Improving the stability of Fe0.8Mn1.54Ni0.66O4 NTC thermistor with nano-powders and N2 annealing

Fe0.8Mn1.54Ni0.66O4 nano-ceramics have been successfully prepared by sol-gel auto combustion. The microstructure and phase of these samples were observed by using SEM and XRD. The diameters of Fe0.8Mn1.54Ni0.66O4 ceramic particles pre-fired at 800 °C range from 52 to 83 nm. The powder sintered at above 1050 °C has the compact and uniform spinel structure. We have investigated the electrical characteristics of these thermistors at different sintering temperatures and concluded that the sample sintered at 1200 °C is sufficient to form the appropriate spinel phase. Moreover, the thermistor annealed for 72 h at 450 ∼ 550 °C in N2 atmosphere has the drift rate of <0.7%.


Introduction
NTC (Negative Temperature Coefficient) thermistors are widely used in industrial applications such as temperature measurement, control and compensation.
One important problem in using of NTC thermistor is to overcome the aging phenomenon [1,2]. Their electrical properties change with time particularly when the temperature increases and it is the main reason why they are unable to use at temperatures above 150°C. Aging of electrical properties attributes to non-equilibrium states that exist inherently in semiconducting NTC ceramics [3].
In order to solve this problem, the previous authors doped the oxides of transition metals such as Zn, Co, Cr and Zr to nickel manganite materials and improved their aging characteristics and stability [4][5][6][7][8][9][10][11][12]. In addition, some authors employed various new powder preparation methods thereby changing parameters of both electrode coating and heat treatment after sintering [13][14][15].
Z B Wang and his colleagues reported that the improved aging behavior of the N 2 -annealed thermistor is explained by the reduction of the concentration of the cation vacancy upon annealing under lower oxygen partial pressure and the suppression of cation redistribution [16]. However, they did not determine the reasonable temperature and time in N 2 annealing for high reliable NTC thermistor.
Wenwen Kong explained about the influence of oxygen atmosphere annealing on the thermal stability of Mn l.2 Co l.5 Ni 0.3 O 4±δ ceramic films fabricated by RF magnetron sputtering [17].
So, the aim of this paper is to research the electrical properties of Fe 0.8 Mn 1.54 Ni 0.66 O 4 nanoceramic material prepared by a more convenient sol-gel auto combustion method and determine the reasonable temperature and time in N 2 annealing. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Experimental procedure
The analytical grade Fe(NO 3 ·were mixed in molar ratio of Fe 0.8 Mn 1.54 Ni 0.66 O 4 and dissolved in deionized water. This solution, citric acid and ethylene glycol were mixed in molar ratio of 1:2:4.4. A small amount of ammonium hydroxide was carefully added into the solution to change the pH value to 7. The solution was heated at 80°C and stirred by using a magnetic stirrer continuously.
Because of evaporation, the viscous solution was turned into a dried gel. When the gel was ignited in the air, it burnt in a self-propagating combustion way and it turned to loose powder.
The prepared mixture powder was pre-sintered at 800°C for 2 h and ball-milled for 36 h in ethyl alcohol slurry and dried at 80°C. Pellets of diameter 4 mm, thickness 2 mm were shaped from the powder under 30 MPa pressure with 5 wt% PVA binder. These pellets were classified into 4 batches and each of them was sintered at 1 050, 1 150, 1 200 and 1 250°C for 2 h in the air, respectively. For metallization, these sintered pellets were coated with Ag paste on their both faces.
In order to determine the annealing temperature and time in the N 2 atmosphere, the N 2 gas was flowed with the rate of 4 l min −1 into the quartz tube furnace (diameter 50 mm, length 400 mm). The specimens of 16 batches were in this furnace and maintained respectably at 350, 450, 550 and 650°C for 24, 72, 96 and 144 h.
The precursor powder was analyzed using x-ray diffraction (D8-Advance diffractometer) for phase identification. The particle morphology in the pre-sintered powder and sintered pellets were observed with a Scanning Electron Microscope (Quantum 200). The electrical resistance was measured with a digital multimeter (DT9208A) at 25°C and 50°C, respectively.
The B value was calculated by the following equation: where R 25 and R 50 are the resistance measured at 25°C and 50°C. Then the specimens were cooled to 25°C. Their electrical parameters were measured and compared with the parameters before annealing.

Results and discussions
3.1. Phase formation and microstructure From x-ray diffraction analysis, we find that the spinel phase was formed in the powders pre-fired at 800°C. Figure 3 is SEM photograph of specimen pre-sintered at 800°C. As seen in this figure, the diameter of power particles ranges from 52 to 83nm. The relative densities of all the specimens were calculated at different sintering temperatures (figure 1).
From figure 1, we find that the relative density gets saturated when sintered at 1 200°C and a high sintering temperature is necessary for densification of ceramics.
The actual density and the calculated density of sample are 3.965 g cm −3 and 4.495 g cm −3 respectively when sintered at 1 200°C. The SEM photograph of the specimen sintered at 1050°C shows that the specimen is comparatively compact and its particles are dispersed uniformly and are fine without any big particle ( figure 4).
The specimens sintered at 1 200°C and 1 250°C were denser than at 1 050°C. ( figure 5 and figure 6) and there are no too big particles. However, it is difficult to get the homogeneous system of ceramic material because the activation energy is too high above 1 200°C (figure 6).
The XRD patterns for the powder after firing at 800°C and specimen sintered at 1 200°C are shown in figures 2 and 7. We can identify from the above XRD patterns that the synthesized powder is a single cubic spinel phase. It can be seen that the amorphous gel is transformed directly into a single spinel phase after firing at 800°C.
There are the some unessential phases in figure 2. However, no other phases were formed after sintering at 1 200°C in figure 7. Figure 8 represents the resistivity at 25°C and the B values of all the specimens.  where a is the number of moles of the separated NiO phase. The resistivity of the sintered sample at 1250°C is suddenly increased because the amount of the insulated NiO phase increases at this temperature.

Observation of the aging phenomenon
The analytical result of the resistivity and B values above shows that the specimen sintered at 1200°C is suitable for the manufacture of temperature sensors.
Therefore, we observe in detail the aging phenomenon of the specimen sintered at 1200°C.  The main problems are to analyze the effects of N 2 annealing temperature and time on the thermistor prepared by sol-gel auto combustion and to determine the reasonable annealing temperature and time.
The drift rate η is calculated according to where R 0 is the resistance at 25°C before annealing and R 1 is the resistance at 25°C after annealing. Figure 8 represents the drift rate of the samples. From figure 9, we find that the sample annealed at 450°C for 72 h has the drift rate of 0.7%. When the annealing temperature is at above 550°C, the drift rate rather decreases, but the specimens show a partial change in mechanical characteristics of the ceramics due to the sudden temperature stress. From this results, it is found that the annealing in N 2 atmosphere is reasonable at 450∼500°C for 72 h.
Finally, we tested to compare the drift rate of two kind of thermistor prepared with nano-powder and micropowder. The testing results are shown that the stability of samples prepared with nano-powder is superior to the samples prepared with micro-powder. The drift rate of the samples prepared with micro-powder is 0.9%-1.4%, but one of the samples prepared with nano-powder is <0.7%.

Conclusions
The nano-ceramics Fe-Ni manganite powders were prepared via a sol-gel auto-combustion process. The nitritecitrate gels can burn in a self-propagating combustion process in air to transform into single phase.
Nano-ceramics manganite particles with spinel crystal structure. The fine-grained Fe 0.8 Mn 1.54 Ni 0.66 O 4 ceramics with homogeneous microstructure were prepared at a lower temperature. During the sintering at high temperature, a small amount of NiO phase was formed due to the decomposition of spinel. The obtained ceramics have high activation energy.
We have investigated the electrical characteristics of these thermistors at different temperatures and concluded that the sample sintered at 1 200°C is sufficient to form the appropriate spinel phases. Moreover, it is observed that the thermistor heat-treated for 72 h at 450∼550°C in N 2 , has the drift rate of <0.7%. These samples will be used for several NTC thermistor applications.