FR2575458A1 - COMPACT FRITTE SILICON CARBIDE BODY, HEATING ELEMENT FOR SUCH A BODY, AND HEATING APPARATUS CONTAINING SAID HEATING ELEMENT - Google Patents

Abstract

BODY IN FRITTED SILICON CARBIDE WITH HIGH MECHANICAL AND THERMAL RESISTANCE. THIS BODY CONTAINS APPROXIMATELY 60 TO 94 BY WEIGHT OF SILICON CARBIDE, ABOUT 0.5 TO 20 BY WEIGHT OF ALUMINUM NITRIDE, AND ABOUT 2 TO 20 BY WEIGHT OF CARBON. THE BODY MAY CONSTITUTE THE INTERNAL TUBULAR HEATING ELEMENT 23 OF A QUARTZ TUBULAR 22 OVEN SURROUNDED BY A HIGH FREQUENCY WINDING 21, TO HEAT FOR EXAMPLE A TUBE 2. APPLICATION: ENGINEERING CERAMIC MATERIAL, HEATING ELEMENT OR STABLE AT HIGH TEMPERATURE.

Description

The present invention relates to a sintered silicon carbide body, which

  has a high mechanical strength and a high resistance to heat, and which can serve as

  ceramic material for engineering and heating element,

  especially for high frequency heaters. The sintered silicon carbide bodies serve as

  parts and heating elements that can withstand

  high temperatures, because of their superior resistance to

  heat and their resistance to thermal shock. We do

  sintering, in order to manufacture the sintered body,

  under pressure or at normal pressure.

  Sintering under pressure gives dense and high purity sintered bodies. However, this process can be applied only to products of simple shapes and, in addition, it

  requires complex and expensive equipment.

  In the case of a conventional carbide sintered body

  of silicon, produced by sintering at normal pressure,

  describes me in Japanese Patent Publication No. 57-40109,

  No. 58-14390 or No. 59-52948, boron (B) or car-

  boron bure [B4C) as sintering aid with carbon dioxide

  ne (C). However, the reaction is a reaction in the

  lide, so that properties of the sintered body are affected.

  by a dispersion of the sintering aid. In addition, the conventional sintered body has a mechanical strength of about 280 MPa at high temperatures, so that it is unsatisfactory in some fields of application

  and in some modes of application. In general, the resistance

  The mechanical strength of the sintered body deteriorates at high temperature because the bond strength between grains of the sintered body decreases. Recently, he applied

  requiring greater mechanical strength at temperatures

  high temperatures equal to or greater than 1400-C.

  In addition, we use, as an element of

  resistance heating of about 14009C,

  heating elements of silicon carbide. The heating element

  silicon carbide is produced by silicifi-

  cation of the mixture of silicon carbide powder,

  silica and carbon powder at about 2400 ° C. The-

  the heating element has a porosity equal to or greater than

  at 20%, and it has a specific electrical resistance of

  0.5 ohm.erm at room or normal temperature.

  However, the classic element of heating risk

  to degrade by oxidation in air at elevated temperatures.

  in other words, the heating element has a low bond strength between the grains, so that the grain boundary is subject to oxidation. Oxidation forms non-conductive silicon dioxide which, at high temperature,

  reacts with silicon carbide to volatilize. AIN-

  if, the properties and integrity of the heating element

  deteriorate a lot. In particular, the heating element

  fage can not be used for high frequency heating.

  Similarly, the conventional heating element has a specific electrical resistance of about 0.1 ohm.em to 1000 QC but it is impossible to vary this value.

according to the desires.

  In addition, as shown in the appended FIG. 4,

  when an object 2, such as a tube, having a circulating section

  re, is heated using the classic heating element

  while this object is inserted in a central tube furnace

  tral 3, it is necessary to assemble at

  the heating elements 1, in the form of rods.

  heated by direct application of the current so as to

  keep a uniform heating. However, the operation of a

  such an assembly of heating elements is complex.

  The present invention therefore aims to provide: - an improved body of sintered silicon carbide; - a sintered silicon carbide body, which is chemically stable, and has a higher mechanical strength and thermal resistance; a heating element made of silicon carbide, which is chemically stable, has characteristics

  superior strength and thermal resistance

  that and can be used for heating-high frequency; and an improved ceramic material made of carbur

d. silciuni, for lngénierle.

  In the present invention, a carbide body of sintered silicon comprises about 60% to 94% by weight of

  silicon carbide, about 0.5% to 21% by weight of nitrile

  of aluminum, and about 2% to 20% by weight of carbon.

  More particularly, this body contains from 0.5% to 5% by weight of aluminum nitride. Aluminum nitride can

  include aluminum oxynitride.

  The invention also relates to a heating element

  Sage, consisting of, or comprising, a sintered silicon carbide body as defined above. This element

  In particular, the heating circuit may have an electrical resistance

  that specific between about 0.01 ohm.cm and 10 ohms.cm.

  This heating element can in particular be used as a high frequency heating element in a heating device

high frequency.

  Other objects, features and benefits of

  the present invention will emerge from the detailed description of

  Hereinafter, certain preferred embodiments

  illustrated by way of illustration and in no way limitative with reference to the accompanying drawings, in which FIG. 1 is a graph showing the flexural strength (in MPa, on the ordinate) as a function of the content

  aluminum nitride (in percentages by weight, in abscis

  its) sintered bodies according to the invention;

  Figure 2 is a graph showing the consumption of

  electrical energy (in kVA, on the ordinate) of the elements

  of the present invention, depending on the temperature

  temperature (in O0, on the abscissa); FIG. 3 is a perspective view of the element

  of the present invention, when used for heating

  wise; and FIG. 4 is a perspective view of the element

  conventional heating, when used for heating.

  The present inventors studied the stresses and stresses appeared. nt. in a silicon carbide body

  sintered. It has thus been found that characteristic properties

  sintered body ticks, such as flexural strength or

  chemical stability, are greatly affected by the

  oxygen neur from silicon carbide used as raw material. The surface of the silicon carbide oxidizes with air,

  at room temperature, and the oxidation is particularly

  pronounced when particles are extremely

  nes, and have for example a diameter equal to or less than I.m.

  In general, the amount of oxygen present in car-

  silicon fiber increases in proportion to the specific surface area

  silicon carbide particles. In-other

  my, the smaller the particle size, and the more

  There is oxygen in the silicon carbide.

  In the present invention, aluminum nitride is used as a sintering aid. Aluminum nitride

  It is preferable to boron. It gives a heavier sintered body.

  as aluminum nitride allows the SiC system to

  AlN-C to advance the reaction in the liquid phase. This-

  during, the nitride is easily affected by the pre-

  smell in silicon carbide. So when we add

  aluminum nitride to silicon carbide containing

  1% by weight of oxygen, it can not have its effect

  superior as sintering aid. That is why he

  comes that the oxygen content of the silicon carbide is

  as low as possible when using aluminum nitride

  as sintering aid. In other words, if aluminum nitride is added to the silicon carbide, it is necessary to keep the size of the silicon carbide particles as large as possible. In terms of robustness

  of the sintered body, it is better that the dimension of

  silicon carbide is as low as possible, but for the above reason, it is not possible to use

  silicon carbide in fine particles.

  The inventors of the present invention have found it possible to combat the annoying effect. exerted by the oxygen gas in the silicon carbide by adding carbon to the reaction system. Aluminum nitride is added,

  as a sintering aid, with carbon carbide

  licium so that the sintering is carried out under ideal conditions. The aluminum nitride added to the silicon carbide becomes a liquid phase at the sintering temperature of

  2000 C or more without being affected by oxygen. The alumina

  of the liquid phase is uniformly replaced by

  silicon, so that the resulting sintered body is homogeneous

  and it has a great mechanical strength and a

high chemical stability.

  In the present invention, the carbon content is between about 2% and 20% by weight. If the proportion of carbon is less than about 2% by weight, the resulting sintered silicon carbide body is dense, but it has low mechanical strength at elevated temperatures and has a high specific electrical resistance, exceeding 10 ohms. cm, which is inconvenient for a heating element. Likewise, if the proportion of carbon exceeds about 20% by weight, the resulting sintered silicon carbide body has low density and low strength.

  between the grains which, therefore, deteriorates the resistance

  mechanical strength. In addition, a surplus of carbon prevents sintering of the silicon carbide and gives a lot of free carbon, which gives the sintered body resistance to

oxidative deterioration.

  On the other hand, the aluminum nitride content is in the present invention between about 0.5% and 20% by weight. If the proportion of aluminum nitride is less than about 0.5% by weight, the resultant sintered silicon carbide body is less dense and has a low density. Such a sintered body has a low resistance

  binding mechanism and may degrade after a

  repeated use as a heating element. Conversely, if the proportion of aluminum nitride exceeds about 20% by weight, the sintering of the silicon carbide is not satisfactorily performed, and the resultant sintered body has a low mechanical strength. Likewise, such a heating element made of silicon carbide does not give off heat in

  because of its high specific electrical resistance. By way of

  more forgiving in terms of mechanical strength of

  fried body '+, the aluminum nitride

  0.5% and 5% by weight. In the present invention,

  the aluminum oxide may include aluminum oxynitride

Niumo

  A theoretical density of silicon carbide

  It is 3.21 g / cm3. In the present invention, the density of the sintered silicon carbide body is desirably between about 70% and 99% of the theoretical value of the density of the silicon carbide. Of

  even more desirable in terms of mechanical resistance

  the sintered body, the density is between

  80% and 98% of the theoretical value for carbide

silicon.

  The invention will be more easily understood by reference

  the following non-limitative examples:

Example 1

  A silicon carbide powder, whose particles have a mean diameter of 1 .mu.m, is reduced using a grinder and acetone without water to obtain a powder of which

  the particles have an average diameter of 0.5 micron. We mix

  the silicon carbide powder with car-

  bone and aluminum nitride powder, according to a report

  mixing port shown in Table I. The average particle size of the aluminum nitride powder is

  3 mm. Phenolic resin is used as binder. We

  The formulated powder is formed and sintered under normal pressure in an argon atmosphere at 2100 ° C. Dense bodies each having a density of 80% by weight are obtained. the theoretical value for silicon carbide (3.21 g / cm3). The flexural strength of the sintered bodies at -, is shown in Table 1. It is also shown in the graph of Figure i (in} Pa, in ordinates) in -1 depending on the aluminum nitride content, in percentages

  by weight (as abscissa). It should be noted that the resistance

  Bending is low when the proportion of aluminum is less than 0.5% by weight or greater than 5% by weight. The above-mentioned characteristic properties suggest that the sintered silicon carbide body, which contains 0.5% to 5% by weight of aluminum nitride, will be able to serve as an engineered ceramic material, for example for

  wire drawing and for heat exchanger ducts

their for waste gases.

TABLE I

  Composition according to (% by weight) bending (Ifa) SiC C A1 N Comparative Example 1 95.0 5.0 O 280

1-1 94.5 5.0 0.5 550

1-2 94.0 5.0 1.0 590

1-3 93.0 5.0 2.0 610

Examples 1-4 92.0 5.0 3.0 620

1-5 91.0 5.0 4.0 620

1-6 90.0 5.0 5.0 570

1-7 89.0 5.0 6.0 500

Example 2

  In Example 2, the sintered silicon carbide bodies are obtained in the same way as in Example 1, according to the mixing ratio shown in Table II. Density and flexural strength at 1400 C are shown in Table II. As can be seen from Table II, the body

  of the present invention has a high mass density

  that, equivalent to 70% to 99% of the theoretical value for silicon carbide. Similarly, the sintered body presents

  High resistance to heat at high temperature

  greater than or equal to 14009C. So the silicon carbide body

  Sintered cumr of the present invention is well suited as

  It can withstand high temperatures.

1 TABLE I I

  Composition Mass Vol. Resistance to Example (% by weight) lumen SiC AlN flexion (g / cm 3 (MPa)

2-1 90.0 5.0 5.0 3.18 550

2-2 85.0 5.0 10.0- 3.05 450

2-3 75.0 5.0 20.0 2.79 410

2-4 92.5 2.5 5.0 3.04 500

2-5 87.5 7.5 5.0 2.92 440

2-6 75.0 20.0 5.0 2.49 380

Example 3

  In the same way as in Example 1, we obtain

  In the mixing ratio shown in Table III, the silicon carbide heaters. Density and specific electrical resistance at room temperature

  are shown in Table III. As is apparent from this

  III, the resistance can be adjusted according to the wishes

  specific electric heating element of the present

  the invention in the range of 0.01 to 10 ohms.cm,

  composition, and this element has a density equivalent to 70% to 99% of the theoretical density.

  silicon carbide. According to the present invention, the

  heated silicon carbide has a density

  high and a low specific electrical resistance. From

  the silicon carbide heating element has a resistance

  bending strength of between 300 MIPa and 550 MPa.

TABLE III

  Composition (% by weight) Mtasse vo- Specific light resistance Example SiC C AIN (g / cm3) (ohm.cm) 3-1 94.0 (5.0 1.0 2.80 8, n

3-2 90.0 5.0 5.0 3.05 0.1

3-3 85.0 5.0 10.0 2.97 0.2

3-4 75.0 5.0 20.0 2.79 10.0

3-5 94.0 1.0 5.0 2.50 26.0

3-6 92.5 2.5 5.0 3.04 3.9

3-7 87.5 7.5 5.0 2.92 0.5

3.8 75.0 20.0 5.0 2.49 0.1

  The carburized heating elements are then used in high frequency heating experiments.

  examples 3-9 to 3-11 and the comparative example

  6 .. 2 presented in Table IVo The results of the experiments s Gnt represented graphically in Figure 2 on which

  Consumption is transferred to electrical energy (kVA, in

  data) as a function of the temperature (in C, on the abscissa).

  As can be seen in FIG. 2, the heating elements of Examples 3-9 to 3-11 are suitable for high temperature high temperature heating. FIG. 3 shows a silicon carbide heating element of the present invention. , which is used for high frequency heating

TABLE IV

Composition (% by weight) Mass

  SiC C AlN light (g / cm 3) Example 3-9 95.0 2.5 2.5 3.04 Example 3-10 90.0 5.0 5.0 3.05 Example 3-11 85.0 7, 5 7.5 2.92

Comparative example

  The high frequency coil 21 is wound outside the quartz tube furnace 22 which contains the silicon carbide heating element 23 in the form of a central cylinder. The cylindrical shape is desirable for high frequency heating. A tube 2 to be heated is

  inserted into the heating element 23. When an electric current

  that is applied to the winding 21 for high frequency, the e-

  the heating element 23 of silicon carbide continuously generates

  heat, and tube 2 is heated to a temperature of

  uniform structure. In addition, the cylindrical structure of the

  heating element 23 makes it possible to heat a long tube 2 of an

  to the other, evenly when the tube is moved in the direction of its length. So, the heating element in

  The silicon carbons of the present invention are

  It is able to produce uniform heating

  Likewise, the heating element is advantageous compared to

  a conventional heating element that requires assembly steps

wiring.

  As described in detail above, according to the present

  In the invention, a sintered silicon carbide body has chemical stability and high mechanical strength.

  due to the presence of an appropriate amount of nitrile

  aluminum and carbon which have been incorporated therein. This high mechanical strength allows a decrease in thickness and therefore a reduction in weight. Similarly, the sintered body has superior heat resistance. Finally, the sintered silicon carbide body of the present invention is suitable for use as a ceramic engineering material, such as a roll

  conveyor or parts of machinery, to serve as

  elements that must withstand high temperatures or heat

heating elements.

1 1

Claims (10)

REVENI) I CAT IONS   1. Sintered silicon carbide body, comprising   silicon carbide, aluminum nitride and   carbon, characterized in that it comprises about 60 $ to 94 -   by weight of silicon carbide, about 0.5% to 20% by weight of alumininum nitride and about 2% to 2% by weight of carblone.   Sintered silicon carbide body according to claim 1, characterized in that the nitride content   of aluminum is between about 0.5% and 5% by weight.   Sintered silicon carbide body according to claim 1, characterized in that the aluminum nitride   comprises 1 aluminum oxynitride.   Sintered silicon carbide body according to claim 1, characterized in that its density is   equivalent to about 70% to 99% of the theoretical density. only silicon carbide.   Sintered silicon carbide body according to claim 1, characterized in that its density   equivalent to about 80% to 98% of the theoretical density. only silicon carbide.   6. Sintered silicon carbide body according to   claim 1, characterized in that it has a resistance   specific electrical current between about 0.01 ohm.cm and 10 ohms.ecm.   7. Heating element, characterized in that it   includes, or includes, a fired silicon carbide body   tee according to any one of the preceding claims.   Heating element according to claim 7,   characterized by having a specific electrical resistance   from about 0.01 ohm.cm to 10 ohm.cm.   9. High frequency heating apparatus comprising   as a high frequency heating element, characterized in that   that this heating element is or includes a heating element ghost according to claim 7.   10. Ceramic potion of ingenidia, characterized   in that it is or involves a body of silicon carbide   ciunm úri'tt sA] any one of claims 1 to. 6.