MIT Amorphous Materials

Transcription

1 MIT Amorphous Materials 2: Classes of Amorphous Materials Juejun (JJ) Hu 1

2 Network formers, modifiers and intermediates Glass network formers Form the interconnected backbone glass network Glass network modifiers Present as ions to alter the glass network Compensated by non-bridging oxygen (NB) in oxide glasses Usually reduce glass network connectivity Intermediates Can function as network formers or modifiers depending on glass composition 2

3 Network formers, modifiers and intermediates Si Si Na 2 Si Bridging oxygen - Na + Na + - Non-bridging oxygen Si Silicon: glass former Sodium: network modifier 3

4 Glass former: high valence state, covalent bonding with Modifier: low valence state, ionic bonding with Network modifiers Glass formers Intermediates 4

5 Silica glass (Si 2 ) A 3-D glass network predominantly consisting of corner-sharing Si 4 tetrahedra interconnected by bridging oxygen (B) High network connectivity: high softening point, low diffusion coefficient, small coefficient of thermal expansion (CTE) Cristobalite Image of vitreous silica removed due to copyright restrictions. 5

6 Volume anomaly in silica glass 6

7 Alkali silicate glass Each alkali ion creates one non-bridging oxygen Reduced network connectivity: viscosity decreases (compared to silica at the same T), diffusion coefficient and CTE increases Increased ionic conductivity, reduced chemical resistance Increasing alkali concentration Invert glass 7

8 Structural determination in alkali silicate glass Y : the average number of corners shared per Si 4 tetrahedron In a glass with molar composition: x Na 2 (1-x) Si 2 Viscosity isotherms Y = 2 Number of NB per mole: 2x Number of B per mole: 2-3x Number of corners shared per mole: (2-3x) 2 = 4-6x Number of tetrahedra per mole: 1-x log 10 (viscosity) Y = (4-6x) / (1-x) nset of inverted glass structure: Y = 2, x = 0.5 Y 8

9 Alkali-alkaline earth-silicate glass Each alkaline earth ion creates two NBs Si Si Ca Si - Ca 2+ - Si Increased network connectivity compared to alkali silicates: stabilized glass network, improved chemical resistance Approximate composition of commercial soda-lime glass (window glass): 16Na 2 10Ca 74Si 2 9

10 Borate glass B 2 3 : the glass former consisting of corner-sharing B 3 triangles connected by bridge oxygen B B B Boroxol rings: basic structural unit in boric oxide glass B 2 3 crystal 10

11 Borate glass B 2 3 : the glass former consisting of corner-sharing B 3 triangles connected by bridge oxygen Alkali borate glass: NB formation B 1/2 Na 2 B - Na + B 1/2 Na 2 B - Na + B 3 B 4 conversion 11

12 The Boron anomaly Fraction of 4-fold coordinated boron B 3 B 4 conversion NB formation Initial addition of alkali ions increases network connectivity, reduces CTE and enhances thermal & chemical resistance Molar fraction of alkali (%) 12

13 Various properties of lithium borate glass More than two structural transformations contribute to the boron anomaly Further reading: Shelby Ch. 5 J. Non-Cryst. Sol. 351, (2005). Courtesy of Elsevier, Inc., Used with permission. Source: Mazurin,.V. "Glass Properties: Compilation, Evaluation, and Prediction." J. Non-Crystalline Solids 351 (2005):

14 (Alkali) borosilicate glass Borosilicate glass: x M 2 y B 2 3 (1 - x - y) Si 2 Si 2 and B 2 3 : glass formers Alkali ions (M + ) converts B 3 to B 4 states (when x / y < 0.5) Each additional alkali ion creates one NB (when x / y > 0.5) The original Pyrex recipe: 4Na 2 13B 2 3 2Al Si 2 Image of space shuttle tile courtesy of the Science Museum London on Wikimedia Commons. License CC BY-SA. Glassware images Pyrex. All rights reserved. This content is excluded from our Creative Commons license. For more information, see Space shuttle tile coating 14

15 (Alkali) aluminosilicate glass Aluminosilicate glass: x M 2 y Al 2 3 (1 - x - y) Si 2 Si 2 : glass former Al functions as a glass former in the form of Al 4 groups (when x > y) Each excess Al atom creates three NBs (when x < y) Al 3+ in an oxygen octahedron Activation energy for DC conductivity y = x y / x 15

16 Chalcogenide glass (ChG) Reduced mechanical strength Low softening temperature Low phonon energy (infrared transparency) Enhanced optical (Kerr) nonlinearity ACerS Bull. 94, (2015). 16

17 Amorphous semiconductors Tetrahedral glasses a-si, a-ge, a-si:h (hydrogenated amorphous silicon) Vapor deposition: plasma enhanced chemical vapor deposition (PECVD), sputtering, electron beam evaporation Dangling bonds 17

18 Metallic glass (amorphous metal, glassy metal) Polycrystalline metal Amorphous metal Grain boundaries Inoue s empirical rules for bulk metallic glass (BMG) formation Multicomponent systems consisting of three or more elements Significant difference in atomic size ratios (> 12%) Negative enthalpy of mixing 18

19 ther glass groups and glass formers Phosphate glass: P 2 5 Heavy metal oxide (HM) and transition metal oxide glass Te 2, Pb, Bi 2 3, V 2 5, Ti 2, etc. Halide glass and alloys e.g. ZBLAN: ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF Chalcohalide, oxyhalide, etc. Amorphous minerals pal, biominerals and many others Amorphous calcium carbonate in lobster carapace The Formula for Lobster Shell, Max Planck Research 19

20 Representation of glass composition In oxide glasses, the convention is to list the glass network modifiers in increasing valence order ending with glass network formers Example: K 2 Ca 5Si 2 In mole fraction: 14.3K Ca 71.5Si 2 By weight: 20.9K Ca 66.7Si 2 (wt%) In metallic glasses, the listing is usually done in decreasing order of content Example: Zr 41.2 Be 22.5 Ti 13.8 Cu 12.5 Ni 10.0 (Vitreloy-1) 20

21 Summary Glass formers, network modifiers, and intermediates Silicate glass chemistry Corner-sharing tetrahedra Bridging and non-bridging oxygens Different modifiers: alkali, alkali earth Borates and boron anomaly Impact of network connectivity on glass properties ther glass systems Chalcogenides: weak bonds Tetrahedral glasses: passivation of dangling bonds Amorphous metals: w/o grain boundaries 21

22 Summary of oxide glass chemistry No modifier Alkali oxide Alkaline earth oxide Alumina (Al 2 3 ) Si 2 (silicate) Structural unit: Si 4 No NBs, low or negative CTE, high softening point, low diffusivity Each ion creates 1 NB; large CTE, low softening point, poor chemical durability, ionic conductivity Each ion creates 2 NBs; similar effects as alkali ions although some network connectivity is preserved Each ion creates 3 NBs; in the presence of alkali ions Al 3 Al 4 conversion occurs B 2 3 (borate) Structural unit: B 3 No NBs, corrugated layered structure Each ion creates 1 B 4 group or 1 NB; extremum in glass properties (boron anomaly) Each ion creates 2 B 4 groups or 2 NBs; extremum in glass properties (boron anomaly) B 2 3 and Al 2 3 both serve as glass formers; glass is stable only with high B 2 3 content 22

23 MIT pencourseware Amorphous Materials Fall 2015 For information about citing these materials or our Terms of Use, visit: