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ZnO nanopowders doped with bismuth oxide, from synthesis to electrical application
Accepted Manuscript ZnO nanopowders doped with bismuth oxide, from synthesis to electrical application A. Boumezoued, K. Guergouri, R. Barille, D. Rechem, M. Zaabat, M. Rasheed PII:
S0925-8388(19)31077-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.03.251
Reference:
JALCOM 50015
To appear in:
Journal of Alloys and Compounds
Received Date: 2 February 2019 Revised Date:
11 March 2019
Accepted Date: 17 March 2019
Please cite this article as: A. Boumezoued, K. Guergouri, R. Barille, D. Rechem, M. Zaabat, M. Rasheed, ZnO nanopowders doped with bismuth oxide, from synthesis to electrical application, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.03.251. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
ZnO nanopowders doped with Bismuth Oxide, from synthesis to electrical application A. Boumezoued1, K. Guergouri1,*, R. Barille2, D. Rechem3, M. Zaabat1 and M. Rasheed2 1
Laboratory of Active Components and Materials, University Larbi Ben M'Hidi of Oum El 2
MOLTECH-Anjou University of Angers/CNRS UMR 62002, Bd Lavoisier, 49045 ANGERS Cedex 01, France
3
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Bouaghi 04000, Algeria
Laboratory of Materials and Structure of Electromechanic systems and their fiability,
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University Larbi Ben M'Hidi of Oum El Bouaghi 04000, Algeria
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*Corresponding Author: (Phone: + 213 6 61 30 91 96) Email: [email protected]
Abstract:
Bismuth doped ZnO nanopowders with doping concentrations between 0 and 7 %mol were successfully synthesized with a soft chemistry method: the sol-gel route. This method allows obtaining powders with a good quality in a nanometric size range. The well-defined optimal conditions for the synthesized powders are given. The hydrated zinc acetate dissolved in
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ethylene glycol is used as a precursor for the zinc oxide (ZnO), with different small amount of Bi concentrations (1%, 3%, 5% and 7%). After calcination at 500°C, the powders are consolidated into pellets and sintered using a conventional furnace and then characterized by: X-ray diffraction, SEM, TEM, and J(E) electrical measurements. Finally, the obtained
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material has been used to prepare varistors.
The XRD patterns indicate that pure and Bi doped ZnO are solid solutions with average grain
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sizes varying between 65 nm to 48 nm. The β-Bi2O3 liquid phase of tetragonal structures appears as a secondary phase for a 5% mol concentration of Bi. High-resolution TEM images of Bi doped ZnO particles indicate that during the sintering of Bi-doped ZnO, the growth of particles leads to a change in the microstructure of ZnO giving semi-transparent cylindrical nanotubes.
J(E) results indicate that the content of Bi increases the nonlinearity coefficient α and the breakdown fields VB of the binary Zn %Bi-O varistors. The 7% doped Bi-ZnO varistor gives the best electrical characteristics for varistors.
ACCEPTED MANUSCRIPT Keywords: ZnO nanoparticles; Bi doping; Grain boundaries; Varistor; Electrical characteristics. 1. Introduction For several decades, the consumption of electric power has been growing constantly, resulting in the extension of the transmission network and the development of very high voltage lines.
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This has led to the fabrication of surge protection devices to ensure a good quality of service and protection of electrical networks. ZnO-based varistors are recommended to play this role. Zinc oxide (ZnO) varistors are polycrystalline ceramics made with pellets of metal oxides, composed essentially of zinc oxide doped with different oxides such as Bi2O3, Pr6O11, SbO3,
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CoO or MnO [1]. One of their most important required properties is the high non-linearity of the current-voltage characteristic [2, 3].
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In that goal, ZnO nanopowders are synthesized by various techniques, the most widely used currently is the sol-gel route, because it produces manufactured varistors with a low cost and a good quality. This is undoubtedly related to the efficient control of the particle size and the homogeneity of their distribution, which are a direct consequence of this method [4, 5]. The electrical characteristics of ZnO varistors are related to the bulk of materials.
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The origin of the varistor effect is firstly essentially due to the microstructure, and secondly to the potential barriers of grain boundaries, the grain conduction and the boundary resistivity [6, 7]. The varistor effect can be absent between two grains. More the density of these grain boundaries increases more the varistor effect is important. It has been demonstrated that the
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decrease in grains and particle sizes in nanometric scale, results in an increase of the breakdown voltage and improves the electrical characteristics of the varistors (coefficient α
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and breakdown voltage) [8]. So, the improved homogeneity of the starting powder mixture and the well-defined optimum conditions are very important for producing high performance ceramic varistors [9]. The varistor electrical properties are closely correlated with the homogeneity of grain size and the homogenous distribution of the additives. Currently, the majority of commercial ZnO varistors are based on Bi doped ZnO and they exhibit excellent ohmic behaviors. However, there is a neccessity to improve these varistors for more powerful characteristics (a higher breakdown voltage and non-linearity coefficient). The aim of our study is to investigate the influence of the doping bismuth on the structural, morphological and electrical behaviors of ZnO varistors and more particularly to determine the effect of different new phases, specifically the Bi-rich phase [10].
ACCEPTED MANUSCRIPT 2. Experimental details 2. 1 Synthesis of powders Pure and Bi-doped ZnO nano-powders have been synthesized with the sol-gel technique. Zinc acetate dehydrate ( C H O Zn.2 H O) (purity>99%, biochem-Chempharma) is used as a
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starting material. An alkoxide: citric acid (C H O . H O) monohydrate is used to keep particles in suspension, mono-ethanolamine MEA ( C H NO) and ethylene glycol are used as a stabilizer and solvent, respectively. The dopant source used in our study is bismuth chloride (BiCl3). Two solutions are put in a bath of paraffin oil heated at a well fixed temperature (130°C). The first solution was prepared by dissolving zinc acetate dehydrate CAZ in ethylene
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glycol, the second one was prepared by dissolving citric acid in ethylene glycol CAC. Then, during stirring, bismuth chloride, with appropriate amount, is added to the first solution to
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obtain the desired concentrations (1 to 7mol% Bi). After a complete dissolution of the precursors and the stabilization of the oil bath temperature the two solutions are mixed. The solution (1) is gradually added to the solution (2) with a molar ratio (CAZ/CAC = 0.06). Then, the obtained solution is stirred at 130°C during 2h to obtain a homogeneous and transparent solution. Finally, after 24h the solution was calcined at 500°C during4h.
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2. 2 Sintering and manufacturing of varistors based on Zn% Bi-O nanopowders To obtain dense varistors with high performances, a conventional sintering study was performed. In this goal, the obtained powders were pressed between discs with a diameter of 11 mm and a thickness of 1.8 mm at a pressure of 2 MPa. The obtained pellets were sintered
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at 1750°C during15 min at a heating rate of 25 °C/min. In order to realize J(E) measurements, gold was evaporated on both faces of the pellets to have ohmic contacts, followed by a
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heating at 500◦ during 10 min in the goal to remove all organic functions. The obtained powders were characterized by several techniques: XRD, using the Cukα (λ = 0.154056 nm) radiation of a BRRUKER AXS, a D8 advance X-ray diffractometer in order to identify the structure and to calculate the grain size. A TEM (Transmission Electron Microscope - X-Max model) was used to identify the morphology and to estimate particle sizes. Finally J(E) electrical characteristics were measured to calculate the breakdown electric field and the nonlinear coefficient α using a high voltage measure unit (KEITHLEY 2401 source meter ).
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Results and discussion
The figure 1 shows the x-ray diffraction pattern of pure and Bi (1%mol to 7% mol) doped ZnO varistors, sintered at 500°C. The spectra exhibit peaks of the würtzite structure for all samples. The intensity of the peaks decreases with the increase of the Bi concentration, whereas the full width at half-maximum (FWHM) of these peaks increases. The peak around
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19.2° corresponding to Bi appears only for concentrations with 7 %mol of Bi, identified as a phase linked with the metallic bismuth. It is mentioned in the literature [11] that bismuth oxoacetate forms bismuth oxide at 340°C. This is the reason why a slightly higher annealing temperature was chosen. Moreover, the X-ray analysis indicates that there is no spinel phase.
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This may be explained by the fact that the calcination temperature (500°C) of the synthesized nano-powders is not enough to allow the complete reaction between Bi2O3 and ZnO and to
(101)
Z nO β -B i 2 O 3
(100)
∇ B i-rich ph ase
(110) (103)
(102)
Z n O :B i 7 %
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∇
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form the spinel phase.
30
40
50
Z n O :B i 3 % Z nO :B i 1%
Z n O :B i 0 %
60
70
80
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20
Z n O :B i 5 %
90
1 00
1 10
1 20
13 0
14 0
15 0
1 60
2 θ (°)
Fig.1 XRD patterns of pure and Bi doped ZnO starting powders
The average grain sizes were calculated using X-ray line broadening from Scherrer’s formula [12]:
=
. ∆
ACCEPTED MANUSCRIPT where D is the grain size, λ the X-ray wavelength (λ=1.5418A°), ∆θ the full width at halfmaximum (FWHM) and θ the Bragg angle. Table 1: XRD data of pure and Bi doped ZnO. θ (°)
∆θ (rad)
D (nm)
ε (GPa)
0% mol Bi
18.18
0.0022
65
+0.00057
1% mol Bi
18.18
0.0026
60
3% mol Bi
18.17
0.0031
50
5% mol Bi
18.18
0.0032
48
7% mol Bi
18.19
0.0033
47
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Zn% BiO
+0.00054
+0.00054 +0.00052
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+0.00051
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The average values of the crystalline size and strain of the samples were calculated using the (110) peak. The results are given in the table 1. The crystalline size decreases with the increasing of Bi concentration; the values vary from 65 to 47 nm (Fig.2) and follow an exponential variation as it can be observed in the loglog plot of the grain size as a function of the dopant concentration. The diffraction peaks of the ZnO matrix are slightly shifted towards
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the small angles after the introduction of Bi atoms indicating that Bi3+ ions go to Zn2+ sites. The ionic radius of Zn2 + is 0.6 Å while that of Bi3 + is larger and is equal to 1.1 Å. We confirm the presence of a solid solution. On the other hand, the calculated strain values decrease as a function of the Bi concentration. Indeed, a small shift of the most intense peaks, of about 0.03°
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for the peak (100) and 0.02° for the peak (101) is recorded, indicating that each crystallite is subjected to a slight dilation of the deformation (Fig.3).
D [nm]
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100
10
1 1
% Bi
8
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4000
6000
(100)
Pure ZnO ZnO 1% Bi ZnO 3% Bi ZnO 5% Bi ZnO 7% Bi
2000
1000
31,5
32,2
32,9
4000
PureZnO ZnO 1% Bi ZnO 3% Bi ZnO 5% Bi ZnO 7% Bi
2000
36,0
2θ (°)
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Intensity(u,a)
Intensity(u,a)
(101) 3000
36,6
2θ (°)
Fig.3 Variation of the width and the position of the most intense peaks
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as a function of Bi concentration.
Furthermore, the spectra (Fig.4) exhibit new peaks for 2θ ranging between 20° - 67° when the
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concentration of bismuth increases from (5% to 7 %mol). The peaks indicate the formation of liquid phase β according to the JCPDS 27-0050. We notice that the β − Bi O tetragonal phase, which is a metastable phase, was obtained during cooling from phase δ. This structure can be observed in a temperature range of 330 ° C to 650°C [13].
The formation of the β -Bi2O3 phase is due to the cooling mode out of equilibrium, where
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Bi2O3 is formed from a liquid phase localized at the grain boundaries. As it is observed in the figure 2 the grain size decreases with increasing the Bi concentration and reaches its lowest value at nearly 47 nm for 7% Bi. This can be explained by the presence
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of a liquid and an inter-granular phase at the higher sintering temperature (1075°C) causing a rapid growth of the grains. An increase of the concentration of Bi above the value of 7%
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would not change the grain size.
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Z nO :B i 0 %
Zn O :B i 5 % Z nO :B i 7 % β -B i 2 O 3
45,29° 43,39°
55,32
29,55°
30
40
2 θ (°)
50
60
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20
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24,18° 29,66°
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β -B i 2 O 3
70
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Fig. 4 X-ray spectra of secondary phase of Zn5%Bi-O and Zn7%Bi-O nanopowders
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b
a
10 µm
10 µm
c
d
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Bi
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R
Liquid phase β
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Z
10 µm
10 µm
e
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a
S
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G
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10 µm
Fig.5 SEM images of pure and Bi doped ZnO varistors sintered at 1075°C : (a): 0%Bi, (b): 1% Bi, (c): 3% Bi, (d): 5% Bi and (e) 7% Bi. Z: ZnO grain, P: Pore, S: Spinel phase, G: inter-granular phase, R: Bi-rich phase
In order to understand the effect of the different phases on the behavior of bismuth-doped ZnO varistors sintered at (1075 °C) it is important to understand the sintering mechanism and the role of the dopant introduced into the pores, because the properties of a varistor depend on several factors as the sintering temperature and the Schottky barrier which is the main cause
ACCEPTED MANUSCRIPT of the varistor effect in addition to the particle geometry [14]. Considering the sintering mechanism which is an essential step for the preparation of pellets, three main stages are associated with the manufacture of ZnO-based varistors [15]. First, after the dissolution of the additive particles in the solid solution during the sintering process, a liquid phase may occur in the grain boundaries. This liquid phase wets the grain boundaries for the formation of the
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secondary phases such as a spinel phase during the consolidation treatment. Then, a grain growth occurs, which is closely related to the uniform additive distribution and optimized sintering processes. Finally, the grain growth and pores are gradually eliminated. The cooling process generates electrical potential barriers at grain boundaries and the crystallization of the
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second phases from the bismuth-rich liquid phase as well [6].
The figure 5 shows the SEM images of Bi doped ZnO varistors for different Bi concentrations
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sintered at 1075°C for 15 min. First of all, the SEM image of pure ZnO presented in the figure 5a shows that grains are uniformly distributed and keep the same shape. The grain boundaries are not yet well defined and the pores form a network between the grains. In this case the high density of pores confirms the poor densification. However, for doped samples (Fig. 5: b, c, d, e) the grains have grown considerably and are well joined on one hand, and on another hand the grain boundaries are well formed between them. Furthermore, the porosity decreases
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when the Bi content increases. This phenomenon is due to the sintering temperature and the addition of bismuth, which makes these doped ZnO varistors denser than pure ZnO varistors [16, 17]. Moreover, it is well known that the Bi-rich phase is responsible for an enhanced densification in the initial stage of sintering [18, 19, 20]. This phase is clearly observed in the
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figure 5b for the Zn3%Bi-O binary varistor.
SEM images allow us also to determine the effect of Bi doping sintered at 1075°C on the
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microstructures based varistors. For each Bi concentration the formation of a new phase is clearly observed. The microstructure exhibits the appearance of new secondary phases identified as: bismuth-rich phase, liquid trace of β-Bi2O3, inter-granular and spinel phases for 1% mol, 3% mol, 5% mol and 7% mol ZnO varistors respectively [21, 22, 23], as it was confirmed by XRD (Fig. 1). Practically no pores are observed in ZnO grains, and most of Bi based particles diffuse into ZnO grain junctions in a uniform and homogeneous way. The microstructure can also contain one or more types of spinel phase. It is also clear from figure 5e that Bi based particles are much more in the spinel phase than in the bi-rich phases, while there is only very little Bi in
ACCEPTED MANUSCRIPT the grains of ZnO. It should be noted that the spinel phase formation temperature is 900 ° C [24]. The average particle size (D) of each phase is determined by the lineal intercept method with SEM images and given by the following equation:
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D =1, 56 L/M,N Where L is the random line length on the micrograph, M the magnification of the micrograph and N the number of grain boundaries intercepted by the lines [25].
From the above results, it possible to quantify the number, distribution and morphology of
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each phase in the microstructure of Zn%molBi-O varistors captured at various locations of SEM images. The figure 6 shows the phase distribution of ZnBi%mol-O varistors doped with
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different Bi concentrations, We noted the following phase distribution: 62% for the pure varistor, 22% for 1% Bi 33% for 3% Bi, 37% for 5% Bi and 45% for 7% Bi, as it is listed in the table 2.
It is reported that the presence of a larger value of phase distribution of ZnO grains improves the homogeneity of the varistor since a phase rich in bismuth prevents the formation of grain boundary between ZnO grains. This is obviously explained by the difference in grain size and
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the applied electric field of the varistor material. The Bi-rich phases are located throughout the structure and especially growth in liquid phase sintering. In addition, intergranular and spinel phase particles are displayed in the ZnO grains and also separate in multiples.
phases
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Sample
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Table 2. Morphological properties of pure and doped ZnO varistors Average particle
Location
size (nm)
Distribution %
0%
ZnO
159
Substitutional
62%
1%
Bi - rich
171
Liquid phase
22%
3%
Metallic Bi
153
ZnO grain
33%
5%
Liquid phase β
-
Triple point
37%
7%
Spinel and inter-
-
Grain
45%
granular phase
boundaries
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0 % mol Bi 1 % mol Bi 3 % mol Bi 5 % mol Bi 7 % mol Bi
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50
40
30
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Distribution of number %
60
20
10
0
Bi-rich Metallic bismuth Liquid phase Spinel and inter-
ZnO
granular phase
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Fig. 6 Phases distribution of ZnBi%mol-O varistors doped with different Bi concentrations
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0%
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(1%mol to 7%mol) and sintered at 1075°C
Average particle size (nm)
200
1%
3%
5%
7%
D SEM D XRD
150
100
50
0 0
1
2
3
4
5
6
7
% Bi Doping
Fig.7 particle size histograms as a function of Bi concentration
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The average particle size (D) of sintered samples obtained from SEM micrographs and TEM images were determined with an image analysis software. More than 300 particles were taken into account for the determination of the average particle size. The results presented in the histogram of the figure 7 indicate that the mean crystallite sizes
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measured from SEM and TEM images are varying in the same direction than those obtained by XRD; they range between 115 to 159 nm. The table 3 summarizes the values obtained from XRD, SEM and TEM. Particle size values are almost twice the size of the grains. The only factor that has a significant effect on the particle size in this observation is the sintering
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temperature because at higher temperatures, the crystal network disappears rapidly. The particle size can then increase easily since the network no longer inhibits the particle growth
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during the sintering cycle. This result is in good agreement with previous researches [26]. The Figure 8 shows TEM observations of the influence of Bi concentrations of ZnO nanopowders. Two different structures of pure and Bi doped ZnO varistors are obtained during this synthesis. For Zn 0%Bi-O, the particles exhibit hexagonal form with an uniform distribution (Fig. 8a), whereas, for all doped samples aligned cylindrical rod shaped particles can be clearly seen in the micrographs (Fig. 8b, c and d). During cooling, a magnification of
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the ZnO particles doped with Bi changes the hexagonal microstructure into transparent cylindrical rod morphologies. The same observation has been reported by S. Anas et al. [27]. The particle size (length and diameter) of pure and Bi doped ZnO varistors obtained from TEM micrographs are summarized in table 4. The sizes varied in the range of 84 to 37 nm in
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lengths and in the range of 118 - 83 nm for the diameters. Consequently, we focused our work on the fabrication of an homogeneous distribution of Zn%mol Bi-O varistors at the nanometer
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scale with the synthesized powders. Thus, the obtained particles after sintering were similar in shape, but these two structures differed in terms of agglomeration.
ACCEPTED MANUSCRIPT Table 3 Grain and particle sizes Zn% BiO
D (nm): XRD
D (nm): SEM
65
159
1% mol Bi
60
149
3% mol Bi
50
120
5% mol Bi
48
118
7% mol Bi
47
115
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0% mol Bi
Table 4 Geometrical dimensions and characteristics of material structures for different
Diameter (nm)
0 mol% Bi
85
1 mol% Bi
169
3 mol% Bi
154
5 mol% Bi
93
7 mol% Bi
37
Lengths (nm)
Structure
118
Hexagonal
118
Cylindrical rod
143
Cylindrical rod
97
Cylindrical rod
83
Cylindrical rod+hexagonal
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Zn%Bi-O
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Zn%Bi-O varistors observed with TEM
However for the sample of 7% Bi-doped ZnO (Fig. 8e), we notice two different particles
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shape; hexagonal and cylindrical tube corresponding to zinc and bismuth particles respectively, which means that Bi+3occupy Zn+2 ions. This result indicates that Bi atoms are located in solid solution, which significates that Bi+3 ions are incorporated into the network
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due to the replacement of Zn+2 ions in substitutional sites. We can also say that the presence of zinc oxide nanoparticles in the reaction medium considerably modifies the morphology of the reaction’s product between the bismuth and the zinc. If we consider the SEM observations one can suggest that the smaller particles are those rich in zinc and the larger ones are rich in bismuth, and the segregation between them is clearly indicated.
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b
100 nm
d
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c
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100 nm
100 nm
100 nm
100 nm
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e
Fig.8 TEM images of pure and Bi doped ZnO varistors prepared from nanopowders
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(a): pure ZnO, (b): 1% Bi, (c): 3% Bi, (d): 5% Bi and (d): 7% Bi.
The electric field variation as a function of the current density for different concentrations is
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given in the figure 9. The curves allow calculating the two main coefficients: the nonlinear coefficient α and the barrier voltage or breakdown voltage VB determining the varistor effect.
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The nonlinear exponent (α) is calculated from the empirical relation: J = KEα, where K is a constant, J and E the current density and electric field, respectively. The nonlinear coefficient α is obtained by the formula: α=∆ ∆J/∆ ∆E, where E and J are the values of breakdown voltage and current density respectively in the non-ohmic region [28]. The breakdown voltage VB is given by the point that sets the transition between the linear and non-linear regime. The results of the figure 9 indicate clearly two regions: the ohmic region, known as a high resistance region and the non-ohmic one, known as a very low resistance region, where E is the electric field (V/cm) and J is the current density (A/cm2). The different values of VB were deduced from curves of J(E) (table 5). They allowed to calculate the breakdown voltage per
ACCEPTED MANUSCRIPT grain boundary (Vgb), given by the equation VB= Vgb/D [29] and listed in Tables 4 and 5 with D the average of grain sizes. An average value of 21 mV ± 0.3 mV is obtained.
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1E-5
1E-6
1E-7
1E-8
0%mol Bi
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Current density J (A/cm2)
1E-4
1%mol Bi 3%mol Bi
1E-9
5%mol Bi
10
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7%mol Bi
1E-10
100
1000
Electric field E (V/cm)
Fig. 9 J (E) Varistor characteristic of the current intensity J as a function of the electric field
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for different Bi concentrations.
The electrical parameter values were characterized (fig. 9). The curves J (E) is plotted in a logarithmic scale in the Figure 9 in order to determine the coefficient of the nonlinearity α.
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We see with the curves of the figure 9 that the current density J in the non-linear region increases strongly when the concentration of Bi is increased. Thus J is depending on the
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particle size and the average number of grain boundaries. Considering the breakdown voltage VB, the measured values indicate that they vary within a wide range in a range varying between 1366.63 V/cm and 1901.50 V/cm (Tab.5). We notice that VB increases by increasing Bi concentration, the maximum value is obtained for 7 mol% Bi sintered at 1075°C, this is obviously due to the increasing in the number of grains and particle sizes of the powder. We know that small grains lead to a decrease of the electric field in grain boundaries [30]. It is clear that the values of VB increase with increasing Bi content, as an example, we have measured the value of 1435 V/cm for Zn-1%Bi-O and 1901 V/mm for Zn-7%Bi-O at the same sintering temperature (1075 °C). Varistor electrical properties are closely correlated
ACCEPTED MANUSCRIPT with the homogeneity of grain size and of barrier distribution and characteristics in the bulk, i.e. the homogeneous distribution of the additives. The Figure 10 shows that the breakdown voltage VB decreases when the grain size increases. The most important result is that we can link the breakdown voltage to the grain size. The reduction in the size of the powder particles leads to the decrease in the grain size of the Zn-
C
Rb
A
Rb
5 % m ol Bi
Rb
1800
7 % m ol Bi
60
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Grain boundary
VB
1900
B
ZnO grain
1700
1600
1500
1400 Grain size (nm) Breakdown voltage V/cm
40
-1
0
1
2
3
Breakdown voltage (V/cm)
Rb
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Rb
Grain size (nm)
Rb
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%Bi-O varistors.
1300 4
5
6
7
8
% Bi doping
Vgb
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Fig.10 a) Electrical current distributions in the material for different topologies of the grain network and distribution of crystalline defects for two different Bi dopants, b) Equivalent electrical circuit representation of a grain boundary according to the equation J = KEα
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c) Variation of the grain size D and breakdown voltage VB as a function of Bi doping.
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Table. 5 Varistor effect parameters of pure and Bi doped ZnO samples synthesized by a sol-gel route.
Sample ZnxBi-O
Breakdown voltage VB (V/cm)
Non-linearity coefficient α
0 mol% Bi
1367
5.56
breakdown voltage per grain boundary Vgb (V) 0.0217
1 mol% Bi
1435
15.65
0.0214
3 mol% Bi
1641
17.32
0.0196
5 mol% Bi
1773
23.25
0.0210
7 mol% Bi
1901
29.18
0.0218
ACCEPTED MANUSCRIPT This coefficient α increases with the increase of the Bi concentration up to 29.18 for 7 mol% Bi as indicated in table 5. So, we see that α increases with the increase of the number of grains and particle sizes. The results demonstrate also that the sample doped with 7 mol% Bi has the best electrical behavior (VB=1901.50 V/cm and α=29.18). If we add to this result the fact that Zn7%Bi-O owns the lower values of grain number and particle sizes we can conclude that
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this varistor has the best characteristics and owns a very important varistor effect. However, Savary et al. [31] report the formation of a nano-sized ceramics by a liquid route made of zinc oxide powders that exhibits nonlinear electrical characteristics. We did not observe the same behavior with pure and bismuth doped zinc oxide grains within a short time: in our case, the
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bismuth introduction was found to be necessary in order to obtain a high breakdown voltage and nonlinearity behavior. In the figure 10a we consider a side grain as one node of the primary mesh corresponding to one cell of a polyhedral mesh. These nodes could be
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considered as the generating germs of the resulting grain structure. The grain boundaries are represented by the generally non-planar interfaces between the neighboring polyhedral cells. The figure 10b give an equivalent circuit with an electrical model for the different topologies of grain structures corresponding to two different dopants. The 7 mol% dopant gives the best value in term of breakdown voltage VB and nonlinearity. We see that for this dopant the
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grains are optimum in sizes and connectivities are increased. So, increasing the number of active boundaries causes the switching breakdown voltage to shift towards higher values while the effective dynamic conductance of the material is increased. The transition between the linear and nonlinear part is due to the maximum current flow through the connected
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network [32].
From SEM images, many phases were observed in the microstructure of the binary varistors Zn%Bi-O obtained from nanopowders (Fig. 5). The rapid growth of the particles of these
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phases at grain boundaries during the sintering and the cooling of samples has a desirable influence on the characteristics of the Schottky double barrier [33].
4.
Conclusion
Nanopowders of pure and Bi doped ZnO were synthesized successfully to manufacture varistor devices by a sol-gel route. XRD spectra confirm the würtzite structure and the presence of new phases for each Bi concentration for all powders and varistors with grain sizes varying from 47.70 - 65.76 nm as a function of the Bi concentration. SEM images confirm the existence of phases observed with the XRD diffraction, with particle size values
ACCEPTED MANUSCRIPT almost twice the size of the grains but varying in the same way. On the other hand, TEM images showed that the presence of Bi atoms contributes to change the hexagonal structure of ZnO into cylindrical transparent rods with different physical dimensions. The electric characteristics J (E) of these varistors indicate a good non-ohmic behavior for all samples. The breakdown electric field VB ranges between 1367 to 1901 V/cm and the
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coefficient of non-linearity between 5.56 to 29.18 whereas, many secondary phases are formed at grain boundaries just after adding bismuth due to the decrease of the grain size average. J(E) results indicate that the content of Bi increases the nonlinearity coefficient α and the breakdown fields VB of the binary Zn %Bi-O varistors. The 7% doped Bi-ZnO varistor
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gives the best electrical characteristics for varistors. A higher value of dopant doesn’t affect the grain geometrical characteristics. The results are twice the obtained values for Co-doped ZnO varistors.
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As it has been successfully demonstrated in our work, microstructural evolution phases of the sintered samples can efficiently modify the electrical parameters on one hand and on the other hand nano-sized pure and Bi-doped ZnO binary varistors.
The results also show that the synthesis method (sol-gel), and the ZnO doping with bismuth give a good quality of powders and a high breakdown electric field, which was the goal of this
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study in order to obtain more performances for a surge protection.
A strong correlation between the grain size and the electric field on one hand, and the type of dopant on another hand allows obtaining a varistor with a good non-ohmic behavior. A very high field against overvoltage should also be taken into consideration.
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Finally, it is recommended now to use nanometric powders for manufacturing varistors as a
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raw material for the development of nano-structured integrated varistors.
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HIGHLIGHTS
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The grain size of pure and Bi doped ZnO nanopowders decreases by increasing Bi concentrations. The material becomes more interesting after doping with Bi. The breakdown electric field increases with increasing sintering temperature and decreasing of grains size. we have recorded higher values of breakdown electric field and non-linearity coefficient.
S0925-8388(19)31077-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.03.251
Reference:
JALCOM 50015
To appear in:
Journal of Alloys and Compounds
Received Date: 2 February 2019 Revised Date:
11 March 2019
Accepted Date: 17 March 2019
Please cite this article as: A. Boumezoued, K. Guergouri, R. Barille, D. Rechem, M. Zaabat, M. Rasheed, ZnO nanopowders doped with bismuth oxide, from synthesis to electrical application, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.03.251. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ZnO nanopowders doped with Bismuth Oxide, from synthesis to electrical application A. Boumezoued1, K. Guergouri1,*, R. Barille2, D. Rechem3, M. Zaabat1 and M. Rasheed2 1
Laboratory of Active Components and Materials, University Larbi Ben M'Hidi of Oum El 2
MOLTECH-Anjou University of Angers/CNRS UMR 62002, Bd Lavoisier, 49045 ANGERS Cedex 01, France
3
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Bouaghi 04000, Algeria
Laboratory of Materials and Structure of Electromechanic systems and their fiability,
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University Larbi Ben M'Hidi of Oum El Bouaghi 04000, Algeria
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*Corresponding Author: (Phone: + 213 6 61 30 91 96) Email: [email protected]
Abstract:
Bismuth doped ZnO nanopowders with doping concentrations between 0 and 7 %mol were successfully synthesized with a soft chemistry method: the sol-gel route. This method allows obtaining powders with a good quality in a nanometric size range. The well-defined optimal conditions for the synthesized powders are given. The hydrated zinc acetate dissolved in
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ethylene glycol is used as a precursor for the zinc oxide (ZnO), with different small amount of Bi concentrations (1%, 3%, 5% and 7%). After calcination at 500°C, the powders are consolidated into pellets and sintered using a conventional furnace and then characterized by: X-ray diffraction, SEM, TEM, and J(E) electrical measurements. Finally, the obtained
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material has been used to prepare varistors.
The XRD patterns indicate that pure and Bi doped ZnO are solid solutions with average grain
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sizes varying between 65 nm to 48 nm. The β-Bi2O3 liquid phase of tetragonal structures appears as a secondary phase for a 5% mol concentration of Bi. High-resolution TEM images of Bi doped ZnO particles indicate that during the sintering of Bi-doped ZnO, the growth of particles leads to a change in the microstructure of ZnO giving semi-transparent cylindrical nanotubes.
J(E) results indicate that the content of Bi increases the nonlinearity coefficient α and the breakdown fields VB of the binary Zn %Bi-O varistors. The 7% doped Bi-ZnO varistor gives the best electrical characteristics for varistors.
ACCEPTED MANUSCRIPT Keywords: ZnO nanoparticles; Bi doping; Grain boundaries; Varistor; Electrical characteristics. 1. Introduction For several decades, the consumption of electric power has been growing constantly, resulting in the extension of the transmission network and the development of very high voltage lines.
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This has led to the fabrication of surge protection devices to ensure a good quality of service and protection of electrical networks. ZnO-based varistors are recommended to play this role. Zinc oxide (ZnO) varistors are polycrystalline ceramics made with pellets of metal oxides, composed essentially of zinc oxide doped with different oxides such as Bi2O3, Pr6O11, SbO3,
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CoO or MnO [1]. One of their most important required properties is the high non-linearity of the current-voltage characteristic [2, 3].
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In that goal, ZnO nanopowders are synthesized by various techniques, the most widely used currently is the sol-gel route, because it produces manufactured varistors with a low cost and a good quality. This is undoubtedly related to the efficient control of the particle size and the homogeneity of their distribution, which are a direct consequence of this method [4, 5]. The electrical characteristics of ZnO varistors are related to the bulk of materials.
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The origin of the varistor effect is firstly essentially due to the microstructure, and secondly to the potential barriers of grain boundaries, the grain conduction and the boundary resistivity [6, 7]. The varistor effect can be absent between two grains. More the density of these grain boundaries increases more the varistor effect is important. It has been demonstrated that the
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decrease in grains and particle sizes in nanometric scale, results in an increase of the breakdown voltage and improves the electrical characteristics of the varistors (coefficient α
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and breakdown voltage) [8]. So, the improved homogeneity of the starting powder mixture and the well-defined optimum conditions are very important for producing high performance ceramic varistors [9]. The varistor electrical properties are closely correlated with the homogeneity of grain size and the homogenous distribution of the additives. Currently, the majority of commercial ZnO varistors are based on Bi doped ZnO and they exhibit excellent ohmic behaviors. However, there is a neccessity to improve these varistors for more powerful characteristics (a higher breakdown voltage and non-linearity coefficient). The aim of our study is to investigate the influence of the doping bismuth on the structural, morphological and electrical behaviors of ZnO varistors and more particularly to determine the effect of different new phases, specifically the Bi-rich phase [10].
ACCEPTED MANUSCRIPT 2. Experimental details 2. 1 Synthesis of powders Pure and Bi-doped ZnO nano-powders have been synthesized with the sol-gel technique. Zinc acetate dehydrate ( C H O Zn.2 H O) (purity>99%, biochem-Chempharma) is used as a
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starting material. An alkoxide: citric acid (C H O . H O) monohydrate is used to keep particles in suspension, mono-ethanolamine MEA ( C H NO) and ethylene glycol are used as a stabilizer and solvent, respectively. The dopant source used in our study is bismuth chloride (BiCl3). Two solutions are put in a bath of paraffin oil heated at a well fixed temperature (130°C). The first solution was prepared by dissolving zinc acetate dehydrate CAZ in ethylene
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glycol, the second one was prepared by dissolving citric acid in ethylene glycol CAC. Then, during stirring, bismuth chloride, with appropriate amount, is added to the first solution to
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obtain the desired concentrations (1 to 7mol% Bi). After a complete dissolution of the precursors and the stabilization of the oil bath temperature the two solutions are mixed. The solution (1) is gradually added to the solution (2) with a molar ratio (CAZ/CAC = 0.06). Then, the obtained solution is stirred at 130°C during 2h to obtain a homogeneous and transparent solution. Finally, after 24h the solution was calcined at 500°C during4h.
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2. 2 Sintering and manufacturing of varistors based on Zn% Bi-O nanopowders To obtain dense varistors with high performances, a conventional sintering study was performed. In this goal, the obtained powders were pressed between discs with a diameter of 11 mm and a thickness of 1.8 mm at a pressure of 2 MPa. The obtained pellets were sintered
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at 1750°C during15 min at a heating rate of 25 °C/min. In order to realize J(E) measurements, gold was evaporated on both faces of the pellets to have ohmic contacts, followed by a
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heating at 500◦ during 10 min in the goal to remove all organic functions. The obtained powders were characterized by several techniques: XRD, using the Cukα (λ = 0.154056 nm) radiation of a BRRUKER AXS, a D8 advance X-ray diffractometer in order to identify the structure and to calculate the grain size. A TEM (Transmission Electron Microscope - X-Max model) was used to identify the morphology and to estimate particle sizes. Finally J(E) electrical characteristics were measured to calculate the breakdown electric field and the nonlinear coefficient α using a high voltage measure unit (KEITHLEY 2401 source meter ).
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Results and discussion
The figure 1 shows the x-ray diffraction pattern of pure and Bi (1%mol to 7% mol) doped ZnO varistors, sintered at 500°C. The spectra exhibit peaks of the würtzite structure for all samples. The intensity of the peaks decreases with the increase of the Bi concentration, whereas the full width at half-maximum (FWHM) of these peaks increases. The peak around
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19.2° corresponding to Bi appears only for concentrations with 7 %mol of Bi, identified as a phase linked with the metallic bismuth. It is mentioned in the literature [11] that bismuth oxoacetate forms bismuth oxide at 340°C. This is the reason why a slightly higher annealing temperature was chosen. Moreover, the X-ray analysis indicates that there is no spinel phase.
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This may be explained by the fact that the calcination temperature (500°C) of the synthesized nano-powders is not enough to allow the complete reaction between Bi2O3 and ZnO and to
(101)
Z nO β -B i 2 O 3
(100)
∇ B i-rich ph ase
(110) (103)
(102)
Z n O :B i 7 %
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∇
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form the spinel phase.
30
40
50
Z n O :B i 3 % Z nO :B i 1%
Z n O :B i 0 %
60
70
80
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20
Z n O :B i 5 %
90
1 00
1 10
1 20
13 0
14 0
15 0
1 60
2 θ (°)
Fig.1 XRD patterns of pure and Bi doped ZnO starting powders
The average grain sizes were calculated using X-ray line broadening from Scherrer’s formula [12]:
=
. ∆
ACCEPTED MANUSCRIPT where D is the grain size, λ the X-ray wavelength (λ=1.5418A°), ∆θ the full width at halfmaximum (FWHM) and θ the Bragg angle. Table 1: XRD data of pure and Bi doped ZnO. θ (°)
∆θ (rad)
D (nm)
ε (GPa)
0% mol Bi
18.18
0.0022
65
+0.00057
1% mol Bi
18.18
0.0026
60
3% mol Bi
18.17
0.0031
50
5% mol Bi
18.18
0.0032
48
7% mol Bi
18.19
0.0033
47
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Zn% BiO
+0.00054
+0.00054 +0.00052
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+0.00051
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The average values of the crystalline size and strain of the samples were calculated using the (110) peak. The results are given in the table 1. The crystalline size decreases with the increasing of Bi concentration; the values vary from 65 to 47 nm (Fig.2) and follow an exponential variation as it can be observed in the loglog plot of the grain size as a function of the dopant concentration. The diffraction peaks of the ZnO matrix are slightly shifted towards
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the small angles after the introduction of Bi atoms indicating that Bi3+ ions go to Zn2+ sites. The ionic radius of Zn2 + is 0.6 Å while that of Bi3 + is larger and is equal to 1.1 Å. We confirm the presence of a solid solution. On the other hand, the calculated strain values decrease as a function of the Bi concentration. Indeed, a small shift of the most intense peaks, of about 0.03°
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for the peak (100) and 0.02° for the peak (101) is recorded, indicating that each crystallite is subjected to a slight dilation of the deformation (Fig.3).
D [nm]
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100
10
1 1
% Bi
8
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4000
6000
(100)
Pure ZnO ZnO 1% Bi ZnO 3% Bi ZnO 5% Bi ZnO 7% Bi
2000
1000
31,5
32,2
32,9
4000
PureZnO ZnO 1% Bi ZnO 3% Bi ZnO 5% Bi ZnO 7% Bi
2000
36,0
2θ (°)
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Intensity(u,a)
Intensity(u,a)
(101) 3000
36,6
2θ (°)
Fig.3 Variation of the width and the position of the most intense peaks
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as a function of Bi concentration.
Furthermore, the spectra (Fig.4) exhibit new peaks for 2θ ranging between 20° - 67° when the
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concentration of bismuth increases from (5% to 7 %mol). The peaks indicate the formation of liquid phase β according to the JCPDS 27-0050. We notice that the β − Bi O tetragonal phase, which is a metastable phase, was obtained during cooling from phase δ. This structure can be observed in a temperature range of 330 ° C to 650°C [13].
The formation of the β -Bi2O3 phase is due to the cooling mode out of equilibrium, where
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Bi2O3 is formed from a liquid phase localized at the grain boundaries. As it is observed in the figure 2 the grain size decreases with increasing the Bi concentration and reaches its lowest value at nearly 47 nm for 7% Bi. This can be explained by the presence
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of a liquid and an inter-granular phase at the higher sintering temperature (1075°C) causing a rapid growth of the grains. An increase of the concentration of Bi above the value of 7%
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would not change the grain size.
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Z nO :B i 0 %
Zn O :B i 5 % Z nO :B i 7 % β -B i 2 O 3
45,29° 43,39°
55,32
29,55°
30
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2 θ (°)
50
60
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24,18° 29,66°
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β -B i 2 O 3
70
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Fig. 4 X-ray spectra of secondary phase of Zn5%Bi-O and Zn7%Bi-O nanopowders
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b
a
10 µm
10 µm
c
d
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R
Liquid phase β
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Z
10 µm
10 µm
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S
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Fig.5 SEM images of pure and Bi doped ZnO varistors sintered at 1075°C : (a): 0%Bi, (b): 1% Bi, (c): 3% Bi, (d): 5% Bi and (e) 7% Bi. Z: ZnO grain, P: Pore, S: Spinel phase, G: inter-granular phase, R: Bi-rich phase
In order to understand the effect of the different phases on the behavior of bismuth-doped ZnO varistors sintered at (1075 °C) it is important to understand the sintering mechanism and the role of the dopant introduced into the pores, because the properties of a varistor depend on several factors as the sintering temperature and the Schottky barrier which is the main cause
ACCEPTED MANUSCRIPT of the varistor effect in addition to the particle geometry [14]. Considering the sintering mechanism which is an essential step for the preparation of pellets, three main stages are associated with the manufacture of ZnO-based varistors [15]. First, after the dissolution of the additive particles in the solid solution during the sintering process, a liquid phase may occur in the grain boundaries. This liquid phase wets the grain boundaries for the formation of the
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secondary phases such as a spinel phase during the consolidation treatment. Then, a grain growth occurs, which is closely related to the uniform additive distribution and optimized sintering processes. Finally, the grain growth and pores are gradually eliminated. The cooling process generates electrical potential barriers at grain boundaries and the crystallization of the
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second phases from the bismuth-rich liquid phase as well [6].
The figure 5 shows the SEM images of Bi doped ZnO varistors for different Bi concentrations
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sintered at 1075°C for 15 min. First of all, the SEM image of pure ZnO presented in the figure 5a shows that grains are uniformly distributed and keep the same shape. The grain boundaries are not yet well defined and the pores form a network between the grains. In this case the high density of pores confirms the poor densification. However, for doped samples (Fig. 5: b, c, d, e) the grains have grown considerably and are well joined on one hand, and on another hand the grain boundaries are well formed between them. Furthermore, the porosity decreases
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when the Bi content increases. This phenomenon is due to the sintering temperature and the addition of bismuth, which makes these doped ZnO varistors denser than pure ZnO varistors [16, 17]. Moreover, it is well known that the Bi-rich phase is responsible for an enhanced densification in the initial stage of sintering [18, 19, 20]. This phase is clearly observed in the
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figure 5b for the Zn3%Bi-O binary varistor.
SEM images allow us also to determine the effect of Bi doping sintered at 1075°C on the
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microstructures based varistors. For each Bi concentration the formation of a new phase is clearly observed. The microstructure exhibits the appearance of new secondary phases identified as: bismuth-rich phase, liquid trace of β-Bi2O3, inter-granular and spinel phases for 1% mol, 3% mol, 5% mol and 7% mol ZnO varistors respectively [21, 22, 23], as it was confirmed by XRD (Fig. 1). Practically no pores are observed in ZnO grains, and most of Bi based particles diffuse into ZnO grain junctions in a uniform and homogeneous way. The microstructure can also contain one or more types of spinel phase. It is also clear from figure 5e that Bi based particles are much more in the spinel phase than in the bi-rich phases, while there is only very little Bi in
ACCEPTED MANUSCRIPT the grains of ZnO. It should be noted that the spinel phase formation temperature is 900 ° C [24]. The average particle size (D) of each phase is determined by the lineal intercept method with SEM images and given by the following equation:
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D =1, 56 L/M,N Where L is the random line length on the micrograph, M the magnification of the micrograph and N the number of grain boundaries intercepted by the lines [25].
From the above results, it possible to quantify the number, distribution and morphology of
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each phase in the microstructure of Zn%molBi-O varistors captured at various locations of SEM images. The figure 6 shows the phase distribution of ZnBi%mol-O varistors doped with
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different Bi concentrations, We noted the following phase distribution: 62% for the pure varistor, 22% for 1% Bi 33% for 3% Bi, 37% for 5% Bi and 45% for 7% Bi, as it is listed in the table 2.
It is reported that the presence of a larger value of phase distribution of ZnO grains improves the homogeneity of the varistor since a phase rich in bismuth prevents the formation of grain boundary between ZnO grains. This is obviously explained by the difference in grain size and
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the applied electric field of the varistor material. The Bi-rich phases are located throughout the structure and especially growth in liquid phase sintering. In addition, intergranular and spinel phase particles are displayed in the ZnO grains and also separate in multiples.
phases
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Sample
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Table 2. Morphological properties of pure and doped ZnO varistors Average particle
Location
size (nm)
Distribution %
0%
ZnO
159
Substitutional
62%
1%
Bi - rich
171
Liquid phase
22%
3%
Metallic Bi
153
ZnO grain
33%
5%
Liquid phase β
-
Triple point
37%
7%
Spinel and inter-
-
Grain
45%
granular phase
boundaries
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0 % mol Bi 1 % mol Bi 3 % mol Bi 5 % mol Bi 7 % mol Bi
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50
40
30
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Distribution of number %
60
20
10
0
Bi-rich Metallic bismuth Liquid phase Spinel and inter-
ZnO
granular phase
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Fig. 6 Phases distribution of ZnBi%mol-O varistors doped with different Bi concentrations
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0%
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(1%mol to 7%mol) and sintered at 1075°C
Average particle size (nm)
200
1%
3%
5%
7%
D SEM D XRD
150
100
50
0 0
1
2
3
4
5
6
7
% Bi Doping
Fig.7 particle size histograms as a function of Bi concentration
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The average particle size (D) of sintered samples obtained from SEM micrographs and TEM images were determined with an image analysis software. More than 300 particles were taken into account for the determination of the average particle size. The results presented in the histogram of the figure 7 indicate that the mean crystallite sizes
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measured from SEM and TEM images are varying in the same direction than those obtained by XRD; they range between 115 to 159 nm. The table 3 summarizes the values obtained from XRD, SEM and TEM. Particle size values are almost twice the size of the grains. The only factor that has a significant effect on the particle size in this observation is the sintering
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temperature because at higher temperatures, the crystal network disappears rapidly. The particle size can then increase easily since the network no longer inhibits the particle growth
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during the sintering cycle. This result is in good agreement with previous researches [26]. The Figure 8 shows TEM observations of the influence of Bi concentrations of ZnO nanopowders. Two different structures of pure and Bi doped ZnO varistors are obtained during this synthesis. For Zn 0%Bi-O, the particles exhibit hexagonal form with an uniform distribution (Fig. 8a), whereas, for all doped samples aligned cylindrical rod shaped particles can be clearly seen in the micrographs (Fig. 8b, c and d). During cooling, a magnification of
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the ZnO particles doped with Bi changes the hexagonal microstructure into transparent cylindrical rod morphologies. The same observation has been reported by S. Anas et al. [27]. The particle size (length and diameter) of pure and Bi doped ZnO varistors obtained from TEM micrographs are summarized in table 4. The sizes varied in the range of 84 to 37 nm in
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lengths and in the range of 118 - 83 nm for the diameters. Consequently, we focused our work on the fabrication of an homogeneous distribution of Zn%mol Bi-O varistors at the nanometer
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scale with the synthesized powders. Thus, the obtained particles after sintering were similar in shape, but these two structures differed in terms of agglomeration.
ACCEPTED MANUSCRIPT Table 3 Grain and particle sizes Zn% BiO
D (nm): XRD
D (nm): SEM
65
159
1% mol Bi
60
149
3% mol Bi
50
120
5% mol Bi
48
118
7% mol Bi
47
115
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0% mol Bi
Table 4 Geometrical dimensions and characteristics of material structures for different
Diameter (nm)
0 mol% Bi
85
1 mol% Bi
169
3 mol% Bi
154
5 mol% Bi
93
7 mol% Bi
37
Lengths (nm)
Structure
118
Hexagonal
118
Cylindrical rod
143
Cylindrical rod
97
Cylindrical rod
83
Cylindrical rod+hexagonal
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Zn%Bi-O
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Zn%Bi-O varistors observed with TEM
However for the sample of 7% Bi-doped ZnO (Fig. 8e), we notice two different particles
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shape; hexagonal and cylindrical tube corresponding to zinc and bismuth particles respectively, which means that Bi+3occupy Zn+2 ions. This result indicates that Bi atoms are located in solid solution, which significates that Bi+3 ions are incorporated into the network
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due to the replacement of Zn+2 ions in substitutional sites. We can also say that the presence of zinc oxide nanoparticles in the reaction medium considerably modifies the morphology of the reaction’s product between the bismuth and the zinc. If we consider the SEM observations one can suggest that the smaller particles are those rich in zinc and the larger ones are rich in bismuth, and the segregation between them is clearly indicated.
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b
100 nm
d
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c
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100 nm
100 nm
100 nm
100 nm
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e
Fig.8 TEM images of pure and Bi doped ZnO varistors prepared from nanopowders
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(a): pure ZnO, (b): 1% Bi, (c): 3% Bi, (d): 5% Bi and (d): 7% Bi.
The electric field variation as a function of the current density for different concentrations is
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given in the figure 9. The curves allow calculating the two main coefficients: the nonlinear coefficient α and the barrier voltage or breakdown voltage VB determining the varistor effect.
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The nonlinear exponent (α) is calculated from the empirical relation: J = KEα, where K is a constant, J and E the current density and electric field, respectively. The nonlinear coefficient α is obtained by the formula: α=∆ ∆J/∆ ∆E, where E and J are the values of breakdown voltage and current density respectively in the non-ohmic region [28]. The breakdown voltage VB is given by the point that sets the transition between the linear and non-linear regime. The results of the figure 9 indicate clearly two regions: the ohmic region, known as a high resistance region and the non-ohmic one, known as a very low resistance region, where E is the electric field (V/cm) and J is the current density (A/cm2). The different values of VB were deduced from curves of J(E) (table 5). They allowed to calculate the breakdown voltage per
ACCEPTED MANUSCRIPT grain boundary (Vgb), given by the equation VB= Vgb/D [29] and listed in Tables 4 and 5 with D the average of grain sizes. An average value of 21 mV ± 0.3 mV is obtained.
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1E-5
1E-6
1E-7
1E-8
0%mol Bi
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Current density J (A/cm2)
1E-4
1%mol Bi 3%mol Bi
1E-9
5%mol Bi
10
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7%mol Bi
1E-10
100
1000
Electric field E (V/cm)
Fig. 9 J (E) Varistor characteristic of the current intensity J as a function of the electric field
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for different Bi concentrations.
The electrical parameter values were characterized (fig. 9). The curves J (E) is plotted in a logarithmic scale in the Figure 9 in order to determine the coefficient of the nonlinearity α.
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We see with the curves of the figure 9 that the current density J in the non-linear region increases strongly when the concentration of Bi is increased. Thus J is depending on the
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particle size and the average number of grain boundaries. Considering the breakdown voltage VB, the measured values indicate that they vary within a wide range in a range varying between 1366.63 V/cm and 1901.50 V/cm (Tab.5). We notice that VB increases by increasing Bi concentration, the maximum value is obtained for 7 mol% Bi sintered at 1075°C, this is obviously due to the increasing in the number of grains and particle sizes of the powder. We know that small grains lead to a decrease of the electric field in grain boundaries [30]. It is clear that the values of VB increase with increasing Bi content, as an example, we have measured the value of 1435 V/cm for Zn-1%Bi-O and 1901 V/mm for Zn-7%Bi-O at the same sintering temperature (1075 °C). Varistor electrical properties are closely correlated
ACCEPTED MANUSCRIPT with the homogeneity of grain size and of barrier distribution and characteristics in the bulk, i.e. the homogeneous distribution of the additives. The Figure 10 shows that the breakdown voltage VB decreases when the grain size increases. The most important result is that we can link the breakdown voltage to the grain size. The reduction in the size of the powder particles leads to the decrease in the grain size of the Zn-
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Rb
A
Rb
5 % m ol Bi
Rb
1800
7 % m ol Bi
60
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Grain boundary
VB
1900
B
ZnO grain
1700
1600
1500
1400 Grain size (nm) Breakdown voltage V/cm
40
-1
0
1
2
3
Breakdown voltage (V/cm)
Rb
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Rb
Grain size (nm)
Rb
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%Bi-O varistors.
1300 4
5
6
7
8
% Bi doping
Vgb
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Fig.10 a) Electrical current distributions in the material for different topologies of the grain network and distribution of crystalline defects for two different Bi dopants, b) Equivalent electrical circuit representation of a grain boundary according to the equation J = KEα
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c) Variation of the grain size D and breakdown voltage VB as a function of Bi doping.
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Table. 5 Varistor effect parameters of pure and Bi doped ZnO samples synthesized by a sol-gel route.
Sample ZnxBi-O
Breakdown voltage VB (V/cm)
Non-linearity coefficient α
0 mol% Bi
1367
5.56
breakdown voltage per grain boundary Vgb (V) 0.0217
1 mol% Bi
1435
15.65
0.0214
3 mol% Bi
1641
17.32
0.0196
5 mol% Bi
1773
23.25
0.0210
7 mol% Bi
1901
29.18
0.0218
ACCEPTED MANUSCRIPT This coefficient α increases with the increase of the Bi concentration up to 29.18 for 7 mol% Bi as indicated in table 5. So, we see that α increases with the increase of the number of grains and particle sizes. The results demonstrate also that the sample doped with 7 mol% Bi has the best electrical behavior (VB=1901.50 V/cm and α=29.18). If we add to this result the fact that Zn7%Bi-O owns the lower values of grain number and particle sizes we can conclude that
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this varistor has the best characteristics and owns a very important varistor effect. However, Savary et al. [31] report the formation of a nano-sized ceramics by a liquid route made of zinc oxide powders that exhibits nonlinear electrical characteristics. We did not observe the same behavior with pure and bismuth doped zinc oxide grains within a short time: in our case, the
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bismuth introduction was found to be necessary in order to obtain a high breakdown voltage and nonlinearity behavior. In the figure 10a we consider a side grain as one node of the primary mesh corresponding to one cell of a polyhedral mesh. These nodes could be
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considered as the generating germs of the resulting grain structure. The grain boundaries are represented by the generally non-planar interfaces between the neighboring polyhedral cells. The figure 10b give an equivalent circuit with an electrical model for the different topologies of grain structures corresponding to two different dopants. The 7 mol% dopant gives the best value in term of breakdown voltage VB and nonlinearity. We see that for this dopant the
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grains are optimum in sizes and connectivities are increased. So, increasing the number of active boundaries causes the switching breakdown voltage to shift towards higher values while the effective dynamic conductance of the material is increased. The transition between the linear and nonlinear part is due to the maximum current flow through the connected
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network [32].
From SEM images, many phases were observed in the microstructure of the binary varistors Zn%Bi-O obtained from nanopowders (Fig. 5). The rapid growth of the particles of these
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phases at grain boundaries during the sintering and the cooling of samples has a desirable influence on the characteristics of the Schottky double barrier [33].
4.
Conclusion
Nanopowders of pure and Bi doped ZnO were synthesized successfully to manufacture varistor devices by a sol-gel route. XRD spectra confirm the würtzite structure and the presence of new phases for each Bi concentration for all powders and varistors with grain sizes varying from 47.70 - 65.76 nm as a function of the Bi concentration. SEM images confirm the existence of phases observed with the XRD diffraction, with particle size values
ACCEPTED MANUSCRIPT almost twice the size of the grains but varying in the same way. On the other hand, TEM images showed that the presence of Bi atoms contributes to change the hexagonal structure of ZnO into cylindrical transparent rods with different physical dimensions. The electric characteristics J (E) of these varistors indicate a good non-ohmic behavior for all samples. The breakdown electric field VB ranges between 1367 to 1901 V/cm and the
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coefficient of non-linearity between 5.56 to 29.18 whereas, many secondary phases are formed at grain boundaries just after adding bismuth due to the decrease of the grain size average. J(E) results indicate that the content of Bi increases the nonlinearity coefficient α and the breakdown fields VB of the binary Zn %Bi-O varistors. The 7% doped Bi-ZnO varistor
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gives the best electrical characteristics for varistors. A higher value of dopant doesn’t affect the grain geometrical characteristics. The results are twice the obtained values for Co-doped ZnO varistors.
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As it has been successfully demonstrated in our work, microstructural evolution phases of the sintered samples can efficiently modify the electrical parameters on one hand and on the other hand nano-sized pure and Bi-doped ZnO binary varistors.
The results also show that the synthesis method (sol-gel), and the ZnO doping with bismuth give a good quality of powders and a high breakdown electric field, which was the goal of this
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study in order to obtain more performances for a surge protection.
A strong correlation between the grain size and the electric field on one hand, and the type of dopant on another hand allows obtaining a varistor with a good non-ohmic behavior. A very high field against overvoltage should also be taken into consideration.
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Finally, it is recommended now to use nanometric powders for manufacturing varistors as a
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raw material for the development of nano-structured integrated varistors.
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HIGHLIGHTS
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The grain size of pure and Bi doped ZnO nanopowders decreases by increasing Bi concentrations. The material becomes more interesting after doping with Bi. The breakdown electric field increases with increasing sintering temperature and decreasing of grains size. we have recorded higher values of breakdown electric field and non-linearity coefficient.