203 research outputs found
Thermodynamics of volume collapse transitions in cerium and related compounds
We present a non-linear elastic model of a coherent transition with
discontinuous volume change in an isotropic solid. The model reproduces the
anomalous thermodynamics typical of coherent equilibrium including intrinsic
hysteresis (for a pressure driven experiment) and a negative bulk modulus. The
novelty of the model is that the statistical mechanics solution can be easily
worked out. We find that coherency leads to an infinite-range density--density
interaction, which drives classical critical behavior. The pressure width of
the hysteresis loop shrinks with increasing temperature, ending at a critical
point at a temperature related to the shear modulus. The bulk modulus softens
with a 1/2 exponent at the transition even far from the critical point. Many
well known features of the phase diagram of Ce and related systems are
explained by the model.Comment: Acta Materialia, in pres
Droplet Fluctuations in the Morphology and Kinetics of Martensites
We derive a coarse grained, free-energy functional which describes droplet
configurations arising on nucleation of a product crystal within a parent. This
involves a new `slow' vacancy mode that lives at the parent-product interface.
A mode-coupling theory suggests that a {\it slow} quench from the parent phase
produces an equilibrium product, while a {\it fast} quench produces a
metastable martensite. In two dimensions, the martensite nuclei grow as
`lens-shaped' strips having alternating twin domains, with well-defined front
velocities. Several empirically known structural and kinetic relations drop out
naturally from our theory.Comment: 4 pages, REVTEX, and 3 .eps figures, compressed and uuencoded,
Submitted to Phys. Rev. Let
Thermodynamic theory of epitaxial ferroelectric thin films with dense domain structures
A Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory is
presented, which enables the thermodynamic description of dense laminar
polydomain states in epitaxial ferroelectric thin films. The theory explicitly
takes into account the mechanical substrate effect on the polarizations and
lattice strains in dissimilar elastic domains (twins). Numerical calculations
are performed for PbTiO3 and BaTiO3 films grown on (001)-oriented cubic
substrates. The "misfit strain-temperature" phase diagrams are developed for
these films, showing stability ranges of various possible polydomain and
single-domain states. Three types of polarization instabilities are revealed
for polydomain epitaxial ferroelectric films, which may lead to the formation
of new polydomain states forbidden in bulk crystals. The total dielectric and
piezoelectric small-signal responses of polydomain films are calculated,
resulting from both the volume and domain-wall contributions. For BaTiO3 films,
strong dielectric anomalies are predicted at room temperature near special
values of the misfit strain.Comment: 19 pages, 8 figure
Martensitic growth in ZrO2--An in situ, small particle, TEM study of a single-interface transformation
An in situ TEM experiment on martensitic growth was performed using submicron ZrO2 particles of a square-platelet shape. The transformation was between the orthorhombic and the monoclinic phases and involved a simple shear plus a dilatation in the shear plane. The o/m interface propagated at a speed of 2 nm/s, while maintaining a sharp habit plane which was stepped on the unit cell scale. The average inclination of this stepped interface obeyed the invariant plane strain condition. While no long-range stresses were present, dislocation-like line contrast was revealed at the steps. These results are analyzed in terms of the coherency dislocation concept. Fundamental properties, such as the interfacial energy, Peierls stress and nucleus size, have been deduced.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28555/1/0000357.pd
- …