Sensitivity of electrical behaviours of ZnO nanoparticle–Bi2O3–Mn2O3 varistor system to various La2O3 doping compositions

The effects of La2O3 on the behaviours of ZnO nanoparticle–Bi2O3–Mn2O3 varistors were determined. The discs with various La2O3 compositions (0.0–3.0 mol%) were prepared via traditional ceramic techniques and their characteristics were investigated. The results demonstrate that La2O3 remarkably affects the varistor samples, in which the size of grain diminished from 29.3 to 8.5 µm according to increasing La2O3 content from 0.0 to 3.0 mol%. The significant response within the varistor sample fabricated from ZnO nanoparticles through sintering process at 1200oC perhaps caused by the surface properties of the powdered ZnO nanoparticles. X-ray diffraction results show that the addition of La2O3 to the varistor resulted in additional peaks, which can be attributed to the construction of La-based phases through the sintering process at high temperature. The spinal phases of the (Bi48ZnO73) and (Mn3O4) diminish with the 3.0 mol% content of La2O3. The varistor samples with 2.0 mol% La2O3 show a superior spinal phase construction of (Bi48ZnO73) and (Mn3O4) and La-based phases, resulting in homogeneous grain size. La2O3 doping substantially enhanced the electrical behaviours of the disc sample with a notable increase in breakdown voltage from 2614.8 to 5730.7 V/mm caused by the increase in La2O3 doping concentration from 0.0% to 3.0 mol%. The improvement was associated with the potential lanthanide ion segregation caused by its greater ionic radius to the boundaries between ZnO grains, thus enhancing its electrical properties. Moderate La2O3 doping concentration at 1.0 mol% improved the non-linear characteristics of the sample (α = 66) and 4400 V/mm breakdown voltage was introduced, while a significant amount of La2O3 doping concentration (more than 1.0 mol%) caused its deterioration (α = 35). The increase in donor concentration Nd to 2.1 × 1020 cm−3 for the varistor doped with 1.0 mol% La2O3 is accompanied by a decrease of interfacial states density NIS to 2.2 × 1012 cm−2 eV−1, in addition to decrease in depletion layer ω to 3.9 nm. The leakage currents of the sample decreased from 0.473 to 0.310 mA/cm2 with the increase of La2O3 doping contents from 0.0% to 2.0 mol% and then increased to 0.596 mA/cm2 at 3.0 mol% La2O3 doping concentration. Therefore, La2O3 may be added to dominate the microstructural characteristics and electrical behaviours of ZnO–Bi2O3–Mn2O3 varistor manufactured using powdered ZnO nanoparticles.


Introduction
The metal oxide ZnO-based varistors are non-linear ceramic resistors which are mainly utilized to protect electric circuits and devices against overvoltages behaviours [1].
Several investigators have studied the microstructural and treatment effects via electrical conduction within varistor samples [2]. Varistor discs with nonhomogenous microstructure lead to substantial deflection in I-V behaviours owing to elevated topical and overload currents caused by singular massive ZnO grains, resulting in prompt degeneration of electrical and electronic properties of varistor sample. Consequently, the dominance of the varistor structure is necessary to attain the values required for the nonlinear exponent and breakdown voltage values.
ZnO grains with a size of 20 nm possess diverse chemical and physical properties in comparison with bulk material. Uniformity, excellent sintering capability and diverse remarkable behaviours are expected from these materials because of their huge areas of particles surface, nano-sized particles and different characteristics of surface [3,4]. Therefore, the behaviours of varistors manufactured using ZnO nanoparticles should be developed.
Various metal oxide materials such as Li 2 O 3 , Bi 2 O 3 , Mn 2 O 3 , and Al 2 O 3 are applied as dopants, but the system depends on the tripartite ZnO-Bi 2 O 3 -Mn 2 O 3 , which is generally utilized for ZnO-based ceramic fabrication process, in which Bi 2 O 3 performs the substantial function in non-linear properties of the varistor [5][6][7]. In the sintering process, Bi 2 O 3 provides a liquid phase to the varistor system and constructs a Birich phases skeleton around the ZnO grains. The metal oxide addition enhances the electrical behaviours of the fundamental binary ZnO-Bi 2 O 3 -Mn 2 O 3 varistor sample through (i) adjusting the grains conductivity, (ii) affecting grain growth through the sintering step with the spinel and pyroclore phases creation, (iii) by the reversal grains boundary formation (IB) between ZnO grains, and (iv) by impacting the intercrystalline phase properties [8][9][10].
Nahm et al. [15] found that R oxide-doped ZnO-Pr 6 O 11 -CoO-Cr 2 O 3 -La 2 O 3 (R = Er, Y, La) varistors with various doping concentrations possess superior nonlinearity with considerable stability under diverse stresses. The influence of rare earth oxides (La 2 O 3 ) on the different characteristics of Zn-Bi-Mn varistor systems derived from ZnO nanoparticle powder has not been investigated.
In our work, the effect of different amounts of lanthanum oxide on the microstructural, morphological, electrical and dielectric behaviours, sintering process and construction of secondary phases within the ZnO nanoparticle-Bi 2 O 3 -Mn 2 O 3 -based varistor was obtained. As a consequence, a superior synthesis was considered.

Sample preparation
ZnO-Bi 2 O 3 -Mn 2 O 3 based varistors ceramics were manufactured via the traditional ceramic process involving ball milling, drying, pressing and sintering. 99.9% purity oxide precursors were utilized. Discs with the next constitutions were presented: (97.0-x) mol% 20-nm ZnO + 1.0 mol% For the construction of each disc, the powder was mixed with polyvinyl alcohol (PVA) via blending with distilled water within a ball milling jar for 6 h. The ZnO slurry was dried at 60 o C in air for 1 h and thereafter was grained by using sieving through a 20-mesh sieve. The producing grains were utilized to fabricate discs via pressing at 4 ton/cm 2 pressure. The green samples were 26 mm in diameter and 2 mm thick. Eventually, the green samples were sintered through 1200 o C in air during 3 h with 2°C/ min heating rate and cooling at room temperature. To obtain the electrical characteristics, silver pastes were coated and toasted on each side for the sintered discs.

Characterization
The varistor structure was obtained by a field emission scanning electron microscopy (FESEM) system (model: JFM -6470 LV). The mean ZnO grain size (d) was calculated via the measure on the FESEM images with the linear intercept technique [16][17][18] utilizing the equation d = 1.56 L/MN, in which L is the random line length on the graph, M is the graph magnification and N is the boundaries number among the grains intercepted by lines.
The crystalline phases were considered by a significant resolution X-ray diffractometer (XRD) system (PANalytical X' Pert PRO MED PW6023) with λ = 1.5406 Å K α radiation.

Electrical testing
The I-V behaviours of the varistors were obtained utilizing a huge source of voltage with a measuring unit (KEITHLEY devices 276 voltage supply). The resistance R was estimated from Voltage (V) -Current (I) behaviours in correspondence with Ohm's Law. The breakdown voltage (V b ) was calculated at the 1.0 mA current. The leakage current (I L ) was calculated at 0.75 V b voltage and the value of nonlinearity coefficient (α) was defined within the range of current between 1.0 and 10 mA. The capacitance-voltage (C-V) behaviours of the pure samples were investigated at 1 kHz by utilized Keithley 4100-SCS recorder. The density of donor (N d ) within grains and the height of barrier ( B ) at the boundary among ZnO grains were defined from the next expression suggested by Mukae et al. [19]. (1) in which C and C o are the correspondence capacitances of the varistors at various used voltages, where C o is the C value when V = 0, and V is the used voltage; t is the varistor thickness, A is the electrode area of the varistor, d is the mean size of ZnO grain, is the ZnO permittivity and e is the electron charge. Capacitance and frequency were investigated via applying the Impedance Analyser. Hioki LCR Meters and Impedance Analysers range from 1 mHz to 3 GHz devices to suit a wide range of applications in the testing of electronic components. The IM3570 is ideal for use in applications requiring low-ESR measurement on the order of several milliohms, for example, testing of functional polymer capacitors, due to its superior low-impedance repeatability. High-speed testing achieves maximum speeds of 1.5 ms (1 kHz) and 0.5 ms (100kHz) in LCR mode. Highaccuracy measurements, basic accuracy of Z parameter are about ± 0.08%, Perfect impedance analyser for testing the resonance characteristics of piezoelectric elements, C-D and low ESR measurement of functional polymer capacitors, DCR and L-Q measurement of inductors (coils and transformers). Figure 1 displays the spectra of X-ray diffraction (XRD) for the pure varistor doped with various La 2 O 3 components. The varistors consist commonly of three major phases: namely, ZnO, Bi-rich and spinel phases and were distinguished via superior essential peaks of (101), (100), (002) and (110) that originated from the ZnO layer as were emphasized over the nature of varistor polygranular. Another peaks for α-Bi 2 O 3 , Bi 48 ZnO 73 and Mn 3 O 4 phases presented as secondary phases are based upon the kind and dopants quantity within grains and the preparing circumstances. The combination of these metal oxides creates molecular impurities within the ZnO granules and interface among the grains, with donor impurities predominant in the depletion layer and acceptor impurities prevailing in the grain boundary regions [20,21]

Field emission scanning electron microscopy
The structure of 20-nm ZnO-Bi 2 O 3 -Mn 2 O 3 varistor samples doped with various quantities of La 2 O 3 components was determined using FESEM microscopy ( Figure  2), and their congruous mean sizes of ZnO grains are presented in Table 1. These photos of FESEM exhibit that the varistors surface morphology was significantly reliant on the amounts of the La 2 O 3 additives, and the mean ZnO granules size had various doping amounts.
These FESEM images show that the average grain size of the varistors decreased from 29.3 to 8.5 μm as the La 2 O 3 doping concentration increased from 0.0 to 3.0 mol% ( Figure 2). The ZnO grains reduced to approximately 29.3, 19.6, 15.2 and 8.5 μm as the La 2 O 3 doping concentration increased to 0.0, 1.0, 1.2, and 3.0 mol%, respectively (Table 1). Figure 2 shows that the porosity is higher for the La 2 O 3 additive system, and fundamental porosity within varistors was observed with the addition of 3.0% mol of La 2 O 3 additives. However, the porosity of sample surface was improved when La 2 O 3 content was incremented because of La 2 O 3 segregation in the boundary region among the ZnO grains that prevents mass transition at the boundary between grains, thus weakening the sintering technique.
Notably, a considerable quantity of spinel phase (1-4 μm) in sample was observed with high La 2 O 3 doping amount as previously shown in XRD analysis. While in varistor with less doping contents, the Bi 2 O 3 rich phase constructed an intercrystalline layer at the interfaces among the grains. The sintering behaviour of the examined varistors can be demonstrated via the formation of phase, such as liquid phase existence. In the sample fabricated from ZnO nanoparticles, in which the sintering of the solid state has already begun during a lower temperature (650°C) because high S/V rate of ZnO nanocrystallines, the starting of sintering step is transferred to greater temperatures at the varistors doped with La 2 O 3 additives. The existence of Bi 2 O 3 leads to the formation of liquid phase through 740°C (the Bi 38 ZnO 58 phase melting point) [22]. The augmentation of La 2 O 3 to the varistor caused the spinel and pyrochlore phase formation.
The presence of secondary phases in the ZnO nanoparticle-Bi 2 O 3 -Mn 2 O 3 -based varistor ceramics remarkably affected grain growth. For the varistor without any La 2 O 3 additive (0.0 mol%), the development of ZnO grains is advanced, and the intragranular porosity is controlled with the existence of the Bi 2 O 3 -liquid phase. The addition of La 2 O 3 within the varistor resulted in significantly smaller mean size of grain with higher intragranular porosity. The sintering process at high temperature (1200°C) provided a considerable driving force into interior atomic propagation and is important for the growth of ZnO grains and pore annihilation. The addition of minimal quantity from La 2 O 3 resulted in spinel phase creation inside the boundary regions between grains, thus affecting grain growth. Therefore, decreased growth of grains within the varistors can be attributed to the effect of La 2 O 3 on the Bi 2 O 3 liquid phase wettability.
The beneficial effect of La 2 O 3 addition in controlling the grain growing arises from the impact of grain boundary pinning by La-rich mixed oxide spinel phases.   The distributions of La secondary phases at the triple junction and grain boundary confirm this observation. A considerable decrease in grain size has been obtained with further higher concentration of La 2 O 3 addition (0.0-3.0 mol.%). Several techniques have been attempted to develop a high-performance ZnO-based varistor by means of grain size reduction [23][24][25]. One of the simple approaches is the reduction of grain size during the sintering of varistor powder mixture via the formation of precipitates that will exert Zener pinning effects [26,27]. Rare earth oxides (Er 2 O 3 , Dy 2 O 3 , Tb 4 O 7 , La 2 O 3 , Y 2 O 3 , etc.) form the spinels that efficiently restrict the grain boundary migration and thereby, diminish grain growing [14,28,29].  Table 1 and illustrated in Figure 4. In comparison with the varistor sample without La 2 O 3 doping, the N d of all the samples doped with La 2 O 3 was promoted. Thus La 2 O 3 addition acts like a donor within ZnO nanoparticle-Bi 2 O 3 -Mn 2 O 3 varistor ceramics [30]. N d increase is accompanied by a decrease of interfacial states density N IS , in addition to decrease in depletion layer ω. The height of barrier ( B ) for the ceramics enhances the initial value and therefore drops with raising La 2 O 3 additive amounts. The height of barrier value B depends on the reality that ZnO grains are electroactive. Considering that the barrier height raise is supportive of the increase of nonlinear coefficient for varistor ceramic [31], it can be predictable that the varistor doped with 1.0 mol% La 2 O 3 will have a relative higher nonlinearity coefficient value, as shown in Table  1. For the varistors doped with more than 1.0 mol% La 2 O 3 concentration, the increment of the capacitance value associated with bias voltage maybe related to the degeneration of the nonohmic behaviours involving the Schottky barrier deformation among ZnO grains. A modification in the concentration of donor could immediately affect the electrical conductivity and capacity for the discs. The diminution in the width of potential barrier and augmentation in the height of barrier are related to the rise of negative particles in the boundaries among the ZnO granulate (N IS ) and donor concentration (N d ) attributed to the La 2 O 3 separation near the interface between the grains along with the induction of positive defects states within the depletion layer and negative defects states interfacial. Therefore, the electronic particles within the grain boundary state modify according to the La 2 O 3 additive concentration.

C-V characteristics
The formation of a narrow and high potential barrier within the ZnO grain interface region is the last situation desired to achieve a varistor sample with a significant value of non-linearity coefficient. The attained results present that doping the ZnO nanoparticle-Bi 2 O 3 -Mn 2 O 3 varistor system with La 2 O 3 at appropriate level creates an advanced barrier within the interface among ZnO grains.

V-I behaviour
The used electric voltage with respect to the electric current for the pure 20 nm ZnO-Bi 2 O 3 -Mn 2 O 3 varistor sample and doped with various La 2 O 3 concentrations is shown in Figure 5(a). The V-I curves in Figure 5 indicate a diode response with an elevation-resistivity area at the left region of V b and a lower-resistivity area at right region of V b . The minimal resistivity area indicates the non-linearity area, while the nonlinear coefficient value (α) can be investigated using the following equation [32]: V 1 and I 1 as well as V 2 and I 2 are the identical value of current and voltage for two points that would be selected capriciously [33]. The description of correlated electrical parameters obtained from the I-V is calculated from Table 1 for various La 2 O 3 concentrations. Table 1 indicates that all examined varistors display non-linearity voltage-current behaviours. The values of α were achieved based on the I-V curves at the current range of 1-10 mA. The maximum value of non-linearity coefficient (α = 66) was recorded when 1.0 mol% La 2 O 3 molar concentration was used to dope the varistor sample, and 4400 V/mm electric breakdown voltage was introduced. The addition of alternating amounts of La 2 O 3 from 1.0% to 3.0 mol% caused a considerable adjustment for the electrical characteristic of the ZnO nanoparticles-Bi 2 O 3 -Mn 2 O 3 varistor ceramics. The electric behaviour of the varistor with 0.0 mol% La 2 O 3 , albeit non-linearity, is extremely resistive. The varistors involving 1.0 mol% La 2 O 3 are further resistive within the boundary of ZnO grains (breakdown voltage near 4,400 V/mm) and has a non-linearity coefficient comparable to 66, as revealed previously. Based on the comparison of results in Table 1, La 2 O 3 addition reduces the size of grains, thus increasing the breakdown voltage and nonlinearity coefficient. Corresponding results were reported via Bueno et al. [34], who explained the DC electrical characteristic of the SCN and SCNCr systems with various molar concentrations of Cr 2 O 3 . SCN displays a sample behaviour with 58 α and 1870 V/mm breakdown electric field (E b ). At 0.05% Cr 2 O 3 , the value of α enhanced to 41, while V b changed to 3990 V/mm [35]. Based on these results, the addition of La 2 O 3 improved the nonlinear properties remarkably. The enhancement for non-linear behaviour and superior I-V arise point within varistors consisting of a limited La 2 O 3 dopant concentration, indicating that the addition of La 2 O 3 increases the grain resistivity. This finding was obtained, because the I-V characteristic of the varistor sample in the arise area follows ohmic I-V properties, and the parameters of I-V are only based upon the grain resistance. When the La 2 O 3 doping amounts exceeded 1.0 mol%, the sample breakdown voltage increased. The V b value of varistor is comparable to the size of ZnO granule and the utilized electrical voltage per boundaries among the grains [36,37], as demonstrated in Figure 5(b). The increased value of breakdown voltage may be demonstrated via the reduced mean ZnO granule size with the increase in doping concentration, resulting in an increase in the grain boundary numbers among the electrodes, causing an increment in "p-n junctions". Superior "p-n junctions" was caused by raised V b of the disc. The non-linearity conducts of the samples are of the boundaries phenomenon among semiconducting ZnO granules. The V b of the samples is related to the number of grain boundaries per thickness unit and the inverse of the ZnO granule size.
The leakage currents of the ZnO nanoparticle-Bi 2 O 3 -Mn 2 O 3 -based varistor decreased with the increase of La 2 O 3 doping contents. However, as the concentration of La 2 O 3 added becomes more augmented, the value of leakage currents in the varistors increased significantly. Figure 6 suggests that the reduction of the varistor leakage currents was caused by the enhancement in their non-linearity coefficients; otherwise, when a significant La 2 O 3 concentration was added, sudden leakage currents increased possibly because of the sharp reduction in the density of samples sintered, where the considerably produced pores play as the hot spots for the currents flow within the sample [38]. The crucial frequency increased, but the crucial capacitance decreased as the additive concentrations of La 2 O 3 were augmented. This phenomenon was possibly caused by two things. First, the trapping within the depletion area near the boundaries among ZnO grains is predicted to be less on elevated frequencies to permit the charge carriers' dynamic commutation within the boundary between the grains, and the secondary phases reduplication within the added sample, which is attributed to the various dopants within the varistor specimen. The structural properties of the sample added with diverse La 2 O 3 amount, such as particles size and orientation of growth, additionally  the adding impact upon the various behaviours, were deemed to clarify the capacitance-frequency property of the sample doped with La 2 O 3 . The crystallization size remarkably affected the varistor's electric characteristic, as indicated in prior research [39,40]. The increment in the concentration of La 2 O 3 added considerably influenced the amount of carriers. The grain boundary density diminished, and their impact upon the charge carrier was decreased and incremented the frequency as the grain size developed with increasing concentration. By contrast, the dissipation of the interfacial state among the grains seemed predominant and subsequently reduced the frequency as the crystallite size decreased. The frequency development did not obey a term precisely proportional to the size of grains, essentially at the major amounts of La 2 O 3 . Figure 7(b) displays the alteration in capacitance at various amounts of La 2 O 3 . The results demonstrate that the capacitance considered the broader relaxation peak and is supposed to refer an elevated traps amounts, which intricate dynamics perhaps associated with collaborative reactions within confused media. Jonscher proposed that a broader relaxation peak may be attributed to several particle reactions between separated traps [41,42]. Moreover, the XRD examination indicates that the doped varistor samples involved various phases, where the initial phase consisting of ZnO is the significant composition that remains within the crystallite granules and the crucial capacitance represented among the ZnO granules and interfacial states between the crystalline grains. This capacitance is perhaps reduced via presenting the second phases that are acquired from the addendum of diverse dopants to the varistor. Consequently, the capacitance diminished at the trap state density raised through the large La 2 O 3 amount.

Conclusion
ZnO-Bi 2 O 3 -Mn 2 O 3 varistors manufactured from 20 nm ZnO powder and doped with different amounts of La 2 O 3 were fabricated via the conventional ceramic processing method. The effect of various La 2 O 3 concentrations (0.0-3.0 mol%) on the sintered density, structural improvement, and nonlinear behaviour of the 20 nm ZnO-Bi 2 O 3 -Mn 2 O 3 varistors was discussed. Different La 2 O 3 concentrations exerted considerable influence on the varistors. The addition of large amounts of La 2 O 3 led to the inhibition of grain growth from 29.3 to 8.5 μm by increasing the amount of the La 2 O 3rich spinel phase. X-ray diffraction analysis showed that the addition of La 2 O 3 to the varistor systems led to the development of La 2 O 3 -rich phases during sintering. The varistor with 2.0 mol% La 2 O 3 shows a superior phase construction, resulting in homogeneous grain size. During sintering, the considerable surface area of the nanoparticle powder resulted in numerous interactions in the surfaces of the ceramics manufactured from 20 nm ZnO powder. The electrical behaviours of the varistors were substantially influenced by La 2 O 3 . V b increased from 2614.8 to 5730.7 V/mm with the increment in doping amount from 0.0% to 3.0 mol%. The superior nonlinearity value is approximately 66 for the varistor with 1.0 mol% La 2 O 3 content, while a more La 2 O 3 doping amount (more than 1.0 mol%) caused its deterioration (α = 35). For the varistor doped with 1.0 mol% La 2 O 3 , the donor concentration N d increased to 2.1 × 10 20 cm −3 , whereas the interfacial states density N IS and depletion layer ω decreased to 2.2 × 10 12 cm −2 eV −1 and 3.9 nm, respectively. The leakage currents I L of the varistors decreased from 0.473 to 0.310 mA/cm 2 with the increase of La 2 O 3 from 0.0% to 2.0 mol%. When La 2 O 3 increased up to 3.0 mol.%, I L was further increased to 0.596 mA/cm 2 . Therefore, the amalgamation of La 2 O 3 may enhance the electric behaviours of ceramics with amended microstructural properties.

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