US3857990A - Heat pipe furnace - Google Patents

Abstract

A furnace having a constant uniform temperature zone is provided by a heat pipe so shaped that the outside walls of the heat pipe form the inside walls of the furnace. A particular embodiment for growing crystals provides one or more annular shaped heat pipes heated at one end with the cylindrical space enclosed by the annular heat pipe at constant uniform temperature for heating crystal growing material, with access to the crystal growing material through the ends of the cylindrical space.

Description

United States Patent [191 Steininger et al.

[451 Dec. 31, 1974 HEAT PIPE FURNACE Inventors: Jacques Steininger, Lexington;

Thomas B. Reed, Concord, both of Mass.

Massachusetts Institute of Technology, Cambridge, Mass.

Filed: June 27, 1973 Appl. No.: 374,223

Related US. Application Data Division of Ser. No. 241,597, April 6, 1972.

Assignee:

US. Cl 13/22, 13/1, 219/397 Int. Cl H05b 3/66, F28d 15/00 Field of Search 13/1, 24, 22,20; 219/399, 219/395, 406, 397, 530, 540, 390, 413; 165/105 References Cited UNITED STATES PATENTS 3/1972 Kirkpatrick 13/22 Primary Examiner-R. N. Envall, Jr. Attorney, Agent, or Firm-Arthur A. Smith, Jr.; Martin M. Santa; Robert Shaw ABSTRACT A furnace having a constant uniform temperature zone is provided by a heat pipe so shaped that the outside walls of the heat pipe form the inside walls of the v furnace. A particular embodiment for growing crystals provides one or more annular shaped heat pipes heated at one end with the cylindrical space enclosed by the annular heat pipe at constant uniform temperature for heating crystal growing material, with access to the crystal growing material through the ends of the cylindrical space.

6 Claims, 11 Drawing Figures CRYSTAL "56 FULLER DRlVE RF POWER souncz u an PATENTEU macs 1 I574 SHEET 20F 4 CRYSTAL PULLER DRIVE NE ALONG AXIS 50 FROM P TO BOTTOM m R 7 EE 4 WC 8 P QR M PW T m R E m H. D 5 5 RAW -H A a 7 FIG '6 PATENTEU 951331 3 4 TEMPERATURE ALONG FURNACE AXIS (C) 3.857. 990 SHEET 3 BF 4 FURNACE WITH l5-cm QUARTZ-ZINC ANNULAR HEAT PIPE 0 HIGH POWER x LOW POWER X l I l 0 5 I0 l5 DISTANCE FROM BOTTOM OF HEAT PIPE cm) PATENTEDUEBIN I914 35857. 990

saw u or TEMP DISTANCE ALONG AXIS 70 FROM TOP TO BOTTOM ssfw w H OO OOQOOOOOO DISTANCE ALONG AXIS 80 FROM TOP TO BOTTOM HEAT PIPE FURNACE filed Apr. 6, 1972.

This invention relates to furnaces, and more particularly to a source of uniform constant temperature'forming the inside walls of a furnace. Heretofore, heat pipes have been used as highly efficient heat exchangers and have been used where the high thermal conductance of the heat pipe can be exploited. The heat pipe is essentially a closed vessel, lined internally witha wick saturated with a volatile liquid. Heat is absorbed by evaporation of the liquid in high temperature regions of the pipe and transferred by vapor transport to colder regions, where it is released by condensation of the vapor. The condensed liquid is then recycled by capillary action in the wick and flows back to the point of high temperature where it is again evaporated. Because of the high latent heats of vaporization of liquids, effective thermal conductivities several orders of magnitude greater than those of the best conducting metals can be obtained and it is this characteristic which has spurred the principal interest in heat pipes in the past.

Heretofore, the operating temperatures of heat pipes been a metal, such as lithium, zinc, or sodium. How- I ever, the principle of operation to provide high thermal conductance can be extended to much lower temperatures employing other liquids to provide the high conductance at-ambient temperatures or lower.

In the flow cycle inside a heat pipe, the evaporation and condensation constitutes a mono-variant reversible phase transformation and so the heat pipe tends to assume a very nearly isothermal profile from one'end to the other. It is this characteristic of a heat pipe which is an important part ofthe present invention. It is an object of this invention'to provide afurnace having at least one zone of constant uniform temperature with access to the zone so that crystals can be grown therein, for example, by pulling the crystals from a melt.

It is another object of the present invention to provide a furnace liner for maintaining a zone of constant uniform temperature inside the furnace.

It is another object to provide a surface of substantial area of constant uniform temperature for heating bodies in contact therewith.

It is another object to provide means for enclosing an object and maintaining the temperature thereof constant.

It is another object to provide a hot plate for use in open counter cooking.

It is another object to provide a furnace having at least two adjacent zones, each of different constant uniform temperature.

It is a further object to provide such a multiple zone furnace with access to the zones through the ends of the furnace to permit moving crystal growing material between the zones and drawing crystals from a melt of the material.

In accordance with the principalfeature of the present invention, a furnace liner is provided enclosing a zone of constant uniform temperature in the furnace, the liner being made at least partially of a heat pipe, the heat pipe being so shaped that the outside walls of the heat pipe form at least a part of the walls of the zone of constant, uniform temperature inside the furnace. In

particular embodiments of the invention, a complete furnace liner is provided by a heat pipe consisting of two concentric cylinders, one inside the other, with the space between the cylinders forming the inside of the heat pipe and the inside of the inner cylinder defining the zone of constant uniform temperature in the furnace.

These and other objects and features of the present invention are revealed by the following specific description of embodiments of the invention, taken in' to provide a hot plate, used for example for counter top cooking;

FIG. 5 is a partially sectioned view of a multi-zone furnace for heating crystal growing'material and growing crystals therefrom;

FIG. 6 is a plot of temperature along the furnace axis v distance along the axis for an embodiment of the present invention, illustrating the uniformity of temperature; I

FIG. 7 is a chart of temperature v distance along the axis of the apparatus in FIG. 5;

FIG. 8 is a partially sectioned view of apparatus with three zones for zone melting processes;

FIG. 9 is a chart of temperature v distance along the I outside cylinders 2 and 3, which are closed at their ends. The annular space 4 between the cylinders is lined or filled with wicks made up of several layers of fine mesh screen 5. A fluid is also contained in the annular space and selected in consideration of the temperature desired.

The liner 1 is enclosed by furnace insulation 6 in a container 7 and an axial opening 8 at one end, concentric with the axis 9 of the liner is provided for access to the constant temperature zone 10 inside the furnace, defined by the inside walls of the cylinder 2. A plug'll may be provided for closing the opening 8.

Heat is delivered to the liner by a heating coil 12, wound around one end of the liner and energized by a power source 13. Heat is carried from the coil to the liner for evaporating the liquid inside the annular space 4 either by radiation, conduction or a combination of both.

In operation, the heat supplied causes evaporation of the liquid in the space 4 in the high temperature region 14 of the space. Due to the pressure differential inside the annular space 4 this vapor is transported at sonic velocity in the direction of arrow I5 toward the colder regions of the pipe at the end adjacent the opening 8 and condenses at the colder end or along the way toward the colder end, releasing absorbed heat of vaporization. The condensed liquid is then recycled to the high temperature evaporating area 14 by capillary action .in the wick. This may be assisted by gravity if the furnace stands with the axis 9 vertical and the high temperature region 14 at the bottom.

The actual rate of heat transfer in the annular space 4 is essentially equal to the latent heat flux in the evaporator, expressed as follows:

where:

Q is heat transfer rate,

rh, is vapor mass flow rate L is latent heat of vaporization of the liquid.

For an annular channel, such as channel 4 in the structure-in FIG. 1, equation 1 becomes:

Q PP

where:

A, is the cross section area of the annular space, p is the vapor density, and V is the average vapor velocity Because of high latent heat of vaporization of the selected liquid, effective thermal conductivities several mum heat transfer rate limited by the maximum flowrate of the condensed liquid through the wick can be expressed for the liner shown in FIG. I by:

where:

ais surface tension of the liquid A is the free flow area of the wick D is the wick pore size 1/ is kinematic viscosity, and

b is capillary geometric constant (about 20 for wire mesh capillary) The furnace liner, such as shown in FIG. 1, can be made and operated of selected materials to provide a constant temperature zone ranging from below C to above 2,000C, depending on the working liquid that is selected. Working liquids can have vapor pressures at their operating temperatures ranging from a few Torrs to several atmospheres. The selection of the container, the wick materials and the liquid is subject to the requirement that the liquid must wet but not react with the other materials. The table below shows combinations of materials and liquids that are suitable for operation at various temperature ranges indicated.

Table l-Continued Fluid Material Temperature Hydrocarbons Mercury Stainless Steel 350 Potassium Nickel 600 Sodium Nickel 900 Stainless Steel 780 Hastelloy 750 Zinc Quartz llOO Lithium TZM Alloy I500 Tungsten l500 The furnace liner illustrated in FIGS. I and 2 for providing the zone of constant uniform temperature in a furnace is generally cylindrical and the zone is cylindrical. This is a convenient shape and permits relatively easy construction of the furnace liner and the furnace from stock materials. However, the liner may be square or rectangular in cross section to provide a square/rectangular shaped zone inside the furnace, such as illustrated by the liner 21 in FIG. 3, enclosed by insulation 22 and heated at one end by a heating coil 23, energized by source 24. Clearly, the liner can be just about any shape, or a portion of an inside wallofa furnace can be a section of heat pipe, taking advantage of the constant uniform temperature profile across the surface of the heat pipe and the intrinsic temperature reliability associated with the temperature of evaporation and condensation of a pure liquid.

FIG. 4 illustrates use of these same qualities to provide a hot plate surface 26, this being one face of a flat heat pipe 27, supported by a thermally insulating base 28, and heated at one end by, for example, a heating coil 29, energized by a power source 30. Inside the flat heat pipe '27, a wick 31 extends throughout the inside and carries condensed liquid back to the hot area adjacent the heating coil 29, where liquid is evaporated absorbing heat and carrying the heat at very high efficiency throughout the surface 26 of the plate. The hollow inside of the plate (or cavity) containing the wick 31 and fluid is preferably supported throughout by structure such as 32 to maintain the mechanical shape even while pressure inside is varied in control of temperature. This embodiment is suggested for use, for example for counter top cooking, as it provides a surface 26 of constant uniform temperature, determined substantially by the selected liquid. Here, high thermal conductivity of the heat pipe is also exploited, as this characteristic insures that heat from the coil is efficiently carried to all points of the surface 26. It is not dependent upon the heat carrying capacity of the liner material per se.

From the thermodynamic point of view, evaporation and condensation of a pure liquid constitute-a monovariant reversible phase transformation. Heat pipes tend to assume nearly isothermal temperature profiles and in practice a small pressure drop is needed" to maintain the flow of vapor from the evaporator (hot end) of the pipe to the condenser (colder end). This pressure drop, however, can be as small as l torr and so quite obviously the temperature gradient from one end to the other is extremely small. It should be noted, however, that the pipes can be operated in two modes: constant volume and constant temperature. If the pipe is sealed as the embodiments described herein, operation is at constant volume and variations in input power are matched by variations in internal vapor pressure and temperature. Thus, the closed or constant volume, furnace liner or hot plate, such as described above, can be operated over a temperature range dependent upon the input power and this is accompanied by an increase in pressure inside the pipe. If input power is monitored by pressure inside the pipe, the power can be controlled by a feedback arrangement to maintain temperature constant. For example, in the hot plate embodiment shown in FIG. 4, pressure inside the pipe could be sensed by a pressure sensing mechanism, producing an output for controlling the electrical energy from the power source 30. This would constitute a feedback circuit and could be selectively set, thereby setting the temperature of the plate 26 over a predetermined temperature range. The temperature range would be determined in consideration of the rupture pressure of the flat heat pipe 27.

It is also possible to operate a heat pipe as an open pipe where the vapor is contained by an inert gas pressure. In that case, variations in power input are matched by variations in the length of the vapor zone in the heat pipe and the pipe operates with an isothermal zone offixed temperature, but of variable length. This fixed temperature is substantially independent of the amount of input power, at least over a significant range.

Use of the annular heat pipe, such as the liner 1 in FIG. 1 to provide a multi-zone temperature profile along the axis of a heating zone is illustrated by the structure in FIG. 5. This is particularly designed for crystal growing or annealing. With several annular heat pipes in series, well defined short zones with extremely high temperature gradients in between 'can be obtained along a crystal growing axisfThe apparatus in FIG. 5 provides such temperature zones for producing a melt of crystal growing material in one zone and drawing crystals from the melt into a lower temperature zone, at a temperature maintained to prevent constitutional supercooling and to minimize thermal strain in the drawn crystal. In this apparatus,.the upper zone 41 is maintained by heat pipe liner 42 and the lower zone 42 is maintained by liner 44. The liners are separated by a thin spacer 45 and heated by an RF coil 46, energized by RF power source 47. Along the axis 50 of the liners is located a pedestal 51 for supporting the melt 52 in zone 43. A crystal 53 is pulled from the melt into zone 41, where the temperature is maintained ideal for cooling. The apparatus for pulling the crystal may include a seed rod 54, with an extension 55, to a crystal puller drive 56.

The heat pipe liners 42 and 44 are held rigidly together and supported by frame 57, which is positioned along the axis 50 by liner drive 58.

FIG. 6 is an approximate chart of temperature along the axis 50, showing the zones 41 and 43 and the transition in between. The constant temperature zones denoted T and T are shown flat in the chart, although they may have a slight gradient on the order of a degree of two per centimeter. However, the transition zone at feet position the pulled crystal 53 for ideal growth as the crystal is pulled from the melt 52.

A typical heat pipe liner, such as 42 or 44, shown in FIG. 5, designed for operating in the temperaturerange of l,l00C may include two concentric cylinder sections 61 and 62, which are of quartz. Quartz is particularly useful because it is workable and transparent, permitting observation of the inside of the liner. The wicks 63 in this liner are ofquartz cloth and the working liquid can be either zinc or cadmium. A feed tube such as 64 at the end of the liner serves to load the liquid and afterwards is sealed off to provide the constant volume heat pipe liner.

Accurate measurements of the temperature profile along the axis of such a quartz heat pipe liner in a furnace at different input power levels are plotted in FIG. 6. These plots were obtained with a double walled quartz pipe 3.4 centimeters outside diameter, 1.9 centimeters inside diameter and 15 centimeters long. The wicks are of quartz woven cloth and the working liquid is zinc. Two temperature profiles measured along the axis at high and low input powers are plotted. As can be seen from the plot, the higher temperature profile remains within a few degrees of l,065C for a distance of 8 centimeters, where it reaches a cold zone at the upper part of the liner, which is attributed to the presence of non-condensable residual gases inside the liner. These gases apparently are swept to the colder section by the pumping action of the zinc vapor and form a low conductivity vapor lock.

Other combinations of materials and fluids for designated temperature operation are listed in the Table 1 above. A liner constructed of nickel is particularly sturdy, containing several layers of mesh nickel wire screen to provide the wick. For a temperature of 900C, the working fluid may be high purity sodium, which is loaded into the liner by vacuum distillation through the feed tubewhich is then pinched and electron beam welded.

FIGS. 8 and I0 illustrate in less detail other apparatus for crystal growth. In FIG. 8, three heat pipe liners 71,

72 and 73 abut end-to-end, separated by spacers 74 and 75, and are heated by IF coil 76 enclosing portions of each and energized by RF source 77. Along the axis 70, the liner 72 provides a short, high-temperature zone, denoted T between two lower temperature zones T and T This is shown in the chart in FIG. 9. A boule of crystal growing material 78 on a pedestal 79 is raised by drive 80 into the center zone T and so zone melting occurs in the crystal, producing upon crystallization the desired crystalline structure along that zone.

The apparatus in FIG. 10 includes two'heat pipe liners 81 and 82 in an RF heated furnace 83, the furnace walls being lined with an RF heating coil 84, energized by RF source 85. Three zones are produced in this furnace, the upper zone 86, the zone inside liner 81 and the zone inside liner 82. A body 87 such as a crystal lowered along the axis of the apparatus by a drive 88 will experience first the lowest temperature T then the highest temperature T and then an intermediate temperature T as shown on the chart in FIG. 11. This sequence of temperatures is desired where crystal growing vapor material is introduced inside the liners while the crystal 87 moves along the axis 80, as in a vapor phase crystal growth process.

The uses of the heat pipe liner in apparatus described herein, are examples of but a few particular uses related to crystal growth. These uses have been foremost, because the heat pipe liner provides a uniform, very even temperature zone, which can be precisely controlled and is so necessary in the growth of pure crystalline materials. Clearly, other heat pipe configurations besides the liner configuration for providing a zone of known dimensions in which temperature is precisely controlled and extremely uniform have application in other areas. For example, small, but uniform temperature gradients over relatively large areas for epitaxial semiconductor growth could be accomplished either with an off axis annular heat pipe or a sandwich configuration of two flat heat pipes maintained at slightly different temperatures. These and other applications of the present invention are within the scope of the invention as set forth in the appended claims, in which:

We claim:

1. in a furnace, for providing at least two zones of different constant uniform temperature and a high temperature gradient therebetween, comprising,

a first and second hollow-walled sealed container defining a first and second zone in their interiors,

a wick inside the hollow wall extending from one end thereof to the other end thereof,

a liquid inside the hollow wall impregnating the wick,

said first and second containers being in axial alignment and having an end of each in proximity to one another,

an insulating spacer between said first and second container ends,

means for heating the liquid in both zones with a heater located adjacent the proximate ends of both hollow walls,

the liquids in the first and second hollow walls are different, having different temperatures of vaporization selected to provide different zone temperatures,

the liquids absorb heat from the heater, evaporate, and fill the inside of the hollow walls with vapor at said constant different temperatures and the vapor condenses, gives up heat to the wall, flows through the wick and is heated again to repeat the cycle,

whereby a high thermal gradient occurs across the insulating spacer between the first and second zones.

2. In a furnace as in claim 1 for use in growing crystalline materials comprising in addition means for introducing crystal growing materials into the interior of one container,

means for moving the crystal growing material along the axis with respect to the heat pipes,

means for pulling a crystal from the introduced material,

said containers including wick and liquid being designated as a heat pipe,

whereby said different temperature zones and the high thermal gradient are effective in the control of crystal growth from the materials.

3. In a furnace as in claim 2 wherein said containers are formed of two concentric cylinders with the annular space therebetween being closed at both ends of the cylinders,

and the axis of the cylinders being the axis of the containers.

4. A furnace as in claim 1 wherein,

the heater is an RF radiation coil enclosing adjacent ends of the hollow walls concentric therewith.

5. A furnace as in claim 2 wherein the means for heating is an RF coil around at least a portion of each of the heat pipes attheir proximate ends.

6. A furnace as in claim 5 wherein the heat pipes and RF coil are movable with respect v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,857,990 Dated December 31, 1974 Inventor(s) Jacques Steininger and Thomas B. Reed It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, after the first sentence, insert the following statement:

The invention herein described was made in the performance of work under a, contract from the United States Air Force, Electronic Systems Division.-

Signed and Scaled this eighteenth D ay Of November 1 9 75 [SEAL] Arrest.-

RUTH C. MASON C. MARSHALL DANN :I HSII'HZ ffl'fi (mnmixsimzer uj'Palenrs and Trademarks I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,857,990 Dated December 31, 1974 Inventor(s) Jacques Steininger and Thomas B. Reed It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, after the first sentence, insert the following statement:

-The invention herein described was made in the performance of work under a contract from the United States Air Force, Electronic Systems Division.

Signed and Scaled this eighteenth D ay 0f November 1 9 75 [SEAL] Arrest.

RUTH C. MASON C. MARSHALL DANN .lmsring ()jfl'cer (mnmissivner uflarenrs and Tradcmurkx

Claims (6)

1. In a furnace, for providing at least two zones of different constant uniform temperature and a high temperature gradient therebetween, comprising, a first and second hollow-walled sealed container defining a first and second zone in their interiors, a wick inside the hollow wall extending from one end thereof to the other end thereof, a liquid inside the hollow wall impregnating the wick, said first and second containers being in axial alignment and having an end of each in proximity to one another, an insulating spacer between said first and second container ends, means for heating the liquid in both zones with a heater located adjacent the proximate ends of both hollow walls, the liquids in the first and second hollow walls are different, having different temperatures of vaporization selected to provide different zone temperatures, the liquids absorb heat from the heater, evaporate, and fill the inside of the hollow walls with vapor at said constant different temperatures and the vapor condenses, gives up heat to the wall, flows through the wick and is heated again to repeat the cycle, whereby a high thermal gradient occurs across the insulating spacer between the first and second zones.

2. In a furnace as in claim 1 for use in growing crystalline materials comprising in addition means for introducing crystal growing materials into the interior of one container, means for moving the crystal growing material along the axis with respect to the heat pipes, means for pulling a crystal from the introduced material, said containers including wick and liquid being designated as a heat pipe, whereby said different temperature zones and the high thermal gradient are effective in the control of crystal growth from the materials.

3. In a furnace as in claim 2 wherein said containers are formed of two concentric cylinders with the annular space therebetween being closed at both ends of the cylinders, and the axis of the cylinders being the axis of the containers.

4. A furnace as in claim 1 wherein, the heater is an RF radiation coil enclosing adjacent ends of the hollow walls concentric therewith.

5. A furnace as in claim 2 wherein the means for heating is an RF coil around at least a portion of each of the heat pipeS at their proximate ends.

6. A furnace as in claim 5 wherein the heat pipes and RF coil are movable with respect to each other along the axis.

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