Positron Porosimetry

of Physics and the Center for Materials Research, Washington State University, Pullman WA 99164-2711, USA 

Positrons and positronium (Ps), the bound state of a positron with an electron, are highly sensitive to open volume in a material. Their sensitivity ranges from monovacancies (positrons) to voids to pores of several 10 nm in cross section (positronium). With a variable energy monoenergetic beam, positrons can be implanted in a sample at a chosen depth (or range of depths for a profile). Dependent on the porosity properties of the sample, positronium may be formed there. When positrons and positronium 

In this chapter it will be shown with numerous examples how information on open versus closed porosity, the total porosity, the pore sizes and a measure of the average length of connected pores can be measured with varying degrees of ease. Some of these parameters can be obtained from alternate methods. However, none can provide depth dependent profiles of the parameter without microtoming the sample. No special sample preparation is required. A sample investigated by positrons could easily be reinserted in a device production line to correlate results from positron measurements with device performance. 

This chapter is organized by the specific property of porosity rather than by positron technique. This should assist a reader interested in answering a particular question in the evaluation of which method best solves the problem. The sequence is organized in order of increasing complexity of the method needed for the problem. The methods are presented briefly. For more detailed discussions of a technique the reader may refer to a listed reference. 

The terminology used here has been defined in preceding chapters. Treaties on positronium can also be found in Berko and Pendleton [2] and Rich [3]. Recent lifetimes are published by Gidley [4], Jean [5] and Yang and Jean [6] discuss positronium in open volume within polymers. Doppler broadening can be found in Saarinen [7] and Krause-Rehberg [8], Some aspects of positron porosimetry are presented rather briefly for lack of space. An upcoming review article will go into more details [9], 

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The vast majority of the work on positron porosimetry shown here was carried out as part of a NIST-ATP grant in cooperation with groups at IBM... 

Characterization of the pore structure of amorphous adsorbents and disordered porous catalysts remains an important chemical engineering research problem. Pore structure characterization requires both an effective experimental probe of the porous solid and an appropriate theoretical or numerical model to interpret the experimental measurement. Gas adsorption porosimetry [1] is the principal experimental technique used to probe the structure of the porous material, although various experimental alternatives have been proposed including immersion calorimetry [2-4], positron... 

Figure 4.6 (a, b) Pore size distributions can be measured through a variety of techniques—including eUipsometric porosimetry (EP) and Positron Annihilation Spectroscopy (PALS) (Reprinted with permission from Gates et al., 2007a, 2009a). 

The pore characteristics of the film were studied using Positron Annihilation Lifetime Spectroscopy (PALS) [4] by Prof. Gidley at University of Michigan. It was fi>und that all pores were intercoimected, and the average pore diameter was 2.2 run. Porosity was measured as 53% using Ellipsometric Porosimetry (EP) [15. /6]atIMEC. 

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