Microstructure and thermoelectric properties of Y

Authors: Satofumi Maruyama Toshiyuki Nishimura Yuzuru Miyazaki Kei Hayashi Tsuyoshi Kajitani Takao Mori

Publish Date: 2014/05/27

Volume: 3, Issue: 3, Pages: 31-

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Abstract

Excellent control in p and ntype transport characteristics was previously obtained for the thermoelectric Y x Al y B14 compounds through Al flux method In this study new attempts were made to reduce their grain sizes to obtain dense samples and to possibly lower the thermal conductivity Introducing the reduction of grain sizes into Y x Al y B14 samples was attempted by two methods one was through mechanical grinding and the other was by synthesizing Y x Al y B14 via Y056B14 denoted as “vYBYAlB14” Mechanical grinding using ball milling with Si3N4 pots and balls was found not to be an efficient way to decrease the grain size because of contamination of Si3N4 In contrast vYBYAlB14 samples were successfully synthesized Through the synthesis of Y056B14 the boron network structure was first formed Afterward Y x Al y B14 was obtained by adding Al in the boron network structure through a heat treatment Due to shorter heating time at lower temperature the grain sizes were discovered to be smaller than that of Al flux method The decrease of grain size was found to be beneficial for the densification of Y x Al y B14 and the decrease of its thermal conductivityThe development of thermoelectric materials has recently been carried out with great intensity because of the possibility for useful energy conversion of waste heat 1 Thermoelectric performance is evaluated by the dimensionless figure of merit ZT = S2T/ρκ where S ρ κ and T are the Seebeck coefficient the electrical resistivity the thermal conductivity and the absolute temperature respectively Boron icosahedra cluster compounds are good candidates for hightemperature thermoelectric materials because they exhibit intrinsic low thermal conductivity and are stable at high temperature 2 3 4 5 Boron carbide is one of the attractive ptype thermoelectric materials for the hightemperature region 6 Metal doped βboron compounds have been also investigated as possible thermoelectric materials 7 8 In addition novel boron icosahedra cluster compounds like RB44Si2 R = rare earth 9 10 11 12 B6S1x 13 are being investigated Rare earth borocarbonitrides RB22C2N RB17CN and RB285C4 were discovered to be the first boron icosahedra cluster containing compounds that exhibit intrinsic ntype thermoelectric materials 14 15 Some of the recently discovered novel borides have been found not to be easy to obtain dense samples and various studies to remedy this have been carried out for example mechanical grinding and sintering the sample through the spark plasma sintering SPS treatment with sintering aids such as metals rare earth tetra brides and carbides 16 17 18 19Recently thermoelectric properties of Y x Al y B14 have been investigated 20 and Y x Al y B14 was found to exhibit excellent p–n control with large absolute values of the Seebeck coefficient through the control of the Al occupancy y To increase figure of merit we attempt to reduce grain size by means of mechanical grinding and also by a change of synthesis method The reduction of grain size is expected to help densification of Y x Al y B14 and decrease thermal conductivity Since Y x Al y B14 is usually synthesized with hightemperature molten Al flux method 21 it is normally difficult to reduce the grain sizes through the synthesis conditions As a result Y x Al y B14 is also difficult to densify Therefore we focused on the boron atomic network structure of Y x Al y B14 Y056B14 YB25 has a similar atomic network structure and is synthesized by the borothermal reduction 22 23 As both compounds have similar boron atomic network structures we aimed to sinter Y x Al y B14 via Y056B14 which possibly can act like a precursor In this work we tried to decrease grain size of Y x Al y B14 samples and investigate effects of the grain size on the thermoelectric propertiesAfter synthesis of Y056B14 Al was added to Y056B14 and the mixture was heated around 1573 K for 4 h in vacuum After heating samples were crushed in a S3N4 mortar and excess Al was dissolved using NaOH All samples were pressed and compacted using SPS treatment at several sintering temperature ranged from 1673 to 1823 KXray diffraction XRD measurements using Rigaku Ultima3 with Cu Kα radiation were performed to characterize the samples and to determine the detailed crystal structure by means of Rietveld refinement using RietanFP software 24 Grain sizes of samples before SPS treatment were checked using a laser diffraction particle size analyzer The microstructures of the SPS sintered samples are observed using a scanning electron microscope SEM The electrical resistivity and the Seebeck coefficient were measured with an ULVAC ZEM2 using the fourprobe method and differential method respectively To determine the thermal conductivity the thermal diffusivity coefficients were measured by the laser flash method and the specific heat was measured by Quantum Design PPMS

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