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Novel Technique for Nanopore Membrane Characterization

Explore the capillary condensation flow porometry technique for nanopore structure analysis, including principles, applications, and results. Understand pore diameter, flow rate, and more.

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Novel Technique for Nanopore Membrane Characterization

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  1. Achema 2009 Characterization of Pore Structure of Nanopore Membranes Dr. Akshaya Jena and Dr. Krishna Gupta Porous Materials, Inc. 20 Dutch Mill Road, Ithaca, NY, USA

  2. OUTLINE ● Introduction ● Capillary Condensation Flow Porometry - Basic Principle - Technique - The Instrument ● Results and Analysis - Equilibrium - Pore Diameter - Flow Rate - Analysis ● Unique Features ● Summary and Conclusion

  3. INTRODUCTION ● Nanoporemembranes are being developed for application in many advanced technologies including - Biotechnology - Power sources - Environmental protection ● The performance of such membranes are governed by their pore structure characteristics like - Diameters of nano through pores - Pore throat diameters - Pore distribution - Permeability ● Our investigation has resulted in the development of a novel technique for such characterization of nanopores ● We have discussed the principles, applications and results of this novel technique, ‘Capillay Condensation Flow Porometry’

  4. CAPILLARY CONDENSATION FLOW POROMETRY BASIC PRINCIPLES ● Vapor below its equilibrium vapor pressure on flat surface G (V  L) > 0, +ve, cannot condense ● Vapor below its equilibrium vapor pressure in pores G (V  L[in pore]) < 0, -ve, can condense

  5. CAPILLARY CONDENSATION FLOW POROMETRY BASIC PRINCIPLES ● Vapor condenses in all pores smaller than D, determined by the pressure of the vapor D = – [4 Vl/v cos / RT] / [ ln (p/po)] p = pressure of vapor p0= equilibrium vapor pressure V = molar volume of condensed liquid l/v = surface tension  = contact angle R= gas constant T = test temperature D= pore diameter

  6. CAPILLARY CONDENSATION FLOW POROMETRY BASIC PRINCIPLES ● Measurement of pressures of vapor equilibrated with the sample yields the largest diameter of pore blocked by condensation

  7. CAPILLARY CONDENSATION FLOW POROMETRY BASIC PRINCIPLES ● A small increase in pressure on inlet side of the sample causes flow ● Measurement of pressure change on outlet side of the sample yields flow rate FSTP = (Vo Ts / T ps)(dp/dt) FSTP = flow rate at standard pressure ps and temperature Ts Vo =volume of sample chamber on the outlet side T= test temperature in Kelvin P= pressure on the oulet side of sample T= time

  8. CAPILLARY CONDENSATION FLOW POROMETRY BASIC PRINCIPLES ● Measurement of -- pressures of vapor equilibrated with the sample -- rates of pressure change yield pore structure characteristics of nanopore membranes

  9. CAPILLARY CONDENSATION FLOW POROMETRY TECHNIQUE ● The instrument is maintained at a constant temperature ● Sample is loaded (It divides the sample chamber in to two parts)

  10. CAPILLARY CONDENSATION FLOW POROMETRY TECHNIQUE ● Both parts are filled with a vapor at the desired pressure below the equilibrium vapor pressure at the test temperature ● Vapors of liquids like water & alcohol are used ● The vapor pressure is monitored with time

  11. CAPILLARY CONDENSATION FLOW POROMETRY TECHNIQUE ● It takes about 30 min for equilibration ● After equilibration with the sample, the pressure is measured ● Pressure of vapor in equilibrium with sample yields the largest diameter of pores blocked by liquid condensation

  12. CAPILLARY CONDENSATION FLOW POROMETRY TECHNIQUE ●Pressure of the vapor in outlet chamber is reduced a little (< 10%) and the time rate of increase of pressure is measured for a few minutes ● The time rate of pressure increase yields the flow rate of vapor through open pores in which condensation has not occurred

  13. CAPILLARY CONDENSATION FLOW POROMETRY TECHNIQUE ● Tests are repeated at a number of pressures of vapor in equilibrium with the sample

  14. CAPILLARY CONDENSATION FLOW POROMETRY THE INSTRUMENT ● The instrument used in the investigation with all of the required features

  15. CAPILLARY CONDENSATION FLOW POROMETRY THE INSTRUMENT ● Uniform temperature is maintained in the instrument by warm air circulation with real time temperature indication in front of the panel ● Powerful vacuum pump used in design to - evacuate the system quickly - remove air from the liquid chamber to have only vapor - remove small amount of vapor at a time to reduce pressure in the sample chamber by a small amount

  16. CAPILLARY CONDENSATION FLOW POROMETRY THE INSTRUMENT ● The cup shaped bottom part of sample chamber supports the sample at the bottom on o-ring ● The top moving part goes inside the cup.

  17. CAPILLARY CONDENSATION FLOW POROMETRY THE INSTRUMENT ● The top part has bottom o-ring and circumferential o-rings to minimize leak ● Application of desired pneumatic pressure on top part of chamber pressurizes o-rings uniformly to ensure a good seal ● The sample is sealed between two high vacuum o-rings

  18. RESULTS AND DISCUSSION EQUILIBRATION ● Sample chamber filled with vapor at a pressure which is a little higher than the desired value ● Pressure is monitored as function of time ● The equilibrium pressure is evaluated

  19. RESULTS AND DISCUSSION • EQUILIBRATION • Equilibrium time varied with equilibrium pressure • and was about thirty minutes • Equilibrium pressure of water vapor with a sample

  20. RESULTS AND DISCUSSION PORE DIAMETERS ● Pore diameters are computed using pressures of vapor in equilibrium with the sample as all parameters are known D = – [4 Vl/v cos  / RT] / [ ln (p/po)] p = pressure of vapor p0= equilibrium vapor pressure V = molar volume of condensed liquid l/v = surface tension  = contact angle R= gas constant T = test temperature D= pore diameter ● Corrections added to the measured diameter due to adsorbed layer thickness

  21. RESULTS AND DISCUSSION MEASURED PORE DIAMETERS ● In a pore vapor first condenses in the throat ● On increase of pressure vapor condenses in wider parts of the pore and throats of other pores ● Condensation in throat prevents vapor flow ● Flow occurs through all pores having throat diameters greater than the throat diameter corresponding to the equilibrium pressure

  22. RESULTS AND DISCUSSION Measured diameters were the through pore throat diameters Condensation in pores with increasing vapor pressure

  23. RESULTS AND DISCUSSION FLOW RATE ● Flow rate computed from pressure decay in the outlet chamber ● Typical pressure decay curves demonstrate ■ High flow rate at pe=4.91 torr ■ Low flow rate at pe =14.73 torr & 20.81 torr

  24. RESULTS AND DISCUSSION • ANALYSIS • ● The technique measures pores less than about • 20 nm under test conditions with mean free path • of molecules of about 550 nm. • ● We may expect molecular flow to be dominant • ● Flow rate through the pores of the sample may • be written as: • FSTP = (Ts/T) (/12ps)(8RT/M)1/2 (p/L) [ANi(Di)3] • FSTP = flow rate in volume at standard D= pore diameter • temperature, Ts and pressure, psn T= test temperature • A= cross-sectional area of sample R=gas constant • p=pressure drop across the sample M= molecular weight • = average tortuosity of capillaries= l/L L= thickness of sample • Ni = number of capillaries per unit area l= length of capillary • Di = diameter of capillary

  25. RESULTS AND DISCUSSION ANALYSIS ● F= cumulative flow per unit pressure drop and unit area F = (FSTP /Ap)cumulativeDmax = (Ts/T) (/12psL)(8RT/M)1/2 [ Ni(Di)3] D Dmax = diameter of the largest pore

  26. RESULTS AND DISCUSSION • ANALYSIS • Expected variation of flow, F  • Variation of flow per unit pressure drop per unit area with diameter

  27. RESULTS AND DISCUSSION ANALYSIS ● Experimental cumulative flow, F

  28. RESULTS AND DISCUSSION • ANALYSIS • ● Pore distribution function: • f = - d((FSTP /Ap)cumulative) /dD • Expected flow distribution • Pore distribution

  29. RESULTS AND DISCUSSION ANALYSIS ● Experimental distribution curve

  30. UNIQUE FEATURES OF THE TECHNIQUE • Pressures used are negligible. • No distortion of pore structure or damage of the sample • Only through nanopores are measured unlike the gas adsorption technique • Throat diameters are measurable • A wide variety of vapors can be used • Measuring technique is simple. Complex instrumentation like RGA is not required

  31. SUMMARY AND CONCLUSIONS ● We have discussed the novel technique, Capillary Condensation Flow Porometry ● It can measure nanopore diameters, pore distribution, and flow rate ● It has many unique features including use of negligible pressure gradients, ambient temperatures, and nontoxic materials ● Work is under progress to automate the instrument to obtain more accurate data

  32. THANK YOU!

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