WO2013113049A1

WO2013113049A1 - Tungsten composite electrode

Info

Publication number: WO2013113049A1
Application number: PCT/AT2013/000012
Authority: WO
Inventors: Ingmar Wesemann; Hannes TRAXLER; Thomas Huber; Alexander Tautermann; Wolfgang Spielmann; Wolfram Knabl; Gerhard Leichtfried
Original assignee: Plansee Se
Priority date: 2012-01-31
Filing date: 2013-01-25
Publication date: 2013-08-08

Abstract

The invention describes an electrode of a high-pressure gas discharge lamp, comprising a core (3) made of tungsten or tungsten doped with potassium, of a diameter di and a shell (4) attached thereto, of an external diameter da, wherein the shell consists of a particle composite material, at least in some regions, with a matrix made of tungsten, and the following condition is met: di ≤ da / 3. The electrode according to the invention has considerably lower arc instability.

Description

TUNGSTEN ELECTRODE COMPOSITE

The invention relates to an electrode of a high-pressure gas discharge lamp having a core with at least partially a diameter d, and an adjoining jacket with at least partially a

Outside diameter d _a includes.

Furthermore, the invention relates to a short arc lamp comprising a quartz glass bulb, a filling gas containing at least one element of group Xe and Hg, and a cathode and an anode for forming an arc.

Electrodes of high pressure gas discharge lamps serve to provide electrons for ionization and excitation of the fill gas. A sufficiently high density of emitting electrons can be achieved only at high temperatures, so that tungsten base materials are usually used as the electrode material.

Changes in the morphology, structure and shape of the electrode as well as an increase / local change in electron work function during use are the major electrode-related failure mechanisms.

The consequences of this are:

Increasing the electrode temperature, which in turn leads to evaporation of electrode material associated with Koibenschwerzung and

Amplification of the electrode-related failure mechanisms leads; - Short-term local change in the arc starting point and associated fluctuations in the luminous flux (arc disturbance or flicker);

- electrode burn-back. The cause of the arc disturbance is that the arc starts where the work of the electron work, compared to the previous one

Light approach point, is lower and thus more favorable conditions for the electron emission are given. Arc wandering and arc disturbance are usually assigned to material-relevant phenomena in the area of the electrode tip.

For high-pressure high-pressure gas discharge lamps, for example

Thoroughbred tungsten (W-Th0 ₂ ) is still preferred as the cathode material because this additive significantly reduces the electron work function (from 4.6 to 5.4 eV for pure softwam on 2, depending on the grain orientation). 4 to 3.0 eV for W-Th0 ₂ ). However, since thorium is a radioactive element that emits alpha rays, efforts have been made for decades to replace this material.

Thus, EP 1 481 418 A1 describes a Woiframkathodenwerkstoff containing La ₂ O ₃ and ZrO ₂ or Hf0 ₂ . In the area of the arc approach, however, melting of these oxides occurs because of their lower thermal stability compared to TnO ₂ . This leads to a local

Change in the degree of coverage of the cathode with the emission-promoting substance, which in turn expresses in an increased arc disturbance. In addition, during use, the cathode coarsen the originally fine particles by, for example, penetration of the melt along

Grain boundaries. Also, the inhomogeneous distribution of the emission promoting element caused thereby can lead to arc instability and arc disturbance.

WO 2008/074361 A1 describes an electrode comprising a core and a cladding. The core is preferably W-ThO ₂ . The

Arc quenching behavior is unaffected by this electrode design.

The object of the present invention is to provide an electrode having the following properties:

Arc disturbance similar to or less than with thoriated tungsten; - Low evaporation of electrode material and thus low

bulb blackening;

- Stable electrode shape and low electrode burn-back;

- No local changes in the arc starting area. Another object of the invention is the provision of a Kurzbogeniampe with the following characteristics:

- High and constant light output over a long period of use;

- Arc disturbance similar to or less than with a lamp with thoriated cathode.

This object is achieved with the features of the independent claims. Advantageous embodiments are the subject of the dependent claims. The electrode of a high-pressure gas discharge lamp in this case comprises a core at least partially made of tungsten or doped with potassium tungsten with at least partially a diameter d, and a thereto

subsequent jacket with at least partially an outer diameter d _a , wherein the jacket at least partially from a

Particle composite with a matrix consists of tungsten and

d; and d _a satisfy the following relationship:

d, <d _a / 3.

Tungsten is used in connection with the invention

Tungsten material understood that at least 95% by mass, preferred

at least 99% by mass, more preferably at least 99.9% by mass of tungsten. The balance to 100% preferably contains at least one element and / or a compound from the group consisting of tantalum, rhenium, molybdenum, oxides and carbides.

In the context of the invention, tungsten doped with potassium is understood to mean a tungsten material which preferably contains from 5 to 150 pg / g of potassium. In addition, the potassium-doped tungsten may contain up to 5% by mass, preferably up to 1% by mass, particularly preferably up to 0.1% by mass of one or more further elements and / or one or more further compounds, preferably from the group consisting of Tantalum, rhenium, molybdenum, oxides and carbides. A composite part of a composite is a composite material in whose matrix particles of another or other element (s) are embedded. In the embodiment according to the invention, at least one element contained in the particles reduces the electron work function of tungsten. The matrix is made of tungsten. In the context of the invention, tungsten in addition to pure tungsten (purity typically 3N or better) is also to be understood as meaning tungsten with a low content of a further or further element (s). The content is <5 a%, preferably <1 Ma%, particularly preferably <0.1 Ma%. So can the tungsten matrix of the

Particle composite tantalum, rhenium, molybdenum and / or up to

150 μ / g potassium. Further, for example, during production, during the activation annealing or during use, elements contained in the particles may be solubilized in the tungsten matrix or segregated at the grain boundaries. This effect is quite desirable because these elements can diffuse to the electrode tip and reduce the electron work function there.

The inventive design of the electrode is achieved that the arc preferably attaches only in the range of tungsten or doped with potassium tungsten. The potassium is in the smallest bubbles

(typically less than 100 nm) that are stable up to high temperatures (2700 K). As mentioned, the emission-promoting component is preferably present in the particles of the particle composite. Thus, there are no components or phases in the area of the arc projection which have a lower melting point than tungsten. The geometry according to the invention (d; <d _a / 3) largely ensures that melting of an electrode constituent is avoided.

Surprisingly, it has now been shown that in the

According to the invention electrodes, the electrode tip temperature is not unduly higher than in electrodes where the arc on the

Teiichenverbundwerkstoff impinges and thus on an area where the emission-promoting substance is provided immediately. It can be assumed that the top area is sufficiently through

Surface and / or grain boundary diffusion is supplied with the emission-promoting substance. If now d,> d _a / 3, the electrode temperature rises successively to areas corresponding to an electrode temperature of pure tungsten. These high temperatures lead to strong evaporations, high burn-back and deformation of the electrode tip. It can be assumed that at dj> d _a / 3 the after-transport paths of the emission-promoting component become too long, as a result of which, as will be explained in more detail in the examples, an increase in the electrode tip temperature results from an insufficiently reduced electron work function. As a result, more evaporation and blackening of the lamp body occurs. It has now further surprisingly been found that in the inventive geometric conditions, the arc disturbance is greatly reduced. This is attributed to the fact that in the area of

Arc approach a melting of the particles is largely prevented. It can also be assumed that this also results in the occurrence of locally preferred conditions (areas with reduced

Electron work function) for the approach of the arc is reduced. dj is chosen in such a way that on the one hand there is no melting of the emission-promoting component, but on the other hand the temperature is high enough to initiate mechanisms leading to the formation of a diffusible, metallic species of the emission-promoting component

Lead component. This metallic species diffuses at the surface or along grain boundaries towards the electrode tip. The design concept according to the invention combines the following advantages:

- By preventing the melting of the emission-enhancing

Component is prevented from excessive evaporation, which in turn leads to blackening of the bulb. - The arc stability is improved because a locally and / or temporarily uneven degree of coverage of the emission-promoting component is prevented on the electrode tip.

- A rumbling of the emission-promoting component by

Melting of the particles followed by penetration of the melt along grain boundaries or the surface is prevented.

- The mechanical stability, in particular the shape stability of the tip, is improved because molten phase at the grain boundaries, which triggers creeping processes controlled by grain boundary diffusion, is reduced / avoided. This allows the electrode to endure higher thermally induced voltages.

Since the tip region is free of second phase particles, the thermal conductivity is improved, which in turn leads to a

Reduction of the electrode tip temperature leads. A lower electrode tip temperature brings a higher light output, since

Blackening phenomena of the lamp bulb can be prevented by condensing vapor of the evaporated component.

Furthermore, it is advantageous if d satisfies the following relationship:

0.1 mm <dj <6 mm. If d is greater than 6 mm, the diffusion paths are too long for a sufficient supply of the electrode tip. If d, smaller than 0.1 mm may be a melting of the emission-promoting component and thus

Bow trouble can not be sufficiently avoided.

Furthermore, d, / d _{a is} preferably greater than 0.01. For many lamps, melting at dj / d _a <0.01 can not be sufficiently reduced. In addition, given a small electrode diameter, this limit is also given by manufacture.

In order to ensure an axis-centric burning of the arc, it is advantageous if the core is cylindrical and the jacket is tubular. Particularly suitable as emission-promoting components are oxides, carbides and borides, it being advantageous if these have a melting point> 1500 K. The particle content of the particle composite is preferably 0.1 to 10% by mass. The salary depends among other things on the type the high pressure gas discharge lamp (short arc lamp,

EtaHhaiogenidlampe, ...} and the chemical consistency of the particles.

Particularly suitable are oxides containing at least one metal of the group of lanthanides Y, Sc, Ti, Zr and Hf. If carbides are used, especially carbides which contain at least one metal of the group W, lanthanides, Y, Sc, Ti, Zr and Hf are suitable. Also advantageous is the use of oxides in combination with the use of one or more carbides. For example, when used with tungsten carbide in combination with an oxide, this promotes a uniform transport of the emission-enhancing

Component of the jacket to the electrode tip. For some types of lamps, on the other hand, the after-transport speed of the emission-promoting system is

Component too high, is the addition of zirconium oxide and / or

Hafnium oxide to an oxide of lanthanides. Thus, the Nachtransport is slowed down and evened out. Even with the setting of the grain size, the post-transport speed can be influenced. At comparatively low electrode temperatures (low Nachtransportgeschwindigkeit), the use of fine-grained material is advantageous because so that the

Subsequent transport across the grain boundaries can be accelerated. At very high electrode temperatures, which are too high

After transport speed can lead to a depletion of the emission-promoting component in the area of the electrode tip, in turn, a coarse-grained structure is better. The outer diameter of the jacket d _a is preferably in a range 0.5 mm <d _a <30 mm. With a smaller outer diameter of the above-described effect of the composite electrode according to the invention is no longer sufficient advantage. For very heavily loaded electrodes with a diameter greater than 30 mm, other concepts are preferred

Prevention of local melting, such as active cooling of the electrode, for use.

Another advantageous concept is that the tubular jacket has an inner region adjoining the core Particle composite and an outer region of tungsten or doped with potassium tungsten. In particular, this concept promotes accurate arc centering, as the outer region of tungsten or potassium doped tungsten reduces the likelihood of arcing in that region. In addition, this design also reduces the evaporation of the emission-promoting component and thus the piston blackening.

Preferred potassium contents are in the region of the core, as well as the shell 5 to 150 pg / g. The use of a potassium doping is particularly advantageous if, over a long period of time, coarsening of the grain structure is to be prevented, since the potassium bubbles prevent or significantly slow grain growth up to high temperatures. Potassium doping is particularly advantageous when the electrode tip temperature is below 2,550K. At higher temperatures, the potassium bubbles coarsen due to the high vapor pressure or by diffusion of the potassium. In addition, at electrode temperatures greater than 2,550 K, it is advantageous if the microstructure is coarse-grained to slow down the outdiffusion of the emission-promoting component.

Furthermore, there is an attractive interaction between the potassium bubbles and dislocations, the creep behavior and thus the

Improve dimensional stability of the electrode tip.

A conical shape of the tip region has also proven itself in the composite electrode according to the invention. Furthermore, with many

Lamp types a carburization of the cone-shaped tip area of advantage. The carburation also has an influence on the

Surface diffusion and oxidation / reduction processes in the area of the electrode tip. Advantageously, the carburization is carried out only in the region of the jacket. The outermost tip area is in

preferably not carburized, since the temperature of the electrode tip can exceed the electrical temperature in the tungsten-carbon system. The subsequent transport of the emission-promoting component in the region of the cone-shaped tip can also be improved by increasing the surface, since more of this emission-promoting component can be transported by surface diffusion. In a simple manner, this can be done by the introduction of grooves, which are preferably aligned in the axial direction.

Since the core in the region of the electrode tip before use of the electrode can have no or little emission-promoting substance, which at the beginning of the use briefly to an increased electrode temperature and

Aging phenomena of the electrode, it is advantageous if the electrode is subjected to thermal activation before use. In a simple way, this is done for example by annealing, which means grain boundary and surface diffusion, the emission-promoting component for

Electrode tip in the region of the core of tungsten or potassium doped tungsten can diffuse. However, the thermal activation can also be effected by a targeted burn-in of the electrode.

Since the core of tungsten or with potassium doped tungsten and the

Particle composite a different tendency to grain coarsening during the preparation, for example by pressing, sintering and

thermomechanical treatment, can by the

inventive concept also targeted the grain size can be set differently in the different areas. It has proved advantageous for many applications if the mean grain size of the core measured in the transverse section is greater than the mean grain size of the shell. A coarse grain also means that there are only a few grain boundaries on the surface that can present weak points exposed to the arc. The finer-grained mantle material compared to the core ensures a sufficiently high transport rate of the emission-promoting component to the electrode tip.

Particularly advantageous, the inventive compound electrode concept can be used for a cathode of a short-arc lamp, which in DC is operated. The short arc lamp comprises a piston

Quartz glass, a filling gas containing at least one element of the group Xe and Hg, and a cathode and an anode for forming an arc. The cathode has at least one previously described feature of the composite electrode. Through experiments d, is chosen so that the arc attaches to the large part only at the core. By grinding investigations can be easily determined whether the particles of the

Particle composite melt during use or not, dj is chosen so that the particles do not melt in the particle composite, d, is to be made as small as possible in order to ensure a sufficiently high temperature for thermal activation of the particle composite during use.

Furthermore, the composite electrode according to the invention can be produced in a simple manner by powder metallurgical methods. Preference is given to isostatic pressing used. In a rubber hose, first a tubular die is positioned centrically. In this matrix, the tungsten powder or potassium doped Woiframpulver is filled. Thereafter, the outer region is filled with the powder of the particle composite material. The die is then lifted exactly centric in the axial direction upwards. The tube filled in this way is positioned in the isostatic press and subjected to a pressure of typically 50 to 250 MPa. The green compact is subsequently sintered at typical tungsten temperatures of 2,050 to 2,650 K, depending on the grain size or composition. The sintered compact can be produced in a conventional manner, for example by rolling,

Forging, hammering and / or pulling, to be deformed. From the

Formed material can be produced in a conventional manner by machining electrodes. As an alternative to the Hersteliweg described above, the electrode of the invention can also by plasticizing the core with the shell by means of soldering, diffusion bonding, friction welding, co-extruding

Powder compounds, hot pressing or HIPen be obtained. Hereinafter, preferred embodiments will be described by way of example. To ensure comparability, has been identified as an emission-enhancing

Component only La ₂ 0 ₃ used. However, the inventive concept can be applied analogously to other oxides, carbides and borides.

FIG. 1 shows a schematic sketch of a high-pressure gas discharge lamp according to the invention.

FIG. 2 shows the electrode according to the invention in a sectional view,

FIG. 3 shows the electrode according to the invention in a sectional view.

FIG. 4 shows an electrode according to the invention with a carburized region.

FIG. 5 shows a plan view of an electrode according to the invention with one

in the cone region running groove structure.

FIG. 6 shows schematically the determination of the arc disturbance.

FIG. 7 shows a scanning electron micrograph of the microstructure and the tip region of a W-3.6 Ma% La ₂ O ₃ electrode according to the prior art (solid electrode).

According to Figure 1, the high-pressure gas discharge lamp with a

Discharge vessel (piston) 7 made of quartz glass, for example, designed as a DC short-arc lamp. The electrodes 1 are formed as cathode 1a and as anode 1 b. Between cathode and anode burns the

Arc 8. The cathode has a rod-shaped core region 3 and a tubular jacket region 4, which are metallurgically bonded together. The arc 8 starts in the edge zone region 3 a of the core 3.

According to FIG. 2, the electrode 1, 1a according to the invention has a cone-shaped tip 5. The diameter of the rod-shaped core d, corresponds to the inner diameter of the rohrformig formed shell 4, which has an outer diameter d _a . FIG. 3 shows an electrode according to the invention with a core region 3 and a cladding region which is in an outer 4b and inner region 4a

is articulated. The outer region 4b consists of tungsten or potassium-doped tungsten. The inner region 4a consists of the

Particle composite. The electrode 1, 1a has a plateau-shaped formed peak region, as is typically used in electrodes for highly loaded lamps. It is advantageous if the Plaieaudurchmesser smaller than d, is. FIG. 4 shows the electrode according to the invention with a region which is carburized in the region of the cone-shaped tip. The carburization is by appropriate masking only in the area of

Teigchenverbundwerkstoffs performed. The carburization can be done for example by a carbon-haitiges gas, eg CH ₄ or by applying graphite or other carbon-containing solid substance.

FIG. 5 shows an electrode 1, 1a with its conical tip region

Introduction of grooves 6 is modified such that a larger surface increases the Nachtransportrate the emässionsfördemden component.

FIG. 6 shows the determination of the arc disturbance. The light output is measured over time. Over a certain period of time, in the example given 100 seconds, the percentage change in the luminous efficacy is determined. The mean value of the individual numerical values gives the measure for the

Arc instability. For highly loaded short arc lamps, this value for thoriated tungsten (tungsten 2 Ma% ThO ₂ ) is typically in the range of 0.5 to 0.9%. The electrode tip temperature for thoriated tungsten is about 2,400 K to 2,800 K. Pure tungsten would indeed be a very small

Arc quench (arc disturbance value at about 0.3 to 0.8), however, due to the lack of emission-promoting components justified, high peak temperature in the range of 3,200 to 3,800 K a strong

Evaporation and deformation of the electrode tip, resulting in a short

Life of the electrode results. For rare earth oxide electrodes (eg W - 2, 5 Ma% La ₂ O ₃ - 0.07 Ma% Zr0 ₂ ), the cathode tip temperature is about 200 to 300 ° C lower than for thoriated tungsten, but the arc disturbance value is at up to 4.5%.

In a cathode according to the invention, the temperature can also be effectively reduced. For cathodes with a jacket of W - 2.5 Ma%

La ₂ 03-0.07 Ma% ZrO ₂ the cathode tip temperature is at d, = d _a / 3 about 400 ° C, at d, = d _a / 10 by about 50 ° C higher than in cathodes, the complete (solid electrode) of W - 2.5 Ma% La ₂ 0 ₃ - 0.07 Ma% Zr0 ₂ were made. The arc disturbance value of the cathode according to the invention corresponds to the value of thoriated tungsten or is below this value. The best

Arc disturbance values could be measured at d, = d _a / 3. The height

However, electrode temperature already leads to a reduction in the service life of the lamp in heavily loaded lamps by increased evaporation and poor dimensional stability of the tip area. FIG. 7 shows a scanning electron micrograph of the microstructure and the tip region of a W-3.6 Ma% La ₂ O ₃ electrode (prior art solid electrode) after use in a Hg-Xe

Short arc lamp. The area marked 3 in FIG. 7 is strongly depleted of La 2 O ₃ . This is followed by an area 2 in which the La ₂ O ₃ particles are heavily coarsened. In area 1, the originally ellipsoid-shaped La20 ₃ particles are spherically formed, which suggests activation phenomena. This is followed by an area (not shown) where the particles are

Ellipsoid-shaped present. The surface also reflects these different areas. The depleted area is strongly faceted (denoted by 4 in FIG. 7). This is followed by an area (denoted by 5 in FIG. 7) where superficial lanthanum can be detected.

The optimum diameter d, can now be determined in a simple manner by Schiäffauswertungen. The areas where

Depletion / coarsening and Particle Injection occurs evaluated. The optimum d value is chosen so that no melting of the particles (no particle coarsening and particle depletion) occurs outside of the area delimited by the smallest possible di. It is advantageous if the region adjacent to the core has spherically shaped particles in order to ensure sufficient thermal activation during use.

In Table 1 this is shown for W - 2.5 Ma% La ₂ O ₃ - 0.07 Ma% Zr0 ₂ . Table 1

Watt in KW 2.5 2.0 2.5 1.5 1.8

Fill gas Hg-Xe Xe Hg-Xe Xe Xe

Electrode diameter d _a [mm] 6 10 12 6 6

Angle of the cone-shaped

40 40 40 40 40 electrode tip [°]

Extension of the impoverished zone in

axial direction 1.5 0.75 1.6 1.5 1.5

[in mm from the electrode tip]

Extension of the zone with molded-in

Particles in the axial direction 4 2,5 2, e 3,2 3

[in mm from the top of the Elektorode]

egg, min. to avoid 1.1 0.5 1.2 1.1 1.1 melting effects [mm]

dj max. for sufficient activation

and diffusion of the emission-promoting 2.9 1.8 2.0 2.3 2.2 component to ensure [mm]

Preferred inner diameter d ₍ [mm] 2.0 1.0 1.5 1.5 1.5 d _a di 3 10 8 4 4

Claims

claims

1. electrode (1) of a high-pressure gas discharge lamp (2) having a

Core (3) with at least partially a diameter d, and an adjoining shell (4) with at least partially an outer diameter d _a ,

characterized,

in that the core (3) consists at least partially of tungsten or tungsten doped with potassium and the cladding (4) at least partially consists of a particle composite with a matrix of tungsten, where dj and d _a satisfy the following condition:

2. Electrode according to claim, characterized in that d * the

meets the following condition:

0.1 mm <d, <6 mm.

3. An electrode according to claim 1 or 2, characterized in that the core (3) is rod-shaped.

4. Electrode according to one of the preceding claims, characterized

characterized in that the jacket (4) is tubular.

5. Electrode according to one of the preceding claims, characterized

in that the particle composite material has a

Particle content of 0.1 to 10 Ma%.

6. Electrode according to one of the preceding claims, characterized

in that the particle composite contains oxide particles.

7. An electrode according to claim 6, characterized in that the oxide

contains at least one metal of the group consisting of lanthanides, Y, Sc, Ti, Zr and Hf.

8. Electrode according to one of the preceding claims, characterized

in that the particle composite contains carbide particles.

9. An electrode according to claim 8, characterized in that the carbide contains at least one metal of the group consisting of W, lanthanides, Y, Sc, Ti, Zr and Hf

10. Electrode according to one of claims 6 to 9, characterized in that the composite material contains an oxide of the lanthanides and at least one compound of the group consisting of tungsten carbide, zirconium oxide and hafnium oxide. , Electrode according to one of the preceding claims, characterized

characterized in that the Teitchenverbundwerkstoff contains Boridteilchen.

12. Electrode according to one of the preceding claims, characterized

characterized in that d _a satisfies the following relationship:

0.5 mm <d _a <30 mm.

13. Electrode according to one of the preceding claims, characterized

in that the tubular sheath (4) has an inner region (4a) of the particle composite material adjoining the core (3) and an outer region (4b) of tungsten or potassium doped tungsten.

14. Electrode according to one of the preceding claims, characterized

characterized in that the potassium-doped tungsten has a potassium content of 5 to 150 pg / g.

15. Electrode according to one of the preceding claims, characterized

characterized in that the electrode has a cone-shaped tip region (5) at one end.

16. An electrode according to claim 15, characterized in that the

cone-shaped tip region (5) at least partially has a carburized zone (9).

17. An electrode according to claim 16, characterized in that the

cone-shaped tip region (5) is at least partially carburized only in the region of the jacket (4).

18. Electrode according to one of claims 15 to 17, characterized

characterized in that the cone-shaped tip region (5) has grooves (6).

19. An electrode according to claim 18, characterized in that the

Grooves (6) are aligned in the axial direction.

20. Electrode according to one of the preceding claims, characterized

in that, measured in transverse section, the mean grain size of the core (3) is greater than the mean grain size of the shell (4).

21. Electrode according to one of the preceding claims, characterized

characterized in that the core (3) and the jacket (4) are metallurgically connected.

22. Electrode according to one of the preceding claims, characterized

in that it comprises a cathode (1a) of a

Short arc lamp (2a) is.

23. A short arc lamp (2a) comprising a quartz glass bulb (7), a cathode (1a) and an electrode (1) functioning as an anode (1b) for forming an arc (8), and a filler gas contains at least one element of the group consisting of Xe and Hg, characterized

in that the cathode (1a) is designed according to one of claims 1 to 22 and dj is chosen such that the arc (8) attaches to the end region (3a) of the core (3).