High specific capacitance capacitor-grade tantalum powder with improved electrical properties and process for making the same

Abstract

A method for providing a tantalum powder with a piece+block structure, comprising the following steps: 1) providing a granulous tantalum powder, and dividing same into a first part and a second part; 2) putting the first part of the tantalum powder in a ball mill for ball milling, taking the powder out after the ball milling and sieving same, and obtaining a tantalum powder in the form of a piece; 3) mixing the tantalum powder in the form of a piece and the second part of the tantalum powder to obtain a mixture, and preferably, the mixing proportion of the tantalum powder in the form of a piece and the granulous tantalum powder being 1:0.11, preferably being 1:0.250.8, and more preferably being 1:0.40.6; and 4) performing the steps of water washing, acid washing, and nodularization on the mixture to finally obtain a tantalum powder with a piece+block structure.

Claims

1. A process of providing tantalum powder having a flake+block structure, comprising the steps of: 1) providing a granular tantalum powder and dividing it to a first portion of tantalum powder and a second portion of tantalum powder; 2) charging the first portion of tantalum powder into a milling machine and milling the tantalum powder, and after the milling, taking the tantalum powder out and screening it to give a flaked tantalum powder; 3) mixing the flaked tantalum powder with the second portion of the granular tantalum powder to give a mixture, wherein the mixing ratio of the flake tantalum powder to the granular tantalum powder is from 1:0.4 to 1:0.9; and 4) subjecting the mixture to water washing, acid washing, and agglomeration to produce a tantalum powder having a flack+block structure wherein the proportion of 400 mesh tantalum powder having a flake+block structure is from 20 to 40%.

2. The process according to claim 1, wherein the tantalum powder in the step 1) is produced by a process of reducing potassium fluorotantalate with sodium.

3. The process according to claim 1, wherein the tantalum powder in the step 1) has a specific capacitance ranging from 30,000 to 80,000 FV/g, and/or has a particle size of from 0.3 to 3.0 m.

4. The process according to claim 1 or 2, wherein prior to the step 1), the tantalum powder is subjected to acid washing, to remove impurities therein.

5. The process according to claim 1, wherein during the milling in the step 2), a stainless steel ball is used as a grinding body, and alcohol is used as a liquid milling agent medium.

6. The process according to claim 5, wherein the weight ratio of the stainless steel ball to the first portion of tantalum powder is in the range of 3-10:1, and the weight ratio of the first portion of tantalum powder to the liquid milling agent is 1:0.5-2.5.

7. The process according to claim 1, wherein the tantalum powder is milled for a period of 5 to 20 hours.

8. The process according to claim 1, wherein prior to the screening in the step 2), the tantalum powder is subjected to water washing and acid washing to remove impurities therein.

9. A tantalum powder having the flake +block structure obtained by the process according to claim 1.

10. A tantalum powder having a flake+block structure, comprising flake tantalum powder and granular tantalum powder, wherein the mixing ratio of the flake tantalum powder to the granular tantalum powder is from 1:04 to 1:0.9, wherein the proportion of 400 mesh tantalum powder having a flake+block structure is from 20 to 40%.

11. The tantalum powder according to claim 10, wherein the mixing ratio of the flake tantalum powder to the granular tantalum powder is from 1:0.4 to 1:0.7.

12. The tantalum powder according to claim 10, wherein the ratio of the flake tantalum powder to the granular tantalum powder is from 1:0.4 to 1:0.8.

13. The tantalum powder according to claim 10, wherein the proportion of 400 mesh tantalum powder having a flake+block structure is from 25 to 35%.

14. A capacitor comprising an anode block wherein the anode block comprises the tantalum powder having the flake+block structure as recited in claim 10.

Description

BRIEF ILLUSTRATIONS TO THE DRAWINGS

(1) FIG. 1 is a diagram to show the variations of the leakage current of the samples in Examples 1-4 with the ratios of the flake powder to granular powder ratio.

(2) FIG. 2 is a diagram to show the variations of the specific capacitance of the samples in Examples 1-4 with the ratios of the flake powder to granular powder ratio.

(3) FIG. 3 is a diagram to show the variations of the loss of the samples in Examples 1-4 with the ratios of the flake powder to granular powder ratio.

EMBODIMENTS OF THE INVENTION

(4) In the art, a physical parameter which is used to describe the fineness of metallic particles is the average Fisher sub-sieve size (FSSS, which is also called Fisher mean particle size) measured by a Fisher sub-sieve meter. The average Fisher sub-sieve size is obtained by measuring the flow rate of powder filled in a metal tube by the gas penetration method in a Fisher sub-sieve meter. On one hand, the physical parameter is relevant to the size of the particles, and on the other hand, it is relevant to the agglomeration strength of powder. As to the same tantalum powder obtained by reducing potassium fluorotantalate with sodium, the lower average Fisher sub-sieve size will lead to a higher specific surface area. Furthermore, as to agglomerated metal powder, the powder having different specific surface area may have similar average Fisher sub-sieve size. As to powder in the same grade, agglomerated powder has a larger average Fisher sub-sieve size.

(5) The Fisher sub-sieve size (FSSS/m) of the tantalum powder in accordance with the invention is measured according to the method prescribed in the standard Standard Method for Determination of Particle Size of Powders of Refractory Metals and CompoundsFisher Method (the serial number of the standard: GB/T3249); the bulk density (SBD) is measured by the method prescribed in the standard Metallic PowderDetermination of Bulk DensityPart I: Funnel Method (the serial number of the standard: GB/T1479); the particle size distribution is measured by the method prescribe in the standard Determination of Particle Size for Metallic PowdersDry sieving (the serial number of the standard: GB/T1480); the sampling process is conducted according to the method as prescribed in the standard sampling for Powder Metallurgical PurposesSampling (the serial number of the standard: GB/T5314).

(6) In the invention, the elements in tantalum powder are detected according to methods as prescribed in National Standard of the People's Republic of China. These standards include GB/T 15076.8-2008 Methods for Chemical Analysis of Tantalum and NiobiumDetermination of Carbon and Sulphur Contents, GB/T 15076.9-2008 Methods for Chemical Analysis of Tantalum and NiobiumDetermination of Iron, Chromium, Nickel, Manganese, Titanium, Aluminum, Copper, Tin, Lead, and Zirconium Contents in Tantalum, GB/T 15076.12-2008 Methods for Chemical Analysis of Tantalum and NiobiumDetermination of Phosphorous Content, GB/T 15076.14-2008 Methods for Chemical Analysis of Tantalum and NiobiumDetermination of Oxygen Content, GB/T 15076.15-2008 Methods for Chemical Analysis of Tantalum and NiobiumDetermination of Hydrogen Content, and GB/T 15076.16-2008, Methods for Chemical Analysis of Tantalum and NiobiumDetermination of Sodium and Potassium Contents.

(7) In the invention, detecting methods and apparatus for the electrical properties of tantalum powder are measured according to the National Standard GB/T 3137-2007 Testing Method for Electrical Properties of Tantalum Powder.

(8) Another physical parameter which is used to describe the fineness of metal particles is the specific surface area (m.sup.2/g) as measured by a BET low-temperature nitrogen absorption.

(9) The inventors further surprisingly find out that if an effective pre-agglomeration process is used after the step 3), the bulk density of samples and the proportion of 400 mesh particles can be improved in a great extent. The inventor surprisingly find out that if the proportion of the 400 mesh particles is too high, the proportion will result in problems of poor flowing property, and poor shaping homogeneity of the tantalum powder; if the proportion of 400 mesh particles is too low, the proportion will result in that the shaped anode block has a rough surface, and its edge is ready to collapse. For the invention, the proportion of 400 mesh particles would be controlled in the range of from 20 to 40%, and in the case, the better effects that the physical properties of samples can be obviously improved; the proportion can be advantageous to the sintering and shaping of powder; and the proportion can increase the applicability of samples to customers. After the pre-agglomeration, the tantalum powder is subjected to thermal treatment agglomeration, deoxygenation (860 C.-960 C./heat preservation 1-4 hours, de-magging under gas evacuation for 1-4 hours), and after the above steps, desirable tantalum powder is finally produced.

(10) In a certain embodiment of the invention, the thermal treatment agglomeration is conducted by keeping at 1000 C. for a period of from 30 to 60 minutes, and then raising the temperature and keeping at 1300 to 1450 C. for a period of from 30 to 90 minutes.

(11) In a certain embodiment of the invention, the deoxygenation (also referred to oxygen-lowering in the art) is conducted by keeping at 860 C. to 960 C. for a period of from 1 to 4 hours, and then de-magging under gas evacuation for a period of from 1 to 4 hours.

(12) In order to further explain the invention, the embodiments of the invention are described by combining the following examples and tables. However, it should be understood that these descriptions are used to further describe the features and advantages of the invention, but not for limitations to the scope of the claims of the invention.

Example 1

(13) Commercially available crude tantalum powder having a high specific capacitance of 40000 FV/g (i.e., the tantalum powder which is not subjected to the thermal treatment agglomeration after the reduction) is used in the example as raw material. The tantalum powder is divided into two portions. Then, the first portion is acid washed to remove impurities therein, and following this, the portion of tantalum powder is subjected flaking treatment. The flaking treatment is conducted by using a stirring milling method. The grinding body is a 2 mm stainless steel ball; the liquid milling agent medium is alcohol; the milling time is 15 hours; the weight ratio of the steel ball to the tantalum powder is 6:1, and the weight ratio of the tantalum powder to the liquid milling agent is 1:1.8; and the rotating speed used in the milling is 100 r/m.

(14) After the milling, the milled powder is separated from the grinding body. Then, the separated powder is subjected to water washing and acid washing to remove impurities therein, and the particles of tantalum powder are dispersed by mesh screening to give a flaked tantalum powder. The resultant tantalum powder is uniformly mixed with the second portion of tantalum powder in the ratio of 1:1, and the mixture is further subjected to pre-agglomeration, thermal treatment agglomeration, and oxygen-lowering process to produce tantalum powder having the flake +block structure. The thermal treatment agglomeration conditions used in the example include keeping at 1350 C. for a period of 40 min, and the oxygen-lowering conditions include keeping at 900 C. for a period of 180 min and gas evacuation for a period of 180 min.

Example 2

(15) Commercially available crude tantalum powder having a high specific capacitance of 40000 FV/g (i.e., the tantalum powder which is not subjected to the thermal treatment agglomeration after the reduction) is used in the example as raw material. The tantalum powder is divided into two portions. Then, the first portion is acid washed to remove impurities therein, and following this, the portion of tantalum powder is subjected to flaking treatment. The flaking treatment is conducted by using a stirring milling method. The grinding body is a 2 mm stainless steel ball; the liquid milling agent medium is alcohol; the milling time is 15 hours; the weight ratio of the steel ball to the tantalum powder is 6:1, and the weight ratio of the tantalum powder to the liquid milling agent is 1:1.8; and the rotating speed used in the milling is 100 r/m.

(16) After the milling, the milled powder is separated from the grinding body. Then, the separated powder is subjected to water washing and acid washing to remove impurities therein, and the particles of tantalum powder are dispersed by mesh screening to give a flaked tantalum powder. The resultant tantalum powder is uniformly mixed with the second portion of tantalum powder in the ratio of 1:0.75, and the mixture is further subjected to pre-agglomeration, thermal treatment agglomeration, and oxygen-lowering process to produce tantalum powder having the flake +block structure. The thermal treatment agglomeration conditions used in the example include keeping at 1350 C. for a period of 40 min, and the oxygen-lowering conditions include keeping at 900 C. for a period of 180 min and gas evacuation for a period of 180 min.

Example 3

(17) Commercially available crude tantalum powder having a high specific capacitance of 40000 FV/g (i.e., the tantalum powder which is not subjected to the thermal treatment agglomeration after the reduction) is used in the example as raw material. The tantalum powder is divided into two portions. Then, the first portion is acid washed to remove impurities therein, and following this, the portion of tantalum powder is subjected to flaking treatment. The flaking treatment is conducted by using a stirring milling method. The grinding body is a 2 mm stainless steel ball; the liquid milling agent medium is alcohol; the milling time is 15 hours; the weight ratio of the steel ball to the tantalum powder is 6:1, and the weight ratio of the tantalum powder to the liquid milling agent is 1:1.8; and the rotating speed used in the milling is 100 r/m.

(18) After the milling, the milled powder is separated from the grinding body. Then, the separated powder is subjected to water washing and acid washing to remove impurities therein, and the particles of tantalum powder are dispersed by mesh screening to give a flaked tantalum powder. The resultant tantalum powder is uniformly mixed with the second portion of tantalum powder in the ratio of 1:0.50, and the mixture is further subjected to pre-agglomeration, thermal treatment agglomeration, and oxygen-lowering process to produce tantalum powder having the flake +block structure. The thermal treatment agglomeration conditions used in the invention include keeping at 1350 C. for a period of 40 min, and the oxygen-lowering conditions include keeping at 900 C. for a period of 180 min and gas evacuation for a period of 180 min.

Example 4

(19) Commercially available crude tantalum powder having a high specific capacitance of 40000 FV/g (i.e., the tantalum powder which is not subjected to the thermal treatment agglomeration after the reduction) is used in the example as raw material. The tantalum powder is divided into two portions. Then, the first portion is acid washed to remove impurities therein, and following this, the portion of tantalum powder is subjected to flaking treatment.

(20) The flaking treatment is conducted by using a stirring milling method. The grinding body is a 2 mm stainless steel ball; the liquid milling agent medium is alcohol; the milling time is 15 hours; the weight ratio of the steel ball to the tantalum powder is 6:1, and the weight ratio of the tantalum powder to the liquid milling agent is 1:1.8; and the rotating speed used in the milling is 100 r/m.

(21) After the milling, the milled powder is separated from the grinding body. Then, the separated powder is subjected to water washing and acid washing to remove impurities therein, and the particles of tantalum powder are dispersed by mesh screening to give a flaked tantalum powder. The resultant tantalum powder is uniformly mixed with the second portion of tantalum powder in the ratio of 1:0.25, and the mixture is further subjected to pre-agglomeration, thermal treatment agglomeration, and oxygen-lowering process to produce tantalum powder having the flake +block structure. The thermal treatment agglomeration conditions used in the invention include keeping at 1350 C. for a period of 40 min, and the oxygen-lowering conditions include keeping at 900 C. for a period of 180 min and gas evacuation for a period of 180 min.

Example 5

(22) Commercially available crude tantalum powder having a high specific capacitance of 70000 FV/g (i.e., the tantalum powder which is not subjected to the thermal treatment agglomeration after the reduction) is used in the example as raw material. The tantalum powder is divided into two portions. Then, the first portion is acid washed to remove impurities therein, and following this, the portion of tantalum powder is subjected to flaking treatment. The flaking treatment is conducted by using a stirring milling method. The grinding body is a 2 mm stainless steel ball; the liquid milling agent medium is alcohol; the milling time period is 18 hours; the weight ratio of the steel ball to the tantalum powder is 6:1, and the weight ratio of the tantalum powder to the liquid milling agent is 1:2.0; and the rotating speed used in the milling is 100 r/m.

(23) After the milling, the milled powder is separated from the grinding body. Then, the separated powder is subjected to water washing and acid washing to remove impurities therein, and the particles of tantalum powder are dispersed by mesh screening to give a flaked tantalum powder. The resultant tantalum powder is uniformly mixed with the second portion of tantalum powder in the ratio of 1:0.45, and the mixture is further subjected to pre-agglomeration, thermal treatment agglomeration, and oxygen-lowering process to produce tantalum powder having the flake +block structure. The thermal treatment agglomeration conditions used in the invention include keeping at 1350 C. for a period of 40 min, and the oxygen-lowering conditions include keeping at 900 C. for a period of 180 min and gas evacuation for a period of 180 min.

Example 6

(24) Commercially available crude tantalum powder having a high specific capacitance of 70000 FV/g (i.e., the tantalum powder which is not subjected to the thermal treatment agglomeration after the reduction) is used in the example as raw material. The tantalum powder is divided into two portions. Then, the first portion is acid washed to remove impurities therein, and following this, the portion of tantalum powder is subjected to flaking treatment. The flaking treatment is conducted by using a stirring milling method. The grinding body is a 2 mm stainless steel ball; the liquid milling agent medium is alcohol; the milling time period is 18 hours; the weight ratio of the steel ball to the tantalum powder is 6:1, and the weight ratio of the tantalum powder to the liquid milling agent is 1:2.0; and the rotating speed used in the milling is 100 r/m.

(25) After the milling, the milled powder is separated from the grinding body. Then, the separated powder is subjected to water washing and acid washing to remove impurities therein, and the particles of tantalum powder are dispersed by mesh screening to give a flaked tantalum powder. The resultant tantalum powder is uniformly mixed with the second portion of tantalum powder in the ratio of 1:0.30, and the mixture is further subjected to pre-agglomeration, thermal treatment agglomeration, and oxygen-lowering process to produce tantalum powder having the flake +block structure. The thermal treatment agglomeration conditions used in the invention include keeping at 1350 C. for a period of 40 min, and the oxygen-lowering conditions include keeping at 900 C. for a period of 180 min and gas evacuation for a period of 180 min.

Comparative Example 1

(26) Commercially available crude tantalum powder having a high specific capacitance of 40000 FV/g (i.e., the tantalum powder which is not subjected to the thermal treatment agglomeration after the reduction) is used in the example as raw material. The tantalum powder is subjected to acid washing, and then the washed powder is subjected to pre-agglomeration, thermal treatment agglomeration, and oxygen-lowering process according to conventional production processes to produce tantalum powder produced by conventional processes. The thermal treatment agglomeration conditions used in the example include keeping at 1350 C. for a period of 40 min, and the oxygen-lowering conditions include keeping at 900 C. for a period of 180 min and gas evacuation for a period of 180 min.

Comparative Example 2

(27) Commercially available crude tantalum powder having a high specific capacitance of 70000 FV/g (i.e., the tantalum powder which is not subjected to the thermal treatment agglomeration after the reduction) is used in the example as raw material. The tantalum powder is subjected to acid washing, and then the washed powder is subjected to pre-agglomeration, thermal treatment agglomeration, and oxygen-lowering process according to conventional production processes to produce tantalum powder produced by conventional processes. The thermal treatment agglomeration conditions used in the example include keeping at 1260 C. for a period of 30 min, and the oxygen-lowering conditions include keeping at 900 C. for a period of 180 min and gas evacuation for a period of 180 min, the oxygen-lowering process being repeated twice.

(28) After tests, individual physical properties in Examples 1-4 and Comparative Example 1 are shown in Table 1

(29) TABLE-US-00001 TABLE 1 Individual physical properties of tantalum powder Samples Fsss (m) SBD (g/cc) +80 (%) 400 (%) Example 1 2.75 1.66 0.10 28.34 Example 2 2.85 1.62 0.36 32.64 Example 3 3.04 1.60 0.24 27.10 Example 4 3.18 1.56 0.16 28.20 Comparative 2.46 1.65 0.08 25.48 Example 1

(30) In the table, the Fsss (m) represents the Fisher sub-sieve size; the SBD (g/cc) represents the bulk density; the +80(%) represents the proportion of tantalum powder with the size of greater than 80 meshes; and the 400(%) represents the portion of tantalum powder with the size of less than 400 meshes.

(31) With comparisons and analyses, it is found the Fisher sub-sieve sizes of tantalum powder in the examples are greater than that of tantalum powder in the comparative example. Furthermore, with the increase of the proportion of the flaked tantalum powder, the Fisher sub-sieve size of the tantalum powder is increased, and electrical properties, e.g., leakage current and loss, can be improved. As to other physical properties, e.g., the bulk density, the proportion of +80 mesh tantalum powder, and the proportion of 400 mesh tantalum powder, the examples are comparable to the comparative example.

(32) After tests, amounts of primary impurities in the tantalum powder of Examples 1-4 and Comparative Example 1 are shown in Table 2

(33) TABLE-US-00002 TABLE 2 Amounts of primary impurities in tantalum powder (unit: ppm) Chemical impurities sample O C N Fe K H Example 1 2750 26 150 17 8 72 Example 2 2630 26 170 15 6 93 Example 3 2560 24 130 14 9 90 Example 4 2450 23 110 15 8 78 Comparative Example 1 2780 20 140 12 12 86

(34) With comparisons and analyses, it is found the oxygen contents in the tantalum powder of the examples are reduced with the increase of the proportion of flaked tantalum powder in the tantalum powder, and as a whole, the oxygen contents in the examples are lower than the oxygen content in comparative Example 1. Not restricted by general theories, the inventor considers that generally, the specific surface area is relevant to the oxygen content, that is, the lower specific surface will be lead to easier controls of a lower oxygen content. The fact shows that the specific surface areas in the examples are lower than the specific surface area in the comparative example. As to the contents of other impurities, such as, C, N, Fe, etc., the examples are comparable to the comparative example.

(35) The above powder sample is compressed. The density of resulting briquette is 5.0 g/cm.sup.3, and the weight of the core powder is 0.25 g. The molder in use is a molder with 4 mm. The compressed powder sample is sintered at 1400 C. for a period of 10 minutes at a vacuum furnace of 10.sup.3 Pa to give a sintered block, and then the sintered block is energized in a 0.1% phosphoric acid solution at 70 V for a period of 120 minutes, in which the energizing temperature is 80 C., and the current density is 150 mA/g. Other properties are measured according National Standard GB/T 3137-2007. The specific capacitances of individual samples are listed in Table 3.

(36) After tests, the electrical properties of the tantalum powder in Examples 1-4 and Comparative Example 1 are shown in Table 3.

(37) TABLE-US-00003 TABLE 3 comparisons of electrical properties of tantalum powder K Samples (nA/FV) CV (FV/g) tg (%) SHD (%) SHV (%) Example 1 0.21 38068 13.8 3.0 8.0 Example 2 0.20 37880 13.5 2.8 7.6 Example 3 0.19 37654 12.6 1.5 2.5 Example 4 0.27 35280 12.2 1.0 1.0 Comparative 0.34 38162 16.8 3.2 8.5 Example 1

(38) In the table, the K(nA/FV) represents the leakage current; the CV(FV/g) represents the capacitance; the tg(%) represents the loss; the SHD represents the radical contraction rate; and SHV (%) represents the volume contraction rate.

(39) As seen from Table 3, the leakage current in the examples is reduced with the increase of the proportion of flaked tantalum powder in the tantalum powder, and as a whole, the leakage current in the examples is lower than the leakage current in comparative Example 1. The fact shows that the tantalum powder obtained by the process has a low leakage current and a low loss, and thus the performances of the product are improved.

(40) As seen from FIG. 1, the leakage current in the examples is reduced with the increase of the proportion of flaked tantalum powder in the tantalum powder, and as a whole, the leakage current in the examples is lower than the leakage current in comparative example. However, when the proportion of flaked tantalum powder is close to or higher than 80% (see Example 4), the leakage current of tantalum powder samples is increased (still superior to the leakage current in Comparative Example 1).

(41) As seen from FIG. 2, the specific capacitances in the examples are slightly reduced, and it is still comparable to that in Comparative Example 1. However, when the proportion of the flaked tantalum powder is 33.33%, the specific capacitance begins to decrease. Hence, in view of the specific capacitance, the ratio of the flaked tantalum powder to the granular tantalum powder should be 1:0.5-1.

(42) As seen from FIG. 3, the loss of tantalum powder has a monotonic relation with the proportion of the flaked powder.

(43) After tests, individual physical properties of tantalum powder in Examples 5-6 and Comparative Example 2 are shown in Table 4.

(44) TABLE-US-00004 TABLE 4 Individual physical properties of tantalum powder Sample Fsss (m) SBD (g/cc) +80 (%) 400 (%) Example 5 2.52 1.76 0.50 25.52 Example 6 2.65 1.72 0.24 26.30 Comparative 2.36 1.75 0.20 24.60 example 2

(45) In the table, the Fsss(m) represents the Fisher sub-sieve size; the SBD(g/cc) represents the bulk density; the +80(%) represents the proportion of tantalum powder with the size of greater than 80 meshes; and the 400(%) represents the portion of tantalum powder with the size of less than 400 meshes.

(46) With comparisons and analyses, it is found the Fisher sub-sieve sizes of tantalum powder in the examples are greater than that of tantalum powder in the comparative example. Furthermore, with the increase of the proportion of the flaked tantalum powder, the Fisher sub-sieve size of the tantalum powder is increased, and the corresponding particle shape becomes simpler. As to other physical properties, e.g., the bulk density, the proportion of +80 mesh tantalum powder, and the proportion of 400 mesh tantalum powder, the examples are comparable to the comparative example.

(47) After tests, the amounts of primary impurities in tantalum powder of Examples 5-6 and Comparative Example 2 are shown in Table 5.

(48) TABLE-US-00005 TABLE 5 Amounts of primary impurities in tantalum powder Chemical impurity Sample O C N Fe K H Example 5 4182 28 310 18 16 160 Example 6 3926 29 280 16 15 170 Comparative Example 2 4312 25 400 15 19 160

(49) With comparisons and analyses, it is found the oxygen contents in the tantalum powder of the examples are reduced with the increase of the proportion of flaked tantalum powder in the tantalum powder, and as a whole, the oxygen contents in the examples are lower than the oxygen content in comparative Example 2. The fact demonstrates that the specific surface areas in the examples are lower than the specific area in the comparative example. As to the contents of other impurities, such as, C, N, Fe, etc., the examples are comparable to the comparative example.

(50) The above powder sample is compressed. The density of resulting briquette is 5.5 g/cm.sup.3, and the weight of the core powder is 0.15 g. The molder in use is a molder with 3 mm. The compressed powder sample is sintered at 1320 C. for a period of 10 minutes in a furnace with 10.sup.3 Pa vacuum to give a sintered block, and then the sintered block is energizeding a 0.15% phosphoric acid solution at 30 V for a period of 120 minutes, in which the energizing temperature is 85 C., and the current density is 150 mA/g. Other properties are measured according National Standard GB/T 3137-2007. The specific capacitances of individual samples are listed in Table 6.

(51) After tests, the electrical properties of the tantalum powder in Examples 5-6 and Comparative Example 2 are shown in Table 6.

(52) TABLE-US-00006 TABLE 6 comparisons of electrical properties of tantalum powder K Sample (nA/FV) CV (FV/g) tg (%) SHD (%) SHV (%) Example 5 0.33 67056 47.8 3.0 8.0 Example 6 0.30 65785 43.5 2.8 7.6 Comparative 0.39 67120 52.8 3.3 8.5 Example 2

(53) In the table, the K(nA/FV) represents the leakage current; the CV(FV/g) represents the capacitance; the tg(%) represents the loss; the SHD represents the radical contraction rate; and SHV (%) represents the volume contraction rate.

(54) As seen from Table 6, the leakage current in the examples is reduced with the increase of the proportion of flaked tantalum powder in the tantalum powder, and as a whole, the leakage current in the examples is lower than the leakage current in comparative Example 1. The fact shows that the tantalum powder obtained by the process has a low leakage current and a low loss, and thus the performances of the product are improved.

(55) The analytical data demonstrates that the process of the invention increase the characteristic of tantalum powder of voltage resistance, reduce the leakage current, and reduce the loss. Thus, the process meets the requirements of capacitor products on tantalum powder in the aspect of voltage resistance and leakage current. The claimed product is suitable for the tantalum powder having a specific capacitance of 30000 to 80000 FV/g.

(56) The description and examples of the invention as disclosed herein are illustrative. Furthermore, it is obvious for a person skilled in the art that the invention may involve other embodiments, and the essential scope and spirit of the invention is determined by the claims.