Abstract:
Methods and systems for electrodeposition of Indium, Gallium, or other metals within a metal acoustic resonator chamber are provided, including for electrodeposition of Indium or Gallium as a strike layer on an interior surface of a cavitation chamber. The resultant strike layer affords an accommodating wetting surface for subsequent filling with a liquid metal and performing cavitation therein. In some embodiments, this allows for the use of liquid metal in an acoustic resonator without the damping effects inherent with oxide boundaries or other defects.

Description:
RELATED APPLICATIONS 
       [0001]    This application incorporates by reference and claims the priority and benefit of U.S. Provisional Patent Application 61/206,661, under 35 U.S.C. Sec. 119(e), filed on Feb. 4, 2009. 
     
    
     STATEMENT OF GOVERNMENT SPONSORED RESEARCH 
       [0002]    The present work was facilitated at least in part by U.S. government support under Contract No. W9113M-07-C-0178, which was awarded by the U.S. Space and Missile Defense Command and subcontracted to the present assignee. Accordingly, the government may have certain rights herein. 
     
    
     BACKGROUND 
       [0003]    Gallium is an element that is used in semiconductor and other industries and more recently in acoustics research. Gallium is generally recovered as a by-product from Bayer-process liquors containing sodium aluminate. Although electrodeposition is a common method to recover bulk Ga out of basic or acidic solutions, or to purify bulk Ga, there have not been many applications for this material where thin films were deposited with controlled uniformity, morphology and thickness. Therefore, only a few electroplating bath chemistries and processes were developed and reported for the deposition of thin layers of Ga on substrates for electronic applications. For example, Ga-chloride solutions with pH values varying between 0 and 5 were evaluated by S. Sundararajan and T. Bhat (J. Less Common Metals, vol. I 1, p. 360, 1966) for electroplating of Ga films. 
       Technical Field 
       [0004]    The present application relates to treatment of surfaces of acoustic resonators such as would be used to cause cavitation in a liquid or liquid metal medium, including the wetting and electrodeposition of materials onto a surface of said resonators. 
         [0005]    The above-mentioned methods and plating baths reportedly achieved Ga film deposition. There are, however, some common problems associated with the prior-art electrochemical deposition processes. These problems include, low cathodic deposition efficiency due to excessive hydrogen generation, poor repeatability of the process, partly due to the poor cathodic efficiency, and the poor quality of the deposited films such as their high surface roughness and poor morphology. These issues may not be important for bulk Ga electroplating or for Ga films deposited for the purpose of investigating scientific topics such as deposition mechanisms. Poor film morphology or inadequate thickness control may also not be important for the electrically inactive applications of Ga layers, such as their use as lubricating coatings etc. However, properties of the Ga films become important for certain new acoustic applications where Ga film plays a role in forming an active boundary layer of an acoustic device, such as a resonator. 
         [0006]    Prior-art Ga electroplating techniques utilizing simple electrolytes operating under acidic or basic pH values are not suitable for the above mentioned applications for a variety of reasons, including that they result in poor plating efficiencies and films with rough morphology (typically surface roughness larger than about 20% of the film thickness). Gallium is a difficult metal to deposit without excessive hydrogen generation on the cathode because Ga plating potential is high. Hydrogen generation on the cathode causes the deposition efficiency to be less than 100% because some of the deposition current gets used on forming the hydrogen gas, rather than the Ga film on the substrate or cathode. 
         [0007]    Hydrogen generation and evolution also causes poor morphology and micro defects on the depositing films due to the tiny hydrogen bubbles sticking to the surface of the depositing film, masking the micro-area under them, and therefore impeding deposit on that micro-area. This causes micro-regions with less than optimum amount of Ga in the film stack. Poor plating efficiencies inherently reduce the repeatability of an electrodeposition process because hydrogen generation phenomenon itself is a strong function of many factors including impurities in the electrolyte, deposition current densities, small changes on the morphology or chemistry of the substrate surface, temperature, mass transfer etc. As at least one of these factors may change from run to run, hydrogen generation rate may also change, changing the deposition efficiency. 
         [0008]    Furthermore, electrodeposition of Ga out of low pH aqueous electrolytes or solutions may suffer from low cathodic efficiencies arising from the presence of a large concentration of H+ species in such electrolytes. Therefore, hydrogen gas generation may be expected to lessen at higher pH values. However, as the pH is increased in the solution, Ga forms oxides and hydroxides, which may precipitate and lead to adverse acoustical or mechanical or chemical conditions. 
         [0009]    Another problem is with which the electroplating art in general is concerned is the necessity for preparing the surface on which an initial layer or strike of metal is to be deposited. In particular, the deposition of an initial nickel or gold strike on a stainless steel surface usually requires preliminary pretreatment of the surface to prepare it to accept similar Group IV metals. 
         [0010]    It has not been possible or practical in the prior art to achieve large acoustic standing waves and high Q&#39;s in acoustic resonators. 
       SUMMARY 
       [0011]    Aspects of the present disclosure are directed to a composition providing an aqueous bath for electrodeposition of Indium and Gallium on a substrate, and with a method of preparing the composition. The invention finds particular application, although it is not necessarily limited thereto, to the provision of an aqueous bath composition for the electrodeposition of Indium and Gallium strike upon a stainless steel substrate. 
         [0012]    Indium is a soft (modified Brinell 0.9 to 1) silvery white metal with a brilliant metallic luster. It has a low melting point (156.7° C.) and a relatively high boiling point (2080° C.), therefore resulting in a low vapor pressure. It is ductile, malleable, crystalline and diamagnetic. In the electromotive series, it lies between iron and tin. 
         [0013]    Indium will generally plate from either acid or alkaline solutions. Acid plating formulations include sulfate, sulfamate, fluoborate and EDTA or NTA complexed acidic baths. Alkaline formulations include the cyanide bath and non-cyanide alkaline baths complexed with ammonium tartrate. 
         [0014]    In some aspects, the present invention to provide a novel bath composition for an aqueous bath which may be prepared for electroplating Indium or Gallium by a simple and efficient method without the necessity for crystallization, precipitation or filtration. 
         [0015]    Further aspects hereof are directed to a novel Indium or Gallium electroplating bath composition, which is particularly suited to provide an initial strike Indium or Gallium on an untreated stainless steel surface. 
         [0016]    In a specific embodiment, a method for preparation of the base metallization for the acoustic resonator includes the steps of: 1) Cleaning and activation. The cleaning step removes oils, grease and other soils from the base metallization surface. The activation step removes oxides from the base metallization surface, which improves electro-deposition. The base metal may be cleaned either by ultrasonic solvent vapor degreasing or by immersion in a commercial alkaline cleaning bath operated at, e.g., 80-90° C. for a time, e.g., 10-15 minutes, followed by a hot water rinse. 2) The base metal then is acid activated by immersion in about a 10-15 percent by volume of sulfuric or hydrochloric acid solution at room temperature for about 3-5 minutes, followed by a quick cold water spray rinse. The quantitative examples provided herein are given for the sake of illustration only, and are not intended to be bounds on the possible embodiments hereof or limitations of the same. 
         [0017]    In an exemplary multistep plating process there may be an intermediate plate or strike, it is necessary to remove the drag-in from the previous process. Immediately following the acid activation or strike plate, the base metal should then be immersed in a 5 percent by weight solution of sulfamic acid solution for 1-3 minutes. This is to ensure that the pH of the base metallization surface remains acidic so no reformation of oxide occurs and also to protect the indium sulfamate plating bath from drag-in of activator chemicals. If there is no intermediate process (ionic contaminates) then a cold-water rinse to remove any activation process products will suffice. The substrate is now ready for indium plating. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    For a fuller understanding of the nature and advantages of the present invention, reference is be made to the following detailed description of preferred embodiments and in connection with the accompanying drawings, in which: 
           [0019]      FIG. 1  illustrates an exemplary resonator prepared for electrodeposition with an Indium or Gallium source; 
           [0020]      FIG. 2  illustrates an exemplary electrodeposition process in an acoustic resonator; 
           [0021]      FIG. 3  illustrates other exemplary aspects of a process for preparing an acoustical resonator for cavitation of a metal fluid; 
           [0022]      FIG. 4  illustrates another exemplary resonator cross section after being prepared and filled with a liquid metal filler; and 
           [0023]      FIG. 5  illustrates another embodiment for treating a portion of an acoustic resonator body. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    As discussed above, it is useful to have a proper surface to allow wetting of the surface for use in an acoustic resonator. For example, in the case of a metal (e.g., stainless steel) acoustic resonator having substantially an interior and an exterior surface thereof, it is useful to properly prepare the wall of the interior surface so that minimal contamination, oxidation, delamination, shape deformities, gas bubble trapping, and other undesirable effects take place during acoustic resonance. Specifically, in the case of enclosures used for acoustic cavitation of a metal liquid, it can be useful to have the enclosures sealed and filled with the liquid metal to achieve the highest quality factor (Q) for the resonator or cavitation chamber. 
         [0025]    Referring to  FIG. 1 , an acoustic resonator  100  is provided. The resonator includes a metal (e.g., stainless steel) shell  110 , which can be spherical in general shape, but is not limited to such a geometry. The shell can be made to resonate using acoustic drivers coupled to shell  110 . The resonators can be driven in a manner known to those skilled in the art, and can be driven at a main frequency and power level as desired to cause vibration, oscillation, movement, or other mechanical and acoustic modes in shell  110 . 
         [0026]    The shell  110  has an outer or exterior surface  112  and an inner or interior surface  114  substantially defining an enclosure or enclosed volume of the resonator  100 . In one or more embodiments, it is desired to electroplate or treat the interior surface  114  to optimize or improve the ability to wet the interior of the shell  110  and achieve a good acoustic coupling to the cavitation fluid within the shell  110 . 
         [0027]    Accordingly, in some embodiments, a plating solution  150  is provided within shell  110 . The plating fluid is a fluid material having certain desired properties and chemical composition as is discussed further herein. 
         [0028]    In addition, an activation element or source  130  is positioned within the shell  110 . The source  130  may be positioned at or near the center of the volume of the resonator  100 . In a spherically-shaped resonator, source  130  may be placed at the geometric center of the sphere. The source  130  may comprise Indium in some embodiments. In other embodiments, the source may comprise Gallium. The source  130  may be in the form of a small spherical source or mass suspended by a platinum wire  120  or similar conducting element. The platinum wire  120  is in turn coupled to a copper wire with a sleeve portion  125 . The wires penetrate into the resonator  100  through a port  115  provided at a chosen spot in the shell  110  and allowing penetration of the anode through the shell  110 . The port  115  may be sealed around copper wire sleeve  125  in a way that preserves the plating solution or fluid  150  inside resonator  100 . 
         [0029]    The electrical plating process is facilitated by an electrical source  140 , which may be a DC current source or voltage source or other driver. The electrical source  140  can have a positive and a negative end to develop an electromotive force to sustain the plating process. In some embodiments, one end  142  of a circuit comprising the electrical source  140  is coupled to a spot on the exterior surface  112  of resonator shell  110 . Current is then driven through the circuit to achieve the desired plating onto the interior surface  114  of shell  110 . 
         [0030]    Once the interior surface  114  is sufficiently plated it is treated to other steps as described below and the resulting resonator may be more suited for taking on and cavitating a metal fluid within the resonator. In one embodiment, the electrodes are conductive wires providing a low resistive electrical pathway between the current source and the anode and cathode. In some embodiments, the electrodes can be any suitable conducting conduit or trace. 
         [0031]    The preparation of the electrode/anode junction for the plating operation may be designed for optimal effect, as the plating bath can be poisoned by ions from other metals, e.g., nickel or copper. In some embodiments, the assembly may be coated with a chemically resistant epoxy/insulator to prevent contamination of the plating bath. 
         [0032]    In one embodiment, the current source  140  is one sufficient to support current densities of 10-20 Amps (Amperes) per square foot of the cathode substrate to be plated. Current densities of 100 Amps per square foot may be used, and the temperature of the plating bath may be maintained between 20-25° C. The use of cooling coils may help to keep the bath at this temperature when operating at higher current densities in some embodiments. Excessive current densities might cause the constituents of the plating bath to break down, and as a result, the current source can be replaced with a battery or any other suitable power supply if desired in some embodiments. 
         [0033]    In the current embodiment of the present invention, the cathode/substrate is a 17-4 stainless steel acoustic resonator in the shape of a spherical shell  110 . In other embodiments, the bulk can be made of other steels, such as 304 or 316 steels. However, one skilled in the art will appreciate that the invention is not limited to stainless steel and that any suitable substrate can be used. 
         [0034]    The present discussion also illustrates one or more methods for achieving the instant electroplating and surface preparation for acoustic resonator chambers. For example, an Indium sulfamate electrolytic solution  150  may be used in the manner below allowing the use of standard plastic or plastic lined plating tanks with continuous filter pump agitation. The plating bath may be operated at room temperature; therefore, no immersion heaters are necessary in some embodiments. 
         [0035]    In some embodiments, sulfamic acid may be used to control the bath pH. The bath may have a pH of 1-3.5 (and more specifically, e.g., a pH of 1.5-2.0). In the present embodiment, 26.4 grams per liter is used. The pH can be adjusted by making additions of sulfamic acid. If the pH gets too high the indium will precipitate out in the form of Indium hydroxide, which causes the solution to assume a milky-white appearance. Over time, the pH of the bath will rise. The pH is frequently monitored using a pH meter. The pH is maintained within this range by small additions of a 10% solution of sulfamic acid dissolved in distilled or deionized water. 
         [0036]    The bath  150  may be further prepared by addition of Indium sulfamate at a concentration of 105 grams per liter of solution. Because the anode efficiency is 100% and the cathode efficiency is 90%, the Indium concentration tends to rise over time, leveling off at about 200-250 g/l. This is a normal situation and the rise in Indium concentration does not necessarily affect the operation of the bath except perhaps for excessive driving currents, which may cause the hydrolysis of water at the anode and oxygen gas to evolve. 
         [0037]    The bath  150  may further include other constituents such as 150 grams per liter of Sodium sulfamate, 46 grams per liter of Sodium Chloride, 8 grams per liter of Dextrose, and 2.3 grams per liter of Triethanolamine. These are used to increase bath conductivity and buffers, respectively. 
         [0038]    In some embodiments, it is preferable to have the surface area of the anode be approximately equal to or greater than the surface area of the work pieces. 
         [0039]    The following describes a preferred embodiment, provided for the purpose of illustration, but not by way of limitation. As an example, the base metal of the shell  110  may be cleaned either by ultrasonic solvent vapor degreasing or by immersion in a commercial alkaline cleaning bath operated at 80-90° C. for about 10-15 minutes, followed by a thorough hot water rinse. The base metal then should be acid activated by immersion in a 10-15 percent volume of sulfuric or hydrochloric acid solution at room temperature for 3-5 minutes, followed by a quick cold water spray rinse. The concentrations and times given are illustrative in nature and can be modified as desired for a given purpose. 
         [0040]    Following the acid activation, the base metal may be immersed in a 5 percent by weight solution of sulfamic acid solution for 1-3 minutes. Depending on the vessel or the work piece, the work piece is filled or placed in the sulfamate plating bath  150 . 
         [0041]    The anode source  130  is lowered into the bath  150  to the proper height in the center of the sphere. Current is applied via the current source in accordance with the current density boundaries. Time dictates the amount of Indium deposited at the rate of 20 amps per square foot resulting in an exemplary rate of deposition of Indium of 0.001486 inches deposited per hour. 
         [0042]    Following deposition, the anode and solution are removed from the sphere. The interior of the sphere is given a thorough rinse in deionized water and air dried in an inert environment (e.g., argon, etc.). Gallium metal is now applied to the surface of the bulk in the form of liquid. In another embodiment, the Gallium is in a solid form. In either event, the Gallium mixes with the Indium creating a Ga/In alloy producing a mirror-like wetted surface at room temperature. The resonator  100  can now be filled in an inert environment with liquid Gallium with no threat or reduced threat of oxide boundary layers with inhibit acoustic resonation or cavitation within the resonator  100 . 
         [0043]    In other embodiment, the Indium anode source  130  may be replaced with one of Gallium and using a high alkaline plating solution  150  instead of the sulfamate bath. It should be noted that at highly alkaline pH values oxides or hydroxides may dissolve as soluble Ga species. Therefore, it may be possible to electrodeposit Ga in a bath of pH greater than 14 containing Ga salts using high concentrations of KOH and NaOH in the bath formulation. High concentrations of alkaline species, however, may cause corrosion of the equipment as well as the cathode material itself. A limit on the amount of Ga that can be dissolved in the form of acidic Ga salts (GaCl3, Ga(NOa)3 etc) in such solutions before Ga starts to precipitate may exist in some embodiments. Accordingly, the pH may be adjusted again by further addition of alkaline species such as NaOH and KOH. As pointed out above, solutions comprising a large molar amount of caustics may be difficult to handle and may also have high viscosity. Therefore, the viscosity and caustic content may be controlled in some embodiments. High viscosity can cause hydrogen bubbles formed on the cathode may adhere more to the cathode making it difficult to remove them by stirring or other means of mass transfer. Such gas bubbles on the cathode surface increase defectivity of the deposited Ga layer. The present techniques take these effects into consideration, and embodiments hereof reduce and resolve these issues so that bubbles are removed or reduced in the system. 
         [0044]    The electrodeposition of Ga, or other suitable materials of similar chemical and/or mechanical, acoustic, hydrodynamic, or electrical property, is performed in a similar manner as described in the above embodiments. Gallium plates to the stainless steel sphere interior  114  resulting in an improved wetting surface for liquid Ga when applied. This effects a boundary layer devoid of oxides which impede transference of acoustic power from the shell and drivers (not shown) to the bulk of the liquid metal. 
         [0045]    Once properly treated, the resonator  100  is better suited to provide cavitation within the resonator, especially for metallic substances or liquid metals such as Gallium, Indium or others. 
         [0046]    By way of overview,  FIG. 2  illustrates an exemplary and simplified process  200  for electrodeposition and preparation of an interior surface of a metal acoustic resonator. 
         [0047]    At step  202  the resonator is provided consistent with the needs of a specific application, and can include for example providing a spherical or cylindrical or other shape of metal walled resonator. 
         [0048]    At step  204  the interior surface of the resonator is cleaned as described above. The cleaning  204  may be followed by a rinsing step  206 , which can include rinsing with water or with another inert or suitable fluid. 
         [0049]    At step  208  the surface is activated and prepared for electrodeposition at step  210 . 
         [0050]    Now referring to  FIG. 3 , an exemplary process  300  for electroplate deposition onto a metal resonator is provided. As before, a metal shell resonator having an interior surface and volume thereof is provided. At least the interior surface is cleaned at step  302 . This is followed by a rinse, such as a hot water rinse in step  304 . It should be appreciated that the rinse may be done using another fluid or gas and does not necessarily need water for said rinsing step. 
         [0051]    At step  306 , the surface being treated is inserted into an acid solution, such as to activate the surface as discussed above. This is followed by another optional rinsing step  308 , which an be in the form of a cold water rinse. 
         [0052]    The resonator is filled or substantially filled with the plating bath solution, which is typically a fluid as described above, at step  310 . 
         [0053]    The anode source is inserted into the volume of the resonator being treated at step  312 . 
         [0054]    Current or electromotive drive is applied at step  314  to achieve the electroplating process and coat the interior surface of the acoustic resonator as needed. The process of electrodeposition is continued as needed, and is stopped at step  316  based on a criterion or set of criteria. The criteria for stopping the electrodeposition may include a predetermined length of time, an integrated time-current calculation, a thickness of deposited material, a temperature preset, a predetermined electrical property of the system, or other criteria. 
         [0055]      FIG. 4  illustrates an exemplary cross-section of a treated resonator  400 . The metal shell  406  of the resonator is for example stainless steel. The shell  406  is coated with an electroplated layer  404  as described above. The fluid  402  then wets along the surface of electroplated layer  404  and substantially fills any voids so that cavitation or other acoustical effects can take place in fluid  402 . 
         [0056]      FIG. 5  illustrates an exemplary embodiment  500  whereby a portion of a resonator body  502  is treated by electrodeposition as described above. The body portion  502  comprises for example a partial spherical (e.g., hemispherical) portion. The system for treating the resonator includes a DC power supply  504  that provides electromotive driving force for the plating steps. 
         [0057]    A cathode  506  is provided in an electro-polish solution  508  and the process plates the interior surface of portion  502  until sufficient treatment or deposition has taken place. The other portions of the system may be similarly treated, and afterwards, the system may be assembled using the plurality of treated portions. So for example, two halves of a substantially spherical acoustical resonator may be treated as described above, and the halves may then be welded, joined, bolted, threaded, adhered, or otherwise coupled to form the resultant acoustic resonator apparatus or cavitation chamber. 
         [0058]    The present invention should not be considered limited to the particular embodiments described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable, will be readily apparent to those skilled in the art to which the present invention is directed upon review of the present disclosure. The claims are intended to cover such modifications.