Glasses Tech – Prescription Glasses, Prescription Eyeglasses

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.　

Starting from a multicomponent lead phosphate glass composition given in the literature, a glass was developed suitable for the protective overclad of heavy- metal fluoride glass fibers. Besides a low glass transition temperature less than 270 degree C, a high thermal expansion coefficient greater than 18 center dot 10** minus **6 K** minus **1, the phosphate glass material developed had a water resistance at 60 degree C for 10 h comparable to that of commercial container glass. The final composition developed was (in mol%): 54.7 P//2O//5, 19.8 PbO, 18.0 Na//2O, 1.2 MgO, 1.2 BaO, 1.2 CaO, 0.4 V//2O//5, 1.5 Al//2O//3, 2.0 Bi//2O//3. Besides the influences of these different components the action of the melting conditions, especially melting temperature and trace SiO//2 content from the crucible material, have to be taken into account. Fiber drawing experiments including the in-situ application of the phosphate glass overclad were successful. (Author abstract) 20 Refs.

Hartmann, Matthias　Bintzer-Schmidt, Sigrid　Heinz Frischat, Guenther

The bioactivity, i.e., bone-bonding ability, of 26 glasses in the system Na2O-K2O-MgO-CaO-B2O3-P2O5-SiO2 was studied in vivo. This investigation of bioactivity was performed to establish the compositional dependence of bioactivity, and enabled a model to be developed that describes the relation between reactions in vivo and glass composition. Reactions in vivo were investigated by inserting glass implants into rabbit tibia for 8 weeks. The glasses and the surrounding tissue were examined using scanning electron microscopy (SEM), light microscopy, and energy-dispersive X-ray analysis (EDXA). For most of the glasses containing < 59 mol % SiO2, SEM and EDXA showed two distinct layers at the glass surface after implantation, one silica-rich and another containing calcium phosphate. The build-up of these layers in vivo was taken as a sign of bioactivity. The in vivo experiments showed that glasses in the investigated system are bioactive when they contain 14-30 mol % alkali oxides, 14-30 mol % alkaline earth oxides, and < 59 mol % SiO2. Glasses containing potassium and magnesium bonded to bone in a similar way as bioactive glasses developed so far. J Biomed Mater Res, 37, 114-121, 1997.

Maria Brink1, Tia Turunen

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.

John Gilchrist

A combination of results from X-ray and neutron diffraction is used to obtain structural information about the metal-oxygen coordination shell in oxide glasses. Two ways to extract structural parameters of the Me-O coordination are presented. The first variant is a direct combination of both distance correlation functions which are considered simultaneously in a least-squares fit procedure. On the other hand a suitable difference of the two structure factors is introduced, which do not contain any O-O correlation. The corresponding distance correlation function directly shows the Me-O peak. The samples are metaphosphate glasses with Me equals Al, Zn, Mg, Ca, Ba and Na and two sodium silicate glasses (76.5 and 67 mol% silicon dioxide). Four oxygens are found in contact to the Mg ion. But two additional, more distant positions are detected. Thus, the sum of all oxygen atoms in the coordination sphere is 6 rather than 4. The Zn cation is located in a real ZnO//4-tetrahedron. The number of oxygens in the environment of the Na ion is of about five both in the metaphosphate glass and in the silicate glasses. But a surprising result is a splitting observed for the Na-O distance peak in case of silicate glasses. (Author abstract) 28 Refs.

Hoppe, U.　Stachel, D.　Beyer, D.

The bioactivity, i.e., bone-bonding ability, of 26 glasses in the system Na//2O-K//2O-MgO-CaO-B//2O//3-P//2O//5-SiO//2 was studied in vivo. This investigation of bioactivity was performed to establish the compositional dependence of bioactivity, and enabled a model to be developed that describes the relation between reactions in vivo and glass composition. Reactions in vivo were investigated by inserting glass implants into rabbit tibia for 8 weeks. The glasses and the surrounding tissue were examined using scanning electron microscopy (SEM), light microscopy, and energy-dispersive X-ray analysis (EDXA). For most of the glasses containing less than 59 mol % SiO//2, SEM and EDXA showed two distinct layers at the glass surface after implantation, one silica-rich and another containing calcium phosphate. The build-up of these layers in vivo was taken as a sign of bioactivity. The in vivo experiments showed that glasses in the investigated system are bioactive when they contain 14-30 mol % alkali oxides, 14-30 mol % alkaline earth oxides, and less than 59 mol % SiO//2. Glasses containing potassium and magnesium bonded to bone in a similar way as bioactive glasses developed so far. (Author abstract) 30 Refs.

Brink, Maria　Turunen, Tia　Happonen, Risto-Pekka　Yli-Urpo, Antti

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

The structures of the two fast ion conducting glass systems (AgI)//x (Ag//2O-2B//2O//3)//1// minus //x and (LiCl)//x (Li//2O-2B//2O//3)//1// minus //x have been examined. The neutron diffraction experiments revealed large similarities in the short range order of the B-O network for the two glass systems. However, remarkable differences are observed for the intermediate range order. This is evident from the low-Q part of the total structure factor; the first sharp diffraction peak (FSDP) at 1.6 angstrom** minus **1 is almost unaffected by LiCl doping, while in the case of AgI doping a new strong low-Q peak appears at about 0.8 angstrom** minus **1. To investigate the underlying structural differences Reverse Monte Carlo (RMC) simulations have been performed. The simulations show that the experimental FSDPs are caused mainly by density fluctuations in the B-O network. The anomalous low-Q peak of the AgI doped glasses is thus explained by that AgI locally expands the B-O network and causes longer range correlations between borate groups. The network have larger density fluctuations and voids, which, in turn, are beneficial for the formation of open pathways in which the cations can move. ( Author abstract) 23 Refs.

Swenson, J.　Borjesson, L.　Howells, W.S.

Various experimental techniques have been used to investigate the effect of mechanical milling on the structural stability of rapidly solidified aluminum-basedmetallic glasses. Using transmission electron microscopy (TEM) and X-ray diffraction methods, the formation of nanocrystalline Al particles in some ball-milledAl-rich metallic glasses (such as Al//90Fe//5Gd//5 and Al//90Fe//5Ce//5) is clearly observed. For other composiions with lower Al concentration such as Al//85i//5Y//10, no such phase transformation can be detected by TEM or X-ray. However,differential scanning calorimetry (DSC) measurements show that the crystallization peaks of the ball-milled Al//85Ni//5Y//10 metallic glass shifted to hiher temperatures, while the crystallization enthalpy associated with the first exothermic peak decreased to a lower vlaue, revealing that certain structural changes have taken place as a result of mechanical deformation. The compositional dependence of the structural stability of Al-based metallic glasses against mechanical deformation suggests that the nanocrystal formation induced by a deformation process is different from that caused by a thermal process. The large plastic strain induced atomic displacements and the enhancement of atomic mobility during thedeformation process, are the possible mechanissm of mechanical deformation-induced crystallization. Our results demonstrate a new way of obtaining nanophase glassy composite alloy powders which are suitable for engineering applications upon

He, Y.　Shiflet, G. J.