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Materials Letters 62 (2008) 335 – 337 www.elsevier.com/locate/matlet
Ceramic system based on ZnO–CuO–glass Jusmar V. Bellini a,⁎, Márcio R. Morelli b , Ruth H.G.A. Kiminami b a
b
UEM, Departamento de Física, Av. Colombo 5790, CEP 87020-900, Maringá-PR, Brazil UFSCar, Departamento de Engenharia de Materiais, C.P. 676, CEP 13565-905, São Carlos-SP, Brazil Received 14 March 2007; accepted 13 May 2007 Available online 18 May 2007
Abstract Mixtures of powders containing ZnO, (CH3COO)2Cu·H2O and glass frit were obtained by freeze-drying with compositions of 99 wt.% [(100 − x) mol% ZnO + x mol% Cu2+ (x = 0.05, 0.5, 5.0)] + 1 wt.% G, where G is a glass powder containing 26% SiO2, 62% PbO, 7% B2O3, 5% ZnO in weight. Pellets were sintered in air at 950 °C/1 h. Ceramic system based on ZnO–CuO–glass originated during heating above 425 °C. Liquid-phase sintering occurred above 850 °C. Results showed that the breakdown electric field, the nonlinearity coefficient (α), average grain size and grain boundary voltage increased; at the same time that the leakage current decreased, densification was enhanced as the Cu-contents increased from 0.05 to 5. An excess Cu-containing segregate phase was observed in ceramics containing 5.0 mol% Cu2+. Samples presented densities above 95% of the theoretical density of ZnO and α N 200. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; Copper(II) acetate; Glass; Freeze-drying; Varistors; Electroceramics
1. Introduction ZnO-based varistors are important technological materials due to their applications as surge arresters in high power electrical systems or electronic circuits. Matsuoka [1] developed a multi-component ZnO-based varistor composition whose densification mainly occurred owing to Bi2O3 through reactive liquid-phase sintering. Excellent reviews on that composition have already been published in the literature [2–6]. Alternatively, addition of 1 wt.% glass frit to Matsuoka's composition showed that ZnO–glass varistors provided higher values of nonlinearity coefficients and smaller leakage currents [7]. Further, 10-wt.% glass addition instead of Bi2O3 improved processing parameters and enhanced electrical stability [8–13]. On the other hand, nonlinear electrical characteristics were observed on polycrystalline ZnO solely doped with Cu [14–19]. In samples sintered at 950 °C, obtained from freeze-dried mixtures of powders containing ZnO + (CH3COO)2Cu·H2O (copper(II) acetate monohydrate, henceforth termed CuAc2·H2O), the gradual addition of Cu promoted grain growth and enhanced electrical properties, albeit the densification was not completely reached ⁎ Corresponding author. Tel./fax: +55 44 32 63 46 23. E-mail address:
[email protected] (J.V. Bellini). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.05.032
[20]. In air, the decomposition solid product of CuAc2·H2O freeze-dried powders above 425 °C was CuO [21]. Therefore, the resulting system from ZnO + CuAc2·H2O freeze-dried powder is ZnO–CuO. Above 800 °C, solid-state reactions involving ZnO– CuO gave rise to Cu-doped ZnO [20–23]. The activation energy for the thermal decomposition of CuAc2·H2O was estimated to be around 154 kJ/mol [24]. Current research explores the addition of 1 wt.% of lead zinc borosilicate glass frit to ZnO + x mol% Cu2+(x = 0.05, 0.5, 5.0) to improve varistor properties. Results of the physical, electrical and microstructural changes of the resulting ceramic system based on ZnO–CuO–glass obtained from freeze-dried mixtures of powders containing ZnO + CuAc2·H2O + (glass frit) are provided below. 2. Experimental 2.1. Preparation of the glass frit SiO2 (99.6%, Aldrich), PbO (99.98%, Fischer), B2O3 (99.98%, Aldrich) and ZnO (99.9%, Uniroyal) powders were used for the preparation of the glass frit. Proportions of SiO2– PbO–B2O3–ZnO were 26–62–7–5 wt.% respectively. Powder, processed by ball-milling, was fused in air at 900 °C and rapidly
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2.3. Physical and microstructural characterization
Fig. 1. J–E curves for 99 wt.% [(100 − x) mol% ZnO + x mol% Cu2+ (x = 0.05, 0.5, 5.0)] + 1 wt.% G samples sintered at 950 °C/1 h.
quenched by flowing in distilled water. Resulting glass was grounded in mortar, sieved (200 mesh) and ball-milled. Resulting glass frit powder (denoted by G) had particles with mean diameter of D50 ∼ 4.5 μm and density of 5.17 ± 0.07 g/ cm3. 2.2. Freeze-drying Initially, at room temperature, powders containing ZnO (99.9%, Uniroyal, D50 ∼ 0.5 μm), CuAc2·H2O (98.7%, Mallinckrodt) and G glass frit, with compositions equivalent to 99 wt.% [(100 − x) mol% ZnO + x mol% Cu2+ (x = 0.05, 0.5, 5.0)] + 1 wt.% G were homogeneously diluted in 150 ml of distilled–deionized water. CuAc2·H2O was chosen as a source for Cu2+. The aqueous mixture was then transferred to an appropriate glass flask and freeze-dried, as previously described elsewhere [24].
The freeze-dried powders were uniaxially pressed into disk shape. Pellets were sintered in an electric furnace (EDG, model 3000), in air, at 950 °C/1 h, with heating and cooling rates at 5 °C/min. Sintered pellets were previously polished with 0.3μm Al2O3 powder to a mirror-like surface. The physical changes of the samples (densification, %ρtheo) were evaluated by measuring the volumetric density (%) in relation to the theoretical density of ZnO (5.67 g/cm3). For microstructural analyses, the samples had been chemically attacked by a solution of 0.2% HF + 0.8% HCl Vol., for 30 s. Microstructures were analyzed by scanning electron microscopy (SEM) (Leica, model StereoScan 440) with a microprobe apparatus (Oxford, model eXL) for Cu-element mapping. Mean grain diameter (Dgrain) was estimated by the maximum value of a Gaussian distribution of grain-diameter frequencies. The dispersion around Dgrain was represented by the full width at half maximum (FWHM) of the distribution. 2.4. Electrical characterization Pellets were polished with SiC abrasive sandpaper (#600, #1200 and #2000). Conductive silver paste electrodes (0.385 cm2) were deposited on the pellets' surfaces (∼1.0 mm in thickness) and heat-treated at 110 °C for 10 min. The I–V characteristics of the samples were measured by using a high-voltage power supply (Keithley, model 248) with a GPIB interface. The I–V relations of ZnO-based varistors may be expressed by the empirical equation I = kV α, where I is the electric current, V the applied voltage and k a constant. From J–E (current density × electric field) curves, for comparison, the nonlinearity coefficient (α) was estimated from the equation α = log(JB /JA) / log(E(JB) / E(JA)) by JA and JB for
Fig. 2. SEM microstructure for 99 wt.% [(100 −x) mol% ZnO +x mol% Cu2+] + 1 wt.% G samples sintered at 950 °C/1 h: (a) x = 0.05; (b) 0.5 and (c) 5.0; including (at side) SEM Cu-element mapping (gray color) for the same region of the sample.
J.V. Bellini et al. / Materials Letters 62 (2008) 335–337 Table 1 Electrical and microstructural properties for 99 wt.% [(100 − x) mol% ZnO + x mol% Cu2+ (x = 0.05, 0.5, 5.0)] + 1 wt.% G samples sintered at 950 °C/1 h x
Ebr
(mol%) (V/cm)
Ileak
α
(μA)
7.5 × 103 132 6.3 × 103 120 2.2 × 104 91
Vgb
Dgrain FWHM ρ
(V/grain) (μm)
% ρtheo
(μm)
(g/cm3)
1.6 3.7 5.6
5.24 ± 0.05 92.4 5.32 ± 0.07 93.8 5.39 ± 0.06 95.1
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inclusions in the microstructure. The existence of such particles was confirmed using a larger counting time in a small region of the sample (not shown).
4. Conclusion
(%)
current densities in the interval 1–10 mA/cm2, respectively [8]. Ebr represents the breakdown electric field of the varistor when the current density is 1 mA/cm2, and Ileak represents the leakage current at room temperature when the voltage is 0.8Ebr [5]. Taking the values of Ebr and Dgrain into consideration, the barrier height was estimated through the relation Vgb = Ebr / Dgrain (V/grain), where Vgb is the grain boundary voltage.
Freeze-drying is a useful technique to produce homogeneous mixtures of powders containing ZnO + (CH3COO)2Cu·H2O + (glass frit). ZnO–CuO–glass-based varistor system originates after sintering at 950 °C, in air. The system densifies with the aid of liquid-phase sintering which occurs above 850 °C. Results showed that the electrical properties were improved and densification was enhanced as the Cu-contents increased from 0.05 to 5. Best results were for samples containing 5.0 mol% Cu2+ (densities above 95% of the ZnO theoretical density and α N 200). The presence of an excess Cu-containing segregate phase is believed to inhibit exaggerated grain growth in such samples.
3. Results and discussion
Acknowledgement
Figs. 1 and 2 show respectively the results of the electrical and microstructural characterization. Fig. 2c presents the microstructure and the SEM Cu-element mapping (at side) for the sample containing 5.0 mol% Cu2+. Table 1 contains some electrical and microstructural parameters of samples. Results indicate that when the Cu-contents are increased from x = 0.05 to 5.0, the Ileak decreases and both Ebr and α increase, while Dgrain increases and densification is enhanced. Ileak decrease indicates that the resistivity of the grain/grain interfaces as well as the resistivity of the grain rise as the Cu-contents increase. It is believed that the presence of complex acceptor-type defects (associated to intrinsic donors as Zni or VO) involving CuZn or Cui may simultaneously contribute to decrease the conductivity of the grains from n-type to n−-type [20]. Further, the contribution of amorphous vitreous phase along the edges and triple point of grains cannot be discarded, albeit such phases could not be detected in the current stage of the research. Although Dgrain increases, Vgb also increases; Ileak decreases as the Cu-contents increase (see Table 1). Thus, the Cu-doping contributes towards an increase in grain and grain boundary resistivity, which also gives rise to higher values of Ebr. Comparison between the previously reported properties for ZnO + x mol% Cu2+ (x = 0.05, 0.5, 5.0) [20] and the properties presented for the samples in current research suggests that the addition of glass frit enhanced the densification of the latter system. Fig. 2 shows homogeneous microstructures with the disappearance of pores and without exaggerated grain growth. Fig. 2c indicates the presence of Cu-doped ZnO grains with slightly higher Cu-concentration near the inclusions (Cu-rich segregated phases). The presence of such inclusions is believed to contribute to inhibit exaggerated grain growth (pinning). The microstructure (Fig. 2c) shows pores (black regions) originating from inclusions (in white) pulled out during polishing. Analysis of the Cu-element (Fig. 2c, at side, in gray) reveals a homogeneous Cudistribution into the ZnO matrix with Cu-rich regions corresponding to
The authors would like to thank Fundação Araucária for the financial support.
0.05 0.5 5.0
17 1.5 12 2.7 204 11.7
2.1 4.3 5.3
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