Porous Problems: Ceramics and Structural Materials

July 8, 2005
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Porosity is one of those properties with "kill or cure" effectiveness. In the right hands and in the right product, it confers exquisite performance. In the wrong hands or inappropriate application, it can stop the process/product dead in its tracks! Porosity should be well understood, easily identified, quantifiable and controllable. Why is it, then, that it can be so easily overlooked or misunderstood?

A Porous Overview

Porous properties can be both positive and negative. Porosity is obviously desirable in lightweight products, thermal insulation, catalyst supports, wicking and filtration uses. However, negative aspects include friability, loss of strength, undesirable fluid absorption etc.

The forming of ceramics from powders necessarily generates porosity by fixing, in three dimensions, the positions and relationships of interparticle voids. Low-pressure forming methods generate higher porosity, while higher pressures generate lower porosity levels. Thermal treatment, be it simultaneous with forming (HIP) or subsequent to it (normal firing), is used to eliminate some, most or all porosity, depending on the end use of the product. The size of the pores depends on the particle size distribution and shape of the starting powders and various additives (binders, liquids). Of course, additives create porosity in the initial stages of firing as they are burned out. As sintering is initiated, small pores may coalesce into larger ones with little or no loss of overall pore volume. Loss of pore volume (porosity) must be accompanied by shrinkage of the piece.

Your Pore Problems Begin

Trouble begins by not putting in place a program of pore volume and size monitoring at all stages of production. From consideration of mass, true density and geometric (envelope) volume, you might believe you have all the information needed...just calculate porosity. Maybe even determine it explicitly by water absorption. But not knowing the pore size distribution can lead to serious misunderstandings. You are probably familiar with looking at particle size distributions and making certain deductions. That's okay for loose powders, but even then it is impossible to deduce how homogeneous the blend is from a size analysis. But when the particles attain a fixed spatial order by pressing, casting, extrusion etc., the resulting pore size distribution plot will reveal the homogeneity of the blend as it exited the forming process. If it's worth proceeding, you will now be able to understand the path along which the sintering process might proceed. Small pores will be the most thermally "reactive," while larger pores will be less so. Therefore, don'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 of initial porosity, but also of pore size.

Measuring Pore Size Quickly and Reliably

The best available measurement method is mercury intrusion porosimetry. It provides rapid analysis (orders of magnitude faster than gas sorption) a very wide dynamic range (orders of magnitude larger than gas sorption - unrivaled for pores over 0.25 microns) and yielding network structure (more evident than gas sorption because the fluid penetration is sequential or serial in nature). Mercury intrudes from the outside in, and while there exists a pressure-pore size relationship, it is subject to the arrangement and connectivity of the different pore sizes in the network of internal voids. Therefore, the result not only yields pore size information, but also structural or network information.

The forced intrusion of non-wetting mercury into a pore network by applying an external force has been used commercially for the best part of 60 years. So why is it not more widely used? Perhaps it was the lack of automation, the lack of interpretation skills or maybe noise and expense? But that is not now the case. Modern mercury intrusion porosimeters offer high throughput, a very wide pore size range, a host of automated features and near-silent operation. Many analyses can be completed in fifteen 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 of ceramics, consider using the mercury porosimetry method. Mercury porosimetry may reveal porosity where there should be none, identify off-spec green ceramics and allow them to be removed from the remaining processes, and to optimize firing conditions. After all, kilns are not inexpensive to operate.

For More Information

For more information about mercury intrusion porosimetry and related measurement instruments, contact Quantachrome Instruments at (561) 731.4999, fax (561) 732.9888, e-mail QC.ceramics@quantachrome.com or visit http://www.quantachrome.com.

SIDEBAR: Mercury Intrusion Porosimetry Facts

  • Pore size is calculated using the Washburn equation:
    Pore diameter = - ( 4 γ cos θ )/ P
    Where γ = surface tension, P = pressure and θ = contact angle.
  • Pore size range is from approximately 1 mm down to 3.5 nm
  • Pore volume resolution better than 0.1 microliter.
  • Pressures required as high as 60,000 psi, achieved using specially designed hydraulic pressure generators.
  • Featured in a number of standard test methods including ASTM D4284, ASTM D4404, ASTM D2873, UOP578-02, BS7591-1.

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