Tantalum powder and preparation method therefor

Abstract

A tantalum powder, a tantalum powder compact, a tantalum powder sintered body, a tantalum anode, an electrolytic capacitor and a preparation method for tantalum powder. The tantalum powder contains boron element, and the tantalum powder has a specific surface area of greater than or equal to 4 m.sup.2/g; the ratio of the boron content of the tantalum powder to the specific surface area of the tantalum powder is 2˜16; the boron content is measured in weight ppm, and the specific surface area is measured in m.sup.2/g; Powder that can pass through a ρ-mesh screen in the tantalum powder accounts for over 85% of the total weight of the tantalum powder, where ρ=150˜170; and the tantalum powder with high CV has a low leakage current and dielectric loss, and good moldability.

Claims

1. A method for preparing tantalum powder, comprising: 1) reducing a tantalum fluoride salt with a reducing agent in a molten diluent to obtain a raw tantalum powder, wherein said raw tantalum powder contains boron in an amount of 30˜300 ppm and has a specific surface area of 5˜13 m.sup.2/g; 2) granulating said tantalum powder to obtain a pre-agglomerated powder, wherein the pre-agglomerated powder has a bulk density of 1˜1.5 g/cm.sup.3; the granulating comprises: mixing raw tantalum powder with water to obtain a powder mixed with water; drying the powder mixed with water to obtain a dried powder; and crushing and sieving the dried powder, wherein the mesh number of the screen for sieving is 120˜170 mesh, and powder that passes through the screen is pre-agglomerated powder; and 3) sequentially subjecting said pre-agglomerated powder to one or more of the following steps: heat treatment, sieving, oxygen reduction treatment, and nitrogen doping; 4) sequentially subjecting the powder obtained in the previous step to one or more of the following steps: acid washing, water washing, drying and sieving, to obtain a tantalum powder product; wherein, the tantalum powder product has an oxygen content of 10000 to 31000 ppm a nitrogen content of 2000 to 3300 ppm; a potassium content of 10 to 50 ppm a boron content of 20 to 150 ppm; and a phosphorous content of 200 to 400 ppm; wherein the tantalum powder product has a specific surface area of 5˜12 m.sup.2/g; and the ratio of the boron content of the tantalum powder to the specific surface area of the tantalum powder is 4˜13, the boron content is measured in unit of ppm by weight, and the specific surface area is measured in unit of m.sup.2/g; wherein, more than 85% of the total weight of the tantalum powder product can pass through 170; from 5 to 40% of the total weight of the tantalum powder product can pass through a 400 mesh screen; the amount of powder that can pass through 170 mesh but can not pass through 400 mesh accounts for 60% to 80% of the total weight of the tantalum powder product.

2. The method of claim 1, wherein the oxygen content of the raw tantalum powder is from 0.8α′ to 1.2α′ ppm, wherein α′=−68.5 x′.sup.3+2197.3 x′.sup.2−18616.4 x′+62307.8, x′ is the specific surface area of the raw tantalum powder in unit of m.sup.2/g; the raw tantalum powder has a nitrogen content of from 0.8β′ to 1.2β′ ppm, wherein α′=98.1 x′−388.8, x′ is the specific surface area of the raw tantalum powder in unit of m.sup.2/g; the raw tantalum powder has a boron content of from 0.8γ′ to 1.2γ′ ppm, wherein α′=−68.5 x′.sup.3+2197.3 x′.sup.2−18616.4 x″=62307.8, x′ the specific surface area of the raw tantalum powder in unit of m.sup.2/g; the raw tantalum powder has a potassium content of from 0.8ε′ to 1.2ε′ ppm, wherein ε′+−9.39E-02x′.sup.3+3.13E+00x′.sup.2 −2.64E+01x′+9.40E+01, x′ is the specific surface area of the raw tantalum powder in unit of m.sup.2/g; the raw tantalum powder has a sodium content of 1 to 10 ppm; and α′, β′, γ′ and ε′ are all positive numbers.

3. The method of claim 1, wherein the tantalum powder product has a bulk density of from 0.8 τ To 1.2 τ g/cm.sup.3, wherein τ is mathematically related to the specific surface area x (in m.sup.2/g) of the tantalum powder product as follows: τ=−6.14E-03x.sup.2+2.53E-02x+1.82E+00, and x is the specific surface area of tantalum powder product in m.sup.2/g.

4. The method of claim 1, wherein the tantalum powder product has a FSSS particle size of 0.8 Ψ to 1.2 Ψ, wherein Ψ is mathematically related to the specific surface area x (in m.sup.2/g) of the tantalum powder product as follows: Ψ=−2.22E-03x.sup.3+4.39E-02x.sup.2−2.93E-01x+1.64E+00, x is the specific surface area of tantalum powder product in m.sup.2/g.

5. The method of claim 1, wherein the powder that can pass through 170 mesh while can not pass 400 mesh in the tantalum powder product accounts for 62% to 78% of the total weight of the tantalum powder product; and 8.84% to 32.22% of the total weight of the tantalum powder product can pass through a 400 mesh screen.

6. The method of claim 1, wherein in step (1), the diluent comprises potassium chloride and potassium fluoride; in the diluent, the weight ratio of potassium chloride to potassium fluoride is 18˜24: 10˜12; the reducing agent comprises metallic sodium; the tantalum fluoride salt comprises potassium fluotantalate (K.sub.2TaF.sub.7); and the weight ratio of the tantalum fluoride salt to the diluent is from 2˜5: 300˜4004.

7. The method of claim 1, wherein in step (1), the diluent also contains an additive K.sub.2SO.sub.4 and the weight ratio of K.sub.2SO.sub.4 to the diluent is 400˜600: 300000˜400000, or 500˜550: 300000˜340000; and the diluent further comprises boron element, and the weight ratio of boron element in the diluent to the diluent is 1˜5:300000˜340000.

8. The method of claim 1, wherein in step (2), the water contains 100 to 1000 ppm phosphoric acid.

9. A tantalum powder prepared by a method comprising: 1) reducing a tantalum fluoride salt with a reducing agent in a molten diluent to obtain a raw tantalum powder, wherein the raw tantalum powder contains boron in an amount of 30˜300 ppm and has a specific surface area of 5˜13 m.sup.2/g; 2) granulating the tantalum powder to obtain a pre-agglomerated powder, wherein the pre-agglomerated powder has a bulk density of 1˜1.5 g/cm.sup.3; wherein granulating comprises: mixing raw tantalum powder with water to obtain a powder mixed with water; drying the powder mixed with water to obtain a dried powder; and crushing and sieving the dried powder, wherein the mesh number of the screen for sieving is 120˜470 mesh, and powder that passes through the screen is pre-agglomerated powder; 3) sequentially subjecting the pre-agglomerated powder to one or more of the following steps: heat treatment, sieving, oxygen reduction treatment, and nitrogen doping; and 4) sequentially subjecting the powder obtained in 3) to one or more of the following steps: acid washing, water washing, drying and sieving, to obtain a tantalum powder product; wherein the tantalum powder product has an oxygen content of from 10000 to 31000 ppm; a nitrogen content of from 2000 to 3300 ppm; a potassium content of from 10 to 50 ppm; a boron content of from 20 to 150 ppm; and a phosphorous content of from 200 to 400 ppm; wherein the tantalum powder product has a specific surface area of 5˜12 m.sup.2/g; and the ratio of the boron content of the tantalum powder to the specific surface area of the tantalum powder is from 4˜13, the boron content being measured in units of ppm by weight, and the specific surface area being measured in units of m.sup.2/g; wherein, more than 85% of the total weight of the tantalum powder product can pass through 170 mesh; from 5 to 40% of the total weight of the tantalum powder product can pass through a 400 mesh screen; and the powder that can pass through 170 mesh but not pass through 400 mesh in the tantalum powder product accounts for 60% to 80% of the total weight of the tantalum powder product.

Description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(1) Embodiments of the present invention will be described in detail below with reference to examples, but it will be understood by a person skilled in the art that the following examples are only illustrative of the present invention and should not be construed as limiting the scope of the present invention. The examples, in which specific conditions are not specified, are performed according to conventional conditions or conditions recommended by the manufacturer. The reagents or instruments, in which specific manufacturers are not specified, are all conventional products which are commercially available.

EXAMPLE 1

(2) S1) Preparation of the raw tantalum powder, including the following steps S1a)˜S1c) S1a) A kg of potassium chloride (KCl), B kg of potassium fluoride (KF), C g of potassium sulfate (K.sub.2SO.sub.4) and D g of boric acid (H.sub.3BO.sub.3) were added into a reaction vessel, the temperature was raised to E° C., and preserved with stirring for 30 minutes.

(3) S1b) F kg of potassium fluotantalate (K.sub.2TaF.sub.7) was added into the reaction vessel. After the temperature returned to G° C., metal sodium in a stoichiometric ratio (namely, a molar ratio of 1:5) was added according to the chemical reaction of K.sub.2TaF.sub.7+5Na=Ta+5NaF+2KF, and the mixture was reacted for a period of time.

(4) S1c) The step 1b) was repeated H times until the cumulative amount of the potassium fluotantalate added was I kg. After the reaction was completed, the product was discharged after it was cooled, the discharged product was crushed into small pieces by a crusher, and subjected to acid washing, water washing, drying and sieving to obtain a raw tantalum powder. The properties of the raw tantalum powder were shown in Table 2.

(5) S2) The raw tantalum powder and deionized water containing J ppm phosphoric acid were added into a water tank equipped with a stirring baffle rotating at a speed of 200 r/min. After being stirred for 30 min, a water-containing powder was obtained. The water-containing powder was dried by a vacuum drying oven at a temperature of 80° C. for a period time of 16 h, and the dried powder essentially did not contain water. The dried powder was crushed, and the crushed powder was sieved by a vibrating sieve with the mesh number of 150 mesh, no extra water being added during being sieved. The sieved powder was pre-agglomerated powder, and the bulk density-SBD of the pre-agglomerated powder was 1.0˜1.5 g/cm.sup.3. Specifics were shown in Table 2.

(6) S3) The pre-agglomerated powder was added into a crucible, the crucible was put into a vacuum heat treatment furnace, heated to K° C. under a vacuum degree of below 1.33 Pa, and the temperature was preserved for L min. The powder obtained from heat treatment was sieved with a 100 mesh screen, and the powder with a particle size of −100 mesh was collected, and thus a heat treated powder was obtained.

(7) S4) Magnesium powder was added into the heat treated powder in an amount of M wt %, then they were put into a crucible, and the crucible was put into a reaction container protected by argon, being kept at a temperature of N° C. for 3 hours.

(8) S5) After the temperature of the reaction container in the previous step decreased from N° C. to O° C., the gas in the container was evacuated until the pressure in the container was close to 0 MPa. Nitrogen (N.sub.2) was charged into the container until the pressure in the container reached 0.105 MPa, this being maintained for 4 h, after which the container was cooled to room temperature, and a passivation treatment was performed by intermittently charging oxygen-containing gas for several times, and thereby an oxygen-reduced and nitrogen-doped powder was obtained.

(9) S6) The oxygen-reduced and nitrogen-doped powder was added into a mixture of 10 wt % hydrochloric acid and 0.5 wt % hydrogen peroxide, stirred and washed for 2 h, and stood for 15 min, after which an acid containing solution was poured out, so as to remove impurities such as residual magnesium, magnesium oxide and the like in the product. The above adding deionized water—stirring for 10 min—allowing to stand for 15 min, were performed one more time, and an acid-washed powder was obtained. The acid-washed powder was put into a filter tank for filtering and washing, where deionized water was used until the electrical conductivity of the filtrate was lower than 5 μs/cm. After filtration, a water-washed powder was obtained. The water-washed powder was dried, sieved by a 100-mesh screen, and the powder of −100 mesh was collected, which was the tantalum powder product.

COMPARATIVE EXAMPLE 1

(10) The procedures of Comparative Example 1 were similar to those in Example 1 except the following step.

(11) S2) The raw tantalum powder and the deionized water containing J ppm phosphoric acid with a weight ratio of 10:1 were added into a tank equipped with a stirring baffle. The stirring baffle rotated at a speed of 300 r/min. After being stirred for 30 min, a water-containing tantalum powder was taken from the tank, then added with deionized water in an amount of 12% by weight of the tantalum powder by spraying, and the sprayed water-containing powder was sieved by a vibrating sieve, and the sieved wet powder was dried in a vacuum drying oven at a temperature of 90° C. for 18 h, and thereby a pre-agglomerated powder was obtained.

COMPARATIVE EXAMPLE 2

(12) The procedures of Comparative Example 2 were similar to those in Example 1 except the following step.

(13) S2) The raw tantalum powder and the deionized water containing J ppm phosphoric acid with a weight ratio of 10:1 were added into a tank equipped with a stirring baffle. The stirring baffle rotated at a speed of 300 r/min. After being stirred for 30 min, a water-containing tantalum powder was taken from the tank, then added with deionized water in an amount of 12% by weight of the tantalum powder by spraying, and the sprayed water-containing powder was sieved by a vibrating sieve, and the sieved wet powder was dried in a vacuum drying oven at a temperature of 90° C. for 18 h, and thereby a pre-agglomerated powder was obtained.

(14) The process parameters for preparing tantalum powders of the Examples and Comparative Examples were shown in Table 1.

(15) The chemical and physical properties of the raw tantalum powder and the pre-agglomerated powder were shown in Table 2.

(16) The chemical and physical properties of the pre-agglomerated powder and the tantalum powder product were shown in Table 3.

(17) The electrical properties of the tantalum powder product were shown in Table 4.

(18) The analysis of the impurities content of the tantalum powder was performed according to the regulation of Chinese standard GB/T15076.1-15076.15.

(19) The physical properties of tantalum powder were tested according to the regulation of industry Standard YS/T573-2007.

(20) The electrical property test of the tantalum powder comprised compressing 30 mg of tantalum powder product into a cylindrical tantalum powder compact with a diameter of 2 mm at a compression density of 5.0 g/cm.sup.3, and a tantalum wire serving as a lead wire was preset in the tantalum powder before pressing. The tantalum powder compact was subjected to vacuum sintering at 1100˜1150° C. for 10˜20 min to form an anode compact (namely a tantalum powder sintered body). The shrinkage rate of the sintered compact was within 5˜10%. The sintering conditions were shown in Table 4.

(21) The anode compact was placed in a H.sub.3PO.sub.4 solution having a concentration of 0.01 wt % and charged at a voltage of 8V˜10V to form an anode compact, with the charging voltage shown in Table 4. The specific capacitance and dielectric loss were measured at a frequency of 120 Hz with the anode compact being in a 30 wt % H.sub.2SO.sub.4 solution at room temperature, and the leakage current was measured with the anode compact being in a 0.1 wt % H.sub.3PO.sub.4 solution.

(22) The matters relating to the electrical performance test of tantalum powder which are not fully described can be seen in Chinese standards GB/T 3137-2007 Testing method for electrical properties of tantalum powder.

(23) In order to characterize the formability of high capacitance tantalum powder more intuitively, the tantalum powder was compressed into a tantalum powder compact with a size of 2.2 mm×1.0 mm×1.7 mm, and the tantalum powder compact was placed under an electronic magnifier with a magnification of 10 times and was observed to check whether apparent cracks exist on its surface. If the cracks exist on the surface, the formability of the tantalum powder is poor; otherwise, if the cracks do not exist on the surface, the formability of the tantalum powder is good.

(24) TABLE-US-00001 TABLE 1 Temper- Tem- Tem- Time of Tem- Tem- ature per- perature Heat perature perature when be- ature Re- K.sub.2TaF.sub.7 Phos- of heat pre- of of ginning to add peating total phorus treat- ser- Magnesium oxygen nitrogen KCl KF K.sub.2SO.sub.4 H.sub.3BO.sub.3 to stir K.sub.2TaF.sub.7 sodium times amount doping ment vation doping reduction doping A/kg B/kg C/g D/g E/° C. F/kg G/° C. H/time I/kg J/ppm K/° C. L/min M/wt% N/° C. O/° C. Ex 240 100 500 20 850 5 830 14 75 300 1100 20 4.8 790 300 ample 1 Ex- 200 120 550 20 850 5 830 14 45 380 1070 20 4.8 780 300 ample 2 Ex- 180 120 550 20 850 2.5 800 13 35 400 1060 20 4.9 770 410 ample 3 Ex- 180 120 550 20 850 2.5 800 9 25 400 1050 20 4.8 770 410 ample 4 Com- 240 100 500 5 850 5 830 14 75 300 1100 20 4.8 790 300 parative Ex- ample 1 Com- 200 120 550 40 850 3 830 14 45 380 1070 20 4.8 780 300 parative Ex- ample 2

(25) As shown in Table 1, for Examples 1˜4, KCl and KF were diluents in the reduction reaction process. The weight ratio of the boric acid to the diluent was 20:300000˜340000. The weight ratio of the K.sub.2SO.sub.4 to the diluent was 500˜550:300000˜340000. The weight ratio of K.sub.2TaF.sub.7 to the diluent was 2.5˜5:300˜340. In the diluent, KCl:KF=18˜24:10˜12 (weight ratio).

(26) TABLE-US-00002 TABLE 2 Physical and chemical properties of the raw tantalum powder and the pre-agglomerated powder pre- Raw tantalum powder agglomerated Chemical components (ppm) BET powder O N K Na B (m.sup.2/g) SBD (g/cm.sup.3) Example 1 14890 180 28 4 58 5.8 1.31 Example 2 22400 450 41 5 96 8.9 1.22 Example 3 34200 840 61 4 164 11.4 1.15 Example 4 39200 760 70 4 212 12.5 1.09 Com- 10120 260 26 4 8 3.89 1.76 parative Example 1 Com- 30180 380 52 3 168 9.2 1.65 parative Example 2

(27) As shown in Table 2, the raw tantalum powder and the pre-agglomerated powder of Examples 1-4 conform to the following relationships.

(28) The mathematical relationship between the O content α′ (in ppm) and the specific surface area x′ (in m.sup.2/g) of the raw tantalum powder conforms to:
α′=−68.5x′.sup.3+2197.3x′.sup.2−18616.4x′+62307.8, R.sup.2=1.

(29) The mathematical relationship between the N content β′ (in ppm) and the specific surface area x′ (in m.sup.2/g) of the raw tantalum powder conforms to:
β′=98.1x′−388.8, R.sup.2=0.9.

(30) The mathematical relationship between the B content ε′ (in ppm) and the specific surface area x′ (in m.sup.2/g) of the raw tantalum powder conforms to:
γ′=0.28x′.sup.3−4.72x′.sup.2+35.11x′−42.02, R.sup.2=1.

(31) The mathematical relationship between the K content γ′ (in ppm) and the specific surface area x′ (in m.sup.2/g) of the raw tantalum powder conforms to:
ε′=−9.39E−02x.sup.3+3.13E+00x.sup.2−2.64E+01x+9.40E+01, R.sup.2=1.

(32) The content of Na is less than 5 ppm.

(33) The mathematical relationship between the bulk density of the pre-agglomerated powder τ′ and the specific surface area x′ (in m.sup.2/g) of the raw tantalum powder conforms to: τ′=−1.39E-03x′.sup.2 −6.30E-03x′+1.39E+00, R.sup.2=0.993.

(34) TABLE-US-00003 TABLE 3 Physical and chemical properties of the tantalum powder product O con- Chemical +100 −100~+150 −150~+170 −170~+400 −400 tent/ B con- properties (ppm) BET FSSS SBD mesh mesh mesh mesh mesh N con- tent/ O N K B P m2/g um g/cm3 (%) (%) (%) (%) (%) tent BET Ex- 10280 2010 21 22 220 5 1 1.79 0 11.32 2.36 77.48 8.84 5.11 4.40 ample 1 Ex- 16500 2420 28 46 300 7.6 0.98 1.65 0 5.42 1.80 73.10 19.68 6.82 6.05 ample 2 Ex- 25700 3100 34 114 340 10.3 0.86 1.43 0 2.58 2.88 66.08 28.46 8.29 11.07 ample 3 Ex- 30210 3250 52 146 330 11.6 0.69 1.28 0 3.56 1.29 62.93 32.22 9.30 12.59 ample 4 Com- 8940 1980 19 6 240 3.74 1.2  1.91 0 38.4 5.18 48.38 8.04 4.52 1.60 parative Ex- ample 1 Com- 19980 2250 43 148 280 8.2 1.1  2.02 0 45.18 6.18 42.62 6.04 8.88 18.05 parative Ex- ample 2

(35) The tantalum powder products of Examples 1-4 shown in Table 3 comply with the following rules: the mathematical relationship between the O content α (in ppm) and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
α=−26.7x.sup.3+802.4x.sup.2−4496.9x+16038.2, R.sup.2=1; the mathematical relationship between the N content β (in ppm) and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
β=197.1x+995.0, R=1.0; the mathematical relationship between the B content γ (in ppm) and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
γ=−0.48x.sup.3+13.95x.sup.2−108.84x+277.20, R.sup.2=1; the mathematical relationship between the K content ε (in ppm) and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
ε=0.45x.sup.3−10.48x.sup.2+79.94x−173.43, R.sup.2=1; the mathematical relationship between the ratio of the O content to the N content and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
λ=0.62x+2.04, R.sup.2=1.00; the mathematical relationship between the ratio χ of the B content to the specific surface area x of the tantalum powder and the specific surface area x (in m 2/g) of the tantalum powder conforms to:
χ=−6.11E−02x.sup.3+1.63E+00x.sup.2−1.25E+01x+3.39E+01, R.sup.2=1.00; the mathematical relationship between the percentage η of powder not passing through 170 mesh in the total weight of the tantalum powder and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
η=−4.55E−02x.sup.3+1.39E+00x.sup.2−1.45E+01x+5.70E+01, R.sup.2=1.00; the mathematical relationship between the percentage ζ of the powder that can pass through 170 mesh while cannot pass through 400 mesh in the total weight of the tantalum powder and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
ζ=−0.11x.sup.2−0.40x+82.33, R=1.00; the mathematical relationship between the percentage θ of powder that can pass through 400 in the total weight of the tantalum powder meshes and the specific surface area x (in m2/g) of the tantalum powder conforms to:
θ=−0.15x.sup.2+6.01x−17.44, R.sup.2=1.00; the mathematical relationship between the bulk density τ and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
τ=−6.14E−03x.sup.2+2.53E−02x+1.82E+00, R.sup.2=1.00; the mathematical relationship between the FSSS particle size Ψ and the specific surface area x (in m.sup.2/g) of the tantalum powder conforms to:
ψ=−2.22E−03x.sup.3+4.39E−02x.sup.2−2.93E−01x+1.64E+00, R.sup.2=1.00.

(36) TABLE-US-00004 TABLE 4 Electrical properties of the tantalum powder product Sintering Charing Specific Leakage Di- condition voltage capacitance current electric Form- ° C./min V μFV/g nA/μFV loss % ability Example 1 1150/20 10 214232 0.32  53.6 Good Example 2 1100/20 10 321424 0.42  32.3 Good Example 3 1100/10  8 405633 0.79  60.8 Good Example 4 1100/10  8 561306 2.8   57.9 Good Com- 1150/20 10 168920 0.42 106.2 Crack- parative ed Example 1 Com- 1150/20 10 264012 4.9  149.2 Crack- parative ed Example 2

(37) As shown in Table 4:

(38) the relationship between the leakage current μ (nA/μFV) and the specific capacitance z (μFV/g) for the tantalum powders of Examples 1 to 4 conforms to:
μ=5.02E−17z.sup.3−2.92E−11z.sup.2+5.61E−06z−3.72E−02, R.sup.2=1.00.

(39) As shown in Table 4, the tantalum powders of Examples 1˜4 have the following electrical properties.

(40) (1a) For the tantalum powder with the specific capacitance of 20000˜35000 μFV/g, the leakage current is 0.32˜0.42 nA/μFV.

(41) (1b) For the tantalum powder with the specific capacitance of 40000˜60000 μFV/g, the leakage current is 0.79˜2.8 nA/μFV.

(42) (2) The dielectric loss tan δ of the tantalum powder is about 30˜60%.

(43) (3) The tantalum powder compact has good formability, which indicates that the tantalum powder compact is smooth and has no cracks.

(44) In contrast, the tantalum powders of Comparative Examples 1˜2 have the following electrical properties:

(45) (1a) For the tantalum powder with the specific capacitance of 15000˜17000 μFV/g, the leakage current is 0.42 nA/μFV.

(46) (1b) For the tantalum powder with the specific capacitance of 20000˜35000 μFV/g, the leakage current is 4.9 nA/μFV.

(47) (2) The dielectric loss tan δ of the tantalum powder is 100˜200%

(48) (3) Cracks exist on the surface of the tantalum powder compact.

(49) It is clear from the results of the examples that the tantalum powder of the above examples has one or more of the following advantages: (1) the specific capacitance is high; (2) the leakage current is low; (3) the dielectric loss is small; (4) good formability.

(50) Without being limited by theory, by adjusting the amount of boric acid added to the reaction system during the reduction process together with subsequent treatments to control the content of boron added in the tantalum powder within a certain range, combined with technologies of pre-agglomeration and heat treatment, the above examples prepare tantalum powders having high specific capacitance, low leakage current and good formability.

(51) Without being limited by theory, by virtue of adjusting the composition (for example, boron content) and the structure (for example, particle size distribution) of the tantalum powder in one or more of the steps, the disclosed method prepares a tantalum powder having high specific capacitance, low leakage current, and good formability.

(52) Although specific embodiments of the invention have been described in detail, those skilled in the art will appreciate, various modifications and substitutions of those details may be made in light of the overall teachings of the disclosure, and such variations are within the scope of the invention. The full scope of the invention is given by the appended claims and any equivalents thereof.