Electret stability related to the crystallinity in polypropylene

Abstract

Through mixing isotactic-polypropylene (i-PP) and atactic-polypropylene (a-PP), we have demonstrated the importance of the crystallinity in polypropylene as an electret material. Samples with crystallinities between 7 % and 47 % were used. A high degree of crystallinity in polypropylene, used as an electret, gives a better charge stability with respect to temperature and humidity changes. The semicrystalline i-PP significantly outperforms a-PP regarding charge stability. a-PP is an amorphous polymer. By mixing a-PP and i-PP, the degree of crystallinity can be controlled, while all other sample preparation processes and characteristics can be identical. This is important since the performance of an electret material is sensitive to its previous process history. Activation energies used for predicting the thermal potential decay are determined from thermally stimulated current and isothermal potential decay experiments. Activation energies from 0.98 eV to 1.41 eV were determined. From these values, a very good agreement was achieved between the experimental potential decay at room temperature for more than 290 days, and a theoretical estimation of the potential decay.

Outstanding performance of parylene polymers as electrets for energy harvesting and high‐temperature applications

“…During thermal stimulation, the corresponding surface potential of the electret over time was measured every second using the electrostatic voltmeter (TREK 347). By deriving the surface potential value V(T) as a function of temperature T, the release current I(T) can be derived as follows: where ε 0 is the vacuum permittivity, ε r and d , respectively, stand as the material dielectric constant and thickness, and β is the heat rate temperature. Assuming that no charge retrapping occurs, the release current I(T) can be expressed, thanks to eq.…”

“…Assuming that no charge retrapping occurs, the release current I(T) can be expressed, thanks to eq. which describes the second‐order reaction kinetic: where n is the total number of carriers contained in the traps having activation energy equal to E a . ν exp(−E a /k b T) is the probability per unit of time that a carrier escapes from the trap, where k b is the Boltzmann constant and ν represents the attempt‐to‐escape frequency typically standing between 10 12 and 10 14 s −1 in polymer electrets.…”

“…During thermal stimulation, the corresponding surface potential of the electret over time was measured every second using the electrostatic voltmeter (TREK 347). By deriving the surface potential value V(T) as a function of temperature T, the release current I(T) can be derived as follows 18,46,47 :…”

“…Assuming that no charge retrapping occurs, the release current I(T) can be expressed, thanks to eq. (2) which describes the second-order reaction kinetic 18,46 :…”

Time and thermal stability improvement of polyethylene ferroelectrets

“…The stability of the piezoelectric coefficient of ferroelectrets is a critical factor from an application point of view. This electromechanical property originates from the charges between electric dipoles and from the cellular structure, which can change the dipole moment in response to an external stress . Obviously, the cellular structure stability is limited by temperature when approaching the polymer's melting point ( T m ).…”