Method for preparing tantalum powder of capacitor grade with high nitrogen content, tantalum powder of capacitor grade prepared thereby, and anode and capacitor prepared from tantalum powder

Abstract

A method for preparing a tantalum power of capacitor grade, comprising: solid tantalum nitride is added when potassium fluotantalate is reduced by sodium. The method increases the nitrogen content in the tantalum powder, and at the same time improves the electrical performance of the tantalum powder. The specific capacitance is increased, and the leakage current and loss is improved. The qualification rate of the anode and the capacitor product is also improved. The method is characterized in that the nitrogen in the tantalum nitride diffuses between the particles of the tantalum powder, with substantially no loss, and thus the nitrogen content is accurate and controllable.

Claims

1. A method for preparing tantalum powder of capacitor grade, comprising: (1) feeding KCl and KF into a reactor, and increasing a reaction temperature; (2) feeding K.sub.2TaF.sub.7 to the reactor, and simultaneously feeding tantalum nitride powder to the reactor depending on desired amount of doped nitrogen; (3) heating the reactor to 880 to 930° C., and maintaining the reaction temperature; (4) cooling the reactor to 800 to 880° C., and feeding sodium to the reactor; (5) maintaining the reaction temperature at 880 to 930° C. until the reaction temperature drops rapidly; and (6) after-treating the tantalum powder resulted from step (5), and obtaining nitrogen-doped tantalum powder product.

2. The method according to claim 1, wherein the tantalum powder for preparing the tantalum nitride powder which is fed in step (2) has the same mean particle diameter as the tantalum powder product resulted from step (6).

3. The method according to claim 1, wherein the amount of doped nitrogen is 1000 to 3000 ppm.

4. The method according to claim 1, wherein the weight ratio between the sodium which is fed in step (4) and the K.sub.2TaF.sub.7 which is fed in step (2) is in a range of 30 to 32:100.

5. The method according to claim 1, wherein step (2) is performed at 900 to 950° C.

6. The method according to claim 1, wherein steps (3) to (5) are performed under agitation.

7. The method according to claim 6, wherein agitator blades are used, and the agitator blade are elevated in step (4).

8. The method according to claim 1, wherein the time for keeping the temperature in step (3) is at least 1 hour.

9. The method according to claim 1, wherein the after-treatment in step (6) includes crushing, water washing, acid washing, pelletizing or a combination thereof.

10. Tantalum powder prepared by the method according to claim 1, wherein the nitrogen content in the tantalum powder is uniform.

11. A tantalum anode made of the tantalum powder according to claim 10.

12. A tantalum capacitor comprising the tantalum anode according to claim 11.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a curve graph in which the electrical performance of the products of the Examples according to the present invention and the Comparative Example change with nitrogen content.

(2) In the FIGURE, DCL represents leakage current K×10.sup.−4 (μA/μFV), and LOT represents series numbers of the samples.

DETAILED DESCRIPTION

(3) The present invention provides a method for doping nitrogen in tantalum powder, comprising the steps of:

(4) Firstly, KCl and KF are charged into a reducing furnace, and temperature is increased according to predetermined program. When the temperature reaches 900 to 950° C., K.sub.2TaF.sub.7 is added through feeding inlet, and tantalum nitride powder made of tantalum powder of the same grade is added simultaneously. At this time the temperature is cooled to about 800° C. Agitation is started after the charging, and the reducing furnace is heated to 880 to 930° C. The temperature is kept, and the time for keeping temperature is recorded. After a period of time, the agitator blade is elevated, and the agitation continues, leading to uniform temperature and composition of the molten salt system. The temperature is decreased to 800 to 880° C., and sodium is added smoothly. Agitation is kept as the sodium is added, to timely spread the resulting heat and keep uniform temperature of the whole molten salt system, and at the same time to timely transfer particles of the formed tantalum powder outside of the reaction zone and avoid rapid growth of the particles which results in poor uniformity. The temperature is kept at a relatively stable level in a range of 880 to 930° C. The weight ratio between sodium and K.sub.2TaF.sub.7 added in the reduction reaction is in a range of 30 to 31:100. When the reaction temperature drops rapidly, it can be determined that the reduction reaction is over. Raw tantalum powder with uniformly doped nitrogen is obtained.

(5) During the above reaction, the raw tantalum powder is collided and contacted with the added tantalum nitride powder, allowing the nitrogen in the tantalum nitride to spread between the particles, resulting in primary particles of the tantalum powder with high nitrogen content and relatively uniform doping.

(6) The tantalum powder with relatively uniform nitrogen content agglomerates, resulting in a bulk material. The material is subject to steps including crushing, water washing, acid washing, pelletizing etc., resulting in tantalum powder product with high nitrogen content.

(7) The tantalum powder with high nitrogen content can be used to produce tantalum anode and tantalum capacitor by the methods in prior art.

(8) In this context, the term “tantalum powder of the same grade” is referred that the tantalum powder for preparing the tantalum nitride powder which is added in the reaction has essentially the same mean particle diameter as the tantalum powder product, and thus the final tantalum powder has uniform particle size.

EXAMPLES

(9) In order to further illustrate the present invention, embodiments according to the present invention are described with reference to the Examples. However, it is understood that the description is for further illustration of the characteristics and advantages of the present invention, rather than limitation to the scope of the claims of the present application.

(10) The Fisher particle diameter referred in the Examples the particle diameter measured with Fisher Sub-sieve sizer, also known as Fisher transmitter. The specific surface area of the particles is obtained according to the height difference (h) between the liquid levels in the two tubes of differential pressure gauge that is caused by the pressure difference generated by the atmosphere passing through the bed of powder. And then he mean particle size is calculated according to the equation: the mean particle size (in micron)=6000/volume specific surface area (in square centimeter/gram).

Example 1

(11) 120 kg of KCl and 100 kg of KF were charged into a reducing furnace, and temperature was increased according to predetermined program. When the temperature reached 920° C., 60 kg of K.sub.2TaF.sub.7 was added through feeding inlet, and 1500 g of tantalum nitride powder made of raw tantalum powder of Fisher particle diameter in a range of 0.3 to 0.45 μm was added simultaneously as seed crystal. After the charging, the temperature was elevated to 930° C., and the time for keeping temperature was recorded. After holding 1 hour at 930° C., the agitator blade was elevated, and the agitation continued. The temperature was decreased to 820° C. with a blower, and sodium was added smoothly. Agitation was kept as the sodium was added. The temperature was kept at about 900° C. The amount of sodium added in the reduction reaction was 18.5 kg. When the reaction temperature dropped rapidly, it was determined that the reduction reaction was over and the reduced material was obtained. Then the reduced material was subject to steps including crushing, water washing, acid washing, pelletizing etc., resulting in nitrogen-doped tantalum powder product.

Example 2

(12) 120 kg of KCl and 100 kg of KF were charged into a reducing furnace, and temperature was increased according to predetermined program. When the temperature reached 930° C., 60 kg of K.sub.2TaF.sub.7 was added through feeding inlet, and 1000 g of tantalum nitride powder made of raw tantalum powder of Fisher particle diameter in a range of 0.3 to 0.45 μm was added simultaneously as seed crystal. After the charging, the temperature was elevated to 930° C., and the time for keeping temperature was recorded. After holding 1 hour at 930° C., the agitator blade was elevated, and the agitation continued. The temperature was decreased to 820° C. with a blower, and sodium was added smoothly. Agitation was kept as the sodium was added. The temperature was kept at about 900° C. The amount of sodium added in the reduction reaction was 18.5 kg. When the reaction temperature dropped rapidly, it was determined that the reduction reaction was over and the reduced material was obtained. Then the reduced material was subject to steps including crushing, water washing, acid washing, pelletizing etc., resulting in nitrogen-doped tantalum powder product.

Example 3

(13) 120 kg of KCl and 100 kg of KF were charged into a reducing furnace, and temperature was increased according to predetermined program. When the temperature reached 930° C., 60 kg of K.sub.2TaF.sub.7 was added through feeding inlet, and 800 g of tantalum nitride powder made of raw tantalum powder of Fisher particle diameter in a range of 0.3 to 0.45 μm was added simultaneously as seed crystal. After the charging, the temperature was elevated to 930° C., and the time for keeping temperature was recorded. After holding 1 hour at 930° C., the agitator blade was elevated, and the agitation continued. The temperature was decreased to 820° C. with a blower, and sodium was added smoothly. Agitation was kept as the sodium was added. The temperature was kept at about 900° C. The amount of sodium added in the reduction reaction was 18.5 kg. When the reaction temperature dropped rapidly, it was determined that the reduction reaction was over and the reduced material was obtained. Then the reduced material was subject to steps including crushing, water washing, acid washing, pelletizing etc., resulting in nitrogen-doped tantalum powder product.

Example 4

(14) 120 kg of KCl and 100 kg of KF were charged into a reducing furnace, and temperature was increased according to predetermined program. When the temperature reached 910° C., 60 kg of K.sub.2TaF.sub.7 was added through feeding inlet, and 600 g of tantalum nitride powder made of raw tantalum powder of Fisher particle diameter in a range of 0.3 to 0.45 μm was added simultaneously as seed crystal. After the charging, the temperature was elevated to 930° C., and the time for keeping temperature was recorded. After holding 1 hour at 930° C., the agitator blade was elevated, and the agitation continued. The temperature was decreased to 820° C. with a blower, and sodium was added smoothly. Agitation was kept as the sodium was added. The temperature was kept at about 900° C. The amount of sodium added in the reduction reaction was 18.5 kg. When the reaction temperature dropped rapidly, it was determined that the reduction reaction was over and the reduced material was obtained. Then the reduced material was subject to steps including crushing, water washing, acid washing, pelletizing etc., resulting in nitrogen-doped tantalum powder product.

Example 5

(15) 120 kg of KCl and 100 kg of KF were charged into a reducing furnace, and temperature was increased according to predetermined program. When the temperature reached 900° C., 60 kg of K.sub.2TaF.sub.7 was added through feeding inlet, and 300 g of tantalum nitride powder made of raw tantalum powder of Fisher particle diameter in a range of 0.3 to 0.45 μm was added simultaneously as seed crystal. After the charging, the temperature was elevated to 930° C., and the time for keeping temperature was recorded. After holding 1 hour at 930° C., the agitator blade was elevated, and the agitation continued. The temperature was decreased to 820° C. with a blower, and sodium was added smoothly. Agitation was kept as the sodium was added. The temperature was kept at about 900° C. The amount of sodium added in the reduction reaction was 18.5 kg. When the reaction temperature dropped rapidly, it was determined that the reduction reaction was over and the reduced material was obtained. Then the reduced material was subject to steps including crushing, water washing, acid washing, pelletizing etc., resulting in nitrogen-doped tantalum powder product.

Comparative Example 6

(16) 120 kg of KCl and 100 kg of KF were charged into a reducing furnace, and temperature was increased according to predetermined program. When the temperature reached 930° C., 60 kg of K.sub.2TaF.sub.7 was added through feeding inlet. After the charging, the temperature was elevated to 930° C., and the time for keeping temperature was recorded. After holding 1 hour at 930° C., the agitator blade was elevated, and the agitation continued. The temperature was decreased to 820° C. with a blower, and sodium was added smoothly. Agitation was kept as the sodium was added. The temperature was kept at about 900° C. The amount of sodium added in the reduction reaction was 18.5 kg. When the reaction temperature dropped rapidly, it was determined that the reduction reaction was over and the reduced material was obtained. Then the reduced material was subject to steps including crushing, water washing, acid washing, pelletizing etc., resulting in a product which we needed.

(17) Nitrogen was not purposely doped in this Comparative Example. The nitrogen in the sample was brought in when the tantalum powder formed oxide film in the air.

(18) The resulting six samples were analyzed. The comparison results of the properties are shown in Table 1.

(19) TABLE-US-00001 TABLE 1 Element contents in the tantalum powder (in ppm) Smaple O C N Fe Si P K Ex. 1 3260 28 3420 10 12 130 30 Ex. 2 3780 35 2300 12 15 120 30 Ex. 3 3920 36 1820 15 16 130 32 Ex. 4 4030 30 1400 13 14 120 33 Ex. 5 4150 28 810 16 12 130 34 Com. Ex. 6 4220 28 520 12 14 126 31

(20) All of the test methods of the element in the tantalum powder are according to Chinese national standards such as GB/T 15076.8-2008, GB/T 15076.9-2008, GB/T 15076.12-2008, GB/T 15076.14-2008, GB/T 15076.15-2008, GB/T 15076.16-2008, and Chemical Analytic Technique for Tantalum and Niobium.

(21) As seen from Table 1, the amounts of doped nitrogen in Examples 1 to 5 are higher than that in Comparative Example 6. It means that nitrogen-doping effect through tantalum nitride powder is better than that of traditional nitrogen-doping method. Tantalum powder with high nitrogen content can be obtained by the method according to the present invention.

(22) TABLE-US-00002 TABLE 2 Physical Properties of the Tantalum Powder Smaple Fsss (μm) SBD (g/cc) +80 (%) −400 (%) Ex. 1 1.60 1.50 0.10 30.16 Ex. 2 1.54 1.52 0.02 36.80 Ex. 3 1.56 1.50 0.06 25.96 Ex. 4 1.50 1.45 0.00 28.92 Ex. 5 1.64 1.56 0.10 30.52 Com. Ex. 6 1.62 1.52 0.12 28.68.

(23) In Table 2:

(24) FSSS represents Fisher particle diameter of tantalum particles.

(25) SBD represents apparent density of powder, referring to tap density measured after the powder freely fills a standard vessel under specified conditions, i.e. mass of the powder per unit volume when packed loosely, expressed in g/cm.sup.3. It is a method property of powder. The measuring method used herein is funnel method in which powder freely drops from a funnel hole at a certain height to fill a vessel.

(26) +80(%) represents the proportion of particles larger than 80 mesh in all particles, and −400(%) represents the proportion of particles smaller than 400 mesh. The mesh refers to mesh number per inch (25.4 mm) on a screen.

(27) TABLE-US-00003 TABLE 3 Comparison of Electrical Performance (sintering condition: 1250° C./20 min, Vf: 20 V, pressed density: 5.0 g/cc) I K × 10.sup.−4 CV tgδ SHV Sample (μA/g) (μA/μFV) (μFV/g) (%) (%) Ex. 1 35.0 3.6 97815 45.7 1.8 Ex. 2 22.0 2.2 101520 40.7 1.2 Ex. 3 24.4 2.4 100108 41.6 1.5 Ex. 4 34.0 3.5 98763 44.0 1.9 Ex. 5 36.0 3.7 96600 43.1 1.3 Com. Ex. 6 44.5 4.7 95060 40.5 2.1

(28) All of the test method and device of the electrical performance of tantalum powder are according to Chinese national standards GB/T 3137-2007, Experiment Technique for Electrical Performance of Tantalum Powder.

(29) In Table 3:

(30) Sintering condition: 1250° C./20 min means that the tantalum powder is sintered at 1250° C. for 20 minutes to obtain anode block.

(31) Vf: 20V means energization at voltage of 20V.

(32) Pressed density: 5.0 g/cc means that the pressed density of the anode block is 5.0 g/cc.

(33) K×10.sup.−4 (μA/μV) represents leakage current, hereinafter referred to as K value. Capacitance media cannot be absolutely non-conducting, so when direct voltage is applied to capacitance, the capacitor may generate leakage current. If the leakage current is too high, the capacitor may heat up and fail. When specified direct working voltage is applied to the capacitor, it will be observed that the change of charging current is initially great, and decreases over time, and reaches a relatively stable state until a final value. This final value is called as leakage current.

(34) CV (μFV/g) represents specific capacity, i.e. electric quantity that can be released by unit weight of cell or active substrate.

(35) tgδ (%) represent capacitor loss. Capacitor loss is actually reactive power consumed by a capacitor. Thus it can be defined as that capacitor loss also refers to the ratio between reactive power consumed under electric field and total consumed power, expressed as: tangent of capacitor loss angle=reactive power/total power, or tangent of capacitor loss angle=reactive power×100/total power (resulting value is a percentage)

(36) SHV (%) represents volume shrinkage of capacitor anode block.

(37) TABLE-US-00004 TABLE 4 Comparison of Electrical Performance (sintering condition: 1300° C./20 min Vf: 20 V, pressed density: 5.0 g/cc) I K × 10.sup.−4 CV tgδ SHV Sample (μA/g) (μA/μFV) (μFV/g) (%) (%) Ex. 1 33.0 3.5 93500 43.0 2.7 Ex. 2 20.0 2.1 96921 38.6 1.6 Ex. 3 28.6 3.0 95169 40.1 1.8 Ex. 4 33.1 3.4 96096 42.8 2.1 Ex. 5 33.2 3.4 96491 43.0 2.2 Com. Ex. 6 33.0 3.5 92500 45.0 2.7

(38) The data in the above tables (especially in Table 3) show that with regard tantalum powder of capacitor grade, specific capacitance (CV value) of the tantalum powder increases, leakage current (K value) decreases, and loss (tgδ%) decreases as the amount of doped nitrogen increases. However, when nitrogen content is more than 3000 ppm (Example 1), specific capacitance (CV value) of the tantalum powder decreases, leakage current (K value) and loss (tgδ%) begin to increase, and the electrical performance degrades.

(39) If the amount of doped nitrogen is relatively low (Example 5), there is problems such as too low specific capacitance (CV value) of the tantalum powder, too high leakage current (K value) and loss (tgδ%), which is not preferred.

(40) FIG. 1 shows a curve graph in which K values of the products of Examples 1 to 5 and Comparative Example 6 change with nitrogen content.

(41) Therefore, with regard to the tantalum powder with high specific capacitance according to the present invention, when nitrogen content is controlled in a range of 1000 to 3000 ppm, leakage current of the sample is relatively low, and electrical performance of the tantalum powder is preferred.

(42) The present invention enable effective control of nitrogen content in tantalum powder by adding TaN with high nitrogen content as seed crystal during the reduction. Also, the tantalum prepared by this method has uniform nitrogen content and relatively small particle diameter of primary particles. The greatest characteristic of this method is that the nitrogen in tantalum nitride diffuses between particles of the tantalum powder, with substantially no loss, and thus the nitrogen content can be accurately controlled.

(43) The description and the Examples according to the present invention disclosed herein are illustrative. It is apparent to a person skilled in the art that the present invention includes other more embodiments, and the actual scope and spirit of the present invention is defined by the claims.