DE102009060939A1 - Electric heating element for high temperature oven, has one-side closed ceramic tube filled with electrically isolating material that is disintegrated into inert and oxygen-binding gas components at high temperatures - Google Patents

Abstract

The element has a molybdenum- or tungsten heating conductor arranged in a one-side closed ceramic tube (1). The ceramic tube is filled with an electrically isolating material e.g. aluminum nitride and silicon nitride, that is disintegrated into inert and oxygen-binding gas components at high temperatures. The ceramic tube is closed at an open end with a copper cap (2). The cap is sealed against the ceramic tube with a press gasket (3). A ceramic hollow cylinder (7) with a spiral duct is arranged within the ceramic tube.

Description

The invention relates to an electrical heating element for high temperatures, at least comprising a molybdenum or tungsten heating conductor.

• • Metallische Heizleiterlegierungen mit den Komponenten Ni, Cr, Fe und/oder Al
• • Hochschmelzende Metalle wie Mo, W und ihre Legierungen
• • Platinmetalle wie Pt, Rh und ihre Legierungen
• • Kohlenstoff/Graphit
• • Keramische Elektronenleiter wie SiC, MoSi₂, LaCrO₃
• • Ionenleitende Keramiken wie stabilisiertes ZrO₂.

In resistance-heated electric furnaces, the electric current flows through a heating conductor, where it releases heat depending on current intensity and ohmic resistance. The most commonly used heating conductors are:

• • Metallic heating element alloys with the components Ni, Cr, Fe and / or Al
• • High-melting metals such as Mo, W and their alloys
• Platinum metals such as Pt, Rh and their alloys
• • carbon / graphite
• • Ceramic electron conductors such as SiC, MoSi ₂ , LaCrO ₃
• • Ion-conducting ceramics such as stabilized ZrO ₂ .

Each heating element has a specific temperature range, in which it can be used without damage. This temperature range is influenced by the surroundings of the heating conductor, in particular the surrounding gas atmosphere to a considerable extent. A classification of gas atmospheres could roughly distinguish chemically from oxidizing, reducing, carborating, inert atmospheres and vacuum. There is no heat conductor that works at temperatures above 1400 ° C in both oxidizing and reducing gas atmospheres.

The refractory metals, especially molybdenum and tungsten, are widely used as high-temperature heating conductors up to temperatures of 1900 ° C. and above. However, their use is limited because they are chemically attacked in an oxidizing atmosphere, at high water vapor partial pressures, in the presence of carbon oxides and hydrocarbons. The oxides are also volatile.

There have been many proposals to extend the field of application of molybdenum and tungsten to oxidizing atmospheres. It has been proposed to provide Mo and W and other oxidation-sensitive heating conductors with a highly heat-resistant coating. As the material of such a coating, aluminum in combination with a low melting point metal such as In, Sn or Ga was considered (Disclosure DE 2014460 from 17.12.1970 or GB 130 1265 A from 29.12.1970). In another publication ( GB 7098 06 A dated 2.6.1954) Si is recommended as a layer material up to operating temperatures up to 1500 ° C in an oxidizing atmosphere. When cracking, this layer should also be self-healing.

In ( GB 490 116 A from 9.8.1938) oxidic protective layers of SiO ₂ , Cr ₂ O ₃ and Al ₂ O _{3 are} preferred.

At an early stage it was also considered to densely encapsulate the metallic heating conductor in a ceramic tube ( GB 5060 98A from 3.3.1939 or GB 499 447A from 24.1.1939). In an improvement of this arrangement, it has been proposed to include getter metals such as Zr, Th, Ba or Na in the ceramic tube simultaneously with the heating conductor wire ( GB 595060 A from 26.11.1947).

• • Bei hohen Temperaturen werden alle oxidischen Materialien sauerstoffdurchlässig. Die Sauerstoffpermeabilität ist eine Eigenschaft der dichten, porenfreien Materialien. So beginnt auch bei gut gesintertem Sinterkorund ab 1400°C eine signifikante Sauerstoffdurchlässigkeit.
• • Bei Schichten sind meist die Differenzen im thermischen Ausdehnungsverhalten Ursache der Ausbildung von Rissen und anderen Defekten, die den Sauerstoffangriff ermöglichen.
• • Beim Aufheizen eingekapselter Heizleiter treten deutliche Druckänderungen auf. Beim Aufheizen auf 1700°C beispielsweise steigt der Druck auf etwa das Siebenfache. Diese Druckschwankungen wirken sich ungünstig auf die Rohrverschlüsse und elektrischen Kontaktdurchführungen aus.

All of these proposals have not led to a sweeping success. Only when combined with Si has a powerful heating conductor emerged, which can no longer be used in a reducing atmosphere and in a vacuum at temperatures above 1400 ° C. The failure of the many proposals to stabilize Mo and W at high temperatures in an oxidizing atmosphere is probably due to three effects:

• • At high temperatures, all oxidic materials become oxygen permeable. Oxygen permeability is a property of dense, nonporous materials. Thus, even with well-sintered sintered corundum starting from 1400 ° C, a significant oxygen permeability.
• • In the case of layers, the differences in the thermal expansion behavior are usually the cause of the formation of cracks and other defects that make the oxygen attack possible.
• • When heating encapsulated heating conductors occur significant pressure changes. When heated to 1700 ° C, for example, the pressure increases about seven times. These pressure fluctuations have an unfavorable effect on the tube seals and electrical contact bushings.

The object of the invention is to provide an electrical heating element with molybdenum or tungsten heating, which overcomes the disadvantages of the known solutions.

The object is solved by the features of claim 1. Here, the electric heating element is arranged in a ventilated aluminum oxide tube, which is additionally filled with an electrically insulating material which decomposes at high temperatures into both inert and oxygen-binding gaseous components.

A ventilated alumina tube is a unilaterally or bilaterally open tube, which is connected to a protective gas reservoir, whereby the pressure change is equalized with temperature change. The gas at the pipe ends can be both stationary and flowing. The filling of the ceramic tube with inert gas does not lead to a permanent protection of the high temperatures Heating conductor before oxidation. Only the additional introduction of thermally unstable compounds, which decompose into predominantly gaseous, on the one hand in inert and on the other hand in oxygen-binding constituents, surprisingly leads to a long-lasting stability of the metallic heat conductor at high temperatures. The higher the temperature, the better the protection appears to be. The upper temperature limit for Mo and W is only limited by the upper service temperature of the pipe material (1900 ° C for aluminum oxide ceramics).

Materials which decompose in this way are the electrically insulating or semiconductive nitrides of some main group elements II; III or IV such as Mg, B, Al, Ga, In, Si. These nitrides decompose at high temperatures into nitrogen and the respective metal. The metals have consistently a higher affinity for oxygen than Mo and W. The decomposition temperatures are different. Mg, Ga and In nitride decompose even at temperatures around 1200 ° C, BN, however, only noticeably above 2000 ° C. In the temperature range of 1500 to 1800 ° C provided for the heating elements, the nitrides of aluminum and silicon are to be preferred. Aluminum nitride has a decomposition pressure of about 10 ^-4 bar at a temperature of 1700 ° C. The decomposition pressure of silicon nitride is slightly higher. At these temperatures, the metals in these small amounts are present as gases. It may just be the vapors of these metals, which lead to an effective protection of the heat conductor. Both Al and Si react with oxygen which permeabilizes through the wall of the ceramic tube. Al forms Al ₂ O ₃ in the reaction, which is chemically identical to the tube wall. Si also reacts with oxygen. Non-oxygen bonded Si, however, combines with Mo and W to form silicides. For their part, these silicides likewise protect the heating conductor metal from oxidation, since they form a dense SiO ₂ layer on the surface of Mo and W, as is known.

The nitrogen gas which is still formed in the decomposition of the nitrides is inert at high temperatures to all constituents of a heating rod. Its effect could be in the volume displacement of oxygen-containing gases.

The fabrication of nitrides can be very different. They can be used as powders, granules or ceramic parts and also with functional tasks such as heat conductor carriers.

The decomposition of the nitrides does not lead to empty volumes in the heating rod, since the major part of the metal component of the nitride is bound as an oxide. However, the shape and distribution of the solid phase changes in the heating element.

On a sketch of the inventive structure of a heating element is explained. One-sided closed ceramic tube 1 99.7% Al ₂ O ₃ is at the open end with a copper cap 2 locked. The cap is against the ceramic tube 1 with a press seal 3 sealed. The cap 2 is with a connection 4 to a gas reservoir and electrically insulated feedthroughs 5 for connection to the heating conductor wire 6 Mistake. Inside the ceramic tube 1 there is a ceramic hollow cylinder 7 made of AlN with ground spiral groove. In this spiral course is the Heizleiterdraht 8th made of molybdenum. The return of the heating conductor wire 8th takes place inside the ceramic hollow cylinder 7 , To the seal on the cap 2 thermally not too high load is the AlN hollow cylinder 7 shorter than the alumina tube 1 , That from the AlN cylinder 7 unfinished end of the pipe 1 is with alumina wool 9 filled for heat insulation.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 2014460A [0005]
• GB 1301265 A [0005]
• GB 709806 A [0005]
• GB 490116A [0006]
• GB 506098 A [0007]
• GB 499447 A [0007]
• GB 595060 A [0007]

Claims (5)

An electric heating element for high-temperature furnaces, comprising at least one molybdenum or tungsten heating conductor, characterized in that molybdenum or tungsten heating conductors together with an electrically insulating, decomposed at high temperatures in an inert and an oxygen-binding gas component material in a ventilated ceramic tube ( 1 ) are arranged. Electric heating element according to claim 1, characterized in that the material with which the ceramic tube ( 1 ) is filled, at least partially, of the nitride of a main group element of Groups II; III or IV exists. An electric heating element according to claim 2, characterized in that the nitride is at least partially aluminum nitride. An electric heating element according to claim 2, characterized in that the nitride is at least partially silicon nitride. Electric heating element according to claim 1, characterized in that the material which is in the ceramic tube ( 1 ), both as a ceramic insert, as a bed; but also at the same time as Heizleitererträger ( 7 ) or electrical insulator is formed.

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