JPS5923072B2 - silicon carbide heating element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved silicon carbide heating element that can be used stably for a long period of time.

Conventional heating elements are formed from recrystallized silicon carbide bodies obtained by adding and mixing silicon carbide powder with an organic binder, shaping the mixture, firing and carbonizing the organic binder, and then silicifying the carbon in the fired body. There is.

However, since this heating element made of recrystallized silicon carbide has a high porosity of 11 to 25%, its surface is oxidized during use to produce silicon dioxide, which cracks and peels off when cooled after use. , the heating element itself becomes thinner,
It becomes unusable in a short period of time. Furthermore, due to the formation of silicon dioxide, the resistivity of the heating element locally increases, causing the heating element to locally generate abnormal heat, further shortening its lifespan. For this reason, recently, after applying a mixture of clay and water lath to the surface of a recrystallized silicon carbide body,
A heating element has been proposed in which a rough protective layer is formed by heat treatment at 00° C., and the surface of the recrystallized silicon carbide body is isolated from the outside air to prevent oxidation. However, since the above-mentioned heating element has a glassy protective layer with a different thermal expansion coefficient on the surface of the recrystallized silicon carbide body, the recrystallized silicon carbide body and the glassy protective layer are disassembled while the power is repeatedly turned on and off. Cracks are generated due to the difference in thermal expansion between the two, and air enters through the cracks and reaches the surface of the recrystallized silicon carbide body, where it is oxidized and becomes unusable in a short period of time, as described above. On the other hand, as a result of intensive research in order to eliminate the above-mentioned drawbacks, the present inventor found that
By forming a dense silicon carbide film of a predetermined thickness on the surface of the recrystallized silicon carbide body with approximately the same coefficient of thermal expansion as the silicon carbide body by the CVD method, even if the power is repeatedly turned on and off, We have found a silicon carbide heating element with extremely long durability that does not cause cracks in the silicon carbide film and can isolate the surface of the recrystallized silicon carbide body from the outside air for a long period of time.

That is, the silicon carbide heating element of the present invention is formed by forming a dense silicon carbide film with a thickness of 10 μm or more on the surface of a recrystallized silicon carbide body by VCCVD.

The recrystallized silicon carbide body of the present invention can be applied to either a tubular or rod shape. The reason why the thickness of the dense silicon carbide film is limited as described above in the present invention is that if the thickness is less than 10μ, the surface of the recrystallized silicon carbide body cannot be sufficiently isolated from the outside air, resulting in a shortened lifespan. , the preferred thickness is 30-500
It is in the range of μ.

In order to obtain the silicon carbide heating element of the present invention, for example, after inserting a recrystallized silicon carbide body inside a cylindrical body, raw material gas consisting of a silicon source and a carbon source is introduced under reduced pressure, and the raw material gas is A silicon carbide heating element is manufactured by heating the silicon carbide to a reaction temperature to gradually precipitate a reactant on the surface of the recrystallized silicon carbide to form a dense silicon carbide film.

Next, an embodiment of the present invention will be described using the silicon carbide film forming apparatus shown in the first diagram.

Embodiment As shown in FIG. 1, a cylindrical graphite electrode 4 with an inner diameter of 90wn1 and an outer diameter of 100mm is placed inside an outer shell 3 which is airtightly installed on a pedestal 2 of a table 1. , a recrystallized silicon carbide tube 5 with a length of 180011st is arranged, and a water-cooled lid 6 is airtightly attached to the upper end of the outer shell 3 via an O-ring 7, and then recrystallization is carried out using a supply pipe 8 inserted into the table 1 and the pedestal 2. 2 cc/min of trichloromethylsilane (raw material gas) and 2 cc/min of hydrogen gas in the silicon carbide tube 5.
．． cc/Min inflow, and table 1 and pedestal 2 at the same time.
The recrystallized silicon carbide tube 5 is connected to the double-tubular supply tube 9 inserted into the
While supplying similar amounts of raw material gas and hydrogen gas to the cylindrical space between the cylindrical space and the graphite electrode 4, the recrystallized silicon carbide tube is forcibly evacuated using the vacuum pump 11 of the exhaust pipe 10 inserted into the water-cooled lid 6. 5
The inner space and the cylindrical space between the tube 5 and the graphite electrode 4 are 30t0rr.
And so.

Next, an induction heater 12VC placed around the outer periphery of the outer shell 3
While energizing, the vertical drive chain 13 fixed to one end of the induction heater 12 is operated by the deceleration motor 14 placed on the table 1, and the induction heater 12 is guided along the shell 3 by the support 15. Move it in the direction of arrow A,
The cylindrical graphite electrode 4 inside the outer shell 3 is heated to 1300° C. in a zone to gradually precipitate reactants of the raw material gas on the inner and outer surfaces of the recrystallized silicon carbide tube 5, resulting in dense carbonization with a thickness of 250 μm on the inner and outer surfaces. A silicon carbide heating element was obtained by forming a silicon film. Comparative Example After applying a mixture consisting of clay and colloidal silica to the inner and outer surfaces of a recrystallized silicon carbide tube having the same dimensions as in the previous example, 13
A heating element was obtained by heat treatment at 00° C. to form a siliceous protective layer with a thickness of 500 μm on the inner and outer surfaces.

After impregnating both ends of the heating elements of the above Examples and Comparative Examples with silicon to form terminal parts, power was applied to these heating elements, and when the surface temperature reached 1400°C, the power was cut off and the temperature was increased to 500°C. The resistance increase rate of each heating element was examined by repeating the power-on/power-off cycle in which the power was turned on again when the temperature dropped to .

The result was as shown in Figure 2. In addition, R1 in Fig. 2
R2 is the resistance increase curve for the heating element of the example, and R2 is the resistance increase curve for the heating element of the comparative example. As is clear from Fig. 2, the resistance of the conventional heating element increases by about 25% after 300 power-on and power-off cycles, whereas the resistance of the heating element of the present invention increases by approximately 25% after 300 power-on and power-off cycles. It can be seen that the resistance hardly changes and it can be used stably for a long period of time. As detailed above, according to the present invention, even if the power is repeatedly turned on and off, cracks do not occur in the dense silicon carbide film as a protective layer, and the surface of the recrystallized silicon carbide body remains intact for a long period of time. It is possible to provide a silicon carbide heating element that can be isolated from the outside air, maintains stable heat generation characteristics for a long period of time, and has an extremely long service life.

[Brief explanation of the drawing]

FIG. 1 is a partial sectional view showing a silicon carbide film forming apparatus used in an example of the present invention, and FIG. 2 is a diagram showing the rate of increase in resistance of each heating element with respect to the number of power-on/power-off cycles. 3... Outer shell, 4... Cylindrical graphite electrode, 5... Recrystallized silicon carbide tube, 8... Throat supply pipe, 9... Double tubular supply pipe,
10... Scarlet tube, 11... Vacuum pump, 12... Induction heater.

Claims (1)

[Claims] 1 The surface of the recrystallized silicon carbide body is coated with a thickness of 10 mm by the CVD method.
A silicon carbide heating element formed by forming a dense silicon carbide film of μ or more.