precipitation of nanometer-sized sno2 crystals and sn depth profile in heat-treated float glass

Precipitation of nanometer-sized SnO2 crystals and Sndepth profile in heat-treated float glass

Satoshi Takeda a,b,*, Ryoji Akiyama a, Hideo Hosono b

a Research Center, Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi 221-8755, Kanagawa, Japanb Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda-cho, Midori-ku, Yokohama 226-8503, Japan

Received 4 September 2001; received in revised form 28 February 2002

Abstract

The effect of oxygen diffusion from the atmosphere on tin depth profile in the bottom face of a soda-lime–silica float

glass at temperatures above Tg was investigated. The heat treatment was performed in 18O2/N2 and argon (Ar) at-

mospheres. The significant diffusion of tin to the surface was observed for the glass heat-treated in 18O2/N2 atmosphere,

resulting in the formation of a tin-enriched layer near the surface region. It was found that the tin was supplied from the

region shallower than the �hump� which is commonly observed in the tin profile of a commercial soda-lime–silica float

glass. No significant change in the tin depth profile was observed for the glass heat-treated in Ar atmosphere. These

results indicate that 18O diffusion into the glass, which causes the change in chemical state of tin from Sn2þ to Sn4þ,

induces the significant diffusion of tin. Furthermore, the precipitation of crystalline SnO2 particles with a diameter of

�1 nm was clearly recognized in the tin-enriched layer. This fact indicates that a phase separation was induced by the

oxygen diffusion into the glass. Consequently, Sn2þ may be supplied to the surface in order to compensate for the

marked decrease in Sn2þ concentration in the glass system. The significant diffusion of tin to the surface was suppressed

by increasing the iron content in the glass. This suppression was ascribed to the increase in Sn4þ concentration as a

result of the redox reaction between tin and iron because the diffusion coefficient of Sn4þ is much smaller than that of

Sn2þ.

� 2002 Elsevier Science B.V. All rights reserved.

PACS: 61.43.F; 64.75; 94.20.Q; 68.35.F

1. Introduction

Float glass is most widely used in industrial uses

for architectures, automobiles and displays such as

liquid crystalline displays or plasma display panels

because of its high productivity and excellent

flatness over a large area. In the float process,

molten glass is floated on a molten tin bath, so that

tin is penetrated into the glass. It is known that the

tin is not uniformly diffused into the glass and the

profile has an anomalous hump [1–5].When the float glass is used for architectural

or automotive applications, it is sometimes tem-

pered or bent by thermal toughening process.

Consequently, the appearance of the glass often

Journal of Non-Crystalline Solids 311 (2002) 273–280

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* Corresponding author. Tel.: +81-45 374 8794; fax: +81-45

374 8892.

E-mail address: [email protected] (S. Takeda).

0022-3093/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0022-3093 (02 )01374-1

mail to: [email protected]

becomes milky or hazy by the process, which iscalled �bloom� [1]. This is a serious problem for the

glass manufacturing industry because the trans-

parency is lost. Therefore, it is important to clarify

the origin of the bloom. This phenomenon is ob-

served only for the bottom face of the glass, indi-

cating that the diffused tin may be closely related

to the bloom. Until now, many researchers have

extensively studied the phenomenon, and reportedthat the tin-enrichment layer and wrinkling struc-

ture were detected near the surface region after the

heat treatment [3,6,7]. These findings suggest that

the bloom may be induced by the formation of the

tin-enrichment layer. However, the mechanism of

the significant tin diffusion is not clearly elucidated

so far.

The purpose of the present study is to clarify themechanism of the significant tin diffusion to the

surface. A part of this work has been reported in

Ref. [8]. Here, we investigated the effect of oxygen

diffusion from the atmosphere on the tin diffusion.

The heat treatment was performed in 18O2/N2 and

argon (Ar) atmospheres. The depth profiles of tin

and 18O for the bottom face of the glass were

measured using secondary ion mass spectrometry(SIMS). In this experiment, the behavior of oxygen

from the atmosphere could be traced in detail even

if much oxygen is present in the glass because the

oxygen tracer (18O) gas was used. Namely, direct

information can be obtained about the oxygen

and tin depth profile. The tin-enriched layer was

explored by transmission electron microscopic

(TEM) observation and selected area diffraction(SAD) analyses.

In addition, we also investigated the effect of

iron on the tin diffusion. It is known that the im-

purity iron is commonly present in the glass, and

that the iron affects the chemical states of tin [9–

11], causing the change in tin depth profile. From

the results obtained, the behavior of tin in the

bottom face of the float glass by the heat treatmentwas discussed.

2. Experimental procedures

Commercial soda-lime–silica float glasses were

used in this study. The main compositions of the

glasses are listed in Table 1. These glasses weremanufactured on the same float line although the

iron content was different. The glass transition

temperature (Tg) of the glasses is 562 �C. A sche-

matic illustration of the experimental apparatus

for the heat treatment was described in Ref. [8].

The bottom face of the glasses was set upward.

The glasses were heat-treated at 740 �C (�2 �C) for

15 min in 18O2/N2 ¼ 1=4 and Ar atmosphere. Priorto the heat treatment, the sample chamber (quartz

glass vessel) was purged with Ar gas (99.999%

purity) flow for 5 h in order to diminish the re-

sidual oxygen. Then, the vessel was evacuated and

then 18O2/N2 (99% purity for 18O and 99.999%

purity for N2) or Ar gas was introduced in the

vessel up to the atmospheric pressure.

Observation of surface morphology and thequantitative analysis of the surface roughness of

the glass were performed using an atomic force

microscope (AFM). The optical transmission

spectra of the glass were measured at room tem-

perature in air using a dual beam spectrometer.

The appearance change of the glass due to the

heat treatment was evaluated as haze value change

using a haze meter. The haze value is defined asTd=Tt � 100% (Td; scattered light, Tt; transmitted

light).

The depth profiles of 120Sn, 18O and 54Fe for the

bottom face of the glass were measured using

SIMS. Positive secondary ions were detected using

an Oþ2 primary ion beam operated at 8 keV, 100

nA. The angle of incidence was 60� to the normal

Table 1

The main compositions of the glass samples

Sample Main compositions (wt%)

Sample Aa 72-SiO2, 13-Na2O, 8-CaO, 4-MgO, 2-Al2O3, 0.15-Total Fe (as Fe2O3)

Sample Ba 72-SiO2, 13-Na2O, 8-CaO, 4-MgO, 2-Al2O3, 0.18-Total Fe (as Fe2O3)

a These glasses were manufactured on the same float line.

274 S. Takeda et al. / Journal of Non-Crystalline Solids 311 (2002) 273–280

of the sample surface and an area of 100 � 100lm2 was sputtered. Negative secondary ions were

detected using a Csþ primary ion beam operated at

6 keV, 20 nA. The angle of incidence was 45� to

the normal of the sample surface and an area of

400 � 400 lm2 was sputtered. The charge neu-

tralization was accomplished using an electron

flood gun. The etching rate was determined by

mechanically measuring the depth of the craterafter SIMS measurements. The accuracy was

within �5 nm. The tin-enriched layer near surface

region of the glass was observed by TEM. The

crystalline phases of the layer was identified by

SAD analyses. The chemical compositions of

the layer were determined by X-ray photoelectron

spectroscopy (XPS).

3. Results

3.1. Effect of heat treatment atmosphere on tin

depth profile

Table 2 shows the haze value for Sample A

heat-treated in 18O2/N2 or Ar atmosphere. It is

found that the haze value significantly increases

after the heat treatment in 18O/N2 atmosphere.

This phenomenon is not observed when heattreating in Ar atmosphere. This result suggests

that the increase in haze value may be induced by

oxygen from the atmosphere.

Fig. 1 shows the AFM images for Sample A

before and after heat treatment in 18O2/N2 or Ar

atmosphere. The wrinkling structure is observed on

the surface of the glass heat-treated in 18O2/N2 at-

mosphere. The surface roughness (Rms) estimatedfrom the image is �50 nm. This result indicates that

the haze increase, as shown in Table 2, is due to

the geometrical light scattering by the wrinkling

structure. The wrinkling structure is not recognized

for the sample heat-treated in Ar, suggesting that

the formation of the wrinkling structure is caused

by oxygen from the atmosphere.

Table 2

The effect of the heat treatment atmosphere on the haze value

for Sample A

Heat treatment atmosphere Haze value

Before heat treatment 0.1

Ar; 100% 0.318O2/N2 ¼ 1=4 3.2

Fig. 1. AFM images of Sample A; before heat treatment (a), after heat treatment in 18O2/N2 (b) and Ar (c) atmospheres.

S. Takeda et al. / Journal of Non-Crystalline Solids 311 (2002) 273–280 275

Fig. 2 shows the SIMS depth profiles for Sam-

ple A before and after heat treatment in 18O2/N2 or

Ar atmosphere. Before the heat treatment, a hump

in tin profile is clearly observed around at thedepth of �3 lm from the top surface. After the

heat treatment in 18O2/N2 atmosphere, the signifi-

cant diffusion of tin to the surface is recognized,

resulting in the formation of a tin enriched layer

near the surface region. The tin profile shallower

than the hump distinctly changes after the heat

treatment. This means that the tin diffused to the

surface is supplied from the region shallower thanthe hump. No significant change in tin depth

profile is observed for the glass heat-treated in Ar

atmosphere.

Fig. 3 shows the SIMS depth profile of 18O and120Sn near surface region for the glass before and

after the heat treatment in 18O2/N2 atmosphere.

The diffusion of 18O into the glass is also observed

and the amount of incorporated 18O is larger bytwo orders of the magnitude than that without the

heat treatment. The diffused depth of 18O, which is

defined as the depth that the secondary ion in-

tensity reach to the same level as that without

the heat treatment, is �300 nm of the surface.

The depth at the maximum concentration of 18O

is almost the same as that of tin.Fig. 4 shows (a) cross-sectional TEM image and

(b) SAD pattern of the tin-enriched layer. The

Fig. 2. SIMS depth profiles for Sample A before and after heat treatment in 18O2/N2 or Ar atmosphere.

Fig. 3. SIMS depth profiles of 120Sn and 18O for Sample A heat-

treated in 18O2/N2 atmosphere.


precipitation of nanometer particles is clearly ob-

served in the tin-enriched layer and the average

size of the particles is �1 nm. The SAD pat-

tern reveals that these particles are crystalline

and identified as SnO2. The chemical compo-

sitions of the tin-enriched layer determined byXPS is (wt%): 58.0-SiO2, 16.9-Na2O, 6.5-CaO,

18.2-SnO2, which significantly differs from that of

the glass before the heat treatment.

3.2. Effect of iron on tin depth profile

Fig. 5 shows the SIMS depth profile for Sam-

ples A and B heat-treated in 18O/N2 atmosphere.These glasses were manufactured on the same float

line, and the iron concentration of Sample B is

larger than that of Sample A, as shown in Table 1.

It is found that the depth at the maximum con-

centration of tin is slightly shifted to the deeper

side for Sample B compared with that of Sample

A. This indicates that the diffusion of tin to the

surface is suppressed by increasing the iron con-

tent in the glass. Furthermore, the surface iron

concentration of Sample B is larger than that of

Sample A. No significant difference was observed

in the transmission spectra of the glasses.

4. Discussion

4.1. Mechanism of significant tin diffusion to the

surface

As shown in Fig. 2, the significant diffusion of

tin to the surface is distinctly observed after the

Fig. 4. Cross-sectional TEM image (a) and selected area dif-

fraction pattern (b) of the glass heat-treated in 18O2/N2 atmo-

sphere.

Fig. 5. SIMS depth profiles of 120Sn, 18O and 54Fe for Samples

A and B heat-treated in 18O2/N2 atmosphere.


heat treatment in 18O2/N2 atmosphere. In addition,the tin depth profile shallower than the hump

position markedly changes. These results suggest

that the tin is supplied from a shallower region

than the hump. As mentioned before, it is known

that the chemical states of tin are changed with

depth, and that the depth profile shows an

anomalous hump [1–5]. According to the research

of Williams et al. [3] using M€oossbauer spectro-scopy, the majority of the tin near the surface exists

as Sn2þ. Nomura [5] also investigated the Sn2þ/

Sn4þ ratio with depth, and reported that almost all

the species were Sn4þ at the deeper layers than the

hump. Furthermore, it is known that the diffusion

coefficient of Sn4þ is much smaller than that of

Sn2þ. These suggest that the diffused tin to the

surface is considered to be stannous tin (Sn2þ).On the other hand, no marked change in the tin

depth profile is observed for the glass heat-treated

in Ar atmosphere, indicating that the significant

diffusion of tin is induced by oxygen diffusion from

the atmosphere. As is seen in Fig. 3, the depth of

diffused 18O is �300 nm of the surface. This fact

suggests that the effect of oxygen diffusion on the

oxidation states of tin is within �300 nm. It isknown that the small amount of Sn2þ is oxidized

to Sn4þ by the heat treatment in air [3].

The precipitation of crystalline SnO2 particles

with a diameter of �1 nm is clearly observed in the

tin-enriched layer, as shown in Fig. 4. This ob-

servation indicates that a phase separation occurs

in the tin-enriched layer. Here, it is known that the

co-ordination structure of tin is different betweenSn2þ and Sn4þ in tin–soda-lime–silica glass system,

and that Sn4þ is in an octahedral co-ordination

(the bond is ionic) and Sn2þ is in tetragonally py-

ramidal co-ordination (the bond is rather cova-

lent) [12]. This suggests that the change in

oxidation state of tin from Sn2þ to Sn4þ by oxygen

diffusion [3] may subsequently cause the change in

the co-ordination structure of tin from tetrago-nally pyramidal to octahedral co-ordination. This

valence change is rather drastic because the va-

lence increases by 2 and the bonding nature

changes from covalent to ionic. Thus, the forma-

tion of non-bridging oxygens is required so as to

meet the local electroneutrality around Sn4þ. As a

consequence, phase separation is induced [13]. In

fact, it is known that SnO2–SiO2 glass system isphase-separated into SnO2 and SiO2: Sn phases

when the SnO2 concentration increases [14]. In

the present study, the tin concentration of the tin-

enriched layer increases markedly due to the up-

diffusion of Sn2þ , so that the phase separation

may be induced, as is seen in Fig. 4.

Based on these analyses, we conclude that the

significant diffusion of tin should result from theconcentration decrease in Sn2þ ions as a result of

elimination of Sn ions from the supercooled liquid

state, i.e., when O2 gases from air are diffused into

the tin-enriched layer to oxidize Sn2þ into Sn4þ,

resulting Sn4þ ions are precipitated as SnO2 crys-

tals, and Sn2þ ions in the region shallower than the

hump are diffused towards the surface layer to

compensate the drop of Sn2þ ions in the layers.Consequently, the wrinkling structure on the sur-

face is formed, as shown in Fig. 1(b), as a result

of mismatching of the thermal expansion coeffi-

cient between the tin-enriched layer and the glass.

Fig. 6 shows a schematic illustration of a mecha-

nism to explain the significant tin diffusion and the

formation of the wrinkling structure on the sur-

face.

4.2. Iron effect on the tin diffusion

As is seen in Fig. 5, it is found that the signifi-

cant diffusion of tin to the surface is slightly sup-

pressed by increasing the iron content in the glass.

It is known that the iron in the glass can react with

tin, as follows [9–11]:

Sn2þ þ 2Fe3þ ! Sn4þ þ 2Fe2þ:

Namely, the increase in iron content causes the

increase of Sn4þ concentration in the glass. It is

considered that increasing the Sn4þ concentration

decreases the mobility of tin because the diffusion

coefficient of Sn4þ is much smaller than that of

Sn2þ. Namely, the suppression of the tin diffusion,

as shown in Fig. 5, is due to the increase in Sn4þ

content as a result of redox reaction betweenSn2þ and Fe3þ. This result suggest that adding

the oxidizing species for Sn2þ such as Fe3þ is useful

for controlling the tin diffusion by the heat treat-

ment.


5. Conclusions

In this paper, we have investigated the effect of

oxygen diffusion from the atmosphere on tin depth

profile of a soda-lime–silica float glass at temper-

atures above Tg by TEM and SIMS with 18O2 gas.

The significant diffusion of tin to the surface was

observed for the glass heat-treated in 18O2/N2

atmosphere, resulting in the formation of tin-

enriched layer near the surface region. It was found

Fig. 6. Mechanism of significant tin diffusion to the surface and formation of wrinkling structure near the surface region of the glass

heat-treated in 18O2/N2 atmosphere.


that the precipitation of nanometer-sized SnO2

crystals was clearly recognized in the tin-enriched

layer, indicating that a phase separation was in-

duced by the oxygen diffusion into the glass. Based

on these analyses, we conclude that the tin (Sn2þ)

was supplied from the region shallower than the

hump in order to compensate for the marked de-

crease in Sn2þ concentration of the glass system.

Furthermore, it was also found that the significantdiffusion of tin was suppressed by increasing the

iron content in the glass. The suppression was due

to the decrease in the mobility of tin, which was

induced by the increase in Sn4þ concentration as a

result of the redox reaction between tin and iron,

because the diffusion coefficient of Sn4þ is much

smaller than that of Sn2þ. Therefore, to add the

oxidizing species for Sn2þ such as Fe3þ is usefulfor controlling the tin diffusion by the heat treat-

ment.

We expect that the present findings offer a novel

clue to control of the tin depth profile and to im-

provement of the thermal durability of float glass.

Acknowledgements

The authors are grateful to Dr S. Ito and Dr K.Yamamoto of Asahi Glass Co., Ltd. for valuable

discussion.

References

[1] L.A.B. Pilkington, Proc. Roy. Soc. Lond. A 314 (1969) 1.

[2] J.S. Sieger, J. Non-Cryst. Solids 19 (1975) 213.

[3] K.F.E. Williams, C.E. Johnson, J. Greengrass, B.P. Tilley,

D. Gelder, J.A. Johnson, J. Non-Cryst. Solids 211 (1997) 164.

[4] K.F.E. Williams, C.E. Johnson, O. Nikolov, M.F.

Thomas, J.A. Johnson, J. Greengrass, J. Non-Cryst. Solids

242 (1998) 183.

[5] K. Nomura, in: M. Miglierini, D. Petridis (Eds.), M€ooss-

bauer Spectroscopy in Material Science, Kluwer Academic,

Dordrecht, 1999, p. 63.

[6] J. Deubener, R. Bruckner, H. Hessenkemper, Glastech.

Ber. 65 (1992) 256.

[7] C.G. Pantano, V. Bojan, Riv. Staz. Speriment. Vetro

XXIII (Suppl.) (1993) 285.

[8] S. Takeda, R. Akiyama, H. Hosono, J. Non-Cryst. Solids

281 (2001) 1.

[9] A. Kumar, S.P. Singh, R. Pyare, Glastech. Ber. 64 (1991)

106.

[10] K.F.E. Williams, M.F. Thomas, C.E. Johnson, B.P. Tilley,

J. Greengrass, J.A. Johnson, Fundamentals of Glass

Science and Technology, 1997, p. 127.

[11] P.D. Townsend, N. Can, P.J. Chandler, B.W. Farmery,

R. Lopez-Heredero, A. Peto, L. Salvin, D. Underdown,

B. Yang, J. Non-Cryst. Solids 223 (1998) 73.

[12] H. Kawazoe, J. Nishii, H. Hosono, T. Kanazawa,

H. Imagawa, J. Phys. C9 (1982) 156.

[13] H. Rawson, Inorganic Glass-Forming System, Academic

Press, London, 1967.

[14] C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic

Press, 1990;

V.V. Sidorchuk, V.M. Chertov, Sov. J. Inorg. Mater.

22 (1986) 1692.


precipitation of nanometer-sized sno2 crystals and sn depth profile in heat-treated float glass

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