Gd-incorporation and luminescence properties in sol-gel silica glasses

A thorough study was performed on Gd-doped sol-gel silica glasses, in the concentration range 0-8 mol% Gd. The analyses were carried out as a function of Gd content, before and after a post-densification thermal treatment. Different results concerning optical, vibrational, magnetic, and structural features were gathered in correlated experiments. The presence of Gd-rich nanoclusters was revealed. The size of nanoclusters increases by increasing the dopant concentration and by performing a rapid thermal treatment (RTT) at 1800 °C in air, which causes also a remarkable intensity increase of the ⁶P_7/2 → ⁸S_7/2 radioluminescence transition of Gd³⁺. Nanoclusters are amorphous, possibly close to a Gd₂SiO₅ stoichiometry as suggested by fast Fourier transform infrared spectroscopy (FTIR). FTIR studies revealed also the presence of Gd-dimers interacting with OH⁻ groups. Moreover, the presence of isolated Gd³⁺ ions was detected by electron paramagnetic resonance investigations. Based on the results obtained with different techniques, the Gd-incorporation in silica glass host and the resulting optical properties are discussed.     

Investigation of silica particle structure containing metallocene immobilized by a sol-gel method

A series of homogeneous metallocenes (Cp₂ZrCl₂, (nBuCp)₂ZrCl₂, (iBuCp)₂ZrCl₂, (tBuCp)₂ZrCl₂, Cp₂TiCl₂, Cp₂HfCl₂, EtInd₂ZrCl₂ and Et(IndH₄)₂ZrCl₂) were immobilized within a silica matrix during its preparation using a non-hydrolytic sol-gel method. Supported metallocenes were characterized by small angle X-ray scattering (SAXS) and nitrogen adsorption. The results show that the silica particle was formed by the aggregation of primary structures having a diameter of about 160 nm. This was a result of the aggregation process, which was affected by the metallocene. This result suggests that the metallocene is surrounded by primary particles deep inside the silica matrix. The low polymer productivity exhibited by the less active metallocene catalysts could be related to the problems of particle fragmentation due to monomer mass transfer limitations along the partially fragmented particle, which would subsequently affect catalytic activity.     

Solubility of glasses in the system P2O5-CaO-MgO-Na2O-TiO2: Experimental and modeling using artificial neural networks

Phosphate glasses in the system P₂O₅-CaO-MgO-Na₂O-TiO₂ for use as degradable implant materials were produced. In order to classify their solubility behavior, dissolution experiments were performed in deionized water for 60 min at 98 °C. Resulting solutions were analyzed using ICP-OES. In addition, pH measurements were carried out in physiological NaCl solution. With increasing phosphorus oxide content, the glasses showed a higher solubility and gave lower pH values in aqueous solution. This was caused by changes in the glass structure, as long phosphate chains are more susceptible to hydration than smaller phosphate groups. These changes in glass structure were followed by ³¹P MAS-NMR experiments. Increasing sodium oxide concentrations in exchange for calcium or magnesium oxide also increased the glass solubility by disrupting ionic cross links between chains. By contrast, addition of titania made the glasses more stable towards dissolution by cross linking smaller phosphate groups. The aim of this study was to find a relationship between glass composition and solubility behavior. As classical linear methods of data analysis were unsuitable due to the complexity of the relationship, preliminary artificial neural networks analyses were performed and were found to be an interesting tool for modeling the solubility behavior of phosphate glasses.     

Preparation of organic-inorganic nano-composites for antireflection coatings

In this research, bi-layer thin film stacks that served as an antireflective coating were developed. The top layer was synthesized using tetraethoxysilane and 3-(trimethoxysilyl) propyl methacrylate by the well-known sol-gel technique. Its refractive index was lower than that of the bottom layer, which was prepared by reaction between tetrabutoxyltitanium and γ-glycidoxy propyl trimethoxysilane. Antireflective coatings were obtained by spin-coating of the synthesized sols on a glass substrate, followed by pre-drying, UV-curing, and post-baking. Fourier transform infrared spectrometer was employed to investigate the evolution of chemical bonds during the UV-curing and the sol-gel processes. The size of the inorganic/organic hybrid particles in the sol was found to be less than 10 nm, as measured by transmission electron microscope and dynamic light scattering. Thermo gravimetric analyzer was used to find out the thermal degradation temperature of the two layers and the effect of post-baking. The results indicated that the thermal stability increased after post-baking at 200 °C for 15 min. The reflectance of the antireflection coating was controlled by the relative refractive indices and the thickness of the top and bottom layers. Under optimal synthesis condition, we obtained an antireflection coating, exhibiting a low reflection, 1% at 550 nm, in the visible range.     

Synthesis of Si-MCM-48 membrane by solvent extraction of the surfactant template

Mesoporous MCM-48 silica membranes have been synthesized on porous supports under hydrothermal conditions, using tetraethyl orthosilicate (TEOS) as silica source, cetyltrimethylammonium bromide (CTAB) as template surfactant. And then the templates were removed by solvent extraction instead of calcination. The results of the TG and FT-IR showed that the solvent of 1 M HCl/EtOH solution was feasible to extract the templates in the MCM-48 materials and over 90% of the templates were extracted at room temperature for 24 h. The results of XRD, SEM indicated that the MCM-48 membranes were prepared on the porous supports and extraction did not destroy the mesostructure of the MCM-48. The compactness of the extracted MCM-48 membrane was evaluated by the permeation of single gas (N₂ and H₂) with transmembrane pressure of 60-220 kPa. The permeance of N₂ was independent of the transmembrane pressure and the ideal separation factor was about 3.45. All the results demonstrate that the method, solvent extraction to remove surfactant template, is more effective to synthesize high quality MCM-48 membranes than calcination.     

Icosahedral and dense random cluster packing in metallic glass structures

It has recently been shown that metallic glass structures can be idealized as inter-penetrating solute-centered atomic clusters that are packed with essentially periodic symmetry. The present work applies the same methodology to explore whether experimental observations can be matched by inter-connected solute-centered clusters that are organized in space via dense random cluster packing, Bergman icosahedral cluster packing or Mackay icosahedral cluster packing. Idealized partial pair distribution functions are developed where the symmetry of the solute positions in the structure is derived from the cluster-packing symmetry and the solute concentration, which establishes occupation of inter-cluster sites, especially β structural sites enclosed by an octahedron of solute-centered clusters. While each of the three models matches major features of the measured solute-solute partial pair distribution functions, the arrangement of clusters with Mackay icosahedral ordering provides the best fit. However, this model is not able to match an essential feature in solute-lean glasses and does not provide the same overall agreement as does periodic cluster packing for solute-rich glasses. Strong similarities between the structure factors in the Mackay icosahedral and periodic cluster-packing models, along with expected deviations from the idealized solute positions studied here, are likely to hinder an unambiguous distinction between these two models.     

Sulfur behavior in silicate glasses and melts: Implications for sulfate incorporation in nuclear waste glasses as a function of alkali cation and V2O5 content

The presence of sulfur in radioactive waste to be incorporated in borosilicate glasses entails difficulties mainly due to the relatively low solubility of sulfates in the vitreous phase. In this work a study is presented on the effects of the ratio R = [Na₂O]/[B₂O₃], the type of sulfate added and the addition of V₂O₅ on the incorporation of sulfates in borosilicate glasses. Glass samples were prepared at the laboratory scale (up to 50-100 g) by melting oxide and sulfate powders under air in Pt/Au crucibles. XRF and ICP/AES chemical analysis, SEM/EDS, microprobe WDS and Raman spectroscopy were employed to characterize the fabricated samples. The main experimental results confirm that the incorporation of sulfates in borosilicate glasses is favored by the network depolymerization, which evolves with the ratio R. The addition of V₂O₅ seems to accelerate the kinetics of sulfur incorporation in the glass and, probably, increase the sulfate solubility by modifying the borate network and fostering the formation of voids of shape and size compatible with the sulfur coordination polyhedron in the glassy network. The kinetics of X₂SO₄ incorporation in the glass seems to be slower when X = Cs.     

Indentation crack initiation behavior of vitreous silica glasses containing different hydroxyl concentration

The indentation crack initiation behavior of eight vitreous silica specimens containing bulk OH concentrations ranging from 0.2 to 754 wt ppm was investigated. A recording microindentation instrument equipped with optical observation and acoustic emission detection was used to study, in situ, the cracking behavior from indentation with a Vickers diamond. No significant differences in the threshold loads for various types of cracking behavior of the specimens were found. In addition, the polishing medium was found to have little influence on the cracking behavior. The lengths of median-radial cracks around indentations varied little between specimens. The Vickers hardness of the specimens measured at 0.98 N ranged from 6.6 ± 0.3 GPa to 7.5 ± 0.7 GPa, and no trend with the OH concentration was apparent. In addition, the Vickers hardness of the specimens measured while under a 9.81 N maximum load (LVH_max), showed little variation, and no apparent trend with the OH concentration.     

Effect of N2/CH4 flow ratio on microstructure and composition of hydrogenated carbon nitride films prepared by a dual DC-RF plasma system

Hydrogenated carbon nitride (a-CN:H) films were deposited on n-type (1 0 0) silicon substrates making use of direct current radio frequency plasma enhanced chemical vapor deposition (DC-RF-PECVD), using a gas mixture of CH₄ and N₂ as the source gas in range of N₂/CH₄ flow ratio from 1/3 to 3/1 (sccm). The deposition rate, composition and bonding structure of the a-CN:H films were characterized by means of X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectrometry (FTIR). The mechanical properties of the deposited films were evaluated using nano-indentation test. It was found that the parameter for the DC-RF-PECVD process had significant effects on the growth rate, structure and properties of the deposited films. The deposition rate of the films decreased clearly, while the N/C ratio in the films increased with increasing N₂/CH₄ flow ratio. CN radicals were remarkably formed in the deposited films at different N₂/CH₄ flow ratio, and their contents are related to the nitrogen concentrations in the deposited films. Moreover, the hardness and Young’s modulus of the a-CN:H films sharply increased at first with increasing N₂/CH₄ flow ratio, then dramatically decreased with further increase of the N₂/CH₄ flow ratio, and the a-CN:H film deposited at 1/1 had the maximum hardness and Young’s modulus. In addition, the structural transformation from sp³-like to sp²-like carbon-nitrogen network in the deposited films also was revealed.     

Making amorphous steel in air by rare earth microalloying

Recently great advancement in the fabrication of structural amorphous steels (SASs) with large cross-section sizes has been obtained. One of the remained key problems for the manufacturability and the application of SASs is that their glass-forming ability is very sensitive to the processing environments. Here we present the discovery that the bulk amorphous alloy Fe₄₄Mn₁₁Cr₁₀Mo₁₂C₁₅B₆Ho₂ has high oxygen resistance and can be prepared by conventional copper-mold casting method in air atmosphere. Rod-shaped amorphous samples of diameter up to 6 mm at least have been fabricated successfully. This amorphous steel alloy showed high glass-forming ability and good manufacturability. The beneficial effects of the addition of holmium on the glass-forming ability and oxygen resistance during the SAS formation are explored.     

MOVPE of AlN-free hexagonal GaN/cubic SiC/Si heterostructures for vertical devices

The heterostructures of GaN/SiC/Si were prepared without using AlN or AlGaN buffer layers (AlN buffers) in the metalorganic vapor phase epitaxy of GaN on SiC. GaN (0 0 0 1) with specular surface was obtained. The AlN buffers are usually used in the conventional growth of GaN on SiC due to the poor nucleation of GaN on SiC. Instead, the nucleation of GaN was controlled by varying the partial pressure of H₂ in the carrier gas, the mixture of H₂ and N₂, during the low-temperature (600 °C) growth of GaN (LT-GaN). After the LT-GaN, the high-temperature (1000 °C) growth of GaN was performed using pure H₂ as the carrier gas. The epitaxial film of cubic SiC (1 1 1) on a Si (1 1 1) substrate was used as the SiC template. Increasing the partial pressure of H₂ in the carrier gas decreased the coverage of SiC surface by LT-GaN. It is suggested that the hydrogen atoms adsorbed on the surface of SiC is preventing the nucleation of GaN.     

Fabrication of MTN-type zeolite by self-assembling of supramolecular compound

MTN-type (Zeolite Socony Mobil Thirty-Nine) zeolite was prepared at 473 K by a novel method through self-assembling of a supramolecular compound called 2,4,6-tris (4-pyridyl) triazine (TPT) in DMF (N,N-dimethylformamide). The effects of fluoride, DMF and germanium on the synthesis of MTN-type zeolite were investigated. The crystallization was facilitated by adding fluoride to the synthesis solution, resulting in the formation of highly crystalline MTN samples, while some amorphous phase was observed in fluoride-free batches. DMF was required to obtain a highly crystalline MTN sample, since TPT dissolves easier in DMF than in water, thus facilitating the self-assembling of TPT into a 3D network to structure the MTN framework. The MTN structure could be synthesized at low germanium content (Ge/Si≤0.18), while AST (AlPO₄-sixteen) as a foreign phase is formed at high germanium substitution (Ge/Si≥0.5).     

Kinetics of the thermally induced precursor curing to polymer via statistical analysis of TEM images

Thermally induced curing of the precursor to the cross-linked polymer was studied with transmission electron microscopy. The cured polymer was observed to have micro-domain morphology. Analysis of the microscopic images resulted in the statistical size distributions of micro-domains described subsequently using principles of irreversible thermodynamics. The statistical distribution parameters were determined to change with the curing temperature. A correlation of micro-structural and macroscopic parameters (the mean size of micro-domains and the dynamic Young’s modulus), both varying during the precursor curing, was found and explained in terms of the kinetic concept of fracture.     

The character of FeMn-1# powder catalyst and its influence on the synthesis of diamond

In this paper industrial diamond crystal was synthesized using FeMn-1# powder catalyst in China-type cubic high-pressure apparatus at 5.7 GPa and 1400–1600 °C. The growth feature of diamond in the graphite–FeMn-1# system was researched. Optical microscope observation showed that all the diamond crystals were light yellow octahedral with grain size of 0.3–0.5 mm. There are also plenty of bubbles in the crystals. By SEM, we can see that the surface of diamond is smooth and the crystal is intact. Mössbauer spectrum was used to detect the impurity in the diamonds.     

Mechanical, chemical and acoustic properties of new hybrid ceramic-polymer varnishes for musical instruments

Novel ceramic-polymer hybrid varnishes were designed to protect the wood surface of musical instruments. These hybrid coatings consist of chemically functionalized silica nanoparticles and synthetic solvent-based acrylic- and alkyd-polyurethanes. The nanoparticles were added to increase the abrasion resistance. An alkoxide was used to increase the number and reactivity of OH’s groups on the wood surface improving the adhesion with the coating through a chemical link between them. The properties of the synthetic coatings were compared with those of a traditional varnish (based on alcohol and natural resins) to obtain a better performance. Two types of woods were used: maple and spruce. The samples were characterized by UV-Vis, mechanical and abrasion tests, water’s absorption, acoustic properties, chemical resistance and SEM.     

Float-reaction between liquid bronze and magnesium aluminosilicate and ZnO-doped magnesium aluminosilicate glass-ceramic-forming glassmelts

Wavelength-dispersive X-ray spectroscopy (WDS) was used to characterize the morphology of the reactions between a liquid bronze alloy (Cu-36 at.%Sn) and two magnesium aluminosilicate, glass-ceramic-forming glassmelts, one of which was doped significantly with ZnO. Two suites of experiments were pursued for each glassmelt, an isochronal series (30-min reactions with temperatures ranging from ∼1300 to 1400 °C) and an isothermal series (1350 °C reactions with durations ranging from 5 to 60 min). The reactions are decidedly complex. Transient behavior sees initially rapid incorporation of Cu^+,2+ into the glassmelts, effected primarily by a redox couple involving the SiO₂ component of the aluminosilicate. This behavior gives way to a more dominant kinetic response in which Sn^2+,4+ is incorporated into the glassmelt in a reaction and chemical diffusion process that, in part, pulls the early-incorporated ionic copper back out of the aluminosilicate. In the case of the ZnO-doped glassmelt, coupled redox and interdiffusion of ionic Sn and ionic Zn is important in the longer-time response, giving rise to a Liesegang-band morphology. The extent of metal-silicate reaction diminishes as the temperature is increased, a thermodynamic effect related to the solution thermodynamics of the liquid bronze alloy. The reaction kinetics are interpreted following the Wagner-Schmalzried formalism for diffusion-effected redox reactions.     

Glass formation ability and mechanical properties of Ti40Cu40Zr10Ni10 alloy

Melt-spun ribbon and bulk samples in cylindrical rod form with diameter ranging from 2 mm to 4 mm of Ti₄₀Cu₄₀Zr₁₀Ni₁₀ alloy were prepared by melt-spinning technique and copper mould casting method, respectively. The microstructure, thermal stability and mechanical properties of the bulk samples were investigated. A completely glassy single phase is formed in the 2 mm rod sample. Increasing the diameter of the rod samples resulted in the formation of CuTi crystalline phase in the 3 mm and 4 mm rod samples. The 2 mm single glassy rod sample exhibited a large supercooled liquid region ΔT_x = 58 K and γ = T_x/(T_g + T_l) is 0.390, which indicated that the alloy possessed a good glass-forming ability. The bulk samples also exhibited good mechanical properties. The 2 mm rod sample showed the highest yield strength of about 2086 MPa. The 3 mm rod sample not only showed high yield strength of about 2000 MPa, but also enhanced plastic strain of about 0.71%.     

Poly(ethyl methacrylate-co-hydroxyethyl acrylate) random co-polymers: Dielectric and dynamic-mechanical characterization

The conformational mobility in poly(ethyl methacrylate-co-hydrxyethyl acrylate) co-polymers was studied by dielectric relaxation spectroscopy, thermally stimulated depolarization currents and dynamic-mechanical experiments. The relaxation strength and the shape of the dielectric relaxation spectra of the homopolymer and co-polymer networks were analyzed using the Havriliak-Negami equation. The dependence of the relaxation strength on co-polymer composition and temperature was analyzed taking into account the merging of the main and the secondary relaxation processes. The shape of the ε″ versus frequency plots led to the conclusion that the distribution of relaxation times was broader in the co-polymers with intermediate composition than in the homopolymers, a feature that can be explained by the inhomogeneity produced at molecular scale by the sequence distribution of the monomeric units along the chain. Master curves were built both for the elastic modulus and the mechanical loss tangent, and the temperature dependence of the relaxation times was deduced from the shift factors. The fit to the Vogel equation permitted the calculation of the strength parameter, which is higher in the co-polymers that the simple average of the values of the homopolymers, a feature that can be related to the broadness of the distribution of relaxation times.     

Electrical characterization of ion conducting biopolymeric gel complexes

Gum Arabica is a naturally occurring conducting biomaterial biopolymer, its various aspects of electrical and structural characteristics have been investigated in our earlier works. In the present work the ionic gel formation of Gum Arabica and its electro-chemical application were experimentally investigated. Hot concentrated aqueous solution of Gum Arabica with phosphoric acid, minimum 16% of solute, was found to form ionic hydrogel on slow cooling. It consists of polyelectrolyte molecules cross-linked by phosphate ions by chemical bonds into an integrated spatial network in water solvent. The increase in surface to volume ratio shows a gel collapse to provide transparent solid material. Gel formation of Gum Arabica was also studied with chromium and molybdenum oxide as cross-linking agent. The various studies carried out on the mentioned materials to investigate its structural and electrical characteristics are XRD, DSC, SEM and impedance spectroscopy. The said gel was found to provide a good electro-chemical cell with suitable redox pair.     

Improvement of the Tm:H4 level lifetime in silica optical fibers by lowering the local phonon energy

The role of some glass network modifiers on the quantum efficiency of the near-infrared fluorescence from the ³H₄ level of Tm³⁺ ion in silica-based doped fibers is studied. Modifications of the core composition affect the spectroscopic properties of Tm³⁺ ion. Adding 17.4 mol% of AlO_3/2 to the core glass caused an increase of the ³H₄ level lifetime up to 50 μs, 3.6 times higher than in pure silica glass. The quantum efficiency was increased from 2% to approximately 8%. On the opposite, 8 mol% of PO_5/2 in the core glass made the lifetime decrease down to 9 μs. These changes of Tm³⁺ optical properties are assigned to the change of the local phonon energy to which they are submitted by modifiers located in the vicinity of the doping sites. Some qualitative predictions of the maximum achievable quantum efficiency are possible using a simple microscopic model to calculate the non-radiative de-excitation rates.