vinodhini b. - mercury porosimetry

8/19/2019 Vinodhini B. - Mercury Porosimetry

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Mercury Porosimetry

By Vinodhini B Vishwanathan S K


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Pores – Classifications-Porosity

Methods of Porosity Measurements

Mercury Porosimetry Method

Hysteresis

Washburn Equation

Advantages – Disadvantages-Applications

Contents


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What are Porous Materials?

Non-porous solid Porous solid

Porous materials have high specific surface area andpore volume.


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Size of Pores (IUPAC Standard)

2 nm 50 nm

Micropores Mesopores Macropores

Zeolite,

Activatedcarbon,Metal organicframework

Mesoporous silica,

Activated carbon

Sintered metals

and ceramics

Pores are classified according to the sizes:< 2 nm micropores,2-50 nm mesopores,>50 nm macrospores.


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Types Of Pores


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Open Vs Closed Pores

Dead end(open)

ClosedInter-connected(open)

Passing(open)

Open pores are accessible whereas closed pores areinaccessible pores. Open porescan be inter-connected, passingor dead end .


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Shapes of Pores

Conical

Interstices

SlitsCylindrical

Spherical orInk Bottle

PoreShapes


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Measurement of Porosity

Pore size &its distribution

Specific Surface Area, m 2/g =

Porosity

Parameters used to measure porosity; specific surface area, specific pore volume ,and pore size and its distribution.

Mass of the solid, g

Total surface area, m 2

Specific Pore volume, cm 3/g

Mass of the solid, g

Total pore volume, cm 3=

Porosity, % =

Volume of solid (including pores )

Volume of pores X 100


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Laboratory MeasurementsDirect Measurement MethodImbibition Method

Mercury Porosimetry MethodGas expansion methodDensity Method

Petrographic MethodBulk Volume Measurement


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Mercury PorosimetryExtremely useful characterization technique for porousmaterials.

Pores b/w 500µm-3.5nm can be investigated.

Only method that provides information aboutPore size distribution,Porosity ,Skeletal & Apparent density andSurface area of the sample.


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Purity of Mercury Hydraulic oil should not be recycled , itcontaminate the mercury and changes its dielectricand flow properties.

Contamination of mercury affects the contactangles and surface tension values.

Mercury-Acid washed, dried and distilledpreferably doubly- or triply-distilled.

It is always advisable to use fresh mercury forevery measurement.


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Sample Preparation Porous materials are prone to adsorb water or otherchemicals, which should be removed during the initialevacuation of the sample.

One of the key parameter here is the sample weight.


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InstrumentationReservoir of mercury to which a graduated barrelwith a piston is attached.

Penetrometer-sample of known weight is placed.

The sample cell is surrounded by hydraulic fluid.

After evacuating the air around the specimenwith the vacuum pump and the penetrometer isfilled with mercury up to the reference level.


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Schematic diagram


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Working Principle At normal pressures Hg will not enter the pores ofmost samples.From the Hg displacement, bulk volume of the

sample is calculated.The pressure on the Hg is then raised further, forcingthe mercury into the pores of the sample .Atsufficiently high pressure, the mercury will invade allthe pores.The amount of mercury lost into the sample providesthe pore volume directly.The porosity can then be calculated from the bulk

volume and the pore volume.


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Pressure RangeLow Pressure System

The first data point is usually taken at a pressure of3000 to 4000 Pa or higher.

High Pressure SystemPressure given to the hydraulic oil surrounding thesample cell in an isostatic way up to 414 Mpa.


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Mercury Injection Apparatus


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Mercury will enter the capillary at the threshold

pressure, but will not advance.The mercury level increase is depend on an excesspressure applied , above the threshold pressureFor the increased level the distance - time relationship is


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The distance - time relationship for mercury in tubes of fivedifferent sizes is shown fig below, E.g., More than 100 secondsare needed to travel 3 cm in a tube of 0.5µm radius.


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Washburn equation

The relationship between the applied pressure (p) andthe pore size (d),

d= –(4γ/ p) cos θ where,

γ -surface tension of mercury,θ -contact angle between solid and mercury

The value of Vi(Hg) at the applied pressure Piapparently gives the cumulative volume of all availablepores of radius ≥ d.


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Hysteresis Wardlaw at 1988 proposed mercury entrapment leadsto hysteresis between the mercury intrusion andextrusion curves.Explanations proposed are:

a) Contact angle hysteresisb) Ink bottle theory

c) Percolation -connectivity model


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Hysteresis


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a) Contact angle hysteresis

The surface roughness or the impurities on themercury or solid surface could certainly affect thecontact angle.


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b) Ink Bottle Theory

The opening of a pore is smaller than the actual cavity.So mercury entering into the pore cavity depends onthe neck size and not the actual cavity size.

c) Connectivity Model

Uses network of pores. An extension of ink bottle theory


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Percent Porosity

Where,

Va -Hg volume intruded at any given pressureVb -Hg volume intruded at a user-defined IntraporeFilling Pressure Limit

Vc -Hg volume intruded at the max. experimentalpressure


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The PMI Mercury Intrusion

PorosimeterEquipment


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PMI Mercury Porosimeter Data

ReportPore Volume vs. DiameterDelta Volume histogramPore Distribution (histogram)Percent Total Pore VolumePore Distribution (pressure and diameter)Particle Size Distribution (histogram)Percent Porosity and DensityCumulative Surface AreaPorosimetry Text Data


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AdvantagesSpeed and simplicity.grain size and pore throat size distribution of thesample can be calculated.

Can analyse lower porosity values than other methods.Low-pressure mercury porosimetry determinesmacropores (pore diameter 14 – 200 µm).

High-pressure porosimetry determines mesopores andmacropores (pore diameter 3 nm – 14 µm).it can be done on small irregular samples.


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Limitations & DisadvantagesMercury porosimetry does not actually measures theinternal pore size, determines the largest connection(throat or pore channel)Cost of distilled Mercury is high.The assumption of a constant value of surface tensionand angle of contact for mercury.

The Pores are not usually circular in shape, so theresults can only be comparativeSample must be disposed of safely after the test.


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ApplicationsCharacterization of carbon blacks, pharmaceuticalsamples.

Distribution of porous bead cellulose.

Determination of Maltose.

Distribution of foamed chitosan.

Determination of Cement Particles.


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Thought for the Day


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vinodhini b. - mercury porosimetry

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