# Improvement of DC Breakdown Strength of the Epoxy/POSS Nanocomposite by Tailoring Interfacial Electron Trap Characteristics

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## Abstract

**:**

_{A}) and large electronegativity that introduces deep-level traps into epoxy resin and restrain the electron transport. In this work, the origin of traps has been investigated by the simulation method. It is revealed that the functional properties of POSS side group can tailor an extensive network of deep traps in the interfacial region with epoxy and enhance the breakdown strength of the epoxy/POSS nanocomposite.

## 1. Introduction

_{2}O

_{3}nanocomposites and found that the DC breakdown strength was dominated by deep trap level, which is improved by the incorporation of nanoparticles [9]. S. Li et al. reviewed numerous research on breakdown characteristics of nanocomposites, and finally established a positive correlation between dielectric strength and interfacial traps [10]. The traps can be tailored by incorporating conventional nanoparticles to enhance the breakdown strength of base polymers [11,12,13]. However, the existing incompatibility between inorganic nanoparticles and organic polymer, dispersion of particles, and uniform distribution is compromised, leading to the agglomeration of fillers. In order to avoid agglomeration and enhance compatibility, the particle’s surface is functionalized. Surfaces are treated with silane, grafting, and plasma treatment is utilized with varying degrees of achievement [14,15,16]. Alternatively, POSS could be a promising nanoparticle for electrical applications. It has a unique structure of inorganic Si–O central core with organic side groups attached to central core. These organic side groups enhance compatibility with multitude of polymers and form an interfacial region [17]. POSS can tailor interfacial traps by restructuration of the base polymer at the nanometric scale and has been recently reported for successful improvement of dielectric properties [7,18]. However, the origin of traps in the interfacial region and the effects of traps on breakdown performances need further investigations.

_{A}), density of states (DOS), and frontier molecular orbital of EP/POSS nanocomposites, i.e., LUMO distribution. Finally, the origin of traps and the effects of traps on breakdown performance are discussed.

## 2. Materials and Experimental Methods

#### 2.1. Materials

#### 2.2. Preparation of Epoxy/POSS Composites

^{3}), and IKA T25 high-speed shearing machine was used to mix the liquid for 15 min with 5200 rpm speed. Then, the liquid is degassed and de-foamed by THINKY mixing machine for 30 min. To ensure the homogenous dispersion of POSS fillers into curing agent, sonication was performed for 15 min at 50 °C using an ultrasonic device of 99W power. Then, the DGEBA (ρ = 1.03 g/cm

^{3}) and DMP-30 accelerator was blended with the liquid. The same stirring and de-foaming procedures were repeated. Steel molds were cleaned by ethanol and kept in a kiln at 60 °C for 2 h before the liquid was cast into the different shapes and sizes of molds. An anti-setting agent was applied on the surface of molds to ensure an easy detachment of backed specimens. The mixed liquid was cured for 4 h at 80 °C and then for 8 h at 120 °C. The samples were then cooled to room temperature using an annealing process to get an adequately cured sample and to avoid cracks. The samples were cleaned with ethanol and then kept at 60 °C for 12 h before the experiments were carried out. A typical stoichiometric ratio for Epoxy, MeTHPA, POSS, and DMP30 was set as 100:80:2.5:1. POSS was added with a filler loading of 2.5 wt.% as an optimal value reported in Reference [19]. The chemical structure of DGEBA, MeTHPA, DMP-30, ECH-POSS, and OG-POSS is shown in Figure 2.

#### 2.3. Breakdown Test

_{i}is expressed in Equation (1).

_{bi}is experimental values of breakdown strength (kV/mm), α is a scale parameter that shows the cumulative failure probability of 63.2% samples in kV/mm, β is a shape parameter that reflects the variation of data. The P

_{i}of experimental data (E

_{bi}) can be approximated according to the IEEE-930 standard [20].

_{bi}. n is total breakdown attempts, which are 16 in this work. The schematic is shown in Figure 3.

#### 2.4. TSDC Experiment

_{B}is the Boltzmann constant (1.3802 × 10

^{−23}J/K). Coefficient A and B are dependent on the model used for the TSDC curve, but independent of the T and H, which are in References [21,22].

_{M}is the corresponding temperature of peaks and β is the heating rate in °C/min. The number of trapped charges Q

_{TSC}at the depolarization stage can be calculated by the integration of the current peak.

_{1}and T

_{2}are the initial and final temperatures of the peak, respectively. From the depolarized current, trap parameters H and Q

_{TSC}were analyzed. The schematic is shown in Figure 4.

#### 2.5. Molecular Modeling and Simulation

#### 2.5.1. Schematics of Chemical Reaction

#### 2.5.2. Quantum Chemical Calculation (DFT Method)

## 3. Results

#### 3.1. DC Breakdown Strength

#### 3.2. Trap Characterization by TSDC

_{1}) and shallow trap peak (α

_{2}), as shown in the inset figure of the EP/ECH-POSS sample. For epoxy/POSS composites, the trap levels are higher than that of neat epoxy. EP/OG-POSS has the highest deep trap energy of 2.15 eV. EP/ECH-POSS and neat epoxy have 2.0 eV, and 1.8 eV deep traps energy, respectively. It has been illustrated that tailoring deeper trap energy lowers the probability of trapped charges to escape. The deep trap has higher retention potency to restrain the trapped charges until some external factors are applied, such as temperature in this case is applied. Additionally, the TSDC peak for EP/OG-POSS occurs at higher temperatures. It shows that incorporating OG-POSS can form a large network of traps that restrain trapped electrons from de-trapping at earlier temperatures, as it requires external energy to escape. The neat epoxy has a smaller peak and a lower trap level (Table 2), indicating that fewer traps have been tailored; therefore, a lower peak has been observed. While the energy of shallow traps weakly links to the breakdown strength does not noticeably change for all samples (0.95 eV to 0.96 eV).

_{d}and Q

_{s}show the quantity of de-trapped charges from deep and shallow traps calculated by Equation (5). The small peak of neat epoxy occurs at low temperatures, which indicates that trapped electrons escape at earlier temperatures. It means that only a low temperature is required to stimulate the trapped electrons to escape. In Table 2, values of Q

_{s}and Q

_{d}of neat epoxy are low, which causes the amount of de-trapped charges to be lower than the other two samples. For EP/ECH-POSS, it has the highest peak with large quantities of Q

_{s}and Q

_{d}, and its trap energy and corresponding temperature of peak are higher than neat epoxy. However, the trap’s energy of EP/OG-POSS is higher than EP/ECH-POSS, and the TSDC peak occurs at higher temperatures. In this case, it is difficult for electrons to escape from deep traps in EP/OG-POSS.

#### 3.3. Density of States (DOS) and Energy Level Distribution

_{F}is located in the middle of the bandgap. There are many peaks in the whole energy level. On the right side of E

_{F}, the first peak (shaded) is recognized as the trapping peak, while the second peak is the bottom of the CB. Hence, a number of traps accumulate near the CB, and the gap distance between the center of the trapping peak and the CBM peak is defined as trap depth. For neat pristine epoxy resin, the trap depth is 2.05 eV. The trap depth is improved by the introduction of POSS to the epoxy matrix. For EP/ECH-POSS, the trap depth increases by 0.05 eV compared to neat epoxy resin, while it increases 0.28 eV for EP/OG-POSS. It is obvious that EP/OG-POSS has a larger trap depth.

_{F}are regarded as the trap density. Referring to Figure 5, the neat structure has three DGEBA chains, while the other two samples have only one DGEBA chain. The trapping region of the neat sample is three times larger than the other two samples. To compare the shaded area for all samples, the area under the curve for neat is multiplied with a factor of 0.3. The outcome shows that the trapping region for EP/OG-POSS is 33.02 A

^{2}, which is larger than the EP/ECH-POSS and pristine sample, i.e., 31.8 A

^{2}and 24.1 A

^{2}, respectively. The trap characteristics of EP/POSS nanocomposites calculated by TDOS are consistent with TSDC results.

_{F}. The vacuum level (VL) is at the zero energy level, and the gap distance between VL and LUMO is the electron affinity (E

_{A}), which is the released energy when the composite acquires an electron and closely associated with the trapping depth in the bulk of the matrix [27]. For neat epoxy resin, the E

_{A}is 1.331 eV. While for EP/ECH-POSS and EP/OG-POSS, the E

_{A}is 1.504 eV and 1.606 eV, which is 0.173 eV and 0.275 eV higher than neat epoxy.

## 4. Discussion

#### 4.1. Relationship between DC Breakdown and Traps

_{A}) increase, the DC breakdown strength increases, which indicates that the trapping mechanism strongly influences the breakdown strength. DC breakdown strength has a positive relationship with the trap level, and E

_{A}is calculated from the energy distribution diagram in Figure 9. The results show that doping of the OG-POSS filler improves the trap level and E

_{A}, which eventually enhances the DC breakdown strength.

#### 4.2. Origin of Traps Introduced by POSS Nanofillers

#### 4.3. The Effect of Traps on the DC Breakdown Performance

_{0}of electrodes and the applied field E, and the potential barrier Φ

_{1}for electrons injection can be calculated by the Schottky effect:

_{1}is a coefficient of Schottky effect, which can be expressed as:

^{−19}C), ε

_{0}is permittivity in vacuum (8.854 × 10

^{−12}F/m) and ε

_{r}is relative permittivity.

_{TSC}improved by POSS reduce the space charge and enhance the DC breakdown strength, which is consistent with the phenomenon in Figure 10.

_{tr}and P

_{de}are the trapping and detrapping probabilities, E

_{t}and N

_{t}are the trap level in eV and trap density in m

^{−3}, μ

_{0}is carrier mobility in m

^{2}·V

^{−1}·s

^{−1}, and υ

_{ATE}is the attempted escape frequency in s

^{−1}. In Equations (8) and (9), the P

_{de}decreases with the trap level, which indicates that the trapped charges in deep trap cannot gain enough energy to detrap unless the applied field is improved, while the increase of trap density improve the P

_{tr}and capture more charges into the trap. Hence, the increases of trap level and density suppress the charge transport process in the dielectric material, reducing the conductivity and improving the dc breakdown strength. In addition, the charge transport process in dielectric material is determined by Poole-Frenkel effect [28]. The potential barrier Φ

_{1}for charge function is the same as Equation (6), but the Φ

_{0}is the trap level and β

_{1}is the coefficient for the Poole-Frenkel effect:

_{st}, and the height of potential barrier E

_{th}, the free volume breakdown criterion can be expressed as:

_{th}is the trap level in this work, which indicates that the breakdown strength is improved by the increase in deep trap level.

## 5. Conclusions

- Incorporation of OG-POSS and ECH-POSS nanofillers to epoxy matrix successfully increases the deep trap level and breakdown strength, the DC breakdown strength of EP/OG-POSS, and EP/ECH-POSS increases 17% and 15% compared to neat epoxy.
- A positive relationship has been established between DC breakdown strength and trap parameters, i.e., deep trap level and electron affinity (E
_{A}). - Traps originated from the interfacial bonded region of EP/POSS. The depth of the trap level has a positive correlation with the electronegativity of atoms in the side groups of POSS. The greater the electronegativity of polymeric composite, the larger would be the trap depth.
- The increment in deep trap level and density suppresses the charge injection and transport process in the dielectric material, restraining the carriers from hopping over the potential barrier and further improves the DC breakdown strength.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent statement

## Data Availability statement

## Conflicts of Interest

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**Figure 2.**Molecular structure of epoxy (DGEBA), Hardener (MeTHPA), Accelerator (DMP-30), ECH-POSS and OG-POSS.

**Figure 7.**TSDC curves for all specimen (Dots for experimental results and lines for fitting curve). Inset: EP/ECH-POSS TSDC.

**Figure 8.**Total density of states (TDOS) of neat epoxy, EP/ECH-POSS, and EP/OG-POSS. Black line represents TDOS. E

_{F}is shown by vertical red dashed line in the mid-point of valance band (VB) and conduction band (CB). The trapping region is shaded. The first peak of CB is regarded as the CBM peak.

**Figure 9.**Energy level distribution of samples. Redline is E

_{F}, blueline is LUMO, and violet line is VL.

**Figure 10.**Relationship between DC breakdown strength and trap characteristics analyzed by TSDC test and molecular simulation.

**Figure 12.**Modified morphology epoxy/POSS composite in the interfacial regions and POSS distribution.

Sample Code | Sample Ingredients |
---|---|

Neat epoxy | Epoxy (DGEBA + MeTHPA + DMP-30) |

EP/OG-POSS | Epoxy+OG-POSS 2.5% |

EP/ECH-POSS | Epoxy+ECH-POSS 2.5% |

α Peak1 (α1) | α Peak2 (α2) | |||||
---|---|---|---|---|---|---|

Samples | Trap Energy/eV | Q_{d} (nC) | T_{m1} (°C) | Trap Energy/eV | Q_{s} (nC) | T_{m2}(°C) |

Neat | 1.8 | 1.12 × 10^{−9} | 108 | 0.95 | 1.58 × 10^{−9} | 96 |

EP/ECH-POSS | 2 | 8.11 × 10^{−9} | 124 | 0.96 | 6.74 × 10^{−9} | 116 |

EP/OG-POSS | 2.15 | 6.69 × 10^{−9} | 127 | 0.956 | 6.83 × 10^{−9} | 116 |

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**MDPI and ACS Style**

Aslam, F.; Li, Z.; Qu, G.; Feng, Y.; Li, S.; Li, S.; Mao, H.
Improvement of DC Breakdown Strength of the Epoxy/POSS Nanocomposite by Tailoring Interfacial Electron Trap Characteristics. *Materials* **2021**, *14*, 1298.
https://doi.org/10.3390/ma14051298

**AMA Style**

Aslam F, Li Z, Qu G, Feng Y, Li S, Li S, Mao H.
Improvement of DC Breakdown Strength of the Epoxy/POSS Nanocomposite by Tailoring Interfacial Electron Trap Characteristics. *Materials*. 2021; 14(5):1298.
https://doi.org/10.3390/ma14051298

**Chicago/Turabian Style**

Aslam, Farooq, Zhen Li, Guanghao Qu, Yang Feng, Shijun Li, Shengtao Li, and Hangyin Mao.
2021. "Improvement of DC Breakdown Strength of the Epoxy/POSS Nanocomposite by Tailoring Interfacial Electron Trap Characteristics" *Materials* 14, no. 5: 1298.
https://doi.org/10.3390/ma14051298