Does a larger dielectric constant mean a stronger insulation performance?
In many popular science materials, we often see such statements as "the higher the dielectric constant, the better the insulation performance of the material". However, in fact, there is no direct and necessary connection between the dielectric constant and the insulation performance of the material.
Dielectric: Insulating material that is polarized
Dielectric is a type of insulating material that can be polarized under the action of an electric field. The word "medium" means to isolate the current. Dielectrics not only have insulating properties, but also can be polarized under the action of an external electric field, thereby storing charge. The specific polarization mechanism includes: electronic displacement polarization of non-polar molecules and orientation polarization of polar molecules. When affected by an external electric field, the two end faces of the dielectric perpendicular to the direction of the electric field will form equal polarized charges of opposite signs. These polarized charges form an electric field in the opposite direction of the external electric field inside the dielectric, weakening the external electric field. The stronger the polarization ability of the dielectric, the stronger the reverse electric field, and the more obvious the weakening effect on the external electric field. Polarized charges are different from free charges in conductors. They cannot be separated from the dielectric and transferred to other objects, nor can they move freely inside the dielectric, so they are called bound charges. Since the displacement of bound electric charges in an external electric field is limited, dielectrics are able to have insulating properties. Polarization is the process of generating an electric field inside a dielectric.
Insulating materials are defined as materials used to prevent conduction between conductive elements. They are designed to use their electrical insulation to isolate conductors of different potentials and limit the flow of current. The electrical properties of insulating materials are mainly reflected in the conductivity, dielectricity and insulation strength under the action of an electric field, covering four aspects: relative dielectric constant, dielectric loss, conductivity (or resistivity), and dielectric strength.
Electrical properties of insulating materials
Relative dielectric constant (εᵣ): This dimensionless physical quantity is always greater than 1, indicating the ratio of the absolute dielectric constant ε of the material to the dielectric constant ε₀ of vacuum (or air), reflecting the polarization ability of the material in an electric field. The larger the dielectric constant, the stronger the polarization ability of the dielectric, the more bound charges are generated, and the stronger the ability to weaken the external electric field and store electrical energy. It is related to the strength of the polarity of the dielectric molecules, and is also affected by factors such as temperature and the frequency of the external electric field. High dielectric constant means better energy storage and electric field homogenization performance.
Dielectric loss: Actual dielectrics always have energy loss under the action of external electric fields, including loss caused by conductivity and loss caused by lossy polarization. In an alternating electric field, when the direction of the external electric field changes, the internal electric field changes accordingly. The directional movement or deflection of charged particles consumes energy and converts it into heat energy, which is polarization loss. Under DC voltage, the dielectric has conductivity loss and ionization loss; under AC voltage, in addition to conductivity loss, polarization loss caused by periodic polarization will also occur. Dielectric loss increases the temperature of the dielectric. If the loss is too large, it will cause material aging, loss of insulation ability or even breakdown. It is an important indicator for measuring the electrical properties of insulating media.
Conductivity: No dielectric is an ideal insulator. There are always some charged particles inside, such as mobile positive and negative ions, electrons, holes and charged molecular clusters. Under the action of an external electric field, some carriers drift in a directional manner to form a conduction current. Conductivity γ or resistivity ρ is a physical quantity that characterizes the conductive properties of dielectrics. The larger the resistivity, the better the insulation performance. The conductivity of dielectric materials is divided into volume conductivity and surface conductivity. The former is determined by the material structure, composition, and impurity content, and is affected by the working environment such as air pressure, temperature, and radiation; the latter is greatly affected by the external environment such as water vapor and dust.
Dielectric strength (breakdown field strength): Under normal circumstances, dielectrics are insulators. When the external electric field is not strong, electric polarization occurs without destroying the insulation performance. When the external electric field is too strong, the positive and negative charges of dielectric molecules may be completely separated, and the electrons become free mobile charges. The insulation performance of the dielectric is destroyed and becomes a conductor, that is, the dielectric breaks down. Dielectric strength is the maximum electric field strength that the dielectric can withstand without breakdown. It is a basic insulation characteristic parameter. The greater the dielectric strength, the better the insulation performance. Dielectric strength is affected by many factors. The dielectric strength of inorganic materials is affected by comprehensive factors such as microstructure, impurity defects, dielectric thickness, temperature, frequency, moisture absorption and electric field distribution. Improving the dielectric strength of high-performance insulating materials requires optimizing material purity, reducing defects and adjusting microstructure. In addition, the dielectric breakdown process and breakdown voltage of the dielectric also depend on the electric field distribution, ambient temperature, heat dissipation conditions, surrounding medium properties, pressurization speed and voltage action continuity.
Insulation resistance measures the insulation performance of a material at low voltage, but the material may have high insulation resistance at low voltage, but cannot withstand high voltage, and the dielectric strength is not high. Therefore, when designing electronic devices or selecting insulating materials, insulation resistance and dielectric strength should be considered comprehensively, especially in high-voltage applications, where dielectric strength is crucial.
The influence of dielectric constant on the application of insulating materials
The core of insulation capacity lies in dielectric strength and resistance. The dielectric constant mainly affects electric field propagation and energy storage rather than directly restricting current flow. The size of the dielectric constant has no direct correlation with the insulation of the material. In practical applications, the influence of dielectric constant on insulating materials is as follows:
Capacitor application: In capacitors, the role of dielectrics is to store electrical energy. The larger the dielectric constant, the larger the capacitance. Since high dielectric constant materials can store more charge, capacitor dielectrics usually require high dielectric constants. The energy storage density of capacitor dielectric materials is proportional to the dielectric constant and the breakdown field strength. Ideally, it is hoped that the dielectric has both high dielectric constant and high breakdown field strength, but in reality the two are often contradictory. High dielectric constant materials have large dielectric loss and low breakdown voltage. The high dielectric loss associated with high dielectric constant has a negative impact on insulation performance.
High-frequency circuit substrate: High-frequency circuits require high signal transmission speeds. The signal transmission speed is closely related to the dielectric constant of the material. The higher the dielectric constant, the longer the signal delay time. To achieve fast signal transmission, a low dielectric constant substrate must be selected. In addition, fluctuations in dielectric constant at high frequencies can lead to impedance instability, phase distortion, and signal loss. Therefore, high-frequency circuits should use materials whose dielectric constants change little with frequency, temperature, and humidity. Materials such as polytetrafluoroethylene, alumina, and AlN, Aluminum Nitride have low dielectric constants, dielectric losses, and high stability, making them perform well in high-frequency circuits and improving circuit performance and reliability.
