Abstract:
Apparatus and method for continuous melt spin casting of homogenous materials, including metals, alloys, and non metals. The material to be cast is provided to a crucible and liquefied. The melt is filtered prior to casting to provide a melt with a high degree of purity. The material temperature is maintained throughout the process to form a solidified product with excellent homogeneity.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method and apparatus for melt spin casting of materials, and more specifically, a continuous, economical method and apparatus for melt spin casting of homogenous materials.  
         BACKGROUND OF THE INVENTION  
         [0002]    A number of techniques are known for the production of homogeneous materials. In the field of producing powdered materials, for example those materials used as electrode material for rechargeable electrochemical cells, property requirements include size, homogenous chemistry, and homogenous crystalline structure. In the field of battery production, a hydrogen storage alloy is commonly formed as a bulk ingot from a melt. One method of producing a hydrogen storage alloy is disclosed in commonly assigned U.S. Pat. No. 4,948,423 to Fetcenko, Sumner, and LaRocca for ALLOY PREPARATION OF HYDROGEN STORAGE MATERIALS, incorporated herein by reference. However, an inherent concern with manufacturing materials as an ingot where homogenous material properties are required is the rate of change of cooling through the ingot. As the ingot solidifies, the cooling rate of the interior material is much less than that of the exterior, resulting in a variation of the material crystalline structure.  
           [0003]    Hydrogen storage negative electrodes utilizing the aforementioned alloys are of relatively high hardness. Indeed, these alloys can typically exhibit Rockwell “C” (“Rc”) hardness of 45 to 60 or more. Moreover, in order to attain the high surface areas per unit volume and per unit mass necessary for high capacity electrochemical performance, the alloy must be in the form of fine particles. In a preferred exemplification, the hydrogen storage alloy powder must pass through a 200 U.S. mesh screen, thus being smaller than 75 microns in size (200 U.S. mesh screen has interstices of about 75 microns). Therefore, the resulting hydrogen storage alloy material is comminuted, e.g., crushed, ground, milled or the like, before the hydrogen storage material is fabricated into electrode form.  
           [0004]    Comminution of bulk ingots of hydrogen storage alloy material is made more difficult because the materials described hereinabove are quite hard, and therefore do not easily fracture into particles of uniform size and shape. In commonly assigned U.S. Pat. No. 4,893,756 to Fetcenko, Kaatz, Sumner, and LaRocca for HYDRIDE REACTOR APPARATUS FOR HYDROGEN COMMINUTION OF METAL HYDRIDE HYDROGEN STORAGE MATERIAL, the disclosure of which is incorporated herein by reference, a hydride-dehydride cycle comminution process was disclosed for initial size reduction of bulk ingots of hydrogen storage alloy material to flakes of about 80-100 mesh size. While this process is effective for the initial size reduction of hydrogen storage alloy, it is inadequate for the task of further comminuting particulate hydrogen storage alloy powder to the required particle size of 75 microns or less (i.e. 200 mesh or less). Furthermore, the process begins an ingot which by default, is subject to the inherent thermal limitations of materials produced in ingot form as described above. Still another concern with initiating a powder formation process with material in the form of an ingot is the size of the ingot with respect to the final size of the powder material. Initiating a powder formation process with a large piece of material in order to form a powder is hardly efficient.  
           [0005]    Any method which can accomplish the objective of providing economical size reduction of the metal hydride material is a potential candidate for commercial processing. However, there are numerous characteristics of the material which require special handling, instrumentation and other precautions. These characteristics include: (1) inherent alloy powder hardness, i.e., approximately Rockwell “C” (“Rc”) 60 hardness. This means that conventional size reduction processes of shear, abrasion and some types of impact mechanisms as ball mills, hammer mills, shredders, fluid energy, and disk attrition, are not very effective for material in the form of an ingot; (2) sensitivity to oxidation, such that comminution must be done under an inert environment to provide a safe environment and maintain acceptable electrochemical performance; (3) requirement of a specific crystalline structure necessary for electrochemical activity; i.e., the microstructure of the material cannot be adversely altered during grinding or atomization to produce powders directly from a melt; and (4) requirement of a broad particle size distribution with a maximum size of 75 microns (200 mesh) which provides optimum packing density and electrochemical accessibility.  
           [0006]    Early attempts to provide a method for size reduction of hydrogen storage alloy materials from ingots proved inadequate due to the extreme hardness of the hydrogen storage alloy materials. Conventional size reduction processes employing devices such as jaw crushers, mechanical attritors, ball mills, and fluid energy mills consistently fail to economically reduce the size of such hydrogen storage materials. Grinding and crushing processes have also proven inadequate for initial reduction of ingots of hydrogen storage alloy material to intermediate sized (i.e. 10-100 mesh) particulate.  
           [0007]    There are numerous methods for preparing metal powders. Since the alloys under consideration are at one stage molten, one might consider ultrasonic agitation or centrifugal atomization of the liquid stream to prepare powders directly. The cost and the product yield are the two main concerns with using this approach. The particle shape is also not optimal. Finally, because it is difficult to provide a completely inert atmosphere; surface layers, which are undesirable from an electrochemical perspective, may be formed on the particulate.  
           [0008]    Attempts to embrittle the hydrogen storage alloy material by methods such as immersion in liquid nitrogen, so as to facilitate size reduction are inadequate because: (1) the materials are not sufficiently embrittled; (2) the methods typically introduce embrittlement agents in the alloys; which have an undesirable effect upon the electrochemical properties of the hydrogen storage alloy material; and (3) as the materials become more brittle, it becomes increasingly difficult to obtain uniform particle size distribution. Other methods for embrittling metals are disclosed, for example, in Canadian patent No. 533,208 to Brown. Brown employs cathodic charging as a size reduction technique.  
           [0009]    Furthermore, material size reduction by an hydrogenation processes is not desirable because of the inherent dangers associated with hydrogen gas. Therefore it is desirous to employ a technique that minimizes subsequent size reduction processes while providing homogeneous material properties.  
           [0010]    One alternative for preparing materials for powder formation is rapid solidification. Rapid solidification refers to a technique for rapidly quenching material from a liquid, or molten, state into a solid state at a rate sufficient to freeze the position of atoms. One rapid solidification technique is a spin cast technique where the molten material into is formed into ribbons. Spin casting is a method of dispersing molten material on a rotating wheel, also commonly known as melt spin casting. The rotating wheel, made of a highly conductive metal, typically copper, is positioned proximal to a reservoir of molten material. The reservoir typically has an orifice or nozzle to direct molten material onto the rotating wheel. The molten material is rapidly solidified because of the mass of the wheel and the significant difference in temperature between the wheel and the molten material. The wheel need not be cooled in order to provide a sufficiently cold moving surface relative to the molten material, however, the wheel may be cooled to achieve higher quench rate if so desired. A wheel is typically between 6″ and 10″ (15.24 and 25.40 centimeters) in diameter and is rotated at a rotational velocity of between 1,000 and 5,000 rpm thereby to obtain a linear velocity, at the point of contact at the material with the cylindrical periphery of the wheel, of 32.81 to 65.62 feet per second (1,000-2,000 centimeters per second).  
           [0011]    Materials produced by melt spin casting techniques of the prior art exhibit material property variations due to a number of factors. Variations such as flow rate, stream diameter and material temperature, and compositional variations including chemistry and impurities within the melt stream contribute to inhomogeneity of the solidified material. Changes in the diameter and flow rate of the molten material stream result in different cooling rates of the material. As explained above, different cooling rates will effect the material&#39;s crystalline structure. Inhomogeneity is the main drawback of the melt spin processes of the prior art. Increased homogeneity requirements of materials make an improved technique especially important.  
           [0012]    Melt spin casting is an attractive method for producing ribbons of material because of the lack of complexity involved. This technique may be employed to form ribbons of material such as metals, metal alloys, or thermoplastics. One method of producing materials by melt spin casting is disclosed in commonly assigned U.S. Pat. No. 4,637,967 to Keem et al for ELECTRODES MADE WITH DISORDERED ACTIVE MATERIAL AND METHODS OF MAKING THE SAME, the disclosure of which is incorporated herein by reference. The &#39;967 patent discloses a heated crucible that is equipped with means for pressurizing the crucible to extrude molten material through a nozzle onto the surface of a chill wheel. Although this method is very good for forming homogeneous materials, the process efficiency is reduced because the crucible must be pressurized to extrude the material. The flow rate is also inconsistent as the material level in the crucible changes, the flow rate of the material changes. This process is not continuous, therefore an hiatus in the production of ribbon material is inevitable, resulting in a reduced capacity.  
           [0013]    Another method and apparatus for spin melt casting of materials is commonly assigned U.S. Pat. No. 4,339,255 to Ovshinsky et al for METHOD AND APPARATUS FOR MAKING A MODIFIED AMORPHOUS GLASS MATERIAL, the disclosure of which is incorporated by reference. Although the teachings of the &#39;255 patent disclose the advantages of spin melt casting over thin film processes for making amorphous materials, the process has inherent limitations. A piston is provided to cause material within a crucible to be ejected onto a rotating wheel. However, once all of the material is driven from the crucible, the process must be halted and the crucible refilled. This method is also susceptible to temperature and chemistry variations because he process must be stopped and started.  
           [0014]    In order to improve process efficiency, a hydrostatic system may be employed to provide the necessary force to extrude the molten material from the crucible. Such a device is disclosed in U.S. Pat. No. 4,485,839 to Ward for RAPIDLY CAST ALLOY STRIP HAVING DISSIMILAR PORTIONS. The &#39;839 patent discloses a planar flow casting technique for drawing thin ribbons. Although this technique includes a hydrostatic system for delivering the molten material, there are shortcomings associated with this invention. The device disclosed is not capable of providing a true, continuous melt casting operation. Furthermore, the disclosed operation relies on heating a crucible to prevent the nozzles from clogging, which is ineffective since it is commonly known that slags and other impurities are present within the crucible. Also, because of the tolerances associated with this device, less than 0.120″ (3.05 mm), the surface of the chill wheel must be constantly maintained.  
           [0015]    Accordingly, there exists a need for an economical, spin melt casting process for producing homogeneous materials.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention disclosed herein is an apparatus for continuous melt spin casting of materials. The apparatus comprises a supply crucible and a casting crucible disposed a chamber. The supply crucible provides molten material to the casting crucible for ejecting at least two streams of molten material upon a chill wheel for solidification. Each stream is ejected through one of at least two orifices in the casting crucible. Monitoring means determine the level of molten material in the casting crucible.  
           [0017]    The apparatus of the present invention includes a selectively actuated flow control valve for controlling the flow of molten material from the supply crucible to the casting crucible to maintain the level of molten material in the casting crucible within a range. Valve control means actuate the flow control valve as a function of material level in the casting crucible as; sensed by the monitoring means. The present invention includes means for filtering the molten material prior to casting and thermal control means for controlling the temperature of material within said casting crucible.  
           [0018]    Each orifice is at an equal distance from a point of contact on the chill wheel. Equal length streams of material at a uniform temperature and flow rate are rapidly solidified form homogenous ribbons by the chill wheel.  
           [0019]    Means for removing accumulated material from an external surface of said orifices is also disclosed.  
           [0020]    Also disclosed herein is a method for continuous melt spin casting of materials. The novel method comprises the steps of providing a material supply crucible and a casting crucible disposed within a chamber, where the material supply crucible is provided to receive molten material to be supplied to the casting crucible; monitoring the level of molten material in the casting crucible; releasing molten material from the supply crucible through a flow control valve; filtering the molten material released from the material supply crucible prior to solidification; maintaining the molten material level in the casting crucible, whereby the molten material flow rate through the orifices is controlled by the static pressure of the molten material in the casting crucible to provide a constant material flow rate; maintaining the temperature of the molten material in the casting crucible to provide material for casting at a consistent temperature; ejecting at least two streams of molten material from the casting crucible, each stream ejected through one of at least two orifices in the casting crucible; and rapidly solidifying the molten material by ejecting equal length and diameter streams of molten material upon the chill wheel.  
           [0021]    At least two nozzles may be provided, each nozzle in communication with one of at least two orifices. accumulated material deposits are removed from the orifices or nozzles by means including a shearing device, the chill wheel or a diamond wheel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a cross sectional view of one embodiment of the present invention taken through a chamber to reveal the operative elements therein.  
         [0023]    [0023]FIG. 2 is a is a cross sectional view showing one operating position of one embodiment of a casting crucible in relation to a chill wheel.  
         [0024]    [0024]FIG. 3 is a sectional view of the casting crucible and chill wheel taken along section A-A of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The present invention is directed toward an apparatus and method for forming ribbons of material by melt spin casting. Referring now to FIG. 1, the apparatus  10  of the present invention includes a chamber  20  containing a supply crucible  30  and casting crucible  40 . The supply crucible  30  is provided to receive molten material and has a selectively actuated flow control valve  70  for releasing molten material into the casting crucible  40 . Referring now also to FIG. 2, the casting crucible  40  has at least two orifices  50 , each orifice  50  for ejecting a stream of molten material upon a chill wheel  110  having a horizontal axis of rotation.  
         [0026]    A loading vessel  130  in communication with the chamber  20  provides material to the supply crucible  30  without exposing the materials within the chamber  20  to contaminants by incorporating a flap valve  135 . It should be noted that although a flap valve is used to seal the chamber  20 , any suitable means known in the art may be substituted for a flap valve.  
         [0027]    The apparatus  10  may also include at least two nozzles (not shown), each nozzle in communication with one of the orifices  50  in the casting crucible  40 . The flow control valve  70  is selectively actuated by valve control means  80  to provide the molten material to the casting crucible  40 . The valve control means  80  may be manually operated or automatically controlled by a controller (not shown). The valve control means  80  are actuated as a function of the material level in the casting crucible  40 .  
         [0028]    Molten material is ejected from the casting crucible  40  onto the chill wheel  110 . The molten material level within the casting crucible  40  provides hydrostatic pressure at the orifices  50  to eject molten material upon the chill wheel  110 . The material level in the casting crucible  40  is maintained in order to maintain a uniform flow rate. The chill wheel  110  is preferably formed of a material having a high thermal conductivity such as copper. The temperature of the chill wheel  110  may be controlled by any suitable cooling means (not shown) known in the art, including a cooling medium such as a water and ethylene glycol mixture. In the preferred embodiment, the chill wheel  110  has a passage to allow the cooling medium to pass through and draw heat away from the casting wheel  110 .  
         [0029]    The supply crucible  30  is heated by thermal control means  100 ; in the preferred embodiment, the thermal control means is an induction heater  180 . The molten material within the supply crucible  30  may be mixed by any suitable means known in the art to maintain homogeneity. The casting crucible  40  is heated by thermal controls means  105 , and in the exemplary embodiment, the thermal control means is an induction heater  190  as well. Likewise, the casting crucible  40  may be stirred by any suitable means known in the art.  
         [0030]    Molten material, raw materials, or ingot material may be provided to the supply crucible  30  by the loading vessel  130 . The atmosphere in the chamber  20  may consist of an insert gas or may be pumped down to a vacuum to prevent contamination. Once the supply crucible has received the materials for casting, heat is provided by the induction heater  180  to maintain viscosity. The material within the supply crucible  30  is mixed to maintain homogeneity. By lowering the frequency of the induction heater  180 , the material may be electrodynamically mixed. In the exemplary embodiment, the induction heaters  180  and  190  are reduced below 1000 Hz, resulting in excellent mixing results. It should be noted that other mixing operations may be substituted for electrodynamic mixing, such as agitation.  
         [0031]    Referring now also to FIG. 2, the chill wheel  110  is shown in one of many potential locations. The chill wheel  110  is movable in the X, Y, and Z-axis, providing many advantages to the present invention. By adjusting the position of the chill wheel  110  along the Z-axis, the length of the streams is changed, changing the exposure time to ambient conditions and ultimately, temperature. Therefore, fine temperature adjustments of the molten material prior to contacting the chill wheel  110  may be made by adjusting the position of the chill wheel  110  along the Z-axis. The form of the ribbons produced by apparatus  10  of the present invention may be altered by moving the chill wheel  110  along the X-axis, which will change the angle of incidence of the material streams on the chill wheel  110 .  
         [0032]    Referring now also to FIG. 3, the chill wheel  110  may be positioned by any combination of orthogonal coordinate changes and then translated along the Y-axis to remove material which has accumulated upon the orifices  50  by shearing the accumulated material with the chill wheel  110 . Furthermore, the orifices  50  or nozzles (not shown) may be heated to reduce material accumulation.  
         [0033]    A thermal screen  220  may also be disposed below the supply crucible  40  to stabilize the flow rate of the molten material exiting the supply crucible  40 . The thermal screen  220  is heated whereby the material temperature is preserved as the material exits the supply crucible  40 . Means for filtering the molten material prior to casting are provided. Referring again to FIG. 3, the means for filtering may be a filter element  230 , such as a ceramic filter capable of high temperature filtering of molten materials for the removal of slags, oxides or other impurities. A common occurrence experienced when melting is pieces of the crucible break away due to thermal cycling, and become inclusions in the melt.  
         [0034]    The temperature of the material within the casting crucible  40  is maintained by the induction heater  190  and stirred to maintain homogeneity. Monitoring means  60 , such as a sight glass for measuring height or a balance for determining material mass, is provided to evaluate the material level in the casting crucible  40 . The flow rate of the material from the casting crucible  40  is governed by hydrostatic pressure. The material level in the casting crucible  40  will determine the rate material is ejected from the casting crucible  40 . The material level must be maintained within a range in order to provide a uniform flow rate of the molten material stream. Once the material stream is ejected from the casting crucible  40  upon the rotating chill wheel  110 , the material is rapidly solidified and is projected from the chill wheel  110  and the resulting ribbons of material are captured in a material collector  210 .  
         [0035]    The size and the form of the ribbons can be modified by changing the rotational speed and diameter of the chill wheel  110 . By increasing the speed of the chill wheel, thin ribbons are formed and the material dwell time is reduced. In the preferred embodiment, the surface of the casting wheel  40  is polished to provide sufficient mechanical and thermal contact with the melt streams.  
         [0036]    By providing multiple orifices  50 , process throughput is increased. The orifices  50  are positioned at a uniform distance from a contact point of the material stream upon the chill wheel  110 . Flow rate, temperature, material purity, and homogeneity must be simultaneously maintained in order to obtain uniform material properties such as crystallite size and homogeneity of the solidified material. The present invention discloses an economical solution to melt spin casting concerns found in the state of the art. The present invention has improved throughput without a need for a mechanical device to provide additional pressure within the casting crucible  40 , such as a piston. Also, an uninterrupted stream of material with excellent homogeneity and purity for rapid solidification upon the chill wheel  110  is provided.  
       EXAMPLE  
       [0037]    In the present example, the chamber  20  is hermetically sealed and operated in a vacuum to prevent oxidizing of the melt, thereby achieving higher purity. An ingot of negative electrode material for a rechargeable electrochemical storage cell is provided to the supply crucible  30  within chamber  20  by the loading vessel  130  and heated to 1550° C. After melting, the melt it is mixed electrodynamically to increase homogeneity.  
         [0038]    Once liquefied, thermal control means  100  sustain the material temperature until the material is released into casting crucible  40 . The molten material within the supply crucible  30  is electrodynamically mixed. By lowering the frequency of the induction heater to below 1000 Hz frequency, more efficient mixing is achieved.  
         [0039]    The molten material in the supply crucible  30  is released to the casting crucible  40  by actuating the valve control means  80  to open the flow control valve  70 . The material released by the control valve  70  passes through the thermal screen  220  where the material temperature is stabilized to avoid cooling. The material then passes through the filter element  140  and into the casting crucible  40 . The material temperature in the casting crucible  40  is stabilized by an induction heater  190 . When the level of the material in the casting crucible  40  rises to about 200 mm, the valve control means  80  close the supply crucible flow control valve  70 .  
         [0040]    The material level in the casting crucible  40  is evaluated by a sight glass disposed within chamber  20  to assure a material height of 200 mm is maintained. In the present example, ten streams of molten material, each stream formed by one of ten calibrated orifices  50 , are ejected onto the rotating chill wheel  110 . Each orifice  50  being uniform in diameter and equidistant from the chill wheel  110 , forms a melt stream that is equal in length and diameter. In this example, the orifices are disposed about 150 mm from the contact point on chill wheel  110 .  
         [0041]    In the present example, a cooling medium is flowed through a passage in the chill wheel  110 . By cooling the chill wheel  110 , ribbons are formed having a constant width and thickness. The ribbons produced by this technique exhibit high homogeneity of properties and uniform crystallite size while increasing the productivity of the process.  
         [0042]    The temperature of the casting crucible is regulated to about 1500° C. and the level of the melt bath to about 200 mm. The casting crucible  40  flow rate is between about 0.15 to 0.32 L/min (1 to 2.5 kg/min). The ten streams have an equal length, less than about 150 mm in the present example, and a diameter between about 1.0 to 2.5 mm. The chill wheel  110  rotates with a linear speed of between about 5 to 25 m/sec.  
         [0043]    The apparatus and method of the present can be used to produce threads, films, ribbons and any variant thereof. Furthermore, although metallic materials and alloys have been specifically referenced, it should become apparent to those skilled in the art that a variety of material, including non metallic materials, such as plastics, may be formed by employing the teachings set forth herein.  
         [0044]    While the invention has been described in connection with preferred embodiments and procedures, it should be understood that it is not intended to limit the invention to the described embodiments and procedures. On the contrary, it is intended to cover all alternatives, modifications and equivalents which may be included within the spirit and scope of the claims appended hereto.