Thermal depolarization of PVDF : anomaly at 180°C

PVDF samples with high β content (80%), prepolarized at room temperature, were heated to higher temperatures. The polarization was measured first under open circuit conditions at the high temperature and then after cooling down again to room temperature For elevated temperatures up to 175°C the polarization was reduced to 2% of the original value, yet recovered roughly to 8% of the original value after cooling down to room temperature In contrast to this, when heating the films to 180°C, a significantly different behavior was observed in cooling down to room temperature again the polarization returned from ~1% at 180°C to ~40% of the original prepolarized value

Effects of the glutamate carboxypeptidase II (GCP2 1561C>T) and reduced folate carrier (RFC1 80G>A) allelic variants on folate and total homocysteine levels in kidney transplant patients

Effects of the glutamate carboxypeptidase II (GCP2 1561C>T) and reduced folate carrier (RFC1 80G>A) allelic variants on folate and total homocysteine levels in kidney transplant patients.BackgroundThe effect of the glutamate carboxypeptidase II GCP2 1561C>T and the reduced folate carrier 1 RFC1 80G>A polymorphisms on folate and total homocysteine (tHcy) plasma levels of kidney transplant patients are unknown.MethodsIn a cross-sectional study of 730 kidney allograft recipients, GCP2 1561C>T, RFC1 80G>A, folate, and tHcy plasma levels were analyzed using linear regression models that allowed dependent covariates to follow a gamma distribution for univariate and multivariate analyses.ResultsThe allele frequency for GCP2 1561C>T was 0.05, and 0.43 for RFC1 80G>A. Heterozygosity or homozygosity for GCP2 1561C>T was associated with higher folate plasma levels compared to patients without mutation (P < 0.0001), while RFC1 80G>A showed no influence. Multiple testing, also including MTHFR 677C>T and MTHFR 1298A>C, revealed no interaction between the different genotypes and the folate plasma concentration. Neither GCP2 1561C>T nor RFC1 80G>A showed an association with tHcy plasma levels.ConclusionWe conclude that GCP2 1561C>T is associated with elevated folate levels. GCP2 1561C>T and RFC1 80G>A are no major determinants of tHcy plasma levels in kidney transplant patients

Effects of TCN2 776C>G on vitamin B12, folate, and total homocysteine levels in kidney transplant patients

Effects of TCN2 776C>G on vitamin B12, folate, and total homocysteine levels in kidney transplant patients.BackgroundControversy exists regarding the possible associations between a single nucleotide polymorphism of the transcobalamin II encoding gene (TCN2 776C>G) and plasma levels of vitamin B12, folate, or total homocysteine.MethodsIn a cross-sectional study of 732 kidney allograft recipients, patients were categorized by TCN2 776C>G genotype. In univariate and multivariate linear regression models that allowed the outcome variables vitamin B12, folate, and total homocysteine plasma levels to follow a gamma distribution, we tested for possible associations of allelic variants of the TCN2 776C>G gene and these three dependent variables.ResultsThe allele frequency for TCN2 776C>G was 0.46. Heterozygosity or homozygosity for TCN2 776C>G was not associated with plasma levels of vitamin B12 (776CG, P = 0.22; 776GG, P = 0.89), folate (776CG, P = 0.91; 776GG, P = 0.84), or total homocysteine (776CG, P = 0.11; 776GG, P = 0.33) even after adjustment for several possible confounders.ConclusionWe conclude from this largest study on the subject thus far that there are no associations between allelic variants of TCN2 776C>G and plasma vitamin B12, folate, or total homocysteine plasma levels in kidney transplant patients

Polarization dynamics of VDF-TrFE copolymers

The polymer polyvinylidenefluoride (PVDF) and its copolymers with trifluoroethylene (TrFE) exhibit a strong piezoelectric effect after poling in high electric fields (100 MV/m). HV impulses of definite duration have been applied to the polymer in order to study the dynamics of the poling process. The dielectric displacement during the HV impulse was recorded. After the impulse, the remanent polarization under short-circuit conditions was also measured. It was thus possible to obtain the minimum poling time dependent on the applied field strength necessary to stabilize the remanent polarization. Comparing the time development of the dielectric displacement with the corresponding remanent polarization revealed a time delay between the orientation of the dipoles and their stabilization. It is concluded that the process of orientation of the dipoles itself is not sufficient to lead to a remanent polarization and that an additional interaction between the trapped charges and the oriented dipoles can explain the stability of the remanent polarization and the observed time delay

Time development of multiple polarization zones in PVDF

The polarization distribution in PVDF which causes the piezo- and pyroelectricity of this polymer depends strongly on the polarization distribution. Applying an electric field of E = 60M/m causes a ca. 10&#956;m thick polarization zone in the middle of the sample. Applying the same field in opposite direction a bimorph structure results. If the same field is applied to a homogeneously prepolarized PVDF sample a trimorph structure is formed with three polarization zones under field. But under short circuit conditions the polarization zone in the new field direction vanishes. The polarization distribution is measured with the acoustic PPS method with a spatial resolution of about 2&#956;m

Influence of poling conditions on the gas emission of PVDF

The influence of the poling time and the polarity of the applied voltage on the time dependence of the gas emission of PVDF is investigated. If PVDF films are poled in electric fields of about 100MV/m, charges are injected and ions are formed by electrochemical reactions. By recombination and by other reactions of these ions, HF gas is evolved, diffusing out of the sample through the porous electrode. This gas emission increases with increasing field strength. At constant field it decreases to a steady state value comparable to the space-charge-limited poling current of PVDF. Free ions are trapped in the polarization zone. Under short-circuit conditions shallow trapped ions are freed and the gas emission increases strongly. The longer the poling time, the broader the polarization distribution. If the polarization zone is located near the nonporous electrode, the recombined gas molecules need longer time to reach the porous electrode. Therefore, the gas emission under short-circuit conditions is delayed by poling with negative polarity compared to positive polarity

GaN heterostructures as innovative x-ray imaging sensors — change of paradigm

Direct conversion of X-ray irradiation using a semiconductor material is an emerging technology in medical and material sciences. Existing technologies face problems, such as sensitivity or resilience. Here, we describe a novel class of X-ray sensors based on GaN thin film and GaN/AlGaN high-electron-mobility transistors (HEMTs), a promising enabling technology in the modern world of GaN devices for high power, high temperature, high frequency, optoelectronic, and military/space applications. The GaN/AlGaN HEMT-based X-ray sensors offer superior performance, as evidenced by higher sensitivity due to intensification of electrons in the two-dimensional electron gas (2DEG), by ionizing radiation. This increase in detector sensitivity, by a factor of 104 compared to GaN thin film, now offers the opportunity to reduce health risks associated with the steady increase in CT scans in today’s medicine, and the associated increase in exposure to harmful ionizing radiation, by introducing GaN/AlGaN sensors into X-ray imaging devices, for the benefit of the patient

Binding energies of trapped charges in PVDF and P(VDF/TrFE)

The remanent polarization in PVDF and P(VDF/TrFE) is stabilized by trapped charges. We measured the remanent polarization P as function of temperature during heating from room temperature to 180°C. The discharge current is then computed as the first derivative -dP/dT. Assuming a Debye relaxation and a continuous distribution of binding energies g(E) of the trapped charges we can show that the discharge current I(T) is proportional to g(E=mT), with m=const. The constant m can either be calculated numerically or by analytical approximation, With this method the distribution g(E) can be determined. These data are consistent with the extraordinary long life time of the remanent polarisation in PVDF and P(VDF/TrFE) at room temperature

Polarization distribution in PVDF obtained by poling under constant current condition

Since the resistivity of PVDE films increases when the films are poled, the voltage across the samples must be increased during the poling process in order to keep the current constant. Under these conditions, the formation of a polarization zone in the center of the film depth is observed. The thickness of the polarization-free zone close to the film surfaces is found to decrease with larger current densities. The results are consistent with a model assuming charge trapping in deep traps associated with the orientation of polar crystallites

Field induced gas emission of polymer films

Poling polyvinylidene fluoride under vacuum conditions causes strong gas emission, which is analyzed with a quadrupole mass spectrometer. The dominant peak in the mass spectrum is HF, which is produced by electrochemical reactions at the sample surface. After the electric field is switched off, the gas emission unexpectedly increases to a value higher than under the field. This can be explained by ionic charge detrappingand charge migratio