Treatment with orthophosphoric acid enhances the thermal stability of the piezoelectricity in low-density polyethylene ferroelectrets

Abstract

Ferroelectrets have been fabricated from low-density polyethylene (LDPE) films by means of a template-based lamination. The temperature dependence of the piezoelectric d33 coefficient has been investigated. It was found that low-density polyethylene ferroelectrets have rather low thermal stability with the piezoelectric coefficient decaying almost to zero already at 100 °C. This behavior is attributed to the poor electret properties of the polyethylene films used for the fabrication of the ferroelectrets. In order to improve the charge trapping and the thermal stability of electret charge and piezoelectricity, LDPE ferroelectrets were treated with orthophosphoric acid. The treatment resulted in considerable improvements of the charge stability in LDPE films and in ferroelectret systems made from them. For example, the charge and piezoelectric-coefficient decay curves shifted to higher temperatures by 60 K and 40 K, respectively. It is shown that the decay of the piezoelectric coefficient in LDPE ferroelectrets is governed by the relaxation of less stable positive charges. The treatment also leads to noticeable changes in the chemical composition of the LDPE surface. Infrared spectroscopy reveals absorption bands attributed to phosphorus-containing structures, while scanning electron microscopy shows new island-like structures, 50–200 nm in diameter, on the modified surface.

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“…Similar improvements were also achieved on low-density polyethylene (LDPE) films used for assembling a templatebased tubular-channel piezoelectret system [12], [19]. In this case, the charge with the lower stability determines the stability of the desired piezoelectricity of the system.…”

“…While XPS (ESCA) spectra reveal that titanium and oxygen have been built into the polymer surface after the treatment with TiCl 4 vapor [18], the scanning-electron-microscope (SEM) images of Figure 14 demonstrate that nano-islands have been formed on the surface treated with orthophosphoric acid (H 3 PO 4 ) [19]. From such images, it seems that a 2-dimensional nanodielectric was generated by self-assembly from the liquid phase and that the nano-islands might provide true "insulation" of the subsequently deposited electret charges.…”

“…Therefore, it is necessary to stabilize positive and negative charges to a similar degree. The goal was achieved by means of a treatment with orthophosphoric acid (H 3 PO 4 ) as demonstrated in Figure 12 where the thermal stability of both, positive and negative, charges increased by approximately 70 • C [19]. Figure 13 illustrates that the desired improvement in the thermal stability of the relevant piezoelectric thickness coefficient has been achieved [19].…”

“…The goal was achieved by means of a treatment with orthophosphoric acid (H 3 PO 4 ) as demonstrated in Figure 12 where the thermal stability of both, positive and negative, charges increased by approximately 70 • C [19]. Figure 13 illustrates that the desired improvement in the thermal stability of the relevant piezoelectric thickness coefficient has been achieved [19]. In a direct comparison between tubular-channel electrets made from LDPE and from FEP films [20] not only the temperature dependences of the respective piezoelectric response, but also its pressure dependences and frequency characteristics are reported and discussed in view of possible device applications in electroacoustic transducers.…”

“…Initial surface potential of all samples: ±6.5 kV. (1)and(2)as-received samples (black); (3) and (4) samples treated with H 3 PO 4 (red)[19].…”