363 research outputs found
Application of Finite Strain Landau Theory To High Pressure Phase Transitions
In this paper we explain how to set up what is in fact the only possible
consistent construction scheme for a Landau theory of high pressure phase
transitions that systematically allows to take into account elastic
nonlinearities. We also show how to incorporate available information on the
pressure dependence of elastic constants taken from experiment or simulation.
We apply our new theory to the example of the high pressure cubic-tetragonal
phase transition in Strontium Titanate, a model perovskite that has played a
central role in the development of the theory of structural phase transitions.
Armed with pressure dependent elastic constants calculated by density
functional theory, we give a both qualitatively as well as quantitatively
satisfying description of recent high precision experimental data. Our
nonlinear theory also allows to predict a number of additional elastic
transition anomalies that are accessible to experiment.Comment: submitted to Phys. Rev. Let
Revealing the pure confinement effect in glass-forming liquids by dynamic mechanical analysis
Many molecular glass forming liquids show a shift of the glass transition Tg
to lower temperatures when the liquid is confined into mesoporous host
matrices. Two contrary explanations for this effect are given in literature:
First, confinement induced acceleration of the dynamics of the molecules leads
to an effective downshift of Tg increasing with decreasing pore size. Secondly,
due to thermal mismatch between the liquid and the surrounding host matrix,
negative pressure develops inside the pores with decreasing temperature, which
also shifts Tg to lower temperatures. Here we present novel dynamic mechanical
analysis measurements of the glass forming liquid salol in Vycor and Gelsil
with pore sizes of d = 2.6, 5.0 and 7.5 nm. The dynamic complex elastic
susceptibility data can be consistently described with the assumption of two
relaxation processes inside the pores: A surface induced slowed down relaxation
due to interaction with rough pore interfaces and a second relaxation within
the core of the pores. This core relaxation time is reduced with decreasing
pore size d, leading to a downshift of Tg in perfect agreement with recent DSC
measurements
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