EP0769790A1 - Process of fabrication of a heating resistor for electric oven - Google Patents

Abstract

The weakly doped phase is formed in a layer between two highly doped layers and two extensions of ceramic material, compatible with the terminal material, are soldered to either end after the heat treatment. The mixture in the mould (10) is of lanthanum III oxide and chromium III oxide and of dopants, calcium carbonate, yttrium III oxide and samarium III oxide. The mixture includes a binding agent which is thermally eliminated prior to the heat treatment stage. The resistor thus obtained has a decreasing concentration of conducting particles in moving from the terminal phase to the resistive phase.

Description

The present invention relates to a method of manufacturing a heating resistor for an electric oven, as well as to a heating resistor obtained by means of such a method.

Heating resistors made of ceramic material are known, such as lanthanum chromite.

Such resistors generally have the form of a tubular bar, possibly bent, consisting of a highly resistive heating part, causing heat dissipation by Joule effect, when it is traversed by a current, and extending at each of its ends by an electrically conductive part ensuring the current supply of the heating part and intended to allow the mechanical fixing of the resistance in the oven.

The resistive and electrically conductive parts are currently manufactured separately by pressing a mixture of particles of ceramic material.

The heating resistive part and the connection terminals have different compositions, and are welded after the sintering operation. Their connection is therefore relatively fragile and therefore unreliable.

The invention aims to overcome this drawback.

• remplissage d'un moule par un mélange de particules de matière céramique dopé destiné à former une partie résistive chauffante,
• pressage des particules contenues dans le moule,
• frittage des particules contenues dans le moule et pressées en vue d'assurer une cohésion entre lesdites particules du mélange, et
• formation de bornes de raccordement électrique et de fixation mécanique pour la partie résistive chauffante,

   caractérisé en ce que ladite étape de formation desdites bornes est réalisée au cours de ladite étape de remplissage du moule, au cours de laquelle on introduit, dans le moule, une couche de formation de la partie résistive chauffante d'un premier mélange de particules de matière céramique faiblement dopé et destiné à former une phase électriquement résistive entre deux couches de formation de bornes d'un deuxième mélange de particules de matière céramique fortement dopé et destiné à former une phase électriquement conductrice et l'on procède à l'opération de frittage pour réaliser simultanément ladite partie résistive chauffante et les parties d'extrémité formant bornes de fixation mécanique et de raccordement électrique.It therefore relates to a method of manufacturing a heating resistor for an electric oven, comprising the steps of:

• filling a mold with a mixture of doped ceramic material particles intended to form a heating resistive part,
• pressing of the particles contained in the mold,
• sintering of the particles contained in the mold and pressed in order to ensure cohesion between said particles of the mixture, and
• formation of electrical connection and mechanical fixing terminals for the heating resistive part,

characterized in that said step of forming said terminals is carried out during said step of filling the mold, during which a layer is formed in the mold for forming the heating resistive part of a first mixture of particles of lightly doped ceramic material intended to form an electrically resistive phase between two terminal forming layers of a second mixture of particles of highly doped ceramic material intended to form an electrically conductive phase and the sintering operation is carried out to simultaneously produce said heating resistive part and the end parts forming mechanical fixing and electrical connection terminals.

• le procédé comporte en outre les étapes consistant à fabriquer des barreaux de matière céramique formant rallonge compatibles avec le matériau constitutif des bornes et à souder, après l'étape de frittage, un desdits barreaux sur chaque extrémité desdites bornes de fixation mécanique et de raccordement électrique pour la partie résistive;
• l'étape de remplissage du moule consiste à disposer dans le moule un mélange de particules d'oxyde de lanthane, d'oxyde de chrome et de particules dopantes de carbonate de calcium, d'oxyde d'yttrium et d'oxyde de samarium ;
• ledit mélange de particules de matière céramique comporte en outre un liant pour assurer une adhésion entre les particules au cours de l'étape de pressage;
• le procédé comporte en outre une étape de traitement thermique d'élimination dudit liant, préalable à l'étape de frittage ;
• le premier mélange comporte de 61 à 65% en poids d'oxyde de lanthane, de préférence 63,1%, de 29 à 32% en poids d'oxyde de chrome, de préférence 31,1%, et de 0 à 3,3% en poids de particules dopantes, de préférence 1%, le reste comprenant ledit liant organique;
• le deuxième mélange comporte de 46 à 62% en poids d'oxyde de lanthane, de préférence 55,7%, de 28 à 36% en poids d'oxyde de chrome, de préférence 32,2%, et de 3,3 à 22,5% en poids de particules dopantes, de préférence 8,4%, le reste comprenant ledit liant organique.

The invention may also include one or more of the following characteristics:

• the method further comprises the steps of making bars of ceramic material forming an extension compatible with the material of which the terminals are made and of welding, after the sintering step, one of said bars on each end of said mechanical fixing and electrical connection terminals for the resistive part;
• the step of filling the mold consists in placing in the mold a mixture of lanthanum oxide particles, chromium oxide and doping particles of calcium carbonate, yttrium oxide and samarium oxide;
• said mixture of particles of ceramic material further comprises a binder to ensure adhesion between the particles during the pressing step;
• the method further comprises a heat treatment step for removing said binder, prior to the sintering step;
• the first mixture comprises from 61 to 65% by weight of lanthanum oxide, preferably 63.1%, from 29 to 32% by weight of chromium oxide, preferably 31.1%, and from 0 to 3, 3% by weight of doping particles, preferably 1%, the remainder comprising said organic binder;
• the second mixture comprises from 46 to 62% by weight of lanthanum oxide, preferably 55.7%, from 28 to 36% by weight of chromium oxide, preferably 32.2%, and from 3.3 to 22.5% by weight of doping particles, preferably 8.4%, the remainder comprising said organic binder.

The invention also relates to a heating resistor for an electric oven, comprising a longitudinal heating resistive part extended at each end by an electrical connection and mechanical fixing terminal for the heating part, characterized in that it is obtained by means of 'A manufacturing process as defined above, each junction zone between said heating part and a mechanical fixing terminal and corresponding electrical connection comprising a decreasing concentration of electrically conductive particles in the direction of the heating part.

• la figure 1 est une vue schématique en coupe d'un moule utilisé pour la fabrication d'une résistance conformément au procédé selon l'invention ;
• la figure 2 est une vue de profil d'une résistance en cours de fabrication, obtenue après l'opération de pressage ;
• la figure 3 est une vue de profil d'une résistance chauffante fabriquée au moyen du procédé selon l'invention; et
• la figure 4 est une vue schématique de profil d'un deuxième mode de réalisation d'une résistance chauffante obtenue au moyen du procédé selon l'invention.

Other characteristics and advantages of the invention will emerge from the following description, given by way of example, with reference to the appended drawings in which:

• Figure 1 is a schematic sectional view of a mold used for the manufacture of a resistor according to the method according to the invention;
• Figure 2 is a side view of a resistance during manufacture, obtained after the pressing operation;
• Figure 3 is a side view of a heating resistor manufactured by the method according to the invention; and
• Figure 4 is a schematic profile view of a second embodiment of a heating resistor obtained by the method according to the invention.

In FIG. 1, there is shown a mold used for the manufacture of a heating element in lanthanum chromite, designated by the reference numeral 10.

In the following description, it will be assumed that the resistors to be produced have a cylindrical shape. However, the invention also applies to the manufacture of heating resistors of straight or bent tubular shape, the mold then having a corresponding shape.

The mold 10 has a cylindrical shape. It comprises a deformable casing 11 in which is placed a central mandrel 12 provided at one of its ends with a centering plug 14 closing a corresponding end of the mold 10, the other end of which is provided with a filling device 16, of known type.

The mold 10 is completed by a support 18 constituted by a sheath for holding the mold 10 in a vertical position for its filling.

The first step in the manufacturing process of the heating resistor consists in preparing a first mixture of particles of ceramic material consisting of a mixture of lanthanum oxide, chromium oxide and doping particles at very low concentration intended to form a electrically conductive phase at very low concentration, and to prepare a second mixture of particles of ceramic material, substantially identical with the first mixture, but comprising a higher concentration of doping particles to form an electrically conductive phase.

It is understood that the second mixture having a higher concentration of doping elements, it has a lower resistivity than that of the first mixture.

Advantageously, the first mixture consists of lanthanum oxide, in a proportion ranging from 61 to 65% by weight of the mixture, preferably 63.1%, of chromium oxide, in a proportion ranging from 29 to 32% by weight, preferably 31.1%, and of doping particles intended to form an electrically resistive phase, consisting of calcium carbonate, yttrium oxide and samarium oxide present in the mixture in a proportion ranging from 0 to 3.3% by weight, preferably 1% the rest of the mixture consists of an organic binder or by filling compounds of known type.

Furthermore, the second mixture consists of lanthanum oxide, in a proportion ranging from 46 to 62% by weight of the mixture, preferably 55.7%, of chromium oxide, in a proportion ranging from 28 to 36% by weight, preferably 32.2%, of doping particles intended to form an electrically conductive phase, also consisting of calcium carbonate, yttrium oxide and samarium oxide in a proportion ranging from 3.3 to 22 , 5%, preferably 8.4%, the remainder consisting of an organic binder or also filler compounds of known type.

The first and second mixtures, forming part of the heating resistance, are introduced into the mold 10 by means of the filling device 16 in the following manner.

The second mixture is first introduced into the mold 10, at a predetermined height, as represented in FIG. 1, in which the reference numeral 19 designates the layer of the second mixture introduced into the mold 10. The first mixture is then introduced, also at a predetermined height, which is covered with another layer of the second mixture . A layer of the first mixture is thus obtained placed between two layers of the second mixture so as to form in the resistance a highly resistive heating part extended at each of its ends by an electrically conductive part forming a mechanical fixing and electrical connection terminal of the part. heating.

It should be noted that during this filling step, there is a diffusion of doping particles from the heavily doped area towards the weakly doped area. There are thus obtained junction zones between the heating part and the mechanical fixing and electrical connection terminals comprising a decreasing concentration of conductive particles in the direction of the heating resistive part.

It is understood that during the heating resistance manufacturing process the doping particles react with the constituent materials of the ceramic material to form electrically conductive particles at different concentrations for the resistive part and the conductive parts.

During the next step, the particles of ceramic material placed in the mold are subjected to a pressing step aimed at shaping the lanthanum chromite cylinder by compression of the particles on the metal mandrel 12 inside the deformable envelope 11.

The filled mold is closed with waterproof material and then immersed in a compression chamber containing oil or, alternatively, water. When the pressure increases in the chamber, the oil transmits this pressure to the mold which deforms and in turn transmits the pressure to the particles. These are compressed, for example to a pressure of 1000 bars.

There is thus obtained, after demolding, a cylinder, the structure of which is shown in FIG. 2, comprising a heating part 20, highly resistive, extending at each of its ends by a terminal of low resistivity, designated by the references 22 and 24, and intended to ensure mechanical fixing of the resistance and the current supply of the heating part 20, with the interposition of junction zones, designated respectively by the references 26 and 28, comprising a decreasing concentration of calcium carbonate in the direction of the heating part 20.

It should be noted that each separation between a resistive part and a conductive part is a fictitious separation which is in fact not visible.

It can also be seen in this FIG. 2 that the mechanical resistance can be provided with a gripping zone 30 extending the free end of one of the mechanical fixing terminals, such as 22, produced from particles of ceramic material, and whose constitution is identical to that of the heating part 20. In this case, a junction zone 32 is interposed between the gripping zone 30 and the corresponding mechanical fixing part 22, having an increasing concentration of calcium carbonate towards it. The gripping zone 30 is provided with a hole 34 for the passage of a pin in order to maintain the resistance during the subsequent stages of the process.

It should be noted that, by the geometry of the mold, the ends of the resistance have, at this stage, a flared shape.

These flared areas will then be cut off as will be described later.

The heating resistance is then subjected to a heat treatment consisting of several stages during which the resistance is subjected to temperatures of up to 1100 ° C. in stages, so as to ensure degradation of the binder in view of its elimination.

The resistance is then subjected to a sintering step making it possible to absorb the porosity of the resistance and to obtain cohesion between the particles entering into its constitution. This sintering step is carried out by fixing a pin to the gripping zone 30 and inserting the resistance in an oven suitable for subjecting the resistance to temperatures up to 1750 ° C. without air.

The resistance thus stiffened is then subjected to a finishing step during which it is cut so as to eliminate on the one hand the gripping zone 30 as well as the corresponding junction zone 32 and on the other hand the free end of the opposite mechanical fixing terminal 24.

A metal deposit is then projected on the two free ends of the resistance terminals to allow efficient electrical connection.

A resistance is thus obtained, represented in FIG. 3 comprising a highly resistive part 36 connected by junction zones 38 and 40 to two mechanical fixing and electrical connection terminals 42 and 44, the free ends 46 and 48 of which are metallized.

As a variant, and as shown in FIG. 4, to increase the size of the resistor, for example to obtain resistors of length greater than 700 mm, it is possible to provide a central heating part 50 of larger size framed by two junction zones 52 and 54 with mechanical fixing terminals 56 and 58 of shorter length.

In this case, extensions are made up of bars of highly doped ceramic material and forming an electrically conductive phase, of composition identical to that of the terminals, by molding, pressing and sintering, as mentioned above.

The mechanical fixing terminals 56 and 58 each form an end piece on which is welded an extension, 60 and 62 whose ends, respectively 64 and 66 are also preferably metallized.

It is understood that the extensions having a composition compatible with that of the terminals, that is to say having the same coefficient of expansion, the weld is very reliable.

Claims (9)

Method for manufacturing a heating resistor for an electric oven, comprising the steps of: filling a mold (10) with a mixture of particles of doped ceramic material intended to form a heating resistive part (20, 36; 50), - pressing of the particles contained in the mold, sintering of the particles contained in the mold and pressed in order to ensure cohesion between said particles of the mixture, and - formation of terminals (22,24,42,44; 56,58) for electrical connection and mechanical fixing for the heating resistive part (20,36; 50), characterized in that said step of forming said terminals is carried out during said step of filling the mold (10), during which a layer is formed in the mold (10) to form the heating resistive part (20 , 26; 50) of a first mixture of particles of lightly doped ceramic material and intended to form an electrically resistive phase between two terminal formation layers of a second mixture of particles of highly doped ceramic material and intended to form a phase electrically conductive and the sintering operation is carried out to simultaneously produce said heating resistive part (20,36;
50) and the end parts forming terminals (22, 24, 42, 44; 56, 58) for mechanical fixing and for electrical connection. A method according to claim 1, characterized in that it further comprises the steps consisting in manufacturing bars of ceramic material forming an extension compatible with the material constituting the terminals and solder after the sintering step, one of said bars (60,62) on each end of said terminals (56,58) for mechanical fixing and electrical connection for the resistive part. Method according to either of Claims 1 and 2, characterized in that the step of filling the mold consists in placing in the mold (10) a mixture of particles of lanthanum oxide (La ₂ O ₃ ), of oxide chromium (Cr ₂ O ₃ ) and doping particles of calcium carbonate (CaCO ₃ ), yttrium oxide (Y ₂ O ₃ ) and samarium oxide (Sm ₂ O ₃ ). A method according to any one of claims 1 to 3, characterized in that said mixture of particles of ceramic material further comprises a binder to ensure adhesion between the particles during the pressing step. A method according to claim 4, characterized in that it further comprises a heat treatment step for removing said binder, prior to the sintering step. Method according to one of claims 4 and 5, characterized in that the first mixture contains from 61 to 65% by weight of lanthanum oxide, preferably 63.1%, from 29 to 32% by weight of lanthanum oxide chromium, preferably 31.1%, and from 0 to 3.3% by weight of doping particles, preferably 1%, the remainder comprising said organic binder. Method according to one of claims 4 and 5, characterized in that the second mixture comprises from 46 to 62% by weight of lanthanum oxide, preferably 55.7%, from 28 to 36% by weight of lanthanum oxide chromium, preferably 32.2%, and from 3.3 to 22.5% by weight of doping particles, preferably 8.4%, the remainder comprising said organic binder. Heating resistor for electric oven, comprising a heating resistive part (20,36; 50) longitudinal extended at each end by a terminal (22,24,42,44; 56,58) for electrical connection and mechanical fixing for the heating resistive part, characterized in that it is obtained by means of a manufacturing process according to any one of claims 1 to 7. Heating resistor according to claim 8, characterized in that each junction zone between said heating resistive part (20,36; 50) and a terminal (22,24,42,44; 56,58) for mechanical fixing and electrical connection corresponding has a decreasing concentration of electrically conductive particles in the direction of the heating part (20, 36; 50).

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