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1.
《Journal of Non》2007,353(18-21):1773-1778
Rare-earth phosphate glasses exhibit exotic optical and magnetic properties, which are dictated by their structure, principally the nature of the closest R  R separation. Conventional structural characterization tools have enabled the comprehensive determination of the radial distribution from the rare-earth ion out to ∼4 Å. At or beyond this distance, however, lies the crucially important minimum R  R separation. This study illustrates three types of non-conventional diffraction experiments that overcome the radial limitations of regular structural probes. The results from such investigations are compiled with all other known estimates of R  R separations in rare-earth phosphate glasses, (R2O3)x(P2O5)1−x, in the range 0.167 < x < 0.25, both from experimental and computational means. Coupled with tabulations of all known minimum R  R separations in analogous meta- and ultra-phosphate crystal structures, a collective comparison of all of these R  R separations permits a hypothesis by which all results are consistent. That hypothesis indicates that where rare-earth phosphate glasses are near meta-phosphate, their minimum R  R distance lies at ∼4.0 Å whilst a second, much broader, nearest-neighbour R  R correlations is centered between 6.0–6.4 Å. However, glasses that contain rare-earth ions at least as heavy as holmium are likely to exhibit a very different structure, with a minimum R  R separation of 5.4–5.6 Å. Structures that lie intermediate between the ultra-phosphate and meta-phosphate composition may display mixed characteristics of these two extremes. If present in high levels, aluminium contamination arising from the synthetic process may also affect the R  R distribution significantly. Overall, there is excellent agreement between experiment and theory.  相似文献   

2.
The crystallization behavior of 30Na2O–25Nb2O5–(45 ? x) SiO2–xAlO1.5 (x = 0, 2.5, and 5) (mol%) glasses was examined and the effect of Al2O3 addition on the formation of perovskite-type NaNbO3 crystals was clarified. It is found from X-ray diffraction analyses and transmission electron microscope observations that NaNbO3 nanocrystals are formed in all glasses and the size of NaNbO3 crystals decreases with the substitution of Al2O3 for SiO2. A crystallized (heat-treated at 684 °C for 5 h) glass with x = 5, which contains NaNbO3 nanocrystals with an average size of 50 nm, shows good optical transparency in the wavelength region of 500–2000 nm and a small hysteresis loop in the polarization–electric field curve. The lines containing NaNbO3 crystals were patterned on the surface of NiO-doped glass with x = 5 by irradiations (power: 1.3–1.4 W, scanning speed: 10 μm/s) of Yb:YVO4 fiber laser (wavelength: 1080 nm). The formation mechanism of NaNbO3 nanocrystals in aluminosilicate glasses was also discussed.  相似文献   

3.
Higher refractive index and higher Abbe value were obtained simultaneously at B2O3/Al2O3 = 1 in the system of 65P2O5xAl2O3–(20 ? x)B2O3–10CaO–5Li2O, although the refractive index generally shows a positive correlation with the dispersion. We investigated the molar volume and the molar refraction by Lorentz–Lorenz’s formula and also studied the coordination number of 11B and 27Al by MAS NMR. By the replacement of Al2O3 with B2O3, the molar refraction constantly decreased, but the molar volume was minimized at B2O3/Al2O3 = 1. The refractive index behavior of the glasses was mainly determined by the molar volume in the system. The coordination number of B3+ was only IV and it did not change if the composition was changed. On the other hand the coordinations IV, V and VI were observed for Al3+. The ratio of Al3+(VI) was maximized at B2O3/Al2O3 = 1. It is considered that the higher coordination of Al3+ brings the improvement of the packing and it leads to high refractive index.  相似文献   

4.
《Journal of Non》2006,352(38-39):4024-4029
High-energy ball milling was used to synthesize aluminum-based alloys containing amorphous and nanocrystalline phases to investigate the compositional effects of transition metals (TM) on the amorphization and crystallization processes of the ball-milled Al85Y7Fe5TM3 alloys (TM = Ni, Co, Cu, and Fe) were investigated. The crystallization kinetics of the ball-milled Al–Y–Fe–TM nanocomposite powders were studied using differential scanning calorimetry (DSC). The DSC results of Al83Y7Fe5Ni5 show that the crystallization temperature and the activation energy of crystallization are 668 K and 310 kJ/mol, respectively. In-situ high-temperature X-ray diffraction showed that the crystallization was a complex process involving growth of the nanocrystalline phase along with crystallization of the amorphous matrix phase.  相似文献   

5.
《Journal of Non》2005,351(43-45):3483-3489
Glasses in the system BaO/Al2O3/B2O3 with and without the addition of platinum were melted. In one sample series, the BaO-concentration was varied while the ratio [Al2O3]/[B2O3] was kept constant. In another sample series, the [BaO]/[Al2O3]-ratio (= 0.9) was kept constant and the B2O3 concentration was varied. The samples were thermally treated at 720 °C for 24 h and subsequently at 780 °C for 4 h. In most thermally treated samples, the crystalline phase BaO · Al2O3 · B2O3 occurred. At some compositions, the platinum-doped samples showed larger concentrations of the crystalline phases. The most remarkable property of the obtained glass–ceramics is their zero or negative thermal expansion coefficient. Here, notable differences were observed: samples with fine grained microstructures showed thermal expansion coefficients approximately zero up to temperatures of around 80 °C. By contrast, samples with coarser microstructures and large spheroidal crystals exhibit negative expansion coefficients up to temperatures of around 280–375 °C. The thermal expansions of these samples were close to those of the mean thermal expansion of the unit cell of the BaO · Al2O3 · B2O3 phase. The thermal expansion of the fine grained samples was approximately equal to that of the crystallographic a-axis of the BaO · Al2O3 · B2O3 phase.  相似文献   

6.
《Journal of Non》2005,351(40-42):3179-3190
Previous diffraction studies of the structures of rare-earth phosphate glasses (R2O3)x(P2O5)1−x are extended to glasses with smaller R3+ ions with R = Sm, Gd, Dy, Er, Yb, Y for x = ∼0.25 and with R = Nd, Sm, Gd for x = ∼0.15. Parameters for the P–O, R–O and O–O first-neighbor peaks were obtained by Gaussian fitting. P–P and R–P distances were estimated from the positions of peak maxima. Effects of residual silica or alumina contents present as a result of glass processing were taken into account for selected samples. The P–O coordination number, NPO, and the P–O, O–O, P–P distances are consistent with the presence of phosphate tetrahedra and are insensitive to the R species and the R2O3 content. Rare-earth coordination numbers, NRO, decrease from ∼8 to ∼6.5 when x is increased from ∼0.15 to ∼0.25. NOO and NPP decrease with increasing R2O3 content indicating the network disintegration. The numbers NRO of the metaphosphate glasses (x = ∼0.25) decreases from ∼7 to ∼6 when R is changed from La to Yb. This change is also indicated by the behavior of the R–O distances and by constant number densities of atoms. The decrease in NRO with increasing R2O3 content is due to the reduction in the number of terminal O (OT) available for coordination of the R3+ ions (six at metaphosphate composition). Especially for smaller R3+ ions sharing OT between two R sites is not favored. The decrease by ∼0.04 nm of the prominent R–R first-neighbor distance with a change of R from La to Yb at the metaphosphate composition is indicated by a shift to higher magnitude of scattering vector of the shoulder occurring in front of the first main diffraction peak.  相似文献   

7.
Pr:Gd3(Ga,Al)5O12 single crystals were grown by the micro-pulling down (μ-PD) method. All grown crystals were greenish and transparent with 3.0 mm in diameter, 15–30 mm in length. Neither visible inclusions nor cracks were observed. Luminescence and scintillation properties were measured. The substitution at the Al3+ sites by Ga3+ in garnet structure has been studied. In these crystals, Pr3+ 5d–4f emission is observed with 340 nm wavelength. Pr1%:Gd3Ga3Al2O12 shows highest emission intensity. The light yield of Pr:Gd3Ga3Al2O12 sample with 3 mmφ×1 mm size was around 4500 photon/MeV. Scintillation decay time was 7.9 ns (0.5%), 46 ns (0.7%) and 214 ns (98.8%).  相似文献   

8.
《Journal of Non》2007,353(30-31):2837-2844
The crystallization of amorphous diphasic Al2O3–SiO2 precursors doped with nickel has been studied by differential scanning calorimetry (DSC), XRD diffraction (XRD) and scanning and transmission electron microscopy (SEM, TEM) equipped with energy dispersive X-ray spectroscopy (EDS). Diphasic gels with constant atomic ratio (Al + Ni)/Si = 3:1, where 0, 1, 2 and 3 at.% of aluminum were replaced by nickel, have been prepared by hydrolyzing of TEOS in aqueous solution of aluminum nitrate. Crystallization of Ni-containing γ-Al2O3 preceded the crystallization of Al–Si spinel. Activation energy of 603 ± 16 kJ mol−1 for crystallization of Ni-containing γ-Al2O3 was obtained in non-isothermal conditions. Ni-incorporated γ-Al2O3 transforms gradually with the temperature increase into Ni aluminate spinel, while Al–Si spinel reacts with amorphous silica forming mullite at about 1200 °C. Rietveld structure refinement of phases present in the samples annealed at 1600 °C and SEM-EDS and TEM-EDS analyses of related phases have shown that nickel predominantly crystallizes as NiAl2O4, but small amount of nickel is incorporated in mullite structure, as well as, dissolved in the glassy phase of the system.  相似文献   

9.
《Journal of Non》2006,352(30-31):3290-3294
Bulk metallic glasses (BMGs), especially Zr-based BMGs, have attracted lot of attention of materials scientists because of their very attractive physical, thermal and mechanical properties and a few unique applications. In the present study, Zr65Cu18Ni9Al8 alloy was designed according to the criterion of conduction electron/atom (e/a ratio) ∼1.395 and average atomic size of alloy (Ra) ∼0.1498 nm. Addition of 2 at.% Er was carried out in the base alloy to investigate its effect on thermal and mechanical properties. Characterization of alloys was performed using the techniques of XRD, DSC, and SEM/EDS. Mechanical properties like Vicker’s microhardness, nanohardness, elastic modulus, density and fracture strength were measured. Average shear angle was found to be ∼35 ± 1° for base alloy and about 31 ± 1° for alloy containing 2 at.% Er. Wide supercooled liquid regions of 129 K and 119 K were found for the base alloy and the alloy containing 2 at.% Er.  相似文献   

10.
《Journal of Non》2006,352(21-22):2254-2258
The aluminum coordination state in bismuth doped silica glass, which has new broad infrared emission at 1.3 μm regions, was investigated by using 27Al NMR, and it is demonstrated that 6-fold coordinated aluminum ions with corundum structure are dominant in bismuth doped silica glass until Bi2O3 concentrations of 1.0 mol% with Al2O3. The aluminum ion efficiently affects the creation of a Bi luminescent center at an intensity of Bi2O3 (1.0 mol%)–Al2O3 (2.3 mol%)–SiO2 (96.7 mol%); the sample is three orders of magnitude larger than the Bi2O3 (1.0 mol%)–SiO2 (99.0 mol%) sample. Aluminum ions with corundum structure in silica glass have a very important role for the configuration of peculiar Bi luminescent centers.  相似文献   

11.
《Journal of Non》2006,352(21-22):2129-2136
Conductivity vs. temperature (σT) at zero and 8-T field, magneto-resistance (Δρ/ρ), magnetization vs. temperature (M–T) and magnetization vs. field (M–H) of Al70.5Pd22Mn7.5 quasicrystal have been studied in the temperature range of 1.4 K to 300 K. The σT variations in both the field conditions show a σT minima. In addition to this the σT variation at 8 T shows a maxima also at ∼6 K. Comparative analysis shows that the observed σT minima arises due to competing inelastic scattering events in the presence of weak-localization effect. These events are e–ph scattering in the dirty-metallic limit (τi  T−2), and the Kondo-type spin-flip scattering (τsf). The maxima observed for the σT variation at 8 T, has been attributed to suppression of the spin-flip scattering in the presence of field. The magneto-resistance is found to be large and positive. It was properly accounted only when the Stoner-enhancement, found in the case of spin fluctuating systems, was taken into consideration.  相似文献   

12.
《Journal of Non》2006,352(26-27):2841-2845
Annealing effect on photoluminescence intensity of Er doped Al2O3–SiO2 prepared from Er doped boehmite (AlOOH) and GPS (3-glycidoxypropyltrimethoxysilane) hybrid was investigated. The emission intensities peaked at 1.54 μm, which correspond to the 4I13/2  4I15/2 transition of the Er3+ ion, are greatly increased by about 8 times between 900 and 1000 °C, than that expected from TGA associated with the elimination of hydroxyl groups which is responsible for the fluorescence quenching. The residual hydroxyl groups for Er doped Al2O3–SiO2 after annealing at high temperature was further analyzed by FT-IR. Finally, fluorescence intensities were compared with the variation of BET surface areas against the annealing temperature. It was found that photoluminescence intensity below 1000 °C was more dependent on surface hydroxyl groups re-adsorbed by a high specific surface area rather than internal hydroxyl groups remained in gel film.  相似文献   

13.
《Journal of Non》2006,352(38-39):4082-4087
Liquids with the base compositions (16  x/2)Na2O · xNaF · 10CaO · 74SiO2 (x = 0, 1, 3, and 4) and (10  x/2) · Na2O · xNaF · 10CaO · yAl2O3 · (80  y)SiO2 (x = 0, 1, 3, 5 and y = 5 and 15) doped with 0.25 mol% Fe2O3 were studied by means of square-wave voltammetry in the temperature range from 1000 to 1500 °C. With increasing temperature, the redox equilibria were shifted to the reduced state. Also while increasing the alumina concentration, the Fe2+/Fe3+-redox equilibrium is shifted to the reduced state. In the soda-lime–silica melt the addition of fluoride shifts the equilibrium to the oxidized state, while in the aluminosilicate melts with 15 mol% Al2O3, the equilibrium is shifted to the reduced state. In the aluminosilicate melts with 5 mol% Al2O3, the equilibrium was not affected by the fluoride concentration. This is explained by the structure of the respective glass compositions.  相似文献   

14.
《Journal of Non》2007,353(24-25):2383-2391
Thermal stability and crystallization kinetics of the glass 21% MgO, 21.36% Al2O3, 53.32% SiO2 and 4.11% TiO2 (mol%) has been studied using differential thermal analysis (DTA), dilatometry and X-ray diffraction (XRD). Glass in both bulk and frit forms were produced by melting in platinum crucible at 1600 °C for 1–2 h. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 340 kJ mol−1 and 498 kJ mol−1 for first and second crystallization exotherms, respectively. Crystallization of bulk glass was carried out at various temperatures and for different time durations in the range of 850–1000 °C. The influence of the addition of TiO2 on the crystallization sequence of the glass was experimentally determined and discussed.  相似文献   

15.
Interactions in the La1?xNdxNi3.5Al1.52 (x = 0.1 and 0.2) system was studied from room temperature up to 950 °C at the initial hydrogen pressure of 5 MPa through differential thermal (DTA) and X-ray phase analyses. Heating two-phase alloys (x = 0.1 and 0.2) in hydrogen results in their disproportionation (at 530 and 560 °С, respectively) and the formation of NiAl and unidentified amorphous products. The single-phase La0.9Nd0.1Ni3.5Al1.5 alloy decomposes in hydrogen at 900 °С into a hydride of rare-earth metals and an Ni3Al intermetallic; traces of NiAl and hydride of a phase of the CaCu5-type structure have also been observed. Heating the disproportionated samples in vacuum to 520–550 °С leads to their recombination into a homogenized phase with a CaCu5-type structure. In other words, the increase of neodymium content shifts the reaction equilibrium of La1?xNdxNi3.5Al1.5 alloys with hydrogen towards recombination.  相似文献   

16.
《Journal of Non》2006,352(28-29):3015-3026
The crystallization behavior of Zr65.0Al7.5Cu27.5 (at.%) metallic glass with 753 and 1053 K annealing treatment and its effect on oxidation resistance around the supercooled liquid region at 623 and 663 K was studied. The crystalline phase of bct-Zr2Cu precipitates for the specimen annealed at 753 K was observed, while duplex structures of bct-Zr2Cu and Zr2Al formed in the specimen annealed at 1053 K. The oxidation resistance of the specimen depended on the amount of crystalline precipitates. Regardless of the exposure temperature, the annealed specimens showed higher oxidation resistance than the melt-spun one, especially for the specimen annealed at 1053 K. The formation of numerous crystalline phases of bct-Zr2Cu and Zr2Al from the matrix was responsible for improving the oxidation resistance due to their higher oxidation resistance and promotion of the development of Al2O3 and oxides of copper. The oxide constituents of the amorphous alloy after long exposure depended on the temperature. The oxide was composed of a large amount of CuO, some tetragonal and monoclinic-ZrO2 and a minor amount of Cu2O as well as a slight amount of Al2O3 for the melt-spun specimen during exposure at 623 K. Under the 663 K exposure, however, the oxide state of Cu3+ in the scale was also detected.  相似文献   

17.
Magnetic and structural properties of nanocrystalline LaCoO3 with particle size ranging from 25 to 38 nm, prepared by the citrate method, were investigated. All nanoparticles exhibit ferromagnetism below TC  85 K. It was found that the unit-cell volume increases monotonically with decreasing particle size and ferromagnetic (FM) moment increases simultaneously with lattice expansion, whereas TC remains nearly unchanged. It appears that both magnetic and structural properties of LaCoO3 nanoparticles are size-dependent due to the surface effect. On the other hand, an applied pressure suppresses strongly the FM phase leading to its disappearance at ~11 kbar. Remarkably, the TC does not change visibly under pressure. Our data reveal that the ferromagnetism in LaCoO3 nanoparticles, likely related to the intermediate-spin (IS) Co3+ state, is simply controlled by the unit-cell volume. Within this scenario, the FM coupled IS states appear/disappear with expanding/compressing the lattice and/or Co–O bonds.  相似文献   

18.
《Journal of Non》2007,353(32-40):3174-3176
The dynamic structure factor S(Q, ω) of the melt of an icosahedral quasicrystal, Al72Pd20Mn8, was measured at 1223 K near the melting point, Tm = 1140 K, for momentum transfers, Q, from 1.5 nm−1 to 5.55 nm−1 by means of inelastic X-ray scattering technique using synchrotron radiation at SPring-8. The composition of Al72Pd20Mn8 is a special one in AlPdMn ternary alloys, since icosahedral AlPdMn quasicrystal is formed directly from the melt. The acoustic mode was observed in the low Q region, and a sound velocity was estimated to be 4170 ± 100 m/s.  相似文献   

19.
《Journal of Non》2007,353(22-23):2295-2300
(1  x)Li2O–xNa2O–Al2O3–4SiO2 glasses were studied for the progressive percentage substitution of Na2O for Li2O at the constant mole of Al2O3 and SiO2. The crystallization temperature at the exothermic peak increased from 898 to 939 °C when the Na2O content increases from 0 to 0.6 mol. The coefficient of thermal expansion and density of these as-quenched glasses increase from 6.54 × 10−6 °C−1 to 10.1 × 10−6 °C−1 and 2.378 g cm−3 to 2.533 g cm−3 when the Na2O content increases from 0 to 0.4 mol, respectively. The electrical resistivity has a maximum value at Na2O · (Li2O + Na2O)−1 = 0.4. The activation energy of crystallization decreases from 444 to 284 kJ mol−1 when the Na2O content increased from 0 to 0.4 mol. Moreover, the activation energy increases from 284 kJ mol−1 to 446 kJ mol−1 when the Na2O content increased from 0.4 to 0.6 mol. The FT-IR spectra show that the symmetric stretching mode of the SiO4 tetrahedra (1035–1054 cm−1) and AlO4 octahedra (713–763 cm−1) exhibiting that the network structure is built by SiO4 tetrahedra and AlO4.  相似文献   

20.
Al2O3–ZrO2 (Y2O3) nanopowders containing 5, 10 and 15 wt% ZrO2 were synthesized by aqueous sol–gel method using aluminum sec-butoxide and zirconium butoxide as precursors. BET analysis shows that, increasing the zirconia content results in a decrease in surface area, 152, 125 and 121 m2/g, and an increase in pore size, 5.63, 9.79 and 11.05 nm for 5, 10 and 15 wt% ZrO2, respectively. Furthermore, a shift toward higher temperatures is observed for transition of transitional aluminas to stable α-alumina phase through increasing the zirconia content. SEM micrograph of calcined nanopowders revealed nanosize spherical particles in the range of 15–75 nm.  相似文献   

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