Evaluation of education scenarios for acquiring digital competences of secondary school students in Slovenia

Central European Journal of Operations Research - Digital competences are among the most important competences in modern society, with the education system playing a key role in the acquisition of...     

Synthesis and gas barrier characterization of poly(ethylene isophthalate)

Poly(ethylene isophthalate) (PEI) was synthesized for this research with essentially a condensation polymerization of isophthalic acid and ethylene glycol catalyzed by zinc acetate and antimony trioxide. Several samples were obtained, and their characteristics were observed and compared with poly(ethylene terephthalate) (PET). The synthesized PEI samples were chemically identified by ¹H NMR. Thermal analysis with differential scanning calorimetry (DSC) yielded results that indicate the samples were primarily amorphous, with a glass‐transition temperature of 55–60 °C. Molecular weights of these PEI samples were also obtained through intrinsic viscosity measurements (Mark–Houwink equation). Molecular weights varied with conditions of the polymerization, and the highest molecular weight achieved was 21,000 g/mol. Finally, the diffusion coefficient, solubility, and permeability of CO₂ gas in PEI were measured and found to be substantially lower than in PET, as anticipated from their isomeric chemical structures. This is because in PET the phenyl rings are substituted in the para (1,4) positions, which allows for their facile flipping, effectively permitting gases to pass through. However, the meta‐substituted phenyl rings in PEI do not permit such ring flipping, and thus PEI may be more suitable for barrier applications. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4247–4254, 2004     

Linearizability of d-webs, d ≥ 4, on two-dimensional manifolds

We find d − 2 relative differential invariants for a d-web, d ≥ 4, on a two-dimensional manifold and prove that their vanishing is necessary and sufficient for a d-web to be linearizable. If one writes the above invariants in terms of web functions f(x, y) and g ₄(x, y),..., g _d (x, y), then necessary and sufficient conditions for the linearizabilty of a d-web are two PDEs of the fourth order with respect to f and g ₄, and d − 4 PDEs of the second order with respect to f and g ₄,..., g _d . For d = 4, this result confirms Blaschke’s conjecture on the nature of conditions for the linearizabilty of a 4-web. We also give the Mathematica codes for testing 4- and d-webs (d > 4) for linearizability and examples of their usage.     

Comparison of the Temperature Effect on the π∗←n and π∗←π Electronic Transition Bands of NO<Subscript>3</Subscript><Superscript>−</Superscript>(aq)

The effect of temperature on the π∗←π transition band in the UV absorption spectrum of NO₃⁻(aq) centered at ≈200 nm was studied in the temperature range 10–70 ^∘C. The observed temperature independence of this band was in contrast to the significant influence of temperature on the nitrate π∗←n transition reported recently by us. However, taking into account the electronic states involved in both the transitions, it was concluded that this finding was in accordance with our previous assumption that interconversion between spectrally distinct (with respect to π∗←n band) nitrate species included the rupture/formation of hydrogen bond(s) in the hydration shell of the nitrate ion.     

Density functional study of the electric hyperfine interactions and the redox-structural correlations in the cofactor of nitrogenase. Analysis of general trends in (57)Fe isomer shifts

The influence of the interstitial atom, X, discovered in a recent crystallographic study of the MoFe protein of nitrogenase, on the electric hyperfine interactions of (57)Fe has been investigated with density functional theory. A semiempirical theory for the isomer shift, delta, is formulated and applied to the cofactor. The values of delta for the relevant redox states of the cofactor are predicted to be higher in the presence of X than in its absence. The analysis strongly suggests a [Mo(4+)4Fe(2+)3Fe(3+)] oxidation state for the S = 3/2 state M(N). Among C(4-), N(3-), and O(2-), oxide is found to be the least likely candidate for X. The analysis suggests that X should be present in the cofactor states M(OX) and M(R) as well as in the alternative nitrogenases. The calculations of the electric field gradients (EFGs) indicate that the small values for DeltaE(Q) in M(N) result from an extensive cancellation between valence and ligand contributions. X emerges from the analysis of the hyperfine interactions as an ionically bonded species. Its major effect is on the asymmetry parameters for the EFGs at the six equatorial sites, Fe(Eq). A spin-coupling scheme is proposed for the state [Mo(4+)4Fe(2+)3Fe(3+)] that is consistent with the measured (57)Fe A-tensors and DeltaE(Q) values for M(N) and identifies the unique site exhibiting the small A value with the terminal Fe site, Fe(T). The optimized structure of a cofactor model has been calculated for several oxidation states. The study reveals a contraction in the average Fe-Fe distance upon increasing the number of electrons stored in the cluster, in accord with extended X-ray absorption fine structure studies. The reliability of the adopted methodology for predicting redox-structural correlations is tested for cuboidal [4Fe-4S] clusters. The calculations reveal a systematic increase in the S...S sulfide distances, in quantitative agreement with the available data. These trends are rationalized by a simple electrostatic model.     

Determination of calcium in silicate rocks by potentiometric titration with ethyleneglycol-bis-(2-Aminoethylether)tetraacetic acid and a calcium-selective electrode

The method reported for the determination of calcium in silicate rocks involves titration with ethyleneglycol-bis(2-aminoethylether)tetraacetic acid (EGTA) to a potentiometric end-point. A Crytur calcium-selective electrode is used; the selectivity constants (K_ca,x) are less than 10^-3 for Mg, Ba, Sr, K and Na. The method can be used for calcium contents of 2% or higher (and exceptionally for lower contents), in the presence of up to 60% Mg, 33% AI, 17% Fe, 5% Ba, 5% Ti, and 2.5% Mn. Al, Fe and Ti are masked with sulphosalicylic acid, barium is precipitated as its sulphate, and manganese is bound as its cyanide complex. If the magnesium content is less than that of calcium, EDTA can be used as titrant, magnesium being masked with acetylacetone.     

Transport in ceria electrolytes modified with sintering aids: effects on oxygen reduction kinetics

Small (2 mol%) cobalt oxide additions to ceria-gadolinia (CGO) materials considerably improve sinterability, making it possible to obtain ceramics with 95–99% density and sub-micrometre grain sizes at 1,170–1,370 K. The addition of Co causes a significant shift of the electrolytic domain to lower pO₂. This modification to the minor electronic conductivity of the electrolyte material has influence on the cathodic oxygen reduction reaction. The impedance technique is shown to provide information not only about polarisation resistance, but also about the active electrode area from analysis of the current constriction resistance. It is demonstrated that this current constriction resistance can be related to the minor electronic contributions to total conductivity in these materials. A simple imbedded grid approach gives control of the contact area allowing the properties of the electrolyte materials to be studied. A much lower polarisation resistance for the Co-containing CGO electrolyte is observed, which can be clearly attributed to an increased three-phase reaction area in the Co-containing material, as a consequence of elevated p-type conductivity.