arXiv subjects

Satyakam Kar

Publications and source records attributed to Satyakam Kar.

3 recordsLinked to original sources

arXiv2026

Epitaxial Films as Model Platform for Understanding Compositionally Complex Electrocatalysts

Compositionally complex solid solutions provide a unique route for engineering high-performance electrocatalysts, where the polyelemental surface composition can be seamlessly tuned to optimize activity, selectivity, and stability. However, the mechanistic understanding of these electrocatalysts remains limited by the lack of a model system with a crystallographically-defined surface that is compatible with correlative, multi-scale characterization. Here, we present epitaxial films as a model platform for studying compositionally complex electrocatalysts. Using magnetron sputtering, we realize (111) epitaxial Ir-Pd-Pt-Rh-Ru films on (0001) sapphire substrate via a (111) Pt buffer layer, confirmed by X-ray diffraction and transmission electron microscopy. The growth approach is applicable across a broad composition range and produces smooth surfaces (root mean square roughness < 1 nm) with micrometer-sized grains in the nanoscale films. With these films, we demonstrate direct structure-activity mapping at the nanoscale through precise co-localization using micro-indents and performing correlative atomic force microscopy, electron backscatter diffraction, and scanning electrochemical cell microscopy. Our work establishes a model platform for fundamental scalebridging characterization and paves the way for rational design of compositionally complex electrocatalysts.

cond-mat.mtrl-sci↗

arXiv2024

Formation of martensitic microstructure in epitaxial Ni-Mn-Ga films after fast cooling

Shape memory alloys have a wide range of applications, including high stroke actuation, energy-efficient ferroic cooling, and energy harvesting. These applications are based on a reversible martensitic transformation, which results in a complex microstructure in the martensitic state. Understanding the formation of this microstructure after fast heating and cooling is crucial, as a high cycle frequency is essential for achieving high power density in the mentioned applications. This article uses an epitaxial Ni-Mn-Ga film as a model system to study the formation of the martensitic microstructure, since the high surface-to-volume ratio of thin films enables rapid heating and cooling, and since the formation of a multi-level hierarchical microstructure during slow cooling of this material system is already well understood. We apply a millisecond flash lamp pulse of varying energy density on the Ni-Mn-Ga film and analyse the hierarchical martensitic microstructure afterwards. Substantial differences compared to slow cooling occur, which we attribute to the limited time available for the microstructure to form and to the thermal stress between film and substrate during rapid temperature changes.

cond-mat.mtrl-sci↗

arXiv2024

Multiferroic Microstructure Created from Invariant Line Constraint

Ferroic materials enable a multitude of emerging applications, and optimum functional properties are achieved when ferromagnetic and ferroelectric properties are coupled to a first-order ferroelastic transition. In bulk materials, this first-order transition involves an invariant habit plane, connecting coexisting phases: austenite and martensite. Theory predicts that this plane should converge to a line in thin films, but experimental evidence is missing. Here, we analyze the martensitic and magnetic microstructure of a freestanding epitaxial magnetic shape memory film. We show that the martensite microstructure is determined by an invariant line constraint using lattice parameters of both phases as the only input. This line constraint explains most of the observable features, which differ fundamentally from bulk and constrained films. Furthermore, this finite-size effect creates a remarkable checkerboard magnetic domain pattern through multiferroic coupling. Our findings highlight the decisive role of finite-size effects in multiferroics.

cond-mat.mtrl-sci↗