Application Cases

Experiment Name: External Space Charge Measurement Test of Polyimide Films

Research Direction: As China enters the critical phase of the "14th Five-Year Plan" and the Long-Range Objectives Through the Year 2035, various industries and fields are increasingly reliant on electrical energy. This high dependence brings with it severe challenges to the reliability and service life of power equipment. With the rapid development of modern electronic technology and ultra-high voltage (UHV) technology, high-performance insulating materials are being widely applied in electronic devices and high-voltage power equipment. According to relevant statistics, one of the main causes of grid accidents is power equipment failure, with damage to insulating materials accounting for the vast majority of these failures.

Polyimide (PI), as a polymer material with excellent electrical properties, high-temperature stability, chemical stability, and relatively high hydrolysis resistance, is widely used in high-temperature electronic components, high-voltage power equipment, aircraft, and other electrical and electronic engineering fields. However, with the rapid development of electrical and electronic equipment, an increasing number of devices need to withstand higher voltages and electric fields, placing greater demands on the insulating performance of polyimide films.

Polyimide is a polymer whose molecular backbone contains an imide ring structure; its typical structural formula is shown in Figure 1.1. The imide ring structure contains two carbonyl groups (C=O) and one imine group (NH), offering high stability and chemical inertness. This polymer material generally exhibits excellent physical properties, such as high strength, high modulus, high melting point, and good corrosion resistance.

Common polyimide films appear light yellow or brown, depending on their preparation process and chemical structure. In terms of thermal performance, polyimide films demonstrate high stability at elevated temperatures and can be used long-term in environments exceeding 400°C. Electrically, they possess good insulating properties and anti-interference capabilities, effectively isolating electrical signals and preventing electromagnetic interference. Mechanically, polyimide films have high tensile strength and Young's modulus, maintaining good mechanical strength under significant stress.

However, in practical applications, polyimide films are prone to space charge accumulation under strong electric fields, leading to electric field distortion and, in severe cases, breakdown. This severely limits their application in fields such as high-voltage power equipment. Studying the space charge effects and breakdown characteristics of polyimide films under high electric fields is highly significant for understanding their breakdown mechanisms, improving their breakdown performance, and enhancing the reliability of electronic devices.

Experimental Purpose: To measure and validate the space charge distribution in unmodified and nano-modified polyimide films with varying nanoparticle content, examining the injection amount, migration direction, and extent of space charge migration under different nano-content conditions, thereby laying the groundwork for subsequent experiments.

Test Equipment: High-voltage amplifier, Signal generator, Piezoelectric sensor, Electrode system, Amplifier, Pulse power supply, High-frequency oscilloscope, High-voltage DC power supply, Protective resistor, Coupling capacitor, Shielding box, etc.

Experimental Process: The piezoelectric sensor, which transmits signals via acoustic waves, is used to detect and reflect pressure waves carrying space charge distribution information. The weak voltage signal output by the piezoelectric sensor is amplified by an amplifier and then collected and tested by an oscilloscope. The excitation signal generated by the signal generator is amplified by a high-voltage amplifier to provide a DC polarization electric field for the PI film sample, enabling the study of the internal space charge distribution under a specific DC electric field. When insulation breakdown or surface flashover occurs in the sample, the protective resistor limits the current and prevents damage to the high-voltage amplifier due to excessive current. A schematic diagram of the measurement system is shown in Figure 1-1.

Figure 1-1: Schematic Diagram of the Measurement System

Experimental Results: After a voltage is applied across the PI film, significant changes occur in the internal space charge distribution due to charge injection and migration. Different amounts of negative and positive charges are injected at the cathode and anode, respectively. As the duration of the externally applied voltage increases, charge injection within the PI film continues, eventually leading to a gradual stabilization of internal space charge accumulation. Test data are shown in Figure 2-2.

The matrix of polyimide films generally lacks a well-ordered structure, and their energy bands often contain numerous trap energy levels. Furthermore, due to their frequent operation under various complex conditions and external factors such as high temperature and high electric fields, electrical aging and thermal aging occur, accompanied by phenomena like molecular chain breakage and void formation. These changes may transform shallow traps within the matrix into deep traps or cause free charges to become trapped (trapped electrons or holes), making carriers more susceptible to trap capture.

Figure 2-2: Peak Space Charge Density Near the Electrode After 30 Minutes of Voltage Application

These changes may also introduce a large number of trapped charges within the polyimide film. These trapped charges capture and bind free charges in the matrix, making it easier for space charges to accumulate at the electrodes of the polyimide film, thereby inducing electric field distortion. Structural modification of polyimide films using nanoparticles significantly impacts their electrical properties. Nano-modification inhibits the accumulation of internal space charges in polyimide films. On one hand, the high specific surface area of nanomaterials enhances the surface recombination efficiency of carriers, reducing charge accumulation and space charge effects. On the other hand, the energy band structure and energy level distribution of nanomaterials differ from those of semiconductor materials, allowing them to modulate the electronic structure and energy band configuration of semiconductor materials, thereby mitigating the impact of space charge effects.

High-Voltage Amplifier Recommendation: ATA-7100

Figure: ATA-7100 High-Voltage Amplifier Specifications

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