Stability and thermoelectric performance of doped higher manganese silicide materials solidi fied by RGS (ribbon growth on substrate) synthesis

Large scale deployment of thermoelectric devices requires that the thermoelectric materials have stable electrical, thermal and mechanical properties under the conditions of operation. In this study we examine the high temperature stability of higher manganese silicide (HMS) materials prepared by the RGS (ribbon growth on substrate) technique. In particular we characterize the effect of element substitution on the structural and electrical changes occurring at the hot side of temperatures of thermoelectric devices relevant to this material (600°C). Only by using suitable substitution (4% vanadium at the Mn site) can we obtain temperature-independent structural parameters in the range 20°C - 600°C, a condition that results in stable electrical properties. Additionally, we show that 4% vanadium substitution at the Mn site offers the best thermoelectric figure of merit among the different compositions reported here with ZTmax=0.52, a value comparable to the state of the art for HMS materials. Our analysis suggests that ionized impurity scattering is responsible for the better performance of this material

Crystallisation of phosphates revisited: a multi-step formation process for SrHPO4_4

SrHPO$_4$ is used in a multitude of applications, including biomedicine, catalysts, luminescent materials, and batteries. However, the performance of these materialsdepends on the ability to control the formation and transformation of strontium phosphates. This work focuses on the application of in situ and exsitu measurements, including synchrotron-based X-ray diffraction (XRD) analysis, luminescence of Ce$^{3+}$ and Eu$^{3+}$ dopants, light transmission, reflectance, and thermogravimetry to track structural changes in SrHPO$_4$ under different experimental conditions. Ex situ analysis of aliquots revealed favourable crystallisation of β-SrHPO$_4$ through the formation of Sr$_6$H$_3$(PO$_4$)$_5$·$_2$H$_2$O as an intermediate. Furthermore, in situ analysis showed that the reaction mechanism evolves via the initial formation of amorphous strontium phosphate and Sr$_5$(PO$_4$)$_3$OH, which subsequently transforms to γ-SrHPO$_4$. Analysis of the luminescence properties of the lanthanide dopants provided insights into the coordination environments of the substituted Sr$^{2+}$ sites