New Synthesis Route for Complex Borides; Rapid Synthesis of Thermoelectric Yttrium Aluminoboride via Liquid-Phase Assisted Reactive Spark Plasma Sintering

Hyoung-Won Son,David Berthebaud,Toetsu Shishido,Akira Yoshikawa,Kunio Yubuta,Takao Mori,Keiko Suzuta

doi:10.1038/s41598-020-65818-z

Abstract

YxAlyB14 ceramics are of high interest as high temperature thermoelectric materials with excellent p, n control. In this study, direct synthesis of dense polycrystalline YxAlyB14 (x ~0.64, 0.52 ≤ y ≤ 0.67) ceramics was successfully carried out by spark plasma sintering using commercially available precursors. YB4, AlB2 and B powders were reactively sintered with an additive AlF3 at 1773 K for 5–60 min in reduced Ar atmosphere. The sinterability was remarkably enhanced by liquid phase sintering comparing to conventional synthesis techniques. Phase composition analysis by X-ray diffraction showed that main peaks belong to YxAlyB14 with the MgAlB14 structure type and no peaks of AlF3 were detected. The thermoelectric behavior was changed from p-type to n-type with increasing Al occupancy. Power factor and ZT values measured in this study were found to be in the same range as the best values previously reported. This original synthesis process is found to be less precursor-consuming as compared to previous synthesis processes, and strikingly, less time-consuming, as the synthesis time, is shortened from 8 h to 5 min for p-type and to 1 h for n-type. The total process time is shortened from ≥3 days to ~4–5 h. This discovery opens the door for more accessible synthesis of complex borides.

Highlights

On the other hand, at the other extreme of very high temperatures above 1000 K, there are various attractive high temperature thermoelectric applications
Different sintering conditions[39,56] and sintering aids were tried for the metal borocarbonitrides which are attractive thermoelectric materials, with some effective sintering aids found, but which were detrimental to the properties
The peaks of secondary phases are still detected on the YA0 composition sample which were even sintered for 1 h

Summary

Introduction

At the other extreme of very high temperatures above 1000 K, there are various attractive high temperature thermoelectric applications. In comparison with conventional sintering, SPS has some advantages such as rapid heating, shorter process time and densification at relatively lower temperatures[47,48,49]. In spite of using SPS, relative densities of obtained n-type YxAlyB14 (x = ~0.56; 0.57 ≤ y ≤ 0.63) samples exhibited quite low values (71.1–89.4%) due to the material’s poor sinterability. The previous synthesis method required a waste of resources by evaporation of large amounts of Al during the synthesis, and more importantly requiring a complicated and time consuming total process[43,44,45,46] This synthesis issue can be said to be a problem for many complex borides discovered with interesting electrical, magnetic, thermoelectric, and mechanical properties[50,51,52,53,54]. Different sintering conditions[39,56] and sintering aids were tried for the metal borocarbonitrides which are attractive thermoelectric materials, with some effective sintering aids found, but which were detrimental to the properties

Methods

Results

Conclusion