NIOBIUM POWDERS AND NIOBIUM ELECTROLYTIC CAPACITORS

Abstract: A flaked niobium powder is disclosed as well as electrolytic capacitors formed from the flaked niobium powders. Niobium powders having a BET surface area of at least about 0.50 m<2>/g are also disclosed and capacitors made therefrom, as well as niobium powders doped with an oxygen content of at least 2,000 ppm. Methods to reduce DC leakage in a niobium anode are also disclosed.

Description: Niobium Powders and Niobium Electrolytic Capacitors BACKGROUND OF THE INVENTION The present invention relates to niobium powders and electrolytic capacitors using the niobium powders as well as methods of making the powders and electrolytic capacitors. For many years, it has been the goal of various researchers to develop niobium electrolytic capacitors because of the high di-electric constant of its oxide and the relativly low cost of niobium compared to a variety of other metals. Initially, researchers in this field considered the possibility of using niobium as a substitute for tantalum capacitors. Accordingly, many studies were conducted to determine the suitability of replacing tantalum with niobium. In some of these studies, however, it was concluded that niobium has serious fundamental deficiencies that needed to be resolved, thus inferring that niobium was not an acceptable substitute for tantalum. (See J. Electrochem. Soc. p. 408 C, Dec. 1977). In another study, one conclusion reached was that the use of niobium in solid electrolytic capacitors seems very unlikely due to various physical and mechanical problems, such as field crystallization. (Electrocomponent Science and Technology, Vol. 1, pp. 27-37 (1974)). Further, in another study, the researchers concluded that anodically formed passive films on niobium were different from electrical properties accomplished with tantalum and that the use of niobium led to complexities which were not present with tantalum. (See Elecrochimica Act., Vol. 40, no. 16, pp. 2623-26 (1995)). Thus, while there was initial hope that niobium might be a suitable replacement for tantalum, the evidence showed that niobium was not capable of replacing tantalum in the electrolytic capacitor market. Besides tantalum electrolytic capacitors, there is a market for aluminum electrolytic capacitors. However, the aluminum electrolytic capacitors have dramatically different performance characteristics from tantalum electrolytic capacitors. A driving force in electronic circuitry today is the increasing move toward lower Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). As IC performance increases with submicron geometry, there is a need for lower power supply voltage and noise margin. At the same time, increasing IC speeds require higher power needs. These conflicting requirements create a demand for better power management. This is being accomplished through distributed power supplies which need larger currents for decoupling noise. Increasing IC speeds also mean lower switching times and higher current transients. The electrical circuit must, therefore, also be designed to reduce the transient load response. This broad range of requirements can be met if the circuit has large enough capacitance but low ESR and ESL. Aluminum capacitors typically provide the largest capacitance of all capacitor types. ESR decreases with increase in capacitance. Therefore, currently a large bank of high capacitance aluminum capacitors are used to meet the above requirements. However, aluminum capacitors do not really satisfy the designers' requirements of low ESR and ESL. Their mechanical construction with liquid electrolyte inherently produce ESR in the 100s of milliohm along with high impedance. SUMMARY OF THE INVENTION A feature of the present invention is to provide flaked niobium powders. A further feature of the present invention is to provide niobium powders, preferably having high surface areas and physical characteristics which permit the niobium powders to be formed into a capacitor having high capacitance. Another feature of the present invention is to provide niobium powders which, when formed into capacitors, have a low DC leakage. An additional feature of the present invention is to provide a method of reducing the DC leakage in a capacitor formed from niobium powder. Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The present invention relates to a flaked niobium powder. Another aspect of the present invention relates to any niobium powder having a BET surface area of at least about 0.15 m 2 /g. The present invention also relates to a niobium powder, which when formed into an electrolytic capacitor anode, the anode has a capacitance of 30,000 CV/g to about 61,000 CV/g. The present invention, in addition, relates to a niobium powder having an oxygen content of at least about 2,000 ppm. Also, the present invention relates to a method to reduce DC leakage in a niobium anode made from niobium powder which comprises the step of doping the niobium powder with a sufficient amount of oxygen to reduce the DC leakage. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the BET surface areas of niobium powders and their respective capacitance when formed into anodes and sintered at a temperature of 1750°C. Figure 2 is a graph depicting the BET surface areas of niobium powders and their respective capacitance when formed into anodes and sintered at a temperature of 1600°C. Figure 3 is a graph depicting the BET surface areas of a niobium powders and their respective capacitance when formed into anodes and sintered at a temperature of 1450°C. Figure 4 is a graph depicting the BET surface areas of niobium powders and their respective capacitance when formed into anodes and sintered at a temperature of 1300°C. Figure 5 is a graph showing various sintering temperatures of niobium anodes and their respective calculated maximum capacitance. Figure 6 is a graph depicting the oxygen doping content of niobium powders as well as their respective DC leakage when formed into anodes and sintered at different temperatures and using a forming voltage of 50 volts. Figure 7 is a graph showing niobium powders having various doping levels of oxygen as well as the respective DC leakage when formed into anodes and sintered at various temperatures and using a forming voltage of 30 volts. Figure 8 is a graph showing the effects of various levels of phosphorus doping in niobium powders and their respective capacitance when formed into anodes. Figure 9 is a graph showing the effects of various phosphorus doping levels of niobium powder and their respective DC leakage when formed into anodes. DETAILED DESCRIPTION OF THE PRESENT INVENTION One aspect of the present invention relates to a flaked niobium powder. The flaked niobium powder can be characterized as flat, plate shaped, and/or platelet. Also, the flaked niobium powder has an aspect ratio (ratio of diameter to thickness) of from about 3 to about 300, and preferably, from about 3 to about 30. The flaked niobium powder permits enhanced surface area due to its morphology. Preferably, the BET surface area of the flaked niobium powder is at least 0.15 m 2 /g and more preferably, is at least about 1.0 m /g and even more preferably, is at least about 2.0 m 2 /g. Preferred ranges of BET surface area for the flaked niobium powder are from about 1.0 m 2 /g to about 5.0 m 2 /g and more preferably from about 2.0 m 2 /g to about 5.0 m 2 /g or from about 2.0 m /g to about 4.0 m 2 /g. The BET ranges are based on pre-agglomerated flaked niobium powders. The flaked niobium powder can be agglomerated. The flaked niobium powder can also be hydrided or non-hydrided. The flaked niobium powder preferably has a Scott Density of less than about 35 g/in 3 , and more preferably less than about 12, and most preferably, less than about 5 g/in 3 . Preferably, the agglomerated flaked niobium powder has a flow of greater than 80 mg/s, more preferably from about 80 mg/s to about 500 mg/s. The flaked niobium powder can be prepared by taking a niobium ingot and making the ingot brittle by subjecting it to hydrogen gas for hydriding. The hydrided ingot can then be crushed into an angular powder, for instance, with the use of a jaw crusher. The hydrogen can then be removed by heating in a vacuum and the degassed angular powder can then be subjected to milling, such as with use of a stirred ball mill where the powder is dispersed in a fluid medium (aqueous or non-aqueous) such as ethanol, to form the flaked powder by the impact of the stainless steel balls moved by the action of rotating bars. Various sizes of flakes can be made by hydrogen embrittlement followed by subjecting the flakes to impact milling, for example with use of a fluidized bed jet mill, Vortec milling, or other suitable milling steps. The flaked niobium powder can optionally have a high oxygen content, such as by doping. The amount of oxygen doping content can be at least about 2,000 ppm. More preferably, the flaked niobium powder has an oxygen content of from about 2,000 ppm to about 20,000 ppm and more preferably from about 2,750 ppm to about 10,000 ppm, and most preferably from about 4,000 ppm to about 9,000 ppm. The doping of the niobium powder with oxygen can be done in a variety of ways including, but not limited to, repeated heating in vacuum at 900 °C and cooling in air. Further, the flaked niobium powder can be also doped with phosphorus alone or with oxygen. The doping of the niobium powder with phosphorus is also optional. In one embodiment of the present invention, the amount of phosphorus doping of the niobium powder is less than about 400 ppm and more preferably less than about 100 ppm, and most preferably less than about 25 ppm. Based on an example set forth herein, the amount of phosphorus doping can be unimportant with respect to the DC leakage and capacitance of an anode formed from a niobium powder having various levels of phosphorus as a dopant. Accordingly, in one embodiment, low amounts of phosphorus and even negligible amounts or no phosphorus is present since phosphorus can have small or no benefits for DC leakage and capacitance with respect to certain anodes formed from niobium powders. In another embodiment of the present invention, the present invention relates to niobium powder (e.g. , flaked, angular, nodular, and mixtures thereof) having a significant level of oxygen present in the niobium powder. The oxygen level can be achieved in the same manner as described above. Preferably, the amount of oxygen in the niobium powder is at least about 2,000 ppm, and more preferably from about 2,000 ppm to about 20,000 ppm. Other preferred ranges of oxygen content in niobium powder are from about 2,750 ppm to about 10,000 ppm and levels of from about 4,000 ppm to about 9,000 ppm. With respect to these niobium powders, like the embodiment relating to the flaked niobium powder only, the phosphorus levels in the niobium powders can be considerably low for certain embodiments. Preferably, in such embodiments, the phosphorus level (as a dopant) is less than about 400 ppm and more preferably less than about 100 ppm, and most preferably less than about 25 ppm. In another embodiment, the present invention relates to niobium powders (e.g. flaked, angular, nodular, and mixtures thereof) having a BET surface area of at least 0.5 m 2 /g and preferably, at least about 1.0 m 2 /g, and more preferably from about 1.0 to about 5.0 m 2 /g, and most preferably from about 2.0 to about 5.0 m 2 /g. The BET ranges are based on pre-agglomerated niobium powders. The niobium powder can be hydrided or non-hydrided. Also, the niobium powder can be agglomerated. The niobium powder in this embodiment can be doped with nitrogen. Also, for certain uses, the niobium powder can have an oxygen content below about 2,000 ppm. With respect to making the flaked niobium powder or the niobium powder having any morphology with the BET surface area, the examples show the preferred steps of forming the niobium powder which can then subsequently be made into a flake or other morphology. In general, the process is as follows and the examples provide specific details as to preferred embodiments of making the niobium powders of the present invention. Generally, in preparing the niobium powders having a BET surface area of at least 0.5 m 2 /g, a niobium ingot is hydrided by heating in a vacuum to form an embrittled ingot which is crushed into a powder. The hydrogen in the powders can optionally be removed by heating the particle in a vacuum. The various BET surface areas can be achieved by subjecting the powder to milling, preferably an attritor milling process. The higher the BET surface area of the powder generally will require a longer milling time. For instance, with a milling time of approximately 60 minutes a BET surface area of approximately 1.0 m 2 /g can be achieved. To obtain even higher BET surface areas, longer milling times will be needed and to achieve the BET surface area of from about 4 to about 5 m 2 /g or greater, milling times on the order of approximately 24 hours or more in an attritor mill is one way of making such niobium powders having high BET surface area ranges. When making such high surface areas, it is preferred to use a 30-SL Union Process attritor mill using 1,000 lbs 3/16" SS media, and approximately 80 pounds of niobium powder with the mill set at a rotation of approximately 130 rpm. Also, the mill will contain a sufficient amount of a medium such as ethanol on the order of 13 or more gallons. After milling, the niobium powders are then subjected to a heat treatment and preferably the niobium powders can have a phosphorus content to help in minimizing the reduction in surface area during the heat treatment. The heat treatment can be any temperature sufficient to generally cause agglomeration and preferably without reducing the surface area. A temperature for heat treatment which can be used is approximately 1100°C for 30 minutes. However the temperature and time can be modified to ensure that the high BET surface area is not reduced. The various niobium powders described above can be further characterized by the electrical properties resulting from the formation of a capacitor using the niobium powders of the present invention. In general, the niobium powders of the present invention can be tested for electrical properties by pressing the niobium powder into an anode and sintering the pressed niobium powder at appropriate temperatures and then anodizing the anode to produce /an electrolytic capacitor anode which can then be subsequently tested for electrical properties. Accordingly, another embodiment of the present invention relates to capacitors formed from the niobium powders of the present invention. Anodes made from some of the niobium powders of the present invention can have a capacitance of from 30,000 CV/g to about 61,000 CV/g. In forming the capacitor anodes of the present invention, a sintering temperature is used which will permit the formation of a capacitor anode having the desired properties. Preferably, the sintering temperature is from about 1200°C to about 1750°C, and more preferably from about 1200°C to about 1400°C, and most preferably from about 1250°C to about 1350°C. The anodes formed from the niobium powders of the present invention are preferably formed at a voltage of less than about 60 volts, and preferably from about 30 to about 50 volts and more preferably at about 40 volts. Preferably, the working voltages of anodes formed from the niobium powders of the present invention are from about 4 to about 16 volts and more preferably from about 4 to about 10 volts. Also, the anodes formed from the niobium powders of the present invention preferably have a DC leakage of less than about 5.0 na/CV. In an embodiment of the present invention, the anodes formed from some of the niobium powders of the present invention have a DC leakage of from about 5.0 na/CV to about 0.50 na/CV. The present invention also relates to a capacitor in accordance with the present invention having a niobium oxide film on the surface thereof. Preferably, the niobium oxide film comprises a niobium pentoxide film. The capacitors of the present invention can be used in a variety of end uses such as automotive electronics; cellular phones; computers, such as monitors, mother boards, and the like; consumer electronics including TVs and CRTs; printers/copiers; power supplies; modems; computer notebooks; and disk drives. The present invention will be further clarified by the following examples, which are intended to be exemplary of the invention. TEST METHODS Anode Fabrication: size - 0.197" dia 3.5 Dp powder wt = 341 mg Anode Sintering: 1300 Deg C * 10' 1450 Deg C * 10' 1600 Deg C * 10' 1750 Deg C * 10' 30V Ef Anodization: 30V Ef @ 60 Deg C/0.1 % H 3 PO 4 Electrolyte 20 mA/g constant current DC Leakage/Capacitance - ESR Testing: DC Leakage Testing — 70% Ef (21 VDC) Test Voltage 60 second charge time 10% H 3 PO 4 @ 21 Deg C Capacitance - DF Testing: 18% H 2 SO 4 @ 21 Deg C 120Hz 50V Ef Reform Anodization: 50V Ef @ 60 Deg C/0.1 % H 3 PO 4 Electrolyte 20 mA/g constant current DC Leakage/Capacitance - ESR Testing: DC leakage Testing — 70% Ef (35 VDC) Test Voltage 60 second charge time 10% H 3 PO 4 @ 21 Deg C Capacitance - DF Testing: 18% H 2 SO 4 @ 21 Deg C 120Hz 75V Ef Reform Anodization: 75V Ef @ 60 Deg C/0.1 % H 3 PO 4 Electrolyte 20 mA/g constant current DC Leakage/Capacitance - ESR Testing: DC leakage Testing — 70% Ef (52.5 VDC) Test Voltage 60 second charge time 10% H 3 PO 4 @ 21 Deg C Capacitance - DF Testing: 18% H 2 SO 4 @ 21 Deg C 120 Hz Scott Density, oxygen analysis, phosphorus analysis, and BET analysis were determined according to the procedures set forth in U.S. Patent Nos. 5,011,742; 4,960,471; and 4,964,906, all incorporated herby in their entireties by reference herein. EXAMPLE 1 This example illustrates an embodiment of this invention comprising angular niobium powder. Electron beam produced niobium ingot was hydrided by heating the ingot in a vacuum of 10 "4 torr to 850°C for 120 minutes. The vacuum was replaced by hydrogen gas purge at 21 kPa for sufficient time to embrittle the ingot. The vacuum was then pumped down to -28" mercury and then backfilled with argon to -5" Hg. The pressure was maintained until the temperature, as measured by a work thermocouple, stabilized. Air was gradually introduced in increasing pressure such that the work temperature did not rise. The embrittled ingot was crushed into angular powder in a jaw crusher and classified by extracting powder which passed through a No. 325 sieve screen (equivalent to a 44 micrometer particle size). Hydrogen was removed from the size-reduced hydrogen-containing particles by heating the particles to 850 °C in a vacuum until pressure was no longer affected by hydrogen being emitted from the particles to provide niobium metal angular powder having a Fisher Sub Sieve Size of 10.6 micrometers, a Scott density of 2.67 g/cc (43.8 g/in 3 ), a pre-agglomerated BET surface area of 0.17 m 2 /g and 1770 ppm oxygen; the ratio of oxygen to BET surface area was 10,400 ppm O/m 2 /g, and the flow was 19 mg/sec. About 0.34 g samples of unagglomerated angular niobium powder were pressed into an anode mold 5 mm in diameter around a 0.6 mm diameter niobium lead wire to a density of 3.5 g/cc. Samples of the pressed niobium powder were sintered in a vacuum (at less than 10 "3 Pa) at four different temperatures, i.e. 1300, 1450, 1600 and 1750°C for 10 minutes, then anodized by applying 20 mA/g constant current at 50 V to the anode immersed in 0.1 weight percent phosphoric acid to produce electrolytic capacitor anodes, which were washed and dried. The capacitor performance characteristics, evaluated by measurements on the anodes immersed in 18 wt% sulfuric acid, are reported in Table 1. Capacitance, determined at a frequency of 120 Hertz, is reported in units of microfarad volts per gram (CV/g) and microfarad volts per cubit centimeter of anode volume (CV/cc); DC leakage, measured after a 1 minute charge of 35 volts, is reported in units of nanoamperes per microfarad-volt (nA/CV). EXAMPLE 2 This example illustrates an embodiment of the powder of this invention comprising agglomerated mixture of angular and flaked powder. 2.5 lbs of degassed angular powder prepared essentially in the manner of Example 1 was processed in a 1-S Union Process attritor stirred ball mill (285 rpm for 90 minutes) where powder dispersed in 2,400 ml ethanol medium and 40 lbs 3/16" 440SS medium was formed into flaked powder by the impact of stainless steel balls moved by the action of rotating bars. After the desired deformation into flake, the niobium powder was then removed and washed to remove any alcohol present. The niobium powder was then washed with a mixture of deionized water, hydrofluoric acid and hydrochloric acid in an amount of 500 ml/lb, 4 ml/lb and 250 ml/lb of niobium respectively (18.6% HC1 containing 22 ml/kg HF) to remove metal contamination (e.g. iron, nickel, chromium and the like transferred from contact with stainless steel balls). Afterwards, the niobium powder was again washed with deionized water and then dried. The acid washed flaked powder was dried in air at 85 °F (30 °C) and had an aspect ratio (determined by observation of micrographs) in the range of 50 to 70. The flaked powder was blended with starting angular powder (in the weight ratio of 30:70) and with a phosphorus containing powder, i.e. NH 4 PF 6 , in an amount to provide 60 ppm phosphorus, which serves as a grain retarding agent to minimize the reduction in surface area during subsequent heat treatment for agglomeration. The pre-agglomerated BET surface area was 0.31 m 2 /g. The mixed powders were agglomerated by heating in a vacuum at 1100°C for 30 minutes to form an agglomerated mass. The agglomeration procedure was performed in a manner such that the material was pumped down to a high vacuum and heated at a ramp rate of 3°C/minute to 700°C and held for outgassing until the high pressure was achieved. The heating continued in the furnace at a ramp rate of 8°C/minute to 1100°C under high pressure and held for 30 minutes. The material was then allowed to cool in the furnace and the material was manually passivated by exposing it to air. The material was then reduced to smaller agglomerated particles by jaw crushing; reduced particles passing a No. 50 sieve size (equivalent to a maximum agglomerated particle size of 300 micrometers) exhibited a Scott density of 1.3 g/cc (21.7 g/in 3 ), a BET surface area of 0.26 m 2 /g, oxygen content of 3693 ppm and phosphorus content of 25 ppm; the ratio of oxygen to BET surface area was 14,000 ppm O/m 2 /g and a flow of 22 mg/sec. The agglomerated powder was fabricated into anodes and tested for electrical properties in the manner of Example 1 which are reported in the Table 1. EXAMPLE 3 This example illustrates an embodiment of the powder of this invention comprising agglomerated flaked powder. Acid leached flaked powder having an aspect ratio of about 50 to 70 was prepared essentially as described in Example 2 (cycle time of 60 minutes) except the niobium powder was hydrided a second time by exposure to hydrogen at 20.7 kPa (3 psig) and 850°C to provide an embrittled flake which was cooled and reduced in size by self impaction in a fluidized bed Jet mill (obtained from Hosokawa Micron Powder Systems, Summit, NJ) to make flaked powder having a median particle size of 6 micrometers (as determined by laser particle size scanning). The pre-agglomerated BET surface area was 0.62 m 2 /g. The reduced-size flaked powder was agglomerated by heating in a hydrogen atmosphere by heating the furnace at a rate of 10°C/minute to 1050°C under a vacuum furnace and holding this temperature until the furnace pressure decreased below 100 microns. Tantalum coarse chips (10-20 mesh) were used as an oxygen getter in a weight ratio of 1 Nb to 1-1.5 Ta. The furnace was then backfilled with hydrogen to obtain a pressure of 360 mmHg and the furnace temperature was then increased to 1200°C and held for 1 hour. The hydrogen was then evacuated until the furnace pressure decreased to less than 1 micron and the furnace was allowed to cool to room temperature. The niobium powder was then passivated in air for 30 cycles wherein the operating pressure was increased by 20 torr for each cycle and held for 2 minutes before starting the next backfill of air. The agglomerated niobium powder was reduced in size to agglomerated particles by a jaw crusher; reduced agglomerated flaked niobium powder was separated by screening through a No. 50 sieve size screen (equivalent to a maximum agglomerated flaked particle size of 300 micrometers) and exhibited a Scott density of 1.21 g/cc (20.4 g/in 3 ), a BET surface area of 0.46 m 2 /g, oxygen content of 8760 ppm; the ratio of oxygen to BET surface area was 19,000 ppm O/M 2 /g, and a flow of less then 1 mg/sec. The agglomerated powder was fabricated into anodes and tested for electrical properties in the manner of Example 1 and reported in Table 1. EXAMPLE 4 This example illustrates another embodiment of the powder of this invention comprising high surface area, low oxygen, agglomerated niobium flaked powder. Niobium powder was prepared in the same manner as in Example 3 except the niobium powder was attritor milled for 90 minutes, and heat treatment was carried out in a vaccum at 1150°C for 30 minutes. The pre-agglomerated BET surface area was 0.85 m 2 /g. The oxygen content of quantities of flaked niobium powder prepared essentially in the manner of Example 3 was reduced by heating niobium powder admixed with 4 to 5 wt% magnesium powder under argon at a temperature in the range of 750 to 850°C for 2 hours. The magnesium content was established as being in the range of 2 to 3 times the stoichiometric amount of oxygen in the niobium powder. After cooling, residual magnesium and oxides were removed from agglomerated flaked niobium by nitric acid leach. Deoxidized flaked niobium was water washed, dried, and separated by screening through a No. 50 sieve size screen. The screened niobium flake exhibited a Scott density of 1.47 g/cc (24.1 g/in 3 ), a BET surface area of 0.96 m 2 /g, an oxygen content of 3130 ppm; the ratio of oxygen to BET surface area was 3260 ppm O/m 2 /g, and a flow of 76 mg/sec. The agglomerated powder was fabricated into anodes and tested for electrical properties in the manner of Example 1 , and reported in Table 1. TABLE 1 Sinter temperature 1300°C 1450°C 1600°C 1750°C Example 1 : Capacitance (CV/g) 8400 7500 6400 5500 (CV/cc) 40900 37000 33400 30000 DC Leakage (na/CV) 53 2.8 2.3 2.4 Sinter Density (g/cc) 4.9 5.0 5.2 5.5 Example 2: Capacitance (CV/g) 13600 11900 10000 8200 (CV/cc) 46000 41600 36900 33400 DC Leakage (na/CV) 25 1.7 2.1 2.5 Sinter Density (g/cc) 3.4 3.5 3.7 4.1 Example 3: Capacitance (CV/g) 32500 21400 13400 7100 (CV/cc) 114100 94300 73600 45800 DC Leakage (na/CV) 5.8 4.1 2.4 2.0 Sinter Density (g/cc) 3.5 4.4 5.5 6.4 Example 4: Capacitance (CV/g) 31,589 21 ,059 12,956 7,254 (CV/cc) 110,562 88,448 64,780 42,799 DC Leakage (na/CV) 5.8 5.3 2.6 1.4 Sinter Density (g/cc) 3.5 4.2 5.0 5.9 EXAMPLE 5 A niobium powder was prepared in the same manner as in Example 4 except the heat treatment occurred in a vaccum at 1250°C for 30 minutes. The pre- agglomerated BET surface area was 0.78 m 2 /g. The powder was formed into an anode as in Example 1 and had the following performance characteristics Cv/g @ 50 Vf 19,600 (1450°C) 31,040(1300°C) Sinter Density, g/cc 4.8 (1450°C) DC Leakage, na/Cv 2.33 (1450°C) BET, m 2 /g 0.80 Oxygen, ppm 2,815 Scott Density, G/in 3 24.0 Flow, mg/sec 97 EXAMPLE 6 A niobium powder was prepared in the same manner as in Example 4 except the niobium powder was in an attritor mill for 150 minutes and the heat treatment was in a vacuum furnace where the pressure was pumped down to 1 micron and then the temperature was increased by 50°C/minute to 950°C and held until the high vacuum was achieved. The temperature was then increased by 15 °C stages until a temperature of 1250°C was reached and that temperature was held for 30 minutes. The material was then allowed to cool to room temperature under vacuum and passivated for 30 cycles, wherein the pressure was increased by 20 torr after each cycle and held for 2 minutes before starting the next backfill of air. The powder was then crushed in a -50 mesh jaw crusher and deoxidized by blending the powder with 4% w/w magnesium metal and placing the material in a retort furnace and pumping down to 100 microns. The pre-agglomerated BET surface area was 1.05 m 2 /g. The furnace was then backfilled with argon to a pressure of 800 torr and the pressure increased to 800 °C and held for 2 hours. The material was then allowed to cool to room temperature and passivated in air for 30 cycles in the same manner mentioned above in Example 3. The material was then washed with a mixture of deionized water (500 ml/lb), hydrofluoric acid (4 ml/lb) and nitric acid (250 ml/lb). The powder was then rinsed with deionized water and dried. The niobium powder was then formed into an anode as in Example 1 and had the following performance characteristics CV/g @ 50 Vf (Sintering Temp.) 24,300 (1450°C) 41,700(1300°C) Sinter Density, g/cc 4.0 (1450°C) DC Leakage, na/Cv 1.5 (1450°C) BET, m 2 /g 1.11 Oxygen, ppm 3,738 Scott Density, g/in 3 24.4 Flow, mg/sec 112 EXAMPLE 7 Niobium powder was prepared in the same manner as in Example 6 except the niobium powder was blended with phosphorus before heat treatment to achieve a phosphorus loading of 56 ppm. The pre-agglomerated BET surface area was 1.05 m 2 /g. The material was hydrided as in Example 3 and crushed, heat treated, and deoxidized as in Example 6. The niobium powder was then formed into an anode as in Example 1 and had the following performance characteristics Cv/g @ 50 Vf (Sintering Temp.) 29,900 (1450°C) 45,400(1300°C) Sinter Density, g/cc 3.7 (1450°C) DC Leakage, na/Cv 1.3 (1450°C) BET, m 2 /g 1.07 Oxygen, ppm 3,690 Scott Density , g/in 3 23.2 Flow, mg/sec 76 EXAMPLE 8 Niobium powder was prepared in the same manner as in Example 4 except the niobium powder was milled in a 30 S attritor mill (130 rpm) for 8 hours by using 1,000 lbs of 3/16" SS media, 80 lbs of niobium powder, and 13 gallons of ethanol. The milled powder was acid leached and washed in the samer manner as described before and the material had the following characteristics BET, m 2 /g 1.39 Oxygen, ppm 8,124 Scott Density, g/in 3 3 EXAMPLE 9 Figures 1 , 2, 3, and 4 show CV/g vs BET for various Nb powders having a range of BETs. Each figure represents the measurement of CV/g for the powders determined at a specific sinter temperature. As the figures show, the higher the sinter temperature the greater is the loss of surface area of the anode and there is also a general reduction in CV/g for any particular powder sample as the sample is tested at higher sinter temperatures (CV/g is proportional to the residual specific surface area of the anode after sintering). As illustrated by Figures 1 through 4, for any given sinter temperature, the CV/g achieved will have a relationship to the starting BET of the sample. As shown, low BET will produce low net CV/g and as BET rises the CV/g will rise. For materials having high BETs the degree of surface area loss during sintering is so great as to obliterate so much surface area that only a small fraction of the original high BET is left to be expressed as CV/g after the sinter so CV/g drops off with the highest BETs. For this reason, the response of CV/g vs BET shows a maximum at a BET value that preserves the most net specific surface area after sintering. In general, as shown in the figures, lower sinter temperature will achieve optimum CV/g with higher BET material; whereas, high sinter temperatures, which are very destructive to small, high BET particles, will achieve optimum CV/g with a lower BET powder. There is generally an optimum BET for use at any given sinter temperature; and, the set of all optimum BETs form a response surface relative to the sinter temperatures. As shown in the figures, the CV/g is generally proportional to the BET, and CV/g shows a relationship to the sinter temperatures. Thus, Figure 5 shows the CV/g for each sinter temperature from Figures 1 through 3 plotted against the sinter temperature. Figure 5 shows the CV/g that would be achieved at the 1300°C sinter to be on the order of about 61 ,000. The preparation of Figure 5 is based on an objective and mathematically correct procedure for determining the maximum CV/g from each of the Figures 1 through 3. Because the response of CV/g vs BET in each of Figures 1 through 3 is observed to exhibit a maximum, the requirement was resolved by finding the maximum of the best functional fit to the data for each figure. The actual response of CV/g to BET is a complex function of the variables; however, the Taylor Series expansion of functions teaches that all functions can be approximated by the first three terms of the Taylor Series within a limited domain of the independent variable (in this case BET). This amounts to approximating the function as a quadratic (F(x)=ax 2 + bx + c) valid within a limited neighborhood of any selected value for x. This calculation is valid as long as the values of x are within the neighborhood. The optimum BET in each case was used as the center of the neighborhood of the BET domain so that the approximation is most valid for BET near this value; and, to therefore take the maximum of the quadratic fit to the data to be the best estimate for the peak CV/g of the data in Figures 1 through 3. For this reason, a best fit of the data was performed in Figures 1 through 3 to a quadratic function using the curve fitting tool in Microsoft Excel v 5.0 which generated the parabolic curves superimposed on the measured data in Figures 1 through 3. The maximum of the fitted parabolae in Figures 1 through 3 were used as the input data to make Figure 5. The set of maximum CV/g vs sinter temperature data in Figure 5 was next fitted to an exponential decay function using the curve fitting tool in Microsoft Excel v 5.0. The reason for selecting exponential decay as the best approximation to the response of maximum CV/g vs sinter temperature is because, as shown in the figures, CV/g will decrease with increasing sinter temperature; however, CV/g can in principal never be lower than 0.0 because the specific surface area cannot be made less than zero (cannot be negative). The exponential function which asymptotically decays to zero is the simplest functional form that can be used with the data in Figure 5 that does not predict negative CV/g. The best fit of an exponential curve as determined by Microsoft Excel v 5.0 was added to the data in Figure 5 and this allowed the calculation of the maximum CV/g that would be achieved with a 1300°C sinter temperature based upon all of the data from Figures 1 through 3 as explained above. Figure 4 is the actual data for the available Nb samples tested at the 1300°C sinter; however, it is seen in Figure 4 that the data does not peak because none of the samples had the optimum BET for the 1300°C sinter. The data was fitted to the quadratic function just as was used in Figures 1 through 3 and the result shown superimposed on Figure 4 shows the peak should exist following the observations of peaks in Figures 1 through 3; and, the peak is shown to be a CV/g > 55,000 and BET> 1.7. It is readily apparent that in the case of Figure 4, the peak CV/g predicted by using the same analysis used to make the data in Figure 5 describes a maximum CV/g very close to the independently derived maximum estimated by Figure 5. The agreement between two separate determinations of the maximum CV/g at the 1300°C sinter agree and make it clear that the materials made with BET > 1.7 (BETs on the order of 2 or more) will exhibit CV/g > 55,000 (CV/g on the order of 60,000) when tested at 1300°C sinter conditions. Table 2 Example data used for Figures 1 through 4 EXAMPLE 10 The effects of oxygen on niobium powders were studied. Five samples of flaked niobium powder (prepared as in Example 5) each weighing 1 pound, were tested. One of the samples was a control and the four remaining samples were processed in such a manner as to increase the oxygen content in the niobium powder. In particular, the four samples were heat treated in a furnace at 900 °C for 30 minutes. The powders were then passivated in air in a manner similar to the air passivation discussed in the proceeding examples. Then, one of the four samples was removed and the three remaining samples heat treated and passivated again in the same manner described above. As before, one of these three samples was then remove and the procedure was repeated again with the two remaining samples. Afterward, again one of the samples was removed and the final remaining sample was again heat treated and passivated as before. Thus, there were five samples prepared wherein either 0, 1 , 2, 3, or 4 cycles of heat treatment were preformed. Prior to testing for the oxygen content in each of these samples, the samples were passed individually through a 40 mesh screen. The powders were then agglomerated and sintered at various temperatures and formed into anodes based on three different forming voltages as indicated in Table 3. The results of the DC leakage are also set forth in Table 3. As can be seen from the results in Table 3 and in Figures 6 and 7, the DC leakage gradually decreased as the oxygen content in the niobium increased. The decrease in DC leakage was especially apparent with lower forming voltages such as 30 and 50 volts. Table 3 Data showing effect of O 2 on na/CV at 30, 50 and 60 Volts 30 Vf 1300 1450 1600 1750 EXAMPLE 11 The effect of phosphorus on niobium powder was then examined. Six samples of niobium powder prepared in a manner like Example 5 were tested. One of the samples was used as a control and the remaining five samples had sufficient phosphoric acid added to achieve phosphorus levels of 5 ppm, 10 ppm, 30 ppm, 100 ppm, and 500 ppm respectively. The phosphoric acid was added as a diluted solution with 150 ml of deionized water. The phosphoric acid solution and powder were mixed and the samples dried in a vacuum oven. After drying, the samples were individually blended and tested for phosphorus levels. The results are set forth in Table 4. As can be seen in Table 4 and Figures 8 and 9, there was a small effect caused by phosphorus doping and it was noticed that higher amounts of phosphorus doping did not necessarily improve the properties of DC leakage. Table 4 P doped Niobium samples data Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims: WHAT IS CLAIMED IS: 1. A flaked niobium powder. 2. The niobium powder of claim 1 , wherein said powder is agglomerated. 3. The powder of claim 1 , wherein said powder has a BET surface area of at least 0.15 m 2 /g. 4. The niobium powder of claim 1 , wherein said powder has a BET surface area of at least about 1.0 m 2 /g. 5. The niobium powder of claim 1 , wherein said powder has a BET surface area of at least about 2.0 m 2 /g. 6. The powder of claim 1 , wherein said powder has a BET surface area of from about 1.0 to about 5.0 m 2 /g. 7. The powder of claim 1 , wherein said powder has a BET surface area of from about 2.0 to about 5.0 m 2 /g. 8. The niobium powder of claim 1 , wherein said powder is not hydrided. 9. The powder of claim 1, wherein said powder has a Scott Density of less than about 35 g/in 3 . 10. The powder of claim 1 , wherein when said powder is formed into an electrolytic capacitor anode, said anode has a capacitance of from about 30,000 CV/g to about 61 ,000 CV/g. 11. A capacitor made from the niobium powder of claim 1. 12. A niobium powder, wherein when said powder is formed into an electrolytic capacitor anode, said anode has a capacitance of from 30,000 CV/g to about 61,000 CV/g. 13. The niobium powder of claim 12, wherein said powder is hydrided. 14. A niobium powder having a BET surface area of at least 0.50 m 2 /g. 15. The niobium powder of claim 14, having a BET surface area of at least about 1.0 m 2 /g. 16. The niobium powder of claim 14, having a BET surface area of from about 1.0 to about 5.0 m 2 /g. 17. The niobium powder of claim 14, having a BET surface area of at least about 2.0 m 2 /g. 18. The niobium powder of claim 14, having a BET surface area of from about 2.0 m 2 /g to about 4.0 m 2 /g. 19. The niobium powder of 14, having a BET surface area of from about 2.0 to about 5.0 m 2 /g. 20. The niobium powder of claim 14, wherein said niobium powder is not hydrided. 21. The niobium powder of claim 14, wherein said niobium powder is agglomerated. 22. A capacitor prepared from a formulation comprising the niobium powder of claim 14. 23. The capacitor of claim 22, wherein said powder is sintered at a temperature of from about 1200°C to about 1750°C. 24. The capacitor of claim 22, wherein said powder is sintered at a temperature of from about 1200°C to about 1450°C. 25. The capacitor of claim 22, wherein said powder is sintered at a temperature of from about 1250°C to about 1350°C. 26. A capacitor prepared from a formulation comprising the niobium powder of claim 15. 27. A capacitor prepared from a formulation comprising the niobium powder of claim 16. 28. A capacitor prepared from a formulation comprising the niobium powder of claim 17. 29. A capacitor prepared from a formulation comprising the niobium powder of claim 21. 30. The niobium powder of claim 14, having a phosphorus level of less than about 400 ppm. 31. The niobium powder of claim 14, having a phosphorus level of less than about 100 ppm. 32. The niobium powder of claim 14, having a phosphorus level of less than about 25 ppm. 33. The capacitor of claim 22, wherein said capacitor is formed at a voltage of from about 30 to about 50 volts. 34. The capacitor of claim 22, wherein said capacitor has a DC leakage of less than about 5.0 na/CV. 35. The capacitor of claim 22, wherein said capacitor has a DC leakage of from about 5.0 na/CV to about 0.50 na/CV. 36. The capacitor of claim 26, wherein said capacitor is formed at a voltage of from about 30 to about 50 volts. 37. A method to reduce DC leakage in a niobium anode made from niobium powder comprising the step of doping said niobium powder with a sufficient amount of oxygen to reduce DC leakage. 38. The method of claim 37, wherein said niobium powder is doped with at least about 2,000 ppm of oxygen. 39. The method of claim 37, wherein said niobium powder is doped with oxygen in an amount of from about 2,000 ppm to about 10,000 ppm. 40. The method of claim 37, wherein said niobium powder is doped with oxygen in an amount of from about 3,000 ppm to about 7,000 ppm. 41. The method of claim 37, wherein said anode is formed at a voltage of from about 30 to about 50 volts. 42. The method of claim 37, wherein said anode is formed at a voltage of about 40 volts. 43. The method of claim 37, wherein said niobium powder has a phosphorous level of less than about 400 ppm. 44. The method of claim 37, wherein said niobium powder has a phosphorous level of less than about 100 ppm. 45. The method of claim 37, wherein said niobium powder has a phosphorous level of less than about 25 ppm. 46. The method of claim 37, wherein said anode is sintered at a temperature of from about 1200°C to about 1750°C. 47. The method of claim 37, wherein said anode is sintered at a temperature of from about 1200°C to about 1450°C. 48. The method of claim 37, wherein said anode is sintered a temperature of from about 1250°C to about 1350°C. 49. A niobium powder having an oxygen doped content of at least about 2,000 ppm. 50. The niobium powder of claim 49, having an oxygen doped content of from about 2,000 ppm to about 20,000 ppm. 51. The niobium powder of claim 49, having an oxygen doped content of from about 2,750 ppm to about 10,000 ppm. 52. The niobium powder of claim 49, having an oxygen doped content of from about 4,000 ppm to about 9,000 ppm. 53. An electrolytic capacitor formed from the niobium powder of claim 49. 54. The capacitor of claim 25, further comprising a niobium oxide film on a surface thereof. 55. The capacitor of claim 54, wherein said film comprises niobium pentoxide film. 56. The niobium powder of claim 14, having an oxygen content of not less than about 2,000 ppm.

Inventors: FIFE JAMES A, LIU JANE JIA, STEELE ROGER W, FIFE, JAMES, A., LIU, JANE, JIA, STEELE, ROGER, W.
Applicants: CABOT CORP, CABOT CORPORATION
Application year: 1997
Main classification: B22F1
IPC codes: B22F1, C22C1, H01G9