Scientific highlights

—–  New quantum number for the many-electron Dirac-Coulomb Hamiltonian  —–

When studying molecules containing heavy elements it is necessary to include relativistic effects in the quantum chemical theory. Particularly challenging are systems with a non-degenerate ground state, so-called paramagnetic systems. These molecules appear in wide variety of scientific fields, for instance in the development of new magnetic materials (data storage), in studies of biological systems (metal-based drugs), and in nuclear waste treatment. One of the many challenges of relativistic quantum chemistry is the characterization of the configuration of quantum states when studying paramagnetic substances. In the non-relativistic domain, this is achieved by referring to the spin multiplicity of the system. However, this option is not available in the relativistic domain, since spin is not a good quantum number any more. One usually resorts, therefore, to the effective spin; or, equivalently, to the formal degeneracy of the ground and excited states. By accident, we have discovered a new quantum number in the relativistic theory of many-electron systems. This newly-discovered fundamental law has the potential to influence whole research fields dealing with heavy-element-containing paramagnetic systems.

Eigenfunctions of the many-electron Dirac-Coulomb Hamiltonian, C, are described by the main quantum number
and the recently discovered quantum number k.

Further information:

  • KOMOROVSKÝ, S. – REPISKÝ, M. – BUČINSKÝ, L. New quantum number for the many-electron Dirac-Coulomb Hamiltonian. In Physical Review A, 2016, vol. 94, p. 052104-1-052104-14. (2.765 – IF2015).

 

—– Ceramic carbon nanostructure composites with high electrical conductivity —-

In this work dense alumina (Al2O3) or silicon carbide (SiC) composites with addition of carbon nanostructures (carbon nanotubes – CNT, or graphene nanoplatelets – GNP) were prepared by a new approach.  This process comprises functionalisation of carbon nanostuctures, their stabilisation in suspension, and subsequent spraying of the stable suspension into liquid nitrogen. The influence of carbon nanostructures addition and effect of graphene layers orientation on the functional properties (electrical and thermal conductivity) of the composites thus prepared was studied. In the case of Al2O3-CNT composites the electrical conductivity significantly increased (by about 13 orders of magnitude) from 10-10 S/m for pure Al2O3 to 1748 S/m for the composite incorporating 12.5 vol.% of CNTs. The SiC-GNP composites exhibited electrical conductivity of up to 1775 S/m, an increase of about 5 orders of magnitude over the base material. The improvement of electrical conductivity can be attributed to the preparation method, which allows the homogeneous distribution of the carbon nanostructures to be preserved. As a result, carbon nanostructures formed conductive networks in the ceramic matrix. Ceramic composites with high electrical conductivity can be machined by the electrical discharge machining (EDM) process. Effective machining of prepared composites was confirmed for composites with ≥ 7.5 vol.% of CNTs.

Fracture surfaces of (a) SiC-GNPs and (b) SiC-GO composites; arrows show the location of GNPs and also the orientation of applied load during sintering

Further information:

Contact person: Ondrej Hanzel, e-mail: ondrej.hanzel@savba.sk
  • HANZEL, O. – SEDLÁK, R. – SEDLÁČEK, J.BIZOVSKÁ, V.BYSTRICKÝ, R. – GIRMAN, V. – KOVALČÍKOVÁ, A. – DUSZA, Ján – ŠAJGALÍK, P. Anisotropy of functional properties of SiC composites with GNPs, GO and in-situ formed graphene. In Journal of the European Ceramic Society 37 (2017) 3731-3739. (3.454 – IF2016)
  • HANZEL, O. – LOFAJ, F. – SEDLÁČEK, J. – KABÁTOVÁ, M. – KAŠIAROVÁ, M. – ŠAJGALÍK, P. Mechanical and tribological properties of alumina-MWCNTs composites sintered by rapid hot-pressing. In Journal of the European Ceramic Society 37 (2017) 4821-4831. (3.454 – IF2016)

 

—– Luminescent materials for potential applications in phosphor-converted white light emitting diodes (pc-WLED) —-

Y2O3-Al2O3 and Y2O3-Al2O3-SiO2 glasses were prepared in the form of glass microspheres from precursor powders by the flame-spraying method. The precursor powders were synthethised by the sol-gel Pechini method to achieve better homogeneity of the system. The prepared glasses were doped with rare-earth and transition metal ions with photoluminescence properties suitable for LED diodes in applications requiring emission of white light, e.g. Ce3+, Mn2+ at low concentration. The glass-ceramics material prepared by controlled crystallization of Ce3+-doped glasses at low concentration exhibits strong green-yellow emission under blue light, comparable with commercial phosphors. Moreover, heat treatment of the glass under suitable time-temperature regimes leads to emission of white light under NUV excitation. Co-doping of the Ce3+ system with Mn2+ ions enables the preparation of phosphors with improved emission in the orange-red region of the spectrum.

The colour coordinates (CIE 1931) of the emitted light from the samples (glass and polycrystalline) heat treated in reducing or oxidizing
atmosphere under NUV excitation. The pictures clearly indicate the emission of the white light when the reduced samples (due to Eu2+) are
irradiated by NUV light at 345 nm.

Further information:

  • HALADEJOVÁ, K. – PRNOVÁ, A. – KLEMENT, R. – TUAN, W.-H. – SHIH, S.J. – GALUSEK, D. Aluminate glass based phosphors for LED applications. In Journal of the European Ceramic Society, 2016, vol. 36, no. 12, p. 2969-2973. (2.933 – IF2015)

 

—– (Oxo)(fluoro)‒aluminates in the KF‒Al2O3 system: thermal stability and structural correlation —-

Potassium oxo- and oxo-fluoro-aluminates are new materials with interesting charge-related and optical properties. Understanding the unusually high transport properties of these materials requires more rigorous characterization of all phases existing in these systems. For this reason, a critical evaluation of the chemistry and the phase stability of the system KF−Al2O3 was performed. The system cannot be interpreted as a simple binary system, but represents the stable diagonal of the ternary reciprocal system KF–Al2O3–AlF3–K2O. It was discovered that this system contains many pure complex compounds – e.g. K3AlF6, K2O × 1, 9 and 11Al2O3, and K2Al2O3F2. The highlight of the work is the identification of a structural coherence between oxyaluminates of mullite type K2xAl2O3 (where x = 3-12) and oxy-fluoro-aluminates, which was determined, in particular, with the help of new NMR techniques. The structural characterization, using various solid state NMR techniques, including MQMAS and D-HMQC, has provided new and clearer insight into the links between the structures of individual existing complexes.

Deconvolution of 27Al and 19F NMR signals of pure K2Al2O3F2 using 27Al–19F D-HMQC HETCOR MAS NMR analysis of 2KF/1Al2O3.

Further information: F. Šimko, uachsim@savba.sk

  • ŠIMKO, F. – RAKHMATULLIN, A. – FLORIAN, P. – KONTRÍK, M.KORENKO, M.NETRIOVÁ, Z. – DANIELIK, V. – BESSADA, C. (Oxo)(fluoro)-aluminates in KF-Al2O3 system: thermal stability and structural correlation. In Inorganic Chemistry, 2017, vol. 56, no. 21, p. 13349-13359. (4.857 – IF2016).

 

—– Resonance energy transfer between laser dyes in smectite colloids: influence of the dye surface concentration —-

Highly efficient Förster resonance energy transfer (FRET) was observed in the hybrid colloids of layered silicate saponite (Sap) and two cationic laser dyes, rhodamine 6G (R6G) and oxazine 4 (Ox4), even at relatively low surface concentrations of the dyes. The bulk dye concentration was kept constant, and surface concentration of the dye was changed by varying the Sap concentration. Dye molecular aggregation, which often leads to luminescence quenching, was eliminated by keeping dye surface concentrations in an appropriate range. The FRET efficiency was found to be dependent primarily on the degree of saturation of the layered silicate surface with dye molecules. FRET can easily be observed in the quenching of green emission from the initially-excited R6G molecules in favour of red emission from Ox4. FRET yields were determined mainly by using the technique of time-resolved fluorescence (TRF) spectroscopy. The values determined experimentally using TRF were not in agreement with the model based on average distances between interacting dye molecules. Therefore, a new theoretical model was proposed considering the statistical distribution of intermolecular distances. Using this model, the FRET efficiency can be predicted with high accuracy. This model helps to explain the fundamental properties of hybrid colloidal systems based on layered silicate and adsorbed laser dyes. The application potential of such hybrid systems includes optical and photovoltaic materials, solid-state lasers, molecular probes, photosensitizers, photocatalysts, and photodisinfection materials.

Increasing dyes surface concentration and its influence on the photophysical properties of the hybrid nanomaterials.

Further information:

  • BELUŠÁKOVÁ, S. – MARTÍNEZ-MARTÍNEZ, V. – LOPEZ ARBELOA, I. – BUJDÁK, J. Resonance energy transfer between dye molecules in colloids of a layered silicate. The effect of dye surface concentration. In Journal of Physical Chemistry C, 2017, 121, p. 8300-8309. (4.536 – IF2016).

 

—– Flash joining of CVD-SiC coated Cf/SiC composites with a Ti interlayer —-

   This is the first work that applies so-called „flash“ (ultra-rapid) processing to joining of ceramic materials (carbon fibre / silicon carbide), thus allowing „flash joining“ of ceramics. During the process, electric current passes directly through the joining assembly and heats it up. The maximum shear strength of the joint was obtained when the process was performed in just 8 s using a maximum heating power of 2.2 kW, raising the temperature to 1237°C. The extremely rapid heating rate (9 600°C/min) and very short processing time (~ 8 s) suppressed diffusion between the base materials and the interlayers. A sound joint was obtained by forming a metallic kind of joint (Ti-based) at the joining temperature, in which the absence of any reaction phase significantly shortened the time necessary for the solid-state diffusion bonding. This has significant potential as a new joining mechanism of fibre-reinforced ceramic matrix composites (CMCs). When compared to the standard joining procedures (with a typical process time of ~ 1 h and temperature of 1700°C for the same interlayer and base materials), this newly developed „flash joining“ technique displays a significant reduction in processing time and maximum temperature required, which in turn leads to energy savings, making it potentially attractive for industry.

Schematic of flash joining of Cf/SiC composites

 

Further information:

Contact person: Peter Tatarko, e-mail: peter.tatarko@savba.sk

  • TATARKO, P. – GRASSO, S. – SAUNDERS, T.G. – FERRARIS, M. – REECE, M.J. Flash joining of CVD-SiC coated Cf/SiC composites with a Ti interlayer. In Journal of the European Ceramic Society, 2017, 37, 3841-3848. (3.454 – IF2016).

 

—– Solar thermal MgO electrolysis process for magnesium production —-

   Our project proposal called for lab development and techno-economic analysis of a new eco-economic process for production of magnesium with solar thermal input. Energy requirements for the process were based on thermodynamics and an extrapolation of laboratory measurements of the electrochemical kinetic and mass transport parameters (electrode potentials, diffusion coefficients, exchange current densities, electrolyte conductivity) via a finite-element numerical model. The result of the project was the design of a completely new technology for the production of primary magnesium, which is environmentally sensitive and economically competitive in relation to the currently-dominant thermal Pidgeon process. The project has recently been converted into an up-scale test (1000 A, 1000 hours), funded by the US DOE and performed at the Materials and Electrochemical Corporation, Tuscon, Arizona.

Schematic concept of the process and current as a function of cell potential

 

Further information: M. Korenko, uachmiko@savba.sk

  • LEONARD, N. – KORENKO, M. – LARSON, C. – BLOOD, K. – VENTROM, L.J. – NUDEHI, S. – DUNCAN, S. – DIVER, R. – ŠIMKO, F.PRIŠČÁK, J. – SCHOER, J. – KISSINGER, P.T. – PALUMBO, R. The thermal electrolytic production of Mg from MgO: A discussion of the electrochemical reaction kinetics and requisite mass transport processes. In Chemical Engineering Science, 2016, vol. 148, p. 155-169. (2.750 – IF2015).
  • KORENKO, M. – LARSON, C. – BLOOD, K. – PALUMBO, R. – NUDEHI, S. – DIVER, R. – BLOOD, D. – ŠIMKO, F. – VENSTROM, L.J. Technical and economic evaluation of a solar thermal MgO electrolysis process for magnesium production. In Energy, 2017, vol. 135, p. 182-194. (4.520 – IF2016).