JP2003073168A - Reactive sintered silicon carbide heating element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively provide a heating element which is composed of a high density and high strength reactive sintered silicon carbide compact high in reliability, and capable of accurately controlling even in a low temperature region by lowering a negative temperature dependency of electric resistance at <=500 deg.C. SOLUTION: The reactive sintered silicon carbide heating element is manufactured as follows. The sintered compact is >=80% in relative density, >=50MPa in bending strength and a N type semiconductor containing nitrogen. Crystal type 3C of silicon carbide occupies >=40% to whole cuptalline part, >=50MPa in bending strength, >=-0.1%/ deg.C in resistance temperature coefficient. Mixed powder of β-SiC powder of 30-80 wt.% and carbon powder of 20-70 wt.% is molded fired at the temperature of 1400-2000 deg.C and then after being buried in Si powder, the remaining Si is removed by heat processing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon carbide type electric resistance heating element used in a high temperature industrial heating furnace, and more specifically, it shows a negative temperature dependence of electric resistance in a temperature range from room temperature to about 500.degree. The present invention relates to a silicon carbide heating element having small characteristics.

[0002]

2. Description of the Related Art Silicon carbide is a compound semiconductor having good electrical conductivity, and since it has excellent heat resistance and chemical stability in terms of material, it has long been used as an electric heating element for high temperature electric furnaces and the like. It's being used.

In general, in a silicon carbide heating element, the specific resistance sharply decreases as the temperature rises due to heat generation by energization, and the resistance changes such that the specific temperature around 500 ° C. becomes a minimum and then rises to the maximum usable temperature range. Tend to show. The reason for this is that since silicon carbide is a semiconductor, the number of conduction electrons that can be excited from the impurity level to the conduction band increases as the temperature rises,
This behavior causes the resistance to decrease from room temperature to about 500 ° C, but it is interpreted that after about 500 ° C, the resistance tends to increase slightly because the conduction electron mobility decreases due to thermal vibration of the lattice. There is. Therefore, when controlling the temperature of the silicon carbide heating element in the range of room temperature to about 500 ° C., it is difficult to perform precise temperature control as compared with the metal heating element.

For this reason, various attempts have been proposed in the past for the purpose of reducing the negative resistance characteristic of the silicon carbide heating element. One of the methods has been proposed by the applicant of the present invention in Japanese Patent Application Laid-Open Nos. 7-53265 and 7-53265. No. 89764 is disclosed. Silicon carbide is essentially a polymorphic material, and has a mixed crystal form such as cubic 3C, hexagonal 2H, 4H, 6H, and rhombohedral 15R. Regarding the relationship between the crystal phase of silicon carbide and the temperature change of resistance, the above invention shows the electric resistance of a nitrogen solid solution type silicon carbide heating element containing a specific amount of β-SiC particles (crystal form 3C). It has been proposed that the temperature dependence, especially the negative characteristics from room temperature to 500 ° C., can be effectively reduced. This has been clarified to be achieved when the 3C crystal form having a low donor level is dominant in the conductive path in addition to the N-type semiconductor in which nitrogen is substituted and solid-solved.

[0005]

[Patent Document 1] Japanese Patent Application Laid-Open No. 7-89764
When manufacturing the silicon carbide heating element disclosed in the publication, any one of a sublimation recrystallization method, a reaction sintering method and an atmospheric pressure sintering method is used. In either method, since β-SiC powder is used as a starting material and nitrogen is solid-dissolved in silicon carbide to form an N-type semiconductor, firing in a nitrogen gas atmosphere is a prerequisite.

The sublimation recrystallization method is a very inexpensive method in which a neck at a contact portion of SiC particles is grown by firing at high temperature to cause sintering without dimensional shrinkage. According to this method, in order to obtain a sufficiently strong heat generating element, the firing temperature must be about 2300 ° C. or higher in a nitrogen gas atmosphere, and 100% β-SiC raw material is used for the raw material silicon carbide. Even about 200
Since a transition between β-α crystals occurs at 0 ° C. or higher, the 3C-type crystal form hardly remains, and the negative characteristics cannot be reduced.

In the normal pressure sintering method, fine β-phase SiC powder having an average particle diameter of 1 μm or less is added with, for example, boron and carbon as a sintering aid, and the mixture is molded and fired, so that firing accompanied by dimensional shrinkage occurs. The sintered body has a higher density than the sublimation recrystallization method. However, it is difficult to increase the density by firing in a nitrogen gas atmosphere, and two-step firing is inevitable, such as once densifying by firing in an Ar gas atmosphere and then firing in a nitrogen gas atmosphere, which is a complicated process. Since the price of the fine grain SiC raw material for producing the sintered body is high, it is considered to be a relatively expensive method. In addition, this method requires a firing temperature of about 2100 to 2200 ° C. to densify by sintering, and even if the starting material is β-SiC 100%, β-α transition results in Since the ratio of 3C type SiC was low, the efficiency was poor.

In the reaction sintering method, a predetermined amount of SiC and carbon are mixed, shaped, and brought into contact with Si at the time of firing to react (silicize) with carbon to generate SiC secondary particles and sinter. This is a method of obtaining a high-density SiC sintered body at a relatively low cost. Since SiC produced by silicidation has a high β phase ratio, it is more suitable than the above two methods for increasing the ratio of 3C type SiC which is a silicon carbide crystal form.

However, the silicon carbide heating element described in Japanese Patent Application Laid-Open No. 7-89764 has not yet sufficiently reduced the negative characteristic which is the temperature dependence of the electric resistance at 500 ° C. or lower.

The problem to be solved by the present invention is 500
By reducing the negative characteristic, which is the temperature dependence of the electrical resistance below ℃, it enables precise control even in the low temperature range, and therefore provides a highly reliable and high-strength heating element at low cost. .

[0011]

A silicon carbide heating element according to the present invention for solving the above problems is an N-type semiconductor containing 0.1% nitrogen in a silicon carbide material having a relative density of 80% or more. The 3C type which is a crystal form of silicon carbide is 40% or more of the whole, the bending strength is 50 MPa or more, and the temperature coefficient of resistance from room temperature to 500 ° C. is −0.1% / ° C. or more.

In the method for manufacturing a silicon carbide heating element according to the present invention for solving the above problems, the nitrogen content is 0.1%.
Above and β-SiC powder 30 having an average particle size of 5 μm or more
~ 80% by weight and carbon powder 20 ~ 70% by weight, after molding a mixed powder, embedded in Si powder 1400 ~ 20
It is characterized in that it is fired at a temperature of 00 ° C., and then the remaining Si is removed by heat treatment.

In the above manufacturing method, β-SiC
The powder is produced by a silica reduction method in which coke reacts with silica, and a powder containing unreacted carbon in an amount of 20 to 70% by weight is used as a mixed powder.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Generally, a heating element used in an industrial furnace controls the temperature inside the furnace by controlling the electric power to be applied, and therefore the temperature dependence of its electric resistance is important. If the absolute value of the temperature coefficient of resistance, which indicates the temperature dependence of the electrical resistance, is small and has a positive characteristic, precise temperature control is possible.

Since silicon carbide has semiconductor characteristics, and the resistance-temperature characteristic in the range of room temperature to 500 ° C. shows a typical negative characteristic, it is considered difficult to precisely control in this temperature range. There is. The cause of the negative characteristics is considered as follows from the band structure of silicon carbide. That is, since the band gap of silicon carbide is as wide as 2 to 3 eV, it is necessary to form a solid solution with a certain element to form a donor or acceptor level in order to reduce the electric resistance to a level at which heat can be generated by conduction. . The resistance temperature characteristic of silicon carbide depends on the donor or acceptor level to be dissolved, and the larger the energy gap between the donor level and the conduction band or the acceptor level and the valence band, the greater the temperature change of the specific resistance. Examples of the element capable of forming a solid solution in silicon carbide include boron, nitrogen, aluminum, and phosphorus. Among them, the donor level formed by nitrogen solid solution has the smallest energy gap with the conduction band. The property of the N-type semiconductor in which nitrogen is solid-dissolved in the reaction sintered silicon carbide heating element of the present invention is a prerequisite for reducing the temperature dependence of resistance. The N-type semiconductor in which nitrogen is dissolved forms a donor level of about 0.03 to 0.1 eV, but the level often depends on the crystal form of silicon carbide. That is, the donor level for each crystal polymorph is about 0.066 to 0.1 eV for the 6H type, while it is as low as 0.03 to 0.05 eV for the 3C type. Therefore, the higher the ratio of 3C-type crystals,
Since the negative characteristic of the resistance temperature characteristic is reduced, in the present invention, β-SiC powder is used as a starting material, and further 40% or more of the 3C type crystal is left in the sintered body, so that the conductive path is sufficiently formed. It is ensured and the electric characteristics of the 3C-type crystal become dominant.

The strength is an important characteristic of a ceramic heating element, and particularly when the solid solution amount of nitrogen is large and the specific resistance is low as in the present invention, heat generation is required to increase the resistance value of the entire heating element. Since it is necessary to form a spiral slit in the portion, a high strength material is further required.
In the present invention, the limit value of the bending strength that can sufficiently withstand as a heating element is investigated even if spiral working is performed, and in order to achieve this limiting bending strength, β-SiC and carbon are blended and melted. Adopting a reaction sintering method in which Si is brought into contact with silicon to sinter and sinter carbon, and by incorporating a coarse particle powder as β-SiC, the compacting density can be improved to achieve high density, and at the same time, a 3C-type crystal It is possible to increase the ratio.

From the above point of view, the silicon carbide heating element of the present invention has β-S which is 3C type crystalline SiC particles as a starting material.
Using iC powder, carbon powder is mixed and molded, and the obtained molded body is embedded in Si powder and fired. From this, Si
By using the reaction sintering method in which C secondary particles are generated and sintered, the ratio of 3C type crystals can be easily increased. Further, a SiC sintered body having a relative density of 80% or more can be easily obtained without dimensional shrinkage after sintering.

As the β-SiC powder, a relatively coarse particle having an average particle diameter of 5 μm or more is used. This is because the reaction sintering method does not cause densification accompanied by dimensional shrinkage, so it is necessary to make the density of the compact as high as possible in order to obtain high strength, and for that purpose, β-SiC which is an aggregate is required. This is because it is desirable to use coarse powder. Also,
In order to leave a large amount of 3C-type crystals in the final sintered body, it is preferable to use β-SiC particles having a particle size as large as possible, having an average particle size of 15 μm or more. In addition, β-Si
The nitrogen content in the C powder is preferably 0.1% or more.

The carbon powder used has an average particle diameter of 1 to 100 μm, and particularly preferably an average particle diameter of 3 to 15 μm. It is desirable to adjust the mixing ratio of the β-SiC powder and the carbon powder in the range of 3: 8 to 7: 2 by weight ratio. If the ratio is out of this range, a sufficient theoretical compact density cannot be obtained, or Si At the time of silicidation in which carbon reacts, unreacted carbon remains inside the fired body.

As the mixed powder, it is also possible to use a raw material which is β-SiC produced by a silica reduction method in which coke and silica are reacted to synthesize SiC and which contains 20 to 70% of unreacted carbon. . By using these raw materials, the mixing process in the manufacturing process can be shortened,
Since silicon carbide and carbon are uniformly dispersed, a fired body having a more uniform structure can be obtained.

Next, these mixtures are molded. Molding means is not particularly limited, for example, a mixture of a binder solution obtained by dissolving an organic binder such as polyvinyl alcohol, methyl cellulose or carboxymethyl cellulose in a solvent such as water or alcohol is kneaded, and a conventional means such as extrusion molding or press molding. To perform pressure molding.
At this time, the relative density of the compact is 85 to 95 of the theoretical compact density.
It is necessary to perform pressure molding so as to be in the range of%.

This compact was embedded in Si powder and exposed to high temperature.
And so on, the so-called silicidation process is performed, and C and Si are
Reacted secondary SiC is coarse-grained β-SiC grains
Are combined and sintered. Processing temperature during silicidation
Is 1500 ° C. or higher at which β can sufficiently react with Si and β-
The temperature is 2000 ° C. or lower at which the β → α transition of SiC does not occur,
The processing atmosphere is vacuum or N which is an inert gas.₂ Gas and A
Although it can be selected from r gas, it is preferable that
3C
It is possible to increase the type crystal ratio. But Si
When is near the melting temperature, it reacts rapidly with carbon,
The phenomenon that the sintered body cracks due to the difference in thermal expansion due to the reaction heat
Many occur. Therefore, the temperature rises near the melting point of Si.
The temperature rate must be slowed down and precise temperature control is required.
This causes poor yield. On the other hand, N ₂ In a gas atmosphere
Is generated because the nitriding reaction of Si proceeds at the same time.₃ 
N_Four A decomposition temperature of 1900 ° C or higher is required.
However, since the silicidation reaction proceeds slowly, cracking occurs after silicidation.
A sintered body can be stably produced without any generation. Well
N₂ By firing in gas, the nitrogen content
It becomes higher and the resistance can be lowered.

Regarding the sintered body obtained by the above method,
Be sure to use it as a heating element, as a few percent of Si remains.
In order to use it, it is necessary to perform Si removal processing. this
The treatment is heat treatment at a high temperature to volatilize and remove Si.
About the atmosphere and temperature
Is N₂ 2100 to 2300 ° C in a gas atmosphere,
In a vacuum atmosphere, in the range of 1800-2000 ° C
To do. In order to suppress the β-α transition, a vacuum atmosphere 18
Treatment at 00 to 2000 ° C is preferable, but N ₂gas
Even if treated at 2100 to 2300 ° C, 3C ratio to some extent
Since it is possible to leave the rate, N suitable for continuous production₂ Moth
In-air heat treatment is also useful.

As described above, according to the present invention, β-SiC powder having a nitrogen content of 0.1% or more and an average particle size of 5 μm or more is used as the raw material silicon carbide powder, and the silicon carbide powder is used as carbon. By manufacturing by the reaction sintering method in which secondary β-SiC generated by the reaction of Si is combined and sintered, the 3C type, which is the crystal form of silicon carbide, is 40% or more of the whole, and the resistance temperature characteristic is negative. Improve the characteristics, room temperature ~ 500
It is possible to provide a reaction-sintered silicon carbide heating element having a resistance temperature coefficient of −0.1% / ° C. or more in a temperature range of C ° C. and further having a bending strength of 50 MPa or more.

[0025]

EXAMPLES Next, the present invention will be described in detail with reference to examples. Example 1 β-having a nitrogen content of 0.13% and an average particle size of 21.5 μm
A common cellulosic binder and water were added to a mixed powder of 55% by weight of SiC powder and 45% by weight of carbon powder having an average particle diameter of 10 μm, and the mixture was kneaded, and the kneaded product was formed into a pipe shape by an extrusion molding machine. The obtained molded product has a pipe shape with an outer diameter of 14 mm, an inner diameter of 8 mm, and a length of 600 mm.
It was about 90% of the theoretical compacted density. The molded body thus obtained was degreased at 600 ° C. in a nitrogen gas atmosphere, embedded in Si powder, and then treated in an electric furnace in a nitrogen gas atmosphere at a treatment temperature of 20.
Sintered by firing at 00 ° C. to react carbon powder with Si
It was made iC and sintered. Then, heat treatment is performed at a temperature of 2200 ° C. in a nitrogen gas atmosphere to remove Si by volatilization, and a relative density of 8
A 7% silicon carbide sintered body was produced.

Examples 2 to 3 The average particle size of β-SiC powder as silicon carbide powder was 11.
Example 1 except that those of 2 μm and 8.0 μm were used.
Manufactured in the same manner as.

Examples 4 to 5 As the silicon carbide powder, the compounding ratio of β-SiC powder and carbon powder was 70% to 30% by weight, respectively.
Manufacture was performed in the same manner as in Example 1 except that 40 wt% -60 wt% was used.

Examples 6 to 8 The same processes as in Example 1 were carried out except that the sintering was performed in a vacuum at 1500 ° C. and an Ar gas atmosphere, and the heat treatment was performed in a N ₂ gas atmosphere at 2200 ° C. and 2000 ° C. vacuum.

Comparative Examples 1 to 3 The average particle size of α-SiC powder as silicon carbide powder was 35 μm.
m and 0.7 μm were used, and manufactured in the same manner as in Example 1.

With respect to the sintered bodies of Examples 1 to 8 and Comparative Examples 1 to 3 produced as described above, three-point bending strength, nitrogen content,
The content ratio of 3C-type crystals, the specific resistance, and the temperature coefficient of resistance (TCR) from room temperature to 500 ° C were measured by X-ray diffractometry, and the results are shown in Tables 1 and 2.

[0031]

[Table 1]

[0032]

[Table 2]

As shown in Tables 1 and 2, the silicon carbide sintered body of the present invention has a negative temperature coefficient of resistance of −0 from room temperature to 500 ° C., as compared with the silicon carbide sintered body of the comparative example. .1%
/ It is less than ℃. Also, regarding strength, 50
The pressure is not less than MPa, and the strength is sufficient to withstand spiral groove processing on the heat generating portion. Further, as seen in Examples 1 to 8, according to the present invention, the electrical resistance can be adjusted by adjusting the blending ratio of β-SiC and C and the average particle size of β-SiC. Temperature coefficient is -0.1% / ℃
Has been maintained above.

Next, with respect to the silicon carbide sintered bodies of Example 1 and Comparative Example 1, a central portion of about 200 mm was grooved into a spiral shape to adjust the resistance value to a desired value, and then the result of energizing and heating up to about 1000 ° C. was obtained. As shown in FIG. Rt / R100 shown on the vertical axis
Regarding 0, the ratio of resistance values at each temperature when the resistance at 1000 ° C. is set to 1 is calculated. As can be seen from FIG. 1, the negative characteristic of the resistance change of the silicon carbide heating element of the present invention is reduced.

[0035]

As described above, according to the present invention, it is a high-density and high-strength reaction-bonded silicon carbide material, and the content of 3C type crystals, which is the crystal form of silicon carbide, is 40% or more, and Temperature coefficient of resistance from 0 to 500 ° C is -0.1%
It is possible to provide a silicon carbide heating element which has a negative characteristic of / ° C or higher and can be precisely controlled in this temperature range, and can be manufactured at low cost. Therefore, it is extremely useful as a ceramic heating element used in a high temperature industrial heating furnace or the like.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a heating element surface temperature and a resistance value ratio (Rt / R1000) of heating elements in Example 1 and Comparative Example 1.

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Claims (3)

[Claims] 1. A silicon carbide material having a relative density of 80% or more, which is an N-type semiconductor containing nitrogen, in which 3C type, which is a crystalline form of silicon carbide, accounts for 40% or more of the whole and has a bending strength of 50 MPa or more. A reaction-sintered silicon carbide heating element having a temperature coefficient of resistance from room temperature to 500 ° C. of −0.1% / ° C. or more. 2. After molding a mixed powder consisting of 30 to 80% by weight of β-SiC powder having a nitrogen content of 0.1% or more and an average particle size of 5 μm or more and 20 to 70% by weight of carbon powder,
2. The method for producing a reaction-bonded silicon carbide heating element according to claim 1, wherein the heating element is embedded in Si powder, fired at a temperature of 1400 to 2000 [deg.] C., and then heat treated to remove residual Si. 3. The β-SiC powder, which is produced by a silica reduction method in which coke and silica are reacted, and which contains unreacted carbon in an amount of 20 to 70% by weight as a mixed powder. A method for manufacturing the above described silicon carbide heating element.