Structure, thermal stability and electrical properties of zinc manganites

Solid State Ionics 128 (2000) 233–242www.elsevier.com/ locate / ssi

Structure, thermal stability and electrical properties of zincmanganites

a , a a a a b*S. Guillemet-Fritsch , C. Chanel , J. Sarrias , S. Bayonne , A. Rousset , X. Alcobe ,c`M.L. Martinez Sarrion

a ´ ´ ´ ˆLaboratoire de Chimie des Materiaux Inorganiques et Energetiques, ESA CNRS 5070, Universite Paul Sabatier, Batiment 2R1-118,Route de Narbonne, 31062 Toulouse Cedex, France

b ´Serveis Cientifico-Tecnics, Universitat de Barcelona, Lluis Sole i Sabaris 1 –3, 08028 Barcelona, SpaincDepartament de Qimica Inorganica, Universitat de Barcelona, Marti i Franques 1 –11, 08028 Barcelona, Spain

Received 9 September 1999; accepted 25 November 1999

Abstract

Zinc manganites Zn Mn O were prepared by the thermal decomposition in air of oxalate precursors. The structure andx 32x 4

the thermal stability of the oxides were determined and correlated with their electrical properties (resistivity, resistivity drift).The substitution of manganese by zinc in the spinel structure has a stabilizing effect against oxidation and phase

21transformation. The strong energetic Zn –O bonding slows down the cationic migration in the lattice. The zinc manganitesare suited to high temperature NTC thermistors. The resistivity range is large and no ageing phenomenon is observed in theceramics. 2000 Elsevier Science B.V. All rights reserved.

Keywords: Zinc manganite; Spinel; NTC thermistor; Oxidation; Electrical resistivity

1. Introduction manganites have been mainly prepared by solid–solid reaction between zinc oxide ZnO and man-

Transition metal manganites Mn M O are im- ganese oxides (usually a-Mn O ). This method32x x 4 2 3

portant technological materials used as negative needs high temperature reactions, often associatedtemperature coefficient (NTC) thermistors owing to with grinding. Usually the well defined compoundtheir interesting electrical properties. Zinc manga- Zn Mn O is obtained at 9008C after a very long1 2 4

nites Zn Mn O are possible candidates for high time [1–7], usually more than 24 h (Table 1). Thex 32x 4

temperature applications. Thus, it is important to ‘Chimie Douce’ technique avoids the difficulties dueknow perfectly the structure, the thermal stability to these mixing processes. Moreover, the temperatureand the electrical properties of these phases. Zinc and the time of preparation can be significantly

reduced. A few zinc manganites have been preparedby calcination of hydroxides [8], by the sol /gel

*Corresponding author. Tel.: 133-5-6155-6283; fax: 133-5-method [9] or by coprecipitation of oxalates [10].6155-6163.

At ambient temperature, the hetaerolithe (mineralE-mail address: [email protected] (S. Guillemet-Fritsch) name, PDF file 71-2499) Zn Mn O is a normal1 2 4

0167-2738/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 99 )00340-9

234 S. Guillemet-Fritsch et al. / Solid State Ionics 128 (2000) 233 –242

Table 1Different solid–solid preparation methods for zinc manganites reported in the literature

Solid–solid Thermal treatment Compound Referencereaction

MnO1ZnO 1 month, 6008C Zn Mn O [1]x 32x 4

24 h, 14508CMnO 1ZnO 9008C (O ) Zn Mn O [2]x 2 1 2 4

MnO 1ZnO 60 h, 9508C (air) (ZnO) (MnO) Mn O [3]2 x 12x 2 3

Mn O 1ZnO 6 to 48 h, 8008, 9008 and 10008C, Zn Mn O [4]2 3 1 2 4

(air)Mn O 1ZnO 12508C (air) Zn Mn O [5]2 3 1 2 4

Mn O 1ZnO 120 h, 11758C (air) Zn Mn O [6]3 4 1 2 4

2 /3Mn O 1ZnO 72 h, 11008C (air) Zn Mn O [7]3 4 1 2 4

spinel with a tetragonal structure, similar to haus- 1.5). The powders have been prepared via themannite, Mn O . The phase diagram ZnO/Mn O oxalate precursor route. X-ray diffraction experi-3 4 3 4

built by Driessens and Rieck [1] shows that the ments and thermogravimetric analyses were per-spinel phase is stable up to 11008C in air. Above formed to understand the structural evolution and the11008C a reversible tetragonal to cubic transforma- thermal stability of these oxides as a function oftion, identical to the one observed in Mn O is temperature. Particular attention was given to de-3 4

noticed [3,10–15]. This transformation may be the termine the electrical properties (resistivity, resistiv-consequence of a change in the cationic distribution ity drift) of the ceramics, in relation to the structureas a function of temperature, i.e. the concentration of and the microstructure.

31the octahedral Mn diminishes while the quantity of21 41Mn and Mn cations increases. This change

anneals the cooperative Jahn Teller distorsion and 2. Experimentalthe cubic structure is observed. The cationic dis-tribution in these oxides has been mainly determined 2.1. Synthesisby X-ray diffraction experiments [1,7,8,10,16](Table 2). Some of the authors [1,10] do not exclude Zinc manganese oxalate precursors,

21some Zn cations on the octahedral sites of the Mn Zn C O ? nH O, were obtained by the co-1-a a 2 4 2

spinel structure. precipitation of an aqueous solution of ammonium21Most of the studies concerning the zinc manganese oxalate (concentration: 0.2 mol l , which corre-

oxides found in the literature were mainly dedicated sponds to the saturation of the solution at ambientto the compound Zn Mn O . The present work deals temperature) and a mixture of manganese and zinc1 2 4

with the elaboration and characterization of a wide nitrates, in the desired proportion (concentration 421range of Zn Mn O solid solutions (0.5 # x # mol l ). The saturated solutions produce particles ofx 32x 4

Table 2Cationic distribution for zinc manganites reported in the literature

Techniques Cationic distribution Compound Reference21 21 21 41 31 22X-ray Zn Mn Zn Mn Mn O Mn Zn O [10]f gt t u u 222u 4 32x x 4

diffraction electrical measurements21 21 41 31 22X-ray diffraction Zn Zn Mn Mn O Mn Zn O [1]f g1 x21 x21 422x 4 32x x 4

In situ X-ray diffraction Zn Mn Zn Mn O Mn Zn O [16]f gx2t 12x2t t 22t 4 32x x 421 31 22X-ray diffraction Zn Mn O Zn Mn O [7]f g1 2 4 1 2 431 21 21 31 22X-ray diffraction Mn Zn Zn Mn O Zn Mn O [8]f g0.04 0.96 0.04 1.96 4 1 2 4

S. Guillemet-Fritsch et al. / Solid State Ionics 128 (2000) 233 –242 235

isotropic morphology [17]. After 30 min, the solution pass through the reactor. During the experiment, thewas filtered, washed several times with water and temperature was linearly increased to 9508C (with a

21dried at 908C in air. heating rate b 558C min ). Every 120 s, the gasThe reaction is described by the following equa- flowing out of the reactor was sampled and analyzed

tion: by gas chromatography (Shimadzu GC-8Achromatograph fitted with a molecular sieve 1331 2 a Mn NO ? 6H O 1 aZn NO ? 6H O 1 NH C O ? H Os d s d s d s d3 2 2 3 2 2 4 2 2 4 2

column and a thermal conductivity detector). These↓1 2 analyses provided the oxygen concentration in theMn Zn C O ? nH O 1 2NH 1 2NO 1 (7 2 n)H O12a a 2 4 2 4 3 2

flowing gas, and the integration of these data over(1)

time gave the total amount of oxygen released duringSeveral compositions were prepared (Table 3). The the experiment.composition (Mn/Zn) was determined in the solidstate (calcined powders). The oxides were obtainedafter a heat treatment either at low temperature 3. Results and discussion(T54008C) or a higher temperature (9008C).

3.1. Structure and thermal stability of the oxide2.2. Characterization techniques powders

The powder morphology was observed with a Jeol At 4008C, all the oxides crystallize in the tetragon-JSM-6400 scanning electron microscope. The ther- al spinel structure. The structural evolution wasmal decomposition of the mixed oxalate precursors followed by in situ X-ray diffraction experimentswas examined by thermogravimetric analyses (TGA) (Table 4). The thermal stability in air of these oxides

26(Setaram TAG 24 apparatus, accuracy 10 g). High has been studied by thermogravimetric analysis. Thetemperature X-ray diffraction data were collected curves corresponding to different oxideswith a Bragg–Brentano u /2u Siemens D500 powder (Zn Mn O and Zn Mn O ) are given in0.51 2.49 4 1.03 1.97 4

diffractometer, using the CuKa radiation (l 5 Figs. 1a and b. Two mass losses are observed during1˚ ˚1.54060 A and l 51.54443 A, l /l 50.50) and the heating process. They were identified as oxygen2 2 1

operating at 40 kV and 30 mA. The step-scan mode loss by chromatography. The first one (I) is noticedwas used (0.028 2u step-size and 1s per step counting at 430,T ,5308C, and the other one (III) occurstime) over the 2u range 2082808. The chemical between 7308C and 8308C, interrupted by an oxida-composition of the oxides was determined by plasma tion reaction (II) (530,T ,6308C). The TG curveemission spectrometry. corresponding to a composition richer in zinc

Temperature-programmed reduction analyses (Zn Mn O ) only shows the first phenomenon1.03 1.97 4

(TPR) were studied by thermogravimetry and gas (I).chromatography. Experiments were performed in a The first mass loss observed on the TG curve (Fig.vertical plug flow differential reactor. The tempera- 1a) corresponds to the loss of the oxygen excess dture of the sample could be linearly increased by a according to:furnace monitored by a shimaden SR25 temperature Zn Mn h O → Zn Mn O 1 d /2O (2)x 32x 3d / 4 41d x 32x 4 2programmer. The sample was first degassed (1 Pa) atroom temperature for 1 h, then the system was filled The experimental mass loss is 2.2% for the com-

3 21with Ar. A flow of 15 cm min was allowed to position Zn Mn O . The theoretical coeffi-0.51 2.49 41d

cient d calculated from Eq. (2) is 0.32, which is veryclose to the experimental one (0.33), determined

Table 3from TPR. The nonstoichiometric coefficient corre-Composition of the synthesized zinc manganese oxalatessponding to the oxygen excess d (in Zn Mn O )Mn Zn C O ?nH O and the corresponding oxides Zn Mn O x 32x 41d1-a a 2 4 2 x 32x 4

diminishes as the zinc content increases (d 50.30 fora 0.17 0.28 0.34 0.36 0.39 0.40 0.47 0.50

x50.51 and d 50.20 for x51.03). The step IIx 0.51 0.84 1.02 1.08 1.17 1.20 1.41 1.50 noticed for the oxide Zn Mn O (T55308C) is0.51 2.49 4


Table 4Structural evolution of zinc manganites Mn Zn O as a function of temperature32x x 4

T x(8C)

0.51 0.84 1.03 1.08 1.41 1.50a400, 1 h T T T T T T

c b b500, 1 h T1a-Mn O T T T ZnMnO 1T ZnMnO 1T2 3 3 3b600, 1 h T1a-Mn O T T T1ZnMnO ZnMnO 1T ZnMnO 1T2 3 3 3 3

b700, 1 h T1a-Mn O T T T1ZnMnO ZnMnO 1T ZnMnO 1T2 3 3 3 3

800, 1 h T1a-Mn O T T T1ZnMnO ZnMnO 1T ZnMnO2 3 3 3 3

900, 4 h T T T T1ZnMnO ZnMnO 1T ZnMnO 1ZnO3 3 3

a T: tetragonal spinel.b Small amount (,10%).c Trace (,2%).

Fig. 1. TGA curve of zinc manganites prepared at 4008C. Zn Mn O (a); Zn Mn O (b).0.51 2.49 4 1.03 1.97 4


21attributed to the oxidation of the tetrahedral Mn zinc content increases (1.08,x,1.41), the cubiccations of the spinel phase, which leads to the phase ZnMnO is observed, in addition to the spinel3

crystallization of a-Mn O : phase. For x51.50, the tetragonal cubic phase has2 3

completely disappeared and the two phases ZnMnO33 and ZnO are observed. The lattice parameters have]Zn Mn O 1 yO → Zn Mn O 1x 32x 4 2 x1y 32x2y 44 been calculated for the different compositions (Tabley 5, Fig. 2). The substitution of Mn by Zn leads to a] a-Mn O (3)2 32 decrease of the c /a ratio, up to the compositionwith 0#y#0.02 and 0.5#x#1.5.As the zinc content x51.08. For a higher zinc content, a stabilization of

21increases in the spinel structure, the number of Mn c /a is observed, which corresponds to the crys-cations diminishes, and no oxidation appears (Fig. tallization of the cubic phase ZnMnO .3

101b). The step III (T56308C) is due to the reduction Due to their electronic configuration (3d ), the31 21of the Mn cations, according to the reverse Zn cations are highly stabilized on the tetrahedral

reaction of Eq. (3). As the zinc content and the sites of the spinel structure [18,19]. They formtemperature increase, the ZnMnO phase is ob- ionocovalent bounding with the oxygen anions3

21served, in addition to the spinel phase. [20,21]. In the case of zinc manganites, the Zn21The structure of the oxides obtained at higher substitute for the Mn tetrahedral cations. The

temperatures (T59008C) is reported in Table 5. The decrease of the lattice parameter up to x51.0 can be21oxides having a low zinc content (x,1.08) crys- explained by the fact that Zn cations have a

21tallize in a single tetragonal spinel structure. As the smaller ionic radius than Mn (r 2155.80 nmZn

Table 5Structure and lattice parameters of zinc manganite oxides Mn Zn O prepared at 9008C, as a function of zinc content (x)32x x 4

x

0.51 0.84 1.03 1.08 1.17 1.20 1.41 1.50aStructure T T T T T1 ZnMnO T1 ZnMnO T1 ZnMnO ZnMnO 1 ZnO3 3 3 3

Lattice parameters24(nm65310 )

a 0.8111 0.8093 0.8098 0.8069 0.8084 0.8085 0.8085 –c 0.9354 0.9294 0.9267 0.9223 0.9231 0.9232 0.9222 –c /a 1.153 1.148 1.144 1.143 1.142 1.142 1.141 –

a T: tetragonal spinel phase.

Fig. 2. Evolution of the c /a ratio for zinc manganite Zn Mn O powders prepared at 9008C as a function of zinc content (x).x 32x 4


and r 2156.55 nm) [22]. This hypothesis has phase is observed during the heating, as it is the caseMn

been confirmed by the work of Nogues and Poix [7] for Mn O , which was heated under the same3 4

who showed that the oxygen–cation distance, on the conditions [23]. The presence of zinc in hausmannite21 21tetrahedral sites, is smaller for Zn than for Mn Mn O stabilizes the spinel structure against oxida-3 4

(d 51.970 nm and d 52.041 nm). For21 21 tion. For the composition Zn Mn O , the phaseZn –O Mn –O 1.03 1.97 4

the composition Zn Mn O , all the tetrahedral ZnMnO appears at 6008C. For higher zinc content1.0 2.0 4 321sites are occupied by the Zn -O cations. For higher (1.03,x,1.20), both the tetragonal spinel and the

21zinc content, the additional Zn cations probably ZnMnO phase are observed at ambient temperature.3

occupy the octahedral sites, up to the composition There is no structural change up to 10008C. Only the21Zn Mn O . In this case the Zn substitute for ratio of the ZnMnO increases. Above 10008C, the1.08 1.92 4 3

31 41the Mn , and an equivalent amount of Mn is phase ZnO is observed whatever the composition.formed, in order to maintain the electrical charge However, the temperature of crystallization of theneutrality. For x.1.08, the crystallization of the zinc oxide decreases as the zinc content increases.second phase indicates that the maximum in the Between 10008C and 12008C, there is a tetragonal to

21 21octahedral Zn has been reached. Zn cations in cubic phase transformation. This phase transition hasexcess form the ZnMnO phase. As the zinc content been described previously (see the Introduction3

increases, the quantity of this phase also increases. section).Thus, we may consider the presence of a small

21quantity of Zn cations on the octahedral sites of 3.2. Structure of zinc manganites ceramicsthe spinel structure, for the oxides Zn Mn O withx 32x 4

1.00,x,1.08. Dilatometric analyses were performed in order toHigh temperature X-ray diffraction and thermo- optimize the thermal treatment of sintering. The

gravimetric analyses have been performed on the results of this study are reported elsewhere [24]. Theoxides synthesized in air at 9008C and cooled at pressed pellets were heated at 12008C in air for 2 h,1508C/h in order to determine their thermal stability and cooled at different rates (68C/h; 1508C/h orat higher temperature. The structures observed at quenched in air). The different structures are reporteddifferent temperatures are reported on Table 6. When in Table 7.the zinc content is lower than x51.0, no a-Mn O The slow cooled ceramics are single phase for a2 3

Table 6Structural evolution of zinc manganites Mn Zn O as a function of temperature for different zinc content (x)32x x 4

T x(8C)

0.51 0.84 1.03 1.20 1.41 1.50a25, 1 h T T T T1ZnMnO ZnMnO 1T ZnMnO 1ZnO3 3 3

500, 1 h T T T T1ZnMnO ZnMnO 1T ZnMnO 1ZnO3 3 3c600, 1 h T T T1ZnMnO ZnMnO 1T ZnMnO 1T ZnMnO 1ZnO3 3 3 3c700, 1 h T T T1ZnMnO ZnMnO 1T ZnMnO 1T ZnMnO 1ZnO3 3 3 3

800, 1 h T T T1ZnMnO ZnMnO 1T ZnMnO 1T ZnMnO 1ZnO3 3 3 3c900, 1 h T T T1ZnMnO ZnMnO 1T ZnMnO 1T ZnMnO 1ZnO3 3 3 3

c c c c1000, 1 h T T T1ZnMnO T1ZnMnO ZnMnO 1ZnO ZnMnO 1ZnO3 3 3 3c d c1100, 2 h T T T 1ZnMnO T 1ZnO C1ZnO C1ZnO3

d1ZnMnO3

b c d1160, 2 h T1C T1C T 1C1ZnO C1ZnO ZnO1C C1ZnOc c c c1200, 2 h T 1C1ZnO T 1C1ZnO C1ZnO C1ZnO ZnO1C C1ZnO

a T: Tetragonal spinel phase.b C: Cubic spinel phase.c Small amount (,10%).d Trace (,2%).


Table 7Structure of the ceramics Zn Mn O sintered at 12008C as a function of the cooling ratex 32x 4

Cooling rate x

0.51 0.84 1.03 1.08 1.17 1.20 1.41 1.50a68C/h T T T1ZnMnO T1ZnMnO T1ZnMnO T1ZnMnO T1ZnMnO 1ZnO T1ZnMnO 1ZnO3 3 3 3 3 3

1508C/h T T T T1ZnMnO T1ZnMnO T1ZnMnO T1ZnMnO 1 ZnO T1ZnMnO 1 ZnO3 3 3 3 3

Quenched in air T T T T T T1ZnO T1ZnO T1ZnOa T: Tetragonal spinel phase.

zinc content up to 0.84. For higher zinc content, the c /a ratio diminishes as the quantity of zinc increasesoxide ZnMnO (cubic phase) is also noticed. And for up to 1.17. We can consider that in the quenched3

x51.41, an additional phase, ZnO, appears. The samples of composition comprised between 1.03 and21ceramics that were cooled at 1508C/h have the same 1.17, some Zn cations are located on the octahed-

structure as the first series (68C/h). However in this ral sites of the spinel structure. For richer zinc oxidecase a single phase is observed up to x51.03. When (x.1.17), the stabilization of the c /a ratio corre-the ceramics were quenched, only the spinel phase is sponds to the crystallization of the ZnO phase in the

21observed when the zinc content is lower or equal to ceramics. In fact, when the quantity of Zn cations1.17. Above that composition, the zinc oxide ZnO is too high, the ZnO phase precipitates beside theappears. The increase of the cooling rate allows spinel structure.single phase ceramics to be obtained up to x51.03.The high cooling rate freezes the high temperature 3.3. Electrical properties of zinc manganitephases at ambient temperature, supposing that the ceramicsrate of the phase transformation is slow enough. Athigh temperature, the cubic ZnMnO phase trans- The resistivity of the ceramic Zn Mn O has3 1.50 1.50 4

forms into the spinel phase, due to the instability of been measured as a function of temperature. It41 31the Mn cations that are reduced to Mn . For the decreases exponentially as the temperature increases.

quenched ceramics, the c /a ratio of the tetragonal The logarithm of r /T as a function 1/T is linearspinel phase is reported in Fig. 3, as a function of (Fig. 4), which is characteristic of the conductionzinc content. As was the case for oxide powders, the process described in the Nernst–Einstein relation:

Fig. 3. Evolution of the c /a ratio for zinc manganite Zn Mn O ceramics (sintered at 12008C and quenched in air), as a function of zincx 32x 4

content (x).


Fig. 4. Logarithm of r /T as a function of 1 /T for Zn Mn O0.50 2.50 4

ceramics (sintered at 12008C and quenched in air). Fig. 5. Resistivity of Zn Mn O ceramics (sintered at 12008Cx 32x 4

and quenched in air), as a function of zinc content (x).

s E1 0 H] ] S]Ds 5 5 NC(1 2 C) exp with In this composition range (0,x,1.0), the value ofr T kT

the resistivity is similar to the one of Mn O (r ¯3 492 2 5.9310 V cm [25]). The number of charge carriersN e d voct 0

17 23]]]s 5 (4)0 (10 cm ) located on the octahedral sites has beenkdetermined by Seebeck coefficient measurements

where N is the concentration of the octahedral [26]. It represents only 0.001% of the octahedraloct

sites, d is the jump distance for the charge carriers, sites participating in the conduction process andn is the lattice vibrational frequency associated with explains the insulating behavior of these ceramics.0

conduction, k is the Boltzmann’s constant, e the The constant diminution of r is due to the decreaseelectronic charge, N is the concentration per for- of the lattice parameters (a) and (c).mulae unit of sites which are available to the charge The oxides with a zinc content x comprisedcarriers, C is a fraction of available sites which are between 1.02 and 1.17 are single phase (tetragonaloccupied by the charge carriers, and E is a hopping spinel). The decrease of the resistivity is possibleH

21energy. only if some Zn cations are located on theThe resistivity was determined for the quenched octahedral sites of the spinel structure. Then an

41ceramics, that were mainly single phase. It could not equivalent proportion of Mn is created on thebe measured for the ceramics of composition x,1, same sites, in order to maintain the electrical neu-since the value is too high (r .1 MV cm). The trality as it is described in Eq. (6):results are reported in Fig. 5. There is a complex

21 21 31 41 22evolution of the resistivity as a function of the zinc Zn Zn Mn Mn O (6)f g1 x21 222(x21) x21 4

content. For x.1.0, the values are between 90 and800 kV cm. For 1.03#x,1.17, a continuous de- The hopping phenomenon is possible then between

31 41crease of r is observed. At x51.17, the resistivity Mn and Mn and the ceramic becomes semi-suddenly increases again. And for x.1.17, another conductive [10]. However the value of the resistivitydecrease is noticed. is high, since the number of charge carriers is still

The ceramics of composition lower than x51.00 low.are single-phase (tetragonal spinel structure). The For x51.20, the resistivity reaches 700 kV cm. At

21Zn cations are located on the tetrahedral sites, and the same composition, the ZnO phase is observed, in31the Mn cations are present on the octahedral sites, addition to the spinel phase. In this case, we consid-

21according to: ered that the Zn cations located on the octahedralsites have migrated outside of the spinel phase to

21 21 31 22Mn Zn Mn O (5) form the ZnO phase. Thus, the number of chargef g12x x 2 4


carriers decreases and should be very close to the value of the resistivity drift after 500 h at 1258C was21one of the oxide Zn Mn O (with all the Zn found to be particularly low, less than 0.1% [24]1.02 1.98 4

cations located on the tetrahedral sites). In fact, the (Fig. 6), whatever the composition. The ageingexperimental value of the resistivity for phenomenon has been explained by cationic migra-Zn Mn O is effectively similar to the one of tion and/or electronic changes between the sublat-1.17 1.83 4

Zn Mn O . The small difference observed can tices of the spinel structure [34] and more recently1.02 1.98 4

be explained by the contribution of the small propor- by a redox process [35]. In the zinc manganites, notion of the ZnO phase. In fact, the resistivity of the oxidation occurs at low temperature (T ,9008C). So,ZnO phase [27,28] is much lower than the one of the no structural modification relative to oxidation statesspinel phase. At this point, both the structure (cation appears during the thermal treatment of the serig-distribution) and the microstructure (spinel1ZnO) raphy process. The details of this process has beencontribute to the resistivity change. described elsewhere [35]. The resistivity is not

For x$1.20, an inverse tendency is observed, the modified and effectively no drift is observed. More-resistivity decreases. In this case, the slope of the over it has been shown that the ageing phenomenoncurve (Fig. 5) is not as steep as it was the case for is related to microstructural defects [36,37], thatlower zinc content (1.02#x#1.17). The global were observed by transmission electron microscopy.decrease of the ceramics resistivity in this com- In fact when these defects are present, they inhibitposition range can be attributed to the contribution of the ionic migration in the spinel sublattice andthe ZnO phase. In fact. As the proportion of ZnO reduce the ageing phenomenon. Numerous micro-phase increases with zinc content, a constant diminu- structural defects (twins, dislocations, . . . ) weretion of r is observed in the composition range evident in zinc manganite ceramics [24]. These(1.20,x#1.50). defects contribute to the small resistivity drift of

The ‘ageing’ behavior of these NTC zinc manga- these compounds.nite thermistors was examined. Generally the ther-mistor’s resistivity increases with time under athermal constraint. This phenomenon is called age- 4. Conclusioning. The resistivity drift (DR /R) depends on thechemical composition, the crystal structure, and the The thermal decomposition at low temperaturethermal history of the ceramics [29–33]. For exam- (4008C) or at higher temperature (9008C) in air ofple, it reaches 20% for the manganites containing oxalate precursors leads to zinc manganite powderscopper or it can represent a few percent for nickel Zn Mn O . At 4008C, the oxides crystallize in thex 32x 4

manganites (Fig. 6). In zinc manganite ceramics, the tetragonal spinel structure, whatever the composition.The oxides synthesized at 9008C are single phase(tetragonal spinel) up to x51.08. Thermogravimetricanalyses have shown that the zinc cations inhibit the

21oxidation of the Mn cations in the spinel structure.21The Zn cations are highly stabilized in the tetra-

hedral sites. However, X-ray diffraction analyses andelectrical properties have shown that for a com-position x between Zn Mn O and1.0 2.0 4

Zn Mn O , some zinc cations are localized on1.08 1.92 4

the octahedral sites. The value of the resistivity ishigh (90,r ,800 kV cm), since the number ofcharge carriers is very low. The lack of oxidationand the presence of numerous microstructural defectsinhibit the ageing phenomenon in these compounds.Fig. 6. Drift of resistivity (DR /R) of nickel manganitesThe resistivity drift after 100 h at 1258C is less than(Ni Mn O [25]) (j) and zinc manganites (Zn Mn O )0.99 2.01 4 1.03 1.97 4

(d) as a function of time (ageing temperature: 1258C). 0.1%.


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Structure, thermal stability and electrical properties of zinc manganites