Patent Application
20240331952
The present invention is related to an improved formation (anodization), electrolyte for the formation of tantalum oxide dielectric on a tantalum anode body. More specifically, the present invention is specific to a formation electrolyte comprising compounds derived from inositol which significantly improves oxide formation.
Solid electrolytic capacitors comprising oxide formed on sintered tantalum as the anode and conductive polymer as the cathode are now utilized extensively throughout the electronics industry in virtually every application necessitating a capacitive couple in the electronic assembly. As would be fully understood by those of skill in the art, tantalum oxide is formed on the tantalum surface wherein the tantalum oxide functions as the dielectric between the tantalum anode and conductive polymer cathode. The process of forming tantalum oxide by subjecting the anode to voltage in the presence of an electrolyte is referred to as anodization.
Dielectric quality is a measure of electrical property of a solid electrolytic capacitor. Poor dielectric may cause failure of the part. One factor that contributes to faulty electrical properties is anomalous charge current (ACC). Parts formed in currently available anodization electrolytes exhibit high ACC wherein the high ACC is now known to be detrimental to electrolytic capacitor quality. This has led to a significant effort to develop an improved forming, or anodizing electrolyte which provides a more stable dielectric resulting in an improved solid electrolytic capacitor, particularly with a lower ACC.
Solid electrolytic capacitors using valve metal as anodes, specifically tantalum, and conductive polymer as cathode, display anomalous charge current (ACC) which exceeds the theoretical value {I(t)) calculated as: I(t)=C*dv/dt with C being the capacitance and dV/dt being the voltage ramp}. ACC might interfere with circuit performance thereby leading to faulty capacitors as described in Y. Freeman and P. Lessner Evolution of Polymer Tantalum Capacitors Appl. Sci. 2021, 11(12), 5514-5521.
Provided herein is an improved electrolyte which is particularly suitable for use in the formation of tantalum oxide on tantalum wherein the tantalum oxide functions as an improved dielectric in a solid electrolytic capacitor.
The present invention is related to an improved formation electrolyte which is particularly suitable for use in the formation of tantalum oxide on tantalum.
More specifically, the present invention is related to a capacitor with improved electrical properties, comprising a tantalum anode and conductive polymer cathode with an improved tantalum oxide dielectric there between.
A particular feature of the present invention is the ability to form a dielectric oxide on a tantalum anode without alteration of the manufacturing steps, equipment or procedures.
These and other advantages, as will be realized, are provided in a formation electrolyte suitable for formation of an oxide on a valve metal anode comprising a derivative of inositol is defined by Formula 1:
Yet another embodiment is provided in a method of forming a solid electrolytic capacitor comprising:
The present invention is related to improved solid electrolytic capacitor comprising an improved tantalum oxide dielectric wherein the properties of the solid electrolytic capacitor, and particularly ACC, are significantly improved relative to the art. More specifically, the present invention is related to an improved formation electrolyte suitable for use in the formation of an improved tantalum oxide on a tantalum anode that yields low wet leakage. The capacitor formed with the improved tantalum oxide provides an improved solid electrolytic capacitor, particularly, when used with solid conductive polymer cathodes.
High ACC in solid electrolytic capacitors is solved by formation of tantalum oxide on a tantalum anode in an electrolyte that comprises derivatives of inositol. Without being limited to theory, it is hypothesized that the derivatives of inositol are capable of bonding with the free hydroxyl groups present on the surface of tantalum or with tantalum oxide thereby stabilizing the interface of the growing dielectric and anode which facilitates improved tantalum oxide growth.
The present invention is related to improved electrical properties in solid electrolytic capacitors and more specifically anomalous charge current (ACC). This invention provides a method of forming a dielectric oxide film on a tantalum anode and of making a solid electrolytic capacitor therewith. Formation in the inventive forming electrolyte, having free terminal O− ion upon ionization, in aqueous solution, forms an improved dielectric oxide layer. Without being limited to theory, it is hypothesized that the geometry of the ions of the inventive forming electrolyte anchor to the tantalum surface, which has free hydroxyl groups. The bonding/anchoring of the ions, facilitates more electrolyte molecule access and in effect forms uniform oxide. Another hypothesis is that a portion of the inventive forming electrolyte remains on the surface of the tantalum oxide and forms a stable interface between dielectric and conductive polymer at the p-n junction. This assists in possible tunneling and significantly reduces anomalous charge current (ACC) in solid electrolytic capacitors.
Forming a stable dielectric interface is crucial for having stable electrical properties. The dielectric is formed by anodically oxidizing the metal anode in the electrolyte that contains the inventive forming electrolyte. Through selective control over a voltage range, the anodization process leads to a dielectric having better electrical properties which in turn leads to a capacitor having improved ACC exhibiting less anomalous charge current when charged at a constant voltage slew rate such as 100 volts/second.
The derivative of inositol is defined by Formula 1
In a preferred embodiment at least one of R1-R6 is PO3R7R8 with at least one of R7 or R8 being H and preferably R7 and R8 are both H. In a more preferred embodiment at least two of R1-R6 is —PO3R7R8 with at least one of R7 or R8 being H and preferably R7 or R8 are both —H. In a more preferred embodiment at least three of R1R6 is PO3R7R8 with at least one of R7 or R8 being H and preferably R7 and R8 are both H. In a more preferred embodiment at least four of R1-R6 is PO3R7R8 with at least one of R7 or R8 being —H and preferably R7 and R8 are both H. In a more preferred embodiment at least five of R1-R6 is PO3R7R8 with at least one of R7 or R8 being H and preferably R7 and R8 are both H. In a more preferred embodiment each of R1-R6 is PO3R7R8 with at least one of R7 or R8 being —H and preferably R7 and R8 are both H.
R7 and R8 can be a cation preferably selected from quaternary amines, ammonium; metal cation, saturated or unsaturated carbon chain of up 35 carbon atoms, preferably 10-17 carbon atoms.
Substituted or unsubstituted carbon chains include alkyl chains and alkene chains which are straight chains, branched chains or cyclic which may be substituted or unsubstituted. Substitutions include ethers, —OH, carboxylic acids, phosphonic acids, phosphinic acids, esters, amines and amides.
Particularly preferred derivatives of inositol are selected from the group consisting of myo-inositol and it's isomers and their respective derivatives namely myo-inositol hexakis-phosphate (phytic acid); pentakis-, tri-, di-phosphates and their isomers; and myo-inositol mono phosphate, myo-inositol trispyrophosphate, 1-phosphatidyl-myo-inositol, 1-phosphatidyl-myo-inositol 3-phosphate, derivatives of ononitol, sequoytol, dombonitol, viscumitol, pinitol, quebrachitol, pinpollitol and brahol, myo-inositol 1,3,4,5,6-pentakis-O-(trimethylsilyl)-, bis(trimethylsilyl) phosphate, phosphatidylinositol 5-phosphate PI(5) diC8 ammonium salt, phosphatidylinositol 5-phosphate diC16 (PI(5)P diC16) sodium salt, 1,2-ciacyl-sn-glycero-3-phospho-(1-D-myo-inositol 4,5-biphosphate), phosphatidylinositol, phosphatidylinositol 3-phosphate, phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-phosphate, di-myo-inositol-phosphate, ciceritol phosphate, fagopyritol phosphate, glycosylinositol phosphoryl ceramide, C25,25-Archeditylinositol, ceramide phophoinositol, D-myo-inositol-4-hydrogen phosphate monoammonium salt and phosphatidylinositol phosphate.
The formation electrolyte may further comprise additives selected from metal salts, salts of organic acids, salts of inorganic acids, organic acids, inorganic acids, organometallic compounds, inorganic solvents, organic solvents, crosslinking agents, surface active agents, buffers and the like.
The metal salts, salts of inorganic acids and salts of organic acids included in the formation electrolyte may comprise halides, nitrides, sulfides, amides, nitrates, sulfates, phosphates, carbonates, chromates, chlorates, perchlorates, oxides, oxychlorides, peroxides, carboxylates, amides and esters.
The organic and inorganic acids included in the formation electrolyte may comprise carboxylic acids, phosphonic acids, phosphinic acids, pyrophosphoric acids, phosphoric acid, phthalic acid, maleic acid, malonic acid and trimesic acid and the like.
The organometallic compounds included in the formation electrolyte may comprise organosilanes, organoboranes, carbonyls, phosphines, crosslinking agents, surface active agents and buffers.
The solvent included in the formation electrolyte may be selected from the group consisting of water, alcohol, ethylene glycol, polyethylene glycol, tetraglyme, propylene glycol, glycol ether and alkanolamines.
The solid electrolytic capacitor comprises an anode, a cathode and a dielectric oxide between the anode and cathode. The anode is a sintered porous tantalum metal which is anodized to form the dielectric oxide. The dielectric oxide layer is covered by solid electrolyte, which is preferably a conductive polymer, which acts as cathode. The dielectric oxide is formed by subjecting the anode to voltage, in the presence of an inventive forming electrolyte in a process referred to in the art as anodization.
The solid electrolytic capacitor comprises an anode, a cathode and a dielectric oxide between the anode and cathode. The anode is a sintered porous tantalum metal which is anodized to form the dielectric oxide. The dielectric oxide layer is covered by solid electrolyte, which is preferably a conductive polymer, which acts as cathode. The dielectric oxide is formed by subjecting the anode to voltage, in the presence of an inventive forming electrolyte in a process referred to in the art as anodization.
After formation (anodization) of the anode, with a dielectric thereon can be washed. In one embodiment the anode, with a dielectric thereon is not washed which allows residual formation electrolyte to remain anchored to the surface which facilitates bonding with the cathode layer.
Formation temperatures of about 60-125° C. are suitable for demonstration of the invention. More preferably an anodization temperature of about 75-90° C. is suitable for demonstration of the invention.
The anomalous charging current (ACC) refers to the ideal current (I ideal) in milliamps (mA) for charging a capacitor Iideal=1000*C(dv/dt), C is the capacitance in Farads and dv/dt is the instantaneous rate of voltage change which is typically about 100V/S. Thus, the actual charging current remains the same or is greater than the ideal charging current (Iideal). Iactual/Iideal=1 or greater than 1.
The structure of a solid electrolytic capacitor is well understood by those of skill in the art and further elaboration is not necessary herein.
The conductive polymer is preferably selected from a group consisting of polyanilines, polypyrroles and polythiophenes each of which may be substituted. A particularly preferred polymer comprises conjugated groups having the structure of Formula 2:
In a particularly preferred embodiment the R1 and R2 of Formula 1 are taken together to represent —O—(CHR4)n—O— wherein:
The conducting polymer can be either a water-soluble or water-dispersible compound. Examples of such a π conjugated conductive polymer include polypyrrole or polythiophene. Particularly preferred conductive polymers include poly(3,4-ethylenedioxythiophene), poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-yl)methoxy)-1-butane-sulphonic acid, salt), poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-yl)methoxy)-1-propane-sulphonic acid, salt), poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-yl)methoxy)-1-methyl-1-propane-sulphonic acid, salt), poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-yl)methoxy alcohol, poly(N-methylpyrrole), poly(3-methylpyrrole), poly(3-octylpyrrole), poly(3-decylpyrrole), poly(3-dodecylpyrrole), poly(3,4-dimethylpyrrole), poly(3,4-dibutylpyrrole), poly(3-carboxypyrrole), poly(3-methyl-4-carboxypyrrole), poly(3-methyl-4-carboxyethylpyrrole), poly(3-methyl-4-carboxybutylpyrrole), poly(3-hydroxypyrrole), poly(3-methoxypyrrole), polythiophene, poly(3-methylthiophene), poly(3-hexylthiophene), poly(3-heptylthiophene), poly(3-octylthiophene), poly(3-decylthiophene), poly(3-dodecylthiophene), poly(3-octadecylthiophene), poly(3-bromothiophene), poly(3,4-dimethylthiophene), poly(3,4-dibutylthiophene), poly(3-hydroxythiophene), poly(3-methoxythiophene), poly(3-ethoxythiophene), poly(3-butoxythiophene), poly(3-hexyloxythiophene), poly(3-heptyloxythiophene), poly(3-octyloxythiophene), poly(3-decyloxythiophene), poly(3-dodecyloxythiophene), poly(3-octadecyloxythiophene), poly(3,4-dihydroxythiophene), poly(3,4-dimethoxythiophene), poly(3,4-ethylenedioxythiophene), poly(3,4-propylenedioxythiophene), poly(3,4-butenedioxythiophene), poly(3-carboxythiophene), poly(3-methyl-4-carboxythiophene), poly(3-methyl-4-carboxyethylthiophene), poly(3-methyl-4-carboxybutylthiophene), polyaniline, poly(2-methylaniline), poly(3-isobutylaniline), poly(2-aniline sulfonate), poly(3-aniline sulfonate), and the like.
Co-polymers composed at least two different copolymerized monomers are contemplated. Co-polymers comprise at least one polymerized monomer selected from the group consisting of polypyrrole, polythiophene, poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-yl)methoxy)-1-butane-sulphonic acid, salt), poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-yl)methoxy)-1-methyl-1-propane-sulphonic acid, salt), poly(N-methylpyrrole), poly(3-methylthiophene), poly(3-methoxythiophene), and poly(3,4-ethylenedioxythiophene).
A particularly preferred conductive polymer is poly-3,4-polyethylene dioxythiophene (PEDOT).
The conductive polymer layer can be formed on the dielectric by any technique commonly employed in the art. The conductive polymer can be formed into a slurry and deposited onto the surface. Alternatively, the conductive polymer can be added as monomer and polymerized in-situ as well known in the art.
It is known in the art to additional layers to the cathode to facilitate soldering of the cathode to a lead frame or circuitry. Carbon containing layers and metal containing layers are well known and well documented in the art and further discussion is not warranted herein.
Organofunctional silanes and organic compounds with more than one crosslinking group, especially more than one epoxy group, are particularly suitable for use in combination with the inventive formation electrolyte. The formation electrolyte may include the organofunctional silanes and organic compounds with more than one crosslinking group as additive in combination with inositol derivative. After formation of the dielectric oxide the anode with dielectric thereon can be washed. In an embodiment the anode with dielectric thereon is not washed which allows residual organofunctional silanes and organic compounds with more than one crosslinking group together with inositol derivative to be present during formation of the cathode.
An exemplary organofunctional silane is defined by the formula:
XR1Si(R3)3-n(R2)n
The organofunctional silane can also be dipodal, define by the formula:
Y(Si(R3)3-n(R2)n)2
wherein Y is any organic moiety that contains reactive or nonreactive functional groups, such as alkyl, aryl, sulfide or melamine; R3, R2 and n are defined above. The organofunctional silane can also be multi-functional or polymeric silanes, such as silane modified polybutadiene, or silane modified polyamine, etc.
Examples of organofunctional silane include 3-glycidoxypropyltrimethoxysilane, 3-aminopropytriethoxysilane, aminopropylsilanetriol, (triethoxysilyl)propylsuccinic anhydride, 3-mercaptopropyltrimethoxysilane, vinyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-trihydroxysilyl-1-propane sulfonic acid, octyltriethyoxysilane, bis(triethoxysilyl)octane, etc. The examples are used to illustrate the invention and should not be regarded as conclusive Examples of organofunctional silane include 3-glycidoxypropyltrimethoxysilane, 3-aminopropytriethoxysilane, aminopropylsilanetriol, (triethoxysilyl)propylsuccinic anhydride, 3-mercaptopropyltrimethoxysilane, vinyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-trihydroxysilyl-1-propane sulfonic acid, octyltriethyoxysilane, bis(triethoxysilyl)octane, etc. The examples are used to illustrate the invention and should not be regarded as conclusive.
A particularly preferred organofunctional silane is glycidyl silane defined by the formula:
wherein R1 is an alkyl of 1 to 14 carbon atoms and more preferably selected from methyl ethyl and propyl; and each R2 is independently an alkyl or substituted alkyl of 1 to 6 carbon atoms.
A particularly preferred glycidyl silane is 3-glycidoxypropyltrimethoxysilane defined by the formula:
which is referred to herein as “Silane A” for convenience.
A particularly suitable organometallic is neoalkoxy titanate with titanium IV 2,2(bis 2-propenolatomethyl)butanolato, tris neodecanoato-O; titanium IV 2,2(bis 2-propenolatomethyl)butanolato, iris(dodecyl)benzenesulfonato-O; titanium IV 2,2(bis 2-propenolatomethyl)butanolato, tris(dioctyl)phosphato-O; titanium IV 2,2(bis 2-propenolatomethyl)tris(dioctyl)pyrophosphatobutanolato-O; titanium IV 2,2(bis 2-propenolatomethyl)butanolato, tris(2-ethylenediamino)ethylato; and titanium IV 2,2(bis 2-propenolatomethyl)butanolato, tris(3-amino)phenylato being representative neoalkoxy titanates and derivatives thereof.
A crosslinker with at least two epoxy groups is referred to herein as an epoxy crosslinking compound and is defined by the formula:
wherein the X is an alkyl or substituted alkyl of 0-14 carbon atoms, preferably 0-6 carbon atoms; an aryl or substituted aryl, an ethylene ether or substituted ethylene ether, polyethylene ether or substituted polyethylene ether with 2-20 ethylene ether groups or combinations thereof. A particularly preferred substitute is an epoxy group.
Examples of epoxy crosslinking compounds having more than one epoxy groups include ethylene glycol diglycidyl ether (EGDGE), propylene glycol diglycidyl ether (PGDGE), 1,4-butanediol diglycidyl ether (BDDGE), pentylene glycol diglycidyl ether, hexylene glycol diglycidyl ether, cyclohexane dimethanol diglycidyl ether, resorcinol glycidyl ether, glycerol diglycidyl ether (GDGE), glycerol polyglycidyl ethers, digylcerol polyglycidyl ethers, trimethylolpropane polyglycidyl ethers, sorbitol diglycidyl ether (Sorbitol-DGE), sorbitol polyglycidyl ethers, polyethylene glycol diglycidyl ether (PEGDGE), polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, di(2,3-epoxypropyl)ether, 1,3-butadiene diepoxide, 1,5-hexadiene diepoxide, 1,2,7,8-diepoxyoctane, 1,2,5,6-diepoxycyclooctane, 4-vinyl cyclohexene diepoxide, bisphenol A diglycidyl ether, maleimide-epoxy compounds, etc.
Wet capacitance and leakage are measured by an LCR meter in 25 wt % Phosphoric acid in water under a test frequency of 120 Hz and test voltage of 70% of formation voltage. For wet leakage, the parts have been tested after a charge time of 120 seconds and DC bias voltage of 2V. ACC (anomalous charging current) is measured at the end of the production process. The finished parts are mounted on a circuit board and measurement is done at 0° C. and 80% Vr and expressed as times theoretical value (xTLV).
Comparative Example: Tantalum anodes (330 microfarads, 16V rated voltage) are prepared by sintering of tantalum powder. First, the anodes are anodized in an electrolyte that works as a control {phosphoric acid about (2-5 wt %), ethylene glycol (50-70 wt %) and water having resistivity of about (100-370) ohm-cm measured at 80° C.} at 35V and 80° C. to form a dielectric oxide on the tantalum anode (1st anodization). The anodes are rinsed, heat-treated at 450° C. for 30 minutes and re-anodized in the original electrolytes. Wet leakage after 1st anodization is given in Table 1. The anodes are then covered in conductive polymer that served as cathode, followed by carbon and silver coatings. Parts get assembled and molded into surface mount finished capacitors using known techniques. ACC is measured at 0° C. on mount parts and reported as current in mA at 12.8V {80% of the rated voltage (Vr)}. Cap and ESR (equivalent series resistance) of the finished part are also reported along with ACC in Table 1.
Inventive Example 1: A series of solid electrolytic capacitors have been prepared in a similar manner to that in comparative example 1 using 300 microfarad parts with rated voltage 16V except that the electrolyte is (5-10 wt %) inositol-6-phosphate in water having resistivity (7-15) ohm-cm at 80° C. Wet leakage after 1st anodization, cap and ESR of finished part are also reported along with ACC as current (mA) at 12.8V (80% of Vr) are given in Table 1.
Inventive Example 2: A series of solid electrolytic capacitors are prepared in similar manner to that in inventive example 1 using 300 microfarad parts with rated voltage 16V except that the anodization electrolyte is a mixture of (5-10 wt %) inositol-6-phosphate and epoxy silane (1:1) in water having resistivity (9-17) ohm-cm at 80° C. Wet leakage after 1st anodization, cap and ESR of finished part are also reported along with ACC as current (mA) at 12.8V (80% of Vr) are given in Table 1.
Inventive Example 3: In this example, a series of solid electrolytic capacitors are prepared in similar manner to that in inventive example 1 using 300 microfarad parts with rated voltage 16V except that the electrolyte has ethylene glycol (EG) added to it. Inositol-6-phosphate (5-10 wt %) is added to ethylene glycol (50-70 wt %) in water having resistivity (75-100) ohm-cm at 80° C. Wet leakage after 1st anodization, cap and ESR of finished part are also reported along with ACC as current (mA) at 12.8V (80% of Vr) are given in Table 1.
Inventive Example 4: In this example, a series of solid electrolytic capacitors are prepared in similar manner using 300 microfarad parts with rated voltage 16V to that in inventive example 2 except that the polymer also has the same electrolyte used in inventive example 2 along with conductive polymer. Wet leakage after 1st anodization, cap and ESR of finished part are also reported along with ACC as current (mA) at 12.8V (80% of Vr) are given in Table 1.
Inventive Example 5: In this example, a series of solid electrolytic capacitors using 300 microfarad parts with rated voltage 16V are prepared in similar manner to that in inventive example 1 except that the electrolyte has inositol (1-6 wt %) and phosphoric acid (1-6 wt %) in water having resistivity of about (10-30) ohm-cm. Wet leakage after 1st anodization, cap and ESR of finished part are also reported along with ACC as current (mA) at 12.8V (80% of Vr) are given in Table 1.
The inventive examples demonstrated the ability to form a capacitor, using the inventive formation electrolyte, having an anomalous charge current less than 4 times the theoretical value. This is otherwise not available in the art.
The invention has been described with reference to preferred embodiments without limit thereto. One of skill in the art would realize additional embodiments which are described and set forth in the claims appended hereto.
