DE3723950A1 - Regenerative heat engine with a hypocycloidal eccentric crank mechanism - Google Patents

Abstract

A regenerative heat engine of crossed-piston construction with a Parsons gear is described. <IMAGE>

Description

The invention relates to a regenerative heating machine, especially simple, working according to the Stirling process or double-acting heat and power machine, heat pump, single or multi-stage chiller, according to the Vuilleu Mier process working heat pump or chiller, one Mixed forms between the Stirling and Vuilleumier processes possible Practicing machine for optional, freely adjustable combinable Ren provision of mechanical and heating or cooling performance in which two are arranged at right angles to each other, opposite pairs of pistons by a gear perform certain harmonic movements in their cylinders ren, whereby the changing work spaces of the regenerati ven heat machine are formed.

Such a regenerative heating machine is republished open patent application P 36 02 634.4 (attorney file PG1139) refer to.  

According to the prior art, different designs are knows the above Machines and their engines concern. These are, for example: machines with rows or V arrangement cylinder, with crosshead gear; Machines in Rei hen or boxer arrangement with rhombus gears; machinery with swashplate or swashplate gear and square parallel arrangement of the cylinders.

Various configurations of crank loops have also been found as well as a variety of rotary and rotary piston designs gene.

All these have the problem in common, the conversion of the thermodynamic working medium in which the gearbox contains as well as the passage of the lubricant in to reliably prevent the thermodynamic working rooms dern, the latter as according to the prior art can be viewed as controllable. It was already in the The past considered the crankcase space with the ar beitsmedium at a pressure that is about the average of the swan kenden working pressure, to fill the Abdich to facilitate the thermodynamic work rooms. At the pressure levels corresponding to the state of the art However, this leads to the fact that the crankcase is essential  share of the total weight of the machine and therefore also make up the design effort. So the benefit an increased working pressure in the form of increased specific performance brings to nothing.

The object of the invention is a regenerative heat to indicate machines of the type mentioned, which are characterized by a extremely compact gearbox design and simple torque ment decoupling and in which the gear housing flawlessly flawless against thermodynami sealed working spaces and if necessary with under a medium pressure thermodynamic work medium can be filled.

This task is done with a regenerative heating machine of the type mentioned solved in that the transmission a hypocycloidal eccentric crank gear, in particular is a Parson transmission.

The compact structure of the one used according to the invention Gearbox allows a correspondingly compact design of the regenerative heating machine, thanks to which the dead volumes the thermodynamic work rooms, and in particular the they keep connecting line volumes very small  that can. This has a positive effect on efficiency the heating machine. At most with swashplate gear ben are similarly compact gearboxes according to the prior art built-up accessible. Opposed to this exists because it is not possible due to thermal similarity, to enlarge the piston dimensions as desired, however invented according to the advantage that machines of the described Design due to the short, very stiff crank waves can be combined into larger units.

There, determined by the gearbox, the movements of the pistons are harmonious is a complete balance of the masses Forces and moments easily possible, for example through a balance weight rotating with the crankshaft.

Especially when using the proposed design on cryogenic refrigerators with multi-stage operation arises according to the invention as a result of the arrangement conditional removal the advantage of good options for thermal insulation of the expansion rooms.

The transmission used according to the invention preferably has a runner with two rigidly connected eccentric gears ben, which are arranged with parallel, offset axes and rotatably sit in cross heads, which with one or other of the Kol arranged at right angles to each other  pairs are coupled.

The eccentric discs are preferably in the axial direction offset and partially overlapping each other connected.

The rotor can be one of the axes of the eccentric discs parallel, in the geometric center between these axes have horizontal axis of rotation.

In a first variant of the Parson gearbox are the cross heads and the piston rods rigidly connected. The vibratory systems are linearly guided through a piston rod bearing or the pistons in the cylinder themselves. The transverse to the direction of movement of the pistons then occurring reaction forces are from the pistons themselves or, if this leads to sealing or other problems, picked up by the piston rods and their bearings. It ver it is clear that the components involved are sufficient must be designed to be rigid so that the forces are jammed can be transferred. Provided that, draws this Parson gearbox has an uncomplicated structure out.  

This takes place in a second variant of the Parson transmission Straight guidance of the vibratory systems to the cross behead. The latter are on linear guideways performed, which preferably on the gear housing ausgebil det. The reaction forces are already in the transmission housing included. So that the pistons with the cross heads due to bending moments articulated on both sides free piston rods are connected. Overall, one is lighter, material-saving design possible.

The same applies to a third variant of the Parson transmission, where the linear guidance of the vibratory systems caused by a gimbal pair of wheels with two gears becomes. One of the gears has external teeth and is firmly connected to the rotor so that its axis with the Axis of rotation of the rotor coincides and the axes of the at the eccentric on the pitch circle of the gear. The other gear is fixed to the gear housing and has an internal toothing with just twice the number of teeth of the first gear. Now the first gear is rolling the rotation of the rotor in the second gear decreases it makes the eccentric disc straight and harmonious. So can here, too, the pistons with articulated, bending moment-free Piston rods connected to the eccentric disc the.  

The axis of rotation of the rotor can be the articulation axis of a crank form a wave parallel to this axis of rotation, the Projective section of the longitudinal axis of both pairs of pistons Crankshaft axis of rotation that cuts vertically is stored. This arrangement allows simple torque decoupling ment on the crankshaft, and also a possibly the required starting of the heating machine turns out very easy.

The length of the crank, i.e. H. the distance between the crank shaft axis of rotation and the articulation axis of the crankshaft The axis of rotation of the rotor is in the Parson gearbox equal to the distance of the latter axis from the axes of the Eccentric disc.

The rotor preferably has an axle bore, and it is mounted on a crank pin of the crankshaft. These Design is particularly easy to manufacture and assemble.

The crankshaft can consist of a main pin, one first crank, the crank pin, one parallel at a distance to the first crank extending second crank and one with the first main pin on the same axis the second main spigot, with all the parts mentioned  are rigidly connected. This design is special practical, especially when it comes to several Coupling heat machines in a cross-piston design.

In the gearbox described, the two eccentrics rotate circular disks around the articulation axis of the crankshaft, its partly with the same frequency in the opposite direction around the crankshaft axis of rotation. The eccentric circle discs are therefore clearly eccentric to the crankshaft axis of rotation. In a preferred design of the Parson-Ge drive is therefore a counterweight provided this eccentricity balanced. You get one like that good balance of the acting mass forces and moments and perfect rotation of the heating machine. The end balance weight can be firmly connected to the crankshaft, its focus being on that of the articulation axis lying side of the crankshaft axis of rotation.

The counterweight can be a cylindrical disk segment or a cylinder with relief bores be disc, the central axis of the crankshaft axis of rotation is. The outer surface of the counterweight is preferred have a guide surface for the purpose of storage. The lead rungsfläche preferably works with one on the housing  Heat engine trained, stationary tread together. The balance weight takes over in an advantageous manner at the same time a management function.

The high temperature resistant components of the fiction Moderate heat machine can not completely or partially oxidic ceramics, in particular silicon carbide, silicon nitrite or reaction sintered silicon infiltrated Silicon carbide (silicon-silicon carbide; SiSiC), or from oxidic ceramics, especially steatite, exist. These ceramic materials are well machinable, corrosion constantly, and they hold extreme pressures and temperatures Stand. The high thermal should also be emphasized Conductivity of SiSiC and the good self-running properties of this material.

At least one piston of the heating machine according to the invention can be made entirely or in part of graphite, and in particular with Reinforced graphite (carbon fiber graphite, CfC) exist, being on its surface with non-oxidic Ceramic, in particular silicon carbide, silicon nitride or SiSiC, or with oxidic ceramics, especially steatite, can be coated. Graphite and CfC stand out good temperature resistance and high mechanical sta  balance with a low specific weight. You grant achieves such small piston masses. Next have graphite and CfC have good thermal insulation properties. A ceramic Coating of the piston made of graphite or CFC body has an extremely high abrasion resistance and temperature temperature resistance safe.

The invention is described below with reference to the drawings gene illustrated embodiments explained in more detail. Some show schematically:

Figure 1 shows the demonstration model of a single-acting Stirling engine in a cross-piston design with Parson transmission, in which the linear guidance of the vibratory systems on piston rod bearings he follows.

Fig. 2 is a thermodynamic schematic of this Stirling engine;

Figure 3 shows the group consisting essentially of two Exzenterkreisscheiben rotor of Parson transmission with a crank hinged thereto.

Fig. 4 the same rotor with the eccentric discs in cross heads, with piston rods crossing at 90 ° on the cross heads; and

Fig. 5 is an axial projection of the arrangement according to Fig. 4.

Fig. 6 is a variant of Parson transmission, wherein the linear guide of the oscillatory systems is carried out to the rim heads;

Provided 7 shows a further variant of Parson-transmission systems in which, for linear guidance of the oscillatory Sys a cardan pair of wheels.

Fig. 8 is a thermodynamic diagram of a double-acting Stirling engine in cross piston construction with Parson gear; and

Fig. 9 shows a thermodynamic diagram of a Vuilleumier heat pump in a cross-piston design with Parson gearboxes.

Fig. 1 and Fig. 2 show a regenerative heat engine in cross piston construction. It is a single-acting Stirling engine with two working systems 10 , 20 and a transmission housing 30 arranged in the center. The Ar beitssysteme 10 , 20 are arranged above and left and below and right of the gear housing 30 . To the first Ar beit system 10 includes the working cylinder 11 with the working piston 12 and the displacement cylinder 13 with the displacement piston 14th The second working system includes the working cylinder 21 with the working piston 22 and the displacement cylinder 23 with the displacement piston 24 . The Arbeitszylin the 11 , 21 are diametrically opposite to each other on the same axis, and also the displacement cylinder 13 , 23rd

The working pistons 12 , 22 are rigidly connected to a crosshead 31 by means of axial piston rods 15 , 25 and thus form an oscillatory kinematic unit. Likewise, the displacement pistons 14 , 24 are rigidly connected to a second crosshead 32 so that they form a second kinematic unit capable of oscillation. The axes of the units cross each other at 90 °, but come to lie in different planes parallel to the plane of the drawing in FIG. 1; in particular, the cross heads 31 , 32 are set accordingly ver.

The cylinder spaces above, ie on the side of the displacement piston 14 , 24 facing away from the transmission housing 30 , are the hot working spaces V _{H 1} , V _{H 2 of} the working systems 10 , 20 . The cold working spaces V _{K 1} , V _{K 2} are formed by the cylinder spaces below the displacement pistons 14 , 24 together with the cylinder spaces above the working pistons 12 , 22 , with which they are connected by channels and lines 17 , 27 . The annular gaps around the displacer pistons 14 , 24 represent thermal regenerators R ₁ , R ₂ , which flow through the thermodynamic working medium as they move. Fig. 2 shows schematically heater E ₁ , E ₂ and cooler K ₁ , K ₂ on the hot or cold side of the re generators R ₁ , R ₂ .

The crossheads 31 , 32 located in the gear housing 30 have the shape of circular rings with diametrically opposite mounting lugs 33 , on which the pistons 15 , 16 , 25 , 26 engage. The piston rods 16 , 26 connected to the displacer 14 , 24 are guided in the transmission housing 30 with a seal. In contrast, no sealed bushing is provided for the piston rods 15 , 25 connected to the working pistons 12 , 22 , since the cold working spaces V _{K 1} , V _{K 2 are} sealed by the working pistons 12 , 22 themselves. The reaction forces occurring in the linear management of the two kinematic units are absorbed by piston rod bearings which are fixedly connected to the gear housing 30 .

The crossheads 31, 32 receive a rotor 34 of the Parsonstown transmission, whose construction in detail FIG. 3 and FIG. 4 can be removed. The rotor 34 has two eccentric circular disks 35 which are in face-to-face contact and which partially overlap one another and are firmly connected to one another. The eccentric discs 35 are rotatably seated in one of the cross heads 31 , 32 . The rotor 34 is rotatably mounted on the crank pin of a crankshaft 36 , which consists of a first main pin 37 , a first crank 38 , the crank pin, a second crank 39, which extends parallel to the first crank 38 , and one with the first main pin 37 second main pin 40 lying on the same axis. The pins of the crankshaft 36 are oriented parallel to the axes of the eccentric disk 35 . The main pin 37 , 40 are out of the gear housing 30 leads and rotatably mounted in the wall of the gear housing 30 . The geometric relationships can be found in the individual NEN Fig. 5. The axes of the kinematic units are designated here with x and y . The pistons 12 , 22 and 14 , 24 and the associated crossheads 31 and 32 move along these axes x , y . K is the crank shaft axis of rotation, ie the central axis of the main journal 37 , 40 of the crankshaft 36 mounted on the transmission housing 30 . The crankshaft axis of rotation K extends perpendicular to the piston system axes x , y through their projective crossing point. A ₁ and A ₂ are the central axes of the eccentric circular discs 35 which are seated centrally and rotatably in the cross heads 31 , 32 . These axes extend parallel to the crankshaft axis of rotation K , and they meet the movement axis x or y of the associated kinematic unit. M is the articulation axis of the crankshaft 36 to the rotor 34 , ie the central axis of the crank pin on which the rotor 34 is rotatably mounted. This articulation axis M extends parallel and exactly in the middle between the central axes A ₁ , A _{2 of} the eccentric disk 35 . The length of the cranks 38 , 39 , ie the distance between the crankshaft axis of rotation K and the articulation axis M is equal to the distance that this articulation axis M from the central axes A ₁ , A _{2 of} the eccentric disc 35 has.

In the case of a harmonic oscillation of the kinematic units that is phase-shifted by 90 °, the articulation axis M of the crankshaft 36 rotates on a crank circuit 41 . This can be easily recognized if the following time dependency is used in the projection in FIG. 5:

With a linear movement of the kinematic units in the direction of arrows 42 , 43 , the crankshaft is rotated in the direction of arrow 44 . The eccentric circular disks 35 rotate in the cross heads 31 , 32 , and the rotor 34 composed of two eccentric circular disks 35 performs a total rotation about the articulation axis M. A corresponding eccentric circle is indicated at 45 , and the direction of rotation is represented by arrow 46 . The eccentric rotation is opposite to the rotation of the articulation axis M , and it takes place at the same frequency.

After all, the described Parson transmission converts the 90 ° phase-shifted harmonic vibration of two piston systems into a crankshaft rotation. The rotor 34 consisting of the two eccentric discs 35 comes to lie eccentrically with respect to the crankshaft axis of rotation K in all phases of the movement. As shown in Fig. 1, it is therefore advisable to attach a counterweight 47 , which can be firmly connected to the crankshaft 36 . The center of gravity of the counterweight 47 lies on the side of the crank rotation axis K opposite the articulation axis M. From the counterweight 47 has a substantially semicircular plan according to FIG. 1. In general, the counterweight can be a cylindrical disk segment, and a cylindrical disk provided with relief bores is also suitable as a counterweight. The center axis of the balance weight is the crankshaft axis of rotation K in each case. Its outer surface can be designed as a guide surface for the purpose of storage and cooperate with a housing 30 formed on the gear housing tread.

A large number of variants of the transmission according to the invention are possible, which can be classified according to the type of straight guidance of the linearly moving oscillatory units, the same kinematics always being used. In the previously discussed embodiment, the straight line is caused by the piston rods 15 , 16 , 25 , 26 store in rods. For this purpose, however, the pistons 12 , 22 , 14 , 24 in the cylinders 11 , 21 , 13 , 23 themselves can also be used, provided that this does not lead to sealing and other problems. The components involved must be designed to be sufficiently rigid so that the forces can be transmitted without jamming.

In a variant shown in FIG. 6, the cross heads 31 , 32 are provided with straight guide surfaces, and slideways 48 are formed on the gear housing 30 , on which the cross heads 31 , 32 are guided. The reac tion forces occurring are thus already absorbed by the gear housing 30 . So that the pistons 12 , 24 with the cross heads 31 , 32 by bilaterally articulated, bending moment-free piston rods 15 , 26 are connected.

In the variant illustrated schematically in FIG. 7, the straight guidance of the articulation points A ₁ , A ₂ for the piston rods is effected by a cardanic wheel pair 49, 50 . For this purpose, a gear 49 provided with external teeth is fixedly connected to the rotor 34 , so that its axis coincides with the axis of rotation M of the rotor, while the axes A ₁ , A _{2 of} the eccentric disc 35 lie on the pitch circle of the gear 49 . The gear meshes with a stationary gear 50 , which is fixedly connected to the gear housing 30 and has internal teeth, which has just twice the number of teeth as the gear 49 with external teeth. If the latter rolls off in the gearwheel 50 with internal toothing, it guides the eccentric circular disks 35 of the rotor 34 in a straight and harmonious manner. Here, too, the pistons can be connected to the eccentric disc 35 with articulated, bending-moment-free piston rods.

Fig. 8 and Fig. 9 illustrate other types of regenerative heat engines in cross piston construction with Parson transmission. As in the case of the single-acting Stirling engine dealt with at the beginning, straight guidance takes place here by means of the piston rods and suitable rod bearings. In Fig. 8 a double-acting Stirling engine is shown which provides double the performance with practically the same space as a single-acting Stirling engine. Four work systems identified by indices 1 to 4 can be seen, each of which is assigned to one of the cylinders 51 , 61 , 71 , 81 . In the cylinders 51, 61, 71, 81 , pistons 52 , 62 , 72 , 82 work , above which there are hot working spaces V _H and below which there are cold working spaces V _K. The hot working space of a cylinder is spatially cyclically connected to the cold working space V _{K of} the adjacent cylinder via a thermal regenerator R. Heater E and cooler K are indicated schematically as in Fig. 2.

InFig. 9 is a Vuilleumier heat pump in cross-piston construction represented wisely. It has above a piston90 one hot workspaceV _H , above an adjacent piston91  a cold work spaceV _K , and above the other two piston92,93 interconnected warm work rooms V _W . The cylinder chambers below the pistons90,91,92,93  are over the gearbox30th connected with each other. From the hot work roomV _H  leads a line over a Heat exchangerW T ₁, on which a drive at high temperature heat flowQ _H  is coupled in, a hot thermal regeneratorR _H  and a second heat exchangerW T _2nd to the inside space of the gearbox30th. About a branch in which a heat exchangerW T _3rd, a cold thermal regenerator R _K  and a heat exchangerW T _4th lie, is the interior of the Gear housing30th at the same time to the cold work areaV _K   connected. On the last-mentioned heat exchangerW T _4th is at  low temperature an ambient heat flow _K  injected pelt while on the heat exchangersW T _2nd and WT _3rd at mitt temperature is a heat flow _W  be removed can.

Mixed forms between the Stirling and Vuilleumier processes can in particular through a lie in the thermodynamic working group ing, adjustable throttle valve arrangement can be realized.  

• List of reference numerals 10 work system
  11 working cylinders
  12 working pistons
  13 displacement cylinders
  14 displacement pistons
  15 piston rod
  16 piston rod
  17 line
  20 work system
  21 working cylinders
  22 working pistons
  23 displacement cylinder
  24 displacement pistons
  25 piston rod
  26 piston rod
  27 line
  30 gear housing
  31 crosshead
  32 crosshead
  33 assembly approach
  34 runners
  35 eccentric disc
  36 crankshaft
  37 main spigot
  38 crank
  39 crank
  40 main spigots
  41 crank circuit
  42 arrow
  43 arrow
  44 arrow
  45 eccentric circle
  46 arrow
  47 Balance weight
  48 slideway
  49 gear
  50 gear
  51 cylinders
  52 pistons
  61 cylinders
  62 pistons
  71 cylinders
  72 pistons
  81 cylinders
  82 pistons
  90 pistons
  91 pistons
  92 pistons
  93 pistons

Claims (17)

1. Regenerative heat machine, in particular working according to the Stir ling process, single or double-acting heat and power machine, heat pump, single or multi-stage chiller, working according to the Vuilleumier process heat pump or chiller, a mixed form between Stirling and Vuilleumier process enlivening machine for the optional, freely adjustable combinable provision of mechanical and heating or cooling power, in which two mutually perpendicular, opposite piston pairs by a gearbox perform certain harmonic movements in their cylinders, thereby changing the working spaces the regenerative heating machine are formed, characterized in that the transmission is a hypocyclic iden eccentric crank transmission, in particular a Parson transmission. 2. Heating machine according to claim 1, characterized in that the gear has a rotor ( 34 ) with two rigidly connected ver eccentric discs ( 35 ) which are arranged with parallel, offset axes ( A ₁ , A ₂ ) and rotatable in crossheads ( 31 , 32 ), which are coupled to one or the other of the piston pairs ( 12 , 22 ; 13 , 23 ) arranged at right angles to one another. 3. Heating machine according to claim 1 or 2, characterized in that the eccentric discs ( 35 ) offset in the axial direction and are connected to each other partially overlapping. 4. Heating machine according to one of claims 1 to 3, characterized in that the rotor ( 34 ) one of the axes ( A ₁ , A ₂ ) of the eccentric disc ( 35 ) parallel, in the geometric center between these axes ( A ₁ , A ₂ ) axis of rotation ( M ). 5. Heating machine according to one of claims 1 to 4, characterized in that the crossheads ( 31 , 32 ) and piston rods ( 15 , 16 , 25 , 26 ) are rigidly connected to one another, and that a linear guide of the vibration-capable units by a piston rod bearing or the pistons ( 12 , 14 , 22 , 24 ) in the cylinders ( 11 , 13 , 21 , 23 ) themselves. 6. Warming machine according to one of claims 1 to 4, characterized in that the cross heads ( 31 , 32 ) on preferably on the gear housing ( 30 ) formed linear guide tracks ( 48 ) are guided. 7. Warming machine according to one of claims 1 to 4, characterized in that a linear guide of the units capable of vibrating is effected by a gimbal pair of wheels ( 49 , 50 ), one of which ( 49 ) has an external toothing and with the rotor ( 34 ) firmly so verbun is that its axis coincides with the axis of rotation M of the rotor ( 34 ) and the axes ( A ₁ , A ₂ ) of the two eccentric discs on the pitch circle of the gear ( 49 ), while the other ( 50 ) arranged fixed to the gearbox is and has an internal toothing with twice the number of teeth. 8. A heating machine according to claim 6 or 7, characterized in that the cross heads ( 31 , 32 ) are connected to the pistons ( 12 , 24 ) via piston rods ( 15 , 26 ) which are articulated on both sides. 9. Heating machine according to one of claims 1 to 8, characterized in that the axis of rotation ( M ) of the rotor ( 34 ) forms the articulation axis of a crankshaft ( 36 ) which is parallel to this axis of rotation ( M ), the longitudinal axis ( x , y ) of both pairs of pistons ( 12 , 22 ; 14 , 24 ) is rotatably mounted in their projective intersection of a vertically intersecting crank shaft rotation axis ( K ). 10. Warming machine according to one of claims 1 to 9, characterized in that the length of the crank ( 38 , 39 ), ie the distance between the crankshaft axis of rotation ( K ) and the pivot axis of the crankshaft ( 36 ) bil end axis of rotation ( M ) of Runner ( 34 ) equal to the latter was the latter axis of rotation ( M ) of the axes ( A ₁ , A ₂ ) of the eccentric discs ( 35 ). 11. Heating machine according to one of claims 1 to 10, characterized in that the rotor ( 34 ) has an axle bore and is rotatably mounted on a crank pin of the crankshaft ( 36 ). 12. Heat machine according to one of claims 1 to 11, characterized in that the crankshaft ( 36 ) from a first main pin ( 37 ), a first crank ( 38 ), the crank pin, one parallel to the first crank ( 38 ) extending second crank ( 39 ) and a second main pin ( 40 ) lying on the same axis with the first main pin ( 37 ). 13. Warming machine according to one of claims 1 to 12, characterized in that the transmission contains a counterweight ( 47 ). 14. Heat machine according to one of claims 1 to 13, characterized in that the balance weight ( 47 ) is fixedly connected to the crankshaft ( 36 ), its focus being on the side of the crankshaft axis of rotation ( K ) opposite the articulation axis ( M ) . 15. Heating machine according to one of claims 1 to 14, characterized in that the balance weight ( 47 ) is a cylindrical disk segment or a cylindrical disk provided with lightening bores, the central axis of which is the crankshaft axis of rotation ( K ), and that the outer surface of the balance weight ( 47 ) forms a guide surface for the purpose of storage. 16. Warming machine according to one of claims 1 to 15, there characterized in that their temperature stresses All or part of components made of non-oxide Ke ramic, in particular silicon carbide, silicon nitrite or reaction sintered silicon infiltrated Silicon carbide; SiSiC), or made of oxide ceramic, especially steatite. 17. Heating machine according to one of claims 1 to 16, characterized in that at least one of their Kol ben ( 12 , 14 , 22 , 24 , 52 , 62 , 72 , 82 , 90 , 91 , 92 , 93 ) in whole or in part consists of graphite, and in particular with carbon fiber-rated graphite (carbon fiber graphite, CfC), where it can be coated on its surface with non-oxidic ceramics, in particular silicon carbide, silicon nitride or SiSiC, or with oxidic ceramics, in particular steatite.