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Evaluation of Electric Properties for Niobium Capacitors

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Evaluation of Electric Properties for Niobium Capacitors Materials Transactions, Vol. 48, No. 12 (2007) pp. 3197 to 3200 #2007 The Japan Institute of Metals RAPID PUBLICATION Evaluation of Electric Properties for Niobium Capacitors 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 capacitor, the present study measured capacitor performance evaluation factors such as leakage current, permittivity loss (tan ), capacitance, impedance, equivalent series resistance, etc. Capacitance decreased significantly from 156 mF in electrolyte (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, dielectric 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 Æ20%. 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.
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