Characterization of Pore Structure of Membranes Akshaya Jena and Krishna Gupta Porous Materials,...

Characterization of Pore Structure of MembranesCharacterization of Pore Structure of Membranes

Akshaya Jena and Krishna Gupta

Porous Materials, Inc.

20 Dutch Mill Road, Ithaca, NY 14850

Akshaya Jena and Krishna Gupta

Porous Materials, Inc.

20 Dutch Mill Road, Ithaca, NY 14850

OutlineOutline

Characterization TechniquesExamples of ApplicationsConclusions

Characterization TechniquesExamples of ApplicationsConclusions

Important Pore Structure Characteristics




The largest through pore diameterMean flow through pore diameter Through pore distributionBreakthrough pressureGas permeabilityLiquid permeabilityWater vapor transmission rate

The largest through pore diameterMean flow through pore diameter Through pore distributionBreakthrough pressureGas permeabilityLiquid permeabilityWater vapor transmission rate

Through pore throat diameterThrough pore throat diameter

Characterization TechniquesCharacterization Techniques

A pressurized gas extrudes liquid from pores

A pressurized gas extrudes liquid from pores

Capillary Flow Porometry

PrinciplePores are filled with a wetting liquid

(s/g > s/l)


PrinciplePores are filled with a wetting liquid

(s/g > s/l)



Principle


PrinciplePressure & gas flow rates of wet &

dry samples measuredPressure & gas flow rates of wet &

dry samples measured


p = 4 cos (dS/dV)p = differential pressure q = contact angleg = surface tension S = pore surfaceV = volume of gas in pore area

p = 4 cos (dS/dV)p = differential pressure q = contact angleg = surface tension S = pore surfaceV = volume of gas in pore area


Principle


PrincipleDifferential pressure related to pore

size Work done by gas = Increase in surface free energy

Differential pressure related to pore size Work done by gas = Increase in surface free energy


Many Characteristics computedMany Characteristics computed


Principle


Principle

Pore diameter and permeability Pore distribution


Test executionData acquisitionData storageData reduction

Test executionData acquisitionData storageData reduction

The Capillary Flow Porometer used in this study

EquipmentFully automated:

EquipmentFully automated:


4.6 0.14.6 0.14.5 0.54.5 0.5Polycarbonate membranePolycarbonate membrane

86.7 4.186.7 4.181.7 5.281.7 5.2Etched stainless steel disc

Etched stainless steel disc

PMI PorometerPMI Porometer

SEMSEMSEM MicrographSEM MicrographSampleSample

Pore diameter, mPore diameter, m

AccuracyAccuracy

Water Intrusion PorosimetryWater Intrusion Porosimetry

Pressure on water is increased - Water intrudeshydrophobic pores

Pressure on water is increased - Water intrudeshydrophobic pores

PrincipleWater is allowed to surround

membranes - Water spontaneously enters all hydrophilic pore

PrincipleWater is allowed to surround

membranes - Water spontaneously enters all hydrophilic pore


p = - g cos q (dS/dV)V = volume of liquid in pore

Intrusion volume gives pore volume

p = - g cos q (dS/dV)V = volume of liquid in pore

Intrusion volume gives pore volume

PrinciplePrinciple

Pressure yields pore sizePressure yields pore size


Equipment

PMI Aquapore (Water Intrusion Porosimeter)PMI Aquapore (Water Intrusion Porosimeter)

Water Vapor Transmission Analyzer


Instrument evacuatedVapor is introduced on one side &

maintained at constant pressure

Instrument evacuatedVapor is introduced on one side &

maintained at constant pressure

PrinciplePrinciple

Sample loadedSample loaded



Increase in pressure on other side measured

Increase in pressure on other side measured

Principle of vapor transmission analyzer

PrinciplePrinciple



EquipmentEquipment

The PMI Water Vapor Transmission Analyzer

Capable of detecting = 10-4 cm3/sCapable of detecting = 10-4 cm3/s

Examples of ApplicationsExamples of Applications

Pore Diameter

What is a pore diameter?Most pore cross-sections irregular

Pore Diameter

What is a pore diameter?Most pore cross-sections irregular

Examples of ApplicationsExamples of Applications

D = +/- 4 cos /pD = +/- 4 cos /p

Pore Diameter

What is a pore diameter?

Pore Diameter

What is a pore diameter?Definition of pore diameter, D:

(dS/dV) pore= (dS/dV)circular opening of diameter, D= 4/D

Definition of pore diameter, D: (dS/dV) pore= (dS/dV)circular opening of diameter, D= 4/D

Examples of ApplicationExamples of Application

Each technique measures certain diameters of the pore

Each technique measures certain diameters of the pore

Multiple diameters of Each PorePore diameter varies along pore path

Multiple diameters of Each PorePore diameter varies along pore pathEach pore has many diametersEach pore has many diameters

Pore Diameter Measured by Flow Porometry


Variations of flow rate with pressure for membranes #3

Variations of flow rate with pressure for membranes #3

Measured pressures and flow ratesMeasured pressures and flow rates



Which diameter of pore is measured?Flow porometry detects the most

constricted pore diameter

Which diameter of pore is measured?Flow porometry detects the most

constricted pore diameter



The largest pore diameter (Bubble Point)Computed from pressure to start flow

through wet sample.

The largest pore diameter (Bubble Point)Computed from pressure to start flow

through wet sample.

Variations of flow rate with pressure for membrane #3



The mean flow pore diameterComputed from mean flow pressure

The mean flow pore diameterComputed from mean flow pressure



Wide range of diameters measurableWide range of diameters measurable

Membrane Bubble Point PoreDiameter, m

Mean flow porediameter, m

Membrane #1 27.123 14.714Membrane #2 17.914 3.812Membrane #3 13.544 0.043Membrane #4 0.073 0.043

Membrane Bubble Point PoreDiameter, m

Mean flow porediameter, m

Membrane #1 27.123 14.714Membrane #2 17.914 3.812Membrane #3 13.544 0.043Membrane #4 0.073 0.043



Pore size distribution (Flow Distribution)Distribution function, f:

f = -d(Fw/Fd)x100)/dD

Pore size distribution (Flow Distribution)Distribution function, f:

f = -d(Fw/Fd)x100)/dD



Narrow bimodal distributionMost of the pores: 5 to 11 m constricted

diameter

Narrow bimodal distributionMost of the pores: 5 to 11 m constricted

diameter

Pore size distribution (Flow Distribution)Pore size distribution (Flow Distribution)

Area under the curve gives percentage flow

Area under the curve gives percentage flow



Define the measurable parameter fi:fi = [1/(4 cos /pi)4]x [(fw,i+1/fd,i+1)-(fw,i/Fd,i)]p = differential pressure

i & i+1 = two successive readings

Define the measurable parameter fi:fi = [1/(4 cos /pi)4]x [(fw,i+1/fd,i+1)-(fw,i/Fd,i)]p = differential pressure

i & i+1 = two successive readings

Pore Fraction DistributionFraction of pores of diameter

Di = Ni/iNi

Pore Fraction DistributionFraction of pores of diameter

Di = Ni/iNi



It has been shown that:Ni/

iNi= fi/ ifi

It has been shown that:Ni/

iNi= fi/ ifi

Pore Fraction DistributionPore Fraction Distribution

Example of fractional pore number distribution

Example of fractional pore number distribution

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.2 0.4 0.6 0.8 1 1.2

Diameter, microns

Ni / S

um

(Ni)

Other Characteristics Measurable by Flow Porosmetry


In any desired unit: Darcy, Fazier, Gurley and Rayle

Liquid permeabilityComputed from liquid flow rate through

sample

In any desired unit: Darcy, Fazier, Gurley and Rayle

Liquid permeabilityComputed from liquid flow rate through

sample

Gas PermeabilityComputed from gas flow rate

through dry sample

Gas PermeabilityComputed from gas flow rate

through dry sample

Cyclic compressionTemperatureChemical environment

Cyclic compressionTemperatureChemical environment


Other Characteristics Measurable by Flow PorosmetryEffects of Service variables on pore

structureCompressive stress

Effects of Service variables on pore structure

Compressive stress

0

2

4

6

8

10

12

14

16

18

0 100 200 300 400

Compressive Stress (KPa)

Bubb

le P

oint

Por

e Di

amet

er(m

icro

ns)

Other Characteristics Measurable by Flow

Porometry


Porometry

Sample orientation (x,y & z directions)

Sample orientation (x,y & z directions)

Pore size of separator determined using KOH solutionPore size of separator determined using KOH solution


Porometry


PorometryPore diameters in various directionsPore diameters in various directions

Material Bubble point, m Mean Flow porediameter, m

Fuel cell componentz-direction 14.1 1.92x-direction 14.6 1.04y-direction 7.60 0.57Printer paperz-direction 12.4 4.20y-y plane 1.11 0.09Liquid filterz-direction 34.5 -x-y plane 15.3 -


Porometry


PorometryIn-situ pore structure of individual

layers of a layered or graded materialIn-situ pore structure of individual

layers of a layered or graded material


Pore VolumePore Volume

Cumulative pore volume & pore diameter of a hydrophobic membrane determined in the PMI Water Intrusion Porosimeter

Cumulative pore volume & pore diameter of a hydrophobic membrane determined in the PMI Water Intrusion Porosimeter


Pressure required was only about 1000 psi for pore diameters down to about 0.01 microns.

Pressure required was only about 1000 psi for pore diameters down to about 0.01 microns.

Pore VolumePore Volume

Water rater than mercury was used for intrusion

Water rater than mercury was used for intrusion

Pore diameterPore diameter

Pore diameter & volume of each part of pore measured

Pore diameter & volume of each part of pore measured

Which diameter of pore measured?Which diameter of pore measured?

Pore distributionPore distribution

Area under the curve is the volume of pores

Bimodal board distribution

Area under the curve is the volume of pores

Bimodal board distribution

Distribution function, F:F = -[dV/ d log D]

Distribution function, F:F = -[dV/ d log D]

Pore distributionPore distribution

Maximum contribution to distribution at 0.22 microns

Maximum contribution to distribution at 0.22 microns

Pore volume distribution by water intrusion porosimetryPore volume distribution by water intrusion porosimetry

Comparison with Mercury Intrusion


Cumulative pore volume measured in mercury intrusion porosimetry

Cumulative pore volume measured in mercury intrusion porosimetry



Pore volume distribution obtained using mercury porosimetryPore volume distribution obtained using mercury porosimetry



Volume distributions are similarRequired pressure 20000 psi

compared with Hg 1000 psi for waterMercury is toxic

Volume distributions are similarRequired pressure 20000 psi

compared with Hg 1000 psi for waterMercury is toxic

Intrusion volumes similarIntrusion volumes similar

Water Vapor Transmission Rate


Vapor transmission Vapor transmission

Change of pressure on the outlet side of two samples of the membrane in the PMI Water Vapor Transmission Analyzer

Change of pressure on the outlet side of two samples of the membrane in the PMI Water Vapor Transmission Analyzer



n = number of moles of vapor transferred across the membrane

F= vapor transmission rate through the sample per unit time in volume of gas at STP

n = number of moles of vapor transferred across the membrane

F= vapor transmission rate through the sample per unit time in volume of gas at STP

(dp/dt) (dn/dt)F (dn/dt)F (dp/dt)

(dp/dt) (dn/dt)F (dn/dt)F (dp/dt)



Incubation period - Transmission rate zero

Small transient zonePressure increases with a decreasing

rate[F (pi - p)]

Incubation period - Transmission rate zero

Small transient zonePressure increases with a decreasing

rate[F (pi - p)]

The variation of pressure with time is almost sigmoial

The variation of pressure with time is almost sigmoial

ConclusionConclusion

Application of these techniques for characterization of membranes have been described with examples.

Application of these techniques for characterization of membranes have been described with examples.

Principle of three characterization techniques, Capillary Flow Porometry, Water Intrusion Porosimetry & Water vapor transmission Analyzer have been explained.

Principle of three characterization techniques, Capillary Flow Porometry, Water Intrusion Porosimetry & Water vapor transmission Analyzer have been explained.

ConclusionsConclusions

The techniques:Capillary Flow PorometryWater Intrusion PorosimetryWater Vapor transmission Analyzer

are appropriate for pore structure characterization of membranes.

The techniques:Capillary Flow PorometryWater Intrusion PorosimetryWater Vapor transmission Analyzer

are appropriate for pore structure characterization of membranes.

Capability of each technique has been discussed.

Capability of each technique has been discussed.

Thank YouThank You

Characterization of Pore Structure of Membranes Akshaya Jena and Krishna Gupta Porous Materials,...

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