WO1991002833A1

WO1991002833A1 - Apparatus and method for containerless directional thermal processing of materials in low-gravity environments

Info

Publication number: WO1991002833A1
Application number: PCT/US1990/004540
Authority: WO
Inventors: Donald Roy Pettit; James Derrick Calaway
Original assignee: United States Department Of Energy
Priority date: 1989-08-18
Filing date: 1990-08-16
Publication date: 1991-03-07
Also published as: AU6284890A

Abstract

Apparatus and method for thermally processing thin, self-supporting samples (12) of material in low-gravity environments. In its simplest form, the apparatus of the present invention includes a holder (14) for the material, movable heaters (10a, 10b) on each side of the sample, and means (25) for measuring the temperature of the surface of the sample. Preferably, an auxiliary heater (20, 22) is utilized in order to maintain the sample at a temperature just below the desired processing temperature until low-gravity conditions are obtained, at which time the movable heaters rapidly bring the sample temperature up to the processing temperature. An example of the advantages of the low-gravity environment is the observed homogeneous mixing of metallic silver with a superconducting material after a sintered composite thereof is brought to melting temperature. Such uniform mixing has not been available through processing in the presence of normal terrestrial gravitational forces. Clearly, other multiphase composite materials may be processed using the present apparatus and method.

Description

APPARATUS AND METHOD FOR CONTAINERLESS DIRECTIONAL THERMAL PRO¬ CESSING OF MATERIALS IN LOW-GRAVITY ENVIRONMENTS

BACKGROUND OF THE INVENTION The present invention relates generally to thermal processing of materials and more particularly to containerless, directional thermal processing of materials in low gravity environments. The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-36 between the U.S. Department of Energy and the University of California. Space Industries, Inc., a Webster, Texas corporation also has rights in the present invention.

Beginning in 1973, a number of melt growth experiments were performed using semiconductors in spacecraft and on sounding rockets in the search for reproducible growth procedures and characterization techniques as well as knowledge of defect structures and dopant segregation behavior. Floating-zone crystallization is a technique which was developed to reduce power requirements and radio frequency usage in spacecraft in which mirrors are used to focus light from halogen lamps onto the sample. The sample is slowly moved through the region of irradiation and volumes thereof are melted and permitted to resolidify away from contact with foreign surfaces. In a double ellipsoid configuration, zone melting of silicon bars up to 12 mm in diameter in a protective atmosphere was achieved. See, e.g., Fluid Sciences and Materials Sciences in Space-A European

Perspective. H.U. Walter, Ed. (Springer-Verlag, 1987), pp. 338-9.

In U.S. Patent No. 4,740,264, "Liquid Encapsulated Float Zone Process and Apparatus," issued to Robert J. Nau ann et al., on April 26, 1988, the inventors describe a technique for growing crystals using float zone techniques. A rod of crystalline materials is disposed in a cylindrical container with a space being left between the rod and the container walls which is filled with an encapsulant material selected to have slightly lower βelting point than the crystalline material. The rod is rigidly attached to a container end cap at one end, and to a shaft at the other end. A piston slides over the rod and provides pressure to prevent loss of volatile components during the melting process. Prior to melting of the rod material itself, the container is heated to melt the encapsulant in order to drive off unwanted gases which are removed from the vicinity of the rod. The container is moved longitudinally through a heated zone to progressively melt sections of the rod as in conventional float zone processes. It should be emphasized that the material to be processed is moved through the region of heating in float zone processing, and that the material itself is in the form o a rod.

In "Quas -Containerless Glass Formation Method And Apparatus," U.S. Patent No. 4,654,065, issued to Robert J. Naumann and Edwin C. Ethridge on March 31, 1987, the inventors describe an apparatus fo forming ultrapure glass rods or fibers using a containerless, doubl float zone technique in which a short section of a polycrystall n rod is heated in a first furnace to form a molten zone of the rod, an a second short section of the rod is heated in a second furnac which is initially separated from the first furnace by a short gap t form a second molten zone of the rod which is initially contiguou with and part of the first molten zone of the rod to form a singl molten zone, the first furnace and the second furnace then bein slowly moved apart forming a rod of ultrapure glass in th increasingly widening gap between the two furnaces. Also describe are a preheating primary furnace as part each furnace, and the fac that the process requires a microgravity environment since the glas rod to be formed must be supported, at least initially, by th surface tension of the two liquid zones. As in the Naumann et al patent described hereinabove, the '065 patent describes technique suitable for rod-shaped articles.

The cost, power, space and weight limitations, and uncertaintie in placing experiments aboard space flights have rendered the lo gravity environment inconsistent with^' both fast-paced terrestrial research and with producing meaningful quantities of advanced low gravity processed materials. The most realistic -_ethod for accessing the pre-space low gravity environment is through aeroparabolas (airplanes flying in repetitive parabolas), since the cost of parabolic flights is relatively low, and although the increment of time available at low gravity per parabola ranges typically between 20 and 60 s, depending upon the type of airplane utilized, the cumulative time achievable is large (i.e., on a ■odified 707, one can fly 40 parabolas per day, for a total of 13 Bin. of low gravity time). However, all known aeroparabolic experiments to date have been either research in nature or have been to test systems for space flight. The common assumption for these tests was that one would have to go into space to conduct microgravity manufacturing given the short period of microgravity available to parabolic low gravity systems. Therefore, the configuration of the material to be processed was quite important. In the past, cylindrical or spherical samples have been employed exclusively. These samples were required to be small due to the difficulty of the center of the samples resolidifying prior to the onset of accelerating forces in the aeroparabola experiments. Thus meaningful quantities were not contemplated to be processible in these experiments.

In what follows, "containerless" refers to processing materials without contact with walls or supporting structure during the processing. When applied to solidification of a liquid melt, containerless means that the liquid solidifies without contact with pre-existing solid. "Directional solidification" refers to a liquid zone which is undergoing continuous, sequential solidification in a particular direction, and may be achieved by employing a heater which melts a region of the material to be processed. Impurities found in precursor solid materials are often excluded in solidification from a melt where there is no contact with walls or supporting structure, the impurities remaining in the liquid zone. Accordingly, it is an object of the present invention to provid an apparatus for rapidly thermally processing materials in lo gravity environments.

It is a further object of our invention to provide an apparatu for rapidly thermally processing significant quantities of material in low gravity environments.

Another object of the present invention is to provide a apparatus for containerless, rapid thermal processing of materials i low gravity environments. Yet another object of the present invention is provide a apparatus for containerless, directional thermal processing o materials in low-gravity environments.

Additional objects, advantages and novel features of th invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upo examination of the following or may be learned by practice of th invention. The objects and advantages of the invention may b realized and attained by means of the instrumentalities an combinations particularly pointed out in the appended claims. SUMMARY OF THE INVENTION

The present invention provides a reliable apparatus and metho for processing generally immiscible substances to form a soli mixture of substances. Such processing results in a mixture having substantially uniform distribution of the intermixed substances. T achieve the foregoing and other objects and in accordance with th purpose of the present invention, as embodied and broadly describe herein, the apparatus for containerless, directional therma processing of materials in low-gravity environments hereof ma include means for holding a self-supporting material to be processe in the vicinity of the edges thereof, a first heater for providin heat to a volume of the material and a second heater for providin heat to a volume of the material located on the opposite side of th material from the first heater, and means for moving the first heate and the second heaters in opposite directions such that localize portions of the material are sequentially heated to processing temperatures substantially in the direction of βotion of the first heater and the second heater, the heated portions being permitted to cool by thermal conduction and radiation to below processing temperatures after the passage thereover of the first heater and the second heater. Preferably, the first heater and the second heater include line heaters for directionally heating the material substantially along a narrow strip thereof. Preferably also, thin samples are employed, the first heater and the second heater being deployed on opposite sides of the thin dimension of the sample. It is preferred that auxiliary heating means be provided in order to maintain the material close to, but below processing temperatures such that the first heater and the second heater can more rapidly bring the material to processing temperatures. In a further aspect of the present invention, in accordance with its objects and purposes, the apparatus for containerless, directional thermal processing materials in a low-gravity environment may include means for holding a self-supporting material to be processed in the vicinity of the edges thereof, first heating means for providing heat to the material including two first movable heaters spaced-apart and located in the proximity of a first side of the material and first means for translating each of the first movable heaters relative to one another along a first line substantially parallel to the first side of the material, and second heating means for providing heat to the material including two second movable heaters spaced-apart and located in the proximity of the opposite side of the material from the first heating means and second means for translating each of the second movable heaters relative to one another along a second line parallel to the first line, such that localized portions of the material are sequentially heated to processing temperatures along a line therein substantially parallel to the first line and the second line, the heated portions being permitted to cool by thermal conduction and radiation to below processing temperatures after the passage thereover of the first heating means and the second heating means. Preferably, thin samples are employed, the first heating means and the second heating means being deployed on opposite sides of the thin dimension of the sample. It is also preferred that auxiliary heating means be provided in order to maintain the material close to, but below, processing temperatures such that the first heating means and the second heating means can more rapidly bring the material to processing temperatures.

In yet a further aspect of the present invention, in accordance with its objects and purposes, the method for preparation of homogeneous mixtures of materials hereof includes the steps of preparing a self-supporting sintered mixture of the materials, reducing the gravitational forces on the sintered mixture, heating the sintered mixture in reduced gravitational forces to above the melting temperature thereof and permitting the melted sample to cool. Preferably, the step of reducing the gravitational forces on the sintered material includes flying the sintered material in an airplane executing parabolic maneuvers.

Benefits and advantages of the present invention include the ability to thermally process materials in a low-gravity environment wherein new and improved materials may be synthesized and produced. For example, solid materials deriving from immiscible liquid phases in gravitational fields may be homogeneously prepared from a melt in low-gravity conditions. Additionally, materials containing fine crystallites which would settle in a gravitational field can similarly be prepared from a melt in low-gravity. In situations where directional solidification is desirable or where containerless processing is mandated, the present invention is valuable. Moreover, the present apparatus permits full advantage to be taken of the limited low-gravity periods available in the execution of aeroparabolas using conventional aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of the specification, illustrate three embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIGURE 1 is a schematic representation of a side view of the two-heater apparatus of the present invention showing the location of a sample of the material to be processed relative to the moving heating lamps. Shown also is microwave means including a waveguide and lens for keeping the sample close to, but below processing temperature, and pyrometer means for determining the temperature of the sample. FIGURE 2a is a schematic representation of a side view of the four-heater apparatus of the present invention showing the location of the thin sample of the material to be processed relative to the moving heating lamps in the situation where a central portion of the sample is being heated. FIGURE 2b shows the apparatus as described in Figure 2a hereof where the heating lamps have been moved away from the central region of the sample and are heating a region closer to the outside thereof.

FIGURE 3a is a photomicrograph of a thin superconducting sample interspersed with metallic silver which was processed in a gravitational field, while FIGURE 3b is a photomicrograph of a similar superconducting sample mixed with metallic silver, but prepared according to the teachings of the present invention at low gravity. FIGURE 3c is a photomicrograph of a thin sample of superconducting material processed according to the technique of the present invention at low gravity showing significant crystal formation in nonsuperconducting phase thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Briefly, the present invention includes an apparatus and method for thermally processing self-supporting samples of material in low-gravity environments. In its simplest form, the apparatus includes a holder for the material, movable heaters on each side of the sample, and means for measuring the temperature of the surface of the sample. Preferably, thin samples are to be used, since cooling times for materials having significant volume may be large. It is also preferred that an auxiliary heater be utilized in order to maintain the sample at a temperature just below the desired processing temperature until low-gravity conditions are obtained at which time the movable heaters rapidly bring the sample temperature up to the processing temperature. An example of the advantages of the low-gravity environment is the observed intimate mixing of metallic silver with . a superconducting material after a sintered composite is brought to melting temperature. Such uniform mixing has. not been available through processing in the presence of ordinary terrestrial gravitational forces, and clearly, immiscible systems in general may be processed according to the teachings of the present invention.

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Similar or identical structure is identified therein by identical callouts. Figure 1 is a schematic representation of a side view of the two-heater apparatus of the present invention showing the location of a thin sample of the material to be processed relative to the moving heating lamps. Movable heating lamps 10a,b are deployed on either side of a generally flat, thin, self-supporting sample 12_. Rods, filaments, etc. may be employed so long as the material has sufficient time to cool. Sample holder 14 is shown supporting the sample from the ends thereof. However, a rectangular holder may be employed to support a rectangular sample along all four edges. Typically, the sample is a sintered composition of two or more materials which will be brought to processing temperatures in a low-gravity environment. The sintering process occurs below processing temperature. In the Example below, processing temperature is simply the melting temperature of the material. The moving heating lamps K) are shown in the vicinity of the center of the sample. As the heating lamps are moved in the direction of the arrows 16a.b, the sample is melted along two liquid float zones which move away from the center of the sample, forming thereby a directionally solidified sample supported by two liquid float zones, with no contact with any containers. Shown also is microwave means 18 including a waveguide £0 and microwave generator £2_. along with lens £4 for keeping the sample close to, but below processing temperature so that the heating lamps can rapidly bring the sample to at or above processing temperature, and pyrometer means £5 for determining the temperature of the sample. In Figure 1, heating lamps \0 are shown to include linear halogen lamps 25a.b and parabolic focusing reflectors 26a.b which can provide a linear float region. The sample may also be preheated by scrolling the heating lamps 10 repetitively over the surface of the sample at lower power levels, or by a combination of the scrolling process and use of a microwave or other heater. When processing temperatures are desired and melt zones are to be established, the lamps are brought to higher power levels. Clearly, the lamps can be moved in the same direction, or equivalently, the sample moved between the two lamps if but a single melt region is desired.

Figure 2a is a schematic representation of a side view of the four-heater apparatus of the present invention showing the location of the sample 1£ of the material to be processed relative to the movable heating lamps lOa-d in the situation where a central portion of the sample is being heated. Shown also are screw means £8,30 for moving lamps 10 on either side of the flat surface of the sample to be processed in opposite directions. The desired motion of the lamps may be obtained by fabricating the screw means £8,30 from oppositely threaded screws 32a,b which are held together, but freely able to rotate, by coupler 34_» and which are coupled to movable stages 36a,b by threaded flanges 38a,b. The lamps JJ) are rigidly affixed to stages 36- Figure 2b shows the apparatus as described in Figure 2a hereof where the heating lamps have been moved away from the central^' region of the sample and are heating a region closer to the outside thereof, creating thereby a directionally-solidified sample supported by two float zones 40a,b in the absence of a container. The sample may be preheated to just below processing temperature by rapidly scrolling the heaters in an oscillatory manner over the sample surface or by use of an auxiliary heater as shown in Figure 1 hereof, or by a combination thereof.

Microwave preheating has several advantages over other methods in that it is a volume heating phenomenon where the bulk of the sample is heated. This may alleviate thermal stresses in the sample as well as permit high temperatures (greater than 2000°C) to rapidly be achieved. Moreover, it is known that microwave heating can enhance diffusion of trace species in glasses, thereby more uniformly dispersing the components of the initial composition before low-gravity processing. See, for example, T.T. Heek et al., "Cation Diffusion In Glass Heated Using 2.45 GHz Radiation," J. Mat. Sci . Letters I, 928-931 (1988) and Thomas T. Meek, "Proposed Model For The Sintering Of A Dielectric In A Microwave Field," by 0. Mat. Sci. Letters. 6_, 638-640 (1987). It should be emphasized that the use of a sintered sample in the form of a sheet provides a processed sample in near finished shape for many articles of interest. For example, large cylinders of semiconductor are cut into sheets for wafer manufacture, while a sheet is the final product shape from the sheet float zone furnace of the present invention. Another aspect of our sheet float zone furnace is that it may be scaled in the horizontal direction (that is, horizontal to the gravitational fields) to as large a dimension as power availability on the aircraft utilized permits. Furnaces have been designed with 6 in. line heaters, permitting samples of up to 5.5 in. in width to be processed. These heaters can be replaced by commercially available heaters up to 38 in. long. The vertical height of the processed sheet is limited more by time available at low-gravity and can be increased by utilizing more powerful heaters. As mentioned above, samples having other geometrical configurations than sheets may be employed. Bars, for example, may be processed according to the teachings of the present invention if sufficient time for cooling is permitted for samples having significant heated internal volumes. Having generally described the present invention, the following example is intended to more particularly point out some of the features thereof.

EXAMPLE It is known that metallic silver and 123 high temperature superconducting materials are immiscible and separate due to large density differences when melted in a gravitational field. A similar situation obtains for 123 superconducting materials and metallic palladium. Moreover, certain glasses which are predicted to have desirable properties, such as lead borate (PbO + B2O3), separate into melts of component compounds, when attempts are made to produce them. Metallic silver and 123 superconducting material were sintereo into sheets and melted in a low-gravity environment using the apparatus of the present invention. Meaningful quantities of the sintered material (4 in. x 1 in. x one-eighth in. sheets) were thermally processed within the 20 s time period of an aeroparabola. Figure 3a is a 15-power photomicrograph of a mixture of 20 volume percent metallic silver and a 123 superconducting material which has been melted in the presence of ordinary gravitational forces. Note the large globules of silver remaining in the composite material . Figure 3b is a 15-power photomicrograph of the same 123 superconducting material mixed with 20 volume percent of metallic silver and melted under low gravitational forces. It is to be noted that the silver is now uniformly dispersed in the matrix. The large globules of silver on the edges of the low-gravity processed sample arise from capillary forces wieking a small portion of the molten silver out of the 123 matrix, while the bulk of the material remains homogeneously mixed. The superconducting material was found to retain its high temperature superconducting properties with a critical temperature of 93 K and with a rejection of external magnetic fields of 63%. Additionally, it has been found that single crystals of 211 phase (nonsuperconducting) material can be grown from the melt in the presence of low gravitational forces as is illustrated in the 15-power photomicrograph of Figure 3c. It is believed that by employing appropriate mixes of starting materials, single crystals of superconducting 123 material can be grown.

The foregoing description of three preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, other uniform composite materials have been prepared according to the teachings of the method of our invention, such as homogeneous palladium/123 superconductor composite materials. Moreover, bismuth germanate which normally exists in a polycrystalline form can be processed in the sheet float zone furnace at low gravity to yield a ceramic glass structure. Additionally, melt texturing of pure 123 materials in low-gravity conditions may increase crystal alignment, thereby increasing conductivity. The absense of wall effects permits the fabrication of bismuth superconductors which are free from the contamination found to occur using other procedures. In fact, the process of the the subject invention can be applied to any multiphase system of normally immiscible materials to generate homogeneous composites. The embodiments were chosen and described in order to best explain the prinicples of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

WHAT WE CLAIM IS:

1. An apparatus for containerless, directional thermal processing of materials in a low-gravity environment, said apparatus comprising in combination: means for holding self-supporting material to be processed in the vicinity of the edges thereof; first heating means for providing heat to a volume of the material; second heating means for providing heat to a volume of the material, said first heating means and said second heating means being disposed on opposite sides of the material; and means for moving said first heating means and said second heating means in opposite directions such that localized portions of the material are sequentially heated to processing temperatures substantially in the direction of motion of said first heating means and said second heating means, the heated portions being permitted to cool by thermal conduction and radiation to below processing temperatures after the passage thereover of said first heating means and said second heating means.

2. The apparatus as described in Claim 1, wherein said first heating means and said second heating means initially heat the same volume of the material.

3. The apparatus as described in Claim 1, wherein said first heating means and said second heating means include line heaters for heating the material substantially along a narrow strip thereof.

4. The apparatus as described in Claim 1, further comprising auxiliary heating means for providing heat to the material in order to keep the material close to, but below processing temperatures such that said first heating means and said second heating means can more rapidly bring the material to processing temperatures, and temperature measurement means for determining the surface temperature of the material.

5. The apparatus as described in Claim 1, wherein said material is generally flat and thin and wherein said first heating means and said second heating means.are disposed on opposite sides of the thin dimension of said material.

6. An apparatus for containerless, directional thermal processing materials in a low-gravity environment, said apparatus comprising in combination: means for holding a self-supporting material to be processed in the vicinity of the edges thereof; first heating means for providing heat to the material, said first heating means comprising two first movable heaters spaced-apart and located in the proximity of a first side of the material and first means for translating each of said first movable heaters relative to one another along a first line substantially parallel to the first flat side of the material; and second heating means for providing heat to the material, said second heating means comprising two second movable heaters spaced-apart and located in the proximity of the opposite side of the material from said first heating means and second means for translating each of said second movable heaters relative to one another along a second line parallel to the first line, such that localized portions of the material are sequentially heated to processing temperatures along a line therein substantially parallel to the first line and the second line, the heated portions being permitted to cool by thermal conduction and radiation to below processing temperatures after the passage thereover of said first heating means and said second heating means.

7. The apparatus as described in Claim 6, wherein said first heating means and said second heating means initially heat the same volume of the material.

8. The apparatus as described in Claim 6, wherein said first movable heaters and said second movable heaters include line heaters for heating the material substantially along a narrow strip thereof substantially perpendicular to the first line and to the second line.

9. The apparatus as described in Claim 6, further comprising auxiliary heating means for providing heat to the material in order to keep the material close to, but below processing temperatures such that said first heating means and said second heating means can more rapidly bring the material to processing temperatures, and temperature measurement means for determining the surface temperature of the material.

10. The apparatus as described in Claim 6, wherein each of said first movable heaters in said first heating means is moved in a coordinated manner with one of said second movable heaters in said second heating means by said first translating means and said second translating means.

11. The apparatus as described in Claim 6, wherein said material is generally flat and thin and wherein said first heating means and said second heating means are disposed on opposite sides of the flat dimension of said material.

12. A method for preparation of homogeneous composites from multiphase component materials, said method comprising the steps of preparing a self-supporting sintered mixture of said component materials, reducing the gravitational forces on the sintered mixture, heating the sintered mixture in the reduced gravitational forces to above the melting temperature thereof, and permitting the melted- mixture to cool .

13. The method as described in Claim 12, wherein said step of reducing the gravitational forces on the sintered mixture includes flying the sintered mixture in an airplane executing parabolic maneuvers.

14. The method as described in Claim 12, wherein said multiphase component materials include silver and a superconducting material.

15. The method as described in Claim 12, wherein said self-supporting sintered mixture of said component materials is a generally-flat, thin mixture.