Modification of Insulating Properties of Surfaces of Dielectric High-Voltage Devices Using Plasma

Abstract

Plasma discharges under atmospheric pressure are applicable for modifying the electrical properties of dielectric surfaces. The aim of the plasma discharge treatment of such surfaces is to design a procedure so that its characteristic parameters improve the resulting levels of the breakdown electrical strength E_b when tested under pulsed or alternating electrical voltages. In this research, a set of functional experiments performed by using plasma in samples of two types of materials (thermoset, thermoplastic) were processed and evaluated, and the resulting effect of the magnitude of the breakdown electrical voltage, electrical intensity, and electrical conductivity of the surface were compared. A slit plasma chamber, previously described and parameterized, was employed to treat the surface of the dielectric samples. The surface structure was modified via plasma discharge without precursors, and methodologies were developed to evaluate these modifications with respect to the change in the electrical strength parameters of the insulator surface. Subsequently, the surface strength of the affected and unaffected samples was measured and evaluated as a function of exposure time, and the stability of the modification was assessed. The first methodical test showed that plasma discharge without precursors improved the long-term surface electrical strength of the dielectric surface. The test and its parameters were carried out with respect to feasibility in an industrial environment.

1. Introduction to Plasma Generation

In the technology of manufacturing and surface modification, plasma [1] finds use in changing the properties of the upper layers of material surfaces, various mechanical surface adjustment procedures, and altering the properties at the molecular and atomic levels [2,3,4,5]. For such purposes and to achieve physical or chemical changes in the target material surface, the plasma must be generated with a suitable generator [6] under repeatable, pre- defined conditions. To date, plasma-enhanced chemical vapor deposition (PECVD) applications have employed vacuum technologies; presently, however, high-frequency discharge generators to produce plasma under an atmospheric pressure are becoming increasingly more widespread. These technologies, unlike vacuum applications, offer several advantages.

Methods to generate a non-thermal high-frequency plasma discharge have been implemented in diverse industries and exhibit a broad application potential [3,4]. The technology is well developed and has proved interesting especially in thin film deposition [5] or surface modification. The advantage of generating plasma discharges under an atmospheric pressure lies in the possibility of easily forming a local structure with liquid precursors [7]. The electromagnetohydrodynamic properties [1] of generated plasma discharges depend on the design of the electrical generator and the chamber employed to excite the plasma [8,9,10]. The basic chamber arrangement may involve a cold plasma device that utilizes an atmospheric pressure (kINPen), with a central RF electrode and an argon atmosphere [7]. Research has been performed on the COST reference microplasma current in He [8] and the use of an RF plasma pencil to generate plasma [9]. An emerging approach relies on a COST reference microplasma current in He, supported by parallel RF and grounded electrodes [8] and the RF plasma pencil (from the category of the barrier torch discharge, BTD) [9].

The plasma generator, also referred to as the plasma pencil, contains an electrical generator and a high-frequency electrode with an operating frequency of f = 13.56 MHz. When the generator is operating, an atmosphere consisting of argon, nitrogen, and other gases, independent or with admixed precursor gases or substances, flows through the capillary [9]. The qualitative and quantitative electromagnetic discharge parameters are determined by the geometrical arrangement of the chamber, electrodes, shape of the excitation signal of the RF generator, and related parameters [10,11,12].

2. Electrical Surface Strength of Insulators

Considering the historical development of the design and manufacture of an HV apparatus [13,14,15] and the design of an electrical apparatus and equipment [16,17,18,19,20], one of the parameters studied is tahe ability of the equipment to withstand the action of an electrical surge. This parameter is tested with both shock waves and a harmonic electric voltage (f = 50 or 60 Hz) for a period of t = 1 s according to the Czech and international standards. In the device or on its surface, during a time period (pulse or t = 1 s), an electrical breakdown can occur through the volume of the insulator and at the insulator interface (dielectric/metal, dielectric/dielectric), Figure 1.

Research into the electrical breakdown during testing is relevant to the design of materials, their modification, and when assembling the device/equipment being tested. The solution focuses on achieving a high electrical strength E_p during the testing or operating the device at the insulator interface, delivering a repeatability of the parameters during voltage immunity tests (Figure 1).

Deficiencies in the electrical strength E_p and the repeatability of stress test parameters have thus far been addressed by standard “macroscopic” methods, focusing on:

(1)

Designing a longer jump tangent component of the electric field strength E_p (Figure 1) and distance d (Figure 2).

(2)

Designing a longer jump tangent component of the electric field strength E_p (Figure 1) and surface distance d_s, while maintaining distance d by setting a longer jump path (Figure 3) d_s > d.

(3)

Designing a longer jump tangent component of the electric field strength E_p (Figure 1) and increasing the surface distance d_s while maintaining the distance d by adjusting the micro/nanosurface properties (Figure 4).

Figure 2. An approach to improve the surface electrical strength E_p of the designed device by changing the jump distance d.

Figure 2. An approach to improve the surface electrical strength E_p of the designed device by changing the jump distance d.

Figure 3. An approach to improve the surface electrical strength E_p of the designed device via changing the surface distance by an additional element (rib) d_s.

Figure 3. An approach to improve the surface electrical strength E_p of the designed device via changing the surface distance by an additional element (rib) d_s.

Figure 4. Increasing the surface electrical strength E_p of the proposed device by changing the micro/nanosurface properties of the insulator.

Figure 4. Increasing the surface electrical strength E_p of the proposed device by changing the micro/nanosurface properties of the insulator.

The testing of the surface and bulk properties of dielectric materials and designed insulators is described with the standard techniques published in the Czech and international standards. Schematically, however, this could be expressed by setting up a method for measuring the values of the breakdown voltages U_d and breakdown intensities E_p (Figure 5). From the traditional empirical measurement experience [21], it can be argued that the relationship between the bulk and the surface electrical strength is described through an approximate expression as

$$\frac{E_{\mathrm{pn},\mathsf{\epsilon }}}{E_{\mathrm{pt},\mathsf{\epsilon }}}=\frac{100·{10}^3}{300},=\frac{1}{3}·{10}^3,$$

(1)

where E_pn,ε is the electrical strength in the normal direction characteristic of a material with relative permittivity ε_r, and E_pt,ε denotes the electrical strength in the tangential direction characteristic of a material with relative permittivity ε_r. The standard treatment of the interface of insulating materials or insulating materials and electrodes of the device under test via method 2 (Figure 3) encounters critical parameters; the essential factor is expressed by the formula

$$\frac{a}{b}\ge \frac{{\epsilon }_{\mathrm{r},\mathrm{d}2}}{{\epsilon }_{\mathrm{r},\mathrm{d}1}}$$

(2)

where a, b are the distances and dimensions of the dielectric shaping, respectively according to Figure 6.

In a system of auxiliary ribs at a finite jump distance d = constant, at a jump voltage U_d (Figure 6), critical Relation (2) is determined by the actual number and geometry of the ribs, which still contribute positively to the increase in the electrical strength E_p of the dielectric–dielectric–electrode interface. If the ratio a/b drops below the critical value of Relation (2), the electrical strength E_p paradoxically decreases, and the opposite resultant effect occurs.

One of the progressive methods for formulating a technical procedure to increase the surface electrical strength E_p in a specific device/apparatus design is applying microscopic (nanoscopic) surface modification of the dielectric–insulator interface. Surface modification techniques, such as sandblasting, facilitate the microscopic roughening of the surface and an increase in the jump distance d. Another progressive method lies in the modification/deposition of a nanosurface on the dielectric to increase the surface electrical strength value E_p. In this context, the effects of the residual electrical charges σ [C/m²] on the surface during impact stress tests are known [22].

3. Current State of the Research and Solutions for the Surface Strength of Insulators

According to well-proven procedures [14,15,16,17,18,19,20], standard devices and HV devices with surface treatment and known materials are designed. The current trends in increasing the surface strength E_p focus mainly on the concept, design, geometric layout of the HV device, and solutions using miniaturization (using microelectronics); one of the related activities then lies in changing the electrical strength of materials, both volumetric and surface ones [23,24,25,26], by choosing the concept and design for the critical part of the device or equipment. The approaches can be characterized and summarized into several categories, as follows:

(a)

Insulator surface modification and shaping, both physical and chemical;

(b)

Application of layers to increase the electrical strength parameters of the insulator;

(c)

Alteration of the nanostructure or the interface surface of the insulator.

Utilizing experiments, the numerical modeling, design, and improvement of the measurement methods of the electrical surface charge σ or the electric field vector E can be carried out.

Further, a more detailed approach to increase insulation properties according to (a) is outlined in [23], for example, where the authors analyze the results of a multiphysics numerical model of a three-pole insulator in a horizontal arrangement, with the operating voltage U_n = ± 800 kV GIL (gas-insulated transmission lines), based on this model; the temperature distribution and gas flow inside the GIL insulator are then determined. A similar issue is addressed, with the support of numerical modeling, by the authors of [25] for the DC-GIL system of the ITER project with U_n = 1 MV.

Another experimental concept [24] focuses on a set of procedures for evaluating parameters involved in the aging of insulators; the authors of the report discuss methods for evaluating the occurrence of surface discharges, observing space charge generation, and measuring the strength of the dielectric.

In the area of space or surface charge generation and distribution, it has been established that the charge accumulation rate depends on the aging time of the nanostructured resin. The problem of formation, distribution, and lifetime of the surface electric charge σ also depends on the method of combining the dielectric and electrode system. Importantly, the insulator design can cause a disproportionate generation of the local surface electric charge σ [25].

If we examine in greater detail the possibilities in the field of increasing the stress resistance achieved through the deposition of layers, we may emphasize procedures such as minimizing the influence of inhomogeneous or non-isotropic environments [26,27,28,29] on this parameter according to (b); for instance, this option is outlined in paper [26]. To increase the dielectric strength and to reduce the probability of electrical breakdown during voltage tests (due to a surface discharge), the concept of a layered insulator with functional gradient dielectric materials is used.

If we further investigate the results in the field of modifying the nanolayer surface (c) to reduce the discharge activity and to increase the voltage resistance of the insulator, we can consult sources such as [15,30]. Numerical modeling was employed in the hypothesis analysis and verification [15,30]; thus, to configure the measurement process, the system was suitably selected and adjusted to the analysis of a numerical model based on finite element methods [13,30], and the subsequent evaluation of the influence of the micro–nanostructure on the physical parameters of the dielectric.

The dependence of the electrical strength E_p and the micro/nano structure of the dielectric interface was already considered in an article from 1955 [31].

In investigating the mechanism of electrical breakdown in liquids, researchers have sought to measure the electrical strength value E_p that is characteristic of the pure liquid itself; ideally, the goal is to find an E_p value that is free from the influence of any external factors such as electrode composition, the condition of the electrode and dielectric vessel surfaces, and the measurement site configuration. In [31], very reproducible results are provided, allowing the correlation of the electrical strength E_p with the molecular structure of two homologous series of liquid hydrocarbons. This line of research was followed by a more recent project from 2010 [32]; the concept addressed the measurement and evaluation of selected electrical characteristics of nanocomposite materials.

The electrical breakdown voltage value U_p and the surface resistance value R_s in PE/SiO₂ nanocomposites were evaluated, and a comparison of the results [32] showed that the dispersion of small numbers of electrical nanocharges could improve the observed properties of the synthetic insulator.

Another stream of research on the nanostructure of materials usable as dielectrics was addressed in [33]. Different contents and particle sizes of Cr₃C₂ in composite materials were tested. The topic of nanocomposite structures was developed in [34]. The electrical strength E_p of nanocomposites containing low-density polyethylene (LDPE) filled with TiO₂ nanoparticles was investigated and also evaluated. The surface modification of the TiO₂ nanoscale was shown to have a significant impact on the electrical strength during stress tests.

The authors’ research focuses on improving the surface electrical parameters of insulators through surface changes in the order of micrometers or at the atomic and molecular levels. Such modification is not expected to radically increase the breakdown strength with intensity E_p through orders of magnitude; rather, the procedure aims to provide the possibility of an additional technological improvement in a device which has approached the limits of breakdown testing. The modification must have a long-term effect and must not be demanding in terms of technology or funding.

Plasma discharge treatment at an atmospheric pressure nevertheless offers such an additional refinement of the insulator–dielectric surface. As shown above, these technologies are employed in other industrial domains; thus, the authors’ research has centered on generating a suitable plasma discharge by a device usable in relevant branches of the industry [35,36].

Current techniques [37,38] deliver some plasma discharge parameters and evaluations that can be used as procedures and approaches to reduce the surface discharge in a device or apparatus. These options include, above all, the precise adjustment of the electromagnetic-electric field vectors [39,40] and the parameters of the dimensions and nature of the materials, observing the manufacturing technology. Among the experimental projects published to date [21,38,41], a very promising direction lies in creating layered materials at the interface of dielectrics and forming nanomaterial composites with a subsequent increase in the surface strength E_p.

4. Precursors and Surface Treatment

To shape the nanosurfaces of insulator samples, a separate plasma discharge or the insertion of precursors into the gases entering the plasma chamber are applicable; the precursors can be inorganic-based or organic-based [42]. To minimally predict the influence and impact of the plasma discharge on the surface of a dielectric material, it is useful to have a basic numerical model of the precursor and to analyze its electrohydrodynamic properties.

One of the precursors considered is benzene; benzene-based molecules are also usable. Benzene and its derivatives can be geometrically modeled in a simplified way as toroids, or toroids augmented with the probability space of the excursion of the envelope atoms (carbon C and hydrogen H) and the bonds between them [42].

With the produced and utilized samples, the plasma discharge and precursor application was performed. Subsequently, the changes in the electrical properties of such treated surfaces were systematically measured and evaluated for repeatability (type A and B uncertainties) and then compared with untreated samples [43].

The evaluation of the numerical model was based on the results outlined in [42]. The outcomes of the analysis were compared with the experimental evaluations; for example, the frequency spectrum of the benzene model for the Z parameter (impedance parameter) is non-primitive, according to [42]. Such impedances interact with the electromagnetic field of the chamber and its excited frequency characteristic. The mutual interactions and the radiating EMG field change the physico-chemical properties of the precursor and form an output nanosurface on the exposed part of the dielectric surface of the sample.

To predict the formation of new bonds on the dielectric–polymer surface via precursors, we need to know the precursors’ electromagnetic properties. Substances based on benzene molecules embody the elementary types of precursors used. When these interact with the plasma discharge, new bonds are formed on the surface to be treated; the treated surface then changes its parameters. For our purposes, the surface strength was monitored. Knowing the results of the analysis of the models allowed us to design an effective measurement method for diagnosing the discharge and the condition of the treated surface.

5. Methods of Adjusting the Electrical Strength of Dielectrics: Surface Strength

In order to achieve the desired surface treatment and to change the parameters, it appears convenient to use the plasma technique under atmospheric conditions, thus modifying not only the mechanical and structural but also the electrical properties of the sample. Such a state of the dielectric and its surface in the structure of the insulator is suitable for changing the surface strength parameters, both with and without the use of precursors. As a basis for testing and verifying the plasma modification of the dielectric surface, we created samples of dielectric insulating materials: an epoxy-based thermoset and a PE thermoplastic polymer, Figure 7.

The surface electrical strength E_p and its management in insulators, together with modifications of the geometrical or surface design of the equipment, are important for outdoor and indoor installations of apparatuses and equipment for the LV, HV, and UHV voltage classes. This requirement is determined by the durability or long-term safety and safe operation of the designed/manufactured equipment, identification, detection, evaluation of initial discharges and discharge activity, and use of surface modification technologies [1,44]. An example of the impact of long-term exposure to increased electrical intensity of the surface discharge E_p on the condition and operation of the equipment is shown in Figure 8; in this context, significant factors include not only the quality of the equipment (aging) in terms of safe operation, parameters and changes/impacts on the insulator surface, but also the design and construction of the equipment itself with respect to minimizing the conditions for generating surface electric field intensity E_t [43].

From previous research [36,37,44] on the properties of precursors of organic origin [36,42], the basic experiments with the effect of the plasma atmospheric discharge treatment [4,5,6,7] on the surface of the dielectric, together with the evaluation of changes in the relevant parameters, have moved to the application level (GA 20-14105S) [37,43]. In a recent article by Polášková et al. [37], the scientifically proven mechanism of surface changes through the action of plasma generated by a slit jet is approached. Interestingly, a part of the report is centered on elucidating the physico-chemical processes during the action of plasma at the interface of the dielectric material in a free atmosphere. The basic principle lies in the creation of electromagnetic conditions at the dielectric/air surface interface, such that the strong action of the electromagnetic field and temperature changes the arrangement and molecular composition of the thermoset and thermoplastic polymers; see Figure 9 and Figure 10. The result of the measurement of the incident plasma spectrum at the dielectric interface is exposed in Figure 11. The design and tests of the chamber have previously been published [9,10,12,36,44]. These chambers can be set up, and they parametrically affect the surface of the samples with plasma in a repeatable manner, monitoring and documenting the impact of plasma matter on the surface and the formation of artifacts (Figure 12 and Figure 13). These changes and new formations vary both from the point of direct action of the atmospheric plasma discharge (Figure 14) and according to the chosen setting of the plasma-generating chamber mode and frequency spectrum (for example, the spectrum presented in Figure 11). A more in-depth qualitative evaluation and approximation of the process to influence the polymer surface via atmospheric plasma discharges was proposed in [45,46]. Such treatment procedures, if performed properly, induce changes not only on the surface of the dielectric in the microscopic region (Figure 12 and Figure 13) but also at the level of the polymer molecules and atoms [45] (Figure 15). The surface is altered in its molecular structure and the newly formed chains cause a macroscopic change in the surface of the material (Figure 15). The expected bonds are those that are potentially able to bind an electric charge, and this results in a macroscopic increase in the interfacial strength of the modified surface, E_p. This hypothesis has already been investigated and tested in other areas of electrochemistry [45,46]. Therefore, the authors attempted to verify it by measurements, whether a change in surface strength properties could also be expected. The authors worked with the hypothesis of an increase in the capture of free electric charges moving along the surface at threshold parameters before the electric discharge effect (Figure 15). These electric charges (negative) can bind to the free hydrogen bonds of the bridges and do not complete the electrode–electrode path of the test electrode system. In its effect, such a mechanism increases the electrical strength, which has been quantitatively tested and demonstrated. These effects then deliver the result schematically outlined in Figure 4. Thus, changes in the parameters can be expected when testing the surface strength of the dielectric.

In this section, the methodology of applying an atmospheric plasma discharge on the surface of a dielectric sample is presented (Figure 7a,b). The process utilized a generator with the output power P_out = 10–1000 W, U_out = 300–600 V, a periodic signal, and the first harmonic f₁ = 13.56 MHz (Figure 14). According to the previously performed experiments (from 1996 until now [4,7,9,10,35,36,45,46]), the parameters (Figure 11) of the electric power of the exciting slot nozzle of the plasma discharge in gases, for example, were obtained based on experiments with Ar, N2, etc., to effectively influence the surface. An example of the impact of changes in the plasma surface is shown in the images of the surface in Figure 12 under different settings of the discharge parameters. Furthermore, the effective state of the discharge on the surface of the dielectric is indicated in Figure 14, with the generator parameters set. Other settings can be applied, but they either achieve the necessary efficiency, significantly change the surface, or the permanence of the change in surface properties is insufficient. Thus, in the laboratory, air pressure was maintained in the range of p = 101.30–102.2 kPa, under daylight (wavelength of 440–780 nm, temperature distribution for white light of 4800–5100 nm), with a luminous flux of 1500 lm, and humidity of 45–55%. These experimental condition data are quite justified for laboratory research. However, for the planned possible deployment of this technology in an industrial application, their achievability in a factory environment must be considered and accessibility taken into account. Therefore, no major precautions were taken in terms of setting up and maintaining very harsh laboratory conditions. The repeatability of the effective plasma action on the sample was ensured by a conveyor belt with a set feed rate of v = 22 mm/s. The samples and their surfaces were subjected to plasma discharge twice on the same surface to ensure good homogeneity of action on the surface. After the first exposure, the samples were rotated 90° and treated again with plasma at the same height of the PSJ mouth above the sample surface equal to h = 10 mm.

Because of the targeted involvement of the technology in the production of HV devices, the authors also decided not to modify and clean the surface of the samples before plasma application. In the anticipated production use, these measures (cleaning of the device surface, air pressure, humidity, lighting conditions, etc.) would be unrealistic to follow repeatedly in several respects. However, test results indicated that measurable changes in electrical surface strength parameters can be expected.

5.1. Detailed Description of the Plasma Surface Treatment

The plasma was generated through a setup including a power frequency generator and a plasma chamber producing a plasma slit beam (PSJ). The effective area of the plasma beam amounted to 150 mm. The plasma was generated in an argon atmosphere conducted into the plasma nozzle at a flow rate of w = 4 m³/h. The high-frequency power generator to produce the PSJ had an operating frequency of the first harmonic at f₁ = 13.56 MHz, and the output was regulated to a power of up to P_out = 600 W. In the first series of experiments for the surface modification of polymer dielectric samples (six samples, two controls) the power was set to P_out = 500 W. The repeatability of the plasma impact on the sample was ensured by a belt conveyor having a set feed rate. The samples and their surfaces were exposed to the plasma discharge twice in order to ensure good homogeneity of the influence on the surface (the feed rate was set to v_d = 22 mm/s, and the height of the PSJ mouth above the sample surface equaled h = 10 mm). The selected sample size to demonstrate the effect was chosen to be a × b = 100 × 100 mm; such a parameter facilitated easy handling and the capture of the homogeneity characteristics of the plasma exposure. All dielectric samples of the second series, both the thermoplastic and the thermoset types, were exposed in this way in a time sequence. In each of the two subsets, a total of N = 8 samples were exposed, while 2 samples were left unexposed to allow a comparison of the measured parameters. The distance to the plasma nozzle mouth from the sample surface was micrometrically set to d = 10.0 mm. For the measurement of the surface parameters (Figure 7) with the probe (Figure 7d), and the required measurement purposes, the standard recommended “suitable” electrode and dielectric arrangement concept (IEC 61340-5-1- [47], ESD STM 11.11-2001- [48], ASTM Standard D 257-99- [49]) (Figure 7d) was chosen. The test probe and specimens were chosen to accommodate a minimal deformation of the field lines due to the geometry of the surrounding environment and to satisfy the accuracy of the testing. The configuration could then be considered as a homogeneous and defined environment.

5.2. Surface Treatment and Measuring Methodologies

The test methodology and relevant parameters were verified as follows:

• In the first step, the proposed methodology was employed for measuring six samples, two without a surface treatment and four with the surface exposed, by an atmospheric plasma discharge. The treatment was performed on samples of both types of material, namely, epoxy resin-based and thermoplastics-based materials.
• After verifying the parameters and repeatability of the discharge exposure, the procedure was conducted again, this time on 10 samples of each category.
• Before and after exposing the sample surface to a plasma discharge, we measured and evaluated the residual surface electric charge σ [C/m²], surface resistance R_p [Ω], and electric current passing through the sample I_s [μA].
• The time dependence of the stability of the parameters R_p and I_s was tested, and the measurement according to point 2 was repeated at pre-defined time intervals T, namely, 24 h, 48 h, 72 h, and from 120 h to 144 h after exposure to the plasma discharge.
• The post-exposure samples were transported in a container with a defined material and surface conductivity, pre-determined dimensions, and material structure/composition. The surface electric charge was measured and adjusted to ensure minimum surface charging of the exposed sample during storage and transport to the measurement laboratory.

Regarding the surface parameters of the samples after treatment, these were measured according to the above-mentioned procedure in the following manner:

• Measuring system calibration: device, probe, environment, unexposed sample.
• Measurement and evaluation of the resistance R_c using a CA-6543 device, Chauvin Arnoux, France.
• Simultaneous measurement of the electric current I_s using a Kethley probe EA10000, Keythley, Tectronix, USA.
• Evaluation of the resistance R_v, electric surface charge density σ, and electric intensity E in exposed and non-exposed samples.
• The entire procedure 1–4 was repeated for the interval T, namely, 24 h, 48 h, 72 h and from 120 h to 144 h.
• Evaluation of the measured data and graphic recording.

The treatment methodology is shown in Figure 16.

6. Experimental Evaluation, Results, and Discussion

Based on the described measurement methodology [35], the evaluation of the time dependence of the monitored treatment parameters and the verification of the influence of the surface conductivity/resistance of the insulator sample were graphically represented. The designing and testing of the PSJ were described in [35]. For the given purpose, a generator [36] with the output power of P_out = 10–1000 W, U_out = 300–600 V, a periodic signal, and a first harmonic of f₁ = 13.56 MHz was used. The graphical patterns of the tests of changes in the surface resistance of the insulator as a function of time (from the exposure time without a precursor) are shown in Figure 9 and Figure 10. The surface resistance of the untreated samples of the second series of materials was R_p,tset = 0.082 TΩ, and R_p,tplast = 0.092 TΩ. According to the previously performed experiments (from 1996 until now [4,7,9,10,35,36,45,46]), the parameters (Figure 11) of the electric power of the exciting slot nozzle of the plasma discharge in gases, for example, were obtained based on experiments with Ar, N2, etc. to effectively influence the surface. An example of the impact of changes in the plasma surface is shown in the images of the surface in Figure 12 under different settings of the discharge parameters. Furthermore, the effective state of the discharge on the surface of the dielectric is indicated in Figure 14, with the generator parameters set. Other settings can be applied, but they either achieve the necessary efficiency, significantly change the surface, or the permanence of the change in surface properties is insufficient.

When setting different parameters of the discharge, for example, the electric power P_out for f = 13.56 MHz, with gas flows such as argon, nitrogen, air-argon, very different changes in the dielectric surfaces can be achieved. As an integral part of the research on the influence of the parameters of dielectric surfaces, the evaluation of their states during the parametric setting of the discharge is necessary. This part of the research is not the subject of this communication, but an example of surface changes on the dielectric material is given in Figure 12 and Figure 13. The images at the corresponding magnification scales are shown there. The structure of the surface testifies to the possible hypothesis of the formation of free bonds on the surface, as shown in Figure 15, where broken hydrogen bonds are indicated in red, which then remain at the surface, and the free hydrogen atom H is removed during the plasma discharge.

In designing the test, it was known from previous work [35,41,42,44] that plasma-treated surfaces changed, their nanochemical composition changed, but at the same time, a remanent electrical charge remained on the surface. This in turn may affect the resulting effect of measuring the long-term change in the electrical strength of the surface. Therefore, it was decided in the measurement methodology to repeat the measurements after some time intervals (Figure 16).

The reason for this was the indicated surface charge, which was only verified initially after plasma application and before the last measurement after 144 h (SIMCO FMX-004, ser.no. FX24585). The prepared tests already included a systematic measurement of the electrical charge. The surface treatment with plasma without precursors showed that in thermoplastic materials and thermosets, the surface resistance R_p rose and did not decrease with time; conversely, it stabilized at a higher level of values (Figure 17). The difference and properties of thermosets and thermoplastics had not been investigated, but both types of insulants demonstrated increased electrical surface strength. The hypothesis of a nano-treatment of the surface and the capture of the free electric charge propagating along the surface before the moment of discharge was verified by quantitative measurements. Furthermore, the effect of the retention of the electrical charge after plasma discharge application was demonstrated using a time-lapse surface strength measurement test after surface treatment. The results were obtained under ambient conditions (pressure, temperature, waviness, illumination, surface cleanliness, etc.) similar to industrial conditions for the expected deployment of the technique. Thus, the fundamental decision was that the dielectric surfaces were not specially treated or cleaned before the actual plasma application. For the measurements at 144 h after plasma discharge application, there was a statistically significant change in the parameters in an otherwise exponential trend (Figure 17). Here, there may have been a sample manipulation error during the indicative (no-)surface charge measurement. This fact will be verified in a forthcoming experiment.

The evaluated surface jump intensities according to the proposed methodology were evaluated from the measured surface resistance shown in Figure 9 and Figure 10. The recalculated magnitudes of E_p intensities moved within a proportional range (E_p = 0.099–0.109 V/mm).

Further research will focus on selecting and applying the precursors; the effects on the surface resistance R_p and stability over time will also be investigated.

7. Conclusions

The proposed methodology for dielectric/insulator plasma surface treatment was repeatedly verified on sets of test samples. The repeatability and desired increase in the surface strength during the plasma treatment was demonstrated, together with the change in the intensity of the electric field on the surface of the sample at the post-treatment times; the effects and quantities were graphically represented. A parametric analysis of the plasma treatment was carried out in order to determine the maximum strength of the surface of the sample, E_p. The tests were repeated on other sets of samples, indicating the actual repeatability of the plasma treatment without the use of precursors. The procedures showed that the first treatment increased the parameters of the dielectric surface resistance (and thus the flashover resistance) within tens of percent for both the thermoset and the thermoplastic dielectrics.

The benefit of the research rests in a systematic, technologically manageable exposure of the insulator surface. In the process, the surface strength increased by an industrially usable value (up to 50% of the surface resistivity), and the gradually improved setting of the atmospheric plasma discharge allowed changing the surface parameters with a proven time stability.

At the next stages, our research will prominently involve applying precursors in plasma and evaluating their influence on the described traceable parameters of the dielectric, the aim being to refine the parameters of the surface properties of insulators.