Glass Aluminum Research Articles

Local Heterogeneity for a Eu3+-Doped Glass Evidenced by Time-Resolved Fluorescence Spectroscopy Coupled to Scanning Near-Field Optical Microscopy

Time-resolved fluorescence spectroscopy (TRFS) was applied to an aluminate glass sample doped with Eu3+ cation as a fluorescent probe of the chemical environment and local symmetry. Conventional far-field experiments revealed the presence of two different phases: an amorphous phase featured by a highly disordered environment surrounding the Eu3+ cation and a more ordered polycrystalline phase that exhibits a significant increase in the Eu3+ fluorescence decay time compared to that of the amorphous phase. Near-field fluorescence spectra and decay kinetics were recorded in the frontier region between the two phases using a home-built scanning near-field optical microscope. SNOM-TRFS experiments confirmed the presence of local heterogeneities in this part of the glass at a sub-micrometric spatial scale. Polycrystalline sites featured an important shear-force interaction with the probing fiber optic tip, a longer fluorescence decay time, and a higher Stark splitting of the 5D0 --> 7FJ (J = 1-4) electronic transitions of the Eu3+ cations.

A molecular dynamics study of the atomic structure of(CaO)x(Al2O3)1−x glasswith x = 0.625 close to the eutectic

Aluminate glasses are difficult to prepare as they do not contain traditional networkformers, but they are promising materials for optical applications. The atomicstructure of calcium aluminate glasses has been studied using several experimentaltechniques. The current study uses molecular dynamics to obtain a model of a(CaO)0.625(Al2O3)0.375 glass close to the eutectic. The glass consists of a tetrahedral alumina network with averagenetwork polymerization of n = 3.3. Ca actsas a network modifier with average coordination of 6.2. Ca is typically coordinated to three bridging oxygens(Ob) and three non-bridgingoxygens (Onb), withCa–Onb bonds noticeablyshorter than the Ca–Ob bonds. A new method of analysing modifier cation coordination ispresented, which specifically shows the distribution of Ca coordinationNCaO in terms ofcombinations of NCaOb and NCaOnb. Ob is most often coordinated to two Al plus two Ca, andOnb is most often coordinated to one Al plus three Ca. The typical coordinations of Ca,Ob,and Onb all have a noticeable similarity to those for the5CaO·3Al2O3 crystal. The Ca–Ca distribution shows a clear similarity to that for(CaO)0.5(SiO2)0.5 glass, and this is attributed to the equal atomic number densities of Ca in these glasses.

Properties and cyclic fatigue of glass infiltrated tape cast alumina cores produced using a water-based solvent

Objective The purpose of this study was to investigate the properties of tape cast alumina composite produced using a water-based solvent and its possible clinical use as an all ceramic crown system in a fixed partial denture. Durability of the system will be measured by fatigue test to simulate the masticating conditions of the oral cavity. Methods The optimal weight ratio of water-based alumina tape was determined by tensile strength, shrinkage ratio and durability. The coefficient of thermal expansion, fracture toughness, biaxial flexural strength and flexural strength after fatigue test of a composite produced from alumina tape at optimal weight ratios were determined and compared to In-Ceram alumina core (control). Results The weight ratio of alumina/(alumina + binder + plasticizer) of 0.84 and binder/(binder + plasticizer) of 0.5 was observed to be the optimal composition for achieving excellent composite properties. Coefficient of thermal expansion of the sintered alumina tape was observed to be 7.3 × 10 −6/°C, and this value was increased to 7.5 ×10 −6/°C after infiltrating the sintered tape with glass. The fracture toughness and biaxial flexural strength of glass infiltrated alumina tape was observed to be 4.6 MPa m 1/2 and 498 MPa, respectively. After cyclic loading for 10 2–10 6 cycles, no significant change in the biaxial flexural strength was observed between the glass infiltrated alumina core and the In-Ceram alumina core ( p > 0.05). Significance The observed properties provide evidence that the water-based tape cast alumina–glass composite is suitable for clinical use as an all ceramic crown system in a fixed partial denture.

Molecular dynamics simulations of calcium aluminate glasses

Molecular dynamics simulations of a series of calcium aluminate glasses have been carried out using empirical potentials with a covalent term. The simulations closely reproduce the total neutron correlation function of glasses with 30 and 38 mol% Al 2O 3 and physical properties such as elastic constants. For compositions close to the eutectic 37 mol% Al 2O 3, aluminum is tetrahedrally coordinated by oxygen, but the proportion of five-fold and six-fold coordination and also edge-sharing tetrahedra gradually increases with increasing Al 2O 3 content. The coordination number for oxygen around calcium atoms is close to 6 and this involves a larger coordination at a shorter, well-defined distance, followed by a smaller coordination with a broader range of distances. When the Al 2O 3 content decreases, the calcium aluminate structure becomes depolymerised and the average ring size increases. Also reported is an X-ray photoelectron spectroscopy measurement on a glass sample with 38 mol% Al 2O 3, which shows that 38.6 ± 0.9% of the oxygens are non-bridging, in excellent agreement with the simulations.

Tm-doped aluminate glass fibers for S-band optical amplification

Thulium-doped fiber amplifiers (TDFAs) have been proposed as practical devices for the amplification of light signals in the so-called S-band (1460–1530 nm) of the transparency window of standard telecommunications fiber. As the quantum efficiency of the desired 3H 4 → 3F 4 luminescence of Tm 3+ is adversely affected by non-radiative decay when high maximum phonon energy (MPE) host glasses are used, a practical TDFA requires an active fiber made from a glass with intermediate to low MPE. We have explored the possibility of using aluminate fibers for this application, as bulk samples of Tm-doped alkaline earth aluminate glass are characterized by a MPE of 780 cm −1 and a quantum efficiency for the 1460 nm fluorescence of ∼35%. Despite the high devitrification tendency of aluminate glass, pure aluminate core fibers with minimum losses of ∼0.5 dB/m have been successfully fabricated by the rod-in-tube technique using viscosity- and expansion-matched alkaline earth aluminosilicate cladding glasses.

Acoustic Emission Characteristics of Single-Edge-Notched Glass Fiber/Aluminum Hybrid Laminates

Acoustic emission (AE) characteristics have been studied for single-edge-notched monolithic thin aluminum (Al) plates and glass fiber/Al hybrid laminates. Traveling microscope was used for observing the plastic deformation and damage zone around the initial notch tip. Frequency characteristics of AE signals processed by fast Fourier transform (FFT) from monolithic Al could be classified into two different types. Type I signal had a relatively low frequency band of 96~260kHz, while Type II signal had broad band frequencies of 192~408kHz. In case of glass fiber/Al hybrid laminates, AE signals with high amplitude (>80dB) and long duration (>2msec) were additionally confirmed on FFT frequency analysis, which corresponded to macro-crack propagation and/or delamination between aluminum layer and glass fiber layer. Also, distributions of the first and the second peaks in frequency spectrum were related with local fracture behaviors of the hybrid laminates. AE source location determined by signal arrival time showed the extent of fracture zones. On the basis of the above AE analysis, characteristic features of fracture processes of single-edge-notched glass fiber/aluminum laminates were elucidated according to different fiber orientations.

Effects of substrate material and annealing temperature on morphology of zinc oxide films

A two stage chemical deposition (TSCD) technique is used to produce ZnO films on quartz glass (QG), soda lime glass (SLG) and high purity alumina (HPA) from an aqueous solution of zinc complex. The effects of the substrate material on the chemical composition and morphology of the deposited layer are investigated. The effects of different annealing temperatures (180, 300, 500 and 800°C) on the morphology and orientation of the ZnO crystallites are also determined. X-ray diffraction diffractograms show that above 300°C, the intensity of (002) peak considerably decreases with increasing temperature. Results indicate that changing the substrate from QG to SLG does not significantly influence the chemical composition of the deposited layer. These variations can be attributed to different factors including (a) physorption of cold complex solution, (b) growth behaviour of the nuclei forming on the substrate and (c) the surface energy of the substrate. Owing to the existence of the morphological differences bound to the molecular nature of the substrate, properties of the coatings are found not to be the same. HPA substrate causes clustering of the ZnO crystallites, while QG and SLG substrates do not distinguishably show this effect. Observations show that clustering is intensified by annealing at temperatures > 180°C.

ZrO2-nucleated calcium aluminate glass-ceramics with mid-infrared transparency

A silica-free MgO–CaO–BaO–Al2O3–ZrO2 glass was converted into glass-ceramic by adding zirconia as a nucleating agent. The crystallization and properties of the glass-ceramics were investigated. It was found that ZrO2 is an efficient nucleating agent for the present calcium aluminate glass. Addition of an appropriate amount of ZrO2 changed the crystallization behavior from surface nucleation to internal nucleation. The primary crystalline phase was Ca3Al2O6, which exhibited bulk nucleation. When the crystallizing time was increased, BaAl2O4 formed as a surface layer that grew from the surface into the interior of the sample. The hardness was increased when the parent glass was converted into glass-ceramics. The glass-ceramics without BaAl2O4 surface layer have visible-to-infrared transparency comparable with the parent glass. The transparency was substantially reduced after the development of the surface layer.

Glass–alumina composite coatings by plasma spraying. Part II: Microstructure-based modeling of mechanical properties

The mechanical properties of composite glass–alumina coatings produced by plasma spraying, as described in Part I, were numerically characterized with relation to the peculiar microstructure. Finite element meshes were created for the most significant coating typologies, starting from SEM acquired microstructures. The coatings elastic properties and fracture behaviour were characterized as a function of relevant microstructural features. The results confirm that the coatings are anisotropic, with a lower elastic modulus in the direction perpendicular to the substrate plane (spray direction), because of the lamellar microstructure; increasing the alumina volume fraction increases the elastic modulus value both in the spray and transverse direction. Moreover, it is found that cracks start from large, irregular pores, and propagate easily through the glass areas, but are stopped by alumina. Smaller individual glass areas hinder crack propagation. The post-deposition thermal treatment described in Part I produces tensile residual stresses in the glass and compressive ones in the alumina; thus, the arresting effect of the latter on cracks propagation is greatly enhanced.

Microstructure-based modelling and experimental investigation of crack propagation in glass–alumina functionally graded materials

The aim of the present work was the determination of the fracture mechanisms in glass–alumina functionally graded materials (FGMs). The investigation was performed by means of a combined approach based on microscale computational simulations, which provided for an accurate modelling of the actual FGM microstructure, and experimental analysis. The numerical results proved that microstructural defects, such as pores, deeply influenced the damage evolution. On the contrary, the minimization of the mismatch in the coefficients of thermal expansion of the ingredient materials allowed to obtain low thermal residual stresses, which did not relevantly affect the crack propagation. In order to support the numerical model, microindentation tests were performed on the cross-section of FGM specimens and the experimentally observed crack paths were compared to the computationally predicted ones.

Absorption, emission and absorption saturation of Cr4+ ions in calcium aluminate glass

Absorption, luminescence and absorption saturation of Cr ions in a calcium aluminate glass are studied. In the absorption spectrum, the absorption bands of Cr3+, Cr4+ and Cr6+ ions are revealed. The emission spectrum presents luminescence of Cr3+ ions centered at 0.82μm and that of Cr4+ ions at 1.3μm. The luminescence signal demonstrates short decay times of 120±10ns and 300±20ns for Cr3+ and Cr4+ ions, respectively. Absorption saturation measurements allowed an estimate of the ground-state absorption cross-sections for Cr4+ ions at 1.06μm of 0.7×10−18cm2 and at 0.69μm of 1.5×10−18cm2.

Anodic bonding of glass to aluminium

Anodic bonding of glass to aluminium may provide a higher degree of freedom in device design. In this paper, a systematic variation of the bonding parameters for the aluminium–glass bond is presented. Hermetic seals with strengths of 18.0 MPa can be achieved using a 50–100-nm-thick bonding aluminium layer, and bonding at 300–400°C applying a voltage of 1,000–1,500 V for 20 min. With these parameters, bond yields above 95.1% were obtained on 17 wafers. The bonds survived extensive thermal ageing without significant degradation. The possibility of bonding glass to an aluminium layer with buried, electrically isolated conductors underneath is also demonstrated.

Er 3 + fluorescence in rare-earth aluminate glass

Er 3 + ion fluorescence was excited with a 980-nm pump laser in Er-doped rare-earth aluminate (REAl) glasses with Er-dopant concentrations from 0.5–30mol% (oxides basis). The spectral and decay characteristics were measured at ∼1550nm from Er3+I13∕24 and at ∼2750nm from Er3+I11∕24. Red and green light emissions were also observed, from Er3+F9∕24 and S3∕24+H11∕22, respectively. The fluorescence decay rates are described by a model that yields an accurate fit of results at Er concentrations from 0.5to7mol%. The radiative lifetime of Er3+I13∕24 in Er:REAl glass is 6.12±0.26ms. Hydroxyl ion quenching occurs at a rate given by 9.88×10−20 aOHnEr Hz, where aOH is the glass absorption coefficient (in cm−1) at a wavelength of 2950nm and nEr is the total Er ion concentration. The I13∕24 upconversion rate constant increases with the Er concentration to 1.35+0.05×10−18cm3∕s at and above 7-mol% Er2O3. Er3+I11∕24 fluorescence decays primarily by multiphonon quenching to I13∕24, at 7700±800Hz, a rate that is slightly less than in tellurite glasses. The addition of 20-mol% silica to the glass has only a small influence on the fluorescence decay rates and greatly improves glass formation from the liquid to allow melting and casting of Er-doped REAl glass from platinum crucibles. The application of these Er-doped glasses in laser and optical device applications is briefly discussed.

NMR Heteronuclear Correlation between Quadrupolar Nuclei in Solids

We show for the first time that it is possible to acquire high-resolution heteronuclear NMR correlation experiments in solid state between second-order-broadened half integer quadrupolar nuclei (i.e., 27Al and 17O) using the scalar J-coupling. The sensitivity of the experiment is dramatically improved at high fields (gain proportional to the fourth power of the principal field) with a combination of signal enhancement techniques. This turns a challenging experiment into a real tool. We apply this experiment to characterize a calcium aluminate glass in which we prove the presence of tricluster mu3 oxygen sites and describe the signature of their directly bonded aluminum sites. Applications involve a large range of possible pairs of quadrupolar nuclei in different materials, such as glasses, porous or mesoporous framework materials, zeolites, hybrid organic-inorganic, and bioinvolved materials.

Thermal lens determination of fluorescence quantum efficiency of3F4level of Tm3+ions in solids

In this work, a method based on thermal lens spectrometry is proposed to determine the fluorescence quantum efficiency (η) of the 3 F 4 level (∼1.8μm emission) of Tm-doped solids. It was applied to Tm-doped water free low silica calcium aluminate glasses. The obtained η value for the glasses with high Tm content was approximately 30 %, which is in agreement with the achieved η values from ratio between radiative and experimental lifetimes.

Fluorescence quantum efficiency of Er 3+ in low silica calcium aluminate glasses determined by mode-mismatched thermal lens spectrometry

The fluorescence quantum efficiency of Er 3+ in low silica calcium aluminate glasses, with nominal composition 58 CaO, 27.1 − x Al 2O 3, 6.9 MgO, 8 SiO 2, x Er 2O 3, 0.1 ⩽ x ⩽ 1.5 (mol%), melted in air and under vacuum, has been measured using the mode-mismatched thermal lens spectrometry, with excitation beam wavelength at 488 nm and 804 nm. From 0.1 up to 1.1 mol% of Er 2O 3 the quantum efficiency is around 0.65 for vacuum-melted samples, and around 0.46 for air-melted ones. The quenching of the quantum efficiency appears above 1.3 mol% of Er 2O 3. The thermal diffusivity was also obtained, with results showing a decrease from 5.69 × 10 −3 cm 2/s (undoped sample) to 4.78 × 10 −3 cm 2/s (for the highest doped sample, 1.5 mol% of Er 2O 3).

Press-Molding of High-Alumina Ceramic Castables. 1. Compaction and Properties of Matrix Systems Based on Mixed HCBS of Composition: Bauxite, Quartz Glass, and Commercial Alumina

Preparation of powders based on bauxite HCBS (Highly Concentrated Ceramic Binding Suspensions) containing high-disperse quartz glass and plasticized with refractory clay is described. The powders, molded at moderate pressure (50 – 100 MPa), give a high-density material with an initial porosity of 20 – 25%. Mixes containing 10 to 43% commercial alumina are tested. The materials can be used as matrices for press-molded high-alumina ceramic castables which, when sintered, show a high mechanical strength and volume constancy.

Microscale computational simulation and experimental measurement of thermal residual stresses in glass–alumina functionally graded materials

Glass–alumina functionally graded materials are new attractive composite materials, that can achieve peculiar mechanical properties due to their gradual compositional variation. Nevertheless, the difference between the coefficients of thermal expansion of the constituent phases may result in significant thermal residual stresses in service or during fabrication. A proper glass formulation can minimize the mismatch in thermo-mechanical properties, thus relevantly reducing the mean value of the resultant thermal stresses. However, it is a crucial requirement to evaluate the effect of microstructural discreteness and randomness on the actual stress distribution in functionally graded materials. With this aim, a computational model which applies the finite element method at the microscale is used. The careful modelling of the real microstructural details enables to accurately predict the local stress values and distribution. In order to verify the reliability of the computational simulations, the residual thermal stresses were also experimentally measured by means of a piezo-spectroscopic technique. The comparison between the numerical and the experimental results validate the microstructure-based model.

Thermal and mechanical properties of rare earth aluminate and low-silica aluminosilicate optical glasses

Aluminate glasses containing 45–71.5 mol% alumina, 10–40 mol% rare earth oxide, and 0–30 mol% silica were synthesized from precursor oxides. The glass transition and crystallization temperatures were determined by differential scanning calorimetry; the structural and mechanical properties were investigated by Raman and Brillouin spectroscopy. The range of the supercooled liquid region varies from ∼40 °C to 200 °C, providing a useful working range for compositions with 5–30 mol% silica. Raman scattering showed the presence of isolated SiO 4 species that strengthen the network-forming structure, enhance glass formation, and stabilize the glass even when they are present at fairly low concentrations. Sound velocities were measured by Brillouin scattering. From these and other values, various elastic moduli were calculated. The moduli increased with both aluminum and rare earth content, as did the hardness of the glasses. Young’s modulus was in the range 118–169 GPa, 60–130% larger than that for pure silica glass.

Effect of network modifiers on spectroscopic properties of erbium-doped phosphate glasses

The integrated absorption cross section Σabs, peak emission cross section σemi, Judd-Ofeld intensity parameters Ωt (t=2,4,6), and spontaneous emission probability ArR of Er3+ ions were determined for Erbium doped alkali and alkaline earth phosphate glasses. It is found the compositional dependence of σemi is almost similar to that of Σabs, which is determined by the sum of Ωt(3Ω2 + 10Ω4 + 21Ω6). In addition, the compositional dependence of Ωt was studied in these glass systems. As a result, compared with Ω4 and Ω6, the Ω2 has a stronger compositional dependence on the ionic radius and content of modifiers. The covalency of Er-O bonds in phosphate glass is weaker than that in silicate glass, germanate glass, aluminate glass, and tellurate glass, since Ω6 of phosphate glass is relatively large. AR is affected by the covalency of the Er3+ ion sites and corresponds to the Ω6 value.