Abstract:
A device for separating high mass particles (M H ) and low mass particles (M L ) from each other includes a laser source for vaporizing a solid target material that contains M H  and M L . The resultant vapor jet is directed along an axis and an injector directs a gas flow along a path through the vapor jet perpendicular to the axis of the vapor jet. This entrains M L  in the gas flow to thereby separate M L  from M H . Collectors are respectively positioned on the axis for collecting M H  from the vapor jet, and on the path for collecting M L  from the gas flow.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains generally to methods and systems for separating the constituents of a composite material from each other. More particularly, the present invention pertains to separating vapor particles from each other according to the respective mass of the particles. The present invention is particularly, but not exclusively, useful as a winnowing process for separating particles in a vapor from each other according to the respective mass of the particles.  
       BACKGROUND OF THE INVENTION  
       [0002]     In a typical winnowing process, a flow of air (i.e. gas) is passed through a stream of particles. Specifically, the purpose of such a process is to separate different types of particles in the stream from each other. In particular, due to the fact that the energy loss of a particle in a winnowing process depends on its mass, the end result is the separation of relatively heavy particles from relatively lighter particles. Stated differently, as particles, atoms or molecules in a stream interact with a gas flow, the heavier particles will lose relatively little energy. On the other hand, the lighter particles will lose all of their directed energy, and will become entrained in the gas flow. As specifically recognized by the present invention, this process can be applied to vaporized materials that include particles of relatively high mass (M H ), and particles of relatively low mass (M L ).  
         [0003]     It is known that when a material is vaporized, all particles of the vapor will depart from the evaporating surface of the material at a substantially same constant velocity. This happens, regardless of the mass of the vaporized particles. In the particular case wherein a laser beam is focused onto a material to vaporize the material, the resulting vapor will expand outwardly in a cone with an angle, θ. The particular methodology for using a laser beam to vaporize a material will depend, in large part, on the nature of the material.  
         [0004]     For example, U.S. application Ser. No. 11/109,137 for an invention entitled “System and Method for Vaporizing a Metal”, and U.S. application Ser. No. 11/131,961 for an invention entitled “System and Method for Vaporizing a Solid Material” respectively disclose systems and methods for using laser systems to vaporize metals and ceramics. Both of these applications are assigned to the same assignee as the present invention, and both are incorporated herein, in their entirety, by reference. In any event, after the vapor has been created, particles in the vapor will behave in a predictable, albeit statistical, manner.  
         [0005]     The efficacy of a winnowing process will, of course, depend on conditions in the vapor as well as the gas flow. Specifically, such a process is most efficacious when, as between each other, the vapor particles are essentially collision-free. On the other hand, the winnowing process must rely on the relative energy losses that occur with collisions between particles in the vapor and gas atoms that are introduced as a gas flow into the vapor. Collisions with the gas atoms will then cause the lighter vapor particles (M L ) in the vapor to lose their directed energy faster than the heavier vapor particles (M H ) that have relatively little loss of energy. Consequently, the lighter vapor particles, M L , will become entrained in the gas flow and will be literally “swept away” by the gas flow. Meanwhile, the heavier vapor particles (M H ) that have relatively little loss of energy will be relatively unaffected. This separates M L from M H .  
         [0006]     With the above in mind, consider the laser vaporization of a composite material that includes particles M H  and M L . Further, consider that vaporization of the material occurs from an evaporating surface having a radius “r 0 ”, and that “n 0 ,” is the density of vapor particles at the evaporating surface. In this case, the density of particles (n) at a radius (r), where r&gt;r 0 , can be expressed as:
 
 n=n   0   r   0   2   /r   2 
 
 As implied above, it is also to be taken that the velocity of all vapor particles at the radius “r” will still be substantially the same as they were at the evaporating surface “r 0 ” (e.g. v 1 ). Under these conditions, the mean collision free path (λ) among the vapor particles is expressed as:
 
λ=[σ n]−   1 
 
         [0007]     In the above expression, σ is the collision cross section of the vapor particles. Thus, when “r” is comparable to “λ” (i.e. r λ ), the vapor particles will no longer collide with each other. Stated differently, when the vapor particles have traveled to the distance “r λ ”, and beyond, they are, thereafter, essentially collision-free. Mathematically, this can be stated as occurring when:
 
λ≅ r   λ   =σn   0   r   0   2 
 
 wherein σ is a collision cross section of the vapor, no is the density of the vapor at the evaporation surface, and r 0  is a radius of the evaporation surface. 
 
         [0008]     Now consider the circumstance wherein a gas having gas atoms of mass “m” is introduced into the vapor. Here, m&lt;&lt;M (L or H) . Also, consider that, in this circumstance, the collision frequency between gas atoms and vapor particles is “ν”. Then, if “N” is the average number of collisions that are required for gas atoms to stop a vapor particle (e.g. for vapor particles M L : N L =M L /m), it can be shown that the average range &lt;R&gt;of a vapor particle as it travels in the gas is:
 
 &lt;R&gt;=[V   1   /ν]N 
 
 The consequence here is that heavy mass particles (M H ) can be separated from the lighter mass particles (M L ) if the minimum range of the heavy mass particles (R H,min ) exceeds the maximum range of the lighter mass particles (R L,max ):
 
 R   H,min   &gt;R   L,max 
 
 Statistically, it can be shown that when a vapor is directed along an axis in a “z” direction, and a gas flow is directed substantially perpendicular to the axis in an “x-y” plane, the gas flow will cause particles in the vapor to lose energy. Specifically, this occurs between z 0 =λ=σn 0 r 0   2  (i.e. the distance from the source where vapor particles become collisionless), and z=h (where h is effectively equal to R L,max ). In this case, the lighter vapor particles of mass M L  will not travel the distance “h” when:
 
 V   1 τ[1 +N   L   −1/2 ]cosθ &lt;h−z   0 
 
 wherein τ=N L /νand θ is the cone angle of the vapor jet, and wherein v 1  is an entry velocity of M H and M L into the gas flow, τ is the time for the gas flow to stop M L , N L  is the number of collisions between M L  and atoms in the gas flow required to stop M L , and ν is the collision frequency. 
 
         [0009]     In light of the above, it is an object of the present invention to provide a system and method for winnowing a vapor to separate vapor particles in the vapor from each other according to their respective masses. Another object of the present invention is to provide a system and method for separating high mass particles (M H ) and low mass particles (M L ) from each other which is relatively easy to use and comparatively cost effective.  
       SUMMARY OF THE INVENTION  
       [0010]     In accordance with the present invention, a system and method for employing a winnowing process to separate high mass particles (M H ) and low mass particles (M L ) from each other requires the vaporization of a solid target material containing both M H  and M L . Preferably, a laser system is used for this purpose. As contemplated for the present invention, the solid target material is vaporized at an evaporation surface to create a vapor jet. In this case, the evaporation surface is considered to be curved and have a radius “r 0 ”. The vapor jet that is thereby created will have a conical shape, wherein θ is the angular spread, and it will be directed substantially along an axis.  
         [0011]     The present invention also includes an injector for directing a gas flow (e.g. hydrogen or helium) along a path through the vapor jet. Importantly, the path of this gas flow will be substantially perpendicular to the axis of the vapor jet. Further, the gas flow needs to intersect the vapor jet on the axis, beyond a predetermined distance “z” from the evaporation surface. Specifically, the gas flow needs to intersect the vapor jet at distances from the evaporation surface that are beyond the mean collision free paths (λ) of the vapor particles. As discussed above, this circumstance occurs beyond a distance from the evaporating surface where θ is comparable to “r” (i.e. at a mean collision free distance “r θ ”). Thus, the gas flow intersects the vapor jet when z is greater than “r θ ”. In this instance, r θ is determined by the expression r θ =σn 0 r 0   2 ; wherein σ is a collision cross section of the vapor, and n 0  is the density of the vapor at the evaporation surface.  
         [0012]     As the gas flow interacts with the vapor jet, and gas atoms collide with vapor particles, the particles M L  will be separated from the particles M H  when their respective ranges of travel (R) through the gas flow are such that R H,min &gt;R L,max . With R H,min  taken to be at a distance “h” from the evaporating surface, it can be mathematically shown that the particles M L  will become entrained in the gas flow, and that separation of M L  from M H  will therefore occur when:
 
 V   1 τ[1 +N   L   −1/2 ]cos θ&lt;h−z   0  with τ=N L /ν
 
 In the above expression, v 1  is the entry velocity of M H  and M L  into the gas flow, τ is the time for the gas flow to stop the particles M L , N L  is the number of collisions with atoms in the gas flow that are required to stop the particles M L , and ν is the collision frequency. 
 
         [0013]     During operation of the system of the present invention a first collector is positioned on the axis at the axial distance “h” from the evaporation surface to collect heavy particles M H  from the vapor jet. Additionally, a second collector is located on the path of the gas flow for collecting M L  from the gas flow. Preferably, the target material will be constituted such that M H  /M L &gt;1.5. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawing, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0015]     The Figure is a schematic representation, not to scale, of a system in accordance with the present invention, showing the path of vapor particles and gas atoms in a typical operation.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     Referring to the Figure, a device in accordance with the present invention for separating high mass particles (M H ) and low mass particles (M L ) from each other is shown, and is generally designated  10 . In overview, the device  10  includes a target material  12  that is to be vaporized, and a laser system  14  for vaporizing the target material  12 . Also included is a gas injector  16  for winnowing the vaporized target material  12 , a heavy collector  18  for collecting high mass particles (M H ) from the vapor, and a light collector  20  for collecting low mass particles (M L ) from the vapor. For purposes of the present invention, the target material  12  may be either metallic or ceramic, and will include both high mass particles (M H ) and low mass particles (M L ).  
         [0017]     As intended for the device  10  of the present invention, when activated, the laser system  14  generates a laser beam  22  that is directed toward, and is focused onto, the target material  12 . The result of this is the creation of a molten portion of the target material  12  that has a generally spherical shaped evaporation surface  24  of radius “r 0 ”. With the evaporation of the target material  12 , a vapor jet  26  is formed that includes the particles  28  having a relatively high mass (M H ), and the particles  30  having a relatively low mass (M L ). Preferably, the relationship between the particles  28  and the particles  30  will be such that M H  /M L &gt;1.5. In any event, the vapor jet  26  will be directed from the target material  12  in a generalized “z” direction along the axis  32 , toward the heavy collector  18 . As so directed, the vapor jet  26  will diverge from the axis  32  within a vapor jet angle “θ”.  
         [0018]     The Figure also shows that a gas flow  34  is to be supplied by the injector  16  using a source of gas, such as a gas bottle  36 . Preferably, the gas in gas flow  34  is either Helium or Hydrogen (note: Helium is to be used when Hydrogen will cause significant chemical reactions with the vapor  26 ). Further, as shown, the injector  16  will include an impeller, such as a fan  38 , that will direct the gas flow  34  in a direction indicated by the arrow  40 . Specifically, the direction that gas flow  34  is directed from the injector  16  should be substantially perpendicular to the axis  32  (i.e. perpendicular to the vapor jet  26 ).  
         [0019]     In the operation of the device  10 , the vapor jet  26  is created by focusing the laser beam  22  onto the target material  12 . The resultant vapor jet  26 , containing M L  and M H , is then directed along the axis  32  toward the heavy collector  18 . Simultaneously, the gas flow  34  is directed (arrow  40 ) toward the light collector  20 . In combination, the orientation of the gas flow  34  relative to the vapor jet  26 , the distant “r λ ” of the gas flow  34  from the target material  12 , and the distance “h” of the heavy collector  18  from the target material  12  are important to the operation of the device  10 .  
         [0020]     As shown in the Figure, the heavy collector  18  is placed on the axis  32  at a distance “h” from the evaporation surface  24  of the target material  12 . Importantly, the distance “h” is determined by the inability of the particles  30  (M L ) to continue travel along the axis  32 , when limited by the influence of the gas flow  34  from injector  16 . Simply stated, because collisions with gas atoms in the gas flow  34  will cause particles  30  (M L ) to loose energy faster than the particles  28  (M H ), the particles  30  (M L ) will be swept from the vapor jet  26  by the gas flow  34  before they have traveled the distance “h” along axis  32 . The particles  30  (M L ) will then come into contact with the light collector  20 . Due to their heavier mass, however, the particles  28  (M H ) will continue in a generally axial direction until they come into contact with the heavy collector  18 .  
         [0021]     It is also important in the operation of the device  10  that the gas flow  34  be, at least, at a distance “r θ ” from the evaporation surface  24  of the target material  12 . Specifically, the gas flow  34  must be more than a mean collision free distance “r ν ” from the evaporation surface  24 . In this case, rν =σn 0 r 0   2 , wherein σ is a collision cross section of the vapor  26 , n 0  is the density of the vapor  26  at the evaporation surface  24 , and r 0  is a radius of the evaporation surface  24 . The consequence here is that the winnowing process needs to take place when particles  28  (M H ) and particles  30  (M L ) no longer collide with each other in the vapor  26  (i.e. at a distance greater than “r λ ”). On the other hand, once they are beyond the distance “r θ ” from the evaporation surface  24 , the particles  28  (M H ) and particles  30  (M L ) will collide with gas atoms in the gas flow  34 , and thereby be separated from each other.  
         [0022]     As envisioned for the present invention, collection of the particles  28  (M H ) on the heavy collector  18 , and collection of the particles  30  (M L ) on the light collector  20 , happens under determinable conditions. Specifically, these conditions will exist when the heavy collector  18  is positioned on the axis  32  at the axial distance “h” from the evaporation surface  24 , and h−r λ  satisfies the condition:
 
 V   1 τ[1 +N   L   8 −1/2 ]cosθ&lt; h−r   ν : with τ=N   L /ν
 
 In the above expression, v 1  is an entry velocity of the particles  28  (M H ) and the particles  30  (M L ) as they enter the gas flow  34 . “τ” is the time for the gas flow  34  to stop the particles  30  (M L ) from further travel in an axial direction, when N L  is the number of collisions between the particles  30  (M L ) and atoms in the gas flow  34  that are required to stop further axial travel of the particles  30  (M L ). Also, in this expression “ν” is the collision frequency between atoms in the gas flow  34  and the particles  30  (M L ) and “θ” is the angular spread of the vapor jet  26 . 
 
         [0023]     While the particular Chafftron as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.