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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.29 No.10 pp.586-591
DOI : https://doi.org/10.3740/MRSK.2019.29.10.586

# Doping Effect of Yb2O3 on Varistor Properties of ZnO-V2O5-MnO2-Nb2O5 Ceramic Semiconductors

Choon-Woo Nahm†
Department of Electrical Engineering, Dongeui University, Busan 47340, Republic of Korea
Corresponding author E-Mail : cwnahm@deu.ac.kr (C.-W. Nahm, Dongeui Univ.)
July 24, 2019 September 9, 2019 September 11, 2019

## Abstract

This study describes the doping effect of Yb2O3 on microstructure, electrical and dielectric properties of ZnO-V2O5-MnO2-Nb2O5 (ZVMN) ceramic semiconductors sintered at a temperature as low as 900°C. As the doping content of Yb2O3 increases, the ceramic density slightly increases from 5.50 to 5.54 g/cm3; also, the average ZnO grain size is in the range of 5.3-5.6 μm. The switching voltage increases from 4,874 to 5,494 V/cm when the doping content of Yb2O3 is less than 0.1 mol%, whereas further doping decreases this value. The ZVMN ceramic semiconductors doped with 0.1 mol% Yb2O3 reveal an excellent nonohmic coefficient as high as 70. The donor density of ZnO gain increases in the range of 2.46-7.41×1017 cm−3 with increasing doping content of Yb2O3 and the potential barrier height and surface state density at the grain boundaries exhibits a maximum value (1.25 eV) at 0.1 mol%. The dielectric constant (at 1 kHz) decreases from 592.7 to 501.4 until the doping content of Yb2O3 reaches 0.1 mol%, whereas further doping increases it. The value of tanδ increases from 0.209 to 0.268 with the doping content of Yb2O3.

## 초록

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Varistor is resistor in which the resistance varies with voltage in a nonohmic relation.1,2) Varistor has high resistance and is excellent resistor, if any voltage is below critical voltage. When the applied voltage exceeds critical voltage, the resistance of the varistor suddenly decreases and the electrical current abruptly increases. Basically, due to these varistor properties, the varistor possesses the ability to absorb various surges and prevent electrical appliances, electronic passive and active components from being damaged by surges.3)

Ceramic varistor is fabricated from metal oxide powder with high purity. Among a lot of materials, the main varistor type is zinc oxide (ZnO), which is nonstoichiometric n-type semiconductor having excess Zn. To change ZnO of low conductivity to ceramic semiconductors of much higher conductivity, the oxides such as bismuth, cobalt, manganese and others should be added to ZnO by small content. Based on usage of varistor, firstly diverse additives are designed to obtain satisfactory solution.

The microstructure structure of ZnO varistor consists of numerous semiconducting grains separated by grain boundaries. As it were, this is like to the microstructure of a polycrystalline. One grain boundary between grain and grain is like to one pn junction.2-,4) As a result, varistors possess three dimensionally a lot of pn junctions, that is, multi-junction. If the junction is not active, the potential barrier will be not generated and then the ohmicity will appear. However there is no how confirm it. Most of varistor are assumed to be active. Owing to multi-junction, the varistor can block or pass the current according to the applied voltage condition.3,4)

Today, commercial multilayered varistors are largely classified into three: bismuth-based, praseodymium-based, and borosilicate-lead-zinc glass-based. These are obtained by sintering at a temperature above 1,000 °C.1,5,6) Therefore, the internal electrode within varistors should be expensive palladium (Pd) or platinum (Pt). By the way, adding V2O5 to ZnO varistors may allow ceramic semiconductors to sinter at low temperatures. That is, ZnO-V2O5-based ceramic semiconductors can be sintered at below 900 °C. This is a big advantage because ZnO-V2O5-based ceramic semiconductors can be cofired with silver (Ag) inner- electrode instead of the expensive Pd or Pt.7,8) However, currently, the ZnO-V2O5-based ceramic semiconductors are not being applied to field due to characteristics, reliability and so on.9-12) Therefore, it is necessary to be continuously researched in diverse aspects such as dopants and sintering conditions for applications.13-19)

In this work, the doping effect of Yb2O3 on microstructure, E-J and C-V, and dielectric properties of ZnO-V2O5- MnO2-Nb2O5 varistors were examined. Furthermore, the mutual correlation between diverse characteristic parameters was analyzed. And the important one of this research is to report the fact that Yb2O3-doped ZnO-V2O5-MnO2- Nb2O5 varistors revealed excellent nonohmic properties (α = 70).

## 2. Experimental procedure

### 2.1 Specimen preparation

Metal oxide powder of high purity (> 99.9 %) for varistors were prepared with the following composition ration: (97.4-x) % ZnO, 0.5 % V2O5, 2.0 % MnO2, 0.1 % Nb2O5, x % Yb2O3 (x = 0.0, 0.05, 0.1, and 0.25 mol). The powder composition was obtained by conventional blending using ZrO2 balls (ϕ 10 mm) and acetone in ppbottle of 250 ml (Nalgene) for 24 h. The blended powder was dried, mixed with polyvinyl butyral binder, and pressed into pellets of disk shape (ϕ 10 mm × t 1.5 mm) at a pressure of 100 MPa. The disk-pellets were sintered at 900 °C in air for 3h and thereafter cooled at 4 °C/min. The disk-pellets were lapped and polished in exact dimension (ϕ 8 mm × t 1.0 mm). A silver paste was painted on both pellet sides through screen printer and thereafter heated at 550°C for 10 min.

### 2.2 Microstructure analysis

The samples for microstructure were lapped and polished using a SiC paper and Al2O3 powder and then chemically etched using an HClO4–H2O solution. The surface was analyzed in a field emission scanning electron microscope (FE-SEM Model Quanta 200, Czech). The average grain size (d) was determined by the lineal intercept method.20) The ceramic phases generated were analyzed by an Xray diffraction (XRD Model X¢pert-PRO MPD, the Netherlands) using CuKα radiation. The ceramic density (ρ) was measured using a density determination kit (238490).

### 2.3 Electrical and dielectric measurement

The V-I measurements of the samples were performed using a high voltage source measure unit (Model Keithley 237, USA). The switching voltage (V1mA) was estimated at a current density of 1.0 mA/cm2 and the density of leakage current (JL) was estimated at 0.80 V1 mA. Furthermore, the nonohmic coefficient (a) was estimated from the following expression, α = 1/(logV2 – logV1), where V1 and V2 are the switching voltage corresponding to 1.0 mA/cm2 and 10 mA/cm2, respectively.

The C-V measurements of the samples were carried out using an LCR meter (Model QuadTech 7600, USA) and an electrometer (Model Keithley 617, USA). The donor density (Nd), potential barrier height (Φb), surface state density (Ns) were determined from the expression proposed by M. Mukae et al..21)

The f-CAPP' relation and dissipation factor (tanδ) were performed by using an LCR meter (Model QuadTech 7600, USA).

## 3. Results and Discussion

The SEM images of the samples with various doping contents of Yb2O3 are indicated in Fig. 1 The images show typical varistor microstructure regardless of the doping content of Yb2O3. The variation trend of the average ZnO grain size and the ceramic density of the samples according to the doping content of Yb2O3 are indicated in Fig. 2. The average ZnO grain size decreased from 5.5 to 5.3 μm until the doping content ofYb2O3 reaches 0.1 mol%, whereas a further doping has reversely increased up to 5.6 μm at 0.25 mol% Yb2O3. The YbVO4 phase (see Fig. 3 in advance) is like to restrict the grain growth, whereas too much YbVO4 phase has rather enhanced the grain growth. On the other hand, the ceramic densities (r) slightly increased from 5.50 to 5.54 g/cm3. The reason why the additive Yb2O3 increases the ceramic density is like to be attributed to the increase of the content of YbVO4 with the doping content of Yb2O3.

The XRD profiles of the samples for various doping contents of Yb2O3 are indicated in Fig. 3. The XRD patterns show the minor phases such as Zn3(VO4)2, ZnV2O4, and VO2 generated in all the samples, with ZnO grains as a major phase. In addition, the samples doped with Yb2O3 revealed YbVO4 as a secondary phase by showing the diffraction peak at a diffraction angle θ = 25.39 and 33.93. The peak of YbVO4 gradually increased with increasing the doping content of Yb2O3. The YbVO4 phase may significantly affect V-I properties as shown in Table 1. The estimated microstructure characteristic parameters are indicated in Table 1.

The modified V-I characteristics (E-J) of the samples for various doping contents of Yb2O3 are indicated in Fig. 4. The varistors have the ability to block or pass currents through its nonohmicity or ohmicity. Current-blocking is done at prebreakdown region and current-passing is done at breakdown region. As can be seen through curve shapes in Fig. 4, the doping of Yb2O3 significantly affects the nonohmic properties of varistors. The switching voltage (V1 mA) increased from 4,874 to 5,494 V/cm when the doping content of Yb2O3 is less than 0.1 mol%, whereas a further doping has rather decreased V1 mA-value up to 4,574 V/cm at 0.25 mol% Yb2O3. The variation trend of V1 mA-value depends on firstly grain size and secondly switching voltage (vgb) of a single grain boundary. As a result, the V1 mA-value will be more affected by microstructure.

The variation trend of the nonohmic coefficient and the the density of leakage current of the samples with according to the doping content of Yb2O3 are indicated in Fig. 5. The nonohmicity in the E-J characteristic curves is characterized by the nonohmic coefficient (α). The α-value markedly increased from 51 to 70 until the doping content of Yb2O3 reaches 0.1mol%, whereas a further doping has abruptly decreased α-value up to 34.4 at 0.25 mol% Yb2O3. The results show that the doping of Yb2O3 had an obvious effect on the nonohmic properties. The varistors doped with 0.1 mol% Yb2O3 revealed the highest α-value (70), compared with existing ZnO-V2O5-based varistors.18,19) The key point of this paper is that an excellent nonohmicty can be obtained from ZVMN-Yb2O3 varistors. The reason why the varistor doped with 0.25 mol% Yb2O3 exhibited lower α-value than that of Yb2O3-free is assumed to be the result of the excess YbVO4 phase as mentioned in XRD analysis. This excess YbVO4 phase may have a significant effect on the potential barrier height at the grain boundaries. As a result, that will decrease the potential barrier height (see Table 2 in advance) and then it is concluded that the α-value abruptly decrease. Meanwhile, as the doping content of Yb2O3 increases, the density of leakage current (JL) increased from 56.8 to 262.2 μA/ cm2. On the whole, although the nonohmic coefficient is high, the leakage current of the samples is high. The leakage current is related to prebreakdown region, of three regions: prebreakdown region (ohmicity), breakdown region (nonohmicity), and upturn region (ohmicity). A high leakage current in prebreakdown region is attributed to the recombination between holes and electrons at the grain boundaries. Furthermore, JL-value is not unrelated to the electron density (see Table 2 in advance). The estimated E-J characteristic parameters are indicated in Table 1.

The C-V characteristics of the samples for various doping contents of Yb2O3 are indicate in Fig. 6. The C-V characteristic parameters, such as donor density (Nd), potential barrier height (Φb), and surface state density at the interfaces of grain boundaries (Ns), are indicated in Table 2. The Nd-value significantly increased in the range of 2.46-7.41 × 1017cm−3, as the doping content of Yb2O3 increases.

The increase of donor density due to doping content of Yb2O3 is like to be ascribed to the generation of electron according to partially substitution between Yb and Zn, on the basis of following chemical-defect reaction expression: $Yb 2 O 3 → ZnO 2Yb Zn . +2O O +1/2O 2 +2e'$. Therefore, Yb is obviously donor. The potential barrier height (Φb) formed at the grain boundaries increased from 1.01 to 1.25 eV until the doping content of Yb2O3 reaches 0.1 mol%, whereas a further doping has rather decreased Φb-value up to 0.76 eV at 0.25 mol% Yb2O3. The variation trend of Ns-value with the doping content of Yb2O3 is similar to that of Φb-value. The a, Φb, and Ns exhibited the same variation trend. As a result, E-J and C-V characteristics are electrically well correlated.

The dielectric properties of the samples with various doping contents of Yb2O3 are indicated in Fig. 7. On the whole, the dielectric constant εAPP' decreased with increasing frequency, whereas not monotonously. This behavior is directly related to the rotation of the electric dipole, which induces the interfacial polarization and the oriental polarization with frequency. The variation trend of the dielectric constant εAPP' (at 1 kHz) with the doping content of Yb2O3 is indicated in Fig. 8. The dielectric constant εAPP' (at 1 kHz) decreased from 592.7 to 501.4 until the doping content of Yb2O3 reaches 0.1 mol%, whereas a further doping has rather decreased εAPP' up to 715.9 at 0.25 mol% Yb2O3. The behavior of dielectric constant εAPP' with the doping content of Yb2O3 completely agreed to the behavior of the average ZnO grain size in microstructure, however it showed the opposite trends to the switching voltage in E-J characteristics. As a result, the dielectric properties are obviously affected by microstructure.

Meanwhile, the dissipation factor (tanδ) exhibited similar and unique curves for all samples with increasing frequency. In other words, all the samples exhibited valley-point and peak-point with increasing frequency. In general, it can frequently observe this phenomenon in varistors. The valley-point shows a minimum dielectric loss, and then εAPP'-value is constant. However, peak-point, that is, absorption peak exhibit a loss peak, and then εAPP'-value shows an abrupt decrease. The behavior of the dissipation factor tanδ (at 1 kHz) with the doping content of Yb2O3 is indicated in Fig. 8. The tanδ-value increased from 0.209 to 0.268 with the doping content of Yb2O3. The variation trend of tanδ-value with the doping content of Yb2O3 is similar to those of JL-value and Nd-value. In light of the variation trend of tanδ, JL, and Nd, it is obvious that JLvalue is affected by Nd-value, and tanδ is affected by JLvalue. Where, on the basis of the related data, the ZVMN-Yb2O3 varistors show that their characteristics are electrically and dielectrically well correlated. A few dielectric parameters with doping content of Yb2O3 are indicated in Table 2.

## 4. Conclusions

The doping effect of Yb2O3 on microstructure and electrical properties of ZnO-V2O5-MnO2-Nb2O5 (ZVMN) varistors was examined. The change of 0-0.25 mol% in the doping content of Yb2O3 has a meaningful effect on varistor properties, in particular, nonohmic coefficient, unlike microstructure. The doping effect of Yb2O3 is not large in microstructure, whereas the slight variation of microstructure has a significant effect on electrical and dielectric properties. The average ZnO grain size and switching voltage is not fully decreased or fully increased in full range (0-0.25 mol%) the doping content of Yb2O3. However, the variation trend between them was the same. The nonohmic coefficient (α) as the core parameter of varistor exhibited a maximum value (α = 70) when the doping content of Yb2O3 is 0.1 mol%. Then potential barrier height (Φb) and surface state density (Ns) also exhibited maximum value. The a, Φb, and Ns exhibited the same variation trend. As a result, it is verified that EJ and C-V characteristics are electrically well correlated. The electron density (Nd) increased with the doping content of Yb2O3. In the light of this result, Yb obviously serves as a donor. The increase of leakage current with the doping content of Yb2O3 is assumed to be ascribed to the increasing electron density. As a result, the increase of leakage current resulted in the increase of dissipation factor (tand). The variation trend of dielectric constant is correlated to average ZnO grain size. Through this study, it was logically analyzed that the microstructure, electrical and dielectric properties are mutually related. Conclusively, the Yb2O3-doped ZnO-V2O5-MnO2-Nb2O5 varistors is believed to have sufficient potential as a promising multilayered chip varistors.

## Figure

SEM images of the samples with various doping contents of Yb2O3.

Average grain size and sintered density of the samples with various doping contents of Yb2O3.

XRD patterns of the samples with various doping contents of Yb2O3: (a) 0.0 mol%, (b) 0.05 mol%, (c) 0.1 mol%, and (d) 0.25 mol%.

E-J characteristics of the samples with various doping contents of Yb2O3.

Nonlinear coefficient and leakage current density of the samples with various doping contents of Yb2O3.

C-V characteristics of the samples with various doping contents of Yb2O3.

Dielectric characteristics of the samples with various doping contents of Yb2O3.

Dielectric constant and dissipation factor of the samples with various doping contents of Yb2O3.

## Table

Microstructure and J-E characteristic parameters of the samples with various doping contents of Yb2O3.

C-V and dielectric characteristic parameters of the samples with various doping contents of Yb2O3.

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