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Abstract

This paper provides a first-ever look at differences of centrality scores (i.e., networks) over time and across research specializations in Korea. This is a much needed development, given the variance which is effectively ignored when Science Citation Index (SCI) publications are aggregated. Three quantitative tests are provided—OLS, two sample t-tests, and unit-root tests—to establish the patterns of centrality scores across Korea over time. The unit-root test is particularly important, as it helps identify patterns of convergence in each region's centrality scores. For all other geographic regions besides Seoul, Gyeonggi, and Daejeon, there appears to be little promise—at least in the immediate future—of being network hubs. For these top three regions, though, there is a pattern of convergence in three-quarters of all research specializations, which we attribute in part to policies in the mid- and late-1990s.

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Abstract  

Oxygen depletion and reoxidation of bismuth molybdates (2∶1; 2∶3) have been studied by means of isothermal thermogravimetry and DTA measurements. From the isothermal curves, Arrhenius energies were obtained between 411 and 683 K. The activation energies for oxygen depletion from Bi2O3·MoO3 were lower than those for Bi2O3·3MoO3. Two kinetically different types of oxygen release were identified for both molybdates. Arrhenius plots were also obtained from reoxidation experiments: Bi2O3·MoO3 was more easily reoxidable than Bi2O3·3MoO3. The substantial closeness of the respective activation energies suggests that depletion and reoxidation follow the same mechanistic steps. Some DTA measurements confirm the existence of at least two types of reoxidation sites for both oxysalts.

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High alumina cement (HAC) concrete undergoes with ageing a change in mineral constitution known as conversion which may be accompanied by a loss in strength. The amount by which this change in composition has gone, measured by the ratio of CaO·Al2O3·10 H2O to Al2O3·3 H2O is known as the degree of conversion (D c ). The faster the conversion goes the greater the loss in strength.

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The chemical and physical processes occurring during grinding of copper hydroxocarbonates mixtures with aluminium were studied. A planetary ball mill was used. A thermogravimetry and X-ray powder diffraction method allowed to determine the composition of solid products after mechanical activation. The amount of the Cu2(OH)2CO3 undecomposed andAl2O3⋅3H2O, CuO, Al2O3, Cu0, CuxAly alloys and remained Al0 in the systems is strongly dependent on the duration of grinding and on the proportion of components. The comparative results are presented.

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Abstract  

4-Methoxy-2-methylbenzoates of Y(III) and lanthanides(III) (La-Lu) were prepared as crystalline anhydrous complexes with general formula Ln(C9H9O3)3 (complexes of La and Pr as monohydrates). Monohydrates heated in air lose crystallization water molecule and then anhydrous complexes decompose directly to oxides. Only La(III) complex decomposes to oxide with intermediate formation La2O2CO3. The carboxylate group in the studied complexes is a tridentate chelating - bridging or bidentate chelating (Y).

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Abstract  

Sintering processes in the Y2O3–Al2O3–B2 O3 system and its subsystems (Y2O3–B2O3 and Al2 O3–B2O3) have been investigated by using combined DTA and XRD measurements to get a better understanding of solid state chemical changes resulting in the formation of yttrium aluminum borate (YAl3(BO3)4, YAB) phase and to study the possible role and contribution of various simple borates formed also in the former processes. Two new exothermic heat effects of YBO3 formation have been detected by DTA in the Y2O3–B2O3 system between 720 and 980C. In the Al2O3–B2O3 system a new experimental XRD profile of Al4B2O9 was observed. Formation of these borates seems to promote the nucleation of double borate YAB below 1000C. Conversion of Al4B2O9 to Al18B4 O33 was observed after a long term (10 h) sintering at 1050C. Similarly, an increased formation of YAB has been observed as a product of the sintering reaction between YBO3 and Al18B4O33 at 1150C. The two latter single borates are found to be identical with the high temperature decomposition products of YAB.

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Abstract

The scientific interest for the Bi2O3-PbO system has increased due to the importance of the PbO in the high-T c superconducting phase formation in the Bi2O3-SrO-CaO-CuO system. Also Bi2O3-PbO system contains compounds with some specific semiconductor and dielectric properties and Bi2O3-based solid solutions are well known as high oxygen ion conductors.

Previously, several low melting defined compounds have been identified in the system: 6Bi2O3·PbO; 3Bi2O3·2PbO; 4Bi2O3·5PbO; 4Bi2O3·6PbO and Bi2O3·3PbO.

This work deals with the phase formation and thermal stability of these compounds. Under non-isothermal conditions, in all mixtures regardless of the Bi2O3/PbO ratio, the compound 6Bi2O3·PbO is preferentially formed, followed by the compound 4Bi2O3·5PbO. The formation of the compound 4Bi2O3·6PbO was not confirmed while the formation of the compound Bi2O3

3PbO occurs through a complex mechanism which includes an intermediate step in which a solid solution with the litharge structure was identified. Under isothermal conditions in the same temperature range the tendency to form the stoichiometric compounds increases. All compounds form, decompose and melt at temperatures between 530–780°C.

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Abstract  

The crystal structures of oxo-centered trineclear cobalt-iron chloroacetate complex [CoIIFe 2 III O(CH2ClCO2)6(H2O)3]·3H2O (1) was compared with that of previously reported trinuclear iron complex [FeIIFe 2 III O(CH2ClCO2)6(H2O)3]·3H2O (2) which has an isomorphous structure to 1. Compound 1 crystallizes in space group P21/n with Z=4 in a unit cell of a=14.826 (4) Å, b=4.536 (8) Å, c=14.000 (4) Å, =100.32 (2)0 and V=2968 (11) Å3. The structure was refined to R=0.75 and Rw=0.82. The coordination geometries of the three iron atoms are observed equivalent in 1 indicating a static disorder of the position among cobalt and iron atoms. Two distinct FeIII doublets observed in Mössbauer spectra of 1 become an indistinguishable broad doublet by dehydration of crystal water. On the other hand, no significant line-broadening is observed after the dehydration in complex 2. The results indicate that the dehydration in 2 induces a local environmental change reordering of an electronic configuration around iron atoms, whereas the remaining disordering is reflected in Mössbauer spectrum after the dehydration in 1.

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Abstract  

The thermal decomposition of FeSO46H2O was studied by mass spectroscopy coupled with DTA/TG thermal analysis under inert atmosphere. On the ground of TG measurements, the mechanism of decomposition of FeSO46H2O is: i) three dehydration steps FeSO46H2O FeSO44H2O+2H2O FeSO44H2O FeSO4H2O+3H2O FeSO4H2O FeSO4+H2O ii) two decomposition steps 6FeSO4 Fe2(SO4)3+2Fe2O3+2SO2 Fe2(SO4)3 Fe2O3+3SO2+3/2O2 The intermediate compound was identified as Fe2(SO4)3 and the final product as the hematite Fe2O3.

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Abstract  

3-Methoxy-4-methylbenzoates of Y(III) and lanthanide(III) (La-Lu) were prepared as crystalline compounds with molar ratio of metal to organic ligand of 1.0:3.0 and general formula Ln(C9H9O3)3nH2O, where n=2 for Y, La-Er and n=0 for Tm-Lu. IR spectra of the prepared complexes suggest that carboxylate groups are bidentate chelating. During heating dihydrated complexes lose crystallization water molecules in one (Y, La, Pr-Er) or two steps (Ce) and then all the anhydrous complexes decompose directly to oxides Ln2O3, CeO2, Pr6O11 and Tb4O7.

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