Effect of temperature on the electrophysical properties of Si–polymer composite varistors
Introduction
Varistors are a voltage dependent electroceramic passive components, whose current–voltage characteristics are Ohmic at pre-breakdown region while its resistance drops drastically at the breakdown region. This behavior makes them suitable to protect electronic devices and electric circuits against surges and over voltages. The best example of varistors is metal oxide varistors which are reputed for their highly nonlinear (I–V) characteristics.
Actually, the nonlinear characteristic of a varistor is controlled by potential barriers which are formed due to subjoining sufficient amount of additives. Many researchers have studied about different properties of varistors such as their microstructure, electronic properties and particle size, as well as the effect of changing ingredients and preparation method on their properties [1], [2], [3], [4], [5], [6], [7].
Likewise other metal oxide varistors, the nonlinearity of ZnO-based varistors is controlled by the electronic defects in polycrystalline zinc oxide ceramics, which are developed through the addition of different oxide additives, e.g., CoO, TiO2, Bi2O3, etc. There are two microstructure models which demonstrate the nonlinearity of ZnO-based varistors:
- (1)
Homojunction model which describes nonlinear properties of the varistors based on interaction between ZnO grains and band bending in grain boundaries because of depletions which occur during sintering process [8].
- (2)
Heterojunction model which describes discrete inter-granular layers which act as acceptors beside n-type ZnO grains. This provides a depletion region that causes electronic band bending at grain boundary interface [9].
Both models are based on oxygen vacancy at high sintering temperature that causes band bending in the grain boundaries. This is because the sintered ZnO ceramic is electrically n-type and twice ionization of Zn makes it the origin of intrinsic donors in ZnO. This can be specified as Frenkel defects (interstitial Zn), [10], [11], or Schottky defects (oxygen vacancy), [12], [13], that controls the electrical conductivity in zinc oxide ceramics [14].
According to the conduction theory of ZnO varistors, [15], [16], breakdown voltage of a varistor is dependents on the number of ZnO grains between two contacts and it is about 3–3.6 V/grain. On the other hand, there are two ways to lower the decrease breakdown voltage: (a) making thin varistors, (b) using some additives like TiO2 to grow the size of ZnO grains to reduce number of grains between contacts. But making low voltage ZnO-based varistors is not very interesting because of low permittivity of ZnO varistors to absorb sparks and low capability of thin varistors to absorb energy. Besides, using high percentage of TiO2 content restricts (I–V) characteristics and reduces nonlinear coefficient [17]. So, application of low voltage ZnO-based varistors is less than high voltage ones.
Sintering temperature is one of the most important parameters in making varistors, because approximately all properties of varistor are affected by temperature, [18]. Actually, finding the best sintering time interval and temperatures related to ingredients to hand in the best varistor performance. Although sintering temperature does not affect hexagonal structure of ZnO ceramics, but the average grain size, nonlinear coefficient and breakdown field strongly take effect from it, e.g. [19], [20].
Since the breakdown voltage of a varistor is highly sensitive to primary contents and preparation procedure, it seems that changing primary materials is a good strategy to manufacture low voltage varistors. To reach this goal, polymer-based varistors have been introduced by some researchers as well as our group [21], [22], [23], [24], [25], [26], [27], [28].
Our previous works were based on using polyaniline (PANI) instead of impurities such as Bi2O3 with both ZnO and GaAs binder which resulted in lower breakdown voltages (100 V for ZnO and 62 V for GaAs varistors). While sintering process of ZnO-based varistors has been fully investigated [17], [20], [33], [34], [35], [36], studying about sintering process of polymer-based varistors is still in its early stages.
Amongst conjugated polymers, PANI gets more attention because of its unique properties due to its various forms leading to different physical properties, [29], [30]. Conductivity of doped form (conductive form) of PANI, which is green in color, is about 1 S cm−1 [31]. This amount of conductivity is comparable with semiconductors’. By dehydrogenating it with ammonia solution, its conductivity reduces and approximately equals to 10−10 S cm−1 [31], [32]. This amount of conductivity introduces undoped PANI as a suitable choice to be used as matrix (inter-granular phase).
Sintering process is fully investigated on ZnO-based varistors and currently is being followed, [17], [20], [33], [34], [35], [36], but sintering process on polymer-based varistors requires more investigation to proceed.
In this paper, in continuation to our previous works, the effect of temperature on electrophysical properties, hysteresis and nonlinear coefficient of Si–polymer composite varistors are investigated. Also the effect of annealing on breakdown voltage and leakage current of the varistors are studied.
Section snippets
Experiment
To prepare the desired varistor, first, doped PANI was synthesized from aniline monomer according to reference, [37]. As mentioned before, because of high conductivity of doped PANI, it should be undoped to become proper for use in varistor structure as second phase which should have high resistivity. In order to get homogeneous undoped PANI, doped PANI was dehydrogenated using 2 M Ammonia solution for 8 h in atmosphere and dried at 70 °C for 96 h. During drying process, the product was ground
The I–V characteristic
(I–V) characteristic deliberation of the prepared Si-PANI–PE composite at 25 °C reveals that this composite has nonlinear behavior and can be classified as a varistor with breakdown voltage which is lower than that of similar ZnO-based varistors’ but whose leakage current is higher (Fig. 1). Its electrical resistance varies from 12.5 KΩ at pre-breakdown zone to about 68 Ω at upturn zone. Within these regions, leakage current increases about two hundred times.
Effect of annealing temperature and potential barrier
In ZnO-based varistors, nonlinear
Hysteresis
Fig. 9 shows hysteresis loops of unannealed sample at different temperatures. This figure shows that by increasing temperature, hysteresis loops tend to shrink. This result is completely natural by looking at the hysteresis mechanisms of varistors. There are two reasons to observe hysteresis on (I–V) diagrams which have effect on lifetime of a varistor: (a) Joule heating and (b) existence of dipole moments. The former is very important here because of lower band gap of Si that causes higher
Conclusions
A Si-PANI–PE composite varistor, with lower breakdown voltage, can be a suitable substitute for ZnO-based varistors for the purpose of protection from low overvoltage. As temperature increases, its breakdown voltage, nonlinearity coefficient and hysteresis decrease, whereas its leakage current increases. Furthermore, since the varistor has low degradation, its lifetime is longer.
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