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Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
SCIENCES
Methods for characterization
of porous materials
Most materials used as battery active materials are porous. Pores can have
various shapes being “passing through, blind or closed”. The volume of a
porous material is the sum of the volumes of the pores (denoted as porosity)
and of the solid skeleton. Because of this, three kinds of densities of porous
materials exist: the skeleton density, the apparent density and the real
density.
Porosimetry counts for the porosity (pore volume and pore size distribution)
of the material, while the object of Porometry is the largest and smallest
pore size measurement.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
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Porous materials, porosity
passing through
“blind”
closed
internal
pore types
V = Vsolid + Vvoid
Porosity:
P = Vvoid/V = 1 –Vsolid/V
Solid part:
S = Vsolid/V = 1 –Vvoid/V
rappar = m/V
rreal = m/Vreal
rskeleton = m/Vsolid
Porosimetry: porosity (pore volume and pore size distribution).
Porometry: largest and smallest pore size measurement.
Source: T.Plachenov and S.Kolosencev, Porometry, Chimija (Rus), Leningrad, 1988.
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
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Methods for characterisation of porous materials
 Total pore voulme (porosity), pore shape and size,
 Pore distribution by size (diameter),
 Pore surface distribution, specific surface
1.
2.
3.
4.
5.
6.
7.
8.
Adsorption (molecular adsorption on solids ) –for extremely porous samples – pore
volume, even the smallest pores, Brunauer, Emmett, Teller (BET) method for specific
surface measurements 9-200oC, 0.1 – 1000 m2/g), range 0.3 – 700 nm.
Pycnometry (density of the solid sample) – total pore volume, pore volume, size and
distribution even of the smallest pores, range 0.2 – 1 nm.
liquid gas – He or a set of gases with known molecular size and adsorption on the sample.
Calorimetry (thermal effect of wetting liquid penetration into the pores) – specific
surface, micropore size and distribution, range 0.5 – 1 nm.
Volumetric (filling up the volume of the pores, weight or volume of gas/liquid ) –
porosity, pore volume and size, range 1 nm – 1000 mm.
Mercury intrusion (under increasing pressure) – pore size and distribution, specific
surface, range 1.5 nm – 800 mm.
Small angle X-rays or neutron dissipation (0 – 2 degrees) – micropore size and
distribution, closed pores, range 0.5 – 700 nm.
Etalon porometry – (comparative volume saturation) - all kind of samples: hard, soft
or elastic, brittle, pore volume and distribution, range 2 nm – 1000 mm.
A comparison with data from direct observation (SEM) is obligatory!
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
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Mercury Intrusion Porosimetry
Mercury intrusion porosimetry belongs to the most frequently used
porometric methods. Reliable devices produced in series are available.
The principle of the method is based on the penetration of liquids into small
cylindrical pores.
The diameter of the pores is calculated from the pressure value taking into
account the surface tension and the wetting angle of the liquid by
Washburn’s equation.
The sample is dried, degassed, precisely weight (~1.0 g). Than it is poured
into a special container, the penetrometer. The column is filled up from its
bottom orifice with mercury by a vacuum pump. Then the pressure is
increased, first using a low pressure pump, later using the high pressure one.
Part of the mercury penetrates into the pores of the sample. This mercury
volume change is measured precisely by capacitance change (between the
mercury column and the metal surrounding), resistance or radiation
intensity.
The method provides data about pore size and distribution, bulk density
and apparent density of the sample.
Source: http://www.quantachrome.com; http://www.pml.tno.nl
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
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Mercury Intrusion Porosimetry
Penetration of liquids into small cylindrical pores.
Washburn’s equation
D = +/- 4s.cosq / P
D – pore diameter, P – pressure applied
s - surface tension (for Hg: 480 dyne cm-1),
Hg wetting angle = 135 - 180o (>90o - non wetting)
penetrometer
sample
mercury
low
pressure
high
pressure
2
bar
pump
4100
bar
pump
vacuum
pump
Sample: dried, degassed, weight (~1.0 g).
vacuum, filling up with mercury
pressure, intrusion, volume measurement
V = f (P)
Penetrometers for
solid and powder
samples (3-5-15 cm3)
mercury volume changes: measured by capacitance
change (between the mercury column and the metal
surrounding), resistance or radiation intensity.
 pore volume
 pore size and distribution
 bulk density
 apparent density
Source: http://www.quantachrome.com; http://www.pml.tno.nl
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
MIP test protocols and data interpretation
LP EQUILIBRATION = +10.0000 SEC
PNTR CONSTANT = +21.6300 MICRO-L/PF
HP EQUILIBRATION = +15.0000 SEC
THETA = +140.0000
SAMPLE WEIGHT = +6.0540 G
GAMMA = +485.0000 DYNES/CM
PNTR WEIGHT = +68.5890 G INITIAL PRESSURE = +0.8425 PSIA
PNTR+SAMPLE WEIGHT = +74.6430 G
STEM VOLUME = +1.1000 CC
PNTR+SAMPLE+MERCURY = +113.3080 G
MERCURY DENSITY = +13.5335 G/CC
PNTR VOLUME = +4.3600 CC
INTRUSION (PRESSURIZATION) DATA SUMMARY
TOTAL INTRUSION VOLUME = +0.1293 CC/G
TOTAL PORE AREA = +4.8186 SQ-M/G
MEDIAN PORE RADIUS (VOLUME) = +0.3650 MICROMETERS
MEDIAN PORE RADIUS (AREA) = +0.0105 MICROMETERS
AVERAGE PORE RADIUS (4V/A) = +0.0537 MICROMETERS
BULK DENSITY = +4.0279 G/CC
APPARENT (SKELETAL) DENSITY = +8.4052 G/CC
% CAPILLARY = +71.1587
PRESSURE
PORE
PSIA
RADIUS
MICRO-M
+0.8
+127.9200
+1.0
+113.1600
+1.9
+55.3038
INTRUSION
VOLUME
CC/G
+0.0000
+0.0001
+0.0013
PORE
SURFACE
SQ-M/G
+0.0000
+0.0000
+0.0000
MEAN
RADIUS
MICRO-M
+127.9200
+120.5400
+84.2318
DV
+0.0000
+0.0001
+0.0012
SCIENCES
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
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Mercury porograms for PAM, NAM, 3BS and 4BS pastes
NAM with
3 additives
SS in m2/g (BET):
3BS – 1
NAM – 0.5 - 1
cumulative
differential
SS in m2/g (BET):
3BS - 1
4BS – <0.5
PAM – 2 - 10
Cumulative porograms
Cumulative and differential curves
vs. pore diameter
Source: A. Gigova, to be published
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
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Capillary Flow Porometry
The method of capillary flow porometry is used to characterise AGM, which
is soft and can’t be studied by MIP.
Principle: (i) Initially the pores are filled spontaneously by a thin liquid with
low surface tension. This happens when the surface free energy on the
surface liquid/sample is smaller than the surface free energy on the surface
gas/sample. (ii) Then gas with increasing pressure is pumped through the
sample. The gas pushes the liquid out of the pores. The total surface
gas/sample increases and the total surface liquid/sample decreases.
The equation for the pore diameter follows Washburn’s equation. The
largest pores are opened by the smallest pressure, called “bubble point”. On
further pressure increase the gas flow increases since more and more
smaller pores are opened.
Sources:
http://www.mplp.com/tech; A. K. Jena and K. M. Gupta, Journal of Power Sources, Volume 80, 1999, pp. 46-52; Vibhor Gupta and A.
K. Jena, Advances in Filtration and Separation Technology, Volume 13b, 1999, pp. 833-844; http://www.ceramicindustry.com
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
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Capillary Flow Porometry
Thus first a “dry curve” is recorded with gas only. Than the “half-wet”
curve is built assuming half of the pores are still filled with fluid. The sample
is filled with the real fluid and the gas pressure starts to increase. The
bubble point is detected when the first portion of gas passes through the
sample. From the cross point of the half-wet and the wet curve the “mean
flow” is determined. The high pressure value at which all pores are open
and the wet curve approaches the dry one is used to calculate the smallest
pore diameter.
Flow porometry measures the diameter of the constricted part of the pore.
Since the glass fibres are oriented during AGM processing, the pore
distribution of the AGM sample (in the right part of the next graph) often
depends on the geometrical direction of measurement – parallel or normal
to the surface.
Two device designs are illustrated - for porosity measurements along the
axis z and along the axes x/y.
Sources:
http://www.mplp.com/tech; A. K. Jena and K. M. Gupta, Journal of Power Sources, Volume 80, 1999, pp. 46-52;
Vibhor Gupta and A. K. Jena, Advances in Filtration and Separation Technology, Volume 13b, 1999, pp. 833-844;
http://www.ceramicindustry.com
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
Capillary flow porometry
pores: large
ACADEMY OF
medium
SCIENCES
small
L
D
D
S
D
Gas flow rate as a function of
differential pressure.
wet curve
dry curve
half – wet curve
mean flow
bubble point
gas
• pores filled spontaneously with a low surface
tension wetting liquid.
• Fs(liquid/sample) < Fs(gas/sample).
• gas displaces the liquid from the pores.
• S(gas/sample) > S(liquid/sample), i.e. Fs increases.
•Main equation for the pore diameter D:
D = 4s/p
s – surface tension, p - differential pressure for
displacement of the wetting liquid in a pore
• the largest pore emptied by the smallest pressure.
•gas flow start: largest pore – bubble point
• gas flow increasing with pressure (smaller pores).
Flow porometry measures the diameter
of the constricted part of the pore.
Sources:
http://www.mplp.com/tech; A. K. Jena and K. M. Gupta, Journal of Power Sources, Volume 80, 1999, pp. 46-52; Vibhor Gupta and A.
K. Jena, Advances in Filtration and Separation Technology, Volume 13b, 1999, pp. 833-844; http://www.ceramicindustry.com
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
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Capillary flow porometers
Flow porometry along the z-direction
(homogeneous materials)
z
x
y
non porous
sample
non porous
Flow porometry along the x-y-directions
(multi-layer and thin layer materials)
non porous
gas
Sources:
http://www.mplp.com/tech; A. K. Jena and K. M. Gupta, Journal of Power Sources, Volume 80, 1999, pp. 46-52; Vibhor Gupta and A.
K. Jena, Advances in Filtration and Separation Technology, Volume 13b, 1999, pp. 833-844; http://www.ceramicindustry.com
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
CFP test protocols and data interpretation
PORE DISTRIBUTION = FILTER FLOW% / (LASTDIA-DIA.)
FILTER FLOW = WET FLOW / DRY FLOW
Example:
SMALLEST DETECTED PORE PRESSURE = 4.275 PSI
SMALLEST DETECTED PORE DIAMETER = 2.1646 mm
MEAN FLOW PORE PRESSURE = 3.816 PSI
MEAN FLOW PORE DIAMETER = 2.4255 mm
BUBBLE POINT PRESSURE = 0.672 PSI
BUBBLE POINT PORE DIAMETER = 13.7697 mm
MAXIMUM PORE SIZE DISTRIBUTION = 2583.6707
DIAMETER AT MAX. PORE SIZE DISTRIBUTION = 2.4075 mm
SCIENCES
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
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Capillary flow porometry of two AGM separator materials
Pore isze distribution, %
100
differential
80
AGM-1: 2,42 mm
60
40
AGM-2: 4,10 mm
20
AGM-1
AGM-2
0
Pore size distribution, %
100
1
3
5
7
9
11
13
cummulative
15
Average pore diameter, mm
80
AGM-1: 2,42 mm
60
40
AGM-2: 4,10 mm
20
AGM-1
AGM-2
0
1
3
5
7
9
11
13
15
2,42 - 4,10 mm
Average pore diameter, mm
Source: A. Gigova, to be published
Centre of Excellence POEMES, IEES (CLEPS), BULGARIAN
ACADEMY OF
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Contributions to VRLAB knowledge
made using porosimetry and porometry
1. Structure and properties of the positive active material (Hg).
2. Influence of additives to the negative plate to its properties
(Hg).
3. Structure and properties of separators (PVC, PE, AGM CFP).
4. Gas transport in the system pos. plate – separator – neg. plate
in VRLAB.