Eunjoo Kim

Origins of thermally stimulated current in polyethersulfone

In order to understand the dominant carrier species in electrical conduction in polyethersulfone (PES), thermally stimulated current (TSC) measurements were carried out under various conditions. It was found that PES has two TSC peaks in the temperature range from 20 to 220/spl deg/C, consisting of an /spl alpha/ peak at /spl sim/210/spl deg/C and a /spl beta/ peak whose peak temperature moves towards a higher temperature with an increasing poling temperature. Even when the sample was not poled, PES shows a spontaneous current with its peak at /spl sim/210/spl deg/C. Both the spontaneous current peak and the /spl alpha/ peak were found to disappear when the sample had been heated to 230/spl deg/C, keeping the external circuit closed. From such similarity of appearance and disappearance between the two peaks, it is concluded that they have the same origin. From the change in the spatial distribution of space charges inside the sample measured simultaneously with the TSC measurements, positive charges, probably due to K/sup +/ ions, existed nonuniformly in the sample from the beginning, and are thought to be responsible for the two peaks. From a similar study, the /spl beta/ peak is considered to be due to polarization of ionic space charges during the poling process.

Ionic behavior of dc conduction in polyetheretherketone

The conduction process in polyetheretherketone (PEEK) is discussed mainly from the results of dc conduction, thermal pulse current (TPC), thermally stimulated current (TSC), and pulsed electroacoustic (PEA) measurements. It was found that dc conduction currents in PEEK are consistent with the values predicted by a theory based on the conventional thermally activated ionic hopping process. The ionic hopping distance was calculated to change from 2.5 to 3.9 nm when the temperature crosses the glass transition temperature. The results of TPC and PEA measurements show the existence of heterocharges in the vicinity of both electrodes. In TSC spectra, a peak appears at a temperature of /spl sim/5 to 20/spl deg/C above the biasing temperature. The peak position approaches to the constant temperature of 155/spl deg/C when the biasing temperature is >150/spl deg/C. The peak height is maximum when the biasing temperature is 150/spl deg/C. It is considered that this peak is related to the glass transition and caused by the depolarization of ionic space charge. From these results, it is concluded that ionic carriers which may come from byproducts of synthesis play an important role in the dc conduction process in PEEK. >

Effect of gamma-ray irradiation on the TSC in polyethersulfone

When polyethersulfone was irradiated by /spl gamma/ rays and then poled at 37/spl deg/C, a new TSC (thermally stimulated current) peak appeared at /spl sim/100/spl deg/C. This new peak and other peaks near 40 and 150/spl deg/C increase with an increase in dose of /spl sim/0.6 kGy, and this is considered to be due to trapped charges and other ionic carriers becoming mobile when rearrangement of scissioned chains occurs. However, if the dose exceeds 0.6 kGy, the peak magnitude decreases. Furthermore, if the TSC was poled at 120/spl deg/C, only the peak near 150/spl deg/C appeared and this peak monotonically decreased with an increase in the irradiation dose. A similar decreasing tendency was also observed in the conduction current and in the residual voltage. These decreasing features are considered to be caused by the radiation-induced change that rearrangement of scissioned chains decreases free volume for ionic transport.

Dominant conduction carriers in polyethersulfone

With the development of modern technology, the working condition for insulating materials has become even severer. Electrical insulating materials are demanded to have even higher reliability. Good thermal endurance has also become very important in recent years. Polyethersulfone (PES) is a new engineering thermoplastic material which can be used continuously at 180/spl deg/C thanks to its high glass transition temperature of 225/spl deg/C. It also has the highest-class resistance for chemicals among amorphous polymers. Therefore, PES is expected to be applied in electrical insulation as well as in various other industrial fields. In this paper, dominant conduction carriers in polyethersulfone are discussed through the experimental results obtained mainly by the thermally stimulated discharging current (TSC) and dc conduction current.

Ionic DC transport in polyetheretherketone

The dc conduction process in polyetheretherketone is discussed from the results of thermally stimulated current (TSC). In TSC spectra, a peak appears at a temperature of about 5-20/spl deg/C above the biasing temperature and approaches to the constant temperature of 155/spl deg/C when the biasing temperature is higher than 150/spl deg/C. This peak shows the maximum value at 155/spl deg/C when the biasing temperature is 150/spl deg/C. It is considered that this peak is related to the glass transition and caused by the depolarization of an ionic space charge. A drastically increasing current is also observed at temperatures over 180/spl deg/C. It is considered that this current is attributed to a spurious current which is spontaneously generated even in a non-biased sample. Both the large current at high temperature and the above mentioned peak in TSC are thought to be related to ionic carriers which may come from by-products of synthesis. This result is consistent with the dc conduction process, which is mainly due to ionic transport.<<ETX>>

Effect of /spl gamma/-ray irradiation on the thermally stimulated current in polyethersulfone

Effect of /spl gamma/-ray irradiation on the charge behavior of polyethersulfone (PES) is investigated through the measurement of thermally stimulated current (TSC). Each sample is irradiated by /spl gamma/-rays in air at room temperature to various total doses less than 30 kGy. In the TSC spectra obtained after a poling process at 37/spl deg/C, /spl beta/ peak at 150/spl deg/C increases first with an increase in the dose up to 0.61 kGy, and then decreases. The /spl gamma/ peak which is not observed in the non-irradiated sample appears at 100/spl deg/C in the irradiated sample. Similarly to the /spl beta/ peak, the /spl gamma/ peak increases in the low dose region and decreases if the irradiation dose exceeds 1 kGy. When the sample is poled at 120/spl deg/C, the /spl gamma/ peak is not observed and the /spl beta/ peak decreases monotonically as the dose increases. It is considered that scission of polymer main chains occurs during the /spl gamma/-ray irradiation and that rearrangement of main chains from a loose packing to a more rigid packing occurs while the sample is heated during the TSC measurement. This molecular rearrangement is considered to be the origin of the /spl gamma/ peak and also related to the decrease of the charge quantity of TSC.

Low-temperature TSC peak of polyethersulfone

Through the measurements of TSC and space charge distribution, it is concluded that the low-temperature /spl beta/ peak in TSC and the conduction current in the amorphous thermoplastic polymer polyethersulfone (PES) is mainly due to impurity ions.

Effect of /spl gamma/-ray irradiation on thermally stimulated current in polyethersulfone

It is shown that when the thermoplastic polymer polyethersulfone (PES) is irradiated by /spl gamma/-rays, the DC conduction current and thermally stimulated current (TSC) decrease. This is probably because the progress of crosslinking induced by the irradiation hinders the transport of ions.

Spontaneous current of polyethersulfone investigated through the measurement of space charge distribution by the pulsed electroacoustic method and thermally stimulated current

In the temperature range from 0/spl deg/C to 230/spl deg/, the authors show that polyethersulfone (PES) has two TSC peaks, /spl alpha/ and /spl beta/. A spontaneous current is also observed even in the sample to which electric field has never been applied. The spontaneous current and TSC /spl alpha/ peak are caused by the non-uniformly distributed space charges in the sample which were created during the manufacturing process.

Origins of thermally stimulated current peaks in polyethersulfone investigated through the measurements of space charge distribution

The mechanism of electrical conduction in polyethersulfone (PES) is investigated through the thermally stimulated current (TSC), dc conduction current, and space charge distribution. PES has two TSC peaks below the glass transition temperature of 225/spl deg/C. The peak temperature of the lower-temperature TSC, /spl beta/, increases with an increase in the poling temperature. Through the measurements of space charge distribution by the pulsed electroacoustic method done in combination with the TSC measurements, /spl beta/ peak is considered to be due to ions. Furthermore, the dc conduction current obeys the ionic hopping conduction model, suggesting that the dominant carriers are ions.