Much Ado About Nothing was : Density of a microporous solid
- From: Jbuch <jbuch@xxxxxxxxxxxx>
- Date: Tue, 31 Oct 2006 09:52:42 -0600
Gregg wrote:
Jbuch wrote:
<snip>
Hi Gregg,
I'm retired from materials science now.
For three widely spaced periods in my life, I was interested in porosity - and specifically the effects of porosity on materials.
After a while, I decided that people actually know relatively little
about porosity, even though the first book "The Nature of Porous Bodies"
or similar title was published in about the 1700's by a prominent chemist.
Even now, people appear to know relatively little about porosity. And
porosity effects.
Your suggestion for the use of X-Ray attenuation is a reasonable one. To
estimate the percentage of solid material in the sample. And thus infer
something about the total volume of the pores in the sample.
One could make reference standards out of stepped thickness bars of the
pore free bulk metal, if available.
But for understanding the actual nature of porosity, and trying to
predict effects of porosity on properties, the X-Ray attenuation route
offers no physical details.
A structural mechanic type would ask a question of the nature "What is (are) the load path(s) in your solid structure?" And generally, you and I might have absolutely no idea of the answer.
What seems needed is the microscopic equivalent of a CT scan. To reveal in three dimensions and two dimensional slices the distribution of solid
and pore phases in a material.
Serial 2D optical microscopy is pretty tedious, and nobody will advocate
it as a cost effective way to characterize pore geomentry.
The chemists are near useless in this area. They defined microporosity about a century ago in terms of chemical reaction parameters - access to the solid surface from the exterior reactive gases.
This has nothing to do with mechanical properties such as stiffness or
strength. On the other hand the average chemist is aware of the
importance of stiffness and strength, but cannot keep straight which is
which.
About 12 years ago, I heard a talk by Dr. Edward Teller who predicted that 3-D X-Ray microscopy would be practical in just a few years. He was a bug on X-Ray stuff, having devoted some years of his life towards the weapons effects of X-Rays. He argued that work was beginning on the detector chips in weapons programs.
In short, how can we understand the effects of the porosity
microstructure when we don't know much about what it is, or what to
measure even if we had the full 3-D microstructure sitting in front of
us in digital form.
A long time ago, one of the technical guys from Leitz told me that one
of the limitations of quantitative microscopy instruments was the USER. He said that technically they can probably figure out how to measure definite things that the USER wants, but actually the USER can usually only describe in quantitative non-mathematical terms what he wants. I saw that clearly over 20 years ago. Probably things haven't changed that much.
"Open" and "Closed" porosity are terms over 100 years old by now, and we
don't know how to go much beyond gross characterization.
As a field of materials specialists, we should be ashamed of ourselves for our lack of attention to the fundamentals of porosity.
Jim
Hi Jim,
You maybe retired in body, but it sounds like your mind is still wrapped around the subject matter.
My previous job was the development of porous bodies mainly for filtration and diffusion.
I was surprised at the lack of understanding in the field. When we worked with hot gas filters, we supplied a lot of samples for testing (through another company) and creep was a big issue. A couple of manufactures (including the company I worked for) supplied filters which were a granular SiC bonded with multiple crystalline phases.
We could see that the creep was anisotropic from the deformation patterns that occurred. (tensile vs compressive between grains). Amazenly, no one would here of it - It would screw up their models and probably their funding.
I don't see any significant progress being made in the near future.
I completely agree with the point - if we could represent porosity digitally - it doesn't mean we'd understand it.
One could hope that the complete digital representation of a porous body would allow it to be modeled - and the proper fudge factors could be used to extrapolate real properties. - maybe someone could make sense of the fudge factors in the future?
Gregg
Sounds as if we have encountered similar people and similar thinking, or the lack of it.
The thinking that needs to be done is to change from "What can we measure?" to "What should we measure?"
About 25 years ago, I did a technical report on a theoretical approach to understanding the effect of pore shape on the elastic modulus. It required introducing simplified ideas of pore geometry - such as isolated ellipsoids of revolution.
The theory could be made to predict exponential decay of modulus with increases of porosity, and the more eccentric ( or non-spherical ) the ellipsoid of revolution, the higher was the exponential constant ( the modulus was increasingly sensitive to porosity ).
The automated quantitative microscopy equipment the company owned gave some pretty odd looking results when pore shape was output - it could only attempt to approximate some kind of minimum to maximum diameter ratio.
When I gave it synthetic images based on known circles, ellipses, rectangles, .... it virtually never produced results compatible with the known images.
It was hard to get a technician to sit down and make minimum and maximum diameter measurements from the SEM images. Nobody had ever actually asked them to make many measurements from photographs in their experience.
I finally wrote the experimental part off as a bad deal.
I have a talk I sometimes give :
"POROSITY : Much Ado About Nothing and Why We Still Fail To Understand It"
Jim
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