DE29909497U1 - Kiln furniture - Google Patents

Description

The invention relates to firing aids for porcelain firing, for sintering metals and hard metals and for sintering electronic components.

Kiln furniture refers to those devices or parts of devices that are required to hold porcelain to be fired or metal or hard metal to be sintered or electronic components to be sintered during the firing or sintering process. This includes the plates on which the porcelain or metal, hard metal parts or electronic components are placed during firing. However, supports that are intended to hold these plates and rollers for roller kilns are also parts of this kind that are summarized under the term kiln furniture.

The material properties of such kiln furniture are increasingly subject to greater demands. For example, porcelain is now fired using modern technology exclusively in so-called fast-firing kilns, with cold/cold cycle times in the order of 5 to 8 hours. Due to the higher

Due to the high thermal loads, the previously used kiln furniture made of clay-bonded silicon carbide or cordierite can no longer be used in conjunction with rapid firing technology. Instead, recrystallized SiC (RSiC) or fine-grained, nitrite-bonded SiC (NSiC) are used. Well-known examples of these materials are "Crystar" and Advancer from St. Gobain/Norton. In addition to plates, supports and columns as well as rollers for roller kilns are also made from these materials.

The main disadvantages of RSiC and NSiC are the porosity (up to 20 percent by volume) and the coarse-grained structure. Due to the open pores, the material is susceptible to oxidation, i.e. a quartz layer grows in the pore spaces over time, which ultimately leads to mechanical destruction of the component. Due to the coarse structure of crystallites and pores, the strength is limited to 100 to 200 MPa. Therefore, plate thicknesses of 4 to 6 mm cannot be undercut. The low strength and high brittleness are responsible for too high a reject rate due to breakage during handling.

The kiln furniture discs made of RSiC and NSiC are coated on one side with a so-called engobe, the purpose of which is to prevent the glaze from penetrating the pores of the kiln furniture. In conjunction with the oxidation of the silicon carbide, this would lead to premature mechanical destruction of the part. Oxide ceramic materials are used as engobes, which, due to their chemical composition, have a strong tendency to adhere to the glaze. For this reason, the manufacturer must rework the kiln furniture discs several times over the course of their service life (grinding and recoating). The average service life for kiln furniture discs made of RSiC and NSiC is 800 to 1500 rapid firing cycles.

In addition to the negative properties such as susceptibility to oxidation and low strength, the internal oxidation leads to further disadvantages in the application. The warping of the discs causes unevenness in the support of the firing material, so that

Manual grinding of the porcelain may be necessary to achieve a level surface. Discarding the plates can also cause silicon carbide particles to break off from the underside of the disc and fall onto the fired item underneath. This creates black spots in the glaze and the affected porcelain piece is rejected. Due to their porous structure, RSiC and NSiC have a comparatively low thermal conductivity, which is seen as a disadvantage with regard to the fastest and most homogeneous heating of the firing aid and the fired item.

The sintering of metals and hard metals (for example tungsten carbide/cobalt) takes place on kiln furniture made of aluminum oxide or graphite. The high density of hard metals of around 14 g/cm ³ , coupled with the low strength of graphite, requires very thick plates. This leads to poor utilization of the combustion chamber, as the majority of the furnace volume is taken up by kiln furniture and not by the material to be fired. In addition, undesirable reactions can occur between the material to be fired and the kiln furniture (for example carburization of the hard metal surface) and caking.

In comparison to non-oxide high-performance ceramics such as silicon carbide and nitrite, aluminum oxide experiences a drastic drop in all mechanical properties at temperatures as low as 700 ^° C. At 1000°C, the strength is less than 50% of the strength at room temperature. Even small loads at working temperatures lead to plate deformations due to creep processes, which render the material unusable. In the worst case, the firing substrates break, so that the entire firing material is rejected. Aluminum oxide is also used in the sintering of electronic components, with the disadvantages mentioned above.

The object of the invention is to provide a kiln furniture that is universally suitable for porcelain firing, for sintering metals and hard metals and for sintering electronic components, whereby it is suitable for the working temperature

■»■· lit* ·· **

temperature, has a high level of strength and, on the other hand, has a longer service life than the known kiln furniture.

According to the invention, this object is achieved by using the materials according to the characterizing part of claim 1. Accordingly, densely sintered solid state silicon carbide (SSiC) is used. Furthermore, densely sintered liquid phase silicon carbide (LPSiC) or silicon nitride (Si ₃ N ₄ ) can also be used. Preferred embodiments of the inventions arise from the subclaims following the main claim.

When it comes to porcelain firing, densely sintered silicon carbide (SSiC) can be described as the ideal material for this application, as it forms a material that has essentially no surface pores and is very strong when firing porcelain, where temperatures of up to 1400 ⁰ C can occur. Due to the smooth surface, the otherwise usual engobe is not required.

Due to the significantly higher strength compared to the RSiC used in the state of the art, the thickness of the plate can be reduced by about half, which of course has a positive effect on the energy balance of the firing process. Since oxidation inside the material is excluded, a significantly longer service life can be expected, whereby in this case the service life can be at least doubled.

The densely sintered silicon carbide can be produced by thermoplastic molding, so that parts with high flatness and dimensional stability can be produced without post-processing. Overall, when using the kiln furniture according to the invention, the waste during porcelain firing can be reduced or eliminated and the quality improved.

For the sintering of metals and hard metals, which takes place at about 1200°C in an inert atmosphere, the three materials SSiC, LPSiC and Si ₃ N ₄ are advantageous.

in question. In this case, the optimization in terms of plate thickness, high-temperature creep resistance and chemical inertness to the metal or hard metal determines the material selection. Depending on the material, the plate thickness can be reduced by a factor of 7 to 10, which significantly increases energy efficiency and the utilization of the furnace space. At the same time, the service life of the new kiln furniture can be expected to be improved compared to graphite and aluminum oxide, as used in the prior art.

When sintering electronic components using kiln furniture made of densely sintered SSiC, LPSiC and S13N4 instead of the aluminum oxide used previously, a significant increase in the service life of the kiln furniture plates can be expected. At the same time, the reject rate of the kiln material can be reduced and the quality of the fired components can be increased.

Further details and advantages of the invention are explained with reference to an embodiment shown in the drawing.

The only figure shows a mobile frame for use in porcelain firing.

The carriage 10 shown in the figure has support plates 12 made of densely sintered solid state silicon carbide (SSiC), which forms a flat support surface. These support surfaces 12 are held by supports 14, which advantageously also consist of densely sintered solid state silicon carbide. Porcelain plates 16, which are to be fired, stand on the flat support surfaces and are fed into the fast-firing furnace (not shown here) by means of the carriage.

In addition to the plates 12 shown here, which are made of densely sintered solid state silicon carbide, rollers for roller furnaces can also be made of this material, as well as the corresponding plates on which the material to be fired is placed and which are transported over the rollers of the roller furnaces.

In the following table, the kiln furniture materials according to the invention are compared with kiln furniture materials according to the state of the art in terms of their physical properties.

Table

Comparison of kiln furniture materials:

""~~--^^^Materials
Properties----^^^ RSiC NSiC SSiC LPSiC _{Si3N4 ​ Al2O3 ​} Type of porosity open open closed closed closed closed Pore volume in % 15-25 15-25 0-5 .0-5 0-5 0-5 Size pores and
Crystals in &mgr;&igr;&eegr; up to 500 up to 250 until 50 until 20 until 20 until 20
I Density in g/cm" ³ 2.60-2.85 2.60-2.85 £3.10 >3.20 >3.20 > 3.80 years Strength in MPa at
RT
1,200 ⁰ C 100
200 200
200 400
450 550
420 800
425 350 ^:
<100 ^; WLK in W/m K 25 25 120 85 35 10 Oxidation resistant
ity* 6 4 2 3 3 1 HT creep behaviour * 2 3 1 3 3 6

* qualitative rating from 1 (excellent) to 6 (very poor)

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From the properties listed in the table, it is clear that the densely sintered materials SSiC, LPSiC and Si ₃ N ₄ according to the invention have particularly good physical properties for the task to be solved here. While RSiC and NSiC already show the disadvantages described above due to their open pores, AI2O3 has very low strength, particularly in the high temperature range, which leads to the problems in use also mentioned above (large plate thicknesses, risk of deformation and breakage). The materials SSiC, LPSiC and Si ₃ N ₄ according to the invention each have physical properties that make them suitable for use as kiln furniture compared to the comparison materials.

Claims (5)

1. Kiln furniture for porcelain firing, for sintering metals and hard metals and for sintering electronic components, characterized in that it consists of densely sintered solid state silicon carbide (SSiC), liquid phase silicon carbide (LPSiC) or silicon nitride (Si ₃ N ₄ ). 2. Kiln furniture according to claim 1, characterized in that the solid state silicon carbide (SSiC), the liquid phase silicon carbide (LPSiC) and the silicon nitride (Si ₃ N ₄ ) are each sintered so densely that they have closed pores. 3. Kiln furniture according to claim 1 or claim 2, characterized in that the density of the solid state silicon carbide (SSiC) is greater than 3 g/cm ³ . 4. Kiln furniture according to claim 1 or claim 2, characterized in that the density of the liquid phase silicon carbide (LPSiC) is greater than 3 g/cm ³ . 5. Kiln furniture according to claim 1 or claim 2, characterized in that the density of the silicon nitride (Si ₃ N ₄ ) is greater than 3 g/cm ³ .