Dielectric constant

The dielectric constant of a material is denoted by the Greek alphabet, kappa (κ). It is defined as:
 

• The ratio of the capacitance of the material to vacuum, C/C₀
• The ratio of the permittivity of the material to vacuum, ε/ ε₀

The dielectric constant is also termed relative permittivity (εr). This is because it is measured relatively from the permittivity of free space (ε₀). Other names include electric permittivity. 

 

The dielectric constant is a dimensionless measure. The formula to calculate the dielectric constant is:

Κ or ε_r  = C/C₀ = ε/ε₀

C₀ = ε₀A/T

where,
 

• Κ or ε_r = dielectric constant of the material
• C = capacitance using the material as the dielectric capacitor
• C₀ = capacitance using vacuum as the dielectric
• ε = permittivity of the substance
• ε₀ = permittivity of free space (8.85 x 10^-12 F/m i.e., Farad per metre)
• A = area of the plate/sample-cross section area
• T = thickness of the sample
   

1. Thermoplastics with Dielectric Constant - View all Products
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The dielectric constant is the ability of plastics to store electrical energy. Typical values of dielectric constant for different materials are:
 

Note: A dielectric constant of 2 indicates that an insulator absorbs twice the electrical charge as the vacuum.

 

• Frequency - Dielectric constant decreases with an increase in frequency.

• Moisture - Dielectric constant increases in the presence of moisture.

• Temperature - Dielectric constant increases with an increase in temperature. This happens till it reaches the transition temperature. Above this, an increase in temperature leads to a decrease in the dielectric constant.

• Voltage - Dielectric constant decreases in the presence of direct current voltage.

• Structure and morphology - It determines the polarization of materials. Thus, this influences the dielectric constant values.

The structure of dielectric polymers determines whether a polymer is polar or non-polar. This in turn decides the electrical properties of the polymer.

 

Polar polymers
 

• In polar polymers, dipoles are created due to an imbalance in the distribution of electrons.
• These dipoles tend to align in the presence of an electric field.
• The dipole polarization of the material is created. This makes the materials as good as insulators.
• Polar plastics absorb moisture from the atmosphere. The presence of moisture raises the dielectric constant and lowers the resistivity.
• With the rise in temperature, there is faster movement of polymer chains and fast alignment of dipoles. This raises the dielectric constant values for polar plastics.
• Example: PMMA, PVC, Nylon, PC, etc.

Non-polar polymers
 

• Non-polar polymers have symmetrical molecules and are covalent.
• There are no polar dipoles present in them. Hence, the presence of an electric field does not align the dipoles.
• But, slight electron polarization occurs due to the movement of electrons in the direction of the electric field. This is instantaneous.
• These polymers have high resistivities and low dielectric constant.
• Non-polar plastics are not affected by moisture and rise in temperature.
• Example: PTFE, PP, PE, PS, etc.

The applications of dielectric constant include:
 

• Use of materials in the production of capacitors. These capacitors are used in radios and other electrical equipment.

• It is used to compare different printed circuit board (PCB) materials.

• Polymer-based dielectric composites are useful for:

  • electronic packaging,
  • embedded capacitors, and
  • energy storage.

  These composites are flexible with a low process temperature. They exhibit a high dielectric constant, low dielectric loss, and high dielectric strength.

The standard tests to calculate the dielectric constant for plastics are:
 

• ASTM D2520: It is a standard test method to measure the dielectric constant. The solid electrical insulating materials are measured.

• ASTM D150: It determines the dielectric constant. The solid and liquid electrical insulating materials are measured.

• IEC 62631-2-1:2018: Part 2-1 — It measures the dielectric constant. It also determines the dissipation factor.

Procedure
 

1. A sample is placed between two metallic plates and capacitance is measured.
2. A second run is measured without the specimen between the two electrodes.
3. The ratio of these two values is the dielectric constant.
4. The test can be conducted at different frequencies, often between the 10Hz and 2MHz range.
5. The sample must be flat and larger than the 50mm (2 in) circular electrodes used for the measurement.