Scientific highlights

A scalable, low cost processing method to prepare porous bioactive glass (BG) microspheres is presented. Glass powder with composition based on 45S5 BG and exhibiting irregularly shaped particles was fabricated by conventional melting. Glass powder was alkali activated to induce pore formation during the following flame synthesis step. Porous microspheres, with diameters ranging between 45 and 75 µm, were successfully prepared and characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. The porous bioactive glass microspheres are promising candidates for applications in bone regeneration, tissue engineering, and as carriers for controlled drug delivery. To the best of our knowledge the paper presents the first successful preparation of porous bioactive (Figure 1) glass microspheres based on 45S5 BG composition by flame synthesis process. The microspheres were prepared by alkaline activation of conventionally melted BG powder, followed by flame spheroidisation. C-S-H and natrite, formed during the alkaline activation release volatile species (water vapour and CO₂) during the flame synthesis, which act as pore forming agents yielding to porous microspheres with diameter ranging between 45 and 75 µm. Only slight reduction in the content of Na₂O, P₂O₅, and CaO was observed after the flame synthesis.e.

Publication:

• KRAXNER, J. – MICHÁLEK, M. – ROMERO, A.R. – ELSAYED, H. – BERNARDO, E. – BOCCACCINI, A. – GALUSEK, D. Porous bioactive glass microspheres prepared by flame synthesis process. In Materials Letters, 2019, vol. 256, p. 126625-1-126625-4. (2018: 3.019 – IF)

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The accurate prediction of electron absorption and emission spectra of heavy-element containing compounds requires proper inclusion of relativistic effects into consideration. When spin-orbit (SO) relativistic effects are included in the description of the system, then otherwise forbidden singlet-triplet transitions become possible. The correct accounting of this phenomenon is, for example, prominent in the description of photochemical properties of organic light-emitting diodes (OLEDs), in the prediction of L-edge X-ray spectra, or other spin-related phenomena in magnetic materials.

In this contribution, we have developed and implemented the relativistic four-component (4c) method for calculation of the excitation energies and transition dipole moments in the framework of time-dependent density functional theory (TDDFT). Four-component methodology treats relativistic effects on an equal footing across the periodic table of elements, and it is therefore ideal for performing accurate benchmark calculations. The implementation of the 4c-TDDFT method in our in-house ReSpect program allows the treatment of systems with up to 100 atoms, which makes the method suitable for routine real-life calculations. Moreover, the 4c-TDDFT method applies to both closed and open-shell reference states. In the case of four-component methods for open-shell systems, this is the first time implementation worldwide. This achievement is an especially remarkable result since up to now the scientific community considered this task as too challenging. Prof. W. Liu, one of the leading scientists in the field, wrote: “… whose [4c-TDDFT] moment adaptation is technically rather difficult, if not impossible. …” [Mol. Phys. 111, 3741 (2013)].

A textbook demonstration of the method on open-shell systems is the calculation of the J_5/2, J_3/2 spin-orbit splitting of the otherwise ten times degenerate ²D state. In the figure, the splitting is visualized as two distinct signals in a series of Coinage-metal atoms. As relativistic effects grow in the magnitude along the series, the splitting increases and is the largest for the gold atom. Note, however, that the intensity of the peaks was scaled, and in practice, they are too small to be detected in the electron absorption spectroscopy (EAS).

Publication:

• KOMOROVSKÝ, S. – CHERRY, P. – REPISKÝ, M. Four-component relativistic time-dependent density-functional theory using a stable noncollinear DFT ansatz applicable to both closed- and open-shell systems. In Journal of Chemical Physics, 2019, vol. 151, no. 18, p. 184111-1-184111-14. (2.997– IF2018).

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Silicon nitride-based ceramics with SiO₂, CaO and Ca₃(PO₄)₂ as sintering additives, have been prepared in order to study the bioactivity. Dense ceramic bodies were oxidized by an oxy-acetylene flame at approx. 1475°C for 60 seconds, in order to modify the surface in terms of bioactivity enhancement and the formation of optimal porosity for cell viability. During oxidation two concurrent processes occurred on the ceramic body surface: (i) formation of thin glassy layer with a composition close to that of grain boundary phase in ceramic body, and (ii) partial decomposition of silicon nitride matrix. The latter one resulted in the formation of gases (N₂ and SiO), which formed bubbles in the viscous surface glassy phase. The best bioactivity was obtained for oxy-acetylene flame treated Si₃N₄ ceramics with Ca₃(PO₄)₂ sintering additive.

Publication:

• HNATKO M. – HIČÁK M. – LABUDOVÁ M. – GALUSKOVÁ D. – SEDLÁČEK J. – LENČÉŠ Z. – ŠAJGALÍK P. Bioactive silicon nitride by surface thermal treatment; In Journal of the European Ceramic Society 40 (2020) 1848-1858; https://doi.org/10.1016/j.jeurceramsoc.2019.12.053

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Micro scale fracture strength of silicon nitride grains and grain boundaries in polycrystalline La-doped ß-Si₃N₄ ceramics were investigated and compared with theoretically predicted values. The fracture behaviour of SiO₂-La₂O₃ intergranular glassy phase (IGP) between ß-Si₃N₄ grains was modelled by ab initio simulations. Microcantilevers, Fig 1 were FIB-milled from polycrystalline samples and single grains of ß-Si₃N₄ and tested in bending on an Agilent G200 NanoIndenter. The fracture strength of microcantilvers were determined by linear beam theory and fracture surfaces were investigated by micro/nano fractography based on the acquired SEM images. In the literature predicted theoretical strength of the intergranular phases decreased from ~20 GPa for pure SiO₂ to 13-16 GPa for La-doped IGP. In our experiments, microcantilevers from single grains of ß-Si₃N₄ that fractured at the fixing with an average strength of 10.6±0.8 GPa did not exhibit any nano/micro defects as fracture origin. The average strength of single grain microcantilevers with defects as fracture origin was 5.9±2.3 GPa. Polycrystalline ß-Si₃N₄ samples with increasing La₂O₃ content in IGP from 0.86 wt% to 4.19 wt% showed intergranular fracture with decreasing strength values in the range of 2.9±0.4 GPa – 2.1±0.5 GPa, Fig 2. The comparison of single grain results with theoretical values in the literature revealed a correlation between theoretical and experimental results, which was used to convert our ab initio simulations from subnano to micro size samples. The converted strength values for IGP showed quantitative agreement with micro-bending experiments, Fig. 3.

More information:

• P. Šajgalík, pavol.sajgalik@savba.sk

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Almost fully dense silicon carbide composites with Y₂O₃ and Al₂O₃ as a sintering additives and with different amount of graphene nanoplatelets (GNPs) from 5 to 15 wt. % or 5 wt. % of graphene oxide (GO) were sintered in rapid hot press (RHP) at 1800°C for 5 min with a pressure of 50 MPa in vacuum.

Composites microstructure anisotropy caused by graphene alignment as a consequence of rapid hot pressing was confirmed by measuring of electrical conductivity and thermal diffusivity. Electrical conductivity increased significantly with increased weight fraction of graphene in both measured directions. Highest value of 2031 S/m was obtained for composites with 15 wt. % of GNPs in parallel direction and only 1246 S/m in perpendicular direction to aligned GNPs. Material removal rate (MRR) of all SiC/GNPs and SiC/GO composites was higher than 1.6 mm³/min during the WEDM tests. MRR was almost doubled and increased to 2.8 mm³/min in composite containing 15 wt. % of GNPs. At the same time surface roughness decrease to 1.5 µm.

Publication:

• HANZEL, O. – SINGH, M.A. – MARLA, D. – SEDLÁK, R. – ŠAJGALÍK, P. Wire electrical discharge machinable SiC with GNPs and GO as the electrically conducting filler. In Journal of the European Ceramic Society, 2019, vol. 39, no. 8, p. 2626-2633. (4.029 – IF2018)

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Embedded catalysts for toluene combustion, featuring iron oxide (FeOx) moieties on the surface of clay derived porous heterostructural supports (PCH) were developed. The nucleation and growth of Fe4Cl0.44O3.56(OH)4.44(H2O)0.2 (akaganeite) in the function of Fe precursor interaction with the support entailed creation of nanostructured Fe oxides on PCH surface. Hydrothermal forcing of ions migration and thermal transformation of as synthesized Fe-PCH altered the surface composition and crystallinity of the formed nanostructured Fe oxides, thereby affecting the advantageous effects of donation gauged by electronic transfer. The polymorphism of Fe oxides created on PCH supports resulted in diverse catalytic activity of toluene combustion. Analysis of XPS Fe 2p core excitations showed that elongation of crystallization time of iron moieties on as synthesized Fe-PCH, as well as shortening of the annealing time, led to highly oxidized Fe species, mainly in the form of ?-Fe2O3 phase. Pressure dependent functionalization induced morphology changes with consequent uncommon “lace-like” morphology of ?-Fe2O3 nanocrystals. The octahedral coordination of Fe3+ species was found to be beneficial in toluene decomposition, while presence of Fe3O4 diminished the catalysts activity. The distinction of the ?-Fe2O3 and Fe3O4 based on DFT calculations and experimental assignments at far infrared (FIR) region is proposed.

Publication:

• ZIMOWSKA, M. – GURGUL, J. – SCHOLTZOVÁ, E. – SOCHA, R.P. – PÁLKOVÁ, H. – LITYNSKA-DOBRZYNSKA, L. – MORKZYCKI, L. – LATKA, K. A precursor approach for the development of lace-like Fe2O3 nanocrystallites triggered by pressure dependent nucleation and growth of akaganeite over clay based composites for toluene combustion. In Journal of Physical Chemistry C, 2019, vol. 123, no. 43, p. 26236-26250. (4.309 – IF2018).

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