Structure and electrical properties characterization of NiMn2O4 NTC ceramics

https://doi.org/10.1016/j.inoche.2021.108856Get rights and content

Highlights

  • The structure, microstructure, electrical and dielectric study of NiMn2O4 is reported.

  • The XRD pattern confirms the formation of spinel crystallized in a cubic structure.

  • SEM micrograph shows agglomerated nanostructure with a grain size of less than 100 nm.

  • The AC conductivity studies confirmed the dominance of the polaron hopping process.

  • NTCR behavior was confirmed by a decrease in temperature-dependent DC resistivity.

  • Cole-Cole plot establishes the dominance of grain boundary on conduction mechanism.

Abstract

Spinel-type NiMn2O4 negative temperature coefficient (NTC) ceramic compound was successfully prepared by the co-precipitation technique. The structure, microstructure, and transport properties are examined for the thermistors application. X-ray diffraction pattern illustrates the formation of spinel structure with cubic crystal symmetry. The scanning electron micrograph shows irregular shape grains with an average size of less than 100 nm. The observed microstructure is mainly due to the low-temperature synthesis and occupancy of Mn3+ at octahedral sites. The dielectric loss was very less in low-temperature regions and increase exponentially at higher temperatures indicating an improvement in electrical conductivity. Similar behavior was observed for relative permittivity and indicates temperature-dependent dielectric polarization along with high-temperature phase transition. The Cole-Cole graph shows a decrease in an area of semicircular with an increase in temperature confirming the improvement of electrical conductivity. The dc-resistivity measurement shows a decrease in resistivity with increasing temperature typically observed in NTC materials.

Introduction

In recent times the metal oxides with spinel (AB2O4) type structure have gained immense research interest due to their wide ranging applications from electronic device manufacturing industries to bio-medical applications [1], [2]. The modified spinels like Ni-Cu-Zn based ferrites are highly useful in spintronics and high frequency device applications [3] whereas pure ferrites are extensively investigated for anti-cancer activities [4]. The overwhelming demand of thermal sensors in various industrial applications motivate researchers to investigate some novel materials with high structural stability for thermistors applications. The high temperature sensitivity is one of the important characteristics of spinels along with its high structural stability. The main objective of this research is to probe a thermally sensitive material and characterize its usability for thermistors application.

Negative temperature coefficient of resistance (NTCR) behavior is one of the most exciting temperature-dependent phenomena, normally observed in spinel oxides where the resistance decreases sharply with the increase in temperature [5], [6], [7]. NTC thermistors are high-performance temperature sensors comprising advantages like high-temperature sensitivity, simple operation, and low cost over other types of sensing technologies [8], [9].

Among studied NTC materials, the Mn-Co-Ni-O based oxide systems are promising materials used as thermistors for temperature measurement in both industrial and domestic applications. The NTCR behavior of Mn-Co-Ni-O based oxide systems are potential candidates for temperature measurement due to high thermal sensitivity, quick response, and good reliability in comparison with other temperature sensor materials [10], [11]. Though several reports are available on the electrical behavior of Mn-Co-Ni-O oxides [12], [13] the reports on Mn-O based systems and its NTC behavior are indeed less.

Mn-O based oxides with the spinel structure AB2O4 have attracted huge attention due to their high thermal sensitivity and NTC behaviour [14], [15]. They are generally used in home appliances for temperature monitoring, control, and compensation [16]. Among many reported Mn3O4 based spinel oxides, copper and nickel substituted Mn3O4 are the most commonly studied stoichiometric compositions for NTC thermistors because of their inexpensive, simple preparation method and superior thermistors properties. They normally crystallize in cubic spinel structure with cations situated equally at the tetrahedral and octahedral site. In these spinels, the electrical conductivity mechanism is well explained by the electron hopping mechanism [17]. Mn3O4 based NTC thermistors are bulk ceramic devices generally synthesized by a well known solid-state synthesis technique. The substantial attempts have been reported for solid-state synthesized Mn3O4 and its optimization of process parameters for fabricating high-performance Mn3O4 based NTC thermistors [18], [19]. However, there are fewer reports on the synthesis of substituted Mn3O4 based ceramics in the compositional form of NiMn2O4 by wet chemical technique and its NTC behavior. Hence, this study reports the phase analysis, structural features, and transport properties of NiMn2O4 NTC materials prepared by a co-precipitation method.

Section snippets

Materials and methods

Spinal type NiMn2O4 was prepared by a simple co-precipitation technique as reported in our earlier studies on Mn3O4 [20]. The main advantages of co-precipitation technique include the use of low cost starting materials and ability to produce large scale ceramic oxide powders with high purity. In brief, the stoichiometric amount of analytically grade 0.1 M MnSO4·H2O and 0.1 M NiCl2·used as starting materials were mixed by magnetic stirrer employing DM water as a solvent at room temperature.

Structure and microstructure studies

The synthesis of NiMn2O4 is carried out using a wet chemical co-precipitation of Mn2+/Ni2+ in the presence of NaOH and at 363°K and allowed to crystallize at 773°K to obtain a spinel structure. Further, the addition of dilute ammonium solution to Mn2+/Ni2+ leads to the formation of M(NH3)42+ (M: Mn and Ni) and the continuous addition of Mn2+/Ni2+ produces MnOOH. Further, it reacts with Mn(NH3)42+ once again to form Mn7O13 and M(NH3)4+. It should be noted that the intermediate of Mn7O13 is

Conclusion

In this study, we successfully report the structure, microstructure, dielectric properties and NTC behavior of NiMn2O4 spinel prepared by the co-precipitation method. The powder XRD pattern confirms the formation of a single-phase spinel crystallized in a cubic structure. The SEM micrograph shows agglomerated and uneven nanostructure with an average grains size less then  100 nm. The frequency-dependent ε′ behavior was observed to follow Koops and Maxwell relations. The mobility of charge

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

Author (RS) gratefully acknowledges Vision Group of Science and Technology (VGST), Dept. of IT, BT and S&T, Govt. of Karnataka, India for the project grant of CESEM, vide GDR No.221 dated 24-02-2014 for the Dept of Physics, MITE Moodbidri. One of the Authors also acknowledges VTU, TEQIP Cell towards the project grant under competitive research funding. All authors acknowledge Mr. Rajesh Chouta, Chairman, Mangalore Institute of Technology & Engineering, Moodbidri, for extending the research

References (42)

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