In the rapidly evolving field of electric vehicles, the drive motor serves as the core power unit for EV cars, directly influencing their performance and reliability. However, under high-frequency pulse voltage conditions, the insulation system in EV car drive motors, particularly the gas-solid interface of insulating paper, is prone to partial discharge, leading to premature aging and failure. Traditional polyimide insulating papers, while offering excellent thermal and mechanical properties, exhibit limited resistance to corona discharge. To address this, a novel polyimide-boron nitride nanocomposite insulating paper has been developed, leveraging materials such as pyromellitic dianhydride and 4,4′-diaminodiphenyl ether. This study investigates the gas-solid interface discharge characteristics and other key properties of this composite material, emphasizing its potential to enhance the durability and efficiency of EV cars. The composite insulating paper is designed to effectively capture high-energy carriers and scatter electrons, thereby impeding discharge channel propagation and delaying electrical tree initiation and growth. Through comprehensive experimental analysis, including thermal stability, dielectric performance, and discharge behavior under simulated operating conditions, this research aims to provide insights into advanced insulation solutions for the next generation of EV cars.
The development of high-performance insulating materials is critical for the advancement of EV cars, as they operate in demanding environments characterized by high-frequency pulses and elevated temperatures. In EV cars, the drive motor’s stator winding insulation is subjected to intense electrical stresses, which can accelerate degradation through mechanisms like partial discharge at gas-solid interfaces. This study focuses on synthesizing a polyimide-boron nitride nanocomposite insulating paper and evaluating its properties compared to conventional materials. The preparation process involves surface modification of hexagonal boron nitride nanosheets, synthesis of polyamic acid prepolymer, and formation of a composite slurry, followed by film casting and thermal imidization. Key performance metrics, such as thermal stability, thermal conductivity, dielectric constant, dielectric loss factor, and partial discharge inception voltage, are analyzed to assess the material’s suitability for EV car applications. The results demonstrate that the composite insulating paper exhibits superior thermal and electrical properties, making it a promising candidate for improving the reliability and lifespan of EV car drive motors. Furthermore, the integration of boron nitride into the polyimide matrix creates a wide-bandgap insulating material that effectively mitigates discharge phenomena, contributing to the overall safety and performance of EV cars.
To fabricate the polyimide-boron nitride nanocomposite insulating paper, several steps were meticulously followed. Initially, hexagonal boron nitride nanosheets were modified using a silane coupling agent in an ethanol/water mixture to enhance dispersion and interfacial adhesion. This step is crucial for ensuring uniform distribution within the polymer matrix, which directly impacts the performance in EV car insulation systems. The mixture was stirred at 70°C for 4 hours, followed by centrifugal washing and drying. Next, the polyamic acid prepolymer was synthesized by dissolving 4,4′-diaminodiphenyl ether in N,N-dimethylacetamide under a nitrogen atmosphere, followed by the gradual addition of pyromellitic dianhydride. After approximately 6 hours of stirring, a viscous polyamic acid solution was obtained. The modified boron nitride nanosheets were then dispersed in N,N-dimethylacetamide via ultrasonication for 1 hour, and this dispersion was slowly incorporated into the polyamic acid prepolymer under mechanical stirring and additional ultrasonication for 2 hours, resulting in a homogeneous composite slurry. Finally, the slurry was degassed and cast onto a clean glass plate using an automatic coating machine. The film underwent a stepwise thermal imidization process in an oven, with temperatures ramping from 80°C to 150°C, 200°C, and finally 300°C, each held for 1 hour, to produce a flexible and dry insulating paper. This method ensures optimal material properties for withstanding the harsh conditions in EV cars.
The performance of the composite insulating paper was evaluated through a series of tests, with comparisons made against pure hexagonal boron nitride and a commercial insulating paper. Thermal properties were assessed using thermogravimetric analysis and laser flash analysis for thermal conductivity. The thermal stability was characterized by the temperature at which weight loss occurs, and thermal conductivity was calculated from thermal diffusivity measurements at 25°C, 100°C, and 150°C. The results are summarized in Table 1, highlighting the enhanced performance of the composite material for EV car applications.
| Material | Thermal Decomposition Temperature (°C) | Thermal Conductivity (W/m·K) |
|---|---|---|
| Pure Boron Nitride | 374.5 | 0.18 |
| Commercial Insulating Paper | 382.6 | 0.32 |
| Polyimide-Boron Nitride Composite | 395.1 | 0.45 |
The dielectric properties were analyzed using a broadband dielectric impedance spectrometer, measuring dielectric constant (ε) and dielectric loss factor (tanδ) over a frequency range of 50 Hz to 1 MHz at room temperature (approximately 26°C). The data, presented in Table 2, indicate that the composite material offers improved dielectric performance, which is essential for reducing energy losses and enhancing insulation reliability in EV cars.
| Material | Dielectric Constant | Dielectric Loss Factor |
|---|---|---|
| Pure Boron Nitride | 3.2 | 0.0020 |
| Commercial Insulating Paper | 3.2 | 0.0020 |
| Polyimide-Boron Nitride Composite | 3.1 | 0.0018 |
Gas-solid interface discharge characteristics were evaluated using a partial discharge detection system with a pulse power source set at 10 kHz and a voltage rise time of 200 ns. The partial discharge inception voltage was recorded when discharge pulses exceeded 5 pC, and long-term aging tests were conducted at a constant voltage slightly above this threshold to measure average discharge magnitude and discharge frequency. The results, shown in Table 3, demonstrate the composite’s superior resistance to discharge, which is critical for the longevity of EV car drive motors.
| Material | Partial Discharge Inception Voltage (kV) | Average Discharge Magnitude (pC) | Average Discharge Frequency (counts) |
|---|---|---|---|
| Pure Boron Nitride | 1.65 | 18.3 | 500.1 |
| Commercial Insulating Paper | 1.77 | 6.2 | 46.3 |
| Polyimide-Boron Nitride Composite | 1.92 | 1.1 | 10.2 |
The enhanced performance of the polyimide-boron nitride composite can be attributed to the formation of a robust interface between boron nitride and polyimide, which introduces deep-level traps for charge carriers. This interface effectively captures high-energy electrons and scatters them, thereby reducing electric field distortion and inhibiting discharge channel propagation. The thermal conductivity improvement is described by the Fourier heat conduction equation: $$q = -k \nabla T$$ where \(q\) is the heat flux, \(k\) is the thermal conductivity, and \(\nabla T\) is the temperature gradient. For EV cars, higher \(k\) values facilitate better heat dissipation, reducing the risk of thermal runaway in drive motors. Similarly, the dielectric behavior can be modeled using the complex permittivity formula: $$\epsilon^* = \epsilon’ – j\epsilon”$$ where \(\epsilon’\) is the dielectric constant and \(\epsilon”\) relates to the dielectric loss. The lower dielectric constant and loss factor in the composite material minimize polarization and energy dissipation under high-frequency conditions, which is advantageous for EV car applications where efficiency is paramount.
In the context of partial discharge, the inception voltage can be correlated with the material’s ability to withstand electric stresses. The composite’s higher inception voltage indicates a greater defect tolerance, which is vital for EV cars operating under variable loads. The discharge magnitude and frequency during aging tests follow a power-law relationship, often expressed as: $$Q = A \cdot t^B$$ where \(Q\) is the discharge magnitude, \(t\) is time, and \(A\) and \(B\) are constants dependent on material properties. For the polyimide-boron nitride composite, the values of \(A\) and \(B\) are lower, signifying slower degradation and extended service life. This is particularly important for EV cars, as it translates to reduced maintenance and improved reliability over time.
Further analysis of the thermal stability reveals that the composite insulating paper maintains its integrity at elevated temperatures, which is common in EV car drive motors during peak operation. The thermal decomposition process can be described by the Arrhenius equation: $$k = A e^{-E_a / RT}$$ where \(k\) is the rate constant, \(A\) is the pre-exponential factor, \(E_a\) is the activation energy, \(R\) is the gas constant, and \(T\) is temperature. The higher thermal decomposition temperature of the composite corresponds to a larger \(E_a\), indicating greater resistance to thermal degradation. This property ensures that the insulation remains effective even under the strenuous conditions encountered in EV cars, such as rapid acceleration or regenerative braking.
The role of boron nitride as a wide-bandgap material cannot be overstated. Its integration into the polyimide matrix creates a composite with a bandgap energy that exceeds that of traditional insulators, effectively trapping charge carriers and preventing avalanche breakdown. This mechanism is crucial for suppressing partial discharge at gas-solid interfaces, a common failure mode in EV car insulation systems. The discharge energy per cycle can be estimated using: $$W = \frac{1}{2} C V^2$$ where \(W\) is the energy, \(C\) is the capacitance, and \(V\) is the voltage. By reducing the discharge magnitude, the composite minimizes energy dissipation, thereby lowering the risk of insulation failure and enhancing the overall efficiency of EV cars.
In long-term aging studies, the composite insulating paper demonstrated a significant reduction in discharge activity compared to reference materials. This is attributed to the homogeneous dispersion of boron nitride nanosheets, which act as barriers to discharge propagation. The discharge frequency data can be fitted to a Weibull distribution: $$F(t) = 1 – e^{-(t/\eta)^\beta}$$ where \(F(t)\) is the cumulative probability of failure, \(t\) is time, \(\eta\) is the scale parameter, and \(\beta\) is the shape parameter. For the composite, \(\eta\) is larger and \(\beta\) is smaller, indicating a longer time to failure and a more gradual aging process. This statistical approach provides a reliable framework for predicting the lifespan of insulation in EV cars, aiding in design and maintenance decisions.
Moreover, the interfacial adhesion between boron nitride and polyimide plays a key role in enhancing mechanical strength and preventing delamination under thermal cycling. This is particularly relevant for EV cars, where temperature fluctuations are frequent. The stress-strain behavior can be modeled using Hooke’s law for composite materials: $$\sigma = E \epsilon$$ where \(\sigma\) is stress, \(E\) is Young’s modulus, and \(\epsilon\) is strain. The composite exhibits a higher modulus due to the reinforcing effect of boron nitride, contributing to dimensional stability and resistance to mechanical wear in EV car drive motors.
In conclusion, the polyimide-boron nitride nanocomposite insulating paper developed in this study offers substantial improvements in thermal stability, dielectric performance, and gas-solid interface discharge characteristics. These advancements are directly beneficial for EV cars, as they enhance the reliability and efficiency of drive motor insulation systems. The composite’s ability to capture high-energy carriers and scatter electrons effectively delays discharge initiation and growth, prolonging the service life of EV cars. Future work should focus on optimizing the filler concentration and exploring other nanomaterials to further push the boundaries of insulation technology for EV cars. As the demand for electric vehicles grows, such innovations will play a pivotal role in ensuring sustainable and high-performance transportation solutions.