FIELD OF THE INVENTION
- BACKGROUND OF THE INVENTION
This invention relates to processes and systems for forming high voltage, anodized valve metal anodes for use in wet electrolytic capacitors. This type of anode is suitable for use in high voltage capacitors particularly for use in implantable medical devices (IMDs).
The term “valve metal” stands for a group of metals including aluminum, tantalum, niobium, titanium, zirconium, etc., all of which form adherent, electrically insulating, metal-oxide films upon anodic polarization in electrically conductive solution, e.g., formation electrolytes.
Wet electrolytic capacitors generally consist of an anode, a cathode, a barrier or separator layer for separating the anode and cathode and an electrolyte. In tubular electrolytic capacitors, anodes are typically composed of wound anodized aluminum foil in which subsequent windings are separated by at least one separator layer. The anodes in flat electrolytic capacitors may consist of stacked sheets of anodized aluminum or of tantalum sintered structures separated from the cathode by at least one separator layer as described further below. Such electrolytic capacitors find wide application in industry including in implantable medical devices (IMDs), such as external and implantable defibrillation apparatuses.
As described in commonly assigned U.S. Pat. No. 6,006,133, a wide variety of IMDs are known in the art. Of particular interest are implantable cardioverter-defibrillators (ICDs) that deliver relatively high-energy cardioversion and/or defibrillation therapy to a patient's heart when a potentially lethal tachyarrhythmia (e.g., ventricular fibrillation, ventricular tachycardia, atrial tachycardia, atrial fibrillation) is detected. Prior to delivering high voltage therapy one or more high voltage capacitors are rapidly charged to a high voltage depending on the type of desired therapy delivery (e.g., ventricular defibrillation, atrial cardioversion, etc.) and the type of device (e.g., external or internal). In practice, typically a relatively low voltage battery operatively couples to a step-up (e.g., a fly-back type) transformer, and the capacitor(s) are discharged across a subject's myocardium. The therapy delivered can include customized waveforms (e.g., bi-phasic polarity, sharp- or ramp-type leading edge, exponential decay, and the like) and is appropriately timed for a patient's then-present heart rhythm. Current ICDs also typically possess single or dual chamber pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia and were referred to previously as pacemaker/cardioverter/defibrillators (PCDs). Earlier automatic implantable defibrillators (AIDs) did not have cardioversion or pacing capabilities. For purposes of the present invention, ICDs are understood to encompass all such IMDs as well as external devices known as automatic external defibrillators (AEDs) having at least high voltage cardioversion and/or defibrillation capabilities.
Energy, volume, thickness and mass are critical features in the design of ICD implantable pulse generators (IPGs) that are coupled to the ICD leads. Given the rising popularity, efficacy, declining prices and recent over-the-counter status of certain AEDs, such devices are almost certainly becoming small and more portable. Both ICDs and AEDs have historically utilized relatively bulky and expensive battery and high voltage capacitor units to provide the energy required for the therapies they provide. Presently, ICDs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams.
It is beneficial to patient comfort and minimizes complications due to erosion of tissue around the ICD to reduce the volume, thickness and mass of such capacitors and ICDs without reducing deliverable energy. Reductions in size of the capacitors may also allow for the balanced addition of volume to the battery, thereby increasing longevity of the ICD, or balanced addition of new components, thereby adding functionality and additional features to the ICD. It is also desirable to provide such ICDs at low cost while retaining the highest level of performance. At the same time, reliability of the high-voltage capacitors cannot be compromised. Aluminum and tantalum based electrolytic capacitors have usually been employed as high-voltage ICD capacitors. An aluminum electrolytic capacitor that is incorporated into an ICD is disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 09/607,830 filed Jun. 30, 2000, for IMPLANTABLE MEDICAL DEVICE HAVING FLAT ELECTROLYTIC CAPACITOR FORMED WITH PARTIALLY THROUGH-ETCHED AND THROUGH-HOLE PUNCTURED ANODE SHEETS filed in the names of Yan et al.
The performance of electrolytic capacitors is dependent upon several factors, e.g., the effective surface area of the anodes and cathodes that can be contacted by electrolyte, the dielectric constant of the oxide formed on the metal surface, the thickness of the oxide layer on top of the metal surface, the conductivity of the electrolyte, etc. In all electrolytic capacitors, the thickness of the anodic oxide layer is approximately proportional to the potential applied to the anode during the formation of the anode, i.e., at the time when the anode is immersed into the formation electrolyte. For aluminum, the oxide grows approximately by 1.2 nm per Volt; for tantalum this “rate” is somewhat higher, approximately 1.7 nm per Volt.
Niobium and tantalum anodes are typically made in the form of a pressed powder pellet or “slug” when used in an electrolytic capacitor. The density of the anode slugs is typically significantly less than the density of the metals themselves, i.e., up to ⅔ of the volume of a given slug may be open or pore space. The final density of the anode slug is largely determined at the time of pressing, when a known amount of powder is pressed into a known volume. For the proper formation of the anode slug it is critical to achieve a fairly homogeneous distribution of pores throughout the anode slug since the forming electrolyte needs to wet even the most “remote” cavities or interstices in the karst-like internal structure of the anode. This is specifically important for comparatively large anodes with volumes of the order 1 cm3 or above.
Furthermore, it is critical that electrolyte may flow fairly readily through the structure because a significant amount of electrical power may be dissipated as heat during the formation process. During formation of oxide layers on the surface and interstices of a valve metal anode, local potential differences of several hundred volts together with local current densities of several tens of milliamperes may be encountered (i.e., 20 to 30 Watts may be dissipated as heat). If not regulated in some way, the dissipated heat can affect the quality and performance of the anodes, as is discussed further herein.
Various methods are used to achieve a homogeneous distribution of pores throughout the anode, as is well known to those skilled in the art. Traditional methods of forming the oxide layers are described in the prior art, e.g., in U.S. Pat. Nos. 6,231,993, 5,837,121, 6,267,861 and in the patents and articles referenced therein. An example of an improved method for forming advance valve metal anodes that may be incorporated-into IMDs is disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 10/692,649 filed Oct. 23, 2003, for ADVANCED VALVE METAL ANODE WITH COMPLEX INTERIOR AND SURFACE FEATURES AND METHODS FOR PROCESSING SAME, the entire contents of which is incorporated herein by reference. Typically, a power source capable of delivering a constant current and/or a constant potential is connected to the anode slug that is immersed in the electrolyte. The potential is then ramped up to a desired final potential while a constant current flows through the anode-electrolyte system.
Regardless of the process by which the valve metal powder was processed, pressed and sintered, valve metal powder structures (e.g., tantalum, niobium, and the like) are typically anodized by the controlled application of formation potential and electrical current while the anode is immersed in a fluidic formation electrolyte. A typical formation electrolyte consists of ethylene glycol or polyethylene glycol, de-ionized water and H3PO4 and has a conductivity anywhere between 50 μS/cm (read: micro-Siemens per cm) to about 20,000 μS/cm at 40° C.
Conventional practice has been to form the anodically polarized valve metal to a target formation potential with a constant electrical current flowing through the anode-electrolyte system. Typically, stainless steel cathodes are used with the glycol-containing electrolytes. The magnitude of the current depends on the electrolyte, the valve metal powder type and the size of the valve metal structure. Most of the electric current flowing through the anode-electrolyte system is used in the process of the anodic oxidation for the electrolysis of water as outlined below:
Therefore, the current setting directly influences the speed of the anodization reaction: using Faraday's laws, it can be readily shown that very low formation currents—for sintered Ta samples, low currents would mean currents of the order of 0.1 μA/μC or approximately 1 μA/cm2—will require long formation times well in excess of 1 week for anode sizes and target formation potentials relevant for ICD capacitors. Adjusting these parameters according to conventional practice is within the knowledge of those skilled in the art. An anode is said to be fully formed when the dielectric layer (e.g., tantalum pentoxide covering a tantalum anode) has reached a certain thickness and structure which enables the anode to hold the electrical charge at the desired operating potential for an appropriate time duration and without an excessive amount of charge leaking out. The amount of charge leaking out per time unit is called the leakage current of the capacitor. A typical formation process may take anywhere between 1 and 250 h, depending upon such factors as the size and porosity of the anode structure and the electrolyte viscosity, temperature and conductivity.
The above-referenced '121 patent discloses use of particular electrolytes and applied potentials and currents that depart from the normal practice of applying a constant potential and current as described in the above-referenced '993 patent. The electrolyte in the '121 patent comprises glycerine solutions of dibasic potassium phosphate which have been heated to 180° C. for 1-2 hours, or to 150° C. overnight. It is reported that such thermally treated electrolytes behave far differently when employed as anodizing electrolytes at 150° C. or above compared to electrolytes that are not thermally treated. It is alleged that the thermally treated electrolytic solutions provide anodic films on tantalum and other valve metals which are not limited in thickness according to the anodizing voltage, but instead continue to grow thicker so long as anodizing potential is applied.
The '121 patent asserts that relatively uniform thick films can be produced within the interstices and on the surface of sintered tantalum powder capacitor anodes if the potential applied to the anode bodies is applied as pulsed direct current (DC). The positive bias pulse is continued for approximately 0.3 seconds or less with an unbiased or open-circuit period of at least 0.3 seconds between pulses. It is also suggested that alternating current (AC), half-wave AC, saw-tooth waveforms, etc., can also be used in place of pulsed DC to obtain uniform anodic films in these electrolytes. However, no other details are provided. Clearly, the goal of applying pulsed potentials in the '121 patent is to support the growth of oxide the thickness of which is not limited by the formation potential.
The above-referenced '993 patent reports that there are problems with conventional valve metal anodization processing due to heating of the electrolyte inside the interstitial pores of the porous tantalum pellet during the anodization process. The heating of the electrolyte is due to the thermal dissipation of electrical power within the anode the structure. The dissipation of power may be non-isotropic, that is, certain local regions or areas within in the anode may become very hot while others remain comparatively cool. In the hot areas of the karst-like structure, which may be likened to an assembly of steam vessels, the electrolyte may decompose and/or the sinter-structure may crack because of the increased internal pressure. As a consequence, instabilities may be introduced into the system, which adversely affect the performance of the capacitor. Such instabilities are, of course, unacceptable. Therefore, thermal management of the anode system during anodization becomes critical in order to anodize large sintered anodes of volumes 1 cm3 and above.
The '993 patent suggests periodically replacing heated electrolyte inside the anodized structure with fresh electrolyte from the anodization electrolyte bath by means of diffusion during periods of time when the applied formation potential is turned off. In other words, the formation potential is periodically dropped to zero for a time long enough to allow the electrolyte inside the pellet to cool and diffuse. Therefore, hot, condensed electrolyte, which, upon prolonged heating would likely be reduced to solid residuals, may be replaced by fresh electrolyte from the anodization bath during the time period in which the formation potential is turned off.
In addition, the current is reduced in a stepwise fashion in conjunction with raising the formation potential, according to the authors of patent '993. In one of the examples listed in patent '993, the current I1 is initially set in a range of about 80 mA for an eight gram anode. The current I1 is maintained until a formation potential V1=75 V is reached. Following this step, the formation potential is turned off for three hours to allow for cooling and electrolyte replenishment inside the anode pellet. The potential is then raised in steps, the size of which decreases with increasing potential while, at the same time, the current, that is allowed to flow through the system, is decreased. In the potential regime just below the target formation potential (e.g., about 231 Volts), the current setting in the above referenced example is just 31 mA, or approximately ⅓ of the initial current setting. In another example, the rest intervals are on the order of one hour, and the formation potential steps are applied for one to three hours. This method clearly can become very time consuming, as can be readily estimated using Faraday's laws. In addition, the application of the method suggested in '993 results in prolonged periods of anodization time during which low currents are used together with high potentials, specifically in the potential regime just below the target formation potential.
According to models from L. L. Odynets (Soviet Electrochemistry 23(12) pp 1591-1594 (1987)), these conditions are favorable for the occurrence of field crystallization: during field crystallization, crystalline tantalum pentoxide grows at the metal-oxide interface, i.e., beneath the amorphous oxide that grows preferably under high current formation conditions. In the long term, the growth of crystalline oxide seeds beneath the previously grown amorphous layer may lead to the destruction of the anode. In the short term, crystalline growth may result in unfavorably high leakage currents. Therefore, high potential, low current, formation conditions should be avoided or kept as short as possible.
The authors of the '993 patent do not mention another important component of the anodization process, namely the agitation of the electrolyte. Yet agitation can advantageously maintain an approximately isotropic temperature profile throughout the anode during anodization. Typically, stirring impellers (e.g., rotating magnets) are placed within the solution and rotated to agitate the electrolyte.
In U.S. Pat. No. 6,235,181, Kinard et al. emphasizes the need for agitation of the electrolyte during the formation process and also suggest ultrasonic agitation of the electrolyte during the anodization process as an alternative to the use of stirring impellers. However, the use of ultrasonic agitation in the '181 patent is explicitly directed to the above mentioned process of non-thickness limited anodizing of sintered Ta anodes, where a very specific electrolyte (dibasic potassium phosphate dissociated in heat treated glycerol) is prescribed. This electrolyte is to be used at temperatures at or above 150° C., a temperature regime in which local temperature fluctuations are difficult to avoid. Ultrasonic agitation is expressly applied to avoid or “drastically reduce” temperature fluctuations within the bulk of the electrolyte at these comparatively high electrolyte temperatures.
In summary, the prior art anodization processes for comparatively large, high voltage wet electrolytic valve metal anodes tend to take a considerable amount of valuable production time and they tend to produce low yields, either because deposits of electrolyte decomposition products may render the anode unusable or because field crystallization has caused unacceptably high leakage currents.
Accordingly, there is a need to define new and improved formation processes for high voltage, electrolytic valve metal anodes. Such processes must deliver a high yield of fully formed anodes, must be more economical than the processes reported in the patent literature so far, i.e., it must be shorter, and/or must allow for the use of comparatively high current settings throughout the entire formation cycle.
Many IMDs generally include a battery and at least one capacitor operatively coupled to microelectronics disposed in a hermetic housing adapted to receive a proximal end of one or more medical electrical leads for therapy delivery. The components disposed within the housing occupy approximately ⅓ of the IMD by volume. For a number of reasons, it is highly desirable to reduce the volume of at least one of these components, allowing for increased capacity or volume of any one or both of the other two components or simply allowing for a decrease in the overall size or volume of the IMD. Accordingly, there is a need to provide methods and apparatus for forming high voltage, valve metal anodes having reduced volume thus resulting in smaller high-energy capacitors for use in ICDs.
Free flow of liquid electrolyte during anode processing (e.g., formation of surface oxide on the anode) and during subsequent operation as an electrochemical cell, traditionally has been used in an attempt to optimize capacitor performance. One reason relates to the fact that the electrolyte used during anodization (which is oftentimes referred to as a “formation electrolyte”) can become overheated within the interstices of the anode. During formation, a power source capable of delivering a constant electrical current of about 100 mA per anode and a constant electrical potential of several hundred volts is connected to the anode slug that is immersed in the electrolyte. Electrical energy as high as 20 to 30 Watts per anode may be dissipated as heat and local differences in applied electrical potential may be encountered. This overheating adversely affects oxide formation and may cause electrolyte residue (polymer-like deposits) to accumulate within the pores or interstices. During operation of the electrochemical cell continued free circulation of the electrolyte, typically referred to as the “working electrolyte,” is required for rapid charge access even in the finer crevices, i.e., ions within the electrolyte must be allowed to rapidly migrate to provide a balance for the charge on the metal electrode. Such charge migration occurs during charge and discharge cycling of the capacitor.
Deposits of such electrolyte residue deleteriously take up void spaces, which preferably should be occupied by either formation electrolyte or working electrolyte, respectively, during anodization or during operation of a electrochemical cell in an IMD. The presence of such residue can negatively impact crystalline structure of oxides during formation or the migration of ions in the forming electrolyte during formation or in the working electrolyte during operation. Further such residue can: decrease the energy density via a reduction of capacitance, compromising performance of completed electrochemical cells; can increase internal resistance of the capacitor (also known as the equivalent series resistance or “ESR”) thereby impeding rapid discharge and recharge of the capacitor. The residue can also result in lower efficiency of a capacitor measured as a ratio of energy out to energy in (EOUT//EIN) of the capacitor. For comparison, capacitors can have an efficiency as high as 85%. In the field of IMDs, any increase in energy density represents a valuable improvement. Without being bound or limited by theory or experimental observation, the inventor has found that it is possible to realize an increase of at least up five percent (5%) in energy density with one or more of the various embodiments of the processes and systems of the invention.
- SUMMARY OF THE INVENTION
Thus, the present invention discloses, describes, depicts and claims methods and apparatus for formation of high voltage, valve metal anodes that, by example and without limitation, result in one or more of the following benefits or advantages: reduced or eliminated formation of electrolyte residue deposits, improved oxide structure, lower ESR, increased capacitance and, in turn, increased energy density, and decreased formation time. One or more of these benefits or advantages can be realized with one or more of the various embodiments of the processes and apparatus of the invention.
The present invention provides various embodiments of processes and systems for formation of a high-voltage, anodic oxide on a valve metal anode. The various embodiments of processes according to the teaching of the invention generally provide a process for forming a high voltage, anodic oxide on a valve metal anode, comprising: immersing a valve metal anode in an electrolyte forming bath comprising a formation electrolyte, performing an anodization step; and maintaining a relatively cool temperature of the formation electrolyte in the forming bath (e.g., at a temperature at or below 40° C.) during performance of the anodization step. In one form of the invention, the temperature of the formation electrolyte is maintained with an accuracy of about +/−2° C. The accuracy in maintaining the temperature of the formation electrolyte is generally provided by controlled cooling of the formation electrolyte. In some embodiments of the invention, the process the anodization step comprises application of electrical potential under a constant electrical current until a target electrical potential is reached and then applying the electrical potential at the target potential until the electrical current falls below a predetermined termination current level. In a variation of embodiments of processes according to the invention, the anodes may be removed from the formation electrolyte, heat-treated and the anodization step is repeated.
In an embodiment of a process for formation of oxide layers according to the invention, a process for forming a high-voltage, anodic oxide on a valve metal anode is provided, comprising: immersing a valve metal anode in electrolyte forming bath, comprising a formation electrolyte; performing an anodization step under a constant current until a target potential is reached and then at the target potential until the current falls below a predetermined termination current level; circulating a flow of formation electrolyte from the forming bath through a heat exchanger to provide a cooled flow of formation electrolyte, and accurately maintaining a relatively cool temperature of the formation electrolyte in the forming bath (e.g., at a temperature at or below 40° C.) during anodization accompanied by a flow of relatively cool formation electrolyte in and about the forming bath.
In another embodiment of a process according to the invention a process for forming a high-voltage, anodic oxide on a valve metal is provided, comprising: providing an electrolyte forming tank configured to receive one or more anodes, the tank containing an electrolyte forming bath, comprising a formation electrolyte; providing an electrolyte circulation subsystem for circulating and cooling the formation electrolyte; immersing one or more anodes in the formation electrolyte; circulating formation electrolyte from the forming bath through the circulation subsystem to provide a cooled flow of formation electrolyte; applying an electrical potential to the one or more anodes, the electrical potential being ramped up to a target voltage under constant current until a target potential is reached; continuing application of the electrical potential to the one or more anodes at the target potential until the current falls below a predetermined termination current level; and regulating electrolyte flow rate and temperature so that the temperature of the formation electrolyte in the forming tank is accurately maintained at a temperature at or below 40° C. during application of the electrical potential.
In an embodiment of a system according to the invention an electrolytic bath system is provided, comprising: a tank configured to receive one or more anodes, the tank containing an electrolyte forming bath comprising a formation electrolyte. The tank may be configured with an fluid inlet and a fluid outlet. An electrolyte circulation subsystem connected in flow-through communication with the tank is provided. The subsystem is configured to receive a flow of electrolyte from the outlet, to lower the temperature of the flow of electrolyte, and to return the flow of electrolyte to the fluid inlet.
In a variation of this embodiment, instead of an electrolyte circulation subsystem, the walls of the tank are in direct contact with a cooling fluid or medium to accomplish heat transfer.
In another embodiment of a system according to the invention an electrolytic bath system is provided, comprising: a tank having a lower level and an upper level in flow-through communication with the lower level, the lower level having an inlet configured to receive a flow of electrolyte into the tank, the upper level having an outlet configured to discharge a flow of electrolyte from the tank, the upper level being configured with a plurality of anode formation slots, the slots being sized to receive at least one anode, the slots each having an opening through which electrolyte flows from the lower level into the upper level; and an electrolyte circulation subsystem, the subsystem being connected to the inlet and the outlet.
In the foregoing and other embodiments of systems according to the invention, the electrolyte circulation subsystem comprises a heat exchanger coupled to a refrigeration unit and at least one pump for circulating electrolyte between the tank and the circulation subsystem. The circulation subsystem can further comprise at least one pump or blower for circulating a cooling fluid through the refrigeration unit. In some embodiments of systems according to the invention, the tank may be an enclosed housing configured with a lid and the system may further comprise a vacuum unit in order to force forming electrolyte even into the smallest crevices of the karst-like sinter structure.
In yet another embodiment of a system according to the invention The electrolytic bath system, comprising: a tank containing an electrolyte forming bath comprised of a formation electrolyte, the tank being configured to receive one or more anodes, the tank having interior and exterior walls spaced to define a plenum through which a cooling fluid can be circulated, an inlet for receiving the cooling fluid into the plenum and an outlet from which the cooling fluid exits the plenum; and a cooling fluid circulation subsystem connected in flow-through communication with the inlet and the outlet.
In order to maintain the temperature of the formation electrolyte or to regulate electrolyte flow rate and temperature in the various embodiments of the invention, the systems further comprise an electronic controller equipped and configured to regulate one or more of electrolyte circulation rate, heat transfer rate, and cooling fluid circulation rate.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is specifically useful for forming high voltage, high capacitance anodes as it allows for managing the thermal energy dissipation during the formation process and provides for a high yield of fully formed anodes with improved energy density and low leakage currents at the operating voltage.
These and other advantages and features of the present invention will be appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiment of the invention when considered in connection with the accompanying drawings, in which like numbered reference numbers designate like parts throughout the figures thereof, and wherein:
FIG. 1 depicts a block diagram showing steps of an embodiment of a process according to the invention.
FIG. 2 depicts a block diagram showing steps of an embodiment of a process according to the invention.
FIG. 3 depicts a block diagram showing steps of an embodiment of a process according to the invention.
FIG. 4 depicts a block diagram showing steps of an embodiment of a process according to the invention.
FIG. 5 depicts a schematic view of an embodiment of a system according to the invention.
FIG. 6 depicts a perspective view of a forming tank useful in the system FIG. 3.
FIG. 7 depicts a graphical depiction of typical prior art formation traces where the potential rises smoothly until the target potential is reached and is then held constant at the target potential for a predetermined hold time during which the current becomes smaller and smaller.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 8 depicts a graphical depiction of formation traces obtained using a pulsed formation potential.
The present invention provides novel processes and systems in forming and manufacturing compact, high voltage, high capacitance and high energy density AVM anodes. As further detailed herein, various embodiments and forms of the present invention provide distinct advantages over the prior art. Also, even though only a few valve metals, which are known for use in conjunction with IMDs are described in detail herein, the invention is not so limited. For example, any valve metal may be used when practicing the present invention. By example and without limitation, the following materials may be used: tantalum, niobium, aluminum, zinc, magnesium, zirconium, titanium, hafnium, palladium, iridium, ruthenium, molybdenum including combinations and/or alloys thereof. The materials used can take the form of etched sheets or in the form of pellets pressed from powdered material. In one aspect, each of the foregoing is susceptible of accurate, predictable control of oxide thickness during formation and resulting oxides that are dense, tightly adhering and electrically insulative (e.g., having high dielectric strength and a high dielectric constant). Finally, while the present invention is described primarily with respect to liquid electrolyte, mixed and/or all-solid-electrolyte may be utilized in the final capacitor component in accordance with the present invention.
FIG. 1 to FIG. 4 depict block diagrams showing steps of certain aspects of embodiments according to the invention. In general, the processes of the embodiments depicted in these figures can be carried out in electrolytic bath systems according to the invention.
Referring now to FIG. 1, a method 100 is depicted in which an anode is immersed in an electrolyte formation bath at 102, and then anodized at 104 while the temperature of the formation electrolyte is maintained at a relatively low temperature 106 (e.g., at or below about 40 degrees Celsius).
Referring to FIG. 2, a method 200 is depicted in which an anode is immersed in an electrolyte formation bath at 202, and then anodized at 204 while under constant electrical current until a target electrical potential is reached. After the target electrical potential is reached the anodization continues until the electrical current falls below a predetermined termination current threshold. During anodization the temperature of the formation electrolyte is maintained at a relatively low temperature 206 (e.g., at or below about 40 degrees Celsius).
Referring to FIG. 3, a method 300 is depicted in which an anode is immersed in an electrolyte formation bath at 302, and then anodized at 304 while under constant electrical current until a target electrical potential is reached. After the target electrical potential is reached the anodization continues until the electrical current falls below a predetermined termination current threshold. At step 306, the formation electrolyte is circulated between the forming bath through a heat exchanger to provide the relatively cool electrolyte in and around the anode units. Thus, during anodization the temperature of the formation electrolyte is maintained at a relatively low temperature 308 (e.g., at or below about 40 degrees Celsius) in conjunction with the active cooling of circulating electrolyte.
With reference to FIG. 4, a method 400 is depicted in which an electrolyte forming tank is provided at step 402, said tank containing a volume of formation electrolyte. At step 406 one or more anodes are immersed in an electrolyte formation bath. At step 408 a formation electrolyte is circulated between the forming bath (tank) through a fluid circulation subsystem to thereby cool the formation electrolyte. At step 210 electrical potential is applied to the one or more anodes to grow oxide on the metallic surfaces thereof. During the foregoing steps, the flow rate and fluid temperature of the formation electrolyte are controlled to control the temperature at a relatively cool temperature at step 412 (e.g., at or below about 40 degrees Celsius).
An example of an electrolytic bath system within the scope of the invention is depicted in FIG. 5 in schematic view. The system 10 includes an electrolyte forming tank 20 and an electrolyte circulation subsystem 30 fluidly coupled to tank 20. Tank 20 may be provided with a lid or cover 21 as shown in FIG. 5 or without as shown in FIG. 6. Regardless, tank 20 can be configured with a fluid inlet 22 and a fluid outlet 24. The electrolyte flowing from a heat exchanger enters forming tank 20 through inlet 22 and exits forming tank 20 through outlet 24. Tank 20 is further configured to receive one or more anodes and contains an electrolyte forming bath comprised of a formation electrolyte such as discussed earlier hereinabove and known generally to those skilled in the art. The system further includes a pump 36 for circulating electrolyte between tank 20 and heat exchanger 32.
Tank 20, heat exchanger 32 and pump 36 are connected with sections of pipe or hoses 38. In FIG. 5 pump 36 is shown located down stream from tank 20 and between tank 20 and heat exchanger 32; however, location of pump 36 is a matter of design choice. Pump 36 may for example be located down stream from heat exchanger 32 and between heat exchanger 32 and tank 20. Further, more than one pump may be utilized. Further, no inlet or outlet may be utilized, if an impeller is intended for agitation and cooling is provided by contacting the outer walls of the bath with a cooling liquid. Further still, the tank may be provided with a plurality of inlets 22 and outlets 24. Single sections of pipe 38 are shown but there may be a plurality of sections of pipe 38 so that each inlet 22 and outlet 24 is connected to a section of pipe 38 which leads either to a plurality of pumps 36 or to a common conduit feeding into a single pump 36. Similarly, section of pipe 38 between pump 36 and heat exchanger 32 may be a single pipe section or may feed into a conduit that branches out into a plurality of sections of pipe 38 passing through heat exchanger 32. Such a configuration may provide for more rapid heat exchange if needed.
Heat exchanger 32 is coupled to a refrigeration unit 34. A cooling fluid flows through refrigeration unit 34 and heat transfers from the formation electrolyte flowing through heat exchanger 32 to the cooling fluid flowing through refrigeration unit 34. The source of cooling fluid may any of a variety of fluids known to those skilled in the art to be suitable for this purpose. For example, the cooling fluid could be heat transfer gas or liquid, e.g., air, water, liquid coolant. Refrigeration unit 34 is connected to a source of cooling fluid (not shown in FIG. 3 via sections of pipe 38 and a pump (also not shown in FIG. 3). The source of cooling fluid could be a dedicated source or it could be part of a larger facility cooling system, such as industrial water system, industrial liquid or gas cooling or refrigeration systems, or building air cooling or conditioning systems.
In operation, a flow of formation electrolyte (represented by directional arrows 41) exits outlet 24, is pumped through pump 36 to heat exchanger 32 where it is cooled and is then returned to inlet 22 of tank 20. The flow of formation electrolyte upon introduction into tank 20 mixes with the formation electrolyte within the batch. The flow of electrolyte circulating within the system should generally be sufficient to ensure uniform temperature distribution within the forming bath to avoid temperature fluctuations. However, uniform temperature distribution may be further enhanced or promoted by agitation during the formation cycle by means known to those skilled in the art, such as with stirring impellers or with ultrasonic agitation. Dissipation of heat may be further aided by configuring tank 20 with a plurality of fins or fans to transfer heat to the ambient air in the external processing environment.
In embodiments of system 10 having a tank 20 with lid 21, system 10 may further comprise a vacuum unit. In such embodiments, tank 20 and lid 21 are formed of material of sufficient strength and configured to withstand vacuum induced pressure differentials within tank 20. Further, when closed lid 21 must be able to form a vacuum tight seal for this purpose. When system 10 includes a vacuum unit, lid 21 and/or tank 20 are configured to form a vacuum seal when lid 21 is closed and a vacuum is induced. The vacuum induced must be sufficient to create a pressure differential capable of forcing formation electrolyte into the pores and interstices of the anodes immersed therein. This is accomplished by inducing a reduced pressure within the tank. A pressure of about 20 mbar has been found generally suitable for this purpose.
One non-limiting example is provided by the embodiment illustrated in FIG. 6. The tank of this embodiment, has a lower level with an inlet 22 configured to receive a flow of electrolyte into tank 20. It also has an upper level in flow through communication with the lower level. The upper level has an outlet 24 through which the flow of electrolyte is discharged from tank 20 to pump 36 or heat exchanger 32 much as is described relative to FIG. 5 above. The upper level is further configured with a plurality of anode formation slots 26 in which anodes may be immersed. Slots 26 are sized to receive at least one anode. Each slot 26 has an opening through which electrolyte rising from the lower level flows into the upper level. The flow of electrolyte 41 enters into the lower level through inlet 22 located in the low part of tank 20. The lower and upper levels are in flow-through communication via the openings of slots 26. The electrolyte rising from the lower level flows through the openings. Tank 20 of FIG. 6 may have 2 or more slots 26. For some processing, the tank may be configured with one or more slots, tens of slots or upwards of one hundred slots or more. The number of slots is a matter of process engineering choice and may practically be limited by the spaces in which the system and its components are to be located or other process considerations.
In order to maintain temperature or regulate temperature and electrolyte flow rate during processing, system 10, in its various embodiments may include an electronic controller. The controller is equipped and configured to regulate electrolyte circulation rate, the circulation rate of a cooling fluid, heat transfer rate or combinations thereof and may include one or more temperature sensors. The sensors may be located in the formation bath in tank 20, in heat exchanger 32, up stream and down stream of heat exchanger 32, upstream and/or downstream of refrigeration unit 34, amongst other locations. The sensors provide temperature reading outputs to the controller, which processes the data against preprogrammed control parameters. The controller then signals one or more subsystems or system components, such as pump 36, to increase or decrease the rate of electrolyte flow 41 or the rate of cooling fluid flow 43. Types of controllers known to those skilled in the art to be suitable for integrated control and variation of temperature and flow rates or for use with heat transfer subsystems can be utilized to vary, monitor and/or adjust process or operating parameters in system 10.
The process of forming a high voltage, anodic oxide on a valve metal can be carried out in embodiments of the systems of the invention described herein above or systems of different configuration. The process may be understood with reference to FIG. 1 to FIG. 4 and embodiments of the invention derived therefrom, as well as from the following description.
Heat generated during anodization in an electrolyte forming bath may negatively impact the quality of the anodic oxide deposited on a valve metal anode. The problems of conventional anodization processing have been noted above in the Background of the Invention. Prior art anodization has typically been carried out at temperatures ranging between 40° C.-80° C. More recently, in commonly assigned, co-pending U.S. application Ser. No. 10/058,437, one of the Applicants carried out anodization at a temperature of 40° with application of potential in a pulsed fashion to achieve thermal management.
Applicants have found that improved anodic oxide deposition can be achieved with processes according to the invention. More specifically, Applicants have found that with the processes of the invention, heat generated during anodization can be managed through controlled transfer of heat from the formation electrolyte in the forming bath, resulting in improved oxide formation. This can be accomplished by circulating a flow of electrolyte from the forming bath to be cooled in a electrolyte circulation subsystem 30 or a heat exchanger 32. This may also be accomplished by circulating a cooling fluid from a cooling fluid circulation subsystem through a plenum of a tank 20 configured with interior and exterior walls defining said plenum or through contacting the exterior walls of tank 20 with a cooling fluid or a cooled environment. In the latter case, cooling may be enhanced if tank 20 is configured with a plurality of fins or fans for dissipating heat into the environment. This environment may simply be the room or space in which tank 20 is housed or a cooling fluid circulated between the exterior walls of tank 20 and another tank or vessel in which tank 20 may reside.
With controlled cooling achieved with the embodiments of processes and systems of the invention, Applicants are able to accurately maintain the temperature of the formation electrolyte at a temperature at or below 40° C. with an accuracy of about +/−2° C. Further, Applicants have found that improved anodic oxide properties are achieved with the temperature control provided. Applicants have achieved these improved properties with anodic oxides deposited at temperatures at or below 40° C., at or below 30° C., at or below 20° C.; and at or below 10° C. Applicants believe that the process according to various embodiments of the invention can be carried out at temperatures as low as 0° C. and further that processing below that temperature is possible. The limitations upon low temperature processing reside in the properties of the electrolyte. At lower temperatures, some electrolytes may become too viscous to adequately wet the surfaces of anodes, particularly the surfaces within pores or interstices of anodes. Vacuum-initiation of processing, can in part compensate for reduced wetting capability of more viscous electrolytes by forcing the electrolyte into the pores. With attention to the properties of electrolytes and development of low temperature electrolytes that provide good surface wetting at low temperatures, the processes of the present application may be carried out at increasingly lower temperatures
In an embodiment of the process of the invention depicted in FIG. 1, an anode is immersed in an electrolyte forming bath comprised of formation electrolyte. An anodization step is performed as in later discussed herein. During the anodization step, the temperature of the formation electrolyte is accurately maintained at a temperature at or below 40° C.
In another embodiment of a process of the invention depicted in FIG. 2, an anode is immersed in an electrolyte forming bath comprised of formation electrolyte and an anodization step is performed under constant current until the target potential is reached. Once reached the anodization step proceeds at the target potential or at constant potential until the current falls below a predetermined level. During the anodization step, the temperature of the formation electrolyte is accurately maintained at a temperature at or below 40° C.
In another embodiment of a process according to the invention, temperature management is accomplished with circulation of a flow of formation electrolyte that is cooled to provide a cooled flow of formation electrolyte. This cooled flow is returned or recirculated and introduced back into the forming bath. With reference to FIG. 3, in this embodiment a valve metal anode is immersed in an electrolyte forming bath comprising a formation electrolyte. An anodization step is performed to form the anodic oxide on the surface of the anodes. The anodization step is performed under constant current until a target potential is reached and continues or proceeds at the target potential until a predetermined termination current level or is reached. During performance of the anodization step, the formation electrolyte will heat up due to the thermal dissipation of electrical power within the anode. In order to thermally manage the temperature of formation bath, applicants circulate a flow 41 of formation electrolyte from the forming bath in tank 2, as shown in FIG. 5. The cooled flow 41 is returned to the forming bath. With the cooling and recirculation of flow 41 of formation electrolyte, the temperature of the forming bath is accurately maintained at a temperature at or below 40° C. during performance of an anodization step.
If desired, the anodic oxide may be modified somewhat by removing the anode from forming tank 20, washing out the forming electrolyte and heat treating or annealing the anode to a temperature of about 350° C. in an oxygen-containing atmosphere followed by another anodization step or a re-anodization step, as described by D. M. Smyth et al. (J. Electrochem. Soc., Vol. 110(12), pp 1264-1270 (1963). One purpose of the heat treatment and a subsequent re-anodization is to improve the dielectric properties of the anode. An other purpose of the heat treatment is to widen fissures and cracks in the oxide so that they can be healed up in a one or more subsequent re-anodization steps. For the re-anodization, the anode is again immersed into the formation electrolyte and an other anodization step is performed, this time at constant potential until a termination current level is reached. Preferably, the re-anodization is performed at a temperature slightly higher than the projected device operating temperature. For devices that will operated at body temperature, the electrolyte bath temperature for the re-anodization may preferably be around 40° C. Though preferable, the re-anodization can be carried out at non-operating temperatures, both lower or higher. Several such annealing and re-anodization steps may be applied. Preferably, one such step is applied after the target potential has been reached and the formation current has fallen below a threshold of about 0.05-0.5 mA per gram of anode weight.
With reference again to FIG. 4, another embodiment of a process according to the invention for forming high voltage anodic oxides on a valve metal anode is depicted as a block diagram. In the method of this embodiment, an electrolyte forming tank 20 is provided. Tank 20 is configured to receive one or more anodes and contains an electrolyte forming bath comprising a formation electrolyte. An electrolyte circulation subsystem is also provided for circulating and cooling the formation electrolyte. One or more anodes, valve metal anodes, are immersed in the formation electrolyte. Formation electrolyte is circulated from the forming bath through the circulation subsystems to provide a cooled flow of formation electrolyte. An electrical potential is applied to the one or more anodes and is ramped up to a target voltage under constant current until a target potential is reached. Once the target potential is reached, the anodization step proceeds or continues at the target potential until the formation current falls below a predetermined termination current level. The flow rate and temperature of the cooled flow of formation electrolyte is regulated so that the temperature of the formation electrolyte in the forming tank is accurately maintained at a temperature at or below 40° C., during the application of the electrical potential. In this and other embodiments of processes of the invention, accurate temperature control can be accomplished by any of the various techniques disclosed herein. This would include recirculation of a cooled flow 41, circulation of a cooling fluid through the plenum of an appropriately configured tank, or through heat transfer to or in a temperature controlled environment or medium.
As previously discussed, one or more heat treatment or annealing steps with a subsequent re-anodization step may be performed for the one or more anodes. Preferably, one such annealing and re-anodization step is performed after both the target potential of the one or more anodes has been reached and the formation current has fallen below a level of, for example, about 0.05 to 0.5 mA per gram of anode weight.
The above embodiments of processes of the invention are provided by way of non-limiting example. The recitation of these steps here or in the claims does not mean that the steps are presented in the only permissible sequence as the sequence of certain steps may be changed and still be within the intended scope of the invention as claimed.
As noted in the above discussion of the process, the anodization step is carried out by applying electrical potential to one or more anodes by ramping up to a target voltage under constant current until a target potential is reached. Once the target potential is reached, the anodization step continues at the target potential until a predetermined termination current level is reached. This can be accomplished by traditional prior art techniques such as illustrated in FIG. 7 or the improved pulse technique of U.S. Application Ser. No. U.S. application Ser. No. 10/058,437, as illustrated in FIG. 8, which also aids in thermal managements. The benefits and advantages of the present invention can be realized with either technique. It should be understood with reference to the discussion herein and in the claims that the phrase “constant current” is used herein to refer to either continuous current (as illustrated in FIG. 7) or pulsed current of constant pulse height (as illustrated in FIG. 8). The process of the invention may be carried out with application of either form of current. Also “termination current level” is used herein to refer to a level or point of current drop at the end of the formation. By way of non-limiting example, a desirable predetermined level may be about 1/10 or about 1/100 of the applied constant current for some applications. The predetermined level may be different for other applications. These levels are readily determined by those skilled in the art.
FIG. 7 represents typical formation traces obtained with traditional formation protocols. The current is set to a constant level, and the voltage rises slowly until the target formation potential (Vf) is reached. The current falls rapidly once the formation potential is reached. Minor modifications to this protocol are described in the above-referenced '993 patent, where it is prescribed that the voltage is to be turned off approximately every three hours in order to allow for electrolyte cooling and diffusion
FIG. 8 schematically represents current and potential traces resulting from the application of a pulsed formation potential. The formation waveform is defined by the waveform period t, which may be constant or variable throughout the formation and a duty cycled, which also may be constant or variable throughout the formation. The ratio of the time width of the applied current or potential pulse to the time of the waveform period t, expressed in percent, is the duty cycle d. Preferably, the duty cycle d would be high during the initial phase of the formation, where the potential and current pulse widths would be long. The duty cycle d and correspondingly the applied potential and current pulse widths would decrease as the formation potential increases toward and reaches the target formation potential. The height of the constant current pulses can be seen to drop off as Vf is reached
The formation method of the present invention in conjunction with the formation potential and current traces illustrated in either FIG. 7 or FIG. 8 accomplishes a comprehensive and accurate thermal management of the sintered valve metal anode as it is anodized, whereby anodization failures due to build-up of electrolyte residue and field crystallization are largely avoided and whereby the dielectric properties of the anodes, namely their capacitance, are significantly improved. The forming process is simply defined by a limited set of parameters that are readily adjusted to various anode sizes and to their internal properties.
In accordance with the present invention, the formation protocol for high voltage anodes using pulsing technique is characterized by the following parameters and is generally illustrated in FIG. 8:
1.) A target formation potential Vf
2.) A formation current If
3.) A formation frequency vf=1/t defining the waveform period of the pulsed application of the formation potential.
4.) A duty cycle d of the rectangular formation potential waveform defining the fraction of the formation potential waveform period during which the potential is applied to the anode and during which the formation current If is flowing through the anode-electrolyte system.
- EXAMPLE 1
5.) A formation bath temperature Tf, accurately maintained at 40.degree. C. or lower for large high voltage electrodes.
A set of 8 capacitors were formed with the pulsed formation technique depicted in FIG. 8
and discussed earlier herein. Anodes 1
were processed in a system without active temperature control, which allowed the bath temperature to fluctuate or climb up to 40° C. at the time of maximum power dissipation, a traditional method. Anodes 5
were processed in a system with accurate temperature control of the formation bath, keeping the temperature constant at 18° C., a method according to the invention. Processing conditions were otherwise the same for all 8 electrodes, with the exception of the use of active temperature control according to the invention in the processing of Anodes 5
to accurately maintain the temperature of the formation electrolyte at 18° C. The target potential was 260 V, initial formation current was 275 mA for four anodes. Formation frequency was about 0.2 mHz with a duty cycle between 95% and 75% depending on the power dissipation. Table 1 illustrates the improvement in capacitance observed on anodes formed in accordance with the present invention:
|TABLE 1 |
| || ||Capacitance |
|Anode Number ||Method ||(micro-Farad) |
|1 ||Traditional at about 40° C. ||408 |
|2 ||Traditional at about 40° C. ||410 |
|3 ||Traditional at about 40° C. ||420 |
|4 ||Traditional at about 40° C. ||412 |
|5 ||According to Invention at 18° C. ||434 |
|6 ||According to Invention at 18° C. ||431 |
|7 ||According to Invention at 18° C. ||430 |
|8 ||According to Invention at 18° C. ||432 |
As indicated by the data of Table 1, capacitance improvements were obtained with forming anodes according to the present invention over anodes formed according to traditional method of prior art without thermal management. The improvement in capacitance is solely due to the process improvement involving actively cooling the formation electrolyte bath and maintaining its temperature accurately at 18° C. The capacitance of the anodes formed at 18° C. is improved by 2-5% over those formed under conditions in which the temperature was allowed to climb up to 40° C. at the time of maximum power dissipation. This capacitance improvement is obtained by comparison of the difference between the capacitance of Anode 3 with that of Anode 7 and that of Anode 1 with that of Anode 5.
Anodization processing of valve metal anodes according to the principles set forth herein has been demonstrated to deliver improved capacitance over anodes processed according to prior art traditional methods. The improved capacitance translates directly into an improved energy density of the capacitor, which, in turn, allows for the design of a smaller overall device. Smaller devices ease the side effects associated with implanting the device and provides for improved patient comfort.
All patents and printed publications disclosed herein are hereby incorporated by reference herein into the specification hereof, each in its respective entirety.
The preceding specific embodiments and examples are illustrative of an anode formation process for anodes usable in capacitors, particularly capacitors incorporated into an IMD, in accordance with the present invention. It is to be understood, therefore, that other expedients known to those skilled in the art or disclosed herein, and existing prior to the filing date of this application or coming into existence at a later time may be employed without departing from the invention or the scope of the appended claims.