Thermal shielding system

Abstract

A thermal shielding system for thermally shielding a batch space of high-temperature furnaces includes at least one shielding element. The shielding element has an encasing wall formed of refractory metal sheet(s) and a ceramic material accommodated in the wall. The ceramic material is present in a particulate and/or fibrous structure and it is based on zirconium oxide (ZrO.sub.2)

Claims

1. A thermal shielding system for thermally shielding a batch space of high-temperature furnaces, the shielding system comprising: at least one shielding element; said shielding element having a encasement made of refractory sheet metal and a ceramic material accommodated in said encasement, said ceramic material having a particulate structure and/or a fibrous structure and being based on zirconium oxide (ZrO.sub.2).

2. The thermal shielding system according to claim 1, wherein said ceramic material is present as a bed of individual particles.

3. The thermal shielding system according to claim 2, wherein said bed of individual particles has a monomodal grain distribution.

4. The thermal shielding system according to claim 1, wherein said ceramic material consists of ≧80% by weight of zirconium oxide (ZrO.sub.2).

5. The thermal shielding system according to claim 1, wherein said refractory sheet metal is formed of ≧98% by weight from tungsten.

6. The thermal shielding system according to claim 1, wherein said refractory sheet metal has a wall thickness d.sub.B where 0.25 mm ≦d.sub.B ≦2.5 mm.

7. The thermal shielding system according to claim 1, wherein said shielding element has a thickness d.sub.E in a range of 8 mm ≦d.sub.E ≦120 mm.

8. The thermal shielding system according to claim 1, which further comprises, in addition to said shielding element, a plurality of spaced-apart radiant plates made of refractory metal.

9. The thermal shielding system according to claim 8, wherein said radiant plates are formed of a material selected from the group consisting of molybdenum, a molybdenum-based alloy, tungsten, and a tungsten-based alloy.

10. A high-temperature furnace, comprising: a furnace body with a batch space for a thermal treatment of parts; and a thermal shielding system according to claim 1 at least partially surrounding said batch space.

11. The high-temperature furnace according to claim 10, wherein said thermal shielding system comprises a plurality of radiant plates made of refractory metal, which are spaced apart in a shielding direction and arranged adjacent said shielding element in the shielding direction.

12. The high-temperature furnace according to claim 11, wherein said radiant plates are arranged on a distal side of said shielding element remote from said batch space.

13. The high-temperature furnace according to claim 11, wherein a number of radiant plates arranged adjacent said shielding element in the shielding direction lies in a range from 1 to 7.

14. The high-temperature furnace according to claim 10, which comprises at least one electrical heating element for heating said batch space, said electrical heating element being disposed inside said thermal shielding system.

15. The high-temperature furnace according to claim 10, wherein at least one shielding element is in the form of a component which is removable from the high-temperature furnace in modular form.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1: shows a schematic cross-sectional view of a shielding element according to the invention according to a first embodiment;

(2) FIG. 2: shows a schematic cross-sectional view of a shielding element according to the invention according to a second embodiment;

(3) FIG. 3: shows a schematic section from a high-temperature furnace for illustrating a third embodiment of a shielding system according to the invention;

(4) FIG. 4: shows a schematic section from a high-temperature furnace for illustrating a fourth embodiment of a shielding system according to the invention; and

(5) FIG. 5: shows a schematic section from a high-temperature furnace for illustrating a fifth embodiment of a shielding system according to the invention.

DESCRIPTION OF THE INVENTION

(6) FIG. 1 shows a shielding element 2 according to a first embodiment of the present invention. The shielding element 2 has a disk-like basic form. The basic form is formed by a closed surround 4 made up of tungsten sheet(s) 5 (the surround 4 in the present case not having a gas-tight form). A disk-like cavity 6 is formed inside the surround 4 and is filled with a ceramic bed 8 of high-temperature-stabilized zirconium oxide (ZrO.sub.2), the size ratio of the particles relative to the surround not being reproduced correctly. The ceramic bed 8 has a monomodal grain size distribution. The mean grain diameter of the bed is 2 mm. The tungsten sheet has a wall thickness d.sub.B of 1 mm. The shielding element 2 has a thickness d.sub.E of 50 mm. The shielding direction r.sub.A for which the shielding element 2 shown is designed runs parallel to the direction of measurement shown for the thickness d.sub.E.

(7) The disk-like basic form is designed in particular for the realization of bottom-side and/or top-side shielding. In this respect, the shielding element 2 can be used alone or alternatively in combination with further shielding components, for example with radiant plates. The disk-like basic form can also have yet further structures and/or openings for adaptation to components of a high-temperature furnace, for example to heating elements, to connection contacts for the heating elements, to further components of the shielding system, etc. By way of example, the shielding element 2 can have a circumferential step or groove for receiving at least one further, lateral shielding element.

(8) In the explanation of the further embodiments which follows, details are provided predominantly in relation to the differences compared to the first embodiment. Where identical components or components corresponding to one another are identified, the same reference signs are used to some extent.

(9) In the embodiment shown in FIG. 2, the shielding element 2′ in turn has a closed surround 4′ made up of tungsten sheet(s) 5′ (the surround 4′ in the present case not having a gas-tight form). In contrast to the first embodiment, the material made of high-temperature-stabilized zirconium oxide (ZrO.sub.2) which is accommodated in the cavity 6′ of the surround 4′ is present in a fibrous structure. In particular, the material is accommodated in the form of a fiber mat 10′ in the surround 4′.

(10) Hereinbelow, three embodiments of a thermal shielding system according to the invention, which in this case forms lateral shielding, will be explained with reference to FIGS. 3 to 5. FIG. 3 shows a section from a high-temperature furnace 12. The section runs along the radial direction and also along the axial direction of a main axis or axis of symmetry 16, the illustration corresponding to a position of use of the high-temperature furnace 12 with an axis of symmetry 16 oriented vertically or in the height direction. The section shown extends in the radial direction from the axis of symmetry 16 arranged centrally inside a batch space 14 via a lateral, thermal shielding system 18 as far as a lateral, outer housing 20. The housing 20 is formed from steel, for example.

(11) A component 21 to be thermally treated is shown schematically in the batch space 14. The thermal shielding system 18 has a lateral shielding element 22. This has a design corresponding to that of the shielding element 2 according to the first embodiment (cf. FIG. 1). In particular, the lateral shielding element 22 has a surround (not shown in detail) made up of tungsten sheet(s), in which a bed (not shown in detail) of high-temperature-stabilized zirconium oxide (ZrO.sub.2) is accommodated.

(12) The thermal shielding system 18 furthermore has a plurality of (here: four) radiant plates 24, which are spaced apart in the shielding direction and are arranged adjacent to the lateral shielding element 22 in the shielding direction r.sub.A.

(13) The radiant plates 24 are arranged on that side of the lateral shielding element 22 which faces away from the batch space 14. The shielding element 22 is formed running around the batch space 14 and extends along the height direction over a predetermined height. The radiant plates 24 are formed correspondingly with a diameter of increasing size. Appropriate additional structures or through-openings can be provided both in the shielding element 22 and in the radiant plates 24, for example for adaptation to further components of the high-temperature furnace 12 (e.g. to heating elements, to connection contacts for the heating elements, to a charge opening, etc.). Since the batch space 14 is already thermally shielded by the shielding element 22, it is sufficient for the radiant plates 24 to be formed from molybdenum. The individual radiant plates 24 each have a thickness of 0.25 mm. The overall thickness d.sub.G of the lateral, thermal shielding system 18 as measured along the shielding direction r.sub.A is approximately 43 mm, the thickness d.sub.E of the lateral shielding element 22 along the shielding direction r.sub.A being approximately 19 mm.

(14) In the explanation of the fourth and fifth embodiments which follows, details are provided predominantly in relation to the differences compared to the third embodiment. The same reference signs are used again for identical components or components corresponding to one another (these reference signs being provided in each case with one or two primes).

(15) In the fourth embodiment, shown in FIG. 4, the radiant plates 24′ (here: four) are arranged on that side of the lateral shielding element 22′ which faces toward the batch space 14′. Of the radiant plates 24′, at least those which are arranged directly adjacent to the batch space 14′ or which are at only a small distance from the batch space 14′ (for example the first three) are formed from tungsten, on account of the high temperatures which arise in this region. The radiant plates 24′ which follow further to the outside in the shielding direction r.sub.A (for example the remaining one) can also be formed in particular from molybdenum. The surround of the lateral shielding element 22′ can also be formed from molybdenum (instead of tungsten), if appropriate.

(16) In the fifth embodiment, shown in FIG. 5, the lateral, thermal shielding system 18″ is formed exclusively from a shielding element 22″, i.e. no additional radiant plates are provided. The shielding element 22″ has a correspondingly large thickness d.sub.E of 43 mm along the shielding direction r.sub.A.

(17) Concerning the various variants in which the shielding system according to the invention can be realized, the following points are to be taken into consideration with respect to the shielding action and to the behavior in use:

(18) A ceramic material based on zirconium oxide (ZrO.sub.2) present in a particulate and/or fibrous structure has a relatively low thermal conductivity (for example compared to refractory metals). Particularly at high temperatures, in particular at temperatures ≧1500° C., more preferably at temperatures ≧1700° C., the shielding action of the shielding element is significantly superior to that of a sequence of radiant plates having a comparable overall thickness. This means that, in use in a steady state at a correspondingly high temperature of the region adjoining the shielding element (e.g. of the batch space), a relatively large drop in temperature can be achieved over the thickness of the shielding element, and that the energy consumption for retaining the steady state is relatively low. The temperature gradient which arises inside the batch space can also thus be minimized, this being advantageous particularly for critical processes. The shielding element has, however, by comparison a higher heat capacity than a sequence of radiant plates extending over a comparable thickness. This leads to delays and possibly to an increased energy consumption during heating phases, if changes in temperature are to be established within short times.

(19) In the light of the properties discussed above, the provision of one or more shielding elements without combining them with radiant plates affords advantages in particular for those high-temperature furnaces in which the temperature has to be kept steady or substantially steady at high values (e.g. ≧1500° C., in particular ≧1700° C.) for relatively long periods of time. This is the case, for example, in the production of sapphire single crystals.

(20) In thermal processes with a relatively short duration which are carried out in succession, and/or when establishing a multi-stage temperature profile, work is performed relatively frequently in the thermally transient regime. This is also the case, for example, for sintering furnaces for refractory metals. For these applications, it is advantageous in terms of reducing the heat capacity of the thermal shielding system if the shielding element is combined with radiant plates. In this way, it is possible to lower the energy consumption which is required for increases in temperature to be carried out within a relatively short time. However, a combination of this type also affords advantages for the conditions of use explained above, in which the temperature has to be kept steady or substantially steady at high values (e.g. ≧1500° C., in particular ≧1700° C.) for relatively long periods of time, and is accordingly also well suited to such conditions of use.

(21) It is to be taken into consideration that, at relatively low temperatures (e.g. 500° C.) and given an identical thickness of the shielding, the heat flux through radiation, which dominates in the case of radiant plates, lies below the heat flux through heat conduction, which dominates in the case of the shielding element. Accordingly, the shielding action of a sequence of radiant plates is superior to that of a shielding element of comparable thickness at relatively low temperatures (e.g. at temperatures in the region of 500° C.). For this reason, too, a combination of shielding element and radiant plates in particular is particularly advantageous. With respect to the relationships explained above, it is advantageous if the shielding element is arranged directly adjacent to the batch space and the sequence of radiant plates is arranged on that side of the shielding element which faces away from the batch space.

(22) It is furthermore to be taken into consideration that, in the case of radiant plate shielding, the heat flux increases to a considerably greater extent with an increasing temperature than is the case for a shielding element. This results in the very good shielding action of the shielding element particularly at high temperatures (e.g. ≧1500° C., in particular ≧1700° C.). Accordingly, when a combination of shielding element and radiant plates is used, it is particularly preferable if the shielding element is arranged adjacent to the region of the high temperatures (e.g. ≧1500° C., in particular ≧1700° C.), in particular adjacent to the batch space.

(23) It was also possible to verify the relationships presented by an analytical and numerical calculation.