The effect of TiO 2 concentration on the electrical and microstructural properties of ZnO-base varistor ceramic prepared from nanosize ZnO particles

Abstract

This study investigated electrical properties and microstructure of zinc oxide varistors containing MnO₂, Cr₂O₃, V₂O₅ and TiO₂. To this end, nanosize ZnO particles were synthesised by the polyacrylamide-gel method and doped with minor amounts of MnO₂ (0.5 mol-%), Cr₂O₃ (0.5 mol-%), V₂O₅ (2 mol-%) and TiO₂ in the range of 2–15 mol-%. The prepared samples sintered at 1050°C in air for 2 h. It was observed that TiO₂ promoted grain growth of ZnO particles. X-ray examination showed hexagonal ZnO structure as a main phase and Zn₂TiO₄ and ZnV₂O₄ as minor secondary phases. The nonlinear coefficient and threshold voltage of the prepared samples were in the range of 3.3–7.9 and 1120–4700 V cm⁻¹ respectively, in which both of them increased with increase in TiO₂ content. Among the prepared samples, the varistor with 15 mol-% TiO₂ exhibited the best performance in terms of nonlinearity. The nonlinear electrical behaviour was explained by theory of formation of the potential barriers at grain boundaries.

Keywords

Ceramic nano ZnO powder varistor nonlinear coefficient

Introduction

Recently, zinc oxide (ZnO) which is used for functional devices (sensor, varistor, etc.), piezotronics, backbone materials, etc. has received a great deal of attention. ZnO is a wide bandgap and n-type semiconductor with a direct optical band gap of ∼3.37 eV and large exciton binding energy of 60 meV [1,2]. ZnO-based varistors are electronic ceramic devices with nonlinearity in their current–voltage characteristics. These ceramics have been extensively used as surge protectors in power systems and also in electronic circuits. These devices which are equivalent to back-to-back Zener diodes, enable a small leakage current to flow below the breakdown voltage. When the voltage exceeds the switch voltage during a voltage transient or surge, the varistor becomes highly conductive and draws the current through it, usually to ground. When the voltage returns to normal, the varistor returns to its highly resistive state [3,4].

Nonlinear current–voltage characteristic of these materials is expressed by I = KV^α, where I is the current, V is the voltage, K is a constant and α is the nonlinear coefficient related to the material's microstructure [3]. Also, in terms of the current density J and the applied electric field E, the above current–voltage relation can be rewritten as J = (E/C) ^α where C is a constant [5]. The basic structure of the ZnO-based varistors consists of a conductive matrix of ZnO grains separated by high insulating boundaries, which is prepared by doping with varistor-forming additives, for example, Bi₂O₃, V₂O₅, etc. Melting point of additive is important in densification and formation of potential barriers at the grain boundaries of particles. With the addition of other metal oxides (for example, MnO₂, Cr₂O₃, and Co₂O₃), high-resistance insulating layers can be formed along the grain boundaries, resulting in highly nonlinear current–voltage properties [6,7]. The important electrical and structural properties of ZnO-based varistors are closely related to the compositions, density, grain size, morphology, porosity and distribution of any secondary phase. Therefore, by controlling these parameters, the nonlinear coefficient and break down field value can be controlled in order to obtain the required electrical performance. Accordingly, the method of preparation and homogeneity of the additive is critical for producing good varistor materials, which can be obtained by modifying the chemical composition, processing and sintering conditions. Thus, understanding the effect of sintering temperature and additives on the properties of the varistor ceramics is important for high performance and wide band applications of these materials [8 -10].

The varistor breakdown voltage is reported to be strongly affected by the grain size. In principle, varistors with larger average grain size are used for low-voltage applications, whereas those with smaller average grain size are implemented for high-voltage applications [10]. Recent studies have shown that V₂O₅ is a varistor former and has encouraged many research to study binary ZnO–V₂O₅ system. Also, the electrical, optical and dielectric properties of the above binary system, mixed with other metal oxides have been investigated.

Studies have shown that the light-metal oxide V₂O₅ is a potential varistor-forming ingredient for ZnO varistors. The advantage of vanadium-doped ZnO is that ceramics can be sintered at a relatively low temperature of about 900°C, which allows the material to be co-fired with a silver inner-electrode for applications in multilayer chip components. Moreover, V₂O₅ is a better sintering aid compared to Bi₂O₃ since V₂O₅-doped ZnO materials can be densiﬁed to the same density at a lower temperature [11]. To develop useful ZnO–V₂O₅-based ceramics, it is very important to fully comprehend the influence of diverse additives and sintering processes on the electrical and microstructural properties of these ceramics [12].

Ceramic powder synthesis technology allows the production of ceramic powder with high purity and much homogeneous grains. Today, it is possible to obtain a ceramic powder with well-controlled particle sizes ranging from nanometer to micrometer by employing suitable solution methods. Nano ZnO particles have various chemical and physical properties compared to bulk materials. Higher homogeneity, better sintering ability, and other unusual properties may be anticipated due to their large surface areas, formation of nanocrystallites, and various surface properties [13,14]. Thus, the fabrication of varistors with ZnO nanoparticle powder can improve their properties such as sinterability of the varistors. It has been reported that fine-grained materials prepared by mechanical activation, strongly affect the microstructure and electrical properties of the varistors. [15].

The nonlinear electrical properties of ZnO ceramics and other systems have been reported by many authors. On the other hand, the additives such as V₂O₅ and TiO₂ play an important role in improving the degree of nonlinearity and other properties of varistors. In this work, the primary aim was to prepare nanosize ZnO particles with uniform distribution. Secondary, the influence of TiO₂ incorporation in the range of 2–15 mol-% on the microstructure and electrical properties of varistor ceramics in ZnO–MnO₂ (0.5 mol-%)–Cr₂O₃ (0.5 mol-%)–V₂O₅ (2 mol-%) system was investigated.

Experimental

The polyacrylamide-gel method is a useful sol-gel method for the preparation of superfine nanoparticles. The quality of the nanoparticles can be improved by appropriate selection of monomer length, cross-linking agent, initiator, pH value, and sintering temperature. In this work, analytical grade and extra pure acrylamide (C₃H₅NO), N,N-methylenebisacrylamide (C₇H₁₀N₂O₂), ammonium persulphate ((NH₄)₂S₂O₈) and zinc sulphate heptahydrate (Zn(SO₄)7H₂O) were used to prepare the nano ZnO powder. Poly (acrylamide) hydrogel was prepared by mixing and polymerising 5 g of acrylamide (which was used as a monomer) dissolved in 3 mL of distilled water in presence of a cross-linker (N,N-methylenebisacrylamide). The mass ratio between acrylamide and N,N-methylenebisacrylamide was 5:1. The aqueous solution of Zn²⁺was made by dissolving zinc sulphate heptahydrate in the water and added to polyacrylamide hydrogel. The aqueous solution was heated to solute completely and polymerised for 1 h at 80°C in presence of ammonium persulphate (as a catalyst) to get the white translucent wet gel. The wet gel was dried for 2 h at 300°C to produce nanosize ZnO powder. Figure 1 shows the synthesis procedure of zinc oxide in this method.

The resultant nano ZnO powder was mixed with various additives according to the general formula: ZnO (97–X) mol-%–MnO₂ (0.5 mol-%)–Cr₂O₃ (0.5 mol-%)–V₂O₅ (2 mol-%)–TiO₂ (X) mol-%, where X = 2, 4, 6, 8, 10, 15. All additives were mixed in ball mill for 4 h. After milling, the obtained powder was pressed into disks of 10 mm in diameter and 1 mm in thickness. The prepared samples were sintered at 1050°C in air for 2 h and were covered by silver paste for electrical measurements.

A JEON 360 Transmission Electron Microscope (TEM) operated at 100 kV was used to observe morphology and size of the synthesised ZnO nanoparticles. Surface of the sintered specimens was observed via scanning electron microscope (SEM; Cam Scan MV2300). The crystalline phases were examined by powder X-ray diffractometer (XRD; PW1710, Philips) with Cu K _α radiation. The nonlinear current–voltage behaviour and breakdown points were measured using DC power supply and an electrometer (Model 617, Keithley, USA).

Results and discussion

Recently, the production of nanoparticles in the networks of hydrogel systems is recognised as an important approach due to its direct applicability in electronic devices. In this approach, hydrogel based on polyacrylamide network has been used successfully to produce ZnO nanoparticles in which, functional groups and cross-link density determine the uniformity and size of nanoparticles. The advanced feature of this methodology is that the nanoparticles were simply prepared uniformly at room temperature. Furthermore, it is recognised that the physical and mechanical properties of nanosize particles differ from those of macroscopic material [16].

Figure 2 shows the SEM and TEM images of the synthesised nano ZnO particles. It is clear that the particles are distributed relatively uniformly in the net and have less chance to agglomerate in the process of drying and decomposing. TEM image clearly indicates that the morphology of the nano ZnO particles has a roughly homogenous hexagonal structure. The average size of ZnO nanoparticles before sintering was near to 160 nm.

Figure 3 shows the X-ray diffraction patterns of nanopowder. The results indicate that the prepared ZnO particles have a polycrystalline hexagonal structure as the 2θ = 31.9, 34.4, 36.3, 47.71 and 56.7 which is in good agreement with the literature values [17,18]. It should be mentioned that the XRD results showed that the ZnO used in this study meet the characteristics of reference ZnO (JCPDS 01-080-0075) [19].

The SEM image of the prepared ceramics with general formula; ZnO (97–X) mol-%–MnO₂ (0.5 mol-%)–Cr₂O₃ (0.5 mol-%)–V₂O₅ (2 mol-%)–TiO₂ (X) mol-%, where X = 2, 4, 6, 8, 10, 15 is shown in Figure 4. It is clear that the microstructure of the ceramics mainly consists of large bulk grains, which are dispersed in a matrix of small ZnO grains. The size of these particles was in the range of 400–1500 nm. The large bulk grains were bigger than 3500 nm as shown in Figure 4. Some studies show that TiO₂ can be used as a grain growth-enhancing additive in ZnO–Bi₂O₃-based varistor ceramics. The enhancement of TiO₂ on ZnO grain growth was explained by an increased reactivity of Bi₂O₃-rich liquid phase with the solid ZnO in the presence of TiO₂. In the prepared ceramics, grain growth of the ZnO nanoparticles can be related to the presence of V₂O₅ and TiO₂. The high reactivity of the Vanadium-rich liquid phase in sintering process caused such a grain growth in which, titanium ions are rapidly distributed into the Vanadium-rich liquid phase and increase the chemical activity of ZnO particles and accelerate the enlargement of ZnO grains [20,21].

Figure 5 shows the X-ray diffraction pattern of prepared ceramics with different TiO₂ content. Inspection reveals that the diffraction peaks of ZnO (main phase) are strong and sharp, which indicate the growth of the ZnO crystals. Furthermore, diffraction peaks of Zn₂TiO₄ and ZnV₂O₄ secondary phases can be detected in these patterns. This may be attributed to the sintering reactions of TiO₂ and V₂O₅ with ZnO, producing Zn₂TiO₄ and ZnV₂O₄ respectively [22 -24]

An examination of varistor behaviour requires the display of current versus voltage. Figure 6 depicts current–voltage (E–J) characteristics of the ceramics with different TiO₂ doping levels. The nonlinear behaviour is observable in the E–J curves which are divided into ohmic (high resistance) and non-ohmic (low resistance) regions [22]. In the standby situation, the varistor exhibits a high resistivity (ρ > 10¹² Ω cm), i.e. it acts as an open circuit; but if the voltage across the terminals exceeds the critical breakdown voltage, the varistor demonstrates a remarkable conductive behaviour (ρ < 1 Ω cm), so that the varistor is a short circuit for the current. The transition occurring in nanoseconds is completely reversible and is without hysteresis. It was found that the E–J curves shift to the higher field as TiO₂ content increases. The better nonlinear electrical behaviour was found for the sample with 15 mol-% TiO₂. The nonlinear electrical parameters and composition of the samples are summarised in Table 1. The nonlinear coefficient (α) is the reciprocal of the slope of the E–J curve. Increase in TiO₂ content enhanced the nonlinear coefficient. The breakdown field decreased from 4700 to 1120 Vcm⁻¹ with a decrease in TiO₂ content.

Table 1

Nonlinear electrical parameters of the samples.

ZnO content (mol-%)	TiO₂ contetnt (mol-%)	Nonlinear coefficient	Breakdown field (volt cm⁻¹)
95	2	3.3	1120
93	4	4	1300
91	6	4.8	2150
89	8	6.6	2430
87	10	6.8	2760
82	15	7.9	4700

The nonlinear characteristic of the samples can be explained by the electron states that are formed on the surfaces of grains or existence of potential barrier at the grain boundaries [23,25]. The grain-boundary is treated as a junction in which the Fermi level of the bulk or grains is different from that of the layer between two grains, i.e. at the grain-boundary region. When the junction is formed and after the equilibrium is reached, the Fermi level is the same line along the junction. Therefore the chemical (binding) energy gained by an electron occupying a trap state is equal to the electrostatic energy spent in moving an electron from the interior of the grains to the boundary. In this situation, electrons trapped in the interface and a sheet of trapped electrons at the boundary leaves behind a layer of positively charged donor sites on either side of the boundary and creates an electrostatic field with a barrier at the boundary junction. This picture is in line with the Schottky diode picture of the potential barriers in which the breakdown electric field depends on the voltage barrier and the average number of electrical barriers formed per unit length during sintering. Therefore, the current–voltage characteristic can be described by the following equation: (1)

In which J is related to electronic transport through the potential barrier or electronic flux and J₀ is a constant. Φ_b is the barrier height, q the elementary charge, K_B Boltzman constant, T temperature, p number of barrier per unit of length of the polycrystalline system, and V the potential of forward bias.

At low voltage or ohmic region, i.e. when the applied bias voltage is low, Equation (1) can be approximated by using the following relationship exp(−qV/pK_BT) = (1 − qV/pK_BT) and it turns into (2)

Equation (2) means a linear response of current–voltage curves at low potential.

For the increment of the current density at high voltage we can write: (3)

In non-ohmic region of I – V (or E − J) curves, the current density increment contains two parts; increment of electric field at constant conductivity and lowering of barrier height. Then the current density increment in non-ohmic material can be written by the semi-empirical equation as (4) where σ₀ is the conductivity of the varistor at low fields. The nonlinearity factor β is then given by (5)

In Equation (5), is the average distance between barrier or average grain size (in the majority of situations). As expected, at low fields (βE << 1) and linear relation, J = σ₀E takes place. At high fields, J is increased with electrical field exponentially. This semi-empirical model was reasonably applied to a different composition of ZnO and SnO₂-based varistor systems [22,24 -26].

From this explanation, electric properties of the zinc-oxide-based ceramics are determined by the value of current, which passes through consequence chains of contacts between the adjacent grains of ZnO. For each such contact, it is conditionally possible to allocate two components of a direct current according to Figure 7.

On this figure, the phase between two adjacent zinc oxides is shaded. This layer could be enriched with additives. Component of current I₁ passes through the border areas where adjacent grains of ZnO closely abut to each other in which thickness of this layer could be about (30–40)Å. In this border area, the energy barrier formed is responsible for high nonlinear part of ceramic's current–voltage curve. Other component of current I₂ is determined by the resistance of border area where adjacent grains ZnO are separated by a thick layer of the internodes phase. For I₂ the weak nonlinear dependence is specific.

According to this explanation, the equivalent circuit of separate contact between grains has been shown in Figure 7 in which R₃ and R₁ are the resistance of ZnO grain and the resistance of the inter-grains phase, respectively. С_k is capacity between the two ZnO grain and R₂ is nonlinear resistance of the contact area.

At low values of the applied voltage, the varistor behaves as dielectric which has the high resistance (R₁). This resistance is caused by the presence of potential barriers between adjacent ZnO grains. More specifically, at low values of an electric field, the value of this resistance is high enough to play the basic role and limit the value of the current through the varistor. In high voltage region, the current value is determined by resistance R₂ which has nonlinear character. With an increase in applied voltage, the value of this resistance decreases sharply and at the same time, a growth of conductivity is observed [27].

Conclusion

Zinc Oxide powder with nanosize particles was prepared by polyacrylamide-gel method and doped with minor amounts of MnO₂, Cr₂O₃, V₂O₅ and TiO₂ according to the general formula: ZnO (97–X) mol-%–MnO₂ (0.5 mol-%)–Cr₂O₃ (0.5 mol-%)–V₂O₅ (2 mol-%)–TiO₂ (X) mol-% where X = 2, 4, 6, 8, 10, 15. Varistor ceramics were prepared with this powder by conventional powder processing route and sintered at 1050°C for 2 h. Characterisations of microstructural and electrical properties of the prepared ceramics reveal that all investigated properties are dependent on the ceramic composition. TiO₂ was found to be already playing their expected role like sintering aids and grain growth promoter. X-ray diffraction patterns of samples revealed the presence of hexagonal ZnO as a main phase and Zn₂TiO₄ and ZnV₂O₄ as minor secondary phases. Nonlinear coefficient and breakdown electric field increased with increase in TiO₂ content. Breakdown electric field lies in the range of 4700–1120 Vcm⁻¹. The varistor with 15 mol-% TiO₂ exhibited the best performance in terms of nonlinearity.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

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