Sealing glass with high absorption in the near infrared region

Journal of Non-Crystalline Solids 52 (1982) 467-477 North-Holland Publishing Company

SEALING GLASS WITH HIGH ABSORPTION I N F R A R E D REGION

467

IN THE NEAR

LIAN Tiejun Baoding No. 3 Glass Plant, Hebei, China

JIANG Chuansong, YAN Guofeng, XU Yueshen and WANG Naizhen Beifing Electron Tube Factory, Beifing China

In order to obtain a high proportion of FeO in glass, experiments were carried out using a series of reductants. Results show that combined reductant is more efficient for this purpose. When Na~O in the glass is substituted for Li20, not only the position of near IR absorption peak shifts toward the shorter wavelengths but the absorption rate also declines. The equiponderant substitution of bivalent oxides has little influence on the near IR spectra. However, when ZnO is added, the transmissivity in the 400/tm region increases conspicuously. Through two groups of experiments the formulation of glass with high resistivity and lower working temperature was found. The ingredients of the formulation do not contain such elements as Pb, CI and F, and the content of K20 is only 1.14%, thus avoiding contamination of devices in the sealing process. Moreover, the mechanism of glass-seed formation during the melting process is explained.

1. Background I n the p a s t decade, I R sealing glass c o n t a i n i n g F e 2+ has b e e n p r e p a r e d at a fairly high level [ 1-6,9], b u t the glass itself has b o t h m e r i t s a n d d r a w b a c k s . F o r e x a m p l e , fig. 1 shows the result of Y a n o a n d Y o k o g a w a ' s w o r k [7]. It shows t h a t even if the C 1 - c o n t e n t of the glass is 0.1%, KC1 will e v a p o r a t e d u r i n g the

lO, O ..~ l.g Ol

ooia,

i¢' ' # ' '#" )00

Fig. 1. Variation of the contact resistance of the reed tube: A, K20-free glass; O, K20-containing glass. 0022-3093/82/0000-0000/$02.75

© 1982 N o r t h - H o l l a n d

468

Lian Tiejun et aL / Sealing glass with high absorption

sealing process and pollute the reed thus making the constant resistance of the reed unstable within 106 operations. As the K20 content is decreased from 13.4% to 1.07%, the contact resistance of the reed becomes stable. However, when the K20 content is decreased, T k - 1 0 0 which is the temperature at which the resistivity of the material is equal to l0 s f l c m - 1 is prone to decrease to 210 ~ 250°C, thus other components of the glass must be modified to obtain the desired resistivity. Moreover, glass with a high working temperature will result in obvious pollution and thermal damage of the devices. Therefore, it is necessary to decrease the working temperature of the glass and at the same time, increase the near IR absorption of the glass. The purpose of this paper is to present the experimental results of the formulation of glasses with favourable characteristics and high IR absorptivity.

2. Experiment On the basis of an eleven-component R20-R203-SiO 2 system, two groups of experiments were performed. In group A, Li20 and N a 2 0 contents were varied as follows L i E O : N a 2 0 = X: (1 - X), where X = 0 -, 1 (mo196),

and the K 2 0 content remained unchanged. In group B, MgO, CaO, SrO, BaO and ZnO were respectively used as RO in the system. The reductants used were sugar, silicon, tin, stannous oxide, tartaric acid and their combinations. Cullet and powdered material were mixed uniformly in the proportions of 0: l, 1 : l, 2 : 1 and 3 : 1 and were melted. Then samples were taken to determine their visible-near IR spectra. The temperature corresponding to the viscosities of 1012"5, 107.6 and 104 P, T k - 100, chemical stability against water, expansion coefficient and density were determined. Technological experiments on the IR sealing machine were carried out. The amounts of Fe203 and A1203 in the quartz sand used were taken in account in the formulation. Melting was carried out in a sealed 30 lb crucible. The refining temperature was about 1320 ~ 1350°C. 3. Results

The spectral curves of the samples from Groups A and B are shown in figs. 2 and 3. Other physical-chemical properties are listed in tables 1 and 2. Results obtained with the selection of reductant can been seen in table 3 and fig. 4.

Lian Tiejun et al. / Sealing glass with high absorption

469

i k

4~.~

3524

Y o4

31o

~

.~

6oo

70o

~

9~ ~.(#m)

Fig. 2. Spectral curves of glasses with LiO 2 substituted for N a 2 0 gradually. Note: The meaning of the numbers on the curves refers to table 1.

90

!O9 I Io.8

N

~5~_~U //

~

~\

--

I

02

~.(#m) Fig. 3. The spectral curves of glasses containing different bivalent oxides. * Visible transmissivity of the ZnO-containing glass is high. Note: The meaning of the numbers on the curves refers to table 2.

Dispensation

3521 3522 3523 3503 3524 3525 3526 3527

IlO,

0:1 0.3:1 0.5:1 0.7:1 1:1 1:0.7 1:0.5 1:0

Li 20 : N 2 0

1 2 3 4 5

No.

3535 3531 3534 3533 3532

no.

Dispensation

MgO CaO SrO BaO ZnO

RO

88.6 88.8 89.6 88.7 87.7

a × 107

Table 2 Physical-chemical properties of group B glass

1 2 3 4 5 6 7 8

No.

Table 1 Physical-chemical properties of group A glass

91.5 90.0 89.2 88.9 88.5 87.6 86.1 83.1

531 529 530 533 532

1012-5 p

T(°C)

aXlO 7

576 535 527 519 518 515 517 527

619 616 614 619 622

10 7.6 p

1012"5 p

T(°C)

656 626 617 610 612 607 608 632

10 7.6 p

853 840 848 843 875

10 4 p

891 865 850 839 840 854 845 853

273 294 287 283 274

(°C)

Tk-lO0

104 p 230 265 274 286 282 279 267 218

2.724 2.748 2.775 2.790 2.778

d ( g / c m 3)

Tk-100 (°C)

6.8 4.8 4.4 4.7 4.0 4.8 4.5 3.8

Against water weight loss (mg)

7.9 3.6 7.4 7.2 6.1

Against water weight loss (mg)

2.789 2.792 2.786 2.785 2.772 2.759 2.744 2.730

d ( g / c m 3)

E"

¢b

5

O

471

Lian Tiejun et al. / Sealing glass with high absorption Table 3 The conditions for the use of reductants No.

Forms of the introduced iron

Reductant

Refining agent

1

Fe203

sugar

2

Fe203

Si

3

Fe203

Sn

4

FeC204.2H20

Sn, Tartaric acid

5

Fe203

SnC12" 2H 2O

6

Fe + +

Sn + +, Tartaric acid

poor reduction, unstable, clarification just so so unstable reduction, serious Sb203 or gray seeds, suspension of black As203 grains can be seen at times N H 4 H C O 3 stable reduction serious gray seeds N H 4 H C O 3 good reduction, serious gray seeds quite good reduction, clarificaN H 2C1 or NH4HCO 3 tion good, smokes at sealing Nil good reduction, good clarification no smoke at sealing

~

,~

,,~

,~

,3,

,,~

Results

Sb203

,~o

,~

,~

~

,~

z~

~,,

z~

2~,

~

90

z~

~

~ o.9

~06

5O 3

~

~5 4oi

N I0 o ?~o

03 02 ot

~ 3~o

~

~

6oo

7o0

8oo

0

900 h(#m) Fig. 4. The effect of different reductants on spectral curves of the glasses with similar composition. 1. Combined reductant. 2. Simple inorganic reductant, S. Simple organic reductant.

472

Lian Tiejun et al. / Sealing glass with high absorption

OD 3 . 0 / I D 2.0 mm glass tube were used to make reeds; sealing power on the automatic sealing machine was 20 V × 9 A × 3.55 (without preheating).

4. Discussion 4.1. Selection of reductant The literature [8,11] describes the effect of glass components on the formation of (Fe 2÷ O6), however the influence of reductant must not be neglected. When Si and Sn are chosen as reductant, reduction effects are not so good, moreover, Si and Sn result in a great deal of gas-bubbles in the glass. When SnC12 and H2C204 are used as combined reductant, reduction effects are improved and gas-bubbles are avoided, but a large amount of C1- is introduced which causes pollution during sealing [7,9]. According to the diffusion equation [10] the glass is melted in the presence of oxygen, Fe 3÷ formed on the surface will diffuse into the inside of the glass. The longer the time of diffusion, the higher is the concentration of Fe 3+ in glass, i.e., a portion of Fe 2+ is oxidized. Evidently, by adding reductants such as silicon or sugar, the IR absorbing capacity of the glass cannot be increased and will not remain stable. When C4H606 and SnO, etc. are mixed and used as a reductant at low temperatures, a large amount of CO and CO 2 which results from the decomposition of C4H606 and raw ingredients lowers the partial pressure of oxygen in the melting atmosphere. Therefore tin and iron tend to enter the glass melt in the low-valence state. At high temperatures, the diffusion of oxygen is enhanced, but the reduction capacity of Sn 2+ is stronger than that of Fe 2+ and the possibility of Sn 2÷ transforming into Sn4÷ which enables Fe 2+ to predominate and remain stable, hence the absorption of the glass in the 1.0 ~ 1.1 /xm region, is promoted. After repeated determinations, the fluctuation of the transmissivity of the glass at 1.05/~ is only 5.7 - 6.3%; whereas, with any single reductant, the results are not so good, for example, 1 5 - 25% for sugar, and 15 ~ 30% for silicon powder (fig. 4). 4.2. Spectral curves The visible-near IR spectra of ion-containing glasses in simple systems have been reported [11-15]. The characteristic absorption band described in these papers is similar to the present work. From fig. 2 we can see that the absorption band of [Fe 2+ 06] is at 1.05 #, the absorption band of [Fe 2+ 04] is at 1.85/~, and a series of absorption bands of [Fe3+O6] and [Fe3+O4] are located at 380~735/~m. As N a 2 0 is gradually substituted by Li20, the position of the IR absorption peak shifts from 1.11 # to 1.03 kt. Meanwhile, transmissivity rises from 5.19% to 9.6% (table4). Kumar [14] has explained why the peak position shifts. To explain the increase in transmissivity, one can

1 2 3 4 5 6 7 8

5.4899 5.2290 5.3714 6.0761 6.3047 6.8922 8.1099 9.7559

1050

3521 3522 3523 3503 3524 3525 3526 3527

1 2 3 4 5 6 7 8

No.

Dispensation no.

No.

5.4365 5.2107 5.2916 6.0705 6.3490 6.9014 8.2014 9.8295

1060

0:1 0.3:1 0.5:1 0.7:1 1:1 1/0.7 1:0.5 1:0

5.3520 5.1237 5.2763 6.0791 6.3139 6.9015 8.2151 9.9287

1070

In composition Li20:Na20

Table 4 Absorption peaks and transmissivity

5.2931 5.1191 5.3050 6.1141 6.3414 6.9548 7.2869 10.0767

1080

6.1949 5.7951 5.6944 6.2728 6.5652 7.1412 8.3769 9.6327

1000

5.2565 5.0764 5.2916 6.1294 6.4558 6.9685 7.3662 10.2323

1090

6.0077 5.6242 5.5738 6.2316 6.4573 7.0890 8.2807 9.6510

1010

Transmissivity ( 1000-1130 tt m)

5.2198 5.1100 5.3587 6.1645 6.5275 7.0845 8.4684 10.3987

1100

5.8409 5.5174 5.4960 6.1691 6.3627 6.9502 8.1770 9.6281

1020

5.1893 5.1496 5.3999 6.2301 6.6420 7.1562 8.5417 10.6123

1110

5.7112 5.4274 5.3877 6.1004 6.3012 6.9349 8.1236 9.5961

1030

5.2153 5.1680 5.4350 6.2805 6.6740 7.2584 8.6775 10.8168

1120

5.5759 5.3175 5.3099 6.0485 6.2910 6.8647 8.1037 9.6540

1040

5.2458 5.2320 5.6433 6.3705 6.8109 7.3717 8.8987 11.0487

1130

r.--.

t".

Lian Tiejun et aL / Sealing glass with high absorption

474

refer to fig. 2 first. As N a 2 0 is substituted by Li20, transmissivity increases gradually in the 1.1# region, this shows that the amount of [Fe2+O6] is decreasing. The downward concave tendency of transmission curves in the 1.85/~ region gradually becomes obvious, this shows the increase in the amount of [Fe 2+ 04]. Meanwhile, there is no regular change in the visible region which shows at least that Fe 3+ has not increased. It follows that as N a 2 0 is substituted by Li20, a portion of [Fe 2÷ 06] changes into [Fe2+O4] instead of Fe 3÷ . This conversion may be the result of the structural contraction and the increase of split energy of Li20, thus, the Li20 content, which is useful in promoting the IR absorbing capacity is limited. From fig. 3, it can be seen that using equiponderant bivalent oxides MgO, CaO, SrO, BaO and ZnO as R O in the basic formulation have no noticeable effect on the transmissivity in near IR region. This agrees with Kumar's conclusions [14]. However, the addition of 2.5 ~ 3.5% ZnO into the glass is more effective than the addition of the same amount of MgO, CaO, SrO and BaO in raising the transmissivity in the 3 8 0 - 550 # m region, by a value as much as 5 ~ 10% [10]. This is due to the fact that FeS4 is substituted by ZnS 4 [16], and Fe 2+ - S has absorption in the 300 m/~ and 410 m~ regions [11]. 4.3. I n v e s t i g a t i o n on the c o m m o n p r o p e r t i e s o f the glass

Under the condition that the K 2 0 content is 1.14%, when N a 2 0 is gradually substituted for Li20, the first group of properties like the temperatures at different viscosities undergo a minimum value, and Tk-100 shows a maximum c 600

~.?=1012.5P

6~Ol-\'z°°[ r~=107.6 P

, 90~

bOO .900

O0

oo t~(~O

200

LizO N&eO

• Li20 N~,O

# # ~ too L(~O

~7 0 20 40 60 80 100 0 ~ 40 60 &# 100 O ~ 40 60 ~ 100 LieO No,O LSzO Na,O LifO

Na,O

Fig. 5. Changes of glass properties due to the substitution of Li20 for Na20.

Li + : N a + - - - 0 : 1

Na +

Na +

R + b o n d e d to [AIO4]- and [BOa]

R + in n e t w o r k interstices

Na +

N a + ~ Li +

Li + :Na + < 1 : 1

Na 4

Li +

Li + : N a + = 1 : 1

Na + ~ Li +

Li +

Li + :Na + >1:1

a n d L i 2 ° o n t h e s t r u c t u r e p o s i t i o n s a f t e r g r a d u a l l y s u b s t i t u t i n g N a 2° b y L i 2 0

proportion

R ÷

Table 5 Distributive variations of Na20

Li ÷

Li +

Li + :Na + =1:0

J

q.

E

~

Lian Tiejun et al. / Seating glass with high absorption

476

value. These minimum and maximum points are in the region of L i 2 0 : N a 2 0 -- 1 : 1. And the second group of properties like the expansion coefficients and the density curves also inflect at the point ( L i 2 0 : N a 2 0 --- 1 : 1), and the curves descend in the shape of an S (fig. 5). The number of Li + corresponding to the point of L i 2 0 : N a 2 0 = 1:1 is equal to the sum total of AI 3+ plus B 3+ in the glass. As Li + can neutralize the negative charge of (A104)- and (BO4)- [17] better than Na + , therefore, when the Na ÷ bound on the tetrahedra are all substituted by Li + , any additional Li ÷ will occupy the position in the network interstices originally occupied by Na ÷ (table 5). When L i 2 0 : N a 2 0 < 1, the exchange of two kinds of ions influences the network directly, so most of the properties change rapidly (fig. 5). When L i 2 0 : N a 2° > 1, the exchange of two kinds of ions takes place in the network interstices, so the characteristic curves change gradually. But the Tk-100 curve is just the opposite (fig. 5). Thus, the theory that the conductivity of the glass results from the transfer of ions is proved. This is favourable to make products with high Tk-100, lower softening point and lower operating temperature.

4.4. The mechanism of gas-seed formation of elemental reductants When simple substances like Si or Sn are chosen as the reductant, a great many gas seeds are always produced in the glass and they are difficult to eliminate. On this point the author proposes the following interpretation: the interface between unoxidized silicon or tin in the glass melt provides a

~-

A~5~ ~'~n~

G

OSe~ d

D

Fig. 6. Mechanism of seed formation: A, co-existence in different phase; B, bubbles of nucleation; C, melted after solid phase oxidation; D, formation of seeds.

Lian Tiejun et al. / Sealing glass with high absorption

477

heterogeneous nucleation center for gases evolved at high temperatures. When silicon or tin is gradually oxidized and enters into the glass, the gases are left in the glass to form the gas-seeds (fig. 6).

5. Conclusions

Combined reductant is effective for promoting iron-containing glasses to absorb 1.1/z light. When the same equivalent Na20 is substituted by Li20 in glass, the IR absorption peak shifts to the direction of the shorter wavelength, and the absorption rate also lowers somewhat. With the presence of alkali metal oxides, the equiponderant substitution of alkaline earth metal oxides does not effect the IR absorption of the glass to any considerable extent. When ZnO is added, it is possible to up-grade the transparency of the glass. Addition of certain amounts of A1203 and B 2 0 3 will help to strengthen the mixed base effect. It is possible to interpret the mechanism of gas-seed formation of elemental reductants by using the heterogeneous nucleation center concept. As a result of the present work, it is possible to obtain a formulation of glass with high sealing performance of IR absorption that can be produced on an industrial scale.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

M. Morgan, U.S. patent 3949335 (1976). M. Morgan, U.S. patent 3961970 (1978). O. Lindig et al., US patent 4001741 (1977). Japanese Patent, 50-50417 (1975). Japanese Patent, 51-21007 (1976). Japanse Patent, 53-22830 (1978). Takao Yano and Toshiki Yokogawa, The effect of impurities sputtering from sealed glass to reed contacts (OKI Electric Industry Company Ltd., Japan). V.M. Firsov et al., Tenth Int. Congr. on Glass No. 1 (1974) p. 778. Lian Tiejun et al., Electron-Vacuum Glass Technology, 1 (1981) p. 92. W.D. Johnston, J. Am. Ceram. Soc. 47 (1964) 199. A. Bishay et al., Proc. Int. Conf. on Physics of Non-Crystalline Solids, Delft (1964) p. 594. F.N. Steele et al., Phys. Chem. Glasses 6 (1965) 251. T. Bates, Modem Aspects of the Vitreous State, Vol. 2, ch. 5 (1962) p. 241. S. Kumar, Glass and Ceramic Bulletin, 6 (1959) 101, 114, 124. E.I. Swarts et al., Proc. Vile Congr. Int. Verre Bruxelles, C. R. 1-2 (1965) p. 23.4. M. Fanderlik, Chemistry of Glass (1958). J.M. Stevels, Philips Technical Review, 22 (1960/1961) 301,310.