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Nondestructive testing of ceramic capacitors by partial discharge method
August 2001
Materials Letters 50 Ž2001. 82–86 www.elsevier.comrlocatermatlet
Nondestructive testing of ceramic capacitors by partial discharge method Byoung-Chul Shin a,) , Sung-Chul Kim b, Choon-Woo Nahm c , Seung-Ju Jang d a
Department of AdÕanced Materials Engineering, Dong-eui UniÕersity, Pusan, South Korea b Department of Physics, Dong-eui UniÕersity, Pusan, South Korea c Department of Electrical Engineering, Dong-eui UniÕersity, Pusan, South Korea d Department of Computer Engineering, Dong-eui UniÕersity, Pusan, South Korea
Received 24 October 2000; received in revised form 17 November 2000; accepted 5 December 2000
Abstract The majority of ceramic capacitors are fabricated with BaTiO 3. Dielectric breakdown is one of the primary drawbacks for ceramic capacitors. In this research, instead of the destructive reliability tests of BaTiO 3 ceramic capacitors, a new nondestructive testing method was proposed. The BaTiO 3 ceramics used in this study were sintered at 13008C, 13508C, 14008C or 14508C to vary pore size distribution. Pore size of barium titanate ceramics increased with sintering temperature. The breakdown field and partial discharge inception field decreased with increase of sintering temperature. This means that the large pores in over-fired BaTiO 3 , having relatively low inception voltage of partial discharge, decrease the breakdown field. Reliability of ceramic capacitors can be nondestructively evaluated by partial discharge testing method. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ceramic capacitors; Partial discharge method; Dielectric breakdown
1. Introduction The majority of ceramic capacitors are fabricated with BaTiO 3 dielectrics. Barium titanate is a ferroelectric materials whose high dielectric constant has made it possible to manufacture small components having high capacitance, such as multilayered ceramic capacitors ŽMLCC.. The rapid increase in use of these MLCC and high voltage ceramic capacitors for which reliability is a major concern requires the study of failure mechanism. Dielectric breakdown is )
Corresponding author. E-mail address: [email protected] ŽB.-C. Shin..
major source of failure of high dielectric materials, such as MLCC and high voltage ceramic capacitors w1–3x. The breakdown voltages have been measured for the reliability statistics of the ceramic capacitors. However, the capacitors cannot be used after the destructive breakdown tests. To investigate a new nondestructive testing method, dominant factors affecting breakdown voltages should be studied. For the ceramic capacitors, large pores can fatally decrease the breakdown voltages w4x. If the pores are dominant factors affecting the breakdown voltages, the partial discharge testing can be a new nondestructive testing method because the partial discharge
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 2 0 - 1
B.-C. Shin et al.r Materials Letters 50 (2001) 82–86
83
phenomena have been observed at the pores by analysis of acoustic emission data w5x. Purpose of this research is to see whether the nondestructive partial discharge testing method can be one of the reliability tests of ceramic capacitors or not. So, breakdown voltage and partial discharge inception voltage were measured for BaTiO 3 ceramics which have different pore size distribution.
2. Experiments Barium titanate powders ŽHPB-Lot 705, TAM. were used to prepare the ceramic capacitors. PVA Žpolyvinyl alcohol, ŽCH 2 : CHCH. n . and DOLAPIXCA ŽZschimmer & Schwarz, Chemische Fabriken. were used for a binder and a deflocculant, respectively. Chemical basis of DOLAPIX-CA Žyellow liquid. is polyelectrolyte, synthetic and free from alkalis. The barium titanate powders, binder and deflocculant were wet milled with zirconia balls in a plastic bottle. After being dried, the powders were granulated with 100-mesh sieve for pressing. The body were uniaxially pressed andror isostatically pressed Ž20,000 psi.. The organic materials in the specimens were burnt out at 350–5008C. After burning out process, the specimens were sintered at 13008C, 13508C, 14008C or 14508C for 2 h to control pore size. The sintered specimens were coated with PdrAu alloy by diode sputtering to form electrodes. Fig. 1 shows the shape and electrode arrangement of the specimens which were designed to prevent a electric field concentration at the electrode margin. The measurement of partial discharge characteristics was performed based on IEC standard 270 Ž1981. using the discharge detector ŽFCR-5.. Before the partial discharge, the discharge magnitude was lower than 5 pC. When the partial discharge happened by increasing the applied voltage, the discharge magnitude jumped from 5 to over 30 pC in this specimens. Therefore, we defined the partial discharge inception field as the lowest field at which the partial discharge magnitude exceed 30 pC abruptly. The breakdown voltages of the specimens were measured using ac voltage Ž60 Hz. which was stepped up in amplitude until breakdown occurred.
Fig. 1. Shape and electrode arrangement of the specimen.
All of the electrical measurements were carried out in a silicone oil bath for insulation. For microstructure observation, the samples were polished with SiC abrasive papers and abrasive Al 2 O 3 powders, and etched with a solution of HCl and HF for 1 or 2 min w6,7x. Then pore size distributions of each sample were determined by using a scanning electron microscope and an image analyzer ŽOmnicon-3500..
3. Results and discussion Fig. 2 shows the microstructure of BaTiO 3 ceramics sintered at 13008C, 13508C, 14008C or 14508C. The pore size increase with sintering temperature as shown in Table 1. Large pores are present almost entirely at the grain corners and have nonspherical shape. Fig. 3 shows a typical pattern of the discharge pulses in BaTiO 3 ceramics. This is a lissajous figure of the cavity breakdown sequence under ac voltage. Panel A of this figure represents individual discharge pulses and panel B shows standard pulses for reading of the discharge amplitude. Fig. 4 shows that BaTiO 3 ceramics sintered at higher temperature have lower partial discharge inception field and lower breakdown field. As shown in Table 1, pore size increased with sintering temper-
B.-C. Shin et al.r Materials Letters 50 (2001) 82–86
84
Fig. 2. Microstructures of BaTiO 3 ceramics sintered for 2 h at Ža. 13008C, Žb. 13508C, Žc. 14008C, Žd. 14508C.
ature. The BaTiO 3 ceramics, which have large pores, exhibited lower partial discharge and breakdown field. Partial discharges at the large pores may cause them to discharge. These discharges can cause local heating or stressing of the remaining solid dielectrics, and lead to thermal breakdown.
Therefore, instead of the destructive breakdown tests, the nondestructive partial discharge tests can be
Table 1 Density and pore size of pure BaTiO 3 ceramics Sintering temp. Ž8C. Apparent density Ž%. Average pore size Žmm. Standard deviation Žmm.
1300
1350
1400
1450
92 0.5
93 3.7
93 12.4
93 14.9
Ž"0.1.
Ž"2.2.
Ž"5.0.
Ž"8.5. Fig. 3. Typical pattern of internal discharge in BaTiO 3 ceramics. ŽA. Discharge pulses, ŽB. calibration pulses.
B.-C. Shin et al.r Materials Letters 50 (2001) 82–86
85
Fig. 4. Breakdown ŽBD. and partial discharge ŽPD. inception field of BaTiO 3 ceramics as a function of sintering temperature.
used for the reliability tests of the BaTiO 3 ceramic capacitors.
4. Conclusions Measurement of the ac breakdown field and the partial discharge inception field of barium titanate ceramics at 60 Hz resulted as follows. Pore size of barium titanate ceramics increased with sintering temperature Ž1300–14508C.. The breakdown field and partial discharge inception field decreased with the sintering temperature. The large pores, having relatively low inception voltage of
partial discharge, contributed to the low breakdown field of BaTiO 3 ceramics sintered at high temperature. Consequently, the reliability of BaTiO 3 ceramic capacitors can be nondestructively tested by partial discharge testing method.
Acknowledgements The authors wish to acknowledge the financial support of the Korea Research Foundation made in the Program Year 1997.
86
B.-C. Shin et al.r Materials Letters 50 (2001) 82–86
References w1x T. Murakami, A. Yamaji, J. Am. Ceram. Soc 55 Ž1976. 572. w2x T. Nozawa, J. Kato, T. Murakami, A. Yamaji, IEEE Trans. Parts, Hybrids, Packag. 13 Ž1977. 174.
w3x H. Doningos, D. Quattra, J. Scaturro, IEEE Trans. Compon., Hybrids, Manuf. Technol. 1 Ž1978. 423. w4x R. Gerson, T.C. Marshall, J. Appl. Phys. 30 Ž1959. 1650. w5x Z.A. Bukhala, J.K. Nelson, Mater. Eval. 57 Ž1999. 1083. w6x G. Petzow, Am. Soc. Met. Ž1978. 93. w7x F. Kulcsar, J. Am. Ceram. Soc. 39 Ž1956. 13.
Materials Letters 50 Ž2001. 82–86 www.elsevier.comrlocatermatlet
Nondestructive testing of ceramic capacitors by partial discharge method Byoung-Chul Shin a,) , Sung-Chul Kim b, Choon-Woo Nahm c , Seung-Ju Jang d a
Department of AdÕanced Materials Engineering, Dong-eui UniÕersity, Pusan, South Korea b Department of Physics, Dong-eui UniÕersity, Pusan, South Korea c Department of Electrical Engineering, Dong-eui UniÕersity, Pusan, South Korea d Department of Computer Engineering, Dong-eui UniÕersity, Pusan, South Korea
Received 24 October 2000; received in revised form 17 November 2000; accepted 5 December 2000
Abstract The majority of ceramic capacitors are fabricated with BaTiO 3. Dielectric breakdown is one of the primary drawbacks for ceramic capacitors. In this research, instead of the destructive reliability tests of BaTiO 3 ceramic capacitors, a new nondestructive testing method was proposed. The BaTiO 3 ceramics used in this study were sintered at 13008C, 13508C, 14008C or 14508C to vary pore size distribution. Pore size of barium titanate ceramics increased with sintering temperature. The breakdown field and partial discharge inception field decreased with increase of sintering temperature. This means that the large pores in over-fired BaTiO 3 , having relatively low inception voltage of partial discharge, decrease the breakdown field. Reliability of ceramic capacitors can be nondestructively evaluated by partial discharge testing method. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ceramic capacitors; Partial discharge method; Dielectric breakdown
1. Introduction The majority of ceramic capacitors are fabricated with BaTiO 3 dielectrics. Barium titanate is a ferroelectric materials whose high dielectric constant has made it possible to manufacture small components having high capacitance, such as multilayered ceramic capacitors ŽMLCC.. The rapid increase in use of these MLCC and high voltage ceramic capacitors for which reliability is a major concern requires the study of failure mechanism. Dielectric breakdown is )
Corresponding author. E-mail address: [email protected] ŽB.-C. Shin..
major source of failure of high dielectric materials, such as MLCC and high voltage ceramic capacitors w1–3x. The breakdown voltages have been measured for the reliability statistics of the ceramic capacitors. However, the capacitors cannot be used after the destructive breakdown tests. To investigate a new nondestructive testing method, dominant factors affecting breakdown voltages should be studied. For the ceramic capacitors, large pores can fatally decrease the breakdown voltages w4x. If the pores are dominant factors affecting the breakdown voltages, the partial discharge testing can be a new nondestructive testing method because the partial discharge
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 2 0 - 1
B.-C. Shin et al.r Materials Letters 50 (2001) 82–86
83
phenomena have been observed at the pores by analysis of acoustic emission data w5x. Purpose of this research is to see whether the nondestructive partial discharge testing method can be one of the reliability tests of ceramic capacitors or not. So, breakdown voltage and partial discharge inception voltage were measured for BaTiO 3 ceramics which have different pore size distribution.
2. Experiments Barium titanate powders ŽHPB-Lot 705, TAM. were used to prepare the ceramic capacitors. PVA Žpolyvinyl alcohol, ŽCH 2 : CHCH. n . and DOLAPIXCA ŽZschimmer & Schwarz, Chemische Fabriken. were used for a binder and a deflocculant, respectively. Chemical basis of DOLAPIX-CA Žyellow liquid. is polyelectrolyte, synthetic and free from alkalis. The barium titanate powders, binder and deflocculant were wet milled with zirconia balls in a plastic bottle. After being dried, the powders were granulated with 100-mesh sieve for pressing. The body were uniaxially pressed andror isostatically pressed Ž20,000 psi.. The organic materials in the specimens were burnt out at 350–5008C. After burning out process, the specimens were sintered at 13008C, 13508C, 14008C or 14508C for 2 h to control pore size. The sintered specimens were coated with PdrAu alloy by diode sputtering to form electrodes. Fig. 1 shows the shape and electrode arrangement of the specimens which were designed to prevent a electric field concentration at the electrode margin. The measurement of partial discharge characteristics was performed based on IEC standard 270 Ž1981. using the discharge detector ŽFCR-5.. Before the partial discharge, the discharge magnitude was lower than 5 pC. When the partial discharge happened by increasing the applied voltage, the discharge magnitude jumped from 5 to over 30 pC in this specimens. Therefore, we defined the partial discharge inception field as the lowest field at which the partial discharge magnitude exceed 30 pC abruptly. The breakdown voltages of the specimens were measured using ac voltage Ž60 Hz. which was stepped up in amplitude until breakdown occurred.
Fig. 1. Shape and electrode arrangement of the specimen.
All of the electrical measurements were carried out in a silicone oil bath for insulation. For microstructure observation, the samples were polished with SiC abrasive papers and abrasive Al 2 O 3 powders, and etched with a solution of HCl and HF for 1 or 2 min w6,7x. Then pore size distributions of each sample were determined by using a scanning electron microscope and an image analyzer ŽOmnicon-3500..
3. Results and discussion Fig. 2 shows the microstructure of BaTiO 3 ceramics sintered at 13008C, 13508C, 14008C or 14508C. The pore size increase with sintering temperature as shown in Table 1. Large pores are present almost entirely at the grain corners and have nonspherical shape. Fig. 3 shows a typical pattern of the discharge pulses in BaTiO 3 ceramics. This is a lissajous figure of the cavity breakdown sequence under ac voltage. Panel A of this figure represents individual discharge pulses and panel B shows standard pulses for reading of the discharge amplitude. Fig. 4 shows that BaTiO 3 ceramics sintered at higher temperature have lower partial discharge inception field and lower breakdown field. As shown in Table 1, pore size increased with sintering temper-
B.-C. Shin et al.r Materials Letters 50 (2001) 82–86
84
Fig. 2. Microstructures of BaTiO 3 ceramics sintered for 2 h at Ža. 13008C, Žb. 13508C, Žc. 14008C, Žd. 14508C.
ature. The BaTiO 3 ceramics, which have large pores, exhibited lower partial discharge and breakdown field. Partial discharges at the large pores may cause them to discharge. These discharges can cause local heating or stressing of the remaining solid dielectrics, and lead to thermal breakdown.
Therefore, instead of the destructive breakdown tests, the nondestructive partial discharge tests can be
Table 1 Density and pore size of pure BaTiO 3 ceramics Sintering temp. Ž8C. Apparent density Ž%. Average pore size Žmm. Standard deviation Žmm.
1300
1350
1400
1450
92 0.5
93 3.7
93 12.4
93 14.9
Ž"0.1.
Ž"2.2.
Ž"5.0.
Ž"8.5. Fig. 3. Typical pattern of internal discharge in BaTiO 3 ceramics. ŽA. Discharge pulses, ŽB. calibration pulses.
B.-C. Shin et al.r Materials Letters 50 (2001) 82–86
85
Fig. 4. Breakdown ŽBD. and partial discharge ŽPD. inception field of BaTiO 3 ceramics as a function of sintering temperature.
used for the reliability tests of the BaTiO 3 ceramic capacitors.
4. Conclusions Measurement of the ac breakdown field and the partial discharge inception field of barium titanate ceramics at 60 Hz resulted as follows. Pore size of barium titanate ceramics increased with sintering temperature Ž1300–14508C.. The breakdown field and partial discharge inception field decreased with the sintering temperature. The large pores, having relatively low inception voltage of
partial discharge, contributed to the low breakdown field of BaTiO 3 ceramics sintered at high temperature. Consequently, the reliability of BaTiO 3 ceramic capacitors can be nondestructively tested by partial discharge testing method.
Acknowledgements The authors wish to acknowledge the financial support of the Korea Research Foundation made in the Program Year 1997.
86
B.-C. Shin et al.r Materials Letters 50 (2001) 82–86
References w1x T. Murakami, A. Yamaji, J. Am. Ceram. Soc 55 Ž1976. 572. w2x T. Nozawa, J. Kato, T. Murakami, A. Yamaji, IEEE Trans. Parts, Hybrids, Packag. 13 Ž1977. 174.
w3x H. Doningos, D. Quattra, J. Scaturro, IEEE Trans. Compon., Hybrids, Manuf. Technol. 1 Ž1978. 423. w4x R. Gerson, T.C. Marshall, J. Appl. Phys. 30 Ž1959. 1650. w5x Z.A. Bukhala, J.K. Nelson, Mater. Eval. 57 Ž1999. 1083. w6x G. Petzow, Am. Soc. Met. Ž1978. 93. w7x F. Kulcsar, J. Am. Ceram. Soc. 39 Ž1956. 13.