Edgar S. Dvilis

Magnesium aluminate spinel is a promising transparent ceramic material. It can be used as transparent armor as windows for spacecraft, in scanners and optical elements in night-vision systems, and as a matrix for scintillation and laser materials. The broad range of applications for ceramics based on MgAl₂O₄ is due to their unique combination of properties. Research aimed at developing transparent ceramic materials with an increased aspect ratio is highly relevant and has significant practical importance. This study gives a comprehensive characterization of a commercial nanopowder of magnesium aluminate spinel. The primary structural properties of the nanopowder are determined. The rheological properties of the nanopowder during its compression and thermal consolidation are investigated based on the principles of a mechanistic pressing model by approximating the experimental densification data using a dimensionless logarithmic equation. The graphical representation of this equation enables the evaluation of the potential for achieving a pore-free state in the consolidated material and the optimization of the parameters for spark plasma sintering. Constructing a densification surface and identifying the optimal path for increasing pressure and temperature during spark plasma sintering on this surface can be used to optimize the consolidation parameters for transparent ceramics. Verified discrete-element modeling of the packing and consolidation processes of magnesium aluminate spinel nanopowder particles is conducted. Both model-based and experimental results show that the identified combination of temperature (1300°C) and static pre-pressing pressure (100 MPa) is sufficient to achieve a pore-free ceramic structure without causing excessive recrystallization. Transparent ceramics based on high-aspect-ratio magnesium aluminate spinel can be fabricated using the spark plasma sintering method at a temperature of 1300°C and a static pre-pressing pressure of 100 MPa. The optical and mechanical properties of the fabricated transparent ceramics are comparable to or exceed those of similar materials. The light transmittance of the samples reaches 41% in the visible region of the spectrum and 64% in the infrared region. The Vickers microhardness value is 15.6 ± 0.5 GPa; the fracture toughness is 4.4 ± 0.4 MPa · m^1/2; the compressive strength is 1.37 ± 0.23 GPa; the Poisson ratio is 0.26 ± 0.01; the Young modulus is 278 ± 6 GPa; and the shear modulus is 110 ± 2 GPa.

Subject of study. This study focuses on multilayer yttrium aluminum garnet/yttria-stabilized zirconia (YAG/YSZ) ceramics doped with rare earth ions. Aim of study. The aim of this study is to produce luminescent multilayer ceramics composed of YAG:CeO₂/YSZ:Eu₂O₃ with an increased shape factor using the spark plasma sintering technique and to investigate their complex optical-luminescent characteristics, which have potential applications as radiation converters. Method. The functionally graded ceramic samples were fabricated using the spark plasma sintering technique. The morphology and spectral-luminescent properties of the materials were analyzed through electron microscopy, photoluminescence, and cathodoluminescence spectrometry. Main results. The study demonstrates the feasibility of producing luminescent multilayer ceramics with the YAG:CeO₂/YSZ:Eu₂O₃ composition via the spark plasma sintering technique, achieving an increased shape factor. The dynamics of relative shrinkage were examined, revealing that the sintering parameters (temperature of 1450°C, pre-pressure of 80 MPa, and isothermal exposure duration of 30 min) are sufficient to obtain relatively dense multilayer ceramics of the selected composition. Studies on the spectral characteristics of YAG/YSZ functionally graded ceramics doped with europium and cerium oxides identified the main types of luminescence centers and elucidated the mutual influence of different layers on the luminescent properties. The integrated cathodoluminescence and photoluminescence spectra were consistent with existing theories regarding luminescence centers in YAG and YSZ materials, confirming the success of the sintering process. Practical significance. The comprehensive analysis of YSZ/YAG multilayer ceramics doped with rare earth ions conducted in this study demonstrates their potential use as converters of high-power laser radiation.

The present study showcases a novel effective technique for the surface modification of micro-arc diatomite coatings using low-energy, high-current electron beams (LEHCEBs). A variety of methods such as scanning electron microscopy, energy-dispersive X-ray spectroscopy, the X-ray diffraction method, scratch testing, the potentiodynamic polarization method, immersion testing in SBF, and flow cytometry have been used to study the coatings. During processing, the electron beams’ energy density ranged between 2.5–7.5 J/cm². After the LEHCEB treatment, the surface morphology of the coatings changed completely. The corrosion resistance of the LEHCEB-treated coated samples increased significantly, as evidenced by the decrease in corrosion current to 4.6 × 10⁻¹⁰ A·cm⁻² and the increase in polarization resistance to 1.4 × 10⁸ Ω·cm². The electron beam treatment also increased the adhesion strength of the coatings to the magnesium substrate by 1.8–2.5 times compared to untreated coatings. Additionally, biological studies have shown the high viability of the NIH/3T3 cell line after contact with the samples of the coating extracts.

Magnesium aluminate spinel is a promising material for optical application. It can be used as transparent armor, spacecraft windows, scanners, optical elements in night vision systems, as well as a base (matrix) for scintillation and laser materials. The wide range of practical applications of MgAl2O4 ceramics is due to the complex of its unique properties. The research is aimed to creating transparent ceramic materials with an increased shape factor, relevant and of high practical importance. In the presented work, a comprehensive characterization of the commercial nanopowder of magnesium aluminate spinel is completed. Its main structural features are defined. Rheological properties of nanopowder in the process of its compression and thermal consolidation are investigated on the basis of the provisions of the mechanistic pressing model by approximating experimental data of compaction during spark plasma sintering with a dimensionless logarithmic equation, the graphical representation of which allows us to evaluate the possibility of achieving a non-porous state of the consolidated material and optimize the modes of spark plasma sintering. Shown, that the structure of the sealing surface and the search for the optimal way to increase pressure and temperature during spark pulse plasma sintering on this surface can be used to optimize the consolidation modes of transparent ceramics. Verified discrete element modeling of the processes of packing and consolidation of particles of magnesium aluminate spinel nanopowder has been performed. Model and field experiments have shown that the revealed combination of temperature (1300 °C) and static pressure (100 MPa) is sufficient to obtain a porous ceramic structure and does not lead to excessive recrystallization. It is shown that transparent ceramics based on magnesium aluminate spinel with an increased shape factor can be produced by spark plasma sintering at temperature of 1300 °C and static pressure of 100 MPa. The optical and mechanical properties of the manufactured transparent ceramics are comparable to their analogues. The light transmission of samples reaches 41% of the visible spectrum and 64% in the infrared spectrum, the Vickers microhardness value is 15.6±0.5 GPa, crack resistance is 4.4±0.4 MPa·m1/2, compressive strength is 1.37±0.23 GPa, Poisson's ratio is 0.26±0.01, Young's modulus - 278±6 GPa, shear modulus - 110±2 GPa.

Poly(vinyl trimethylsilane) (PVTMS) films were subjected to direct surface fluorination in liquid medium (perfluorodecalin). The samples were investigated using several techniques: SEM-XEDS, XPS, ATR-IR, and contact angle measurement. The methods used allowed us to estimate chemical changes occurring because of the treatment. ATR-IR showed that most of the changes occurred in the Si(CH₃)₃ group. Monofluorinated Si(CH₃)₃ groups formed in the near-surface layer (Ge crystal, 0.66 µm penetration) after 30 min of fluorination, and then di- and trifluorinated groups appeared. Oxidation of the film with oxygen was also shown with the use of ZnSe crystal (2 µm penetration). The XPS method allowed an assessment of the ratio of the main elements at the surface of the fluorinated film. Two different exponential models were proposed to fit the experimental data of SEM-XEDS. Based on the model with the intercept, the depth of fluorination was estimated to be ≤1.1 µm, which is consistent with the result from the literature for the gas-phase fluorination. Contact angle measurements showed that oxidation of the PVTMS surface prevailed for the first 45 min of fluorination (surface hydrophilization) with a subsequent fluorine content increase and hydrophobization of the surface upon 60 min of fluorination.

Yttrium aluminum garnet (YAG) ceramics doped with Ce3+ was prepared by dry compaction using the common uniaxial pressing under powerful ultrasound assistance (PUA). The structure and luminescent properties of sintered ceramics were investigated. It was demonstrated that the powerful ultrasonic assistant during the dry pressing has a positive effect on the structural and luminescent characteristics of YAG: Ce3+ ceramics. The sintered material consolidation is very intensive accompanied by a decrease in the number of pores and grain growth at a sintering temperature of 1650 ºC. The luminous efficiency of YAG:Ce3+ ceramics vary from 120 to 219 lm/W (without ultrasound treatment) and from 120 to 250 lm/W (prepared with ultrasound conditions) closely related with the phase purity and implicitly with sintering temperature. It was demonstrated the positive effect of PUA on the thermal properties of ceramics.

A direct fluorination technique was applied for the surface treatment of PIM-1 films in a liquid phase (perfluorodecalin). The fluorinated samples were analyzed by various instrumental techniques. ATR-IR spectroscopy showed that the fluorination predominantly takes place in methylene- and methyl-groups. Cyano-groups, aromatic hydrogens and the aromatic structure of the PIM-1 repeat unit were shown to be relatively stable at the fluorination conditions. XPS confirmed that the concentration of fluorine, as well as oxygen, in the near surface layer (~1 nm) increases with fluorination time. C1s and O1s surface spectra of the fluorinated PIM-1 samples indicated an appearance of newly-formed C-F and C-O functional groups. Scanning electron microscopy and X-ray energy-dispersive spectroscopy of the fluorinated PIM-1 samples showed an increase of the fluorine concentration at the surface (~0.1–1 μm) with the treatment duration. Analysis of the slices of the PIM-1 films demonstrated a decline of the fluorine content within several microns of the film depth. The decline increased with the fluorination time. A model of fluorine concentration dependence on the film depth and treatment duration was suggested. A change in the specific free surface energy as a result of PIM-1 fluorination was revealed. The fluorination time was shown to affect the surface energy (γ_SV), providing its shift from a low value (25 mJ∙m⁻²), corresponding to tetrafluoroethylene, up to a relatively high value, corresponding to a hydrophilic surface.