US2431887A

US2431887A - Electric device and method for heating materials

Info

Publication number: US2431887A
Application number: US380728A
Authority: US
Inventors: Penning Frans Michel
Original assignee: Individual
Current assignee: Individual
Priority date: 1940-01-16
Filing date: 1941-02-26
Publication date: 1947-12-02
Anticipated expiration: 1964-12-02
Also published as: GB544326A; GB614861A; FR870717A

Description

Dec. 2, 1947. F. M. PENNING 2,431,837

ELECTRIC m-zvxcs AND METHOD FOR HEATING MATERIALS Filed Feb. 26, 1941 2 Sheets-Sheet 1 j'zvnewral? 17/11.

1, 1947- F. M. PENNING 2,431,887

ELECTRIC DEVICE AND METHOD FOR HEATING MATERIALS Filed Feb 26, 1941 2 Sheets-Sheet 2 4f fzvrzm rol? 1-7/14. Pea lad/ y a closed glas's space. may have a low pressure.

Patented Dec- 2, 1947 uNrrEo STATES PATENT, omcs ELECTRIC DEVICE AND METHOD FOR HEATING MATERIALS Frans Michel Penning, .Elndhoven, Netherlands: vested in the Attorney General of the United States Application February 26, 1941, Serial No. 380,728 In the Netherlands January 16, 1940 Claims. .(Cl. 219-35) Electric furnaces which materials are heated to high temperatures in a-low pressure gaseous atmosphere by high frequency currents are known. However, such furnaces are rather ex-,-

pensive and as a consequence thereof there exists the need of .aless expensive electric furnace with which such heating operatlonsc'an be effected.

According, to the invention, for heating materials above 750'C. the source of heat is furnished by a glow-discharge which takes place in a. magnetic field which greatly lengthens the electron paths. Thismethod of treatingprmits temperatures of the order of 2000" C. to be obtained with equipment which is relatively inexpensive.

The magnetic field may be provided either .by-

an electromagnet or by a permanent vmagnet I while theheating operation may take place in- I The gas in this space It is already known to effect heating operations by meansof a glow discharge. For example, it has, previously been proposedto .de-gas electrodes by vantage that theglow-discharge can be more means of a glow-discharge; However when heating to high temperatures is attempted the glow discharge too readily passes into an arc discharge and the heating becomes concentrated at a single point on the surface of the body to be heated.

Furthermore,- the heating energy greatly decreases since the arc discharge hasa 'much lower I operating voltage than a glow-discharge.

If now the paths along which the electrons travel from the cathode to the anode are greatly lengthened by a magnetic field, this will be found to hav a stabilizing eifect on the glow-discharge and heating of the cathode for longer periods and to higher temperatures is possible. A further advantage is that with the same supply of energy as otherwise would be required for the glow- -discharge lower gas-pressures and lower-voltages may be utilized. It is obvious that the strength of the magnetic field must be efllciently chosen in'connection with the voltage existing between the electrodes and with the pressure of th gas.

It is a further requirement that the radius of.

the arc according to' which the electrons are deflected under the influence of the magnetic field should be of the same order of magnitude as or smaller than the length of the free path between the gas molecules.

.The above-mentioned method may be carried out in different ways. Thus, the heating device may be constructed as a furnace and the body to be heated may be given the shape of a vessel in the hollow interior of which are provided the materials to be heated, melted or vaporised. It

in a space of reduced gas-pressure. .ment should preferably be such that the gasis also possible to utilize the body to be heated as'aradiator of light and to use the device as a lamp. Finally .the method may also be utilised for heating electrodes for a long time to a high temperature in order to remove occluded gases to a satisfactory extent.

In the above-mentioned cases the body to be heated-or the furnace may be arranged, if desired,

The arrangepressure in this space may be varied and may be adjusted duringlth glow-discharge to the most favourable value. This affords the further adeasily ignited at a higher pressure whilst, once ignited, it may be continued at a low pressure. Furthermore, with such an arrangement, the glow discharge may be conveniently extinguished by pumping oil the gas and the heated object kept at a high temperature by the vacuum. For the purpose of regulating the pressure it is often efllcacious to cause the gas continually to flow-in on the one side of the tube and to pump on the other side. In this way a continual renewal of gas takes place, which may be desired in those cases wherein during the heating many contamihating gases are set free.

According to one form of realisation of the method according to the invention the glowdischarge is caused to take place in helium of low pressure for as compared with most other gases the cathode volatilisation in helium is very slight.

.The invention will be explained more fully with reference to the accompanying drawings in which F'ig.- 1 represents diagrammatically a concentrically arranged cathode and anode and the Fig. 5 represents a cylindrical cathode, an

anode arranged outside thereof and a magnetic field which is perpendicular to the axis of the cathode.

Fig. 6 shows a cathode formed as a vessel and surrounded by a. cylindrical anode.

Fig. 7 represents a furnace according to the invention.

Fig. 8 is a different form of construction of the cathode in the furnace according to Fig. '7.

Fig. 9 represents a discharge tube having arranged in it an auxiliary electrode by which the removal of occluded gases from the anode is rendered possible.

Fig. 1 diagrammatically represents a cylindrical cathode l and a cylindrical anode II which are coaxially arranged in a gaseous atmosphere of low pressure. If a voltage is applied to these electrodes the electrons proceeding from the cathod travel in vacuo according to the radially extending lines. In the presence of gas they collide with the gas atoms; with a low-gas pressure the path of an electron may assume, for example, due to a collision at A, the shape indicated by a dotted line.

I If the anode voltage is high enough ionisation may occur upon these collisions; the positive ions formed are drawn to the cathode and may set free therefrom not only the electrons required for their neutralisation but also further electrons. Assuming that an electron starting from the oathode forms on an average n ions on its way to the anode, an independent discharge will occur when these n ions set free together one additional electron from the cathode. During this discharge there occurs in the cathode a luminous phenomenon from which this discharge derives the name of glow-discharge. This luminous phenomenon is due to the lighting-up of the atoms hit by electrons. Owing to the frequent collisions of ions against the cathode the latter is heated more and more. When the energy supplied to the discharge increases, the possibility of the discharge passing from a glow-discharge into an arc discharge steadily increases. During the arc discharge the heating-will be concentrated rather at a single point whilst the energy greatly decreases since the arc discharge has a much lower operating Voltage than the glow discharge. The cathode is therefore heated to a much lower temperature.

In Fig. 2 the course of the electron path in vacuo is indicated by a curve B with the same cathode l0 and anode H as are shown in Fig. l. The difierence is, however, that this cathode and this anode are arranged in a magnetic field whose lines of force are perpendicular to the plane of the drawing. Due to this magnetic field the electrons undergo a strong deflection from the rectilinear path which they would attempt to follow under ordinary conditions. They are defiected according to a curve B whose radius of curvature depends at any point on the voltage between the electrodes and on the strength of the magnetic field. If the space is filled with gas of a very low pressure, these factors may advantageously be so chosen that this radius of curva- -ture becomes of the same order of magnitude or small with respect to the length of the free paths between the gas atoms. Owing to this, the electron, for example after having left the cathode,

' will take through the magnetic field a path towards the anode as is diagrammatically shown in Fig. 2 by a dotted line. It may be seen how the electron successively collides in its path with gas atoms C, D and E. Since in these collisions the electron loses energy it can no longer reach the cathode whilst, on the other hand, it is prevented by the magnetic field fromattaining the anode. In this way it may take a very long period of time before the electron has lost suflicient energy to reach the anode. This path is considerably longer than that in Fig. 1 so that also con.

rounded by a coil 4.

. 4 siderably more collisions with the gas atoms take place and consequently with lower voltages and a lower gas-pressure the same heating of the oathode may be attained and, besides, it is achieved that the glow discharge is stable and has little tendency to pass into the arc discharge.

Fig. 3 shows in lateral elevation the arrangement of Fig. 2. Moreover, the magnetic lines of force are indicated by arrowed lines I! which extend parallel to the cathode. When an electron acquires a speed component parallel to the magnetic field this component is not influenced by the magnetic field. So long as the electron makes no collisions it consequently moves in a spiral path around a magnetic line of force and thus it may leave the discharge space. In order to prevent this Flg. 4 shows a shape of cathode ll which is provided with flanges 15 and is arranged coaxially with respect to an anode ii. The ma netic lines of force are parallel to the axis 0! the cathode and cut the operative surface at the point I! where they enter into the discharge space and at the point l8 where they leave said space. Owing to the latter fact the electrons which should have the tendency indicated in Fig. 3 will, when approaching the cathode wall, travel again in the opposite direction while moving around the lines of the magnetic held and travel in a zig-zag line between the flanges of the cathode to the anode. It is clear that the paths of these electrons are considerably lengthened and that consequently the possibility of collisions with atoms is appreciably increased.

Fig. 5 represents a further form of construction wherein an anode 20 is arranged outside a .cylindrical cathode 2|. The lines of force 22 of the magnetic field are perpendicular to the axis of the cathode. If desired, the cathode may also be given the shape of two plates arranged opposite one another, the magnetic field being perpendicular to these plates.

Fig. 6 represents a cathode 25 which essentially is formed with flanges as is the cathode M of Fig. 4 but, in addition, it has a central cavity 26 in which may be introduced a material which is heated by heat delivered by the cathode. Fig. 7 represents a heating furnace equipped with a cathode which oifers these possibilities. The discharge tube consists of a wall 30 of Pyrex having sealed into it a stem 31 on which is mounted a cathode 32 provided with a cavity. The cathode is surrounded by an annular anode 33 which is led out of the tube by a conductor 34 and is maintained in its position by supporting wires 35. The stem 3| and the conducting wire 35 of the oathode are further surrounded by a tube 31 of quartz which safeguards the stem against arcing. The material to be heated 38 is introduced through an aperture 39 into the cathode 32 whereupon the interior space of the tube 30 is filled with gas of low pressure. The tube 30 is placed in a cooling vessel 4i provided with a supply conduit 42 and a discharge conduit 43 and is further sur- In connection with the gas-pressure prevailing in the tube and with the voltage to be applied between leading-in wires 34 and 40 the magnetic field set up by the said coil may be so chosen that the paths of the electrons are considerably lengthened. Due to the glowdischarge produced the cathode is very strongly heated and temperatures above 1000 C., if desired even temperatures of about 2000 C., may be attained, this heat being delivered to the material 38 which iiquefies or vaporises. In order to ensure that the cathode can sustain these temperatures it is made, for example, of graphite. It is efiicaceous to utilise in this heating a filling of helium, preferably at a pressure of 5 mms. at the most since in helium volatilisation of the operative cathode wall occurs to a less extent. The arrangement of Fig. '7 is utilised, for example, for applying the material vaporised in the cathode 32 by vaporisation to a body 45 which is also mounted in the tube by means of a supporting wire 46. It is obvious that the tube must be of other construction if it is intended to eilect fusions of metals within the cathode 32 for in such cases it is necessary that the molten ma-- terial can be easily removed from the cathode 32 or that the whole of this cathode is detachably mounted on the supporting rod 36 and may be removed through the aperture 39 from the tube 30. It is also possible to modify the pressure of the gas within the tube 30. For example, at the outset a higher pressure may be taken in order to effectuate a more easy ignition of the glowdischarge. When this discharge has once been ignited the pressure may be reduced, for example, by a vacuum pump 49 connected to the tube 30 at the aperture portion 39. It is also possible to provide the tube 30 in addition to the outlet aperture 39, with an inlet aperture 48, by which renewal of the gas may be realised by the continual supply and discharge of gas.

Fig. 8 represents a cathode 50 which constitutes a variant of the cathode 32.

Fig. 9 represents a discharge tube comprising an incandescent cathode 5| and two plate-shaped anodes 52. When it is desired to de-gas these anodes 52 in the manner indicated above, this may be effected by means of a magnetic field 54 which is directed perpendicularly to the plates 52. By temporarily connecting the cathode 5| as an anode and the plates 52 as a cathode, the latter plates will be strongly heated, during which heating degassing may take place.

What I claim is:

1. An apparatus for electrically heating materials to temperatures exceeding 750 0., comprising a discharge tube having an envelope containing an anode electrode and a heat generating cathode electrode spaced from said anode to form a discharge space and a gaseous medium within said envelope, means to increase the length of the path of electrons between said anode and cathode electrodes and to thereby increase the heating of the cathode electrode comprising a source of a magnetic field having lines of force extending through said discharge space and intersecting the paths of the electrons of the discharge, and means to confine the electrons to paths within the discharge space, said latter means comprising a portion of said cathode extending towards the anode electrode into the discharge space and intercepting a portion of the magnetic lines of force.

2. An apparatus for electrically heating materials to temperatures exceeding 750 0., comprising a glow discharge tube having an envelope containing an anode electrode, a heat-generating cathode electrode spaced from the anode to form a discharge space and a gaseous medium within the envelope, said cathode having an operative surface exposed to the discharge, and means to increase the length of the path of electrons between the anode and cathode electrodes and to thereby increase the heating of the cathode elec- .'trode comprising a source of a magnetic field having lines of force extending through the discharge space with at least a portion of the lines 6 substantially parallel to the operative surface of said cathode.

3. An apparatus for electrically heating materials to temperatures exceeding 750 0., comprising a glowdischarge tube having an envelope provided with an opening and containing an anode, a heat generating cathode spaced from said anode to form a discharge space and a gaseous medium within the envelope, means to increase the length of the path of electrons between the anode and cathode and to thereby increase the heating of the cathode comprising a source of a magnetic field having lines of force extending through said discharge space and intersecting the paths of the electrons of the discharge, and means connected to said opening to regulate the pressure of the gaseous medium.

4. An apparatus for electrically heating materials to temperatures exceeding 750 C., comprising a glow-discharge tube having an envelope provided with inlet and outlet openings and containing an anode, a heat generating cathode spaced from said anode to form a discharge space and a gaseous medium within the envelope, means to increase the length of the path of electrons between the anode and cathode and to thereby increase the heating of the cathode comprising a source of a magnetic field having lines of force extending through said discharge space and intesecting the paths of the electrons of the discharge, and means connected with said inlet and outlet openings to circulate the gaseous medium through said envelope.

5. An apparatus for electrically heating materials to temperatures exceeding 750 0., com-- prising a glow discharge tube having an envelope containing an anode, a heat generating cathode spaced from the anode to form a discharge space and a gaseous medium within the envelope, and means to increase the length of the path of electrons between the anode and cathode and to thereby increase the heating of the cathode comprising a source of a magnetic field having lines of force extending through the discharge space and intersecting the paths of the electrons of the discharge, said gaseous medium consisting substantially of helium at a pressure less than 5 millimeters of mercury.

6. An apparatus for electrically heating materials, comprising a discharge tube having an envelope containing an anode and a heat generating cathode spaced from the anode to form a discharge space and a gaseous medium within the envelope, said cathode comprising an electron-emitting body provided with a cavity adaptedto receive the material to be heated and with edge portions extending towards the anode into the discharge space, and means to increase the length of the path of electrons between the anode and cathode and to thereby increase the heating at the cathode comprising a source of a magnetic field having lines of force intersecting the paths of the electrons of the discharge and extending through the discharge space and said edge portions.

'7. A device for heating a body to temperatures exceeding 750 C. by ionic bombardment, comprising a glow-discharge tube having an envelope, said body being arranged within said envelope and acting as a cathode during the operation of the device, an anode electrode spaced from the body to form a discharge space and a gaseous filling within the envelope, and means to increase the length of path of electrons between the anode and cathode and to thereby increase the heating of the body comprising a source of a magnetic field having lines of force extending through said discharge space and intersecting the electron paths between the body and said anode.

8. An apparatus for heating an electricallyconducting body to temperatures exceeding 750 0., comprising a glow-discharge tube having an envelope containing the body, an anode electrode spaced from the body to form a discharge space and a gaseous filling, and means to increase the heating of the body comprising a source of magnetic field having lines of force extending through the discharge space and intersecting the electron paths between the body and said anode, the pressure of the gaseous filling and the intensity of the magnetic field having values at which the electron paths in the discharge are greatly lengthened.

9. In apparatus for heating a substance to a high temperature, the combination of, means to maintain a gaseous medium in a discharge zone, means to produce a glow discharge in said zone by causing a flow of electrons, means to support the substance to be heated in the vicinity of the glow discharge, and means to maintain a magnetic field with the lines of flux intersecting the paths of the electrons, whereby the substance is heated by the heat produced by the successive impacting between the electrons and the atoms of gas, and the gaseous medium provides efiective heat insulation for the substance.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,219,612 Berghaus et a1 Oct. 29, 1940 2,217,187 Smith Oct. 8, 1940 2,137,198 Smith Nov. 15, 1938 1,137,964 Goddard May 4, 1915 2,206,020 Berghaus et al July 2, 1940 2,085,450 Rohn June 29, 1937 1,840,130 Rashevsky' Jan. 5, 1932 1,710,747 Smith Apr. 30, 1929 1,584,728 Case May 18, 1926