Characteristics and local structure of hafnia-silicate-zirconate ceramic nanomixtures.

Zirconate systems having the composition 3HfO2·15SiO2·xY2O3·(82 - x)ZrO2, where x = 2, 7 and 12 mol% Y2O3, were synthesized by a sol-gel method. The analysis of X-ray diffraction data showed the presence of the t-ZrO2, m-ZrO2, m-HfO2, Y2SiO5 and Y2Si2O7 crystalline phases in a ceramic nanomixture. Spectroscopic data show that the increase of the Y2O3 content of samples determines the increase of the t-ZrO2, m-HfO2 and silicate crystalline phases. Gap energy values decrease almost linearly with increasing Y2O3 content of samples. A detailed study of XANES data does not show a significant difference with increasing Y2O3 content of the samples suggesting an appreciable stability of the hafnium ions +4 oxidation state and their microvicinity. EXAFS results show that the local structure around the Hf cation is similar to that from the monoclinic crystalline HfO2 where the Hf-O coordination number tends to 7. The bond lengths of Hf-O shells show small deviations from ∼2.12 Šand the Hf-metal paths become more structured by increasing the Y2O3 content of the samples.

Advanced zirconia ceramics stabilized with yttria and magnesia: Structure and Vickers microhardness.

The samples 8YSZ containing 70, 75, 80 mol% ZrO2 and other oxides were prepared by. a high temperature solid state reaction process. Using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and spectroscopic methods were evaluated the effect of oxide addition on the properties of yttria-doped zirconia. X-ray data shows the formation of zirconia (Y-doped), YSZ and/or tetragonal zirconia crystalline phases in all samples. IR data indicate the stretching vibrations of the Si-O bonds from tetrahedral [SiO4] units and the stretching vibrations of the Fe-O bonds from [FeO6] structural units, respectively. EPR results indicate the destroying of the local ordering of the Fe+3 ion vicinities situated in the rhombic distorted octahedral geometries by the increasing of ZrO2 content in the host ceramic. The highest values of Vickers hardness were recorded for the ceramics containing 70 and 75 mol% ZrO2. This superior performance can be explained considering the presence of a smaller amount of monoclinic ZrO2 crystalline phase in the ceramic structure. The analysis of the X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) data shows the presence of Fe+2 and Fe+3 ions. In the ceramics with higher ZrO2 contents the iron atoms adopt a randomly structure due to the highly distorted [FeO6] structural units and non-equivalent Fe-O distances in the first coordination shell.

Toward modeling phosphate tellurate glasses: the devitrification and addition of gadolinium ions behavior.

Glasses in the xGd2O3 x (100 - x)[7TeO2 x 3P2O5] system with 0 < or = x < or = 20 mol % have been prepared using the melt quenching method. The influence of gadolinium ions on structural behavior of the phosphate tellurate glass has been investigated using infrared spectroscopy and density functional theory (DFT) calculations. The addition of gadolinium ions into the host glass matrix leads to an increase of the glass network polymerization due to the replacement of P-O-P bonds by the more resistant P-O-Te bonds having as result the improvement of the chemical durability of the glass. The structural evolution of the studied glasses with the gradual increase of the gadolinium oxide content up to 20 mol % could be explained by considering that the excess of oxygen may be accommodated by the conversion of some orthophosphate structural units into metaphosphate or/and pyrophosphate units. X-ray diffraction and IR spectra revealed that heat treatment of the samples also causes an increase of the glass network polymerization for heat treatment times, t, up to 36 h, while for 36 h > t > or = 48 h showed a drastic structural modification which lead to the apparition of the Te4P2O13 crystalline phase. DFT calculations show that tellurium atoms occupy two different sites in the proposed model. In the first case the tellurium atom is coordinated with four oxygen atoms giving a trigonal bipyramide arrangement, while in the second case the tellurium atom is coordinated with three O atoms. The calculated IR absorption spectrum of the proposed model for phosphotellurite glasses is in good agreement with the experimental absorption data.

An optical spectroscopic study of borate glasses containing uranium ions.

An optical spectroscopic investigation has been performed on some borate glasses containing 1mol % UF(4), UO(2) and UO(3). The optical data confirm the presence of uranium ions in valence stages of +6, +5 and +4. The influence of the composition of the glass matrix and of the gamma-irradiation on the redox equilibrium of uranium ions was discussed.

The network modifier and former role of the bismuth ions in the bismuth-lead-germanate glasses.

The present work is focused on the enhancement of network former environment in lead-germanate glasses by bismuth ions doping. A series of bismuth-lead-germanate glasses with the xBi2O3·(100-x)[7GeO2·3PbO] composition glass where 0≤x≤30 mol% Bi2O3 were synthesized by melt-quenching method. The FTIR, UV-VIS spectroscopy and cyclic voltammetry were conducted on these samples to evaluate the doping effect of structure of the host matrix network. Our results indicate that direct incorporation of Bi2O3 into the lead-germanate network modifies the lead-germanate network and the internal structure of glass network is rearranged. The structural flexibility of the lead-germanate network is possible due to its incapacity to accommodate with the excess of oxygen atoms and the creation of bridging oxygen ions. Optical gap energy and refractive index were obtained as a function of Bi2O3 content. Gap energy values decrease as Bi2O3 content increased from 0 to 10 mol%. Further increase of Bi2O3 concentration beyond 10 mol% increased the gap energy values. These behaviors of the glass system can be explained by two mechanisms: (i) for x≤10 mol% Bi2O3--increase of degree of disorder of the host matrix because Bi2O3 is network modifier and (ii) for x>10 mol%--Bi2O3 acts as a network former. Cyclic voltammetry measurements using the glass system with 10Bi2O3·90[7GeO2·3PbO] composition as working electrode show the mobility of the lead ions, in agreement with UV-VIS data.

Effect of aluminum oxide codoping on copper-lead-germanate glasses.

Glasses from xCuO⋅(100-x)[7GeO(2)⋅3PbO(2)⋅0.05Al(2)O(3)] system where x=0, 1, 5, 10, 20 and 30 mol% CuO were studied by FT-IR, UV-VIS and ESR spectroscopy in order to obtain information about the structural correlations and the relationship between structure and optical properties in these materials. The analyses of these IR spectra reveal that the accommodation of the network with the excess of oxygen ions is possible by the depolymerization of the germanate network in shorter chains, especially ortho- and/or pyrogermanate structural units. UV-VIS absorption spectrum of the sample with x=1% CuO begins with a rising absorption band situated at about 250 nm. This band can be assigned to the GeOGe wrong bonds such as the Ge(+2) centers. Optical study is performed to calculate the refractive index and optical band gap using UV-VIS spectra in the wavelength range 250-1,000 nm. The increase in optical band is explained on the basis of the average bond energy of the system and the number of non-bridging oxygen ions. ESR spectra of CuO substituted samples are characterized by broad peaks probably because of the formation of Cu(+2)-Ge(+2) exchange pairs which are weakly coupled though the oxygen atom.

Dinuclear manganese centers in the manganese-lead-tellurate glasses.

FTIR, UV-VIS and EPR spectra of manganese doped lead-tellurate glasses with composition xMnO·(100-x)[4TeO2·PbO2] where x=0, 1, 5, 10, 20, 30, 40mol% have been studied. The FTIR spectra show the formation of the Mn-O-Pb and Mn-O-Te bridging bonds by increasing of MnO concentration. The UV-VIS spectra show the Mn(+3) species exhibit pronounced absorption, which masks the Mn(+2) spin-forbidden absorption bands when Mn(+2) ions are in high concentrations in these glasses. The EPR spectra exhibit resonance signals characteristic of Mn(+2) ions. The resonance signal located at g≈2 is due to Mn(+2) ions in an environment close to octahedral symmetry, whereas the resonance at g≈4.3 and 3.3 are attributed to the rhombic surroundings of the Mn(+2) ions. The increase in the MnO content gives rise to absorption at g≈2.4 and the paramagnetic ions are involved in dinuclear manganese centers.

Structural study of ternary iron-lead-germanate glass ceramics.

Glass ceramics with the composition xFe(2)O(3)·(100-x)[7GeO(2)·3PbO(2)] where 0≤x≤60 mol% were obtained and studied using XRD, FTIR and UV-vis spectroscopy investigations. Heat treatment of glass samples at 400°C for 8 h led to the formation of α, γ-PbGe(4)O(9), Pb(3)Fe(2)Ge(4)O(14) and PbO(1.44) crystalline phases. The content of these crystalline phases depends of Fe(2)O(3) concentration. FTIR spectroscopy data suggest that the lead ions have a pronounced affinity towards [GeO(5)] structural units containing non-bridging oxygens and [FeO(4)] anions producing formation of the Pb(3)Fe(2)Ge(4)O(14) crystalline phase. The introduction of low concentrations of Fe(2)O(3) into the host matrix results in the formation of new absorption UV bands between 320 and 450 nm. These bands arise from to the d-d transitions of the Fe(+3) ions. The light absorption in the range from 250 to 600 nm increases with increasing iron oxide content in matrix network, accompanied with the changes on color from white to brown yellow and darker brown.

The heterogeneity of segmental dynamics of filled EPDM by (1)H transverse relaxation NMR.

Residual second moment of dipolar interactions M(2) and correlation time segmental dynamics distributions were measured by Hahn-echo decays in combination with inverse Laplace transform for a series of unfilled and filled EPDM samples as functions of carbon-black N683 filler content. The fillers-polymer chain interactions which dramatically restrict the mobility of bound rubber modify the dynamics of mobile chains. These changes depend on the filler content and can be evaluated from distributions of M(2). A dipolar filter was applied to eliminate the contribution of bound rubber. In the first approach the Hahn-echo decays were fitted with a theoretical relationship to obtain the average values of the (1)H residual second moment and correlation time <τ(c)>. For the mobile EPDM segments the power-law distribution of correlation function was compared to the exponential correlation function and found inadequate in the long-time regime. In the second approach a log-Gauss distribution for the correlation time was assumed. Furthermore, using an averaged value of the correlation time, the distributions of the residual second moment were determined using an inverse Laplace transform for the entire series of measured samples. The unfilled EPDM sample shows a bimodal distribution of residual second moments, which can be associated to the mobile polymer sub-chains (M(2) ≅ 6.1 rad (2) s(-2)) and the second one associated to the dangling chains M(2) ≅ 5.4 rad(2) s(-2)). By restraining the mobility of bound rubber, the carbon-black fillers induce diversity in the segmental dynamics like the apparition of a distinct mobile component and changes in the distribution of mobile and free-end polymer segments.

Infrared spectroscopic and DFT investigations of the vanadate-tellurate glasses structures.

Vanadate-tellurate vitreous systems with composition (1-x)TeO(2).xV(2)O(5) where x=0.3 and 0.4 have been prepared by the conventional melt-quench method. The structural aspects have been investigated using FTIR spectroscopy and the density functional theory (DFT) calculations. The present study provides the interesting information concerning devitrification behavior of the vanadate-tellurate vitreous system which occur Te(2)V(2)O(9) crystalline phase. The structure of the heat-treated glasses was found to consist mainly of rings containing [TeO(3)], [TeO(4)], [VO(4)] and some [VO(5)] structural units.

Structural properties of molybdenum-lead-borate glasses.

Glasses and glass ceramics in the system xMoO3·(100 - x)[3B2O3·PbO] with 0 ≤ x ≤ 30 mol% have been prepared from melt quenching method and characterized by means of X-ray diffraction, FTIR, UV-VIS and EPR spectroscopy. We have examined and analyzed the effects of systematic molybdenum ions intercalation on lead-borate glasses and glass ceramics with interesting results. The observations present in these mechanisms show the lead ions bonded ionic have a strong affinity towards [BO3] units containing non-bridging oxygens and [MoO4]²â» molybdate units. The pronounced affinity towards molybdate anions yields the formation of the PbMoO4 crystalline phase. Then, the excess of oxygen can be supported into the glass network by the formation of [MoO6] and [Mo2O7] structural units. Pb²(+) ions with 6s² configuration show strong absorption in the ultraviolet due to parity allowed s²-sp transition and yield an absorption band centered at about 310 nm. The changes in the features of the absorption bands centered at about 310 nm can be explained as a consequence of the appearance of additional absorption shoulder due to photoinduced color centers in the glass such as the formation of borate-molybdate and lead-molybdate paramagnetic defect centers in the glasses. The concentration of molybdenum ions influences the shape and width of the EPR signals located at g ∼ 1.86, 1.91 and 5.19. The microenvironment of molybdenum ions in glasses is expected to have mainly sixfold coordination. However, there is a possibility of reduction of a part of molybdenum ions from the Mo6(+) to the Mo5(+) and Mo4(+) to the Mo³(+) states.

Dual role of the six-coordinated molybdenum and lead ions in novel of photochromic properties of the molybdenum-lead-borate glasses.

Transparent glasses were prepared by conventional melting-quenching method in the xMoO(3).(100-x)[3B(2)O(3).PbO] system where 0

Distributions of transverse relaxation times for soft-solids measured in strongly inhomogeneous magnetic fields.

The single-sided NMR-MOUSE sensor that operates in highly inhomogeneous magnetic fields is used to record a CPMG (1)H transverse relaxation decay by CPMG echo trains for a series of cross-linked natural rubber samples. Effective transverse relaxation rates 1/T(2,short) and 1/T(2,long) were determined by a bi-exponential fit. A linear dependence of transverse relaxation rates on cross-link density is observed for medium to large values of cross-link density. As an alternative to multi-exponential fits the possibility to analyze the dynamics of soft polymer network in terms of multi-exponential decays via the inverse Laplace transformation was studied. The transient regime and the effect of the T(1)/T(2) ratio in inhomogeneous static and radiofrequency magnetic fields on the CPMG decays were studied numerically using a dedicated C++ program to simulate the temporal and spatial dependence of the CPMG response. A correction factor T(2)/T(2,eff) is derived as a function of the T(1)/T(2) ratio from numerical simulations and compared with earlier results from two different well logging devices. High-resolution T(1)-T(2) correlations maps are obtained by two-dimensional Laplace inversion of CPMG detected saturation recovery curves. The T(1)-T(2) experimental correlations maps were corrected for the T(1)/T(2) effect using the derived T(2)/T(2,eff) correction factor.