Diffusion Through Polymer Membranes - PowerPoint PPT Presentation

1 / 16
About This Presentation
Title:

Diffusion Through Polymer Membranes

Description:

Diffusion Through Polymer Membranes. Doug Crowson and Brian Grimsley ... To predict the leak rate of He and O2 gas through a Polyethylene Terephthalate membrane. ... – PowerPoint PPT presentation

Number of Views:828
Avg rating:3.0/5.0
Slides: 17
Provided by: rebecca215
Category:

less

Transcript and Presenter's Notes

Title: Diffusion Through Polymer Membranes


1
Diffusion Through Polymer Membranes
  • Doug Crowson and Brian Grimsley
  • Department of Materials Science and Engineering
  • Virginia Polytechnic Institute
  • and State University

2
Outline
  • Background
  • X-33 Fuel Tank Failure
  • Proposed Solutions
  • Diffusion Defined
  • Ficks Law
  • Mathematica Solutions
  • Tabulated Results
  • Conclusion

3
X-33 Space Plane
  • Lockheed-Martin/NASA Venturestar Program
  • Half-Scale Demonstrator Vehicle
  • Aerospike Engines
  • Thermal Protection
  • CFRP LH2 Fuel Tanks

4
LH2 Fuel Tanks
  • Accounts for half of the vehicle airframe
  • 29,000 gallons of LH2 at -423F(-250C)
  • Sandwich structure
  • (Skin)Quasi Lay-up of 977-3 epoxy with IM-7
    carbon fiber reinforcement
  • (Core) Spectra reinforced honeycomb

5
Tank Failure
  • Microcracking of inner composite skin
  • Delamination of inner skin plys
  • Poor interfacial bond at core to skin interface

6
Thermally Induced Microcracks
  • Polymer matrix has a higher coefficient of
    thermal expansion
  • Fibers resist matrix shrinkage, causing
    internal stresses
  • If stress is high enough, microscopic cracks
    form within matrix

7
Proposed Solutions
  • Develop a polymer matrix with CTE closer to fiber
  • Develop a toughened polymer matrix to resist
    microcracking
  • Investigate and develop polymer films which will
    act as a barrier to diffusing hydrogen gas

8
Project Statement
  • To predict the leak rate of He and O2 gas through
    a Polyethylene Terephthalate membrane.
  • To determine the effects of crystallinity,
    temperature, and film thickness on the leak rate.

9
Definitions and Assumptions
  • Diffusion is the process by which matter is
    transported from one part of a system to another
    as a result of random molecular motions.
  • The leak rate is defined as the volume of gas per
    time(cc/sec) that reaches the boundary where
    concentration is zero.
  • We assume that the diffusion coefficient,
    D(cm2/sec), remains constant.

10
Ficks Laws
  • One dimensional diffusion in a medium bounded by
    two parallel planes
  • Ficks First Law (steady state) gives the
    relation between concentration gradient and rate
    of transfer
  • Integrating from x0 to L, and CC1 to C2
  • This allows the Diffusion Coefficient, D, to be
    determined from an observed value of flux.

11
  • Ficks Second Law (unsteady state) gives the
    relation between concentration gradient and the
    rate of change of concentration caused by
    diffusion
  • When D is independent of concentration
  • The solution of this PDE may be found by assuming
    that the variables are separable

12
  • Substitution into Ficks equation yields
  • Both sides must equal a constant
  • The solution of these two equations leads to the
    general solution of Ficks Second law

13
Temperature Dependence
  • An empirical equation was found in the literature
    for the temperature dependence of the diffusion
    coefficient
  • Where
  • ED is the activation energy
  • R is the gas constant
  • D0 is the pre-exponential factor

14
Tabulated Results

15
Effect of Crystallinity and Temperature for Helium

16
Summary
  • Due to the nature of the X-33 composite tank
    failure, a possible solution may be found in the
    use of polymer film as a barrier to diffusing
    gases.
  • As part of this investigation, a solution was
    generated in Mathematica of the Ficks equation
    for unsteady state.
  • The solution predicts the gas leak rate, given
    the diffusion coefficient for commercially
    available films.
  • These predicted values will be compared with
    results found in future testing of these films.
Write a Comment
User Comments (0)
About PowerShow.com