Aging of NTC ceramics investigated by magnetic measurements

JOURNAL OF MATERIALS SCIENCE LETTERS 18 (1999)1233 – 1235Aging of NTC ceramics investigated by magnetic measurementsW. A. GROEN, V. ZASPALISPhilips Research Laboratories, Weisshausstrasse 2, D-52066 Aachen, GermanyE-mail: groenwa@pfa.research.philips.comS. SCHUURMANPhilips Components Industrial s.a., Avenue Franz Guillaume 70, B-1040 Brussels, BelgiumNegative temperature coefficient (NTC) ceramics orthermistors are widely used for temperature sensing andcontrol. These ceramics are mostly based on mixed oxidesof Mn, Ni, Fe, Co and Cu which crystallize in aspinel structure. A main problem with these materialsis aging. This is a drift in resistivity with time whichoccurs at elevated temperatures, i.e., 150 ◦ C. A numberof models have been reported to explain the agingphenonema. Zurbruchen and Case [1] report thatbesides contact degradation, the degree of inversion(between tetrahedral and octahedra sites in the spinel)changes upon aging. This is concluded from changes inthe crystal structure. We have observed similar resultsin the system Mn 2−x In 0.1 Ni x O 4 [2]. Feltz [3] reportstwo models for aging of NTC ceramics. The first is anoxygen uptake or release, the second is also the redistributionof the cations over the sublattices. Mössbauermeasurements on Mn 3−x Fe x O 4 (x = 0.58 and 1.05) beforeand after aging have shown that migration occursof Fe 3+ from the A-site to the B-site [4].Recently, we reported on aging in the seriesMn 2.46−x Ni 0.54 Fe x O 4 [5]. It was shown that aging isstrongly dependent on the iron content. For sampleswithout iron the aging (R/R) is lower than 1% (after1000 h at 150 ◦ C). When iron is introduced the agingstrongly increases to 20% for x = 0.75. However, athigher iron contents the aging decreases again to

polycrystalline materials used the decisive factor fordetermining the anisotropy might not be the bulk crystalstructure itself but the nature of the grain boundariesand the associated stresses. In this case the initial permeabilityis given by the relation:(µ i − 1)/4π = (2/9)Ms 2 /(|λ||σ |)where λ is the magnetostriction tensor and σ is thestress tensor of the grain boundaries. The magnitude ofthe initial permeability, µ i , is in principle proportionalto the square power of the saturation magnetization,M s , and inversely proportional to the anisotropic energy(∝λ · σ ), either originated from the crystal latticeor from mechanical stresses at grain boundaries. Thisenergy changes with the temperature so that the initialpermeability as a function of temperature can be acomplex function. In general it increases with temperatureup to the Curie-point as shown in Figs. 1 and 2.The magnitude of this increase strongly depends onhow the samples have been cooled. At the Curie point,in an ideal single crystal, the magnetic dipoles are freeto move, because of the thermal energy they possess.They are not restricted to their motion by any kind ofanisotropy factors. The anisotropy fields almost vanish.In other words, an infinitively small field is enough tochange the direction of the dipoles. Just at this pointFigure 2 Magnetic permeability as a function of temperature for aircooledand stoichiometric-cooled and air cooled aged samples.Figure 1 Magnetic permeability as a function of temperature for aircooledand stoichiometric-cooled samples.the “ideal” magnetic permeability will reach infinity.This is not the case in real situations as in polycrystallinematerials because the grain boundaries generateanisotropy. This imposes restrictions to the dipolemovements. Furthermore, the presence of grain boundariesmay also generate extra stresses.Stoichiometrically cooled samples apparently indicatelow energetic restrictions to the dipole movementand the increase of the intitial permeability with temperatureis significant and sharp. On the other hand,air-cooled samples will have stresses due to oxidationat their grain boundaries which induce an anisotropyfield and energies which are high enough to limit theincrease of the magnetic permeability with temperatureat low levels. We therefore attribute the differencein the permeability vs. temperature behavior to differencesin the chemical and/or mechanical nature of thegrain boundaries of the samples.A second observation is that the differences betweenthe air cooled and stoichiometric cooled samples ismuch larger in case of the samples with the lowest ironcontent: Mn 1.71 Ni 0.54 Fe 0.75 O 4 . The results of the magneticpermeability measurements after aging of the aircooledsamples for 1000 h at 150 ◦ C are presented inFig. 2. The data for the air-cooled and stoichiometriccooled samples are also included. The µ-T behavior ofthe aged samples are nearly the same as observed for1234

Figure 3 Schematic presentation of the processes which occur duringcooling and aging of NTC ceramics.the stoichiometric-cooled samples. This indicates thatupon aging the stressed grain boundaries which areformed during cooling in air, disappear.These results are summarized in Fig. 3. The stressesor defects at the grain boundaries which are formed duringcooling in air are cationic vacancies [5]. During thesintering and while cooling stoichiometrically, homogeneousceramics are produced with no cationic vacancies.As a consequence, these ceramics will be stablein the aging tests. When the ceramics are cooled in airafter the sintering, cationic vacancies are produced atthe grain boundaries. At sufficiently high temperaturesdiffusion from the grain boundaries into the bulk is possible.However, at lower temperatures the cationic vacancieswill predominantly be at the grain boundaries.So, when cooled in air, ceramics with inhomogeneouslydistributed cationic vacancies are produced which willnot be stable in the aging tests. When such ceramics areaged for a long time the cationic vacancies can diffusefrom the grain boundaries into the bulk and again a homogeneousmaterial is formed. Note that this situationis also achieved by very slow cooling in air, as has beenreported [8].In conclusion, magnetic permeability measurementscan be used to investigate the nature of the grain boundariesin NTC ceramics. It has been shown that the defectswhich are formed during oxidation, the cationicvacancies, play a dominant role in the aging mechanism.When the formation of cationic vacancies duringcooling after the sintering is inhibited, as achieved bystoichiometric cooling, stable NTC ceramics with respectto aging can be obtained.References1. J. M. ZURBUCHEN and D. A. CASE, in Proceedings of theSixth International Symposium on Temperature, Its Measurementsand Applications, Washington DC, March 1982, Part 2, edited byJ. F. Schooley, p. 889.2. CH. METZMACHER, P. HUPPERTZ and W. A. GROEN,submitted.3. A. FELTZ, in Proceedings Electroceramics IV, Aachen, September1994, edited by R. Waser, p. 677.4. T. BATTAULT, R. LEGROS, M. BRIEU, J. J. COUDERC,L. BERNARD and A. ROUSSET, J. Phys. III 7 (1997) 979.5. W. A. GROEN, S. SCHUURMANS, CH. METZMACHERand P. HUPPERTZ, J. Electroceram., submitted.6. R. MORINEAU and M. PAUKUS, IEEE Trans. Magn. 11 (1975)1312.7. H. VERWEIJ and W. H. M. BRUGGINK, J. Phys. Chem.Solids 50 (1989) 75.8. S. FRITSCH, J. SARRIAS, M. BRIEU, J. J. COUDERC,J. L. BADOUR, E. SNOECK and A. ROUSSET, Solid StateIons. 109 (1998) 229.Received 25 November 1998and accepted 23 February 19991235