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8UDC 537.635:621.3.03
RESEARCH OF THE DIELECTRIC PENETRATION (s), ELECTRICAL CONDUCTIVITY(a) AND SPECIFIC RESISTANCE(p) OF COMPOSITE VARISTORS BASED ON VARIOUS CERAMICS AND POLYMERS
AHADZADE SHAFAG MiRBABA
Leading researcher and associate professor of the Department «Physics and Techniques of High Voltages», Azerbaijan Republic Ministry of Science and Education Institute of Physics, Baku,
Azerbaijan.
NURUBEYLi TARANA KAMiL12
1. Leading researcher, associate professor and a head of the laboratory of the Department «Physics and Techniques of High Voltages», Azerbaijan Republic Ministry of Science and Education
Institute of Physics, Baku, Azerbaijan.
2. Associate Professor of the Department «Energy» Azerbaijan State Oil and Industry University,
Baku Azerbaijan.
iMANOVA ALMAZ YAGUB
Scientific secretary (PLP) and associate Professor and of the Department «Applied Mechanics», Azerbaijan State Marine Academy, Baku, Azerbaijan.
HASANOVA SABiNA iLHAM
Leading researcher and associate professor of the Department «Physics and Techniques of High Voltages», Azerbaijan Republic Ministry of Science and Education Institute of Physics, Baku,
Azerbaijan.
Annotation. In the article, dielectric penetration (s), electrical conductivity (a) and specific resistance (p) of composite varistors based on various ceramics and polymers was researched. It was revealed that the regardless of the type of ceramic phase (ZnO, Si, GaAs, InAs), the value of electrical conductivity increases in all composite varistors. The main reason for this is that the flow of electric current between two conductors is possible not only by their direct contact, but also by the presence of a thin dielectric layer between them. In this case, conductivity is caused by the tunneling of charge carriers through the potential barrier. It was also found that in composites made on the basis of various types of ceramics and polymers, the specific resistance decreases with the volume percentage of the filler, and the dielectric penetration of the composites increases, and this is related to the processes occurring at the interphase boundary.
Keywords: ceramica, composite, polymers, varistors, dielectric penetration, electrical conductivity, specific resistance.
1. INTRODUCTION
It is known that as electronics develops, the fields of application of electronic devices are
expanding. Practically every complex technical system consists of different types of electronic
devices. There is no technological process in which management is carried out without the use of
electronic devices. The effective and reliable operation of these devices is highly dependent on their
schematic design.
The main reason for the development of electronics is the continuous complexity of the
functions performed by electronic devices. At a certain stage, it becomes difficult to solve new
problems with the help of existing electronic tools and element base. Therefore, there is a need to
improve the current element base. The need to develop electronic devices on a new element base
arises due to the main factors such as increasing reliability, reducing dimensions, mass, cost and
power consumption, along with the complexity of functions [1, pp. 3043-3049].
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Microelectronics is a branch of electronics, a field dealing with development, research and application principles of integrated circuits of qualitatively new electronic devices. Semiconductor devices are the basis of modern electronic systems. The most widely used elements made of semiconductor materials are: semiconductor diodes, bipolar transistors, unipolar transistors (field transistors), controlled valves (thyristors), optoelectronic devices, operational amplifiers, analog and digital information systems [2, pp.408-417]. The main elements of electronic circuits are connections, switches, passive and active elements. As mentioned, passive elements include resistors, capacitors, coils and transformers, and active elements include electronic lamps, special types of diodes, transistors and microcircuits.
It should be noted that the protection of microelectronics, electronic devices and their functional elements from switching and lightning voltages is one of the important problems. Various types of varistors are used in the electronics industry all over the world to protect electrical networks and electronic devices from extreme electrical impulses of the desired type. For this purpose, ZnO-based ceramic varistors, which differ from other materials by a number of advantages, have found a wider field of application. It should be noted that recently, one of the promising directions for the development of protection devices and their elements is the creation of two- and multi-phase composite materials based on ceramic varistors. By preparing these composites on the basis of ceramic phase and polymers, it is possible to obtain composite varistors that are cheaper and of higher quality than those available so far in both high-voltage equipment and low-voltage devices of electric power[3, pp. 14350-14354].
The aim of the work is to study dielectric permeability (s), specific electrical conductivity (a) and resistances (p) of semiconductor-polymer composite varistors with different ceramic phases (ZnO, Si, GaAs, InAs). In the article, the dependence of the dielectric permeability (s), specific electrical conductivity (a) and specific resistance (p) of the composite varistor obtained on the basis of ZnO, Si, GaAs, InAs ceramics and various polymers on the volume percentage of the filler was experimentally determined.
2. EXPERIMENTAL RESULTS AND THEIR DISCUSSION
The initial stage of the process of obtaining composites is the preparation of a ceramic and polymer press. The initial stage of obtaining a mixture is crushing and mixing the components. Nowadays, a simpler and universal ceramic mixing method is used to mix the primary product (ceramic-polymer), and as a result, a homogeneous mixture of components is obtained. The mixing process is carried out dry in micromills. The diameter of the particles of the ceramic phase was changed from 60 цт to 350 цт, depending on the application. Semiconductor phase (ZnO, Si, GaAs, InAs) and polymer powders (polyethylene (PE), polypropylene (PP), (polyvinylidene fluoride (PVDF) intended for industrial purposes with high dielectric properties were taken as the polymer phase[4, pp. 17-23].
The individual components of the mixture were first dried in the form of powder at 413 K, and at 523 K ZnO, Si, GaAs, and InAs ceramics. The structure and electrophysical parameters of individual components of the press unit depend on the demand placed on the varistor to be made on its basis [5, pp. 22-29].
It should be noted that in order for any material to be a varistor, it is important to have regions in its physical structure with distinctly different electrophysical, thermal and physical properties - that is, crystalline and amorphous phases. If we consider composites with this basic requirement, we will see that polymer-ceramic and polymer-semiconductor composites meet this requirement. That is, the role of the crystalline phase is played by the ceramic or semiconductor, and the role of the amorphous phase is played by the polymer matrix. However, a large number of microscopic and structural studies show that it is the third phase that strongly influences the formation of the varistor effect in composite materials and its electrophysical properties, supramolecular structure, and electronic processes at the interphase boundary. This phase is formed at the polymer-semiconductor boundary[6, pp. 58-62].
It should be noted that obtaining a homogeneous mixture is of particular importance. Because interphase interactions are important in the formation of the varistor effect in the semiconductor-
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polymer composite. Therefore, the uniform distribution of the processes at the interphase boundary in the volume of the sample significantly affects its electrophysical parameters.
The study of electrical conductivity is one of the main methods used to determine the purity of materials, mainly metals and semiconductors. In addition, electrical conductivity allows to clarify the dynamics of current charge carriers in a macroscopic object, the characteristics of their interaction with each other and with other objects in the object.
When the mobility of charge carriers is very small, the mechanism of current passing through a thin dielectric layer is not related to contact events, but to events occurring in the dielectric. For samples received
I l a =--
U S
specific electrical conductivity through the formula,
1
P =
Q-1m-1
(1)
= —, Qm
a
(2)
and the values of specific resistance were determined by means of the formula.
Where, U- is the voltage applied to the composite (V), I- is the electric current corresponding to the applied voltage (A), l- is the thickness of the composite (m), 5-is the contact area (m2).
The values of electrical conductivity (a) and specific resistance (p) were calculated for the studied samples using formulas (1 and 2).
In figure 1 dependence of the electrical conductivity of the composite on the type of ceramics, figure 2 showes dependence of electrical conductivity on the electric field in the Poole-Frenkel coordinate[7, pp.88-91].
Figure1. Dependence of the electrical conductivity of the composite on the type of
ceramics(50%C+50%PP)
Figure 2. Dependence of electrical conductivity on the electric field in the Poole-Frenkel coordinate
(40%ZnO+60%PE)
From the pictures (figure 1 and 2), regardless of the type of ceramic phase (ZnO, Si, GaAs, InAs), the value of electrical conductivity increases in all composite varistors.
Based on numerous experimental results, it can be said that the conductivity of the composite is a function of the average number of contacts corresponding to one particle. In addition, based on the theory of electrical contacts, it can be said that the flow of electric current between two conductors is possible not only by their direct contact, but also by the presence of a thin dielectric layer between them. In this case, the conductivity is caused by tunneling of charge carriers through the potential barrier [8, pp.992-997].
The height of the potential barrier at the boundary of the phase separation is determined by the following formula:
<P
2 2 e ndr
2^0
(3)
where, e- is the electron charge, nd - is the concentration of donors, r- is the width of the potential barrier, Sk - is the dielectric permeability of the composite, and so - is the dielectric constant. The distance (b) between the ceramic particles in the polymer is calculated as follows [8, pp.992997].
b = d
(1 + Ф) 6Ф
л1/3
1
(4)
where O- is the volume fraction of the filler in the polymer, d- is the diameter of the particles of the dispersant (d=40 ^m).
We calculated the dependence of the dielectric permittivity and the distance between ceramic particles on the filler volume percentage for Si-polymer based composites using formula (4). The results of the report are given in table 1.
Table 1.
Dependence of the dielectric permittivity and the distance between Si particles (b) on the
filler volume percentage
PP PVDF
e (f=100 Hs) b, |im e (f=100 Hs) b, |im
10% Si + 90% polimer 8 31,69 12,95 31,69
15% Si+ 85% polimer 9,5 23,54 22 23,54
20% Si + 80% polimer 11 18,57 30 18,57
25% Si + 75% polimer 12 15,12 41 15,12
30% Si + 70% polimer 13 12,55 51.04 12,55
35% Si + 65% polimer 19 10,55 80 10,55
40% Si + 60% polimer 26 8,94 119,7 8,94
45% Si + 55% polimer 30 7,61 190 7,61
50% Si + 50% polimer 33 6,49 310,9 6,49
It can be seen from the table that as the volume percentage of the dispersant increases, the distance between the ceramic particles (b) decreases by approximately 4 times, which leads to a decrease in resistance and an increase in conductivity.
Figures 3-7 also show graphs of dependence of specific volume resistance of composites with different ceramic phase (ZnO, Si, GaAs, InAs) on the percentage of filler.
Figure 3. Dependence of the specific resistance of Si-PP based composites on the volume percentage of the dispersant (U=60V), 1; 2 are conditional distributions.
Figure 4. Dependence of the specific resistance of Si-PVDF based composites on the volume percentage of the dispersant (U=60V), 1; 2 are conditional distributions.
Figure 5. Dependence of the specific resistance on the volume percentage of the GaAs-based disperser( U=100V), 1; 2 are conditional distributions.
Figure 6. Dependence of the specific resistance on the volume percentage of the InAs-based disperser ( U=30V), 1; 2 are conditional distributions.
Figure 7. Dependence of the specific resistance of ZnO-PE based composites on the volume percentage of the dispersant (U=60V), 1; 2; 3 are conditional distributions.
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Depending on the volume percentage of the filler in the ZnO-Pe composite, the change in specific resistance can be conventionally divided into 3 parts (figure 7). As the filler volume percentage increases in the first part, the value of the specific resistance (p) decreases sharply (up to C=30-40%), and remains almost constant in the second part (up to C=40-60%). In the third part, p increases sharply when the volume percentage of ceramics is >60%.
The obtained experimental results are explained by the mechanism of transfer of current charge carriers in polycrystalline structures. So that:
1) Tunneling of charge carriers through the potential barrier created by the dielectric-polymer layer between the filler and polymer particles;
2) Emission of electric charge carriers from the barrier existing between polymer and ceramic particles;
3) transfer of electric charge carriers along the chain formed as a result of direct contact of filler particles in the composite.
It is known that the electrical conductivity of composites is a function of the average number of contacts per particle. We know that electric current can be transferred between two conductors not only when they are in direct contact, but also when there is a thin dielectric layer between them. In this case, conduction occurs due to the passage of electric charges through the potential barrier. This is due to the fact that during the tunneling effect, electric charge carriers whose energy is smaller than the height of the potential barrier have a non-zero probability of crossing the barrier. Also, the tunnel resistance is exponentially dependent on the width of the potential barrier[9, pp. 150-152; 10, pp.7074].
Based on the above, the dependence of the specific resistance of the composite on the volume percentage of the filler can be explained as follows. When the volume percentage of the filler in the composite is low, the average number of contacts per ceramic particle is low, so the conductivity of the composite is determined by the hopping mechanism on local levels. In other words, due to the thickness of the polymer layer between the ceramic particles, it is unlikely that the electric charge carriers will be transferred by the tunneling mechanism, and the role of the tunneling conductivity in the first part of the dependence p=f(C) is very small. Therefore, the resistance of the composite is mainly determined by the resistance of its polymer phase. As the volume percentage of the ceramic phase increases, on the one hand, the average number of interparticle contacts increases, and on the other hand, the width of the potential barrier decreases, and the probability that electric charge carriers can cross the potential barrier increases. As a result, the conductivity of the composite increases and its resistance decreases accordingly. The increase in resistance with the increase in the volume percentage of the ceramic phase of the composite based on ZnO ceramics leads to the formation of a circuit consisting of particles, and therefore the conductivity of the composite is determined by the conductivity of the particles of the filler, and its resistance is determined by the resistance of its polymer phase. As the volume percentage of the ceramic phase increases, on the one hand, the average number of interparticle contacts increases, and on the other hand, the width of the potential barrier decreases, and the probability that electric charge carriers can cross the potential barrier increases. As a result, the conductivity of the composite increases and accordingly, its resistance decreases [11, pp. 8-12; 12, pp.18-21].
Note that the value of the potential barrier ф is influenced by both the width of the potential hole formed at the border of the polymer layer and the disperser, as well as the change in the dielectric penetration (e). Let's consider this experimentally. The results of the experiment are shown in figures 8- 12.
Figure 8. Dependence of the theoretical and experimental values of the dielectric penetration of Si-
PP based composites on the volume percentage of Si. 1- theoretical;2- experimental (100 Hs);3- experimental (1kHs)
Figure 9. Dependence of the theoretical and experimental values of the dielectric penetration of Si-PVDF based composites on the volume percentage of Si.
1- theoretical;2- experimental (100 Hs);3- experimental (1kHs)
23-
From the comparison of figure 8 and 9, it can be seen that the dielectric penetration of the composites increases as the filler volume percentage increases. In addition, the value of dielectric constant in polar (PVDF) matrix composites is 1,5 times higher than that of non-polar (PP) matrix composites at small filler volume percentages, and increases up to approximately 8 times with increasing filler volume percentage (figure 8). Most likely, one of the reasons for this is the presence of primary dipoles in polar polymers relative to non-polar polymers.
Taking into account the above, it can be said that the main reason for the observed change in the height of the potential barrier in polymer-based composites is the high value of the dielectric penetration due to the presence of initial dipole moments in polar polymers.
Figure 10. Dependence of dielectric penetration on volume percentage of InAs-based dispersant. 1-theoretical;2- experimental (1 kHs);3- experimental (100 Hs)
Figure 11. Dependence of dielectric penetration on volume percentage of GaAs-based dispersant. 1-theoretical;2- experimental (1 kHs);3- experimental (100 Hs)
As can be seen from the figures 10 and 11, there is a gap between the theory and practice of dielectric loss. Thus, this difference will increase as the percentage of the dispersant increases. This difference is related to the fact that according to the Maxwell-Wagner model, a 2-phase system consisting of a matrix with dielectric constant sx and a disperser with dielectric constant s2 is
considered from the point of view of the static distribution of the dispersant and the influence of the polarization of the polymer layer is not taken into account [13, pp. 2871-2875; 14, pp.992-997].
. Therefore, the experimental value of the dielectric penetration of the composite is greater than the value calculated according to the Maxwell-Wagner formula.
2(l -Q)g, +(1 + 2Q)g2
(2 + Ф>1 -(l -Ф>2
Q _ scs0S = d
(3)
(4)
^c =
Here sx - the dielectric constant of the polymer, s2 - the dielectric constant of the dispersant; Ф -volume percent of dispersant (%); C-capacity of the composite (pF); S- contact area (mm2); d-the thickness of the composite (prn); s0 - electric constant; sc - is the dielectric penetration of the
composite.
Figure12. The dependence of the dielectric penetration of the volume content of the filler (in the ZnO-PE composite) (1 - not processed sample, 2- sample processed in an electric discharge for 3 minutes;, 3 - sample processed in an electric discharge for 10 minutes).
In figure 12 shows the dependence of the dielectric penetration on the volume content of Zinc oxide for samples treated under the action of electric discharge plasma. It can be seen that for all volumetric contents, the treatment with electric discharge plasma leads to a notice able increase in. This parameter increases with increasing duration of the discharge. The obtained experimental results show that the size of the particles of the inorganic phase significantly affects the CVC of the composite varistor. By at certain (constant) thickness of the composite varistor, the operation voltage decreases markedly, and the nonlinearity coefficient increases. Numerous experimental results obtained by us show that the impact of electric discharge plasma on the polymer- zinc oxide composite leads to a significant change in the permittivity and the concentration of local levels at the interface of the composite [15, pp.72-76 ; 16, pp. 166-171].
The observed decrease in the dielectric penetration of the composite can be explained as follows. Firstly, the dielectric constant of the semiconductor is higher than the dielectric penetration of the polymer. Therefore, the impact of the semiconductor on the dielectric penetration of composites is greater than that of polymers. Secondly, with the increase of the volume percentage of the semiconductor, its particles are closer to each other and the thickness of the polymer layer between the semiconductor particles decreases. This, in turn, leads to the formation of local levels in these layers, or rather, to the polarization of this layer, which, accordingly, leads to an increase in the dielectric penetration of the composite.
3. CONCLUSIONS
It should be noted that the electrical properties of many polycrystalline semiconductors and semiconductor ceramics are determined by the potential difference created at the crystallite boundary. Because the electrical properties are determined by the Schottky-type double potential coefficient between crystallites, it is of great importance from the point of view of studying the processes taking place at the boundary between crystallites at the microscopic level under the influence of an electric field.
From the conducted experiments, it was found that in composites made on the basis of polymer-ceramics, with an increase in the volume percentage of ceramics, its particles get closer to each other, and the thickness of the polymer layer between the semiconducting particles decreases.
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This, in turn, leads to the creation of liquid levels in these layers, that is, to the polarization of this layer. This causes the electrophysical parameters of the composites to change. As a result, the specific volume resistance of composites decreases, conductivity and dielectric penetration increase.
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