New LED fluorescence technology for the development of ceramic fibers
Spinning preparations are mixtures of various chemical components that are applied to the surface of fibers. Specific characteristics of fibers or yarns can be influenced in this way. It is common, for example, to the fibers a high slip resistance or antistatic properties. Spinning preparations reduce fiber damage in the further processing procedure and enable high production speeds.
Spinning preparation also plays an important role in the development of ceramic fibers at the DITF. Here, the green fibers, i. e. the still unfired precursor fibers, are treated with a preparation in order to keep the mechanical stresses during further processing to a minimum. This is because ceramic-forming green fibers do not yet exhibit high mechanical stability. Their processing is therefore challenging and requires coordinated process control. For the quality of the subsequent process steps, it is important to apply the spinning preparation evenly, but checking this application is difficult: the translucent preparation on the equally light-colored fibers is barely visible under the light microscope. And the thin films on the fibers are also difficult or impossible to image under an electron microscope.
This is where fluorescence microscopy can help: it is based on the fact that certain molecules (fluorophores) irradiated light of a certain wavelength at a different (longer) wavelength. Using suitable filter combinations, it is possible to filter out only this emitted light. The bright excitation light is excluded from the imaging. The fact that fluorescence microscopy is performed against a dark background means that even low concentrations of fluorophores can clearly imaged.
The spinning preparations used in ceramic fiber production do not fluoresce by themselves. A suitable dye (fluorochrome) must be added to them. Very low concentrations are sufficient here, but the fluorochrome must be distributed homogeneously in the spinning preparation. The dye must therefore meet special requirements. For decades, the very bright light used for fluorescence excitation in fluorescence microscopes generally came from mercury vapor lamps. These emit light across the entire visible spectrum, right into the UV range. Suitable filter combinations can be used to filter out the necessary range for the required excitation and emission from this broad spectrum.
Recently, this solution has been replaced by modern LED technology. The DITF have also retrofitted an LED fluorescence device (Colibri) manufactured by Carl Zeiss. The advantages are immense: exactly the right wavelength for excitation of the fluorescent dye can be selected from the outset. In addition, the brightness of the excitation can be regulated. It is even possible to mix different color channels and thus enable optimal imaging of the sample. For, pure UV excitation can be mixed with light from the visible range. For example the green fibers, the Carl Zeiss Colibri device on a new Axioscope 5 reflected light microscope has proven to be ideal. Under UV excitation, the distribution of the spinning preparation over the entire green fiber bundle can be easily visualized. This allowed any inhomogeneities to be made visible and the preparation distribution to be optimized.
