- Email: [email protected]
Combustion synthesis-derived tantalum powder for solid-electrolyte capacitors
Journal of Alloys and Compounds 478 (2009) 716–720
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Combustion synthesis-derived tantalum powder for solid-electrolyte capacitors H.I. Won ∗ , H.H. Nersisyan, C.W. Won Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Yuseong, Daejeon 305-764, South Korea
a r t i c l e
i n f o
Article history: Received 6 August 2008 Received in revised form 26 November 2008 Accepted 28 November 2008 Available online 6 December 2008 Keywords: Ta SHS Capacitor
a b s t r a c t In this paper, the synthesis of capacitor-grade tantalum (Ta) powder via the self-propagating hightemperature synthesis (SHS) method is described. In addition, the sintering aspects and electrical characteristics of the powder are discussed. Ta powder was prepared via the combustion of a Ta2 O5 –xMg–kNaCl mixture, under argon pressure. The morphology and size of the final powder particles was controlled by adjusting the Mg–NaCl concentration. The final powder particles had nodular shapes and sizes ranging from 0.1 to 0.5 m. A leakage current of a sintered Ta sample containing the smallest particle size was 10 A; its capacitance was 92,738 CV, when a 40-V voltage was applied to the sample. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Tantalum (Ta) is a reactive metal with a high-melting point, highcorrosion resistance, and great ductility. This metal has been used in many industrial fields, such as the chemical, medical, machinery, and electronic industries [1,2]. In the electronic industry, particularly, Ta is widely used to produce Ta electrolyte capacitors, which provide the highest capacitance (CV) per unit volume and are more thermally stable than other capacitors [3]. The Hunter process is the conventional method widely used to produce Ta powder [4,5]; it uses the metallothermic reduction (MR) of K2 TaF7 in molten salt. Control of the morphology and size of Ta powder particles in the metallothermic reduction of K2 TaF7 process can be achieved by varying both the injection and stirring speeds of the molten raw materials. However, in the MR process for obtaining Ta powder in a batch-type operation, it is difficult to control morphology and the location of Ta deposits. An MR–EMR (electronically mediated reaction) combination process [6,7] was developed to overcome these challenges; in that process, the total charge passed through the external circuit and the average particle size was controlled by changing the reduction temperature. Even so, the Ta yield can be improved only from 65% to 74%. A new and promising synthesis route for Ta powder that was recently developed by researchers from the H.C. Stark Company [8] involves the direct reduction of Ta2 O5 by controlling magnesium (Mg) vapor. In the Ta2 O5 –Mg reduction process, Mg was vaporized by heating it in a container; the vaporized Mg directly reached the
∗ Corresponding author. Tel.: +82 42 821 6587; fax: +82 42 822 9401. E-mail address: [email protected] (H.I. Won). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.11.140
Ta2 O5 , converting it to metallic Ta powder. This process has many advantages, in comparison with MR, including the absence of both a liquid-phase and a contamination risk from the container, as well as the relatively easy control of the morphology and size of the Ta powder particles. However, the filtration process of evaporating Mg into a Ta2 O5 bed is not easy to control and can result in homogeneous product formation. Recently, a method called MSA-CS (molten salt-assisted combustion synthesis), was reported as an attractive synthesis route to overcome the filtration problems inherent in evaporating Mg into Ta2 O5 [9]. In the MSA-CS method, a Ta2 O5 + Mg mixture diluted with NaCl is combusted under argon (Ar) gas pressure to produce well-dispersed and ultra-fine Ta powder. The yield of Ta powder produced by MSA-CS method is >90%. In this study, the synthesis of capacitor-grade Ta powder via the SHS method is described; the sintering and electrical characteristics are also analyzed, to demonstrate prospects for future application. 2. Experimental process 2.1. Powder synthesis Ta penta-oxide (Ta2 O5 ), Mg, and sodium chloride (NaCl) were used as raw materials. The characteristics of the initial powders are listed in Table 1. The green mixture was prepared with a ball-mill containing zirconia balls, for 24 h. The mixed powder was put into a steel glass and gently compacted by hand. The green pellet was 4.0–5.0 cm in diameter and 4–5 cm in height, and it was placed in a reaction chamber under an Ar atmosphere of 2.5 MPa. The combustion reaction was ignited by heating the top surface of the pellet with an electric arc for 1–2 s. Two preliminary tungsten–rhenium thermocouples (W/Re-5 vs. W/Re-20) were inserted into the reaction sample, to record temperature profiles and calculate combustion temperature (Tc ).
H.I. Won et al. / Journal of Alloys and Compounds 478 (2009) 716–720
717
Fig. 1. Combustion temperature of (a) Ta2 O5 –7Mg–kNaCl and (b) Ta2 O5 –xMg–2.5NaCl.
Fig. 2. Microstructure of tantalum powders: (a) zone I (Ta2 O5 –7Mg–1.5NaCl), (b) zone II (Ta2 O5 –7Mg–3.5NaCl), (c) zone I (Ta2 O5 –6Mg–2.5NaCl), (d) zone II (Ta2 O5 –9Mg–2.5NaCl), and (e) zone boundary (Ta2 O5 –7.5Mg–2.5NaCl).
718
H.I. Won et al. / Journal of Alloys and Compounds 478 (2009) 716–720
Table 1 The characteristics of raw materials.
3. Results and discussion
Reactant
Manufacturer
Particle size (m)
Purity (%)
Tantalum penta-oxide Magnesium
Alfa Aesar, USA Daejung Chemical and Metals, Korea Samchun Pure Chemical, Korea
<1 100–200
99.5 99
<50
99.5
Sodium chloride
The combusted product was washed with distilled water; it was then treated with sulfuric acid and hydrochloric acid, to eliminate NaCl and MgO. After removing the NaCl and MgO, the Ta powder was dried at 300 ◦ C in vacuo. Additional de-oxidation of the prepared Ta powder was carried out at 1100–1200 ◦ C for 2 h. The final products were characterized using Cu K␣, a Siemens X-ray diffractometer (D-76181, karlsruhe, Germany), field emission scanning electron microscopy (FESEM; JSM 6330F JEOL Ltd., Tokyo, Japan), an Inductively Coupled Plasma Optical emission spectrometer, a carbon–sulfur determinator (ELTRA CS-800), and a nitrogen–oxygen determinator (ELTRA ON-900). 2.2. Ta preparation and its characteristics The final Ta powder (0.15 g) was pressed into a cylindrical pellet (3.23 mm diameter) with Ta wire (0.23 mm diameter). The green density of the pellet was 5.0 g/CC. The pellet was first sintered at 1350 ◦ C for 30 min in vacuo (<1.0 × 10−6 Torr) and then treated with a 0.1-vol% of H3 PO4 solution at 60 ◦ C (40 mA/g and 15–40 V were applied to the pellets), to form a Ta oxide film on the particle surface. The CV, dissipation factor (DF), and leakage current (LC) were measured with an LCR meter (MIT 9216A) and digital multimeter (Keithley, 2000 61/2). To measure CV and DF (tan ı), the pellet was immersed for 60 s at 25 ◦ C in a 10-vol% of H2 SO4 solution. The applied frequency of alternative current and voltage was 120 Hz and 1.0 V, respectively. The LC was measured in a 10-vol% of H3 PO4 at 25 ◦ C, and 28.0 V and 50 mA were applied for the LC. The average equivalent serial resistance (ESR) for the three pellets was measured, and tan ı was deduced from tan ı = 2fc (ESR) where f and c are frequency in Hz and capacity in F, respectively.
3.1. Chemical and morphological analysis of SHS Ta powder In a former study of MSA-CS (molten salt assisted combustion synthesis) [9], ultra-fine Ta powder with a round particle shape was produced from a Ta2 O5 + 5Mg + kNaCl mixture. However, because the round shape of the Ta particles has a low specific surface that makes it unsuitable for capacitor applications, it is necessary to control Ta particle morphology. To address this problem, the use of excess Mg was investigated for nodular morphology in Ta particle formation. Fig. 1 shows the combustion temperature dependence of Mg–NaCl concentrations. In the present study, 2.5 MPa of Ar pressure was applied to increase the effect of the gas phase, and Mg was reduced by preventing the emission of evaporated Mg from the sample during combustion. A drop-line of combustion temperature in the system of both Ta2 O5 –xMg–2.5NaCl and Ta2 O5 –7Mg–kNaCl system had a horizontal part that was supposedly created by the NaCl phase-change from gas to liquid, in the range of k = 1–2 and x = 6–7. As seen in Fig. 1a and b, that range can be divided into two zones, according to the morphology of the final products: roundshaped Ta particles were found in zone II, and irregular-shaped Ta particles – the latter of which are favorable for making a spongeform sintered body – was found in zone I. In zone I, because of the high-combustion temperature above the boiling temperature of Mg (assumed at 1300 ◦ C), a sufficient amount of evaporated Mg participated in the gas-phase reduction of Ta2 O5 , resulting in the formation of irregular particle shapes. Here, the Mg boiling point was lowered by using high-pressure Ar in the chamber. The microstructures of the final Ta powders synthesized from zone I, zone II, and the zone boundary are listed in Fig. 2. From that figure, one can see that the proportion of round-shaped particles decreased when the combustion temperature changed from zone I
Fig. 3. Microstructure of tantalum powders synthesized from the mixture of Ta2 O5 –7Mg–kNaCl: (a) 0.5, (b) 1.5, (c) 2.5, and (d) 3.5.
H.I. Won et al. / Journal of Alloys and Compounds 478 (2009) 716–720
Fig. 4. The distribution of granulated tantalum powders.
719
Fig. 6. The leakage current data of tantalum powders sintered at 1350 ◦ C.
to zone II. Therefore, it is clear that an excess of Mg influences the morphology of Ta particles, making it irregular and coral-like. Fig. 3 provides micrographs obtained by scanning electron microscopy (SEM), to show the morphological changes in the Ta powder from the Ta2 O5 –7Mg–kNaCl mixture. As seen, four different sizes of Ta powder particles were synthesized from different concentrations of NaCl (k = 0.5, 1.5, 2.5, and 3.5). Here, in Fig. 3d, the powder from a large concentration of NaCl (3.5) contains many round-shaped particles, due to the involvement of a low combustion temperature. It has been clearly demonstrated that the SHS process can control the particle size of Ta powder by controlling the NaCl concentration; it also has the ability to produce sub-micrometer-sized powder particles. The overall combustion process can be presented as shown: Ta2 O5 + (5 + m)Mg + kNaCl → 2Ta + 5MgO + kNaCl + mMg where m and k are the mole concentrations of excess Mg and NaCl, respectively. Note that the shape and the size of the Ta particles can be varied by changing the values of m and k. Table 2 lists the general values of impurity levels of different-size Ta powder particles prepared with a given technique. In general, the
impurity levels are low, with the exceptions of carbon and sulfur. However, the levels of these impurities are within acceptable limits (i.e., carbon and sulfur content in capacitor-grade Ta powder can vary from 30 to 70 parts per million [ppm]). Other powder features to be taken into account are the flowability and the size distribution of the clusters. The flowability of Ta powder is important in manufacture capacitors in large-scale production centers, because the powder should completely fill every crevice of an elaborate mold when a certain quantity of Ta powder is dropped into a mold and pressed into a pellet. In addition, the flowability can decide the quantity of powder, which can in turn affect the density of sintered pellets. In terms of the size distribution of clusters, a powder with a broad particle-size distribution containing a high-percentage of fine particles will not flow and will have low crushing strength. Comparable data regarding the particle size distribution and SEM of granulated Ta powder are shown in Figs. 4 and 5. The results show that the commercial powder had a very broad distribution and a number of small particles that were smaller than 20 m, and the particles were loosely agglomerated in the cluster. On the other hand, the powder synthesized by the SHS process had a narrow
Table 2 Chemical analysis of tantalum powder (ppm). Sample
Ta-2 Ta-3 Ta-4
Impurity (ppm)
Flow-ability (s/20 g)
Mg
Ca
Ni
Fe
S
C
K
Na
O
<10 <10 <10
<10 <10 <10
12 12 14
<10 <10 <10
50 44 49
68 62 72
<10 <10 <10
<10 <10 <10
2870 2520 2350
Fig. 5. Micrograph of granulated powders: (a) commercial powder and (b) SHS powder.
33 32 34
720
H.I. Won et al. / Journal of Alloys and Compounds 478 (2009) 716–720
Table 3 The capacitance (mF/g) and dielectric loss (%) of tantalum powders. Sample Ta-1 Ta-2 Ta-3 Ta-4
15 V 65,124 81,567 69,851 48,581
30 V 73,784 89,986 75,235 53,290
40 V 75,482 92,738 80,309 54,710
tan ı 33 32 34 34
distribution and was well condensed in the cluster; that powder (Ta-2) therefore showed greater flowability (Table 2). 3.2. Electrical analysis of SHS Ta powder As mentioned above, it is very important to ensure that Ta powder comprises irregular-shaped particles, because irregular shapes – which can have a larger surface area than round ones – leads to higher CV values. This irregular shape was provided by an excess of Mg powder incorporated into the initial mixture. The CV and dielectric loss (tan ı) values of the Ta powders are shown in Table 3. Four different particle-size powders (Ta2 O5 –7Mg–kNaCl; k = 0.5 (Ta-4), 1.5 (Ta-3), 2.5 (Ta-2), and 3.5 (Ta-1)) were made into pellets; different voltages (15, 30, and 40 V) were then applied to the pellets, to measure CV per unit of mass. Dielectric loss was similar across all samples, while the CV of each sample generally increased with a decrease in particle size (i.e., from Ta-4 to Ta-2). However, Ta-1 showed a low CV value, in spite of its small particle size; the low CV of the Ta-1 powder can be explained by its plentiful number of round-shaped particles. Fig. 6 illustrates the tendency for LC to decrease as a function of time; each Ta sample showed a similar tendency. The final LC value for each Ta sample was <10 A. A crystalline Ta2 O5 layer had partially formed when the Ta powder contained impurities such as oxygen, carbon, and iron; this crystalline layer caused an increase in
LC [10,11]. This is because the current “focuses” on the contaminants during anodizing, and so higher oxygen and carbon concentrations lead to a higher LC. Therefore, it can be assumed that the amorphous Ta2 O5 layer in each sample was uniformly created during anodizing, and this result corresponds with those of chemical analyses. 4. Conclusion Ta powder with uniform coral-like shape was synthesized with the SHS process, using a Ta2 O5 –(5 + m)Mg–kNaCl mixture. A narrow size distribution among clusters and high-flowability of Ta powder were obtained with different concentrations of NaCl and an excess of Mg. The LC of each Ta sample was <10 A, and the CV of the Ta powder having the smallest particle size was 92,738 CV when 40 V of voltage was applied. Ta capacitors prepared with the SHS process are very promising for electronics, due to the higher CV and low LC values involved. The SHS process demonstrated in this study is highly efficient and therefore suitable for large-scale production. References [1] G. Winter, International Symposium on Tantalum and Niobium, Goslar, Germany, September 1995, pp. 485–515. [2] S.H. Yoo, T.S. Sudarshan, K. Setheram, G. Subhash, R.J. Dowding, Nanostruct. Mater. 12 (1999) 23–28. [3] J.F. Muller, P.M. Dinh, 2nd International Conference on Tungsten and Refractory Metals, McLean, USA, 1994, pp. 573–586. [4] M.A. Hunter, J. Met. 5 (1953) 130–132. [5] E.G. Hellier, G.L. Martin, US Patent 2,950,185 (1960). [6] I. Park, T.H. Okabe, Y. Waseda, J. Alloys Compd. 280 (1998) 265–272. [7] J. Yoon, I.S. Bae, H.H. Park, S. Goto, B. Kim, J. Jpn. Inst. Met. 68 (12) (2004) 1031–1038. [8] L.N. Shekhter, T.B. Tripp, L.L. Lanin, US Patent 6,171,363 (2001). [9] H.H. Nersisyan, J.H. Lee, S.I. Lee, C.W. Won, Combust. Flame 135 (2003) 539–545. [10] T. Izumi, Mater. Jpn. 33 (1) (1994) 78–80. [11] R.H. Lorenz, A.B. Michael, Electrochem. Technol. 2 (5/6) (1964).
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Combustion synthesis-derived tantalum powder for solid-electrolyte capacitors H.I. Won ∗ , H.H. Nersisyan, C.W. Won Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Yuseong, Daejeon 305-764, South Korea
a r t i c l e
i n f o
Article history: Received 6 August 2008 Received in revised form 26 November 2008 Accepted 28 November 2008 Available online 6 December 2008 Keywords: Ta SHS Capacitor
a b s t r a c t In this paper, the synthesis of capacitor-grade tantalum (Ta) powder via the self-propagating hightemperature synthesis (SHS) method is described. In addition, the sintering aspects and electrical characteristics of the powder are discussed. Ta powder was prepared via the combustion of a Ta2 O5 –xMg–kNaCl mixture, under argon pressure. The morphology and size of the final powder particles was controlled by adjusting the Mg–NaCl concentration. The final powder particles had nodular shapes and sizes ranging from 0.1 to 0.5 m. A leakage current of a sintered Ta sample containing the smallest particle size was 10 A; its capacitance was 92,738 CV, when a 40-V voltage was applied to the sample. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Tantalum (Ta) is a reactive metal with a high-melting point, highcorrosion resistance, and great ductility. This metal has been used in many industrial fields, such as the chemical, medical, machinery, and electronic industries [1,2]. In the electronic industry, particularly, Ta is widely used to produce Ta electrolyte capacitors, which provide the highest capacitance (CV) per unit volume and are more thermally stable than other capacitors [3]. The Hunter process is the conventional method widely used to produce Ta powder [4,5]; it uses the metallothermic reduction (MR) of K2 TaF7 in molten salt. Control of the morphology and size of Ta powder particles in the metallothermic reduction of K2 TaF7 process can be achieved by varying both the injection and stirring speeds of the molten raw materials. However, in the MR process for obtaining Ta powder in a batch-type operation, it is difficult to control morphology and the location of Ta deposits. An MR–EMR (electronically mediated reaction) combination process [6,7] was developed to overcome these challenges; in that process, the total charge passed through the external circuit and the average particle size was controlled by changing the reduction temperature. Even so, the Ta yield can be improved only from 65% to 74%. A new and promising synthesis route for Ta powder that was recently developed by researchers from the H.C. Stark Company [8] involves the direct reduction of Ta2 O5 by controlling magnesium (Mg) vapor. In the Ta2 O5 –Mg reduction process, Mg was vaporized by heating it in a container; the vaporized Mg directly reached the
∗ Corresponding author. Tel.: +82 42 821 6587; fax: +82 42 822 9401. E-mail address: [email protected] (H.I. Won). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.11.140
Ta2 O5 , converting it to metallic Ta powder. This process has many advantages, in comparison with MR, including the absence of both a liquid-phase and a contamination risk from the container, as well as the relatively easy control of the morphology and size of the Ta powder particles. However, the filtration process of evaporating Mg into a Ta2 O5 bed is not easy to control and can result in homogeneous product formation. Recently, a method called MSA-CS (molten salt-assisted combustion synthesis), was reported as an attractive synthesis route to overcome the filtration problems inherent in evaporating Mg into Ta2 O5 [9]. In the MSA-CS method, a Ta2 O5 + Mg mixture diluted with NaCl is combusted under argon (Ar) gas pressure to produce well-dispersed and ultra-fine Ta powder. The yield of Ta powder produced by MSA-CS method is >90%. In this study, the synthesis of capacitor-grade Ta powder via the SHS method is described; the sintering and electrical characteristics are also analyzed, to demonstrate prospects for future application. 2. Experimental process 2.1. Powder synthesis Ta penta-oxide (Ta2 O5 ), Mg, and sodium chloride (NaCl) were used as raw materials. The characteristics of the initial powders are listed in Table 1. The green mixture was prepared with a ball-mill containing zirconia balls, for 24 h. The mixed powder was put into a steel glass and gently compacted by hand. The green pellet was 4.0–5.0 cm in diameter and 4–5 cm in height, and it was placed in a reaction chamber under an Ar atmosphere of 2.5 MPa. The combustion reaction was ignited by heating the top surface of the pellet with an electric arc for 1–2 s. Two preliminary tungsten–rhenium thermocouples (W/Re-5 vs. W/Re-20) were inserted into the reaction sample, to record temperature profiles and calculate combustion temperature (Tc ).
H.I. Won et al. / Journal of Alloys and Compounds 478 (2009) 716–720
717
Fig. 1. Combustion temperature of (a) Ta2 O5 –7Mg–kNaCl and (b) Ta2 O5 –xMg–2.5NaCl.
Fig. 2. Microstructure of tantalum powders: (a) zone I (Ta2 O5 –7Mg–1.5NaCl), (b) zone II (Ta2 O5 –7Mg–3.5NaCl), (c) zone I (Ta2 O5 –6Mg–2.5NaCl), (d) zone II (Ta2 O5 –9Mg–2.5NaCl), and (e) zone boundary (Ta2 O5 –7.5Mg–2.5NaCl).
718
H.I. Won et al. / Journal of Alloys and Compounds 478 (2009) 716–720
Table 1 The characteristics of raw materials.
3. Results and discussion
Reactant
Manufacturer
Particle size (m)
Purity (%)
Tantalum penta-oxide Magnesium
Alfa Aesar, USA Daejung Chemical and Metals, Korea Samchun Pure Chemical, Korea
<1 100–200
99.5 99
<50
99.5
Sodium chloride
The combusted product was washed with distilled water; it was then treated with sulfuric acid and hydrochloric acid, to eliminate NaCl and MgO. After removing the NaCl and MgO, the Ta powder was dried at 300 ◦ C in vacuo. Additional de-oxidation of the prepared Ta powder was carried out at 1100–1200 ◦ C for 2 h. The final products were characterized using Cu K␣, a Siemens X-ray diffractometer (D-76181, karlsruhe, Germany), field emission scanning electron microscopy (FESEM; JSM 6330F JEOL Ltd., Tokyo, Japan), an Inductively Coupled Plasma Optical emission spectrometer, a carbon–sulfur determinator (ELTRA CS-800), and a nitrogen–oxygen determinator (ELTRA ON-900). 2.2. Ta preparation and its characteristics The final Ta powder (0.15 g) was pressed into a cylindrical pellet (3.23 mm diameter) with Ta wire (0.23 mm diameter). The green density of the pellet was 5.0 g/CC. The pellet was first sintered at 1350 ◦ C for 30 min in vacuo (<1.0 × 10−6 Torr) and then treated with a 0.1-vol% of H3 PO4 solution at 60 ◦ C (40 mA/g and 15–40 V were applied to the pellets), to form a Ta oxide film on the particle surface. The CV, dissipation factor (DF), and leakage current (LC) were measured with an LCR meter (MIT 9216A) and digital multimeter (Keithley, 2000 61/2). To measure CV and DF (tan ı), the pellet was immersed for 60 s at 25 ◦ C in a 10-vol% of H2 SO4 solution. The applied frequency of alternative current and voltage was 120 Hz and 1.0 V, respectively. The LC was measured in a 10-vol% of H3 PO4 at 25 ◦ C, and 28.0 V and 50 mA were applied for the LC. The average equivalent serial resistance (ESR) for the three pellets was measured, and tan ı was deduced from tan ı = 2fc (ESR) where f and c are frequency in Hz and capacity in F, respectively.
3.1. Chemical and morphological analysis of SHS Ta powder In a former study of MSA-CS (molten salt assisted combustion synthesis) [9], ultra-fine Ta powder with a round particle shape was produced from a Ta2 O5 + 5Mg + kNaCl mixture. However, because the round shape of the Ta particles has a low specific surface that makes it unsuitable for capacitor applications, it is necessary to control Ta particle morphology. To address this problem, the use of excess Mg was investigated for nodular morphology in Ta particle formation. Fig. 1 shows the combustion temperature dependence of Mg–NaCl concentrations. In the present study, 2.5 MPa of Ar pressure was applied to increase the effect of the gas phase, and Mg was reduced by preventing the emission of evaporated Mg from the sample during combustion. A drop-line of combustion temperature in the system of both Ta2 O5 –xMg–2.5NaCl and Ta2 O5 –7Mg–kNaCl system had a horizontal part that was supposedly created by the NaCl phase-change from gas to liquid, in the range of k = 1–2 and x = 6–7. As seen in Fig. 1a and b, that range can be divided into two zones, according to the morphology of the final products: roundshaped Ta particles were found in zone II, and irregular-shaped Ta particles – the latter of which are favorable for making a spongeform sintered body – was found in zone I. In zone I, because of the high-combustion temperature above the boiling temperature of Mg (assumed at 1300 ◦ C), a sufficient amount of evaporated Mg participated in the gas-phase reduction of Ta2 O5 , resulting in the formation of irregular particle shapes. Here, the Mg boiling point was lowered by using high-pressure Ar in the chamber. The microstructures of the final Ta powders synthesized from zone I, zone II, and the zone boundary are listed in Fig. 2. From that figure, one can see that the proportion of round-shaped particles decreased when the combustion temperature changed from zone I
Fig. 3. Microstructure of tantalum powders synthesized from the mixture of Ta2 O5 –7Mg–kNaCl: (a) 0.5, (b) 1.5, (c) 2.5, and (d) 3.5.
H.I. Won et al. / Journal of Alloys and Compounds 478 (2009) 716–720
Fig. 4. The distribution of granulated tantalum powders.
719
Fig. 6. The leakage current data of tantalum powders sintered at 1350 ◦ C.
to zone II. Therefore, it is clear that an excess of Mg influences the morphology of Ta particles, making it irregular and coral-like. Fig. 3 provides micrographs obtained by scanning electron microscopy (SEM), to show the morphological changes in the Ta powder from the Ta2 O5 –7Mg–kNaCl mixture. As seen, four different sizes of Ta powder particles were synthesized from different concentrations of NaCl (k = 0.5, 1.5, 2.5, and 3.5). Here, in Fig. 3d, the powder from a large concentration of NaCl (3.5) contains many round-shaped particles, due to the involvement of a low combustion temperature. It has been clearly demonstrated that the SHS process can control the particle size of Ta powder by controlling the NaCl concentration; it also has the ability to produce sub-micrometer-sized powder particles. The overall combustion process can be presented as shown: Ta2 O5 + (5 + m)Mg + kNaCl → 2Ta + 5MgO + kNaCl + mMg where m and k are the mole concentrations of excess Mg and NaCl, respectively. Note that the shape and the size of the Ta particles can be varied by changing the values of m and k. Table 2 lists the general values of impurity levels of different-size Ta powder particles prepared with a given technique. In general, the
impurity levels are low, with the exceptions of carbon and sulfur. However, the levels of these impurities are within acceptable limits (i.e., carbon and sulfur content in capacitor-grade Ta powder can vary from 30 to 70 parts per million [ppm]). Other powder features to be taken into account are the flowability and the size distribution of the clusters. The flowability of Ta powder is important in manufacture capacitors in large-scale production centers, because the powder should completely fill every crevice of an elaborate mold when a certain quantity of Ta powder is dropped into a mold and pressed into a pellet. In addition, the flowability can decide the quantity of powder, which can in turn affect the density of sintered pellets. In terms of the size distribution of clusters, a powder with a broad particle-size distribution containing a high-percentage of fine particles will not flow and will have low crushing strength. Comparable data regarding the particle size distribution and SEM of granulated Ta powder are shown in Figs. 4 and 5. The results show that the commercial powder had a very broad distribution and a number of small particles that were smaller than 20 m, and the particles were loosely agglomerated in the cluster. On the other hand, the powder synthesized by the SHS process had a narrow
Table 2 Chemical analysis of tantalum powder (ppm). Sample
Ta-2 Ta-3 Ta-4
Impurity (ppm)
Flow-ability (s/20 g)
Mg
Ca
Ni
Fe
S
C
K
Na
O
<10 <10 <10
<10 <10 <10
12 12 14
<10 <10 <10
50 44 49
68 62 72
<10 <10 <10
<10 <10 <10
2870 2520 2350
Fig. 5. Micrograph of granulated powders: (a) commercial powder and (b) SHS powder.
33 32 34
720
H.I. Won et al. / Journal of Alloys and Compounds 478 (2009) 716–720
Table 3 The capacitance (mF/g) and dielectric loss (%) of tantalum powders. Sample Ta-1 Ta-2 Ta-3 Ta-4
15 V 65,124 81,567 69,851 48,581
30 V 73,784 89,986 75,235 53,290
40 V 75,482 92,738 80,309 54,710
tan ı 33 32 34 34
distribution and was well condensed in the cluster; that powder (Ta-2) therefore showed greater flowability (Table 2). 3.2. Electrical analysis of SHS Ta powder As mentioned above, it is very important to ensure that Ta powder comprises irregular-shaped particles, because irregular shapes – which can have a larger surface area than round ones – leads to higher CV values. This irregular shape was provided by an excess of Mg powder incorporated into the initial mixture. The CV and dielectric loss (tan ı) values of the Ta powders are shown in Table 3. Four different particle-size powders (Ta2 O5 –7Mg–kNaCl; k = 0.5 (Ta-4), 1.5 (Ta-3), 2.5 (Ta-2), and 3.5 (Ta-1)) were made into pellets; different voltages (15, 30, and 40 V) were then applied to the pellets, to measure CV per unit of mass. Dielectric loss was similar across all samples, while the CV of each sample generally increased with a decrease in particle size (i.e., from Ta-4 to Ta-2). However, Ta-1 showed a low CV value, in spite of its small particle size; the low CV of the Ta-1 powder can be explained by its plentiful number of round-shaped particles. Fig. 6 illustrates the tendency for LC to decrease as a function of time; each Ta sample showed a similar tendency. The final LC value for each Ta sample was <10 A. A crystalline Ta2 O5 layer had partially formed when the Ta powder contained impurities such as oxygen, carbon, and iron; this crystalline layer caused an increase in
LC [10,11]. This is because the current “focuses” on the contaminants during anodizing, and so higher oxygen and carbon concentrations lead to a higher LC. Therefore, it can be assumed that the amorphous Ta2 O5 layer in each sample was uniformly created during anodizing, and this result corresponds with those of chemical analyses. 4. Conclusion Ta powder with uniform coral-like shape was synthesized with the SHS process, using a Ta2 O5 –(5 + m)Mg–kNaCl mixture. A narrow size distribution among clusters and high-flowability of Ta powder were obtained with different concentrations of NaCl and an excess of Mg. The LC of each Ta sample was <10 A, and the CV of the Ta powder having the smallest particle size was 92,738 CV when 40 V of voltage was applied. Ta capacitors prepared with the SHS process are very promising for electronics, due to the higher CV and low LC values involved. The SHS process demonstrated in this study is highly efficient and therefore suitable for large-scale production. References [1] G. Winter, International Symposium on Tantalum and Niobium, Goslar, Germany, September 1995, pp. 485–515. [2] S.H. Yoo, T.S. Sudarshan, K. Setheram, G. Subhash, R.J. Dowding, Nanostruct. Mater. 12 (1999) 23–28. [3] J.F. Muller, P.M. Dinh, 2nd International Conference on Tungsten and Refractory Metals, McLean, USA, 1994, pp. 573–586. [4] M.A. Hunter, J. Met. 5 (1953) 130–132. [5] E.G. Hellier, G.L. Martin, US Patent 2,950,185 (1960). [6] I. Park, T.H. Okabe, Y. Waseda, J. Alloys Compd. 280 (1998) 265–272. [7] J. Yoon, I.S. Bae, H.H. Park, S. Goto, B. Kim, J. Jpn. Inst. Met. 68 (12) (2004) 1031–1038. [8] L.N. Shekhter, T.B. Tripp, L.L. Lanin, US Patent 6,171,363 (2001). [9] H.H. Nersisyan, J.H. Lee, S.I. Lee, C.W. Won, Combust. Flame 135 (2003) 539–545. [10] T. Izumi, Mater. Jpn. 33 (1) (1994) 78–80. [11] R.H. Lorenz, A.B. Michael, Electrochem. Technol. 2 (5/6) (1964).