Thermally Stimulated Discharge Research Articles

A glass-assisting thermally stimulated discharge technique

Thermally stimulated discharge (TSD) technique is a traditional method in dielectric research, especially for electrets. However, in conventional open-circuit and short-circuit TSD techniques, it is difficult to distinguish the surface charge and body charge of dielectric materials. In particular with the test of polymer electrets, the deformation of the polymer film may take place during the measurement process, which will affect the accuracy of the experiment results. In this paper, a glass-assisting TSD (GA-TSD) technique is proposed to solve the above problems. The feasibility of the experimental technique is verified with the GA-TSD spectra of fluorinated ethylene-propylene copolymer electret films. In addition, their theory analysis is also accomplished. The influences of glass thickness, glass dielectric property, and metallizing on the glass on GA-TSD spectra are investigated. The results prove that the GA-TSD spectra can clearly distinguish the difference between surface charge and body charge according to the current direction. The quartz glass with the lowest dielectric constant is best suitable for the GA-TSD technique. The influence of the glass thickness and metallizing on the glass on GA-TSD spectra is little.

Parylene as an electret for condenser microphones

Electrets in condenser microphones are typically made from varieties of Teflon. This work evaluates different formulations and thicknesses of vapor deposited Parylene electrets as potential alternatives to Teflon. Parylene-coated condenser backplates used in this study were obtained from two different suppliers, as a third variable in addition to formulation and thickness. Long-term charge stability of the electrets was tested with a thermally stimulated discharge (TSD) method. Charge stability was also evaluated by building the charged backplates into condenser microphones and running them through environmental accelerated-life testing. Standard PTFE and FEP Teflon backplates were used as controls in these tests. The experiments showed clear differences in the long-term charge stability of the Parylene electrets due to thickness, formulation, and supplier. Several combinations of thickness, formulation, and supplier were found to perform comparably to traditional Teflon electrets and appear suitable for use in high-end condenser microphones.

Thermally stimulated discharge (TSD) study of low-temperature transitions in chlorine-containing polymers

Thermally stimulated discharge current (TSD) tests on PVC, chlorinated polyethylene, chlorinated PVC, and chlorinated paraffins always show a relatively low-intensity transition in the − 100 to − 80 °C range. Several chlorine-containing polymers and one chlorinated paraffin were tested up to 70% chlorine content by short-circuited TSD method. The intensities of these transitions show a maximum at about 40% chlorine content except for PVC. Knowing the random nature of chlorination, it is assumed that this transition origins from the rotational movement of neighboring –CH2–CHCl– monomeric units.

Biodegradable hydro-charging polylactic acid melt-blown nonwovens with efficient PM0.3 removal

Polylactic acid (PLA) melt-blown nonwovens are attractive candidates for existing non-degradable air filtration materials due to their unique balance between durability in use and biodegradability at their end-of-life. However, owing to the charge instability caused by conventional corona electret treatment, the filtration performance of PLA melt-blown nonwovens is insufficient to be utilized in practical applications. Herein, we presented an effective hydro-charging treatment strategy to prepare biodegradable PLA melt-blown nonwovens with efficient PM0.3 removal. In this process, a large amount of unstable charge was generated by friction between pressurized deionized water and PLA melt-blown microfibers under the influence of negative pressure suction. Then, the unstable charge on microfibers was transformed into stable charge by hot air drying. Through the adjustment of water pressure, conveyer belt speed and drying temperature, we optimized the hydro-charging treatment process. The resulting hydro-charging PLA melt-blown nonwovens (HC-PLA) shown improved PM0.3 removal efficiency (94.63%), a lower pressure drop (11.74 Pa) and a reasonable quality factor (0.249 Pa−1). After experiencing a hydro-charging treatment, PLA melt-blown nonwovens appear to carry more stable and plentiful charges, according to thesurface potential and thermally stimulated discharge (TSD). In addition, plausible charge generation and stable storage mechanisms were proposed based on the optimal performance, which provide promising benefits for the development of long-term and highly efficient filtration materials.

Combined thermal analysis of plant oils

The paper deals with the study of seven selected natural plant oils. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and thermally stimulated discharge (TSD) methods were used. It has been found that most of the oils tested are in a glassy state at low temperature and have multiple transitions in the low temperature range. DSC shows complex melting-like processes or glass transition. For both DMA and TSD, the scaffold supportive method was used and found as a suitable one. DMA and TSD proved more sensitive than DSC and revealed at least two transitions between − 120 and − 40 °C. In the case of three oils (argan, avocado and sunflower), current reversal was observed by TSD; this symptom cannot be fully explained at the moment.

High performance fluorinated polyethylene propylene ferroelectrets with an air-filled parallel-tunnel structure

Fluorinated polyethylene propylene (FEP) ferroelectrets with an air-filled parallel-tunnel structure and regularly distributed bipolar space charges can stimulate integration of ferroelectrets in transducers due to their unique piezoelectric and mechanical properties. Such FEP ferroelectret films exhibit not only large longitudinal piezoelectric activity but also a strong transverse piezoelectric effect which allows for application in flexible transducers either working in 33 mode or in 31 mode. In this article, the transducer relevant properties of FEP ferroelectret films have been characterized. The experimental results show that the directly measurable, quasi-static piezoelectric d 33- and g 31-coefficients of the FEP films reach up to 4700 pC N-1 and 4.3 Vm N-1, respectively, and dynamic values in the frequency range from 10 to 100 Hz are 2600–1500 pC N-1 and 1.2–0.78 Vm N-1, respectively. No significant reduction of d 33-coefficient is observed at strains up to 2.1%, showing a high elasticity of the film. Both isothermal decay of d 33-coefficient and short circuit thermally stimulated discharge (TSD) current spectra show that the ferroelectric FEP films exhibit excellent thermal stability. After 14-day of storage in an atmosphere with a relative humidity of 99%, 75% of the initial piezoelectric d 33-coefficient was still retained. Also a mechanical test with 1.6 million cycles does not harm the piezoelectric coefficients indicating that air-filled parallel-tunnel FEP ferroelectret films are promising candidates for a variety of flexible transducer applications.

Combined thermal analysis of fluid plasticizers

The paper deals with the study of plasticizers using different thermal methods. The literature data on the melting points of plasticizers proved uncertain; we intended to gather the data by other methods, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and thermally stimulated discharge (TSD). Results of ten plasticizers are demonstrated. During this work, we found that most of plasticizers have no well-defined melting point, and the solidification of plasticizer is similar to the glass transition of polymers. Only the di-n-butyl-phthalate showed regular crystallization. Thermally stimulated discharge current (TSD) method revealed that these compounds have several transitions –dispersion ranges assigned to different molecular motions.

Compound electret-system with improved hydrophobicity and charge stability

In this work, we propose a compound electret-system with improved hydrophobicity and charge stability in water and at elevated temperatures by bonding a solid fluorinated-ethylene-propylene (FEP) film to a porous polytetrafluoroethylene (PTFE) film. Two bonding approaches are investigated, the electrostatic-bonding and melting-bonding, to laminate the FEP and PTFE layers. Experimentally, the compound system shows improved hydrophobicity and charge stability both in water and elevated temperatures compared with the single layer FEP. The contact angles of the compound samples with electrostatic-bonding and melting-bonding are 139 and 124°, respectively. After immersion in water for 16 hours, surface potentials of −406 and −105 V are retained in the samples with electrostatic-bonding and melting-bonding, respectively, which exceeds the single layer FEP. The open circuit thermally stimulated discharge (TSD) current spectra show that the negatively charged electrostatic-bonding sample has a broad positive current peak at 202 °C, while for the melting-bonding sample charged under the same conditions, shows two current peaks at 231 °C and 269 °C. The enhancement of hydrophobicity and charge stability in water for compound systems must be attributed to the specific geometry on the surface of porous PTFE layer, and the improved thermal stability of charges in melting-bonding compound system could be due to the deep charge traps at the interface between FEP and porous PTFE layers generated during heat-pressing process.

Air-coupled ultrasonic transducers based on laminated fluorinated ethylene propylene and porous polytetrafluoroethylene ferroelectrets

Laminated fluorocarbon films with an artificially controlled periodic microstructure consisting of solid fluorinated ethylene propylene (FEP) and porous polytetrafluoroethylene (PTFE) layers were successfully fabricated by using a patterning and fusion bonding method. They were made piezoelectric and turned into ferroelectrets by contact charging. The acoustic impedance of the fabricated ferroelectrets determined from dielectric spectra is about 0.012 MRayl. This provides a good match to air. Air-coupled transducers based on such films exhibit a broad bandwidth. For a transducer with a diameter of 20 mm, the transmitting sensitivity is 1.6 mPa/V at 60 cm distance and 50 kHz, while the receiving sensitivity is about 1.8 mV/Pa in a broad frequency range from 25 to 35 kHz. These laminated films, designed in such a way that only positive charges are deposited into the porous PTFE layers, show a significantly improved thermal stability. This arises from the good thermal stability of positive charges in porous PTFE, which is demonstrated by the open circuit thermally-stimulated discharge current spectra.

Evaluations of dielectric property and drug release profile of 5-FU patches based on plasma charged electrets

In the present study, the electret 5-fluorouracil patch was developed, the effective surface potential, piezoelectric coefficient d33, open-circuit thermally stimulated discharge (TSD) current spectra and shear adhesion of the patch were measured. The drug release profile of the patch was determined by using high performance liquid chromatography method. A stable potential difference which was positively dependent on the surface potential of the electret was generated on two sides of the patch. The measurements of d33 coefficient, TSD current spectra and adhesion performance showed that the electrostatic field of the electret could cause polarization and cohesive strength decreasing of the matrix molecules, change the distribution and interaction of the drug molecules in patch, therefore to increase the release of drug from the transdermal patch.

Depolarization current relaxation process of insulating dielectrics after corona poling under different charging conditions

As an insulating dielectric, polyimide is favorable for the application of optoelectronics, electrical insulation system in electric power industry, insulating, and packaging materials in space aircraft, due to its excellent thermal, mechanical and electrical insulating stability. The charge storage profile of such insulating dielectric is utmost important to its application, when it is exposed to electron irradiation, high voltage corona discharge or other treatments. These treatments could induce changes in physical and chemical properties of treated samples. To investigate the charge storage mechanism of the insulating dielectrics after high-voltage corona discharge, the relaxation processes responsible for corona charged polyimide films under different poling conditions were analyzed by the Thermally Stimulated Discharge Currents method (TSDC). In the results of thermal relaxation process, the appearance of various peaks in TSDC spectra provided a deep insight into the molecular status in the dielectric material and reflected stored space charge relaxation process in the insulating polymers after corona discharge treatments. Furthermore, the different space charge distribution status under various poling temperature and different discharge voltage level were also investigated, which could partly reflect the influence of the ambiance condition on the functional dielectrics after corona poling.

Photo-stimulated discharge current in polyimide films

Photo-stimulated discharge (PSD) method is one of the ways to measure the trap level of space charge in the dielectric materials. Compared with the thermally stimulated discharge (TSD) method, it has no observable destroy to the sample's microstructure during the procedure of the light scanning and can therefore be considered to be a more accurate measuring method. However, the light scanning procedure may possibly lead to the occurrence of the external photoelectric effect between the electrode-dielectric interfaces or the internal photoelectric effect in the sample bulk, where the extra current due to the two photoelectric effects can seriously distort the PSD current. In this paper, the total current excited by the scanning light on the polarized or being polarized polyimide films are studied. The influence of the applied field intensity, the electrode structures, the evaporated electrode thickness and the isothermal decaying current of the polarized films on the stimulated current is evaluated, which is comprehensively analyzed to extract the space charge dependent PSD current from the distorted current. The charge trap level based on the PSD method is finally obtained.

Chemical modification with orthophosphoric acid enhances surface-charge stability on polypropylene electrets

The low surface-charge stability of polypropylene (PP) frequently limits its application as an electret material. In this paper, we demonstrate how the treatment of PP-film surfaces with orthophosphoric acid (H3PO4) enhances their charge stability. To discriminate between the effects of chemical modification and thermal treatment, as-received and annealed PP films are used as reference samples. The electret properties of treated and non-treated PP films are characterized with thermally stimulated discharge (TSD) and isothermal surface-potential decay (ISPD) experiments, from which considerable improvement in thermal and temporal charge stability is observed for samples modified with H3PO4. The half-value temperature (T1∕2) observed on TSD curves of chemically treated PP increases to 131 and 145 °C for positive and negative charges, respectively. The enhancement might be attributed to the phosphoric compounds detected on the H3PO4-modified surfaces via attenuated-total-reflection infrared spectroscopy. Deeper surface traps formed at the “foreign” phosphorus-containing structures are able to capture the charges over longer time periods and at higher temperatures, thus leading to significant improvements in the temporal and thermal surface-charge stabilities of PP electrets.

N-Alkyl Pyrrolidones for Plasticizing Polyurethane Elastomers

Polyurethane (PU) products produced from the reaction of isocyanates and polyols have one of the most extensive consumption of plastics. After the invention in 1937 PU had to wait until the 50’s to spread worldwide and since then their development have been continuous. In PU systems are used varied additives, including plasticizers, which are use by several large companies.In our work a new class of plasticizers, namely N-alkyl-pyrrolidones were used in polyurethane systems. Their trade names are Flexidone® 100, Flexidone® 300 and Flexidone® 500 containing 8, 12 and 18 C-atom long alkyl chain, respectively. These plasticizers showed excellent results in our previous works and the literature has also confirmed their great properties. We supposed that this chemical structure can form a strong interaction with the urethane group.The effect was monitored by hardness and dynamic mechanical (DMA) measurements. Migration and thermally stimulated discharge (TSD) measurments are in progress.

Charge storage and transport in oriented and non-oriented polytetrafluoroethylene films

The charge storage and transport in oriented and non-oriented polytetrafluoroethylene (PTFE) films, charged by the corona method at room temperature, were investigated by isothermal decay and thermally stimulated discharge (TSD) techniques. The crystallinity of the PTFE films was determined by differential scanning calorimetry (DSC). The results obtained from isothermal decay of the surface potential and the spectra of the TSD charge and current indicate that the thermal stability of both positive and negative charges in oriented PTFE films is significantly better than that of non-oriented PTFE films. This is probably due to the higher crystallinity in oriented PTFE films. The transport of the de-trapped positive charges in oriented PTFE films, which occurred due to thermal excitation, can be explained by a model that takes thermal ion emission into consideration.

Study of electret effect of rat skin by thermally stimulated discharge analysis

Abstract The thermally stimulated discharge (TSD) spectra of three types of rat skin treated and untreated by negative polypropylene (PP) electret were measured. The natural rat skin shows an excellent electret effect due to the lamellar lipid structure, reticular structure of collagen and protein space structure related hydrogen bond network and non-covalent bond system. Hydration intensifies the skin electret effect. Electret could change the lamellar lipid structure and charge state of skin surface by alteration of dipolar orientation, intermolecular non-covalent action and protein transformation from α helix to β sheet. The regulation of skin electret effect depends on tightly bound water.

Polarization and piezoelectricity in polymer films with artificial void structure

Laminated polymer-film systems with well-defined void structures were prepared from fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE) layers. First the PTFE films were patterned and then fusion-bonded with the FEP films. The laminates were subjected to either corona or contact charging in order to obtain the desired piezoelectricity. The build-up of the “macro-dipoles” in the laminated films was studied by recording the electric hysteresis loops. The resulting electro-mechanical properties were investigated by means of dielectric resonance spectroscopy (DRS) and direct measurements of the stress-strain relationship. Moreover, the thermal stability of the piezoelectric d33 coefficient was investigated at elevated temperatures and via thermally stimulated discharge (TSD) current measurements in short circuit. For 150 μm thick laminated films, consisting of one 25 μm thick PTFE layer, two 12.5 μm thick FEP layers, and a void of 100 μm height, the critical voltage necessary for the build-up of the “macro-dipoles” in the inner voids was approximately 1400 V, which agrees with the value calculated from the Paschen Law. A quasi-static piezoelectric d33 coefficient up to 300 pC/N was observed after corona charging. The mechanical properties of the film systems are highly anisotropic. At room temperature, the Young’s moduli of the laminated film system are around 0.37 MPa in the thickness direction and 274 MPa in the lateral direction, respectively. Using these values, the theoretical shape anisotropy ratio of the void was calculated, which agrees well with experimental observation. Compared with films that do not exhibit structural regularity, the laminates showed improved thermal stability of the d33 coefficients. The thermal stability of d33 can be further improved by pre-aging. E.g., the reduction of the d33 value in the sample pre-aged at 150°C for 5 h was less than 5% after annealing for 30 h at a temperature of 90°C.

Performance of Piezoelectrets Made of Non-Porous Polytetrafluoroethylene and Fluoroethylenepropylene Layers

The thermal stability of piezoelectricd33coefficients and charge dynamics in the piezoelectret films with tailored microstructure, made of non-porous polytetrafluoroethylene (PTFE) and fluoroethylenepropylene (FEP) layers, are investigated by the measurements of the isothermal decay of piezoelectricd33coefficients at elevated temperatures and the analysis of thermally stimulated discharge (TSD) current spectra in short circuit, respectively. The results show that the quasi-static piezoelectricd33coefficient up to 300 pC/N is achieved and its Young's modulus is about 0.28 MPa. Thed33values in the present studied films show improved thermal stability compared with the films without regular microstructure. The drift path for the most of detrapped charges in the films is through the solid dielectric layer.

Mechanical and electrical properties of laminated polytetrafluoroethylene films

Laminated polytetrafluoroethylene (PTFE) films, which are made of compact and porous PTFE layers, are prepared through the process of sintering. The corona charging technique is utilized to make the laminated PTFE films piezoelectric, thus transforming them into piezoelectrets. The crystallinity, Young's modulus in compression mode, stress-strain behavior, charge dynamics, and thermal stability of the fabricated films are investigated by particular techniques, such as differential scanning calorimetry (DSC), dielectric resonance spectra, dynamic mechanical analysis (DMA), thermally stimulated discharge (TSD) spectra, and isothermal annealing, respectively. The results reveal that the crystallinity of the fabricated PTFE films with three and five-layer systems are 79.5 and 59.8 %, respectively. The compressive and tensile moduli at room temperature are 7.4 and 167 MPa for the three-layer system samples. The resulting temperature peak increases by 20°C as the heating rate increases from 2 to 4°C in TSD measurement. Two charge drift mechanisms exist in the films when the samples are thermally stimulated and discharged. With the increase of corona charging voltage from -10 to -25 kV, more and more detrapped charges from the deeper traps in the laminated PTFE films are released, corresponding to the current peaks identified in the temperature range from 130 to 140°C, which prefer to drift through the solid PTFE layers. However, charges also escaped from the relevant shallow traps, corresponding to the current peaks identified in the temperature that range from 80 to 95°C. The charge drift along the surface of the PTFE fibers is always a dominant mechanism, showing resistance of the corona charging voltage under the experimental-study conditions. The sample shows a stable piezoelectric d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> coefficient of 50 pC/N at 120°C after one day annealing at the same temperature.

Significantly improved charge stability of cellular polypropylene films by fluorination and subsequent annealing

An extremely effective and practical approach is proposed and used for improving charge stability of cellular polypropylene (PP) films. The approach is composed of fluorination and subsequent annealing of the PP films. Surface charge stability is significantly improved by the approach, as revealed by the measurements of open-circuit thermally stimulated discharge (TSD) current, charge TSD and isothermal charge decay at 90 °C. As an example, after 7 h at 90 °C, the amount of surface charge of the treated PP film by the approach still remains about 72% of its initial value, while the corresponding value is only 27% for the virgin PP film. The improvement of charge stability is attributed to chemical composition change and structure modification of the PP films due to the fluorination and annealing treatments, as indicated by attenuated total reflection infrared analyses and wide angle X-ray diffraction measurements. The treatments eliminate the shallow charge traps almost completely and produce much deeper ones.