This review is concerned with the properties and structure of silica glass. The following topics are treated: Types of silica glasses; The vitreous state of silica glasses: thermodynamical approach, atomistic approach; Optical properties; absorption and fluorescence, refractive index and homogeneity; Mechanical and thermal properties: specific volume, volume relaxation, volume and pressure, elastic and internal friction behaviour, heat capacity and heat conduction, strength, crystallization.

/pdf/properties-and-structure-of-vitreous-silica-i-3zzb68vkny.pdf

Properties and structure of vitreous silica. I

In recent years, highly sensitive pressure sensors that are flexible, biocompatible, and stretchable have attracted significant research attention in the fields of wearable electronics and smart skin. However, there has been a considerable challenge to simultaneously achieve highly sensitive, low-cost sensors coupled with optimum mechanical stability and an ultralow detection limit for subtle physiological signal monitoring devices. Targeting aforementioned issues, herein, we report the facile fabrication of a highly sensitive and reliable capacitive pressure sensor for ultralow-pressure measurement by sandwiching MXene (Ti3C2Tx)/poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) composite nanofibrous scaffolds as a dielectric layer between biocompatible poly-(3,4-ethylenedioxythiophene) polystyrene sulfonate /polydimethylsiloxane electrodes. The fabricated sensor exhibits a high sensitivity of 0.51 kPa-1 and a minimum detection limit of 1.5 Pa. In addition, it also enables linear sensing over a broad pressure range (0-400 kPa) and high reliability over 10,000 cycles even at extremely high pressure (>167 kPa). The sensitivity of the nanofiber-based sensor is enhanced by MXene loading, thereby increasing the dielectric constant up to 40 and reducing the compression modulus to 58% compared with pristine PVDF-TrFE nanofiber scaffolds. The proposed sensor can be used to determine the health condition of patients by monitoring physiological signals (pulse rate, respiration, muscle movements, and eye twitching) and also represents a good candidate for a next generation human-machine interfacing device.

Wearable Capacitive Pressure Sensor Based on MXene Composite Nanofibrous Scaffolds for Reliable Human Physiological Signal Acquisition.

The 630-nm loss band recently identified in unclad fibers made from low-OH-content vitreous silica is shown to be introduced in the fiber-drawing process. The band has an absorptive component as well as resonance fluorescence whose peak intensity amounts to about 16% of the total losses. A red fluorescence was observed when the same fibers were illuminated with the 325-nm line of a HeCd laser. The loss band could be eliminated by annealing for 3 h at 700 °C. Spontaneous annealing at room temperature was also observed in some cases. It is proposed that the 630-nm band is associated with drawing-induced defects of the silica network.

Drawing-induced coloration in vitreous silica fibers

Fibre-hybrid composites are composed of two or more fibre types in a matrix. Such composites offer more design freedom than non-hybrid composites. The aim is often to alleviate the drawbacks of one of the fibre types while keeping the benefits of the other. The hybridisation can also lead to synergetic effects or to properties that neither of the constituents possess. Even though fibre-hybrid composites are attractive, they also pose more challenges in terms of materials selection than conventional, single fibre type composites. This review analyses the mechanisms for synergetic effects provides guidance on the fibre and matrix selection and describes recent opportunities and trends. It finishes by describing the current applications, and by contrasting how the industrial use is different from the academic research.

Recent advances in fibre-hybrid composites: materials selection, opportunities and applications

Frozen-in stresses and strains can significantly impact both the optical and mechanical performance of optical fibers, enabling unique functionalities or leading to serious impairments. Frozen-in strains can be grouped into two general categories: those directly inducing strong birefringence, which are associated with residual elastic stresses; and those inducing a substantially isotropic index perturbation, which can be described as inelastic frozen-in strains. Both types of frozen-in strains can be simultaneously present in the same optical fiber. Frozen-in viscoelasticity was only recently found to be an important draw-induced inelastic strain that can significantly perturb the refractive index profile and hence the waveguiding properties of optical fibers. The optical and mechanical effects of both types of frozen-in stresses and strains are reviewed. Both practical applications as well as impairments are discussed.

Optical and mechanical effects of frozen-in stresses and strains in optical fibers

The measured strength of a glass fiber is associated with the special circumstances attending its formation. Experimental evidence is presented which shows that, contrary to generally accepted belief, the strength of a fiber does not depend on fiber diameter. When fibers of different diameter are formed under controlled, nearly identical conditions, the breaking strengths are identical within the experimental limits and there is no significant effect of diameter as such.

Summary
(1) The properties of glass fibers, including strength, are associated with the special circumstances attending their formation.

(2) In the past, fibers of different diameters were inevitably made under different forming conditions; it was found necessary in hand drawing to heat the glass to higher temperatures and to pull faster to obtain fibers of small diameter.

(3) In the past, therefore, fibers of different diameters were reported as having different strengths (i.e., breaking stresses, not breaking loads), and this difference was ascribed to diameter as such without considering whether it might be due to different forming conditions.

(4) When fibers of different diameter are formed under controlled, nearly identical conditions, the breaking strengths are identical within the experimental limits and there is no significant effect of diameter as such.

Relationship of Tensile Strength of Glass Fibers to Diameter

Glass fibers as commercially produced have physical and mechanical properties which differ from those of the massive form of the same glass. When these fibers are subjected to subsequent heat-treatments, the values of the measured properties tend to return to those of the massive glass. This phenomenon is called compaction because in the process density increases. The effect of compaction on the physical properties of fibers is discussed and the mechanism of this phenomenon is described briefly.

William H Otto

Papers

Relationship of Tensile Strength of Glass Fibers to Diameter

Compaction Effects in Glass Fibers

Method of coating glass filaments with metal

Metal coated glass fibers

Method and apparatus for applying metal to glass