Temperature Gradient Solution Growth and Seebeck Coefficient of β-FeSi2 Bulk Material. Anisotropy and Self-similar Fractal Structure in the Thermoelectric Crystals J.M. Redondo 1 , M. Kuramitsu 1,2 , M. Takeda 2 , J.M. Sanchez 1,3 1 Dept. Fisica aplica, UPC, Barcelona Tech. B5 Campus Nord 08034 Barcelona, Spain. 2 Department of Mechanical Engineering, Nagaoka University of Technology 1603-1 Kamitomiokamachi Nagaoka, Niigata 940-2188, Japan 2 BEROTZA, Pol. Noain-Ezkirotz, Nave 1, 31110 Noain, Navarra , Spain E-mail: redondo@fa.upc.edu, kuramitu@stn.nagaokaut.ac.jp Abstract We study the complex crystal structure of β-FeSi2 crystal growth material in order to understand and to improve the thermoelectric properties. Anisotropy in the thermoelectric properties of β-FeSi2 is expected because of its compli- cated and highly anisotropic crystal structure. To examine the anisotropy and scaling of Seebeck coefficient of the β- FeSi2, single crystals were grown during different times using the temperature gradient solution growth method with a Ga solvent. Laue back reflection photographs taken from the specimens showed diffraction patterns, indicating high quality β-FeSi2 single crystals and the crystal orientation was determined by the Laue patterns. To improve the meas- urements of the Seebeck coefficient, we developed at the Takeda Lab.an apparatus that is capable of measuring the Seebeck coefficient of a small specimen down to a few mm in size. With the apparatus compared also with PASCO analyzer and micrography we found a strong anisotropy of Seebeck coefficient of the β-FeSi2 and also compared the different fractal dimensions for the samples analyzed. INTRODUCTION Semiconducting iron di-silicide, β-FeSi2, has received attention in recent years from both eco- logical and environmental points of view because of the abundance of resources and its non-toxicity and high recyclability. One of the potential application fields for the β-FeSi2 is thermoelectric con- version of energy at high temperature, which enables to recover electrical energy from waste heat. The β-FeSi2 has several interesting features as a thermoelectric material, mainly a large Seebeck coefficient whose polarity can be controlled by doping, low costs of its constituent elements, and a good stability in air at high temperature. Although the β-FeSi2 has these advantages in practical use, its moderate thermoelectric performance so far limits its application as sensors. Other Bi-Sb, Bi2Te3 and Sb2Te3 based thermoelectric materials, which are currently used for the Peltier cooling devices, have an even larger anisotropy in their thermoelectric properties because of their complex crystal structure. [1-3] The β-FeSi2 is an orthorhombic phase with lattice parameters of a=0.9863 nm, b=0.7791 nm, and c=0.7833 nm. [4] The unit cell contains 48 atoms, and the structure can be seen as an alternative stacking of Si and Fe layers along the crystallographic a-axis. Anisotropy in its thermoelectric properties is therefore expected. However, experimental results are not conclu- sive, because most of the studies on the thermoelectric properties of the β-FeSi2 have been carried out in polycrystalline specimens prepared by conventional solidification or sintering. Recently, β-FeSi2 bulk single crystals were grown by temperature gradient solution growth method using Ga, Zn or Sb as a solvent. The crystal structure of β-FeSi2 is a source of anisotropy in the thermoelectric properties of β-FeSi2, which is expected because of its complicated and highly anisotropic crystal structure. To examine the anisotropy of Seebeck coefficient of the β-FeSi2, sin- gle crystals were grown by temperature gradient solution growth method using Ga solvent. Laue back reflection photographs taken from the specimens showed clear diffraction patterns, indicating high quality β-FeSi2 single crystals without twins. The crystal orientation of the single crystals was determined by the Laue pattern. For measurements of the Seebeck coefficient, we developed an ap- paratus that is capable of measuring the Seebeck coefficient of a small specimen of a few mm in size. With the apparatus, we found the anisotropy of Seebeck coefficient of the β-FeSi2. The under- standing of the strongly directional properties of certain thermoelectric materials needs further un- derstanding of the subtle carrier and thermal electron paths that take place in complex natural and custom made materials. The way in which the material structure is built sometimes leads to strong