Anodizing Electrolytes Using A Dual Acid System For High Voltage Electrolytic Capacitor Anodes

Abstract

An improved formation electrolyte and method for anodizing valve metal anodes used in electrolytic capacitors, particularly for high voltage sintered tantalum powder anode, is described. The anodizing electrolyte composition is comprised of 1) a phosphorus oxyacid and/or its salt, such as phosphoric acid and ammonium phosphate; 2) a weak inorganic acid/salt (such as boric acid, ammonium borate) or a weak carboxylic acid/salt; 3) water; and 4) a protic solvent or a mixture of two or more protic solvents. The weak mono-carboxylic acid/salt has 2 to 7 carbon atoms and the weak di- or poly-carboxylic acid/salt has 3 to 13 carbon atoms. The present electrolytes have high anodizing breakdown voltage capability and the formed dielectric oxides have improved oxide quality including good oxide hydration resistant ability, and result in more stable capacitor performance. These properties are particularly important for critical applications such as implantable cardioverter defibrillators (ICDs). Significantly, this means that fewer capacitors are needed to meet an ICD's operating voltage.

1 . An anodizing electrolyte for providing a dielectric oxide on a valve metal, the anodizing electrolyte comprising: 
 a) water;    b) a phosphorus oxyacid or salt thereof; and    c) at least one of the group consisting of an inorganic acid, a salt of the inorganic acid, a carboxylic acid, a salt of the carboxylic acid, and mixtures thereof.    
   
   
       2 . The anodizing electrolyte of  claim 1  wherein the water is present in a range of from about 5% to about 80%, by weight.  
   
   
       3 . The anodizing electrolyte of  claim 1  wherein the phosphorus oxyacid is in a range of from about 0.1% to about 15%, by weight.  
   
   
       4 . The anodizing electrolyte of  claim 1  wherein the phosphorus oxyacid is selected from the group consisting of phosphoric acid, hypophosphoric acid, phosphorous acid, hypophosphorous acid, metaphosphoric acids, and mixtures thereof.  
   
   
       5 . The anodizing electrolyte of  claim 1  wherein the phosphorus oxyacid salt is selected from the group consisting of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, sodium phosphates, potassium phosphates, and mixtures thereof.  
   
   
       6 . The anodizing electrolyte of  claim 1  wherein the inorganic acid or the carboxylic acid is in a range of from about 0.5% to about 15%, by weight.  
   
   
       7 . The anodizing electrolyte of  claim 1  wherein the inorganic acid is boric acid and the inorganic acid salt is selected from the group consisting of ammonium borate, sodium borate, ammonium tetraborate, ammonium pentaborate, and mixtures thereof.  
   
   
       8 . The anodizing electrolyte of  claim 1  wherein the carboxylic acid is a mono-carboxylic acid having from 2 to 7 carbon atoms.  
   
   
       9 . The anodizing electrolyte of  claim 1  wherein the carboxylic acid is a di-carboxylic acid or a poly-carboxylic acid having from 3 to 13 carbon atoms.  
   
   
       10 . The anodizing electrolyte of  claim 1  wherein the carboxylic acid is selected from the group consisting of acetic acid, propanoic acid, butyric acid, iso-butyric acid, trimethyl acetic acid, cyclohexanecarboxylic acid, dicyclohexylacetic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, brassylic acid, dodecanedioic acid, 1,2-cyclohexanedicarboxylic acid, cyclohexyldicarboxylic acid, methylmalonic acid, dimethylmalonic acid, 2,2-dimethylsuccinic acid, 2-methylglutaric acid, 2,2-dimethylglutaric acid, 3-(tert-butyl)adipic acid, citric acid, tartaric acid, 1,3,5-cyclohexanetricarboxylic acid, and mixtures thereof.  
   
   
       11 . The anodizing electrolyte of  claim 1  wherein the valve metal is selected from the group consisting of tantalum, vanadium, niobium, aluminum, titanium, zirconium, hafnium, and mixtures thereof.  
   
   
       12 . The anodizing electrolyte of  claim 1  wherein the electrolyte further includes a protic solvent.  
   
   
       13 . The anodizing electrolyte of  claim 12  wherein the protic solvent is present in at up to about 90%, by weight.  
   
   
       14 . The anodizing electrolyte of  claim 12  wherein the protic solvent is selected from the group consisting of alkylene glycols, polyalkylene glycols, alkylene glycol monoethers, polyalkylene glycol monoethers, and mixtures thereof.  
   
   
       15 . The anodizing electrolyte of  claim 12  wherein the protic solvent is selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, trimethylene glycol, dipropylene glycol, glycerol, 2-methyl-1,3-propanediol, 1,4-butanediol, 2,3-butanediol, 2,2-dimethyl-1,3-propanediol, 2,4-pentanediol, 2,5-hexanediol, polyethylene glycols, polypropylene glycols, polyethylenepropylene glycol copolymers, ethylene glycol methyl ether, ethylene glycol ethyl ether, ethylene glycol butyl ether, diethylene glycol ethyl ether, diethylene glycol methyl ether, diethylene glycol butyl ether, dipropylene glycol methyl ether, tripropylene glycol methyl ether, and mixtures thereof).  
   
   
       16 . The anodizing electrolyte of  claim 1  having a conductivity of about 20 μS to about 10,000 μS at 40° C.  
   
   
       17 . The anodizing electrolyte of  claim 1  having a conductivity of about 100 μS to about 1,000 μS at 40° C.  
   
   
       18 . An anodizing electrolyte for providing a dielectric oxide on a valve metal, the anodizing electrolyte consisting essentially of: 
 a) water;    b) a phosphorus oxyacid; and    c) a second acid selected from the group consisting of boric acid, acetic acid, propanoic acid, butyric acid, iso-butyric acid, trimethyl acetic acid, cyclohexanecarboxylic acid, dicyclohexylacetic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, brassylic acid, dodecanedioic acid, 1,2-cyclohexanedicarboxylic acid, cyclohexyldicarboxylic acid, methylmalonic acid, dimethylmalonic acid, 2,2-dimethylsuccinic acid, 2-methylglutaric acid, 2,2-dimethylglutaric acid, 3-(tert-butyl)adipic acid, citric acid, tartaric acid, 1,3,5-cyclohexanetricarboxylic acid, and mixtures thereof.    
   
   
       19 . The anodizing electrolyte of  claim 18  wherein the water is present in a range of from about 5% to about 80%, the phosphorus oxyacid is in a range of from about 0.1% to about 15%, and the second acid is in a range of from about 0.5% to about 15%, by weight.  
   
   
       20 . The anodizing electrolyte of  claim 18  having a conductivity of about 20 μS to about 10,000 μS at 40° C.  
   
   
       21 . A method for anodizing a valve metal structure, comprising the steps of: 
 a) providing the valve metal structure selected from the group consisting of tantalum, vanadium, niobium, aluminum, titanium, zirconium, hafnium, and mixtures thereof;    b) providing an anodizing electrolyte comprising water, a phosphorus oxyacid, and a second acid selected from the group consisting of boric acid, acetic acid, propanoic acid, butyric acid, iso-butyric acid, trimethyl acetic acid, cyclohexanecarboxylic acid, dicyclohexylacetic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, brassylic acid, dodecanedioic acid, 1,2-cyclohexanedicarboxylic acid, cyclohexyldicarboxylic acid, methylmalonic acid, dimethylmalonic acid, 2,2-dimethylsuccinic acid, 2-methylglutaric acid, 2,2-dimethylglutaric acid, 3-(tert-butyl)adipic acid, citric acid, tartaric acid, 1,3,5-cyclohexanetricarboxylic acid, and mixtures thereof; and    c) applying a current to the valve metal immersed in the anodizing electrolyte until a target formation voltage is reached.    
   
   
       22 . The method of  claim 21  including providing the valve metal structure being selected from the group consisting of an etched foil, an unetched foil, and a sintered powder body.  
   
   
       23 . The method of  claim 21  including periodically turning off the formation current and letting the valve metal structure rest in the anodizing electrolyte.  
   
   
       24 . The method of  claim 23  including turning off the formation current for at least 10 minutes.  
   
   
       25 . The method of  claim 21  including providing the current in a range of from about 5 mA/gram to about 100 mA/gram amount of the valve metal.  
   
   
       26 . The method of  claim 21  including raising a formation voltage in intervals of from about 10 volts to about 100 volts toward the target formation voltage between periodically turning off the formation current and letting the valve metal structure rest in the anodizing electrolyte.  
   
   
       27 . The method of  claim 21  including anodizing the valve metal structure using a constant wattage protocol where wattage increases with time at constant current within each formation step between when the formation current is turned off, but the maximum wattage for each formation step is kept constant throughout the entire formation process.  
   
   
       28 . The method of  claim 21  including providing the anodizing electrolyte having the water in a range of from about 5% to about 80%, the phosphorus oxyacid in a range of from about 0.1% to about 15%, and the second acid in a range of from about 0.5% to about 15%, by weight.  
   
   
       29 . The method of  claim 21  including providing the electrolyte having a conductivity of about 20 μS to about 10,000 μS at 40° C.