1997 TMS Annual Meeting: Tuesday Session

CHEMISTRY AND PHYSICS OF NANOSTRUCTURES AND RELATED NONEQUILIBRIUM MATERIALS: Session III: Thermal and Thermodynamic Properties

Session Chairperson: Robert Shull, NIST, Bldg. 223, Rm B152, Gaithersburg, MD 20899

9:00 am INVITED

STRUCTURE, MECHANICAL PROPERTIES, AND FRACTURE IN NANOPHASE SILICON NITRIDE: MILLION ATOM MOLECULAR DYNAMICS SIMULATIONS ON PARALLEL COMPUTERS: Rajiv K. Kalia, Aiichiro Nakano, Andrey Omeltchenko, Kenji Tsuruta, Priya Vashishta, Concurrent Computing Laboratory for Materials Simulations, Physics and Astronomy, Computer Science, Louisiana State University, Baton Rouge, LA 70803

Structure, mechanical properties, and dynamic fracture in nanocluster-assembled silicon nitride are investigated with molecular-dynamics (MD) simulations involving 1.08 million particles. The simulations reveal that intercluster regions in the consolidated nanophase Si3N4 are highly disordered with 50% undercoordinated atoms. These disordered interfacial regions deflect cracks and give rise to local crack branching. As a result, the nanophase system is able to sustain an order-of- magnitude larger external strain than the crystalline Si3N4. We also investigate the morphology of fracture surfaces in nanophase Si3N4. The MD results for roughness exponents are very close to experimental values [P. Daguier et al., Europhys. Lett. 31, 367 (1995)] even though the materials and length scales are very different.

9:30 am INVITED

THERMAL CONDUCTIVITY OF NANOPHASE CERAMICS: Paul G. Klemens, Physics, University of Connecticut, Storrs, CT 06269-3046

Heat conduction of insulators is by lattice waves (phonons), which have a wide frequency spectrum. The mean free path of phonons is limited by anharmonic intereactions and by scattering, principally scattering by point defects, and by grain boundaries and other etended imperfections. Point defects reduce the mean free path and the contribution to the conductivity of high frequency phonons; grain boundaries that of low frequency phonons. These reductions in conductivity will be discussed. They are additive in materials of micron-sized grains, but not in nanophase materials, where the frequency ranges of phonons affected by point defects and by grain boundary scattering may overlap. The effect of phonon scattering on the thermal conductivy will be discussed with reference to solutes, non-stoichiometry, grain size, and radiation damage.

10:00 am

A SIMULATION STUDY ON THE MELTING OF NANOCRYSTALLINE PLATES AND SPHERICAL CLUSTERS: J.K. Lee, Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931; B.K. Cheong, W.M. Kim, S.G. Kim, Materials Design Laboratory, Korea Institute of Science and Technology, Seoul, Korea

As an emerging mass-data storage technology, phase change (PC) optical recording makes use of the difference in reflectivity between the amorphous and crystalline state of chalcogenide thin films. The amorphous state is obtained via laser melting and subsequent rapid cooling. For a better understanding of the melting behavior of such nanocrystalline plates, a Lennard-Jonesian fcc solid is studied using the method of molecular dynamics. Unlike the bulk case, it is shown that a plate melting is accompanied by a gradual change in both the volume and enthalpy. The melting point of a plate decreases as a function of h**(-n), where h is the thickness and n is 0.7. If the thickness becomes 15 lattice parameters, the melting point reaches a plateau. (111) plates show slightly higher melting points than those for (100) plates. The melting points of spherical clusters with radii ranging from 1.4 to 6.4 lattice parameters show a r**(-n) dependence, where r is the radius and n is 0.75.

10:20 am BREAK

10:35 am

EVIDENCE FOR THERMODYNAMIC STABILIZATION OF GRAIN BOUNDARIES IN Pd1-xZrx: C.E. Krill, H. Ehrhardt, R. Birringer, Unversität des Saarlandes, FB10 Physik, Postfach 151150, Gebäude 43, D­66041 Saarbrücken, Germany

The rate of grain growth in a crystalline material is proportional to both the mobility and energy of its grain boundaries. Standard techniques for hindering grain growth, such as solute drag, are kinetic in nature: that is, they aim to reduce the mobility term. A thermodynamic approach, however, should be just as effective: a reduction in grain-boundary energy would be accompanied by a corresponding decrease in the driving force for grain growth. According to the Gibbs adsorption theorem, grain-boundary energies can be reduced by introducing a component that tends to segregate to the grain boundaries. We have investigated ball-milled solid solution of Pd1-xZrz for signs of improved stability against grain growth with increasing Zr concentration; the growth rate is observed to drop to very low values for x greater than 0.2. Differential scanning calorimetry has been used to estimate the concentration dependence of the grain-boundary energy, thereby isolating the themodynamic contribution to grain-growth stability from that due to solute drag.

10:55 am

VIBRATIONAL DENSITY OF STATES OF NANOCRYSTALLINE Ni3Fe PREPARED BY HIGH ENERGY BALL MILLING: H. Frase, L.J. Nagel, J.L. Robertson, B. Fultz, California Institute of Technology, mail 138-78, Pasadena, CA 91125; Oak Ridge National Laboratory, Solid Sate Physics Division, P.O. Box 2008, Oak Ridge, TN 37831

We performed inelastic neutron scattering experiments on two states of Ni3Fe: 1) as-milled, when the material had a characteristic nanocrystallite size of 9 nm, and 2) annealed, when the material had a characteristic crystallite size of 30 nm. The nanophase material showed an enhancement by a factor of 2 in its density of states at energies below 15 meV, and some broadening of its longitudinal peak at 33 meV. The large enhancement in the density of states at these low energies appears to require coupling between inter- and intra-crystallite vibrational motions. The inter-crystalline modes would be associated with the vibrations of the crystallites themselves. These measured changes in vibrational DOS predict a difference in vibrational entropy of the bulk and nanocystalline Ni3Fe of about 0.18 kB/atom at high temperatures, with the nanocrystalline materials having the larger vibrational entropy. This work was supported by the U.S. Department of Energy under contract DE-FG03-86ER45270.

11:15 am INVITED

FUNCTIONAL CERAMICS USING NANOMETER-SIZED MATERIALS: Manu Multani, Tata Institute of Fundamental Research, Bombay 400 005, India

Working with a range of current ceramic materials with a wide variety of applications, it is shown that considerable improvements can be obtained when one starts with Small Solid State Systems (S4) Materials studied range from doped ZnO varistors, PZT piezoelectrics, YIG microwave materials, and the new superconductors. General principles underlying these changes are shown to be dependent on the dispersion relations of phonons and wavevectors (for ferroelectrics) and magnons and wavevectors (for ferromagnetics). Another S4 rule that we have found applicable for most oxides is the tendency towards higher symmetry and increasing unit-cell-volume with decreasing size of the S4s. These changes may usher in phase transformation(s) and size dependent phase transition temperatures.