Determination of Axial Stress in Clad Glass Fibers

The axial stress in clad glass fibers was measured by photoelastic techniques and compared with calculated values. In general, there was good agreement when the hydrostatic stress development was included in the calculation. The systems studied were primarily those which would produce a surface compression. Fibers of a soda-lime-silica glass clad with a high-silica glass gave the largest surface compression, 46,000 psi.     

Diameter Dependence of the Refractive Index of Melt-Drawn Glass Fibers

The diameter dependence of the refractive index of as-formed glass fibers is examined. It is shown that the measured results on fibers of a soda–lime–silica glass can be satisfactorily explained by combining three factors: the diameter dependence of the cooling rate, the cooling rate dependence of the fictive temperature, and the dependence of glass properties on its fictive temperature.     

Strengthening of Glass Fibers: I,Cladding *

Experiments on development of internal stress in fibers by cladding borosilicate glass with 96% silica glass are reported. The axial compression in the cladding, determined photo-elastically, agreed well with the increased strength determined in axial tension fracture experiments. In turn, these results agreed well with the axial compressive stress predicted by analysis when proper allowance was made for stresses which can develop when the core is fluid and the cladding rigid.     

Modeling birefringence in SiO2 glass fiber using surface stress relaxation

The retardance of silica glass fibers was evaluated using photoelastic techniques. Here, surface birefringence in glass fibers is shown to be a consequence of surface stress relaxation for as-received fibers drawn from Suprasil II. The surface features of the birefringent fibers were compared to a model of the residual axial stress profile resulting from a diffusion-controlled surface stress relaxation. Additionally, a uniform birefringence in the fiber equivalent to a constant tensile stress was recognized and attributed to structural anisotropy produced during fiber drawing. The contribution of structural anisotropy to the observed birefringence remained constant as the surface features were successively etched away. Surface compressive stress generation was also observed, as retardance corresponding to a surface compressive stress was found to increase with applied tensile stress during short heat treatments. Significant features of the retardance profile in as-received silica glass fibers, with a thin surface compressive stress layer and compensating interior tensile stress, agreed with the residual stress profiles predicted by the surface stress relaxation model after correcting for this observed structural anisotropy.     

Nonlinear Elasticity of Silica Glass

In situ Brillouin light scattering measurements with a spatial resolution of ~1 μm have been carried out to study the elastic moduli of silica glass fibers in a single two‐point bend experiment to nominal strains of 7% in both tensile and compressive regions. Such data are necessary in order to convert the failure strains obtained from two‐point bend experiments into failure stress. For the first time, the neutral axis shift in a bent silica glass fiber was observed in our measurements, with more of the fiber deforming in compression than in tension, resulting from the nonlinear elastic behavior of silica glass. Understanding the neutral axis shift will improve the accuracy of strain and stress calculations in bent fibers. This study shows that an expression including a fifth‐order term is required to capture both the minimum in compression and the maximum in tension in the strain‐dependent Young's modulus of silica glass. A stress versus strain relation over a broad range of compressive and tensile strains was established for silica glass in this study, which will significantly improve our understanding of its deformation behavior.     

Use of CsCl to Enhance the Glass Stability Range of Tellurite Glasses for Er3+-Doped Optical Fiber Drawing

Tellurite glasses are important as a host of Er³⁺ ions because of their good solubility and because they present broadband optical gain compared with Er³⁺-doped silica, with the potential to increase the bandwidth of communication systems. However, the small glass stability range (GSR) of tellurite glasses compromises the quality of the optical fibers. We show that the addition of CsCl to tellurite glasses can increase their GSR, making it easier to draw good-quality optical fibers. CsCl acts like a network modifier in glass systems, weakening the network by forming Te–Cl bonds. We show that the thermal expansion coefficient mismatch is in the right direction for optical fiber fabrication purposes and that the Bi₂O₃ content can be used to control the refractive index of clad and core glasses. Single-mode and multi-mode Er³⁺-doped optical fibers were produced by the rod-in-tube method using highly homogeneous TeO₂–ZnO–Li₂O–Bi₂O₃–CsCl glasses.     

Electron energy loss spectra from silica glass optical fibers

To investigate the possible structural differences between silica glass fibers and bulk silica glasses, electron energy loss spectroscopy (EELS) has been used to study the short-range and medium-range structures of both forms of silica glasses. The short-range structure of silica glass, such as the coordination and symmetry, was investigated by the energy loss near edge structure (ELNES) of Si L_2,3-edges. The ordering structure in the medium-range was analyzed by the exponential optical absorption edge also known as the Urbach edge of the glasses. The optical absorption data were obtained from the low energy loss spectrum of EELS through Kramers-Kronig analysis. The results show that silica fiber has the same short-range structure as the bulk specimen, but is significantly more disordered than the bulk glasses.     

Dry Powder Processing of Fibrous Fumed Silica Compacts for Thermal Insulation

This study proposes a new dry powder processing method to produce fiber reinforced fumed silica compacts for highly efficient thermal insulation. First, mechanical forces such as shear stress and compression were applied into a fumed silica powder (BET SSA 300 m²/g) and glass fibers (3 mm length, 11 μm diameter) using an attrition type milling apparatus without any media balls. In the resultant powder mixture, fumed silica porously coated glass fiber composites were successfully prepared. SiC particles were also integrated as opacifiers. Then, the fibrous fumed silica porous compact was formed by dry pressing of the powder mixture and exhibited fracture strength of 1.58 MPa at room temperature and thermal conductivity of 0.0282 W/mK at 400°C, when its porosity was 80.1%.     

Photoelastic confirmation of surface stress relaxation in silica glasses: Fiber bending and rod torsion

Silica glass samples were given various heat treatments under stress at low temperatures and subsequently their residual stress distributions in terms of retardance were observed using a polarized light microscope, confirming previously reported fast surface stress relaxation while providing more detailed characterization. Retardance profiles of silica glass fibers heat-treated under a constant bending strain in the presence of atmospheric water vapor were measured and fit to a previously developed diffusion-based relaxation model. The retardance of a cross-section of a silica glass rod heat-treated at 650°C in lab air under applied torsional shear strain was also measured to confirm the presence of residual surface shear stress which was predicted by the decrease of torque with time for the rod. Together, these results confirm the low-temperature fast surface stress relaxation which occurs when water vapor is present for both bending and shear stresses.     

Characterization of biodegradable core–clad borosilicate glass fibers with round and rectangular cross-section

Here, we report on core–clad bioactive borosilicate fibers, that we have prepared both with round and rectangular cross-section profile. The exposed approach, which relies on the stacking and drawing of glass slabs, demonstrates our ability to develop bioactive-based glass fibers with tailored cross-section profiles. Tens-of-meters-long fibers were successfully drawn, although suffering from elevated losses in the case of the rectangular ones. The response of the fibers in simulated body fluid was studied for both geometries. We found that a round cladding can act as protective layer, tempering effects of the corrosion. We also noticed that rectangular fibers are more prone to degradation, the enhanced corrosion beginning from their sharp corners as they accumulated residual tensile stress during drawing. To the best of our knowledge, this is the first report on the effect of residual tensile stresses from surface tension deformations applied to the corrosion of rectangular fibers. As geometry plays a critical role on the biodegradation behavior of the fiberglass, we believe the enclosed results could lead to the design of fiber devices with tailored cross-section profile in order to tune their rate of degradation on solely based geometrical effects.     

Fabrication and Characterization of Glass‐Ceramic Fiber‐Containing Cr3 + ‐Doped ZnAl2O4 Nanocrystals

Glass‐ceramic fibers containing Cr³⁺‐doped ZnAl₂O₄ nanocrystals were fabricated by the melt‐in‐tube method and successive heat treatment. The obtained fibers were characterized by electro‐probe micro‐analyzer, X‐ray diffraction, Raman spectrum and high‐resolution transmission electron microscopy. In our process, fibers were precursor at the drawing temperature where the fiber core glass was melted while the clad was softened. No obvious element interdiffusion between the core and the clad section or crystallization was observed in precursor fiber. After heat treatment, ZnAl₂O₄ nanocrystals with diameters ranging from 1.0 to 6.3 nm were precipitated in the fiber core. In comparison to precursor fiber, the glass‐ceramic fiber exhibits broadband emission from Cr³⁺ when excited at 532 nm, making Cr³⁺‐doped glass‐ceramic fiber a promising material for broadband tunable fiber laser. Furthermore, the melt‐in‐tube method demonstrated here may open a new gate toward the fabrication of novel glass‐ceramic fibers.     

Reduction in internal friction in silica glass with high OH content

The aim of this article was to investigate processes occurring during annealing of silica glass classified as being of type III (J Non Cryst Solids. 1970;5(2):123-75). This is an inexpensive silica glass produced by many manufacturers across the globe. However, it can be successfully used for fabrication of high-Q mechanical resonators. The relationship between residual internal stress and internal friction is elucidated. Quantitative analysis of the structural relaxation kinetics is presented. The influence of the cooling process for structural transformation is also discussed. On the basis of our results, we suggest optimal annealing conditions for minimizing internal friction type III silica glass. The results will be useful for further improvement of the Q-factor of mechanical resonators, including the test masses of the next generation of gravitational wave detectors. Our approach might, in addition, be used for studying the modification of atomic structure in multicomponent glasses.     

The Heterogeneity of Glass Surfaces Revealed by Temperature Programmed Desorption

Adsorption of butanol and pyridine on E‐glass fibers with three different compositions, as well as on powders of silica and the crushed fibers, was studied by temperature programmed desorption (TPD) with a mass‐sensitive detector. In the case of butanol, there are two types of desorbing molecules: at lower temperatures butanol desorbs, but in the range 450°C–600°C, 1‐butene desorption is also observed. It is shown that 1‐butene desorption is due to thermal decomposition of butanol chemisorbed to OH groups on both the glass and silica surfaces. The binding energy distributions of adsorption sites for butanol and pyridine are similar on all three glass compositions, but they are much more heterogeneous compared to silica; this difference is most evident for pyridine and is attributed to the presence Al and B in the glasses. The decomposition temperature of chemisorbed butanol is highest for silica and depends on glass composition for the fibers and powders. Interestingly, the glass which does not contain boron shows a well‐defined peak for the decomposition of chemisorbed butanol, suggestive of unique adsorption sites on this boron‐free surface; but they are much less temperature stable than the chemisorption sites on silica. In situ exposure to water vapor increased the number of active sites for chemisorption.     

Surface Crystallization and Water Diffusion of Silica Glass Fibers: Causes of Mechanical Strength Degradation

Pristine silica glass fiber is well‐known to become mechanically weaker when heat‐treated in air but the cause of such weakening is not presently known. The time dependence of mechanical degradation of various silica glass fibers containing varying impurity contents were studied in the range from 500°C to 1000°C. Two possible sources of strength degradation were considered: surface crystallization and water diffusion. Surface crystallization kinetics of silica glass fibers were investigated in a wide temperature range, including nanoscale surface nucleation at low temperatures via scanning electron microscopy. From the comparison of the strength degradation, surface crystallization, and water diffusion data in literature, it was concluded that surface crystallization may be responsible for the mechanical weakening observed in silica glass fiber surface during heat‐treatment at temperatures above ~800°C, whereas water diffusion into the glass surface may be responsible for the strength degradation at lower temperatures.     

Stresses Arising During the Leaching of a Two-Phase Glass

The stresses which arise when one phase of a two-phase alkali borosilicate glass rod is chemically removed are examined. Three mechanisms which may account for these stresses are described: thermal expansion coefficient differences between the phases, hydration of the porous glass, and ion exchange within the leached layers. The first two effects induce eompres-sive stresses, whereas the latter causes tension in the clads of partially leached glasses. Expressions for the stresses are developed in terms of glass properties and specimen geometry. Experimental photoelastic data on stress profiles, the effects of clad thickness, and heat-treatment conditions in two compositions are presented and compared with theoretical predictions. The stresses, whether tensile or compressive, are constant as a function of the radius within the clad, but are strongly dependent on clad thickness. At a given heat-treatment temperature, compressive stresses increase with time until a constant value is reached. This constant value increases with decreasing heat-treatment temperature.     

Fabrication of homogeneous mullite-based fiber porous ceramics with high strength and porosity

Silica sol is widely used in the preparation of mullite-based fiber porous ceramics (MFPCs), but it aggregates at the top surface of MFPCs during the drying process. This leads to the decrease in mechanical strength and porosity. To overcome the problem and fabricate homogeneous MFPCs, the sodium silicate solution and glass fibers were applied in the fabrication process of MFPCs. The effects of concentrations of sodium silicate solution and silica sol, amounts of glass fibers and sintering temperatures on the properties of prepared MFPCs were studied. The sodium silicate solution consolidated the silica sol and mullite fibers, forming a homogeneous structure and ensuring the even distribution of silica sol. Compared with other reported MFPCs, this process required low sintering temperature while maintaining high compressive strength (2.14 MPa) and porosity (75.93%). This study provides an effective method for preparing MFPCs with high strength, uniformity and porosity.     

Effect of Stress on Water Diffusion in Silica Glass

The diffusion of water in silica glass was measured as a function of applied uniaxial stress and hydrostatic pressure at selected temperatures. It was found that the diffusion coefficient of water increased exponentially with increasing tensile stress and decreased with increasing compressive stress and increasing hydrostatic pressure. The activation volume for water diffusion in silica glass was found to be ∼170 cm³/mol at 192°C and ∼72 cm³/mol at 350°C. The solubility of water in glass showed a trend opposite to the diffusion coefficient, namely, it decreased exponentially with increasing tensile stress and increased with increasing compressive stress and hydrostatic pressure. These stress (or pressure) dependences were attributed to the shift of the glass-water reaction equilibrium.     

Thermal conductivity of glass fiber-reinforced silica aerogels using molecular dynamics simulations

Silica aerogels are low-density, low-thermal conductivity, and highly porous solids used in a wide range of applications. In the present work, the glass fibers-reinforced silica aerogels nanocomposite models were investigated for thermal conductivity using molecular dynamics (MD) simulations. Here, the glass fibers weight percentage (% wt) was varied from 2.14 to 21.20%, i. e., the density of the silica aerogel nanocomposite ranged from 252 to 307 kg m^?3. The thermal conductivity increases with the % wt of glass fibers for the considered range. However, the increase was insignificant, i. e., from 0.0357 to 0.048 W m^?1 K^?1. Moreover, the thermal conductivity varies with the density of nanocomposite according to a power-law with an exponent of 1.90 ± 0.18. Furthermore, the obtained thermal conductivity values of silica aerogels and their nanocomposites are in good agreement with experimental and computational studies. The outcome shows that the glass fibers-reinforced silica aerogels nanocomposites can be used for low-thermal conductivity applications.     

High Temperature Resistant Environment-Friendly Silica Fibers and Materials

Methods for the preparation of silica fibers by chemical leaching of glass fibrous are described. The materials in question are composed of high-melting oxides (94 – 96 wt.% SiO₂ and 3.5 – 4.0 wt.% Al₂O₃) and display superior properties: service temperature up to 1000°C, melting point 1650 – 1700°C, heat conductivity 0.082 – 0.11 W/(m · K) at 50°C. The materials are stable on exposure to water, hydrocarbons, ammonia hydroxide, hydrochloric, nitric and sulfuric acids, high temperature, pressure, and radiation. The silica materials are commercially available as fabrics, gauzes, ribbons, filaments, fibers, and blankets.__________Translated from Novye Ogneupory, No. 12, pp. 51 – 53, December, 2004.     

Differences in indentation and wear behaviors between the two sides of thermally tempered soda lime silica glass

Thermal tempering is an industrial process widely used to make soda lime silica (SLS) glass panels stronger and tougher. During the tempering process, the upper and bottom sides of the glass may experience different cooling rates, and thus, their properties could be different. This study characterized changes in surface composition and subsurface glass network structures as well as indentation and wear resistance properties of the air- and tin-sides of 6-mm-thick SLS window panels faced toward the upper and sliding roller sides during thermal tempering. The results showed that although the chemical and structural differences detected with X-ray photoelectron spectroscopy and specular reflection infrared spectroscopy are subtle, there are large differences in nanoindentation behaviors and mechanochemical wear properties of the SLS glass surface. The findings of this study provide further insights into the performance difference between the air- and tin-sides of the SLS glass panel treated with thermal tempering.