D. Hiilsenberg et al Composites: Part B 39(2008)362-373 30mmN8650matⅸ 5 nm BN+30 nm Tio2: N- SK4 matrix wavelength [nm Fig. 17. Comparison of the optical transmittance of composites made om Nextel 440fibers and N-SK 4 or 8650 matrix-glass. The fiber volume Fig. 18. Optica oscopic picture(dark field) of the polished surface of the Nextel 440/40 nm BN/N-SK 4 composite. from 0.2 to 20 um. Rayleigh scattering will occur in the case of nanoheterogeneities Now, which types of heterogeneities can be expected to occur in the composite? interfaces between the fibers and the matrix; they should be invisible if the refractive indices are identical or the coating of the fibers is transparent and thin enough; bubbles in the glass matrix if the glass powder was not completely densified during hot pressing: such bubbles are of a size of some um, which can be concluded from the size of the glass powder and the gases caught between the particles. Their formation is probable bubbles resulting from the reactions which take place between BN and the matrix glasses; diameters ranging from 50 to 500 nm are expected As both types of bubbles may probably develop in the Fig. 19. Optical microscopic picture(dark field)of one BN-coated fiber in composite,both Mie and Rayleigh scattering must be a Nextel 440/40 nm BN/N-SK 4 composite taken into consideration, which are predominant in the case of shorter wavelengths and lead to a deterioration in transmission the long-wavelength range, the transmission of the trans- As can clearly be seen from Figs. 18 and 19, micro- and parent composite represented in Fig 17 will reach 60%at nanobubbles are present, both for the composite Nextel a wavelength of 2 um 440/40 nm BN/N-SK4. In particular, Fig. 18 shows bubbles in the glass matrix, whereas bubbles at the interface between 5 Summary BN and the matrix can be recognized in both figures The protection of bn through TiO,(double layer) -It is possible to prepare a transparent, damage-tolerant against reactions with the matrix glass will reduce the composite on the basis of Nextel 440 fibers and the anobubbles; a higher pressure(15 MPa)applied during Schott glass type 8650 hot pressing will have a reducing effect on microbubbles. The transmission of this composite reaches 45% in the A sophisticated time-temperature-pressure-atmosphere visible wavelength range. and 65% in near infrared regime during hot pressing could bring about good trans- parency. Exactly adapted optical refractive indices and thermal If the investigations are carried out again with comp expansion coefficients are the key to optically transpar ites with double-coated Nextel 440 fibers and the 8650 glass matrix, better transmission should be achieved in the visi-- Bending strengths of 200 MPa and a fracture toughness ble wavelength range as most probably no nanobubbles (not described here)of up to 6 MPa m 2 were measured will develop. By the way, if the curve is extrapolated into for several different damage-tolerant compositesfrom 0.2 to 20 lm. Rayleigh scattering will occur in the case of nanoheterogeneities. Now, which types of heterogeneities can be expected to occur in the composite? – interfaces between the fibers and the matrix; they should be invisible if the refractive indices are identical or the coating of the fibers is transparent and thin enough; – bubbles in the glass matrix if the glass powder was not completely densified during hot pressing; such bubbles are of a size of some lm, which can be concluded from the size of the glass powder and the gases caught between the particles. Their formation is probable; – bubbles resulting from the reactions which take place between BN and the matrix glasses; diameters ranging from 50 to 500 nm are expected. As both types of bubbles may probably develop in the composite, both Mie and Rayleigh scattering must be taken into consideration, which are predominant in the case of shorter wavelengths and lead to a deterioration in transmission. As can clearly be seen from Figs. 18 and 19, micro- and nanobubbles are present, both for the composite Nextel 440/40 nm BN/N-SK4. In particular, Fig. 18 shows bubbles in the glass matrix, whereas bubbles at the interface between BN and the matrix can be recognized in both figures. The protection of BN through TiO2 (double layer) against reactions with the matrix glass will reduce the nanobubbles; a higher pressure (15 MPa) applied during hot pressing will have a reducing effect on microbubbles. A sophisticated time-temperature-pressure-atmosphere regime during hot pressing could bring about good trans￾parency. If the investigations are carried out again with compos￾ites with double-coated Nextel 440 fibers and the 8650 glass matrix, better transmission should be achieved in the visi￾ble wavelength range as most probably no nanobubbles will develop. By the way, if the curve is extrapolated into the long-wavelength range, the transmission of the trans￾parent composite represented in Fig. 17 will reach 60% at a wavelength of 2 lm. 5. Summary – It is possible to prepare a transparent, damage-tolerant composite on the basis of Nextel 440 fibers and the Schott glass type 8650. – The transmission of this composite reaches 45% in the visible wavelength range, and 65% in near infrared range. – Exactly adapted optical refractive indices and thermal expansion coefficients are the key to optically transpar￾ent composites. – Bending strengths of 200 MPa and a fracture toughness (not described here) of up to 6 MPa m1/2 were measured for several different damage-tolerant composites. Fig. 17. Comparison of the optical transmittance of composites made from Nextel 440-fibers and N-SK 4 or 8650 matrix-glass. The fiber volume content is near 16%. Fig. 18. Optical microscopic picture (dark field) of the polished surface of the Nextel 440/40 nm BN/N-SK 4 composite. Fig. 19. Optical microscopic picture (dark field) of one BN-coated fiber in a Nextel 440/40 nm BN/N-SK 4 composite. 372 D. Hu¨lsenberg et al. / Composites: Part B 39 (2008) 362–373