\"First Cut\" estimation of the amount of HF (etc) needed to make a

\"rocket engine\" defect:

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A typical \"rocket engine\" defect is (very roughly) a sphere with a

diameter of 50 microns. \[\"First, assume a spherical cow\...\", as the

famous shaggy-aggie professor story ends.\] Thus its volume is 0.52

micro-cc. I will ignore the presence of the Ge doping in this

\"first cut\" estimate, and pretend that the entire fiber is vitreous

silica for the purposes of estimating the amount of fluorine needed

to excavate the defect. The density of vitreous silica is 2.2 g/cc,

so the etched-out mass is about 1.1 microgram. One mole of SiO2 is

60.1 gram. So the missing material is 19 nanomoles of SiO2. The

chemical reaction by which HF etches the vitreous silica is (I

suppose)

4 HF + SiO2 \--\> SiF4 + 2H2O.

Thus we need 75 nanomoles of hydrofluoride (HF) to etch the cavity.

The resulting materials are gases, and diffuse away from the

cavity as it forms. (There can be a bit of glass re-formed when

some SiF4 dissolves in the ubiquitous water film; inspection of SEM

images shows some of this, but these same SEMs show that there is

not much solid material left inside the rocket engine defect. So I

will ignore any redepositing materials.)

In summary:

diameter of cavity = 50 microns

volume of cavity = 0.52 micro-cc

mass of removed material = 1.1 micrograms

\# of moles of removed material = 19 nanomoles of SiO2

and we need 75 nanomoles of HF to etch out a 50 micron-diameter

rocket engine defect.

The required HF can be made from an appropriate fluorine-containing

gas that reacts with an ambient water film. Possibly this is the

C2F4 and / or the C3F6 that outgases from the FEP? We would need

\[75 nanomoles\]/6 of C3F6, which would weigh 2.0 micrograms. C3F6 is

the dominant species released during thermal decomposition of FEP

under about 525 degC.

But David DiGiovanni noted that it is the COF2 that \"hydrolyzes

quite readily\", and so perhaps the HF comes predominantly from the

COF2. We would need \[75 nanomoles\]/2 of COF2, which would weigh

2.5 micrograms. However, thermal decomposition of FEP does not give

large amounts of COF2 until the temperature is above about 525

degC, which is substantially hotter than the extrusion temperature

(404 degC) reported by BIFF.

(The web site I quoted last week,

http://www.nist.gov/cstl/div836/836.03/papers/, and the sequence of

files beginning with \*/NistTNIntro.html, give a detailed discussion

of the various fluorine reactions, including those in the presence

of water. It will take me some days to read these, but the right

sort of chemist would know this stuff already.)

Using an image of the cross section of the fiber provided in Craig

Mueller (et al.)\'s 10 Nov 1999 \"ISS Fiber Investigation\", I estimate

that the buffer tube has an average OD of 0.67 mm, and an average

wall thickness of 0.14 mm. The density of FEP is close to 2.1 g/cc.

Thus, the buffer has a mass of 19 milligrams per centimeter of

length.

Let us suppose the FEP buffer is at 404 degC for 10 seconds.

(Perhaps Dale Zevotek can give us a more realistic duration?) The

decomposition rate of FEP in air at 404 degC is (dM/dt)/M =

1.5\*10\^(-5)/second, so \"ten seconds\" gives dM/M = 1.5\*10\^(-4), or

2.9 micrograms per centimeter. Of course, half of this would

diffuse outward and would never enter the cavity within the buffer

tube; the other half would diffuse inward and would (eventually)

fill the buffer tube cavity. Thus, we have 1.8 micrograms of

decomposition product per centimeter of buffer tube. For

decomposition below some 525 degC, the decomposition product is

dominantly C3F6.

In the cable P7, there were \"rocket engine\" defects at the rate of

10 per 160 feet of cable, which scales to 1 per 16 feet of cable.

Sixteen feet of buffer tube (held at 404 degC for ten seconds) would

provide 880 micrograms of C3F6, or 5.9 micromoles. If all the

fluorine becomes available as HF, then this provides 35 micromoles

of HF. Since only 75 nanomoles of HF is needed to make a \"rocket

engine\" defect, then this would make some 470 of them.

Certainly, we need a realistic reaction pathway to make the

fluorine in the C3F6 available as HF before we can conclude that

this is how these defects are etched into the fiber. Or perhaps it

is the trace amounts of COF2 produced below 525 degC that are

responsible?

Henning Leidecker