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orosity is one of those properties with "kill orcure" effectiveness. In the right hands, in theright product, it confers exquisite performance.
In the wrong hands or inappropriate application it canstop the process/product, dead in its tracks! Porosityshould be well understood, easily identified,quantifiable and controllable. Why is it then, that itcan be so easily overlooked or misunderstood?
A Porous OverviewPorous properties impact/confer on a ceramic, bothpositive and negative. Porosity is obviously desirablein light-weight products, thermal insulation, catalystsupports, wicking and filtration uses. Negative aspectsinclude friability, loss of strength, undesirable fluidabsorption etc.
The forming of ceramics from powders necessarilygenerates porosity by fixing, in 3-dimensions,positions and relationships of interparticle voids.Low pressure forming methods generate higherporosity and higher pressures produce less. Thermaltreatment, be it simultaneous with forming (HIP) orsubsequent to it (normal firing) is used to eliminatesome, most or all porosity depending on end use.The size of the pores depends on the particle sizedistribution and shape of the starting powders andvarious additives (binders, liquids). Of course, thela creates porosity in the initial stages of firing dueto their loss or burning out. As sintering is initiatedsmall pores may coalesce into larger ones with eor no loss of overall pore volume. Loss of porevolume (porosity) must be accompanied byshrinkage of the piece.
Your Pore Problems BeginTrouble begins by not in place a program ofpore volume and size monitoring at all stages ofproduction. From consideration of mass, true densityand geometric (envelope) volume one might believeone has all the information needed… just calculateporosity. Maybe even determine it explicitly by waterabsorption. But not knowing the pore sizedistribution can lead to serious misunderstandings.You are probably familiar with looking at particle sizedistributions and making certain deductions. That'sOK for loose powders, but even then one cannotdeduce how homogeneous the blend is from a sizeanalysis. But when the particles a ain a fixed spatialorder by pressing, casting, extrusion etc., the resultingpore size distribution plot will reveal the
homogeneity of the blend as it exited the formingprocess. If it's worth proceeding, you will now beable to understand the path along which the sinteringprocess might take. Small pores will be the mostthermally "reactive", larger pores less so. Thereforedon't be surprised if the final piece still has porosity -if it started out with predominantly large pores.Remember, the final porosity is not just a function ofinitial porosity but also of pore size.
Measuring Pore Size Quickly and ReliablyThe best available measurement method is MercuryIntrusion Porosimetry: rapid (orders of magnitudefaster than gas sorption) very wide dynamic range(orders of magnitude larger than gas sorption -unrivaled for pores over 0.25 microns) and yieldingnetwork structure (more evident than gas sorptionbecause the fluid penetration is sequential or serial innature). Mercury intrudes from the outside, in, andwhile there exists a pressure-pore size relationship, itis subject to the arrangement and connectivity of thedifferent pores sizes in the network of internal voids.Therefore, the result not only yields pore sizeinformation but also structural or network information.
The forced intrusion of non-we mercury into apore network by applying an external force has beenused commercially for the best part of sixty years. Sowhy is it not more widely used? Perhaps it was thelack of automation, the lack of interpretation skills ormaybe noise and expense? But that is not now thecase. Modern mercury intrusion porosimeters offerhigh throughput, very wide pore size range, a host ofautomated features and near-silent operation. Manyanalyses can be completed in een minutes or less -on two samples at one time.
So, when you need pore size information for R&D,process control and quality assurance in all fields ofceramics, consider using the mercury porosimetrymethod. Mercury porosimetry may reveal porositywhere there should be none, identify off-spec greenceramics and allow them to be removed from theremaining processes, and to optimize firing conditions.A all, kilns are not inexpensive to operate.
For more information about fuel cells and relatedmeasurement instruments, contact QuantachromeInstruments by phone: (561) 731.4999, fax: (561) 732.9888,email: [email protected] or visitwww.quantachrome.com.
P
&Structural MaterialsCeramics
Pore size is calculated using theWashburn equation:Pore diameter = - (4 γ cos θ )/ PWhere γ = surface tension, P = pressure and θ = contact angle.
Pore size range is from approximately1mm down to 3.5nm
Pore volume resolution be than 0.1 microlitre.
Pressures required as high as 60,000 psi, achieved using specially designed hydraulic pressuregenerators.
Featured in a number of standard testmethods including ASTM D4284,ASTM D4404, ASTM D2873, UOP578-02, BS7591-1.
MercuryIntrusionPorosimetryFacts
Porous Problems:Porous Problems:
by: Martin A. Thomas Ph.D., Director of Business DevelopmentQuantachrome Instruments ([email protected])
Rid Yourself of Unwanted orUnexplained Porosity In YourFinal Product.The PoreMaster GT® uses the extremelyeffective rapid mercury intrusion technique.With automatic pore size and pore volumedistribution analyzers, you will never haveporosity inconsistency in your products again.
For answers to your porosity related questionsand to speak to a Quantachrome consultant,call us at 561.731.4999 or email us at
Advanced Features:-Automatic air purge
-Vapor trap
-Extremely quiet
Three Operating Modes:-Pressure Scanning
-Autospeed
-Step-wise
PoreMaster GT®
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