Thermoelectrical degradation processes in NTC ... - Semantic Scholar
Microelectronics Reliability 41 (2001) 773±777
Research note
www.elsevier.com/locate/microrel
Thermoelectrical degradation processes in NTC thermistors for in-rush current protection of electronic circuits O. Mrooz a,*, A. Kovalski a, J. Pogorzelska b, O. Shpotyuk a, M. Vakiv a, B. Butkiewicz b,1, J. Maciak b a
Scienti®c Research Company ``CARAT'', 202 Stryjska, Lviv, UA-290031, Ukraine Warsaw University of Technology, 15/19 Nowowiejska, 00-665 Warsaw, Poland
b
Received 30 October 2000; received in revised form 15 February 2001
Abstract Degradation processes, caused by the extreme values of current pulses or thermal burn-in, are studied in thermistors with negative temperature coecient of resistance. The drift of thermistor resistance at 25°C is observed. These changes depend on the value and the number of current pulses. The ®rst 100 cycles are the basic ones for the future exploitation parameters of thermistors. Similar eect, rise of thermistor resistance, was observed after burn-in. The investigated thermistors are used for in-rush current protection of electronic circuits. Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Experimental
Thermistors with negative temperature coecient of resistance (NTC thermistors) are used in electronics as elements for suppression of in-rush current, relay delay, temperature control and sensing, fan control, etc. [1±3]. Low costs of manufacturing, reliability at normal conditions of exploitation (when the temperature of ceramics is lower than the critical degradation temperatures for ceramics and contact materials), simplicity are the main advantages of these devices. At the same time the problem of minimization of thermistor dimensions (at the guaranteeing of maximum exploitation load) is actual up to now. In this connection the problems of reliability, lifetime of thermistors, determination of degradation mechanisms in ceramics, interaction of contact material and ceramic matrix, resistance to the attacks by heat, etc. are very important.
The eect of cyclic switching of extreme impulse current loads (according to the method of accelerated degradation of thermistors [4]) on the power disk NTC thermistors, used for in-rush current protection of electronic circuits, is studied in this work. The changes of thermistor parameters are caused by the in¯uence of electric ®elds and high temperatures, appearing due to the current heating up. Power NTC thermistors, which are used for investigations, are manufactured from the semiconductor materials on the basis of transition metal (nickel, manganese, copper, and cobalt) oxides by the traditional ceramic technology [6±14]. Experimental samples of NTC thermistors are in the form of tablets (diameter D 10 mm, thickness d 1 mm) with applied silver contact surfaces and soldered copper leads. For the protection from the in¯uences of external factors the elements are covered by dielectric noncombustible enamel based on silicon±organic lacquer. The nominal resistance of the investigated NTC thermistors at 25°C is 16 X (admissible deviation within one lot is 20% to 40%). The circuit diagram of the set, which is used for the test of NTC thermistors to the action of cyclic impulse
* Corresponding author. Tel.: +380-322-652283; fax: +380322-632228. E-mail addresses: [email protected] (O. Mrooz and O. Shpotyuk), [email protected] (J. Pogorzelska), [email protected] (B. Butkiewicz). 1 Also corresponding author.
0026-2714/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 1 ) 0 0 0 2 7 - 0
774
O. Mrooz et al. / Microelectronics Reliability 41 (2001) 773±777
Fig. 1. Circuit diagram of the set used for the investigation of NTC thermistors to the action of cyclic current loads.
current loads, is shown in Fig. 1. These investigations are realized providing the multiple charging of 270 lF capacitor by the alternating current at the input voltage Ueff 250 V with the f 50 Hz frequency through NTC thermistor and rectifying bridge. Two parallel circuits (top), commanded by rotary switch, were used to accelerate investigations of 52 thermistors, and to leave sucient time (160 s) to cool thermistors. Multivibrator (bottom) produce signals for rotary switch. The maximum value of the current leaking through NTC thermistor at the moment of switching on is 22 A. This value depends strongly on the ambient temperature, because relation between thermistor resistance R and temperature T can be written as R RN exp B=T B=TN ), where RN , TN are resistance and temperature in nominal conditions. Thermistor constant B equals 3000 K. In our case the investigations are carried out at 0:5 0:5°C. Resistance was measured at 25 0:1°C. The parameters of the chosen testing conditions are considerably more rigid than the nominal working regime of these NTC thermistors (in the exploitation regime envisaged by the speci®cations the value of capacitance is 200 lF) [15]. The testing experiments are conducted using 26 samples of NTC thermistors chosen from one technological lot. Moreover, thermal investigations were performed, without electrical stress. Sample of 11 thermistors was placed in thermostat. Every 24 h temperature was elevated up to 125°C during 10 h. After 2 h thermistor resistances were measured at 25 0:1°C (with HP34401A) and compared with another (``control'') sample of thermistors not submitted to this ``burn-in''.
3. Results and discussion Percent change in resistance DR=R0 of NTC thermistors in dependence on the number N of cycles of current impulse supply (the average data for all investigated samples) is presented in Fig. 2. Three sections for the DR=R0 N curve can be seen clearly. The ®rst section (0±200 cycles) is characterized by the sharp rising of DR=R0 value up to 3±5%. The second section (200±700 cycles) has the relatively stable values of resistance, and the third one (more than
Fig. 2. Percent change in resistance (R=R0 ) of NTC thermistors in dependence on the number N of cycles of current pulse supply (average data, measured at 25°C, for investigated samples).
O. Mrooz et al. / Microelectronics Reliability 41 (2001) 773±777
700 cycles) reveals the new sharp R increasing leading to the irreversible destruction of NTC thermistors. The analogous investigations were carried out at dierent conditions of testing. Capacitor values were changed (240, 270, 330 lF), ambient temperature (13°C, 25°C), and voltage (250, 350 V). The character of DR=R0 N dependence did not change essentially. The dierence consists only in the value of ratio of separate sections of this dependence. So, the analysis of obtained results testi®es the conclusion that the process of resistance degradation under the in¯uence of current impulses has complex, not elementary, character and depends on the number of cycles. Let us analyze the initial section of DR=R0 N curve (0±200 cycles). Visual observations of outward appearance of NTC thermistors after the action of few cycles N 5±20 of electric load show the presence of bulking regions on the enamel surface of some thermistors. It can be assumed that such damages are formed owing to local electrical breakdowns of ceramic matrix. These bulking regions appear, ®rst of all, on the edges of contact lands where the strength p of electric ®eld (mean maximal value is equal to 2Ueff =d 354 kV/m) may attain local maximum. At the further increasing of number of cycles (up to 100±200) the expansion of existing damaged regions and the formation of new ones is observed (Fig. 3). Microstructural investigations of ceramics (on the sections etched in 3% solution of NaCl) before and after the action of 200 cycles of electric current pulses do not show, however, the essential dierences of the grain dimensions and the character of internal pores distribution.
Fig. 3. Outward appearance of NTC thermistor after the in¯uence of 200 cycles of current pulse loads.
775
The following section of DR=R0 N dependence (from 200 up to 700 cycles) is characterized by the relative stabilization of the observed structural transformations. At the increasing of number of cycles up to more than 700 the appeared damages expand (their dimensions in certain cases sharply increase due to the coalescence of a few damaged regions), new defects arise along all surface of thermistor. As the consequence, the nominal resistance of these NTC thermistors leaves the limits of speci®ed values. The occurring changes are seen especially well at the comparison of microstructure fragments for the initial and damaged NTC thermistors (Fig. 4). We can see that essential mass-transfer processes take place in the material of ceramic matrix. These processes are accompanied by the increasing of crystallite blocks and pores dimensions, as well as the extending of intergrain boundaries in the regions of surface neighboring to the regions of electrical breakdowns (black ®eld in Fig. 4b pointed by arrows).
Fig. 4. Microstructure of NTC thermistor samples (a) before and (b) after the in¯uence of 1000 cycles of current pulse loads. Arrows point damage region.
776
O. Mrooz et al. / Microelectronics Reliability 41 (2001) 773±777
pulses or thermal cycles. Initial rise of resistance is caused probably by oxidation process or modi®cation of ionic distribution. It must be veri®ed by additional investigations. Thermistor destruction is connected with electrical breakdown of ceramic matrix on the edges of contact land and has the main role for the further changes of thermistorÕs electrical parameters. At the sharp increasing of number of impulses (more than 700) the irreversible mass-transfer processes in ceramics are essential. It leads to the considerable increasing of NTC thermistor resistance.
References Fig. 5. Change of relative resistance, measured at 25°C, during thermal investigations. Sample submitted to stress at 125°C (top), sample not submitted to stress (bottom).
Result of thermal investigations (mean relative resistance from two measurements) is presented in Fig. 5. Dependence on number of temperature cycles is clearly visible. After the ®rst 5±8 cycles, resistance rises about 4%, similarly as after investigations with current pulses. These experiments were performed independently in dierent scienti®c centers. Curves in Figs. 2 and 5 show very similar growth of resistance in the beginning of experiment. Current pulse causes short-time increasing of internal temperature of thermistorÕs body. Thus, the rise of temperature is one of degradation factors at cyclic loads as in the case of thermal burn-in. The mechanism of this degradation can be connected with the modi®cation of ionic distribution or the oxidation state of ceramics due to the metallization process (serigraphy) observed previously by Fritsch et al. [5] in NTC thermistors. Derivatographic investigations (DTA) are planned to ®nd possible reasons. Other factors of the electro-thermal degradation, as it was mentioned above, are the electrical breakdowns of ceramic matrix. This phenomenon is fully responsible for the next rise of resistance, observed after about 700 cycles of the current pulses. Material of electrodes evaporates, growth of grains and pores is observed near these places. Eective surface of cross-section leading current decreases and resistance of thermistor rises.
4. Conclusions Process of degradation of NTC thermistors has complex character and is, in considerable degree, determined by the total number of applied current im-
[1] Lavenuta G. Negative temperature coecient thermistors. Sensors 1997;14:46±55. [2] Gavrilov V, Tiuh V. Power NTC thermistors. Radio 1996;1:61±2 [1996;2:57 [in Russian]]. [3] Edwards L. Selecting NTC thermistors for control applications. Sensor Rev 1988;8:205±6. [4] Clarc JA, Garganese US, Swarz RS. An approach to designing accelerated life-testing experiments. Proceedings of the Annual Reliability and Maintainability Symposium, 1997. p. 242±8. [5] Fritsch S, Sarrias J, Brieu M, et al. Correlation between the structure, the microstructure, and the electrical properties of nickel manganite negative temperature coecient (NTC) thermistors. Solid State Ionics 1998;109(3±4):229± 37. [6] Castelan P, Loubiere BH, Loubiere A, Rousset A, Hegros R. A geeing study of NTC thermistors by thermopower measurements. Sensors and Actuators 1992;33:119±22. [7] Polyakov AA. Technology of ceramic radioelectronic materials. Radio i Sviaz. Moscow, 1989 [in Russian]. [8] Vakiv MM, Veremeychuk MS, Hadzaman IV, Kovalskiy AP, Mrooz OYa, Shpotyuk OI. Electrophysical properties and microstructure of semiconducting oxide ceramics of MnCo2 O4 ±CuMn2 O4 ±NiMn2 O4 system. Bull Lviv State Univ 1994;33:8±11 [in Ukranian]. [9] Gadzaman IV, Kovalsky PM, Mrooz OYa, Shpotyuk OI. Features of the electrical conductance setting of thermoresistor ceramics on the basis of cubic manganese spinels. Funct Mater 1996;3:104±7. [10] Hadzaman IV, Kovalsky AP, Mrooz OYa, Shpotyuk OI. Thermal modi®cation of ceramic composites based on manganese-containing cube spinels. Mater Lett 1996;29: 195±8. [11] Mrooz OYa, Hadzaman IV, Shpotyuk OI. Complex manganese spinels for NTC thermistors application. Proceedings of the 21st Conference of ISHM Poland, Ustron. 1998, p. 217±20. [12] Mrooz O, Shpotyuk O, Hadzaman I, Kravtsiv M. Optimization of NTC thermistors for current protection of incandescent lamps (240 V, 100±160 W). Proceedings of the 22nd Conference of IMAPS Poland, 1999. p. 259±62. [13] Vakiv MM, Shpotyuk OI, Mrooz OYa, Hadzaman IV, Kovalskiy AP. Electrical conductivity studies of temperature sensitive materials based on Mn, Ni, Co and
O. Mrooz et al. / Microelectronics Reliability 41 (2001) 773±777 Cu-containing spinels. Bull Lviv State Univ 1998;31:142±4 [in Ukranian]. [14] Vakiv MM, Kovalsky AP, Mrooz OYa, Hadzaman IV, Shpotyuk OI. Selection guidelines of oxide ceramic materials based on Ni, Co, Mn, Cu-contained cubic spinels for thermistor applications. In: Samuneva B, Dimitriev Y, editors. Proceedings of the 12th Conference
777
on Glass and Ceramics. Publ House Science Invest; 1997. p. 380±3. [15] Vakiv MM, Maciak J, Mrooz OYa, Pogorzelska J, Szpotiuk OI. Degradation of ceramic thermoresistors under impulse current loads. Technologia i Konstruirowanie w Elektronnoj Apparature 2000;2±3:39±41 [in Russian].
Research note
www.elsevier.com/locate/microrel
Thermoelectrical degradation processes in NTC thermistors for in-rush current protection of electronic circuits O. Mrooz a,*, A. Kovalski a, J. Pogorzelska b, O. Shpotyuk a, M. Vakiv a, B. Butkiewicz b,1, J. Maciak b a
Scienti®c Research Company ``CARAT'', 202 Stryjska, Lviv, UA-290031, Ukraine Warsaw University of Technology, 15/19 Nowowiejska, 00-665 Warsaw, Poland
b
Received 30 October 2000; received in revised form 15 February 2001
Abstract Degradation processes, caused by the extreme values of current pulses or thermal burn-in, are studied in thermistors with negative temperature coecient of resistance. The drift of thermistor resistance at 25°C is observed. These changes depend on the value and the number of current pulses. The ®rst 100 cycles are the basic ones for the future exploitation parameters of thermistors. Similar eect, rise of thermistor resistance, was observed after burn-in. The investigated thermistors are used for in-rush current protection of electronic circuits. Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Experimental
Thermistors with negative temperature coecient of resistance (NTC thermistors) are used in electronics as elements for suppression of in-rush current, relay delay, temperature control and sensing, fan control, etc. [1±3]. Low costs of manufacturing, reliability at normal conditions of exploitation (when the temperature of ceramics is lower than the critical degradation temperatures for ceramics and contact materials), simplicity are the main advantages of these devices. At the same time the problem of minimization of thermistor dimensions (at the guaranteeing of maximum exploitation load) is actual up to now. In this connection the problems of reliability, lifetime of thermistors, determination of degradation mechanisms in ceramics, interaction of contact material and ceramic matrix, resistance to the attacks by heat, etc. are very important.
The eect of cyclic switching of extreme impulse current loads (according to the method of accelerated degradation of thermistors [4]) on the power disk NTC thermistors, used for in-rush current protection of electronic circuits, is studied in this work. The changes of thermistor parameters are caused by the in¯uence of electric ®elds and high temperatures, appearing due to the current heating up. Power NTC thermistors, which are used for investigations, are manufactured from the semiconductor materials on the basis of transition metal (nickel, manganese, copper, and cobalt) oxides by the traditional ceramic technology [6±14]. Experimental samples of NTC thermistors are in the form of tablets (diameter D 10 mm, thickness d 1 mm) with applied silver contact surfaces and soldered copper leads. For the protection from the in¯uences of external factors the elements are covered by dielectric noncombustible enamel based on silicon±organic lacquer. The nominal resistance of the investigated NTC thermistors at 25°C is 16 X (admissible deviation within one lot is 20% to 40%). The circuit diagram of the set, which is used for the test of NTC thermistors to the action of cyclic impulse
* Corresponding author. Tel.: +380-322-652283; fax: +380322-632228. E-mail addresses: [email protected] (O. Mrooz and O. Shpotyuk), [email protected] (J. Pogorzelska), [email protected] (B. Butkiewicz). 1 Also corresponding author.
0026-2714/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 1 ) 0 0 0 2 7 - 0
774
O. Mrooz et al. / Microelectronics Reliability 41 (2001) 773±777
Fig. 1. Circuit diagram of the set used for the investigation of NTC thermistors to the action of cyclic current loads.
current loads, is shown in Fig. 1. These investigations are realized providing the multiple charging of 270 lF capacitor by the alternating current at the input voltage Ueff 250 V with the f 50 Hz frequency through NTC thermistor and rectifying bridge. Two parallel circuits (top), commanded by rotary switch, were used to accelerate investigations of 52 thermistors, and to leave sucient time (160 s) to cool thermistors. Multivibrator (bottom) produce signals for rotary switch. The maximum value of the current leaking through NTC thermistor at the moment of switching on is 22 A. This value depends strongly on the ambient temperature, because relation between thermistor resistance R and temperature T can be written as R RN exp B=T B=TN ), where RN , TN are resistance and temperature in nominal conditions. Thermistor constant B equals 3000 K. In our case the investigations are carried out at 0:5 0:5°C. Resistance was measured at 25 0:1°C. The parameters of the chosen testing conditions are considerably more rigid than the nominal working regime of these NTC thermistors (in the exploitation regime envisaged by the speci®cations the value of capacitance is 200 lF) [15]. The testing experiments are conducted using 26 samples of NTC thermistors chosen from one technological lot. Moreover, thermal investigations were performed, without electrical stress. Sample of 11 thermistors was placed in thermostat. Every 24 h temperature was elevated up to 125°C during 10 h. After 2 h thermistor resistances were measured at 25 0:1°C (with HP34401A) and compared with another (``control'') sample of thermistors not submitted to this ``burn-in''.
3. Results and discussion Percent change in resistance DR=R0 of NTC thermistors in dependence on the number N of cycles of current impulse supply (the average data for all investigated samples) is presented in Fig. 2. Three sections for the DR=R0 N curve can be seen clearly. The ®rst section (0±200 cycles) is characterized by the sharp rising of DR=R0 value up to 3±5%. The second section (200±700 cycles) has the relatively stable values of resistance, and the third one (more than
Fig. 2. Percent change in resistance (R=R0 ) of NTC thermistors in dependence on the number N of cycles of current pulse supply (average data, measured at 25°C, for investigated samples).
O. Mrooz et al. / Microelectronics Reliability 41 (2001) 773±777
700 cycles) reveals the new sharp R increasing leading to the irreversible destruction of NTC thermistors. The analogous investigations were carried out at dierent conditions of testing. Capacitor values were changed (240, 270, 330 lF), ambient temperature (13°C, 25°C), and voltage (250, 350 V). The character of DR=R0 N dependence did not change essentially. The dierence consists only in the value of ratio of separate sections of this dependence. So, the analysis of obtained results testi®es the conclusion that the process of resistance degradation under the in¯uence of current impulses has complex, not elementary, character and depends on the number of cycles. Let us analyze the initial section of DR=R0 N curve (0±200 cycles). Visual observations of outward appearance of NTC thermistors after the action of few cycles N 5±20 of electric load show the presence of bulking regions on the enamel surface of some thermistors. It can be assumed that such damages are formed owing to local electrical breakdowns of ceramic matrix. These bulking regions appear, ®rst of all, on the edges of contact lands where the strength p of electric ®eld (mean maximal value is equal to 2Ueff =d 354 kV/m) may attain local maximum. At the further increasing of number of cycles (up to 100±200) the expansion of existing damaged regions and the formation of new ones is observed (Fig. 3). Microstructural investigations of ceramics (on the sections etched in 3% solution of NaCl) before and after the action of 200 cycles of electric current pulses do not show, however, the essential dierences of the grain dimensions and the character of internal pores distribution.
Fig. 3. Outward appearance of NTC thermistor after the in¯uence of 200 cycles of current pulse loads.
775
The following section of DR=R0 N dependence (from 200 up to 700 cycles) is characterized by the relative stabilization of the observed structural transformations. At the increasing of number of cycles up to more than 700 the appeared damages expand (their dimensions in certain cases sharply increase due to the coalescence of a few damaged regions), new defects arise along all surface of thermistor. As the consequence, the nominal resistance of these NTC thermistors leaves the limits of speci®ed values. The occurring changes are seen especially well at the comparison of microstructure fragments for the initial and damaged NTC thermistors (Fig. 4). We can see that essential mass-transfer processes take place in the material of ceramic matrix. These processes are accompanied by the increasing of crystallite blocks and pores dimensions, as well as the extending of intergrain boundaries in the regions of surface neighboring to the regions of electrical breakdowns (black ®eld in Fig. 4b pointed by arrows).
Fig. 4. Microstructure of NTC thermistor samples (a) before and (b) after the in¯uence of 1000 cycles of current pulse loads. Arrows point damage region.
776
O. Mrooz et al. / Microelectronics Reliability 41 (2001) 773±777
pulses or thermal cycles. Initial rise of resistance is caused probably by oxidation process or modi®cation of ionic distribution. It must be veri®ed by additional investigations. Thermistor destruction is connected with electrical breakdown of ceramic matrix on the edges of contact land and has the main role for the further changes of thermistorÕs electrical parameters. At the sharp increasing of number of impulses (more than 700) the irreversible mass-transfer processes in ceramics are essential. It leads to the considerable increasing of NTC thermistor resistance.
References Fig. 5. Change of relative resistance, measured at 25°C, during thermal investigations. Sample submitted to stress at 125°C (top), sample not submitted to stress (bottom).
Result of thermal investigations (mean relative resistance from two measurements) is presented in Fig. 5. Dependence on number of temperature cycles is clearly visible. After the ®rst 5±8 cycles, resistance rises about 4%, similarly as after investigations with current pulses. These experiments were performed independently in dierent scienti®c centers. Curves in Figs. 2 and 5 show very similar growth of resistance in the beginning of experiment. Current pulse causes short-time increasing of internal temperature of thermistorÕs body. Thus, the rise of temperature is one of degradation factors at cyclic loads as in the case of thermal burn-in. The mechanism of this degradation can be connected with the modi®cation of ionic distribution or the oxidation state of ceramics due to the metallization process (serigraphy) observed previously by Fritsch et al. [5] in NTC thermistors. Derivatographic investigations (DTA) are planned to ®nd possible reasons. Other factors of the electro-thermal degradation, as it was mentioned above, are the electrical breakdowns of ceramic matrix. This phenomenon is fully responsible for the next rise of resistance, observed after about 700 cycles of the current pulses. Material of electrodes evaporates, growth of grains and pores is observed near these places. Eective surface of cross-section leading current decreases and resistance of thermistor rises.
4. Conclusions Process of degradation of NTC thermistors has complex character and is, in considerable degree, determined by the total number of applied current im-
[1] Lavenuta G. Negative temperature coecient thermistors. Sensors 1997;14:46±55. [2] Gavrilov V, Tiuh V. Power NTC thermistors. Radio 1996;1:61±2 [1996;2:57 [in Russian]]. [3] Edwards L. Selecting NTC thermistors for control applications. Sensor Rev 1988;8:205±6. [4] Clarc JA, Garganese US, Swarz RS. An approach to designing accelerated life-testing experiments. Proceedings of the Annual Reliability and Maintainability Symposium, 1997. p. 242±8. [5] Fritsch S, Sarrias J, Brieu M, et al. Correlation between the structure, the microstructure, and the electrical properties of nickel manganite negative temperature coecient (NTC) thermistors. Solid State Ionics 1998;109(3±4):229± 37. [6] Castelan P, Loubiere BH, Loubiere A, Rousset A, Hegros R. A geeing study of NTC thermistors by thermopower measurements. Sensors and Actuators 1992;33:119±22. [7] Polyakov AA. Technology of ceramic radioelectronic materials. Radio i Sviaz. Moscow, 1989 [in Russian]. [8] Vakiv MM, Veremeychuk MS, Hadzaman IV, Kovalskiy AP, Mrooz OYa, Shpotyuk OI. Electrophysical properties and microstructure of semiconducting oxide ceramics of MnCo2 O4 ±CuMn2 O4 ±NiMn2 O4 system. Bull Lviv State Univ 1994;33:8±11 [in Ukranian]. [9] Gadzaman IV, Kovalsky PM, Mrooz OYa, Shpotyuk OI. Features of the electrical conductance setting of thermoresistor ceramics on the basis of cubic manganese spinels. Funct Mater 1996;3:104±7. [10] Hadzaman IV, Kovalsky AP, Mrooz OYa, Shpotyuk OI. Thermal modi®cation of ceramic composites based on manganese-containing cube spinels. Mater Lett 1996;29: 195±8. [11] Mrooz OYa, Hadzaman IV, Shpotyuk OI. Complex manganese spinels for NTC thermistors application. Proceedings of the 21st Conference of ISHM Poland, Ustron. 1998, p. 217±20. [12] Mrooz O, Shpotyuk O, Hadzaman I, Kravtsiv M. Optimization of NTC thermistors for current protection of incandescent lamps (240 V, 100±160 W). Proceedings of the 22nd Conference of IMAPS Poland, 1999. p. 259±62. [13] Vakiv MM, Shpotyuk OI, Mrooz OYa, Hadzaman IV, Kovalskiy AP. Electrical conductivity studies of temperature sensitive materials based on Mn, Ni, Co and
O. Mrooz et al. / Microelectronics Reliability 41 (2001) 773±777 Cu-containing spinels. Bull Lviv State Univ 1998;31:142±4 [in Ukranian]. [14] Vakiv MM, Kovalsky AP, Mrooz OYa, Hadzaman IV, Shpotyuk OI. Selection guidelines of oxide ceramic materials based on Ni, Co, Mn, Cu-contained cubic spinels for thermistor applications. In: Samuneva B, Dimitriev Y, editors. Proceedings of the 12th Conference
777
on Glass and Ceramics. Publ House Science Invest; 1997. p. 380±3. [15] Vakiv MM, Maciak J, Mrooz OYa, Pogorzelska J, Szpotiuk OI. Degradation of ceramic thermoresistors under impulse current loads. Technologia i Konstruirowanie w Elektronnoj Apparature 2000;2±3:39±41 [in Russian].
