Electrical Properties of Zn_xMn_3-xO₄ Ceramics for Application as IR Detectors

Abstract

Zn_xMn_3-xO₄ (0.95≤x≤1.20) specimens were prepared using a conventional solid state reaction method. All specimens were sintered in air at 1,200℃ for 12 h, cooled at a rate of 2℃/min to 800℃, and subsequently quenched to room temperature. We investigated the electrical properties of Zn_xMn_3-xO₄ specimens with various amounts of ZnO for use as IR detectors. At a composition of x≥1.15, the ZnO phase precipitates beside the spinel structure. The electrical resistivity at room temperature, activation energy, responsivity, and detectivity of a Zn_1.10Mn_1.90O₄ specimen are 653.2 kΩ-cm, 0.392 eV, 0.016 V/W, and 7.52×10³ cmHz^1/2/W, respectively.

1. INTRODUCTION

Transition metal manganite MxMn3-xO4 (M=Cd, Mg, Zn, Mn) spinels show a tetragonally distorted structure owing to the cooperative Jahn-Teller distortion associated with the Mn(III) ions that fill the octahedral sites of closely packed oxygen ions [1]. They are important technological materials used as negative temperature coefficient (NTC) thermistors owing to their interesting electrical properties [2-4]. The electrical conduction mechanism of MxMn3-xO4 spinel materials is described as a hopping mechanism resulting from electron movement between neighboring Mn ions in the octahedron-adjacent sites. The electrical properties of such materials are highly dependent on their composition and the process by which they are manufactured.

A thermal infrared detector has a lower response speed and sensitivity than does a quantum type detector. On the other hand, the thermal infrared detector has been subjected to a number of studies to assess its usefulness in civilian devices because it has the advantage of being operable at room temperature. Recently, many studies have been done on vanadium oxide [5,6] and Mn-Co-Ni-Oxide [7] materials for application as bolometer-type uncooled infrared detectors.

In general, zinc manganese oxides have high NTC properties and excellent high-temperature stability, and are possible candidates for high temperature devices. However, most previously published studies concerning zinc manganese oxides were dedicated to the compound ZnMn2O4 for use in NTC thermistors [8]. In this study, the influence of composition on the electrical properties of ZnxMn3-xO4 (0.95≤x≤1.20) materials was investigated for application in uncooled infrared detectors.

 

2. EXPERIMENTS

The investigated specimens of ZnxMn3-xO4 (0.95≤x≤1.20) were prepared using the solid state reaction method. High-purity ZnO and Mn2O3 powders were weighed and ball-milled for 24 hours in a polyethylene container using zirconia balls as the milling medium. The mixture was then dried at 100℃ in an oven for 24 hours, and calcined for 2 hours at a temperature of 900℃. The calcined powders were granulated through a 200-mesh screen. The granulated powders were pressed using a hand press at a pressure of 1000 psi to prepare pellets with a diameter of 12 mm. The green compacts were sintered at 1,200℃ at a rate of 5℃/min for 12 hours in air.

The microstructure and morphology of the prepared specimens were characterized by field-emission scanning electron microscopy (FE-SEM). To evaluate the electrical properties, an Ag electrode was formed on both sides of the specimen by a screen printing method, and the electrical properties were measured during a temperature change from - 10℃ to 60℃ using an electrometer. The experimental set-up used for IR detection and measurement is shown in Fig. 1. The temperature of the black-body source, the chopper frequency, and the bias voltage were 500℃, 10Hz, and 3 V, respectively.

Fig. 1.IR detection performance evaluation system.

 

3. RESULTS AND DISCUSSION

Figure 2 shows an FE-SEM image of a Zn1.15Mn1.85O4 specimen and EDS layer imaging scans for Zn, Mn and O. All specimens exhibited a very dense microstructure, and the average grain size increased with increasing concentration of Zn ion. This can be explained by the fact that the size of the crystal grains increased and the movement of charge carriers was accelerated due to an increase in the Mn4+/Mn3+ ion ratio resulting from a subset of Zn2+ ions being located at the octahedral sites [9,10]. However, for specimens with a composition of x≥1.15, the second phase, ZnO, was observed between grain boundaries, which is thought to be due to addition of Zn ions over the solubility limit being segregated in the grain boundary.

Fig. 2.(a) FE-SEM surface microstructure image and (b) EDS layer image of Zn1.15Mn1.85O4 specimens.

Figure 3 shows the resistivity at room temperature and B-value of ZnxMn3-xO4 specimens with various composition ratios. The B-value, which represents the sensitivity of electrical resistance to temperature change, can be calculated by using Eq. (1), where R298 and R323 are the resistances measured at 298 K and 323 K, respectively.

Fig. 3.Resistivity and B-value of ZnxMn3-xO4 specimens with various concentrations of Zn at room temperature.

Resistivity at room temperature and B-value decreased with an increase in the concentration of Zn ion. These properties had minimum values of 653.2 kΩ-cm and 4545 K when x=1.10, and then increased when x≥1.15. For x<1.00, ZnxMn3-xO4 specimens have a single phase and a tetragonal structure, and Zn2+ and Mn3+ ions are distributed at tetrahedral and octahedral sites, respectively. In general, such a composition of materials has a very high electrical resistance, because of the small number of charge carriers at the octahedral sites available for hopping conduction. For 1.00≤x≤1.10, some Zn2+ ions probably occupy the octahedral sites. Then, Zn2+ ions substitute for the Mn3+ ions and an equivalent proportion of Mn4+ ion is created at the same site in order to maintain electrical neutrality. The hopping phenomenon is then possible between Mn3+ and Mn4+ ions, and electrical resistivity rapidly decreases [11,12]. However, for x≥1.15, as previously considered in the EDS analysis of Fig. 2, the electrical resistivity increases again owing to formation of the ZnO phase between the grain boundary layers.

Figure 4 shows the activation energy of ZnxMn3-xO4 specimens with varying Zn content. The activation energy decreased with an increase in the concentration of Zn ion, and the Zn1.10Mn1.90O4 specimen shows the minimum value of 0.392 eV. For x≤1.00, as previously considered in the resistivity properties shown in Fig. 3, the Mn4+/Mn3+ ion ratio increased with an increase in the concentration of Zn ion, so that the activation energy required for the hopping conduction of ions was reduced. For x≥1.15, the activation energy increased again. This result suggests that the amount of Mn4+ ion on the octahedral sites decreased as the proportion of the ZnO phase increased with increasing zinc content. Zn-Mn oxides exhibited higher activation energy characteristics than did Ni-Mn oxides [13].

Fig. 4.Activation energy of ZnxMn3-xO4 specimens with varying Zn content.

Figure 5 shows the responsivity of ZnxMn3-xO4 specimens with varying Zn content. Responsivity, which indicates the output voltage characteristic for the incident infrared radiation, increased with an increase in the composition of Zn ion, and the Zn1.10Mn1.90O4 specimen exhibited a maximum value of 0.016 V/W. This phenomenon may be explained by the fact that the Zn1.10Mn1.90O4 specimen had a minimum value of activation energy for electrical conduction, and thus exhibited a high change in resistance for the incident infrared signal.

Fig. 5.Responsivity of ZnxMn3-xO4 specimens with varying Zn content.

Figure 6 shows the noise voltage of ZnxMn3-xO4 specimens with varying Zn content. Noise voltage of bolometer-type IR detectors is caused by the Johnson noise due to input resistance, thermal noise due to the irregular movement of electrons, and current/ voltage noise due to the amplifier device [14]. In particular, thermal noise due to structural defects plays the dominant role in the noise characteristics. Noise voltage decreased with an increase in the concentration of Zn ion, and the Zn1.10Mn1.90O4 specimen had the minimum value of 6.51×10-5 V. For x≤1.10, Zn ions were properly distributed at tetrahedral and octahedral sites. However, for x≥1.15, the concentration of Zn2+ ions is too high, the ZnO phase precipitates between the grain boundaries.

Fig. 6.Noise voltage of ZnxMn3-xO4 specimens with varying Zn content.

Figure 7 shows the detectivity of ZnxMn3-xO4 specimens with varying Zn content. Detectivity, which is the minimum value of infrared radiation that can be detected, depends on the activation energy and noise voltage characteristics. The Zn1.10Mn1.90O4 specimen shows the maximum value of 7.52×103 cmHz1/2/W. This can be explained by the fact that the Zn1.10Mn1.90O4 specimen had the lowest activation energy and noise voltage properties, as presented in Fig. 4 and Fig. 6.

Fig. 7.Detectivity of ZnxMn3-xO4 specimens with varying Zn content.

 

4. CONCLUSIONS

In this study, we investigated the electrical properties of ZnxMn3-xO4 specimens with varying Zn content for application in uncooled IR detectors. EDS analysis revealed that, for x≥1.15, ZnO phases were observed between the grain boundaries. The Zn1.10Mn1.90O4 specimen exhibited the lowest resistivity and activation energy and, for x≥1.15, these properties increased again due to formation of the ZnO phase. ZnxMn3-xO4 materials exhibited higher activation energy characteristics than did Ni-Mn oxides. The voltage responsivity and detectivity properties were highly dependent on the activation energy and noise voltage characteristics.