Enhancement of Thermoelectric Behavior of La 0.5 Co 4 Sb 12x Te x Skutterudite Materials SUHANA MOHD SAID, MOHAMED BASHIR ALI BASHIR, MOHD FAIZUL MOHD SABRI, YUZURU MIYAZAKI, DHAFER ABDUL AMEER SHNAWAH, ABBAS SAEED HAKEEM, MASANORI SHIMADA, AKOLADE IDRIS BAKARE, NIK NAZRI NIK GHAZALI, and MOHAMED HAMID ELSHEIKH In this work, the effects of Te doping on the microstructure and thermoelectric properties of the partially filled skutterudite La 0.5 Co 4 Sb 12 compounds have been examined. La 0.5 Co 4 Sb 12x Te x skutterudite compounds were synthesized by a combination of the mechanical alloying technique and spark plasma sintering processing, which resulted in partial substitution of Te atoms in Sb sites. The XRD results showed that all the Te-doped bulk samples were composed of a major phase of the Co 4 Sb 12 skutterudite with a small amount of CoSb 2 and Sb as the secondary phases. Thermoelectric measurements of the consolidated samples were examined in a temperature range of 300 K to 800 K (27 °C to 527 °C). With the La 0.5 Co 4 Sb 11.7 Te 0.3 sample, the highest absolute Seebeck coefficient of 300 lV/K was obtained at 404 K (131 °C) and the lowest lattice thermal conductivity of 2 W/mK was achieved at 501 K (228 °C). Moreover, the minimum electrical resistivity of 19.7 lXm was recorded at 501 K (228 °C) for La 0.5 Co 4 Sb 11.5 Te 0.5 sample. The effect of the secondary phases was negligible for the electrical resistivity, and between 0.5 to 1.6 pct for the thermal conductivity. Thus, the highest figure of merit, ZT = 0.47, was obtained at 792 K (519 °C) for La 0.5 Co 4 Sb 11.5 Te 0.5 sample due to a significant reduction in electrical resistivity and a moderate increase in the absolute Seebeck coefficient. DOI: 10.1007/s11661-017-4058-1 Ó The Minerals, Metals & Materials Society and ASM International 2017 I. INTRODUCTION RECENTLY, thermoelectric generation (TEG) sys- tems that directly convert a temperature gradient into electricity has gained a lot of interest due to advances in materials development. Families of chalcogenides, skut- terudites, and the introduction of low-dimensional structures such as quantum dots and nanowires have contributed toward the improvement of the thermoelec- tric figure of merit. [1–4] The performance of TEG is determined by a dimensionless figure of merit ZT: [5] ZT ¼ S 2 rT  ðkÞ ; ½1 where T is the absolute temperature, S is the Seebeck coefficient, r is the electrical conductivity, and k is the thermal conductivity. Consequently, high Seebeck coef- ficient and high electrical conductivity with very low thermal conductivity have been desired for a good thermoelectric material. A skutterudite material possesses the general formula MX 3 , where M is (Co, Ni, Fe) and X is (P, Sb, As) which possess a cubic structure of the space group Im3. In its unfilled state, the skutterudite crystal structure is akin to a series of cubic ‘cages,’ which possess voids that can be filled with heavy atoms. However, the unfilled CoSb 3 binary skutterudite system is not favorable in thermo- electric applications due to its very low carrier concen- tration and very high electrical resistivity and thermal conductivity. [6] Filling the voids in the skutterudite crystal lattice results in a manifestation of the Phonon Glass Electron Crystal concept (PGEC), where good electrical conductivity can be enabled along the thermal lattice, while thermal conductivity is attenuated due to the presence of the filler atoms in the skutterudite voids. This strategy thus enables a potentially high thermo- electric figure of merit. [7] A filled Skutterudite material is considered as a promising candidate for thermoelectric power generation application in the middle range of temperatures [300 K to 800 K (27 °C to 527 °C )]. It also SUHANA MOHD SAID is with the Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. MOHAMED BASHIR ALI BASHIR, MOHD FAIZUL MOHD SABRI, DHAFER ABDUL AMEER SHNAWAH, and NIK NAZRI NIK GHAZALI are with the Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. Contact e-mail: faizul@um.edu.my YUZURU MIYAZAKI and MASANORI SHIMADA are with the Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. ABBAS SAEED HAKEEM and AKOLADE IDRIS BAKARE are with the Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Kingdom of Saudi Arabia. MOHAMED HAMID ELSHEIKH is with the Department of Mechanical Engineering, University of Bahri, 13104 Khartoum, Sudan. Manuscript submitted July 30, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A