TWI491581B - Methods of forming ceramic fibers - Google Patents

Description

Method for forming ceramic fiber

This invention relates to ceramic fibers, and more particularly to ultrafine ceramic fibers and methods of forming same.

Several methods for fabricating ceramic fibers have been used in the manufacture of piezoelectric ceramic fibers. There is a Viscous suspension spinning process (VSSP), for example, US Pat. No. 6,451,059 and US Pat. No. 5,486,497, and World Patent Publication No. WO2008125271.

There is also a piezoelectric ceramic fiber produced by a sol-gel method. For example, U.S. Patent No. 5,945,029, U.S. Patent No. 3,760,049 and U.S. Patent No. 4,921,328, German Patent No. DE 433 283 831, Japanese Patent No. 5,132,320, and Chinese Patent No. CN101190845.

There are also methods for producing ceramic fibers by a templating method, such as the world patents WO2008056891, Japanese patents JP9117964 and JP6340475, and Korean patent KR100806296.

The invention proposes a new method for forming ceramic fibers, which is easy to handle and can produce ceramic fibers with high solid content.

An embodiment of the present invention provides a method for forming a ceramic fiber, comprising: mixing about 100 parts by weight of a polar solvent with about 10 to 30 parts by weight of an alcohol-based polymer to form a binder; mixing about 70 parts by weight to 95 parts by weight of the ceramic The powder and about 30 parts by weight to 5 parts by weight of the binder form a green germ slurry; the green germ slurry is extruded into an alkaline aqueous solution containing boric acid to form a green fiber slurry to form a ceramic fiber green embryo: and a sintered ceramic fiber Raw embryos form ceramic fibers.

The present invention provides a method of forming ceramic fibers. First, about 100 parts by weight of a polar solvent (for example, water, ethanol, propanol, or acetone) and about 10 to 30 parts by weight of an alcohol-based polymer are mixed to form a binder. If the proportion of the alcohol-based polymer is too high, the binder itself is not easily stirred uniformly and a porous hole is generated in the subsequent sintering process. If the proportion of the alcohol-based polymer is too low, the insufficient viscosity of the binder will reduce the strength of the extruded fiber embryo. The alcohol-based polymer may be polyvinyl alcohol, cellulose (e.g., methyl cellulose, etc.), and the average molecular weight of the alcohol-based polymer is between 5,000 and 100,000. In one embodiment of the invention, the alcohol-based polymer is a polyvinyl alcohol having a weight average molecular weight of between about 30,000 and 100,000. In another embodiment of the invention, the alcohol-based polymer is methylcellulose having a weight average molecular weight of between 10,000 and 30,000. Different alcohol-based polymers have different weight average molecular weights and concentration ranges. If the weight average molecular weight of the alcohol-based polymer is too high, the viscosity of the binder is too high to facilitate uniform agitation. If the average molecular weight of the alcohol-based polymer is too low, the viscosity of the binder is too low to shape the green body. In another embodiment of the present invention, the binder further comprises about 0.1 to 5 parts by weight of boric acid. If an alcohol-based polymer having a small molecular weight is used, a small amount of boric acid can be added, which has an easy stirring function and provides appropriate in the green embryo. Viscosity. Next, 70 parts by weight to 95 parts by weight of the ceramic powder and 30 to 5 parts by weight of the above binder are mixed to form a green germ slurry. If the proportion of the ceramic powder is too high, the fiber green embryo cannot be continuously extruded. If the proportion of the ceramic powder is too low, holes are formed in the subsequent sintering process of the fiber green. The ceramic powder may be zirconium titanate lead, barium titanate, or titanium oxide, and the like, and the ceramic powder has a particle size of about 0.1 μm to 0.6 μm. If the particle size of the ceramic powder is too large, the plug hole may be formed when the small pore mold cavity is extruded. In an embodiment of the invention, the mixing step of the ceramic powder and the binder has a temperature of between about 25 ° C and 50 ° C and a mixing time of between about 3 minutes and 20 minutes. If the mixing time is too long and/or the mixing temperature is too high, the viscosity of the green pulp slurry will be too high to increase the pressure of the extrusion process described later. If the mixing time is too short and/or the mixing temperature is too low, the raw germ slurry is not uniformly stirred and cannot be extruded. Through a suitable mixing process, the viscosity of the green germ slurry can be between about 10,000 cps and 38000 cps to facilitate the subsequent extrusion process.

Next, the green germ slurry is extruded into an alkaline aqueous solution containing boric acid to form a green fiber slurry to form a ceramic fiber green embryo. When boric acid is generally used as a crosslinking agent for the green germ slurry, the crosslinking agent is intuitively an acidic solution. In one embodiment of the invention, however, the pH of the aqueous solution of boric acid of from about 1 M to about 5 M is adjusted with aqueous ammonia to form an alkaline solution having a pH of greater than about 7 and less than or equal to 12. Mainly because boric acid in aqueous solution mainly exists in the form of unionized B(OH) ₃ , only a few combine with OH ^{- in} water to form [B(OH) ₄ ] ^- , and [B(OH) ₄ ] ^- will be combined with alcohol A condensation reaction is carried out and the water molecules are removed such that the polyvinyl alcohol molecules are crosslinked together. Therefore, to increase the concentration of [B(OH) ₄ ] ^- , a base is added in an appropriate ratio to increase the concentration of [B(OH) ₄ ] ^- . If the alkaline solution does not contain boric acid, the alcohol-based polymer molecules in the binder will only dehydrate and will not cause polymerization. If the aqueous solution containing boric acid is acidic and not adjusted to be basic, the boric acid remains mainly in the form of unionized B(OH) ₃ and is difficult to polymerize with the alcohol group of the alcohol-based polymer. The boric acid molecule in the alkaline aqueous solution forms [B(OH) ₄ ] ^- , which reacts with the alcohol-based polymer in the green germ slurry to form a crosslinked polymer film layer, and is progressively polymerized toward the center of the ceramic fiber embryo. . In one embodiment, the alkaline solution is adjusted to use aqueous ammonia which does not have an alkali metal cation to prevent the alkali metal cation from remaining in the fiber to cause dielectric loss. After the above polymerization reaction releases water, the ceramic fiber green embryo can be simultaneously densified to densify. In an embodiment of the invention, the alkaline solution containing the aqueous boric acid solution has a temperature between about 25 ° C and 80 ° C. If the temperature of the alkaline solution containing the aqueous boric acid solution is too high, rapid local polymerization may occur. If the temperature of the alkaline solution containing the aqueous boric acid solution is too low, the polymerization reaction may be too slow, and the slurry may not be molded.

It can be understood that the cross-sectional shape of the ceramic fiber green embryo depends on the shape of the extrusion outlet. The skilled artisan can employ different shapes of extrusion outlets to provide ceramic fiber embryos with different cross-sectional shapes such as circles, triangles, squares, or other polygons, depending on the needs. In an embodiment of the invention, a circular extrusion outlet having a diameter of between about 40 μm and 500 μm or between about 40 μm and 250 μm is used, and a pressure of between about 2 kg/cm ² and 50 kg/cm ² is required. A ceramic fiber green body having a circular cross section having a diameter of between about 30 μm and 600 μm or between about 30 μm and 400 μm can be formed. Compared to the pressure required for conventional dry extruded ceramic fibers (>400 kg/cm ² ), the pressure required for the process is significantly reduced, ie, the process cost is reduced. Obviously, the reason why the pressure of the above extrusion process is low is that the viscosity of the green germ slurry of the present invention is low. In addition, the reel can be used to collect ceramic fiber greens such that the ceramic fiber greens have a particular configuration such as a spring-like shape, a fixed length line segment, or a configuration that other linear materials may have.

The ceramic fibers are then dried prior to low temperature, such as from about 25 ° C to 100 ° C, as appropriate. Finally, the ceramic fibers are sintered to form ceramic fibers. In an embodiment of the invention, the sintering conditions are ovens at about 1000 ° C to 1300 ° C in a typical atmosphere. After the above sintering step, the organic product in the ceramic fiber embryo, such as the crosslinked product of the alcohol-based polymer and boric acid, will oxidize and escape to the atmosphere, so that the diameter of the ceramic fiber is smaller than that of the ceramic fiber. After the above steps, for example, a circular extrusion outlet having a diameter of between about 40 μm and 500 μm or between about 40 μm and 250 μm can be formed to have a diameter of between about 30 μm and 600 μm or about 30 μm. A ceramic fiber having a circular cross section of between 400 μm. It can be seen from the micrograph of the cut surface that the ceramic fiber of the present invention has a dense structure and has almost no pores therein.

The above and other objects, features and advantages of the present invention will become more apparent and understood.

[Examples]

Example 1

First, the zirconium titanate lead (PZT) piezoelectric material is used as the ceramic powder, and the average particle size is 0.1 to 0.6 μm. Next, 22 parts by weight of polyvinyl alcohol (weight average molecular weight 37,500), 77.85 parts by weight of water, and 0.15 parts by weight of boric acid were mixed as a binder.

80 parts by weight of the ceramic powder and 20 parts by weight of the binder were respectively taken and mixed to form a green germ slurry having a viscosity as shown in Fig. 1.

Next, the pH of the 3 M aqueous boric acid solution was adjusted to 8.2 with ammonia water, and the green germ slurry was extruded into an alkaline aqueous solution containing boric acid. The extrusion outlets are circular cross-section conical tubes having diameters of 100 μm and 150 μm, respectively. When the slurry extrusion speed was adjusted to 2.5 cm/sec, the relationship between the extrusion pressure and the ceramic powder content is shown in Fig. 2. Smaller extrusion outlets have higher extrusion pressures, while slurries with higher levels of ceramic powder also require higher extrusion pressures.

Since the green germ slurry and the alkaline aqueous solution containing boric acid are reacted in the liquid phase, the boron hydroxide anion group in the alkaline aqueous solution containing boric acid is formed to form a crosslinked polymer with the polyvinyl alcohol in the extruded slurry. The film layer is progressively polymerized toward the fiber center. In addition to releasing moisture, the above polymerization also shrinks the ceramic fiber raw embryos to obtain a denser circular ceramic fiber green embryo.

The ceramic fiber raw embryos are cut into appropriate lengths and placed in an oven for low-temperature drying, followed by high-temperature sintering (excessive lead oxide is placed around the fiber embryos for high-temperature sintering as a compensation atmosphere). The ceramic fiber preform having a circular cross section of about 120 μm (extrusion outlet is 150 μm) and 45 μm (extrusion outlet is 100 μm) is sintered at 1250 ° C / 2 hr, and the SEM microstructure of the formed ceramic fiber is as shown in the third and Figure 4 shows. It can be seen from Figures 3 and 4 that the appearance of the ceramic fiber maintains the roundness of the original ceramic fiber embryo, and the diameter after sintering shrinks from 120 μm and 45 μm to about 90 μm and 39 μm, respectively, and the shrinkage ratio is 16 to 21%. It is close to the shrinkage rate of traditional ingot block sintering (18~20%), which shows that the fiber solid density of the fiber made by this method can be further increased to the extent of the raw ceramic block, which is beneficial to maintain the microfiber. Electrical properties after sintering.

As shown in Figure 5, the density of the ceramic fibers is positively correlated with the solids content of the green germ slurry. When the solid content of the green germ slurry is 80%, the density of the ceramic fiber is about 7.2 g/cm ³ , which is slightly smaller than the density of sintering of the ingot (7.7 g/cm ³ ).

Since the ceramic powder of this embodiment is a piezoelectric material, the piezoelectric properties of the ceramic fiber can be further measured. The piezoelectric ceramic fiber preform having a solid content of 80% by weight has piezoelectric properties such as dielectric constant (Dk) and piezoelectric coefficient (D33) as shown in Fig. 6 after sintering.

Example 2

85 parts by weight of the ceramic powder described in Example 1 and 15 parts by weight of the binder described in Example 1 were respectively taken and mixed to form a green germ slurry having a viscosity as shown in Fig. 1.

Next, the pH of the 3 M aqueous boric acid solution was adjusted to 8.2 with ammonia water, and the green germ slurry was extruded into an alkaline aqueous solution containing boric acid. The extrusion outlets are circular cross-section conical tubes having diameters of 100 μm and 150 μm, respectively. When the slurry extrusion speed was adjusted to 2.5 cm/sec, the relationship between the extrusion pressure and the ceramic powder content is shown in Fig. 2.

Since the green germ slurry and the alkaline aqueous solution containing boric acid are reacted in the liquid phase, the boron hydroxide anion group in the alkaline aqueous solution containing boric acid is formed to form a crosslinked polymer with the polyvinyl alcohol in the extruded slurry. The film layer is progressively polymerized toward the fiber center. In addition to releasing moisture, the above polymerization also shrinks the ceramic fiber raw embryos to obtain a denser circular ceramic fiber green embryo.

Similar to Example 1, the above-mentioned ceramic fiber green embryos were cut into appropriate lengths and placed in an oven for low-temperature drying, followed by high-temperature sintering (excessive lead oxide was placed around the fiber embryos for high-temperature sintering as a compensation atmosphere). The ceramic fiber embryo with a circular cross section of about 135 μm (extrusion outlet is 150 μm) and 47 μm (extrusion outlet is 100 μm) is sintered at 1250 ° C / 2 hr, and the appearance of the ceramic fiber is maintained to maintain the original ceramic fiber embryo. The roundness and the diameter after sintering shrink from 135 μm and 47 μm to about 107 μm and 39 μm, respectively, and the shrinkage ratio is 20.7 to 17%. It is close to the shrinkage rate of traditional ingot block sintering (18~20%), which shows that the fiber solid density of the fiber made by this method can be further increased to the extent of the raw ceramic block, which is beneficial to maintain the microfiber. Electrical properties after sintering.

As shown in Figure 5, the density of the ceramic fibers is positively correlated with the solids content of the green germ slurry. When the solid content of the green germ slurry was 85 wt%, the density of the ceramic fiber was about 7.6 g/cm ³ , which was slightly smaller than the density of sintering of the ingot (7.7 g/cm ³ ).

Since the ceramic powder of this embodiment is a piezoelectric material, the piezoelectric properties of the ceramic fiber can be further measured. The piezoelectric ceramic fiber having a solid content of 85 wt% is subjected to piezoelectric properties such as dielectric constant (Dk) and piezoelectric coefficient (D33) of the ceramic fiber after sintering as shown in Fig. 6.

Example 3

90 parts by weight of the ceramic powder described in Example 1 and 10 parts by weight of the binder described in Example 1 were respectively taken and mixed to form a green germ slurry having a viscosity as shown in Fig. 1. As shown in Fig. 1, the viscosity of the slurry increases as the content of the ceramic powder increases. When the ceramic content is as high as 90% by weight, the viscosity of the slurry is about 25,000 cps.

Next, the pH of the 3 M aqueous boric acid solution was adjusted to 8.2 with ammonia water, and the green germ slurry was extruded into an alkaline aqueous solution containing boric acid. The extrusion outlets are circular cross-section conical tubes having diameters of 100 μm and 150 μm, respectively. When the slurry extrusion speed was adjusted to 2.5 cm/sec, the relationship between the extrusion pressure and the ceramic powder content is shown in Fig. 2. Smaller extrusion outlets have higher extrusion pressures, while slurries with higher levels of ceramic powder also require higher extrusion pressures. In any case, even if the diameter of the extrusion outlet is as small as 100 μm, and the ceramic powder content in the slurry is as high as 90% by weight, the extrusion pressure is only 22 kg/cm ² , which is much smaller than the pressure commonly used for mold injection, 400 to 1500 kg/cm ² . From the above, it can be seen that the colloidal crosslinked ceramic powder used in the present invention can greatly increase the content of the ceramic powder in the green embryo, and can continuously extrude the ceramic fiber green embryo at an extremely low pressure.

Since the green germ slurry and the alkaline aqueous solution containing boric acid are reacted in the liquid phase, the boron hydroxide anion group in the alkaline aqueous solution containing boric acid is formed to form a crosslinked polymer with the polyvinyl alcohol in the extruded slurry. The film layer is progressively polymerized toward the fiber center. In addition to releasing moisture, the above polymerization also shrinks the ceramic fiber raw embryos to obtain a denser circular ceramic fiber green embryo.

Similar to Example 1, the above-mentioned ceramic fiber green embryos were cut into appropriate lengths and placed in an oven for low-temperature drying, followed by high-temperature sintering (excessive lead oxide was placed around the fiber embryos for high-temperature sintering as a compensation atmosphere). The ceramic fiber preform with a circular cross section of about 150 μm (extrusion outlet is 150 μm) and 83 μm (extrusion outlet is 100 μm) is sintered at 1250 ° C / 2 hr, and the appearance of the ceramic fiber is maintained to maintain the original ceramic fiber embryo. The roundness and the diameter after sintering shrink from 150 μm and 83 μm to about 122 μm and 70 μm, respectively, and the shrinkage ratio is 18.7 to 18.6%. It is close to the shrinkage rate of traditional ingot block sintering (18~20%), which shows that the fiber solid density of the fiber made by this method can be further increased to the extent of the raw ceramic block, which is beneficial to maintain the microfiber. Electrical properties after sintering.

As shown in Fig. 5, when the ceramic powder content of the ceramic fiber green embryo reaches 90% by weight, the density after sintering is 7.65 g/cm ³ , which is no significant compared with the density of sintered compact (7.7 g/cm ³ ). gap. Therefore, not only the ultrafine wire diameter ceramic fiber can be produced by the above method, but also the ceramic powder content in the ceramic fiber green embryo can be greatly increased, and the density of the ceramic fiber formed after sintering can be improved.

Since the ceramic powder of this embodiment is a piezoelectric material, the piezoelectric properties of the ceramic fiber can be further measured. The piezoelectric ceramic fiber having a solid content of 90% has a piezoelectric property such as a dielectric constant (Dk) and a piezoelectric coefficient (D33) as shown in Fig. 6 after sintering. At the 1KHz resonance frequency, the higher the PZT piezoelectric ceramic powder content, the higher the dielectric constant and piezoelectric coefficient. When the ceramic powder content in the green pulp slurry is 90 wt%, the dielectric constant and piezoelectric coefficient after sintering are as high as 4500 and 600 (pC/N), respectively. It is shown that the higher the content of the PZT piezoelectric ceramic powder in the green pulp, the denser the solid after sintering, and therefore the better the piezoelectric characteristics.

While the invention has been described above in terms of several preferred embodiments, it is not intended to limit the invention, and the invention may be modified and modified without departing from the spirit and scope of the invention. The scope of the invention is defined by the scope of the appended claims.

1 is a diagram showing the relationship between the viscosity of a green pulp and the content of a ceramic powder contained therein in an embodiment of the present invention;

Figure 2 is a graph showing the relationship between the extrusion pressure and the content of ceramic powder in the green pulp slurry in an embodiment of the present invention;

Figure 3 is a cross-sectional micrograph of a ceramic fiber having a circular cross section in an embodiment of the present invention;

Figure 4 is a cross-sectional micrograph of a ceramic fiber having a circular cross section in an embodiment of the present invention;

Figure 5 is a graph showing the relationship between the density of ceramic fibers and the content of ceramic powder in the green germ slurry in an embodiment of the present invention;

Fig. 6 is a graph showing the relationship between the dielectric constant of the ceramic fiber of the piezoelectric material and the piezoelectric coefficient and the content of the ceramic powder in the green pulp in an embodiment of the present invention.

Claims (11)

A method for forming a ceramic fiber, comprising: mixing 100 parts by weight of a polar solvent with 10 to 30 parts by weight of an alcohol-based polymer to form a binder, wherein the alcohol-based polymer comprises polyvinyl alcohol, or cellulose, and the alcohol The weight average molecular weight of the base polymer is between 5,000 and 100,000; mixing 70 parts by weight to 95 parts by weight of the ceramic powder and 30 to 5 parts by weight of the binder to form a green germ slurry, wherein the viscosity of the raw germ slurry is Between 10000 cps and 38000 cps; extruding the green germ slurry into an alkaline aqueous solution containing boric acid to form a green fiber raw embryo; and sintering the ceramic fiber to form a ceramic fiber The temperature at which the ceramic fiber green body is sintered is between 1000 ° C and 1300 ° C. The method for forming a ceramic fiber according to claim 1, wherein the ceramic powder comprises lead zirconium titanate, barium titanate, or titanium oxide. The method for forming a ceramic fiber according to claim 1, wherein the ceramic powder has a particle size of between 0.1 μm and 0.6 μm. The method for forming a ceramic fiber according to claim 1, wherein the alkaline aqueous solution containing boric acid has a pH of more than 7 and less than or equal to 12, and a boric acid concentration of between 1 M and 5 M. The method for forming a ceramic fiber according to claim 1, wherein the step of extruding the green germ slurry into an alkaline aqueous solution containing boric acid is performed by using an extrusion outlet having a circular shape and a diameter of 40 μm. Up to 500μm. The method for forming a ceramic fiber according to claim 4, wherein the step of extruding the green germ slurry into an alkaline aqueous solution containing boric acid is carried out at a pressing pressure of from 2 kg/cm ² to 50 kg/ Between cm ² . The method for forming a ceramic fiber according to claim 4, wherein the cross section of the ceramic fiber green body is a circle having a diameter of between 30 μm and 600 μm. The method for forming a ceramic fiber according to claim 4, wherein the ceramic fiber has a cross section of a circle having a diameter of between 30 μm and 600 μm. The method for forming a ceramic fiber according to claim 1, wherein the alkaline aqueous solution containing boric acid has a temperature of between 25 ° C and 80 ° C. The method for forming a ceramic fiber according to claim 1, further comprising drying the ceramic fiber green body at a low temperature between 25 ° C and 100 ° C before the step of sintering the ceramic fiber green body. The method for forming a ceramic fiber according to claim 1, wherein the binder further comprises about 0.1 to 5 parts by weight of boric acid.