Polarization Relaxation Effects Research Articles

Regulating N-doping strategies to construct 3D interconnected abundant porous N-doped carbon for polarization loss-enhanced microwave absorbers: Theory, design, and experiments

The widespread proliferation of modern portable electronic devices has necessitated the development of functional materials with high-efficiency microwave absorption properties. In this study, we utilized biomass-derived jujube shells as a carbon source and employed a nitrogen-doping process to modify carbon materials, resulting in the fabrication of outstanding microwave-absorbing biomass-based nitrogen-doped porous carbon. Additionally, we discussed the influence of different nitrogen sources on material structure and microwave absorption performance. These N-doped porous carbon materials exhibited excellent specific surface areas, particularly Org-N, which facilitated the formation of conductive networks and the multi-stage reflection and scattering of microwave. Moreover, N-doping created numerous defect sites and heterogeneous structures, improving impedance matching, thus promoting dipole polarization and interface polarization effects, which accelerated electron migration and interlayer hopping, contributing to the attenuation of microwave energy. The lowest reflection loss for Org-N, Inorg-N, and Mixed-N reached −27.3 dB, −40.9 dB, and −45.6 dB, respectively. Mixed-N exhibited optimal microwave absorption performance at 9.9 GHz with d = 2.4 mm and achieved an effective absorption bandwidth (EAB10) of 3.0 GHz (11.0 GHz–14.0 GHz) at d = 2.0 mm. Additionally, the enhancement of polarization relaxation effects was elucidated by examining the charge density distribution in molecular models featuring various N-doping forms. Consequently, biomass-derived N-doped porous carbon materials emerge as lightweight and highly efficient functional materials for microwave absorption purposes.

Synthesis of effective microwave absorber C/ZnS/Cu composites via molten salt route

A novel multi-component composites C/ZnS/Cu is synthesized via molten salt method (MSM) for the microwave absorbing (MA) materials. The Cu acts as conductive centers to build a conductive system with carbon and the heterostructures among the ZnS, Cu and C promote the polarization relaxation effect. The result indicates that the value of reflection loss (RL) of C/ZnS/Cu can reach – 63.64 dB and the effective absorption band (EAB) reach 6.48 GHz, which is superior to the MA properties of many current reports. The contrast tests explore the effect of holding times on the MA performance of C/ZnS/Cu composites. This research expands the application of MSM in the MA field, and provides a reference for the MSM preparation of MA materials.

Relaxation Induced by Imprint Phenomena in Low-Temperature (400 °C) Processed Hf0.5Zr0.5O2-Based Metal-Ferroelectric-Metal Capacitors

Ferroelectricity in Hf0.5Zr0.5O2 (HZO) has garnered increasing interest due to its potential applications in neuromorphic and nonvolatile memory devices. However, with time, the adverse shift in the coercive voltage (Vc) observed in a prepoled HZO ferroelectric capacitor can lead to insufficient polarization switching from one polarity owing to the development of a built-in voltage, causing reliability concerns. Another consequence of developed built-in voltage is polarization relaxation, that is, the rapid loss of stored polarization with time (t < 6 s). In this work, the correlation between the data retention issues in ferroelectric films, namely, imprint (Vc shift with time) and polarization relaxation, is studied carefully by pulse measurement techniques in HZO-based metal-ferroelectric-metal capacitors. In a nonaged ferroelectric, there are no significant relaxation effects for a switching time delay up to 6 s, even at higher measurement temperatures until 125 °C. After a baking process was performed at higher temperatures (25–125 °C) to a prepoled ferroelectric capacitor, an apparent systematic imprint was observed with increasing baking time (up to 200 h). Conjunctively, we also see that the imprinted ferroelectric capacitor shows a strong polarization relaxation effect even for short time periods due to the generated built-in voltages. For the 2.0 V operation, based on the delay time between the re-WRITE pulse and READ pulses, the lifetime extracted for a 65 °C bake can show a stable 10-year lifetime using a 10 μs delay time compared to a 0.5 year lifetime when using a delay time of 6 s. This work explores the drastic relaxation effects due to imprint in ferroelectric HZO films and the need to mitigate defects/space charges causing these issues.

Investigation and optimization of Fe/ZnFe2O4 as a Wide-band electromagnetic absorber

In this research, a facile in-situ growth method was applied to load ZnFe2O4 nanoparticles on carbonyl iron (Fe) flakes. These loaded ZnFe2O4 exhibited cone shape with an average size of ∼200 nm. The results revealed that the frequency region with reflection loss <−10 dB (fE) was up to 6.2 GHz (d = 1.5 mm), suggesting excellent wideband electromagnetic absorption (EM) properties. The electromagnetic absorption mechanism was discussed in depth which attributed to the synergetic effect of Fe and ZnFe2O4. The loaded ZnFe2O4 played a key role on suppressing inverse electromagnetic radiation, eddy effect, simultaneous maintaining moderate magnetic loss ability. Besides, the formed interface of ZnFe2O4/Fe could induce interface polarization relaxation effect at external electromagnetic field, which greatly boosted the effective dielectric loss ability (ε''E). Meanwhile, the interface polarization intensity was controllable by tuning the weight ratio of Fe.

Carbon foams modified with in-situ formation of Si3N4 and SiC for enhanced electromagnetic microwave absorption property and thermostability

High-performance electromagnetic (EM) absorbing property and excellent thermostability of three-dimensional (3D) hierarchical carbon foams (CFs) modified with in-situ formation of Si3N4 and SiC were herein fabricated for the first time. The crystallinity of the carbon films could be tunable from amorphous carbon to graphite-shape carbon nanosheet by heat treatment of the as-received hybrid foams. Compared with those of pristine CFs, the multiscale hybrid CFs with in-situ formation of Si3N4 and SiC could achieve stronger EM microwave absorption performance with a minimum reflection loss (RL) value of − 43.2 dB at 4.7 GHz at 5.0 mm, and the maximum effective absorption bandwidth (EAB) could be up to 4 GHz (11.8–15.8 GHz) at a thickness of 2.0 mm. The absorption mechanism of the hybrid foams is discussed in detail from the point of the structure and composition transformation, of interfacial polarization and multiple reflections effects and of polarization relaxation effects. The hybrid foams could be thermostable in air atmosphere beyond 1000 °C with above 93% weight retentions, suggesting that the multiscale hybrid CFs could be used as promising absorber materials with strong EM wave absorption performance in harsh environments.

Ultralight lamellar amorphous carbon foam nanostructured by SiC nanowires for tunable electromagnetic wave absorption

Three-dimensional hybrid foams of SiC nanowires (SiC NWs) coated and bonded with nanostructured lamellar carbon films are, herein synthesized for the first time, by combination of unidirectional freeze drying and carbonization techniques. The crystallinity of the carbon films was tunable from amorphous carbon to graphene-like carbon by heat treatment of the as-received hybrid foams to different temperatures within the range of 600–1500 °C. The SiC NWs-nanostructured lamellar carbon (SiC NWs/C) hybrid foams exhibited electromagnetic (EM) wave absorbing properties highly superior to current state-of-the-art counterparts, at filler loadings as low as 9.2 mg/cm3. Effective absorption bandwidth of the SiC NWs/C foam was observed to cover the whole X-band frequency range of 8.2–12.4 GHz, with reflection loss values reaching −31 dB for an optimum thickness of 3.3 mm. The absorption efficiency of the SiC NW/C hybrid foams is discussed in the text in view of the highly absorbing layered morphology of the developed material, of interfacial polarization and multiple reflections effects and of polarization relaxation effects.

Impact of secondary structure and hydration water on the dielectric spectrum of poly-alanine and possible relation to the debate on slaved versus slaving water.

Using extensive molecular dynamics simulations of a single eight-residue alanine polypeptide in explicit water, we investigate the influence of α-helix formation on the dielectric spectrum. For this, we project long equilibrium trajectories into folded and unfolded states and thereby obtain dielectric spectra representative for disordered as well α-helical conformations without the need to change any other system parameter such as pH or temperature. The absorption spectrum in the α-helical state exhibits a feature in the sub-GHz range that is significantly stronger than in the unfolded state. As we show by an additional decomposition into peptide and water contributions, this slow dielectric mode, the relaxation time of which matches the independently determined peptide rotational relaxation time, is mostly caused by peptide polarization correlations, but also contains considerable contributions from peptide-water correlations. In contrast, the peptide spectral contribution shows no features in the GHz range where bulk water absorbs, not even in the peptide-water correlation part, we conclude that hydration water around Ala8 is more influenced by peptide polarization relaxation effects than the other way around. A further decomposition into water-self and water-collective polarization correlations shows that the dielectric response of hydration water is, in contrast to electrolyte solutions, retarded and that this retardation is mostly due to collective effects, the self relaxation of hydration water molecules is only slightly slowed down compared to bulk water. We find the dynamic peptide-water polarization cross correlations to be rather long-ranged and to extend more than one nanometer away from the peptide-water interface into the water hydration shell, in qualitative agreement with previous simulation studies and recent THz absorption experiments.

Enhanced interfacial polarization relaxation effect on microwave absorption properties of submicron-sized hollow Fe3O4 hemispheres

Bowl-like Fe3O4 hemispheres with a hollow structure have been synthesized with an average outer diameter of 250 nm and a shell thickness of 80 nm using tiny ∼25 nm Fe3O4 nanoparticles. The complex permittivity, complex permeability and reflection loss of the samples in weight percentages of 60% of the hollow Fe3O4 hemispheres were studied by using an Agilent E5071 C vector network analyzer in the frequency range of 2–18 GHz. Results show that as-prepared samples exhibit improved microwave absorbing properties compared with our previously reported ball-like Fe3O4 microspheres and Fe3O4 hemispheres/r-GO composites. Two Cole–Cole semicircles reveal that there are dual dielectric relaxation losses: one is interfacial polarization relaxation, the other is dipolar relaxation. X-ray photoelectron and Fourier transform infrared spectra of the products calcinated at different temperatures reveal the enhanced interfacial polarization relaxation effect due to the higher concentration of Fe2+ ions in the surface layer and increased interfaces among the Fe3O4 nanoparticles that form the submicron-sized hollow Fe3O4 hemispheres. In addition, the fitted results of complex permeability demonstrate that the magnetic loss is mainly caused by the Kittel natural resonance.

Polarization relaxation and charge injection in thin films of poly(vinylidene fluoride/trifluoroethylene) copolymer

Polarization relaxation effects and charge injection in poly(vinylidene fluoride/trifluoroethylene) thin films are investigated in a thickness range between 60 nm and 400 nm. In time domain measurements of polarization reversal, the switching transients exhibit two stages: a rapid increase of polarization followed by a continuous slow increase lasting for several decades. This continuous increase of the polarization corresponds to a t−α (Kohlrausch law) behaviour of the current density in the long time range. For a constant electric field, these currents in the long time range are independent of the sample thickness as expected for a relaxational volume polarization effect. Furthermore, the isochronal polarization and depolarization currents saturate at high fields. We described the experiments using an asymmetric double well potential model with a distribution of relaxation times in which dipoles are fluctuating thermally activated between the two minima. As temperature increases, charge injection sets in. The charging currents then deviate from the t−α behaviour and become constant in time. The discharging currents are found to be a superposition of dipole relaxational currents and space charge relaxational currents. In the short time range, only the dipole relaxational currents are found. After several seconds, a space charge relaxation current with a pronounced minimum in time sets in, whose time constant shifts to shorter values with increasing temperature.

Long-term polarization relaxation effects in semicrystalline PVDF films

Abstract The long-term polarization relaxation of semicrystalline PVDF near room temperature has been studied. The relationships were found to be similar to those of free relaxation in ferroelectric ceramics. There are several mechanisms for the relaxation effects for different stimulation conditions. On the basis of the similarity between the behavior of semicrystalline PVDF and ferroelectric ceramics, these phenomena are assumed to be due to space charges existing in the PVDF.

Activation and relaxation of charges in ferroelectric ceramics

By application of the digital integral charge measuring method, long-lasting pyroelectric, piezoelectric, and polarization relaxation effects have been discovered in ferroelectric ceramics. Theoretical models of the relaxation were established. The free relaxation and random relaxation laws corresponding to different conditions agreed well with the experimental results.