Thermal conductivity features of ZnO-based varistors using the laser-pulse method

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1 Materials Science and Engineering A 371 (2004) Short communication Thermal conductivity features of ZnO-based varistors using the laser-pulse method C.M. Barrado a, E.R. Leite a, P.R. Bueno b,, E. Longo a, J.A. Varela c a Department of Chemistry, Interdisciplinary Laboratory of Electrochemistry and Ceramics, UFSCar Federal University of São Carlos C. Postal 676, São Carlos, SP, Brazil b Department of Physics, FFCLRP University of São Paulo, Av. Bandeirantes, Ribeirão Preto, SP, Brazil c Instituto de Química, UNESP, C. Postal 355, Araraquara, SP, Brazil Received 13 June 2003; received in revised form 12 September 2003 Abstract The thermal conductivity of several commercial ZnO-based varistor systems was determined based on the laser-pulse method, a technique that proved extremely useful and easy to apply. Using this technique, the thermal conductivity was found to be dependent on the microstructural features of the devices, involving the mean grain size and phase composition. Among the phases existing in commercial ZnO-based varistors, Zn 7 Sb 2 O 12 and Bi 2 O 3 were found to contribute strongly to the thermal conductivity of the devices Elsevier B.V. All rights reserved. Keywords: ZnO; Varistors; Thermal conductivity 1. Introduction Varistors or nonohmic resistors are polycrystalline ceramics that display considerably strong dielectric behavior in the grain boundary region. The main application of these ceramics is based on their nonohmic electrical behavior, which allows for their use as devices to protect against overvoltage caused by handling and/or atmospheric upsurges. Commercial metallic oxide varistors (MOV), which are based on ZnO [1,2], possess a highly nonohmic property that can be evaluated by the results of the nonlinear coefficient, such as deviation from ohmic properties or the nonlinear relation between current and applied tension. Commercial samples are composed of ZnO and other metal oxides in small concentrations, e.g. Bi 2 O 3, CoO, Sb 2 O 3,Cr 2 O 3 and MnO [3 5]. During the sintering process, mainly as a result of the addition of Bi 2 O 3, different phases whose composition is very important to control the nonohmic properties segregate in the grain boundary region. Corresponding author. Tel.: ; fax: addresses: prbueno@dfm.ffclrp.usp.br, paulo@iris.ufscar.br (P.R. Bueno). The nonlinear feature of a varistor s voltage and current is temperature-dependent in the low current region. The resistance decreases as the temperature increases. The heat generated in the varistor blocks of a lightning rod due to the passage of leakage current causes the temperature to rise, thereby augmenting the leakage current and further increasing the temperature. When the voltage is moderate and the varistor block is in good conditions, the temperature will stabilize at a certain level at which the generated heat is equivalent to the dissipated heat. However, if the applied voltage exceeds the optimal level, or if the blocks are in any way impaired, the leakage current and temperature fail to reach a state of equilibrium, increasing progressively until the lightning rod undergoes thermal runaway. In other words, when a lightning rod is subjected to a handling or atmospheric upsurge, the elements absorb a large amount of energy and the temperature of the blocks rises almost instantaneously. This abrupt increase in temperature takes place while the lightning rod is continually subjected to the voltage, altering the balance between generated heat and dissipated heat and paving the way for a possible thermal runaway. Varistors should therefore be sufficiently stable from the thermal standpoint to ensure proper balance during the normal operation of a device. A good device should be able to /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.msea

2 378 C.M. Barrado et al. / Materials Science and Engineering A 371 (2004) absorb and dissipate excess energy in the event of an eventual upsurge, thus avoiding the thermal runaway effect. The work discussed here attempts to analyze the thermal behavior of commercial ZnO-based varistors by means of the pulsed laser technique [6] at temperatures ranging from ambient to 600 C and to find a correlation between temperature and microstructure. 2. Experimental procedure The varistor samples used here were obtained from three different commercial manufacturers and were dubbed C1, C2 and C3. These blocks were sliced into disk shapes having smaller dimensions than the original ones. Pure bismuth oxide and Zn 7 Sb 2 O 12 pellet-shaped samples were also prepared for purpose of comparison with the commercial varistor samples. The Zn 7 Sb 2 O 12 powders were prepared by the chemical Pechini [7] route which is based on ability of certain alpha-hydroxycarboxylic acids, such as citric acid, to form polybasic acid chelates with certain metallic ions. Several cation sources are employed, such as carbonates, citrates, oxides, alkoxides, nitrates or any other salt that assures purity, solubility in solution and easy elimination of the anion. The cation source when heated with a polydroxylic alcohol such as ethylene glycol, can undergo the polyesterification reaction. The combustion of this polymeric network produces an oxide with a highly controllable stoichiometry in which the cations are homogeneously distributed. The heat capacities of all the systems were calculated using a DSC-404C-Pegasus equipment. Sapphire was used as the reference and a STA409-NETSCH dilatometer was used to measure the thermal expansion of these systems. LFA-427 NETSCH equipment, which directly determines the value of thermal diffusivity, was used to analyze the thermal properties. This device consists of a laser source whose pulse impacts the face of the cylindrical sample, which is kept at a constant temperature. On the opposite face of the sample, the temperature, which increases over time, is measured by an infrared sensor triggered simultaneously with the laser beam. A mathematical analysis of the temperature versus time plot enables one to determine the thermal diffusivity, according to Eq. (1): α = I2 t 0.5, (1) where α is the thermal diffusivity expressed in cm 2 s 1, I the sample s thickness (in cm) and t 0.5 is the time in seconds corresponding to 50% of the increase in temperature measured on the opposite face of the sample. The varistors thermal conductivity as a function of the temperature was determined from room temperature to 600 C, based on Eq. (2): k = αρc p (2) where k is the thermal conductivity in W K 1 cm 1, α the aforementioned thermal diffusivity, ρ the material s density in g cm 3 and c p is the heat capacity in J g 1 K 1. Scanning electron microscopy (SEM/ZEISS DSM-Model 940 A) and EDS were used to analyze the materials microstructure. Silver contacts were used as electrodes for the electrical measurements. Current voltage measurements were taken using a High Voltage Measuring Unit (KEITH- LEY Model 237). 3. Results and discussion The relative densities given in Table 1 were calculated by the method of Archimedes [8] based on the theoretical densities of ZnO. As can be seen, the relative densities of ZnO commercial systems are very similar, although C3 shows the best result. Table 2 lists the nonohmic features obtained from the I V plots. The nonlinear coefficient values obtained by linear regression, starting from 1 ma cm 2 on a logarithmic scale, were the breakdown electric field obtained at this same current density. Fig. 1 illustrates the electric field versus current density curves. The characteristic behavior of varistor systems is well evidenced in the electric field against current density curves in Fig. 1, revealing a marked nonlinearity, mainly in the commercial system, C3, which presents the highest density value. The photomicrographs in Fig. 2a c show the typical and expected microstructure of multicomponent ZnO varistors [9,10]. As can be observed, the varistors consist of three different phases: a ZnO matrix doped with Co and Mn and the spinel (Zn 7 Sb 2 O 12 ) and bismuth oxide ( and Bi 2 O 3 ) phases, which are clearly visible in Fig. 3. These phases act as true barriers that form islands and are related directly to the nonohmic effects shown by the electric field versus current density curves in Fig. 1. Table 1 Relative density of the commercial and pure phases of the systems studied System Relative density (%) C1 96 C2 96 C3 99 Zn 7 Sb 2 O Bi 2 O 3 99 Table 2 Results obtained for the systems nonohmic features System Nonlinear coefficients, α C C C Breakdown electric field (V cm 1 )

3 C.M. Barrado et al. / Materials Science and Engineering A 371 (2004) J /ma.cm C1 C2 C E / V.cm -1 Fig. 1. Characteristic I V plots of commercial ZnO-based systems. Fig. 4 shows the values of thermal diffusivity as a function of temperature of all the commercial systems, of pure ZnO and of individual phases such as (Zn 7 Sb 2 O 12 ) and bismuth ( and Bi 2 O 3 ). Note that the thermal conductivity of zinc oxide decreases as its microstructure is modified through the addition of dopant during the varistor s manufacture. Zn 7 Sb 2 O 12 and Bi 2 O 3 phases display very low thermal conductivity. Therefore, the content of each phase is an extremely important parameter, which was determined using the Link Isis software program coupled with SEM. Table 3 shows the results of this analysis. As can be seen from Table 3, the amount of Zn 7 Sb 2 O 12 and Bi 2 O 3 phase in each system is a decisive factor for the mean free path traveled by the phonons, as indicated in Fig. 4. These phases act as thermal insulators due to their very low thermal conductivity in relation to the system as a whole. The commercial C2 sample presented the most unfavorable thermal behavior, possibly owing to its larger Zn 7 Sb 2 O 12 and Bi 2 O 3 content of and 3.29%, respectively. This may be a disadvantage, because the dopants concentrated in the grain boundary region, which is the site of the potential barrier, are responsible for the varistor s behavior. On the other hand, still with regard to the analysis of Fig. 4, it can be concluded that the commercial C3 system shows the best thermal conductivity at room temperature. However, as the temperature increases, the thermal conduc- Fig. 2. Photomicrographs of (a) commercial ZnO varistor sample C1; (b) commercial ZnO varistor sample C2, and (c) commercial ZnO varistor sample C3. Table 3 Content of the phases found in the commercial varistor samples studied Phase Phase content in commercial systems (%) C1 C2 C3 Zn 7 Sb 2 O Bi 2 O ZnO tivity reaches the same plateau of system C2, especially in the region of higher temperature. The C3 pattern as a function of temperature is similar to that of the pure ZnO phase and can be explained by its lower content of other phases such as Bi 2 O 3 (2.5%) compared with the other compositions studied here. This also means that the composition of the C3 sample is more similar to pure ZnO phase.

4 380 C.M. Barrado et al. / Materials Science and Engineering A 371 (2004) Fig. 3. EDS analysis showing the main phases and their shape in a typical microstructure of commercial ZnO samples. (a) EDS analysis of the region containing Zn 7 Sb 2 O 12 phase; (b) EDS analysis of ZnO grain phase; (c) EDS analysis of Bi 2 O 3 phases, and (d) typical SEM micrograph of ZnO varistors. Fig. 4. Thermal conductivities of systems as a function of temperature. of these devices. Thermal conductivity is a very important parameter to predict the degradation behavior and, in this study, we have taken the first step toward understanding the correlation between the microstructure and thermal dissipation capability of multi-junction polycrystalline ceramics. The Zn 7 Sb 2 O 12 and Bi 2 O 3 phases are crucial factors for the thermal conductivity of systems and their content and distribution in varistors probably act as heat conducting barriers. The technique employed to calculate thermal conductivity proved very easy, with the additional advantage of requiring small samples (12 mm diameter, 2.5 mm thick pellets), thus contributing to the viability of the technique, in which the lightning rod blocks are very small to enable them to be tested by other techniques such as hot wire, calorimeter or hot plate. 4. Conclusions Knowledge of the thermal conductivity of varistor devices is very useful to make inferences about the microstructure Acknowledgements This work was financially supported by the Brazilian research funding agencies CNPq and FAPESP/CEPID.

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