MXPA01001974A - Phosphate anodizing electrolyte and its use to prepare capacitors valve metal anodes produced from very fine metal powders - Google Patents

Abstract

Electrolytes containing water, at least one organic solvent, and at least one alkali metal phosphate salt can be used for anodizing valve metals prepared from metal powder having a surface area of least .35 m2/g or 35,000 CV/g. The alkali metal phosphate salt should be relatively insoluble in the organic portion of the electrolyte, but highly soluble in a water solution of the organic solvent. The anodizing electrolytes have relatively high conductivity and are capable of being used at high anodizing currents. The anodic film produced by these electrolytes on valve metals is of substantially uniform thickness, has improved electrical parameters, and requires a shorter stabilization time at voltage.

Description

PHOSPHATE ANODIZATION ELECTROLYTE AND ITS USE FOR PREPARE CAPACITOR VALVE METAL ANODES PRODUCED FROM VERY FINE METALLIC POWDERS FIELD OF THE INVENTION The present invention is directed to electrolytes suitable for anodizing metal anodes of valves prepared from fine powders, to methods for preparing capacitors, and to capacitors prepared with the appropriate fine powder and electrolyte anodes.

BACKGROUND OF THE INVENTION Since its development in 1950, the use of solid tantalum capacitors having high volumetric efficiency and reliability, has increased in the current world market of several billion solid tantalum capacitors per year. The market continues to demand reliable capacitors with even higher volumetric efficiency and more restrictive parameters (eg, lower series equivalent resistance, lower direct current leak, etc.) at lower costs. The electrochemical anodization processes currently used for the fabrication of powder metallurgy tantalum capacitors typically employ electrolytes containing water, ethylene glycol or polyethylene glycol, and phosphoric acid. The resistivities of these electrolytes are usually too high to anodize low-voltage anodes manufactured from metal powders with a high surface area (the surface area is greater than 35,000 microfarad-volts / g) unless the phosphoric acid content exceed approximately 1% by volume. The relatively high percentage of phosphoric acid in protic acid solutions (ie, water and glycols or polyglycols) results in relatively low pH values (eg, a pH value of less than about 2) and the limited life of the electrolyte due to the development of corrosion products in the electrolyte as well as the limited current carrying capacity of the electrolyte due to the precipitation of phosphate residues in the smaller pores of the powder metallurgy anode bodies manufactured from metallic valve powders. The British patent G.B. 2 168 383 describes the use of amine phosphate salts, soluble in both organic solvents and water for use in anodization electrolytes.

Unfortunately, amines that form soluble phosphate salts (eg, tri-N-propylamine, pyridine and substituted pyridines) tend to be toxic and volatile at standard anodization temperatures (from about 80 to about 90 ° C). In addition, these amines form incompletely ionized phosphate salts resulting in lower electrolyte conductivities than phosphoric acid alone. US Pat. 5,716,511 describes the use of phosphoric acid or acid phosphate salts in electrolytes containing up to 75% by volume of polyethylene glycol dimethyl ether for the anodization of tantalum powder metallurgy at temperatures below about 50 ° C for the purpose of of reducing the incidence of bubble-type oxide defects in higher voltage films (above 100 volts for anodization) formed on mechanically damaged tantalum surfaces. Since the surface area of the metallic powder used for capacitor anodes has increased above approximately 0.35 square meters per gram (35,000 microfarad-voits per gram), the stabilization time required for the voltage to obtain a uniform oxide thickness is It has increased steadily due to the voltage drop through the fine pores of the anode. In addition, electrolytes containing acid phosphate leave very troublesome phosphate residues in the fine anode pores since the surface area of the powder has increased over the years. The required stabilization time in voltage and the accumulation of phosphate residues in the fine pores of anodes are further aggravated as the size of the anode bodies increases.

COMPENDIUM OF THE INVENTION The present invention addresses the above-described disadvantages of traditional anodizing electrolytes for preparing capacitors from anodes made of fine metal powders. The present invention is directed to a capacitor prepared from an anode of fine metallic powder, wherein the surface area of the fine metallic powder is greater than 0.35 m2 / g. The capacitor is prepared by anodizing the fine metallic powder electrolyte using an electrolytic solution comprising at least one alkali metal phate salt, water and at least one organic solvent. The electrolytic solution preferably contains one or more glycols, polyethylene glycols, polyethylene glycol monomethyl ethers, or polyethylene dimethyl ethers. In addition, the present invention is directed to an electrolytic solution, wherein the sum of the sum of the alkali metal phate salts is less than about 10% by weight of the total weight of the electrolyte solution. The present invention is also directed to a method for anodizing an anode of fine metallic powder, wherein a film is formed on the anode of fine metallic powder using the electrolytic solution described above. The fine metal powder is preferably a metallic valve powder, such as tantalum, having a surface area greater than 0.35 m2 / g. The film is formed at a temperature of about 60 ° C to about 100 ° C. The electrolytes of the present invention, in contrast to the electrolytes of the prior art, provide a reduction in the deposition of metal phates / polyphoric acid in the fine anode pores. This results in a more uniform and superior capacitance combined with a lower dissipation factor when anodizing powder metallurgy anodes made of high surface area metal powder. It should be understood that both the foregoing general description and the following detailed description are illustrative and only explain and do not restrict the present invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is represents capacitance values for capacitors described in example 4. Figure 2 is the presentation of the values of dissipation factor (DF) for capacitors described in Example 4. Figure 3 represents the values of resistance in equivalent series (ESR) for capacitors described in Example 4. Figure 4 represents the current leakage values for capacitors described in Example 4. Figure 5 is a tabular description of the capacitance, the dissipation factor, the ESR and the heats of current leakage for capacitors described in Example 5. Figure 6 represents the capacitance values for the capacitors described in Example 5.

Figure 7 represents the dissipation factor values for capacitors described in Example 5. Figure 8 represents the ESR values for capacitors described in Example 5. Figure 9 represents the current leakage values for capacitors described in the Example 5.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES It is desirable to manufacture powder metallurgy capacitor anodes with the highest possible density in order to achieve maximum capacitance for a given device volume and rated voltage (ie, maximum volumetric efficiency). However, the use of electrolytes of the prior art in the anodization of capacitor anodes made of metal powders of high surface area results in an increased precipitation of phate solids in the fine pores of the anode as both the density of the Anode as the surface area of metallic powder are increased. These precipitated phate solids remain in the anode pores after anodization, thereby preventing both uniform anodic oxide formation and contact with counter electrode materials (eg, manganese dioxide). The effects of this impedance include the loss of potential capacitance in addition to an increased dissipation factor and increased current leakage.

A major improvement made with the electrolytes of the present invention when compared to electrolytes of the prior art, is a reduction in the deposition of metal phosphates / polyphosphoric acid in the fine anode pores. This results in a more uniform and higher capacitance combined with a lower dissipation factor when anodizing powder metallurgy anodes manufactured from metal powder of high surface area. The powder of high surface area is defined as that powder having at least 35 m2 / g or 35,000 CV / g, preferably at least 0.5 m2 / g or 50,000 CV / g. The electrolytic solution of the present invention employs alkali metal phosphate salts in combination with water and an organic solvent to give an alkaline or neutral electrolyte solution. The alkali metal phosphate salt and the organic solvent must be selected so that the salt is substantially insoluble in the organic solvent, but relatively and highly soluble in the water / organic solvent solution. In accordance with the present invention, a capacitor is prepared by anodizing a fine metallic powder electrode using an electrolytic solution. The alkali metal phosphate salt is insoluble in the organic solvent, but soluble in the water solution and organic solvent. The alkali metal phosphate salt is preferably a water soluble dibasic salt, such as dibasic potassium phosphate and dibasic sodium phosphate, most preferably dibasic potassium phosphate. The amount of alkali metal phosphate salt is preferably from about 0.1% by weight to about 10% by weight of the total weight of the electrolyte solution. Preferably, it is from about 0.5% by weight to about 5% by weight of the total weight of the electrolyte solution. Most preferably, it is from about 0.5% by weight to 2.5% by weight of the total weight of the electrolyte solution. The organic solvent is preferably at least one solvent selected from glycols, polyethylene glycols, polyethylene glycol monomethyl ethers, or polyethylene dimethyl ethers. The amount of organic solvent must be less than the solubility limit of the organic solvent in water. The organic solvent should be above about 1% by volume and less than about 50% by volume of the electrolyte solution. Preferably, the organic solvent is from about 5% by volume to about 35% by volume of the electrolyte solution. Most preferably, the organic solvent is from about 5% by volume to 25% by volume of the electrolyte solution. It is believed, but the applicants do not wish to be bound to any particular theory, that as the internal temperature of the anode increases due to the heat produced from the anodization reaction, the electrolyte of the present invention separates the phase into a phase of water and an organic phase. During this phase separation, the water phase retains the alkali metal phosphate, since the alkali metal phosphate is insoluble in the organic phase. The insolubility of the phosphate salt (s) in the organic solvent only provides an automatic limit internal heating of the anode bodies during the anodization due to the reduced conductivity of the electrolyte under drying conditions such as occurs inside the anodes at high current densities. Due to this phase separation, the resistivity of the electrolyte solution also increases, thus showing the anodization reaction and the cooling of the electrolyte solution. The now cold electrolyte solution becomes a phase again as the water phase condenses in the organic phase and the anodization reaction rate increases. The pH of the neutral or slightly basic electrolyte of the present invention results in increased resistance to the precipitation of acidic poiiphosphates in the fine anode pores that are formed during the desiccation of the phosphoric acid. The almost neutral pH value also results in a generally lower rate of anodization tank corrosion and the collection of metals through the electrolyte. The pH is preferably from about 7 to 9, very preferably around 7. The high degree ionization of alkali metal phosphates in solution gives rise to electrolytes which have a higher conductivity than traditional electrolytes. This high conductivity reduces the stabilization time in voltage required to anodize metallic powders of high surface area. The benefit of the present invention over the prior art in this regard is that the alkali metal phosphate does not form a viscous mixture with the organic phase as does the phosphoric acid of the electrolytes of the prior art. The viscous mixture formed by the electrolytes of the prior art covers the fine pores of the metallic valve powder. In addition, this viscous mixture does not emerge to an individual phase once the water condenses due to the cooling of a reduced rate of anodization reaction. Therefore, the fine pores of the valve metal continue to be blocked and anodized minimally. This reduced anodization of the fine pores reduces the capacitance of the resulting capacitor. The present invention is further directed to a method for anodizing a metal whereby a film is formed on a metal with an electrolytic solution described above. The metal is preferably a valve metal such as tantalum or niobium.

EXAMPLES The invention will now be described with reference to the following examples. These examples should not be constructed in any way as limiting the invention.

Example 1 Two groups of anodes (330 microfarads / 10 volts) were anodized at 32 volts at 80 ° C. The first of these groups used a conventional electrolyte, while the second group used an electrolyte of the present invention. Table 1 provides the composition of the electrolytes. The anodes were rinsed, heat-treated at 375 ° C for 30 minutes, and anodized again in the original electrolytes. The anodes were then processed to capacitor bodies painted with silver, attached to lead frames, and cast in finished surface mount capacitor using known techniques. In Table 1 a comparison of the two anodes is provided, where it was observed that the stabilization time for the electrolyte of the present invention is half the stabilization time of a traditional electrolyte. The capacitance stability values were higher and the values for the dissipation factor (DF) and current leakage were greatly reduced for the electrolyte of the present invention as compared to the values of the traditional electrolyte. Finally, the porosimetry data for the anodes indicate that the anodized anodes have fewer finer pores when anodized in the electrolyte of the present invention. Although not wishing to be bound by any theory, one possible explanation for the few finer pores is the reduced deposition of phosphate solids within the pores of the anode under conditions with a high pH value.

Table 1 Comparison between a Traditional Electrolyte and an Electrolyte of the Present Invention EXAMPLE 2 Extremely low corrosion and metal collection rates observed with the electrolytes of the present invention were illustrated by the metal analysis of an electrolyte (10% by volume of polyethylene glycol 300/2% by weight of K2HPO4), which was in service in a stainless steel anodization tank for a period of 6 months (more than 4,000 + hours). The metal content was analyzed using ICP (inductively coupled plasma spectrophotometer) and resulted in the metal contents of: Chromium = less than 0.12 parts per million (ppm) Iron = 0.010 ppm Nickel = less than 0.070 ppm Aluminum = 0.190 ppm Typical values of the metal content for a traditional electrolyte after only about 500 hours of service are: Chromium = greater than 5 ppm Iron = greater than 15 ppm Nickel = greater than 5 ppm Aluminum = from at least 30 ppm to more of approximately 100 ppm The extremely high aluminum content in the traditional electrolyte was due to the contact between the electrolyte and the aluminum rods used to support the anodes for anodization.

In order to more directly test the metallumination of the electrolyte of the present invention, an aluminum anode process bar was placed in the electrolyte at 80 ° C. After 3 days, it was found that the aluminum content of the electrolyte is 0.39 ppm, and after 12 days of 0.76 ppm. In a traditional acid electrolyte having the same volume, the dissolution of an aluminum anode process rod requires approximately 24 hours and gives the aluminum a content of approximately 300 ppm.

Example 3 The applicability of the claimed electrolyte for valve metals in addition to tantalum is illustrated by the performance of high surface area niobium powder anodes anodized at 20 volts at 80 ° C in an electrolyte with an almost neutral pH (10 % by volume of polyethylene glycol 300/2% by weight of K2PO4) as compared to niobium powder anodes of high surface area anodized at 20 volts at 80 ° C in dilute aqueous phosphoric acid. Clearly, as can be seen later in Table 2, the electrolyte of the present invention had superior performance with respect to both capacitance and current leakage with niobium anodes when compared to traditional electrolyte solutions.

Table 2 Comparison between Traditional Electrolytes and the Electrolyte of the Invention Claimed in Niobium Anodes Note. D. C. leakage measured at 14 volts after 60 seconds of charging time. nACV = nanoamperes / CV = 10"9 Amp / microfarads volts Example 4 A batch of anodes was manufactured from commercially available high surface area tantalum powders (50,000 CV / g or square meter per gram) with a scale of 470 microfarads / 10 volts The anodes were then divided into three groups and anodized at 32.5 volts at 80 ° C, at a current density of approximately 1.5 microamperes per microchoulium The first group was anodized using a conventional electrolyte consisting of approximately 20 % by volume of ethylene glycol and 2% by volume of phosphoric acid The second group was anodized using an electrolyte consisting of about 10% by volume of ethylene glycol 300 and about 2% by weight of dibasic potassium phosphate. confirm that the improvement in Example 1, with the electrolyte of the present invention, was due to the use of the alkali metal phosphate salt and not due to the low Organic electrolyte, a third group was anodized at a reduced organic content of conventional electrolyte consisting of about 10% by volume of ethylene glycol and about 2% by volume of phosphoric acid. The anodes of the three groups were then processed into capacitor bodies painted with plaque, attached to the lead frames, and molded into surface-mounted assembly capacitors through known methods. The electrical parameters were then measured after molding without any additional electrical classification in order to determine the inherent uniformity and stability (eg resistance of the capacitors to the molding tension, etc.) of the electroprocessed anodes. Figure 1 is a graph of capacitance values for a random sample of capacitors from each of the three groups, expressed as a percentage of deviation from the nominal capacitance value of 470 microfarads. The claimed electrolyte of the invention had a much more uniform capacitance value than any of the electrolytes described above. Figure 2 is a graph of the dissipation factor (DF) values, expressed as a percentage of DF, for a random sample of capacitors for each of the three groups. With the electrolyte of the present invention, it was found that the factor of dissipation is significantly lower than electrodes containing phosphoric acid, without considering the content of ethylene glycol. Figure 3 is a graph of the equivalent series resistance (ESR), expressed in ohms, for a random sample for each group of capacitors. The values of the ESR for the capacitors of the three groups are substantially similar. Figure 4 is a graph of current leakage values, expressed in microamps, for a random sample of capacitors for each of the three groups. The current leak is substantially similar for all three groups.

Example 5 A batch of anodes classified at 375 microfarads / 10 volts, at a density of 6.0 g / cm 3, was made from commercially available high surface area tantalum powder (with a CV of approximately 50,000 microfarad-volts / g 0.5 m2 / g). The anodes were then divided into two groups and anodized at 32 volts at 80 ° C. The first group was anodized with a conventional electrolyte containing 20% by volume of ethylene glycol and 2% by volume of phosphoric acid. The second group was anodized with an electrolyte of the present invention (10% by volume polyethylene glycol 300 and 2% by weight dibasic potassium phosphate). Both groups were then further processed, through methods well known to those skilled in the art of capacitor fabrication., to capacitor bodies impregnated with manganese dioxide, coated with graphite and silver paint. The capacitance, the dissipation factor (DF), the equivalent series resistance (ESR) and the current leakage values for both groups are shown in Figures 5, 6, 7 and 8. It was found that the average capacitance is 17% higher for anodized capacitors using the electrolyte of the present invention. It was found that the dissipation factor is approximately 4 times higher for capacitors anodized with the conventional electrolyte as compared to capacitors anodized with the electrolyte of the present invention. The ESR for both groups was substantially similar, but current leakage for anodized capacitors with traditional electrolytes was approximately 300 times greater than current leakage for anodized capacitors with the electrolyte of the present invention. It will be apparent to those skilled in the art that various modifications and variations may be made in the compositions and methods of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided since they are within the scope of the appended claims and their equivalents.

Claims (19)

1. A capacitor comprising an anode prepared from metallic powder having a surface area of at least 35 m2 / g or 35,000 CV / g, and anodized with an anodizing electrolyte comprising at least one metal phosphate salt alkaline, water and at least one organic solvent, wherein the alkali metal phosphate salt is insoluble in the organic solvent only but soluble in a solution of water and the organic solvent, wherein the total volume of organic solvent is above about 1% by volume and less than about 50% by volume of the total volume of the electrolyte solution, and wherein the anode is anodized at a temperature of about 60 ° C to about 100 ° C.

2. The capacitor according to claim 1, wherein said organic solvent is selected from the group consisting of glycols, polyethylene glycol, polyethylene glycol monomethyl ether, polyethylene dimethyl ether, and mixtures thereof.

3. The capacitor according to claim 1, wherein the alkali metal phosphate salt is dibasic potassium phosphate.

4. The capacitor according to claim 1, wherein the total weight of the alkali metal potassium salt is between about 0.1% by weight and about 10% by weight of the total weight of the electrolytic solution.

5. The capacitor according to claim 4, wherein the total weight of the alkali metal phosphate salt is from about 0.5% by weight to about 5% by weight of the total weight of the electrolyte solution.

6. The capacitor according to claim 5, wherein the total weight of the alkali metal phosphate salt is from about 0.5% by weight to about 2.5% by weight of the total weight of the electrolytic solution. The capacitor according to claim 1, wherein the total volume of the organic solvent is from about 5% by volume to about 35% by volume of the total volume of the electrolyte solution. 8. The capacitor according to claim 7, wherein the total volume of the organic solvent is from about 5% by volume to about 25% by volume of the total volume of the electrolytic solution. 9. A method for anodizing a metal anode prepared from the metallic powder having a surface area of at least 35 m2 / g or 35,000 CV / g, comprising anodizing the metallic anode with an electrolyte comprising at least an alkali metal phosphate salt, water and at least one organic solvent, wherein the alkali metal phosphate salt is soluble in the organic solvent only, but soluble in a solution of water and the organic solvent, wherein the The total volume of organic solvent is above about 1% by volume and less than 50% by volume of the total volume of the electrolytic solution, and wherein the anode is anodized at a temperature of about 60 ° C to about 100 ° C. 10. The method according to claim 9, wherein said metal is a valve metal. 11. The method according to claim 10, wherein the valve metal is tantalum. 12. The method according to claim 10, wherein the valve metal is niobium. 13. The method according to claim 9, wherein the organic solvent is selected from the group consisting of glycols, polyethylene glycol, polyethylene glycol monomethyl ether, polyethylene dimethyl ether, and mixtures thereof. 14. The method according to claim 9, wherein the alkali metal phosphate salt is dibasic potassium phosphate. 15. The method according to claim 9, wherein the total weight of the alkali metal phosphate salt is between about 0.1% by weight and about 10% by weight of the total weight of the electrolyte solution. 16. The method according to claim 15, wherein the total weight of the alkali metal phosphate salt is from about 0.5% by weight to about 5% by weight of the total weight of the electrolyte solution. 1

7. The method according to claim 16, wherein the total weight of the alkali metal phosphate salt is from about 0.5% by weight to about 2.5% by weight of the total weight of the electrolyte solution. 1

8. The method according to claim 10, wherein the total volume of the organic solvent is from about 5% by volume to about 35% by volume of the total volume of the electrolyte solution. 1

9. The capacitor according to claim 18, wherein the total volume of the organic solvent is from about 5% by volume to about 25% by volume of the total volume of the electrolytic solution.