Ruzha Harizanova

The preparation of new magnetic materials is important because of their potential application in various electronic components. In the present work, the synthesis of glass-crystalline materials in the system Na₂O-MnO-SiO₂-Fe₂O₃ prepared by applying melt-quenching is reported. The phase composition as studied by X-ray diffraction and Raman spectroscopy reveals the precipitation of monophase Mn_xFe_3−xO₄ based solid solutions. The microstructure is studied by scanning electron and optical microscopy and shows bulk crystallization and the presence of polygon-shaped as well as of dendritic crystals, depending on the iron oxide concentration and used raw materials. Mössbauer spectra show that in the amorphous matrix the Fe ions are mainly present as Fe³⁺ in tetrahedral coordination and as Fe³⁺ in a solid solution with the composition Mn_xFe_3−xO₄. The simultaneous presence of MnFe₂O₄ (jacobsite) and a Mn-containing solid solution based on Fe₃O₄ (magnetite) is suggested. The room temperature magnetic properties were studied by vibrating sample magnetometer and reveal ferrimagnetic properties for all investigated glass-crystalline materials.

Applying the melt quenching method (cooling rate 10¹–10² K/s), new multicomponent vanadate glasses were synthesized, containing different amounts of MoO₃ at the expense of B₂O₃ with the composition 20Li₂O:(30 − x)B₂O₃:50V₂O₅:xMoO₃, x = 10, 20 mol%. The obtained samples were characterized by X-ray diffraction, infrared spectroscopy, differential scanning calorimetry and impedance spectroscopy. The density of the glasses was measured by the Archimedes method, on the basis of which the physicochemical parameters molar volume, oxygen molar volume and oxygen packing density were calculated. It was found that the replacement of B₂O₃ with MoO₃ leads to changes in electrical conductivity, which are a consequence of the increase in non-bridging oxygen atoms in the amorphous structure. The electrochemical characterization of the 20Li₂O:(30 − x)B₂O₃:50V₂O₅:20MoO₃ glass obtained was performed by assembling an all-solid-state cell, employing 20Li₂O:(30 − x)B₂O₃:50V₂O₅:20MoO₃ glass as a cathode active material. The obtained results show that the studied glass compositions are interesting in view of their potential application as cathode materials in all-solid-state lithium-ion batteries.

The interest in synthesizing new dielectric materials is caused by their potential application in various electronic and sensor devices as well as in a large variety of electronic components. The present work reports the synthesis of glasses in the Na₂O/Al₂O₃/BaO/ZrO₂/TiO₂/B₂O₃/SiO₂ system prepared by melt-quenching. These glasses were then crystallized to glass–ceramics by a controlled thermal treatment. X-ray diffraction experiments reveal the precipitation of Ba₂TiSi₂O₈ (fresnoite) and BaTiO₃, which probably forms a BaZr_xTi_1−xO₃ solid solution. The microstructure is studied by scanning electron microscopy and shows the presence of mulberry-shaped, crystallized structures with a densely-branching morphology. Microcomputed X-ray tomography is used to gather information on the volume fraction and average size of the crystallized volume as an effect of the applied temperature–time schedule. Longer annealing times lead to a higher volume fraction and increasing average size of the crystallization structures obtained. The dielectric properties analyzed by impedance spectroscopy are insulating and show relatively high dielectric constants ≥ 100 and moderate loss tangent values at 10 kHz.

Applying the melt quenching method (cooling rate 101-102K/s), new multicomponent vanadate glasses were synthesized, containing different amounts of MoO3 at the expense of B2O3 with the composition 20Li2O:(30-х)B2O3:50V2O5:xMoO3, x = 10, 20 mol%. The obtained samples were characterized by X-ray diffraction, infrared spectroscopy, differential scanning calorimetry and impedance spectroscopy. The density of the glasses was measured by the Archimedes method, on the basis of which the physicochemical parameters molar volume, oxygen molar volume and oxygen packing density were calculated. It was found that the replacement of B2O3 with MoO3 leads to changes in electrical conductivity, which are a consequence of the increase in non-bridging oxygen atoms in the amorphous structure. The electrochemical characterization of the 20Li2O:(30-х)B2O3:50V2O5:20MoO3 glass obtained was performed by assembling an all-solid-state cell, employing 20Li2O:(30-х)B2O3:50V2O5:20MoO3 glass as a cathode active material. The obtained results show that the studied glass composition is interesting in view of their potential application as cathode materials in all-solid-state lithium-ion batteries.

Glasses with the compositions (23.1-x)Na 2 O/23.1BaO/23TiO 2 /9.8B 2 O 3 /21SiO 2 /xAl 2 O 3 x = 3, 7, 11 and 15 mol% were melted using a traditional melt-quenching technique. The thermophysical properties of the obtained glasses, i.e., glass transition temperature, T g , and crystallization peak maximum temperature, T c , were determined by differential scanning calorimetry and showed that the increasing alumina concentration leads to an increase of both the T g and T c values, while the difference T g-T c , used as a glass stability criterium, changes in the opposite manner, being highest for the glass with 3 mol % Al 2 O 3 and smallest for the composition with 15 mol % Al 2 O 3. The structure of the prepared series of glasses was studied by infrared spectroscopy and showed that the main structural units present are isolated and interconnected SiO 4 tetrahedra. Both BO 4 and BO 3 structural units, which number initially increases with the increasing alumina concentration and then starts to decrease, are also present. The conducted depth-sensing indentation on the obtained glasses allowed to determine mechanical properties such as universal (HMV) and indentation (Hit) hardness, elastic modulus and elastic part of indentation work. The change of the Na 2 O/Al 2 O 3 ratio resulted in increasing HMV and H it up to 11 mol% Al 2 O 3 and then a slight decrease of the respective parameters was observed.

The high temperature synthesis of glasses in the system (100‐x)(0.16Na ₂ O/0.10CaO/0.74SiO ₂ )/xFe ₂ O ₃ , x = 5 ÷ 20 mol% is reported. For Fe ₂ O ₃ concentrations ≤15 mol%, glasses are formed while the sample with 20 mol% crystallizes during cooling the melt. X‐ray diffraction shows the crystallization of magnetite. The microstructure of the glass‐crystalline sample is investigated by optical microscopy and scanning electron microscopy and two types of iron‐rich crystals corresponding to magnetite and hematite are detected. The refractive indices as determined by the Becke line method are in the 1.567 ‐ 1.639 range and. increase with increasing Fe ₂ O ₃ concentration. The structure is characterized using Infra‐red spectroscopy. The presence of symmetric stretching, asymmetric stretching and bending vibrations of Si‐O‐Si is detected and attributed to the occurrence of SiO ₄ tetrahedral units with varying numbers of nonbridging oxygens. Also, the increasing Fe ₂ O ₃ concentration results in occurrence of Fe‐O‐Si bonds indicating the glass network depolymerization due to Fe ₂ O ₃ addition. In all samples, the presence of Fe ³⁺ and Fe ²⁺ and the existence of iron ions in tetrahedral and octahedral coordination, as well as a very small amount of Fe ⁰ and the precipitation of hematite and magnetite in the glass‐crystalline sample is revealed by Mössbauer spectroscopy.

SEM micrograph with the IPF + IQ-maps of the polygon and dendritic MnFe₂O₄ crystals and an EBSD pattern acquired from a large jacobsite crystal.