Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-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
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-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
  • H01G9/048—Electrodes or formation of dielectric layers thereon characterised by their structure
  • H01G9/052—Sintered electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-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
  • H01G9/048—Electrodes or formation of dielectric layers thereon characterised by their structure
  • H01G9/052—Sintered electrodes
  • H01G9/0525—Powder therefor

Definitions

• the present invention relates to substrates for high dielectric constant capacitors and more particularly powder substrates based on tantalum and/or niobium fabricated into porous masses that are electrolytically "formed” to establish a thin oxide of tantalum and/or niobium (normally tantalum and/or niobium pentoxide) as the dielectric layer. These are utilized with well known solid or wet electrolyte systems.
• tantalum/niobium powder substrates (primarily tantalum) have been utilized for over half a Century as materials of choice for highest capacitance, compact capacitors with low leakage, low electrical series resistance and high voltage breakdown levels, standing up well to demanding usage and quality control life tests of military, computer and telecommunications markets.
• the state of the art capacitance level for electrolytic capacitors has moved up in the last decade from under 10,000 micro-farad volts per gram to over 50,000 through shrinkage of powder substrate size (with greater surface area of formed oxide in relation to weight and volume of the anodes, anode porosity control for greater effective access to the expanded area, sinter controls, doping of the substrate with phosphorous and in some instances nitrogen, silicon, or sulfur. Improvements in lead wire production, lead wire to anode bonding, forming routines, electrolytic systems and packaging have also been made.
• the objects of the invention are met through new tantalum-silicon and niobium-silicon systems preferably formed as mixtures of 90-98 wgt-% Ta, Nb and 2- 10 wgt-% of Si powders mixed together.
• Electrolytic porous anode capacitors made with such systems can afford stable performance at high voltage formations, and under conditions of high frequency usage.
• the benefits of the present invention can also be realized in Ta/Nb-nitride systems and in systems of Si with Ta/Nb, Ta/Nb-nitride doped with known capacitance enhancing impurities such as P, Si, S.
• the benefits of silicon addition include pore size control of sintered anodes and optimized porosity with generally larger pores and greater uniformity of pore size to enable a more certain effective electrolyte precursor access, effective electrolyte conduction paths and less degradation of capacitor performance associated with varying porosity.
• One method to distribute Si homogeneously throughout produced Ta or Nb is by use of liquid organo-silicon compounds. Due to the desire for reduced oxygen and carbon content, the preferred organo-silicon compound would be in the silicone family. These compounds which are primarily made up of SiOH bonds will decompose during the high temperature treatment of the powders to si in a reducing atmosphere.
• the reducing atmosphere may be provided in the standard technology of the field but it is preferred to be Mg or H 2 , or NH to minimize contamination.
• FIG. 1 is a graph of capacitance Ta-Si vs. high capacitance type of Ta (50K) capacitor with sintering at various temperatures from 1300 to 1550°C;
• FIG. 2 compares similar materials as to bias dependence at various test bias voltages;
• FIGS. 3-4 trace capacitance and leakage vs. sinter temperature (similarly to FIG. 1) comparing Ta-Si with Ta and also with TaN+Si;
• FIGS. 5-6 compare (similarly to FIG. 2) bias dependence of Ta, Ta-Si, Ta+Si 3 N 4 , TaN-Si 3 N 4 and TaN-Si; and
• FIGS. 7-8 compare incremental volume vs. pore diameter characteristics for Ta vs. Ta-Si, and TaN vs. TaN-Si.
• the present invention starts from a separate path of recognizing, from the work of T. Tripp et al. USP 4,957,541 (capacitor grade tantalum powder; see also, references cited therein), the proper role of tantalum nitride in affording a new series of useful powder substrates.
• Example 1
• the Ta was a standard product 50K-9010 made from sodium reduced potassium heptafluorotantalate with artifacts of leaching, fine sizing, doping and deoxidization well known in the art.
• the Ta-Si was prepared by blending 0.333 gm of 60 mesh 99.999% pure Si powder with 9.667 gm of the 50K-9010 Ta powder, to approximate Ta 9 Si 2 .
• Powders of both systems were pressed into pellets and sintered at 1500°C for first sets of pellets formed at 16, 30, 40, 50, 80 and 100 volts-formation Voltage, Vf, and second sets sintered at various temperatures from 1,350 to 1,550°C.
• FIGS. 3-8 show TaN and TaN-Si with lowest cap' loss within varying sinter temperature, but with leakage enhancement (lowering) for TaN-Si at increasing sinter temperatures. A favorable balance of characteristics of Ta-Si is also shown.
• FIGS. 5-6 show (on 1450°C and 1350°C sintered test products) that at various bias voltages from 0 to 20 volts capacitance declines most at increasing bias for Ta, much less for Ta-Si and still less for Ta-Si 3 N 4 and lease for TaN-Si.
• FIGA. 7-8 with porosmetry testing results show incremental volume vs. pore diameter benefit for Ta-Si vs. Ta (FIG. 7) and TaN-Si (FIG. 8). This can lead to a reduction of electrical series resistance and improved performance in high frequency usage.
• Niobium Silicon Nb-Si
• Nb-Si_ systems were also processed as for Ta above. These behaved differently than the Ta-Si system. There wasn't an improvement on thermal stability and bias dependence, but something different was observed. There was an overall increase in capacitance with the addition of about 1% Si. There was also a decrease in leakage. The % increase in capacitance arose with increasing sinter temperature, decreased in L/C and remained stable generally.
• the present invention establishes uniquely and surprisingly a distinct change of Ta-Si (and/or TaN-Si) powder substrate sinter characteristic vs. Ta (or TaN) that can be tied to higher quality sinter temperature to emerge with beneficial high capacitance, low leakage capacitors with various areas of enhanced stability as to voltage bias, ESR frequency, heat treatment.
• Example 4 Silanes were used to add silicon to tantalum as described below in parts (a) and (b), below, and the resultant silicon doped tantalum tested with results as indicated at (c).
• APST Tantalum powders were wet with an aqueous solution of APST - amino propyl silane, triol, i.e. C 3 H u NO 3 Si, as a means of adding silicon and nitrogen dopants to the powder.
• the doping was done at a level necessary to generate 500 ppm of silicon.
• the tantalum used was a typical 50,000 CV/gm class powder (50 K). This level of doping, theoretically should have generated an additional 249 ppm of nitrogen to the powder, a desired result.
• APST is water soluble, and hence can be added with conventional phosphorous additive using techniques well known to those skilled in the art.
• the powder was in fact simultaneously doped with 100 ppm phosphorous dissolved in the same solution. After doping addition, the powder was dried, and then thermally treated (agglomerated) at 1320°C for 30 minutes under vacuum.
• Tantalum powders were wet with an aqueous solution of THSMP - sodium 3- trihydrosilylmethylphosphonate, i.e. PC 4 H 12 NaO 6 Si, as a means of adding silicon and phosphorous dopants, at a level to generate about 500 ppm of silicon.
• the tantalum powder used was a typical 50,000 CV/gm class powder. This level of dopant would be expected to provide an additional level of 550 ppm phosphorus, a relatively high level of phosphorous for this type powder. Hence, no additional phosphorous was added.
• THSMP is water soluble, and also can be added using the typical methods to add phosphorous known to those skilled in the art. After addition and drying, the powder was thermally treated under the same conditions as the APST sample.
• the doped powders of (a) and (b) were tested for surface area (SA, sq. cm./gm), Scott Bulk Density (SBD, cc/gm), Fisher Average Particle Diameter (FAPD, microns), Flow (gm/sec), carbon (C) content in ppm and similarly content of nitrogen (N), oxygen (O), phosphorus (P) and silicon (Si) and the results are shown in Table X, below for APST and THSMP treated powders, with the base 50K tantalum powder as a control similarly tested.
• the Na present in the sample 50K + THSMP was comparable to the control. It can also be noted that even though the silicon is introduced in a compound form, it is converted to elemental form in the course of thermal treatment for agglomeration and alloyed with the host tantalum.
• silicon doping is applied to niobium, alloys of either tantalum or niobium, including alloying with each other, and compounds of one or both of these metals including nitrides and sub-nitrides.
• silicon containing compounds and solutions e.g., water glass
• the agglomerated particles can be subjected to known per se deoxidation treatments such as exposure to vapors of alkali or alkaline earth metals or aluminum, preferably magnesium or calcium, while heating the powders at 600-1200°C preferably above 800°C as taught, e.g. in W.W. Albrecht et al., U.S. Patents 4,483,819, granted July 19, 1982, and 4,537,641, granted August 27, 1985.
• the deoxidation heating also provides a way of advancing the conversion of silicon compounds to elemental silicon and its alloying with the host refractory metal. Deoxidation can be applied during the thermal agglomeration (reactive agglomeration).
• the deoxidation is followed by a treatment with an inorganic acid to remove residue of the reduction reaction (e.g. magnesium oxide).
• residue of the reduction reaction e.g. magnesium oxide
• other impurities of the host refractory metal can be removed by the deoxidation process and that thermal agglomeration temperatures can be reduced because of such process.
• the combination of chemical and thermal factors of the doping, agglomeration, deoxidation and eventual sintering stops can be optimized for each situation of doping with silicon, alone or with other additives, to improve physical and electrical properties of the capacitors made with porous anode compacts made from such agglomerated powders.

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