WO2007005914A2 - Capacitor electrolyte - Google Patents
Capacitor electrolyte Download PDFInfo
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- WO2007005914A2 WO2007005914A2 PCT/US2006/026106 US2006026106W WO2007005914A2 WO 2007005914 A2 WO2007005914 A2 WO 2007005914A2 US 2006026106 W US2006026106 W US 2006026106W WO 2007005914 A2 WO2007005914 A2 WO 2007005914A2
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- WIPO (PCT)
- Prior art keywords
- capacitor
- voltage
- electrolyte
- anode
- cathode
- Prior art date
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- 239000003990 capacitor Substances 0.000 title claims abstract description 105
- 239000003792 electrolyte Substances 0.000 title claims abstract description 85
- 230000015572 biosynthetic process Effects 0.000 claims abstract description 35
- QTBSBXVTEAMEQO-UHFFFAOYSA-N Acetic acid Chemical compound CC(O)=O QTBSBXVTEAMEQO-UHFFFAOYSA-N 0.000 claims abstract description 20
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 claims abstract description 15
- 239000004615 ingredient Substances 0.000 claims abstract description 13
- 229910000147 aluminium phosphate Inorganic materials 0.000 claims abstract description 8
- USFZMSVCRYTOJT-UHFFFAOYSA-N Ammonium acetate Chemical compound N.CC(O)=O USFZMSVCRYTOJT-UHFFFAOYSA-N 0.000 claims abstract description 7
- 239000005695 Ammonium acetate Substances 0.000 claims abstract description 7
- 229940043376 ammonium acetate Drugs 0.000 claims abstract description 7
- 235000019257 ammonium acetate Nutrition 0.000 claims abstract description 7
- XTHFKEDIFFGKHM-UHFFFAOYSA-N Dimethoxyethane Chemical compound COCCOC XTHFKEDIFFGKHM-UHFFFAOYSA-N 0.000 claims abstract description 4
- 238000000034 method Methods 0.000 claims description 15
- 230000015556 catabolic process Effects 0.000 claims description 8
- GUVRBAGPIYLISA-UHFFFAOYSA-N tantalum atom Chemical compound [Ta] GUVRBAGPIYLISA-UHFFFAOYSA-N 0.000 claims description 8
- ZUHZGEOKBKGPSW-UHFFFAOYSA-N tetraglyme Chemical compound COCCOCCOCCOCCOC ZUHZGEOKBKGPSW-UHFFFAOYSA-N 0.000 claims description 8
- 239000000203 mixture Substances 0.000 claims description 7
- 229910052715 tantalum Inorganic materials 0.000 claims description 7
- 238000002474 experimental method Methods 0.000 description 8
- 239000000843 powder Substances 0.000 description 8
- 238000012360 testing method Methods 0.000 description 8
- 229910052751 metal Inorganic materials 0.000 description 7
- 239000002184 metal Substances 0.000 description 7
- 238000009472 formulation Methods 0.000 description 5
- 230000007704 transition Effects 0.000 description 5
- 230000008569 process Effects 0.000 description 4
- 238000002560 therapeutic procedure Methods 0.000 description 4
- 230000000694 effects Effects 0.000 description 3
- 230000006870 function Effects 0.000 description 3
- 230000009467 reduction Effects 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 229960000583 acetic acid Drugs 0.000 description 2
- 239000000654 additive Substances 0.000 description 2
- 238000007743 anodising Methods 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- -1 hydronium ions Chemical class 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 230000003071 parasitic effect Effects 0.000 description 2
- LWIHDJKSTIGBAC-UHFFFAOYSA-K tripotassium phosphate Chemical compound [K+].[K+].[K+].[O-]P([O-])([O-])=O LWIHDJKSTIGBAC-UHFFFAOYSA-K 0.000 description 2
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 description 1
- QGZKDVFQNNGYKY-UHFFFAOYSA-O Ammonium Chemical compound [NH4+] QGZKDVFQNNGYKY-UHFFFAOYSA-O 0.000 description 1
- 241000237858 Gastropoda Species 0.000 description 1
- 230000002159 abnormal effect Effects 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 239000010405 anode material Substances 0.000 description 1
- 229940067597 azelate Drugs 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000007599 discharging Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- DQYBDCGIPTYXML-UHFFFAOYSA-N ethoxyethane;hydrate Chemical compound O.CCOCC DQYBDCGIPTYXML-UHFFFAOYSA-N 0.000 description 1
- 239000012362 glacial acetic acid Substances 0.000 description 1
- 125000000350 glycoloyl group Chemical group O=C([*])C([H])([H])O[H] 0.000 description 1
- 230000004217 heart function Effects 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 230000003116 impacting effect Effects 0.000 description 1
- 239000012212 insulator Substances 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 230000002107 myocardial effect Effects 0.000 description 1
- BDJRBEYXGGNYIS-UHFFFAOYSA-N nonanedioic acid Chemical compound OC(=O)CCCCCCCC(O)=O BDJRBEYXGGNYIS-UHFFFAOYSA-N 0.000 description 1
- 239000003960 organic solvent Substances 0.000 description 1
- 229960004838 phosphoric acid Drugs 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 229910000160 potassium phosphate Inorganic materials 0.000 description 1
- 235000011009 potassium phosphates Nutrition 0.000 description 1
- 230000008439 repair process Effects 0.000 description 1
- 230000033764 rhythmic process Effects 0.000 description 1
- 150000003839 salts Chemical class 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
- H01G9/004—Details
- H01G9/04—Electrodes or formation of dielectric layers thereon
- H01G9/042—Electrodes or formation of dielectric layers thereon characterised by the material
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
- H01G9/004—Details
- H01G9/022—Electrolytes; Absorbents
- H01G9/035—Liquid electrolytes, e.g. impregnating materials
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
- H01G9/145—Liquid electrolytic capacitors
Definitions
- the present Invention generally relates to implantable medical devices and, more particularly, to capacitors.
- IMDs Implantable medical devices detect and deliver therapy for a variety of medical conditions in patients.
- exemplary IMDs include implantable pulse generators (IPGs) or implantable cardioverter-defibrillators (ICDs).
- ICDs include a circuit that detects abnormal heart rhythms and automatically delivers therapy to restore normal heart function.
- An ICD circuit includes, inter alia, a battery, a capacitor, and a control module.
- the battery supplies power to the control module and the capacitor.
- the control module controls electrical therapy delivered to a patient.
- the control module signals a switch, coupled to the capacitor, to open or close, which controls whether energy is released by the capacitor.
- the capacitor delivers bursts of electric current through a lead that extend from the ICD to myocardial tissue of the patient.
- Electrolytic capacitors e.g. tantalum, aluminum etc.
- a tantalum electrolytic capacitor's formation voltage is typically three to four times the capacitor's rated voltage. See, John Gill, Basic Tanatlum Capacitor Technology, AVX Journal, p. 3.
- Figure 1 is a cross-sectional view of an electrolytic capacitor
- Figure 2 is a graph that depicts exemplary electrolyte voltage regions
- Figure 3 is a graph that depicts leakage current trends versus voltage
- Figure 4 is a graph that depicts energy input versus voltage
- Figure 5 is a graph that depicts output energy trends versus voltage
- Figure 6 is a graph that depicts efficiency trends versus voltage
- FIG. 7 is a graph that depicts delivered energy versus equivalent series resistance (ESR);
- Figure 8 is a graph that depicts capacitance versus voltage
- Figure 9 is a graph that depicts delivered energy trends versus applied voltage
- Figure 10 is a graph that depicts efficiency trends versus applied voltage
- Figure 11 is a graph that depicts efficiency trends versus applied voltage for discharge to 25% of the voltage.
- Figure 10 is a graph that depicts efficiency trends versus voltage
- Figure 13 is a flow diagram for producing a capacitor capable of an operating voltage that is 90% or more of its formation voltage.
- One embodiment of the present invention is directed to an electrolytic capacitor that exhibits an operating voltage of 90% or more of the formation voltage.
- Operating voltage is the maximum voltage to which a capacitor is charged.
- Formation voltage is a voltage maintained during a process in which an oxide layer, an insulator, is grown on a surface of the anode.
- formation voltage is the voltage above which the anode begins to draw significantly higher current as substantial amounts of oxide are grown across the surface of the anode to support the higher voltages.
- operating voltage of 90% or more of the formation voltage is based upon the electrolyte of the electrolytic capacitor. More specifically, the present invention's rated voltage is about 98% of the formation voltage.
- An exemplary electrolyte includes ingredients comprising acetic acid, ammonium acetate, phosphoric acid, tetaethylene glycol dimethyl ether, and water.
- Another aspect of the present invention relates to designing an electrolytic capacitor capable of achieving an operating voltage of 90% or more of the formation voltage.
- design considerations For example, one consideration is dielectric breakdown of an electrolytic capacitor.
- ESR equivalent series resistance
- Skilled artisans will appreciate that the present invention achieves 15-18% improvement in energy density for the electrolytic capacitor.
- FIG. 1 is a cross-sectional view of an electrolytic capacitor 10.
- Electrolytic capacitor 10 includes an anode 11, a cathode 18, a separator 16 therebetween, and an electrolyte 17.
- Anode 11 includes a metal layer 12 and an oxide layer 15.
- Metal layer 12 is an anodic material such as tantalum (Ta), or other suitable material.
- Oxide layer 15 forms in pores 14 on metal layer 12. Formation of oxide layer 15 occurs when metal layer 12, exposed to a different electrolyte in a bath, has high voltage (e.g. up to 390 volts (V)) applied between metal layer 12, which is the positive terminal and a negative terminal (not shown).
- V 390 volts
- One embodiment of the invention relates to an electrolyte that includes glacial acetic acid (HC 2 HsO 2 ), ammonium acetate (NH 4 (C 2 H 3 O 2 )), phosphoric acid (HsPO 4 ), water, and tetraethylene glycol dimethyl ether (CH 2 (OCH 2 CH 2 ) 4 OCH3).
- HC 2 HsO 2 glacial acetic acid
- NH 4 (C 2 H 3 O 2 ) ammonium acetate
- HsPO 4 phosphoric acid
- water tetraethylene glycol dimethyl ether
- CH 2 (OCH 2 CH 2 ) 4 OCH3 tetraethylene glycol dimethyl ether
- each electrolyte formula included a variety of different molarities for each ingredient.
- tetraethylene glycol dimethyl ether ranged from about 0.9 molarity for electrolyte 1 to about 1.43 molarity for electrolyte 4. Water is the remaining portion.
- the best electrolyte formulations depend on the desired operating voltage of capacitor 10.
- electrolyte 8 in Table II is the best electrolyte 17 formula for an operating voltage of 255V.
- FIG 2 a voltage curve from an electrolyte voltage test is shown which allows a designer of capacitor 10 to visually assess the performance of electrolyte 17 across a range of voltages.
- a constant current is applied to a tantalum wire in a sample of electrolyte.
- the voltage is monitored as a function of time.
- the observed voltage typically rises linearly for some time, then rises somewhat more slowly with increasing noise as parasitic processes absorb more of the applied current.
- the voltage of electrolyte 17 has an initial linear rise (Region A), a transition region (Region B) and a relatively flat, noisy region characterized by continuous steady- state breakdown and repair (Region C).
- Electrolytic capacitor 10 is least likely to degrade over time while operating in the linear region (Region A) up to point D where the transition region begins.
- the highest voltage in this region is a nominal working voltage 20 or nominal electrolyte voltage.
- a trade-off occurs between the efficiencies of a higher operating voltage and charge time stability of capacitor 10.
- charge time begins to show some instability due to greater oxide susceptibility to reaction with electrolyte 17.
- actual breakdown occurs of oxide layer 15.
- capacitor 10 is designed by consideration of the working voltage and the nominal working voltage. Nominal working voltage measures the ability of electrolyte 17 to withstand voltage whereas working voltage is the capacitor-level assessment of the highest voltage at which electrolyte 17 may be used. In one embodiment, a designer of capacitor 10 may set working voltage above the nominal working voltage at a value that balances competing design goals. To illustrate, by setting working voltage at a level above the nominal working voltage, battery size of an implantable medical device (IMD) may increase due to increased charge time and leakage losses but ESR-type heating losses may be reduced. Reduction of ESR is due to a more conductive electrolyte 17, which also reduces the size of capacitor 10.
- IMD implantable medical device
- Electrolyte 17 can be used 50 volts (V) higher than the nominal electrolyte voltage. Additionally, electrolyte 17 achieved an operating voltage that is above 90% of anode slug formation voltage. In one embodiment, capacitor 10 has an operating voltage that is above 93% of anode slug formation voltage. In another embodiment, capacitor 10 has an operating voltage that is above 95% of anode slug formation voltage. In still yet another embodiment, capacitor 10 has an operating voltage that is above 98% of anode slug formation voltage.
- Capacitance decreases and operating voltage increases when oxide layer 15 is formed to a higher voltage (and consequently to a greater thickness).
- the increase in energy density from these factors may outweigh possible limitations associated with lower conductivity (higher required porosity, lower powder density, thinner anodes, and/or reduced high frequency performance, etc.), increased leakage current, or charge time.
- Increased energy density which depends on the increased operating voltage, reduces the size of the ICD.
- additives may be added to the mixture of electrolyte 17 ingredients listed in Tables I and II.
- Organic solvents or salts e.g. ammonium azelate, potassium phosphate etc.
- increased conductivity may be achieved by addition of hydronium ions, hydroxide ions, or other suitable additive(s).
- improving characteristics associated with oxide layer 15 may also improve the operating voltage. Oxide susceptibility to leakage and breakdown may be significantly reduced, allowing operation to occur beyond the nominal working voltage.
- the selection of ingredients for electrolyte 17 may synergistically enhance capacitor 10 (e.g. the higher voltage capacitor, a desirable therapy voltage with a three- capacitor system, higher energy density capacitor etc.) in combination with suitably processed capacitor oxides.
- Oxide properties and thickness are affected by oxide formation conditions.
- Exemplary oxide formation conditions include temperature o metal layer 12 in an electrolytic environment (e.g. electrolyte bath etc.), hold time at final formation voltage, and the final electrical current density through metal layer 12 at termination of the formation process.
- electrolyte 17 achieves an operating voltage of 90% or more of the formation voltage.
- the two experiments also explored the manner in -which competing design goals were affected.
- electrolytes 1 through 8 in Tables I and II were formulated to provide nominal electrolyte voltages between 175 V and 225 V for both experiments.
- Figures 3-5 relate to the first experiment whereas Figures 7-12 relate to the second experiment.
- Several general observations were made based upon the experiments. For example, capacitor 10 does not become abruptly nonfunctional at a higher voltage. Electrolyte 17 is usable for somewhat higher voltages, with some possibility of increasing charge times over many charging cycles. The primary trade-off is between leakage current and ESR. These properties may change over the life of capacitor 10.
- Electrolyte 17 should be sufficiently conductive to allow access to charge in the interior of the anode slug on the timescale of the application charge/discharge cycle.
- All capacitors 10 were constructed from slugs prepared under the same formation conditions and formed to 225 V. Conservative working voltages were used for formulations of electrolyte 17.
- Typical formation conditions and/or processes are disclosed in US 5,716,511 entitled “Anodizing electrolyte and its use and US 6,480,371 entitled, "Alkanolamine-phosphoric acid anodizing electrolyte” both of which are incorporated by reference in relevant parts.
- Low leakage current powders with CV in the range of 10,000 to 20,000 microfarad* volt/gram (uF V/g) are appropriate for this testing, where CV represents a standard rating of a capacitor powder, the expected capacitance multiplied by the voltage. Suitable powders are available from H C Starck Incorporated or Cabot Corporation.
- All exemplary electrolytes 17 show a clear progression in capacitance and ESR. As the electrolyte resistivity increases, average ESR progresses from 1.1 ⁇ to 1.5 ⁇ to 2.0 ⁇ , nearly doubling as resistivity is doubled. Additionally, capacitance drops from 344 ⁇ F to 335 ⁇ F to 328 ⁇ F, reflecting the reduced electrical access to the slug interior at 120 Hz, which represents the relevant time scale of ICD function.
- Figure 3 compares average leakage currents for the three electrolytes of Table III. On Figure 3, traces are labeled relative to the nominal electrolyte voltage of capacitor 10. Leakage current rises rapidly upon approaching the formation voltage. The lower nominal voltage electrolyte may exhibit higher leakage current; however, the small sample size lacks sufficient statistical data.
- leakage current is measured after five minutes at a particular voltage, according to standard practices. Results are averaged over the available parts. Capacitance, ESR and leakage current all contribute to energy density. Energy density is a significant consideration for an ICD.
- the present invention achieves 15-18% improvement in energy density for the electrolytic capacitor.
- FIG. 4 shows the variations in input energy with charge voltage of electrolyte 17.
- Electrolytes 1-3 are tabs 101,102, and 103, respectively.
- Figure 5 represents the output energy trends for capacitor 10 discharged into a 16.7 ⁇ load, which generally correlates to a nominal 50 ⁇ patient load for a three capacitor system in an IMD. These output energies are measured on discharging to 0 volts. The lower nominal voltage, higher conductivity electrolytes have consistently higher output energies. At 225 V, there is a 0.5 joules(J) difference between the electrolyte 1 and electrolyte 3. Within this data set, electrolyte 1 for capacitor 10 delivers the most energy. It was also observed that ESR losses outweigh leakage current effects.
- Figure 6 depicts the general downward slope in efficiency primarily related to charging losses.
- the gap between the curves is primarily related to ESR differences.
- Figure 7 depicts delivered energy for each of capacitors 10 charged to 225 V as a function of ESR.
- ESR measured at 120 Hz
- electrolyte 17 is effectively used at 50 V above its nominal breakdown voltages.
- leakage current losses do not compete with ESR losses.
- conductivity outweighs leakage current in choosing an electrolyte formulation. The impact of slower ionic access to the interior of the slug was assessed to be less than 2% of delivered energy efficiency.
- capacitors 10 were used to examine the dependence of delivered energy and charge/discharge efficiency on electrolyte conductivity.
- Capacitors were made of two types of anode powder, Type 1 and Type 2. Four different electrolytes of increasing conductivity and decreasing nominal working voltage were used.
- Capacitors were tested at various voltages, as shown on Table IV.
- Capacitance is displayed in Figure 8.
- Capacitor 10 made with either Type 1 or Type 2 powder but with the same electrolyte 17 have similar ESR.
- the Type 2 capacitors are lower capacitance, but their ability to be charged to a higher voltage provides a similar total available energy.
- Figure 9 relates delivered energy for discharge into a 16.7 ohm load to 25% of voltage for capacitors made with Type 2 powders.
- Input and discharge energies are combined in Figure 10 as overall efficiency, defined as discharge energy divided by input energy.
- the figure indicates two runs ("D" and "U") with indicated electrolyte voltages. Full discharge efficiency starts at about 90% for Type 2 powder capacitors charged to 150 V. It falls off with applied voltage and higher nominal electrolyte voltage.
- Figure 12 shows the marginal efficiency loss of operating at 25% discharge instead of at full discharge of capacitor 10. This representative data is taken at 150 V. The dotted line indicates the efficiency loss expected from the energy remaining on the capacitor. The loss above this threshold is related to losses due to slow access to the slug interior compared to the time scale of the discharge. These losses are less than 2 % for this set of capacitors.
- An implantable cardioverter-defibrillator (ICD) application involves capacitor operation at a very stable temperature (e.g. about 37 0 C) with a small number of cycles (e.g. less than 200) and a very short time (e.g. less than 15 minutes) at a certain voltage over the capacitor 10 lifetime. These conditions, prevent a higher level of parasitic activity from significantly impacting the capacitor lifetime. Additionally, the electrolyte allows creation of a higher energy density capacitor which in turn allows for production of a smaller implantable medical device (IMD).
- IMD implantable medical device
- Figure 13 is a flow diagram for designing an electrolytic capacitor.
- the design criteria includes voltage breakdown, ESR, conductivity of an electrolyte associated with the electrolytic capacitor, and leakage current.
- operating voltage is determined relative to formation voltage.
- an electrolyte is formulated such that an operating voltage is greater than 90 percent of a formation voltage for a capacitor. Skllled artisans will appreciate that alternative embodiments may be formed by implementation of the claimed invention. For example, electrolyte 17 ingredients are selected for certain design conditions but different ingredients and/or quantities may be appropriate for different designs. Additionally, adjustment of nominal working voltage may generally be performed around any desired capacitor operating or surge voltage.
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Abstract
A capacitor for an implantable medical device is presented. The capacitor includes an anode, a cathode, a separator therebetween, and an electrolyte over the anode, cathode, and separator. The electrolyte includes ingredients comprising acetic acid, ammonium acetate, phosphoric acid, and tetaethylene glycol dimethyl ether. The capacitor has an operating voltage ninety percent or greater of its formation voltage.
Description
CAPACITOR ELECTROLYTE
FIELD OF THE INVENTION
The present Invention generally relates to implantable medical devices and, more particularly, to capacitors.
BACKGROUND OF THE INVENTION
Implantable medical devices (IMDs) detect and deliver therapy for a variety of medical conditions in patients. Exemplary IMDs include implantable pulse generators (IPGs) or implantable cardioverter-defibrillators (ICDs). ICDs include a circuit that detects abnormal heart rhythms and automatically delivers therapy to restore normal heart function. An ICD circuit includes, inter alia, a battery, a capacitor, and a control module. The battery supplies power to the control module and the capacitor. The control module controls electrical therapy delivered to a patient. For example, the control module signals a switch, coupled to the capacitor, to open or close, which controls whether energy is released by the capacitor. The capacitor delivers bursts of electric current through a lead that extend from the ICD to myocardial tissue of the patient.
Electrolytic capacitors (e.g. tantalum, aluminum etc.) are typically used since these capacitors attain high energy density in a low volume package. Generally, a tantalum electrolytic capacitor's formation voltage is typically three to four times the capacitor's rated voltage. See, John Gill, Basic Tanatlum Capacitor Technology, AVX Journal, p. 3.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
Figure 1 is a cross-sectional view of an electrolytic capacitor; Figure 2 is a graph that depicts exemplary electrolyte voltage regions; Figure 3 is a graph that depicts leakage current trends versus voltage; Figure 4 is a graph that depicts energy input versus voltage;
Figure 5 is a graph that depicts output energy trends versus voltage; Figure 6 is a graph that depicts efficiency trends versus voltage;
Figure 7 is a graph that depicts delivered energy versus equivalent series resistance (ESR);
Figure 8 is a graph that depicts capacitance versus voltage;
Figure 9 is a graph that depicts delivered energy trends versus applied voltage;
Figure 10 is a graph that depicts efficiency trends versus applied voltage;
Figure 11 is a graph that depicts efficiency trends versus applied voltage for discharge to 25% of the voltage.
Figure 10 is a graph that depicts efficiency trends versus voltage;
Figure 13 is a flow diagram for producing a capacitor capable of an operating voltage that is 90% or more of its formation voltage.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, similar reference numbers are used in the drawings to identify similar elements.
One embodiment of the present invention is directed to an electrolytic capacitor that exhibits an operating voltage of 90% or more of the formation voltage. Operating voltage is the maximum voltage to which a capacitor is charged. Formation voltage is a voltage maintained during a process in which an oxide layer, an insulator, is grown on a surface of the anode. Operationally, formation voltage is the voltage above which the anode begins to draw significantly higher current as substantial amounts of oxide are grown across the surface of the anode to support the higher voltages. Formation voltage and formation temperature follow the relationship TfVf=constant, where Tf is the formation temperature and Vf is the formation voltage. [A F Torrisi, "Relation of Color to Certain Charatcteristics of Anodic Tantalum Films," Journal of the Electrochemical Society, Vol. 102, No 4, ppl76-180, April 1955.] For oxides formed at different
temperatures, the formation voltage should be corrected to the application voltage for appropriate comparison to the operating voltage.
In another embodiment, operating voltage of 90% or more of the formation voltage is based upon the electrolyte of the electrolytic capacitor. More specifically, the present invention's rated voltage is about 98% of the formation voltage. An exemplary electrolyte includes ingredients comprising acetic acid, ammonium acetate, phosphoric acid, tetaethylene glycol dimethyl ether, and water.
Another aspect of the present invention relates to designing an electrolytic capacitor capable of achieving an operating voltage of 90% or more of the formation voltage. There are several design considerations. For example, one consideration is dielectric breakdown of an electrolytic capacitor. Another exemplary consideration is equivalent series resistance (ESR) related to movement of charge between the anode and the cathode of the electrolytic capacitor. Skilled artisans will appreciate that the present invention achieves 15-18% improvement in energy density for the electrolytic capacitor.
Figure 1 is a cross-sectional view of an electrolytic capacitor 10. Electrolytic capacitor 10 includes an anode 11, a cathode 18, a separator 16 therebetween, and an electrolyte 17. Anode 11 includes a metal layer 12 and an oxide layer 15. Metal layer 12 is an anodic material such as tantalum (Ta), or other suitable material. Oxide layer 15 forms in pores 14 on metal layer 12. Formation of oxide layer 15 occurs when metal layer 12, exposed to a different electrolyte in a bath, has high voltage (e.g. up to 390 volts (V)) applied between metal layer 12, which is the positive terminal and a negative terminal (not shown).
One embodiment of the invention relates to an electrolyte that includes glacial acetic acid (HC2HsO2), ammonium acetate (NH4(C2H3O2)), phosphoric acid (HsPO4), water, and tetraethylene glycol dimethyl ether (CH2(OCH2CH2)4OCH3). Exemplary quantities for the electrolyte ingredients are presented in Tables I and II below. These electrolyte 17 formulations were evaluated after being introduced to a set of 0.095 inch thick anode slug in electrolytic capacitors. Electrolyte 17 formulations were selected based on differing conductivities and/or breakdown voltages.
As shown in Table I, each electrolyte formula included a variety of different molarities for each ingredient. For example, tetraethylene glycol dimethyl ether ranged
from about 0.9 molarity for electrolyte 1 to about 1.43 molarity for electrolyte 4. Water is the remaining portion.
Table I: Molarities of ingredients to form the electrolyte
In one embodiment, the best electrolyte formulations depend on the desired operating voltage of capacitor 10. For example, electrolyte 8 in Table II is the best electrolyte 17 formula for an operating voltage of 255V.
Turning now to Figure 2, a voltage curve from an electrolyte voltage test is shown which allows a designer of capacitor 10 to visually assess the performance of electrolyte 17 across a range of voltages. In this test, a constant current is applied to a tantalum wire in a sample of electrolyte. The voltage is monitored as a function of time. The observed voltage typically rises linearly for some time, then rises somewhat more slowly with increasing noise as parasitic processes absorb more of the applied current.
The voltage of electrolyte 17 has an initial linear rise (Region A), a transition region (Region B) and a relatively flat, noisy region characterized by continuous steady- state breakdown and repair (Region C). Electrolytic capacitor 10 is least likely to degrade over time while operating in the linear region (Region A) up to point D where the transition region begins. The highest voltage in this region is a nominal working voltage 20 or nominal electrolyte voltage. Upon increasing voltage into the transition region (Region B), a trade-off occurs between the efficiencies of a higher operating voltage and charge time stability of capacitor 10. In the transition region, charge time begins to show some instability due to greater oxide susceptibility to reaction with electrolyte 17. Beyond the transition region, actual breakdown occurs of oxide layer 15.
Numerous factors may be contemplated to design capacitor 10. In one embodiment, capacitor 10 is designed by consideration of the working voltage and the nominal working voltage. Nominal working voltage measures the ability of electrolyte 17 to withstand voltage whereas working voltage is the capacitor-level assessment of the highest voltage at which electrolyte 17 may be used. In one embodiment, a designer of capacitor 10 may set working voltage above the nominal working voltage at a value that balances competing design goals. To illustrate, by setting working voltage at a level above the nominal working voltage, battery size of an implantable medical device (IMD) may increase due to increased charge time and leakage losses but ESR-type heating losses may be reduced. Reduction of ESR is due to a more conductive electrolyte 17, which also reduces the size of capacitor 10. Other design considerations may include the lifetime of capacitor 10, operating temperature of capacitor 10. charge time of capacitor 10, battery life, overall IMD size, and operating voltage of capacitor 10. Electrolyte 17 can be used 50 volts (V) higher than the nominal electrolyte voltage. Additionally, electrolyte 17 achieved an operating voltage that is above 90% of anode slug formation voltage. In one embodiment, capacitor 10 has an operating voltage that is above 93% of anode slug formation voltage. In another embodiment, capacitor 10 has an operating voltage that is above 95% of anode slug formation voltage. In still yet another embodiment, capacitor 10 has an operating voltage that is above 98% of anode slug formation voltage.
Operating at a higher fraction of formation voltage increases energy density, since E = (1/2) CV2. where E is energy, C is capacitance, and V is operating voltage of capacitor
10. Capacitance is defined as C= (KA)/d where K is the dielectric constant, A is the surface area of oxide layer 15, and d is the thickness of oxide layer 15.
Capacitance decreases and operating voltage increases when oxide layer 15 is formed to a higher voltage (and consequently to a greater thickness). The increase in energy density from these factors may outweigh possible limitations associated with lower conductivity (higher required porosity, lower powder density, thinner anodes, and/or reduced high frequency performance, etc.), increased leakage current, or charge time. Increased energy density, which depends on the increased operating voltage, reduces the size of the ICD.
In order to further enhance the performance of electrolyte 17, additives may be added to the mixture of electrolyte 17 ingredients listed in Tables I and II. Organic solvents or salts (e.g. ammonium azelate, potassium phosphate etc.) may be used to adjust (increase or decrease) the nominal working voltage while reducing the conductivity associated with electrolyte 17. For electrolyte not at a pH 7.0, increased conductivity may be achieved by addition of hydronium ions, hydroxide ions, or other suitable additive(s).
In another embodiment, improving characteristics associated with oxide layer 15 may also improve the operating voltage. Oxide susceptibility to leakage and breakdown may be significantly reduced, allowing operation to occur beyond the nominal working voltage. The selection of ingredients for electrolyte 17 may synergistically enhance capacitor 10 (e.g. the higher voltage capacitor, a desirable therapy voltage with a three- capacitor system, higher energy density capacitor etc.) in combination with suitably processed capacitor oxides.
Oxide properties and thickness are affected by oxide formation conditions. Exemplary oxide formation conditions include temperature o metal layer 12 in an electrolytic environment (e.g. electrolyte bath etc.), hold time at final formation voltage, and the final electrical current density through metal layer 12 at termination of the formation process.
Experimental data from two separate experiments confirmed that electrolyte 17 achieves an operating voltage of 90% or more of the formation voltage. The two experiments also explored the manner in -which competing design goals were affected. As
a preliminary manner, electrolytes 1 through 8 in Tables I and II were formulated to provide nominal electrolyte voltages between 175 V and 225 V for both experiments.
Figures 3-5 relate to the first experiment whereas Figures 7-12 relate to the second experiment. Several general observations were made based upon the experiments. For example, capacitor 10 does not become abruptly nonfunctional at a higher voltage. Electrolyte 17 is usable for somewhat higher voltages, with some possibility of increasing charge times over many charging cycles. The primary trade-off is between leakage current and ESR. These properties may change over the life of capacitor 10.
Increasing conductivity of electrolyte 17 increases the accessibility of charge stored in the interior of the anode slug. Electrolyte 17 should be sufficiently conductive to allow access to charge in the interior of the anode slug on the timescale of the application charge/discharge cycle.
Referring to Table 3, nine capacitors were examined to determine the effects of a particular electrolyte 17 formula from Table I.
Table III: Capacitor inventory
All capacitors 10 were constructed from slugs prepared under the same formation conditions and formed to 225 V. Conservative working voltages were used for formulations of electrolyte 17. Typical formation conditions and/or processes are disclosed in US 5,716,511 entitled "Anodizing electrolyte and its use and US 6,480,371 entitled, "Alkanolamine-phosphoric acid anodizing electrolyte" both of which are incorporated by reference in relevant parts. Low leakage current powders with CV in the range of 10,000 to 20,000 microfarad* volt/gram (uF V/g) are appropriate for this testing, where CV represents a standard rating of a capacitor powder, the expected capacitance multiplied by the voltage. Suitable powders are available from H C Starck Incorporated or Cabot Corporation.
All exemplary electrolytes 17 show a clear progression in capacitance and ESR. As the electrolyte resistivity increases, average ESR progresses from 1.1 Ω to 1.5 Ω to 2.0Ω, nearly doubling as resistivity is doubled. Additionally, capacitance drops from 344 μF to 335 μF to 328 μF, reflecting the reduced electrical access to the slug interior at 120 Hz, which represents the relevant time scale of ICD function.
Figure 3 compares average leakage currents for the three electrolytes of Table III. On Figure 3, traces are labeled relative to the nominal electrolyte voltage of capacitor 10. Leakage current rises rapidly upon approaching the formation voltage. The lower nominal voltage electrolyte may exhibit higher leakage current; however, the small sample size lacks sufficient statistical data. A tradeoff between leakage current and ESR/capacitance may exist. Leakage currents are measured after five minutes at a particular voltage, according to standard practices. Results are averaged over the available parts. Capacitance, ESR and leakage current all contribute to energy density. Energy density is a significant consideration for an ICD. The present invention achieves 15-18% improvement in energy density for the electrolytic capacitor.
Figure 4 shows the variations in input energy with charge voltage of electrolyte 17. The dashed line is a projection of the 125 V energy to higher voltages, assuming an ideal scale with V2. of E = (1/2) CV2 For capacitor 10, about a joule of additional energy is required at 225 V due to energy losses. Electrolytes 1-3 are tabs 101,102, and 103, respectively.
Figure 5 represents the output energy trends for capacitor 10 discharged into a 16.7 Ω load, which generally correlates to a nominal 50 Ω patient load for a three capacitor system in an IMD. These output energies are measured on discharging to 0 volts. The lower nominal voltage, higher conductivity electrolytes have consistently higher output energies. At 225 V, there is a 0.5 joules(J) difference between the electrolyte 1 and electrolyte 3. Within this data set, electrolyte 1 for capacitor 10 delivers the most energy. It was also observed that ESR losses outweigh leakage current effects.
Figure 6 depicts the general downward slope in efficiency primarily related to charging losses. The gap between the curves is primarily related to ESR differences.
Figure 7 depicts delivered energy for each of capacitors 10 charged to 225 V as a function of ESR. As shown, data below 1.3 Ω represents electrolyte 1; between 1.3 Ω and 1.8 Ω represents electrolyte 2; and electrolyte 3 capacitors appear above 1.8 Ω. As expected, there is a reduction in delivered as the internal resistance of capacitor 10 increases, and the reduction is more pronounced with smaller loads. This effect is due to the ESR (measured at 120 Hz) acting as a voltage divider in series with the load resistance.
From the first experiment, a number of observations were made. For example, electrolyte 17 is effectively used at 50 V above its nominal breakdown voltages. Moreover, leakage current losses do not compete with ESR losses. Additionally, conductivity outweighs leakage current in choosing an electrolyte formulation. The impact of slower ionic access to the interior of the slug was assessed to be less than 2% of delivered energy efficiency.
For the second experiment, capacitors 10 were used to examine the dependence of delivered energy and charge/discharge efficiency on electrolyte conductivity. Capacitors were made of two types of anode powder, Type 1 and Type 2. Four different electrolytes of increasing conductivity and decreasing nominal working voltage were used.
Capacitors were tested at various voltages, as shown on Table IV.
Table IV:Test voltages for capacitors by electrolyte
At each voltage, a series of tests were performed including 5 minute leakage current test, ESR, and capacitance, charge energy, charge time, and discharge energy. For the first two voltages, 25 Ω, 16.7 Ω, and 5 Ω loads were used. For other testing, only 16.7-Ω nominal loads were used, corresponding to a 50-Ω load for a 3-capacitor system. Discharge energies were determined for full discharge and for discharge to 25% of charge voltage. Tests were performed at 37 degree Celsius (0C).
Capacitance is displayed in Figure 8. Capacitor 10 made with either Type 1 or Type 2 powder but with the same electrolyte 17 have similar ESR. The Type 2 capacitors are lower capacitance, but their ability to be charged to a higher voltage provides a similar total available energy.
Figure 9 relates delivered energy for discharge into a 16.7 ohm load to 25% of voltage for capacitors made with Type 2 powders. Input and discharge energies are combined in Figure 10 as overall efficiency, defined as discharge energy divided by input energy. The figure indicates two runs ("D" and "U") with indicated electrolyte voltages. Full discharge efficiency starts at about 90% for Type 2 powder capacitors charged to 150 V. It falls off with applied voltage and higher nominal electrolyte voltage.
Similar trends are seen in Figure 11 for discharge to 25% of applied voltage. This figure also indicates two runs ("D" and "U") with indicated electrolyte voltages. Initial efficiencies are slightly lower, in part due to 1/16 of the energy remaining on the capacitors; ideal efficiency would be only 94%. Inefficiencies increase as applied voltage and electrolyte resistivity are increased.
Figure 12 shows the marginal efficiency loss of operating at 25% discharge instead of at full discharge of capacitor 10. This representative data is taken at 150 V. The dotted line indicates the efficiency loss expected from the energy remaining on the capacitor. The loss above this threshold is related to losses due to slow access to the slug interior compared to the time scale of the discharge. These losses are less than 2 % for this set of capacitors.
An implantable cardioverter-defibrillator (ICD) application involves capacitor operation at a very stable temperature (e.g. about 370C) with a small number of cycles (e.g. less than 200) and a very short time (e.g. less than 15 minutes) at a certain voltage over the capacitor 10 lifetime. These conditions, prevent a higher level of parasitic activity from significantly impacting the capacitor lifetime. Additionally, the electrolyte allows creation of a higher energy density capacitor which in turn allows for production of a smaller implantable medical device (IMD).
Figure 13 is a flow diagram for designing an electrolytic capacitor. At block 100, at least one capacitor design criteria is considered. The design criteria includes voltage breakdown, ESR, conductivity of an electrolyte associated with the electrolytic capacitor, and leakage current. At block 110, operating voltage is determined relative to formation voltage. At block 120, an electrolyte is formulated such that an operating voltage is greater than 90 percent of a formation voltage for a capacitor.
Skllled artisans will appreciate that alternative embodiments may be formed by implementation of the claimed invention. For example, electrolyte 17 ingredients are selected for certain design conditions but different ingredients and/or quantities may be appropriate for different designs. Additionally, adjustment of nominal working voltage may generally be performed around any desired capacitor operating or surge voltage.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
Claims
1. A tantalum capacitor for an implantable medical device comprising: an anode; a cathode; a separator coupled to the anode and the cathode; an electrolyte over the anode, cathode, and separator; and wherein an operating voltage being greater than 90 percent of a formation voltage for the capacitor.
2. The capacitor of claim 2, wherein the electrolyte includes ingredients comprising acetic acid, ammonium acetate, phosphoric acid, and tetaethylene glycol dimethyl ether.
3. The capacitor of claim 2, wherein the acetic acid includes a molarity which ranges from about 1.5 to about 2.5.
4. The capacitor of claim 2, wherein the ammonium acetate includes a molarity which ranges from about 1.2 to about 2.1.
5. The capacitor of claim 2, wherein the phosphoric acid includes a molarity which ranges from about 0.02 to about 0.5.
6. The capacitor of claim 2, wherein the tetraethylene glycol dimethyl ether includes a molarity which ranges from about 0.8 to about 1.5.
7. The capacitor of claim 1, wherein a working voltage being at least equivalent to a nominal working voltage.
8. The capacitor of claim 1, wherein a working voltage being greater than a nominal working voltage.
9 A method for forming a capacitor for an implantable medical device comprising: formulating an electrolyte such that an operating voltage is greater than 90 percent of a formation voltage for a capacitor.
10. The method of claim 9, wherein the electrolyte includes ingredients comprising acetic acid, ammonium acetate, phosphoric acid, and tetaethylene glycol dimethyl ether.
11. The method of claim 10, wherein the acetic acid includes a molarity which ranges from about 1.5 to about 2.5.
12. The method of claim 10, wherein the ammonium acetate includes a molarity which ranges from about 1.2 to about 2.1.
13.. The method of claim 10, wherein the phosphoric acid includes a molarity which ranges from about 0.02 to about 0.5.
14. The method of claim 10, wherein the tetraethylene glycol dimethyl ether includes a molarity which ranges from about 0.8 to about 1.5.
15. The method of claim 9, wherein quantities of the ingredients being selected based upon conductivity of a mixture of the ingredients.
16. The method of claim 9, wherein formulating the electrolyte being based upon consideration of at least one of equivalent series resistance (ESR), and at least one characteristic associated with an oxide layer over an anode of the electrolytic capacitor.
17. A method of designing an electrolytic capacitor for an implantable medical device comprising: considering one of voltage breakdown, ESR, conductivity of an electrolyte associated with the electrolytic capacitor, and leakage current; and determining operating voltage relative to formation voltage.
18. The method of claim 17 further comprising: determining electrolyte voltage.
19. The method of claim 20 further comprising: setting a nominal working voltage greater than a working voltage.
20. A tantalum capacitor for an implantable medical device comprising: an anode; a cathode; a separator coupled to the anode and the cathode; an electrolyte over the anode, cathode, and separator; and wherein an operating voltage being greater than 95 percent of a formation voltage for the capacitor.
21. A tantalum capacitor for an implantable medical device comprising: an anode; a cathode; a separator coupled to the anode and the cathode; an electrolyte over the anode, cathode, and separator; and wherein an operating voltage being greater than 98 percent of a formation voltage for the capacitor.
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US9548166B2 (en) | 2005-06-30 | 2017-01-17 | Medtronic, Inc. | Capacitor electrolyte |
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US5716511A (en) * | 1996-08-07 | 1998-02-10 | Kemet Electronics Corporation | Anodizing electrolyte and its use |
EP0989572A2 (en) * | 1998-08-28 | 2000-03-29 | Wilson Greatbatch Ltd. | Electrolyte for use in a capacitor |
US6788523B1 (en) * | 2003-05-30 | 2004-09-07 | Kemet Electronics | Electrolyte for electrolytic capacitor |
US20040243183A1 (en) * | 2003-05-30 | 2004-12-02 | Norton John D. | Wet tantalum capacitor usable without reformation and medical devices for use therewith |
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2006
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US5716511A (en) * | 1996-08-07 | 1998-02-10 | Kemet Electronics Corporation | Anodizing electrolyte and its use |
EP0989572A2 (en) * | 1998-08-28 | 2000-03-29 | Wilson Greatbatch Ltd. | Electrolyte for use in a capacitor |
US6788523B1 (en) * | 2003-05-30 | 2004-09-07 | Kemet Electronics | Electrolyte for electrolytic capacitor |
US20040243183A1 (en) * | 2003-05-30 | 2004-12-02 | Norton John D. | Wet tantalum capacitor usable without reformation and medical devices for use therewith |
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Publication number | Priority date | Publication date | Assignee | Title |
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US9548166B2 (en) | 2005-06-30 | 2017-01-17 | Medtronic, Inc. | Capacitor electrolyte |
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