Optimization of technological processes in the manufacturability of varistors based on recycled ZnO product, with emphasis on environmental sustainability

Abstract

The main objective described in this paper is the optimization of technological parameters in the production of varistors based on recycled zinc oxide (ZnO). The content of this paper builds on our previous research aimed at proving that hydrometallurgically recovered ZnO material, from electric arc furnace dust, is suitable for applications in these semiconductors, an issue which has received very little attention at present. The focus of this research also corresponds to and supports the visions and strategies of environmental sustainability and the circular economy. Samples of ZnO varistors manufactured for this purpose were analyzed by scanning electron microscopy and then the electrical parameters were measured and calculated. Based on the results of the microstructural analysis and electrical properties, prerequisites for the optimal adjustment of technological parameters such as sintering temperature and sintering time were derived, which will be the starting point for further research.

Graphical abstract

Highlights

• •Manufacture of varistors from recycled zinc oxide (ZnO) product.
• •Optimization of technological parameters such as sintering temperature and sintering time.
• •Characterization of electrical properties of recycled ZnO varistors.
• •Towards the recovery and reuse of industrially significant materials.

1. Introduction

Zinc oxide (ZnO) is a special multifunctional material whose electro-physical properties can be relatively easily tuned [1]. Therefore, ZnO has found extensive applications in the electrotechnical industry in the production of varistors used in surge protectors for various electronic systems, primarily due to its highly nonlinear current-voltage characteristics [2]. ZnO varistors provide surge protection for appliances and systems with nominal voltages ranging from a few volts to several thousand volts [3]. Due to the presence of insulating regions at grain boundaries, a significant amount of free charges is trapped, leading to the formation of potential barriers where critical voltages in grain boundaries are generated. The overall breakdown voltage is then proportional to the number of such grain boundaries between two electrodes [4].

Significant attention has been devoted to technological procedures and innovations in the production of ZnO varistors. This is evident from several studies conducted over the past 20 years. The main processing operations in the production of ZnO varistors involve the formation of powders from multi-element oxides and subsequent pressing (formation) of a predominantly cylindrical-shaped compact body. Emphasis is mainly on the physical and chemical properties of metal oxides to create a varistor component with optimal structure and electrical parameters. Various parameters are observed and studied in the production technologies of ZnO (various types) varistors, such as the varistor preparation method, sintering temperature and time, grain size, structure, chemical composition, and the quantitative-qualitative impact of dopants on the electrical properties of varistors.

Sintering is a process in which the interparticle pores in a granular material are eliminated by atomic diffusion driven by capillary forces. It is the preferred method of producing industrial ceramics [5]. The optimization of thermal processes and the study of the influence of sintering temperature in the processing of various types of ZnO varistors have been investigated in studies by Duran et al. [6], Choon-W. Nahm, [7], [8], H. Bidadi et al. [9], Leach et al. [10], Roy et al. [11], Simo et al. [12], and Pengkang Xie et al. [13]. The methods and parameters of two-step sintering and their effects on the microstructure and electrical properties of ZnO varistors have been studied in several published sources, [5], [14], [15], [16]. Progressive methods in this field include rapid microwave sintering by Egorov et al. [17], the preparation of thin-film varistors using the Sol-gel method, offering advantages in composition regulation, microstructure, and functional properties [18], [19], [20], research involving ceramic quenching at different stages of the sintering process to distinguish critical transformations, microstructure evolution, electrical properties and double Schottky barrier defects during the sintering process of ZnO varistor [13], or the step sintering technology for densification of nanocrystalline ZnO [21]. Authors Kelleher and Hashmi also focused on the impact of vibrational milling using cylindrical zirconia media on the physical and chemical properties of commercially produced ZnO varistors [22].

ZnO is characterized by n-type conductivity, where charge carriers are moving electrons in the conduction band. The basic characteristic of ZnO varistors is the nonlinear dependence of the interlayer resistance on the applied voltage [23], [24], [25]. The I-V characteristic and the nonlinearity coefficient [26] are used to define the basic operation of a ZnO varistor in an electrical circuit.

Doping ZnO with other metal oxides (e.g. Bi, Co, Mn, Ba, Pr, Sb, Ni, Si, Al) improves their electrical properties, making ZnO the dominant varistor material, which has gradually replaced SiC-based varistors. Scientific works have pointed out that doping ZnO varistors with trivalent ions of some oxides can increase the conductivity of ZnO grains and reduce the residual voltage of [27]. In addition, some metal oxides and the sample preparation method have a significant effect on the varistor characteristics of ZnO [28]. Dopant addition methods, namely dry mixing method of elemental oxide admixtures and mixing ZnO powder with admixtures in solution and then drying it, have been studied in a paper by Banerjee et al. [29]. The impact of Na₂CO₃ addition on the microstructure and electrical properties of ZnO varistor ceramics, synthesized via conventional solid-state sintering at a low temperature of 900 °C is reported in [30]. There are many researches focused on the influence of metal oxide doping on the microstructure and electrical properties of ZnO-based varistors, such as Eu₂O₃ in [31], Co₂O₃ in [32], Co₃O₄ in [33], Sc₂O₃ in [34], Ho₂O₃ in [35], NiO in [36], SiO₂ in [37], Er₂O₃ in [38] and many other scientific papers.

From the analysis of the available literature, it can be stated that very little attention and research is devoted to the application use of recycled ZnO products. A few exceptions are contributions by Monteiro et al. [39], in which the authors investigated the use of ZnO-rich waste dust coming from the brass melting industrial process in the field of optoelectronics and photonics. Authors Shangguan et al. [40], investigated the possibility of recycling Zn-C batteries and the subsequent use of Zn and Mn for the production of cathodes for zinc-ion accumulators. Ferreira et al. [41] described the use of industrial waste containing ZnO for the production of UV sensors from ZnO nanotubes. Another source partially corresponding to the presented contribution is the patent from 2004 [42], which describes a method of regenerating defective and waste varistor ceramics and enabling the reuse of regenerated ZnO powder in its pure form and also as an additive to the production mixture prepared from pure oxides.

In our previous research [43], we have established the suitability of using ZnO, obtained by hydrometallurgical recycling of electric arc furnace dust, for the production of varistor semiconductor components. This method of ZnO material reuse is also registered as a utility model [44] by the Patent and Industrial Property Office of the Slovak Republic. The aim of the research in this contribution is to find the optimal technological parameters in the manufacture to achieve the necessary electrical properties of such ZnO varistors.

2. Materials and methods

2.1. Prologue and raw material

The input material is the output of hydrometallurgical recycling process of EAF dust, and its in the form of ZnO powder with impurities (Mg, Si, Fe, Ca, K), which was developed at domestic institute. The hydrometallurgical treatment consisted of the following steps:

• •neutral leaching in H₂O,
• •alkaline leaching in (NH₄)₂CO₃,
• •multiple-stage pressure filtration,
• •cementation with Zn dust,
• •calcination.

This process is still under development, it also includes a variant of acid leaching and ZnFe₂O₄ treatment. Approximately 96% - 97% of the intermediate product consisted of ZnO, with the remaining 3% - 4% being impurities. The impurity level is approximately similar to the level of functional dopants found in commercially produced ZnO-based varistors. This ZnO material was used without the addition of further dopants.

The concentration and type of these impurities are influenced by the different input batches of electric arc furnace (EAF) dust from various suppliers. The hydrometallurgical processing technology for industrial EAF dust containing ZnO is performed in a semi-industrial facility at the Institute of Recycling Technologies. The regulation of impurities is ensured by maintaining consistent operating parameters and processing conditions, specifically the concentration of ammonium carbonate solution, constant stirring speed, pH of the solution, and the amount of cementing agent. The quality and quantity of impurities are subsequently analyzed after the hydrometallurgical processing and obtaining recycled ZnO using XRF, SEM, and EDX analysis to provide information about the chemical composition.

2.2. Powder and microstructure characterization

The morphology and size of the starting powder material were determined by SEM. The elemental chemical composition of the materials was determined through energy-dispersive X-ray spectroscopy (EDX), see Fig. 1. EDX mapping of the starting powder is depicted in Fig. 2. Specifically, SEM observations were performed on a TESCAN MIRA 3 FE and EDX analyses were performed on an OXFORD INSTRUMENTS and Philips X'Pert PRO MRD (Co-Kα) diffractometer. Analysis and characterization of the surface of the manufactured varistor samples were carried out using a scanning electron microscope (Jeol, Japan).

Figure 1.

SEM photo and EDX spectra of the starting powder material.

Figure 2.

EDX mapping of the starting powder. The distribution of all elements (left), individual distribution: zinc, magnesium, silicon (top), oxygen, sulfur, calcium (bottom).

The average grain size (see Table 1) on the varistor surfaces was determined using standard statistical methods. The grain size was revealed by micrographic examination of surfaces by evaluation of interceptions on measured straight test lines applied randomly at various surface locations.

Table 1.

Manufacture and microstructural parameters of ZnO varistor samples.

Samples	Sintering temperature	Sintering time	Grain size
	T [°C]	t [min]	d [μm]
Var 1	–	–	4.48
Var 2	600	30	3.96
Var 3	800	30	3.78
Var 4	1000	30	3.82
Var 5	1000	60	3.45
Var 6	1000	90	3.52
Var 7	1000	120	3.49
Var 8	1200	30	6.00

2.3. Procedure for the manufacture of ZnO varistor samples

ZnO varistors are commonly produced by conventional ceramic technology. The preparation process of ZnO varistors is basically the same as for general ceramics, whose primary processes include raw materials preparation, mixing, stoving and sieving, pre-sintering, smashing, prilling, molding, sintering, silvering, and testing. The preparation begins with weighing, mixing, andmilling of oxide powders in ball mills [45], [46]. In the whole process, raw materials preparation using various powder metallurgy techniques that involve pressing powders and sintering are the two key steps that determine the electrical properties of ZnO varistors [47], [48]. The solid-state method (mechanical homogenization/milling of powders) is a classical method applied on a laboratory scale and in the industry [49]. Finally, electrodes are prepared on both surfaces. The best electrodes are painted In–Ga alloys or evaporated Al films. Conventional silver electrodes painted and fired at 400 °C – 800 °C are also used [50]. The solid-state-based ceramic processing route still remains the preferred method of manufacturing because of the simplicity, cost, and availability of the metal oxide additives. A major disadvantage of this route is the difficulty in obtaining additive homogeneity at a microscopic level, which is especially important for the manufacture of miniaturized electronic equipment [49].

The procedure for the manufacture of our ZnO varistor samples consists of several steps and is depicted in the flowchart on Fig. 3. The recycled ZnO material was milled in a vibratory disc mill to a grit size below 50 μm. Then, approximately 1 g of the milled material, was pressed using a hand press at a pressure of 7 MPa for every individual sample. This results in pressed discs with a diameter of ≈12 mm and a thickness of ≈1.5 mm. Pressed discs were later sintered in a muffle furnace LAC LH 15/13 with Ht40AL regulator at various temperatures and sintering times. After sintering, samples were left to cool down to a room temperature. The final step in the production of the varistor samples was the application of silver conductive paste and re-sintering samples at 400 °C. The parameters for the fabrication of ZnO varistor samples were selected based on assumptions and knowledge obtained by studying the scientific and technical literature. Table 1 shows selected ZnO varistor samples, technological parameters during their manufacture, as well as average grain sizes of samples measured by the linear method of intersection. Example of a pressed sample in the form of a tablet, after sintering with applied conductive silver paste, is shown in Fig. 4a).

Figure 3.

The schematic procedure for the manufacture of ZnO varistor samples.

Figure 4.

a) Manufactured ZnO varistor samples; b) Keithley's 2410 Source Measure Unit.

2.4. Electrical measurements

The V-I characteristics were measured using Keithley's 2410 Source Measure Unit (see Fig. 4b)) in the Sweep mode for a DC voltage source with a current limit of 20 mA.

Measurements of V-I characteristics were automated and computer controlled using a Python coded program. The principle of measurement was to increase the voltage by 1 V and measure the magnitude of the current flowing through the sample. Before the measurement, the varistor is inserted between the electrodes and after entering the command through the PC, the measurement is started. All samples were measured in the laboratory at room temperature. The measurement is still stopped when either 1000 V or 20 mA is reached. From the measured V-I characteristics, the values of the breakdown voltage V_br, the nonlinearity coefficient α and the leakage current I_L at an electric current density of 0.5 mA cm⁻² are calculated. The values of the nonlinearity coefficient are calculated according to the equation (1), where V₁ and V₂ are the voltages corresponding to the currents 0.1 mA and 1 mA, respectively. The breakdown voltage corresponds to the voltage value at current 1 mA. The magnitude of the leakage current I_L is calculated as the current corresponding to the voltage value of 0.8 V_br.

$$\alpha =\frac{1}{\log (V_2)-\log (V_1)}$$ (1)

3. Results and discussion

3.1. Microstructural analysis

ZnO varistor is fabricated by the traditional ceramic sintering technology, and the temperature control of the sintering process has a critical influence on the performances of the final products. During the warm-up and constant-temperature periods several chemical phase transformations take place, which result in a complete rearrangement of the microscopic particles and the creation of a dense polycrystalline matrix of ZnO grains and other phases, which are incorporated therein [45]. The characteristics of ZnO varistor ceramics are closely related to their microstructure, which is characterized by the following parameters [51]:

• •ZnO grain size and the grain size distribution,
• •grain boundaries,
• •secondary phases (spinel-type phase, pyrochlore-type phase),
• •distribution of secondary phases along the grain boundaries,
• •size and distribution of pores.

Comparison of the surface character of the analyzed samples of VAR1, VAR2, VAR3, VAR4, VAR5, VAR6, VAR7 and VAR8 (Fig. 5) documented by scanning electron microscopy shows that the surface exhibited a fine relief with a relatively low degree of porosity. The surface of sample VAR16 (unsintered sample) was composed of grains or shapes with irregular morphology, showing indications of forming globular to polyhedral grains with an average size of 4.5 μm.

Figure 5.

Microstructure and parameters of ZnO varistor samples.

Subsequent processing by sintering at temperatures ranging from 600 °C to 1200 °C for 30 min, gradual changes in the surface character of the analysed ZnO varistor samples were observed. The surfaces of VAR2 (600 °C/30 min), VAR3 (800 °C/30 min), and VAR4 (1000 °C/30 min) exhibited very similar character with a fine relief and a relatively low degree of porosity, similar to the VAR1 sample (without sintering). The average size of irregular shapes, formed into globular to polyhedral grains, reached around 4 μm. In the case of VAR4 (1000 °C/30 min), a lower surface relief was observed compared to samples sintered at lower temperatures (VAR2 and VAR3). For VAR8 (1200 °C/30 min), sintered at the highest temperature, a significant change in the surface character of the analyzed varistor sample was observed. This change has an influence by the formation of grains respectively shapes of granular polyhedral morphology with a relatively low degree of porosity. The average size of these grains was around 6 μm.

From the comparison of the surfaces of varistor samples sintered at 1000 °C for 30 to 120 min, it can be inferred that there was no significant change in the character of their surfaces. The surfaces of VAR4 (1000 °C/30 min), VAR5 (1000 °C/60 min), VAR6 (1000 °C/90 min), and VAR7 (1000 °C/120 min) exhibited a very similar character with fine relief and relatively low degree of porosity. In the case of the sample with the longest sintering time (120 min, VAR7), showed a higher proportion of pores compared to samples with shorter sintering time. The surface of these samples consisted of irregular formations shaped into globular grains, with average size up to 4 μm.

The evaluation of the surface character of the manufactured and processed ZnO varistors samples results that the sintering temperature has a significant influence on their structure, especially at 1200 °C, where there is significant the formation of polyhedral grains from the irregular formations, compared to at lower temperatures (600 °C, 800 °C and 1000 °C). The sintering time at the sintering temperature did not have a significant effect on the surface character of the analyzed varistor samples. However, at 1000 °C with sintering time of 120 min, higher porosity was observed.

Theoretically, it was expected that the grain size would increase with rising sintering temperature and time. The used ZnO material with impurities behaves stably up to a sintering temperature of 1000 °C, and we believe that this is related to its material characteristics. The coalescence and growth of grains only occurs at temperatures higher than 1000 °C.

3.2. Electrical measurement results

To determine the electrical characteristics of eight varistor samples, the parameters used to characterize ZnO multicomposite polycrystalline sintered ceramics were measured, namely: voltage-current (V-I) characteristic and dielectric properties. From the measured dependences (Fig. 6), a nonlinear characteristic typical of voltage-dependent resistors - varistors is evident, which is clear indication of the non-ohmic properties of the examined varistors.

Figure 6.

V-I characteristics of ZnO varistor samples.

The breakdown voltage values (V_br) range from 101 V to 316 V. In practice, it is possible to vary electrical parameters such as breakdown voltage by changing various technological and manufacturing parameters, such as sintering temperature and sintering time, as shown in Figure 7, Figure 8.

Figure 7.

Effect of sintering temperature on electrical properties of ZnO varistor samples: a) nonlinear coefficient α; b) breakdown voltage V_br; c) leakage current I_L.

Figure 8.

Effect of sintering time on electrical properties of ZnO varistor samples: a) nonlinear coefficient α; b) breakdown voltage V_br; c) leakage current I_L.

Sources [7], [8] show that increasing the sintering temperature up to 950 °C results in an grain size increase and nonlinearity coefficient, while simultaneously decreasing structural density. When increasing the sintering temperatures from 950 °C to 1100 °C, it is observed the thickening of the grain structure decreases the breakdown voltage and the nonlinear coefficient [11]. In the examined samples VAR1-8, the similar dependence on the increase in grain size was observed. With the increase in sintering temperature from 600 °C to 1200 °C, there was an increase in grain size from 3.96 μm to 6.00 μm. However, the nonlinear coefficient indicated the opposite trend compared to Roy et al. [11], as it increased from 1.43 to 2.09 with increasing sintering temperature. The sintering temperature of 1200 °C (Fig. 7a), c)) proved to be the most effective in terms of achieving the highest nonlinearity coefficient α = 2.09 and the lowest leakage current I_L = 0.518 mA.

The effect of sintering time on the electrical properties of the samples was observed in the investigated VAR 4 - 7 samples. In Fig. 7a), c), it can be seen that the highest nonlinearity coefficient α = 1.64 and the lowest leakage current I_L = 0.611 mA are achieved with sintering time of 120 min. However, by sintering time of 30 min, there was slight increase in the nonlinearity coefficient α from 1.43 to 1.64 and decrease in the leakage current I_L from 0.645 mA to 0.611 mA. These observations create the potential for experimentation to achieve the desired properties with shorter sintering times. Breakdown voltage can be varied depending on the sintering temperature and sintering time, as depicted in Fig. 7b), 8b). These results indicating the possibility of tuning the desired V_br for various voltage applications. Table 2 summarizes the measured and calculated electrical parameters of the examined samples.

Table 2.

Electrical parameters of ZnO varistor samples.

Samples	Non-linear coefficient	Breakdown voltage	Leakage current
	α	Vbr [V]	IL [mA]
Var 1	–	–	–
Var 2	–	–	–
Var 3	1.43	316	0.573
Var 4	1.56	131	0.645
Var 5	1.47	115	0.669
Var 6	1.29	101	0.704
Var 7	1.64	162	0.611
Var 8	2.09	307	0.518

In the study by Leach et al. [10], dependence of decreasing nonlinearity coefficient when increasing the sintering temperature from 1000 °C to 1300 °C was observed by doping (Bi and Mn). In general, any change in the ratio of electrically active grain boundaries is related to some other aspect of the interface structure. Another example is the stabilization of switching field strength of ZnO-V₂O₅ based varistors, which was caused by the reduction of grain size and homogenization of the structure with the addition of PrMnO₃ dopant [52].

4. Conclusions

In this article, attention is devoted to recycled ZnO varistor ceramics, with a focus on the processes of optimized sintering to improve microstructure and tune electrical properties. Several observations and conclusions arise from the results of this research, specifically:

• •For sintered samples, an increase in grain diameter is observed from 3.96 μm to 6.0 μm with sintering temperature ranging from 600 °C to 1200 °C. This indicates that the grain size of zinc oxide increases with the rising sintering temperature.
• •From the measured electrical parameters of samples sintered from 800 °C to 1200 °C (VAR3 - VAR8), a nonlinear dependence typical for ZnO-based varistors is evident, clearly demonstrating the varistor effect.
• •A sintering temperature of 1200 °C has proven to be the most effective, yielding the highest nonlinearity coefficient α = 2.09 and the lowest leakage current I_L = 0.518 mA.

Wastes containing zinc, especially industrial ones like ashes, dust, slags, etc., constitute a significant source of secondary zinc. Zinc recycling deserves increasing attention not only from a material profit perspective but also because specific wastes, such as electric arc furnace dust, pose potential environmental risks. When melting steel mill wastes in electric arc furnaces, approximately 10 kg - 20 kg of dust is generated per ton of produced steel, with the average zinc content in EAF dust being 22%, several times higher than in primary zinc ores.

Our previous research [43] highlighted the suitability of using recycled ZnO in potential ZnO-based varistor production. The novelty compared to the past research in this paper is focused on the study of the optimization of technological parameters for the production of varistors, based on recycled ZnO, such as sintering temperature and sintering time. At the end of the paper, we also present partial results focusing on the doping of varistor samples with Mn and Bi admixtures. The application of recycled ZnO in the electrical industry can contribute to future strategies:

• •Supporting a circular economy and sustainable development.
• •Reducing environmental burdens associated with limiting the landfilling of industrial wastes containing Zn.
• •Conserving primary Zn raw materials and thereby reducing environmental impact associated with their extraction.

This research is ongoing and the effect of dopants as well as the combination of mixtures of commercially produced and recycled ZnO in different ratios will be further investigated in more detail.

5. Future challenges

In this study, the effect of Mn and Bi doping on the microstructure, electrical nonlinear properties, and stability to sintering temperature changes was not investigated. In our cases, an increase in the nonlinearity coefficient depending the increased sintering temperature is observed, even without doping with selected elements. However, future plans include exploring this aspect, leaning on information in the study by Cui et al. [53], where the authors emphasize the need for further research on the effect of dopants (especially Mn and Bi) on ZnO-based varistor ceramics sintered at 900 °C to ensure environmentally friendly varistor production. Additionally, studies by the authors [54], [55] confirm that Mn cations at grain boundaries can strengthen the in-situ Schottky barrier and electrical nonlinearity.

Several samples were continuously produced with additives Bi₂O₃ and MnO₂, specifically:

• •RZnO+MnO₂ – a mixture of 98.5 wt% recycled ZnO and 1.5 wt% MnO₂,
• •RZnO+Bi₂O₃, MnO₂ – a mixture of 97 wt% recycled ZnO, 1.5 wt% MnO₂ and 1.5 wt% Bi₂O₃,
• •RZnO+Bi₂O₃ – a mixture of 98.5 wt% recycled ZnO and 1.5 wt% Bi₂O₃.

The results show that the nonlinearity coefficient in this case increased to a value of 2.75 compared to the original best value of 2.09 for the ZnO varistor sample without dopants (see Fig. 9). This result can be considered as progress in the right direction, however, further research will be needed to optimally fine-tune the recycled ZnO-based varistors by increasing the content of Bi₂O₃ and MnO₂ in the range of 3 wt% to 5 wt% (approximately 0.5 mol % to 1 mol %).

Figure 9.

Effect of dopants on nonlinear coefficient α of ZnO varistor samples.

Preliminary results with these dopants, which we are still analyzing, show improvement in the values of the leakage current. We have moved from mA level to a level of approximately 100 μA. In the future, our goal is to achieve a leakage current below 10 μA. After optimizing the nonlinearity coefficient and the leakage current, in the next step, we will test the ZnO varistor samples with current impulses to determine the impulse response and stability of the varistor.

As noted in the article by Simo et al. [12], that there is currently no single defined method for the production of ZnO varistors that would guarantee an optimal set of its technical parameters. Therefore, from our point of view, it is necessary to continue to study the physicochemical processes and their dependence on the formation of crystalline structures of varistors. A controlling the grain growth at various sintering temperatures still remains a challenge for research in the field of varistor ceramics, since on the basis of previous scientific and professional studies, empiricism is clearly evident in this area. Research and experimental activities in this field generally focus on the investigation of the optimization of key parameters of the production process (such as sintering temperature, pressing pressure, sintering time) as well as the concentration of the chemical composition (dopant control) of the product for predefined applications.

Sintering to full density with sub-micrometer or nanometer grain size appears as another experimental-research effort for tuning and improving electrical properties, from the point of view of future challenges.

CRediT authorship contribution statement

Pavol Liptai: Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Šimon Nagy: Writing – review & editing, Visualization, Software, Resources, Project administration, Investigation, Formal analysis, Data curation. Bystrík Dolník: Validation, Supervision, Methodology, Investigation. Miloš Matvija: Validation, Methodology, Investigation, Data curation. Jana Pirošková: Project administration, Funding acquisition, Formal analysis.

Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jana Piroskova reports financial support was provided by Ministry of Education, Science, Research and Sport of the Slovak Republic. Pavol Liptai reports financial support was provided by Ministry of Education, Science, Research and Sport of the Slovak Republic. Pavol Liptai, Bystrik Dolnik has patent #Industrial Property Office of the Slovak Republik, Utility Model Number 9685 issued to Technical university of Kosice. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.