**HF SiO~2~ Chemistry**

Classically

4 HF + SiO~2~  SiF~4~ + H~2~O

This is stoichiometry [not]{.underline} mechanism or kinetics.

With excess aqueous HF

6 HF + SiO~2~  H~2~SiF~6~

These are macroscopic descriptions suitable for defining endpoints.

To describe the kinetic rate behavior we need microscopic detail.

The detail level must include the mechanistic detail.

The order of the reaction will be dependant upon the rate limiting
reaction and the number of reactants in this step.

The rate law will also include all of the side reactions, reverse
reactions, and alternate reaction paths.

Known reaction properties.

1.  In the complete absence of water

> HF + SiO~2~ → No Reaction

2.  HF Oligomerizes

> HF  H~2~F~2~  H~3~F~3~  H~4~F~4~

3.  HF + SiC → No Reaction

4.  Addition of strong acid (HCl, HNO~3~) accelerates HF + SiO~2~
    reaction.

Equilibrium Review

1.  Equilibria are dynamic and activity based.

2.  Equilibria are based on population statistics, infinite time and
    Maxwell-Boltzman energy distribution assumptions, requiring large
    numbers of individual molecules.

3.  First year Chemistry assumes concentration (moles/L)= activity,
    reality is much more complex.

4.  Vapor pressure = γ \* P~0~ (assuming equilibrium)

P~water~≈ 10^-12^ atm, P~o~ ≈ 10^-2^ atm → γ~water~ = 10^-10^

This drastically affects the behavior of reactions involving water.

![](media/image1.png){width="6.935416666666667in"
height="7.936111111111111in"}

Known Steps in HF/ SiO~2~ Reaction

1.  The silica lattice must be severed by acid, neutral or base
    hydrolysis.

2.  Stepwise Substitution of F^-^ for O-R^-^ either concurrent with
    hydrolysis or subsequent of hydrolysis.

3.  Silica subtypes

Silicic Acid  SiO~2~ I  SiO~2~ II  SiO~2~ III  SiO~2~ IV

K~1~ K~2~ K~3~

Assuming each type is in equilibrium ( a bad assumption due to dynamic
time dependant system)

aSiO~2~ I = ( K~1~K~2~K~3~\*a~SiO2\ IV~\* a~H2O~^3^)^1/2^

aSiO~2~ II = ( K~2~K~3~\*a~SiO2\ IV~\* a~H2O~^2^)^1/2^

aSiO~2~ III = (K~3~\*a~SiO2\ IV~\* a~H2O~)^1/2^

Silica/ HF Reaction Kinetics

  -------------------- ----------- ----------- ----------- ----------- -----------
                       Silicic     SiO~2~ Type SiO~2~ Type SiO~2~ Type SiO~2~ Type
                       Acid        I           II          III         IV

  HF                   K~r0~       K~r1~       K~r2~       K~r3~       K~r4~

  HF~2~^-^             K~r5~       K~r6~       K~r6~       K~r7~       K~r8~

  H~2~F~2~             K~r8~       K~r9~       K~r10~      K~r11~      K~r12~

  H~2~F~3~^-^          K~r13~      K~r14~      K~r15~      K~r16~      K~r17~

  H~3~F~3~             K~r18~      K~r19~      K~r20~      K~r21~      K~r22~

  H~3~F~3\ (cyclic)~   K~r23~      K~r24~      K~r25~      K~r26~      K~r27~

  H~3~F~4~^-^          K~r28~      K~r29~      K~r30~      K~r31~      K~r32~

  H~4~F~4~             K~r33~      K~r34~      K~r36~      K~r37~      K~r38~

  H~2~SiF~6~           K~r39~      K~r40~      K~r41~      K~r42~      K~r43~

  HsiF~6~^-^           K~r44~      K~R45~      K~r46~      K~r47~      K~r48~

  H~2~O                K~r49~      K~r50~      K~r51~      K~r52~      K~r53~
  -------------------- ----------- ----------- ----------- ----------- -----------

These reaction rate constants only represent only the forward reactions,
and assume only one reaction path.

To define the reaction rate, one must also consider the reverse
reactions, and the alternate paths.

This requires knowledge of each species activity and the ability to
discriminate between the reaction pathways.

Each pathway will have its own activation energy and kinetics.

Cable Insults:

Limited insult with Tesla coil, 0.27 femtomoles detected

![](media/image3.png){width="2.85in" height="6.183333333333334in"}

Tesla coil assault generated active fluorine, assault at 0.5 m from
instrument inlet.

Conclusions:

1.  HF chemistry is complex

2.  Silica and Germanium chemistry are not well defined.

3.  We are not at equilibrium.

4.  Many reaction pathways exist.

5.  All required reactants are present.

6.  The carbonaceous coating localizes the reactions.

7.  Mechanically generated ESD makes breaches in the coating and HF
    (assisted by C Coating).

8.  Uncoated Fiber is highly resistant to ESD.

9.  Ab initio rate modeling is not feasible.

10. HF is the bad actor

11. Pyrolytic carbon enhances the rate of reaction.

12. The reaction is HF limited

13. The absence of C coating will stabilize fiber life by spreading out
    the reaction.

14. We are at or beyond the statistical predictive limits of chemistry.

15. We can make measurements and observations and then make estimations
    ( with reservations.)