Джон Вернер Кан: Американ ғалымы және материалтануға қосқан үлесі

American scientist (1928–2016)

John Werner Cahn (January 9, 1928 – March 14, 2016) was an American scientist and recipient of the 1998 National Medal of Science. Born in Cologne, Weimar Germany, he was a professor in the department of metallurgy at the Massachusetts Institute of Technology (MIT) from 1964 to 1978. From 1977, he held a position at the National Institute of Standards and Technology (formerly the National Bureau of Standards). Cahn had a profound influence on the course of materials research during his career. One of the foremost authorities on thermodynamics, Cahn applied the basic laws of thermodynamics to describe and predict a wide range of physical phenomena.

Hans Werner Cahn was born in Cologne, Germany, to a Jewish family.

In the theory of crystal growth, Cahn concluded that the distinguishing feature is the ability of the surface to reach an equilibrium state in the presence of a thermodynamic driving force (typically in the form of the degree of undercooling). He also concluded that for every surface or interface in a crystalline medium, there exists a critical driving force, which, if exceeded, will enable the surface or interface to advance normal to itself, and, if not exceeded, will require the lateral growth mechanism. Thus, for sufficiently large driving forces, the interface can move uniformly without the benefit of either a heterogeneous nucleation or screw dislocation mechanism. What constitutes a sufficiently large driving force depends upon the diffuseness of the interface, so that for extremely diffuse interfaces, this critical driving force will be so small that any measurable driving force will exceed it. Alternatively, for sharp interfaces, the critical driving force will be very large, and most growth will occur by the lateral step mechanism.

In 1977, Cahn published a simple mathematical treatment of the thermodynamics of wetting: the interaction between a liquid in contact with a solid surface. This paper laid out a simple formulation for describing the wetting transition—the point at which a liquid changes from forming a droplet on a surface to spreading out evenly as a liquid film over the surface. This theory had wide ranging implications for many materials processing techniques.

In 1982, Dan Shechtman observed a new crystalline structure with puzzling features. Cahn contributed to the theory of how such a structure could be thermodynamically stable and became co author of the seminal paper which introduced quasicrystals.

In 2004, Cahn and Bendersky presented evidence that an "isotropic non crystalline metallic phase" (dubbed "q glass") can be grown from the melt. This is the "primary phase," to form in the Al Fe Si system during rapid cooling. Experimental evidence indicates that this phase forms by a first order transition. TEM images show that the q glass nucleates from the melt as discrete particles, which grow spherically with a uniform growth rate in all directions. The diffraction pattern shows it to be an isotropic glassy phase. Yet there is a nucleation barrier, which implies an interfacial discontinuity (or internal surface) between the glass and the melt.

In his retirement, Cahn accepted a position at the University of Washington as an affiliate professor in the Departments of Materials Science and Engineering and Physics.

He had three children and six grandsons. In retirement, he lived in Seattle, Washington, with his wife, Anne Hessing Cahn. He died from leukemia in Seattle on March 14, 2016.