Abstract:
This system and method for producing nanomaterials allows for the production of relatively high concentrations of nanoparticles with a minimum of expense, time and energy. Ultrasonic waves, produced at a power of approximately 50 W with a frequency of 26.23 kHz, are projected on a material sample while, simultaneously, a fluid stream jet is projected on the material sample. The ultrasonic waves, in the presence of the fluid jet, create cavities that explode at the surface of the solid material, leading to creation of cracks in the material surface. With the increase in the number of cracks in the material, the solid material erodes. The eroded material, which is on the nanometer scale, is collected on a suitable substrate, such as silicon. This method allows for the preparation of nanoparticles from any solid material, in particular very hard materials, such as diamond, silicon carbide and the like.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to nanoparticles, and particularly to a system and method for producing nanomaterials through an erosion process created by a combination of pressurized fluid and ultrasonic waves focused on a material sample that can be used to form nanoparticles, even from hard materials, such as diamonds. 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 4  illustrates a typical fluid jet polishing system  100 . System  100  includes a part holder  112 , which securely holds a component  113  during the erosion process, within a contained area of an erosion chamber  116 . The part holder  112  can be fixed within the erosion chamber  116 , rotatable relative to the erosion chamber  116  or form part of a moveable platform. Rotating the part holder  112  facilitates the production of annular or arcuate profiles in the component  113 , if desired. 
         [0005]    A nozzle  117  directs a pressurized fluid jet stream of a working fluid  118  at a surface of the component  113 . The working fluid  118  contains a carrier fluid; e.g. water, glycol, oil or other suitable fluids, and small abrasive particles made from harder materials, such as aluminum oxide, diamond and/or zirconium oxide. Varying the type and size of the abrasive particles can be practiced in order to optimize the surface roughness and/or removal rate. The properties of the working fluid  118 , including fluid density, viscosity, pH and rheological properties, can be altered in order to optimize the surface roughness and removal rate. In particular, it is advantageous to have a dilatant fluid in order to increase the removal rate. The viscosity of dilatant fluids increases with increasing shear forces, as compared to normal fluids, in which viscosity is independent of shear forces. Thus, when a fluid jet stream, including a dilatant fluid, impacts on the component  113 , the working fluid  118  experiences high shear forces, and therefore has an increase in viscosity, in particular at an interface between the pressurized stream of working fluid  118  and the surface of the component  113 . 
         [0006]    Abrasive particles that normally have very little effect on the component  113  work much better when a dilatant additive; e.g., corn starch or poly vinyl alcohol, is added. Poly vinyl alcohol is a long chain molecule that can be cross linked to form larger molecules, all with varying degrees of dilatant property. Multiple axis ( 3 ,  4 ,  5  or  6 ) motion systems may be used to process a wide variety of component shapes. A mechanical linkage  120  may also be added to maintain the angle of the nozzle  117  over spherical or aspheric components  113 , and thereby reduce the need for multi-axis motion control systems. During erosion, the end of the nozzle  117  and the component  113  are preferably submerged within the working fluid  118 , such that ambient air is not introduced into the closed loop of working fluid slurry. Any air bubbles that are present in the system simply bubble to an air pocket  115  at the top of the erosion chamber  116  and are not re-circulated, thereby producing surfaces with very smooth surface finishes. 
         [0007]    The air pocket  115  can be vented continuously or at time intervals. A drain pipe  119  at the bottom of the erosion chamber  116  evacuates the erosion chamber  116  and passes the working fluid  118  with eroded particles from the component  113  to a pump  121 , which re-pressurizes the working fluid  118 . Plumbing pipes  122  are used to return the working fluid  118  back to the nozzle  117 . 
         [0008]    A motion system  123 , which is typically computer-controlled, e.g., by computer  150 , directs the nozzle  117  in the x-y directions, or in any suitable manner (such as three-dimensionally, rotationally, etc.) over the component  113  in accordance with the desired pattern and smoothness on the surface of the component  113 . Alternatively, in systems in which the nozzle  117  is fixed and the part holder  112  is movable, the motion system  123  directs the movable platform of the part holder  112  as desired to obtain the required surface shape and roughness. 
         [0009]    A property controller  124 , including switch  125  and a pair of bypass pipes, may be added to control any one or more of the various properties of the working fluid  118 , e.g., temperature, fluid density, viscosity, or pH. If temperature control is required, a temperature sensor in the switch  125  determines the temperature of the working fluid  118  and reroutes all or a portion of the working fluid  118  through the property controller  124  via the bypass pipe, where the temperature of the working fluid  118  is adjusted higher or lower using suitable heating or cooling means. The thermally altered working fluid is passed back to the plumbing  122  via the return bypass pipe. The temperature of the working fluid  118  can be adjusted in order to optimize the removal rate of the component particles and/or the surface roughness of the component  113 . 
         [0010]    In particle heating or cooling, the tip of the nozzle  117  can affect the properties of the working fluid slurry, thereby increasing or decreasing the removal rate, i.e., cooling the working fluid  118  will lead to a stiffer slurry and an increased removal rate. The property controller  124  can alternatively or also include means for altering the pH of the working fluid  118  by adding high or low pH materials thereto for optimizing the removal rate of component material and the surface roughness of the finished product. 
         [0011]    The pump  121  maintains a constant pressure during a single stroke of the fluid jet nozzle  117 , and reverses direction after completion of a stroke. The pump  121  includes first and second pumping chambers  132  and  133 , respectively, each with a diaphragm  134  and  135  for expanding and/or contracting the volume of the respective pumping chamber  132  and  133 . The diaphragms  134  and  135  may be driven electrically, pneumatically or hydraulically. The direction of the pump  121  is coordinated with the fluid jet polishing to ensure that the pressure at the nozzle  117  is constant during a single translation of the nozzle  117  over the workpiece  113 . 
         [0012]    The pump  121  includes a hydraulic (or pneumatic) actuator pump  137 , which drives a hydraulic (or pneumatic) working fluid  139  from the upper part of the first pumping chamber  132 , actuating the first diaphragm  134  to expand the volume of the lower part of the first pumping chamber  132 . The hydraulic working fluid  139  is forced into the upper part of the second pumping chamber  133 , forcing the second diaphragm  135  to contract the volume of the lower part of the second pumping chamber  133 , pressurizing and forcing the abrasive fluid  118  through an output conduit  141  to the nozzle  117 . 
         [0013]    When the hydraulic actuator pump  137  is actuated in the aforementioned direction, a valve assembly  140  is set in a first position (shown in dotted lines) in which the abrasive fluid  118  flows from the drain  119  to the bottom of the first pumping chamber  132 , and abrasive fluid  118  flows from the lower part of the second pumping chamber  133  through the output conduit  141  to the nozzle  117 . On the next stroke, the hydraulic actuator pump  137  pumps the hydraulic working fluid  139  in the reverse direction, i.e., from the top of the second pumping chamber  133  to the top of the first pumping chamber  132 , and the valve assembly  140  ensures that the abrasive fluid  118  flows from the drain  119  to the bottom of the second pumping chamber  133 , and from the bottom of the first pumping chamber  132  to the nozzle  117  via the output conduit  141  (shown by solid curved arrows). 
         [0014]    The second diaphragm  135  rises to increase the volume of the lower part of the second pumping chamber  133 , creating a suction force on the abrasive fluid  118 , while the first diaphragm  134  is lowered to decrease the volume of the lower part of the first pumping chamber  132 , thereby pressurizing the abrasive fluid  118 . Such a typical fluid jet polishing system is shown in U.S. Pat. No. 7,455,573, which is hereby incorporated by reference in its entirety. In such fluid jet polishing systems, the fluid jet stream is highly controllable and produces a controlled polished surface, but the waste products are generally disposed of. Such waste products, however, with some processing, may include valuable materials, and it would be desirable to modify such a fluid jet polishing system to create highly desirable products, such as nanoparticles, from what the polishing system considers as waste. 
         [0015]    Thus, a system and method for producing nanomaterials solving the aforementioned problems is desired. 
       SUMMARY OF THE INVENTION 
       [0016]    This system and method for producing nanomaterials allows for the production of relatively high concentrations of nanoparticles with a minimum of expense, time and energy. Ultrasonic waves, produced at a power of approximately 50 W with a frequency of 26.23 kHz, are projected on a material sample while, simultaneously, a fluid stream jet is projected on the material sample. The ultrasonic waves, in the presence of the fluid jet, create cavities that explode at the surface of the solid material, leading to the creation of cracks in the material surface. With the increase in the number of cracks in the material, the solid material erodes. The eroded material, which is on the nanometer scale, is collected on a suitable substrate, such as silicon. This method allows for the preparation of nanoparticles from any solid material, in particular very hard materials, such as diamond, silicon carbide and the like. 
         [0017]    The system includes a housing having an upper wall, a lower wall and at least one sidewall, the housing defining an open interior region therein. An ultrasonic transducer is mounted to an inner surface of the lower wall for generating focused ultrasonic waves. The material sample is mounted to an inner surface of the upper wall for impingement thereon by the focused ultrasonic waves. 
         [0018]    A nozzle is mounted adjacent the material sample and the pressurized fluid is selectively projected through the nozzle and onto the material sample. A slurry containing a mixture of the fluid and nanoparticles is created by the erosion of the material sample under the influence of the focused ultrasonic waves and the pressurized fluid. The slurry is then collected, and the nanoparticles are removed therefrom, such as by precipitation onto the silicon substrate. 
         [0019]    These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a diagrammatic view of a system for producing nanomaterials according to the present invention. 
           [0021]      FIG. 2  is a block diagram of the components of a controller of the system for producing nanomaterials of  FIG. 1 . 
           [0022]      FIGS. 3A ,  3 B,  3 C,  3 D,  3 E and  3 F illustrate the nanoscopic-scale steps of generating nanoparticles using the system for producing nanomaterials of  FIG. 1 . 
           [0023]      FIG. 4  is a diagrammatic view of a typical prior art system for fluid jet polishing. 
       
    
    
       [0024]    Similar reference characters denote corresponding features consistently throughout the attached drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]      FIG. 1  illustrates a system for producing nanomaterials  10 . A hollow housing  12  is provided, the housing  12  being formed from stainless steel or any other suitable material that will not rust, corrode or react with the fluids and nanomaterials to be described in detail below. The housing  12  includes a lower wall  32 , at least one sidewall  30  and an upper wall  34  forming an enclosure that defines an open, interior region therein. 
         [0026]    A material sample  38  is releasably secured to the inner surface of the upper wall  34  by a sample holder  36 , which may be a clip, a clamp or any other suitable releasable holder for grasping a material sample. Sample  38  is the raw material sample from which the nanomaterials will be produced. The sample holder  36  may be fixed with respect to upper wall  34 , may be selectively and controllably rotatable relative to upper wall  34 , or may form part of a movable platform. Preferably, the sample holder  36  is rotatable, allowing for user control over the size of the nanoparticles produced by the system  10 . The sample holder  36  may be manually rotated or may be driven by any suitable rotation drive system, such as an external motor, controlled by a controller  18  (to be described in detail below). 
         [0027]    A nozzle  28  directs a pressurized fluid jet stream of a working fluid F at the exposed surface of the solid material sample  38 , as illustrated. The working fluid F is initially a fluid in its pure form, such as water, glycol, oil or any other suitable fluid. Following erosion of the solid material sample  38 , the eroded material is mixed with the fluid, leading to the formation of viscous slurry. 
         [0028]    Although the shape and relative dimensions of housing  12  may be varied, in the preferred embodiment, the dimensions of the housing  12  are linearly dependent upon the wavelength λ (or, alternatively, the frequency f) of the ultrasonic wave (to be described in detail below). For example, if the frequency f of the ultrasonic wave U is 26.32 kHz, then dimensions are preferably given by some multiple n of λ (or, given frequency f, using the relation that 
         [0000]    
       
         
           
             
               λ 
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             , 
           
         
       
     
         [0000]    where c is the speed of sound), such that, in this example, 
         [0000]    
       
         
           
             
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         [0000]    This leads to a housing diameter of 8.55 cm (where n=1.5 for a cylindrical housing), a height of 42.75 cm (with n=7.5), and a thickness of approximately 2 mm. These dimensions permit the solid material sample to receive the maximum intensity of the ultrasonic wave U. As noted above, the material forming housing  12  is selected to be chemically inert with respect to the material to be eroded (from sample  38 ), as well as the working fluid F. 
         [0029]    A pump  20  is provided to pressurize the working fluid F. As shown, the slurry resulting from fluid F impinging upon the sample  38  and formed as a mixture of the fluid F and the eroded material is preferably recycled, the slurry passing through an outlet port  26  formed through sidewall  30 , and driven by pump  20  via conduit  24  to be forced through a second conduit  22 , which terminates in nozzle  28 . 
         [0030]    A piezoelectric transducer  14  is attached to the inner surface of the lower wall  32 , as shown, and may be driven at 26.32 kHz, as given in the above example. It should be understood that any suitable piezoelectric transducer capable of delivering focused ultrasonic waves U may be utilized. 
         [0031]    In use, the ultrasonic waves are focused on the exposed surface of the material sample  38 . A controller  18  scans suitable values of ultrasonic wave intensity and/or frequency, along with optimal values of pressure for the fluid jet, for producing the desired nanoparticle sizes. As shown in  FIG. 2 , the controller  18  includes a processor  40 , which may be any suitable type of computer processor, such as that associated with a separate personal computer or the like, or may be any other suitable type of processor or logic controller, such as a programmable logic controller (PLC) or the like. The processor  40  initiates functioning of the piezoelectric acoustic transducer  14  via triggering of a function generator  42 , which may be any suitable type of function generator. 
         [0032]    The processor  40 , in conjunction with function generator  42 , establishes a time axis, which may be visually represented on a digital oscilloscope  48 , and searches for the optimal values of ultrasonic intensity within housing  12  to produce nanoparticles of a predetermined, optimal size. The solid material sample  38  is positioned horizontally within the housing  12 , as shown, at a calibrated position selected such that cavitation and the pressure field are not disturbed due to axis-symmetric geometry. The function generator may have an associated signal amplifier  44 , which feeds control signals, ultimately generated by processor  40 , to the acoustic piezoelectric transducer (APT)  14  and pump  20 . 
         [0033]    The housing  12  is initially calibrated before measuring cavitation noise therein. In order to accomplish this, the housing  12  may be completely filled with water (from a tap or any other suitable source), and the water is allowed to stand undisturbed in the basin for approximately two hours to avoid interference from large bubbles. Following this, a hydrophone  46 , or the like, is utilized (via the reflector of the preferably concave piezoelectric transducer  14 ), to find the positions of the maxima of ultrasonic intensities within housing  12 . 
         [0034]    Typically, the maxima of intensity are found to lie at distances of (n+0.5)λ from the transducer  14 ; i.e., from the example given above, the intensity maxima are found at 2.85 cm, 8.55 cm, 14.25 cm, 19.95 cm, 25.65 cm, 31.34 cm and 37.00 cm, respectively, above the center of the concave transducer  14 . The hydrophone  46  is gradually moved along the housing  12  and is finally positioned in the location of maximum ultrasonic wave intensity. The distance between the final position of hydrophone  46  and the bottom wall  32  remains fixed for all subsequent measurements. The material holder  36  is positioned at this experimentally found location of maximum ultrasonic wave intensity. 
         [0035]    Cavitation noise is then recorded with a broad band hydrophone, preferably with a flat frequency-response curve up to 500 kHz. This frequency response of the hydrophone allows for detection of the first harmonics of the fundamental component with equal sensitivity. The signal received by the hydrophone  46  is fed to the input of the digital oscilloscope  48 . Preferably, the oscilloscope memory (or memory of an associated component, such as a personal computer containing processor  40 ) has a storage capacity allowing for the recordation of approximately two million data points. 
         [0036]    The pressure of the slurry produced by the mixture of fluid F and the eroded material from sample  38  may be varied, as desired, in order to control the size of the produced nanoparticles. With the aid of the hydrophone  46  and the oscilloscope  48 , the cavitation noise is measured, the cavitation noise decreasing as the viscosity of the slurry increases. The concentration of the nanoparticles within the slurry increases with time and/or viscosity of the slurry. At the desired concentration of nanoparticles, the slurry is collected from the bottom of housing  12 , via tape  16 , or any other suitable type of collector or filter, and the nanoparticles are precipitated therefrom by sedimentation on a suitable substrate, such as a silicon substrate. As time increases, the nanoparticle concentration increases, along with the viscosity of the slurry, which increases the removed material rate. It should be understood that both the nozzle  28  and the position of the acoustic piezoelectric transducer (APT)  14  may be varied, depending upon the nanomaterial production needs, such as nanoparticle size and yield. 
         [0037]    Due to acoustic energy loss within housing  12  (in the form of heat), the housing  12  is preferably constantly cooled to prevent effervescence and evaporation of the liquids contained therein. The controller  18  may have a temperature controller integrated therein for maintaining the fluid F at a constant desired temperature. 
         [0038]    The material removal rate from sample  38  depends on a number of different parameters. In order to properly model the formation of the nanomaterials, the fluid jet is represented as being composed of N perfect and equal spheres, each having a radius r, which is measured in meters. Each of these spheres has equal velocity v and kinetic energy density given by 
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         [0000]    where ρ is the fluid density (for purposes of modeling, the fluid F is selected to be water). The total kinetic energy is given by 
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         [0000]    The fluid stream starts at the nozzle  28  and ends at the surface of the material sample  38 , with the trajectory from the nozzle  28  to the material surface being approximately l=2.3×10 −2  m. This provides an approximate volume of the fluid as πr 2 l=1.67×10 −7  m 3 , which leads to a kinetic energy of approximately 0.2 J. Thus, the surface energy is given as 
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         [0000]    MPa. Next, conservation of energy is applied to both ends of the fluid stream and Bernoulli&#39;s equation is applied: 
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         [0000]    where ΔE USB  is the impact energy of the ultrasonic beam U on the surface of the material  38 , and ΔE surface  is the surface binding energy of the atoms in the solid material surface. P 0  and P final  are the initial and final pressure of the slurry at the nozzle and at the surface of the sample  38 , respectively. It is well-known that nanocrystalline aggregated diamond nanorods have a fracture toughness of approximately 11.1±1.2 MPa, which exceeds that of natural and synthetic diamond (which varies from 3.4 to 5.0 MPa) by 2 to 3 times. Thus, ΔE Surface  is considered to be in the range of approximately 22 to 33 MPa, and ΔE USB  is estimated to be at a maximum of 33 MPa. Thus, the intensity of ultrasonic beam U is sufficient to perform, at least, the first fracture in the solid surface. Generally, the first fracture in a material caused by an external energy source depends on the amount of energy applied to the surface, the rate at which it is applied and the manner in which it is applied. 
         [0039]    Cavitation, referenced above, is the phenomenon of sequential formation, growth and collapse of millions of microscopic vapor bubbles (or voids) in a liquid. The collapse or implosion of these cavities creates high localized temperatures, roughly on the order of 14,000 K, with a pressure of approximately 10,000 atmospheres, resulting in short-lived and highly localized hot spots in a cold liquid. Thus, cavitation serves as a method of concentrating the diffused fluid energy locally, in very short durations, creating a zone of intense energy dissipation. 
         [0040]    Cavitation is induced by passing high frequency sound waves (on the order of 16 kHz-100 MHz; i.e., ultrasonic waves) through liquid media. When ultrasonic waves pass through the liquid media, in the rarefaction region, local pressure falls below the threshold pressure for the cavitation (typically the vapor pressure of the medium at the operating temperature), and millions of cavities are generated. In the compression region, the pressure in the fluid rises and these cavities are collapsed. The collapse conditions depend on the intensity and frequency of the ultrasonic waves, as well as the physical properties of the liquid, the temperature of the liquid, and any gases dissolved therein. 
         [0041]    During cavitation, the relatively low average energy density of the acoustic field is transformed into a high energy density field inside and near the bubble. During the collapse of cavitation bubbles in the liquid medium, pressures on the order of several MPa and temperatures on the order of 10 4  K are generated.  FIGS. 3A-3F  illustrate the erosion of material from sample  38  to form the resultant nanomaterials. In  FIG. 3A , ultrasonic waves W are directed toward the solid material surface. The intense ultrasonic energy is suddenly stopped by the atoms at the surface of sample  38 , thus dividing the energy into three parts: the first part is reflected ultrasonic waves (having a relatively low frequency, illustrated as LFU in  FIG. 3C ), the second part is absorbed by the surface atoms, and the third part of the ultrasonic energy is absorbed by the fluid adjacent the surface. 
         [0042]    As shown in  FIG. 3B , the fluid adjacent the surface is almost instantaneously evaporated, leading to the formation of a cloud of bubbles C. The reflected ultrasonic waves LFU interfere with the incident waves U, leading to bubble cloud forced oscillations, which leads to the formation of a shock wave with a very high energy (illustrated in  FIG. 3D ). This energy is absorbed by the surface of the material, thus crushing the material.  FIG. 3E  illustrates the shockwave SW propagating inwardly, compressing the hemispherical bubble cloud C, with the pressure being so great as to create nanoparticles NP at the surface of material  38  (shown in  FIG. 3F ). These nanoparticles NP are washed away with the fluid F to form the slurry, collected by tape  16 . 
         [0043]    The first crack in material sample  38  occurs with the application of approximately 22 to 33 MPa, created by the inwardly propagating shockwave. Such pressures are ordinarily very difficult to generate (which is why nanoparticles in general are extremely difficult to produce), but system  10  allows for the generation of relatively large concentration of nanoparticles NP through the usage of only pressurized fluid and projected ultrasound U. 
         [0044]    It is to be understood that the present invention is not limited to the embodiment described above, but encompasses any and all embodiments within the scope of the following claims.