Correlation Between Composition and Electrodynamics Properties in Nanocomposites Based on Hard/Soft Ferrimagnetics with Strong Exchange Coupling

Dielectric properties of biomass by-products generated from wood and agricultural industries in Finland

Knowledge of the dielectric properties (complex permittivities) of biomasses is critical for understanding their behaviors in a microwave field and for designing large-scale microwave systems. The present research was focused on determining the dielectric properties of different types of biomasses (sawdust, bark, fiber reject, grass, and straw) at temperatures from 25 to 700 °C and frequencies in the range of 397 to 2985 MHz, using cavity perturbation technique. The dielectric properties decreased during the drying (25 to 200 °C) and the pyrolysis stages (200 to 400 °C), but sharply increased during the biochar formation stage (400 to 700 °C). At 912 MHz, straw, grass, and fiber reject exhibited the greatest half-power depths at approximately 300 °C, and sawdust and bark at approximately 350 °C, suggesting that from room temperature to 350 °C, larger material volumes can reduce costs; above 500 °C, the sample size must not exceed the microwave half-power depth to prevent hot spots or uneven heating. The interaction mechanisms of microwaves with biomass can be explained as follows, during biomass drying, the dielectric changes are driven by dipolar polarization of water molecules; during pyrolysis, by polar molecules and functional groups; and during carbonization, by scattering and interface polarization within the biochar. Furthermore, the addition of the produced biochar to the raw biomass could increase the loss tangent up to 400 °C, enabling faster heating and reducing energy consumptions and residence times. The dielectric properties data provided in this study can be used to design large-scale microwave systems, including selection of column diameter, sample size, and microwave frequency.

Lattice-charge imbalance and redox catalysis over perovskite-type ferrite- and manganite-based mixed oxides as studied by XRD, FTIR, UV-Vis DRS, and XPS

In the present investigation, two sets of pure and substituted ferrite- and manganite-based mixed oxides were prepared within the stoichiometric formula[Formula: see text], where A = Bi or La, A^' = Sr, B = Fe or Mn, B' = Co, x = 0 or 0.2, by calcination at 700 °C (for 1 h) of corresponding metal citrate xerogels. Materials thus obtained were examined for bulk and surface characteristics using X-ray diffractometry, ex situ Fourier transform infrared spectroscopy, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and N₂ sorptiometry. Their redox catalytic activity was evaluated towards a 2-propanol dehydrogenation reaction in the gas phase by employing in situ Fourier transform infrared spectroscopy. The results obtained could help reveal that (1) the presence of Bi (versus La) and Mn (versus Fe) facilitated the formation of polymeric crystalline phases assuming lattice-charge imbalance (due to excess positive charge), (2) the surface exposure of the excess positive charge was manifested in the generation of Mn sites having various oxidation states ≥ 3+, (3) the consequent development of visible light absorptions at 498-555 nm suggested occurrence of electron double-exchange facilitated by the formation of Mnⁿ⁺-O^2--Mn⁽ⁿ⁺¹⁾⁺ Zener-type linkages, and (4) the exposure of such linkages at the surface warrants the establishment of the electron-mobile environment necessitated by the redox catalytic activity. Moreover, the relationship between the alcohol dehydrogenation activity and the magnitude of the lattice-charge imbalance (i.e., the net excess positive charge) of the catalysts was highlighted.

Antiferromagnetic to Ferrimagnetic Phase Transition and Possible Phase Coexistence in Polar Magnets (Fe1-xMnx)2Mo3O8 (0 ≤ x ≤ 1)

In the present work, the magnetic properties of a single crystal (Fe_1-xMn_x)₂Mo₃O₈ (0 ≤ x ≤ 1) have been studied by performing extensive measurements. A detailed magnetic phase diagram is built up, in which the antiferromagnetic state dominates for x ≤ 0.25 and the ferrimagnetic phase arises for x ≥ 0.3. Meanwhile, a sizeable electric polarization of spin origin is commonly observed in all samples, no matter what the magnetic state is. For the samples hosting a ferrimagnetic state, square-like magnetic hysteresis loops are revealed, while the remnant magnetization and coercive field can be tuned drastically by simply varying the Mn content or temperature. A possible coexistence of the antiferromagnetic and ferrimagnetic phases is proposed to be responsible for the remarkable modulation of magnetic properties in the samples.

Mechanism of bubbles formation and anomalous phase separation in the CoNiP system

This study announces the anomalous phase separation in CoNiP alloy electroplating. The observed phenomenon of the formation of magnetic bubbles was described for the first time for this triple CoNiP system. This study briefly covers all stages of magnetic bubble formation, starting from the formation of an amorphous phosphor-rich sublayer, followed by nucleation centers, and finally cobalt-rich bubbles. An explanation for the anomalous mechanism of bubble formation was found in the effects of additives and the phenomena of depolarization and superpolarization.

Tough, Injectable Calcium Phosphate Cement Based Composite Hydrogels to Promote Osteogenesis

Osteoporosis is one of the most disabling consequences of aging, and osteoporotic fractures and a higher risk of subsequent fractures lead to substantial disability and deaths, indicating that both local fracture healing and early anti-osteoporosis therapy are of great significance. However, combining simple clinically approved materials to achieve good injection and subsequent molding and provide good mechanical support remains a challenge. To meet this challenge, bioinspired by natural bone components, we develop appropriate interactions between inorganic biological scaffolds and organic osteogenic molecules, achieving a tough hydrogel that is both firmly loaded with calcium phosphate cement (CPC) and injectable. Here, the inorganic component CPC composed of biomimetic bone composition and the organic precursor, incorporating gelatin methacryloyl (GelMA) and N-Hydroxyethyl acrylamide (HEAA), endow the system with fast polymerization and crosslinking through ultraviolet (UV) photo-initiation. The GelMA-poly (N-Hydroxyethyl acrylamide) (GelMA-PHEAA) chemical and physical network formed in situ enhances the mechanical performances and maintains the bioactive characteristics of CPC. This tough biomimetic hydrogel combined with bioactive CPC is a new promising candidate for a commercial clinical material to help patients to survive osteoporotic fracture.

Electro-oxidation reconstitution of aluminium copper MOF-derived metal oxyhydroxides for a robust OER process

It is common knowledge that the O₂ evolution reaction (OER) is a crucial half-reaction in the electrolysis of water. However, it is currently difficult to create inexpensive OER electrode materials in a way that is efficient, simple, and environmentally friendly. In this research, metal oxy-hydroxides with numerous oxygen defects (M-OOHv) are created at surface of Cu foam (CF) using a unique, straightforward electro-oxidation reconstitution (ER) process. Different spectroscopic and microscopy methods are used to analyse the electrode characteristics of Al₂Cu-MOF@M-OOHv-ER/CF; electrochemical measurements display a lower overpotential (η) of 366 mV @ 10 mA cm^-2 and a Tafel slope of 95.2 mV dec^-1 in 1.0 M KOH. X-Ray diffraction (XRD), scanning electron microscopy (SEM), and Raman studies confirm the phase transition of the metal-organic framework (MOF) to the M-OOH, which acts as the active site to boost the OER activity. Through spectroscopic and microscopic investigations, it is determined that the efficiency of bimetallic electrode materials and oxygen vacancies in the M-OOHv have an impact on the electron power density. The manufactured electrode material additionally showed good durability for 50 hours. As a result, the newly developed Al₂Cu-MOF@M-OOHv-ER/CF nanomaterial has greater potential for both electrolysis of water and other energy storage equipment.

The influence of saccharin adsorption on NiFe alloy film growth mechanisms during electrodeposition

This article deals with the effects of current modes on saccharin adsorption during NiFe electrodeposition, and, as a consequence, its effect on chemical composition, crystal structure, and microstructure of deposited films. For this purpose, we obtained NiFe films using direct, pulse, and pulse-reverse electrodeposition modes. The deposit composition, crystal structure, and surface microstructure are studied. Direct current (DC) and pulse current (PC) films have a smooth surface, while a pulse-reverse current (PRC) film surface is covered by a volumetric cauliflower-like microstructure. The mechanism of the film surface development was considered from the point of view of saccharin adsorption and its action as an inhibitor of vertical grain growth during different current modes. During the DC and PC modes, saccharin is freely adsorbed on the growth centers and restrains their vertical growth. Whereas in the case of the PRC electrodeposition, saccharin adsorbs during cathodic pulses and desorbs during anodic pulses. Therefore, its inhibiting action decreases, vertical grain growth rises, and a rougher surface develops.

Hardening of Cobalt Ferrite Nanoparticles by Local Crystal Strain Release: Implications for Rare Earth Free Magnets

In this work, we demonstrate that the reduction of the local internal stress by a low-temperature solvent-mediated thermal treatment is an effective post-treatment tool for magnetic hardening of chemically synthesized nanoparticles. As a case study, we used nonstoichiometric cobalt ferrite particles of an average size of 32(8) nm synthesized by thermal decomposition, which were further subjected to solvent-mediated annealing at variable temperatures between 150 and 320 °C in an inert atmosphere. The postsynthesis treatment produces a 50% increase of the coercive field, without affecting neither the remanence ratio nor the spontaneous magnetization. As a consequence, the energy product and the magnetic energy storage capability, key features for applications as permanent magnets and magnetic hyperthermia, can be increased by ca. 70%. A deep structural, morphological, chemical, and magnetic characterization reveals that the mechanism governing the coercive field improvement is the reduction of the concomitant internal stresses induced by the low-temperature annealing postsynthesis treatment. Furthermore, we show that the medium where the mild annealing process occurs is essential to control the final properties of the nanoparticles because the classical annealing procedure (T > 350 °C) performed on a dried powder does not allow the release of the lattice stress, leading to the reduction of the initial coercive field. The strategy here proposed, therefore, constitutes a method to improve the magnetic properties of nanoparticles, which can be particularly appealing for those materials, as is the case of cobalt ferrite, currently investigated as building blocks for the development of rare-earth free permanent magnets.

Terahertz Faraday Rotation of SrFe12O19 Hexaferrites Enhanced by Nb Doping

The magneto-optical and dielectric behavior of M-type hexaferrites as permanent magnets in the THz band is essential for potential applications like microwave absorbers and antennas, while are rarely reported in recent years. In this work, single-phase SrFe_12-xNb_xO₁₉ hexaferrite ceramics were prepared by the conventional solid-state sintering method. Temperature dependence of dielectric parameters was investigated here to determine the relationship between dielectric response and magnetic phase transition. The saturated magnetization increases by nearly 12%, while the coercive field decreases by 30% in the x = 0.03 composition compared to that of the x = 0.00 sample. Besides, the Nb substitution improves the magneto-optical behavior in the THz band by comparing the Faraday rotation parameter from 0.75 (x = 0.00) to 1.30 (x = 0.03). The changes in the magnetic properties are explained by a composition-driven increase of the net magnetic moment and enhanced ferromagnetic exchange coupling. The substitution of the donor dopant Nb on the Fe site is a feasible way to obtain multifunctional M-type hexaferrites as preferred candidates for permanent magnets, sensors, and other electronic devices.

Features of Galvanostatic Electrodeposition of NiFe Films with Composition Gradient: Influence of Substrate Characteristics

NiFe films with a composition gradient are of particular interest from the point of view of fundamental science and practical applications. Such gradient magnetic structures may exhibit unique functional properties useful for sensory applications and beyond. The issue surrounds the anomaly concerning the compositional gradient formed near the substrate in electrolytically deposited binary and ternary iron-containing alloys, which has not previously been clearly explained. In this work, light is shed on this issue, and a clear relationship is found between the structure and surface properties of the substrate, the initially formed NiFe layers and the film composition gradient.

Modeling and X-ray Analysis of Defect Nanoclusters Formation in B4C under Ion Irradiation

In the presented work, B4C was irradiated with xenon swift heavy ions at the energy of 167 MeV. The irradiation of the substrate was done at room temperature to a fluence of 3.83 × 1014 ion/cm2. The samples were then analyzed with the X-ray diffraction technique to study the structural modification, as it can probe the region of penetration of xenon atoms due to the low atomic number of the two elements involved in the material under study. The nano-cluster formation under ion irradiation was observed. Positron lifetime (PLT) calculations of the secondary point defects forming nanoclusters and introduced into the B4C substrate by hydrogen and helium implantation were also carried out with the Multigrid instead of the K-spAce (MIKA) simulation package. The X-ray diffraction results confirmed that the sample was B4C and it had a rhombohedral crystal structure. The X-ray diffraction indicated an increase in the lattice parameter due to the Swift heavy ion (SHI) irradiation. In B12-CCC, the difference between τ with the saturation of H or He in the defect is nearly 20 ps. Under the same conditions with B11C-CBC, there is approximately twice the value for the same deviation.

Synthesis of flexible Co nanowires from bulk precursors

This work reports a method of producing flexible cobalt nanowires (NWs) directly from the chemical conversion of bulk precursors at room temperature. Chemical reduction of Li6CoCl8 produces a nanocomposite of Co and LiCl, of which the salt is subsequently removed. The dilute concentration of Co in the precursor combined with the anisotropic crystal structure of the hcp phase leads to 1D growth in the absence of any templates or additives. The Co NWs are shown to have high saturation magnetization (130.6 emu g-1). Our understanding of the NW formation mechanism points to new directions of scalable nanostructure generation.

Exploring the High Frequencies AC Conductivity Response in Disordered Materials by Using the Damped Harmonic Oscillator

The AC conductivity response of disordered materials follows a universal power law of the form σ′(ω)∝ωn at the low frequency regime, with the power exponent values in the range 0 < n < 1. At the high frequency regime, in many experimental data of different disordered materials, superlinear values of the power exponent n were observed. The observed superlinear values of the power exponent are usually within 1<n<2, but in some cases values n>2 were detected. The present work is based on the definitions of electromagnetic theory as well as the Havriliak–Negami equation and the damped harmonic oscillator equation, which are widely used for the description of dielectric relaxation mechanisms and vibration modes in the THz frequency region, respectively. This work focuses mainly on investigating the parameters that affect the power exponent and the range of possible n values.

Influence of Dy3+ Ions on the Microstructures and Magnetic, Electrical, and Microwave Properties of [Ni0.4Cu0.2Zn0.4](Fe2-x Dy x )O4 (0.00 ≤ x ≤ 0.04) Spinel Ferrites

[Ni0.4Cu0.2Zn0.4](Fe2-x Dy x )O4 spinel ferrite nanoparticles with different Dy3+ concentrations (0.00 ≤ x ≤ 0.04) were prepared by a citrate sol-gel auto-combustion technique. A strong correlation among Dy concentration, structural parameters, and magnetic, electrical, and microwave properties was established. An increase in the Dy3+ concentration is the reason for a rise in the crystal structure parameters (due to different ionic radii of Fe and Dy ions) and a slight increase in the average particle size with a minor reduction in the specific surface area. It was observed that Dy3+ ions prefer to occupy the octahedral B site due to their large ionic radius (0.91 Å). The explanation of the electrical and magnetic properties was given in terms of the features of Dy3+-O2--Fe3+ dysprosium-oxygen-iron indirect exchange. The occurrence of the intensive changes in amplitude-frequency characteristics was observed from 1.6 to 2.7 GHz. The explanation of electromagnetic absorption was given in terms of the peculiarities of the microstructure (resonance of domain boundaries). The results open perspectives in the utilization of [Ni0.4Cu0.2Zn0.4](Fe2-x Dy x )O4 spinel ferrite nanoparticles as functional materials for targeted drug delivery and hyperthermia applications.

Functional Sr0.5Ba0.5Sm0.02Fe11.98O4/x(Ni0.8Zn0.2Fe2O4) Hard-Soft Ferrite Nanocomposites: Structure, Magnetic and Microwave Properties

This paper reports the correlation between the composition of the functional Sr0.5Ba0.5Sm0.02Fe11.98O19/x(Ni0.8Zn0.2Fe2O4) hard-soft nanocomposites (SrBaSmFe/x(NiZnFe) NCs), where 0.0 ≤ x ≤ 3.0, and their structural features, magnetic, and microwave properties. SrBaSmFe/x(NiZnFe) hard/soft ferrite NCs are produced using the one-pot citrate combustion method. According to the XRD analysis, all samples showed the co-existence of both SrBaSmFe and NiZnFe phases in different ratios. Magnetic properties are measured in a wide range of magnetic fields and temperatures (10 and 300 K) and correlated well with the composition of the investigated samples. The microwave properties (frequency dispersions of the magnetic permeability, and electrical permittivity) are discussed by using the co-axial method in the frequency range of 0.7-18 GHz. Non-linear dependences of the main microwave features were observed with varying of composition. The microwave behavior correlated well with the composite theory. These results could be used in practice for developing antenna materials.

Dipole Relaxation in Semiconducting Zn2-xMgxInV3O11 Materials (Where x = 0.0, 0.4, 1.0, 1.6, and 2.0)

This paper reports on the electrical and broadband dielectric spectroscopy studies of Zn_2-xMg_xInV₃O₁₁ materials (where x = 0.0, 0.4, 1.0, 1.6, 2.0) synthesized using a solid-state reaction method. These studies showed n-type semiconducting properties with activation energies of 0.147-0.52 eV in the temperature range of 250-400 K, symmetric and linear I-V characteristics, both at 300 and 400 K, with a stronger carrier emission for the matrix and much less for the remaining samples, as well as the dipole relaxation, which was the slowest for the sample with x = 0.0 (matrix) and was faster for Mg-doped samples with x > 0.0. The faster the dipole relaxation, the greater the accumulation of electric charge. These effects were analyzed within a framework of the DC conductivity and the Cole-Cole fit function, including the solid-state density and porosity of the sample. The resistivity vs. temperature dependence was well fitted using the parallel resistor model. Our ab initio calculations also show that the bandgap increased with the Mg content.

Pseudo-superparamagnetic behaviour of barium hexaferrite particles

The effect of hexadecyltrimethylammonium bromide (CTAB) addition on the crystal structure, morphology, and magnetic properties of co-precipitated hexagonal barium ferrite was investigated. For a fixed amount of surfactant, different Fe3+ concentrations and Fe3+/Ba2+ ratios were used to optimize the formation of single-phase barium ferrite particles. The results indicated that the obtained ferrite particles exhibited coercivity changes similar to those of superparamagnetic particles with larger than theoretically calculated particle sizes. This results from the softening of the material due to the size reduction of the grains and incorporation of excess barium, localized on the surface of the particles. Therefore, lowering the energy barrier required to reverse the magnetization was observed, while high magnetization saturation was preserved. The precipitation of barium ferrite particles from a surfactant-rich solution allowed control of BaFe12O19 magnetic properties without introducing any modifications inside the crystal structure.

Microwave Absorption Properties of Magnetite Particles Extracted from Nickel Slag

The utilization of nickel slag has attracted much attention due to its high-content of valuable elements. As a part of these efforts, this work focuses on whether magnetite crystals, obtained from nickel slag via molten oxidation, magnetic separation, and ball-milling can be used as a microwave absorber. The composition, morphology, microstructure, magnetic properties, and microwave absorption performance were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and vector network analysis (VNA). The results reveal that the magnetite crystals exhibit excellent microwave absorption properties because of the synergistic action between dielectric loss and magnetic loss. The minimum reflection loss (RL) of the particles obtained after 6 h ball-milling reaches −34.0 dB at 16.72 GHz with thickness of 5 mm. The effective frequency bandwidth (RL ≤ −10 dB) is 4.8–5.4 GHz and 15.9–17.6 GHz. Interfacial polarization of the particles could play a crucial role in improving absorbing properties because several components contained in the particles can dissipate electromagnetic wave effectively. The current study could show great potential in the preparation of magnetite crystals and utilization of nickel slag.

Peculiarities of the Crystal Structure Evolution of BiFeO3-BaTiO3 Ceramics across Structural Phase Transitions

Evolution of the crystal structure of ceramics BiFeO₃–BaTiO₃ across the morphotropic phase boundary was analyzed using the results of macroscopic measuring techniques such as X-ray diffraction, differential scanning calorimetry, and differential thermal analysis, as well as the data obtained by local scale methods of scanning probe microscopy. The obtained results allowed to specify the concentration and temperature regions of the single phase and phase coexistent regions as well as to clarify a modification of the structural parameters across the rhombohedral–cubic phase boundary. The structural data show unexpected strengthening of structural distortion specific for the rhombohedral phase, which occurs upon dopant concentration and temperature-driven phase transitions to the cubic phase. The obtained results point to the non-monotonous character of the phase evolution, which is specific for metastable phases. The compounds with metastable structural state are characterized by enhanced sensitivity to external stimuli, which significantly expands the perspectives of their particular use.

Synthesis of niobium substituted cobalt-nickel nano-ferrite (Co0.5Ni0.5NbxFe2-xO4 (x ≤ 0.1) by hydrothermal approach show strong anti-colon cancer activities

The combination of two or more nanoparticles found to be effective strategy to synthesize nanocomposites for better drug delivery and treatment. In the present study, we have combined cobalt (Co), nickel (Ni), niobium (Nb), and iron oxide (Fe₂O₄) and prepared niobium substituted cobalt-nickel nano-ferrite nanocomposites (Co_0.5Ni_0.5Nb_xFe_2-xO₄ (x ≤ 0.1) by using hydrothermal approach. We have characterized the structure and morphology of nanocomposites by using XRD, EDX, TEM and SEM methodologies. We have examined the impact of nanocomposites (Co_0.5Ni_0.5Nb_xFe_2-xO₄ (x ≤ 0.1) on cancerous cells (human colorectal carcinoma cells, HCT-116) by using MTT assay. We have also checked the impact of nanocomposites on normal and non-cancerous cells (human embryonic kidney cells, HEK-293) to confirm the specificity of their actions. Post- 48 h treatment of Co_0.5Ni_0.5Nb_xFe_2-xO₄ (x ≤ 0.1) led to dose-dependent inhibition of cancer cells growth and proliferation. However, no cytotoxic effect was observed on the normal cells (HEK-293). In addition, DAPI stained nuclear DNA staining analysis demonstrates that the Co_0.5Ni_0.5Nb_xFe_2-xO₄ (x ≤ 0.1) treatment also caused nuclear DNA disintegration which is the marker for programmed cell death. These results demonstrate that synthesized nanocomposites Co_0.5Ni_0.5Nb_xFe_2-xO₄ (x ≤ 0.1) selectively target the colon cancer cells and induce cancer cell death.Communicated by Ramaswamy H. Sarma.

Flux Single Crystal Growth of BaFe12−xTixO19 with Titanium Gradient

Titanium substituted barium hexaferrite BaFe12−xTixO19 single crystal was grown by the top seeded solution growth method from flux on the seed with controlled cooling below 1175 °C. Titanium substitution level gradient in the single crystal in the vertical and horizontal directions was studied. Two planes were cut and polished. A justification for the linear gradient of Ti substitution in a BaFe12−xTixO19 single crystal is proposed; substitution levels in the center and periphery were determined. It was shown that upon growth by the top seeded solution growth method, crystals with a linear Ti substitution level gradient from x = 0.73 to x = 0.77 for a distance of 11 mm along pulling direction were obtained. The study led to the conclusion about the relationship of the gradient and changes in the composition of the nutrient solution.

Investigation of AC-Measurements of Epoxy/Ferrite Composites

A pure ferrite and epoxy samples as well as the epoxy/ferrite composites with different 20 wt.%, 30 wt.%, 40 wt.%, and 50 wt.% weight ferrite contents have been prepared by the chemical co-precipitation method. AC-conductivity and dielectric properties such as the dielectric constant and dielectric loss of the prepared samples have been studied. The obtained results showed that the samples had a semiconductor behavior. The dielectric constant of the composites has been calculated theoretically using several models. For the composite sample that contains 20 wt.% of ferrites, these models give satisfactory compliance, while for the composite samples with a higher percentage of nanofillers, more than 30 wt.% theoretical results do not coincide with experimental data. The investigated polymer has very low conductivity, so this type of polymer can be useful for high-frequency applications, which can reduce the losses caused by eddy current. Thus, the prepared samples are promising materials for practical use as elements of microwave devices.

Peculiarities of the microwave properties of hard–soft functional composites SrTb0.01Tm0.01Fe11.98O19–AFe2O4 (A = Co, Ni, Zn, Cu, or Mn)

Herein, we investigated the correlation between the chemical composition, microstructure, and microwave properties of composites based on lightly Tb/Tm-doped Sr-hexaferrites (SrTb_0.01Tm_0.01Fe_11.98O₁₉) and spinel ferrites (AFe₂O₄, A = Co, Ni, Zn, Cu, or Mn), which were fabricated by a one-pot citrate sol–gel method. Powder XRD patterns of products confirmed the presence of pure hexaferrite and spinel phases. Microstructural analysis was performed based on SEM images. The average grain size for each phase in the prepared composites was calculated. Comprehensive investigations of dielectric properties (real (ε′) and imaginary parts (ε′′) of permittivity, dielectric loss tangent (tan(δ)), and AC conductivity) were performed in the 1–3 × 10⁶ Hz frequency range at 20–120 °C. Frequency dependency of microwave properties were investigated using the coaxial method in frequency range of 2–18 GHz. The non-linear behavior of the main microwave properties with a change in composition may be due to the influence of the soft magnetic phase. It was found that Mn- and Ni-spinel ferrites achieved the strongest electromagnetic absorption. This may be due to differences in the structures of the electron shell and the radii of the A-site ions in the spinel phase. It was discovered that the ionic polarization transformed into the dipole polarization.

Paper presents the correlation between the composition, microstructure, and microwave properties of composites based on Tb/Tm-doped Sr-hexaferrites and spinel ferrites (AFe₂O₄), which were fabricated by a one-pot citrate sol–gel method.

Functional Magnetic Composites Based on Hexaferrites: Correlation of the Composition, Magnetic and High-Frequency Properties

The paper describes preparation features of functional composites based on ferrites, such as "Ba(Fe_1-xGa_x)₁₂O₁₉/epoxy," and the results of studying their systems; namely, the correlation between structure, magnetic properties and electromagnetic absorption characteristics. We demonstrated the strong mutual influence of the chemical compositions of magnetic fillers (Ba(Fe_1-xGa_x)₁₂O₁₉ 0.01 < x < 0.1 solid solutions), and the main magnetic (coercivity, magnetization, anisotropy field and the first anisotropy constant) and microwave (resonant frequency and amplitude) characteristics of functional composites with 30 wt.% of hexaferrite. The paper presents a correlation between the chemical compositions of composites and amplitude-frequency characteristics. Increase of Ga-content from x = 0 to 0.1 in Ba(Fe_1-xGa_x)₁₂O₁₉/epoxy composites leads to increase of the resonant frequency from 51 to 54 GHz and absorption amplitude from -1.5 to -10.5 dB/mm. The ability to control the electromagnetic properties in these types of composites opens great prospects for their practical applications due to high absorption efficiency, and lower cost in comparison with pure ceramics oxides.

Fabrication, characterization, and magnetic properties of exchange-coupled porous BaFe8Al4O19/Co0.6Zn0.4Fe2O4 nanocomposite magnets

Fabrication of exchange-coupled nanocomposite magnets has been considered to be the most effective method to achieve the high energy product for advanced permanent magnet applications. In this work, we report a facile auto-combustion synthesis to prepare porous exchange-coupled hard-soft ferrite-based magnetic BaFe8Al4O19-x wt% Co0.6Zn0.4Fe2O4 nanocomposites (where x = 10, 20, 30 and 40), which realize an effective exchange-coupled interaction when the x value is less than 30. Compared with BaFe8Al4O19, the optimized nanocomposite with 20% Co0.6Zn0.4Fe2O4 shows a 70.3% increase in Ms and a 60.4% enhancement in Mr and maintains a high Hc value of 8.8 kOe. The work demonstrates that the auto-combustion synthesis is a promising approach for the fabrication of high-performance ferrite-based permanent magnets.

Impact of Nitrogen Foamed Stimulation Fluids Stabilized by Nanoadditives on Reservoir Rocks of Hydrocarbon Deposits

The first objective of this experiment was to improve the stabilization of N₂ based foam with nanoparticles as an alternative to typical fracturing fluid, which consists of a gelling agent (HPG—hydroxypropyl guar). The second objective of the project was to investigate the damage caused by nanoparticle–based nitrogen foamed fracturing fluids (F.F) on a reference sandstone, using permeability and porosity tests, optical microscope with a Profilometer, and a scanning electron microscope (SEM). The properties of F.F with two types of SiO₂ nanoparticles (hydrophilic fumed silica Areosil 300 and silica sol U-2 obtained by the sol-gel method), such as rheology and core damage, were investigated. The discussion of this research results is based on the stability tests carried out with the use of rheology and the foam half-life, formation damage ratio, and observation of exposed samples using SEM and the Profilometer. The permeability and porosity damage ratios of the damaged core samples were found to decrease when nitrogen foamed fluids were used. These results were confirmed with the Profilometer and SEM images. The experimental data showed that the foam stability increased when silica (SiO₂) nanoparticles were added. SiO₂ nanoparticle-surfactant-stabilized foam for fracturing is superior to traditional water-based fracturing fluids and causes lower core permeability damage than a traditional F.F.