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Crystal growth and anisotropy of high temperature thermoelectric properties of yttrium borosilicide single crystals
Author’s Accepted Manuscript Crystal growth and anisotropy of high temperature thermoelectric properties of yttrium borosilicide single crystals M. Anwar Hossain, Isao Tanaka, Takaho Tanaka, A. Ullah Khan, Takao Mori www.elsevier.com/locate/yjssc
PII: DOI: Reference:
S0022-4596(15)30194-8 http://dx.doi.org/10.1016/j.jssc.2015.10.006 YJSSC19116
To appear in: Journal of Solid State Chemistry Received date: 18 July 2015 Revised date: 30 September 2015 Accepted date: 3 October 2015 Cite this article as: M. Anwar Hossain, Isao Tanaka, Takaho Tanaka, A. Ullah Khan and Takao Mori, Crystal growth and anisotropy of high temperature thermoelectric properties of yttrium borosilicide single crystals, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2015.10.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Crystal growth and anisotropy of high temperature thermoelectric properties of yttrium borosilicide single crystals M. Anwar Hossaina,b, Isao Tanakab, Takaho Tanakaa, A. Ullah Khana, Takao Moria,c* a International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japan b Center for Crystal Science and Technology, University of Yamanashi, Miyamae 7-32, Kofu, Yamanashi 400-8511, Japan c Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8671, Japan Corresponding: [email protected]
ABSTRACT We studied thermoelectric properties of YB41Si1.3 single crystals grown by the floating zone method. The composition of the grown crystal was confirmed by electron probe micro-analysis. We have determined the growth direction for the first time for these borosilicides, and discovered relatively large anisotropy in electrical properties. We measured the electrical resistivity and Seebeck coefficient along [510] (the growth direction) and [052] directions and we found that this crystal exhibits strong electrical anisotropy with a maximum of more than 8 times. An interesting layered structural feature is revealed along [510] with dense boron cluster layers and yttrium layers, with conductivity enhanced along this direction. We obtained 3.6 times higher power factor along [510] compared to that along [052]. Although the ZT of the present system is low, anisotropy in the thermoelectric properties of a boride was reported for the first time, and can be a clue in developing other boride systems also.
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The growth direction ([510]) was determined for the first time in YB41Si1.3 single crystals and revealed an interesting layered feature of boron clusters and metal atoms, along which the electrical conductivity and thermoelectric power factor was strongly enhanced.
Keywords: Inorganic compound; Crystal growth; Borides; Electrical anisotropy; Thermoelectric materials 1. Introduction The direct conversion of waste heat to electricity is a large incentive to find viable thermoelectric (TE) materials [1]. Many efforts are being made to find routes to enhance thermoelectric properties. One very recent interesting development was reported in Nature on crystals of tin selenide which showed the best thermoelectric properties ever reported, in one direction of the crystal [2]. Boron cluster compounds are attractive candidates as high temperature thermoelectric materials for their stability and generally large Seebeck coefficients [3,5]. Furthermore, they typically exhibit low thermal conductivity [6-9], which is an inherent advantage for thermoelectrics, despite being strongly covalently bonded solids with high sound velocity. Several mechanisms have been proposed to be the origin of this intrinsic low thermal conductivity [5,9,10]. Many of the boron cluster compounds take the variable range hopping transport 2
(VRH) mechanism in which both electrical conductivity and Seebeck coefficient increase with temperature, which is an advantage for thermoelectric performance [4,5,11]. Another attractive feature of boron cluster compounds in general is that the network structures and physical properties have been found to be controllable to some degree through incorporation of metal atoms in the voids of clusters, and also addition of third elements like C, N, Si which can act as bridging sites of the cluster framework [12,13]. Although many borides are being developed as thermoelectric materials [4,14-24], to our knowledge there has been no report regarding the anisotropy of thermoelectric properties of borides. In this work we investigate the thermoelectric anisotropy of a rare earth borosilicide single crystal for the first time. It has previously been reported that two new phases exist between the previously known phases of YB12 and YB66 [25] with composition of about [B]/[Y]=25 and 50, respectively. Tanaka et al. has reported that crystal growth of the YB50 phase become possible by adding Si in the floating zone method and growing YB41Si1.2 crystals [26]. The crystal structure of YB41Si1.2 belongs to the orthorhombic system (space group Pbam) and is composed of B12 icosahedra and B12Si3 polyhedral units. After the successful growth of YB41Si1.2 crystals, a lot of research has been carried out on rare earth borosilicides, REB44Si2,to study physical properties [4,8,9,27-37]. The REB44Si2 (RE = Tb, Er, Yb) compounds are p-type like boron carbide and have been found to exhibit Seebeck coefficients in excess of 200 µVK-1 at high temperatures above 1000 K and also possess a low thermal conductivity of ~0.02 Wcm-1K-1[8,29]. A beneficial zinc doping effect was discovered for arc-melted yttrium borosilicides, YB44Si2 [4]. Tanaka et al. investigated electrical resistivity and Seebeck coefficient of YB41Si1.2crystal from 77 K to room temperature and they also estimated the figure of merit using thermal conductivity of YB66 at room temperature and obtained very poor values [28]. There is no literature available on high temperature thermoelectric properties of YB44Si2 single crystals. Regarding ani3
sotropy, T. Mori has found magnetic anisotropy to be indicated in Tb11B44Si2 and YbB45.6Si1.0 single crystals but with no information obtainable on crystal orientations [32,36].In this manuscript, we report high temperature thermoelectric properties of YB41Si1.3 single crystals grown by floating zone method. For the first time, we have determined crystallographic orientations of a borosilicide crystal and discovered the relatively large anisotropy in electrical properties along the determined directions.
2 Experimental details Floating zone (FZ) crystal growth was employed to grow yttrium borosilicide YB41Si1.3 single crystals using a four-mirror-type infrared image furnace (Crystal System Inc., FZ-T-10000-HIII-VPR) equipped with four 2.5 kW xenon lamps as heat source. The preparation process of the polycrystalline feed rods for FZ crystal growth is as follows; we mixed YB 4 (New Metals Co. 99%), B (SB Boron 99.9%),and Si(Wako 99.9%) powders to obtain a desired final composition, then, pressed it into a rod at a hydrostatic pressure of 300 MPa. Rods having nominal composition YB44Si2 were prepared for both the feed rod and the seed rod to grow crystals by the FZ method. The pressed rod was reacted in a boron nitride (BN) crucible fixed inside a graphite susceptor that was further covered with graphite wool for induction heating. The synthesis was carried out in an RF inductive furnace at 1400 °C for 8 h in vacuum. To obtain high density feed rods, the synthesized rods were grinded and formed again into rods and sintered at the same condition. FZ crystal growth was carried out by driving downward both feed and seed rods at 10 and 8mm/h, respectively, with counter-rotating at 16 rpm under Ar atmosphere (2.5 L/min). The grown crystals were characterized by high-resolution powder X-ray diffraction (XRD) and electron probe microanalysis (EPMA).Powder XRD measurements with CuK radiations 4
(Rigaku Ultima-3) were performed to confirm the required phase formation in the grown crystals, where parts of the crystals were crushed using a stainless steel mortar and then the obtained powder was washed with HCl solution and rinsed with water to remove stainless steel contamination. Rietveld refinement was performed using FullProf Suite software (2.05).Peak shape was refined with the modified Lorentzian function and 6 coefficients polynomial function was used for background refinement. EPMA was carried out in wavelength-dispersive mode using JEOL JXA-8200 instrument. Standard sample used for Y and B were Y3Al15O12 and LaB6, respectively. The crystallographic orientation of the YB41Si1.3 grown crystals were characterized by pole figure measurements using an X-ray 2D-detector (Bruker Corp., model D8 DISCOVER VANTEC-500). We took two cross-sections, one was approximately perpendicular to growth direction and another one was parallel to growth direction of the grown crystal, for pole figure measurement. We cut the crystal using a diamond wire cutter and then polished the pieces with a diamond solution. The crystallographic orientations were determined from the obtained pole figure data by Multex 3 software. For high temperature thermoelectric materials the stability at high temperature is essential. Therefore, to check the stability of the grown crystals at high temperature, we also carried out thermogravimetric analysis (TGA, Rigaku, model Thermo plus TG 8120) from 300 K to 1080 K in Ar flow as well as in air flow. Resistivity and Seebeck coefficient were measured with an ULVAC ZEM-2 in the temperature range of 330–1000 K in He atmosphere. To determine the thermal conductivity values, first of all, the room temperature specific heat was measured by using PPMS (Quantum Design, physical property measurement system). Then, the relative specific heat and thermal diffusivity coefficient were measured by laser flash method (ULVAC
5
TC-7000) from 300 K to 1080 K. The thermal conductivity is determined as the product of the density, specific heat, and thermal diffusivity coefficient.
3 Results and Discussions 3.1
Crystal growth and characterizations
The boron-rich boride compound YB50 which has an orthorhombic crystal structure starts to decompose above 2100 K into phases like YB 12 and YB66 without melting [38]. Tanaka et al. first demonstrated that the melt growth method is applicable to grow an yttrium borosilicide crystal, YB44Si1.0 that is iso-structural to YB50 by adding a small amount of silicon [26,27]. Another interesting Si addition effect was the enlargement of the lattice parameters which enabled synthesis of GdB44Si2, although GdB50 does not form due to the relatively large size of Gd [39]. A view of the crystal structure of REB44Si2 is shown in Fig. 1. The structure is orthorhombic with space group Pbam. Fig.2 shows the picture of a grown crystal. To grow high quality crystals, we used the zone pass technique. The first zone pass was made to obtain uniform composition and the second zone pass was made to grow the crystal.
B12 B12Si3 RE
6
Fig. 1.View of the crystal structure of REB44Si2 from a direction slightly tilted along [001] as indicated by the labels, two kinds of polyhedra are shown: B 12 icosahedra and B12Si3 polyhedra. Only two of the five structurally independent B 12 icosahedra are drawn for clarity. The circles indicate rare-earth (in this case, yttrium) atoms which are aligned along [001] in a ladder-like configuration.
Fig. 2.YB41Si1.3 single crystal grown by the floating zone method EPMA results revealed the atomic concentration of Y, B and Si to be 2.3±0.2 at%, 94.4±0.4 at% and 3.1±0.2 at%, respectively. The composition of the grown crystal was determined to be YB41Si1.3. Fig.3 shows powder X-ray diffraction pattern with Rietveld refinement of the grown crystal.
7
Fig. 3.Rietveld refinement of powder XRD pattern of YB41Si1.3 single crystal, exhibiting good agreement between the observed and the calculated patterns. Rietveld refinement yielded 1.3(0.1) vol. % of YB6 phase.
Rietveld refinement for the YB41Si1.3 crystal data confirmed YB44Si2 structure type with RF=0.019 and RB = 0.031, and a very little amount of YB6 phase was found (2nd row of black Bragg-position lines in Fig. 3).The lattice parameters were obtained to be a = 16.6368 Å, b = 17.5846 Å, and c= 9.5079 Å, which are quite similar with YB44Si1.0 [26]. Fig. 4 (a, b) represents the SEM images ofYB41Si1.3 grown crystal for cross-sections parallel to the growth direction and perpendicular to the growth direction and it was observed that in both cases there were no grain boundaries but some micro-cracks. Apparently, it is not possible to avoid microcracks in grown single crystals of this borosilicide phase. The grown crystals also contained a very small amount of inclusion (white spots) and we also confirmed by EDX that this inclusion is YB6. Amount of YB6 phase obtained from the Rietveld refinement was consistent with the amount of this phase observed in SEM images. In order to determine the growth direction of the YB 41Si1.3 grown crystal, we also took Laue XRD, however, due to the complicated structure and the weakness of the reflections, we were not able to determine the particular alignment of the crystals by Laue photography techniques.
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Fig. 4. SEM images for cross-sections of YB41Si1.3 single crystal along: (a) parallel to growth direction and (b) perpendicular to growth direction
Figs. 5(a-d) represent the measured and simulated pole figures, from the measurements using an X-ray 2D-detector,of the cross sections perpendicular to growth direction (a, b) and parallel to the growth direction (c, d) for YB41Si1.3 grown crystal. As shown in Figs. 1(a) and (b), the orientation of the cross-section perpendicular to growth direction is 11° tilted from [100] and toward [010]. By following the same procedure, the orientation of the cross section parallel to the growth direction was estimated to be 38° tilted from [010] toward [001]. The orientations of the cross-sections perpendicular to the growth direction and parallel to the growth direction were estimated by Q-Laue to be [510] and [052], respectively. The obtained directions [510] and [052] were not orthogonal and the angle between them was 81.4 degrees. This lack of orthogonality is likely due to cutting the cross-sections of YB41Si1.3 grown crystal at a slightly tilted position with respect to the cross-section of the perpendicular to growth direction.
b
a
Psi=11°
Phi=14°
c
d
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Fig.5. Measured and simulated pole figures of the cross sections perpendicular to growth direction (a, b) and parallel to the growth direction (c, d) for YB41Si1.3 crystal. The direction of rotation (Phi) was rotated 14°and 59°.
a
b 10
Fig. 6. Views of the crystal structure along: (a) [510] (the growth direction) and (b) [052] of YB41Si1.3. Dark blue represents Y, green represent Si and red indicates B atoms. Determination of crystallographic orientation is the most important part prior to study directional physical properties of a crystal. Similar to yttrium, rare-earth metals from Gd to Lu can form REB41Si1.2-type borides. To our knowledge, nobody has reported about the orientation of these borosilicide single crystals and we have determined the crystallographic orientation in YB41Si1.3 for the first time. Obviously, the orientation in YB41Si1.3 would be greatly helpful to explore directional properties of REB41Si1.2 borides. Figs. 6 (a) and (b) show views of the crystal structure along [510] (the growth direction) and [052] of YB41Si1.3. The growth direction shows an interesting structure feature in that an almost layered structure is revealed, with dense boron cluster layers separated by the rare earth atom layers. We also investigated the thermal stability of the borosilicide single crystal. Fig.7 shows TGA data of YB41Si1.3 crystal. We performed Thermogravimetric analysis (TGA) in Ar flow as well as in air flow and in both cases, two different small pieces of the crystal were used. TGA data in Fig. 7 (a) reveals that there was no 11
change in weight during heating in Ar flow. However, in the case of air flow, the crystal was oxidized slightly as shown in Fig. 7(b) and the weight increased by 0.3% up to 1073K. Therefore, TGA data implies that at higher temperature, the grown crystal was excellently stable in Ar flow and moderately stable in air flow.
Fig.7 (a) Thermogravimetric analysis (TGA) of YB41Si1.3 single crystal in Ar and air flow.
Fig. 7 (b) EDX mapping of the YB41Si1.3 single crystal used for TGA in air flow showing the oxidized surface and inside of the crystal.
3.2 Thermoelectric properties We measured the thermoelectric properties of the YB41Si1.3 single crystal along the two determined directions [510] (the growth direction) and [052], and compared them with the properties 12
of arc-melted YB44Si2 [4]. The electrical resistivity data are plotted in Fig. 8 as the logarithm of the resistivity against
. The electrical resistivity of many boron-rich borides exhibit Mott’s
variable range hopping mechanism for three dimensional systems where the resistivity ρ follows: ,
(1)
WithT0 being the so-called characteristic temperature [11,40]. The resistivity for YB41Si1.3 along [510] and [052] generally appear to have linear dependence with T-0.25 (K-0.25).
Fig.8. Temperature dependence of electrical resistivity of YB41Si1.3 single crystals along: (I) [510] and (II) [052] orientations. Red triangles represent the resistivity of arc-melted YB44Si2 [4].
By fitting equation (1), we obtained the values of T0 along [510] and [052] to be 7.4×105 K and 9.1×105 K, respectively. The smaller value of T0 along [510] compared to [052] indicates that it is less localized or has higher density of states at the Fermi level along [510] [29]. In general the values for T0 for YB41Si1.3 crystal are comparable with 6×105 K, 1×106 K and 5×106 K for TbB44Si2, ErB44Si2, and YbB44Si2, respectively [9,12]. At 330 K, the resistivity along [510] and 13
[052] directions were 1.6×10-2 m and 13.2×10-2 m respectively, i.e. the resistivity along the [510] direction was 8 times lower than that along [052] direction. These differences in resistivity signify that YB41Si1.3 crystal is strongly anisotropic with the electrical conductivity larger along [510]. As noted above (Fig. 6(a)), an interesting layered structural feature is revealed along [510] with dense boron cluster layers and yttrium layers. We can speculate that the enhancement may be due to good conduction along the metal layers, which is quite interesting and an unusual structural feature for higher borides. These resistivity values in general are lower than reported for the RB66 compounds [12], for example, and indicate the suitability of YB41Si1.3 among the higher borides as thermoelectric materials. The micro-cracks undoubtedly make the measured values of resistivity higher, and if crystal growth techniques to remove these up-to-now unavoidable cracks can be realized, the thermoelectric properties can assumedly be further improved, since such cracks (with no special design for selective phonon scattering) typically more strongly affect electrical conductivity compared to thermal conductivity. This higher electrical conduction along the metal layers found in YB41Si1.3 is intuitively understandable, but interestingly in contrast to what was observed recently for the non-cluster 2D-boron-net compound AlB2 [10]. AlB2 has a prominently layered structure composed of graphene-like 2D-boron-net layers sandwiching Al atom layers. Quite different from graphitebased materials, the thermal conductivity of AlB2 (and assumedly electrical conductivity because it is a good metal) took higher values in the cross-plane direction compared to the inplane direction [10]. This can be assumed to be derived from the unique bonding of boron compounds, and further analysis of the origin of the anisotropy should be closely coupled with theoretical bonding analysis of these compounds. The Seebeck coefficients , of YB41Si1.3 are plotted in Fig. 9. They exhibit positive values and increase with increase in temperature. Throughout the whole range of measurement tempera14
ture, a large difference in Seebeck coefficient was observed between [510] and [052] directions. The Seebeck coefficient along [052] was higher than [510]. This agrees with the orientation dependence, we observed in the electrical resistivity. The values of Seebeck coefficients of the YB41Si1.3 single crystal along both [510] and [052] were significantly higher than that of arc-melted YB44Si2 sample, and showed a larger plateau behavior (i.e. less temperature dependence) over a wide range of temperature. This behavior is beneficial for high temperature thermoelectric materials, since in applications, large temperature differences will be applied to the material and the average performance over a wide temperature range is important.
Fig.9. Temperature dependence of Seebeck coefficient of YB 41Si1.3 single crystals along: (I) [510] and (II) [052]. Red triangles represent Seebeck coefficient of arc-melted YB44Si2 sample.
Fig.10 demonstrates the temperature-dependent power factor of YB41Si1.3. The power factors along [510] of FZ crystal are larger compared to that along [052] of FZ crystal and arcmelted sample, but the gap in performance widens towards higher temperatures. This is related to the plateau-like behavior noted for the Seebeck coefficient, and is especially apparent for the power factor along [510], which exhibits a good rise above 700 K. At 1000 K, the power
15
factor of YB41Si1.3 single crystal along [510] was 13.8
Wm-1K-2, which is 3.6 and 1.8
times higher than that along [052] and the arc-melted YB44Si2 sample, respectively. To gauge the performance of thermoelectric application of compounds, it is necessary to determine the dimensionless figure of merit ZT=PT/k, where k is thermal conductivity. We measured the thermal conductivity of YB41Si1.3 single crystal (Fig. 11) along [510] only due to considerably high power factor in this direction.
Fig.10. Temperature dependence of power factor of YB41Si1.3 single crystals along: (I) [510] and (II) [052]. Red triangles represent power factor of the arc-melted YB44Si2 sample.
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Fig.11. Temperature dependence of thermal conductivity of YB41Si1.3 single crystals along [510] (pink squares). Red diamond and green circles represent thermal conductivity for YbB44Si2 and ErB44Si2 crystals, respectively.
The thermal conductivity of YB41Si1.3 took values from 3.7 to 4.4 Wm-1K-1 in the temperature range. We also compared the measured thermal conductivity of YB 41Si1.3 crystal with those of YbB44Si2 and ErB44Si2 FZ crystals as shown in Fig.11. The magnitude of thermal conductivity was highest for YB41Si1.3 because Y is lighter than Er and Yb and hence there is reduced phonon scattering. Fig.12 shows the temperature dependence of ZT value of YB41Si1.3 single crystals along [510] and the maximum value 0.033 was found at 990 K. Although, this value is not high compared to values such as ZT~0.8 for SiGe at 900K [41], the experimental results discussed in this article were obtained for an unmodified yttrium borosilicide.
Fig. 12. Temperature dependence of ZT value of YB41Si1.3 single crystals along [510] direction.
As mentioned, the grown crystal contains micro-cracks and by optimizing the growth condition, the quality of the grown crystal could be improved and the thermoelectric performance assumedly enhanced. Doping with transition metals is another way to more dramatically modify the properties of REB44Si2 [30]. A beneficial hybrid effect was previously obtained for yttrium boro17
carbonitride with a combination of transition metal doping and heat treatment. Seebeck coefficients, α, of YB22C2N, n-type counterpart to boron carbide, could be increased by up to 220% while resistivity was reduced by x100 [42-44]. In addition to applying such doping effects, we aim to clarify the anisotropy effect in this work, to further develop the borosilicides, which are one of the few thermoelectric materials which can be used at high temperatures such as 1000 o
C and above.
4. Conclusions We have grown YB41Si1.3 single crystals by the floating zone method. To our knowledge, there has been no report on the determination of crystallographic orientation and electrical anisotropy in REB44Si2 crystals. For the first time, we have determined crystallographic orientation and also discovered large anisotropy in the thermoelectric properties of YB41Si1.3 single crystals. The electrical conductivity was significantly higher along [510] (growth direction), a maximum of more than 8 times compared to [052], where it was revealed that the crystal structure has a layered character, with dense boron cluster and metal layers. It can be speculated that conduction is enhanced in the metal layers. This is intuitively understandable, but in contrast to the anisotropic behavior observed recently for the non-cluster 2D-boron-net compound AlB2. These behaviors can be assumed to be derived from the unique bonding of boron compounds, and further theoretical insight including bonding analysis should be of use to quantitatively evaluate the origin of the anisotropy discovered in the YB41Si1.3 single crystal. We also measured the thermal conductivity of YB41Si1.3 single crystal along [510] and then estimated figure of merit to be 0.033, which is not high, but acceptable as a starting point in the development. Future efforts will be made to modify the borosilicides utilizing beneficial doping effects which have been found for other boride systems. Furthermore, this is the first report of 18
existence of such a strong anisotropy in the thermoelectric properties of a boride, and such anisotropy should be investigated in other prospective boride systems also. Thereby, we hope to improve the suitability of this borosilicilde and other borides as possible high temperature thermoelectric materials, one of the few materials systems which can be used at 1000 ºC and higher temperatures.
Acknowledgements The authors are grateful to Dr. Hitoshi Morioka of Bruker AXS K.K. for help of pole figure analysis. This work was partly supported by Green Energy Conversion Science and Technology program, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Japan.
References [1]
Thermoelectric Nanomaterials, ed. K. Koumoto and T. Mori, Springer Series in Materials Science (Springer, Heidelberg, 2013) pp. 1-373.).
[2]
Li-Dong Zhao, Shih-Han Lo, Yongsheng Zhang, Hui Sun, Gangjian Tan, CtiradUher, C. Wolverton, Vinayak P. Dravid&Mercouri G. Kanatzidis, Nature, 508, 373-377 (2014).
[3]
Wood, C.; Emin, D. Phys. Rev. B 1984, 29, 4582–4587.
[4]
Mori, T.; Berthebaud, D.; Nishimura, T.; Nomura, A.; Shishido, T.; Nakajima, K. Dalton Trans.2010, 39, 1027–1030.
[5]
Mori, T. in: Modules, Systems, and Applications in Thermoelectrics, ed. Rowe, D. M, CRC Press Taylor and Francis, London 2012, 14.
[6]
Slack, G. A.; Oliver, D. W.; Horn,F. H. Phys. Rev. B 1971, 4, 1714–1720.
[7]
Cahill,D. G.; Fischer, H. E.; Watson, S. K.; Pohl, R. O.; Slack, G. A. Phys. Rev. B 1989, 40, 3254–3260.
[8]
Mori, T. Physica B 2006, 383, 120–121.
[9]
Mori,T.; Martin,J.; Nolas,G. J. Appl. Phys.2007,102, 073510.
[10]
Wang,X. J.; Mori,T.; Kuzmych-Ianchuk,I.; Michiue,Y.; Yubuta,K.; Shishido,T.; Grin,Y.; Okada,S.; Cahill,D. G. APL Mater. 2014, 2, 046113. 19
[11]
Mott,N. F. J. Non-Cryst. Solids1968,1, 1–17.
[12]
[13]
Mori,T. “Higher Borides” in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner Jr., J. -C. Bunzli, and V. Pecharsky (North-Holland, Amsterdam)2008, vol 38 p. 105-173. Mori,T. J. Phys. Conf. Ser. 2009, 176, 012036.
[14]
Mori, T.; and Nishimura, T. J. Solid State Chem., 2006, 179, 2908-2915.
[15]
Balaz, S.; Dimov, D. I.; Boag, N. M.; Nelson, K.; Montag, B.; Brand, J. I.; Dowben, P. A. Appl. Phys. A 2006, 84, 149–159.
[16]
Gong, Y.; Zhang, Y.; Dudley, M.; Zhang, Y.; Edgar,J. H.; Heard, P. J.; Kuball,M., J. Appl. Phys. 2010, 108, 084906.
[17]
Maruyama, S.; Miyazaki, Y.; Hayashi, K.; Kajitani, T.; Mori, T. Appl. Phys. Lett. 2012, 101, 152101.
[18]
Sologub, O.; Matsushita, Y.; Mori, T. Scr. Mater. 2013, 68, 289–292.
[19]
Frye, C.D.; Edgar, J.H.; Ohkubo, I.; Mori, T. J. Phys. Soc. Jpn. 2013, 82, 095001.
[20]
Wan, L. F.; Beckman, S. P. Phys. B Condens. Matter 2014, 438, 9–12.
[21]
Miura, S.; Sasaki, H.; Takagi, K.;Fujima, T. J. Phys. Chem. Solids,2014, 75, 951-953.
[22]
Mori, T.; Nishimura,T.; Schnelle, W.; Burkhardt, U.; Grin, Y. Dalton Trans.,2014, 43, 15048-15054.
[23]
Slack, G. A.; Morgan, K. E. J. Phys. Chem. Solids 2014, 75, 1054–1074.
[24]
Gürsoy, M,; Takeda, M,; Albert, B. J. Solid State Chem.2015, 221, 191–195.
[25]
Tanaka,T.; Okada,S.; Ishizawa,Y.J. Alloys Comp. 1994, 205, 281-284.
[26]
Tanaka,T.; Okada,S.; Ishizawa,Y. J. Solid State Chem. 1997, 133, 55–58.
[27]
Higashi,I.; Tanaka,T.; Kobayashi,K.; Ishizawa,Y.; Takami,M.J. Solid State Chem. 1997, 133, 11–15.
[28]
Ishizawa,Y.; Tanaka,T. J. Solid State Chem. 2000, 154, 229-231.
[29]
Mori,T.J. Appl. Phys. 2005, 97, 093703.
[30]
Berthebaud, D.; Sato, A.; Michiue, Y.; Mori, T.; Nomura, A.; Shishido, T.; Nakajima, K. J. Solid State Chem. 2011, 184, 1682–1687.
[31]
Mori, T.;Shishido, T.; Nakajima, K. J. Electron. Mater. 2009, 38, 1098-1103.
[32]
Mori,T. Z. Kristallogr. 2006, 221, 464–471.
[33]
Mori,T.; Tanaka,T. J. Solid State Chem. 2000, 154, 223-228.
[34]
Mori,T.; Tanaka, T.J. Alloys and Comp. 1999, 288, 32-35. 20
[35]
Mori, T.; Tanaka, T. IEEE Trans. Magn. 2001, 37, 2144-2146.
[36]
Mori, T.; Tanaka, T. J. Alloys and Comp.2003, 348, 203–207.
[37]
Yuan, J.; Zhang, H.; Tang, J.; Shinya, N.; Lin,Y.; Lu-Chang Qin, J. Mater Sci 2013, 48, 1555–1561.
[38]
Tanaka,T.; Okada,S.; Ishizawa,Y. J. Alloys Compd. 1994, 205, 281–284.
[39]
Mori, T.; Tanaka, T.; Mat. Res. Bull., 2001, 36 2463-2470.
[40]
Electron-electron interactions in disordered systems,edited by A. L. EfrosandM. Pollak, (North-Holland, Amsterdam, 1985), P-409.
[41]
CRC Handbook of Thermoelectrics, ed. D. M. Rowe (CRC, Boca Raton, 1995).
[42]
Prytuliak, A.; Mori, T.; Journal of Electronic Materials, 2011, 40, 920-925.
[43]
Prytuliak, A.; Maruyama, S.; Mori, T. Materials Research Bulletin, 2013, 48, 1972-1977.
[44]
Mori, T.; Hara, T.; Scripta Materialia, 2015, in press.
Highlights:
1. We have grown YB41Si1.3 single crystals by the floating zone method. 2. We determined the growth direction for the first time in a REB41Si1.2 borosilicide, which was [510], and revealed an interesting layered structural feature with dense boron cluster and ytrrium layers. 3. The electrical resistivity was strongly anisotropic between [510] and [052] orientations, with conductivity speculated to be enhanced by conduction along metal layers. 4. The obtained power factor along [510] is 3.6 times higher than that along [052].
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PII: DOI: Reference:
S0022-4596(15)30194-8 http://dx.doi.org/10.1016/j.jssc.2015.10.006 YJSSC19116
To appear in: Journal of Solid State Chemistry Received date: 18 July 2015 Revised date: 30 September 2015 Accepted date: 3 October 2015 Cite this article as: M. Anwar Hossain, Isao Tanaka, Takaho Tanaka, A. Ullah Khan and Takao Mori, Crystal growth and anisotropy of high temperature thermoelectric properties of yttrium borosilicide single crystals, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2015.10.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Crystal growth and anisotropy of high temperature thermoelectric properties of yttrium borosilicide single crystals M. Anwar Hossaina,b, Isao Tanakab, Takaho Tanakaa, A. Ullah Khana, Takao Moria,c* a International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japan b Center for Crystal Science and Technology, University of Yamanashi, Miyamae 7-32, Kofu, Yamanashi 400-8511, Japan c Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8671, Japan Corresponding: [email protected]
ABSTRACT We studied thermoelectric properties of YB41Si1.3 single crystals grown by the floating zone method. The composition of the grown crystal was confirmed by electron probe micro-analysis. We have determined the growth direction for the first time for these borosilicides, and discovered relatively large anisotropy in electrical properties. We measured the electrical resistivity and Seebeck coefficient along [510] (the growth direction) and [052] directions and we found that this crystal exhibits strong electrical anisotropy with a maximum of more than 8 times. An interesting layered structural feature is revealed along [510] with dense boron cluster layers and yttrium layers, with conductivity enhanced along this direction. We obtained 3.6 times higher power factor along [510] compared to that along [052]. Although the ZT of the present system is low, anisotropy in the thermoelectric properties of a boride was reported for the first time, and can be a clue in developing other boride systems also.
1
The growth direction ([510]) was determined for the first time in YB41Si1.3 single crystals and revealed an interesting layered feature of boron clusters and metal atoms, along which the electrical conductivity and thermoelectric power factor was strongly enhanced.
Keywords: Inorganic compound; Crystal growth; Borides; Electrical anisotropy; Thermoelectric materials 1. Introduction The direct conversion of waste heat to electricity is a large incentive to find viable thermoelectric (TE) materials [1]. Many efforts are being made to find routes to enhance thermoelectric properties. One very recent interesting development was reported in Nature on crystals of tin selenide which showed the best thermoelectric properties ever reported, in one direction of the crystal [2]. Boron cluster compounds are attractive candidates as high temperature thermoelectric materials for their stability and generally large Seebeck coefficients [3,5]. Furthermore, they typically exhibit low thermal conductivity [6-9], which is an inherent advantage for thermoelectrics, despite being strongly covalently bonded solids with high sound velocity. Several mechanisms have been proposed to be the origin of this intrinsic low thermal conductivity [5,9,10]. Many of the boron cluster compounds take the variable range hopping transport 2
(VRH) mechanism in which both electrical conductivity and Seebeck coefficient increase with temperature, which is an advantage for thermoelectric performance [4,5,11]. Another attractive feature of boron cluster compounds in general is that the network structures and physical properties have been found to be controllable to some degree through incorporation of metal atoms in the voids of clusters, and also addition of third elements like C, N, Si which can act as bridging sites of the cluster framework [12,13]. Although many borides are being developed as thermoelectric materials [4,14-24], to our knowledge there has been no report regarding the anisotropy of thermoelectric properties of borides. In this work we investigate the thermoelectric anisotropy of a rare earth borosilicide single crystal for the first time. It has previously been reported that two new phases exist between the previously known phases of YB12 and YB66 [25] with composition of about [B]/[Y]=25 and 50, respectively. Tanaka et al. has reported that crystal growth of the YB50 phase become possible by adding Si in the floating zone method and growing YB41Si1.2 crystals [26]. The crystal structure of YB41Si1.2 belongs to the orthorhombic system (space group Pbam) and is composed of B12 icosahedra and B12Si3 polyhedral units. After the successful growth of YB41Si1.2 crystals, a lot of research has been carried out on rare earth borosilicides, REB44Si2,to study physical properties [4,8,9,27-37]. The REB44Si2 (RE = Tb, Er, Yb) compounds are p-type like boron carbide and have been found to exhibit Seebeck coefficients in excess of 200 µVK-1 at high temperatures above 1000 K and also possess a low thermal conductivity of ~0.02 Wcm-1K-1[8,29]. A beneficial zinc doping effect was discovered for arc-melted yttrium borosilicides, YB44Si2 [4]. Tanaka et al. investigated electrical resistivity and Seebeck coefficient of YB41Si1.2crystal from 77 K to room temperature and they also estimated the figure of merit using thermal conductivity of YB66 at room temperature and obtained very poor values [28]. There is no literature available on high temperature thermoelectric properties of YB44Si2 single crystals. Regarding ani3
sotropy, T. Mori has found magnetic anisotropy to be indicated in Tb11B44Si2 and YbB45.6Si1.0 single crystals but with no information obtainable on crystal orientations [32,36].In this manuscript, we report high temperature thermoelectric properties of YB41Si1.3 single crystals grown by floating zone method. For the first time, we have determined crystallographic orientations of a borosilicide crystal and discovered the relatively large anisotropy in electrical properties along the determined directions.
2 Experimental details Floating zone (FZ) crystal growth was employed to grow yttrium borosilicide YB41Si1.3 single crystals using a four-mirror-type infrared image furnace (Crystal System Inc., FZ-T-10000-HIII-VPR) equipped with four 2.5 kW xenon lamps as heat source. The preparation process of the polycrystalline feed rods for FZ crystal growth is as follows; we mixed YB 4 (New Metals Co. 99%), B (SB Boron 99.9%),and Si(Wako 99.9%) powders to obtain a desired final composition, then, pressed it into a rod at a hydrostatic pressure of 300 MPa. Rods having nominal composition YB44Si2 were prepared for both the feed rod and the seed rod to grow crystals by the FZ method. The pressed rod was reacted in a boron nitride (BN) crucible fixed inside a graphite susceptor that was further covered with graphite wool for induction heating. The synthesis was carried out in an RF inductive furnace at 1400 °C for 8 h in vacuum. To obtain high density feed rods, the synthesized rods were grinded and formed again into rods and sintered at the same condition. FZ crystal growth was carried out by driving downward both feed and seed rods at 10 and 8mm/h, respectively, with counter-rotating at 16 rpm under Ar atmosphere (2.5 L/min). The grown crystals were characterized by high-resolution powder X-ray diffraction (XRD) and electron probe microanalysis (EPMA).Powder XRD measurements with CuK radiations 4
(Rigaku Ultima-3) were performed to confirm the required phase formation in the grown crystals, where parts of the crystals were crushed using a stainless steel mortar and then the obtained powder was washed with HCl solution and rinsed with water to remove stainless steel contamination. Rietveld refinement was performed using FullProf Suite software (2.05).Peak shape was refined with the modified Lorentzian function and 6 coefficients polynomial function was used for background refinement. EPMA was carried out in wavelength-dispersive mode using JEOL JXA-8200 instrument. Standard sample used for Y and B were Y3Al15O12 and LaB6, respectively. The crystallographic orientation of the YB41Si1.3 grown crystals were characterized by pole figure measurements using an X-ray 2D-detector (Bruker Corp., model D8 DISCOVER VANTEC-500). We took two cross-sections, one was approximately perpendicular to growth direction and another one was parallel to growth direction of the grown crystal, for pole figure measurement. We cut the crystal using a diamond wire cutter and then polished the pieces with a diamond solution. The crystallographic orientations were determined from the obtained pole figure data by Multex 3 software. For high temperature thermoelectric materials the stability at high temperature is essential. Therefore, to check the stability of the grown crystals at high temperature, we also carried out thermogravimetric analysis (TGA, Rigaku, model Thermo plus TG 8120) from 300 K to 1080 K in Ar flow as well as in air flow. Resistivity and Seebeck coefficient were measured with an ULVAC ZEM-2 in the temperature range of 330–1000 K in He atmosphere. To determine the thermal conductivity values, first of all, the room temperature specific heat was measured by using PPMS (Quantum Design, physical property measurement system). Then, the relative specific heat and thermal diffusivity coefficient were measured by laser flash method (ULVAC
5
TC-7000) from 300 K to 1080 K. The thermal conductivity is determined as the product of the density, specific heat, and thermal diffusivity coefficient.
3 Results and Discussions 3.1
Crystal growth and characterizations
The boron-rich boride compound YB50 which has an orthorhombic crystal structure starts to decompose above 2100 K into phases like YB 12 and YB66 without melting [38]. Tanaka et al. first demonstrated that the melt growth method is applicable to grow an yttrium borosilicide crystal, YB44Si1.0 that is iso-structural to YB50 by adding a small amount of silicon [26,27]. Another interesting Si addition effect was the enlargement of the lattice parameters which enabled synthesis of GdB44Si2, although GdB50 does not form due to the relatively large size of Gd [39]. A view of the crystal structure of REB44Si2 is shown in Fig. 1. The structure is orthorhombic with space group Pbam. Fig.2 shows the picture of a grown crystal. To grow high quality crystals, we used the zone pass technique. The first zone pass was made to obtain uniform composition and the second zone pass was made to grow the crystal.
B12 B12Si3 RE
6
Fig. 1.View of the crystal structure of REB44Si2 from a direction slightly tilted along [001] as indicated by the labels, two kinds of polyhedra are shown: B 12 icosahedra and B12Si3 polyhedra. Only two of the five structurally independent B 12 icosahedra are drawn for clarity. The circles indicate rare-earth (in this case, yttrium) atoms which are aligned along [001] in a ladder-like configuration.
Fig. 2.YB41Si1.3 single crystal grown by the floating zone method EPMA results revealed the atomic concentration of Y, B and Si to be 2.3±0.2 at%, 94.4±0.4 at% and 3.1±0.2 at%, respectively. The composition of the grown crystal was determined to be YB41Si1.3. Fig.3 shows powder X-ray diffraction pattern with Rietveld refinement of the grown crystal.
7
Fig. 3.Rietveld refinement of powder XRD pattern of YB41Si1.3 single crystal, exhibiting good agreement between the observed and the calculated patterns. Rietveld refinement yielded 1.3(0.1) vol. % of YB6 phase.
Rietveld refinement for the YB41Si1.3 crystal data confirmed YB44Si2 structure type with RF=0.019 and RB = 0.031, and a very little amount of YB6 phase was found (2nd row of black Bragg-position lines in Fig. 3).The lattice parameters were obtained to be a = 16.6368 Å, b = 17.5846 Å, and c= 9.5079 Å, which are quite similar with YB44Si1.0 [26]. Fig. 4 (a, b) represents the SEM images ofYB41Si1.3 grown crystal for cross-sections parallel to the growth direction and perpendicular to the growth direction and it was observed that in both cases there were no grain boundaries but some micro-cracks. Apparently, it is not possible to avoid microcracks in grown single crystals of this borosilicide phase. The grown crystals also contained a very small amount of inclusion (white spots) and we also confirmed by EDX that this inclusion is YB6. Amount of YB6 phase obtained from the Rietveld refinement was consistent with the amount of this phase observed in SEM images. In order to determine the growth direction of the YB 41Si1.3 grown crystal, we also took Laue XRD, however, due to the complicated structure and the weakness of the reflections, we were not able to determine the particular alignment of the crystals by Laue photography techniques.
8
Fig. 4. SEM images for cross-sections of YB41Si1.3 single crystal along: (a) parallel to growth direction and (b) perpendicular to growth direction
Figs. 5(a-d) represent the measured and simulated pole figures, from the measurements using an X-ray 2D-detector,of the cross sections perpendicular to growth direction (a, b) and parallel to the growth direction (c, d) for YB41Si1.3 grown crystal. As shown in Figs. 1(a) and (b), the orientation of the cross-section perpendicular to growth direction is 11° tilted from [100] and toward [010]. By following the same procedure, the orientation of the cross section parallel to the growth direction was estimated to be 38° tilted from [010] toward [001]. The orientations of the cross-sections perpendicular to the growth direction and parallel to the growth direction were estimated by Q-Laue to be [510] and [052], respectively. The obtained directions [510] and [052] were not orthogonal and the angle between them was 81.4 degrees. This lack of orthogonality is likely due to cutting the cross-sections of YB41Si1.3 grown crystal at a slightly tilted position with respect to the cross-section of the perpendicular to growth direction.
b
a
Psi=11°
Phi=14°
c
d
9
Fig.5. Measured and simulated pole figures of the cross sections perpendicular to growth direction (a, b) and parallel to the growth direction (c, d) for YB41Si1.3 crystal. The direction of rotation (Phi) was rotated 14°and 59°.
a
b 10
Fig. 6. Views of the crystal structure along: (a) [510] (the growth direction) and (b) [052] of YB41Si1.3. Dark blue represents Y, green represent Si and red indicates B atoms. Determination of crystallographic orientation is the most important part prior to study directional physical properties of a crystal. Similar to yttrium, rare-earth metals from Gd to Lu can form REB41Si1.2-type borides. To our knowledge, nobody has reported about the orientation of these borosilicide single crystals and we have determined the crystallographic orientation in YB41Si1.3 for the first time. Obviously, the orientation in YB41Si1.3 would be greatly helpful to explore directional properties of REB41Si1.2 borides. Figs. 6 (a) and (b) show views of the crystal structure along [510] (the growth direction) and [052] of YB41Si1.3. The growth direction shows an interesting structure feature in that an almost layered structure is revealed, with dense boron cluster layers separated by the rare earth atom layers. We also investigated the thermal stability of the borosilicide single crystal. Fig.7 shows TGA data of YB41Si1.3 crystal. We performed Thermogravimetric analysis (TGA) in Ar flow as well as in air flow and in both cases, two different small pieces of the crystal were used. TGA data in Fig. 7 (a) reveals that there was no 11
change in weight during heating in Ar flow. However, in the case of air flow, the crystal was oxidized slightly as shown in Fig. 7(b) and the weight increased by 0.3% up to 1073K. Therefore, TGA data implies that at higher temperature, the grown crystal was excellently stable in Ar flow and moderately stable in air flow.
Fig.7 (a) Thermogravimetric analysis (TGA) of YB41Si1.3 single crystal in Ar and air flow.
Fig. 7 (b) EDX mapping of the YB41Si1.3 single crystal used for TGA in air flow showing the oxidized surface and inside of the crystal.
3.2 Thermoelectric properties We measured the thermoelectric properties of the YB41Si1.3 single crystal along the two determined directions [510] (the growth direction) and [052], and compared them with the properties 12
of arc-melted YB44Si2 [4]. The electrical resistivity data are plotted in Fig. 8 as the logarithm of the resistivity against
. The electrical resistivity of many boron-rich borides exhibit Mott’s
variable range hopping mechanism for three dimensional systems where the resistivity ρ follows: ,
(1)
WithT0 being the so-called characteristic temperature [11,40]. The resistivity for YB41Si1.3 along [510] and [052] generally appear to have linear dependence with T-0.25 (K-0.25).
Fig.8. Temperature dependence of electrical resistivity of YB41Si1.3 single crystals along: (I) [510] and (II) [052] orientations. Red triangles represent the resistivity of arc-melted YB44Si2 [4].
By fitting equation (1), we obtained the values of T0 along [510] and [052] to be 7.4×105 K and 9.1×105 K, respectively. The smaller value of T0 along [510] compared to [052] indicates that it is less localized or has higher density of states at the Fermi level along [510] [29]. In general the values for T0 for YB41Si1.3 crystal are comparable with 6×105 K, 1×106 K and 5×106 K for TbB44Si2, ErB44Si2, and YbB44Si2, respectively [9,12]. At 330 K, the resistivity along [510] and 13
[052] directions were 1.6×10-2 m and 13.2×10-2 m respectively, i.e. the resistivity along the [510] direction was 8 times lower than that along [052] direction. These differences in resistivity signify that YB41Si1.3 crystal is strongly anisotropic with the electrical conductivity larger along [510]. As noted above (Fig. 6(a)), an interesting layered structural feature is revealed along [510] with dense boron cluster layers and yttrium layers. We can speculate that the enhancement may be due to good conduction along the metal layers, which is quite interesting and an unusual structural feature for higher borides. These resistivity values in general are lower than reported for the RB66 compounds [12], for example, and indicate the suitability of YB41Si1.3 among the higher borides as thermoelectric materials. The micro-cracks undoubtedly make the measured values of resistivity higher, and if crystal growth techniques to remove these up-to-now unavoidable cracks can be realized, the thermoelectric properties can assumedly be further improved, since such cracks (with no special design for selective phonon scattering) typically more strongly affect electrical conductivity compared to thermal conductivity. This higher electrical conduction along the metal layers found in YB41Si1.3 is intuitively understandable, but interestingly in contrast to what was observed recently for the non-cluster 2D-boron-net compound AlB2 [10]. AlB2 has a prominently layered structure composed of graphene-like 2D-boron-net layers sandwiching Al atom layers. Quite different from graphitebased materials, the thermal conductivity of AlB2 (and assumedly electrical conductivity because it is a good metal) took higher values in the cross-plane direction compared to the inplane direction [10]. This can be assumed to be derived from the unique bonding of boron compounds, and further analysis of the origin of the anisotropy should be closely coupled with theoretical bonding analysis of these compounds. The Seebeck coefficients , of YB41Si1.3 are plotted in Fig. 9. They exhibit positive values and increase with increase in temperature. Throughout the whole range of measurement tempera14
ture, a large difference in Seebeck coefficient was observed between [510] and [052] directions. The Seebeck coefficient along [052] was higher than [510]. This agrees with the orientation dependence, we observed in the electrical resistivity. The values of Seebeck coefficients of the YB41Si1.3 single crystal along both [510] and [052] were significantly higher than that of arc-melted YB44Si2 sample, and showed a larger plateau behavior (i.e. less temperature dependence) over a wide range of temperature. This behavior is beneficial for high temperature thermoelectric materials, since in applications, large temperature differences will be applied to the material and the average performance over a wide temperature range is important.
Fig.9. Temperature dependence of Seebeck coefficient of YB 41Si1.3 single crystals along: (I) [510] and (II) [052]. Red triangles represent Seebeck coefficient of arc-melted YB44Si2 sample.
Fig.10 demonstrates the temperature-dependent power factor of YB41Si1.3. The power factors along [510] of FZ crystal are larger compared to that along [052] of FZ crystal and arcmelted sample, but the gap in performance widens towards higher temperatures. This is related to the plateau-like behavior noted for the Seebeck coefficient, and is especially apparent for the power factor along [510], which exhibits a good rise above 700 K. At 1000 K, the power
15
factor of YB41Si1.3 single crystal along [510] was 13.8
Wm-1K-2, which is 3.6 and 1.8
times higher than that along [052] and the arc-melted YB44Si2 sample, respectively. To gauge the performance of thermoelectric application of compounds, it is necessary to determine the dimensionless figure of merit ZT=PT/k, where k is thermal conductivity. We measured the thermal conductivity of YB41Si1.3 single crystal (Fig. 11) along [510] only due to considerably high power factor in this direction.
Fig.10. Temperature dependence of power factor of YB41Si1.3 single crystals along: (I) [510] and (II) [052]. Red triangles represent power factor of the arc-melted YB44Si2 sample.
16
Fig.11. Temperature dependence of thermal conductivity of YB41Si1.3 single crystals along [510] (pink squares). Red diamond and green circles represent thermal conductivity for YbB44Si2 and ErB44Si2 crystals, respectively.
The thermal conductivity of YB41Si1.3 took values from 3.7 to 4.4 Wm-1K-1 in the temperature range. We also compared the measured thermal conductivity of YB 41Si1.3 crystal with those of YbB44Si2 and ErB44Si2 FZ crystals as shown in Fig.11. The magnitude of thermal conductivity was highest for YB41Si1.3 because Y is lighter than Er and Yb and hence there is reduced phonon scattering. Fig.12 shows the temperature dependence of ZT value of YB41Si1.3 single crystals along [510] and the maximum value 0.033 was found at 990 K. Although, this value is not high compared to values such as ZT~0.8 for SiGe at 900K [41], the experimental results discussed in this article were obtained for an unmodified yttrium borosilicide.
Fig. 12. Temperature dependence of ZT value of YB41Si1.3 single crystals along [510] direction.
As mentioned, the grown crystal contains micro-cracks and by optimizing the growth condition, the quality of the grown crystal could be improved and the thermoelectric performance assumedly enhanced. Doping with transition metals is another way to more dramatically modify the properties of REB44Si2 [30]. A beneficial hybrid effect was previously obtained for yttrium boro17
carbonitride with a combination of transition metal doping and heat treatment. Seebeck coefficients, α, of YB22C2N, n-type counterpart to boron carbide, could be increased by up to 220% while resistivity was reduced by x100 [42-44]. In addition to applying such doping effects, we aim to clarify the anisotropy effect in this work, to further develop the borosilicides, which are one of the few thermoelectric materials which can be used at high temperatures such as 1000 o
C and above.
4. Conclusions We have grown YB41Si1.3 single crystals by the floating zone method. To our knowledge, there has been no report on the determination of crystallographic orientation and electrical anisotropy in REB44Si2 crystals. For the first time, we have determined crystallographic orientation and also discovered large anisotropy in the thermoelectric properties of YB41Si1.3 single crystals. The electrical conductivity was significantly higher along [510] (growth direction), a maximum of more than 8 times compared to [052], where it was revealed that the crystal structure has a layered character, with dense boron cluster and metal layers. It can be speculated that conduction is enhanced in the metal layers. This is intuitively understandable, but in contrast to the anisotropic behavior observed recently for the non-cluster 2D-boron-net compound AlB2. These behaviors can be assumed to be derived from the unique bonding of boron compounds, and further theoretical insight including bonding analysis should be of use to quantitatively evaluate the origin of the anisotropy discovered in the YB41Si1.3 single crystal. We also measured the thermal conductivity of YB41Si1.3 single crystal along [510] and then estimated figure of merit to be 0.033, which is not high, but acceptable as a starting point in the development. Future efforts will be made to modify the borosilicides utilizing beneficial doping effects which have been found for other boride systems. Furthermore, this is the first report of 18
existence of such a strong anisotropy in the thermoelectric properties of a boride, and such anisotropy should be investigated in other prospective boride systems also. Thereby, we hope to improve the suitability of this borosilicilde and other borides as possible high temperature thermoelectric materials, one of the few materials systems which can be used at 1000 ºC and higher temperatures.
Acknowledgements The authors are grateful to Dr. Hitoshi Morioka of Bruker AXS K.K. for help of pole figure analysis. This work was partly supported by Green Energy Conversion Science and Technology program, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Japan.
References [1]
Thermoelectric Nanomaterials, ed. K. Koumoto and T. Mori, Springer Series in Materials Science (Springer, Heidelberg, 2013) pp. 1-373.).
[2]
Li-Dong Zhao, Shih-Han Lo, Yongsheng Zhang, Hui Sun, Gangjian Tan, CtiradUher, C. Wolverton, Vinayak P. Dravid&Mercouri G. Kanatzidis, Nature, 508, 373-377 (2014).
[3]
Wood, C.; Emin, D. Phys. Rev. B 1984, 29, 4582–4587.
[4]
Mori, T.; Berthebaud, D.; Nishimura, T.; Nomura, A.; Shishido, T.; Nakajima, K. Dalton Trans.2010, 39, 1027–1030.
[5]
Mori, T. in: Modules, Systems, and Applications in Thermoelectrics, ed. Rowe, D. M, CRC Press Taylor and Francis, London 2012, 14.
[6]
Slack, G. A.; Oliver, D. W.; Horn,F. H. Phys. Rev. B 1971, 4, 1714–1720.
[7]
Cahill,D. G.; Fischer, H. E.; Watson, S. K.; Pohl, R. O.; Slack, G. A. Phys. Rev. B 1989, 40, 3254–3260.
[8]
Mori, T. Physica B 2006, 383, 120–121.
[9]
Mori,T.; Martin,J.; Nolas,G. J. Appl. Phys.2007,102, 073510.
[10]
Wang,X. J.; Mori,T.; Kuzmych-Ianchuk,I.; Michiue,Y.; Yubuta,K.; Shishido,T.; Grin,Y.; Okada,S.; Cahill,D. G. APL Mater. 2014, 2, 046113. 19
[11]
Mott,N. F. J. Non-Cryst. Solids1968,1, 1–17.
[12]
[13]
Mori,T. “Higher Borides” in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner Jr., J. -C. Bunzli, and V. Pecharsky (North-Holland, Amsterdam)2008, vol 38 p. 105-173. Mori,T. J. Phys. Conf. Ser. 2009, 176, 012036.
[14]
Mori, T.; and Nishimura, T. J. Solid State Chem., 2006, 179, 2908-2915.
[15]
Balaz, S.; Dimov, D. I.; Boag, N. M.; Nelson, K.; Montag, B.; Brand, J. I.; Dowben, P. A. Appl. Phys. A 2006, 84, 149–159.
[16]
Gong, Y.; Zhang, Y.; Dudley, M.; Zhang, Y.; Edgar,J. H.; Heard, P. J.; Kuball,M., J. Appl. Phys. 2010, 108, 084906.
[17]
Maruyama, S.; Miyazaki, Y.; Hayashi, K.; Kajitani, T.; Mori, T. Appl. Phys. Lett. 2012, 101, 152101.
[18]
Sologub, O.; Matsushita, Y.; Mori, T. Scr. Mater. 2013, 68, 289–292.
[19]
Frye, C.D.; Edgar, J.H.; Ohkubo, I.; Mori, T. J. Phys. Soc. Jpn. 2013, 82, 095001.
[20]
Wan, L. F.; Beckman, S. P. Phys. B Condens. Matter 2014, 438, 9–12.
[21]
Miura, S.; Sasaki, H.; Takagi, K.;Fujima, T. J. Phys. Chem. Solids,2014, 75, 951-953.
[22]
Mori, T.; Nishimura,T.; Schnelle, W.; Burkhardt, U.; Grin, Y. Dalton Trans.,2014, 43, 15048-15054.
[23]
Slack, G. A.; Morgan, K. E. J. Phys. Chem. Solids 2014, 75, 1054–1074.
[24]
Gürsoy, M,; Takeda, M,; Albert, B. J. Solid State Chem.2015, 221, 191–195.
[25]
Tanaka,T.; Okada,S.; Ishizawa,Y.J. Alloys Comp. 1994, 205, 281-284.
[26]
Tanaka,T.; Okada,S.; Ishizawa,Y. J. Solid State Chem. 1997, 133, 55–58.
[27]
Higashi,I.; Tanaka,T.; Kobayashi,K.; Ishizawa,Y.; Takami,M.J. Solid State Chem. 1997, 133, 11–15.
[28]
Ishizawa,Y.; Tanaka,T. J. Solid State Chem. 2000, 154, 229-231.
[29]
Mori,T.J. Appl. Phys. 2005, 97, 093703.
[30]
Berthebaud, D.; Sato, A.; Michiue, Y.; Mori, T.; Nomura, A.; Shishido, T.; Nakajima, K. J. Solid State Chem. 2011, 184, 1682–1687.
[31]
Mori, T.;Shishido, T.; Nakajima, K. J. Electron. Mater. 2009, 38, 1098-1103.
[32]
Mori,T. Z. Kristallogr. 2006, 221, 464–471.
[33]
Mori,T.; Tanaka,T. J. Solid State Chem. 2000, 154, 223-228.
[34]
Mori,T.; Tanaka, T.J. Alloys and Comp. 1999, 288, 32-35. 20
[35]
Mori, T.; Tanaka, T. IEEE Trans. Magn. 2001, 37, 2144-2146.
[36]
Mori, T.; Tanaka, T. J. Alloys and Comp.2003, 348, 203–207.
[37]
Yuan, J.; Zhang, H.; Tang, J.; Shinya, N.; Lin,Y.; Lu-Chang Qin, J. Mater Sci 2013, 48, 1555–1561.
[38]
Tanaka,T.; Okada,S.; Ishizawa,Y. J. Alloys Compd. 1994, 205, 281–284.
[39]
Mori, T.; Tanaka, T.; Mat. Res. Bull., 2001, 36 2463-2470.
[40]
Electron-electron interactions in disordered systems,edited by A. L. EfrosandM. Pollak, (North-Holland, Amsterdam, 1985), P-409.
[41]
CRC Handbook of Thermoelectrics, ed. D. M. Rowe (CRC, Boca Raton, 1995).
[42]
Prytuliak, A.; Mori, T.; Journal of Electronic Materials, 2011, 40, 920-925.
[43]
Prytuliak, A.; Maruyama, S.; Mori, T. Materials Research Bulletin, 2013, 48, 1972-1977.
[44]
Mori, T.; Hara, T.; Scripta Materialia, 2015, in press.
Highlights:
1. We have grown YB41Si1.3 single crystals by the floating zone method. 2. We determined the growth direction for the first time in a REB41Si1.2 borosilicide, which was [510], and revealed an interesting layered structural feature with dense boron cluster and ytrrium layers. 3. The electrical resistivity was strongly anisotropic between [510] and [052] orientations, with conductivity speculated to be enhanced by conduction along metal layers. 4. The obtained power factor along [510] is 3.6 times higher than that along [052].
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