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  Anodizing valve metals by controlled powerRelated Patent Categories: Electrolysis: Processes, Compositions Used Therein, And Methods Of Preparing The Compositions, Electrolytic Coating (process, Composition And Method Of Preparing Composition), Depositing Predominantly Single Metal Or Alloy Coating On Single Metal Or Alloy Using Specified Waveform Other Than Pure DcThe Patent Description & Claims data below is from USPTO Patent Application 20060191796. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] In general, an electrolytic capacitor comprises an anode, a cathode and a separator material. The separator material separates the anode and the cathode. Impregnated in the separator material is an ionically conductive electrolyte. The electrolyte typically comprises water, organic solvent(s), and salt(s) of weak inorganic and/or organic acids. [0002] The anode is a valve metal. Valve metals include and are not limited to aluminum, tantalum, niobium, titanium, zirconium, hafnium and alloys thereof. The valve metals can be in any conventional form. Examples of such forms include and are not limited to etched foil, sintered powders, or other porous structure forms. As an anode, the valve metal has its exterior surface coated with a film of its corresponding oxide to serve as a dielectric. [0003] The oxide film is formed by anodizing the valve metals in an appropriate anodizing electrolyte. The oxide film thickness increases in accordance with the anodizing voltage applied to it during the anodization process. The desired oxide film thickness is determined by numerous factors. Some of those factors include and are not limited to the capacitor working voltage, operating temperature and other performance requirements. [0004] The maximum anodizing voltage and the quality of the oxide formed depend on the valve metal material, the anodizing electrolyte composition, and the anodizing protocol. The anodizing protocol refers to a series of voltage/current sequences and profiles. [0005] It is believed the anodization protocols that promote locally excessive heating and insufficient material transport in porous valve metal anode bodies cause breakdown during anodization or poor anode electrical properties. The poor anode electrical properties include high DC leakage. There have been numerous attempts to solve these problems by altering the anodization protocol. Some of the prior anodization protocols that have been historically used and disclosed in recent patents include: controlling the anodizing current density; mechanical, sonic, or ultrasonic agitation of the electrolyte; anodizing by combining control of voltage/current and controlled rest steps (U.S. Pat. No. 6,231,993 to Stephenson et al.); and controlled pulses of the voltage/current (U.S. Pat. No. 6,802,951 to Hossick-Schott). [0006] FIG. 1 illustrates an example of a prior anodization protocol that is disclosed in U.S. Pat. No. 6,802,951. In FIG. 1, the current (2) is maintained and the power (4) increases over time. The anodizing protocol described in the '951 patent is adequate for low voltage anodization because there are few oxide defects, gray-outs, and early anodizing breakdown is generally not a problem. The same results are not always obtainable for high voltage anodization protocols. High voltage anodization is when the anodizing voltage increases to over 200 volts during the course of the anodization protocol. During a high voltage anodization protocol, temperature in the porous valve metal anode increases. The locally excessive temperature promotes oxide defects, gray-out, and early anodizing breakdown in the anodes. Such results are undesirable. [0007] Another anodization protocol that is an obvious variation of FIG. 1 is also disclosed in the '951 patent. In the '951 patent, Hossick-Schott disclosed and claimed an anodization protocol having (1) the voltage rise to a predetermined level; (2) when the voltage rises the current remains constant, (3) when the voltage reaches a predetermined level the current decreases; and (4) when the current and/or voltage are rising, being maintained or decreasing, the electrolyte composition is agitated. [0008] What has been needed is another method for manufacturing a valve metal anode such as the kind typically used in an electrolytic capacitor. Particularly, it is desirable to provide valve metal anodes with dielectric coatings having improved oxide quality and high breakdown voltages. [0009] An alternative anodization (formation) protocol for high voltage sintered tantalum anodes is disclosed by Stephenson et al. in U.S. Pat. No. 6,231,993. The '993 patent is assigned to Wilson Greatbatch Ltd., the assignee for this application. Stephenson et al. disclose (bracketed material added for clarity) the following anodization protocol, which is partially illustrated in FIG. 2: [0010] An exemplary formation protocol for a sodium reduced tantalum powder pellet is as follows. Exemplary sodium reduction tantalum pellets are available from H.C. Starck Inc., Newton, Mass. under the "NH" family designation. In this exemplary protocol, the pellet has a weight of about eight grams and the desired target formation voltage is 231 volts. The formation electrolyte is of polyethylene glycol, de-ionized water and H.sub.3PO.sub.4 having a conductivity of about 2,500 .mu.S[/cm] to about 2,600 .mu.S[/cm] at 40.degree. C. The formation protocol is as follows: [0011] 1. The power supply is turned on at an initial current [2a] of 80 mA until the voltage [4a] reached 75 volts. The power supply is then turned off [5a] for about three hours. [0012] 2. The power supply is turned back on [with the current 2b set] at 80 mA, [and the voltage 4b which is initially at] 75 volts and [rises] to 115 volts. The power supply is then turned off [5b] for about three hours. [0013] 3. The power supply is turned back on [with the current 2c set] at 49 mA, [and the voltage 4c which is initially at] 115 volts and [rises] to 145 [volts]. The power supply is then turned off [5c] for about three hours. [0014] 4. The power supply is turned back on [with the current 2d set] at 49 mA, [and the voltage 4d which is initially set at] 145 volts and [rises] to 175 [volts]. The power supply is then turned off [5d] for about three hours. [0015] 5. The power supply is turned back on [with the current 2e set] at 40 mA, [and the voltage 4e which is initially set at] 175 volts and [rises] to 205 [volts]. The power supply is then turned off [5e] for about three hours. [0016] 6. The power supply is turned back on [with the current 2f set] at 36 mA, [and the voltage 4f which is initially set at] 205 volts and [rises] to 225 [volts]. The power supply is then turned off [5f] for three hours. [0017] 7. The power supply is turned back on [with the current 2g set] at 36 mA, [and the voltage 4g is which is initially set] at 205 volts and [rises] to 231 [volts]. The pellet is held at 231 volts for about one hour to complete the formation process. The anodized pellet is then rinsed and dried. [0018] If desired, the formation process is periodically interrupted and the anodized pellet is subjected to a heat treatment step. This consists of removing the anode pellet from the anodization electrolyte bath. The anode pellet is then rinsed and dried followed by heat treatment according to the procedure described by D. M. Smyth et al., "Heat-Treatment of Anodic Oxide Films on Tantalum", Journal of the Electrochemical Society, vol. 110, No. 12, pp. 1264-1271, December 1963. [0019] The anodization protocol illustrated in FIG. 2 controls the current and decreases the heat generated in comparison to the protocol illustrated in FIG. 1. By decreasing the heat, the FIG. 2 anodization protocol obtains an anode having decreased DC leakage. [0020] However, there is a desire to find alternative anodization protocols to obtain similar results in relation to the FIG. 2 protocol, but that can be obtained at a faster rate. SUMMARY OF THE INVENTION [0021] The present invention is directed to an anodization protocol for valve metal structures. The anodization protocol calls for valve metal structures to be positioned in an anodizing electrolyte, and a supply system is electrically interconnected to the valve metal structures. The supply system provides a source voltage, a source current, and a controlled power. The supply system can be any device that provides the controlled power. The anodizing protocol subjects the valve metal structure to (a) the source current, (b) the formation (source) voltage and (c) the controlled power level. The power is controlled to decrease excessive heating in the structure during anodization. The present invention also includes the components for performing the method. [0022] The anodization protocol of the present invention provides acceptable anodizing voltage and oxide quality at a desired rate in relation to the prior art protocols. Continue reading... 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