Preparation and optical properties of nanoscale MgAl2O4 powders doped with Co ions

Co²⁺-doped MgAl₂O₄ nanocrystalline powders were prepared by co-precipitation method. The gels and/or calcined samples were characterized by means of thermogravimetry and differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared (FTIR) spectrum and near-infrared absorption spectrum. MgAl₂O₄ nanocrystals were produced by calcining the gel above 800 °C, with the crystallite size of 10-30 nm in the temperature range of 800-1100 °C. The influence of pH value of precipitant solution on the dispersing of powders was studied and the result showed that Co:MgAl₂O₄ nanocrystalline powders exhibited good dispersion when pH = 11. The absorption spectrum of Co²⁺-doped MgAl₂O₄ exhibited a broad absorption band in the wavelength range of 1200-1600 nm, which indicated that Co²⁺ ions substituted for the tetrahedrally coordinated Mg²⁺ ions in the MgAl₂O₄ lattice.     

Crystal growth and characterization of K2B4O11H8

Single crystals of K₂B₄O₁₁H₈, a non-centrosymmetric borate material, have been prepared by slow evaporation of water solution at room temperature. The crystals were colorless and transparent with smooth faces. The single-crystal X-ray diffraction analysis showed that K₂B₄O₁₁H₈ crystallized in the orthorhombic space group P2₁2₁2₁ with a=6.8556(6) Å, b=11.7787(12) Å, c=12.8949(14) Å, Z=4, R₁=0.0188, and wR₂=0.0464. The powder X-ray diffractometry (PXRD), infrared spectrum, transmittance spectrum, TG–DTA curves and second harmonic generation properties of title compound have been determined.     

Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties - Part II: Physical properties

The effect of the variation in phosphate (P₂O₅) content on the properties of two series of bioactive glasses in the quaternary system SiO₂-Na₂O-CaO-P₂O₅ was studied. The first series (I) was a simple substitution of P₂O₅ for SiO₂ keeping the Na₂O:CaO ratio fixed (1:0.87). The second series (II) was designed to ensure charge neutrality in the orthophosphate (), therefore as P₂O₅ was added the Na₂O and CaO content was varied to provide sufficient Na⁺ and Ca²⁺ cations to charge balance the orthophosphate present. Network connectivity’s of the glasses were calculated, and densities and thermal expansion coefficients predicted using the Appen and Doweidar models, respectively. Theoretical densities were measured using the Archimedes principle. Characteristic temperatures, namely the glass transition temperature, T_g, and crystallization temperatures, T_x, were obtained using differential analysis (DTA). Two crystallization exotherms were observed for both glass series (T_xi and T_xii). Both T_g and T_x decreased with P₂O₅ addition for both series. The working range of the glasses, T_x-T_g was shown to increase to a maximum at around 4 mol% P₂O₅ then decrease at higher P₂O₅ contents for both series. Thermal expansion coefficients were measured using dilatometry increasing with P₂O₅ addition and showed good agreement with the Appen values. Dilatometric softening points, T_s, were also measured, which increased with P₂O₅ addition. X-ray diffraction (XRD) was performed on the glasses to confirm their amorphous nature. The glass containing 9.25 mol% P₂O₅ from series I exhibited well-defined peaks on the XRD trace, indicating the presence of a crystalline phase.     

Fabrication of (1 1 1)-oriented Si layers on SiO2 substrates by an aluminum-induced crystallization method and subsequent growth of semiconducting BaSi2 layers for photovoltaic application

We have prepared (1 1 1)-oriented Si layers on SiO₂ (fused silica) substrates from amorphous-Si(a-Si)/Al or Al/a-Si stacked layers using an aluminum-induced crystallization (AIC) method. The X-ray diffraction (XRD) intensity from the (1 1 1) planes of Si was found to depend significantly on growth conditions such as the thicknesses of Si and Al, deposition order (a-Si/Al or Al/a-Si on SiO₂), deposition technique (sputtering or vacuum evaporation) and exposure time of the Al layer to air before the deposition of Si. The crystal orientation of the Si layers was confirmed by θ−2θ, 2θ XRD and electron backscatter diffraction (EBSD). The photoresponse properties of semiconducting BaSi₂ films formed on the (1 1 1)-oriented Si layers by the AIC method were measured at room temperature. Photocurrents were clearly observed for photon energies greater than 1.25 eV. The external quantum efficiencies of the BaSi₂ were also evaluated.     

Three-dimensional structure of fast ion conducting 0.5Li2S + 0.5[(1 − x)GeS2 + xGeO2] glasses from high-energy X-ray diffraction and reverse Monte Carlo simulations

A high-energy X-ray diffraction study has been carried out on a series of 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses with x = 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8. Structure factors were measured to wave vectors as high as 30 Å⁻¹ resulting in atomic pair distribution functions with high real space resolution. The three dimensional atomic-scale structure of the glasses was modeled by reverse Monte Carlo simulations based on the diffraction data. Results from the simulations show that at the atomic-scale 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses may be viewed as an assembly of independent chains of (Li^+-S)₂GeS_2/2 and (Li^+-O)₂GeO_2/2 tetrahedra as repeat units, where the Li ions occupy the open space between the chains. The new structure data may help understand the reasons for the sharp maximum in the Li⁺ ion conductivity at x ∼ 0.2.     

Influence of pulsed laser deposition rate on the microstructure and thermoelectric properties of Ca3Co4O9 thin films

The effects of deposition rate on the microstructure and thermoelectric (TE) properties of Ca₃Co₄O₉ thin films fabricated by pulsed laser deposition (PLD) technique were investigated. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) revealed that a fast deposition rate resulted in not only low crystallinity but also the existence of the Ca_xCoO₂ secondary phase. Formation of Ca_xCoO₂ was inevitable during the thin film growth, and this was discussed from both structural and compositional point of view. With longer deposition interval or with sufficient oxygen at a lower deposition rate, the Ca_xCoO₂ phase was able to transit into the desired Ca₃Co₄O₉ phase during the coalescence process. The quality of the thin films was further analyzed by electrical properties measurements. The Ca₃Co₄O₉ thin film fabricated at a slower deposition rate was found to exhibit a low electrical resistivity of 9.4 mΩ cm and high Seebeck coefficient of 240 μV/K at about 700 °C, indicating a good quality film.     

Development and characterizations of BaO-CaO-Al2O3-SiO2 glass-ceramic sealants for intermediate temperature solid oxide fuel cell application

Several compositions based on BaO-CaO-Al₂O₃-SiO₂ (BCAS) glass system have been studied in this investigation to see their applicability as sealant for solid oxide fuel cell (SOFC). The glasses as well as the corresponding glass-ceramics have been systematically characterized by differential thermal analysis, dilatometry, X-ray diffractometry, electron microscopy and impedance analysis to examine their suitability as sealant. While the glass transition temperature (T_g) determined from DTA are within 600-665 °C, the coefficient of thermal expansion (CTE) can be tailored between 9.5 and 13.0 × 10⁻⁶ K⁻¹. These glasses are found to be well adhered with metallic interconnects, such as commercial ferritic steel (Crofer22APU), at an optimum sealing temperature of 850 °C. The shrinkage behavior of the developed glasses in their pellet form has also been investigated. The resistivities of the glass-ceramics, as obtained from impedance analysis, are found to be within 10⁴-10⁶ Ω cm at 800 °C. Under sandwiched condition between two metals, some of the developed compositions are found to maintain this high resistivity even after 100 h of operation. One of the glass compositions has shown a low leak-rate of the order of ∼10⁻⁷ Pa m² s⁻¹.     

Vitrification and crystallization in the system of K2O-B2O3-TiO2

The vitrification and crystallization behavior of melts produced at 1400 °C in the ternary system of K₂O-B₂O₃-TiO₂ is investigated. It is shown that there are two fields of compositions (indicated in mol%) which allow obtaining the glass-ceramic materials with continuous glassy matrix after the cooling of molten compositions. In the first field [TiO₂] = 25-57, [K₂O] = 30-50 and [B₂O₃] = 0-25, the glass-ceramics consisted of the potassium-titanium-borate glassy phase and different crystalline potassium titanates (K₄Ti₃O₈, K₂Ti₂O₅, K₂Ti₄O₉, K₂Ti₆O₁₃). The ratio of TiO₂:K₂O in the obtained titanates increases with [TiO₂] and [B₂O₃]. In the second field, [TiO₂] = 7-37, [K₂O] = 0-25 and [B₂O₃] = 52-93, the obtained glass-ceramics consisted of a similar vitreous phase, as mentioned above, and TiO₂ crystals. During the cooling of the melts, short whiskers-like crystals of anatase formed in the compositions with relatively low [TiO₂] and relatively high [K₂O], whereas long fiber-shaped crystals of rutile appeared with the compositions characterized with relatively high [TiO₂] and relatively low [K₂O]. The possible application of the obtained glass-ceramic materials as a source of fibrous TiO₂, for composite reinforcement, and as solid lubricants is discussed.     

Thermal and some physical properties of glasses in the La2O3−CaO−B2O3 ternary system

Glasses in the La₂O₃−CaO−B₂O₃ ternary system were studied. The glass forming range as determined by the appearance of the annealed cast was found to match previously published findings. Clear glasses were formed in the composition range of 5.7−19.1 mol% La₂O₃ with constant B₂O₃ content of 71.4 mol%, and in glasses of constant La₂O₃:CaO ratio of 1:4 with B₂O₃ content in the range of 71.4-55.0 mol%. The non-linear optical crystalline phase La₂Ca₂B₁₀O₁₉ was crystallized from the clear glasses after heat treatments, as determined by powder XRD. Two types of the LaBO₃ crystalline phases were detected in the partially and the fully crystallized glass compositions outside the glass forming range. Data are reported for the glass transition temperature (T_g), dilatometric softening point (T_d), linear coefficient of expansion (α), onset crystallization temperature (T_x), exothermal peak temperature (T_P), density (ρ) and index of refraction (n_D) in the clear glasses.     

Density-structure correlations in Na2O-CaO-P2O5-SiO2 bioactive glasses

Relations for Na₂O-CaO-SiO₂ glasses have been developed to calculate the density of Na₂O-CaO-P₂O₅-SiO₂ bioactive glasses. The calculation makes use of NMR results of O’Donnell et al. indicating that P₂O₅ forms a separate phase, containing Na₃PO₄ and Ca₃(PO₄)₂, in the investigated glasses. The volume of the silicate units is the same as that found in Na₂O-CaO-SiO₂, Na₂O-SiO₂, and CaO-SiO₂ glasses. Similarly, the volume of PO₄ units is equivalent to that in Na₃PO₄ and Ca₃(PO₄)₂. Calculated densities are consistent with the experimental data.     

EPR study of RLi2O.V2O5, RNa2O.V2O5, RCaO.V2O5, and RBaO.V2O5 modified vanadate glass systems

Experimental EPR spectra in several modified vanadate glass systems reveal hyperfine structure (hfs) lines whose widths vary with the molar ratio of modifier to vanadium pentoxide, R. In the RNa₂O.V₂O₅ system, for example, hfs lines show no resolution at low R values (near 0.1); by contrast, these lines exhibit dramatic narrowing as R approaches 0.5. In the model proposed here, this narrowing is due to an increase in hopping time for small polarons associated with V⁴⁺ ions in these systems. Increases in polaron hopping times are accompanied by increases in electron spin-spin relaxation times T₂'s, and, an associated narrowing of EPR linewidths. Experiments confirm that spectral widths are limited by electron T₂'s due to the fact that EPR linewidths do not vary with temperature down to 4.2 K. Resolved spectra in RNa₂O.V₂O₅ at R = 0.5 reveal a hyperfine coupling parameter of 0.0177 ± 0.0008 T, corresponding to an upper-limit polaron hopping frequency of 487 ± 20 MHz. By similar analyses, the systems of RCaO.V₂O₅, RBaO.V₂O₅, and RLi₂O.V₂O₅ exhibit comparable polaron hopping frequencies limits of 480 ± 20 MHz, 469 ± 20 MHz, and 468 ± 20 MHz, respectively, when R is near 1.0. In addition to the relaxation effects discussed here, results of modeling of resolved spectra to obtain hyperfine coupling constants A_|| and A_┴, and g_┴ values g_|| and g_┴ are presented and discussed.     

Elasticity, thermal expansion and compressive behavior of Mg65Cu25Tb10 bulk metallic glass

The existence of special covalently bonded short-range ordering structures in a Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) is confirmed by thermal expansion and compression behavior. Under ambient conditions the linear thermal expansion coefficient obtained is almost constant in the glassy state with a value of 4.0 × 10⁻⁵ K⁻¹. By fitting the static equation of state at room temperature under ambient conditions we find the value for bulk modulus B of 48.7 GPa, which is in excellent agreement with the experimental study by pulse-echo techniques of 44.7 GPa. Unlike many bulk metallic glasses, such as Zr- and Pd-based, which bulk modulus is much larger than 100 GPa, the value B of Mg₆₅Cu₂₅Tb₁₀ BMG falls into the range of SiO₂ and fluorozirconate glass ZBLAN. Moreover, the elastic constant of the Mg₆₅Cu₂₅Tb₁₀ BMG is almost the same as those of ZBLAN. No evidence for the high-pressure phase transitions of the Mg₆₅Cu₂₅Tb₁₀ BMG has been found up to 31.19 GPa at room temperature.     

Y2O3掺杂WO3光催化剂的制备及其性能

采用浸渍法合成了Y2O3 -WO3光催化剂,通过XRD和UV-Vis DRS测试手段对其进行了表征.以对苯二甲酸为探针分子,结合化学荧光技术研究了Y2O3 -WO3表面羟基自由基的生成.以溴百里香酚蓝(BTB)催化降解实验评价了其光催化活性.结果表明:与纯WO3相比,1.2％ Y2O3-WO3样品的吸收带边发生明显红移、在200～800 nm 区域有更强的光吸收.Y2O3-WO3样品的紫外和可见光催化活性随Y2O3含量增加先增加,当Y2O3含量为1.2％时分别达到最大值,然后随Y2O3含量的进一步增加而减小.与纯WO3相比,1.2％ Y2O3-WO3样品明显具有更高的紫外和可见光催化活性以及更大的羟基自由基生成速率.     

Effect of carbon on the microstructural evolution of Zr66.7−xNi33.3Cx (x = 0, 1, 3) alloys during mechanical alloying

In the present paper, the effect of carbon on the microstructural evolution of Zr_66.7−xNi_33.3C_x (x = 0, 1, 3) alloys during mechanical alloying has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that these three alloys undergo similar amorphization and crystallization processes, and the final milling product is a metastable fcc-Zr_66.7−xNi_33.3C_x phase. The carbon addition can shorten the milling time for the complete amorphization reaction and enhance the stability of the formed amorphous alloy, which can suppress the mechanically induced amorphous-crystalline phase transformation with further increasing milling time.     

Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties - Part I: NMR

The effect of the variation in phosphate (P₂O₅) content on the structure of two series of bioactive glasses in the quaternary system SiO₂-Na₂O-CaO-P₂O₅ was studied. The first series (I) was a simple substitution of P₂O₅ for SiO₂ keeping the Na₂O:CaO ratio fixed (1.00:0.87). The second series was designed to ensure charge neutrality in the orthophosphate , therefore as P₂O₅ was added the Na₂O and CaO content was varied to provide sufficient Na⁺ and Ca²⁺ cations to charge balance the orthophosphate present. The glass network connectivity (NC) was calculated for each glass and a modification for the presence of a separate P₂O₅ phase was included (NC′). ³¹P and ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was performed on glass series I and II to determine the structural units present and their relation to glass properties. ³¹P MAS-NMR spectra of series I resulted in a broad resonance around 9 ppm corresponding to orthophosphate in an amorphous environment. The 9.25 mol% P₂O₅ glass shown to be partially crystalline by X-ray diffraction was heat treated, and the ³¹P MAS-NMR spectrum showed a sharp peak around 3 ppm corresponding to calcium orthophosphate or sodium pyrophosphate and overlapping broader peaks at 8.5, 10.5 and 14 ppm possibly corresponding to two mixed calcium-sodium orthophosphate phases and amorphous sodium orthophosphate respectively. ³¹P MAS-NMR spectra of series II resulted in a broad resonance around 10.5 ppm corresponding to orthophosphate in an amorphous environment. ²⁹Si MAS-NMR spectra of glasses from series I showed a shift in the resonance peak from around −78 to −86 ppm indicating an increase in Q³ species in the glass and a reduction in Q² with phosphate addition confirming the presence of orthophosphate. The heat treated sample showed a sharp ²⁹Si-NMR resonance at −88 ppm, indicating a crystalline Q² six-membered combeite (Na₂O · 2CaO · 3SiO₂) silicate-type phase, which was confirmed by powder X-ray diffraction. ²⁹Si MAS-NMR spectra of glasses from series II showed no shift in the resonance at around −78 ppm across the series, confirming an orthophosphate environment.     

Thermal conductivity and specific heat of bulk amorphous chalcogenides Ge20Te80−xSex (x = 0,1,2,8)

Bulk amorphous chalcogenide samples of Ge₂₀Te_80−xSe_x (x = 0, 1, 2, 8) have been prepared using a melting-quench method, and characterized by the differential scanning calorimetry, X-ray powder diffraction, high-resolution transmission electron microscopy, specific heat and thermal conductivity measurements. The low temperature specific heat measurements identified some localized low-frequency oscillation modes (Einstein modes) in conjunction with a Debye-like behavior. It was found that with increasing Se concentration the characteristic Debye temperature did not change whereas the Einstein temperature slightly decreased. The lattice thermal conductivity of all Ge₂₀Te_80−xSe_x samples exhibited typical amorphous heat conduction behavior, which has been discussed in connection with the phonon mean free path and in the context of a phenomenological model of heat conduction for highly disordered system.     

Crystallization behavior and hydrophilic performances of V2O5–TiO2 films prepared by sol–gel dip-coating

Homogeneous and transparent V₂O₅–TiO₂ composite nanometer thin films were prepared on glass substrates by sol–gel processing and dip-coating technique. The films as well as the dried powder of bulk gel were characterized by different techniques like X-ray diffraction (XRD), high-resolution scanning electron microscopy (HRSEM), atomic force microscope (AFM) and thermogravimetry–differential thermal analysis (TG–DTA). The hydrophilicity of the films was determined by measuring the water contact angles on the films. The results showed that the dopant of V₂O₅ on TiO₂ thin films could produce a visible-light response to the films, and the introduction of V₂O₅ could suppress the structural phase transition and crystal growth of TiO₂ crystal. Finally, the relationship between crystalline size and hydrophilicity under sunlight was investigated in this article.     

Characterisation of semiconducting V2O5–Bi2O3–TeO2 glasses through ultrasonic measurements

Tellurite containing vanadate (50−x)V₂O₅–xBi₂O₃–50TeO₂ glasses with different bismuth (x=0, 5, 10, 15, 20 and 25 wt%) contents have been prepared by rapid quenching method. Ultrasonic velocities (both longitudinal and shear) and attenuation (for longitudinal waves only) measurements have been made using a transducer operated at the fundamental frequency of 5 MHz in the temperature range from 150 to 480 K. The elastic moduli, Debye temperature, and Poisson’s ratio have been obtained both as a function of temperature and Bi₂O₃ content. The room temperature study on ultrasonic velocities, attenuation, elastic moduli, Poisson’s ratio, Debye temperature and glass transition temperature show the absence of any anomalies with addition of Bi₂O₃ content. The observed results confirm that the addition of Bi₂O₃ modifier changes the rigid formula character of TeO₂ to a matrix of regular TeO₃ and ionic behaviour bonds (NBOs). A monotonic decrease in velocities and elastic moduli, and an increase in attenuation and acoustic loss as a function of temperature in all the glass samples reveal the loose packing structure, which is attributed to the instability of TeO₄ trigonal bipyramid units in the network as temperature increases. It is also inferred that the glasses with low Bi₂O₃ content are more stable than with high Bi₂O₃ content.     

Effect of pressure on the refractive index and relative density of the CaO · Al2O3 · 6SiO2 glass

The refractive index for glass of the CaO · Al₂O₃ · хSiO₂ system with х = 6 in the range of pressures up to 6.0 GPa was measured using a polarization-interference microscope and an apparatus with diamond anvils. The changes in the relative density characterizing the compressibility of glass were estimated from the measured refractive indices within the framework of the theory of photoelasticity. The data were compared with the previous data for glasses of the same system with х = 2 and 4. The most compressible of the three glasses in the range 2.0-6.0 GPa was the CaO · Al₂O₃ · 6SiO₂ glass. For glasses with х = 2, 4 and 6 we calculated the degrees of polymerization of silicon-aluminum-oxygen network, NBO/T (NBO - non-bridging oxygen), which are determined as the ratio of the number of gram-ions of non-bridging oxygen atoms to the total number of gram-ions of network formers. The structure-chemical parameter NBO/T was calculated with due regard for the formation of triclusters and highly coordinated aluminum. The degree of polymerization of the CaO · Al₂O₃ · хSiO₂ glasses increases with increasing х, which agrees with the change of their relative density under pressure.     

Preparation and physical properties of amorphous V2O5 containing TiO2, Na2O or Li2O

V₂O₅ gels containing up to 18 mol% of TiO₂ were obtained through the simultaneous hydrolysis of alkoxides in ethanol solution. V₂O₅ gels containing Na₂O or Li₂O were obtained through the ion exchange method. The crystallization temperature, T_cr, of the gels increased and the H₂O content of the gels decreased by the addition of TiO₂ or Na₂O. These additives seem to stabilize the amorphous state of the gels. On the other hand, T_cr and the H₂O-content slightly varied with the addition of Li₂O. No ionic polarization was observed in coating films of the gels dried at temperatures below T_cr. The dc conductivity of the films was anisotropic, and increased with the addition of Li₂O or Na₂O. However, it decreased with increasing TiO₂ content. The fiber-like structure of gels was observed by TEM. The gels obtained from alkoxides were thin and short in comparison with the gels obtained through the ion exchange method.