Method for heat treating materials at high temperatures, and a furnace bottom construction for high temperature furnaces

Abstract: A method for heat-treating material at high temperatures where the material from which the bottom of the furnace chamber is made forms a eutectic with the material to be heat-treated at a temperature lower than the heat treatment temperature. The invention is characterized by causing a part (20) of the furnace chamber bottom (24), which part may comprise the whole of or a portion of the furnace chamber bottom, on which the material to be heat-treated shall rest, to be comprised at least in part of a material that has the same chemical composition as or a chemical composition similar to the chemical composition of the material to be heat-treated; positioning said bottom part (20) so that said part will have no physical contact with the remaining furnace lining (7) in the furnace chamber (15); and arranging the bottom part (20) in the furnace construction at a position such that the contact location (21) between said bottom part (20) and said furnace lining material (10) at which said bottom part is arranged will assume during the heat-treatment process a temperature which is lower than the temperature at which a molten phase will be formed between the materials that are in contact with one another at said contact location (10).

Description: The present invention relates to a method for heat treating materials at high temperatures, and a furnace bottom construction for high temperature furnaces. More specifically, the invention relates to a method and to the construction of a furnace chamber floor and furnace bottom insulation that makes possible the heat treatment of a material which is very likely to react with the material from which the furnace chamber floor is made at the heat treatment temperatures concerned. In some industrial applications, there is a need to heat-treat material at very high temperatures. At times, it is necessary to heat-treat material at temperatures close to their melting points. The diffusion coefficient of some substances is very high at such high temperatures. This means that atoms from an object A that consists to 100% of the material A that is being heat-treated will diffuse into underlying material B that consists to 100% of the material B, which may be a furnace bottom or a crucible that forms the furnace bottom and that is intended to receive the material to be heat-treated. The material B will, of course, also diffuse into the material A. The maximum temperature that can be used in the heat treatment process is limited by the temperature at which said materials first begin to form a smelt. Monocrystalline and polycrystalline oxidic material such as aluminium oxide and other aluminium-oxide based materials, such as YAG, i.e. 3Y 2 O 3 * 5Al 2 O 3 or TiO 2 , are examples of materials that it is desirable to heat-treat at temperatures very close to their melting points. A common feature of these materials is that they have a very high melting point when in very pure states. One furnace alternative for heat-treating such materials are furnaces that are heated by means of electrical resistance elements of zirconium dioxide, these furnaces also being the only alternative when the heat treatment shall be carried out in an oxidising atmosphere. Furnaces based on zirconium-dioxide elements are described in U.S. Patent Specifications 4,041,236 and 3,440,322, and also in Swedish Patent Specification 9502475-8. When the furnaces are constructed as box furnaces or elevator furnaces, the floor of the furnace chamber will be made of the same material as the walls and ceiling or roof of the chamber, which may be zirconium dioxide, magnesium oxide, calcium oxide or some other oxidic material, or a combination of oxidic materials. With the view of highlighting the problems that exist, it is assumed that aluminium oxide shall be heat-treated at 2000°C. In this regard, Al 2 O 3 -MgO forms a eutectic at 1995°C, CaOAl 2 O 3 forms a eutectic at about 1600°C, and ZrO 2 -Al 2 O 3 forms a eutectic at 1845°C. None of these material combinations can therefore be used to heat-treat aluminium oxide at 2000°C. The present invention solves this problem and enables heat treatment to be carried out at temperatures higher than the temperature at which a eutectic is formed between the material to be heat-treated and the material from which the floor of the furnace chamber is made. The present invention thus relates to a method of heat-treating material at high temperatures where the material from which the bottom of the furnace chamber is comprised forms a eutectic with the material to be heat-treated at a temperature that is lower than the heat treatment temperature, and is characterised by causing a part of the furnace-chamber bottom on which the material to be heat-treated shall rest, said part comprising the whole of said bottom or a portion thereof, to comprise at least in part of a material that has the same chemical composition as or a similar chemical composition to the material to be heat-treated; by causing the said bottom part to have no physical contact with the remaining portion of the furnace bottom in the furnace chamber; and by arranging said bottom part in the furnace construction at a position at which the contact location between said bottom part and a part of the bottom at which said bottom part is disposed takes, during said heat treatment process, a temperature which is lower than the temperature required to form a molten phase between those materials that are in mutual contact at said contact location. Positioning as intermediate member consisting of other material than that of the bottom lining between the bottom and the article to be heat treated belongs to prior art. However, in this art the contact between the bottom and the intermediate member does not take place outside the furnace chamber. The invention also relates to a furnace bottom construction of the kind defined in the following Claims and having the features set forth therein. The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, together with the graph, where Figure 1 is a schematic, vertical sectioned view of a furnace according to a first embodiment in which the invention is applied; Figure 2 is a phase diagram between two materials; and Figure 3 is a schematic, vertical sectioned view of a furnace in which a second embodiment of the invention is applied. The basic construction of the furnace shown in Figure 1 is described in more detail in Swedish Patent Specification 9502475-8. The furnace includes an inner furnace chamber 15 and an outer furnace chamber 13. The inner furnace chamber is delimited by a roof or ceiling 6, a bottom 7, and side walls 1. The side walls, roof and bottom are ideally made of ceramic material, preferably stabilised zirconium dioxide. The inner furnace space rests on beams and columns of zirconium dioxide material 10. The inner furnace space is also supported at its four corners by aluminium oxide corner posts. The roof and the bottom of the inner furnace space include holes through which lead-ins 3 pass to zirconium dioxide elements whose glow zones 2 are located in the inner furnace space. The conductors 3 are made of the same material as the glow zones 2, i.e. zirconium dioxide stabilised with yttrium oxide, while platinum/rhodium wires 4 are provided for conducting electrical energy. The wires are wound around the lead-ins 3 at the place where said lead-ins pass through the roof of the outer furnace space, and the platinum wires extend from there out of the furnace. The outer furnace space is delimited by a self-supporting roof 11, a bottom 16 and walls 14. The walls delimiting the outer furnace space to the surroundings are comprised of one of the materials aluminium oxide brick and aluminium oxide fibre material. Located in the outer furnace space are resistance elements 17, which preferably consist of a molybdenum disilicide material. The lead-ins leading to these elements extend out through the roof 11 of the outer furnace space and are typically U-shaped. Arranged in the outer furnace space 13 is a thermoelement 18 for sensing the temperature of the outer furnace chamber. This thermoelement is used to regulate the temperature in the outer furnace chamber. The temperature of the inner furnace chamber is regulated with the aid of an optical pyrometer which measures the temperature with the aid of a fibre optic. The outside of the furnace is insulated with fibre material 5. The furnace opening consists in an outer door and an inner door. A furnace of this description is a box-like furnace. The furnace construction is also appropriate for an elevator furnace, when the furnace opening is moved to the bottom of the furnace. The aforedescribed furnace is a suitable type of furnace for practising the present invention, although the invention can also be applied to other existing types of high temperature furnace. The present invention relates to a method for the heat treatment of material at high temperatures, where the material from which the bottom of the furnace chamber is made forms a eutectic with the material to be heat-treated at a temperature which is lower than the heat treatment temperature. Figure 2 is a schematic phase diagram between the substances A and B, where L signifies a liquid phase. The encircled area includes a eutectic, i.e. the lowest temperature at which the substances A and B will assume a liquid phase when brought together. It is this temperature that constitutes the aforesaid limiting heat-treatment temperature. According to the invention, a part 20 of the bottom 7 of the furnace chamber, said part comprising the whole or a portion of the furnace chamber bottom, on which the material to be heat-treated shall rest, is caused to consist at least in part of a material that has the same chemical composition as or a similar chemical composition to the chemical composition of the material to be heat-treated. The bottom part 20 is placed so as not to be in physical contact with other furnace lining in the furnace chamber 15. Furthermore, the bottom part 20 of the furnace construction is disposed at a location such that the position of contact 21 between the bottom part 20 and the part 10 (furnace lining material) of the bottom 7 at which the bottom part is arranged is adapted to take during the heat treatment process a temperature which is below the temperature at which a molten phase is formed between the materials that are in contact with each other at the contact location. The bottom part may comprise a monocrystalline or polycrystalline material. According to one highly preferred embodiment, that portion of the bottom part which consists of a material having the same chemical composition as or a chemical composition y similar to the material to be heat-treated is given the form of a body 22 whose upper surface 23 is located at a level above the remaining portion 19 of the furnace chamber bottom and which extends through a hole 24 in said remaining portion 19 of the furnace bottom. A gap 25 is present between the body 22 and the hole 24. The bottom surface 26 of said body is caused to rest on a part 10 of the bottom 7 that is located beneath the remaining portion 19 of the furnace bottom 7. According to one preferred embodiment, the body 22 is a layered body, as shown in Figure 1. The body includes a plug 27, support legs 28 and a plate 29. In this embodiment, at least the upper part of the body, i.e. the plate 29, is made of a material that has the same chemical composition as or a chemical composition similar to the material to be heat-treated. The support legs 28 may be comprised of monocrystalline or polycrystalline aluminium oxide, for instance. The plug 27 is suitably comprised of a high-temperature insulating material, such as a material based on aluminium oxide. According to one preferred embodiment, the uppermost part of said body forms a crucible 30. The plates 29 and the crucible 30 are made of mutually the same material and thus have the same chemical composition as or a chemical composition similar to the material to be heat-treated. According to another embodiment, shown in Figure 3, the body 31 is an homogenous body. Although not shown, the body 31 may also include a crucible 30 standing thereon. Irrespective of the embodiment in other respects, it is preferred that said body has the form of a pillar. It was mentioned in the introduction that aluminium oxide cannot be heat-treated at 2000°C when the furnace chamber is comprised of zirconium dioxide, magnesium oxide or calcium oxide. Al 2 O 3 -MgO forms a eutectic at 1995°C, CaO-Al 2 O 3 forms a eutectic at about 1600°C, and ZrO 2 -Al 2 O 3 forms a eutectic at 1845°C. However, the present invention makes such heat treatment possible. With regard to aluminium oxide, there is used a body 22, 31 comprised of monocrystalline or polycrystalline aluminium oxide, meaning that Al 2 O 3 meets Al 2 O 3 at the point of contact between the body 22, 31 and the material to be heat-treated. The furnace chamber has a temperature of 2000°C, as illustrated in Figure 3. Pure aluminium oxide forms a molten phase at 2050°C. The temperature immediately beneath the furnace floor 19 is, e.g., 1900°C, see Figure 3. The body 21, 31 has no physical contact with the portion 19 of the furnace bottom 7, because of the presence of the gap 21. The temperature of the bottom surface of the body is, e.g., 1800°C. This lies against the furnace material and consequently the temperature at which a eutectic will form between the material of said body and the furnace material shall be taken into account. Thus, the furnace space and the furnace construction may be comprised of MgO, since Al 2 O 3 -MGO have a melt eutectic at 1995°C, which is higher than 1800°C. ZrO 2 can also be used, since ZrO 2 -Al 2 O 3 form a eutectic at 1845°C. When using CaO as the furnace material, the body 22, 31 must extend further down in the furnace construction, where the temperature is lower than the temperature at which CaO-Al 2 O 3 form a eutectic, namely at a temperature of about 1600°C. It will be evident that the present invention enables different materials to be heat-treated at very high temperatures. It will also be evident that different material combinations between furnace chamber lining material, body material, the material against which the bottom side of the body rests, and the material to be heat-treated can exist when practising the present invention. The person skilled in this art will be capable of choosing material combinations which will not form a melt during the heat-treatment process. Various material combinations and various types of furnaces have been mentioned in the aforegoing. However, the invention is not restricted to these material combinations or furnaces. It will therefore be understood that the invention is not restricted to the aforedescribed exemplifying embodiments thereof, and that variations can be made within the scope of the following Claims.

Claims: A method for heat-treating material at high temperatures where the material from which the bottom (7) of the furnace chamber (15) is made forms a eutectic with the material to be heat-treated at a temperature lower than the heat treatment temperature, characterised by causing a part (20) of the furnace chamber bottom (7), which part may comprise the whole of or a portion of the furnace chamber bottom (7), on which the material to be heat-treated shall rest, to be comprised at least in part of a material that has the same chemical composition as or a chemical composition similar to the chemical composition of the material to be heat-treated; positioning said bottom part (20) so that said part will have no physical contact with the remaining portion (19) of the furnace bottom (7) in the furnace chamber (15); and arranging the bottom part (20) in the furnace construction at a position such that the contact location (21) between said bottom part (20) and a part (10) of the bottom (7) at which said bottom part (20) is arranged will assume during the heat-treatment process a temperature which is lower than the temperature at which a molten phase will be formed between the materials that are in contact with one another at said contact location (21). A method according to Claim 1, characterised in that the portion of the bottom part (20) that is comprised of a material having the same chemical composition as or a chemical composition similar to the material to be heat-treated is given the form of a body (22, 31) whose upper surface (23) is located at a level above the remaining portion (19) of the furnace chamber bottom (7), wherein said body extends through a hole (24) in said remaining portion (19) of the furnace bottom (7); in that a gap (25) is present between said body (22, 31) and said hole (24); and in that the bottom surface (26) of said body (22, 31) is caused to rest on a part (10) of the furnace bottom (7) that is located beneath said remaining portion (19) of the furnace bottom (7). A method according to Claim 2, characterised in that said body (31) is homogenous. A method according to Claim 2, characterised in that the body (22) is a layered body where at least its upper part is comprised of a material whose chemical composition is the same as or similar to the chemical composition of the material to be heat-treated. A method according to Claim 2, 3 or 4, characterised in that the uppermost part of said body is comprised of a crucible (30). A furnace bottom construction for high temperature furnaces for the heat treatment of material, wherein the material from which the bottom (7) of the furnace chamber (15) is made forms a eutectic with the material to be heat-treated at a temperature lower than the heat treatment temperature, characterised in that a part (20) of the furnace chamber bottom (7), which part includes the whole of or a portion of the furnace chamber bottom (7), on which the material to be heat-treated shall rest, is comprised at least in part of a material that has the same chemical composition as or a similar chemical composition to the material to be heat-treated; in that the bottom part (20) is arranged so as to have no physical contact with the remaining portion (19) of the furnace bottom (7) in the furnace chamber (15); and in that said bottom part (20) is arranged in the furnace construction at a position such that the contact location (21) between said bottom part (20) and a part (10) of the bottom (7) at which said bottom part (20) is arranged will assume during the heat-treatment process a temperature which is lower than the temperature at which the materials that are mutually in contact at said contact location (21) will form a molten phase. A furnace bottom construction according to Claim 6, characterised in that the portion of said bottom part (20) that is comprised of a material having the same chemical composition as or a chemical composition similar to the material to be heat treated has the form of a body (22, 31) which is arranged so that its upper surface (23) will be located on a level above the remaining portion (19) of the furnace chamber bottom (7), wherein said body (22, 31) extends through a hole (24) in said remaining portion (19) of the furnace bottom (7); in that a gap (25) is present between the body (22, 31) and the hole (24); and in that the bottom surface (26) of the body (22, 31) rests on a part (10) of the bottom (7) that is located beneath said remaining portion (19) of the furnace bottom (7). A furnace bottom construction according to Claim 7, characterised in that the body (31) is homogenous. A furnace bottom construction according to Claim 7, characterised in that the body (22) is a layered body, where at least its upper part is made of a material that has the same chemical composition as or a chemical composition similar to the material to be heat-treated. A furnace bottom construction according to Claim 7, 8 or 9, characterised in that the uppermost part of said body (22, 31) is comprised of a crucible (30).

Inventors: AXELSSON NICLAS, AXELSSON, NICLAS
Applicants: KANTHAL AB
Application year: 1998
Main classification: F27D1
IPC codes: F27D5, F27D1, F27B5