Atomic-resolution interfacial structures and diffusion kinetics in Gd/Bi_0.5Sb_1.5Te₃ magnetocaloric/thermoelectric composites

Highlights

• The atomic-resolution interfacial structures of composites are characterized by STEM.

• The interfacial phases, including GdTe₂, GdTe₃, and intermediate phases GdTex, are identified.

• The diffusion kinetics of Gd and Te is studied based on the Boltzmann-Matano analysis.

Abstract

The demand of a full solid-state cooling technology based on magnetocaloric and thermoelectric effects has led to a growing interest in screening candidate materials with high-efficiency cooling performance, which also stimulates the exploration of magnetocaloric/thermoelectric hybrid cooling materials. A series of Gd/Bi_0.5Sb_1.5Te₃ composites was fabricated in order to develop the hybrid cooling technology. The chemical composition, phase structure and diffusion kinetics across the reaction layers in Gd/Bi_0.5Sb_1.5Te₃ composites were analyzed at different reaction temperatures. Micro-area elemental analysis indicates that the formation of interfacial phases is dominated by the diffusion of Gd and Te while the diffusion of Bi and Sb is impeded. The interfacial phases, including GdTe₂, GdTe₃, and intermediate phases GdTe_x, are identified by atomic-resolution electron microscopy. The concentration modulation of Gd and Te is adapted by altering the stacking of the Te square-net sheets and the corrugated GdTe sheets. Boltzmann-Matano analysis was applied to reveal the diffusion kinetics of Gd and Te in the interfacial layers. The diffusion coefficients of Te in GdTe₂ and GdTe₃ are much higher than that of Gd while in GdTe the situation is reversed. This study provides a clear picture to understand the interfacial phase structures down to an atomic scale as well as the interfacial diffusion kinetics in Gd/Bi_0.5Sb_1.5Te₃ hybrid cooling materials.

Introduction

Vapor-compression refrigeration has been the dominating cooling technique since the early 20th century. The system relies on a compressor to pressurize a vaporized refrigerant, which unfortunately has ozone depletion potential to our earth's atmospheric environment. Therefore, developing an environmental friendly cooling technology has become a mandatory objective. In the past decade, two mainly solid-state cooling technologies, including thermoelectric (TE) cooling [1] and magnetocaloric (MC) cooling [2], have gained worldwide attention as potential candidates to compete with traditional vapor-compression refrigeration. TE cooling, based on the Peltier effect, can realize fast, accurate, noiseless, and coolant-free refrigeration, which is widely used in laser diodes, micro-chips, high-sensitive sensors, and portable refrigerators [3]. Commercial TE devices use state-of-the-art bismuth tellurides (p-type Bi-Sb-Te alloys and n-type Bi-Te-Se alloys) as key materials because of their unparalleled room-temperature TE performance and excellent manufacturability in mass production [4]. However, the commercial materials have a limited figure of merit (ZT) around 1.0 [[5], [6], [7]]. Since a high ZT of 3.0 is required to compete with the traditional vapor-compression cooling technology, the low ZT accompanied with the low efficiency (<5%) has restricted TE cooling to a narrow application range in the huge cooling industry. Therefore, TE cooling is still facing tremendous challenges to bring about a solid-state cooling revolution [8].

MC cooling represents a high-efficiency, low-noise, low-vibration, and environmental-friendly cooling technique, which has been considered as another potential candidate to replace the traditional vapor-compression cooling in the field of room-temperature refrigeration. The principle of this technique is based on the MC effect [9]. It is related to the change of magnetic moment and magnetic entropy of MC materials under the cycles of an applied magnetic field. When MC materials are magnetized under an applied magnetic field, the magnetic moments become more ordered and the magnetic entropy is consequently reduced, which leads to heating of the MC materials due to an increase of lattice entropy and heat release to the environment. When the MC materials are demagnetized, the magnetic moments are less ordered and the magnetic entropy is increased, which leads to cooling of the MC materials and heat absorption from the environment. The heat release and absorption processes are usually accompanied by magnetic transitions or phase transitions in the MC materials. The most widely studied MC materials for room-temperature cooling are Gd and its alloys, e.g. LaFeSi [10,11], MnAs [11,12]. Among them, metallic Gd is one of the most commonly used MC materials to design an active magnetic regenerator (AMR) [13] because of its large MC effect near room temperature at which it undergoes a second-order magnetic phase transition. However, a wide application of MC cooling is still limited due to an inefficient heat exchange during the complex cycling procedures [14,15]. As most of the reported MC refrigerators use fluids as main heat exchangers, the heat exchange between MC materials and the fluid by means of conduction and convection leads to large circulation losses. Therefore, a search of high-efficiency heat exchange materials is critical to improve the efficiency of MC cooling [16,17].

The combination of TE and MC opens a new avenue to solve the heat exchange problem faced by the MC cooling technique. Many researchers have focused on employing TE devices as thermal diodes to replace the traditional working fluids in AMRs because of the powerful capability of TE cooling in rapid heat dissipation [[18], [19], [20]]. It has been theoretically demonstrated that full solid-state cooling improves the heat exchange and that a high-frequency cycle could simultaneously be realized. However, until now most of the work is limited to a theoretical design without convincing experimental verification.

As inspired by the idea of a combined TE and MC cooling, Gd/Bi_0.5Sb_1.5Te₃ (Gd/BST) composites are designed to develop an MC/TE hybrid cooling technology. The composites could be potentially applied to fabricate MC/TE hybrid refrigeration devices with simultaneously good MC cooling and TE cooling performance. It is expected that the heat exchange of MC materials can be improved by TE materials if the interface between these two different cooling materials can be well engineered. Therefore, in this work, the evolution of the interfacial chemical composition, phase structure and diffusion kinetics across the reaction layers of Gd/BST composites synthesized by spark plasma sintering (SPS) has been investigated. It is found that the reaction layers between Gd balls and BST in Gd/BST composites are composed of Gd and Te without Bi and Sb, and that the thickness increases with sintering temperature. Using high-resolution aberration-corrected transmission electron microscopy (TEM), interfacial phases including GdTe, GdTe₂, GdTe₃, and several GdTe_x phases have been identified down to an atomic scale. The diffusion kinetics at Gd/BST interfaces has been studied and the diffusion coefficients of Gd and Te in the different interfacial phases were determined based on a Boltzmann-Matano analysis. This information was extremely helpful to understand the diffusion mechanism in the interfacial phases for this kind of MC/TE hybrid cooling materials.