The charge storage process of
capacitors involves polarization phenomena within the dielectric at the micro - level. When a voltage is applied across the two plates of a capacitor, atoms or molecules in the dielectric experience displacement polarization or orientation polarization.
- Displacement polarization
- In dielectrics composed of non - polar molecules, such as hydrogen, methane, etc., under the action of an external electric field, the centers of positive and negative charges in the molecules shift relative to each other, forming electric dipoles. These dipoles are arranged in the direction of the external electric field, and polarization charges appears on the surface of the dielectric.
- Orientation polarization
- For dielectrics composed of polar molecules, such as water, organic glass, etc., although the molecules have an inherent electric dipole moment, in the absence of an external electric field, due to the thermal motion of the molecules, the orientation of the electric dipole moment is disordered, and the dielectric as a whole is electrically neutral. When an external electric field is applied, the electric dipole moment of polar molecules turns towards the direction of the external electric field, resulting in a macroscopic polarization phenomenon.
Starting from the Gauss's law of electric field, the formula for the capacitance of a parallel - plate capacitor can be derived, where is the vacuum permittivity, is the relative permittivity of the dielectric, is the plate area, and is the plate spacing. This formula reflects the relationship between capacitance and the structural and material parameters of the capacitor.
- Low - frequency characteristics
- At low frequencies, the capacitance value of the capacitor remains basically unchanged and can store and release charges according to the static capacitance value.
- High - frequency characteristics
- When the frequency increases, due to the influence of the capacitor's own parasitic parameters (such as lead inductance, inter - plate capacitance, etc.), its equivalent capacitance value changes. For capacitors with high dielectric constant, such as some ceramic capacitors, dielectric losses occur at high frequencies, resulting in a decrease in capacitance value.
- Energy loss
- ESR causes joule heat during the charging and discharging process of the capacitor, and the power loss , where is the current passing through the capacitor. In applications with large - current charging and discharging, such as the power management system of electric vehicles, capacitors with high ESR generate a large amount of heat, affecting the efficiency and stability of the system.
- Impact on circuit performance
- In some circuits with high requirements for signal integrity, such as high - speed digital circuits and radio - frequency circuits, ESR affects the waveform and transmission quality of signals. Capacitors with low ESR can better filter out the ripples in the power supply and maintain the purity of the signal.
- Electrical double - layer capacitor (EDLC)
- Its principle is based on the double - layer formed at the electrode - electrolyte interface. At the surface of the electrode, ions in the electrolyte are electrostatically attracted and form charge layers similar to capacitor plates, and the distance between the two charge layers is extremely small (usually nanoscale), so a large capacitance value can be obtained in a small volume.
- Electrical double - layer capacitors have characteristics such as high power density and long cycle life, and can be used in occasions of fast charging and discharging, such as the start - up and braking energy recovery system of electric buses.
- Pseudocapacitor
- The charge storage mechanism of pseudocapacitors involves rapid reversible redox reactions on the surface of the electrode. For example, when transition metal oxides (such as ruthenium dioxide) or conductive polymers (such as polyaniline) are used as electrode materials, redox reactions occur during the charging and discharging process to store and release charges.
- Pseudocapacitors have higher energy density than electrical double - layer capacitors, but their cycle life is relatively short. Currently, they are applied in fields such as power - assisted systems of hybrid vehicles.
- Structural characteristics
- MLCC is composed of multiple layers of ceramic dielectric and metal inner electrodes alternately stacked and sintered. This structure can achieve a large capacitance value in a small volume and has good high - frequency characteristics.
- Technical challenges and applications
- With the miniaturization and high - performance of electronic devices, MLCC needs to develop towards higher capacitance and higher voltage withstand capacity. In 5G communication base station equipment, MLCC is used in radio - frequency circuits, which puts forward extremely high requirements for its performance stability and reliability.
- Electric vehicles
- Capacitors plays a crucial role in electric vehicles. In addition to the supercapacitors used for energy recovery and auxiliary power mentioned above, in the battery management system (BMS) of electric vehicles, film capacitors are used to protect the circuit and prevent damage to electronic components caused by high voltage generated during battery over - charging and over - discharge.
- Renewable energy storage
- In the storage systems of renewable energy such as solar energy and wind energy, capacitors can form a hybrid energy storage system with batteries and supercapacitors. Capacitors can quickly respond to changes in load power and provide or absorb a large amount of electrical energy in a short time, while the battery is used for long - term energy storage, improving the performance and efficiency of the entire energy storage system.
- Control and coupling of quantum bits
- In some quantum computing schemes, such as quantum bit systems based on superconducting Josephson junctions, precise control and coupling of quantum bits are required. Capacitor can be an important component in superconducting quantum bit circuits, used to store and regulate charge, and realize control and operation of the quantum bit state.
- Suppression of quantum circuit decoherence
- The decoherence of quantum systems is a key issue affecting the reliability of quantum computing. By rationally designing quantum circuits containing capacitors, the interaction between quantum bits and the environment can be suppressed to a certain extent, prolonging the coherence time of quantum bits, and improve the accuracy and stability of quantum computing.