Atmospheric pressure plasma jet deposition of TiO₂ layer on alumina/epoxy to improve electrical properties

1.   Introduction

Epoxy Resin (EP) exhibits high stability, corrosion resistance, insulating properties, and excellent plasticity [1]. New composite materials consisting of epoxy resin as the matrix and incorporating a certain amount of micron-sized fillers often achieve significant enhancements in thermal [2, 3], mechanical [4, 5], and electrical properties [5, 6]. Al₂O₃/EP composite materials, which are typical micron dielectric materials, can have their thermal and mechanical properties enhanced by doping with Al₂O₃ particles [7]. Additionally, the insulating ability of these materials under non-uniform electric fields can be improved [8]. These materials are commonly utilized in the preparation of porcelain insulators within industrial Gas Insulated Metal-enclosed Transmission Line (GIL) [9]. However, under DC electric fields, the charge dissipation capacity of Al₂O₃/EP composite materials is poor. This limitation leads to charge accumulation and distortion of the electric field on the surface of porcelain insulators. In severe cases, surface flashover may be triggered, posing a serious threat to the safe and stable operation of power systems [10]. Therefore, it is of significant importance to investigate methods to retain the advantages of Al₂O₃/EP composite materials while simultaneously improving their surface charge accumulation and enhancing the DC flashover withstand level.

Studies have demonstrated that at the gas-solid interface, the charge accumulation and dissipation of porcelain insulators are influenced by various factors. These factors include surface morphology [11], element types [12], and electrical parameters [13‒15]. By regulating the surface physicochemical properties through surface modification methods, it is possible to specifically adjust the charge migration without affecting the internal properties of the material [16]. The most straightforward physical modification method involves changing the surface roughness through physical etching. This change in surface roughness can increase or decrease the smoothness of the electron migration path at the microscopic level. Yu et al utilized a CO₂ laser to etch lines and adjust the surface condition of alumina ceramics. This etching process created continuous and uniform line-shaped pits on the surface, which improved the flashover voltage [17]. Surface coating, radiation exposure, surface fluorination, and other technologies can alter the chemical composition of the composite material’s surface, allowing for broader regulation. Wu et al coated the surface of epoxy resin with a fluorinated nano-SnO₂ layer. This coating resulted in a 66% increase in the charge dissipation rate with increasing SnO₂ content, ultimately improving the flashover voltage by 15% [18]. Shen et al employed electron beam irradiation to treat epoxy resin. This treatment caused epoxy molecular chains to form a large number of side groups, end groups, and free radicals. The formation of these groups increased the deep trap energy level and density on the surface of the epoxy resin, inhibiting the migration of interfacial charges and enhancing the along-surface flashover performance [19]. Li et al proposed a one-step surface functionally graded modification method based on the surface dielectric barrier discharge system. This method significantly enhanced the surface flashover voltage of the modified epoxy resin compared to uniform modification [20]. In this regard, it is possible to adjust the overall electrical performance of composite materials to some extent by modifying morphology and chemical composition.

TiO₂ exhibits strong corrosion resistance, aging resistance, high temperature resistance, and dielectric properties. These properties make TiO₂ widely used in the regulation of mechanical [21] and dielectric [22] properties of polymer composites. As a typical oxide semiconductor material, the lower bandgap characteristic of TiO₂ results in fewer trap levels, which is beneficial for charge dissipation [23]. Li et al investigated the surface charge accumulation and dissipation characteristics of nano-TiO₂/EP materials under a DC electric field. It was discovered that the charge dissipation rate is related to the doping ratio. When the mass fraction of nano-TiO₂ reaches 8%, the charge dissipation time significantly decreases [24]. Zeng et al enhanced the surface deep trap levels and surface resistivity of polyetherimide by doping with nano-TiO₂, thereby increasing the vacuum flashover voltage [25]. Zhu et al prepared Titanium Dioxide Nanosheets (TNSs)/Polyimide (PI) nanocomposite films with different compositions by in-situ polymerization. This preparation method obtained nanocomposite materials with low permittivity and high breakdown field strength [26]. It is evident that current research often utilizes TiO₂ filler doping to form micro-nano composite materials. The purpose of this doping is to enhance the electrical properties of the matrix. However, compared with Al₂O₃/EP composite materials, TiO₂/EP composite materials exhibit more fragile mechanical properties [27]. Additionally, TiO₂/EP composite materials have lower specific surface area characteristics [28, 29] and higher production costs [27], making their engineering practicality far behind that of Al₂O₃/EP. Therefore, by combining the excellent mechanical and thermal properties of Al₂O₃/EP with the superior electrical properties brought by TiO₂ modification through specific surface treatments, a more comprehensive insulation material can be developed.

Atmospheric Pressure Plasma Jet (APPJ), as an innovative plasma source, has emerged as a prominent topic in thin film deposition due to its simple design, ease of operation, and lower production costs [30]. However, current research on the plasma deposition processes of specific precursors, controlling material surface parameters via plasma surface treatment, and consequently studying the impacts of traps and charge accumulation on the space charge behavior and flashover characteristics of materials is not yet sufficiently comprehensive. In this work, Titanium Tetraisopropoxide (TTIP) is employed as a precursor in APPJ technology to deposit TiO₂ on the surface of Al₂O₃/EP composites. The interaction of high-energy plasma with the material surface is utilized to alter the physicochemical properties of the Al₂O₃/EP materials. Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscopy (XPS), and Fourier Transform Infrared Spectroscopy (FT-IR) techniques were used to conduct a detailed characterization of the physicochemical properties of the composite material surfaces with different APPJ durations. The surface withstand voltage performance of Al₂O₃/EP after APPJ treatment was investigated under DC voltage and industrial frequency AC voltage. The performance was analyzed by examining changes in charge accumulation and dissipation characteristics, as well as the trap parameters. By investigating the impact of the modified Al₂O₃/EP surface’s physicochemical characteristics on electrical parameters, the mechanism by which TiO₂ deposition via APPJ enhances the insulation performance of materials was revealed. Furthermore, the optimal treatment time corresponding to the best overall electrical performance of Al₂O₃/EP was determined.

2.   Experiment

2.1   Experimental setup

2.1.1   Raw materials

The resin used is bisphenol A type epoxy resin (E-51) produced by Shanghai Resin Factory; methyl-tetrahydrophthalic anhydride (MTHPA) and 2, 4, 6-tris(dimethylaminomethyl)phenol (DMP30) from the same factory are used as resin curing agent and accelerator, respectively; the alumina with a particle size of 10 μm is produced by Shanghai Superway Nano Technology Company; titanium tetraisopropoxide (TTIP) with a purity of 97% is produced by Shanghai McLin Reagent Company.

2.1.2   Preparation of micron Al₂O₃/EP composite material

The micron alumina, epoxy resin, curing agent and accelerator were weighed for use with a mass fraction ratio of 300:100:80:1 respectively, for later use. First, the micron alumina is mixed with the epoxy resin and preheated at 120 °C for 10 min to reduce viscosity. After preheating, the curing agent and accelerator are quickly added, and the mixture is stirred mechanically, maintaining the temperature at 120 °C using an electric heating jacket for an additional 10 min. The fully mixed Al₂O₃/EP solution is then placed in a 60 °C constant temperature box for vacuum deaeration. After 5 min, the solution is removed and poured into a mold. The mold is then placed in a 120 °C constant temperature box for curing over 2 h to obtain alumina/epoxy resin samples with a diameter of 4 cm and a thickness of 2 mm.

2.1.3   Surface modification of Al₂O₃/EP composite material

The atmospheric pressure plasma jet (APPJ) treatment system shown in figure 1 is used to deposit a TiO₂ layer on the surface of the Al₂O₃/EP samples. The treatment system mainly consists of a high-frequency AC power supply (CTP2000K), an oscilloscope, a rotating lifting platform, an air circuit, and a jet electrode. Before deposition, the surface of the samples is cleaned with anhydrous ethanol and dried in a 50 °C oven for 30 min. Then, the TTIP solution is heated in a water bath to 70 °C and preheated for 10 min. Next, the valve is opened to directly introduce argon gas into the jet electrode at a flow rate of 3 L/min, while the flow rate of argon gas carrying the TTIP is set to 0.3 L/min. The sample is placed at the center of the rotating lifting platform, positioned 1 cm from the jet nozzle. Finally, the high-frequency AC power supply is turned on, the voltage is set to 7 kV, the duty cycle is adjusted to 30%, and the current is slowly increased until the jet flame stabilizes, with the oscilloscope showing the current at 0.6 mA.

Figure  1.  Atmospheric pressure plasma jet treatment system.

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2.2   Methods for testing surface characteristics of samples

In the study, various testing methods were employed to comprehensively analyze the modifications on the samples. The microscopic morphology was examined using a TESCAN MIRA LMS field emission scanning electron microscope, revealing changes in surface morphology before and after modification. The chemical composition and valence states were assessed with a Thermo SCIENTIFIC ESCALAB 250Xi X-ray photoelectron spectrometer (XPS), and the characteristic functional groups within the range of 500‒4000 cm⁻¹ were identified using a Shimadzu IRTracer 100 Fourier-transform infrared spectrometer in Attenuated Total Reflectance (ATR) mode. Surface conductivity measurements were conducted using a DW-P303-1ACH2 high voltage DC power supply, a TH2691A LCR meter, and a ZBT-1017 three-electrode box. Additionally, the static charge distribution and charge dissipation rate on the surface were determined using a Trek P0865 electrostatic voltmeter and a Trek 6000B electrostatic probe. Lastly, the surface flashover voltage under both DC and AC electric fields was captured with a Tektronix MDO 3012 oscilloscope using a needle electrode configuration with a 14 mm separation distance, highlighting the impact of the modifications on the material’s electrical performance.

3.   Results

3.1   Surface characteristics of TiO₂ deposited modified materials

3.1.1   Surface morphology

The surface morphology and cross-section of Al₂O₃/EP composite materials after different treatment durations were observed using an electron microscope, as shown in figure 2. Without surface treatment, the Al₂O₃/EP material’s surface exhibited a noticeable spotted distribution, which are micrometer-sized alumina particles uniformly dispersed within the epoxy resin. The cross-section also showed no significant clumping, resulting in a relatively smooth surface. After APPJ deposition of TiO₂, the material’s surface was covered with a denser network of particles, which intertwined and overlapped, significantly increasing the roughness and forming a film as shown in figure 2(f). These are TiO₂ deposition layers formed by the breaking and recombination of titanium tetraisopropoxide under the bombardment of plasma. When the treatment time is short, the generated TiO₂ particles are small in size and closely arranged without entanglement, presenting a dotted distribution. As the treatment time increases, the TiO₂ particle size gradually enlarges, clumping and adhering to each other, displaying a layered structure that progressively densifies. With further increased treatment time, although TiO₂ particles continue to aggregate and enlarge, obvious gaps between adjacent particles can be observed in figure 2(d), leading to numerous cracks in the deposition layer.

Figure  2.  SEM images of Al₂O₃/EP at different modification times.

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3.1.2   Chemical composition

Based on the surface morphology observed with the scanning electron microscope, we selected a typical area on the material surface for Energy Dispersive X-ray Spectroscopy (EDS) testing to more closely observe the distribution of elements on the surface of the Al₂O₃/EP composite material. As shown in figure 3, alumina particles with a size of about 5 μm are uniformly embedded in the epoxy resin matrix, and there is no mutual adhesion between the particles. This indicates that most of the oxygen elements are closely connected with aluminum elements, forming agglomeration areas, while the remaining small amount of oxygen elements are uniformly distributed with titanium elements throughout the material surface, collectively forming a relatively dense deposition layer.

Figure  3.  EDS images before and after TiO₂ deposition by APPJ.

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Figure 4 shows the XPS spectra of the surface elements of the composite material at different deposition times, and table 1 was derived from this analysis. The surface of the unmodified sample is mainly composed of C and O elements, while the content of Si element accounts for 22.7%. The source of Si comes from two parts: one part is the silicon coupling agent doped in micron alumina, and the other part is the release agent used during the curing process. After APPJ treatment, significant changes occurred in the C 1s peak, O 1s peak, and Si 2s, Si 2p peaks. The content of C and Si elements on the sample surface noticeably decreased, while the content of O, Ti, and N elements increased. This indicates that APPJ treatment not only increases the content of Ti elements but also increases the content of O elements. The proportion of increase varies at different treatment times, with the highest content at 3 min, reaching 58.06%. Prolonging the treatment time further reduces the content of C elements, while the content of Ti gradually increases. The presence of N elements is related to plasma discharge from the air, so their content also increases with the increase in treatment time.

Figure  4.  XPS spectra before and after TiO₂ deposition by APPJ.

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Table  1.  Relative content of main elements before and after APPJ treatment.

Relative elemental content (%)	C	N	O	Al	Si	Ti
Untreated	37.58	1.6	37.29	0.84	22.7	0
1 min	29.34	1.18	52.97	0.26	2.89	13.35
3 min	21.57	2.53	58.06	0.26	1.5	16.08
5 min	17.82	3.55	56.64	0.06	2.36	19.53

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To further investigate the chemical composition of the surface deposition layer of Al₂O₃/EP, peak fitting of the Ti and O elements in the XPS spectra was performed using a symmetric Gaussian/Lorentzian function, showing in figure 5. The Ti 2p peak consists of a double peak of Ti 2p_1/2 and Ti 2p_3/2, with binding energies of 458.9 eV and 464.7 eV, respectively. After peak fitting of the Ti 2p peak, four peaks were obtained, located at 458.17 eV, 458.98 eV, 463.73 eV, and 464.97 eV. Among them, the stronger peaks with binding energies of 458.98 eV and 464.97 eV correspond to the 2p_3/2 and 2p_1/2 orbitals of Ti⁴⁺, while the Ti 2p peaks at 458.17 eV and 463.73 eV come from the 2p_3/2 and 2p_1/2 orbitals of Ti³⁺. It can be found that both too short (1 min) or too long (5 min) treatment times introduce a certain amount of Ti³⁺ defects. Based on the area of each peak, the ratio of Ti³⁺ to Ti⁴⁺ content can be roughly estimated; when the treatment time is 1 min, the ratio of Ti⁴⁺/Ti³⁺ is approximately 3.66; when the treatment time increases to 5 min, the ratio of Ti⁴⁺ to Ti³⁺ is approximately 4.14. No apparent Ti³⁺ defects were found when the treatment time was 3 min.

Figure  5.  Ti 2p and O 1s peak deconvolutions at different treatment times.

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The O 1s peak can be divided into three components at 533.5 eV, 531.9 eV, and 530.2 eV, respectively attributed to O–Ti, O–C, and O=C. Analyzing the peak areas, the proportions of Ti–O bonds at treatment times of 1, 3, and 5 min were 14.5%, 22.8%, and 20%, respectively. This indicates that when the treatment time exceeds 3 min, the content of Ti³⁺ increases, leading to a reduction in the number of O atoms bonded to Ti atoms.

From the infrared spectroscopy graph of Al₂O₃/EP shown in figure 6, it is evident that the infrared absorption peaks of the material before and after treatment are basically similar. The absorption peak at 3395 cm⁻¹ is attributed to the stretching vibration of O–H, and the absorption peaks near 2959 cm⁻¹, 2922 cm⁻¹, and 2862 cm⁻¹ are due to the stretching vibrations of C–H. The absorption peak at 1730 cm⁻¹ is for the carbonyl group C=O. The absorption peak at 1238 cm⁻¹ is attributed to the stretching vibration of C–O, and the peak at 1175 cm⁻¹ is due to the stretching vibration of C–O–C. It can be observed from the graph that after APPJ treatment, the intensity of the C–O and C–O–C absorption peaks has weakened. This is because titanium forms ionic bonds with oxygen, thereby affecting the intensity of these absorption peaks.

Figure  6.  FT-IR spectra before and after TiO₂ deposition by APPJ.

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3.2   Electrical characteristics of TiO₂ deposited modified materials

3.2.1   Surface conductivity

The surface conductivity of Al₂O₃/EP samples at different treatment times is shown in figure 7. The surface conductivity of the unmodified composite material is 2.32×10⁻¹³ S/m, while the APPJ surface modification increased the material’s surface conductivity by two orders of magnitude. Within the range of 0–3 min, the surface conductivity increases with the treatment time, reaching a maximum of 4.01×10⁻¹¹ S/m. However, further extending the treatment time decreases the material’s surface conductivity; the surface conductivity under APPJ treatment for 4 min is only one-tenth of that at 3 min, which may be related to the introduction of Ti³⁺ defects.

Figure  7.  Surface conductivity of samples at different treatment times.

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3.2.2   Charge distribution and dissipation

To further investigate the effect of APPJ TiO₂ deposition on the surface charge distribution of Al₂O₃/EP, this study used a needle electrode to simulate an extremely non-uniform electric field and subjected the sample surface with a negative polarity 3 kV DC corona charging. After the treatment, the surface potential distribution of the samples is shown in figure 8. After the corona charging treatment, the surface potential of Al₂O₃/EP without APPJ treatment showed a ring-shaped distribution. Starting from the tip of the needle electrode, the potential gradually increased from the inside to the outside, with the lowest potential at the tip, reaching −1286 V. The TiO₂ layer can effectively enhance the surface potential of Al₂O₃/EP under negative polarity charging conditions. This enhancement effect varies with different treatment times. When the treatment time is 3 min, the potential drop reaches the minimum, only 331 V, which reduces the potential decay by 74.2% compared to the unmodified Al₂O₃/EP.

Figure  8.  Surface charge distribution diagrams: X-Y distribution (a) and X-Z distribution (b).

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The X-Z plane diagram can show the smoothness of the peak where the charge accumulates. On the surface of the untreated Al₂O₃/EP, the charge peaks formed after charging exhibit sharp characteristics. In contrast, the surface of Al₂O₃/EP treated with APPJ effectively makes the peaks smoother, reducing the distortion of the electric field, and this smoothing effect is still influenced by the treatment time. According to the literature, the sharpness of the potential at the charge peak can reflect the uniformity of the charge distribution [12]. From this perspective, we defined a new parameter: the area of uniform charge distribution, which is used to quantify the decay rate of the potential at the peak. The physical significance of this parameter is the surface area of the sample where the surface potential remains between 0.9 and 1 times the peak potential after corona charging. The area of uniform charge distribution under different treatment time is shown in table 2. It is easy to find that when the treatment time is 3 min, the area is the largest, and the material has the smoothest surface charge distribution. However, too long treatment time will weaken the uniformity of charge distribution in the deposition layer.

Table  2.  Area of uniform charge distribution.

Treatment time	Area of uniform charge distribution (mm2)
0 min	52
1 min	248
2 min	476
3 min	676
4 min	568
5 min	156

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The decline in surface potential can serve as an approximate indicator of the rate of surface charge dissipation. Figure 9 shows the decay process of surface potential within 30 min for composite materials treated for different durations. The results demonstrate that compared to untreated samples, the surface potential of Al₂O₃/EP treated with APPJ decreases faster, accelerating the dissipation of charges. With the increase in treatment time, this dissipation effect first improves and then weakens, especially when the treatment time is 3 min, the final surface potential drops to 66% of the initial value, which is the lowest.

Figure  9.  Single-point surface potential decay curve.

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3.2.3   Flashover voltage

The along-surface withstand voltage performance of Al₂O₃/EP was evaluated through tests with negative direct current voltage and industrial frequency alternating current voltage. The test temperature is 26.6 °C, with an air humidity of 46%. Additionally, the flashover voltage test results were analyzed using the Weibull distribution, as shown in figure 10. The shape parameters and scale parameters corresponding to different treatment times are detailed in table 3. The scale parameter represents the flashover voltage of the sample when the cumulative probability reaches its maximum, while the shape parameter reflects the slope of the fitting curve, indicating the sensitivity of the composite material to the applied voltage.

Figure  10.  Surface flashover voltage distribution.

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Table  3.  Scale and shape parameters of flashover distribution.

Treatment time	DC			AC
	Shape parameter β (%)	Scale parameter γ (kV)		Shape parameter β (%)	Scale parameter γ (kV)
0 min	27.09	8.96		30.03	13.83
1 min	19.18	9.23		30.01	14.36
2 min	18.28	9.60		31.65	15.06
3 min	17.75	11.61		82.63	16.00
4 min	15.18	10.49		37.62	15.22
5 min	16.54	9.37		25.90	14.80

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In DC electric field, with the increase in treatment time, the along-surface withstand voltage performance of Al₂O₃/EP is enhanced. The peak value is reached at a treatment time of 3 min, where the surface flashover voltage of the composite material increases by 29.6%. However, further extending the treatment time actually reduces the flashover voltage of the material. TiO₂ layer on the surface leads to a decrease in the shape parameter of flashover voltage, i.e., the sensitivity of Al₂O₃/EP to voltage increases. However, it is worth noting that despite being more sensitive to voltage, the probability of flashover under various voltage levels shows a downward trend.

The pattern of flashover voltage under AC electric field is similar to that under DC, increasing with treatment time before decreasing, reaching its highest at 3 min. Compared to DC flashover voltage, the AC flashover voltage is higher and less sensitive to voltage changes. This is caused by the different flashover characteristics of the composite materials under DC and AC.

3.3   Adhesion strength of TiO₂ layer

The micro-scratch test can measure the adhesion between the TiO₂ layer and the Al₂O₃/EP composite. By increasing the vertical load, cracks appear in the TiO₂ layer and gradually propagate until the film delaminates. The load at which a sudden change in friction force or acoustic signal occurs characterizes the adhesion strength. The sample of Ti-3 min was tested using the Anton Paar RST³. The results, as shown in figure 11, indicate a critical load of 38 N, demonstrating good adhesion between the layer and the substrate.

Figure  11.  Surface flashover voltage distribution.

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4.   Discussion

4.1   Mechanism of the impact of TiO₂ layer on the physicochemical properties

Integrating the characterization and analysis of the material surface properties mentioned above, we have revealed the mechanism by which surface-deposited titanium affects the physicochemical properties of Al₂O₃/EP. During the APPJ modification process, TTIP is transported to the jet tube by Ar carrier gas and undergoes a transformation in material state under the influence of a high-frequency, high-voltage power supply. The high energy of the plasma state causes the TTIP to decompose into titanium dioxide and other gaseous products [31], as shown in equation (1).

Under low-temperature conditions, the properties of the gaseous products are stable, allowing them to be effectively removed from the reaction system. During this process, titanium dioxide particles form a deposition layer on the surface of Al₂O₃/EP, significantly altering the chemical composition of the surface by introducing a large amount of Ti and O elements. According to the peak fitting analysis results of XPS, the duration of the treatment affects the quality of TiO₂ in the deposition layer; especially when the treatment time is too short or too long, the deposition layer may be doped with Ti₂O₃ defects. This phenomenon is partly because Ar ion sputtering has a reducing effect on titanium oxides, which has been confirmed by some literature [32]. Additionally, we observed that as the treatment time extends, the carbon content in the surface deposition layer gradually decreases. This indicates that during the plasma modification process, the deposition layer retains certain organic substances, which continue to react, transform into gas, and detach from the surface of the composite material after prolonged plasma treatment.

From a physical property perspective, the thickness and density of the deposition layer vary with different treatment times. Short-duration treatments result in TiO₂ particles that fail to join over a large area, distributing across the composite material’s surface and forming a hilly distribution. When the treatment time is appropriately extended, the deposition layer becomes denser, and the surface exhibits a coral-like layered covering effect. However, further extending the treatment time, while promoting the joining of particles to some extent, also leads to cracks in the deposition layer due to the continuous bombardment by high-energy plasma. This results in a blocky distribution, reducing the density, and ultimately forming an island-like surface distribution.

Therefore, although extending the plasma treatment time has its advantages, such as promoting the agglomeration of TiO₂ particles, increasing the proportion of titanium elements, and removing organic impurities from the surface deposition layer, it also comes with some adverse effects. These include an increase in the proportion of Ti₂O₃ defects and the formation of cracks on the surface of the deposition layer, all of which are factors that need to be comprehensively considered during the treatment process.

4.2   Mechanism of the impact of TiO₂ layer on electrical properties

Regulating the surface physicochemical properties of Al₂O₃/EP through APPJ can change its electrical performance. The flashover occurring on the composite material is related to the characteristics of charge accumulation and dissipation on the surface, while the trap characteristics of carriers to some extent determine the accumulation and dispersion of charges. According to the Isothermal Surface Potential Decay (ISPD) theory, the single-point decay curve of material surface potential can be converted into the material’s trap energy level and trap density according to equations (2) and (3):

Where ${k_{\text{B}}}$ is the Boltzmann constant; $T$ is the ambient temperature; $v$ is the electron escape rate; $t$ is the decay time; ${\varepsilon _0},{\text{ }}{\varepsilon _{\text{r}}}$ are the vacuum permittivity and the relative permittivity, respectively; $e$ is the elementary charge; $L$ is the thickness of the Al₂O₃/EP samples; ${\phi _{\text{s}}}(t)$ is the surface potential, which can be fitted according to equation (4) as follows:

Among them, A, B, m, n are parameters related to the fitting curve. The trap energy level distribution curve obtained from the calculations is shown in figure 12. APPJ treatment significantly altered the trap energy levels within Al₂O₃/EP. Compared to the unmodified material, new shallow trap peaks appeared in the treated material, generally reducing the depth of trap energy levels. Specifically, within the treatment time of 1‒3 min, extending the treatment time effectively reduced the overall trap depth on the material surface and enhanced the energy density of shallow traps. Particularly when the treatment time reached 3 min, the main trap depth in the Al₂O₃/EP decreased from 1.08 eV to 0.98 eV, the trap energy level shifted from deep to shallow, and a new shallow trap with a depth of 0.91 eV emerged, marking the most significant manifestation of trap shallowing on the material surface at this time. However, when the treatment time exceeded this limit, not only did the overall trap energy level increase, but the content of shallow traps also decreased. Additionally, with the continued extension of treatment time, the shallowing effect of TiO₂ layer on surface traps gradually weakened, and the trap depth experienced a “rebound”. Combined with the observations of changes in the surface physicochemical properties, this phenomenon might be related to the cracking of the surface deposition layer caused by prolonged sputtering. As the treatment time increased, high-energy plasma sputtering damaged the integrity of the surface deposition layer, thereby affecting the distribution and density of trap energy levels, weakening the effect of TiO₂ modification.

Figure  12.  Surface trap energy level distribution curve.

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Analysis of the potential decay curves shows that the introduction of shallow traps helps accelerate the dissipation of surface charges on Al₂O₃/EP. This is because deeper trap energy levels have a strong binding effect on free electrons, causing the charges to be deeply trapped, thereby inhibiting the free movement of surface charges and reducing the rate of charge dissipation. In contrast, the presence of shallow traps allows captured free electrons to accumulate energy more quickly and escape, thereby promoting the rapid dissipation of surface charges.

In summary, we can summarize the mechanism by which TiO₂ layer enhances the electrical performance of Al₂O₃/EP as shown in figure 13: through plasma modification, the chemical composition of Al₂O₃/EP surface changes, especially with the introduction of a titanium and oxygen-containing deposition layer. This not only overall reduces the surface trap energy levels but also introduces new shallow traps. These changes promote the migration of accumulated charges on the material surface. Moreover, the TiO₂ layer significantly increases the surface conductivity of Al₂O₃/EP by two orders of magnitude. This higher surface conductivity accelerates the movement of surface charges without compromising the insulating properties of the material. The combined effect of these two factors accelerates the dissipation of charges in the air and on the material surface. Furthermore, by observing the changes in the area of uniform charge distribution, we also discovered that TiO₂ layer not only promotes the dissipation of surface charges but also significantly improves the potential distribution of Al₂O₃/EP under extremely non-uniform electric fields. This homogenization effect notably reduces the external electric field distortion caused by charge accumulation, especially the reduction in the maximum charge accumulation density at the triple junction of the needle electrode, Al₂O₃/EP surface, and air, thereby increasing the along-surface flashover voltage of Al₂O₃/EP [33, 34].

Figure  13.  The mechanism of regulating the electrical properties of Al₂O₃/EP through TiO₂ deposition.

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The pattern of flashover on the surface of Al₂O₃/EP under AC electric field differs from that under DC. Due to the continuous change of the AC electric field, the accumulation of surface charges contributes little to the formation of flashover channels. Therefore, the dominant factor for AC flashover is no longer the accumulation of surface charges. The reason why TiO₂ layer can simultaneously improve the material’s flashover withstand voltage under AC electric field mainly relies on the higher permittivity of TiO₂ [35] and the increased surface roughness [36]. According to the Secondary Electron Emission Avalanche (SEEA) theory, inserting a layer of dielectric with a higher permittivity or conductivity than the main insulator between the electrode and insulator can suppress the electric field of the triple junction and reduce seed electron emission, makes flashover formation more difficult; meanwhile, the grooves on the surface of the TiO₂ layer increase the creepage distance, hindering the movement of secondary electrons from the high voltage electrode to the ground electrode, and both factors together increase the flashover voltage.

The treatment duration has a decisive impact on the effectiveness of modification. The comprehensive analysis results show that at a treatment time of 3 min, the TiO₂ layer formed on the surface of Al₂O₃/EP through plasma jet technology not only avoided the introduction of Ti³⁺ defects but also maximized the density of the deposition layer. The treatment at this specific time also maximized the enhancement of the surface conductivity of Al₂O₃/EP and most effectively homogenized the distribution of charge. Under the joint influence of these factors, the surface flashover voltage of Al₂O₃/EP was significantly increased, reaching 11.61 kV under DC electric field and 16.00 kV under AC electric field.

5.   Conclusion

In this study, the impact of APPJ TiO₂ deposition modification on Al₂O₃/EP is explored by analyzing changes in surface and cross-sectional morphology, chemical composition, surface conductivity, adhesion strength, charge behavior, flashover voltage, and trap distribution before and after modification. The research demonstrates that employing TTIP as a precursor for plasma TiO₂ deposition effectively coats the surface of Al₂O₃/EP with a TiO₂ layer. The density and bonding structure of this layer are influenced by the treatment duration. An optimal time of 3 min yields the strongest bond while avoiding the Ti³⁺ defects and surface cracking associated with inappropriate treatment times. This modification significantly improves the surface conductivity of Al₂O₃/EP and alters trap energy levels to introduce numerous shallow traps. These changes enable faster charge dissipation and prevent charge accumulation in non-uniform electric fields. Specifically, at the optimal treatment duration of 3 min, notable enhancements include a 200-fold increase in surface conductivity, a 33% faster charge dissipation rate post-corona charging, and increases of 29.6% and 15.7% in flashover voltage under negative DC and AC conditions, respectively. These findings underscore the potential of plasma TiO₂ deposition to enhance the electrical performance and insulation capabilities of Al₂O₃/EP in highly uneven electric fields. This novel approach offers a means to mitigate charge accumulation on insulators and boost surface flashover voltage. Moreover, the TiO₂ layer exhibits excellent adhesion to the substrate, making it advantageous for engineering applications.