Induction furnace for melting semi-conductor materials

An induction furnace includes an induction coil, an electrically non-conductive crucible having an inner diameter disposed within the induction coil, and an electrically conductive member disposed below the crucible and having an outer diameter which is further from the induction coil than is the inner diameter of the crucible. Due to the non-conductive nature of material disposed within the crucible at lower temperatures, the induction coil initially inductively heats the conductive member, which transfers heat to the material to melt a portion of the material. Once the material is susceptible to inductive heating (usually upon melting) the susceptible material is inductively heated by the induction coil. During the process, inductive heating of the material greatly increases as inductive heating of the conductive member greatly decreases due to low resistivity of the molten material and due to the molten material being closer to the coil than is the conductive member.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to induction heating and an improved induction furnace. More particularly, the invention relates to an induction furnace for melting materials not susceptible to inductive heating at lower temperatures but which are susceptible to inductive heating at higher temperatures, especially upon melting. Specifically, the invention relates to an induction furnace having an electrically conductive susceptor disk which is inductively heated whereby heat is transferred from the disk to such materials to make them susceptible to inductive heating whereby the materials are then inductively heated to melt them.

2. Background Information

Induction furnaces are well known in the art. However, there are a variety of difficulties related to the inductive heating and melting of materials that are initially non-conductive or which have particle sizes sufficiently small so that they are not susceptible to inductive heating. Many prior art induction furnaces utilize a conductive crucible such that an induction coil couples with the crucible to transfer energy directly to the crucible to heat the crucible. Heat is then transferred from the crucible to the material to be melted via thermal conduction. In certain cases, the induction frequency and the thickness of the crucible wall may be selected so that a portion of the electromagnetic field from the coil allows coupling with any electrically conductive material inside the crucible to inductively heat the material directly. However, the direct inductive heating in such cases is quite limited. Because direct inductive heating of the material to be melted is far more effective than the method described above, a system to effect such direct inductive heating is highly desirable.

In addition, the conductive crucibles of the prior art may react with the material to be melted which causes unwanted impurities in the melt and thus requires the use of a non-reactive liner inside the crucible to prevent formation of such impurities. Typically, such liners are electrically non-conductive and thermally insulating. As a result, the transfer of heat from the crucible to the materials to be melted is greatly impeded, thus substantially increasing melting times. To expedite the transfer of heat from the crucible to the material to be melted, the crucible must be heated to undesirably high temperatures which can decrease the life of the crucible and liner.

In addition, there remains a need for an induction furnace capable of producing a continuous melt in an efficient manner, especially for semi-conductor materials. An efficient continuous melt induction furnace is particularly useful for continuous formation of semi-conductor crystals, which are highly valued in the production of computer chips.

U.S. Pat. No. 6,361,597 to Takase et al. teaches three embodiments of an induction furnace especially intended for melting semi-conductor materials and adapted to supply the molten material to a main crucible for pulling of semi-conductor crystals therefrom. Unlike the prior art discussed above, Takase et al. uses a quartz crucible which is electrically non-conductive along with a susceptor which is in the form of a carbon or graphite cylinder. In each of the three embodiments of Takase et al., the carbon or graphite cylinder susceptor is initially inductively heated by a high frequency coil whereby heat is transferred from the susceptor to raw material inside the crucible in order to begin the melting process. Once the raw material is melted, it is directly inductively heated by the high frequency coil in order to speed up the melting process. While this is a substantial improvement over the previously discussed prior art, the induction furnace of Takase et al. still leaves room for improvement, as discussed below.

The first embodiment of Takase et al. involves the use of a pipe extending upwardly into the quartz crucible whereby the pipe receives molten material from within the crucible by overflow and transmits it to a main crucible from which semi-conductor crystals are pulled. The carbon cylinder susceptor encircles the quartz crucible and is moveable in a vertical direction. Prior to melting the material in the crucible, the carbon cylinder is positioned so it covers the entire side wall of the crucible. Once some of the material is melted, the carbon cylinder is moved upwardly so that the molten material is inductively heated by the coil. Once the raw material is fully melted, additional raw material is added and the carbon cylinder is moved downwardly to cover the upper half of the side wall of the crucible so that the carbon cylinder is inductively heated and transfers heat therefrom to aid in melting the added raw material.

While the first embodiment of Takase et al. permits the susceptor to be substantially removed from the electromagnetic field of the induction coil so that it is not further inductively heated or so that the inductive heat is minimized therein, this process still has some disadvantages. One disadvantage to this configuration is the need to provide a mechanism to move the cylindrical susceptor upwardly and downwardly. Another disadvantage of the configuration is the need for a mechanism to monitor the melt in order to determine the proper time to move the susceptor away from the crucible side wall. Because direct inductive heating of the molten materials is more effective than inductive heating of the susceptor and subsequent transfer of heat from the susceptor to the material, any time that the susceptor is left in place after the molten material is susceptible to inductive heating, it prevents the more efficient direct inductive heating of the melt.

The second embodiment in Takase is similar to the first embodiment except that the pipe for transferring molten material from the quartz crucible to the main crucible does not extend upwardly into the quartz crucible. A mass of the initial raw material is disposed over the opening of the pipe and effectively serves as a stopper until the stopper portion is itself melted. In order to prevent the stopper from being melted too soon, the carbon cylinder initially only covers about two thirds of the upper portion of the side wall of the crucible so that heat transferred from the carbon cylinder is transmitted only to about the upper two thirds of the raw material. As the raw material is melted, the carbon cylinder is moved downward to cover the entire side wall of the crucible. Then the carbon cylinder is moved upwardly to cover the upper half of the side wall of the crucible whereby continued inductive heating of the carbon cylinder allows heat transfer from the carbon cylinder to raw material that is added to the melt. Induction heat is also generated in the melt at this point.

The second embodiment similarly suffers from the need for moving the cylindrical susceptor in a vertical fashion. The process must also be monitored in order to determine when to move the susceptor cylinder downwardly to maintain a reasonably high efficiency. Further, the susceptor interferes with the inductive heating of the molten material when positioned around the crucible while there is still unmelted raw material within the crucible.

In the third embodiment, Takase et al. provides a pipe which extends upwardly into the crucible as in the first embodiment to provide overflow of the molten material to the main crucible. In this embodiment, the susceptor has a crucible-like configuration whereby the susceptor cylindrical portion covers the sidewall of the quartz crucible and the bottom of the susceptor covers the lower surface of the quartz crucible. In this embodiment, the susceptor is not vertically moveable. Instead, the thickness of the susceptor sidewall and the frequency applied by the coil are selected so that the penetration depth of the induction current will extend beyond the susceptor into the quartz crucible so that it can inductively heat material inside. As with the prior embodiments, the susceptor is inductively heated and then transfers heat to the raw material to begin the melting process. Once the melting process has begun, inductive heating of the melt also occurs and the melt continues as a result of both inductive heating directly of the molten material as well as transferred heat from the inductively heated susceptor. In addition, the frequency applied to the coil is preferably initially at a relatively high frequency and then once the melting has begun is shifted to a relatively low frequency to better focus inductive heating of the molten portion of the material.

This third embodiment primarily suffers from the fact that the cylindrical susceptor remains in place and thus prevents inductive heating from being focused more effectively on the raw material within the crucible. Instead, the coil continues to inductively heat the carbon cylinder so that energy which might be applied to the material is absorbed by the carbon cylinder, which transfers heat to the raw material in the crucible in a far less effective manner.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an induction furnace comprising an electrically non-conductive crucible defining a melting cavity; an electrically conductive member disposed adjacent the crucible; an induction member for inductively heating material within the melting cavity; and a portion of the melting cavity being closer to the induction member than is the conductive member.

The present invention also provides an induction furnace for melting material, the furnace comprising an electrically non-conductive crucible defining a melting cavity; an electrically conductive member disposed adjacent the crucible in a fixed relation with respect to the crucible; an induction member for creating an electromagnetic field to inductively heat material within the melting cavity and to inductively heat the conductive member; each of the conductive member and the material within the melting cavity absorbing energy from the electromagnetic field whereby the conductive member and material together absorb a combined energy from the electromagnetic field; the crucible, conductive member and induction member being positioned with respect to each other so that inductive heating via the induction member occurs initially within the conductive member and occurs in the material within the melting cavity when the conductive member has transferred sufficient heat to the material to make the material susceptible to inductive heating so that at a certain time during inductive heating the conductive member absorbs no more than thirty percent of the combined energy absorbed by the conductive member and material.

The present invention further provides an induction furnace for melting material, the furnace comprising an induction member for creating an electromagnetic field; an electrically non-conductive crucible defining a melting cavity containing the material to be melted; the material absorbing over time a varying amount of energy created by the magnetic field; an electrically conductive member disposed adjacent the crucible in a fixed relation with respect to the crucible; the conductive member absorbing over time a varying amount of energy created by the magnetic field; and the crucible, conductive member and induction member being positioned with respect to each other so that during heating and melting of the material the amount of energy from the electromagnetic field absorbed by the conductive member to create inductive heating therein is substantially inversely proportional to the amount of energy from the electromagnetic field absorbed by the material in the melting cavity to create inductive heating therein.

The present invention also provides a method of heating comprising the steps of placing material within a melting cavity of an electrically non-conductive crucible; positioning an electrically conductive member and an induction member so that a portion of the melting cavity is closer to the induction member than is the conductive member; heating the conductive member inductively with the induction member; transferring heat from the conductive member to the material; and heating a portion of the material inductively with the induction member.

The present invention also provides a method of heating a material comprising the steps of placing a material within a melting cavity of an electrically non-conductive crucible; positioning a conductive member and an induction member so that a portion of the melting cavity is closer to the induction member than is the conductive member; heating the conductive member resistively; transferring heat from the conductive member to the material; and heating a portion of the material inductively with the induction member.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the induction furnace of the present invention is indicated generally at10inFIGS. 1–2, and a second embodiment is indicated generally at100inFIG. 17. Furnaces10and100are configured to melt material which is electrically non-conductive at relatively lower temperatures and electrically conductive at relatively higher temperatures or upon melting, such as semi-conductor materials, or to melt material having particle sizes sufficiently small so that they are not susceptible to inductive heating even if of an electrically conductive material. The invention is particularly useful for melting semi-conductor materials and while reference may be made to semi-conductor materials in the application, this should not be deemed to limit the scope of the invention. Furnaces10and100may also be used with fibrous materials or other materials having geometries which are particularly difficult to melt via inductive heating. Heating liquids is also an option, as detailed further below. While the invention is thus widely applicable, the exemplary embodiment describes the heating and melting of solid material in particulate form.

Furnace10is shown inFIG. 1in an environment for continuous or intermittent melting and production of semi-conductor crystals wherein furnace10is adapted to utilize a feed mechanism12, a transfer or pouring mechanism14and a receiving crucible or tundish16for receiving molten material from furnace10via pouring mechanism14.

With reference toFIGS. 1–3, furnace10includes an induction member or induction coil18connected to a power source20. Coil18is substantially cylindrical although it may taken a variety of shapes. Coil18defines an interior space19and has an interior diameter D1as shown inFIG. 2. Furnace10also includes a crucible22and an electrically conductive member referred to in the induction heating industry as a susceptor24. Furnace10is configured so that electrical current passing through coil18creates an electromagnetic field which couples initially with susceptor24to inductively heat susceptor24and thereby transfers heat by conduction and radiation from susceptor24to unmelted raw material26(FIG. 3) in order to melt a portion of raw material26. Furnace10is further configured so that the portion of material26which is molten is inductively heated by coil18so that the inductive heating of molten material26far exceeds the inductive heating of susceptor24.

Crucible22includes a bottom wall28and a cylindrical sidewall30extending upwardly therefrom. Bottom wall defines an exit opening29. Sidewall30has an inner surface32defining an inner diameter D2, as shown inFIG. 2. Bottom wall28and sidewall30define a melting cavity34there within. Crucible22is formed of an electrically non-conductive material. While a variety of materials may be suitable for different applications, quartz is usually preferred for use with melting of semi-conductor materials, especially silicon.

Susceptor24may take a variety of shapes, but preferably is in the form of a cylindrical disk having an outer perimeter36and defining a hole37. Outer perimeter36defines an outer diameter D3(FIG. 2) which is smaller than diameter D2of crucible22. Susceptor24is formed of an electrically conductive material suitable for inductive heating, such as graphite. Susceptor24is disposed below crucible22closely adjacent bottom wall28and preferably in abutment therewith. An insulator38encircles sidewall30of crucible22and a refractory material40surrounds a substantial portion of crucible22and is seated on a support45. Material40defines a hole43and support45defines a hole47. Exit opening29of crucible22and holes37,43, and45are aligned to allow molten material to flow via pouring mechanism14into tundish16.

Alternately, susceptor24may be replace with one or more heating elements connected to power source20(FIG. 2). Thus, the heating elements may be resistively heated via an electrical current from power source20. In addition, these resistive heating elements may be inductively heated by induction coil18. As a result, the conductive member may be heated by induction, by resistance or both, depending on the material used and the configuration thereof.

In accordance with one of the main features of the invention, outer perimeter36of susceptor24is further away from coil18than is inner surface32of crucible22sidewall30as shown by the difference of diameters D1, D2and D3inFIG. 2. More particularly, some of the space within melting cavity34is closer to coil18than is susceptor24so that a portion of molten material may be disposed within said space, indicated at41inFIG. 2, and thus be closer to coil18than is susceptor24. Space41is disposed between inner surface32of sidewall30and an imaginary cylinder defined by lines X (FIG. 2) extending upwardly from outer perimeter36of susceptor24. Preferably, coil18, inner surface of sidewall30and outer perimeter36of susceptor24are all concentric about an axis Z (FIG. 2).

In operation, and with reference toFIGS. 2–8, furnace10functions as follows.FIG. 2shows furnace10prior to being charged with raw material26.FIG. 3shows an initial charge of raw material26having been placed into melting cavity34of crucible22. While a greater amount of material26may be placed initially in crucible22, additional material26hinders the initial melting process by dispersing heat over a greater amount of material. Once material26has been added to crucible22, electrical power is provided from power source20to coil18to create an electromagnetic field around coil18which flows in the direction of Arrows A inFIGS. 4–8. Prior to the melting of any of material26, the electromagnetic field from induction coil18produces induction heating within susceptor24. In the initial phase, material26is not susceptible to inductive heating. As previously noted, this may be because material26is not electrically conductive at a relatively low temperature, or it may be because material26is of sufficiently small particles to prevent the flow of electrical current as a result of the small contact area between particles, or both. Once susceptor24is inductively heated, susceptor24transfers heat by conduction and/or radiation through crucible22in order to melt a portion of material26, a molten portion42being shown inFIGS. 4–7.

Alternately, where conductive member (24) is one or more resistive heating elements, power source20provides electrical power to resistively heat the heating elements, which in turn transfer heat conductively and radiantly in the same manner as described above with regard to susceptor24after being inductively heated. If desired, the heating elements may also be simultaneously inductively heated by induction coil18. Whether heated only resistively or in combination with inductive heating, a portion of material26is thus heated and melted. Where only resistive heating is used to melt the initial portion of material26so that it becomes inductively heatable, power to the heating elements for heating by resistance is then halted and induction coil18is powered to inductively heat the susceptible portion of material26, as described below. The operation with respect to the use of susceptor24below is essentially the same for the use of resistive heating elements, although there may be some variations within the scope of the inventive concept. For instance, the configuration of the heating elements may lend themselves to inductive heating to a greater or lesser degree, and thus a certain configuration may act very similarly to susceptor24with regard to the inductive heating of the heating elements whereas another configuration may not be nearly as susceptible to inductive heating. To the extent that the heating elements are inductively heatable, the concepts discussed below regarding the inductive heating aspects of susceptor24also hold true for such heating elements.

Molten portion42is electrically conductive and is susceptible to inductive heating by coil18. Thus, coil18begins to inductively heat molten portion42while simultaneously inductively heating susceptor24. In general, as the molten portion within crucible22grows, inductive heating of the molten portion increases and inductive heating of susceptor24decreases.FIG. 4shows molten portion42having an outer perimeter which extends laterally outwardly to approximately the same distance as outer perimeter36of susceptor24. At this point, inductive heating of molten portion42is occurring, but is not as pronounced as inFIG. 5where the molten portion has extended outwardly to inner surface32of crucible side wall30. At the stage shown inFIG. 5, inductive heating of molten portion is substantially increased due to the molten portion extending closer to coil18than does outer perimeter36of susceptor24. As a result, inductive heating of susceptor24is decreasing as the inductive heating of the molten material is increasing.FIG. 5also shows additional material44being added to melting cavity34. The addition of such material may occur while there is still unmelted material in the crucible or once all the material is molten.

FIG. 6shows a further stage of melting wherein the inductive heating continues to increase within the molten material and decrease within susceptor24. Additional material44is also being added inFIG. 6.FIG. 7shows raw material26almost fully melted and at a stage where the inductive heating of susceptor24is minimal and most of the inductive heating is occurring within the molten material.FIG. 8shows all the raw material26having been melted and at a stage where the inductive heating of susceptor24is quite minimal.

In the earlier stages of the heating/melting process, heat was being transferred by conduction and radiation from susceptor24into raw materials26via crucible22. However, a reversal occurs wherein the inductive heating of susceptor24is sufficiently reduced and the inductive heating of molten material42sufficiently increased so that heat from molten material42in crucible22is being transferred through crucible22into susceptor24. This is illustrated in part inFIG. 9, which shows the temperature of susceptor24over time. Susceptor24is referred to inFIGS. 9–10as “conductive disk”. The graph ofFIG. 9illustrates that the temperature of the conductive disk increases relatively steeply until it reaches a peak and then drops off fairly substantially and then gradually increases. The sharp increase in the temperature of the disk is related to the inductive heating thereof which peaks about the point when materials within the crucible begin to melt and become inductively heatable by the coil. As direct inductive heating of the raw material increases and inductive heating of the susceptor or conductive disk drops off rather sharply, the temperature likewise drops a fairly substantial amount. Then, once the molten material increases in heat and volume, the heat within the molten material is transferred by conduction and radiation back through crucible22to the conductive disk, thereby heating it back up gradually to a certain level. This latter increase in heat is due almost entirely to the transfer of heat from the molten material, as inductive heating of the conductive disk becomes fairly minimal once the material is fully molten or fairly shortly before the fully molten stage.

FIG. 10shows the energy absorbed from the electromagnetic field induction coil18by both the conductive disk and the load material or raw mater to be melted during the melting process. More particularly,FIG. 10shows the energy absorbed by the disk from the electromagnetic field which is transferred by direct inductive coupling of the disk with coil18and the energy absorbed by the load material from the electromagnetic field which is transferred by direct inductive coupling of the load material with coil18. Such energy is hereinafter “direct inductive heating energy”. As clearly illustrated, the conductive disk absorbs essentially all of the energy that is going toward inductive heating in the initial stage of the inductive heating process and then decreases sharply as the load melts and becomes more conductive so that it is consequently inductively heatable. Once the materials are fully molten and even prior to that, the direct inductive heating energy being absorbed by the conductive disk through inductive heating is minimal in comparison to the direct inductive heating energy being absorbed by the material. By contrast, the load material receives essentially no direct inductive heating energy through inductive heating at the beginning of the process when the material is at lower temperatures.

With continued reference toFIG. 10, once the raw material becomes sufficiently hot to conduct electricity, which may be at the time of melting or at some point prior, the direct inductive heating energy absorbed by the load material increases fairly sharply and in substantially inverse relation to the direct inductive heating energy going to the conductive disk as the material melts and becomes more conductive. Once the material is almost fully melted, and after it is fully melted, nearly all of the energy going to inductive heating is being absorbed by the molten load material. In effect then, the conductive disk has nearly “disappeared” to the electromagnetic field of coil18in the sense that virtually all of the direct inductive heating energy being absorbed by the load material and the conductive disk in combination, is being absorbed by the load material as opposed to the conductive disk once the material is fully molten or nearly fully molten. This process happens automatically due to the nature of inductive heating whereby the magnetic field tends to be attracted to electrically conductive materials that are closer to the coil.

With further reference toFIG. 10, of the combined direct inductive heating energy being absorbed by the susceptor and by the material susceptible to inductive heating (hereinafter “the combined energy”), the percentage of direct inductive heating energy being absorbed by the susceptor reaches values lower than possible with known induction furnaces. While the percentage of the combined energy being absorbed by the susceptor is initially 100 percent or very close thereto, that percentage drops drastically during the melting process. The percentage of the combined energy absorbed by the susceptor at a given time during the melting process may be as low as 1 (one) percent or even less. However, under certain circumstances, depending on the particular material to be melted and in order to create overall optimal conditions of power consumption, it may not be possible to obtain such a low percentage. Nonetheless, for many practical applications, percentages for the direct inductive heating energy absorbed by the susceptor may at a given time be no more than 5 (five) percent of the combined energy. This is possible in the melting of semi-conductor materials, for example. The direct inductive heating energy absorbed by the susceptor easily reaches 30 percent or less of the combined energy at a given time during the melting process. This is less than any known stationary susceptor in the prior art. It is noted that the lower percentages are often only reached once the material in the crucible is fully molten or nearly so.

With reference toFIGS. 11–14, the pattern of the electromagnetic field produced by coil18is discussed along with the stirring patterns created within the molten material in crucible22. With reference toFIG. 11, lines46indicate the pattern of the electromagnetic field produced by coil18. As seen inFIG. 11, lines46are bent outwardly from the central portion of crucible22in the region of susceptor24, in accordance with the natural tendency of the electromagnetic field to couple with an electrically conductive material, and particularly with the portion of that material closest to the coil producing the electromagnetic field. At the stage shown inFIG. 11, material26within crucible22does not affect the electromagnetic field or does so to such a minimal degree that it is not appreciable. At this point, inductive heating produced by coil18is for practical purposes within susceptor24only.

FIG. 12shows a further stage of the process wherein a portion of the material has been melted as shown at48. As clearly seen, lines46of the electromagnetic field are moved further outwardly and begin to concentrate on the outer perimeter of molten portion48and tend to follow along the upper surface of portion48as well. Simultaneously, the amount of energy as represented by lines46which passes through susceptor24, has been reduced.FIG. 12also shows the early stage of currents indicated by Arrows C, being formed within molten material48, which are partly due to convection within molten material48. Electromagnetic forces increasingly affect the stirring patterns, as discussed in further detail hereafter.

FIG. 13shows yet a further stage of melting wherein a substantial portion of the material has been melted. Once again, the electromagnetic field as indicated by lines46, has moved outwardly along the periphery of molten material48. At this stage, the vast majority of energy used for inductive heating is being absorbed by molten material48and a relatively minimal amount is being absorbed by susceptor24, as indicated by lines46. In addition, eddy currents within the molten material are further indicated by Arrows E inFIG. 13. As indicated by Arrows E, the current within molten portion48is generally divided into an upper portion and a lower portion. In the upper portion, the molten material flows inwardly and upwardly towards the central upper portion of molten portion48. In the lower portion, the material flows inwardly and downwardly towards the lower central portion of molten portion48. As noted previously, electromotive forces are primarily responsible for the currents within portion48, which is further detailed hereafter. The current flow pattern shown inFIG. 13is known in the art as a “quadrature” flow pattern.

FIG. 14shows all of the material in crucible22in a molten state and further shows the amount of energy being absorbed by susceptor24as being minimal and the amount of energy being absorbed by molten material as having substantially increased.FIG. 14also shows that eddy currents (Arrows F) within the molten material follow the quadrature flow pattern.

As noted above, and with reference toFIG. 15, the electromotive forces created by the electromagnetic field of coil18push on molten material48in the direction of Arrows G. The electromotive forces indicated by Arrows G in the in central region, that is, those that are about halfway up the molten portion48, exert a stronger force than those toward the top or the bottom portion of molten portion48. This creates an electromagnetic force pinch effect whereby the molten material is literally moved inwardly away from side wall30of crucible22. In addition, the difference in the strength of the electromagnetic forces as noted, causes the molten material to flow in the directions indicated by Arrows H, that is, in the quadrature pattern discussed above. Convection plays a role in these currents as well. As shown inFIG. 15, the electromotive forces and the currents produced in molten materials48create a positive meniscus50which can be fairly substantial. While the type currents produced and the positive meniscus described is generally known in the prior art, the increased effect of the electromotive forces on the molten material due to the configuration of susceptor24, increases the velocity of the flow and the height of the meniscus. The increased velocity helps with the drawing of raw materials into the melt and helps produce a more uniform temperature throughout the melt. In addition, the higher meniscus creates a greater surface area atop the melt, and thereby provides greater opportunity for direct contact between molten material and solid material being added to the melt to expedite the drawing of raw material into the melt.

FIG. 16shows the basic concept of induction heating as well as the transfer of heat from susceptor24. In particular, Arrows I inFIG. 16indicate the direction of the electromagnetic field which produces electrical currents shown by Arrows J in accordance with the well-known right-hand-rule regarding inductive currents. As previously discussed, once heat has been inductively produced in susceptor24, heat is transferred as shown by Arrows K, by conduction and radiation through crucible22into materials26in order to initially melt the material. Of course, positioning the susceptor beneath the crucible is advantageous in that heat naturally rises.

Furnace100, the second embodiment of the present invention, is shown inFIG. 17. Furnace100is similar to furnace10except that susceptor24is located inside melting cavity34of crucible22and is seated on bottom wall28thereof, although susceptor24may also be disposed upwardly from bottom wall28if desired. An optional protective liner102encases susceptor24to protect against the contamination of the melt by susceptor24. In addition, refractory material140is altered in accordance with the changed location of susceptor24and defines a hole143through which molten material may flow, as with hole43of refractory material40of furnace10.

Furnace100operates in the same manner as furnace10other than some relatively minor variations. For instance, the configuration of melting cavity34is effectively altered by the presence of susceptor24therein, which consequently varies the melting pattern somewhat. Where protective liner102is used, transferring heat from susceptor24to material within melting cavity34is hampered to some degree in comparison to using susceptor24without liner102. However, even with liner102, heat transfer to the material may be more effective in comparison to furnace10because heat need not be transferred through bottom wall28of crucible22. In addition, where there is no concern of contaminating the melt with susceptor24, protective liner102may be eliminated and heat transfer from susceptor24to the material is then direct. Locating susceptor24inside crucible22does expose susceptor24to higher temperatures due to the inductive heating of the molten material, which may shorten the life of susceptor24. On the other hand, where susceptor24is seated on bottom wall28, susceptor24may insulate bottom wall28from the heat from the molten material to some degree, thus adding to the life of the crucible.

A variety of changes may be made to furnaces10and100without departing from the spirit of the invention. For instance, coil18need not be substantially cylindrical in shape in order to properly function. However, the generally cylindrical coil in combination with the cylindrical side wall of crucible22and disk shape of susceptor24, provides an efficient configuration for inductively heating susceptor24and material26in crucible22. Further, the induction coil or induction member need not surround the crucible22in order for the basic concept of the invention to work. As long as an electromagnetic field is able to inductively heat susceptor24and materials26within crucible22, and the induction member is closer to the material to be inductively heated than it is to susceptor24, the basic process works in accordance with the inventive concept. Thus, the induction member need not be in the form of an induction coil, but may be any member which is capable of producing an electromagnetic field when an electric current passes through it. The illustrated configuration may be more pertinent for certain materials such as semi-conductor materials, which are highly refractory and require a substantial amount of energy to melt.

In addition, susceptor24or a similar susceptor may be positioned above the material to be melted. However, contamination of the melt with the susceptor itself may be an issue in certain circumstances. In addition, where there is a desire to prevent contact between the susceptor and the molten material, positioning the susceptor close enough to material to effect sufficient heat transfer becomes an issue. Further, a susceptor extending over a substantial portion of the material may inhibit adding additional material to the crucible. Also, since heat rises, positioning the susceptor above the material to be melted diminishes efficiency of heat transfer.

As noted previously, the susceptor is an electrically conductive material and is preferably graphite, although it may be formed of any suitable material. Further, the susceptor may be of a wide variety of shapes such as, for example, a cylinder, a doughnut, a sphere, a cube, or any particular shape in which an electrical circuit and heat may be formed by induction. Most importantly, the susceptor should be disposed farther from the induction coil than is the susceptible material within the melting cavity. Similarly, the crucible can also take a variety of shapes although the cylindrical shape is preferred as noted above.

Furnaces10and100show a very simplified bottom flow or bottom pouring concept. This is intended to represent any suitable configuration of a pouring mechanism through which molten material may flow from the crucible, whether a bottom flow, overflow or any other pouring mechanism known in the art.

Induction furnaces10and100thus provide efficient means for inductively heating materials which are not susceptible to inductive heating at generally lower temperatures and which become inductively heatable at higher temperatures, typically when the material is molten. As discussed earlier, semi-conductor materials, for example, silicon and germanium fall within this group. In addition, this process works well with materials which are normally electrically conductive at lower temperatures but which are in the form of sufficiently small particles whereby electricity will not flow from particle to particle due to the small contact point between adjacent particles. While it is generally desired to use particulate material, furnaces10and100may also be used to melt or heat larger pieces of material. As noted above, the present invention may also be used with fibrous materials or other materials having geometries which are particularly difficult to melt via inductive heating.

Certain liquids are also particularly suited to heating with the present invention, for example, those liquids which are not susceptible to inductive heating at a relatively lower temperature but which are susceptible to inductive heating at a relatively higher temperature. The invention is also suitable for heating liquids which are susceptible to inductive heating at relatively higher frequencies (i.e., higher frequency electrical current to the induction coil) at a relatively lower temperature and which are susceptible to inductive heating at relatively lower frequencies at a relatively higher temperature due to the corresponding lowered resistivity of the liquid at the higher temperature. This may include scenarios wherein such liquids are simply not inductively heatable at the relatively lower frequency when the liquid is at the relatively lower temperature. This may also include scenarios wherein such liquids are susceptible to inductive heating to some degree at the lower frequency and lower temperature, but only at a relatively lower efficiency, while this efficiency increases at the lower frequency when the temperature of the liquid is sufficiently raised. Thus, the invention is particularly useful in that the conductive member can heat such liquids to bring them into a temperature range where commercially feasible lower frequencies can be used to inductively heat the liquids, substantially increasing the efficiency of heating such liquids.