In an effort to unify the various phase-field models that have been used to study solidification, we have developed a class of phase-field models for crystallization of a pure substance from its melt. These models are based on an entropy functional, as in the treatment of Penrose and Fife, and are therefore thermodynamically consistent inasmuch as they guarantee spatially local positive entropy production. General conditions are developed to ensure that the phase field takes on constant values in the bulk phases. Specific forms of a phase-field function are chosen to produce two models that bear strong resemblances to the models proposed by Langer and Kobayashi. Our models contain additional nonlinear functions of the phase field that are necessary to guarantee thermodynamic consistency.

Thermodynamically-consistent phase-field models for solidification

Shape-Memory Materials

The interaction of Li(+) with single and few layer graphene is reported. In situ Raman spectra were collected during the electrochemical lithiation of the single- and few-layer graphene. While the interaction of lithium with few layer graphene seems to resemble that of graphite, single layer graphene behaves very differently. The amount of lithium absorbed on single layer graphene seems to be greatly reduced due to repulsion forces between Li(+) at both sides of the graphene layer.

http://ma.ecsdl.org/content/MA2010-02/6/375.full.pdf

The Interaction of Li+ with Single-Layer and Few Layers Graphene

It has been ten years since the discovery of the giant electrocaloric effect in ferroelectric materials showed that it is possible to employ this effect for substantial cooling applications. This last decade has been marked by increasing research interest, especially in characterizing and measuring the electrocaloric effect using both the so-called indirect and direct approaches. In this context, a comprehensive summary and careful reexamination of these approaches are very timely and of great importance to justify the assumptions used in different measurement techniques. This review is therefore dedicated to cover recent important and rapid advances from both the indirect and direct measurements and provides critical insights relevant for quantifying the electrocaloric effect. It involves electrocaloric materials from normal ferroelectrics, antiferroelectrics, and relaxors, and it fundamentally focuses on how the electrocaloric entropy changes in response to electric field in these typical electrocalorics. The article addresses recent developments, especially during the past three years, such as technical selection of proper polarization-electric field loops, negative electrocaloric effect in antiferroelectrics and relaxors, the controversial debate on the indirect method in relaxors, the important role of field dependence of specific heat, kinetic factors, and so on. Moreover, this review also is concerned with extracting reliable data by direct measurements. Four typical techniques and devices used recently, such as thermocouples, differential scanning calorimeters, specifically designed calorimeters, and scanning thermal microscopy, are briefly reviewed, while infrared cameras are emphasized. We hope that our review will not only provide a useful background to understand fundamentally the electrocaloric effect and what one really measures but also may act as a practical guide to exploit and develop electrocalorics towards the design of suitable devices.

/pdf/direct-and-indirect-measurements-on-electrocaloric-effect-2gpa4hs0sb.pdf

Direct and indirect measurements on electrocaloric effect: Recent developments and perspectives

Despite interlayer binding energy is one of the most important material properties for graphite, there is still lacking report on its direct experimental determination. In this paper, we present a novel experimental method to directly measure the interlayer binding energy of highly oriented pyrolytic graphite (HOPG). The obtained values of the binding energy are 0.27($\\pm $0.02)J/m$^{2}$, which can serve as a benchmark for other theoretical and experimental works.

/pdf/interlayer-binding-energy-of-graphite-a-mesoscopic-4ymya4k7yk.pdf

Interlayer binding energy of graphite: A mesoscopic determination from deformation

Pseudo-first-order phase transition for ultrahigh positive/negative electrocaloric effects in perovskite ferroelectrics

Taming martensitic transformation via concentration modulation at nanoscale

The electrocaloric effect (ECE) of ferroelectric materials, which occurs significantly in a narrow temperature region near the first-order paraelectric/ferroelectric transition (FOPFT) Curie temperature, has great potential in solid-state refrigeration. Most ferroelectric materials, however, bear the second-order paraelectric/ferroelectric transition (SOPFT). In the present study, we demonstrate a size-dependent pseudo-first-order phase transition (PFOPT), associated with ultrahigh ECE and Curie temperature in ferroelectric nanoparticles with degradation layers by employing phase field modeling. The PFOPT behavior of the polarization component P3 along the applied electric field direction x3 versus temperature is similar to the classical FOPFT behavior. The results indicate that the ultrahigh ECE and PFOPT occur at temperatures below the Curie temperature of a given nanoparticle size. The adiabatic temperature change is 3.347 K in the simulated barium titanate nanoparticle of 10 × 10 × 8 normalized size under a 96.502 kV cm−1 applied electric field change. The concept of PFOPT should be general, applicable to all ferroelectric perovskite materials. Therefore, the current results provide a novel physical perspective for experiments and for lower power/higher efficiency solid-state cooling devices.

Size-dependent ultrahigh electrocaloric effect near pseudo-first-order phase transition temperature in barium titanate nanoparticles

Phase field study of the copper precipitation in Fe-Cu alloy

Phase field modelling and thermodynamic analysis are employed to investigate depolarization and compression induced large negative and positive electrocaloric effects (ECEs) in ferroelectric tetragonal crystalline nanoparticles. The results show that double-hysteresis loops of polarization versus electric field dominate at temperatures below the Curie temperature of the ferroelectric material, when the mechanical compression exceeds a critical value. In addition to the mechanism of pseudo-first-order phase transition (PFOPT), the double-hysteresis loops are also caused by the abrupt rise of macroscopic polarization from the abc phase to the c phase or the sudden fall of macroscopic polarization from the c phase to the abc phase when the temperature increases. This phenomenon is called the electric-field-induced-pseudo-phase transition (EFIPPT) in the present study. Similar to the two types of PFOPTs, the two types of EFIPPTs cause large negative and positive ECEs, respectively, and give the maximum absolute values of negative and positive adiabatic temperature change (ATC ΔT). The temperature associated with the maximum absolute value of negative ATC ΔT is lower than that associated with the maximum positive ATC ΔT. Both maximum absolute values of ATC ΔTs change with the variation in the magnitude of an applied electric field and depend greatly on the compression intensity.

Jiaming Zhu

Papers

Pseudo-first-order phase transition for ultrahigh positive/negative electrocaloric effects in perovskite ferroelectrics

Taming martensitic transformation via concentration modulation at nanoscale

Size-dependent ultrahigh electrocaloric effect near pseudo-first-order phase transition temperature in barium titanate nanoparticles

Phase field study of the copper precipitation in Fe-Cu alloy

Double Hysteresis Loops and Large Negative and Positive Electrocaloric Effects in Tetragonal Ferroelectrics