Ta-Nb ALLOY POWDER AND ANODE ELEMENT FOR SOLID ELECTROLYTIC CAPACITOR

Abstract

A Ta-Nb alloy powder which has provides a capacitor having a higher capacitance than a Ta capacitor and a better thermal stability in terms of an oxide film is better than a Nb capacitor, the Ta-Nb alloy powder being a Ta-Nb alloy powder produced by a thermal CVD method, wherein a content of Nb is 1 to 50 mass %, and an average particle diameter of primary particles is 30 to 200 nm, preferably, a CV value per unit mass of the powder (μF.Math.V/g) is 250 kμF.Math.V/g or more, or further, a CV value per unit volume (μF.Math.V/mm.sup.3) in terms of a molded body whose molding density ρ (g/cm.sup.3) is ρ.sub.c (g/cm.sup.3)=−0.012R.sub.Nb+3.57, wherein R.sub.Nb: Nb content (mass %) in an alloy, is 900 μF.Math.V/mm.sup.3 or more, and an anode element for a solid electrolytic capacitor using the alloy powder.

Claims

1-5. (canceled)

6. A Ta-Nb alloy powder produced by a thermal CVD method, wherein a content of Nb is 1 to 50 mass %, and an average particle diameter of primary particles is 30 to 200 nm.

7. The Ta-Nb alloy powder according to claim 6, wherein a CV value per unit mass (μF.Math.V/g) when an anode element is formed therefrom is not less than 250 KμF.Math.V/g.

8. The Ta-Nb alloy powder according to claim 6, wherein a CV value per unit volume (μF.Math.V/mm.sup.3) when an anode element is formed therefrom is not less than 900 μF.Math.V/mm.sup.3, wherein the CV value is a value when an anode element whose molding density ρ (g/cm.sup.3) is ρ.sub.c defined by the following formula (1):
ρ.sub.c(g/cm.sup.3)=−0.012R.sub.Nb+3.57   (1) wherein R.sub.Nb: Nb content (mass %) in an alloy is formed therefrom.

9. The Ta-Nb alloy powder according to claim 7, wherein a CV value per unit volume (μF.Math.V/mm.sup.3) when an anode element is formed therefrom is not less than 900 μF.Math.V/mm.sup.3, wherein the CV value is a value when an anode element whose molding density ρ (g/cm.sup.3) is ρ.sub.c defined by the following formula (1):
ρ.sub.c(g/cm.sup.3)=−0.012R.sub.Nb+3.57   (1) wherein R.sub.Nb: Nb content (mass %) in an alloy is formed therefrom.

10. The Ta-Nb alloy powder according to claim 6, wherein a leakage current when an anode element is subjected to reflow treatment of holding under an Ar atmosphere at 260° C. for 30 minutes after the anode element is formed is not more than 8 times a leakage current before the reflow treatment.

11. The Ta-Nb alloy powder according to claim 7, wherein a leakage current when an anode element is subjected to reflow treatment of holding under an Ar atmosphere at 260° C. for 30 minutes after the anode element is formed is not more than 8 times a leakage current before the reflow treatment.

12. The Ta-Nb alloy powder according to claim 8, wherein a leakage current when an anode element is subjected to reflow treatment of holding under an Ar atmosphere at 260° C. for 30 minutes after the anode element is formed is not more than 8 times a leakage current before the reflow treatment.

13. The Ta-Nb alloy powder according to claim 9, wherein a leakage current when an anode element is subjected to reflow treatment of holding under an Ar atmosphere at 260° C. for 30 minutes after the anode element is formed is not more than 8 times a leakage current before the reflow treatment.

14. An anode element for a solid electrolytic capacitor using the Ta-Nb alloy powder according to claim 6.

15. An anode element for a solid electrolytic capacitor using the Ta-Nb alloy powder according to claim 7.

16. An anode element for a solid electrolytic capacitor using the Ta-Nb alloy powder according to claim 8.

17. An anode element for a solid electrolytic capacitor using the Ta-Nb alloy powder according to claim 9.

18. An anode element for a solid electrolytic capacitor using the Ta-Nb alloy powder according to claim 10.

19. An anode element for a solid electrolytic capacitor using the Ta-Nb alloy powder according to claim 11.

20. An anode element for a solid electrolytic capacitor using the Ta-Nb alloy powder according to claim 12.

21. An anode element for a solid electrolytic capacitor using the Ta-Nb alloy powder according to claim 13.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0033] FIG. 1 is a graph showing the influence of a Nb content on a CV value per unit mass (μF.Math.V/g) of Ta-Nb alloy powders.

[0034] FIG. 2 is a graph showing the influence of a Nb content on a CV value per unit volume (μF.Math.V/mm.sup.3) of Ta-Nb alloy powders.

[0035] FIG. 3 is a graph showing the influence of a Nb content on leakage current of Ta-Nb alloy powders.

[0036] FIG. 4 is a graph showing the influence of reflow treatment on a CV value per unit volume (μF.Math.V/mm.sup.3) of Ta-Nb alloy powders.

[0037] FIG. 5 is a graph showing the influence of reflow treatment on leakage current of Ta-Nb alloy powders.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

[0038] First, it is important that the Ta-Nb alloy powder of the invention is produced by a thermal CVD method (gas phase reduction method) because as previously described, the thermal CVD method is suitable for producing a fine metal powder, and is the only method by which a fine Ta-Nb alloy powder can be stably produced at present, it is also easy to adjust alloy components, and an alloy having a narrow composition range can be produced. The specific method and conditions of the above thermal CVD method are not particularly limited, and, for example, a method disclosed in JP-A-2004-52026 can be preferably used.

[0039] Moreover, it is necessary that the Ta-Nb alloy powder (primary particles) of the invention has an average particle diameter in the range of 30 to 200 nm. When the average particle diameter is less than 30 nm, the strength of the bonding portions (neck portions) of the particles formed when the Ta-Nb alloy powder is sintered is weak, and the bonding portions are ruptured by an anodic oxide film formed by chemical conversion treatment, causing a decrease in conductivity and a decrease in capacitance. On the other hand, when the average particle diameter exceeds 200 nm, the primary particle diameter is too large, and therefore the surface area of the Ta-Nb alloy powder decreases, and it is difficult to stably ensure the target CV value (not less than 250 kμF.Math.V/g). In order to stably achieve a high capacitance of not less than 250 kμF.Math.V/g, the primary particles of the Ta-Nb alloy powder preferably have an average particle diameter in the range of 50 to 150 nm, more preferably in the range of 60 to 120 nm. Here, the average particle diameter of the above Ta-Nb alloy powder (primary particles) is a number-based average particle diameter when not less than 1000 particle diameters from a particle image taken by a scanning electron microscope SEM or the like are measured using image analysis type particle size distribution software (Mac-View manufactured by Mountech Co., Ltd.).

[0040] Next, it is necessary that the Ta-Nb alloy powder of the invention has a Nb content in the range of 1 to 50 mass %. Experiments that form the basis of the limitation will be described below.

Experiment 1

[0041] A Ta powder, a Nb powder, and Ta-Nb alloy powders in which the Nb content is variously changed (hereinafter the above three types of powders will be collectively referred to as “metal powders”) are produced by a thermal CVD method. In the production of the above metal powders, the production conditions are adjusted so that the average particle diameter of the primary particles is within the range of 60 to 120 nm. Then, each of the above metal powders is pressure-molded into a pellet having a diameter of 3 mmφ and a length of 4 mm. At this time, the molding density ρ (g/cm.sup.3) of the pellet after the above pressure molding is adjusted so as to fall in the range of ±0.10 g/cm.sup.3 with respect to ρ.sub.c obtained from the following formula (1);

ρ.sub.c=−0.012R.sub.Nb+3.57   (1)

according to the content of Nb in the Ta-Nb alloy, R.sub.Nb, (mass %). Here, the molding density is a value excluding that of the wire.

[0042] Here, the reason why the molding density is adjusted as described above is as follows. The true densities of Ta and Nb differ greatly as Ta=16.69 g/cm.sup.3 and Nb=8.57 g/cm.sup.3, and therefore the true density of the Ta-Nb alloy powder varies depending on the composition. Therefore, the appropriate molding density (mass per unit volume) of the Ta-Nb alloy also varies depending on the composition, and as the content of Nb increases, the appropriate molding density decreases.

[0043] In fact, in the molding in the above experiment, when the molding density exceeded +0.20 g/cm.sup.3 with respect to ρ.sub.c obtained from the above formula (1), cracks (fractures) are likely to occur after the molding or sintering, and conversely, when the molding density is less than −0.20 g/cm.sup.3 with respect to ρ.sub.c obtained from the above formula (1), it is difficult to ensure strength as the molded body. More preferable molding density is in the range of ±0.10 g/cm.sup.3 with respect to ρ.sub.c obtained from the formula (1).

[0044] Next, the pellet after the above molding is sintered under a vacuum atmosphere at a temperature of 900 to 1200° C. to form an anode element.

[0045] The above element is then subjected to chemical conversion treatment at a voltage of 10 V for 2 hour in a 0.05 mass % phosphoric acid solution having a temperature of 80° C. to form an anodic oxide film to be a dielectric on the metal particle surfaces, and thereafter the capacitance CV and the leakage current LC are measured in accordance with methods described in EIAJ RC-2361A. The capacitance CV is measured in a 40 mass % sulfuric acid solution at a voltage of 1V, a bias voltage of 1.5 Vdc, and a frequency of 120 Hz. For the measurement of the leakage current LC, a voltage of 7V is applied, and the leakage current after a lapse of 2 min is measured.

[0046] As reference, the capacitance CV and the leakage current LC are also measured with regard to a Ta-Nb mixed powder obtained by blending a Ta powder and a Nb powder so that the Nb content is 30 mass% under the same conditions as the above.

[0047] The measurement results are shown in Table 1. The leakage current is shown by a leakage current per unit capacitance (nA/μF.Math.V).

TABLE-US-00001 TABLE 1 CV value CV value Nb per unit mass per unit volume Leakage current LC per unit content Primary particle Molding (kμF .Math. V/g) (μF .Math. V/mm.sup.3) capacitance (nA/μF .Math. V) (mass diameter density Before After Before After Before After After reflow/ No Type of metal powder %) (nm) (g/cm.sup.3) reflow reflow reflow reflow reflow reflow before reflow 1 Ta simple substance powder 0 60 3.60 234 238 848 862 0.95 2.18 2.3 2 Ta—Nb alloy powder 1.7 71 3.62 252 259 913 935 1.01 2.35 2.3 3 Ta—Nb alloy powder 14.4 63 3.37 290 314 994 1069 1.30 5.62 4.3 4 Ta—Nb alloy powder 26.9 65 3.19 334 359 1068 1149 1.25 5.53 4.4 5 Ta—Nb alloy powder 47.1 80 3.01 355 427 1074 1293 1.13 7.90 7.0 6 Ta—Nb alloy powder 62.0 72 2.80 402 572 1128 1604 1.24 13.41 10.8 7 Nb simple substance powder 100.0 81 2.44 376 520 921 1273 2.10 22.05 10.5 8 Ta—Nb mixed powder 30.0 66 3.25 272 304 884 988 1.40 13.30 9.5

[0048] FIG. 1 shows a relationship between the Nb content and the CV value per unit mass (μF.Math.V/g) in the above metal powders. As seen from the figure, the CV value of the element using the Nb powder (also referred to as the “Nb powder element”) is about 1.6 times the CV value of the element using the Ta powder (also referred to as the “Ta powder element”), and the CV value of the element using the Ta-Nb mixed powder obtained by blending 30 mass % of a Nb powder in a Ta powder (also referred to as the “Ta-Nb mixed powder element”) is substantially the same as a CV value obtained by interpolation from the CV values of the above Nb powder element and Ta powder element (also referred to as an “interpolated value”), whereas the CV values of the elements using the Ta-Nb alloy powders (also referred to as the “Ta-Nb alloy powder elements”) are higher than the above CV value obtained by interpolation. Therefore, it is seen that the capacitances of the Ta-Nb alloy powder elements show capacitance characteristics different from those of the Ta powder element, the Nb powder element, and the Ta-Nb mixed powder element obtained by only mixing a Ta powder and a Nb powder. However, it is difficult to say that the CV values per unit mass (μF.Math.V/g) of the above Ta-Nb alloy powder elements are better than that of the Nb powder element.

[0049] On the other hand, FIG. 2 shows the CV values per unit mass (μF.Math.V/g) in the above FIG. 1 converted into CV values per unit volume (μF.Math.V/mm.sup.3).

[0050] From this figure, the difference in the CV value per unit volume (μF.Math.V/mm.sup.3) between the Ta powder element and the Nb powder element is greatly reduced and is only a difference of a little less than 9%. This difference corresponds to a difference in capacitance when elements are made by filling each of a Ta powder and a Nb powder under the same conditions with a fixed volume size, and is practically an important indicator when a capacitor is designed. In addition, also in this case, the CV value of the Ta-Nb mixed powder element is on a straight line connecting the CV values of the Ta powder element and the Nb powder element and is the same as an interpolated value obtained from the CV values of the Nb powder element and the Ta powder element.

[0051] On the other hand, the CV values of the Ta-Nb alloy powder elements having a Nb content in the range of 14 to 62 mass % are about 17 to 30% higher than the interpolated value obtained from the CV values of the Nb powder element and the Ta powder element, and even for the Ta-Nb alloy containing only 1.7 mass % of Nb, a CV value higher than the interpolated value by as much as about 8% is obtained. From these, it is seen that the Ta-Nb alloys produced by the thermal CVD method show capacitance characteristics different from those of the powders of Ta and Nb simple substances, and the Ta-Nb mixed powder obtained by simply only mixing a Ta powder and a Nb powder and are materials with which capacitors having higher capacitance than conventional capacitors can be made.

[0052] Patent Document 10 of a conventional art discloses a Ta-Nb alloy having a Nb content of about 75 mass % and a primary particle diameter of about 400 nm produced by a Mg reduction method, and its CV value per unit mass is 290 kμF.Math.V/g. This value is not less than 250 kμF.Math.V/g but is only a value lower than the interpolated value of the Ta-Nb mixed powder having the same composition shown in FIG. 1. From this, it is considered difficult to produce a high capacitance Ta-Nb alloy powder by a method other than the CVD method.

[0053] A problem of the Ta-Nb alloy powders produced by the thermal CVD method is that they have poor fluidity due to being fine powders, but this point can be solved by the improvement of a granulation technique described later.

[0054] FIG. 3 shows the relationship between the Nb content and the leakage current LC of the above Ta powder element, Nb powder element, and Ta-Nb alloy powder elements. From this figure, the leakage current of the Nb powder element is about not less than 2 times that of the Ta powder element, that is, not less than 2.0 nA/μF.Math.V, and is not preferable for a capacitor. But, it is seen that when the Nb content is not more than 62 mass %, the leakage current is hardly influenced by the Nb content and is at the same level. Moreover, the leakage current of the Ta-Nb mixed powder element is also shown in the figure for reference and is at the same level as that of the Ta-Nb alloy powder elements.

[0055] As seen from the above results, the Ta-Nb alloy powders having a Nb content in the range of 1 to 62 mass % produced by the thermal CVD method have a better CV value per unit volume (μF.Math.V/mm.sup.3) than the Ta powder, the Nb powder, and the Ta-Nb mixed powder and are anode materials suitable for a reduction in size and an increase in capacitance of capacitors.

Experiment 2

[0056] Next, in order to examine the thermal stability of the Ta-Nb alloy powders, the inventors examine changes in the CV value (μF.Math.V/mm.sup.3) and the leakage current LC by subjecting each of the elements after the chemical conversion treatment used in the above experiment to heat treatment at 260° C. for 30 minutes in an argon gas atmosphere in a simulation of reflow treatment during mounting. The CV value and the leakage current LC are measured under the same conditions as the above-described <Experiment 1>. Moreover, the Ta-Nb mixed powder is also similarly examined as reference.

[0057] FIG. 4 shows the CV values per unit volume (μF.Math.V/mm.sup.3) before and after the reflow treatment in comparison. From this figure, the CV value per unit volume after the reflow treatment is increased with an increase in the content of Nb, and the increase rate (%) with respect to the CV value before the reflow treatment is about 20% when the Nb content is not more than 47 mass %. When the content of Nb is 62 mass %, however, it can be seen that the increase rate increases to close to 40%, the same as the Nb powder, and the thermal stability is poor. The CV value of the Ta-Nb mixed powder element after the reflow treatment is also on a straight line connecting the CV values of the Ta powder element and the Nb powder element and is the same as an interpolated value obtained from the CV values of the Nb powder element and the Ta powder element.

[0058] Similarly, FIG. 5 shows the leakage current LC before and after the reflow treatment in comparison. It is seen that as described above, the influence of the Nb content is hardly recognized on the leakage current before the reflow treatment, while the leakage current after the reflow treatment increases greatly with an increase in the Nb content, and the increase ratio defined by (LC after the reflow treatment/LC before the reflow treatment) is not more than 8 when the Nb content is not more than 47 mass %, but the increase ratio is not less than 10 and deteriorates to the same level as that of the Nb powder when the Nb content is not less than 62 mass %. The leakage current of the Ta-Nb mixed powder element after the reflow treatment increases greatly compared with that of the Ta-Nb alloy powder elements, and the increase ratio deteriorates to the same level as that of the Nb powder element.

[0059] From the results of the above <Experiment 1> and <Experiment 2>, for the Ta-Nb alloy powder of the invention, the Nb content is set to be not less than 1 mass % from the viewpoint of setting the CV value per unit volume (μF.Math.V/mm.sup.3) higher than an interpolated value obtained from the CV values of a Nb powder and a Ta powder, and the Nb content is limited to not more than 50 mass % from the viewpoint of making the deterioration of the capacitance CV and the leakage current LC due to reflow treatment smaller than that of a Nb powder. A preferable Nb content is in the range of 1 to 40 mass %.

[0060] The reason why the capacitance CV and the leakage current LC are increased when the Nb content is increased as described above is not sufficiently clear yet at present but is considered to be that a niobium oxide film Nb.sub.2O.sub.5 has a low thermal stability, and the oxygen in the niobium oxide film diffuses and disappears by reflow treatment, and the thickness of the oxide film forming the dielectric decreases, and due to the disappearance of the above oxygen, the oxide film thins, and film defects become apparent, and the insulating Nb.sub.2O.sub.5 changes to conductive NbO, and the like.

[0061] When a Ta-Nb alloy powder is used as the anode material of a capacitor, generally, the Ta-Nb alloy powder is compression-molded into the shape of an anode element by an automatic molding machine or the like. A Ta-Nb alloy powder (primary particles) produced by the thermal CVD method, however, has a fine particle diameter and a low bulk density as it is, and the gap is large, and therefore the density of the molded body to be an anode element is likely to be nonuniform. In addition, the Ta-Nb alloy powder has poor fluidity as it is, and therefore automatic charging into a molding die is difficult. Accordingly, it is important that the Ta-Nb alloy powder is granulated to improve fluidity before being used as an anode material.

[0062] The particles after the above granulation desirably have a fluidity within the range of 0.5 to 5 g/s as measured according to JIS Z2502 because when the above fluidity is less than 0.5 g/s, the fluidity is poor, and the amount of the particles introduced into a molding machine die is not stable, and therefore variations in anode element weight after compression molding increase, and on the other hand, when the fluidity exceeds 5 g/s, the particle diameter is too large, and it is difficult to obtain an anode element having uniform density by compression molding. Preferably, the fluidity is in the range of 1 to 4 g/s. The above fluidity in the invention is a value obtained by dividing a falling time (s) measured by a funnel having an orifice diameter of 2.63 mm by the mass (g) of the powder used in the measurement.

[0063] The Ta-Nb alloy powder (secondary particles) after granulation preferably has a size within the range of 10 to 500 μm in terms of a volume-based median diameter d.sub.50 because when the d.sub.50 is less than 10 μm, the fluidity and the moldability are worsened, and it is difficult to mold the Ta-Nb alloy powder, and on the other hand, when the d.sub.50 exceeds 500 μm, it is difficult to uniformly fill a molding die with the Ta-Nb alloy powder, and therefore the molded body density is nonuniform. A more preferable median diameter d.sub.50 is in the range of 15 to 300 μm. Here, the above volume-based median diameter d.sub.50 is a value obtained by measuring a particle image taken at 100× magnification using a scanning electron microscope, using image analysis type particle size distribution software like the primary particles.

[0064] The Ta-Nb alloy powder (secondary particles) after granulation preferably has a powder bulk density in the range of 1.00 to 4.00 g/cm.sup.3 because when the powder bulk density is less than 1.00 g/cm.sup.3, it is difficult to increase the density of the molded body, and the capacitance per unit volume when an anode element is formed decreases, and on the other hand, when the powder bulk density exceeds 4.00 g/cm.sup.3, it is difficult to impregnate dioxide manganese MnO.sub.2 or a conductive polymer material to be a cathode after sintering. A more preferable bulk density is in the range of 1.50 to 3.80 g/cm.sup.3. Here, the above powder bulk density in the invention refers to “loose bulk density” measured in accordance with JIS Z2504.

[0065] The method for obtaining a Ta-Nb granulated powder (secondary particles) from a Ta-Nb alloy powder (primary particles) produced by the thermal CVD method is not particularly limited as long as a granulated powder satisfying the above conditions is obtained. For example, a method in which an acrylic, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), methyl cellulose, carboxyl cellulose, or the like is added as a granulating agent (binder) to Ta-Nb alloy particles and then the mixture is subjected to tumbling granulation by a rotary drum or the like, a high speed rotation granulation method, a fluidized bed granulation method, and a spray drying method can be used.

[0066] Also, a thermal granulation method in which a Ta-Nb alloy powder is granulated using inorganic matter such as water or a phosphoric acid aqueous solution as a binder, then heated and sintered, and ground, and then impurities such as oxygen are removed can also be appropriately used.

[0067] A granulated powder of a Ta-Nb alloy powder whose bulk density, particle size distribution, and median diameter are adjusted in appropriate ranges by the above various granulation methods is usually dry-molded, subjected to binder removal, fired, and then subjected to chemical conversion treatment to form an anode element. At this time, the molding density in dry molding may be appropriately selected according to the required electrostatic characteristics.

[0068] For the making of an anode element, a method of direct molding from a primary powder may be adopted without using the above dry molding process. For example, it is possible to add a binder and a solvent such as water to a primary powder, knead the mixture into kneaded clay, mold it into a sheet by an extrusion molding machine, then subject the sheet to binder removal, vacuum-fire the sheet, and then join wire by welding or the like to form an anode element.

EXAMPLES

[0069] Ta-Nb alloy powders (primary particles) having a Nb content of 30 mass % and an average particle diameter variously changed in the range of 20 to 250 nm as shown in Table 2 are produced by a thermal CVD method, then each of the above alloy powders is water-washed and dried, a cellulosic binder is added, and then the mixture is granulated using a rotary drum to form granulated particles (secondary particles) having a median diameter d.sub.50 of 30 to 50 μm.

[0070] Then, a sintered element is made with the above granulated particles in accordance with test conditions of 100 kCV powder defined in Table 1 in the appendix to Standard of Electronic Industries Association of Japan EIAJ RC-2361A “Test methods of tantalum sintered anodes for electrolytic capacitors.” At this time, the molding density of the element (pellet) is adjusted so as to fall within ±0.10 g/cm.sup.3 with respect to ρ.sub.c obtained by the above-described formula (1). Generally, the optimum sintering temperature of the element depends on the particle diameter and increases as the particle diameter increases, and therefore a preliminary experiment is performed in the temperature range of 950 to 1150° C., and the temperature obtaining the highest capacitance is adopted.

[0071] Then, the above element is subjected to chemical conversion treatment at a voltage of 10 V for 2 hour in a 0.05 mass % phosphoric acid solution having a temperature of 80° C. to form an anodic oxide film on the metal particle surfaces, and then the capacitance CV and the leakage current LC are measured in accordance with methods described in EIAJ RC-2361A. The capacitance CV is measured in a 40 mass % sulfuric acid solution at a voltage of 1 V, a bias voltage of 1.5 Vdc, and a frequency of 120 Hz. In addition, for the leakage current LC, a voltage of 7 V is applied, and leakage current after a lapse of 2 min is measured.

[0072] Further, the element after the above measurement is subjected to heat treatment simulating reflow treatment at 260° C. for 30 minutes in an Ar gas atmosphere, and the capacitance CV and the leakage current LC are measured under the same conditions as the above.

[0073] The results of the above measurement are shown together in Table 2. From these results, it is seen that a CV value per unit volume (μF.Math.V/mm.sup.3) of not less than 900 μF.Math.V/mm.sup.3 can be stably achieved by controlling the average diameter of the primary particles of the Ta-Nb alloy powder in the range of 30 to 200 nm.

TABLE-US-00002 TABLE 2 CV value CV value per unit mass per unit volume Leakage current LC per unit Primary Molding Sintering (kμF .Math. V/g) (μF .Math. V/mm.sup.3) capacitance (nA/μF .Math. V) Type of Nb content particle density temperature Before After Before After Before After After reflow/ No metal powder (mass %) diameter (g/cm.sup.3) (° C.) reflow reflow reflow reflow reflow reflow Before reflow 1 Ta—Nb alloy 30 25 3.12 950 160 172 496 534 1.25 5.6 4.5 2 Ta—Nb alloy 30 50 3.26 1000 341 367 1088 1171 1.28 5.8 4.5 3 Ta—Nb alloy 30 100 3.23 1050 318 342 1056 1136 1.20 5.6 4.7 4 Ta—Nb alloy 30 150 3.18 1100 301 324 1020 1097 1.14 5.1 4.5 5 Ta—Nb alloy 30 250 3.29 1150 225 242 810 871 1.05 5.0 4.8