Materials Transactions, Vol. 48, No. 12 (2007) pp. 3197 to 3200 #2007 The Japan Institute of Metals RAPID PUBLICATION

Evaluation of Electric Properties for

Sung Man Jung1, In Sung Bae2;*, Jae Sik Yoon3;*, Shoji Goto4 and Byung Il Kim1;3

1Department of Material Science and Metallurgical Engineering, Sunchon National University, Sunchon 540-742, Korea 2Korea Institute of Rare Metals, Sunchon 540-742, Korea 3Korea Basic Science Institute, Sunchon Branch, Sunchon 540-742, Korea 4Department of Materials Science and Engineering, Faculty of Engineering and Resource Science, Akita University, Akita 010-8502, Japan

In order to evaluate the applicability of niobium powder, which was manufactured by the external continuous supply method, as a , the present study measured capacitor performance evaluation factors such as current, loss (tan ), capacitance, impedance, equivalent series resistance, etc. Capacitance decreased significantly from 156 mF in (wet cap) to 130 mF after carbon (C)/silver (Ag) solution coating, and around 105 mF after aging, falling within the capacity tolerance of tantalum capacitors. If capacitance was converted to CV/g, it was around 81,000 CV/g. Permittivity loss (tan ) decreased significantly from 12.9% after C/Ag coating to 7.7% after aging, satisfying the general standard level of 10% or less. Leakage current was 2.41 mA after C/Ag coating and 2.93 mA after aging, both less than the standard level of 6.3 mA. As a whole, the niobium capacitor showed somewhat more unstable characteristics than commercial tantalum capacitors but can be considered applicable as a substitute for tantalum capacitors in the future. [doi:10.2320/matertrans.MRP2007619]

(Received July 23, 2007; Accepted October 17, 2007; Published November 7, 2007) Keywords: niobium powder, external continuous supply method, capacitance, leakage current, permittivity loss, impedance, equivalent series resistance

1. Introduction and these properties are determined by the purity and granularity of the powder, aging temperature in the capacity Niobium (Nb) has a small neutron absorbing cross- manufacturing process, formation, etc. sectional area, outstanding ductility, oxidation-resistance Thus, the present study manufactured niobium powder by and impact-resistance, and high transition temperature the external continuous supply method and after manufactur- (8.2 K), so the metal is used widely in nuclear fusion, nuclear ing capacitors with the powder, we measured performance industry, space development, high-power transmission, steel evaluation factors such as leakage current, permittivity loss and super-conductor.1–5) factor, capacitance, impedance and equivalent series resist- The melting point of niobium is as high as 2,740 K, its ance. An experiment was conducted to determine the specific gravity is 8.55 g/cm2 similar to that of iron, and its applicability of the niobium capacitor by assessing it under dielectric dissipation is higher than tantalum. Particularly as the same test criteria and conditions for tantalum capacitors. niobium is superior in the rectification of oxidized film, which is obtained from the oxidation of anode, in dielectric 2. Experiment Method dissipation and in electric stability, it has all desirable properties as a material of capacitors for electronic appli- 2.1 Preparation of samples ances and thus is spotlighted as a substitute to tantalum. In Niobium powder used in this study was manufactured addition, because the price of niobium is merely 30% of that through reduction using the external continuous supply of tantalum, if a niobium capacitor is developed it will be method. Through post-processes such as washing, pickling, highly competitive in price. deoxidation and annealing, we obtained uniform powder with Niobium powder can be manufactured by reducing oxide purity of over 99.9% and average grain size of around (Nb2O5) with aluminum, by reducing chloride (NbCl5) with 110 mm. The chemical composition and a SEM photograph of hydrogen, or by reducing fluoride (K2NbF7) with fused-salt the niobium powder are shown in Table 1 and Fig. 1. electrolysis or metal sodium. In the present study, we The concentration of impurities in Nb powder was manufactured niobium powder using external continuous measured using Inductively Coupled Plasma-atomic emis- supply method,6) which is metallothermic reduction (MR).7,8) sion spectroscopy (Thermo Jarrel Ash Co., IRIS-Advantage) In general, solid electrolyte capacitors are manufactured and the concentration of oxygen, nitrogen and carbon were by sintering power into porous anode pellet, and it shows measured using oxygen-nitrogen determinator (LECO, electric properties such as high capacitance, low leakage TC500) and carbon-sulfur determinator (LECO, CS200). current and low permittivity loss. A solid electrolyte capacitor is made by sintering powder with large surface area into porous anode pellets. The performance of the capacitor can be assessed by measuring capacitance, leakage Table 1 Chemical compositions of niobium powder. current, tan , equivalent series resistance, impedance, etc. (unit: ppm) Impurity Fe Cr Ni Si Ca Na K C N O * Corresponding author, E-mail: [email protected], raremetal@fixoninc. Concentration 43 46 29 42 20 15 17 77 395 5,563 com 3198 S. M. Jung, I. S. Bae, J. S. Yoon, S. Goto and B. I. Kim

180

160 F) µ 140 Solder

Silver paint 120 Graphite Capacitance ( 100 MnO2

Nb2O5 80 Wet cap C/Ag coating Aging Niobium Fig. 3 Distribution of capacitance for niobium capacitors.

Fig. 1 SEM photograph of niobium powders prepared by external continuous supply methods. Fig. 2 Schematic diagram representa- tion of niobium capacitor.

2.2 Capacitor manufacturing process (C)/silver (Ag) solution coating, and molding and aging First, around 70 mg of powder mixed with binder and treatment. In this study, it was measured in three processes, lubricant was loaded onto the molding device, and pellets excluding the molding process. Wep cap is a process for were made by pressing and molding at pressure of 10 MPa. measuring capacitance by settling niobium pellets in HNO3 At that time, a tantalum wire was inserted as an anode. The solution before the formation of MnO2 layer, colloidal tantalum wire stuck out upward functions as the anode of the graphite (C) is applied in order to minimize contact resistance capacitor. Binder and lubricant were removed by heating between the MnO2 layer composed of irregular particles and under vacuum at 423 K for 30 minutes. the Ag layer in the form of flake, and the Ag layer is applied Then, the pellets were sintered in a high-vacuum sintering to enlarge the contact area of the lead frame and to lead out furnace at 1,503 K for 30 minutes, and porous pellets with the cathode. Aging, which is for the stabilization of high strength, high density and large surface area were capacitance, is done by applying current for removing manufactured. defects such as fine cracks in the dielectric layer. In the next step, chemical conversion treatment (0.7 The results of measuring capacitance (mF) at 120 Hz, mass%H3PO4) was done for 2 hours for forming the 0.5 Vrms + 0.5–2.0 V DC and series equivalent circuit after dielectric (Nb2O5) layer. Then, in order to produce cathode, manufacturing a niobium device (3:3 mm 4:0 mm the niobium pellets were submerged in electrolyte manga- 1:8 mm) with rated voltage of 6.3 V and capacitance of nese nitrated solution (Mn(NO3)2), and a manganese dioxide 100 mF are shown in Fig. 3. It showed the tendency of (MnO2) layer was formed over the Nb2O5 dielectric layer by considerable decrease from 156 mF in electrolyte (wet cap) to heating at 523 K. The device on which a MnO2 layer was 130 mF after carbon (C)/silver (Ag) solution coating and formed was coated with carbon (C) and silver (Ag) for the around 105 mF after aging. As shown in the figure, capacitor complete connection to the cathode terminal. decreases while going through wet cap, C/Ag coating and After that, in order to lead out anode and cathode aging process probably because dielectric formation is terminals, the anode was welded to the frame, and conductive measured in nitric acid (HNO3) solution that functions Ag was assembled as the cathode using adhesive. Lastly, a temporarily as the cathode before the formation of MnO2 case was made with epoxy molding resin, and a niobium layer, in the Ag layer (in a solid state) that absorbed a small capacitor was manufactured through aging for one hour at amount of water after C/Ag coating, and in a state with no 398 K. Figure 2 shows a schematic diagram of the niobium water at all due to molding after aging. This is believed to capacitor. come from the decrease of dielectric dissipation resulting from the abnormal change of the Nb2O5 dielectric layer in the 2.3 Electric and frequency properties MnO2 sintering (523 K) range. In general, the standard In order to evaluate the electric and frequency properties of capacity tolerance of M-class tantalum capacitors is 20%. the niobium capacitor manufactured through the process In the case, because the capacitance is 100 mF, the capacity described above, we measured capacitance, permittivity loss should be between 80–120 mF and the final capacity after factor (tan ), impedance and equivalent series resistance aging fell within the range. (ESR) using a LCR meter, and measured leakage current (LC) using a digital multi-meter and DC power supply. 3.2 Permittivity loss factor (tan ) In case of an ideal capacitor, if AC voltage is applied 3. Experiment Results and Discussion between the terminals, there happens phase angle ’ of 90 (/2) between current and voltage. In real capacitors, 3.1 Capacitance however, phase angle () smaller than 90 happens in current In general, capacitance is measured after each of capacitor by resistance components (electric resistance of electrode, manufacturing processes such as dielectric formation, carbon electrolyte, etc.), and the delay of current by is called loss Evaluation of Electric Properties for Niobium Capacitors 3199

20 10 120

100 6.3 15

F) 80 µ

60 A) (%) µ

10 δ 40 LC ( tan Capacitance ( 20 Nb 5 Ta(A107D) 0 0.1 1 10 100 Frequency (KHz) 0 1 C/Ag coating Aging C/Ag coating Aging Fig. 6 Change of capacitance with frequency. Fig. 4 Distribution of dielectric dissipation fac- Fig. 5 Distribution of leakage current for niobi- tor (tan ) for niobium capacitors. um capacitors. angle (tan ). Accordingly, dielectric loss is expressed in % capacitor manufactured in this research and a commercial indicating how much current has deviated from loss angle (A107D). namely, tan . The result of measuring the variation of capacitance The result of measuring permittivity loss (tan ), which according to frequency (0.12–100 kHz) is shown in Fig. 6. In was measured under the same condition as capacitance is general, capacitance according to the frequency of tantalum shown in Fig. 4. As shown in the figure, tan showed capacitors shows almost no change up to 1 kHz and then distribution over 10% (average 12.9%) wider after C/Ag decreases sharply with the increase of frequency, and the coating, but decreased all considerably to below 10% variation of capacity grows larger with the increase of (average 7.7%) after aging. The decrease of tan after aging capacitance. As shown in the figure, the niobium capacitor treatment is considered to result from the decrease of electric manufactured in this study did not show a large variation of resistance due to the crushing of resistance components, capacitance between 0.12 kHz (103 mF) and 1 kHz (96 mF), which occurred by the irregularity of the electrolyte layer but as frequency went up to 10–100 kHz capacitance (pore, particle size), through the molding process. The decreased significantly to 64–25 mF. The tendency was tolerance of tan is different according to capacitor manu- similar to that of the commercial tantalum capacitor but the facturer, but in general it is below 10% if capacitance is decrease in capacity was larger. Because niobium powder higher than 100 mF. used as the material of the capacitor is finer than tantalum powder, it is not easy to impregnate manganese nitrate 3.3 Leakage current (L.C.) solution and as a result there happen pores in the sites of Leakage current is a current flowing through the capacitor MnO2 and the pores work as electric resistors and increase after a lapse of time when DC voltage is applied. Leakage the rate of decrease in capacitance. In addition, at low current is determined by impurities and the uniformity of frequency, the effect of pores, though many in number, is not grain size when Nb2O5 dielectric is formed after the molding significant, but their effect on capacitance grows higher at of niobium powder.9–12) When dielectric is formed, impuri- high frequency. ties form semi-conductive oxide between Nb2O5 and The result of measuring the variation of tan according to niobium. The oxide results in dielectric loss and lowers the frequency (0.12–100 kHz) is shown in Fig. 7. As shown in sinterability of the powder and consequently increases the figure, tan increased almost linearly with the increase of leakage current. frequency in both the commercial tantalum capacitor and the In our experiment, in order to measure leakage current, we niobium capacitor manufactured in this study. applied a rated voltage (6.3 V) by passing a 1 K resistor This result is clear by eq. (1) below. through the capacitor and measured current after 5 minutes. tan ¼ 2fCEsr ð1Þ The result of measuring leakage current of the sample is shown in Fig. 5. As shown in the figure, leakage current was where, f is frequency, C is capacitance and Esr is equivalent 2.41 mA after C/Ag coating and 2.93 mA after aging. Leakage series resistance. current increased after aging treatment probably because part As in eq. (1), tan is proportional to frequency (f ). This is of dielectric membrane was damaged by pressure during consistent with the result of this experiment that tan molding after C/Ag coating and electric current concentrated increases linearly along with the increase of frequency. At on the damage and generated heat. In general, the leakage 0.12–10 kHz, tan was lower in the commercial tantalum current of tantalum capacitors should meet the leakage capacitor, but at over 10 kHz, it was lower in the niobium current standard of 0.01 CV or less. In our experiment, the capacitor. standard level is below 6.3 mA because the rated voltage The results of measuring the variation of impedance and 6.3 V and the capacitance was 100 mF. When final leakage ESR according to frequency (0.12 kHz–100 MHz) are shown current after aging was measured, the result was lower than in Fig. 8 and Fig. 9. As shown in the figure, both the the standard level 6.3 mA. commercial tantalum capacitor and the niobium capacitor manufactured in this study showed similar tendencies, but the 3.4 Variation of properties according to frequency commercial tantalum capacitor had lower impedance and In order to examine the variation of properties according to ESR. frequency, we conducted an experiment on the niobium In general, it is ideal that the impedance and ESR of 3200 S. M. Jung, I. S. Bae, J. S. Yoon, S. Goto and B. I. Kim

1000 100 10 Nb Nb Nb Ta(A107D) Ta(A107D) Ta(A107D) 10 100 ) 1 Ω (

(%)

δ 1 tan 10 ESR (¥Ø) 0.1 Impedance 0.1

1 0.1 1 10 100

Frequency (KHz) 0.01 0.01 0.1 1 10 100 1000 10000 0.1 1 10 100 1000 10000 Frequency (KHz) Frequency (KHz) Fig. 7 Change of dissipation factor (tan ) with frequency. Fig. 8 Change of impedance with frequency. Fig. 9 Change of ESR with frequency. condensers decrease linearly with the increase of frequency, tantalum capacitors. but in reality, tendencies as in Fig. 8 and Fig. 9 were (2) Permittivity loss (tan ) decreased considerably from observed because of resistant components. In general, 12.9% after C/Ag coating to 7.7% after aging, the impedance is expressed by the equation below. general standard level 10% or less. (3) Leakage current was 2.41 mA after C/Ag coating and ImpedanceðZÞ¼ðESR2 þðwL 1=wCÞ2Þ1=2 ð2Þ 2.93 mA after aging, both less than the standard level Where, wL is inductance (reactance of electrode and leads), 6.3 mA. 1=wC is reactance of capacitance, C is capacitance and ESR (4) The variation of properties according to frequency is equivalent series resistance. showed similar tendencies between the commercial As in eq. (2), impedance depends on the size of ESR, and tantalum capacitor and the niobium capacitor and, as a ESR is inversely proportional to frequency as in eq. (1). By whole, the niobium capacitor showed somewhat un- eq. (1) and (2), impedance and ESR should decrease with the stable characteristics compared to the commercial increase of frequency and this is consistent with the result of tantalum capacitor. this experiment. In addition, the reason that the impedance (5) As a whole, niobium capacitor is somewhat inferior to and ESR of niobium capacitor are higher than those of tantalum capacitor in performance but as it satisfies the tantalum capacitor is probably that, as explained above, criteria for assessing the performance of tantalum niobium powder is finer than tantalum powder and as a result capacitor it is considered possible to replace current adequate MnO2 impregnation caused the occurrence of a commercial tantalum capacitors with niobium capaci- large number of pores, which worked as resistance compo- tors in the future. nents. REFERENCES 4. Conclusions 1) G. Gauje and R. Brunetaud: Rev. Phys. Appl. 5 (1970) 513. In order to evaluate the electric and frequency properties of 2) F. J. Cadieu and N. Chencinski: Inst. Phys. Conf. Ser. 39 (1978) 642. niobium powder manufactured by the external continuous 3) J. E. Kunzler et al.: Phys. Rev. Lett. 6 (1961) 89. supply method, the present study manufactured niobium 4) K. Mendelssohn and J. L. Olsen: Proc. Phys. Soc. 63 (1950) 2. capacitors through molding, sintering, formation, C/Ag 5) M. Bidault and J. Dosdat: Rev. Phys. Appl. 5 (1970) 505. 6) J. S. Yoon, H. H. Park, I. S. Bae, S. Goto and B. I. Kim: J. Japan Inst. coating, aging, etc. and examined the applicability of the Met. 68 (2004) 247. niobium powder as a capacitor by measuring capacitance, 7) S. C. Jain, D. K. Bose and C. K. Gupta: Trans. Indian Inst. Met. 24 tan , leakage current, impedance, equivalent series resist- (1971) 1. ance, etc. 8) K. Wayne, H. Waban and J. P. Matin: U. S. Pat. 2,927,855. Mar. 8 (1) Capacitance decreased considerably from 156 mFin (1960). 9) F. C. Aris and T. J. Lewis: J. Phys. 6 (1973) 1067. electrolyte (wet cap) to 130 mF after carbon (C)/silver 10) M. W. Jones and D. M. Hughes: J. Phys. 7 (1974) 112. (Ag) solution coating and around 105 mF after aging, 11) G. P. Klein: J. Electrochem. Soc. 113 (1966) 348. showing that it falls within the capacity tolerance of 12) V. Trifonova and A. Girginow: J. Electroanal. Chem. 107 (1980) 105.