Gel-glass dispersed liquid crystal (GDLC) films may be used as electro-optic devices. Films scatter light according to the number of droplets and the relative refractive indices of the LC and the silica matrix. LCs are birefringent; therefore their refractive index depends on the LC orientation and the optical angle of incidence. If the film is coated with transparent electrodes, and an electric field is applied, a reorientation of the LC director in the droplet occurs, producing a variation of the LC refractive index as `seen’ by the incoming light. If the refractive index of the sol – gel substrate matches the new LC index, the material changes from an opaque, scattering state to a transparent state. This feature can be used for preparing devices for visual presentation, i.e. displays. Unaltered GDLCs switch from white opaque to colourless transparent states. The contrast ratio, viewing angle and switching time of GDLC displays are presented. If these materials are to be used for displays, colour needs to be incorporated for many applications. The use of GDLCs associated to gel-glasses doped with fluorescent dyes for preparation of emissive `all sol – gel’ displays is presented, with the GDLC acting as an optical shutter for the RGB emission of the fluorescent dyes located at the backplane. A number of fluorescent dyes with high quantum yield for the chosen UV excitation, and appropriate colour coordinates, have been investigated.
Thermally stimulated luminescence has been observed in X-ray irradiated alkali borate and sodium silicate glasses doped with Ce**3** plus . The emission wavelength is about 360 nm which corresponds to the 5d-4f transition of Ce**3** plus . For the alkali borate glasses doped with Ce**3** plus , oxygen hole center and electron trapped nearby alkali ion induced by X-ray irradiation contribute to the thermally simulated luminescence, while Sm**3** plus acts as an electron acceptor in the thermally stimulated luminescence of alkali borate glasses codoped with Ce**3** plus and Sm**3** plus . In both cases, Ce**3** plus is ionized by X-ray to form Ce**4** plus and an electron which is trapped by an impurity level (oxygen hole center or Sm**3** plus in glasses with or without Sm**3** plus , respectively). The thermal stimulation excites the electron (electron trapped nearby alkali ion or Sm**3** plus ), which transfers to the 5d level of Ce**3** plus . The electronic transition from the 5d to the 4f levels gives rise to the emission observed. The temperature which gives the maximum emission intensity in the glow curve tends to decrease as the concentration of Na//2O increases in the sodium borate glasses doped with Ce**3** plus and Sm**3** plus . In contrast, the temperature corresponding to the maximum emission intensity is independent of the glass composition for the sodium silicate glasses doped with Ce**3** plus and Sm**3** plus . These facts suggest that the compositional dependence of local basicity for Sm**3** plus ion site in sodium borate glasses is different than that in sodium silicate glasses. (Author abstract) 15 Refs.
Tanaka, K. Hirao, K. Tanaka, H. Soga, N.
Various silica glasses were engraved deliberately by excimer laser radiation using wavelengths of 308 and 248 nm. The ablation of different samples was investigated by systematic variation of the processing parameters. The ablation rates were determined using profilometry and gravimetric measurements by evaluating the processing quality and the morphology of the processed surfaces was considered. The phenomenon of ablation is explained as a non-linear interaction of the laser beam and the glass. The experimental results show that the ablation behaviour of silica glass depends on the wavelength and the intensity of the laser radiation, on the surface quality and the degree of purity of the glass. Although high ablation rates were obtained, the suitability of excimer lasers for micromachining is restricted due to the rough surface morphology and poorly defined edges. (Author abstract) 18 Refs.
Hornberger, Helga Weissmann, Rudolf Lutz, Norbert
The specific heat C//v of Pr^3+ – and Eu^3+ -doped silicate glasses has been determined between 0.1 and 10 K. Below about 5 K, C//v∝T//1+v with v=0.0-0.3. This deopendence is characteristic for the two-level-system (TLS) contribution to the specific heat, C . The values of C and v noticeably vary with the chemical natureand teh concentration of the glass modifies. The number of TLS in samples of almost identical composition is significantly larger for the Pr^3+ doped thanfor the Eu^3+ doped and the undoped glass. Above 5 K, the T^3-phonon term and the T^5 term associated with soft localized vibrations also contribute to C//v. The results do not point to a correlation between the T^1+v and the higher-order terms.
Van de Straat, D. A. Baak, J.
Disordered dielectrics exhibit a variety of nonlinear responses at temperatures below 4 K, sometimes in remarkably small fields. Four characteristic types of behaviour are identified. Two of these are well understood in terms of the `standard′; two-level system model, but two require further experimental study and theoretical development. One type is similar to piezoelectric ceramic behaviour.
The repair of an osteochondral defect in rabbit femur was studied with three kinds of bioactive glasses (BG), hydroxyapatite (HA), and hydroxyapatite-glass (HAG) composite. Seventy-two osteochondral defects were created in 18 rabbits. Sixty-four cylinders were implanted and eight defects were left empty as controls. Histomorphometry, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) were used for evaluation. Small osteochondral defects in rabbit femur found to heal themselves by regeneration. The three BGs, HA, and HAG led to direct lamellar bone repair of subchondral bone and restoration of articular surfaces mostly with hyalinelike cartilage in 12 weeks. However, the composition of the materials affects their behavior. Chondrogenesis took place earlier with the BGs than with HA. HAG degraded too much, glass 14 was too reactive and brittle, and the high alumina content in glass 11 disturbed its bone-bonding ability. Glass 7 and HA were the most balanced in the repair process. A special preparation method was used to retain soft tissues fairly unchanged and enable them to the observed together with hard tissues in SEM analysis.
Erkki Suominen1, Allan J. Aho