Intrinsic resistivity of sintered nickel manganite vs. powder activation time and density

Nickel manganite oxides, are very interesting ceramics widely used as negative temperature coefficient (NTC) mainly in electronics as elements of temperature control and compensation, time delay, voltage regulation, fan control etc. Nickel manganite has an intermediate (partially inverse) cubic spinel structure. The values of cation inversion parameter is calculated (between 0.8 and 0.88). Mechanism, responsible for conduction in nickel manganite, is described by a phonon-assisted electrons jump (so–called hopping) between Mn3+ and Mn4+ cations placed in octahedral sites. In our earlier papers we have been investigated the influence of the time and temperature of sintering on thermal, optical and some electrical properties of this material [12,13,14]. The main purpose of this investigation is to show the unnegligible influence of mechanical activation of starting mixed oxides on electrical properties (direct current (DC) resistivity in our case) of originating nickel manganit.


Introduction
Nickel manganite oxides, consisting of these 3d transition metals are very interesting ceramics widely used for negative temperature coefficient (NTC) thermistors [1][2][3].NTC thermistors have been in use for a long time, mainly in electronics as elements for temperature control and compensation, time delay, voltage regulation, fan control etc. [4][5][6].Their application is limited by a low Currie point (around 150 o C).Much work has been done recently, in order to develop different material contents, for high temperature NTC application [7,8].In order to achieve better understanding of thermistor behavior (its sensitivity and response depends on physical properties) it is significant to characterize the material and analyze more closely its physico-chemical properties.
It is well known that nickel manganite has an intermediate (partially inverse) cubic spinel structure.Values of the cation inversion parameter were calculated to be between 0.8 and 0.88 [9] and this parameter has a direct influence on all physical properties of this material [10].The mechanism, responsible for conduction in nickel manganite, is commonly described as phonon-assisted electron jumping (so-called hopping) between Mn 3+ and Mn 4+ cations placed in octahedral sites.The resistivity of NTC thermistors decreases exponentially with temperature and can be represented with an Arrhenius equation: where  is the resistivity of the material at infinite temperature, T is the absolute temperature, and B is the temperature coefficient of resistivity, given in Kelvin.Resistivity is strongly influenced by the composition and preparation of transition metal oxides.
Many authors have attempted to define a relation between the structure, microstructure, thermal and electrical properties of the sintered material used as the starting material for the production of sensitive NTC thermistors [11].In our earlier papers we investigated the influence of the time and temperature of sintering on thermal, optical and some electrical properties of this material [12][13][14].Electrical properties were also described in [15,16,9].Recently, the influence of oxygen stoichiometry on the magnetic properties of spinel sublattices has been investigated [17].
The main purpose of this investigation is to show the non-negligible influence of mechanical activation of the starting mixed oxides on electrical properties (direct current (DC) resistivity in our case) of the formed nickel manganite.Mechanical activation has great advantages, producing a more homogeneous material, compared to other processes.This technique increases the number of carriers and intensifies transport processes.Besides the time and temperature of sintering, it is very important to optimize the time of mechanical activation, because short mechanical activation doesn't give enough energy to improve material properties, but a long activation time leads to the formation of microstructure defects influencing the number of carriers, conductivity (resistivity) of materials.A very long activation time finally leads to higher nanoparticle agglomeration that causes higher porosity in the material obtained.

Experimental
Samples of the investigated NTC thermistor material (NiMn 2 O 4 ) were obtained following the classical procedure for the preparation of NTC thermistor powder [4].Mixtures of starting MnO and NiO (containing 0.5 wt % CoO and Fe 2 O 3 ) powders were calcinated for 1h at 1050 0 C.After vibratory milling in an ultra-fast ball mill for 2h, an average powder particle size of 0.9 m was achieved.Mechanical activation was done by grinding in a continual regime in a Fritsch Pulversette 5 planetary ball mill for 5,15,30,45 and 60 minutes.Iron grinding balls was used, and the powder to ball mixture mass ratio was 1:10.The powders obtained were uniaxially pressed with 196 MPa into disc shape pellets 8 mm in diameter, and then sintered at 900, 1050 and 1200C for 60 minutes.
DC resistivity measurements were performed on a HP 4194A impedance/gain phase analyzer.For these measurements, sample contacts (electrodes) were prepared by spreading a one component epoxy paste with silver filler.After that, the samples were annealed for 15 minutes at 150 o C, achieving a homogeneous electrode on the sample surface.The thickness of the electrode was about 100m.DC measurements were performed on three different temperatures, at room temperature (25 o C), 50 o C and 80 o C. The coefficient of temperature sensitivity B 25/80 can be calculated using the well known equation which verifies R-T thermistor characteristics: where T 1 and T 2 are the room temperature and 80 o C respectively, and R 2 and R 1 are the resistivities at those temperatures.
The activation energy (E a ) for electrical conduction can be calculated from the following expression: , where k is the Boltzman constant.
Crystalline structures of the nonactivated and activated samples were recorded using a Philips PW 1050 X-Ray Difractometer, with Cu K  radiation and scans were taken with a step of 0.05/s.

Results and Discussion
Figure 1 gives a typical XRD pattern of NiMn2O4, activated for 15 minutes and then sintered at 1200 o C for 60 minutes and.For all analyzed samples X-ray analysis showed that a single phase spinel structure was obtained (Tab.I).Very slightly changes in the lattice parameter parameter a, were noticed, for different activation times.The activation energy (shown in Tab.II) is the energy of conduction, and represents the energy for the hopping process from Mn 3+ to Mn 4+ situated on the octahedral sites, and thus the mobility of small polarons.

Tab. I
It is obvious, from Tab. II, that the electrical properties for thermistors differ, (the smallest B value and thus E a was obtained for the sample sintered at 1200 o C for one hour and activated 5 min) and can be controlled by changing the conditions of sintering and the time of mechanical activation.It can be noted (Fig. 3.) that the intrinsic resistivity falls with the increase of the sintering and environment temperature, as a consequence of carrier number growth, which provides a higher conductivity and hence decrease of resistivity.Mechanical activation for shorter times produces a reduced grain size and porosity, an increase of sample density, leading to a reduction in electrical resistivity.For longer activation times, it leads to an increase of porosity and thus, decrease of sample density.That is in accordance with the resistivity rising.

Conclusion
DC resisitivity of sintered NiMn 2 O 4 was studied as a function of mechanical activation of the starting powder.The activation energy (energy of conduction) and the coefficient of temperature sensitivity B 25/80 , were also calculated.
It is very well known that electrical properties of ceramics are strongly influenced by the material composition, sintering conditions and the time of mechanical activation.It was shown that electrical properties of a thermistor material sintered and mechanically activated for different times differ and can be controlled by variation of these parameters.That way broad application of these thermistors can be achieved.

Fig. 2 Fig. 3
Fig. 2 Variations of density vs. change of activation time and sintering temperature Tab. II Resistivity values at 25 o C and 80 o C, B constant and activation energy E a for NiMn 2 O 4 samples sintered at different temperatures and with different mechanical activation times of the starting powder