Glen A Slack 1928-2019: a giant and friend

Glen was already a giant when I first met him at GE in 1984. The year I was born he established isotope scattering as the main source of thermal resistance at low temperatures in Silicon. Isotope scattering is a special case of mass fluctuation scattering, without which we’d probably have no useful thermoelectric materials at all. 

Slack, Glen A. “Effect of isotopes on low-temperature thermal conductivity.” Physical Review 105, no. 3 (1957): 829.

Glen’s 1979 review of thermal conductivity of nonmetals introduced me to the central ideas in lattice thermal conductivity, and the key idea that each solid has a minimum thermal conductivity. For the same reason that solids conduct sound, so too they must carry heat.

Slack, Glen A. “The thermal conductivity of nonmetallic crystals.” In Solid state physics, vol. 34, pp. 1-71. Academic Press, 1979.

In 1992 Glen first introduced us to the skutterudites, described independently and virtually simultaneously by Caillat and Fleurial of JPL. 

Slack, Glen A. “Some new materials for thermoelectric cooling.”  In: Horn SB, editor. Proceedings of the 1992 1st National Thermogenic Cooler Conference. Fort Belvoir, VA: Center for Night Vision and Electro-Optics (unpublished), Horn, SB editor, 1992.

And in 1997 he brought us his Phonon Glass Electric Crystal (PGEC) idea. The idea is brilliant, not because it’s entirely new. After all, even Ioffe taught us the importance of low lattice thermal conductivity and high mobility. Glen’s genius was in distilling the ideas down to simple, core concepts. The best possible thermal conductivity is in a glass-like material where a phonon is barely even a phonon, travelling as little as one interatomic spacing. You can’t do better. And the best possible charge carrier is one that travels as far as possible, as in a perfect crystal. The two ideas are, and were, somewhat at odds with each other. But now we have a four letter rallying cry: PGEC. And we have Glen to thank for the insight and inspiration.  

Slack, Glen A. “Design concepts for improved thermoelectric materials.” MRS Online Proceedings Library Archive 478 (1997).

It was my privilege to have Glen as mentor and friend. One of my fondest memories was at an MRS meeting in Boston. A well known and successful member of the TE community had just completed his talk to a good sized audience. Polite questions arose. Glen, seated near the back, held his hand up for attention. One couldn’t ignore Glen, even if you wanted to.

When called on, Glen sort of steeled himself, cleared his throat in preparation and delivered this pronuncement in that deliberate, punctuated manner of his.

“I counted. At least. TEN things. WRONG in your talk!”

The speaker remained calm, deferential to Glen, and held his own pretty well, I thought. I almost felt sorry for him. Almost.

But that was so Glen. He held everyone, but especially himself, to high standards. He made us all better for it.

I shall miss him and this evening I shall celebrate his life.