Elsevier

Applied Surface Science

Volume 257, Issue 23, 15 September 2011, Pages 9825-9829
Applied Surface Science

Study on the preparation and electrical properties of NTC thick film thermistor deposited by supersonic atmospheric plasma spraying

https://doi.org/10.1016/j.apsusc.2011.06.029Get rights and content

Abstract

In the present work, a series of thick Ni0.6Mg0.3Mn1.5−xAl0.6+xO4(x = 0, 0.1, 0.2, 0.4, 0.6) films (50 ± 10 μm) with negative temperature coefficient (NTC) were firstly deposited by newly developed high efficiency supersonic atmospheric plasma spray (SAPS) method. The phase, microstructure and electrical properties of films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and two-probe technique. The results showed that all the films were composed of cubic spinel structure, and the MgAl2O4 phase increased with increasing the Al2O3 content in the original powders. In addition, the films showed a dense and smooth surface with some pores in the grain boundaries. All the as-sprayed films showed a linear relationship between ln resistivity and reciprocal of absolute temperature (1/T) in the temperature range from 25 °C to 220 °C, which indicated a NTC characteristic.

Highlights

► The Ni0.6Mg0.3Mn1.5−xAl0.6+xO4 (0≤ x  0.6) films were first deposited by SAPS method. ► The as-sprayed films were composed of nanostructured grains. ► All the as-sprayed films showed NTC thermistor characteristic.

Introduction

The resistivity of thermistors with negative temperature coefficient (NTC) decreases with increasing temperature. Most of the NTC thermistors are based on the solid solution of transition metal oxides, such as NiO, Mn3O4, and Co3O4 [1]. These oxides typically exhibit the spinel-type crystal structure with the general formula as AB2O4 [2], [3]. In the spinel structure, oxygen ions are densely packed with different metal cations occupying the octahedral and tetrahedral sites of anions. The conductivity mechanism for this group of material is usually explained by the generally accepted phonon assisted hopping theory. The NTC property of these materials makes them a suitable choice for temperature sensing in applications like fire warning or over-heat detection [4].

As the bulk ceramic devices, the NTC thermistors are usually prepared by the conventional solid-state processing routes, and considerable efforts have been devoted to optimize the process parameters in order to produce high-performance NTC thermistors [3], [5], [6]. But due to the high porosity and incomplete inter-granular contact, the bulk NTC thermistors shows the poor stability and reproducibility [7], [8]. However, such problems should be less severe in dense films, since the porosity was decreased in the films and the parameters were easy to control in the deposition process [8]. Therefore, some researchers fabricated the film NTC thermistors using functional material, glass binder and organic vehicle [9], [10], [11], [12], [13], [14], [15]. Because the glass binder contains high percentage of lead, cadmium and glass forming agents (boron, silicon, and aluminum), it is harmful to environment and human being [16]. In addition, glass addition can lead to cracks during soldering due to the different thermal expansion coefficients between glasses and ceramics [17]. Meanwhile, some thin films were also deposited by Electron-Beam Evaporation (EBE) method [7], [8], but the disadvantage of this method is associated with the low deposition efficiency and high cost.

Recently, supersonic atmospheric plasma spray (SAPS) process has been successfully used to deposit ceramic and metallic coating with a lower porosity and higher performance [18], [19], [20], [21], [22]. During SAPS, the feedstock is heated to a molten state in the plasma jet. The molten droplets are propelled to the substrate or previously deposited lamella structures to form the coating.

In the present work, a series of Ni0.6Mg0.3Mn1.5−xAl0.6+xO4(x = 0, 0.1, 0.2, 0.4, 0.6) NTC thermistor films were firstly deposited by SAPS method. The phase composition, structure and electrical performance of films were studied in details.

Section snippets

Materials

High-purity (analytical grade) NiO, MgO, MnO2 and Al2O3 (Sinopharm Chemical Reagent Co. Ltd., China) powders were weighed respectively according to the stoichiometric composition of Ni0.6Mg0.3Mn1.5−xAl0.6+xO4(x = 0, 0.1, 0.2, 0.4, 0.6). The weighed powders and alcohol were mixed and milled for 24 h in a plastic jar using agate balls as grinding media. The ball-milled slurry was dried at 80 °C in an oven for 12 h. Then the dried powder was calcined in alumina crucible at 1300 °C for 2 h, and the

X-ray diffraction analysis

The XRD patterns of as-sintered Ni0.6Mg0.3Mn1.5−xAl0.6+xO4(x = 0, 0.1, 0.2, 0.4, 0.6) powders are shown in Fig. 1. As seen from it, the original Ni0.6Mg0.3Mn1.5−xAl0.6+xO4 powders were composed of only cubic spinel including the Ni–Mg–Mn–Al–O oxides and MgAl2O4 phases. Fig. 2 shows the XRD patterns of the as-sprayed Ni0.6Mg0.3Mn1.5−xAl0.6+xO4(x = 0, 0.1, 0.2, 0.4, 0.6) films. As shown in it, all the films contained the solid solutions of Ni–Mg–Mn–Al–O oxides with a cubic spinel structure and a

Conclusion

In the present work, the Ni0.6Mg0.3Mn1.5−xAl0.6+xO4(x = 0, 0.1, 0.2, 0.4, 0.6) films with negative temperature coefficient were firstly deposited by high efficiency supersonic atmospheric plasma spray method. The experimental findings are as follows:

  • (1)

    The as-sprayed Ni0.6Mg0.3Mn1.5−xAl0.6+xO4(x = 0, 0.1, 0.2, 0.4, 0.6) films were composed of cubic spinel structure and some amorphous structures, and the MgAl2O4 phase increased with increasing the Al2O3 content in the original powders.

  • (2)

    All the

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant no. 51072157, 50821140308), and by Doctoral Fund of Ministry of Education of China (grant no. 20100201110036).

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