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A new method for fabrication of stable and reproducible yttria-based thermistors
Sensors and Actuators 87 Ž2000. 60–66 www.elsevier.nlrlocatersna
A new method for fabrication of stable and reproducible yttria-based thermistors Ayan Banerjee, Sheikh A. Akbar ) Center for Industrial Sensors and Measurements (CISM), The Ohio State UniÕersity, 291 Watts Hall, 2041 College Road, Columbus, OH 43210-1178, USA Received 10 December 1999; received in revised form 22 March 2000; accepted 4 May 2000
Abstract Yttria has been considered as a potential candidate for a high-temperature thermistor. Two of the main challenges with regard to commercialization of high-temperature thermistors are Ži. aging of these thermistors at high temperatures and Žii. lack of suitable lead wire attachment techniques. Densification of thermistors near theoretical density seems to eliminate the aging problem. A new technique has been developed that improves the attachment of lead wire with the sensor material Žyttria. leading to a significantly reduced time of manufacturing of thermistors. This technique is compared with two other techniques, namely the conventional method and in-situ lead wire attachment method ŽISAM., used earlier for fabrication of high temperature thermistors. The new technique also helps achieve good density leading to the elimination of the aging problem. Aging tests had been conducted at 10008C in different environments ŽO 2 , N2 , air. to see the effect of environment on aging behavior. Experimental results show no evidence of aging of the thermistors manufactured by the new method. Thermistors developed via this new technique also show good reproducibility. Analysis of the thermistorrPt lead wire interface shows no evidence of any new phase or compound formation at the interface. The SEM micrograph of the interface shows good contact between Pt lead wire and yttria. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Thermistor; Yttria; Yttrium oxide ŽY2 O 3 .; Electrical property
1. Introduction Thermistors are thermally sensitive resistors. Most thermistors exhibit a decrease in resistance with an increase in temperature and are referred to as NTC type. Low-temperature Žy1008C to 5008C. thermistors made of transition metal oxides are widely used in industries for temperature control and compensation in electronic devices w1x. Although oxides of rare earth elements Že.g., Sm and Tb. have been considered for use at higher temperatures, reliable thermistors for applications near and above 10008C are yet to be developed. Typical problems associated with thermistors are stability and reproducibility, which become even more pronounced at higher temperatures. Therefore, commercialization of high temperature thermistors is yet to be realized.
) Corresponding author. Tel.: q1-614-292-6725; fax: q1-614-6884949.
Electrical characteristics of NTC thermistors are essentially due to bulk conduction that can be described by the following equation w2x:
s s s 0 exp Ž yEarkT . PO"2 1r n
Ž 1.
where s 0 is a constant, Ea is the activation energy for conduction, and the value and sign of 1rn are determined by the type of dominant bulk defect involved in the equilibrium process. Where positive value of 1rn corresponds to p-type conduction and negative value corresponds to n-type conduction. The sensitivity of NTC thermistors can be defined as follows w3x:
a T s Ž 1rr . Ž E rrET . s yBrT 2
Ž 2.
where r is the resistivity Žs 1rs . and T is the temperature. B s Eark is known as the beta factor of an NTC thermistor. This factor shows that the larger the activation energy the higher the sensitivity of a thermistor. a T has a strong dependence on temperature, which decreases with temperature with a factor of Ty2 . This suggests that at high temperatures there is a gradual decrease in sensitivity w3x. Therefore, it is required that thermistors should have
0924-4247r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 0 0 . 0 0 4 6 6 - 0
A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
high activation energy to maintain high sensitivity at high temperatures. Yttria-based thermistors have been studied in authors’ laboratory for applications near and above 10008C w3,4x. One of the main challenges is the so-called aging behavior at high temperatures. It has been observed that at high temperatures, the resistance of an yttria-based thermistor changes dramatically over days. In the Y2 O 3-based thermistors, doping by ZrO 2 seems to eliminate the aging problem w5,6x. The doping also eliminates the effect of humid environment and minimizes the oxygen partial pressure dependence w5,6x. However, it has been observed that doping with ZrO 2 causes a significant decrease in sensitivity of the thermistors w5,6x. In this study, it is observed that densification of the thermistor near its theoretical density seems to eliminate the problem of aging without affecting sensitivity dramatically. Dependence of aging behavior on various environments has also been studied and is reported here. Another challenge lies in selecting appropriate electrical contacts and lead wire materials as well as developing a technique to attach them to the sensor. It has been ob-
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Fig. 2. Aging behavior of fabricated thermistors using three different methods.
served in the past that Pt electrodes attached with Pt lead wire assembly causes in electrode detachment after a long period of testing. Moreover, the method takes significantly long processing time to produce these types of thermistors. In this paper, a new approach for thermistor fabrication is described, which practically eliminates the above mentioned problems.
2. Experimental Three types of thermistor designs have been studied using the following methods: Ža. conventional method, Žb. in-situ lead wire attachment method ŽISAM. w6x, and Žc. a new method reported here. 2.1. ConÕentional method Disk-shaped samples of yttria Ž99.99% pure. were made from commercial powders ŽJohnson Matthey. of an average particle size of less than 1 mm. Preweighed powder of 1 g was cold-pressed under 2 tons of pressure in a stainless steel die by single-end compaction. The green compacts were sintered at 15508C in air for 24 h. A Lindberg
Fig. 1. Schematic of the new technique for fabrication of thermistors with inserted lead wires. Legends: Ž1. Insertion of measured length Ž0.5 in.. of platinum lead wires into two drilled holes in the lower ram, Ž2. measured weight of powder is filled, Ž3. single end compaction through upper ram, Ž4. ejection of compacted ceramic, Ž5. sintering at 15508C for 24 h, and Ž6. final product.
Fig. 3. Reproducibility of thermistors prepared by the conventional method.
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A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
Fig. 6. SEM micrograph of aged Žsintered for 24 h at 15508C. sample fabricated by ISAM. Fig. 4. Aging behavior of samples prepared by the ISAM method under different sintering conditions and varying particle size.
furnace was used for sintering. The sintered pellets were 2 mm thick and 10 mm in diameter. The faces of the disk sample were polished and painted with a thin layer of platinum paste to form electrodes. Platinum lead wires were also attached with platinum electrodes. After the organic binder in the paste was dried in room temperature for 24 h, the whole assembly was cured at 12008C for 24 h. 2.2. In-situ lead wire attachment method The ISAM, details discussed elsewhere w6x, used high purity Ž99.99%. commercial ŽJohnson Matthey. yttria powder of less than 1 mm or high purity Ž99.99%. commercial yttria nanoparticles ŽNanoPhase.. A plastic body that consists of 15 wt.% polystyreneracrylic copolymer, 19 wt.% H 2 O, and 66 wt.% yttria powder was made. Preweighed plastic body of 1 g was cold-pressed at 1 kpsi using a steel die by single-end compaction. Two platinum lead wires were then inserted into this green disk-shaped plastic body. Heat treatment of the assembly was done at 2008C for 2 h
Fig. 5. SEM micrograph of sintered Ž24 h at 15508C. sample fabricated by ISAM.
and at 5008C for 2 h for adequate binder burnout. After the binder burnout heat treatment, three different sintering conditions were carried out: 15508C for 24 h, 16258C for 24 h, and 15508C for 168 h. Since ISAM samples have relatively low green density, sintering conditions were varied in a wide range to see the effect on densification and its influence on the aging behavior. 2.3. New method High purity Ž99.99%. commercial ŽJohnson Matthey. yttria powder was used to make thermistor assembly. Two platinum lead wires Ž32-gauge diameter and 1 mm long. were placed into two drilled holes of the lower ram of the die body. Preweighed powder of 1 g was placed in the die and cold pressing of the powder was done along with the platinum lead wires at 2 tons of pressure. The lower ram was ejected from the die body after the compaction. The thermistor and lead wire assembly was then carefully removed. The assembly was then sintered using one of the following conditions to achieve varied densification, 15508C for 24 h, 15508C for 8 h, and 15508C for 48 h. The sintered samples were each 2 mm thick and 10 mm in diameter. Fig. 1 shows the schematic of the new approach for manufacturing high-density thermistors. A Philips XL-30 scanning electron microscope ŽSEM. was used to obtain the microstructural information of all
Fig. 7. Reproducibility of thermistors prepared by ISAM.
A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
Fig. 8. Aging behavior of thermistors, prepared by new method, in different environments.
the sintered samples and samples after aging. SEM operating at 15 kV was used to examine the fractured surfaces of the samples coated with conductive gold. For electrical measurements, platinum lead wires were used to connect the thermistor to an HP 34401A multimeter for DC resistance measurements. Aging tests were conducted at 10008C in air. Samples were kept in the tube furnace for 11 days and aged. Average resistance was recorded every day using the HP 34401A multimeter and data were collected using Benchlink data acquisition software ŽHewlett Packard.. Resistance changes were recorded as a fraction of starting resistance. Porosity of each sample was measured using a BET adsorption method and was also confirmed using the Archimedes method. To measure the different resistance components of a thermistor lead wire assembly, two-probe AC electrical
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data were collected using a Solartron ŽSI 1260. impedance-gain phase analyzer at 10008C. A signal voltage of 1 Vrms was used. The resistance and capacitance were measured as a function of frequency from 1000 Hz to 33 MHz using a computer software program ŽZ60.. Another software program ŽZView. was used to analyze the data in the four complex planes Žimpedance, complex capacitance, modulus, and admittance.. Impedance analysis was conducted for thermistors prepared by ISAM and the new method. SEM and EDS analysis were carried out on the thermistors fabricated by the new method in order to study the thermistor lead wire interface. The objectives of the interface analysis were to determine Ži. any new compound formation at the interface and Ž2. the integrity of the interface.
3. Results and discussion Fig. 2 shows the aging behavior of three samples prepared by the three methods and sintered at 15508C for 24 h. It is clear that the lowest porosity was achieved using the new method, which in turn did not show aging. The highest porosity sample was prepared by using ISAM and showed a large amount of resistance change Ži.e., aging.. 3.1. ConÕentional method Fig. 3 shows the reproducibility of thermistors prepared by the conventional method. All the samples were sintered
Fig. 9. Resistance of Y2 O 3 as a function of % O 2 at 6508C.
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A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
at 15508C for 24 h. Tests were conducted on six independent samples and resistance values were measured at 7508C, 8508C, 9508C, and 10008C in air. Samples did not show good reproducibility and showed varying resistance values from sample to sample. Varying electrode shape and lead wire attachments from sample to sample may be 2 possible reason for poor reproducibility. 3.2. In-situ lead wire attachment method Fig. 4 shows the aging behavior of samples prepared by ISAM under different sintering conditions and also when nanoparticle yttria powder was used for fabrication. The tests were conducted for 11 days at 10008C in air in a tube furnace. Samples with different sintering temperatures and sintering times were compared for their aging behavior. Samples sintered at 15508C for 24 h showed maximum aging. Considerable improvement was achieved when the sample was sintered at 16258C due to lower porosity Ž20%. compared to the sample sintered at 15508C for 24 h Žporosity; 45%.. The use of nanoparticles instead of micron-size particles also showed a considerable improvement when the sample was sintered at 15508C for 24 h. The use of nanoparticles improved the density of the sample to 77%, which in turn resulted in significant improvement in the aging behavior. When the sample was sintered at 15508C for 168 h, density increased to 85% and the aging practically disappeared. Fig. 5 shows the SEM micrograph of sintered sample manufactured by ISAM and heat-treated for 24 h at 15508C. Fig. 6 shows the same sample after it was aged at 10008C for 10 days. Evidently, ISAM sample did not have good density, therefore a significant amount of aging occurred. Post-aged microstructure suggests that the aging behavior was not because of any microstructural changes during aging, rather it was due to oxygen loss from the bulk, as discussed later. Fig. 7 shows the reproducibility of samples prepared by ISAM. All the samples were sintered at 15508C for 24 h. Tests were conducted on six independent samples and resistance values were measured at 7508C, 8508C, 9508C,
Fig. 10. Reproducibility of thermistors prepared by the new method.
Fig. 11. Aging behavior of samples prepared by the new method under two different sintering conditions. R 0 is the resistance at time zero.
and 10008C in air. These samples also showed lack of reproducibility. Varying lead wire spacing from sample to sample may be the possible reason for poor reproducibility.
Fig. 12. SEM micrographs of the sintered samples fabricated by the new method: Ža. 24 h at 15508C and Žb. 8 h at 15508C.
A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
3.3. New method Fig. 8 shows the aging behavior of thermistors prepared by the new method under different environmental conditions, i.e., air, nitrogen, and oxygen. The reason for minimal aging in these samples is attributed to the higher densities Ž85%. achieved by the new method. While in air and oxygen environments, the samples practically showed no aging, in nitrogen it showed some aging. As reported by Norby and Kofstad w7x, yttria is a p-type material. In nitrogen environment, oxygen is lost from the sample and this loss of oxygen causes release of electrons leading to the increase in resistance. Fig. 9 shows that resistance change is much more rapid in lower levels of PO 2 as it becomes easier to release oxygen to the atmosphere.
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Fig. 10 shows reproducibility of thermistors prepared by the new method. All the samples were sintered at 15508C for 24 h. Tests were conducted on six independent samples and resistance values were measured at 7508C, 8508C, 9508C, and 10008C in air. Samples showed very good reproducibility as compared to samples prepared by the conventional method and ISAM. Fixed lead wire spacing, which was due to the fixed space between two drilled holes in the die, from sample to sample might be the possible reason for very good reproducibility. Fig. 11 shows the aging behavior of two types of samples prepared by the new method but under different sintering conditions. Samples were sintered at 15508C for 24 and 8 h, respectively. Fig. 12a and b show the SEM micrographs of the 24- and 8-h sintered samples, respec-
Fig. 13. Ža. SEM micrograph of the inserted platinum lead wireryttria interface. Žb. A magnified SEM micrograph of the interface.
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A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
tively. The sample sintered for 8 h showed considerable amount of aging as compared to the sample sintered for 24 h. The reason is due to low density Ž55%. in 8-h sintered samples, as evident from the SEM micrograph. No evidence of any noticeable microstructural change during aging suggests that aging is caused by other reasons, possibly due to loss of oxygen from the bulk which is easier in the case of highly porous samples. When the 8-h-sintered sample was exposed to pure oxygen the resistance dropped to its starting value and did not show aging in pure oxygen. This behavior suggests that aging in air is mainly due to the loss of oxygen from the bulk of the sample. Therefore, as soon as pure oxygen is introduced the sample does not show any aging. The densification, therefore, helps eliminate the oxygen loss from the bulk, which in turn minimizes aging. The impedance plane plot based on the AC data of a thermistor fabricated by the new method, revealed only one semicircular relaxation. The modulus plane plot also revealed only one semicircular relaxation. This observation is not unusual and has been reported in the literature w8x. A single semicircular relaxation in both the planes suggests that the electrical properties of the grain and grain boundaries are lumped together. This is consistent with the fact that the resistance calculated from the fit in the impedance plane agreed well with the measured DC resistance. The presence of a small depression angle indicates a small degree of heterogeneity in the electrical path. Fig. 13a shows the SEM micrograph across the platinum lead wireryttria interface. It can be seen that there is a good contact between yttria and platinum. A magnified image of the area is shown in Fig. 13b and it can be seen that the yttria grains are directly in contact with platinum. The EDS analysis of the platinumryttria interface showed no evidence of any new phase or compound formation at the interface. In all the figures, the area marked by ‘a’ is the platinum side and the area marked by ‘b’ is the yttria side.
w1x H.B. Sachse, Semiconducting Temperature Sensors and Their Applications, Wiley, New York, 1975. w2x C.C. Wang, S.A. Akbar, W. Chen, V. Patton, A review on electrical properties of high-temperature materials: oxides, borides, carbides, and nitrides, J. Mater. Sci. 30 Ž1995. 1627. w3x C.C. Wang, S.A. Akbar, W. Chen, J.R. Schorr, High-temperature thermistors based on yttria and calcium zirconate, Sens. Actuators, A 58 Ž1997. 237. w4x S.A. Akbar, W. Chen, V.D. Patton, C.C. Wang, High-Temperature Thermistor Device and Method, patent a 5,681,111, October 28, 1997. w5x C.C. Wang, V.D. Patton, S.A. Akbar, M.A. Alim, Effect of zirconia doping to the electrical behavior of yttria, J. Mater. Res. 11 Ž2. Ž1996. 422. w6x C. Holt, M. Rahman, S.A. Akbar, P. Bulusus, C.C. Wang, Electrode attachment, aging and thermal cycling of yttria-based thermistors, Mater. Let. 40 Ž1999. 213. w7x T. Norby, P. Kofstad, Electrical conductivity of Y2 O 3 as a function of oxygen partial pressures in wet and dry atmospheres, J. Am. Ceram. Soc. 69 Ž1986. 784. w8x J. Ross Macdonald, Impedance Spectroscopy — Emphasizing Solid Materials and Systems, Wiley, 1987, p. 222.
4. Conclusions
Biographies
The primary cause of aging in a porous sample is due to loss of oxygen from yttria during exposure to elevated temperatures. Densification of thermistors near theoretical density can eliminate the aging problem. The new technique reported here improves the attachment of lead wire with the sensor material Žyttria. leading to a significantly reduced time of manufacturing of thermistors. This technique also helps achieve higher densities that in turn
eliminates the aging problem. Aging tests had been conducted at 10008C in different environments ŽO 2 , N2 , air. to study the effect of environment on the aging behavior. The thermistors developed through this new technique have also shown good reproducibility. Analysis of the ceramicrplatinum interface showed indication of good contact and no evidence of any new phase or compound formation at the interface. Therefore, the reported method is a viable approach to produce reproducible thermistor probes with inserted lead wires and minimized aging. Acknowledgements This work was supported through the Center for Industrial Sensors and Measurements ŽCISM. funded by the National Science Foundation grant, EEC-9523358. References
Ayan Banerjee received his MS degree in Materials Science and Engineering from The Ohio State University in December of 1999. This paper is based on his thesis work. Sheikh A. Akbar is a Professor of Materials Science and Engineering and Founding Director of NSF Center for Industrial Sensors and Measurements ŽCISM. at The Ohio State University. He obtained his PhD from Purdue University in 1985. His current research interests are in the development and characterization of electronic ceramics such as solid electrolytes, dielectric materials, sensors, high-temperature lead wire and insulation materials.
A new method for fabrication of stable and reproducible yttria-based thermistors Ayan Banerjee, Sheikh A. Akbar ) Center for Industrial Sensors and Measurements (CISM), The Ohio State UniÕersity, 291 Watts Hall, 2041 College Road, Columbus, OH 43210-1178, USA Received 10 December 1999; received in revised form 22 March 2000; accepted 4 May 2000
Abstract Yttria has been considered as a potential candidate for a high-temperature thermistor. Two of the main challenges with regard to commercialization of high-temperature thermistors are Ži. aging of these thermistors at high temperatures and Žii. lack of suitable lead wire attachment techniques. Densification of thermistors near theoretical density seems to eliminate the aging problem. A new technique has been developed that improves the attachment of lead wire with the sensor material Žyttria. leading to a significantly reduced time of manufacturing of thermistors. This technique is compared with two other techniques, namely the conventional method and in-situ lead wire attachment method ŽISAM., used earlier for fabrication of high temperature thermistors. The new technique also helps achieve good density leading to the elimination of the aging problem. Aging tests had been conducted at 10008C in different environments ŽO 2 , N2 , air. to see the effect of environment on aging behavior. Experimental results show no evidence of aging of the thermistors manufactured by the new method. Thermistors developed via this new technique also show good reproducibility. Analysis of the thermistorrPt lead wire interface shows no evidence of any new phase or compound formation at the interface. The SEM micrograph of the interface shows good contact between Pt lead wire and yttria. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Thermistor; Yttria; Yttrium oxide ŽY2 O 3 .; Electrical property
1. Introduction Thermistors are thermally sensitive resistors. Most thermistors exhibit a decrease in resistance with an increase in temperature and are referred to as NTC type. Low-temperature Žy1008C to 5008C. thermistors made of transition metal oxides are widely used in industries for temperature control and compensation in electronic devices w1x. Although oxides of rare earth elements Že.g., Sm and Tb. have been considered for use at higher temperatures, reliable thermistors for applications near and above 10008C are yet to be developed. Typical problems associated with thermistors are stability and reproducibility, which become even more pronounced at higher temperatures. Therefore, commercialization of high temperature thermistors is yet to be realized.
) Corresponding author. Tel.: q1-614-292-6725; fax: q1-614-6884949.
Electrical characteristics of NTC thermistors are essentially due to bulk conduction that can be described by the following equation w2x:
s s s 0 exp Ž yEarkT . PO"2 1r n
Ž 1.
where s 0 is a constant, Ea is the activation energy for conduction, and the value and sign of 1rn are determined by the type of dominant bulk defect involved in the equilibrium process. Where positive value of 1rn corresponds to p-type conduction and negative value corresponds to n-type conduction. The sensitivity of NTC thermistors can be defined as follows w3x:
a T s Ž 1rr . Ž E rrET . s yBrT 2
Ž 2.
where r is the resistivity Žs 1rs . and T is the temperature. B s Eark is known as the beta factor of an NTC thermistor. This factor shows that the larger the activation energy the higher the sensitivity of a thermistor. a T has a strong dependence on temperature, which decreases with temperature with a factor of Ty2 . This suggests that at high temperatures there is a gradual decrease in sensitivity w3x. Therefore, it is required that thermistors should have
0924-4247r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 0 0 . 0 0 4 6 6 - 0
A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
high activation energy to maintain high sensitivity at high temperatures. Yttria-based thermistors have been studied in authors’ laboratory for applications near and above 10008C w3,4x. One of the main challenges is the so-called aging behavior at high temperatures. It has been observed that at high temperatures, the resistance of an yttria-based thermistor changes dramatically over days. In the Y2 O 3-based thermistors, doping by ZrO 2 seems to eliminate the aging problem w5,6x. The doping also eliminates the effect of humid environment and minimizes the oxygen partial pressure dependence w5,6x. However, it has been observed that doping with ZrO 2 causes a significant decrease in sensitivity of the thermistors w5,6x. In this study, it is observed that densification of the thermistor near its theoretical density seems to eliminate the problem of aging without affecting sensitivity dramatically. Dependence of aging behavior on various environments has also been studied and is reported here. Another challenge lies in selecting appropriate electrical contacts and lead wire materials as well as developing a technique to attach them to the sensor. It has been ob-
61
Fig. 2. Aging behavior of fabricated thermistors using three different methods.
served in the past that Pt electrodes attached with Pt lead wire assembly causes in electrode detachment after a long period of testing. Moreover, the method takes significantly long processing time to produce these types of thermistors. In this paper, a new approach for thermistor fabrication is described, which practically eliminates the above mentioned problems.
2. Experimental Three types of thermistor designs have been studied using the following methods: Ža. conventional method, Žb. in-situ lead wire attachment method ŽISAM. w6x, and Žc. a new method reported here. 2.1. ConÕentional method Disk-shaped samples of yttria Ž99.99% pure. were made from commercial powders ŽJohnson Matthey. of an average particle size of less than 1 mm. Preweighed powder of 1 g was cold-pressed under 2 tons of pressure in a stainless steel die by single-end compaction. The green compacts were sintered at 15508C in air for 24 h. A Lindberg
Fig. 1. Schematic of the new technique for fabrication of thermistors with inserted lead wires. Legends: Ž1. Insertion of measured length Ž0.5 in.. of platinum lead wires into two drilled holes in the lower ram, Ž2. measured weight of powder is filled, Ž3. single end compaction through upper ram, Ž4. ejection of compacted ceramic, Ž5. sintering at 15508C for 24 h, and Ž6. final product.
Fig. 3. Reproducibility of thermistors prepared by the conventional method.
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A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
Fig. 6. SEM micrograph of aged Žsintered for 24 h at 15508C. sample fabricated by ISAM. Fig. 4. Aging behavior of samples prepared by the ISAM method under different sintering conditions and varying particle size.
furnace was used for sintering. The sintered pellets were 2 mm thick and 10 mm in diameter. The faces of the disk sample were polished and painted with a thin layer of platinum paste to form electrodes. Platinum lead wires were also attached with platinum electrodes. After the organic binder in the paste was dried in room temperature for 24 h, the whole assembly was cured at 12008C for 24 h. 2.2. In-situ lead wire attachment method The ISAM, details discussed elsewhere w6x, used high purity Ž99.99%. commercial ŽJohnson Matthey. yttria powder of less than 1 mm or high purity Ž99.99%. commercial yttria nanoparticles ŽNanoPhase.. A plastic body that consists of 15 wt.% polystyreneracrylic copolymer, 19 wt.% H 2 O, and 66 wt.% yttria powder was made. Preweighed plastic body of 1 g was cold-pressed at 1 kpsi using a steel die by single-end compaction. Two platinum lead wires were then inserted into this green disk-shaped plastic body. Heat treatment of the assembly was done at 2008C for 2 h
Fig. 5. SEM micrograph of sintered Ž24 h at 15508C. sample fabricated by ISAM.
and at 5008C for 2 h for adequate binder burnout. After the binder burnout heat treatment, three different sintering conditions were carried out: 15508C for 24 h, 16258C for 24 h, and 15508C for 168 h. Since ISAM samples have relatively low green density, sintering conditions were varied in a wide range to see the effect on densification and its influence on the aging behavior. 2.3. New method High purity Ž99.99%. commercial ŽJohnson Matthey. yttria powder was used to make thermistor assembly. Two platinum lead wires Ž32-gauge diameter and 1 mm long. were placed into two drilled holes of the lower ram of the die body. Preweighed powder of 1 g was placed in the die and cold pressing of the powder was done along with the platinum lead wires at 2 tons of pressure. The lower ram was ejected from the die body after the compaction. The thermistor and lead wire assembly was then carefully removed. The assembly was then sintered using one of the following conditions to achieve varied densification, 15508C for 24 h, 15508C for 8 h, and 15508C for 48 h. The sintered samples were each 2 mm thick and 10 mm in diameter. Fig. 1 shows the schematic of the new approach for manufacturing high-density thermistors. A Philips XL-30 scanning electron microscope ŽSEM. was used to obtain the microstructural information of all
Fig. 7. Reproducibility of thermistors prepared by ISAM.
A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
Fig. 8. Aging behavior of thermistors, prepared by new method, in different environments.
the sintered samples and samples after aging. SEM operating at 15 kV was used to examine the fractured surfaces of the samples coated with conductive gold. For electrical measurements, platinum lead wires were used to connect the thermistor to an HP 34401A multimeter for DC resistance measurements. Aging tests were conducted at 10008C in air. Samples were kept in the tube furnace for 11 days and aged. Average resistance was recorded every day using the HP 34401A multimeter and data were collected using Benchlink data acquisition software ŽHewlett Packard.. Resistance changes were recorded as a fraction of starting resistance. Porosity of each sample was measured using a BET adsorption method and was also confirmed using the Archimedes method. To measure the different resistance components of a thermistor lead wire assembly, two-probe AC electrical
63
data were collected using a Solartron ŽSI 1260. impedance-gain phase analyzer at 10008C. A signal voltage of 1 Vrms was used. The resistance and capacitance were measured as a function of frequency from 1000 Hz to 33 MHz using a computer software program ŽZ60.. Another software program ŽZView. was used to analyze the data in the four complex planes Žimpedance, complex capacitance, modulus, and admittance.. Impedance analysis was conducted for thermistors prepared by ISAM and the new method. SEM and EDS analysis were carried out on the thermistors fabricated by the new method in order to study the thermistor lead wire interface. The objectives of the interface analysis were to determine Ži. any new compound formation at the interface and Ž2. the integrity of the interface.
3. Results and discussion Fig. 2 shows the aging behavior of three samples prepared by the three methods and sintered at 15508C for 24 h. It is clear that the lowest porosity was achieved using the new method, which in turn did not show aging. The highest porosity sample was prepared by using ISAM and showed a large amount of resistance change Ži.e., aging.. 3.1. ConÕentional method Fig. 3 shows the reproducibility of thermistors prepared by the conventional method. All the samples were sintered
Fig. 9. Resistance of Y2 O 3 as a function of % O 2 at 6508C.
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at 15508C for 24 h. Tests were conducted on six independent samples and resistance values were measured at 7508C, 8508C, 9508C, and 10008C in air. Samples did not show good reproducibility and showed varying resistance values from sample to sample. Varying electrode shape and lead wire attachments from sample to sample may be 2 possible reason for poor reproducibility. 3.2. In-situ lead wire attachment method Fig. 4 shows the aging behavior of samples prepared by ISAM under different sintering conditions and also when nanoparticle yttria powder was used for fabrication. The tests were conducted for 11 days at 10008C in air in a tube furnace. Samples with different sintering temperatures and sintering times were compared for their aging behavior. Samples sintered at 15508C for 24 h showed maximum aging. Considerable improvement was achieved when the sample was sintered at 16258C due to lower porosity Ž20%. compared to the sample sintered at 15508C for 24 h Žporosity; 45%.. The use of nanoparticles instead of micron-size particles also showed a considerable improvement when the sample was sintered at 15508C for 24 h. The use of nanoparticles improved the density of the sample to 77%, which in turn resulted in significant improvement in the aging behavior. When the sample was sintered at 15508C for 168 h, density increased to 85% and the aging practically disappeared. Fig. 5 shows the SEM micrograph of sintered sample manufactured by ISAM and heat-treated for 24 h at 15508C. Fig. 6 shows the same sample after it was aged at 10008C for 10 days. Evidently, ISAM sample did not have good density, therefore a significant amount of aging occurred. Post-aged microstructure suggests that the aging behavior was not because of any microstructural changes during aging, rather it was due to oxygen loss from the bulk, as discussed later. Fig. 7 shows the reproducibility of samples prepared by ISAM. All the samples were sintered at 15508C for 24 h. Tests were conducted on six independent samples and resistance values were measured at 7508C, 8508C, 9508C,
Fig. 10. Reproducibility of thermistors prepared by the new method.
Fig. 11. Aging behavior of samples prepared by the new method under two different sintering conditions. R 0 is the resistance at time zero.
and 10008C in air. These samples also showed lack of reproducibility. Varying lead wire spacing from sample to sample may be the possible reason for poor reproducibility.
Fig. 12. SEM micrographs of the sintered samples fabricated by the new method: Ža. 24 h at 15508C and Žb. 8 h at 15508C.
A. Banerjee, S.A. Akbarr Sensors and Actuators 87 (2000) 60–66
3.3. New method Fig. 8 shows the aging behavior of thermistors prepared by the new method under different environmental conditions, i.e., air, nitrogen, and oxygen. The reason for minimal aging in these samples is attributed to the higher densities Ž85%. achieved by the new method. While in air and oxygen environments, the samples practically showed no aging, in nitrogen it showed some aging. As reported by Norby and Kofstad w7x, yttria is a p-type material. In nitrogen environment, oxygen is lost from the sample and this loss of oxygen causes release of electrons leading to the increase in resistance. Fig. 9 shows that resistance change is much more rapid in lower levels of PO 2 as it becomes easier to release oxygen to the atmosphere.
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Fig. 10 shows reproducibility of thermistors prepared by the new method. All the samples were sintered at 15508C for 24 h. Tests were conducted on six independent samples and resistance values were measured at 7508C, 8508C, 9508C, and 10008C in air. Samples showed very good reproducibility as compared to samples prepared by the conventional method and ISAM. Fixed lead wire spacing, which was due to the fixed space between two drilled holes in the die, from sample to sample might be the possible reason for very good reproducibility. Fig. 11 shows the aging behavior of two types of samples prepared by the new method but under different sintering conditions. Samples were sintered at 15508C for 24 and 8 h, respectively. Fig. 12a and b show the SEM micrographs of the 24- and 8-h sintered samples, respec-
Fig. 13. Ža. SEM micrograph of the inserted platinum lead wireryttria interface. Žb. A magnified SEM micrograph of the interface.
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tively. The sample sintered for 8 h showed considerable amount of aging as compared to the sample sintered for 24 h. The reason is due to low density Ž55%. in 8-h sintered samples, as evident from the SEM micrograph. No evidence of any noticeable microstructural change during aging suggests that aging is caused by other reasons, possibly due to loss of oxygen from the bulk which is easier in the case of highly porous samples. When the 8-h-sintered sample was exposed to pure oxygen the resistance dropped to its starting value and did not show aging in pure oxygen. This behavior suggests that aging in air is mainly due to the loss of oxygen from the bulk of the sample. Therefore, as soon as pure oxygen is introduced the sample does not show any aging. The densification, therefore, helps eliminate the oxygen loss from the bulk, which in turn minimizes aging. The impedance plane plot based on the AC data of a thermistor fabricated by the new method, revealed only one semicircular relaxation. The modulus plane plot also revealed only one semicircular relaxation. This observation is not unusual and has been reported in the literature w8x. A single semicircular relaxation in both the planes suggests that the electrical properties of the grain and grain boundaries are lumped together. This is consistent with the fact that the resistance calculated from the fit in the impedance plane agreed well with the measured DC resistance. The presence of a small depression angle indicates a small degree of heterogeneity in the electrical path. Fig. 13a shows the SEM micrograph across the platinum lead wireryttria interface. It can be seen that there is a good contact between yttria and platinum. A magnified image of the area is shown in Fig. 13b and it can be seen that the yttria grains are directly in contact with platinum. The EDS analysis of the platinumryttria interface showed no evidence of any new phase or compound formation at the interface. In all the figures, the area marked by ‘a’ is the platinum side and the area marked by ‘b’ is the yttria side.
w1x H.B. Sachse, Semiconducting Temperature Sensors and Their Applications, Wiley, New York, 1975. w2x C.C. Wang, S.A. Akbar, W. Chen, V. Patton, A review on electrical properties of high-temperature materials: oxides, borides, carbides, and nitrides, J. Mater. Sci. 30 Ž1995. 1627. w3x C.C. Wang, S.A. Akbar, W. Chen, J.R. Schorr, High-temperature thermistors based on yttria and calcium zirconate, Sens. Actuators, A 58 Ž1997. 237. w4x S.A. Akbar, W. Chen, V.D. Patton, C.C. Wang, High-Temperature Thermistor Device and Method, patent a 5,681,111, October 28, 1997. w5x C.C. Wang, V.D. Patton, S.A. Akbar, M.A. Alim, Effect of zirconia doping to the electrical behavior of yttria, J. Mater. Res. 11 Ž2. Ž1996. 422. w6x C. Holt, M. Rahman, S.A. Akbar, P. Bulusus, C.C. Wang, Electrode attachment, aging and thermal cycling of yttria-based thermistors, Mater. Let. 40 Ž1999. 213. w7x T. Norby, P. Kofstad, Electrical conductivity of Y2 O 3 as a function of oxygen partial pressures in wet and dry atmospheres, J. Am. Ceram. Soc. 69 Ž1986. 784. w8x J. Ross Macdonald, Impedance Spectroscopy — Emphasizing Solid Materials and Systems, Wiley, 1987, p. 222.
4. Conclusions
Biographies
The primary cause of aging in a porous sample is due to loss of oxygen from yttria during exposure to elevated temperatures. Densification of thermistors near theoretical density can eliminate the aging problem. The new technique reported here improves the attachment of lead wire with the sensor material Žyttria. leading to a significantly reduced time of manufacturing of thermistors. This technique also helps achieve higher densities that in turn
eliminates the aging problem. Aging tests had been conducted at 10008C in different environments ŽO 2 , N2 , air. to study the effect of environment on the aging behavior. The thermistors developed through this new technique have also shown good reproducibility. Analysis of the ceramicrplatinum interface showed indication of good contact and no evidence of any new phase or compound formation at the interface. Therefore, the reported method is a viable approach to produce reproducible thermistor probes with inserted lead wires and minimized aging. Acknowledgements This work was supported through the Center for Industrial Sensors and Measurements ŽCISM. funded by the National Science Foundation grant, EEC-9523358. References
Ayan Banerjee received his MS degree in Materials Science and Engineering from The Ohio State University in December of 1999. This paper is based on his thesis work. Sheikh A. Akbar is a Professor of Materials Science and Engineering and Founding Director of NSF Center for Industrial Sensors and Measurements ŽCISM. at The Ohio State University. He obtained his PhD from Purdue University in 1985. His current research interests are in the development and characterization of electronic ceramics such as solid electrolytes, dielectric materials, sensors, high-temperature lead wire and insulation materials.