Patent Document

RELATED APPLICATION 
   This patent application is a divisional of U.S. patent application Ser. No. 10/366,121 filed Feb. 12, 2003, now U.S. Pat. No. 6,881,039 the contents of which are hereby incorporated by reference. 
   The parent U.S. patent application Ser. No. 10/366,121 claims priority under 35 U.S.C. 119(e) of the co-pending U.S. Provisional Patent Application Ser. No. 60/413,194 filed Sep. 23, 2002, and entitled “MICRO-FABRICATED ELECTROKINETIC PUMP”. The Provisional Patent Application Ser. No. 60/413,194 filed Sep. 23, 2002, and entitled “MICRO-FABRICATED ELECTROKINETIC PUMP” is also hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to an apparatus for cooling and a method thereof. In particular, the apparatus is for an improved electrokinetic pump having substantially straight and very small pore apertures and lengths. The pump is manufactured by a process using semiconductor processing techniques. 
   BACKGROUND OF THE INVENTION 
   High density integrated circuits have evolved in recent years including increasing transistor density and clock speed. The result of this trend is an increase in the power density of modern microprocessors, and an emerging need for new cooling technologies. At Stanford, research into 2-phase liquid cooling began in 1998, with a demonstration of closed-loop systems capable of 130 W heat removal. One key element of this system is an electrokinetic pump, which was capable of fluid flow on the order of ten of ml/min against a pressure head of more than one atmosphere with an operating voltage of 100V. 
   This demonstration was all carried out with liquid-vapor mixtures in the microchannel heat exchangers because there was insufficient liquid flow to capture all the generated heat without boiling. Conversion of some fraction of the liquid to vapor imposes a need for high-pressure operation, and increases the operational pressure requirements for the pump. Furthermore, two phase flow is less stable during the operation of a cooling device and can lead to transient fluctuations and difficulties in controlling the chip temperature. The pump in that demonstration was based on porous glass filters that are several mm thick. A disadvantage of these structures is that the pore density, structure, and mean diameter is not uniform and also not easily reproduced in a low-cost manufacturing process. Furthermore, the fluid path in these structures is highly tortuous, leading to lower flow rates for a given thickness of pump. Porous ceramic structures with nominally the same character were shown to exhibit pumping characteristics which varied by large amounts. 
   What is needed is an electrokinetic pumping element that would provides a relatively large flow and pressure within a compact structure and offer much better uniformity in pumping characteristics. 
   SUMMARY OF THE INVENTION 
   An electrokinetic pump for pumping a liquid includes a pumping body having a predetermined thickness, preferably in the range of 10 microns and 1 millimeter. The body includes a plurality of pore apertures for channeling the liquid through the body, wherein each pore aperture extends from the first outer surface to the second outer surface and are preferably 0.1-2.0 microns in diameter. The pores are preferably narrow, short and straight. The pumping body is preferably oxidized. A pair of electrodes for applying a voltage differential are formed on opposing surfaces of the pumping body at opposite ends of the pore apertures. The pumping body is formed on a support structure to maintain a mechanical integrity of the pumping body. 
   A method of fabricating an electrokinetic pump preferably uses conventional semiconductor processing techniques and includes providing a first material for a pumping body having a first surface and a second surface. A plurality of pore apertures are formed through the first material. The pumping body including the interior of the pore apertures is oxidized. An electrode is formed on the first and second surfaces. A voltage potential is coupled across the electrodes to move a liquid to flow through the plurality of pore apertures. 
   Another method of fabricating an electrokinetic pump includes providing a substrate having a first surface. A plurality of etch stop alignment marks is formed on the first surface. A pumping element material is formed on the first surface. A plurality of pore apertures are formed through the pumping material. A support structure is formed under the etch stop alignment marks by removing remaining material. The resulting structure is oxidized including within the pore apertures wherein a voltage differential applied across the pumping element drives liquid through the plurality of capillaries. 
   Other features and advantages of the present invention will become apparent after reviewing the detailed description of the preferred embodiments set forth below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  illustrates a perspective view of the pumping element of the preferred embodiment of the present invention. 
       FIG. 1B  illustrates a perspective view of the pumping element of an alternative embodiment of the present invention. 
       FIG. 2  illustrates a cross sectional view of the pump of the preferred embodiment of the present invention. 
       FIGS. 3A-3F  illustrates a preferred method of fabricating the pump of the preferred embodiment of the present invention. 
       FIG. 3A  illustrates a first step in fabricating the pump of the preferred embodiment. 
       FIG. 3B  illustrates a second step in fabricating the pump of the preferred embodiment. 
       FIG. 3C  illustrates a third step in fabricating the pump of the preferred embodiment. 
       FIG. 3D  illustrates a fourth step in fabricating the pump of the preferred embodiment. 
       FIG. 3E  illustrates a fifth step in fabricating the pump of the preferred embodiment. 
       FIG. 3F  illustrates a sixth step in fabricating the pump of the preferred embodiment. 
       FIGS. 4A-4H  illustrates an alternative method of fabricating the pump in accordance with the present invention. 
       FIG. 4A  illustrates a first step in an alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 4B  illustrates a second step in an alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 4C  illustrates a third step in an alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 4D  illustrates a fourth step in an alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 4E  illustrates a fifth step in an alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 4F  illustrates a sixth step in an alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 4G  illustrates a seventh step in an alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 4H  illustrates a eighth step in an alternative method of fabricating the pump of the preferred embodiment. 
       FIGS. 5A-5D  illustrates another alternative method of fabricating the pump in accordance with the present invention. 
       FIG. 5A  illustrates a first step in another alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 5B  illustrates a second step in another alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 5C  illustrates a third step in another alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 5D  illustrates a fourth step in another alternative method of fabricating the pump of the preferred embodiment. 
       FIG. 6  illustrates an alternate embodiment of a fit having non-parallel pore apertures in accordance with the present invention. 
       FIG. 7  illustrates a closed system loop including the pump of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred and alternative embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it should be noted that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
   The basic performance of an electrokinetic or electro-osmotic pump is modeled by the following relationships: 
   
     
       
         
           
             
               
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   As shown in equations (1) and (2), Q is the flow rate of the liquid flowing through the pump and ΔP is the pressure drop across the pump and the variable a is the diameter of the pore aperture. In addition, the variable ψ is the porosity of the pore apertures, ζ is the zeta potential, ε is the permittivity of the liquid, V is the voltage across the pore apertures, A is the total Area of the pump, τ is the tortuosity, μ is the viscosity and L is the thickness of the pumping element. The terms in the parenthesis shown in equations (1) and (2) are corrections for the case in which the pore diameters approach the size of the charged layer, called the Debye Layer, λ D , which is only a few nanometers. For pore apertures having a diameter in the 0.1 mm range, these expressions simplify to be approximately: 
   
     
       
         
           
             
               
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   As shown in equations (3) and (4). The amount of flow and pressure are proportional to the amount of voltage potential that is present. However, other parameters are present that affect the performance of the pump. For example, the tortuosity (τ) describes the length of a channel relative to the thickness of the pumping element and can be large for pumps with convoluted, non-parallel channel paths. The length (L) is the thickness of the pumping element. As shown in equations (3) and (4), the tortuosity τ and thickness L of the pumping element are inversely proportional to the flow equation (4) without appearing at all in the pressure equation (4). The square of the diameter a of the pore apertures is inversely proportional to the pressure equation (4) without appearing at all in the flow equation (3). 
   The pump of the present invention operates at significantly reduced voltages in relation to the prior electrokinetic pumps, but still generate the same or more flow without significant reductions in pressure. Existing pumps have average pore aperture diameters in the range of 0.8 to 1.2 microns. In addition, existing ceramic pump elements have thicknesses of 3-4 mm and a tortuosity of 1.4-2.0. A typical prior electrokinetic pump having a thickness of 2.5 mm produces flow of 25 ml/min at a voltage of 100 V and have a max pressure of 1.00 Atm. 
   In contrast, the thickness of the pumping element is reduced by 100 times; the tortuosity is improved by a factor of more than 3; and the pore diameter is reduced by 3 times. The reduction in these three factors allows the pump of the present invention to be operated at 10 times reduced voltage and yet be capable of more than 10 times more flow. The pump of the present invention is able to perform under such conditions by reduction: in the diameter of the pore aperture; the thickness of the pumping element; and the tortuosity of the pump apertures. 
     FIG. 1A  illustrates a preferred embodiment of the pump  100  in accordance with the present invention. The pump  100  includes a pumping element or body  102  and a support element  104 . Preferably, the pumping element  102  includes a thin layer of silicon with a dense array of cylindrical holes, designated as pore apertures  110 . Alternatively, the pumping element  102  is made of any other appropriate material. It is preferred that the pumping element has a thickness range of 10 microns to 1 millimeter and the pore apertures  110  have a diameter of 0.1-2.0 microns. As shown in  FIGS. 1 and 2 , the pumping element  102  is supported by the support element  104  having a less dense array of much larger holes or support apertures  108 . The support element  104  provides mechanical support to the pumping element  102  and a thickness of at least 300 microns. Preferably the support element  104  has a thickness of 400 microns whereby the support apertures  108  are at least 100 microns in diameter, although other thicknesses are contemplated. The illustration of the support structures  108  in  FIG. 1A  is only one type of configuration and it should be noted that other geometric structures can alternatively be used to balance mechanical strength with ease of fabrication. Such alternative structures include a honeycomb lattice of material, a square lattice of material, a spiderweb-lattice of material, or any other structural geometry that balances mechanical strength with ease of fabrication.  FIG. 1B  illustrates an example of a square lattice structure  100 ′. 
     FIG. 2  illustrates a cross sectional view of the pump  100  of the present invention. As shown in  FIG. 2 , the pumping element  102  includes a dense array of pore apertures  110  and the support element  104  attached to the pumping element  102 , whereby the support element  104  includes an array of support structures  106 . The pore apertures  110  pass through the pumping element  102  from its bottom surface  114  to its top surface  112 . In particular, the pore apertures  110  channel liquid from the bottom surface  114  to the top surface  112  of the pumping element  102 , as shown in  FIG. 2 . Preferably, the liquid used in the pump  100  of the present invention is water with an ionic buffer to control the pH and conductivity of the liquid. Alternatively, other liquids may be used including, but not limited to, acetone, acetonitrile, methanol, alcohol, ethanol, water having other additives, as well as mixtures thereof. It is contemplated that any other suitable liquid is contemplated in accordance with the present invention. 
   The support structures  106  are attached to the pumping element  102  at predetermined locations to the bottom surface  114  of the pumping element  102 . These predetermined locations are dependent on the required strength of the pump  100  in relation to the pressure differential and flow rate of the liquid passing through the pumping element  102 . In between each support structure  106  is a support aperture  108 , whereby the liquid passes from the support apertures  108  into the pore apertures  110  in the bottom surface  114  of the pumping element  102 . The liquid then flows from the bottom pore apertures  110  through the channels of each pore apertures and exits through the pore apertures  110  opening in the top surface  112  of the pumping element  102 . Though the flow is described as liquid moving from the bottom surface  114  to the top surface  112  of the pumping element  102 , it will be apparent that reversing the voltage will reverse of the flow of the liquid in the other direction. 
   The liquid passes through the pumping element  102  under the process of electro-osmosis, whereby an electrical field is applied to the pumping element  102  in the form of a voltage differential. Preferably, electrodes  316  ( FIG. 3F ) are placed at the top surface  112  and bottom surface  114  of the pumping element  102 , whereby the voltage differential between the top surface  112  and the bottom surface  114  drives the liquid from the support apertures  108  up through the pore apertures  110  and out through top surface  112  of the pumping element  102 . Alternatively, the electrodes  316  are applied a predetermined distance away from the top surface  112  and bottom surface  114  of the pumping element  102 . Although the process of electro-osmosis is briefly described here, the process is well known in the art and will not be described in any more detail. 
   Preferably, as shown in  FIG. 2 , the pore apertures in the pumping element are short (10-20 microns), straight, and narrow (0.2-0.5 microns). Alternatively, the pore apertures are non-parallel and are not straight, as shown in  FIG. 5B . The configuration of the pore apertures  110  allows the pump of the present invention to produce a relatively large amount flow and pressure with a much lower required voltage than prior art electrokinetic pumps. 
   It is theorized that, the flow rate and pressure differential increases are due to the reduction in the pore diameter a, tortuosity τ, and thickness in the pumping element  102 . This is shown with regard to equations (3) and (4). As shown in equation (3), the reduction in tortuosity τ in the pore apertures  110  increases the overall flow rate of the liquid passing through the pore apertures  110 . In addition, the reduction in thickness, L, of the pumping element  102  also increases the overall flow rate of the liquid passing through the pore apertures according to equation (3). Further, as shown in equation (4), reduction of the pore aperture diameter a substantially increases the amount of pressure differential of the liquid flowing through the pumping element  102 . Although the flow rate, Q, and pressure differential, ΔP, increase due to the configuration of the present pump  100 , the flow rate and pressure differential can be maintained at a suitable amount while reducing the voltage required to operate the pump  100  accordingly. 
   The pump of the present invention can be fabricated in several different ways.  FIGS. 3A-3F  illustrates the preferred embodiment of fabricating the pump  300  in accordance with the present invention. As shown in  FIGS. 3A-3F , the pump is made by a series of lithographic/etching steps, such as those used in conventional integrated circuit manufacturing. In the preferred embodiment, a substrate wafer is provided  302 , whereby the wafer is preferably a SOI wafer, as shown in  FIG. 3A . Alternatively, the wafer is made of standard silicon substrate with pre-formed layers of oxide and polysilicon. Alternatively, as discussed below, a combination of oxide and nitride patterns is used instead of the oxide layer, whereby the combination layer offers differing resistance to the etching process. In such a case, the etching step can be carried out without a carefully-timed termination, producing a bond layer with easily-controlled dimensions. 
   As shown in  FIGS. 3A-3F , the preferred process of fabricating the pump  300  proceeds with forming the support structures  306  and support apertures  308  by patterning and etching the features in the support element  304 , as shown in  FIG. 3B . The pattern  FIG. 3B  preferably uses conventional photo resist deposit, expose, develop and pattern steps. Because the use of photo resist to form predetermined patterns is well known, such steps will not be discussed herein. In  FIG. 3C , hydrofluoric acid etching is performed on the wafer  301  to clear any oxide  303  located between the support structures  306  and the bottom surface  312  of the pumping element  302 . It is appropriate that the HF etch step be properly timed to be sufficiently long to allow the exposure of the surface of the pumping element adjacent the support structures  306  to be exposed but not be excessively long to prevent the pumping element  302  from separating from the support structures  306 . 
   In  FIG. 3D , shown in  FIG. 3 , the pore apertures  310  and corresponding channels are formed by a plasma etching technique. The plasma etching technique forms the pore apertures  310  to preferably be parallel and straight. 
   Once the pore apertures  310  are formed, a diffusion oxidation step is performed on the pump  300  whereby all surfaces of the pump  300 , including surfaces of the pumping element  302  and support element  304  are oxidized with an oxide layer  318 . The oxide layer  318 , preferably SiO 2 , forms a passivation oxide which prevents current from bypassing the electrokinetic osmotic pumping effect caused by the voltage differential between the openings of the pore apertures  310 . In addition, the step of growing the oxide layer  318  serves to narrow the channels of the pore apertures  310 , because SiO2 forms from oxidized silicon at a high-temperature with O 2  gas, as shown in  FIG. 3F  Thus, narrower pore apertures can be formed by this oxidation step than can be etched photo lithographically using a plasma etch. In one embodiment, the pore apertures are less than 0.4 μm in diameter after the oxide is formed, whereby the pumping element  302  has a high porosity due to the dense amount of pore apertures  310  within. 
   The support element  304  has large support apertures  308  which offer very little resistance to the flow of liquid through the pump body  302  while still providing adequate structural support. Therefore, the formation of 0.25 microns of this oxide in a silicon pore with a diameter of 1 micron serve to reduce the pore diameter to almost 0.5 microns. This process can be carried out with excellent thickness control, as the growth of gate oxides in silicon is very thoroughly characterized and determinable in the art. As a final step, an electrode is formed on both surfaces of the pumping element  102 . Details concerning the electrodes are discussed below. 
     FIGS. 4A-4H  illustrates an alternative process of fabricating the electrokinetic pump  400  in accordance with the present invention. The alternative process in  FIGS. 4A-4H  is designed such that the HF etch step is self-terminated. Because this step self-terminates, this alternative process eliminates any timing issues regarding attachment of the support structures  406  to the pumping element  402 . The alternative process begins with providing a standard silicon wafer or substrate  401 , as shown in  FIG. 4A . The next step includes forming a bond layer by depositing a predetermined amount of bonding material  403  such as 0.5 microns of Silicon Nitride, onto the top surface of the substrate  401 , as shown in  FIG. 4B . Alternatively, any other appropriate bonding material is used instead of Silicon Nitride. The Silicon Nitride layer is then patterned and etched from the top surface of the substrate  401  at predetermined locations dependent on the structure support required for the pump  400 . Once etched, the remaining portions of the bonding material  403  are used as alignment marks  405  to align the support structures  406  to their appropriate locations, as shown in  FIG. 4C . In addition, a Chemical-Mechanical Polishing (CMP) process is optionally carried out to smooth the upper surface of the bonding material  403 . 
   As shown in  FIG. 4D , in  FIG. 4 , an oxide layer  407  is applied to the top surface  414  of the substrate  401 , whereby the oxide layer  407  is grown over the alignment marks  405 . Alternatively, the oxide layer  407  is applied at a thickness less than the height of the alignment marks  405 , whereby the oxide layer  407  is not applied over the alignment marks  405 . The polysilicon layer  409  is formed on the surface oxide layer  407  and is used to form the pumping element  402 , as shown in  FIGS. 4E and 4F . The polysilicon layer  409  preferably grows in an epitaxial process. Preferably, the thickness of the polysilicon layer  409  is in the range of 10-20 microns. 
   Next, the plurality of pore apertures  410  are formed in the polysilicon layer  409 , as shown in  FIG. 4F . The pore apertures  410  can be formed using the plasma etch teaching recited in the first method. Once the pore apertures  410  are formed in the polysilicon layer  409 , the process proceeds by forming the support apertures  408  and support structures  406  by plasma etching the support structures  406  and apertures  408  out of the substrate  401 . From  FIG. 4G , a support structure  406  is formed at each alignment mark  405  in the bond layer. Alternatively, the support structures  406  and support apertures  408  are formed before the pore apertures  410  are formed. Once the pore apertures  410  and support structures  406  are formed, the entire pump  400  is preferably dipped in HF to remove all oxide between the polysilicon layer  409  and the top surface of the substrate  401 , as in  FIG. 4H . This HF etch  FIG. 4H  also opens the interface between the pore apertures  410  and the support apertures  408 . As stated above, an advantage of this process is that the HF etch step is self-terminated, because the bonding material is not attacked by the HF during the etching process. Therefore, the support structures  406  are ensured to stay attached to the pumping element  402  regardless of how long the pump  400  is exposed to the HF. 
   Next, the structure is oxidized to form an oxide layer  318  on all the surfaces of the pumping element  402  and support structure  404  to passivate the surfaces and to reduce the diameters of the pore apertures  410 . 
     FIGS. 5A-5D  illustrates another alternative method of fabricating the pump in accordance with the present invention. In the alternate process, a standard silicon wafer substrate  501  is provided, as shown in step  30 . In addition, as shown in step  32 , a frit  502  is bonded to one side of the wafer  501 , preferably on the top side of the wafer  501 . In this embodiment, the frit  502  is preferably made of a glass or ceramic material that is insulates against the transfer of current. Such material preferably includes Silicon Nitride or Borosilicate glass. It is contemplated that other materials or types of ceramics and glass are alternatively used. The frit  502  is bonded to the wafer  501  using a high temperature fusing process, although other methods are alternatively used. In addition, a Chemical-Mechanical Polishing (CMP) process or any other method is performed on the frit  501  to grind and smooth the surface of the flit  501  down to a predetermined thickness, which is approximately 100 microns. Alternatively, the frit  502  may be polished or smoothed to any other appropriate thickness. 
   As shown in  FIG. 5C , the support structures  506  are formed into the wafer  501  by an etching process, such as plasma etching. Alternatively, any other process can be used to form the support structures  506 . Specifically, the support structures  506  are formed by turning the substrate  501  and bonded frit  502  upside down, whereby the substrate  501  faces upward. Next, the etching process is performed to the substrate  501 , whereby the support structures  506  and corresponding support apertures  508  are formed. It should be noted that the steps of polishing and forming the support structure may be done in any order, whereby the polishing is performed either before or after the support structures are formed. Following, the pore apertures  510  may be formed by a plasma etching process, whereby the pore apertures  510  are formed between the top and bottom surfaces of the frit  512  and have straight, parallel configurations. Alternatively, as shown in  FIG. 6 , non-parallel, complex shaped pore apertures may already be present in the frit  502  and the pore apertures  510  need not be formed by the etching process. 
   Once the pumping element  302  and support element  304  are formed by any of the above processes, metal is preferably deposited on the outside surfaces of the pumping element  302 , thereby forming electrodes  316  on surfaces of the pumping element, as shown in  FIG. 3F . The electrodes  316  are fabricated from materials that do not electrically decompose during the electrolysis process. Preferred materials for the electrodes  316  include Platinum and Graphite; although other materials may serve as well, depending on the composition of the fluid being pumped. The electrodes  316  are formed on the outside surfaces of the pumping element  302  in a variety of ways. Preferably, the electrodes  316  are formed on the outside surfaces of the pumping element  302  by evaporation, chemical vapor deposition (CVD), or plasma vapor deposition (PVD). Alternatively, the electrodes  316  are formed on the outside surfaces of the pumping element  302  by screen or contact printing. Alternatively, an electrode screen (not shown) may be positioned in a close proximity to the outside surfaces of the pumping element  302 . Alternatively, a wire is coupled to each outside surface of the pumping element. It should be noted that the electrodes coupled to the pumping element of the present invention are not limited to the methods described above. 
     FIG. 7  illustrates a cooling system for cooling a fluid passing through a heat emitting device, such as a microprocessor. As shown in  FIG. 7 , the system is a closed loop whereby liquid travels to an element to be cooled, such as a microprocessor  602 , whereby heat transfer occurs between the processor and the liquid. After the leaving the microprocessor  602 , the liquid is at an elevated temperature of 59° C. and enters the heat sink  604 , wherein the liquid is cooled within to a temperature of 44° C. The liquid leaves the heat sink  604  at the lower temperature of 44° C. and enters the pump  600  of the present invention. Again, referring to  FIG. 2 , within the pump  100 , the cooled liquid enters the support apertures  108  and is pumped through the pore apertures  110  by the osmotic process described above. In particular, the voltage applied to the pumping element  102  causes the negatively electrically charged ions in the liquid to be attracted to the positive voltage applied to the top surface of the pumping element  102 . Therefore, the voltage potential between the top and bottom surface of the pumping element drives the liquid through the pore apertures  110  to the top surface, whereby the liquid leaves the pump  100  at substantially the same temperature (44° C.) as the liquid entering the pump (44° C.). 
   The pump of the present invention produces enough flow that sufficient heat rejection with a single-phase fluid is possible. Existing pumps that operate with 100 Watt heat sources require 2-phase heat rejection, whereas single-phase fluids can capture and reject heat at lower temperatures and thereby eliminate possible problems associated with stability and phase change in a 2-phase system. In addition, the reduction in operating voltage to very low levels allows the use of existing voltages in all electronic systems without conversion between phases. 
   The pump of the present invention is able to operate with complicated fluids, such as antifreeze or water having additives to improve the heat capture and rejection properties. As stated above, current passes into the fluid through a chemical reaction, whereby the current passes through the electrodes  316  ( FIGS. 3A-3F ) in the electrokinetic pump  100 . In pure H 2 O, this reaction results in electrolysis, which produces pure H 2  gas at one electrode  316  and pure O 2  gas at the other electrode  316 . In more complicated fluids, this reaction results in much more complicated byproducts, many of which cannot be efficiently recombined in a sealed system. The chemical reaction at the electrodes  316  takes place if there is enough energy available, in the form of potential difference between the electrodes, to overcome the affinity of the charges for the electrodes. For H 2 O, these potentials, called overpotentials, add a voltage of approximately 4 Volts. For other chemicals and additive, these overpotentials vary and are accordingly different. 
   If an electrokinetic pump operates at high voltage, the overpotentials are so small that they are neglected in the analysis. However, for low-voltage operation, the overpotentials subtract from the voltage being applied to the pumping element  102 , thereby causing the actual potential difference within the pumping medium to be reduced by an amount equal to the sum of the overpotentials for the reactions at the 2 electrodes. For a multi-component fluid, the electrochemical reactions will involve all the constituents of the fluid if the applied voltage is large enough to overcome the overpotentials of all the reactions. However, operation at low voltages may allow the electrochemistry to take place with only some of the constituents of the fluid. 
   For example, if H 2 O includes additives which inhibit freezing at low temperatures, the overpotentials of the additives are significantly higher than the overpotentials of pure H 2 O. For the exchange of ions in the electro-osmosis process in regard to H 2 O, there is a range of applied voltages which are low enough that only the H 2 O participates in the reactions at the electrodes. The advantage of this circumstance is that the electrochemistry can be kept simple (involving only H 2  and O 2 ) even in a fluid that has a complicated chemical makeup. An important advantage of the low-voltage operation enabled by the pump  100  of the present invention is that it becomes possible to generate adequate flow and pressure for high-power device cooling at voltages that are below the overpotentials of some useful additives, such as antifreeze. Some examples of additives which serve the purpose of depressing the freezing point of the liquid being pumped are Cyclohexanol and Acetonitrile. These additives are soluble in water at low concentrations and are well-characterized. 
   The electrode potentials for these additive chemicals are calculated from theory. However, the overpotentials are typically 2-3 times larger than the theoretical minimum electrode potentials. In addition, the overpotentials are generally a function of chemistry, geometry, roughness, and current density at electrode/electrolyte interface. The values of overpotentials are estimated for a given electrode material/electrolyte pair and depend on the behavior of the type of additive; specific concentration of the additive and the type of specific system within which the additive is used. 
   Like most thermophysical properties, the electrolytic currents of mixtures are not a linearly superposable or weighted effect of the components of the mixture. Instead, an additive at low concentration tends to have negligible effect on the current of the cell up to some critical concentration. The situation is analogous to a circuit with two diodes in parallel where the threshold potential of each is a function of its concentration in the mixture. The lower threshold diode tends to use all of the current. In the present invention, a low-concentration additive with a higher overpotential than water will only divert a small part of the current in the pump, even if the applied potentials are greater than the overpotentials of the additives. The operating voltage of the pump can still be relatively high, and the electrochemical reactions will still tend not to involve the additives if their overpotentials are higher than the water. 
   In addition, the effect of the additives on the cryoscopic constants appear not to correlate with the critical concentration. Therefore, cyclohexanol or acetonitrile or some other additive at low concentrations is added and has a beneficial effect on the freezing point without affecting the electrochemical reactions at the electrodes. Therefore, the best additives are soluble chemicals with high cryoscopic constants that are effective at low concentrations. 
   The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention.

Technology Category: 2