Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/02—Induction heating
  • H05B6/22—Furnaces without an endless core
  • H05B6/24—Crucible furnaces
  • H05B6/26—Crucible furnaces using vacuum or particular gas atmosphere
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/02—Induction heating
  • H05B6/36—Coil arrangements

Definitions

• 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.
• 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.
• 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.
• Plasma formation may occur between points where a potential difference is present, therewith creating an electrical field.
• 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.
• 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.
• 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.
• 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.
• 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.
• polyimide insulation materials are capable to withstand the highest temperatures.
• insulation materials are indicated that comprise polyimide or that consists of polyimide.
• these respective insulation materials consist partially or completely of the specific material.
• the induction heating components can be positioned either inside or outside 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.
• polyimide insulation material 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:
• 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.
• the insulation material may wear down and may finally crack.
• plasma's 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.
• ICP inductively coupled plasma's
• CCP capacitively coupled plasma's
• 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.
• a thermal insulation for an induction heating component.
• the thermal insulation is provided at least between a component and the object that is to be heated by induction heating.
• the thermal insulation is preferably applied as a second layer around the insulation material.
• 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.
• 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.
• 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:
• the magnesium oxide based ceramic material is preferably applied as a paste to the carrier material, which allows for an easy application.
• a trapping system for free charge carriers for an induction heating component.
• a trapping system could be a system comprising a conductive element following at least part of the turn or turns of the induction coil.
• 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.
• conductive 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.
• 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.
• 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.
• fig.1 shows schematically the fields induced by an induction coil fed from an
• fig.2 shows a diagram representing the breakdown voltage for a non-coated induction coil and a coated induction coil in various gas environments
• fig.3A,B,C shows schematically a trapping system for free charge carriers around an induction coil.
• 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 ⁇ -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 ⁇ -field.
• CCP capacitively coupled plasma's
• ICP inductively coupled plasma's
• 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.
• 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.
• 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.
• 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.
• the polyimide layer was covered with an additional layer consisting of glass wool impregnated with MgO-paste.
• 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.
• ICP inductively coupled plasma
• the trapping system for free charge carriers comprises a number of non- conductive elements 8, fabricated from materials such as concrete, BN, A1 2 0 3 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.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• General Induction Heating (AREA)
• Plasma Technology (AREA)

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• 2014
  • 2014-02-27 WO PCT/EP2014/000514 patent/WO2014131519A1/en active Application Filing
  • 2014-02-27 KR KR1020157022069A patent/KR102192273B1/ko active IP Right Grant
  • 2014-02-27 CN CN201480010530.2A patent/CN105027669B/zh not_active Expired - Fee Related
  • 2014-02-27 EP EP14707652.5A patent/EP2962528A1/de active Pending

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