Frits and Sealing

Home , Frit

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

• 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 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

Stable low temperature frits with good aqueous durability

• 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

• 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

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.

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

Micro-reactors

Requirements: Max. sealing temp: 800° CTE: 35-40 Service temp: -50º to +50º Other: high chemical durability (acids and bases)

Straight Slt. Rounding Rounding Flowed Puddled >109 P ~108 P 107 P 104-105 P <103 P

Optimum viscosity range for sealing

Straight Slt. Rounding Rounding Flowed Puddled >109 P ~108 P 107 P 104-105 P <103 P

• 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

Frit A

Frit B

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

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

• 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

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

• 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

• 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

. . • 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

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

90:10 60

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)

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

0.8(CoP2O7)-0.2(MgP2O7)

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

As-fired (850º-1hr)

Frit A Frit B Frit C

After aging (167hrs at 725º)

Frit 6253, fired 1300-hr

7760 7702

Thermal aging samples (120hrs @1500°)

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

Dispensed After organic After frit softening paste burn-out and flow (500-600o) (300-325o)

Frit flow and sealing Temperature

Binder burn-out

Laser sealing (after organics burn-out) Frit flow and sealing

Furnace-sealing Temperature

Organics burn-out

(cooling cycle once laser passes) Cooling

550

500

450

400

350

Temperature,C 300

250

200 14.7 14.8 14.9 15.0 15.1 15.2 15.3 Time,s

Near-IR laser, 5-50mm/sec

Substrate glass is transparent to Near-IR, so all laser energy reaches bottom of frit

Laser energy penetrates several microns into frit

600o Heat is transmitted to top surface of frit

. 46 Science & Technology © 2015 Corning Incorporated 46 . . Acknowledgements • I wish to acknowledge the contributions of my many colleagues at Corning Incorporated whose assistance, hard work, and ideas went into the development of the materials in this presentation.