous solution of SiO2 particles as an SiO2 precursor, were placed in glass beakers. Furthermore, a 5 wt% aqueous mullite species suspension at 6 wt% relative to the amount of Locron L® was added. Different amounts of zirconium dichloride oxide octahydrate (ZrOCl2·8H2O) were dissolved in deionized water in a second glass beaker with stirring, and optionally, a stabilizer up to 1 equivalent relative to the amount of zirconium dichloride oxide octahydrate was added. The solution was stirred for several minutes to ensure that the stabilizer was fully dissolved. The precursor solutions were then combined. Low molecular weight PVP K30 (Sigma Aldrich) was combined with high molecular weight PVP (Sigma Aldrich, M w Along with approximately 1,300,000, it was added while stirring in a ratio of 95:5% by weight. The spinning solution was continued to stir at room temperature using a KPG stirrer until a homogeneous spinning solution was obtained.

[0057] For fiber production, the amounts of Al2O3 and SiO2 precursors were set so that a molar ratio of 3:2 Al2O3 / SiO2 was established during processing. The molar ratio of oxide to polymer in the spindle was 70:30.

[0058] Formation of a spindle The resulting spinning solution was concentrated in a rotary evaporator under reduced pressure and rotated slowly until a viscous spinning mass was obtained at room temperature (zero shear rate viscosity η0 of 222 Pa·s at 25°C, determined using an Anton Paar Physica MCR 301 rheometer with a plate-plate geometry having a plate diameter of 25 mm and a gap distance of 0.5 mm).

[0059] The resulting spun yarn was then tested in a winding test. For this purpose, a few grams of spun yarn were placed on a watch glass. The yarn, pulled vertically upward from the spun yarn, was placed on a spool driven by a winding machine. The distance between the spun yarn and the spool core was 82 cm. The maximum possible winding speed and the time until the yarn broke were determined.

[0060] The results of these decisions, as well as the ZrO2 percentages of various spindles, are reproduced in Table 1.

[0061] [Table 1] Table 1 shows that satisfactory winding speeds can be achieved at various ZrO2 concentrations. Further improvements in winding speeds can be achieved, particularly at very high ZrO2 precursor concentrations, and even compared to lower concentrations, by adding glycine. [Examples]

[0062] Formation of spinning solution and spinning mass Similar to Example 1, spindles were produced with amounts of Al2O3 and SiO2 precursors set during processing to establish a molar ratio of 3:2 Al2O3 / SiO2. The weight ratio of oxides to polymers in the spindles was 70:30, and the proportion of ZrO2 to Al and Si oxides was 3% by weight. The ratio of low molecular weight PVP K30 to high molecular weight PVP was 95:5% by weight. Using an Anton Paar Physica MCR 301 rheometer, the spindles were prepared so that the zero shear rate viscosity, determined by a plate-plate geometry with a plate diameter of 25 mm and a gap distance of 0.5 mm, was 222 Pa·s at 25°C.

[0063] Green fiber production The spinning ball is subjected to pressure and a spinning pump is used to spin it 2.4 cm across the entire 468-hole nozzle plate. 3 Extrusion was performed at a discharge rate of / r. The diameter of each nozzle hole was 100 μm, and the channel length was 200 μm. The filament bundle was taken out vertically downward by a heated spinning shaft. The fibers were then received at a maximum speed of 160 m / min. To achieve thread adhesion of the individual filaments, a spinning preparation was applied before receiving the endless filaments. The resulting green fibers were stored at the specified temperature and relative air humidity (22°C, 36-38% rh).

[0064] The resulting green fibers had a water content in the range of 16–18% by weight. The water content was determined by Karl Fischer titration after evaporation at 140°C.

[0065] Ceramic fiber production The generated green fibers were first continuously fired in a continuous heating furnace at a temperature in the range of 700-1000°C for a period of 320 minutes. Next, the fired fibers were converted into ceramic fibers in a 4m long tubular furnace at a temperature in the range of 1200-1600°C at a throughput rate of 2.5m / min or 2m / min. The residence time of the fibers in the sintering furnace was approximately 1.6 minutes.

[0066] The X-ray diffraction pattern of the fiber obtained in this manner is shown in Figure 1. All detected reflections could be correlated using diffraction patterns from the literature. Mullite can be identified as the dominant phase based on its high-intensity characteristic reflections at 2θ = 16.4, 25.9, 26.2, 40.8, and 60.5°. A reflection characteristic of the tetragonal transformation of ZrO2, at 2θ = 30.2°, was also detected, which is not present in monoclinic and cubic ZrO2.

[0067] For the fibers obtained in this way, various Weibull strengths σ0 and relative Weibull strengths σ 0,rel The Weibull coefficient (m) and modulus of elasticity were determined. The results of these tests are reproduced in Table 2. For comparison, the corresponding values ​​for pure mullite fiber (i.e., without ZrO2) are given.

[0068] [Table 2] Comparisons of various fibers show that ZrO2-containing fibers exhibit slightly improved mechanical properties.

[0069] The results of the bending test are reproduced in Table 3.

[0070] [Table 3] As can be seen from Table 3, ZrO2-reinforced mullite fibers are distinguished from ZrO2-free fibers by significantly improved mechanical properties (breaking load and elongation at break). These have a positive effect, in particular, on the fabric processability of the fibers. The increase in breaking load and elongation at break results in a considerable increase in work at break.

Claims

1. ZrO in the form of an endless filament ２ Reinforced mullite fiber containing at least 0.1% by weight of crystalline ZrO ２ A ZrO2 material characterized by containing at least 80% by weight of crystalline mullite and having fibers with a diameter of more than 5 μm. ２ Reinforced mullite fiber.

2. The ZrO(O)(O)(O)(O)(O)(O)(O)(O))(O)))))) ２ Reinforced mullite fiber.

3. 2-15% by weight of crystalline ZrO ２ The ZrO according to claim 1 or 2, characterized in that it contains ２ Reinforced mullite fiber.

4. The ZrO according to any one of claims 1 to 3, characterized in that it contains at least 85% by weight of crystalline mullite. ２ Reinforced mullite fiber.

5. Crystalline ZrO ２ characterized in that it has a particle size within the range of 10 to 60 nm, the ZrO according to any one of claims 1 to 4 ２ reinforced mullite fiber.

6. Al in the mullite phase ２ O ３ The theoretical ratio of Al ２ O ３ and SiO ２ The ZrO according to any one of claims 1 to 5, characterized in that it is 71 to 80% by weight of the total amount. ２ Reinforced mullite fiber.

7. The remaining phase is Al ２ O ３ Contains or Al ２ O ３ The ZrO according to any one of claims 1 to 6, characterized in that it consists of ２ Reinforced mullite fiber.

8. A ZrO(468 filament fiber bundles having a thickness of 10 μm is characterized in that, at a test speed of 5 mm / min, the breaking load is at least 12.0 N and / or the elongation at breaking is at least 1.5%, as determined by the diameter of the fixed deflection of 2.5 mm. This is the ZrO(468 filament fiber bundles having a thickness of 10 μm) according to any one of claims 1 to 7. ２ Reinforced mullite fiber.

9. ZrO according to any one of claims 1 to 8 ２ A method for producing reinforced mullite fibers, (i) Ceramic-forming SiO ２ Precursor, ceramic-forming Al ２ O ３ Precursor, water-soluble organic polymer, ceramic-forming ZrO ２ A step of generating a spinning solution from a precursor and a stabilizer, wherein ceramic forming SiO ２ Precursor, ceramic-forming Al ２ O ３ Precursor, ceramic-forming ZrO ２ ZrO ２ After forming a mullite precursor solution, a water-soluble organic polymer is added to produce a spinning solution. (ii) A step of partially evaporating the water in the spinning solution in order to produce a spinning mass having a zero shear viscosity of at least 150 Pa·s (determined at 25°C), (iii) A step of subjecting the spinning mass to a dry spinning process to produce green fibers, (iv) A step of firing green fibers to form inorganic precursor fibers, wherein volatile components are removed by thermal decomposition, and (v) ZrO ２ To obtain reinforced mullite fibers, inorganic precursor fibers are sintered to form a mullite phase. A method characterized by the following.

10. Water-soluble ZrO ２ The precursor is given by formula Zr(O) ｏ X （４－２ｏ） [In the formula, o = 0 to 2, and X is a halide ion, an organic anion, or NO] ３ － The method according to claim 9, characterized in that the zirconium compound is a zirconium compound that exists as zirconium oxide sol when o=2.

11. ZrO ２ Mullite precursor solution is water-soluble ZrO ２ The precursor is formed from the precursor solution. ２ Al ２ O ３ and 2 to 15% by weight of ZrO relative to the total amount of the mullite phase. ２ The method according to claim 9 or 10, characterized in that it contains an amount equivalent to [a certain value].

12. Water-soluble Al ２ O ３ The precursor is, formula Al ｎ (OH) ｍ X （３ｎ－ｍ） [In the formula, X is a halide ion, NO ３ － The method according to any one of claims 9 to 11, characterized in that it is an aluminum salt of an organic anion, n = 1 or 2, and m = 0 to 5.

13. Mullite precursor solution is water-soluble Al ２ O ３ The precursor is Al in the mullite phase. ２ O ３ and SiO ２ The method according to any one of claims 9 to 12, characterized in that it contains in an amount equivalent to 71 to 80% by weight of the total theoretical proportion.

14. Water-soluble SiO ２ The method according to any one of claims 9 to 13, characterized in that the precursor is colloidal silicon dioxide or a water-soluble or dispersible Si-containing organic compound.

15. Mullite precursor solution is water-soluble SiO ２ The precursor is formed from the precursor solution in the mullite phase, Al ２ O ３ and SiO ２ The method according to any one of claims 9 to 14, characterized in that it contains in an amount equivalent to 20 to 28% by weight of the total amount of the theoretical proportion.

16. The method according to any one of claims 9 to 15, characterized in that poly(vinylpyrrolidone), poly(vinyl alcohol), and poly(ethylene oxide) are used as water-soluble organic polymers.

17. The method according to any one of claims 9 to 16, characterized in that the aqueous spinning solution contains a water-soluble organic polymer in a weight ratio of polymer to total oxide content of 20:80 to 40:

60.

18. The method according to claim 9 or 17, characterized in that the water-soluble organic polymer comprises a first polymer having a molecular weight Mw of less than 200,000 g / mol, and a second polymer having a molecular weight Mw of greater than 1,000,000 g / mol.

19. Water-based ZrO ２ The method according to any one of claims 9 to 18, characterized in that the precursor solution contains a carboxylic acid as a stabilizer.

20. The method according to any one of claims 9 to 19, characterized in that the green fibers are fired at a temperature of 700 to 1000°C for a period of 240 to 600 minutes to remove volatile components by thermal decomposition.

21. The inorganic precursor fibers are calcined at a temperature of 1200 to 1600°C to produce ZrO ２ The method according to any one of claims 9 to 20, characterized by obtaining reinforced mullite fibers.

22. The ceramic-forming SiO obtained by method steps (i) to (iii) according to any one of claims 9 to 21 ２ Precursor, the ceramic forming Al ２ O ３ Precursor, the water-soluble organic polymer, the ceramic-forming ZrO ２ A ceramic-forming green fiber in the form of a filament, containing a precursor, stabilizer, and water.

23. A fiber-matrix composite material, according to any one of claims 1 to 8, ZrO ２ Use of reinforced mullite fiber.

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