Frits and Sealing R. Morena Presented at Lehigh University as part of IMI-NFG Series on Glass Processing 14 April 2015 Lectures available at: www.lehigh.edu/imi Powdered glasses (frits) • Sealing Frits – • Coatings glaze (with opacifiers, colorants) glazes – ceramic porcelain enamel - ground coat + cover coat metal . Science & Technology © 2015 Corning Incorporated 2 . Starting point • Frits – finely powdered glasses that when re-heated will sinter, soften, and flow to form a seal or a coating (heat) At start: At finish: unconsolidated frit powder strong, hermetic seal or impervious coating . Science & Technology © 2015 Corning Incorporated 3 . Overview • Brief introduction, frits vs bulk glasses • Selection criteria for design of frit composition • Processing of frits . Science & Technology © 2015 Corning Incorporated 4 . Bulk glass vs frit behavior • By definition, glass is a thermodynamically unstable structure. • Converting a bulk glass to a fine powder only increases its instability. • Relatively few bulk glasses are stable as frits, but will crystallize to at least some extent BULK GLASS ? FRIT PROPERTIES PROPERTIES . Science & Technology © 2015 Corning Incorporated 5 . SnO-ZnO-P2O5 system Stable low temperature frits with good aqueous durability . Science & Technology © 2015 Corning Incorporated 6 . Classifications of frits • Vitreous – remains glassy throughout sealing cycle; will soften and flow if re-heated • Devitrifying – initially glassy, but at some point during sealing cycle will crystallize to form a hard, rigid bond. If reheated, will not flow and soften. • Typically, vitreous frits are desired where seals are subjected to repeat thermal cycling; devitrifying frits are preferred for applications in which the seal will be exposed to high operating temperatures . Science & Technology © 2015 Corning Incorporated 7 . Selection criteria for design of frit composition • Maximum permissible sealing temperature • CTE of materials to be bonded (CTE compatibility) • Use temperature of sealed assembly • Other – durability, strength, hermeticity, required lifetime, IR absorbance . Science & Technology © 2015 Corning Incorporated 8 . Frit-sealing: SOFC (solid oxide fuel cells) Frit seal Requirements: Max. sealing temp: 900° CTE: 110-120 Service temp: 750º Other: alkali-free, hermetic during extended (10Y) service time in H2 environment. Science & Technology © 2015 Corning Incorporated 9 . Frit-sealing: OLED devices Frit seal Requirements: Max. sealing temp: 450° CTE: 30-35 Service temp: RT Other: hermetic during 5Y lifetime; laser-sealable since temp during sealing must be <100º 1mm from seal edge . Science & Technology © 2015 Corning Incorporated 10 . Frit-sealing: Micro-reactors for chemical synthesis Micro-reactors Requirements: Max. sealing temp: 800° CTE: 35-40 Service temp: -50º to +50º Other: high chemical durability (acids and bases) . Science & Technology © 2015 Corning Incorporated 11 . Sealing temperature For a frit, flow is everything – main determinant of sealing temperature Straight Slt. Rounding Rounding Flowed Puddled >109 P ~108 P 107 P 104-105 P <103 P . Science & Technology © 2015 Corning Incorporated 13 . For a frit, flow is everything Optimum viscosity range for sealing Straight Slt. Rounding Rounding Flowed Puddled >109 P ~108 P 107 P 104-105 P <103 P . Science & Technology © 2015 Corning Incorporated 14 . Selection criterion: Flow • For strongest seal, seal temperature should correspond to the softening range of the frit Softening range viscosity of several 80CTE frits for 700º sealing 9.5 9 just right at 8.5 sealing temp 8 too stiff at sealing temp 7.5 too fluid at sealing temp 7 6.5 Ca-Zn-silicate log(viscosity, P) log(viscosity, 6 V-borate Sr,Ca alumino borosilicate 5.5 5 300 400 500 600 700 800 900 1000 Temperature, degC . Science & Technology © 2015 Corning Incorporated 15 . Flow evaluation Frit A Frit B . Science & Technology © 2015 Corning Incorporated 16 . Viscosity curves in softening range 9.5 9.5 9.0 9.0 8.5 8.5 8.0 8.0 7.5 7.5 7.0 7.0 Log (Viscosity, P) (Viscosity, Log 6.5 P) (Viscosity, Log 6.5 6.0 6.0 1070 1080 1090 1100 700 725 750 775 Temperature (ºC) Temperature (ºC) Frit A Frit B after firing @ 900º-1hr after firing @ 800º-1hr . Science & Technology © 2015 Corning Incorporated 17 . Temperature interval between Tg and Tx (onset) is best comparative predictor of flow . Science & Technology © 2015 Corning Incorporated 18 . Tg is highly composition sensitive 450 Tg for SnO-ZnO-P2O5 frits at 33% P2O5 400 C) o 350 Tg ( 300 250 0 10 20 30 40 50 8 SnO/ZnO molar ratio at 33% P2O5 . Science & Technology © 2015 Corning Incorporated 19 . Expansion compatibility Expansion compatibility • Mismatch stresses from CTE differences can result in seal failure either during sealing, or in subsequent service. • Measurement of mismatch Butt seal Inverse sandwich seal Expansion mismatch (δ) = α – α ) residual stress, ∆T ( f g residual strain . Science & Technology © 2015 Corning Incorporated 21 . Selection criterion: Expansion compatibility: microreactor frit • Perfect expansion match between a frit and a substrate is unlikely (different curvatures to expansion curve) 200 butt seal, BM-5 frit on EAGLE glass 150 100 50 Frit in Tension 0 CTE Mismatch, ppm 0 100 200 300 400 500 600 700 800 900 Frit in -50 Compression -100 Temperature, °C . Science & Technology © 2015 Corning Incorporated 22 . Expansion compatibility K-borosilicate frits fired on EAGLETG at 800deg 160 140 frit cracked 120 100 80 60 40 20 (frit in tension) 0 Mismatch, ppm -20 -40 -60 0 100 200 300 400 500 600 700 Temperature, degC . Science & Technology © 2015 Corning Incorporated 23 . CTE compatibility: use of fillers • Fillers are added to adjust CTE of base fit • Types of fillers – Additive – depend on Rule of Mixtures – Inversion • depend on volume change associated with phase transformation to adjust CTE • difficult to retain hermeticity because of localized microcracking • PbTiO3, [Co,Mg]2·P2O7 . Science & Technology © 2015 Corning Incorporated 24 . Effect of filler additions • Will adjust CTE, α = α1K1V1 + α2K2V2 (for additive fillers) K1V1 + K2V2 • Will affect flow, and raise viscosity (and most likely, sealing temperature) V-phosphate frit with different filler loadings fired at 380°-1h . Science & Technology © 2015 Corning Incorporated 25 . Possible additive-fillers . • β-eucryptite, LAS glass-ceramic, Li2O Al2O3 2SiO2 (intrinsic) CTE = -10x10-7/degC • Stuffed β-quartz, ZnAS glass-ceramic, CTE = 0x10-7/degC • β-spodumene, LiZnMgAS, CTE = +10x10-7/degC -7 • ZWP, Zr2(WO4)(PO4)2, CTE = -30x10 /degC -7 • Leucite, K2O·Al2O3·2SiO2, CTE = 200x10 /degC -7 • (Stabilized) ZrO2, CTE = 110x10 /degC . Science & Technology © 2015 Corning Incorporated 26 . Additive filler – β-eucryptite Figure 1 - Effect of beta-eucryptite filler level on CTE of OLED frit CTE of frit as function ofblend β-eucryptite level (wt basis) 80 100:0 70 90:10 60 C) o / 85:15 -7 50 80:20 CTE CTE (10 75:25 40 70:30 30 o (data measured in cooling on bars first fired to 400 -1hr in N2) 20 50 75 100 125 150 175 200 225 250 Temperature (oC) . Science & Technology © 2015 Corning Incorporated 27 . Expansion compatibility with display glass 700 (onset of cracking?) 600 500 400 25% filler 300 (badly cracked at RT) 30% filler 200 35% filler 100 Expansion Mismatch, ppm (frit in tension, 1737 in compression) 0 0 50 100 150 200 250 300 350 400 450 500 550 (frit in compression, -100 1737 in tension) (samples fired in situ; measurements made during cooling) -200 Temperature, deg C . Science & Technology © 2015 Corning Incorporated 28 . Inversion filler - (Co,Mg)2·P2O7 0.8(CoP2O7)-0.2(MgP2O7) . Science & Technology © 2015 Corning Incorporated 29 . Inversion filler in frit 500 mismatch for butt seal, 400 (estimated mismatch frit blend on fused silica curve, no filler) 300 200 (frit in tension) 100 225deg inversion 0 0 100 200 300 400 500 600 700 -100 (frit in compression) Expansion Expansion Mismatch in Frit, ppm -200 -300 Temperature, oC . Science & Technology © 2015 Corning Incorporated 30 . Service temperature Service temperature • Considerations – Frit viscosity (should be ≥1010 P for dimensional stability) – Stable chemistry and microstructure needed for long survival times – No reaction with substrate – Soft vs hard seal –stress relief during cooling vs. dimensional stability . Science & Technology © 2015 Corning Incorporated 32 . Frit microstructure may change at high service temperatures As-fired (850º-1hr) Frit A Frit B Frit C After aging (167hrs at 725º) . Science & Technology © 2015 Corning Incorporated 33 . High temp frit coating for Pt (must withstand exposure at 1200deg) Frit 6253, fired 1300-hr . Science & Technology © 2015 Corning Incorporated 34 . Coating for SiC furnace tubes 7760 7702 Thermal aging samples (120hrs @1500°) . Science & Technology © 2015 Corning Incorporated 35 . Other criteria Selection criterion: aqueous durability (frits for insulating windows) . Science & Technology © 2015 Corning Incorporated 37 . Near-IR Absorbance . Science & Technology © 2015 Corning Incorporated 38 . Selection criterion: compressive surface layer for strength Na & K diffusion profiles - for frit on 2318 (fired 650o-30min) 10.0 14.0 9.0 13.0 8.0 7.0 Na2O 12.0 O O 6.0 2 2 approx. surface of 5.0 11.0 substrate glass 4.0 Wt. % K Wt. % 10.0 3.0 Na Wt. % K O 2.0 2 9.0 1.0 0.0 8.0 0 50 100 150 200 250 300 350 Distance from frit surface into substrate glass (um) . Science & Technology © 2015 Corning Incorporated 39 . Processing of frits Frit pastes • Although frits may be applied by a variety of techniques (screen- printing, doctor-blading, nozzle dispensing, etc), they are always applied as a paste or ink rather than as a powder. • Paste = frit powder + solvent + binders + dispersants + surfactants • The paste is designed to exhibit a specific rheology needed for the dispensing method . Science & Technology © 2015 Corning Incorporated 41 . Changes in frit paste with temperature Dispensed After organic After frit softening paste burn-out and flow (500-600o) (300-325o) . 42 Science & Technology © 2015 Corning Incorporated 42 . Furnace sealing Frit flow and sealing Temperature Binder burn-out Elapsed Time .