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MSE 5090: Case Studies in Material Selection

Week 3 - Material Selection Process

 Weighted Property Index Methods - Problem 2

This is a partial selection from Lewis's excellent book on the selection of Engineering materials. I commend it to your attention. It is comprehensive in its coverage of all material classes.

Halide Glasses³

 

Halide glasses fall into three subcategories. Single-component halide glasses (SCHGs) have low attenuation due to monovalent ion behavior; for example, BeF₂- and ZnCl₂-based halide glasses have a of 10^-2 to 10^-5 dB km^-1. These glasses have some drawbacks; for example, beryllium in BeF₂ is highly toxic, while ZnCl₂ highly deliquescent.

Single-anion multicomponent or heavy metal fluoride glasses (HMFGs)* have a wide transparency range (from 0.3 mm to about 9 mm, that is, mid-infrared to near ultraviolet). Attenuation losses of these glasses are on the order of 5 to 8 x 10^-3 dB km^-1 at l = 2.4 mm. This high loss is mainly because of impurities—such as three-dimensional transition metal additions (mainly iron and nickel), which absorb in the 2 - 3 mm range, and rare earths—and because of a fundamental stretching of O-H absorption at about 3 mm. These glasses have a tendency to recrystallize from the surface as a result of attack by moisture and oxygen, so special jackets or hermetic coatings are required for fibers made of these glasses. HMFGs have relatively narrow glass-forming regions because viscosities are low -(for example, 0.4 Pa at 490° C). HMFGs also have thermal instability tendencies, leading to devitrification; they also have a high susceptibility to attack in aqueous environments (see Fig. C9.2, Chapter C9 in Part III).

Heavier halide glasses are based on chloride, bromide, and iodide mixtures, although the latter two types give rise to glasses of low stability and hence are unsuitable for fiber-optic applications. The chloride-based glasses have major problems with impurity ions and are water-soluble.

Chalcogenide Glasses

Chalcogenide glasses contain the elements Se, S, or Te, together with one or more of Ge, Si, As. Sb, and some others. Attenuation loss is on the order of 10^-2dB km^-1; this is primarily due to impurity absorption (from H₂S, H₂Se oxides of the original cationic elements, water molecules, etc.). These glasses have large glass-forming regions, high stability against attack by moisture, and reasonably long wavelength cutoff of 0.5 to about 15 mm.

Fused Silica

The losses in this glass amount to about 0.2 dB km^-1, making it less attractive than either of the other two types of glass already discussed.

Some characteristics of three examples of these glasses are presented in Table Q57.2 and Figs. Q57.3 and Q57.4.

Sources of Attenuation in Optical Glasses

The optical attenuation in glasses arises from three main sources. Scattering and absorption losses can result from extrinsic factors such as mechanical defects (splicing. microdeformations). material inhomogeneities (such as voids and dissolved submicron inclusions), and impurities (such as oxides, hydrides, rare earths, and transition metal ions).

Intrinsic absorption may be due to absorption from the electronic (or Urbach) and edge tails, together with elastic (or Rayleigh) scattering and multiphonon absorption. All these losses are dependent on wavelength l. Such dependence for Urbach, multiphonon, and weak absorption tails for some chalcogenide glasses are given in Table Q57.3.

TABLE 057.2 SOME PROPERTIES OF THREE CANDIDATE OPTICAL FIBER GLASSES

Source: P. Klocek. M. Roth, and R. D. Rock, "Chalcogenide glass optical fibers and image bundles: Properties and applications," Optical Engineering 26 (2). pp. 88—95. 1987 (all data except fracture toughness).

Figure Q57.3 Transmission versus wavelength for various optical glasses.

Figure Q57.4 Infrared spectra of Ge₂₈Sb₁₂Se₆₀ (TI 1173 glass). 9.08mm thick (courtesy of Texas Instruments, Dallas. Texas).

The Rayleigh scattering loss is also dependent on wavelength and is given by B/l^4. where B is a material constant. The total minimum intrinsic loss in a given glass is obtained by the interaction of the scattering loss and the vibrational edge absorption curves (Fig. Q57.5). This yields the so-called theoretical V curve for the material (Fig. Q57.6).

Attenuation may also stem from a number of material-related nonlinear effects, such as Raman and Brillouin scattering, which occur above a critical power level [2]. In the case of Raman scattering, the associated loss is reflected in the value of  F_R which is the ratio of the spontaneous Raman scattering loss to the density fluctuation loss. The values of F_R and the Rayleigh material constant B for three named glasses are given in Table Q57.4.

The trend in reduction in the total optical losses in optical fibers over the period 1972-86 is shown in Table Q57.5. Much of this improvement is directly related to advances in fiber fabrication processing. For mono- or multimode long-distance telecommunications applications, optical fibers are best made by vapor techniques employing energy supplied in a flame, in a heated tube, or in a plasma. Fibers made using these processes have low values of a. Direct melt or liquid phase techniques produce fibers with higher values of a (on the order of 3 x 10³ to 2 x 10⁴ dB km^-1). Such fibers are best suited for use in optics systems for short-distance multimode applications. A third fabrication technique, sol-gel, is currently receiving serious research and development attention.

TABLE Q57.3 SOME LOSS CHARACTERISTICS OF THREE CHALCOGENIDE GLASSES
(in dB km^-1)

l is in mm; glass composition is in wt %.
Source: T. Kanamori. et at., "Chalcogenide fibers for mid infrared transmission," J. LightwaveTechnology 2 (5), pp. 607—613, 1984.

	Figure Q57.5 Scattering and absorp-tion optical losses in an optical fiber (graded-index silica glass).
	Figure Q57.6 Projected minimum optical losses in glasses [1].

TABLE Q57.4 VALUES OF F_R AND B AT VARIOUS WAVELENGTHS FOR THREE TYPES OF GLASS

Source: M. E. Lines. "Scattering losses in optic fiber materials: II. Numerical results." J. Appi. Phys. 55(11). pp. 4058—4063. 1984.

TABLE Q57.5 REDUCTION IN OPTICAL LOSSES IN OPTICAL MATERIALS. 1972—86

Source: N. Poulain. "New glasses for optical fibers." Endeavour II(1), p.27. 1987.

In addition to low attenuation, other desirable properties for an optical fiber material are as follows:

·  Low pulse dispersion (or broadening); this determines the information-carrying capacity of the fibers
·  High strength
·  High chemical durability
·  Ease of fabrication in long length
·  Absence of toxic and/or exotic starting materials

1. S. Shibata, et al., "Prediction of loss minima in infrared optical fibers." Electronics Letters 17(21), p. 776. l98l.
2. R. H. Stolen. "Nonlinearity in fiber transmissions." Proc. IEEE 68(10), pp. 1232—1236, l980.
3. Lewis , G. Selection of Engineering Materials, Prentice Hall , 1990, pp. 421-426 This is a selection of a long suggested study on fiber optics.Your choices of parameter weightings will perforce be somewhat arbitrary. However they should have some logic behind them


Last update 9-12-98