. 25, 009-621 Xnd. Eng. Chem. Prod. Res. D ~ v1986,
Table IV. Tensile and Flexural Strength and Modulus of Some Stryene/Maleimide Copolymers init visc property w t % (PSI) MI 4 CP 8 CP 12 CP 2930 6240 5230 0 ten str 1.4 0.64 1.79 % elong ten str % elong ten mod flex str flex mod
5
ten str % elong ten mod flex str flex mod
18
ten str % elong ten mod flex str flex mod
25
ten str % elong ten mod flex str flex mod
33
ten str % elong ten mod flex str flex mod
48
5390 0.70 2.63 X lo6 4850 5.08 X lo6
6690 1.5 2.47 X lo6 9610 5.18 X lo6
4340 0.82 2.89 X 5950 5.87 X
lo6 lo6
6870 1.29 2.89 X lo6 10800 6.46 x 105
5653 1.1 6.02 X lo6 8910 5.25 X lo5
3810 0.64 3.07 X lo6 5950 6.95 X lo6
6280 1.07 3.03 X lo6 9710 6.52 X lo6
6631 1.2 6.57 X lo6 12800 5.96 105
2930 0.4 6.8 X lo6 8160 6.62 X lo6
5740 0.8 7.05 X 20600 6.93 X
lo6 lo6
3110 0.45 7.75 x 106 9490 7.49 x 106
data are limited, tensile modulus is also seen to increase with MA content. Tensile strength data have been collected on compression-molded specimens for some of the S/MI samples. These data are shown in Table IV. There is a trend to increase flexural strength and both tensile and flexural modulus as MI content is increased. This is in spite of an apparent decrease in molecular weight that is occurring at the same time (Table 11). Trends in tensile strength are not as clear. The expected increase in flexural and tensile properties with solution viscosity increase is also seen.
609
Summary A series of well-characterized S/MA samples have been reacted with ammonia, by gaseous diffusion and subsequent heating, to form their maleimide derivatives. The resulting S/MI copolymers have been found to have large increases in heat resistance over the already heat-resistant parent S/MA copolymers. They also have corresponding increases in melt viscosities. Thermal gravimetric analysis shows a significant increase in thermal stability. The solubility parameter is increased linearly with MI content and is significantly higher than for an equivalent level of MA. Density is shown to increase with MI content, but the coefficient of thermal expansion decreases sharply as MI content is increased. The refractive index decreases in a linear fashion as MI is increased, but at a much slower rate than the corresponding MA copolymers. Some data are presented that show an increase in flexural strength and both tensile and flexural modulus as MI level is increased. Acknowledgment Acknowledgment is made to the following: L. C. Chamberlain, A. W. Hanson, R. F. Boyer, W. Brown, and W. Rupprecht, Jr., for their generous support and encouragement during the early stages of this work; W. E. O'Connor, H. Mashue, R. Carlson, L. Ciezek, R. Owens, R. Salisbury, M. Fryer, V. Cook, and C. Pawloski for synthesis assistance; L. E. Smith I11 and T. R. Wayt for characterization; W. Charlesworth for melt viscosity and physical properties; E. T. Wagoner for thermal gravimetric analysis, differential thermal analysis, glass transition temperature, and gel permeation chromatography;C. Boyd for high-speed osmometry measurement of M,;K. Dennis and W. Alexander for solution viscosity measurements and other helpful assistance. Literature Cited Karam, H. J.; Cleereman, K. J.; Williams, J. L. Moo'. f l a s t . 1855,3 4 . Moore, E. R. Id.Eng. Chem. prod. Res. D e v . 18813,25,315. Moore. E. R.; Nakamura, M. US. Patent 3537885. Nov 3. 1970. Weast, R. C.; Selby, S. M. Handbook of Chemistry and fhysics, 47th ed.; Chemical Rubber Co.: Cleveland, OH, 1986.
Received for review December 12, 1985 Accepted July 29, 1986
Reduction of OH Absorption in Optical Fibers by OH Exchange
-
OD Isotope
Jullan Stone Crawford Hill laboratoty, ATBT Bell Laboratorles, Holmd81, New Jersey 07733
The effects of the interaction of deuterium with silica optical fibers are reviewed. Experiments have shown that it is practical to use deuterium to accomplish isotope exchange in which the hydroxyl in the fiber Is converted to deuterioxyl efficiently. The result is to reduce attenuation in the transmission and dispersion window between 1.3 and 1.6 pm of optlcai fibers, thereby improvlng fiber performance and reducing the cost of fabrication.
I. Introduction Present-day commercially available optical fibers for communications purposes have attained a high degree of perfection. The impurity level has been reduced to so low
a value that the attenuation of light is due now almost exclusively to intrinsic Rayleigh scattering by the pure glass. However, there remains one significant impurityOH bonded into the glass lattice-that seriously limits the
O198-4321/86/~225-O609801.50/0 0 1986 American Chemical Society
810
Ind. Eng. Chern. Prod. Res. Dev., Vol. 25, No. 4, 1986 11
-
o.2 , m r E
I 2
WAVELENGTH, F m 1 3 14 151617181920
RAYLEIGH SCATTERING LOSS
ULTRAVIOLET
FREQUENCY (cm-’) 6000 5000 4000 3000 2000
LOSS DUE TO
I
IMPERFECTIONS OF WAVEGUIDE
0.03
1
c------\
0.02 1.2
1.1
\
I
+ +
2Y3, v1 u2 + u3 3Vl, u1 UQ + vq 2u3 + u4, 2vl + 2u4 4u1, 2 Y 3 + 2u,, 2Vl + 3u3, u1 + u2 + 2u3
4 x 106 1 x 106 6 X lo5 6 X lo4 5 x 104
4.5 4.2 3.8 3.2 3.0
4
INFRARED ABSORPTION LOSS
Table I. Positions and Intensities of Intrinsic Absorption Bands in Fused Silica intensity, wavelength, @m dB/km assignment 21.3 5 x 109 v4 2 x 109 12.5 VI 1 x 10’0 9.1 v.3 8 X lo7 6.2 2% Y 3 + Y4 1 x 108 V l + Yg, Y2 + Yg + Y3, 2Yl + u2 5.3 6 X lo7 5.1 3v2 + YQ, v1 + v p + 2Y4
I
I
-*-
I
1.0 0.9 0.E 07 PHOTON ENERGY, sV
06
-
I
I
I
I
ug
1000
+ w4
0
I
NONDOPED SILICA E203 (5%) DOPED SILICA
109
Figure 1. Sources of optical attenuation in a Ge-doped optical fiber, showing UV and IR tails, the intrinsic Rayleigh scattering level in the glass, and the loss spectrum of a fiber containing OH.
choice of operating wavelength in optical fibers due to its associated absorption. The result of this limitation is to restrict the use of optical fibers to two fairly narrow “windows”, one near 1.3 pm and the other near 1.55 pm, with widths that depend upon the residual OH level. The region of OH absorption is centered near 1.4 pm, between these windows. If the OH absorption could be reduced to a sufficiently low level, the entire spectral range between 1.3 and 1.55 pm could be used. It is the purpose of this paper to show that there is significant experimental evidence to indicate that it may be commercially feasible to reduce the level of OH contamination in optical fibers sufficiently to permit their use in the entire spectral region between 1.3 and 1.55-pm wavelength by using isotope exchange in which OH is converted to OD, which does not have absorption in this portion of the spectrum. This improvement can be achieved by the introduction of lowcost isotope-exchange techniques during fiber manufacture through the use of deuterium in conjunction with conventional drying techniques presently in use. In addition to isotope exchange, a number of important and interesting interactions exist between hydrogen and silica. A review of these effects is presented elsewhere (Stone, 1986). 11. Properties of Optical Fibers Two properties of optical fibers are relevant to this discussion, attenuation and dispersion. I shall outline these properties briefly as they relate to standard commercialquality silica-based optical fibers used principally for long-haul telecommunications purposes. Such fibers are made of high-silica glasses containing various percentages of Ge, F, and P, depending upon the type of fiber and method of manufacture. The ways in which OH is formed and exists in the fiber, as well as the ways in which OH OD conversion occurs, are strongly dependent upon the exact composition and method of fabrication. Hence, any process for reducing the OH level by isotope conversion would have to be optimized for a particular product. However, the exchange process is always determined by the same set of principles. A. Attenuation. The attenuation of light transmitted in a fiber consists of two parts, absorption and scattering.
-
I-
Ge02-Si02 CORE FIBER
102
b : 0203-Ge02-SiO2 CORE FIBER 100
2
3 4 5 6 8101520 WAVELENGTH ( p m )
Figure 2. IR absorption spectrum for several types of bulk silica and optical fibers. See Table I for assignments of the observed bands.
Figure 1 (Miya et al., 1979) shows the attenuation spectrum of a typical fiber and indicates the sources of attenuation common to all present-day optical fibers. I. Absorption. The absorption has both intrinsic and extrinsic (or contamination-dependent) components: a. Intrinsic Absorption. Figure 1shows that intrinsic absorption arises in both the short-wavelength and longwavelength regions of the spectrum. In the UV, there is intrinsic absorption that falls off exponentially with increasing wavelength (Urbach, 1953; Keck et al., 1973), whereas in the IR (Izawa and Shibata, 1977) there are silica vibronic absorptions at frequencies ul, u2, u3, and u4, whose loss contributions decrease with decreasing wavelength (Figure 2). The IR assignments are shown in Table I. As a consequence of the UV and IR “tails”, there is an absorption loss minimum near 1.5 pm. b. Extrinsic Absorption. Also shown in Figure 1 is the contribution from OH absorption. This is caused by overtone absorption due to anharmonicity of the fundamental OH-stretch vibration near 2.7 pm (Keck et al., 1973). Its magnitude and spectral shape depend on the glass composition, the method of manufacture, and the thermal and physical history of the fiber (Figure 3) (Stone et al., 1984). As indicated above, contributions to absorption from other impurities are not significant. Other sources of absorption loss can occur due to environmental effects, principally, exposure to high-energy radiation (Friebele et al., 1982) and hydrogen (Stone et al., 1982;
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 611 ENERGY (cm- l ) 4000
3000
3500
2500
FIBERS
I
I
, /
/
,/
//
/
/
/
13.5% GeOp
0 BULK DATA X
2.5
3.0
3.5
4.0
WAVELENGTH @m)
Figure 3. Example of OH fundamental absorption for silica fibers with different dopants and with different exposures to hydrogen: (a) 3100 psi, 200 O C , 3 days; (b)6-10% P-doped glass Ge-P-doped, Hz, 1atm, 900 O C , 48 h; (d) treated as in a; (c) same glass as in b in Hz, as in c except with D,; (e) untreated Amersil TO-8.
s
;
E
* 2.0
E
m
0
I-
z
W p
I S
LL LL
8 0 f
1.0
w a
s
0.7
5
0
I-
PULSE TRANSMISSION MEASUREMENTS IN FIBERS
Figure 5. Zero-dispersion wavelength of various doped bulk silicas.
persion process as it occurs in optical fibers. It is only important to know that it is possible to design fibers of the single-modetype that have minimum dispersion, hence maximum information-carrying capacity, from a wavelength as short as about 1.27 pm to beyond 1.6 pm by varying the dopants and fiber geometry. Figure 5 shows the dependence on composition (Lin and Cohen, 1978). The dispersion minimum can be shifted to longer wavelengths by varying the index of refraction of the core relative to the cladding (Cohen et al., 1979) or by changing the core diameter. At the present time, fibers of more or less conventional design can be made with dispersion miniia anywhere within these limits, including the region of OH absorption. Since the attenuation minimum occurs at 1.55 pm, the most desirable situation would be ,to have fibers with a dispersion minimum at that wavelength. However, it becomes increasingly difficult to obtain minimum loss as the dispersion minimum is shifted to longer wavelengths. Thus, it is usual that fibers operating at 1.55 pm have dispersion minima at 1.3 pm, although there is considerable work being done to make fibers that have minimum loss, as well as minimum dispersion, at 1.55 pm.
1 J 2.0
0
4.0
A% Figure 4. Rayleigh scattering coefficient va. index differencerelative to pure silica for several doped silica glasses. Dopants are (0) GeGe02-P205; (v)PzO,; (X) GeO,; OZ-PzO5-B2O3;(A)GeO2-B2Oa;(0) (m) GeOZ-PzO5.
Uchida et al., 1983; Pitt and Marshall, 1984). 2. Scattering. High-quality optical fiber is free of bubbles and occlusions and is a locally homogeneous glass without any microcrystals. The scattering loss is due to Rayleigh scattering, which is an intrinsic property of the particular glass. Figure 4 shows the level of the contribution of Rayleigh scattering to fiber attenuation for various dopants (Murata, 1985). The Rayleigh scattering loss is obtained by dividing the Rayleigh scattering coefficient by (~avelength)~. In summary, it can be seen from Figure 1that there is a minimum in the attenuation spectrum of high-quality optical fiber which occurs near 1.5 pm. Except for the presence of OH, the loss between 1.3 and 1.6 pm is primarily due to intrinsic Rayleigh scattering, and losses as low as 0.16 dB/km at 1.57 pm and 0.3 dB/km at 1.3 pm have been obtained (Csencsits et al., 1984). This loss level cannot be reduced except, possibly, by changing the composition of the glass. The only significant reduction in the attenuation of optical fibers that can be made without changing the fiber design is by reducing the OH absorption. 3. Dispersion. Dispersion is the property of a fiber that limits the rate a t which information can be transmitted. Space does not permit a detailed description of the dis-
111. Fiber Drying with Chlorine If optical fiber were manufactured by any of the standard techniques in use today without any attempt to reduce to the OH level, that level would be so high as to make the fibers useless, obliterating even the 1.3- and 1.55-pm windows. A typical figure for that absorption coefficient at the OH overtone absorption peak (1.4 pm) is about 50 dB/km.(ppm w t OH). An empirical figure used in fiber manufacture is -1.2 dB/km.(ppm OH)(Garrett and Todd, 1982) for the added loss at 1.3 pm. At 1.55 pm the OH contribution is somewhat smaller. Therefore the OH level must be kept below -25 ppb in order to avoid a significant increase in attenuation above the intrinsic Rayleigh scattering at these two wavelengths. However, OH levels up to several hundred parts per million may occur if the preform and the starting materials are not dried (Inada, 1982). The standard drying techniques involve the use of chlorine during the manufacture of the preform (Murata, 1985; Nagel et al., 1985). The details of the processing involved vary with the type of fiber fabrication. OH can be incorporated into the fiber in a variety of ways and at a number of stages in the process, from the starting materials to the fiber-drawing operation. In addition to its use in purifying the starting materials there are two principal ways in which the presence of chlorine acts to reduce the OH level in the glass. In the MCVD (Nagel et ai., 1985) process, when Clz is present along with H20during deposition or preform collapse, Clz reacts with the water to form HC1.
612
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 DISTANCE IN DEPOSITED TUBE BEFORE COLLAPSE, pm
-200-100-50-10 W
I
s!
1
1
I
0 IO 50 100 200 l l I I I I
1 200
THERM0
l
I I 1 -2.0 -1.5 -1.0 -0.5
0‘
Clp+He+02
Figure 6. Examples of a furnace used to chlorine-dry and consolidate VAD soot preforms.
I
I
l
J
l
1.0 1.5 DISTANCE IN ROD PREFORM, mm
0
0.5
2.0
Figure 8. OH profiles in a single-mode rod preform made by MCVD, showing diffusion of OH from the support tube into the deposited glass, before collapse (top scale) and after collapse (bottom scale). This shows the need for a deposited cladding with a diameter about 5 times the core diameter. The factor for present-day fibers is about 5. CORE 8 p m DIA
.. E \ .x
DEPOSITED C L A D D l NG 5 0 p m DIA
m
-0
$
..
5
2 OUTER CLADDING 425pm DIP
0 0.8
1.0
1.2
1.4
1.6
1.8
2.0
/
Figure 9. Schematic drawing of the structure of a basic-design single-mode fiber, showing core, deposited cladding, and outer cladding (substrate tube).
WAVELENGTH, p m
7
Figure 7. Loss spectrum of an experimental VAD fiber with a very low OH level. The OH loss peak is 0.05 dB/km; it cannot be seen because a liner scale is used for loss instead of the customary logarithmic scale needed when loss is low.
The reaction products are vaporized and do not enter the glass. The second way in which chlorine reduces the OH level is by drying the silica soot before it is consolidated. This is the drying process used in both the VAD (Murata, 1985) and the OVD (Morrow et al., 1985) processes. The porous preform, consisting of silica soot particles about 0.05-0.2 km in size, is heated in an atmosphere of SOClz or C12 (Figure 6) (Inada, 1982). Hydroxyl, either present at the surface or diffusing out from within the soot particles, reacts with the gas to form HC1, which evaporates, removing the H atoms needed to form OH. Since OH diffusion is rather slow (Moulson and Roberts, 1961; Philen, 198.3, this technique is applicable only to fine particles and thus cannot be used to dry the consolidated glass. These techniques have been used to produce very low OH levels in carefully controlled experiments-as low as 0.04 dB/km at the OH peak for VAD (Moriyama et al., 1980; Hanawa et al., 1980) (Figure 7) and 0.05 dB/km for MCVD (Nagel et al., 1985). However, for manufactured fibers the OH absorption is approximately 1-2 dB/km (Murata, 1985; Nagel et al., 1985), which is too high to permit utilization of the OH portion of the spectrum for transmission. One further significant sources of OH contamination should be mentioned. The OH level in the substrate tube is extremely high, several parts to several hundred ppm wt OH. When the fiber perform is fabricated by MCVD,
6
5
4
-
Rf
Tf
a
2a
3
2 I 0 1.5
2
2.5
V
Figure 10. Radius R, required to contain a percentage f of the optical power in a single-mode fiber of core radius a. T,is the corresponding light-penetration thickness. V is the normalized frequency of the waveguide.
a small fraction of the OH ions diffuse from the substrate tube into the very dry core region, causing a substantial increase in the OH level in the outer region of the core (Figure 8) (Kawachi et al., 1977). When light propagates in the fiber, the outer portion of the optical field is exposed to the OH and the attenuation is increased. In order to avoid this problem a layer of low-OH glass is deposited on the inner wall of the substrate tube to form a buffer or protective layer against OH diffusion for the light-propagating region of the fiber (Figure 9) (Kapron and Lukowski,
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
613
Table 11. Positions and Assignments of OH Absorption Bands in Fused Silica wavelength, nm assignment 1370 1270 1120 1030 950 880 825 775 725 685 585
2u(OH) 2 ~ ( 0 H+ ) ~1 2u(OH) + 2u1 24OH) + 3u1 3u(OH) 3u(OH) + u1 3u(OH) + 2u1 3 ~ ( 0 H+ ) 3Ul 4u(OH) 4 ~ ( 0 H+ ) ~1 5u(OH)
first overtone
- io5 \'v$/
MEASURED ABSORPTION
E
CALCULATED ABSORPTION
f IO' 0
second overtone
third overtone fourth overtone
Table 111. Spectral Assignments for OH and OD Components in Silica for Fundamental and Overtones OH vibration OD vibration suggested frequency, cm-' frequency, cm-l assignment 775 f 10 OH torsion 980 f 10 1595 f 10
5
to3
ii
2
IO'
5
40'
---J,
I00
.-
4n-4
ZOO
400
600
SOO
4000
--
4400
4200
1600
WAVELENGTH h n l
Figure 11. Attenuation spectrum of a fiber showing absorptive contribution, including UV absorption and OH overtones, and calculated Rayleigh scattering. The OH overtone absorption assignments are given in Table 11.
4100 4450 4520 I2
I 3 I4 15 WAVELENGTH Ipml
12
1. I
7920 8065
I3 I4 i WAVELENGTH lpml
s
I 2
I3 I 4 15 WAVELENGTH I p n l
Ibl
(Ll
Figure 12. High-resolution spectra of the first OH overtone absorption in (a) pure fused silica fiber, (b) multimode graded-index Ge-doped silica fiber (0-14% GeO,), and (c) single-mode Ge-doped silica fiber (-5% Ge02). The resolved components of each are labeled, and the assignments are given in Table 111.
11470
Si02 vibration 280 432 800 1060
1977). This must be done for both single-mode and multimode fiber, resulting in an increase in the cost of fabrication. The deposited layer must be much thicker in singlemode fibers due to the greater penetration of light into the fiber cladding (Figure 10).
IV. Spectral Composition of the OH and OD Absorption in Silica-Glass Fibers Before describing how isotope exchange can be accomplished in a practical way in fiber manufacture, I first explain the reasons for doing so. Since optical fibers are, in effect, very long absorption cells, very weak absorptions can be observed and are significant. Therefore not only can the fundamental OH-stretch absorption band near 2.7 pm in silica (Figure 3) (familiar from extensive IR absorption studies (Hetherington and Jack (1962)) be observed, but a whole series of overtones of this absorption as well as combinations of the fundamental or overtones with silica-network vibrations are also seen. Figure 11 shows the absorption spectrum from the IR through the visible as observed using silica-based fibers (Izawa and
085
0.95 4.00 WAVELENGTH (pin1
0.90
La)
0.90 0.95 1.00 WAVELENGTH (pm)
(bl
Figure 13. Second overtone high-resolution spectra for (a) pure fused silica fiber and (b) multimode graded-index fibers. The fibers are the same as in parts a and b in Figure 15, and the numbers labeling the components are also the same.
Shibata, 1977),and Table 11gives the original assignments. Table 111 lists more recent assignments (Walrafen and Samantha, 1978; Stone and Walrafen, 1982) of all the spectral components observed at high resolution for the first, second, and third overtones (Figures 1 2 and 13) (Stone and Walrafen, 1982). It is found that each absorption band actually consists of a set of overlapping Gaussian components associated with different sites in the glass lattice at which the OH is attached. Note also from Figure 11that the overtone strength decreases rapidly with increasing overtone number. In particular, the decrease
814
,
Ind. Eng. Chem. Prod. Res. D e v . , Val. 25, No. 4, 1986
40
1
"id1
COMBINATION
- 2o ~, 2nd loOVERTONE a E
\
-.L
-$
OVERTONE IO
20
30
1-
I
COMBINATION TONE
1400 560
T(C) 282 143
68
5
30
2'
1 L
10-6
-
OD
50
t
c 1.5
1.0
2.0
lo-42gt
WAVELENGTH, p n
Figure 14. Comparison of the positions of the peaks of the OH and OD overtone absorptions showing approximate absorption coefficients.
from the first to the second overtone is about a factor of 50. Once again, the overtone ratio, as well as the shape of the absorption band, depends on composition and fabrication technique. Now consider the next column in Table 111, where the measured bands for the OD fundamental are listed along with assignments. The important feature is the isotopic frequency shift of the OD-stretch absorption from that of OH. The shift is to longer wavelengths by the expected approximate 2'12 factor due to the doubling of the mass of one of the components (H to D). The result is shown in Figure 14, which schematically compares the measured wavelengths of the OH and OD absorption maxima in the spectral region of interest for telecommunications, i.e., where fiber attenuation and dispersion are low. Between 1.3 and 1.6 pm, the two sources of absorption are the first OH overtone at 1.4 pm and the tail of the OH-overtone silica-vibration combination tone at 1.24 pm. The corresponding OD components are at 1.87 and 1.66 pm, outside the region of low intrinsic loss for silica fibers. The only relevant source of absorption loss is the second OD overtone at 1.26 pm. However, since this is a second overtone, it is about l / % as strong as the first OH overtone. (In fact, the OD overtone may be weaker than the OH overtone (Kumar et al., 1981; Burn and Roberts, 1970).) Thus, if isotope exchange could be accomplished, the entire window between 1.3 and 1.6 pm would be unaffected by OH or OD absorption except for relatively small effects at the ends due to the 1.26- and 1.66-pm OD bands. V. Interaction of H2 and D2 with Silica It has long been known that when silica is exposed to a hydrogeq or deuterium atmosphere, the gas diffuses into the glass with relative ease and may undergo a chemical reaction $th the glass if energy in the form of heat or radiation is added to the system. The steady-state diffusion of hydrogen or deuterium obeys the classical diffusion equation, with a diffusivity given by (Lee, 1963; Shelby, 1977a) DH2= 5.65
X
DD,= 5.01 X
-34
[ kcc/mol 1 [ kcc/mol
exp -
exp -
10.4
10.5
1
cm2 s-l
(2)
cm2 s-l
(3)
,h Ib
24
42
T+x104
Figure 15. Diffusion coefficient vs. 1/T for H2and D2 in silica (left-hand scale) and characteristic time for an H2molecule to diffuse 1 mm (right-hand scale). 1 H2
f i
(DIFFUSION1
OH- ___) OD(EXCHANGE)
(DIFFUSION)
SILICA
2 OH + D2t-
2 OD t H p t
-
Figure 16. Schematic illustration of isotope exchange OH OD when silica is exposed to deuterium. Deuterium diffuses in and exchanges, and evolved hydrogen diffuses out and escapes from the system.
where R is the gas constant and T is the absolute temperature in kelvin. These diffusivities are plotted vs. 1/T in Figure 15. The characteristic diffusion time t for a particle to travel a distance 1 is given by
t = 12/4D (4) and the time in hours for a hydrogen molecule to diffuse a distance 1 is shown in Figure 15. The chemical reaction that may occur is one of the following: a. If H2/Dzis used and the glass is relatively dry, additional OH/OD may be formed in the glass (Bell et al., 1962; Hartwig, 1977; Shelby, 1977b, 1980; vandersteen and Papanicholau, 1975). The reaction rates for the two reactions are nearly equal (Bell et al., 1962). b. If the glass has measurable OH and Dz is used, a (usually large) fraction of the OH is converted to OD by the reaction D2(g) + 2SiOH Hz(g) + 2SiOD (5)
-
(see Figure 16), and additional OD may be formed (Fry et al., 1960; Faile and Roy, 1971; Lee, 1964) at other sites. c. If the glass has measurable OD and H2 is used, the reaction inverse to (b) occurs (6) H,(g) + 2Si:OD D,(g) + 2SiOH
-
and additional OH may be formed (Fry et al., 1960; Shelby et al., 1979) at other sites.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 615
-
0.020 0015
,GRADED INDEX CORE 5 0 r m DIP
/
0 040 0005
dg
0 0020
16mm
TO8 OUTER CLADDING 1 1 0 p DIA
___-___________
0015
0.010
0
&,OP
@
zol’”’”
SPECIMEN HEATED IN DRY (02FREE) HYDROGEN AT
0 005
o r ’ 0
10
’ ’
I
I
’
I
x) 30 40 50 60 70
I
I
’
Figure 20. Cross section of a multimode fiber with graded-index core, barrier layer, and TO-8outer cladding.
80 90 100
TIME (h)
Figure 17. Example of hydroxyl fromation in different thicknesses of IR vitreosil for different treatment times and temperatures. Note that an equilibrium concentration is attained which depends on the temperature and is higher at the lower temperature. 400
I-
z
0 W
a w
a W’
z a + b
50
z a
a
F
I00
200
300
400
500
600
700
800
TEMPERATURE, (OC)
0
2
3
4
5
WAVELENGTH, MICRONS
Figure 18. Occurrence of isotope exchange in silica treated in deuterium at 1000 OC for (B)0.5 h, (C) 5 h, and (D) 25 h with (A) the original sample. The OH level decreaseswith exposure time and the OD level increases. TOTAL 0H;PERMANENT OH ^N
.‘
120
”
400
600 700 800 900 I000 1100 TEMPERATURE IN DEGREES CENTIGRADE
500
Figure 19. Example of the total OH and residual OH in GE204 silica. Permanent OH cannot be removed by vacuum degassing at 1000 “C. Metastable OH can be added above 500 “C by hydrogen reaction and removed. Residual OH is the amount remaining in the glass following H2exposure and vacuum degassing at that temperature.
Characteristically, the extent of any of these reactions depends on the t y p e of silica used, i.e., its method of manufacture, and the conditions of treatment with the gas (Figures 17 and 18) (Bell et al., 1962; Fry et al., 1960). Futhermore, the OH or OD formed is partially permanent and partially metastable; i.e., some of the newly formed
Figure 21. Change in OH and OD levels vs. treatment time of fiber of Figure 19 heated to 800 OC in Dz.
OH or OD disappears when the glass is heated (Figure 19) (Lee, 1964). However, it is not possible for OD formed by isotope exchange to revert back to OH, since the hydrogen needed has escaped the system as hydrogen gas. Regardless of the complexity of the process, one salient feature stands out: if OH-bearing silica is exposed to deuterium gas, the OH level will be reduced and an equal or greater amount of OD will appear in its place; the OH level cannot increase again unless the glass is exposed to a source of hydrogen atoms (such as hydrogen gas).
VI. Isotope-Exchange Experiments on Fibers With the above considerations in mind, an isotope-exchange experiment was performed on a standard multimode fiber (Stone and Burrus, 1980). The experiment, which appeared at first glance to be unsuccessful, actually showed it was likely that isotope exchange would work and serve to reduce the OH level in fibers. The fibers had the structure shown in Figure 20. It consisted of a GeOz-dopedsilica core and an Amersil TO-8 cladding. The initial OH level was -0.2 ppm in the core and -100-200 ppm in the cladding. The core diameter was 50 pm, and the fiber OD was 110 pm. In the experiment, lengths of the fiber were stripped of their protective coating and heated at 800 “C in a Dz atmosphere for times ranging from 30 s to 64 min. The changes in OH and OD levels were determined by measuring the magnitude of the OH and OD attenuations at -1.4 pm (the first OH overtone) and 1.26 pm (the second OD overtone), respectively, in the fibers. The results are shown in Figure 2 1 (Stone and Burrus, 1980). What is seen to occur is a rapid increase in the OH level during short treatment time, followed by a rapid decrease to an asymptotic level higher
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LOW INITIAL OH 7
REDUCED B Y FACTOR OF 6 4
CA
6
w
I
I
& - I
'
I
I\ I
I1
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1 1.1
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1.0
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Figure 22. OH absorption level at 1.4 N r n in an Amersil TO-8 fiber before and after exposure to D2 at 1000 "C.
than the initial level and a lagging increase in the OD level to a steady asymptotic level. The explanation for these results is that initially the Dz gas diffuses into the high-OH cladding, where it reacts to form OD, resulting in the release of hydrogen. Some of this hydrogen diffuses into the core before the arrival of significant amounts of D2 and rapidly forms a large amount of OH in the core. As the isotopic conversion in the cladding approaches completion, the D, gas diffuses into the core and reacts with the OH to form OD, reducing the OH level; hydrogen is evolved and diffuses out. The experiment shows a number of important results. It shows that isotope exchange occurs in the TO-8 cladding, that H2is released and diffuses away, that some of the Hz diffuses into the core and forms a large amount of OH in the very dry core silica, that the reaction in the cladding approaches an equilibrium after which the Dz gas can diffuse freely into the core (where it reduces the OH level by exchange), and that the level of OD formation in the core glass also approaches an equilibrium value. In short, it demonstrates virtually all of the effects that occur in the chemical reaction of H2 or D2 in high- and low-OH silica glass. It also clearly demonstrates another result: this is definitely not the way to accomplish OH OD exchange in a fiber, since neither the final OH nor OD levels are low. In fact, for any post-treatment process the proper method would be as follows: start the fiber fabrication using a substrate tube that has been treated in D2 to convert the OH to OD, use this tube in making the fiber preform and final fiber, and then expose the finished fiber to a hydrogen atmosphere for a limited time. Following through the above sequence of steps, we can see that the result would be an initial reduction of the OH level in the core if the exposure to hydrogen does not continue too long. Two additional experiments (Stone and Burrus, 1980) were carried out on existing fibers. In the first, a solid silica fiber drawn from an Amersil TO-8 rod (- 100-200 ppm OH) was treated for 3 min at 1000 "C in Dz. The fiber length was 3.6 m, and the 0.d. was 110 pm. After treatment, the OH absorption was too small to measure, indicating a reduction in the OH level by at least a factor of 1801. The expected OD peak at 1.26 pm was too small to measure (Figure 22). In the second measurement, a single-mode fiber consisting of a TO-8 substrate cladding, a 75-pm-diameter
-
0
10
20
30
40
50
60
70
D p SOAK T I M E I N MINUTES
Figure 23. OH absorption level in core and cladding for a singlemode fiber as a function of temperature when exposed to D,.
borosilicate deposited cladding, and a 10-pm pure silica core was used. Initially the peak OH absorption for the cladding was 8 dB/m and for the core (light-guiding region), 2 dB/m. Lengths of fiber (1-3 m) were treated in D, for 10 min at temperatures ranging from 100 to 700 "C. The OH attenuation of both the core and the cladding was measured as a function of treatment temperature, and the results are shown in Figure 23. No change is observed below 400 "C; then there is a small OH increase in the core without a change in the cladding, indicating that a small amount of exchange in the light-guiding-region cladding produces hydrogen which reaches the light-guidingregion. At 500 "C both core and cladding OH levels decrease with temperature. At 600 "C the core OH level is no longer measurable (less than 0.2 dB/m), and at 700 "C both core and cladding show no measurable OH. The relatively small amount of H2 developed in the cladding does not reach significant levels in the core, whereas some of the much larger amounts of diffusing D2 reaches the core and reacts to convert the OH to OD.
VII. Isotope-Exchange Experiments during Preform Fabrication Although the experiments described in Section VI show that isotope exchange could be a viable method of reducing OH absorption in fibers, it is much more practical to accomplish this result during the fabrication of the preform. In this section we describe experiments carried out at various stages of preform fabrication which demonstrate that the use of isotope exchange can produce high-quality fibers with low OH absorption while simplifying the manufacturing process. As indicated earlier, the sources of OH contamination in the light-guiding region of a fiber arise both from diffusion of OH out of the substrate tube and from water- and hydrogen-containing contaminants in the starting materials used in the deposition and consolidation steps. A. One-Step Process: Treatment of the Substrate Tube in MCVD (Stone and Lemaire, 1981; Modena and Roba, 1981). One source of OH contamination in the MCVD process is the substrate tube. The standard way in which this contamination is avoided, as stated above, is to deposit a buffer layer on the inner tube wall before
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 617
t DEPOSITION TUBE
-1
FURNACE
Figure 24. Experimental arrangement to deuteriate a substrate (deposition) tube. Tube is placed within a seasoned (well-exposed to D2)silica tube, and the two are inserted in a tube furnace. Dzis trickle-flowed in large tube, exposing both sides of substrate tube to the gas. 5.0 I 4.0
2
3.0
1
I
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I
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I
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n I\
I
I
(
i STANDARD
4
3
I 0
0S n 2 4 6 5 4 0 4 2 1 4 BARRIER LAYER THICKNESS ( p )
0
WAVELENGTH, pm
Figure 25. Comparison of loss spectra of multimode fibers made without a barrier layer: (solid line) standard Amersil TO-8 tube; (dot-dash) with deuteriated Amersil TO-8 tube. Residual OH absorption in latter is mainly due to deposited core. (- - -) is proportional to (wavelength)4, implying Rayleigh scattering level.
depositing the layers that will form the light-guiding core of the fiber. These buffer layers are deposited in the fabrication of both single-mode and multimode fibers. For multimode fibers the layer thickness must be 5-10 pm, depending on the OH content of the substrate tube, in order to completely eliminate any OH diffusion into the light-guiding region. For single-mode fibers it is necessary to make the deposited cladding-core ratio about 51. Thus for both types of fibr, a large amount of high-cost material must be used to avoid the problem of OH diffusion from the substrate tube. The alternative is to treat the substrate tube with Dzto convert the OH to OD. If this is done, a thinner barrier layer, or none at all, may be required for multimode fibers, resulting in economy of manufacture. Several experiments using this one-step treatment have been carried out (Shang et al., 1983; Kosinski et al., 1982). In these experiments the substrate tube is heated to a temperature of 900-1000 "C in Dzat 1 atm for 20 h or longer (Figure 24). The flow of D2 need only be high enough to maintain a Dzatmosphere, and only 1-2 L of gas is consumed during the treatment time (Shang et al., 1983). No experiments were made to see if the treatment time and temperature could be reduced. These treated tubes were used in the fabrication of multimode preforms as though they were standard untreated tubes. The only difference in the process was that the barrier layer was omitted. It was found that the OH level and loss spectra of the fibers made in this way were better than those for fibers made in the conventional way including a barrier layer (Figure 25) (Shang et al., 1983). OH attenuation at 1.4 pm as low as 0.2 dB/km was achieved (Kosinski et al., 1982) by using substrate tubes having initial levels of -250 ppm OH and 2-pm-thick barrier layer. A barrier layer of about 8-pm thickness was needed to achieve the same fiber attenuation at 1.4 pm with untreated tubes (Figure 26). These experiments demonstrate that fiber manufacturing costa can be reduced considerably by off-line deuteriation of standard substrate tubes at very little cost, followed by the usual MCVD preform processing but without the barrier layer. The experiments described
Figure 26. OH absorption peak vs. barrier layer thickness for multimode fiber with standard substrate tubes and for a fiber made with a deuteriated tube and a 2-wm-thick barrier layer.
0 1400 W
OAS FLOW 4
d
LL
P
1000
STEP 2 FURNXE+
W
0 4
z a
900 0
4
8
12 16 20 24 28 32
PREFORM P O S I T I O N (cm)
P R E F O R M P O S I T I O N (cm)
Figure 27. (a) Furnace temperature profile for treatment of substrate tube (step 1 furnace) and deposited glass (step 2 furnace),and (b) OH and OD absorption loss levels in fiber drawn from corresponding parts of the preform.
above all involved multimode fiber, and no experiments have yet been performed on single-mode fiber. It should be possible to significantly reduce the deposited cladding thickness for matched-index-cladding single-mode fibers if deuteriated substrate tubes are used. B. Two-step Treatment of Preforms: Deuteriation of Substrate Tube and Deposited Layers (Stone and Lemaire, 1982). In the one-step treatment described above there is no treatment of any of the deposited layers of the preform (in MCVD), so that any OH contamination in the core structure remains. In this section I describe an experiment in which both the substrate tube and the
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Ind. Eng. Chem. Prod. Res.
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L DEPOSITED CORE LAYERS
D2
1
vvvovvvv
DEUTERATED TUBE
SPLIT FURNACE
(a)
(bl
Figure 28. Experimental arrangement for second-step treatment shown in Figure 27. Split furnace is placed around finished uncollapsed preform, and D, is flowed through the tube. 100
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o.2 0.1
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1.0
I
1.4
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1.2
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4.3 1.4 1.5 WAVELENGTH, pm
I
1,6
I
0.7
Figure 29. Comparison of loss spectra for lengths of fiber treated a~ in Figure 30a showing progressive reduction from initial untreated fiber to step 1, substrate tube treatment only, to steps 1 and 2 treatment.
deposited layers were treated. In this experiment the substrate tube was deuteriated as previously described. In order to provide direct comparison between treated and untreated fibers, the preform contained both treated and untreated parts. Figure 27a shows the temperature profile of the furnace used to treat the substrate tube (step 1 furnace). The ends of the tube were at room temperature, and the center of the furnace was at a temperature of -1100 "C. The Dz gas flowed both inside and outside the substrate tube, which was contained within a larger deuteriated silica tube in the tube furnace (Figure 27). The treated tube was removed from the furnace and placed on the MCVD glass lathe, and barrier layers of P-doped silica were deposited (this experiment preceded the ones on eliminating the barrier layer), followed by Ge-P-doped layers, to give a step-index profile, with a fiber numerical aperture of 0.14. After all layers were consolidated but before preform collapse, a split furnace was placed around the preform tube, with the temperature profile shown in Figure 27a, and D2gas was flowed through the inside of the tube for about 2 h (Figure 28). The preform was then collapsed and drawn into a fiber. The loss spectra for the differently treated portions of the fiber were then measured, and the results are shown in Figure 29. The com-
(cl
(dl
Figure 30. Sample preparation and measurement (a) piece of MCVD-made preform; (b) core of preform with wet cladding etched away after which H2was diffused in; (c) segment of core sliced out of the middle of the core rod after treatment: (d) mask rearrangement used to scan core to obtain OH profile.
X 116 3.44
p"l
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
619
n I
E
1.0
I-
-
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$
N
I
0.5-
DEPTH OF PENETRATION
Figure 34. Schematic illustration of OH profiles observed at high and low temperature showing greater penetration at lower temperature due to slowing of the reaction. 0.2
0
0.6
0.4
1.0
0.8
r /a
Figure 32. Calculated hydrogen profiles corresponding to curves in Figure 31 (solid line), and hydrogen profile which would occur corresponding to (1) in the absence of reaction (dashed line). The curves show drastic retardation of gas flow due to chemical reaction. ,MUCH
NO 02
I
D2 PENETRATES OEPOSITEO LAYERS
PENETRATES
HERE, NO OD
SUBSTRATE CLADDING
00 FORMED
LAYERS \
\DEPOSITED LAYERS
LESS OD FORMED OH +OD THROUGHOUT
(0)
(b)
H IO H TEMPERATURE
LOW TEMPERATURE
Figure 33. Schematic illustrations of depth of isotope exchange for (a) high-temperature and (b) low-temperature cases. At high temperature the process is reaction controlled, and at low temperature it is diffusion controlled.
in the fiber drawn from that part of the preform from the center of the step 2 furnace occurs because there has not been enough time for the D2 to penetrate far into the deposited core layers of the preform, which are 2-3 mm thick, so that most of the OH is as yet unexchanged. On the other hand, since some OH OD conversion has occurred in the near-surface core layers and additional OD is formed at new sites, a sizable amount of OD is observed (Figure 33). (Note that the OD level plotted in Figure 27b is for the second overtone.) This serves to explain the high OH and OD levels observed in the fiber that came from the hottest part of the step 2 furnace. Next we consider reasons why the OH level reaches a minimum in the cooler part of the step 2 furnace (Figure 27a). In the second set of recent experiments (Stone et al., unpublished results), which are still in progress and have not yet been reported, the OH concentration profile was measured as a function of temperature and treatment time. It has been found that, as the temperature decreases, the OH formation process changes from being reaction controlled to being diffusion controlled. The reaction rate for OH formation decreases with decreasing temperature more rapidly than the diffusion rate, so that the gas is not held back by being consumed in the reaction and can penetrate further than at the higher temperature. This means that the OH profiles a t high and low temperatures have the shapes shown schematically in Figure 34. The total amount of OH formed is reduced at lower temperature, but there is more formed at a greater depth than at the higher temperature, for the same treatment time. Now I relate this to the results of the exchange experiment
-
showing the low values of OH and OD, assuming a similar mechanism to that described above. As the treatment temperature is reduced, D2 penetrates deeper into the deposited core and reacts to convert OH to OD (Figure 33b). (Comparing Figure 15 and Figure 31, we see that for the second-step-treatment time of -2 h at 900 OC the OH is formed to a depth of only -1 mm, since the formation rate is reaction controlled, whereas for a diffusion-controlled process (Figure 27a) there can be OH formed at 1mm in less than 2 h for temperatures as low as -400 "C.) Since there is only a relatively small amount of OH, it does not require a large amount of D2to convert most of the OH. On the other hand, the amount of OD formed is decreased from that at higher temperatures since the reaction rate (and probably the equilibrium level) are lower, so that the amount of OD becomes too small to measure. At still lower temperature, the exchange-reaction rate and equilibrium level decrease further, so that for the treatment time used there is less and less conversion with decreasing temperature, hence more OH and less OD. Therefore, if we make the assumption that the isotope-exchange process is similar to that observed for OH formation, it changes with decreasing temperature from a slowly advancing reaction-controlled exchange to a relatively rapid diffusion-controlled exchange, and the features observed in the exchange experiment are qualitatively explained.
VIII. Drying of VAD Silica Soot As pointed out earlier, soot preforms obtained in the VAD process are dried with chlorine or a chlorine-bearing compound (see Figure 6). The drying process is relatively slow since it is a surface reaction and it is necessary for the OH inside a soot particle to migrate to the surface in order for it to be reacted and removed (Keck et al., 1973). Therefore the soot preforms must be dried for a few hours in order to reach very low OH levels. It would appear that prior reduction in the amount of OH by isotope exchange would be very helpful. Since the soot particles are very small, the reaction with deuterium should occur quickly. Some preliminary experiments (Stone and Presby, unpublished results) have been done to determine the effectiveness of the isotope-exchange process. In the experiments silica soot was produced by the VAD process. Some of this soot was placed inside a deuteriated silica tube in a furnace. The soot was first dried in a He atmosphere to remove the adsorbed moisture which formed during handling. The soot was then exposed to D2while the temperature was increased over a period of about 10 min until it reached the consolidation temperature of 1500 OC (see Figure 35). A control sample was made in the same way except that no Dzwas used and He was flowed until consolidation. A comparison of the OH levels showed that the Dz treatment reduced the OH level by a factor of 151 (Figure 36). This result, which is a very prelim-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
I
HELIUM
ORAWING FURNACE,
\
L-
EXHAUST
Figure 35.
E x p e r i m e n t a l arrangement used t o consolidate VAD soot p l u g in a D2atmosphere to reduce O H level by isotope exchange.
0.25
1
J
I
preforms, such as those obtained by VAD and OVD, can have their OH levels reduced by deuteriation. The isotope-exchange technique should be viewed as a process complementary to the standard technique of drying with chlorine. Isotope exchange is the only way in which the OH level can be reduced in a preform that is already in the glassy state. The chlorine process requires that the OH be exposed to the chlorine, which means that the OH ions within the glass must diffuse to the surface. This is a very slow process and is practical only for soot particles. On the other hand, the isotopeexchange process involves diffusion of the Dz gas into the interior of the glass combined with reaction, which can be fairly rapid. Thus it can work in the glassy state. This can be very useful in reducing OH contamination which occurs during the consolidation stage. At present, any OH which is consolidated into the glass cannot be removed, so that extreme care must be taken to avoid the introduction of hydrogen-bearing compounds. Much experimental work needs to be done in order to better define the parameters for optimizing the role of isotope exchange in reducing OH in fiber fabrication. There are many ways in which it can be used for various types of fibers, but limits of its utility are yet to be defined. The level of OH can be reduced by the deuteriation process, thereby permitting utilization of a larger fraction of the window in intrinsic attenuation and dispersion. Registry No. OH, 3352-57-6; DP,7782-39-0; C12, 7782-50-5; silica, 60676-86-0.
Literature Cited
WITH D2 TREATMENT
2.65
2.7
2.75
2.8
2.85
2.9
2.95
WAVELENGTH (pin)
Figure 36. IR spectra of O H absorption for solidated in D2 or H e atmosphere.
VAD soot plugs con-
inary one, suggests that the isotope-exchange process for reducing OH absorption may also be useful in the VAD process.
IX. Summary and Conclusions In this paper I have presented an overview of the main features of OH OD exchange in high-silica optical fiber manufacture through the use of deuterium. Clearly the process is complex and not yet fully explained. However, in its main outline we do have a qualitative understanding of the isotope-exchange effect in typical fibers. Experimenta have demonstrated that use of isotope exchange can lead to some simplification and cost reduction in the fiber-making process, explicitly in the case of the fabrication of multimode fibers by the MCVD process, for which definitive experiments have been carried out. We have learned that substrate tubes can be inexpensively deuteriated off-line (Shang et al., 1983). These tubes can be used to make multimode fibers without barrier layers that are at least as good as state-of-the-art fibers made with barrier layers. This results in a reduction in the required amount of deposited high-purity glass of up to 50%. A two-step process has shown that the OH level in the deposited layers of a MCVD preform can be significantly reduced by the use of isotope exchange before preform collapse. Also, tentative experiments indicate that soot
-
Bell, T.; Hetherington, G.; Jack, K. H. Phys. Chem. Glasses 1982, 3, 141. Burn, I.; Roberts, J. P. Phys. Chem. Glasses 1970, 11, 106. Cohen, L. G.; Lin, C.; French, W. G. Nectron. Lett. 1979, 15, 334. Csencsits, R.; Lemaire, P. J.; Reed, W. A,; Shenk, D. S.; Walker, K. L. Presented at the Conference on Optical Fiber Communication, Jan 23-25, 1964, New Orleans, LA; paper TU13. Faiie, S. P.; Roy, D. M. J . Am. Ceram. SOC. 1971, 5 4 , 533. Friebele, E. J.; Glngerich, M. E.; Long, K. J. Appl. Opt. 1982, 2 1 , 547. Fry, D. L.; Mohan. P. V.; Lee, R. W. J. Opt. SOC. Am. 1980, 5 0 , 1321. Todd, C. J. Opt. Quant. Electron. 1982, 14, 95. Garrett, I.; Hanawa, F.; Sudo, S.; Kawachl, M.; Nakahara, M. Electron. Lett. 1980, 76, 699. Hartwlg, C. M. J . Chem. Phys. 1977, 6 6 , 227. Hetherington, G.; Jack, K. H. Phys. Chem. Glasses 1982, 3, 129. Inada, K. Jpn. Annu. Rev. Electron., Comput. Telecommun. 1982, 3, 241. Izawa, T.; Shlbata, N. Appl. Phys. Lett. 1977, 3 7 , 33. Kapron, F. P.; Lukowski, T. I.Appl. Opt. 1977, 16, 1465. Kawachl, M.; Horiguchi, M.; Kawana, A.; Miyashita, T. Nectron Lett. 1977, 13, 247. Keck, D. B.; Maurer, R. D.; Schultz, P. C. Appl. Phys. Lett. 1973, 2 2 , 307-309. Kosinski, S. G.; Nagel, S. R.; Lemaire, P. J.; Stone, J. Am. Ceram. SOC. Bull. 1982, 61, 822. Kumar, 6.; Fernellus, N.; Detrio, J. A. J . Am. Ceram. SOC. 1981, 178. Lee, R. W. J . Chem. Phys. 1983, 3 8 , 448. Lee, R. W. Phys. Chem. Glasses 1984, 5 , 35. Lin, C.; Cohen, L. G. Electron. Lett. 1978, 14, 170. Modena, E.; Roba, G. Electron. Lett. 1981, 17, 815. Miya. T.; Terunuma, Y.; Hosaka, H.;Mlyashita, T. Electron. Lett. 1979, 15, 106. Moriyama, T.; Fukuda, 0.; Sanada, K.; Ineda, K.; Edahiro. K.; Chida, K. Electron. Lett. 1980. 16. 698. Morrow, A. J.; Sarkar, A.;Schultz, P. C. I n Optical Fiber Communication;Li, T., Ed.; Academic: Orlando, FL, 1985; Vol. 1. Moulson, A. J.; Roberts, J. P. Trans. Faraday Soc. 1981, 5 7 , 1208. Murata. H. I n Optical Fiber Communication; Li, T., Ed.; Academic: Orlando, FL, 1985; Vol. 1. Nagel, S. R.; MacChesney, J. B.; Walker, K. L. I n Optical Fiber Communication; Li, T., Ed.; Academic: Orlando, FL, 1985; Vol. 1. Phllen, D. L. Bell Syst. Tech. J. 1982, 6 1 , 283. Pitt, N. J.; Marshall, A. Nectron. Lett. 1984, 2 0 , 512. Shang, H.-1.; Stone, J.; Burrus, C. A. Electron. Lett. 1983, 79, 95. Shelby, J. J . Appl. Phys. 1977a, 4 8 , 3387. Shelby, J. E. J . Appl. Phys. 1977b. 5 0 , 3702. Shelby, J. E. J . Appl. Phys. 1980, 51, 2589. Shelby. J. F.; Mattern, P. L.: Ottesen, D. V. J . Appl. Phys. 1979, 5 0 , 5533. Stone, J. J . Lightwave Techno/. 1988, submitted for publication. Stone. J.; Burrus, C. A. Bell Syst. Tech. J . 1980, 59, 1541. Stone, J.; Lemaire. P. J. CLEO '81. June 10-12, 1981, Washington, DC; postdeadllne paper WC5. Stone, J.; Lemaire. P. J. Electron. Lett. 1982, 18, 78. Stone, J.; Walrafen, G. E. J . Chem. Phys. 1982, 7 6 , 1712. Stone, J.; Presby, H. M., unpublished results.
Ind. Eng. Chem. Prod. Res. Dev. 1986, 2 5 , 621-627
Stone, J.; Burrus, C. A.; Wiesenfeld, J. M. Appl. fhys. Left. 1984, 4 5 , 212. Stone, J.; Chraplyvy, A. R.; Burrus, C. A. Opt. Lett. 1982, 7 , 297. Stone, J.; Wiesenfeld. J. M.; Marcuse, D.; Burrus, C. A.; Yang, S. Appl. fhys. Lett. 1985, 4 7 , 328. Stone, J.; Wlesenfeld, J. M.; Marcuse, D.; Burrus, C. A,; Yang, S., unpublished results. Uchida, N.; Uesugl, N.; Murakaml, Y.; Nakahara, M.; Tanifiji. T.; Inagaki, N. Ninth European Conference on Optical Communications, Geneva, Swttzer-
62 1
land, Oct 23-28, 1983; postdeadline paper. Urbach, F. fhys. Rev. 1959, 92, 1324. vanderSteen, G.H. A. M.; Papanlcholau, E. Wl//psRes. Rep. 1975, 30, 192. Wakafen, 0. E.; Samantha, S. R. J . Chem. Phys. 1978, 69, 493.
Received for review March 27, 1986 Accepted May 14,1986
Determination of the High-Temperature Antioxidant Capability of Lubricants and Lubricant Components Stefan Korcek;
Mllton D. Johnson, Ronald K. Jensen, and Mlklo Zlnbo
Research Staff, Ford Motor Company, Dearborn, Mlchigan 48 12 1
A laboratory procedure for assessment of the antioxidant capabilities of engine oils, base oils, and additives at elevated temperatures under conditions simulating those encountered in Internal combustion engines has been developed. The method includes gradual addition of hydroperoxides or hydroperoxide-producing species during the test in order to simulate their continuous formation in engine oils as a result of the interaction of combustionderived free radicals with the lubricant during engine operation. The procedure detects both radical-trapping and hydroperoxidedecomposing antioxidant species. Examples from evaluations of antioxidants (primary and secondary alkyl zinc dialkyl dithiophosphates and 2,6di-fe~-butyl-4-methylphenol),base oils, and new and used engine oils are presented.
Introduction Oxidation is one of the most important processes causing degradation of engine oils during service. Oil oxidation leads to formation of acidic products, insoluble materials, and sludge, depletion of additives, loss of dispersancy, increase of viscosity, etc. All of these undesirable changes are, however, also affected by other concurrent processes occurring in an operating engine such as thermal degradation, mechanochemical reactions, metal catalysis, and interactions with combustion products which result in nitration and hydrolysis. Contributions of such processes to degradation are the main reason that correlation of engine test results with results of laboratory oxidation tests is not always successful. Nevertheless, proprietary laboratory oxidation tests are often used by engine oil formulators in predicting directional trends and approximate engine test performance since improvement of engine oil resistance toward oxidation leads directionally to improved performance in engine testing. This work was not directed toward development of another of these tests; rather, the focus is understanding oxidation processes occurring in engines and using this knowledge in development of procedures that would evaluate various parameters contributing to oxidation stability of engine oils. Oxidation properties of an engine oil are determined by its composition. In this respect, contributing factors are compositions of base oils, additives, and additive diluent oils. Particularly important are the presence and antioxidant properties of synthetic antioxidant additives and of natural inhibitors in base and diluent oils. Consequently, antioxidant capability is one of the most important technological parameters determining the oxidation stability of engine oils. A procedure for determining the radical-trapping antioxidant capacity of new and used lubricants was published previously (Mahoney et al., 1978). That method 0196-432 lI86/1225-0621$01.50/0
provides useful information and has been applied to investigations of antioxidant consumption in engine oils during laboratory and service evaluations (Korcek et al., 1979,1981;Mahoney et al., 1980; Murray et al., 1982; Hsu et al., 1982). The method, however, measures only radical-trapping capacity at low temperature and, thus, does not reflect the contribution of peroxide-decomposing species or natural inhibitors that can be formed from oil components during oxidation; these types of species can play important roles at elevated temperatures. A new technique that is capable of detecting all types of antioxidants and which is conducted under oxidizing conditions simulating those in internal combustion engines has been developed and is described in this paper. A model for oxidation of oils in internal combustion engines on which this method is based will be discussed, principles of the experimental procedure will be presented, and application of the method to assessment of high-temperature antioxidant capabilities (HTAC) of antioxidants, base oils, and fully formulated engine oils will be described.
Oxidation Model A model for oxidation of oils in internal combustion engines is shown in Figure 1 (Johnson et al., 1983). Engine oil oxidation is initiated by free radicals, which are derived either from combustion or from decomposition of primary oxidation produds such as hydroperoxides, ROOH. These free radicals react with the oil, RH, to abstract hydrogen and form alkyl radicals, R',which in the presence of oxygen form peroxy radicals, ROz'. In the absence of antioxidants, peroxy radicals further react with additional oil to form hydroperoxides (ROOH) and alkyl radicals (R*). This continues in a chain reaction process, which can result in the formation of a high concentration of hydroperoxides. The chain reaction process can be inhibited by adding radical-trapping antioxidants (AH) to the oil. In that case, 0 1986 American Chemical Society
