Ceramic Materials. Chapter 5: Glass

Transcription

1 Ceramic Materials Chapter 5: Glass F. Filser & L.J. Gauckler ETH-Zürich, Departement Materials SS 2007 slide 1

2 Persons in Charge of this Lecture I. Akartuna, HCI G 538, phone 36842, ilke.akartuna@mat.ethz.ch F. Krauss HCI G 538, phone , franziska.krauss@mat.ethz.ch Dr. F. Filser, HCI G 529, phone 26435, frank.filser@mat.ethz.ch Prof. Dr. L.J. Gauckler HCI G 535, phone 25646, ludwig.gauckler@mat.ethz.ch Dipl.-Ing. J. Kübler EMPA Dübendorf, phone , jakob.kuebler@empa.ch slide 2

3 Materials Science I: Ceramics Introduction on ceramic materials, technology, applications Crystal structures of ceramic materials Potential well of bonding and physical properties & Examples of Structural ceramic materials Examples of structural ceramic materials slide 3

4 Overview & schedule Mar 19, 07 term start Mar 20, 07 Glass and Glass-Ceramics (FF) Mar 27, 07 Toughness (JK) Apr 03, 07 Strength & Weibull statistics (JK) Apr 10, 07 Sub-critical crack growth, SPT-Diagrams (JK) Apr 17, 07 Proof-testing, creep, thermal properties (JK) Apr 24, 07 polymer part (Prof. D. Schlüter) Jun 22, 07 term finish slide 4

5 Learning Targets Definition of glass as a frozen liquid Viscosity temperature behavior Structure of Glass, Network Network formers, network modifiers, and Zwischenoxide Optical properties (refraction, dispersion, transmission) Chemical resistance and corrosion Fabrication of glass Glass ceramics, nucleation rate und crystal growth rate slide 5

6 V-T-Diagram for Crystal and for Glass Formation Melt Volume fast slow frozen liquid Glass Crystal supercooled liquid ΔV solidification Vol melt -Vol cryst Tg Tg T f Temperature Glasses are frozen liquids. They don t posses a (distinct) melting temperature but a glass transition temperature. The glass transition temperature and the density of a glass depend on the cooling rate. We don t observe a energy of formation during the solidification of a glass melt. In glass state the disorder of the melt is preserved. slide 6

7 History: W. H. Bragg and W. Lawrence Bragg W.H. Bragg (father) and William Lawrence Bragg (son) developed a simple relation for scattering angles, now call Bragg s law. d λ = n 2 sin θ slide 7

8 Another View of Bragg s Law nλ = 2d sinθ slide 8

9 Laue - Powder X-ray Diffraction Film Tube Powder slide 9

10 Bragg diffraction of glassy and crystalline material (PbF 2 ) slide 10

11 Laue-Diagram for a Single Crystal film (Points) aperture single crystal slide 11

12 Zachariasen s Network-Hypothesis 2-dimensionel visualization of a - regular SiO 4 -network (SiO 2 - quartz, left) and of a - irregular network (glass, right) slide 12

13 [SiO 4 ] - Network [SiO 4 ] - network with network modifiers Na and Ca slide 13

14 Glass Glass Short range order = Tetrahedron Si 4+ O 2- [SiO 4 ] 4- Tetrahedron O 2- Si 4+ O 2- O 2- O 2- O 2- Si 4+ z.b. Na +, K + slide 14

15 Network Former SiO 2 B 2 O 3 P 2 O 5 α thermocycling T g η mech. strength α thermocycling T g η acid resistance mech. strength UV transparency IR transparency chem. resistance Network former and their influence on the glass properties. slide 15

16 Network Modifier Li 2 O Na 2 O K 2 O CaO η η Glass becomes longer a chem. resistance a. Glasses are termed long or short glasses according to their Viscosity-Temperature-Function. slide 16

17 Zwischenoxide MgO Al 2 O 3 PbO TiO 2 ZrO 2 ZnO CdO Glass becomes longer Glass becomes longer mech. strength chem. resistance T g n electrical resistance Absorption of x-rays (40-80 weight-%) n resistance against acids chem. resistance tarnish for enamels hardness absorption of thermal neutrons (30-60 weight-%) slide 17

18 Typical Glass Compositions I pure vitreous silica (quartz glass) Vycor Glass Commercial window glass chem. laboratory glass (aluminium-borosilicate glass) >99.9 weight-% SiO 2 impurities in the ppm-range α= K weight-% SiO 2, 3 weight-% B 2 O 3, rest alkali, impurities in the ppm-range α= K -1 SiO 2 72, Al 2 O 3 1.5, MgO 3.5 CaO 8.5, Na 2 O 14.5 (weight-%) SiO 2 72, B 2 O 3 10 Al 2 O 3 3, CaO 1, MgO 1, Na 2 O 5 (weight-%) slide 18

19 Typical Glass Compositions II optical glass (heavy flint glass) glass for X-ray protection X-ray transparent glass hydrofluoric acid resisting glass SiO 2 28, PbO 70, Na 2 O 1, K 2 O 1 (weight-%) SiO 2 29, PbO 62, BaO 9 (weight%) B 2 O 3 83, BeO 2, Li 2 O 15 (weight%) P 2 O 5 72, Al 2 O 3 18, ZnO 10 (weight%) slide 19

20 Viscosity of Glass Melts η = dpa s B logη = A + T T Vogel-Fulcher-Tamman Equation 0 Glass softens steadily but doesn t melt (at a distinct temperature)! The viscosity of glass is a steady curve which spans 16 to 18 orders of magnitude. Melt exists for temperature higher than Tg (= infexion point of its viscosity curve), solid exists for a temp. lower than Tg. VFT-equation describes the viscosity for temperatures higher than Tg. slide 20

21 long Glass short Glass long glass η in log dpas short glass T in C η-t-function of Dow-Corning s glasses ( long" glass, short" glass) slide 21

22 Elongation of Glass I α th = 1/ l o Δl i ΔT Dilatometry for the determination of T g and α of a glass. slide 22

23 Cooling Rate Influence of the cooling rate on the formation of glass. volume melt fast glass normal slow cooling T g s T g n T g f temperature Tg and density of a glass depend on the cooling rate. A low cooling rate (i.e. slow cooling) leads to higher density than a high cooling rate. slide 23

24 Elongation of Glass II Contraction and elongation curves of a glass. 0: equilibrium curve; 1: normal cooling; 2: slow cooling; 3: fast cooling; 3 : normal heating slide 24

25 Density as function of the alkali amount Density of binary alkalisilicate glasses slide 25

26 Density as a function of the network former and its amount Density variation of a binary alkalisilicate glass in function of the replacement of SiO 2 by other oxides in the same weight amount. slide 26

27 Reduction of the strength caused by flows strength theoretic strength strength of glass fibers strength of brand new acid polished glass strength of normal glass ware strength of damaged glass ware Flaws in glass structure Micro flaws in the surface Macro flaws in the surface Strength of glass and its reasons slide 27

28 Pre-stressing tension compression surface a) b) surface Stress profiles for the a) thermal and the b) chemical method of pre-stressing. slide 28

29 Chemical Resistance of Glasses hydrofluoric acid HF attack (corrosion) diluted aqueous acids attack base attack combined water and acid / base attack slide 29

30 Chemical Resistance of Glass against Hydrofluoric Acid HF SiO 2 + 6HF H 2 SiF 6 + 2H 2 O Hydrofluoric acid breaks up the SiO 2 structures of silicate based glasses. Easy soluble silicon hexaflourid SiF 6 (hexafluoro silcia) is formed during the reaction. slide 30

31 Chemical resistance of glass against aqueous acids Attack by aqueous acids: ion exchange reaction -Si-O-Na + + H + -Si-OH + Na + Leaching time Distance from the surface Gel layer at the surface which is proton saturated This gel layer serves as a diffusion barrier. The thicker this gel-layer the more difficult for ions (Na +, H + ) to diffuse through this layer. -> Passivation! slide 31

32 Chemical resistance of glass against bases -Si-O-Si- + X-OH -Si-OH + SiO-X (X=Li, Na, K) Bases attack directly in a detrimental way the network of the glass in contrast to acids. Bases destroy the network. The SiO 2 molecules are dissolved and remain in the solution as poly-silicates. No protection layer is formed in this type of corrosion process BUT always a new surface forms which is continuously attacked by the hydroxide-ions of the solution. slide 32

33 Chemical resistance of glass against water and combined acid / base exposure Pure water at ph 7-9 corrodes glass because of free protons (H 3 O + ) by auto protolysis. The glass corrodes by leach-out of alkaline cat ions. The alkaline cat ions increase the ph of the water by formation of hydroxyl ions (OH - ). The hydroxyl ion start to solubilise the glass network and /or the gel layer. Thus, hydroxyl ions are consumed, and the proton concentration in the solution increases and the cycle starts over. slide 33

34 Time dependence of glass corrosion Type I: A surface layer is formed which acts as a protection layer (adsorption protection layer). vitreous silica in neutral salt solution accounts for this type. Type II: A protection layer is formed by leaching out of alkalines. The glass network remains stable. Attack of acids at silicate glasses accounts for this type. Type III: Leaching and reaction at the surface form two protection layers of different composition. The glass network remains stable. slide 34

35 Time dependence of glass corrosion Type IV: Leaching and wear (material removal) take place at the same time. A new leached out layer is formed at the surface which will be continously shifted more and more inside the bulk glass by the wear of the glass network (i.e. material removal). Alkalisilicate glasses in water account for this type. Type V: Constant wear of the glass network and material removal. No leaching layer is formed! Hydofluoric acid and strong base attack at silicate glasses account for this type. slide 35

36 Time dependence of glass corrosion Case 1: For the types I, II and III corrosion stops after formation of a protection layer (passivation), i.e. the corrosion front c moves on with a decreasing speed. dc dt te αt Case 2: For type IV corrosion involves two competing reactions: (1) the diffusion: (2) the dissolution: c c t t Case 3: For type V reaction or corrosion rate has a constant value: dc dt = a Case 4: progressive corrosion takes place when we have the solution deficiency on the glass surface. Then, the corrosion reaction changes the ph-value of the solution which leads to a stronger corrosion at a higher rate. The wear or corrosion rate increases with reaction time: dc dt = at slide 36

37 Reaction progress for the corrosion of glasses progressive (Case 4) limited (Case 2) passivated (Case 1) constant (Fall 3) time t Reaction rates for the different cases. slide 37

38 Transmission of a light beam through glass reflected light reflected light transmitted light incident light beam air glass air Way of a light beam transmitting a interface of two media (air, glass) slide 38

39 Influence of Alkaline content on the refractive index n D Variation to the refractive index as a function of the alkaline content of binary alkalisilicate glasses. slide 39

40 Influence of the network former on the refractive index Variation of the refractive index n D of a Na 2 O - SiO 2 - glass (20-80 weight%) if SiO 2 (network former) is substituted according to weight by other network former (Al 2 O 3, B 2 O 3 ) or elements. slide 40

41 Fraunhofer Lines h g F F e d C C r Hg purple Hg blue Cd blue H blue Hg green He yellow Cd red H red He red slide 41

42 What is Dispersion? The dependence of the refractive index from the light-wavelength. h g F F e d C C r Hg purple Hg blue Cd blue H blue Hg green He yellow Cd red H red He red prism screen white light Dispersion splits white light in its colours!!!! That effect is fatal for optical applications of glass!!! slide 42

43 What is Dispersion? The dependence of the refractive index from the light-wavelength. h g F F e d C C r Hg purple Hg blue Cd blue H blue Hg green He yellow Cd red H red He red n F n C ϑ re l = n D 1 ν = n d n F n C slide 43

44 Dispersion und Refractive index Flint glasses Crown glasses n D -ν-diagram of optical glasses ν = n d n F n C slide 44

45 Dispersion and Refractive Index n D -ν-diagramm optischer Gläser ν = n d n F n C slide 45

46 Chromatic Aberration Chromatic aberration in case of a bi-convex and bi-concave lens. slide 46

47 Achromatic Lens Crown Glass Flint Glass Correction of the chromatic aberration by combining two suitable lenses. slide 47

48 Transmission through Glass Wave Number Transmission Wave Length Transmissions spectrum of a commercial float glass (thickness 1mm) slide 48

49 UV-Rim in Glasses Transmission Wave Length UV rim of glasses of various composition 1: SiO 2 -Glass very pure, 2: SiO 2 -Glass normal, 3: Na 2 O - 3 SiO 2 Glass very pure, 4: Na 2 O-3 SiO 2 -Glass, normal slide 49

50 Transmission spectrum of Glasses I Spectral Transmission τ(λ) Wave Length λ [nm] Transmission spectra of coloured glasses. slide 50

51 Transmission spectrum of Glasses II Transmission spectra of colored glasses. slide 51

52 Colorizing Ions and their Effect I Valency Ti(III) V(III) V(V) Cr(III) Cr(VI) Mn(II) Mn(III) Fe(II) Fe(III) Coordination Color purple green colorless green yellow colorless purple blue yellow slide 52

53 Colorizing Ions and their Effect II Valency Co(II) Co(II) Co(III) Ni(II) Ni(II) Cu(I) Cu(II) Coordination ? 6 Color blue pink green blue yellow colorless blue slide 53

54 Float Glass Process slide 54

55 The Float Glass Process slide 55

56 Float Glass process slide 56

57 Float Glass Process Batch Charger Furnace Raw Materials Tin Bath (Float Furnace) Annealing Lehr Cutting System slide 57

58 Composition of commercial float glass Material Glass Composition Reason for Adding Sand (SiO 2 ) Soda Ash (Na 2 CO 3 ) Limestone (CaCO 3 ) Dolomite(CaMg(CO) 2 ) Alumina (Al 2 O 3 ) Others Easier melting Durability Working & weathering properties - - slide 58

59 Composition Float Glass slide 59

60 Chemistry of Glass slide 60

61 Glass-Ceramics Structure Nucleation and crystal growth Fabrication Examples next: mechanical properties of ceramics (J. Kübler, EMPA) slide 61

62 Glass-Ceramics Glass-ceramic refers to materials which are fabricated from glass melts by a process of controlled crystalisation. Glass-ceramis is a partially crystalline material which is fabricated by an incomplete crystallisation ( Ceraming ) of suitable glasses. Brands: Ceran, Zerodur, Robax, Neoceram, Macor slide 62

63 Glass-Ceramics Glass Short-range order Grain with grain boundary SiO 4 tetrahedron O 2- O 2- Si 4+ O 2- O 2- slide 63

64 Nucleation energy and radius of the nucleus Glass (amorph) to Crystal Energy r 3 Surface Energy = r 2 r Dependence of the nucleation energy from its nucleus radius slide 64

65 Critical Nucleus Radius 4π Δ G = r Δ G + 4π r ν σ -> 1. Derivative must be zero for r * 16π 3 σ ( ΔG ν ) 3 Δ G = = π r 2 σ r ( T) 1 ( T T) S ΔG ( T) 1 ( T T) S 2 slide 65

66 Nuclei Formation Rate (Nucleation Rate) ν KB = νnν exp ( E +ΔG ) kt slide 66

67 Crystal Growth Rate (KG) N '' = N ν exp [ Δ G+ E] kt Crystal Growth Rate ν KG N' = N ν exp E kt ( N' N'') E ΔG = a0 = a0 ν exp 1 exp N kt kt slide 67

68 Temperature Dependence of ν KB and ν KG T S KG, Crystal Growth KB, Nucleation T E Nucleation and Crystal Growth rate Temperature Dependence of ν KB and ν KG ; T E : Freezing Temperature T S : Melting Temperature slide 68

69 Temperature-Time (T-t)-Curve for the Fabrication of a typical Glass-Ceramics slide 69 Melt Forming Cooling Checking, Shaping Nucleation Crystallisation Checking, Shaping Time

70 Examples for Glass-Ceramic-Systems System Li 2 O-Al 2 O 3 -n SiO 2, n= 4-6 Crystalline Phase H- Quartz mixed crystal TEC 10-7 K Application cooktop, fireproof cookware fire protection glass, reflecting telescope, gyroscope Li 2 O - Al 2 O 3 -n SiO 2, n= 4-10 ß-Spodume fireproof cookware, heat exchanger Na 2 O - Al 2 O 3-2 SiO 2 Nepheline 11 plates 2 MgO- 2 Al 2 O3 5 SiO 2 Cordierite car exhaust filter, substrates for microelectronics, composite materials Li 2 O- 2 SiO 2 Lithiumdisilicate 12 fabrication of microstructures (Na,K) Mg 3 AlSi 3 O 10 F 2 Phlogopite 8-12 bioceramics (dental implants, bone substitutes) slide 70

71 Properties of Glass-Ceramics thermal expansion: -20 to K -1 strength: MPa optical property: transparency opaque chemical resistance: water soluble inert electrical property: semi-conducting isolating mech. machinability: brittle machinable slide 71

72 Fabrication of Ceran cooktop panels (Schott) 1 st step 2 nd step slide 72

73 Advantages of glass-ceramics over ordinary ceramics Advantages: Disadvantages: forming is as simple as in case of glass limited to certain crystalline phases transparent pre-forms good reproducibility of the materials properties free of pores mono-size crystals economic fabrication process slide 73

74 Summarizing: Glass & Glass Ceramics Glass can be regarded as frozen melt (liquid) - which solidified without crystallisation - which has a short range order but no long-range order. The glass structure can be described using Zachariasen s network model. Glass solidifies at the glass transition temperature T g, - for T > T g glass is a (liquid) melt, - for T <T g glass is a solid. The viscosity as a function of temperature is a smooth (steady) curve spanning a range of 16 to 18 orders of magnitude. Glass-ceramics is fabricated from glass by using controlled crystallisation process, and it has special properties. slide 74

75 Ersatz slide 75

76 Overview on the composition of important glass types Glass type SiO 2 Al 2 O 3 Na 2 O K 2 O MgO CaO B 2 O 3 PbO TiO 2 F As Se Ge Te quartz glass 100 Kalk-Natron-glass (ordinary glass) float glass 72 1,5 13,5-3,5 8, lead crystal glass , ,5 - - Laboratory glass , , E-glass ,5 17, enamel 40 1, chalcogenide glass 1 chalcogenide glass All amounts in weight -%. slide 76

77 slide 77

78 The basic idea of the float glass process Molten glass poured onto a bath of clean molten tin, will spread out in the same way that oil will spread out if poured onto a bath of water. Gravity and surface tension will result in the top and bottom surfaces of the glass becoming approximately flat and parallel. slide 78

79 The basic idea of the float glass process The equilibrium thickness (T) 2 2ρt = ( g + gt + t) i gρg ρt ρg T S S S ( ) where S g, S gt, and S t are the values of surface tension at the three interfaces. For standard soda-lime-silica glass under a protective atmosphere and on clean tin the equilibrium thickness is approximately 7 mm. slide 79

80 The Production Process slide 80