WO2014131519A1

WO2014131519A1 - Protective system for use in induction heating

Info

Publication number: WO2014131519A1
Application number: PCT/EP2014/000514
Authority: WO
Inventors: Colin COMMANDEUR; Edzo ZOESTBERGEN; Theodorus Franciscus Jozef Maalman
Original assignee: Tata Steel Nederland Technology Bv
Priority date: 2013-02-27
Filing date: 2014-02-27
Publication date: 2014-09-04
Also published as: EP2962528A1; CN105027669B; CN105027669A; KR20150123235A; KR102192273B1

Abstract

The invention relates to a protective system for the protection of induction heating components, wherein the induction heating components include an induction coil and connecting elements to connect the induction coil to a power source, the protective system comprising an insulation material chosen from the group of insulation materials consisting of polyimide insulation materials, silicon rubber insulation materials, PTFE insulation materials and PVC insulation material and optionally thermal insulation means.

Description

PROTECTIVE SYSTEM FOR USE IN INDUCTION HEATING

Field of the invention

The invention relates to a protective system for components used in the induction heating of substances, for instance metals, to prevent electrical sparking and arcing as well as to prevent wear due to plasma attack and irradiative heat.

Background of the invention

The induction heating of substances and more in particular the heating of metals, such as the heating and melting of one or more metals or metal alloys in physical vapour deposition (PVD) processes, may give rise to undesirable sparking and arcing between equipment components with different potentials. The occurrence of sparking and arcing is dependent on the voltages and currents used in the induction heating, the geometry of the process parts, the process gas pressure and composition, and the partial gas pressures and compositions related to volatile system components. With sparking a disruptive discharge of electricity is meant that takes place between two places having a large potential difference. The spark is preceded by ionization of the path. With arcing a luminous electrical gas discharge is meant with high current density and low potential gradient. The ionization necessary to maintain the large current is provided mainly by the evaporation of some of the material of the equipment components between which the arcing takes place. With PVD both sparking and arcing may damage the equipment and will result in process stops and are thus disadvantageous for productivity.

Another factor that may give rise to damage to equipment components is the generation of plasma's. Plasma formation may occur between points where a potential difference is present, therewith creating an electrical field. The higher the field strength, the conductivity of the gas and the sensitivity of the gas to ionisation, the easier it is to form a plasma. The conductivity of the gas and its sensitivity to ionisation depend on gas composition, pressure of the gas and the distance between which the potential difference is present. A plasma may wear down protective coatings applied to prevent sparking or arcing therewith increasing the risk for their occurrence.

Finally, the temperatures realized with induction heating, for instance the temperature of a molten metal in a crucible, can give rise to damage caused by irradiation. Such damage could for instance be damage to a protective coating applied to prevent sparking or arcing.

One approach to minimise the risk for sparking of the induction coil is to position the crucible and induction coil outside the vacuum chamber, under atmospheric conditions. In this situation sparking will only occur at extremely high potential difference levels of many kilovolts, see also F. Paschen, 'Ueber die zum

Funkeniibergang in Luft, Wasserstoff und Kohlensaure bei verschiedenen Drucken erforderliche Potentialdifferenz, Annalen der Physik 273 (5): 69-75'. However, placing the induction coil with the crucible outside the vacuum chamber complicates the process considerably by introducing the requirement of vacuum seals that can cope with high temperature differences of the crucible compartment and the rest of the vacuum chamber. The resulting setup will be more capital-intensive than with the improved coil system.

Objectives of the invention

It is an objective of the present invention to provide a protective system for components used in the induction heating of substances to prevent sparking.

It is another objective of the present invention to provide a protective system for components used in the induction heating of substances that prevents arcing.

It is another objective of the present invention to provide a protective system for components used in the induction heating of substances to prevent or minimise the formation of plasma's.

It is another objective of the present invention to provide a protective system for components used in the induction heating of substances that prevents damage caused by irradiative heat.

It is another objective of the present invention to provide a protective system for components used in the induction heating of substances that can be applied easily.

It is still another objective of the present invention to provide a low cost protective system for components used in the induction heating of substances.

Description of the invention

In a series of tests it was determined that the breakdown voltage at which sparking occurs is dependent on the pressure in the chamber used for PVD, the set-up of the induction heating system, the shape and size and the material type of the heating components and type (AC/DC) of the electrical field. It was further determined in experiments with the physical vapour deposition of zinc to a substrate that the breakdown voltage at which sparking occurs in a chamber because of a small pressure rise in the vacuum chamber during the evaporation process. Nevertheless, in order to generate a large enough continuous supply of a metal vapour or metals vapour the power needs to be at a level at which without any protective measures sparking and arcing could easily happen.

Sparking could draw a large amount of energy from the power supply even such that the overloading system of the power supply would be tripped shutting down the power supply all together. The risk of sparking is enhanced by the emission of thermionic electrons from a heated component surface, which results in an increase of the conductivity of the gas at this spot.

According to a first aspect of the invention one or more of the objectives of the invention are realized by providing a protective system for the protection of induction heating components, wherein the induction heating components include an induction coil and connecting elements to connect the induction coil to a power source, the protective system comprising an insulation material applied to a component, wherein the insulation material is chosen from the group of insulation materials consisting of polyimide insulation materials, silicon rubber insulation materials, polytetrafluorethyleen (PTFE) insulation materials and polyvinyl chloride (PVC) insulation materials.

From these insulation materials the polyimide insulation materials are capable to withstand the highest temperatures. With the term polyimide insulation materials insulation materials are indicated that comprise polyimide or that consists of polyimide. For the other mentioned insulation materials the same indication applies in that these respective insulation materials consist partially or completely of the specific material.

In induction heating systems using a vacuum chamber, such as used with the PVD system under consideration, the induction heating components can be positioned either inside or outside the vacuum chamber. With the induction heating components inside the vacuum chamber feedthrough means are provided to connect the induction heating components inside the vacuum chamber through the vacuum chamber wall to a power supply. These feedthrough means are part of the connecting elements. By using one of the insulation materials as an insulation material to the surface of a component the occurrence of sparking and arcing is prevented to a large extent.

In tests it has appeared that by using polyimide insulation material very good results are achieved. The properties of polyimide insulation are such that it provides a:

- low electrical conductivity that prevents sparking/arcing,

- high thermal conductivity as compared to the insulating layer on top,

- good thermal contact with the induction heating component,

- flexibility which prevents cracking or fatigue of the insulation material, and

- high breakdown voltage or dielectric strength.

According to a further aspect of the invention the polyimide is a thermosetting polyimide. The polyimide insulation can be applied as an insulation tape but preferably the polyimide is applied as a thermosetting coating. In this manner a polyimide insulation coating is obtained that closely fits to the induction heating component without leaving any free spaces and which has good thermal contact with the induction heating component.

A coil coated with the polyimide insulation coating has proved to be capable to withstand an applied AC potential difference of 1000 V RMS with currents of up to 6kA in an air pressure range of 0.001 Pa - 2kPa, and with a spacing of the connectors of 8 mm. That is more than double the potential difference that could be achieved without insulation material on the surface of the coil.

Because of the exposure of the insulation material to irradiative heat coming from the melting and evaporation of the metal or metals used in the PVD process the insulation material may wear down and may finally crack. Also because of the high potential differences plasma's will be generated which may either be inductively coupled plasma's (ICP), caused by the azimuthal electrical field of the induction coil, or capacitively coupled plasma's (CCP), caused by an axial electrical field of the induction coil. The plasma's and more in particular the ICP will eventually also give rise to degradation of the insulation material, resulting in sparking/arcing again.

To prevent wear by irradiative heat and plasma attack a thermal insulation is provided for an induction heating component. Preferably the thermal insulation is provided at least between a component and the object that is to be heated by induction heating. However, the thermal insulation is preferably applied as a second layer around the insulation material. According to a further aspect of the invention the thermal insulation comprises a heat resistant material and a carrier for the heat resistant material. The carrier for the heat resistant material is not very critical as long as it at least able to withstand elevated temperatures. Although the induction coil is made of a hollow tube through which a cooling liquid is circulated the temperature at the outside may increase to well above the temperature of the cooling liquid. The carrier may comprise a mineral wool, wherein the mineral wool could be glass wool or stone wool.

According to a further aspect of the invention the heat resistant material is a ceramic material applied to the carrier. The ceramic material is for instance a magnesium oxide based ceramic material. The characteristics of such a magnesium oxide based ceramic material include:

- high temperature capability, and

- low electrical conductivity.

The magnesium oxide based ceramic material is preferably applied as a paste to the carrier material, which allows for an easy application.

It has appeared that even with a layered protective system with a first layer of insulation material and a second layer providing a thermal insulation still inductively coupled plasma's could be generated when high voltages are used.

According to a still further aspect of the invention a trapping system for free charge carriers is provided for an induction heating component. Such a trapping system could be a system comprising a conductive element following at least part of the turn or turns of the induction coil.

According to a further aspect of the invention the free charge carrier trapping system comprises one or more conductive elements positioned in radial direction of the induction coil and parallel to the axis of the induction coil. With such elements, for instance flat plates, it turned out that the voltage and current applied to the induction coil could be increased considerably before an inductively coupled plasma occurred again.

By grounding the free charge carrier trapping system the effect of the trapping system is further improved.

As an alternative to this carrier trapping system non-conductive elements may be used that physically occupy the free space directly around the induction coil and as such trap any free charge carriers moving along the circumference of the coil to prevent the ignition of a plasma.

According to a further aspect of the invention the induction heating components are used in a chamber with a reduced pressure in the range of O.OOlPa - 2.5kPa and wherein a current of maximum 6kA at a voltage of maximum IkV and at a frequency of maximum 20kHz is supplied to the induction coil.

The protective system as described above can be used effectively in induction coil systems that operate on AC as well as DC. Brief description of the drawings

The invention will be further explained by means of the example shown in the drawing, in which:

fig.1 shows schematically the fields induced by an induction coil fed from an

AC power supply,

fig.2 shows a diagram representing the breakdown voltage for a non-coated induction coil and a coated induction coil in various gas environments, and

fig.3A,B,C shows schematically a trapping system for free charge carriers around an induction coil.

Detailed description of the drawings

In fig.1 an axial cross section of an induction coil 1 is shown at the left side and a radial cross section of the induction coil is shown at the right side. The induction coil 1 has a limited number of turns 2 with in this example a potential difference of about 600V between the last and first turn of the coil 1.

The field lines 3 of the magnetic B-field induced by the current through the induction coil have schematically been indicated in the drawing. Further there is an axial electrical field Ez and an azimuthal electrical Εθ-field with the respective field lines 4 and 5. The capacitively coupled plasma's (CCP) are associated with the axial electrical Ez-field and the inductively coupled plasma's (ICP) are associated with the azimuthal Εθ-field. At the right side the direction of the current 6 in the induction coil is indicated with an interrupted line and arrows. The induction coil 1 and the connecting parts thereof are made from copper because of the good conductivity of the material. The copper is in the form of a hollow tube which allows for an adequate cooling of the coil by means of a cooling liquid pumped through the coil.

Fig.2 shows a diagram representing the minimum breakdown voltage for a non- coated induction coil and a coated induction coil in various gas environments with the voltage plotted on the vertical axis and the different coils and gas environments along the horizontal axis. The minimum was determined in the gas pressure range from ~10 mBar to 10^"4 mbar. Typical PVD processes may take place somewhere in this regime.

With the non-coated induction coils, which in the present case is a bare copper induction coil, the spark free regime varies from less than 200V for a coil used in an Argon gas environment to about 400V for air. Above these spark free regimes there is a small zone in which sparking may occur and above this small zone sparking will definitely occur. From this plot it can be seen that the breakdown voltage is relatively high in a Zn atmosphere as occurs in a vacuum chamber during a Zn PVD process.

For a coated coil, that is a coil provided with a layer of insulation material but without a thermal insulation, the breakdown voltages in the same gas environments are very much higher in comparison with those for a bare copper induction coil.

The use of various insulation materials or combinations of materials was investigated and it appeared that Silicon-rubber in combination with a PVC based tape or a ceramic cover gives good results. However, the use of polyimide tape gave the best performance concerning break down voltage and time. The same or even better results are obtained with the use of a polyimide applied as a coating which will seamlessly enclose the induction coil. It was established that with a polyimide insulation coating with a thickness of 65 μιη the breakdown voltage is in the order of 6kV.

Since heat irradiates from the object heated by means of induction preferably a thermal insulation is provided on the coating. The thermal insulation comprises a heat resistant material and a carrier for the heat resistant material. This thermal insulation also provides protection against plasma attack to a certain extent.

For thermal insulation the polyimide layer was covered with an additional layer consisting of glass wool impregnated with MgO-paste. During a Zn PVD test this protective system successfully withstood a crucible temperature of 750-770 °C under vacuum pressures at least up to 10 Pa, operating the coil at AC voltages in the range 550-600V RMS, for several hours of time. The coil showed no signs of damage.

Fig.3A,B show respectively a perspective view and a top view of an induction coil provided with a protective system, wherein the induction coil 1 is provided with an insulation material and a trapping system for free charge carriers to further suppress plasma attack. The insulation of the coil comprises a polyimide coating and a thermal insulation applied over the polyimide coating. The trapping system comprises two grounded metal plates 7 or as shown in Fig.3B three grounded metal plates 7 distributed around the induction coil and parallel to the central axis of the coil.

Trials were run at about 18 kHz with an empty crucible in the coil, in which the induction voltage was increased in steps. Without plates 7 and using nitrogen gas breakdown occurred at 740 V RMS, coil current being 2150A. Adding one grounded plate 7 to the setup this voltage could be increased up to 820V RMS, coil current 2450A. before an inductively coupled plasma (ICP) occurred. After adding a third grounded plate the setup remained ICP-free up to a voltage level of more than 890V RMS, with a coil current of over 2580A.

In fig. 3C the trapping system for free charge carriers comprises a number of non- conductive elements 8, fabricated from materials such as concrete, BN, A1₂0₃ or other insulating materials, positioned around the circumference of the induction coil 1. With these elements 8 placed adjacent to the coil, movement of free charge carriers in the field around the coil is prevented and therewith also the sustaining of a plasma.

Claims

1. Protective system for the induction heating components, wherein the induction heating components include an induction coil and connecting elements to connect the induction coil to a power source, the protective system comprising an insulation material applied to a component, characterized in that the insulation material is chosen from the group of insulation materials consisting of polyimide insulation materials, silicon rubber insulation materials, PTFE insulation materials and PVC insulation materials.

2. Protective system according to claim 1, wherein the insulation material is a polyimide insulation material.

3. Protective system according to claim 2, wherein the polyimide insulation material is applied as a thermosetting coating.

4. Protective system according to one or more of claims 1-3, wherein a thermal insulation is provided for a heating component.

5. Protective system according to claim 4, wherein the thermal insulation is provided at least between a component and the object that is to be heated by induction heating.

6. Protective system according to claim 5, wherein the thermal insulation comprises a heat resistant material and a carrier for the heat resistant material.

7. Protective system according to claim 6, wherein the carrier for the heat resistant material comprises a mineral wool.

8. Protective system according to claim 7, wherein the mineral wool is glass wool.

9. Protective system according to one or more of claims 4 - 8, wherein the heat resistant material is a ceramic material.

10. Protective system according to claim 9, wherein the ceramic material is a magnesium oxide based ceramic material.

11. Protective system according to one or more of claims 1 - 10, wherein a trapping system for free charge carriers is provided for a component used in the induction heating.

12. Protective system according to claim 11, wherein the trapping system comprises a conductive element following at least part of the turn of the induction coil.

13. Protective system according to claim 11, wherein the trapping system comprises one or more conductive elements positioned in radial direction of the induction coil and parallel to the axis of the induction coil.

14. Protective system according to claim 11, wherein the trapping system comprises non-conductive elements positioned along the circumference of the induction coil.

15. Protective system according to one or more of claims 1 - 10, wherein the induction heating components are used in a chamber with a reduced pressure in the range of O.OOlPa - 2.5kPa and wherein current of maximum 6kA at a voltage of maximum lkV and at a frequency of maximum 20kHz is supplied to the induction coil.