A METHOD OF INCREASING THE USEFUL LIFE OF A MOLYBDENUM SILICIDE HEATING ELEMENT

Abstract: A method of increasing the useful life of heating elements that consist essentially of molybdenum silicide and alloys of this basic material, said elements operating at high temperatures in heat treatment processes and resting against the floor and/or the ceiling of a furnace.The invention is characterised by causing the heating element to lie in direct abutment with aluminium oxide brick material, wherein the heating element material contains molybdenum silicide and alloys thereof, and wherein said material is caused to contain aluminium to an extent sufficient to maintain a stable, slowly growing layer of aluminium oxide on the surface of the heating element.

Description: A method of increasing the useful life of a molybdenum suicide heating element . The present invention relates to a method of increasing the useful length of life of heating elements when heat treating electronic ceramics. Such ceramics may be fer- rites (magnetic ceramics) and titanates (e.g. BaTi0 3 ). More specifically, the invention pertains to elements of the types molybdenum suicide and molybdenum tungsten suicide, including different alloys of these basic materials. Such elements are manufactured by applicants in a number of designs. Electronic ceramics are at present used in a number of applications in, e.g., Optronics, mobile telephony and vehicle electronics. Electronic ceramics that have a perovskite structure have dielectric, piezoelectric and ferroelectric properties, among other things. Examples of such materials include BaTi0 3 and Pb (Zr, Ti)0 3 . Components that are typically manufactured from such materials are resonators, filters and capacitors. The capacitors are produced in the form of so-called multi layer capacitors (MLCC) for example. The ceramics are often brought into contact with a metallic conductor, such as nickel, in the heat treatment stage. When such heating elements are used in the manufacture of so-called electronic ceramics, the elements are often placed on a supportive surface and heated respectively from the floor and ceiling of continuous sintering furnaces . A long preferred design of heating elements in such furnaces, for instance furnaces of the so-called Riedhammer type for so-called feπite sintering furnaces are so-called 4-arm meander elements measuring 6/12 mm on the heating zone and connection part respectively. Such elements rest typically on a supportive gravel layer of aluminium-silicate par- tides, which, in turn, rests on a brickwork of purely aluminium oxide. The aluminium silicate, which is compatible (does not react chemically) with heating elements spectively. Sillimannite and mullite can be used for a long period of time before a reaction takes place with the SiU 2 -layer that develops on the surface of MoSi 2 elements. This is because the aluminium silicate reacts more slowly with the Si0 2 - layer than the uminium oxide brick. This reaction causes aluminium to be alloyed in the Siθ 2 -layer, therewith weakening the ability of the layer to protect the element material, resulting in a shorter life length of the element. Subsequent to this reaction with.the surface layer a reaction also takes place in. the bulk material of the heating element, resulting in further corrosion and weakening. In the absence of said particle layer, or gravel bed, a quicker reaction will take place by virtue of the Si0 2 on the surface of the element being in direct contact with the Al 2 0 3 -brick. Many electronic ceramics are heat treated at furnace temperatures ranging from 1200 to 1500°C and higher. In addition to nitrogen gas, the atmosphere also typically contains about 5% hydrogen gas that has a varying dew point. For instance, the dew point may be +20°C. It has been noted that in many cases the useful life length of said elements falls significantly beneath the expected 3-5 year span, and in some cases only extends to a few months. The cause of local life length problems is the aggressive hydrogen gas that reacts with the gravel bed such as to form a smelt that contains chiefly aluminium silicate. The heating element sinks down in the smelt, wherewith an increase in temperature takes place with accelerated corrosion and temperature increase of the elements as a result. Moreover, the gravel bed sinters firmly into the aluminium oxide brick. The problem can also occur in atmospheres that do not contain hydrogen gas, wherewith the element buries down into the particle layer as a result of temperature changes and therewith dimensional changes. The gravel bed, or particle layer, also makes manufacture, transportation and furnace servicing more difficult to carry out. It is therefore desired to eliminate the gravel be from the process. The present invention fulfils this desideratum. The present invention thus relates to a method of increasing the useful life of heating elements that are comprised essentially of molybdenum disilicide and of alloys of this basic material, where said elements operate at high temperatures in heat treatment processes, and where said element rests on the floor and/or against the ceiling of a furnace, wherein the method is characterised by causing said heating elements to lie in direct abutment with the aluminium oxide brick; and in that the heating element material contains molybdenum silicide and alloys thereof, wherein said material is caused to contain aluminium in an amount sufficient to maintain a stable, slowly growing layer of uminium oxide on the surface of respective heat- ing elements. The invention will now be described in more detail, partly with reference to the accompanying drawing, where - Figure 1 illustrates a furnace design in which the invention is applied. Figure 1 illustrates part of a furnace of the kind concerned. The drawing represents a segment, e.g. a heating zone, of a so-called through-pushing furnace in which the material passes through the space between the heating elements on, for instance, a ceramic tray that is pushed forwards on rails, for instance. For heating the material from above, a gravel layer or bed of particles 1 is placed on an alumimum oxide plate 2. A four-arm element 3 is placed on top of the particle bed. A "heating cassette" including aluminium oxide brick-gravel bed-elements is pushed into a compartment/shelf plane 4 in surrounding brick insulation 5. The same procedure is used for elements that are heated from above and from below, with the difference that when heating from below there is used an upper aluminium oxide plate 6. Naturally, other furnace constructions can be used that apply the principle in which the element rests on a gravel bed. The present invention relates to a method of increasing the useful life of heating elements that consist essentially of molybdenum disilicide and alloys of this basic material, where said elements operates in the heat treatment process at high temperatures in, e.g., a corrosive atmosphere that contains hydrogen gas, and where said elements rest against the furnace floor and/or lie in abutment with the furnace ceiling. In the case of the illustrated embodiment, the heating elements 3 are in direct abutment with aluminium oxide brick. This means that the gravel bed 1 in Figure 1 is excluded in accordance with the invention. The material from which the heating element 3 is comprised includes molybdenum suicide and alloys thereof, said material being caused to contain sufficient aluminium to maintain a stable, slowly growing layer of aluininium oxide on the surface of the heating element. In the case of this embodiment there is thus obtained a slowly growing layer of aluminium oxide that is stable in the temperature range relevant in respect of the heat treatment of electronic ceramics. According to one preferred embodiment, the material from which the heating ele- ment is comprised is caused to include Mo(Siι_ x Al x ) 2 , and is also caused to contain an aluminium sufficiency. According to one embodiment, x is in the order of 0.2 - 0.6. It is preferred that x lies in the range of 0.40 - 0,50. This results in a stable oxide layer while, at the same time, obtaining a composition that is highly temperature durable and has good mechanical properties. According to one preferred embodiment of the invention, the heating element material contains up to 40 % by volume AI 2 O 3 . The aluminium oxide constitutes an element mechanical stabilising phase and counteracts abnormal particle size enlargement of the aluminosilicide phase. The inventive method thus obviates the need for a gravel layer, therewith facilitating manufacture, transportation and furnace servicing! The reason why a gravel layer is not required, is because uminium oxide lies against aluminium oxide. Furthermore, the risk of corrosion of the system, heating element/gravel layer/aluminium oxide brick, is reduced, therewith greatly extending the useful life of the heating elements and thereby rninimising operational disturbances. The suitability of the element material in the aforesaid context is based on the inherent property of the AI2O 3 ceramic to resist corrosion in corrosive environments up to very high temperatures. The present invention shall not be considered to be limited to the described type of furnace, but can be applied with other types of furnaces where the aforesaid problems exist.

Claims: Claims 1. A method of increasing the useful life of heating elements that consist essentially of molybdenum sihcide and alloys of this basic material, said elements operating at high temperatures in heat treatment processes and resting against the floor and/or the ceiling of a furnace, characterised by causing the heating element to he in direct abutment with aluminium oxide brick material, wherein the heating element material contains molybdenum suicide and alloys thereof, and wherein said material is caused to contain aluminium to an extent sufficient to maintain a stable, slowly growing layer of aluininium oxide on the surface of the heating element. 2. A method according to Claim 1, characterised in that the heating element material contains Mo(Siι_ x Al x ) 2 , said material being caused to contain a sufficiency of aluminium. 3. A method according to Claim 1 or 2, characterised in that x lies in the range of 0.2 - 0.6. 4. A method according to Claim 1, 2 or 3, characterised in that x lies in the range of O.40 - 0.50. 5. A method according to Claim 1, 2, 3 or 4, characterised in that the heating element material contains up to 40 % by volume A1 2 0 3 .

Inventors: SUNDBERG MATS, SUNDBERG, MATS
Applicants: SANDVIK AB, SUNDBERG MATS, SUNDBERG, MATS
Application year: 2001
Main classification: H05B3
IPC codes: C04B35, C22C27, C22C29, F27B9, F27D1, H05B3