Structural, microstructure and electric properties of SnO2-Sb2O5-Cr2O3 varistor ceramics doped with Co2SnO4 spinel phase previously synthesized

Accepted Manuscript Structural, microstructure and electric properties of SnO2-Sb2O5-Cr2O3 varistor ceramics doped with Co2SnO4 spinel phase previously synthesized M.B. Hernández, S. García-Villareal, R.F. Cienfuegos-Pelaes, C. Gómez-Rodríguez, J.A. Aguilar-Martínez PII:

S0925-8388(16)34348-1

DOI:

10.1016/j.jallcom.2016.12.419

Reference:

JALCOM 40342

To appear in:

Journal of Alloys and Compounds

Received Date: 13 November 2016 Revised Date:

28 December 2016

Accepted Date: 30 December 2016

Please cite this article as: M.B. Hernández, S. García-Villareal, R.F. Cienfuegos-Pelaes, C. GómezRodríguez, J.A. Aguilar-Martínez, Structural, microstructure and electric properties of SnO2-Sb2O5Cr2O3 varistor ceramics doped with Co2SnO4 spinel phase previously synthesized, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2016.12.419. 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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Structural, microstructure and electric properties of SnO2-Sb2O5-Cr2O3 varistor ceramics doped with Co2SnO4 spinel phase previously synthesized

Aguilar-Martínez1*,

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M.B. Hernández1, S. García-Villareal2, R.F. Cienfuegos-Pelaes1, C. Gómez-Rodríguez3, and J.A.

Universidad Autónoma de Nuevo León, Facultad de Ingeniería Mecánica y Eléctrica, Centro de

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Investigación e Innovación en Ingeniería Aeronáutica (CIIIA); Carretera a Salinas Victoria km. 2.3, C.P. 66600, Apodaca, N.L., Mexico

Universidad Autónoma de Coahuila, Facultad de Metalurgia, Carr. 57, Km 4.5, C.P. 25710,

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2

Monclova, Coah., Mexico 3

Universidad Politécnica de Apodaca (UPAP); Av. Politécnica cruz con la carretera Miguel

Alemán, km 24.5, C.P. 66600 Apodaca, N.L., México

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E-mail address: [email protected]

[email protected]

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Abstract

The present investigation reports the variations of the microstructure, structure and electrical

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properties due to a change in the Co2SnO4 content of SnO2-Cr2O3-Sb2O5 ceramic samples. The ceramic system was (99.9-X) % SnO2- X% Co2SnO4- 0.05%Sb2O5-0.05 % Cr2O3, where X= 0, 0.5, 1, 2 and 4 mol %. In order to obtain a comprehensive knowledge of the samples, characterization techniques such as Thermal Analysis, X-ray Powder Diffraction (XRD) with Rietvled refinement, and Scanning Electron Microscopy where carried out. On the subject of the electrical properties of the studied system, the samples showed a tendency to decrease values

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of breakdown electric field (from 6219.30 to 1155.72 Vcm-1) as the spinel content increased. Authors suggest the ceramic system may found application as medium-voltage varistors.

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Keywords: X-ray diffraction; electronic properties; ceramics; sintering. 1. Introduction

Varistors are well known to be effective and economic devices to provide protection against high

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voltage transient and surges. The history of varistors with high nonlinearity values goes back to the introduction of Zinc oxide based system by Matsuoka, early in the 1970’s [1]. Since then, the

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ZnO-based system has been the most frequently used material as the base for ceramic varistors for commercial production [2]. Research on alternative ceramic systems have led to the study of several oxides, such as TiO2 [3], SrTiO3 [4], WO3 [5], CeO2 [6], ZnSnO3 [7], CaCu3Ti4O12 [8], Tb4O7 [9], BaTiO3 [10], and SnO2 [11], to mention few examples. Varistors are devices also

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known as electrical resistors whose resistance is dependent on the applied voltage. Accordingly to this definition, varistors exhibit a highly non-linear electrical current-voltage behavior, that is, they do not fulfill Ohm’s Law [12]. This electrical behaviour makes varistors reliable for the

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protection of electronic devices in both electronic circuits and electric shock devices in power distribution networks. The expression to describe the current (I)-voltage (V) characteristic of a

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varistor system is given by the empirical power-law relation that relates the current density and the electrical field parameters [3]: j = kE α ,

(1)

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where j is the current density, E is the electrical field, k is a constant that depends on the microstructure related to the electrical resistance of the material, and the degree of nonlinearity

α=

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is represented by the dimensionless number α. This α value can be calculated by [3]:

log( J 2 J 1 ) log(E 2 E1 )

(2)

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where E1 and E2 are the values of electric field when the current densities are J1 and J2,

respectively. J and E are calculated by means of i/s and V/t, where i is the electric current, s is the

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area of silver electrode, and t is the thickness of the tested sample.

Lately, several researchers have become interested in studying Tin dioxide (SnO2)-based ceramic systems as possible candidates for varistor applications [13-16]. The mineral form of the Tin dioxide is known as cassiterite; it crystallises with the tetragonal rutile-type crystal structure with

[P4 2 / mnm] [17]. One of the main characteristics of pure tin dioxide is its low

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14 a space group D4h

densification rate during sintering. This effect is due to the predominance of non-densifying mechanisms during mass transport, such as surface diffusion (occurring at low temperatures) and

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a process of evaporation-condensation (at high temperatures) which promote only grain growth

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and the formation of necking between adjacent particles [18]. The easy evaporation of the SnO2 at high temperatures stimulates the evaporation-condensation mechanism; this can be represented by the reaction

1 SnO2 ( s ) → SnO + O2 ↑ 2

(3)

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As can be concluded, all the factors mentioned above have an effect on the highly porous characteristic of SnO2 ceramics, which are of special interest for applications where high porosity is required. On the contrary, for varistors applications this feature is not desirable and

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some actions to increase density must be taken. Researchers have reported that the introduction of densifying agents such as Li2O, CuO, CaO, ZnO, MnO2, and CoO [19-23], or the hot isostatic pressing technique [24] can be used in order to obtain SnO2 ceramic samples with increased

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density as high as almost its theoretical value. The main objective of these densifying agents is the creation of oxygen vacancies. They also, apparently, promote the densification of pure SnO2.

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As for the electrical properties, it has been reported that suitable addition of modifying agents can significantly increase the electrical conductivity of SnO2 based systems. Some oxides such as Ta2O5, Sb2O5, and Nb2O5, have been the most frequently reported dopants used for this purpose [11, 21, 25]. That is, for example, without the addition of Sb oxide, SnO2-based samples behave

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as electrical insulators since grains are highly resistive. An interesting effect happens when Sb+5 ' ions are added to the SnO2-based ceramic system: concentration of e and VSn'''' is produced which

in turn, increases the electronic conductivity of the SnO2 lattice, thus grains become

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semiconductive. The effect of Cr2O3 has been widely studied and reported, originally on

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multicomponent ZnO-based ceramic varistors [26]. The most important contribution of this oxide is the enhancement of the electrical properties of the ceramic samples. However, as the content of Cr2O3 increases, a detrimental effect on the potential barriers located at the grain boundaries is observed. According to Brankovic’s work, the authors have suggested that in SnO2-based varistors the grain growth and densification of samples is inhibited by the presence of Cr2O3 [27]. Another effect of chromium oxide is the formation of grains that are different in their size and their morphology. Lustosa et. al. recently reported the improvement of oxygen 4

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species at the grain boundaries due to the addition of Cr2O3 and observed an increase of the performance of the non-ohmic characteristics of the SnO2-based varistor system [25] they

of Sn+4 ions by Cr+3 ions according to the following reaction:

Cr2 O3 SnO  2 → 2CrSn′ + VO′′ + 2OOx + 12 O2 ↑

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studied. It has been stated that the addition of Cr2O3 to the SnO2 lattice permits the substitution

(4)

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In the present work, authors continue in the reporting of the thorough study they have been doing in the past years on the SnO2 ceramic-based system to be used as a suitable varistor material. The

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effect of high contents of Co3O4 on the structure, the morphology, and the electrical properties (Cr,Sb)-doped SnO2 ceramic samples has already been published elsewhere [28]. In the cited paper the added content of Co3O4 to the base system was 0.0, 0.5, 1.0, 2.0 and 4.0 mol %. In that study, it was found that as the cobalt oxide content into the system increased, the grain size

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decreased, thus increasing the breakdown electric field. Also, it was found that the oxide reacted with the SnO2 and the spinel phase Co2SnO4 formed in situ during sintering. Ceramic samples obtained are candidates for high voltage applications. In the present study, the dopant was added

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in the form of spinel. The Co2SnO4 was first synthetized in the laboratory and later added into

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the SnO2 system. Thus, a comparison between the two different ceramic systems is available.

2. Experimental Procedure Analytical-grade starting materials (Aldrich) to prepare ceramic samples were SnO2, Sb2O5, and Cr2O3. On the other hand, Co2SnO4 spinel was synthesized in the lab as this component is not commercially available. After obtaining the spinel, it was added to the system along with the rest of the oxides. Figure 1 displays a schematic representation of the steps followed to obtain the Co2SnO4 spinel; details of formation mechanism and optimization parameters have been 5

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published elsewhere [29, 30]. The molar composition of the ceramic system was (99.9 – X)%SnO2–X%Co2SnO4–0.05%Sb2O5–0.05%Cr2O3, where X = 0, 0.5, 1, 2, or 4 mol%. Each powder composition was mixed in a high-energy planetary ball mill for 20 minutes at 450 rpm

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(Pulverisette 7, Fritsch GmbH) using agate vials and balls. The resulting powders were

uniaxially pressed into the form of tablets (9.5 mm in diameter and about 1 mm thick) at 150 MPa without using any kind of binder. The tablets were sintered in an ambient atmosphere at

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1623 K for 1 hour with heating and cooling rates of 3 and 6 Kmin-1 respectively in a tube furnace (Carbolite CTF 17/300). The density of the sintered samples was determined by the Archimedes

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method; the residual porosity and the linear shrinkage γ were calculated as explained elsewhere [31].

The microstructure of the samples was characterized by scanning electron microscopy (SEM; model Nova NanoSEM 200, FEI). Mean grain size was evaluated from SEM micrographs with

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image analysis software (Image-Pro Plus 4) according to ASTM-E112 standard procedure. The presence of ceramic phases was determined by X-ray diffraction (XRD; PANalytical Empyrean

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model) with CuKα radiation (λ = 1.5406 Å) operated at 45 kV and 40 mA with a Pixel detector in Bragg-Brentano geometry. The scans were performed in the 2θ range from 10 to 120° with a

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scan step of 0.016° and 80 seconds per step in continuous mode. Lattice parameters and quantitative determination of the phase composition of samples was achieved by using the Rietveld method. This method is a least squares refinement procedure where the experimental step-scanned values are adapted to the calculated ones. It is assumed that the profiles are known and a model for the crystal structure is available. The weight fraction (Wi) for each phase was obtained from the mathematical relationship for refinement [32]:

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Wi =

S i ( ZMV ) i ∑ S j (ZMV ) j

(5)

j

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where i is the value of j for a particular phase among the N phases present, Sj is the refined scale factor, Z is the number of formula units per unit cell, M is the molecular weight of the formula unit, and V is the unit cell volume.

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Rietveld refinement was done by adjusting major parameters like the scale factor, flat

background, zero-point shift, lattice parameters, orientation parameters, peak width parameters

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(U, V, W), asymmetry parameter, and peak shape. Peak profiles were fitted with the pseudoVoigt function. Goodness of fit (χ2) and R weighted profile (Rwp) values were monitored to ensure accurate fit between the observed and calculated data.

In order to study the reaction temperatures of SnO2 and Co2SnO4 within samples, they were

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thermally analysed in the range of 303 to 1773 K in simultaneous TG–DSC–DTA equipment (TA Instruments model SDT Q600). Samples of 20 to 30 mg were placed in a platinum crucible

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and heated at a rate of 10 Kmin-1 in static air, with alumina as the reference material. For electrical characterization, equal silver electrodes were placed on both faces of the sintered

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ceramic samples and then thermal treatment was carried out at 1073 K for 6 minutes according to the specifications of the conductive paint (Heraeus C8717B paint). Current-voltage measurements were performed with a High Voltage Source-Measure Unit (Keithley 2410). The nonlinear coefficient α was estimated at a current density of 1 mAcm-2 with Equation (2). The breakdown field EB was measured at 1 mAcm-2 and leakage current density values (JL) measured at 0.8 EB mAcm-2. The grain voltage (Vb) was calculated as explained elsewhere [31]. 7

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Temperature dependence of dc electrical conductivity was measured between 293 and 500 K using a Keithley 6517B computer controlled electrometer. Measurements were performed in air at a heating and cooling rate of 3 Kmin-1. The barrier height ϕB was estimated from the

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temperature dependence of conductance in the Ohmic region according to G=G0exp(-ϕB/kT) where G0 is a constant, k is the Boltzmann’s constant and T is the absolute temperature. Thus, the barrier height can be obtained from the slope of the fitting straight line of the plot log G vs. the

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reverse of temperature function.

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Capacitance and conductance as a function of frequency were measured in the range of 20 - 1M Hz and a voltage with an amplitude of 1 mA, using a LCR meter (Instek LCR-8101G). The dielectric constant ε was calculated at 1 kHz by using the formula ε=Cd(ε0A)-1, where C is the capacitance of sample, ε0 the permittivity for free space, d and A are the thickness and the cross-

from g=dGA-1.

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sectional area of the flat surface of samples, respectively. The ac conductivity (g) was calculated

3. Results and Discussion

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All compositions were thermally analysed before sintering and results are presented in Figure 2. In the temperature interval studied, no reaction can be detected in any of the samples indicating

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the thermal stability of the system. For the reference sample (0% spinel content) this can be attributed to the chemical stability at high temperatures of SnO2, which is the component with the highest percentage within the sample. Although Sb2O5 and Cr2O3 are present, their content is too low that their effect on the thermal behaviour of the sample could not be detected by the analysis. This statement is more obvious for reference and 0.5 % Co2SnO4 samples since both thermograms completely overlap. Samples with concentrations higher than 0.5% Co2SnO4 8

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displayed in the same Figure exhibit a similar behaviour as that shown by the reference sample. This behaviour indicates that the spinel phase is stable in the range of the working temperature

form Co2SnO4 at 1193 K according to the following reaction: C 2CoO + SnO2 920  → Co2 SnO4 (spinel) o

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and that it does not react with tin dioxide. Moreover, Co3O4 does chemically react with SnO2 to

(6)

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XRD patterns of ceramic samples are shown in Figure 3. In this set of patterns, the peaks from reference sample and the samples doped with 0.5 and 1% Co2SnO4 matched the standard pattern

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of the tin dioxide (ICDD PDF # 04-005-5929). Samples doped with 2 and 4% Co2SnO4, show the presence of the rutile-type SnO2 and the spinel-type phases according to the standard ICDD PDF # 04-005-5929 (marked as *) and ICDD PDF # 04-008-2461 (marked as •), respectively. The XRD pattern of the sample doped with 0.5 and 1% Co2SnO4 showed no evidence of spinel,

the XRD equipment.

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because the content of this phase within the sample is so small that it could not be detected by

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In order to determine the lattice parameters and the amount of phase present in the samples, the Rietveld refinement method was performed on the XRD patterns. Table 1 summarizes the

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information obtained after the refinement of all the samples; a satisfactory fitting value of χ2 is reported.

Micrographs of ceramic samples (Figures 4) doped with the Co2SnO4 spinel at different concentrations are shown. Grain size and morphology of samples after sintering were analysed by SEM. In order to simplify the comparison between the samples, an array of column and row has been established. Micrographs are organized in two columns; the low-magnification (2500x) 9

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set is found at the left while at the right column is located the high magnification (10000x) set. On the other side, in the same row are those samples that contain the equal percentage of dopant. Reference sample is shown in Figures 4a and 4b where it can be seen that both grain size and

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morphology are not homogeneous. Some grains have grown in faceted shape while others remained almost spherical and small. Shrinkage, density, porosity and grain size values are listed in Table 2. The porosity observed is in accordance with that reported on literature and as stated

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before, it is a feature that is no desirable within a ceramic varistor. Low magnification images show that as the spinel is added, the microstructure changes even for the smallest content of 0.5

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mol %. As the dopant content increases, the porosity of samples decreases and the grain size increases. This effect is yet more clearly observed at high magnification micrographs, from where it has been evaluated that grain size increases from 2.36 a 5.66 µm, for samples with 0.5 and 4 mol %, respectively. For the sample with the highest dopant content, additionally to larger

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and rhombohedral-type grains, it is observed a more dense distribution of grains, that is, the voids between grains decreased as grains started to grow.

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Main features of this ceramic system can now be compared to that obtained with the addition of Co3O4 and differences can be determined. For the first case, that is, when the dopant agent is the

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cobalt oxide, the main process that takes place during the sintering process is the in situ formation of the spinel phase. On the other hand, when the cobalt is added as spinel, no chemical reaction occurs, grain growth is promoted, densification is enhanced, and a homogeneous microstructure is obtained.

Figure 5 show the results obtained from the electrical characterization of ceramic samples. The electrical parameters calculated from the J-E curves are listed in Table 3, that is, nonlinear 10

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coefficient (α), breakdown electric field (E1) and leakage current (JL). Non-linear behaviour is evident for all samples. Values of breakdown field decreases as the spinel content increases. This is related to a change in the grain size due to an increase in the dopant content within the ceramic

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system. The electrical behaviour of this ceramic system can also be compared to that observed in samples doped with Co3O4. Leakage current (JL) and barrier height (ϕB) values are listed in Table 3. The highest value of leakage current corresponds to the reference sample, the same with the

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lowest non-linearity coefficient and the highest barrier height. As it can be seen from SEM images, reference sample is porous and therefore it is assumed that potential barriers within this

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sample are not quite efficient and leakage current is high. On the other hand, leakage current values remains almost constant and a decrease in the barrier height is observed as the spinel addition is increased in ceramic samples. As the breakdown field decreases as the barrier height decreases it is suggested that the barriers become more effective for samples doped with spinel

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than reference sample. Dielectric constant (ε) and ac conductivity (g) plots vs. frequency are plotted in Figure 6. The permittivity values presented in Table 3 show an increase as the spinel content increases. For samples 1 and 4 % spinel content, dielectric permittivity seems to behave

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independently of frequency at values above 1 kHz. The ac conductivity increases as the frequency also increases. Moreover, samples 1 and 4 % spinel content show an almost linear

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dependence on frequency.

Conclusions

The results of the present investigation were used to determine the effect of dopant content of previously synthetized Co2SnO4 spinel on the microstructure and electrical properties of SnO2based ceramic system. Additionally a comparison of results obtained in the present investigation 11

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and those previously published was possible. Cobalt addition into the system may be possible through two different paths. On the first one, the dopant agent was introduced in the form of Co3O4 and the formation of spinel phase occurred in situ. In the present investigation the spinel

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phase was first synthetized and later added as dopant agent. The chemical stability of the phase was demonstrated by thermal results. Rietveld refinement and XRD analysis allowed to

determine lattice parameters, phase content, and the confirmation of the presence of the spinel

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phase at high contents. The spinel phase decreases the porosity of the samples as it turns the morphology and the grain size into a more homogeneous microstructure. Another effect of spinel

suitable for medium voltage applications.

Acknowledgements

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content increase is a decrease in barrier height and breakdown field thus making these varistors

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This study was supported by the National Science and Technology Council of Mexico (CONACYT) under the project "Convocatoria Ciencia Basica 2014” (project no. 238054) and PAICYT-2015-IT457-15 program of the Universidad Autónoma de Nuevo León. The authors are

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also grateful to Dr. Alexander Bondarchuk for technical assistance with electrical measurements.

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[29] J.A. Aguilar-Martínez, M.I. Pech-Canul, M. Esneider, A. Toxqui, S. Shaji, Synthesis, structure parameter and reaction pathway for spinel-type Co2SnO4, Mater. Lett., 78 (2012) 28-

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[30] J.A. Aguilar-Martínez, M.A. Esneider-Alcala, M.B. Hernández, M.I. Pech Canul, S. Shaji, Optimal parameters for synthesizing single phase spinel-type Co2SnO4 by sol–gel technique: Structure determination and microstructure evolution, J. Alloy. Compd., 574 (2013) 278-282.

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[31] J.A. Aguilar-Martínez, M.I. Pech Canul, M.B. Hernández, A.B. Glot, E. Rodríguez, L. García Ortiz, Effect of sintering temperature on the electric properties and microstructure of SnO2–Co3O4–Sb2O5–Cr2O3 varistor ceramic, Ceram. Int., 39 (2013) 4407-4412.

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[32] D.L. Bish, S.A. Howard, Quantitative phase analysis using the Rietveld method, J. Appl.

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Crystallogr., 21 (1988) 86-91.

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Table Captions

value (χ2), and lattice constant.

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Table 1. Weighted profile value (Rwp), expected value (Rexp), profile value (RP), goodness of fit

Table 2. Shrinkage ( γ ), theoretical density (ρtheoretical), measured density (ρ), relative density (ρr),

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residual porosity (P), and average grain size of sintered samples.

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Table 3. Nonlinearity coefficient (α), electric field at fixed current density (E1), grain voltage

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(Vb), leakage current (JL), barrier height (ϕB), and dielectric constant (ε) of sintered samples.

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Figure Captions

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Figure 1. Schematic diagram of Co2SnO4 spinel synthesis Figure 2. DSC curves of the decomposition of (a) powder reference sample and (b) powder samples with different Co2SnO4 content.

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Figure 3. XRD diagrams of the as-sintered surfaces of SnO2-based varistors with different Co2SnO4 content.

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Figure 4. SEM images of the as-sintered surfaces of SnO2-based varistors at magnifications of 2500× and 10,000×: (a), (b) reference sample, (c), (d) 0.5% Co2SnO4, (e), (f) 1% Co2SnO4, (g), (h) 2% Co2SnO4, (i) and (j) 4% Co2SnO4.

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Figure 5. J-E characteristic plots of sintered samples.

Figure 6. (a) The relative dielectric permittivity (ε); (b) ac conductivity (g) versus frequency of

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sintered samples.

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Table 1 Phase

Lattice parameter (Å)

Sample c

Rwp(%)

Rexp(%)

Rp(%)

χ2

3.44

7.74

8.75

α=β=γ

Present

a

0% Co2SnO4

SnO2

4.738

4.738

3.187

90

100

10.19

0.5% Co2SnO4

SnO2

4.738

4.738

3.187

90

100

11.28

3.89

7.43

8.38

1% Co2SnO4

SnO2

4.738

4.738

3.187

90

100

11.22

3.92

7.28

8.18

SnO2

4.738

4.738

3.187

90

97.1

11.52

3.87

7.27

8.85

Co2SnO4

8.645

8.645

8.645

90

2.9

SnO2

4.738

4.738

3.187

90

92

6.31

6.44

Co2SnO4

8.639

8.639

8.639

90

8

9.56

Table 2

γ

ρ(theoretical)

ρ

(%)

(g/cm3)

0% Co2SnO4

9.45

0.5%Co2SnO4

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2% Co2SnO4

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b

wt(%)

P

Grain size

(g cm3)

(%)

(%)

( µm )

6.94

6.01

86.59

13.40

1.63

8.33

6.93

6.53

94.22

5.77

2.36

1% Co2SnO4

8.11

6.92

6.79

98.12

1.87

3.02

2% Co2SnO4

7.98

6.91

6.90

99.85

0.14

3.66

4% Co2SnO4

8.01

6.88

6.87

99.85

0.14

5.66

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ρtr

Sample

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Table 3 E1

Vb

JL

ϕB

ε

(V/cm)

(V/b)

(mA/cm2)

(eV)

(1 kHz) 30.58

α

2.36

7429.22

1.21

0.33

1.04

0.5%Co2SnO4

10.94

6219.30

1.46

0.11

0.51

1% Co2SnO4

9.67

2524.12

0.76

0.12

0.43

2% Co2SnO4

11.05

2277.61

0.83

0.10

0.34

4% Co2SnO4

10.19

1155.72

0.65

0.11

0.20

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30.44

68.82

163.08 237.73

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0% Co2SnO4

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Co2SnO4 doped SnO2 based ceramics may be materials used as medium voltage varistors

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As spinel addition increases, so does the densification and grain size As spinel addition increases, the electric field, the barrier height decreases

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The Co2SnO4 spinel phase was synthetized in the laboratory to be used as dopant