Effect of dust on glass fiber strengthRobert D. Maurer Citation: Applied Physics Letters 30, 82 (1977); doi: 10.1063/1.89296 View online: http://dx.doi.org/10.1063/1.89296 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/30/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Strength of long optical glass fibers J. Appl. Phys. 62, 719 (1987); 10.1063/1.339750 Strength Characteristics of Glass Fibers Under Dynamic Loading J. Appl. Phys. 41, 1657 (1970); 10.1063/1.1659088 Crossbending Tests of Glass Fibers and the Limiting Strength of Glass J. Appl. Phys. 29, 1263 (1958); 10.1063/1.1723419 Automatic Balance for Measurement of the Strength of Glass Fibers Rev. Sci. Instrum. 27, 34 (1956); 10.1063/1.1715359 The Effect of Water on the Strength of Glass J. Appl. Phys. 17, 179 (1946); 10.1063/1.1707703

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havior shown in Fig. 3 indicates that at least two spin configurations for the 3He have been established.

The nonexponential magnetization recovery was ob­served earlier in life, but the apparent difference in T j

was not nearly as great as observed in Fig. 3. The point here is that nonexponential recovery does not correlate with the appearance of a signal at low tem­perature. This observation indicates that the defect structure generated early in life tends to isolate groups of nuclear spins, but these groups are not sufficiently large to behave as bulk 3He.

Nuclear resonance data, presented here, support4

the following picture for an aging metal ditritide. Ini­tially the 3He is contained in small defects, perhaps even isolated as individual atoms during very early life. At least some of these defects become larger and contain more 3He atoms with age. A variety of environ­ments for the individual 3He occur, leading to nonexpo­nential T2 plots and, in addition, the variability of local atomic conditions are such that appreciable percentages of the spins are sufficiently isolated that a common spin temperature cannot be established. Evidence that some defects are growing is found in the lowering of the mini­mum temperature for detecting the 3He NMR. Ultimate­ly, large bubbles form. 3He NMR is detected in these bubbles at 1.2 K. The amount of 3He contained in these macroscopic bubbles remains a small percentage of the helium contained in the entire lattice. Since the majority of the 3He remaining in the crystal is not in large bubbles, it is either (1) trapped in a configuration intermediate to bubble formation (perhaps microbub­bles) or (2) providing local lattice strain such that newly generated 3He is immediately detrapped into large bubbles.

The time for large-bubble formation (signal detect­able at 4 K) corresponds closely to the sample age at which rapid helium release from the lattice occurs. Consequently, the inference that large bubbles are the immediate precursor stage to helium release is ines-

capable. This is also consistent with the appearance of blisters3 on tritide surfaces at release time.

In earlier work, percolation theory4 was used to estimate the age at which release (or bubble formation) would occur. This approach is supported by the present picture in that critical densities of the early life defects apparently lead to precipitation of large bubbles. Un­fortunately, experiment has yet to provide sufficiently detailed information about the nature of these defects to allow quantitative calculations. It remains, however, that percolation theory with the assumption of initially isolated 3He, which is quite likely an oversimplification in view of the present work, provides qualitatively ac­curate predictions of release time.

The samples were produced by Lynn Provo of General Electric and R. C. Bowman of MOUND Laboratories. The technical assistance of Barry Hansen is appreciated.

lW.G. Perkins, W.J. Kass, andL.C. Beavis, in Radiation Effects and Tritium Technology for Fusion Reactors, USERDA CONF-750989, edited by J. W. Watson and F. W. Wiffen (USERDA, Oak Ridge, Tenn., 1976), Vol. IV, p. 83.

2A.M. RodinandV.V. Surenyants, Zh. Fiz. Khim. 45, 1094 (1971).

3L.C. Beavis and C.J. Miglionico, J. Less-Common Met. 27, 201 (1972).

4H. T. Weaver and W. J. Camp, Phys. Rev. B 12, 3054 (1975). 50ne of the samples was provided by Dr. R. C. Bowman from a larger sample on which he has carried out NMR work. This sample was essentially identical to the original TiH1•8 which was prepared by the General Electric Co. in St. Petersburg, Fla.

6S. Meiboom and D. Gill, Rev. Sci. lnstrum. 29, 1688 (1958). lWe observe T2 in the msec range. This type of signal is usu-ally associated with motionally narrowed lines. Again, de­tails are discussed in Ref. 4, together with pertinent references.

8 A. Abragam, Principles of Nuclear Magnetism (Oxford, London, 1962), p. 133.

Effect of dust on glass fiber strength Robert D. Maurer

Research and Development Laboratories. Corning Glass Works. Corning. New York 14830 (Received 12 August 1976; in final form 5 November 1976)

A model is developed for the effect of atmospheric dust on fiber strength. It is assumed that dust striking the fiber as it is formed sticks to form flaws. Comparison of the model with experimental data shows that it provides a reasonable description and can be useful in interpreting fiber strength.

PACS numbers: 42.80.Lt, 46.30.Nz, 42.80.Sa, 42.80.Mv

The strength of large pieces of freshly made glass depends primarily upon foreign matter rather than pure material properties. This is especially true for optical waveguides where long lengths are subject to stress. 1

The region of foreign matter generates a flaw in the

82 Applied Physics Letters, Vol. 30, No.2, 15 January 1977

material which acts as a stress intensifier leading to fracture. Dust from the air which sticks to the hot glass surface can be a Significant source of these imperfec­tions. Inorganic refractory particles in the dust that do not completely dissolve in the glass generate the flaw

Copyright © 1977 American I nstitute of Physics 82

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from thermal expansion stresses that arise around the particle as the glass is cooled. In some cases, dust which dissolves could have a similar effect. This paper proposes a model for such strength degradation.

A previous formulation of optical waveguide strength variation showed that the cumulative failure probability is given in general by2

F = 1- exp[ - L l' n(a) da], D

(1)

where L is the fiber length and n(a) da is the number of flaws per unit length which will break at an applied tensile stress between a and a + da. The distribution function, n(a), thus has a purely operational definition. However, Eq. (1) is useful for modeling since expres­sions can be calculated for n(a) based on reasonable physical approximations. In the following, each inorgan­ic dust particle is assumed to form a flaw of the same size as the particle and the strength of the flaw is given byl

a~a'o-l/2, (2) with 0 the flaw size (or particle size) and a' == 10.4 N mm-3 / 2 •

Consider a narrow slice of a waveguide "blank" (or "preform") of length 1 and radius r which will be fabri­cated into a much longer length of waveguide (Fig. 1). The acquisition of dust particles during the time the slice is hot and sticky will be the only acquisition con­sidered. The extension to many other cases is straight­forward. The volume of the slice, 1rrl = VI' is a con­stant of the forming process although both rand 1 are functions of time. Let y be the fraction of dust particles which strike the surface and stick to form a flaw. As­sume that the particle denSity in the air, p(o) do, near the fiber remains unchanged because particles which stick to the glass are replaced. Then, the number of particles acquired per unit time on the surface of the slice is

v, (0) do == [yp (0) do ]27TYlvp dt,

where vp is the velocity of the air carrying dust parti-

. '. > •

83

' .. FIG. 1. Schematic showing acquisition of dust particles by a preform being drawn into a fiber. The preform seg­ment of length 1 is transformed into a fiber of length L.

Appl. Phys. Lett., Vol. 3D, No.2, 15 January 1977

99 98 0 96 (] 90

80

60

40

'" Q: ::::l ..J <i ... 20 I-Z

'" U Q:

'" 10 "-8

6

4

2 0

STRESS

FIG. 2. Weibull plot of strength data from a fiber drawn in the presence of dust. Gauge length was 20 m. The straight line drawn through some of the data has a slope m = 4. 4.

cles. The total number acquired during the fiber form­ing process is

v(o) do == 2[yp(0) dO]VpV{P ;(~!)dr,

with R the preform radius, p the waveguide radius, and the velocity of the air assumed constant with time. The number of flaws per unit length of size 0 or larger is

N(o)/L = (2yvp V/L) J: (l/r) dt ~"p(o) do.

There is an average distribution of particles in the air given by3

~ .. p(o) do ~ 8.12 X lO-sc 0-2·2,

where the constant has dimensions cm-3 (J..un)2.2 and C is the class.

N(o)/L == (YVp V JL)16. 2 x 10-6Co-2• 2 J (1/r) dt. (3)

The flaw size distribution is independent of the forming history with the assumptions above.

A comparison is possible between the strength dis­tribution from this model and experimental results. The left-hand side of Eq. (3) is equivalent to the integral in Eq. (1) using the transformation Eq. (2). If Eq. (1) is written in the Weibull form

F = 1-exp[ - (L/Lo)(a/ao)m],

then from Ref. 1, Eq. (5)

N(o)/L = (a'1)"l/2)m/aWLo.

(4)

(5)

From this and Eq. (3), the dust model predicts a Wei­bull form of strength distribution with m ==4.4.

We can compare this with a fiber knowingly made in a dusty atmosphere in which the dust was generated from zirconium refractory processing. Figure 2 shows the

Robert D. Maurer 83

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strength data4 on a Weibull plot with a straight line drawn through the weakest fibers with a slope of 11'1 = 4. 4 . The weak flaw population adequately fits this model but it must be inferred that some other explana­tion is required for the higher strength data. In prac­tice, dust distributions vary somewhat from the average and the model can only give the median of 111.

This data can also be compared in absolute magnitude of strength, although absolute comparison with theory is usually more difficult and tenuous. The main contri­bution to the integral in Eq. (3) comes when r is small. Therefore, this will be approximated by

j(1/r) dt ",,(1/ p) j dt = A/ PVd'

where A is the length of the hot zone and Vd is the veloc­ity of draw. Dust measurements in the vicinity of the forming process gave C -30 000 and y = 0.03. The air velocity is estimated by noting that there is no dust in still air. Therefore, the updraft currents will be taken as approximately equal to the settling velocity of a 10-j.J.m-diam particle, or 0.3 cm/sec. With p=63 x 10-4

cm, A - 5 cm, and v d - 50 cm/sec, N(6)/L"" O. 8 km-1

for flaw sizes greater than 1 j.J.m. Using the compari­son, as in Ref. 1, with Eq. (5) and the strength data gives N(6)/L "" 2 km-1• This close agreement is fortu­itous because of uncertainty of the numerical values used to compute it but is generally confirmatory.

Dust in the forming environment can affect glass strength beyond the case of optical waveguides and can be assumed to have general relevance. The increase in glass strength with etching is at least partly attributable to removing flaws caused by dust. In particular, the density of flaws increases as the flaw size decreases, Eq. (3), which suggests that high strength samples are also markedly affected. However, some complicating factors like dissolution of the particle may ameliorate the detrimental effects. Further work is needed.

In conclUSion, attention has been drawn to the impor­tance of dust in the forming atmosphere on glass fiber strength. A model has been devised which shows that the effects are easily detectable and is in approximate agreement with some observations. Analyses based on models can be helpful in understanding glass strength, espeCially when used to augment experimental evidence.

lR.D. Maurer, Appl. Phys. Lett. 27, 220 (1975). 2R. Olshansky and R.D. Maurer, J. Appl. Phys. 47, 4497 (1976).

3P.R. Austin and S. W. Timmerman, Design and Operation of Clean Rooms (Business News Publ. Co., Detroit, 1965).

4Data obtained by B. Justice, Corning Glass Works.

Picosecond optoelectronic switching in GaAs t Chi H. Lee"

Electrical Engineering Department. University of Maryland. College Park. Maryland 20742 (Received 1l August 1976; in final form I November 1976)

Picosecond optical pulses are used to switch a GaAs slab in a charged line pulser. The repetition rate of the device is I GHz. The effect of the Ridley-Watkins-Hilsum mechanism on the device performance is discussed.

PACS numbers: 85.60.Me, 84.40.Qp, n.60.+g. 06.60.Jn

Picosecond optoelectronic switching and gating in silicon have recently been demonstrated by Austonl and his co-workers. 2 Using a microstrip transmission-line structure, they were able to switch the line "on" and "off" with picosecond 0.53- and 1.06-j.J.m pulses, re­specti vely. Although the switching time is ultrafast, the repetition rate of the device is rather slow due to the slow recombination processes in Si. In other words, even after the switch is turned "off", it cannot be re­used immediately. Furthermore, the use of the Si sub­strate itself as an insulator in the microstrip structure limits the power handling capability of the device be­cause of possible electric breakdown of the semiconduct­ing substrate. We report here the use of a piece of GaAs as the switching element in an ordinary charged line pulser configuration. In this configuration, a high strength dielectric, instead of the semiconductor switching material, is used as the insulator. This will

84 Applied Physics Letters. Vol. 30, No.2. 15 January 1971

increase the power handling capability of the device many-fold. Since the carrier lifetime in GaAs is less than 100 psec, 3,4 this device only requires an optical pulse to switch it "on" and will turn "off" automatically.

"ON"

O.53f'm GoA.

R

IN INSULATOR OUT

GROUND PLATE

FIG. 1. The structure of the charged line pulser with a slab of GaAs as the switching element. The pulse is switched on with a O. 53-/.lm picosecond pulse. It turns "off' automatically due to fast relaxati.on of the photOinduced charge carriers.

Copyright © 1971 American Institute of Physics 84

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