Seunggu Kang

Optically Transparent Polymethyl Methacrylate Composites made with Glass Fibers of Varying Refractive Index

The dependence of the optical and mechanical properties of optically transparent polymethyl methacrylate (PMMA) composites on the annealing temperature of BK10 glass fibers was investigated. Annealing was used to modify the refractive index (R.I.) of the glass fiber so that it would more closely match that of PMMA. Annealing increased the refractive index of the fibers and narrowed the distribution of refractive index of the fibers, but lowered their mechanical strength so the mechanical properties of composites reinforced with annealed fibers were not as good as for composites containing as-pulled (chilled) glass fibers. The refractive index of as-pulled 17.1 μm diameter fibers (R.I. = 1.4907) increased to 1.4918 and 1.4948 after annealing at 350 °C to 500 °C for 1 h or 0.5 h, respectively. The refractive index of glass fibers annealed at 400 °C/1 h best matched that of PMMA at 589.3 nm and 25 °C, so the composite reinforced with those fibers had the highest optical transmission. Because annealed glass fibers had a more uniform refractive index than unannealed fibers, the composites made with annealed fibers had a higher optical transmission. The mechanical strength of annealed fiber/PMMA composites decreased as the fiber annealing temperature increased. A composite containing fibers annealed at 450 °C/1 h had a tensile strength 26% lower than that of a composite made with as-pulled fibers, but 73% higher than that for unreinforced PMMA. This decrease was avoided by treating annealed fibers with HF. Composites made with annealed and HF (10 vol. %)-treated (for 30 s) glass fibers had a tensile strength (∼200 MPa) equivalent to that of the composites made with as-pulled fibers. However, as the treatment time in HF increased, the tensile strength of the composites decreased because of a significant reduction in diameter of the glass fiber which reduced the volume percent fiber in the composite.

Measurement of the Refractive Index of Glass Fibers by the Christiansen-Shelyubskii Method

The Christiansen principle was employed to measure the refractive index of borosilicate glass fibers (13–41 μm diameter) over the visible range. The refractive index for glass fibers at 589.3 nm was measured by temperature and wavelength scan and values obtained were in close agreement. The refractive index for glass fibers as a function of wavelength was measured to an accuracy of < 10−4. The uniformity of the refractive index for a bundle of fibers of slightly different diameter was calculated using the modified Shelyubskii method and compared to experimental values. Theoretical calculations of the transmission by the present work suggest that, for high optical clarity and transmission of Christiansen cell (or transparent composite consisting of glass fiber and polymer), the refractive index must be controlled to the fifth decimal place. For example, the maximum transmission of a fiber/liquid mixture cell at 25°C can increase from 89 to 97% when the standard deviation is reduced from 13 × 10−5 to 9 × 10−5.

Microstructure of Glass-ceramics Made from Bottom Ash Produced at a Thermal Power Plant

Glass ceramics were made from coal bottom ash by adding CaO and Li O as glass modifiers and TiO as a nucleating agent in a process of melting and quenching followed by a thermal treatment. The surface of the glass ceramics has 1.6 times more Li O compared to the inner matrix. When TiO was not added or when only 2 wt% was added, the surface parts of the glass ceramics were crystalline with a thickness close to 130 µm. In addition, the matrixes showed only the glass phase and not the crystalline phase. However, doping of TiO from 4 wt% to 10 wt% began to create small crystalline phases in the matrix with an increase in the quantity of the crystalline. The matrix microstructure of glass ceramics containing TiO in excess of 8 wt% was a mixture of dark-gray crystalline and white crystalline parts. These two parts had no considerable difference in terms of composition. It was thought that the crystallization mechanism affects the crystal growth, direction and shape and rather than the existence of two types of crystals.

Mechanical properties of optically transparent, multi-layered laminates

The objective of this investigation was to study how the mechanical properties of an optically transparent composite varied with the geometrical arrangement, stacking sequence, of polymethylmethacrylate (PMMA) (designated as P) and composite (designated as C) layers. The multi-layered composites (about 6.63 mm thick) were highly transparent between 22 to 46°C in the visible region. As expected, the sandwich structure, (CCPP)s had the highest Youngs modulus while (PCCP)s and (PPCC)s composites had the highest flexural strength and work of fracture, respectively. The flexural strength of these laminated composites, which contained only 0.8 vol % fibre without any coupling agent, was up to 21% higher than that of pure PMMA.The stress distribution through the thickness at the midpoint of a sample loaded in three-point bending was computed by the finite element method (FEM). The computed stress distribution allowed the expected point of failure to be established. The relationship between the stacking sequence, stress level under a given load, and strength was also investigated. The observed fracture modes were complex and the maximum stress failure criterion did not fit these composites. The fracture was always complex (tensile and shear), starting with tensile failure followed by shear mode (delamination) and another tensile mode. The first crack always commenced at a PMMA layer adjoining the composite layer which contained the highest stress. The optimum stacking sequence when such composites are used as a window is concluded to be (PCCP)s, since this sequence had the highest flexural strength (141 MPa) and a moderate work of fracture (37 kJ m−2).

Influence of Li 2 O Addition on Physical Properties of Glass-Ceramics Fabricated Using a Coal Bottom Ash

Glass-ceramics were fabricated by heat-treatment of glass obtained by melting a coal bottom ash with Li2O addition. The main crystal grown in the glass-ceramics, containing 10 wt% Li2O, was β-spodumene solid solution, while in Li2O 20 wt% specimen was mullite, identified using XRD. The activation energy and Avrami constant for crystallization were calculated and showed that bulk crystallization behavior will be predominant, and this expectation agreed with the microstructural observations. The crystal phase grown in Li2O 10 wt% glass-ceramics had a dendrite-like shaped whereas the shape was flake-like in the 20 wt% case. The thermal expansion coefficient of the Li2O 10 wt% glass-ceramics was lower than that of the glass having the same composition, owing to the formation of a β-spodumene phase. For example, the thermal expansion coefficient of Li2O 10 wt% glass-ceramics was 20 × 10 -7, which is enough for application in various heatresistance fields. But above 20 wt% Li2O, the thermal coefficient expansion of glass-ceramics, on the contrary, was higher than that of the same composition glass, due to formation of mullite.