Abramov, Pavel A

Kovalev A., Vinnik D., Gudkova S., Zherebtsov D., Zhivulin V., Taskaev S., Zhukova E., Ahmed A., Abramov P., Talanov M.

Hexagonal ferrites with the formula SrxBa(1−x)Fe12O19 (x = 0; 0.3; 0.5; 0.7; and 1) were prepared using the citrate method. The main feature of this synthesis is a relatively low calcination temperature of 700 °C. An X-ray diffraction study revealed a single-phase material. According to SEM, the particles were 50−70 nm in diameter. The Curie temperature of the samples that were determined using the DSC method varied in a very narrow range of 455−459 °C. Analysis of the magnetic hysteresis loops obtained at 300 K and 50 K indicated all samples as magnetically hard materials in a single-domain state. The maximal magnetic characteristics encompass strontium hexaferrite. The terahertz spectra of complex dielectric permittivity and the spectra of infrared reflectivity were measured at room temperature in the range of 6–7000 cm−1. The obtained broad-band spectra of the real and imaginary parts of permittivity reveal significant changes associated with structural distortions of the (Sr,Ba)O12 anti-cuboctahedron caused by the substitution of Ba2+ with Sr2+ in the same crystallographic positions.

Abramov P.A., Ivankov O.I., Mostert A.B., Motovilov K.A.

Pancake bonding phenomenology explains the long-term inconsistency between electron paramagnetic resonance, muon spin relaxation and conductivity data for biopigment eumelanin.

Abramov P.A., Zhukov S.S., Savinov M., Mostert A.B., Motovilov K.A.

Doping with copper slows down the low-frequency relaxation and decreases electrical conductivity in eumelanin.

Gorshunov B., Abalmasov V., Uskov V., Chan Y.T., Uykur E., Abramov P., Dressel M., Thomas V., Savinov M.

Quasistatic dielectric permittivity of D2O type I molecules (electric dipole moment perpendicular to the crystallographic c‐axis) within hydrothermally grown beryl crystals characterized by different internal pressure and content of D2O type II molecules (dipole moment parallel to the c‐axis) is measured at temperatures 4–300 K. All crystals are found to display quantum paraelectric behavior of the D2O‐I molecular subsystem permittivity, that is, permittivity growth while cooling from room temperature followed by saturation below 15–40 K. Processing the data with the Barrett expression shows that excess internal pressure and excess content of D2O‐II molecules lead to an increase in quantum temperature T1 and a decrease in the Curie constant C, with the Curie temperature TC remaining unchanged. The discovered strengthening of quantum effects (growth of T1) within an ensemble of dipole–dipole‐coupled D2O‐I molecules is associated with an enhanced azimuthal tunneling of these molecules within the hexagonal localizing potential. The data indicate the possibility of using crystal growth conditions to “tune” the strength of quantum effects in the network of polar water molecules, which provides a workbench for further studies of exotic phases of a lattice of coupled “point” electric dipoles.

Abramov P.A., Zhukov S.S., Bedran Z.V., Gorshunov B.P., Motovilov K.A.

Being a family of biodegradable materials with natural origin, melanins are widely used for development of model bioelectronic devices. However, the mechanism of their electric conductivity is still a matter of discussions. Current study is devoted to the room temperature impedance measurements of pure and copper-doped synthetic eumelanin at different values of humidity in frequency range 0.1–5 $$\cdot$$ 106 Hz. To analyze the obtained impedance spectra, we utilize density relaxation times (DRT) methodology. The performed analysis demonstrates an absence of significant difference in relaxation times in the studied materials. At the lowest frequencies, the doped material has about 30 times lower conductance than pure material. Possible origins of the observed phenomena are discussed in terms of copper ions activity as complexing agent for water molecules and semiquinone groups of melanin.

Bedran Z.V., Zhukov S.S., Abramov P.A., Tyurenkov I.O., Gorshunov B.P., Mostert A.B., Motovilov K.A.

Eumelanin is a widespread biomacromolecule pigment in the biosphere and has been widely investigated for numerous bioelectronics and energetic applications. Many of these applications depend on eumelanin’s ability to conduct proton current at various levels of hydration. The origin of this behavior is connected to a comproportionation reaction between oxidized and reduced monomer moieties and water. A hydration-dependent FTIR spectroscopic study on eumelanin is presented herein, which allows for the first time tracking the comproportionation reaction via the gradual increase of the overall aromaticity of melanin monomers in the course of hydration. We identified spectral features associated with the presence of specific “one and a half” C𝌁O bonds, typical for o-semiquinones. Signatures of semiquinone monomers with internal hydrogen bonds and that carboxylic groups, in contrast to semiquinones, begin to dissociate at the very beginning of melanin hydration were indicated. As such, we suggest a modification to the common hydration-dependent conductivity mechanism and propose that the conductivity at low hydration is dominated by carboxylic acid protons, whereas higher hydration levels manifest semiquinone protons.

Bedran Z.V., Zhukov S.S., Abramov P.A., Tyurenkov I.O., Gorshunov B.P., Bernard Mostert A., Motovilov K.A.

Here we report our study of the mid-infrared absorption spectra of a melanin thin film performed with hydration level variation. We demonstrate the direct signatures of the proposed earlier comproportionation reaction induced by continuous hydration along with the fingerprints of three water sub-phases in hydrated melanin.

Melentev A.V., Zhukov S.S., Balos V., Hoffman G., Alom S., Belyanchikov M., Zhukova E., Dressel M., Bacanu G.R., Abramov P., Levitt M.H., Whitby R.J., Gorshunov B., Sajadi M.

Time-domain terahertz and Fourier-transform infrared spectroscopy techniques are employed for investigating of the low-frequency dynamics of H 2 O@C 60 endofullerene at temperatures 6–300 K. A number of absorption lines associated with rotational transitions of water molecules entrapped inside C 60 cage are observed and assigned. Fitting absorption lines with Lorentzians allowed us to obtain the temperature dependences of the lines’ parameters – frequency position, strength and damping.

Melentev A., Zhukov S., Balos V., Hoffman G., Alom S., Belyanchikov M., Zhukova E., Dressel M., Bacanu G., Abramov P., Levitt M., Whitby R., Gorshunov B., Sajadi M.

Abstract Terahertz time-domain and infrared Fourier-transform spectroscopy techniques are used to investigate the properties of H2O@C60 endofullerene in 6 K – 300 K temperature interval. A number of absorption lines associated with rotational transitions of water molecule entrapped inside C60 cages are observed and assigned. Fitting the resonances with Lorenzian lineshapes allowed us to obtain temperature dependencies of absorption lines parameters – frequency, strength and damping.

Zhukov S.S., Kopylova D.S., Tsapenko A.P., Mogorychnaia A.V., Abramov P.A., Zhukova E.S., Nasibulin A.G., Gorshunov B.P.

Time-domain terahertz and infrared Fourier-transform spectroscopy techniques are used to study the temperature-dependent conductivity spectra of free-standing macroscale films composed of disordered single-walled carbon nanotubes. Comparative experiments are performed on films composed of nanotubes a) treated with oxygen plasma and b) having different lengths. We observe and analyze transformation of the terahertz conductivity of the films from metal-type to semiconductor-like. We trace evolution of the terahertz conductivity peak during the change of the temperature and the length of nanotubes. In films treated with oxygen plasma, the temperature dependence of the terahertz conductivity of only the semiconductor-type was observed and it decreased with exposure time.

Belyanchikov M.A., Abramov P.A., Ragozin A.L., Fursenko D.A., Gorshunov B.P., Thomas V.G.

This article reports on the uneven distribution of water molecules of first (D2O-I) and second (D2O-II) types in a D2O-containing beryl crystal grown hydrothermally on a non-singularly oriented fla...

Zhukov S.S., Kopylova D.S., Tsapenko A.P., Abramov P.A., Melentiev A.V., Zhukova E.S., Nasibulin A.G., Gorshunov B.P.

Time-domain terahertz and Fourier-transform infrared spectroscopy techniques are used to study temperature-dependent conductivity spectra of free-standing macroscale films composed of disordered single-walled carbon nanotubes. Films composed of nanotubes a) treated with oxygen plasma and b) having different lengths are used. We observed evolution of the so-called terahertz conductivity peak during the change of the temperature and the length of nanotubes. We analyzed changes of the terahertz conductivity of the films from metal-like to semiconductor-like due to oxygen plasma treatment and shortening length of the nanotubes.

Zhukov S.S., Balos V., Hoffman G., Alom S., Belyanchikov M., Nebioglu M., Roh S., Pronin A., Bacanu G.R., Abramov P., Wolf M., Dressel M., Levitt M.H., Whitby R.J., Gorshunov B., et. al.

AbstractWe resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

Ambrico M., Mattiello S., Mostert B., Phua J.W., Aceto D., Ambrico P.F., Guzzini A., De Stradis A., Liuzzi F., Santulli C., Lupidi G., Del Giudice A., Gunnella R.

Abstract In the framework of the Circular Economy this study provides a detailed analysis of water-based suspensions of two biopolymers derived by sustainable processes: eumelanin from insect farming and keratin from chicken feathers. The latter material was obtained via two different extraction procedures. Colloidal-like suspensions were produced in water either as a single component system or a mixture of both in selected ratios, taking advantage of their high solubility. The suspensions were examined using a comprehensive set of chemical, structural and dielectric techniques to gather information on their properties. Small-Angle X-ray Scattering (SAXS) results provided insights into the elemental polymer sections within the suspension, while Transmission Electron Microscopy (TEM) images indicate that keratin is the component driving the shape of the aggregation structure in a colloidal environment, and, in some cases, eumelanin internalization. Furthermore, the co-presence of both polymers in water determines the aggregation dimensions and shapes. The discussion focuses on the influence of the aggregation on the dielectric proper-ties by comparing the former to the AC dynamic response returned by Broadband Dielectric Spectroscopy (BDS). Within the BDS framework various items are highlighted including dielectric relaxations, screening effects, counterion condensation and ionic charge transport. The results shown in this work let to foresee the adoption of water or biofriendly aqueous Eum-BSF:Ker suspensions in the production of devices and sensors with low environmental impact

Solizoda I.A., Zhivulin V.E., Zirnik G.M., Gudkova S.A., Chouprik A.A., Taskaev S.V., Alyabyeva L.N., Zhivetev K.V., Terentiev A.V., Trukhanov A.V., Vinnik D.A.

Potapov D.O., Kondratyuk N.D.

Torii H., Sadai S., Hashikawa Y., Murata Y., Ikemoto Y.

Viggiano S., Laura Alfieri M., Panzella L., Crescenzi O., Napolitano A.

Zheng Y., Yu X., Xu B., Guo H., Zhao S., Tang J., Yan Y., Gao Y.

Mostert A.B., Mattiello S., Li S., Perna G., Lasalvia M., Ambrico P.F., Paulin J.V., Lima J.V., Graeff C.F., Phua J.W., Matta M., Surman A.J., Gunnella R., Ambrico M.

Demonstrated is the first example of dihydroxyindole carboxylic acid presence within an insect eumelanin, a conductive biomaterial material for the circular economy.

Sharma D., Duglet R., Dogra A., Singh M.

The improvement of magnetic parameters through dopant introduction has opened up new possibilities for tailored applications of M-type hexaferrite materials. This study investigates the effects of Al3+ and Ni2+ doping on M-type Sr-hexaferrite prepared via sol-gel technique with calcination at 1200 °C for 4 hrs. The x-ray diffraction analysis confirms the presence of high-purity hexaferrite without any secondary phases. The results highlight the significant impact of Al3+ doping on coercivity, leading to an increase from 4500 Oe to 10490 Oe in SrFe12O19 to SrAl2Fe10O19. The concentration of 1% wt. of SiO2 and CaO increases the remanence from 65.16  to 67.05 emu/g. The incorporation of Ni2+ via doping leads to a notable enhancement in magnetic permeability from 1.75 in SrFe12O19 to 2.75 in SrNi0.5 Fe11.5O19, coupled with minimal losses. Two techniques are used to analyze the magnetic permeability of samples: one with vector network analyzer (VNA), and the other with vibrating sample magnetometer (VSM). The values of magnetic permeability with the addition of various substituted elements exhibited the same upward and downward pattern for both approaches. These findings provide valuable insights for optimizing the magnetic performance of M-type hexaferrites.

Wang L., Wang Y., Ouyang Z., Ye J., Xiong W., Zhou Z.

Brizuela Guerra N., Lima J.V., Paulin J.V., Nozella N.L., Boratto M.H., Nogueira G.L., Bufon C.C., de Oliveira Graeff C.F.

AbstractMelanins are macromolecular pigments widely spread in many living organisms, with unique physical and chemical properties. Specifically, their conductive properties have drawn attention for applications in different devices; however, the electrochemical response is equally necessary for designing technologies in sustainable bioelectronics. In this sense, we report a comparative study of the redox electrochemical properties of non‐functionalized and sulfonated melanins in different pH environments. The electrochemical response was investigated using cyclic voltammetry, electrochemical impedance spectroscopy, dielectric permittivity and AC/DC conductivity at pH 3, 5 and 7. The voltammetric currents were higher at low pH, in agreement with the known proton transport properties of melanins. The effect of pH on electrochemical properties was slightly more significant in non‐functionalized pigments. Melanins with a higher 5,6‐dihydroxyindole carboxylic acid/5,6‐dihydroxyindole ratio showed high DC current and low impedance. No significant difference was observed in the dielectric relaxation process between the different samples. © 2024 Society of Chemical Industry.

Motovilov K.A., Mostert A.B.

We analyse and compare the physico-chemical properties of melanins with other biological macromolecular systems and conclude that they are the fourth fundamental class of bioorganic materials alongside nucleic acids, proteins, and polysaccharides.

Zhang S., Zheng Z., Zhang M., Zhang T., Zhang Z., Huang H.

AbstractThe application of terahertz time-domain spectroscopy (THz-TDS) in the quantitative analysis of major minerals in Bayan Obo magnetite ore was explored. The positive correlation between the optical parameters of the original ore and its iron content is confirmed. The detections of three main iron containing minerals, including magnetite, pyrite, and hematite, were simulated using corresponding reagents. The random forest algorithm is used for quantitative analysis, and FeS2 is detected with precision of R2 = 0.7686 and MAE = 0.6307% in ternary mixtures. The experimental results demonstrate that THz-TDS can distinguish specific iron containing minerals and reveal the potential application value of this testing method in exploration and mineral processing fields.

Potapov D.O., Kondratyuk N.D.

The structural and dynamic properties of a water molecule undergo changes if it is located inside a fullerene (H2O@C60). In this work, the atomistic simulation method including nuclear quantum effects is used for the first time to describe the low-temperature dynamics and changes in the structure of the water molecule in the fullerene at 5 K. A machine-learning potential on density functional theory trajectories is used to calculate the interactions in this system. Zero-point energy and delocalization of nuclei are taken into account using path integral molecular dynamics.

Soltani S., Roy A., Urtti A., Karttunen M.

Melanin is a widely found natural pigment serving multiple physiological functions and having numerous applications in industries and pharmaceuticals. Due to the diverse structural properties of melanin, drug molecules exhibit...

Orr L., Roy P.

Marjeghal M.A., Sedghi A., Baghshahi S.

Strontium hexaferrite is a hard ferrite with the chemical formula of SrFe12O19 and has a magnetoplumbite structure. Due to high anisotropy, high saturation magnetization, chemical stability, high corrosion resistance, and appropriate Curie temperature, it has been widely used as a permanent magnetic material. A novel sol-gel combustion process was used to synthesize ultrafine particles of strontium hexaferrite. The weight ratio of citric acid to the total weight of strontium and iron salts nitrate (CA/MN) is among the parameters affecting the purity of SrFe12O19 nanoparticles produced by the sol-gel method, which can improve the purity of the final product by controlling this parameter. M-type SrFe12O19 nanostructures with different citric acid to metal nitrates molar ratios (0.5, 1, 2, and 4) were prepared. The results showed that nitrate-citrate gels exhibit self-ignition behavior after combustion in the air at room temperature. The samples were then characterized by XRD, TGA/DTA, FTIR, FE-SEM, VSM. The results showed that the crystallite size of strontium hexaferrite is significantly affected by the molar ratio of citric acid to metal nitrates. As the molar ratio of citric acid to metal nitrates increases, the size of nanostructures decreases. By increasing the molar ratio of citric acid from 0.5 to 1, the saturation magnetization and magnetic remanence magnetization increase, but the coercive field decreases.

Zulkimi M.M., Azis R.S., Ismail I., Mokhtar N., Ertugrul M., Hamidon M.N., Hasan I.H., Yesilbag Y.O., Tuzluca F.N., Ozturk G., Hasar U.C.

A strategy of a highly feasible method to achieve a broad bandwidth of radar absorbing materials (RAM) is reported. Herein the magnetic Sr-hexaferrite were prepared using a conventional sintering process at 900 °C and later hybridized with coiled carbon nanotubes (CNT) via a chemical vapour deposition (CVD) method. X-ray diffraction (XRD) detected two phases of compounds after sintering which were 36 % of SrFe2O4 and 64 % of SrFe12O19. Two groupings of nanoparticle size showed no significant effect on reflection loss (RL) performance. Interestingly after hybridization of coiled CNT with the magnetic materials, the permittivity was increased tremendously hence enhancing the RL. Multiple relaxations of dielectric and eddy current losses were responsible for the enhancement. The RL was increased as the thickness was increased from 1 mm to 3 mm. 6 % of coiled CNT/Sr-hexaferrite hybrid resulted in low RL of -19 dB with the broadest bandwidth of 3GHz over X-band frequency. The report is important for paving future work in obtaining a desired broad bandwidth RAM.

S T A., Ezhil Vizhi R., V H.

Exchange-coupled hard/soft ferrite nanoparticles are prospective to squeeze out a part of expensive magnets based on rare earth elements. However, the known exchange-coupled composite ferrite nanoparticles often suffer from the lack of a powerful hard core, high defective synthesis of magnetic phases and a poor interface between them. This work is featured by the use of highly coercive Ba0.5Sr0.5Fe12O19 magnets prepared by sol–gel combustion. Here in we demonstrate the synthesis of magnetic nanocomposites comprising Ba0.5Sr0.5Fe12O19 and Zn0.5Co0.5Fe2O4 hard/ soft ferrites by gradually increasing the share of spinel ferrites in the hard ferrites. Four samples of nanocomposites were prepared with the increasing order of hard ferrites using simple solgel auto-combustion method. Thermal breakdown examinations have been performed using TGA/DSC on the as-synthesised material from 0 °C to 1250 °C, and the temperature stability of the sample was detailed. The XRD analyses proved the presence of hexagonal and cubic crystal structure in each nanocomposite. Transmission Electron Microscopy was used to examine the morphology and topology of the nanocomposites. Hard and soft ferrites were detected in rod and non-spherical microstructures, correspondingly. The bending and the stretching vibrations in the sample was studied by using Fourier Transform Spectroscopy. The magnetic response of the synthesised samples was studied by using the Vibrating Sample Magnetometry. The prepared Nano powders displayed a highest coercivity Hc of 6307 Oe for pure hard ferrite while the pure spinel ferrite displayed a coercivity of 80 Oe. All the synthesized samples showed smooth M−H curves and single peak on dM/dH against H curves this indicates that complete exchange-coupled effects between the phases are achieved. The saturation magnetization increased with the increase in the content of spinel ferrite whereas the coercivity decreased with the increase in the spinel content.

Alyabyeva L., Zhukova E., Zhukov S., Ahmed A., Vinnik D., Gorshunov B.

First detailed measurements of broad-band (2–8000 cm−1, i.e. 0.06-240 THz) temperature-dependent (5–300 K) spectra of complex permittivity of single-crystalline barium-lead hexaferrites Ba1-xPbxFe12O19 covering full interval of lead concentrations, x(Pb)=0…1, were performed to study the effect of Ba for Pb substitution on electrodynamic response. It is found that variation of Ba-Pb relative content allows “tuning” of dielectric losses ε"(0.3 THz; 300 K) from 0.1 to 45, and dielectric constant ε'(0.3 THz; 300 K) from 10 to 50. To account for the discovered concentration evolution of terahertz dielectric properties, a microscopic model is suggested that considers interaction of 6s2 lone pair of Pb2+ with nearest FeO5 clusters. Pronounced anomaly in concentration-dependent oscillator strength of terahertz bands is observed at x(Pb)=0.5–0.6 and interpreted by considering redistribution of Fe2+/Fe3+ ions over five iron sublattices. It is pointed out that the revealed “tunability” of terahertz characteristics of hexaferrites makes them promising materials for designing devices operating at terahertz frequencies.

Abramov P.A., Zhukov S.S., Savinov M., Mostert A.B., Motovilov K.A.

Doping with copper slows down the low-frequency relaxation and decreases electrical conductivity in eumelanin.

Rahman M.L., Rahman S., Biswas B., Ahmed M.F., Rahman M., Sharmin N.

AbstractM-type strontium hexaferrite (SrM) were successfully synthesized from Sr2+ and Fe3+ precursor salt through co-precipitation technique. Different higher sintering temperatures (800–1000 °C) were used to get the desired SrM with variation of Fe3+/Sr2+ mole ratio as well. The characterization of SrM and its properties were investigated using modern instrumental techniques viz. X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer, Scanning Electron Microscopy, Vibrating Sample Magnetometer, UV–Visible NIR Spectrometer, Impedance Analyzer and Thermal Conductivity Meter. The phase of the synthesized SrM were confirmed by comparing the XRD patterns with the standard ICDD data and Reitvelt Refinement for the SrM having Fe3+/Sr2+ ratio 10 was performed. The structural parameters, particle size (75 nm–318 nm) and shape of the as prepared samples were changed with calcination temperature as well as mole ratio. The saturation magnetization (73.77–24.27 emu/g), coercivity (3732.28–642.10 Oe) and remanant magnetization (39.15–8.86 emu/g) were varied with calcination temperature and composition. The dielectric properties, optical properties and thermophysical properties were measured for the SrM keeping Fe3+/Sr2+ ratio 10 calcined at 1000 °C. The synthesized SrM can be applied in magnetic recording media and as photocatalyst due to its low coercivity (2764.48 Oe), high saturation magnetization (73.77 emu/g) and low band gap energy (Eg-2.04 eV) respectively.

Abhilash G.P., Sharma D., Bose S., Shivakumara C.

We report electromagnetic interference (EMI) shielding efficiency in the PANI-wrapped BaFe12O19 and SrFe12O19 with rGO composites. Barium and strontium hexaferrites were synthesized using the nitrate citrate gel combustion method. These hexaferrites were polymerized in situ with aniline. The PANI-coated ferrite-based composite materials were developed along with reduced graphene oxide (rGO) in acrylonitrile butadiene styrene (ABS) polymer, and their shielding effectiveness was assessed in X-band frequency range (8.2-12.4 GHz). The reflection (SER) and absorption (SEA) mechanism of shielding effectiveness was discussed with the different rGO concentrations. The results reveal that 5 wt% of rGO with PANI-coated barium and strontium hexaferrite polymer composites exhibit shielding efficiency of 21.5 dB and 19.5 dB, respectively, for 1 mm thickness composite. These hexaferrite polymer-based composite materials can be used as an attractive candidate for EM shielding materials in various technological applications.

Mishra S., Pratap V., Chaurasia A.K., Soni A.K., Dubey A., Dixit A.K.

Radar absorbing structural (RAS) composites are laminate with a low reflection coefficient for the electromagnetic illumination in microwave frequency range, and thus it can be used in the civil and stealth applications. In this study, a series of Exfoliated Graphite (EG) in combination with U-type barium hexaferrite based epoxy composites was fabricated with varying weight percentages (0.25, 0.50, 0.75 and 1.00 wt%) of EG and fixed wt.% (60 wt%) of U-type hexaferrite through wet mixing and compression moulding technique. Here, Raman spectra shows the EG to be defect-free characteristics. Subsequently, prepared composites were measured for microstructure and electromagnetic (EM) properties in 8.2–12.4 GHz frequency range (X-band). Further, complex permittivity (εr = ε′−jε″) and complex permeability (μr = μ′−jμ″) of the designed composites were measured using Vector Network Analyzer (VNA). Designed EG/hexaferrite-epoxy composite shows maximum real permittivity and permeability values (ε′ = 18.90 and μ′ = 0.91) for 3.3 mm thickness with minimum reflection loss −8.43 dB with 0.75 wt % EG filled (Sample-3)in X-band. While, it can withstand the different mechanical forces and shown the better absorption mechanism.EG plays important role due to their unique properties with optimize loading percentages in ferrite-epoxy composites. It may be concluded that, optimized EG/hexaferrite based composites determine as good microwave absorber in 8.2–12.4 GHz frequency ranges. Consequently, the prepared structural composites can be used to design the microwave absorbers and electromagnetic interference (EMI) shields for stealth applications.

Liu R., Wang L., Yu X., Xu Z., Gong H., Zhao T., Hu F., Shen B.

The study on the magnetocrystalline anisotropy (MA) of La–Co co-substituted strontium hexaferrite (La–Co SrM) shows a joint effect of Fe 2+ and Co 2+ ions in the enhancement of MA. Since the role of Fe 2+ single ion has been studied with La-substituted strontium ferrite, in this work, Co-substituted strontium hexaferrite SrFe 12- x Co x O 19 (Co-SrM) single crystals were successfully grown for 0 ≤ x ≤ 0.31 by the Na 2 CO 3 flux method to elucidate the anisotropy of Co 2+ single ions. Co-substitution in this preparation condition has a limit solubility of 0.31 and enhances uniaxial magnetic anisotropy field H A by 19% for 0.03 = x ≤ 0.11, with a mere loss of 7% of saturation magnetization M S at 5 K. The enhanced H A of Co-SrM is reported for the first time, even higher than that of La–Co SrM, which is suitable to be used as permanent magnets in this concentration range. But with the further substitution of Co, the planar anisotropy of x = 0.31 was observed at 5 K. The potential nonlinear magnetic structure of Co-SrM remains to be discovered for magnetoelectric effects. This work is also of great significance as a complement to the magnetocrystalline anisotropy study of La–Co SrM.

Pan J., Xia Z., Deng N., Chen L., Zhang H., Lu Y., Liu Y., Gao H.

• The preparation methods and chemical structures of the eumelanin-inspired nanomaterials are introduced. • The current works of eumelanin-inspired nanomaterials in energy storage are summarized from the perspective of mechanism. • The applications of eumelanin-inspired nanomaterials in rechargeable batteries and supercapacitors are introduced. • The challenges and future perspectives focus on eumelanin-inspired nanomaterials in order to provide a reference for advanced energy storage applications. At present, rechargeable batteries are researched due to the increasing demand for electricity. For these energy storage devices, their energy density or power density are highly dependent on electrode materials including organic, inorganic and their composite substances. Organic electrode materials with structural diversity and tunability begin to attract attention, because it can better adapt to different sizes of cations. As a kind of organic electrode material, eumelanin-inspired nanomaterials (eumelanin-inspired nanomaterials mean natural eumelanin and artificial synthetic eumelanin-PDA in this review) have been widely used in rechargeable batteries and supercapacitors (SCs) including anodes, cathodes, separators and electrolytes. Eumelanin-inspired nanomaterials can mainly use to gain/lose electrons with reversible bonding of metal ions in order to form an electric current, and the performance of energy storage devices can be improved due to its many advantages, such as nanometer size, adhesion and hydrophilia. However, the in-depth summarization of the current work from the perspective of energy storage mechanisms is still lacking at present. In this review, the preparation methods and energy storage mechanisms of eumelanin-inspired nanomaterials are systematically summarized. The applications and current challenges of eumelanin-inspired nanomaterials on rechargeable batteries and SCs have been introduced, and the future perspectives in the rational fabrication of higher performance energy storage devices by the eumelanin-inspired nanomaterials have been proposed.

Mahmoud M.H., Hassan A.M., Said A.E., Taha T.A.

• Nano - crystallites Zinc ferrites were prepared via microwave-combustion/ball-mill. • The crystallite size decreased with mill, while the strain increased. • The magnetization of saturation decreased when the mill time increased to 330 min. • The samples showed robust activity and high selectivity towards propene and acetone formation. By utilizing the microwave-combustion approach, it was possible to synthesize nanocrystals made of zinc ferrite from their stoichiometric metal nitrates and glycine mixtures. The magnetic and structural traits were assayed via FTIR, XRD, SEM, TEM and VSM. Results demonstrated that the as-prepared sample is a monophase zinc ferrite with noticeable crystallinity and 27 nm crystallite proportions. A trace of the α-Fe 2 O 3 phase is observable and the crystallite size diminished with milling. The SEM micrographs show that the samples have a mesoporous structure. TEM images showed agglomerated particles, while the as-prepared samples electron diffraction patterns maintained high crystallinity. The as-prepared ZnFe 2 O 4 with a saturation magnetization value of 47 emu/g at room temperature was observed. After 330 min of milling, the aforementioned value was reduced to 29 emu/g. The catalytic activity of all samples was analyzed via dehydrogenation-dehydration of isopropyl alcohol. The outcomes showed the robust activity and high selectivity of the samples towards propene and acetone formation.

Wang H., Yeh H., Kamenev A.

Many discrete optimization problems are exponentially hard due to the underlying glassy landscape. This means that the optimization cost exhibits multiple local minima separated by an extensive number of switched discrete variables. Quantum computation was coined to overcome this predicament, but so far had only a limited progress. Here we suggest a quantum approximate optimization algorithm which is based on a repetitive cycling around the tricritical point of the many-body localization (MBL) transition. Each cycle includes quantum melting of the glassy state through a first order transition with a subsequent reentrance through the second order MBL transition. Keeping the reentrance path sufficiently close to the tricritical point separating the first and second order transitions, allows one to systematically improve optimization outcomes. The running time of this algorithm scales algebraically with the system size and the required precision. The corresponding exponents are related to critical indexes of the continuous MBL transition. There are several proposals for quantum algorithms solving optimisation problems, but so far none of them has provided a clear speedup. Here, the authors propose an iterative protocol featuring periodic cycling around the tricritical point of a many-body localization transition.

Gouda A., Masson A., Hoseinizadeh M., Soavi F., Santato C.

Biosourced and biodegradable organic electrode materials respond to the need for sustainable storage of renewable energy. Here, we report on electrochemical capacitors based on electrodes made up of quinones, such as Sepia melanin and catechin/tannic acid (Ctn/TA), solution-deposited on carbon paper engineered to create high-performance interfaces. Sepia melanin and Ctn/TA on TCP electrodes exhibit a capacitance as high as 1355 mF cm−2 (452 F g−1) and 898 mF cm−2 (300 F g−1), respectively. Sepia melanin and Ctn/TA symmetric electrochemical capacitors operating in aqueous electrolytes exhibit up to 100% capacitance retention and 100% coulombic efficiency over 50,000 and 10,000 cycles at 150 mA cm−2 (10 A g−1), respectively. Maximum power densities as high as 1274 mW cm−2 (46 kW kg−1) and 727 mW cm−2 (26 kW kg−1) with maximum energy densities of 0.56 mWh cm−2 (20 Wh kg−1) and 0.65 mWh cm−2 (23 Wh kg−1) are obtained for Sepia melanin and Ctn/TA. Biosourced and biodegradable organic electrode materials are investigated for environmentally benign energy storage, but their performance at higher current density is often poor. Here, the authors construct electrodes with quinone-based species from Sepia melanin and tannins on treated carbon paper and observe electrode capacitance as high as 1355 mF cm−2 at current densities up to 10 A g−1.

Camus A., Reali M., Rozel M., Zhuldybina M., Soavi F., Santato C.

Melanins (from the Greek μέλας, mélas, black) are bio-pigments ubiquitous in flora and fauna. Eumelanin is an insoluble brown–black type of melanin, found in vertebrates and invertebrates alike, among which Sepia (cuttlefish) is noteworthy. Sepia melanin is a type of bio-sourced eumelanin that can readily be extracted from the ink sac of cuttlefish. Eumelanin features broadband optical absorption, metal-binding affinity and antioxidative and radical-scavenging properties. It is a prototype of benign material for sustainable organic electronics technologies. Here, we report on an electronic conductivity as high as 10 −3 S cm −1 in flexographically printed Sepia melanin films; such values for the conductivity are typical for well-established high-performance organic electronic polymers but quite uncommon for bio-sourced organic materials. Our studies show the potential of bio-sourced materials for emerging electronic technologies with low human- and eco-toxicity.

Chan Y.T., Uykur E., Belyanchikov M.A., Dressel M., Abalmasov V.A., Thomas V., Zhukova E.S., Gorshunov B.

We measure the real part ${\ensuremath{\epsilon}}^{\ensuremath{'}}$ of the dielectric permittivity of beryl crystals with heavy water molecules ${\mathrm{D}}_{2}\mathrm{O}$ confined in nanosized cages formed by an ionic crystal lattice. The experiments are performed at a frequency of 1 MHz in the temperature interval from 300 down to 4 K under different hydrostatic pressures up to $P=6.3$ GPa. At high temperatures, a Curie-Weiss-like increase of ${\ensuremath{\epsilon}}^{\ensuremath{'}}(T)$ is observed upon cooling. Application of pressure leads to flattening of ${\ensuremath{\epsilon}}^{\ensuremath{'}}(T)$ at low temperatures due to quantum effects, i.e., tunneling of deuterium atoms in the hexagonal localizing potential. Analyzing the temperature behavior of ${\ensuremath{\epsilon}}^{\ensuremath{'}}$ with the Barrett expression allows us to obtain pressure dependencies of the quantum temperature ${T}_{1}$, the Curie-Weiss temperature ${T}_{C}$, and the Barrett constant $C$. The increase of ${T}_{1}$ observed up to 4 GPa is associated with an enhanced azimuthal tunneling of the confined water molecules through the barriers of the potential. For $P>4$ GPa, ${T}_{1}(P)$ levels off since the barriers disappear. Any further pressure increase does not affect the tunneling rate because of the absence of a barrier. The behavior is modeled by solving the Schr\"odinger equation for the water molecule in the azimuthal potential numerically. Small negative values of ${T}_{C}\ensuremath{\approx}\phantom{\rule{4pt}{0ex}}\ensuremath{-}10$ K obtained for $P<4$ GPa indicate the antiferroelectric ordering tendency of the water dipoles localized in the crystalline nanochannels. For higher pressure, a strong decrease of ${T}_{C}$ toward negative values is observed that would correspond to the enhanced interdipole coupling strength, which is however hard to explain in the present case, and thus calls for additional theoretical and experimental studies.