KR20200028874A - Stick-type ceramic fiber electric heating element - Google Patents

Abstract

The present invention provides a method for manufacturing a stick type ceramic fiber electric heater which is formed by uniform alignment and inter-fiber coupling induction of SiC fiber, which can be stably used at a high temperature of 1,000°C or higher, to be quickly heated at 1,000°C or higher in a moment within a few dozens of seconds, and a stick type ceramic fiber thereof. According to the present invention, the method comprises in preparing and cutting a bundle of green fiber by a predetermined, aligning the acquired fiber bundle in a mold with both opened sides and an opened upper part in one direction, covering the opened upper part of the mold with an upper cover having a constantly controlled weight; performing thermal oxidation stabilization for the mold charged with the fiber, performing heat treatment in an inert gas atmospheric after the stabilization is completed, and performing cooling after the heat treatment. Moreover, the stick type ceramic fiber electric heater having the ceramic fiber aligned in one direction comprises: one end and the other end disposed on both sides respectively, to come in contact with an electrode; and the ceramic fiber extended from the one end of the electric heater to the other end.

Description

STICK-TYPE CERAMIC FIBER ELECTRIC HEATING ELEMENT

The present invention relates to a stick-type ceramic fiber heating element, and more particularly, to a stick-type ceramic fiber electric heating element using SiC fibers.

SiC fiber is a reinforcing material of a typical ultra-high temperature ceramic fiber reinforced composite, and is a material that maintains structural properties of high strength, high toughness, corrosion resistance, and high reliability in harsh environments of high temperature and high pressure. In particular, long fiber-reinforced composite materials exhibit the greatest toughness-enhancing effect compared to other composite materials, such as strengthening of graininess and needle-like crystallinity, so aerospace, defense, and nuclear industries require high reliability. It is an essential material. Recently, its application is expanding not only as a composite material, but also in the defense industry, automobile and aerospace industries.

In addition, iodine-doped nano-structured SiC fibers are activated by microwaves and can be rapidly heated to 1400 ° C in air, and the heat generated is very economical because of its good radiation characteristics, so recently, a heating system using SiC fibers as a heating element This is getting attention.

SiC fibers are pre-ceramic polymers, commonly known as polycarbosilanes, made into fibrous (polycarbosilane fibers) by melt spinning (MELT SPINNING, MELT BLOWING) or electrospinning (ELECTROSPINNING), and then heat treated again to form organic (polymers) ) Can be produced by converting from fibers to inorganic (ceramic) fibers.

However, the melting point of the polycarbosilane is usually 150 to 300 ° C, but if the polycarbosilane fiber is heat treated immediately after spinning, it will melt without maintaining the fiber in the melting temperature range. In order to prevent this phenomenon and maintain the fibrous phase during heat treatment, to bond the polymer molecules present on the surface of the fiber through so-called cross-linking (CROSS-LINK) to obtain the final SiC fiber, the melting point of the cross-linked surface portion is higher than the inside, thereby causing thermal decomposition of the polymer Because the temperature is higher than 400 ℃, it does not melt during the heat treatment process and maintains the fibrous form, and is finally made of SiC fiber.

Such a process is called a stabilization (CURING) or infusion (INFUSIBLIZATION) process, and various stabilization methods have been proposed. Among these, the thermal oxidation stabilization (THERMAL OXIDATION CURING) method and the electron beam stabilization (E-BEAM CURING) method are typical. In the former case, when the polycarbosilane fibers obtained by spinning are kept in an oxidizing atmosphere at 150 to 250 ° C. for a long time (typically an air atmosphere at 200 ° C.), weak bonds among the bonds of polymer molecules in which oxygen in the atmosphere is present on the fiber surface are caused by Si-H. The bond and other Si-CH3 bonds are broken and Si-O-Si bonds are formed. In this process, crosslinking between molecules occurs. This method is the most representative stabilization method, and NICALON-based CERAMIC GRADE fibers and TYRANNO fibers in Japan use oxidation stabilization methods.

However, this method requires a long time for stabilization (usually takes 4 to 10 hours in consideration of the heating rate), and 10 to 20% of oxygen mixed by crosslinking during stabilization is not stable at a high temperature of 1200 ° C or higher and decomposes again. Because there is a disadvantage in that the high-temperature physical properties of SiC fibers are greatly reduced.

In addition, when viewed from the fiber spinning side, the spinning temperature of the fiber must be at least 30 to 60 ° C higher than the normal stabilization temperature of 200 ° C to maintain the fiber intact during the oxidation stabilization process, and the oxidation effect can be seen. On the other hand, when the melt spinning temperature is higher than 270 ~ 290 ℃, the stability of the melt is poor, it is not easy to control the spinning conditions, and it is difficult to fine-separate and frequent disconnection due to insufficient stretching during spinning.

For this reason, it is necessary to precisely control the flow and deformation of the raw precursor polymer, which makes the production process of the raw material more complicated and causes the raw material price to rise. In the case of electron beam stabilization, when the electron beam is irradiated to the fiber, intermolecular bonds on the surface are broken, thereby forming radicals. Since these radicals are not in a stable state in the atmosphere, they tend to recombine and return to a stable state. In this recombination process, crosslinks between polymer molecules are formed. Since there is no mixing of oxygen, heat treatment is possible at 1600 ° C or higher, and thus SiC fibers having excellent physical properties can be obtained. However, since it is difficult to control the process and the manufacturing cost is high, there is a limit to apply it to a general continuous mass production process.

In the Republic of Korea Patent Publication No. 10-1209110, in order to solve the above problems, the step of manufacturing a polycarbosilane fiber by spinning polycarbosilane in relation to the method of manufacturing SiC fiber; Stabilizing the polycarbosilane fiber by gas fumigation using a halide-based gas; And a calcination step of heat-treating the stabilized polycarbosilane fiber.

In addition, in the case of the prior art using SiC fibers made of a heating element, for example, in Korean Patent Publication No. 10-2017-0087380, any sublimation raw material and carbon fibers selected from silicon or silicon dioxide or mixtures thereof are vacuum or inert gas. It is also disclosed for a silicon carbide fiber heating element that is prepared from carbon fibers by gas infiltration reaction by sublimation of a sublimation raw material, placed in an atmosphere and a high temperature state, and generates heat by applying microwaves.

In addition, the conventional electric resistance heating element is manufactured by sintering ceramic powder particles at a temperature of 1500 ° C or higher in a rod shape or other structure, and since the shape and size of the particles are different and resistance heating is generated by point contact between particles, quality control is difficult. Since it requires high power, there is a problem in that it consumes a lot of electricity. Figure 1 is a photograph showing an electric heating element and its microstructure prepared using conventional SiC ceramic powder particles showing the above-described particle shape.

Republic of Korea Patent Registration No. 10-1209110 Republic of Korea Patent Publication No. 10-2017-0087380

The present invention is a method of manufacturing a stick-type ceramic fiber electric heating element capable of rapidly increasing temperature to 1000 degrees or more in a few seconds within a few seconds through uniform orientation and induction of inter-fiber SiC fibers that can be stably used at high temperatures of 1000 ° C. or higher, For this purpose, it is an object to provide a stick-type ceramic fiber electric heating element using SiC fibers.

To this end, the present invention prepares a green fiber bundle, cuts it to a specific size, orients the obtained fiber bundle in one direction to the mold with both sides open and the top open, and the top cover with constant weight control over the open top of the mold Provided is a method of manufacturing a stick-type ceramic fiber electric heating element that is covered with a furnace, performs thermal oxidation stabilization for a mold loaded with fibers, and performs heat treatment in an inert gas atmosphere after stabilization is completed, and cools to room temperature after heat treatment.

Here, the heat treatment may be performed for more than 0.5 hours in the temperature range of 1000-1900 ℃.

Further, it is preferable that the electric heating element has one end and the other end capable of contacting the electrode, and the green fiber bundle has a size extending from one end to the other end.

Further, the present invention is a stick-type ceramic fiber electric heating element in which ceramic fibers are oriented in one direction, and the electric heating element has ceramic ends extending from one end to the other end of the electric heating body, with one end and the other end capable of contacting the electrode on both sides. It provides a stick-type ceramic fiber electric heating element comprising a.

According to the present invention, there is provided a method for manufacturing a stick-type ceramic fiber electric heating element capable of rapidly increasing temperature to 1000 degrees or more in a few seconds within a few seconds through uniform orientation and induction of inter-fiber bonding of SiC fibers that can be stably used at high temperatures of 1000 ° C. or higher. In addition to this, it is possible to provide a stick-type ceramic fiber electric heating element using SiC fibers for this purpose.

1 is a photograph showing an electric heating element and its microstructure prepared using conventional SiC ceramic powder particles.
Figure 2 is a photograph showing a uniformly oriented SiC ceramic fiber electric heating element and its microstructure prepared according to the present invention.
Figure 3 is a schematic view showing a manufacturing process for producing a stick-type SiC ceramic fiber electric heating element according to the present invention.
Figure 4a is a schematic view showing an example of a mold capable of simultaneously producing a stick-type SiC ceramic fiber electric heating element according to the present invention, Figure 4b is a plurality of stick-type SiC ceramic fiber electric heating element manufactured through the mold of Figure 4a It is a picture of.
The left side of FIG. 5 is a transmission electron microscope microstructure photo of the fiber when heat-treated at less than 1000 ° C, and the right side of FIG. 5 is a transmission electron microscope microstructure photo of the fiber when heat-treated at 1000 ° C or higher.
6 is a result of thermogravimetric analysis of polycarbosilane fibers.

The present invention uniformly orients the ceramic long fibers uniformly controlled within a size deviation of ± 10 μm in 10 to 50 μm in one direction, and secures the contact resistance stability and uniformity at the same time, and at the same time uses a fibrous specific surface area The present invention provides a method for manufacturing a heating element capable of rapid heating and cooling to a temperature higher than 1000 degrees at a low power by raising the temperature, and a stick-type ceramic fiber electric heating element for this.

First, the manufacturing method according to the present invention can be performed as follows, and the process is schematically and schematically shown in FIG. 3:

Ⓐ A precursor fiber (green fiber) bundle prepared by spinning polycarbosilane (including various silicone-based polymers such as polysilazane, polysiloxane, etc.) as a fiber into fibers is prepared.

Ⓑ Cut the prepared green fiber bundle uniformly into a specific size.

Ⓒ As shown in the drawing, the fiber bundle thus obtained is oriented in one direction to the mold with both sides open and the top open.

Ⓓ Cover the open upper part of the mold with the upper cover whose weight is constantly controlled.

Ⓔ For the thermal oxidation stabilization of the fiber surface, the mold loaded with the fiber is maintained in an oven where the temperature is kept constant in a temperature range of 150-200 ° C for 1 to 10 hours.

Ⓕ After the stabilization is completed, the mold is charged into a high-temperature electric furnace with controlled atmosphere and heat-treated in an inert gas atmosphere (nitrogen, argon, etc.) at a temperature in the range of 1000-1900 ° C for 0.5 hour or more.

Ⓖ After the heat treatment, the specimen obtained by cooling to room temperature is recovered.

The stick-type SiC fiber electric heating element obtained through this process is schematically shown on the far right of FIG. 3.

More specific conditions of the present invention are described below.

The fiber size of the precursor fiber is larger than 10 mm, preferably larger than 100 mm, and the size of the heating element can be determined according to a system to which the ceramic fiber electric heating element manufactured according to the present invention is applied as described below. After heat treatment, it is possible to cut a larger size than the required size, such as a length of 1.5 times the required size in consideration of shrinkage. One of the biggest features of the present invention is that the electric heating element obtained as the final product has one end and the other end capable of contacting the electrode on both sides, and the electric heating element obtained as the final product includes ceramic fibers extending from one end of the electric heating element to the other end. It is in doing. The size of the obtained electric heating element can be determined according to the system to be applied, and after the process conditions are completely determined, the length of the green fiber can be adjusted, or the obtained electric heating element can be cut to a length suitable for the system to be applied. However, the important thing is that the SiC ceramic fibers constituting the electric heating element extend from one end to the other. More detailed process conditions and reasons for this are described in more detail below.

The fiber diameter is larger than 1 um, preferably larger than 10 um, more preferably larger than 30 um, but nano-sized fibers are also possible. The performance of the heating element according to the diameter of the precursor fiber within a predetermined size of the electric heating element is inversely proportional to the resistance value of the precursor fiber. In other words, the smaller the diameter of the precursor fiber in the same volume, the larger the resistance value, so the amount of charge transfer decreases and the heating effect decreases.

The mold consists of an engraved rectangular mold with both sides open and a load-controlled top cover, as shown schematically in the figure. The load applied to the precursor fiber bundle can be controlled by the weight of the top cover.

In addition, it is possible to manufacture a plurality of heating elements at the same time by placing the molds in parallel as shown in FIG. 4A, and FIG. 4B shows the plurality of heating elements thus obtained.

When following the manufacturing method according to the present invention as described above, the following should be considered.

When the precursor fiber is oriented in the mold, the length of the precursor fiber group is different, and when one end in the middle of the mold contacts the middle of the other precursor fiber, after the formation of the heating element, due to the potential difference at the end of the fiber that is not in contact with the electrode Sparking may occur. That is, when the final resultant electric heating element is connected to the electrode, it is desirable that all precursor fibers are shorted to the electrode as much as possible. Therefore, in order to determine the size of the heating element in accordance with the system to which the SiC ceramic fiber electric heating element manufactured according to the present invention is applied and to produce the determined heating element, the precursor fibers are oriented and stacked in a mold produced in consideration of the shrinkage rate of the precursor fibers. It is preferable to heat-treat by applying pressure.

In the case of heat treatment during the manufacturing process of silicon carbide fibers, polycarbosilane, an organic material, is converted into an inorganic material between 500-1000 ° C, and silicon carbide crystal formation starts at 1000 ° C or higher. Therefore, when heat treatment is performed at less than 1000 ° C, silicon carbide crystals are not formed, and only an amorphous (SiOC) phase is present, which significantly reduces electrical conductivity. On the other hand, the electrical conductivity characteristics are improved and the exothermic effect is exhibited by the silicon carbide crystal formed at 1000 ° C or higher. 5 is a microscopic structure of the transmission electron microscope of the fiber. In the left side of FIG. 5, crystals are not observed when heat treatment is performed at less than 1000 ° C, but when heat treatment is performed at 1000 ° C or higher, silicon carbide nanocrystals are formed as shown in the right side of FIG. It can be seen that it is observed.

When the heat treatment temperature is raised to 1900 ° C, the silicon carbide fibers are completely crystallized, so that the electrical properties are also improved. However, when the temperature exceeds 1900 ° C, the silicon carbide crystals sublimate, so that the crystal shape cannot be maintained. FIG. 6 shows that as a result of thermogravimetric analysis of polycarbosilane fibers, continuous weight loss due to sublimation occurs over 1900 degrees.

According to the present invention, there is provided a method for manufacturing a stick-type ceramic fiber electric heating element capable of rapidly increasing temperature to 1000 degrees or more in a few seconds within a few seconds through uniform orientation and induction of inter-fiber bonding of SiC fibers that can be stably used at high temperatures of 1000 ° C. or higher. In addition to this, it is possible to provide a stick-type ceramic fiber electric heating element using SiC fibers for this purpose.

Claims (1)

SiC ceramic fiber is a stick-type ceramic fiber electric heating element oriented in one direction,
The ceramic fiber electric heating element has one end and the other end capable of contacting the electrode on both sides,
The SiC ceramic fiber constituting the ceramic fiber electric heating element extends from one end to the other end of the ceramic fiber electric heating element, and all SiC ceramic fibers are short-circuited at both electrodes.

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