STRUCTURAL, ELECTRICAL PROPERTIES AND APPLICATIONS OF Bi2Se3xTe3(1−x), Bi2xSb2(1−x)Te3, AND Bi0.4Sb1.6Se3xTe3(1−x) THERMOELECTRIC BULK MATERIALS GROWN USING SOLID STATE MICROWAVE SYNTHESIS

 

ABSTRACT

This thesis aims to improve the output power (Pmax) of the thermoelectric generation module (TEG) through cost reduction. The preparation of Bi2Se3xTe3(1−x), Bi2xSb2(1−x)Te3 and Bi0.4Sb1.6Se3xTe3(1−x) bulk thermoelectric materials (ingots) (x for each formulae = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) via a faster, simpler, and more economically feasible method called the solid state microwave synthesis is presented. This thesis also focuses on the structural and electrical characterization of the three sets after being cold pressed into disk form. The structural characterizations of the resulting samples are measured using several techniques such as scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and X-ray diffraction (XRD). The electrical characterizations include electrical conductivity (σ), the Hall effect, and the Seebeck coefficient (S) at temperatures ranging from 300 K to 523 K, and the voltage‒current (V-I) characteristics of the TEG modules. The influence of x on the surface morphology, elemental compositions, and lattice constants of the Bi2Se3xTe3(1−x), Bi2xSb2(1−x)Te3, and Bi0.4Sb1.6Se3xTe3(1−x) samples was studied. Furthermore, the alloying effects on σ, carrier concentration (p), carrier mobility (), S, and the power factor (S2σ) were thoroughly investigated. The Se‒substitution for Te adjusts p to obtain an optimum value of 1.25 × 1020 cm-3 for the Bi2Se3xTe3(1−x) samples (x=0.2). The maximum S2σ is 2.85 mW/mK2 at 403 K based on the increased value of S for the Bi2Se0.6Te2.4 sample (176.3 μV/K). Increasing the Bi‒substitution for Sb in the Bi2xSb2(1−x)Te3 samples caused a decrease in p and σ and an increase in S. A maximum S2σ value of 4.96 mW/mK2 was obtained at 373 K for the Bi0.4Sb1.6Te3 sample. Novel hexagonal rods were obtained based on the Bi0.4Sb1.6Te3 ingots and dispersed with Se in the Bi0.4Sb1.6Se3xTe3(1−x) samples. The alloying of Se caused a decrease in p and σ and an increase in S. The maximum S2σ measured was 7.47 mW/mK2 at 373 K for the Bi0.4Sb1.6Se2.4Te0.6 samples. Given the three sets, the highest S2σ was obtained for the p-type Bi0.4Sb1.6Se2.4Te0.6, Bi0.4Sb1.6Te3, and Bi2Se0.6Te2.4 samples. About 70% of the Te element was excised in the Bi2Se0.6Te2.4 composition as an n-type alloy. Two 9 p–n couples TEG modules were fabricated. The first module (D1) was based on the p–Bi0.4Sb1.6Te3 and n–Bi2Se0.6Te2.4 samples, whereas the second module (D2) was based on the p–Bi0.4Sb1.6Se2.4Te0.6 and n-Bi2Se0.6Te2.4 samples to determine the module with the highest Pmax. The D2 module could generate a Pmax (88.49 mW) larger than that of the D1 module (39.4 mW) at a hot-side temperature TH of 523 K and at a temperature difference ΔT of 184 K. As a result, 18 couples TEG module (D3) based on the p–Bi0.4Sb1.6Se2.4Te0.6 and the n–Bi2Se0.6Te2.4 samples were fabricated. This module generated a Pmax of up to 273.2 mW, which was larger than those of the D1 and D2 modules in the same thermal conditions.

SIFAT STRUKTUR, ELEKTRIK, DAN PENGGUNAAN Bi2Se3xTe3(1-x), Bi2xSb2(1-x)Te3 DAN Bi0.4Sb1.6Se3xTe3(1-x) BAHAN PUKAL TERMOELEKTRIK DITUMBUH MENGGUNAKAN SINTESIS GELOMBANG MIKRO KEADAAN PEPEJAL

ABSTRAK

Atas tujuan menambahbaik kuasa keluaran (Pmax) oleh modul penjana termoelektrik (TEG) menggunakan strategi pengurangan kos, tesis ini mempersembahkan penyediaan ternari Bi2Se3xTe3(1-x) dan Bi2xSb2(1-x)Te3, dan kuaternariBi0.4Sb1.6Se3xTe3(1-x) bahan pukal termoelektrik (jongkong) (x bagi setiap formula = 0.0, 0.2, 0.4, 0.6, 0.8 dan 1.0) menggunakan satu kaedah penumbuhan yang pantas, mudah dan lebih berekonomi, yang dikenali sebagai sintesis gelombang mikro keadaan pepejal. Tesis ini juga memberi penumpuan kepada pencirian struktur dan elektrik bagi tiga set bahan di atas selepas serbuk mereka ditekan dingin menjadi bentuk cakera. Pencirian struktur sampel yang terbentuk diukur menggunakan beberapa peralatan, yang merangkumi mikroskop imbasan elektron (SEM), analisa sebaran tenaga sinar-X (EDX) dan pembelauan sinar-X (XRD). Pencirian elektrik merangkumi kekonduksian elektrik (σ), kesan Hall dan pekali Seebeck (S) dalam julat suhu 303‒523 K, dan pencirian voltan‒arus (V-I) bagi modul TEG yang difabrikasikan. Pengaruh x di dalam sampel Bi2Se3xTe3(1-x), Bi2xSb2(1-x)Te3 dan Bi0.4Sb1.6Se3xTe3(1-x) ke atas morfologi permukaan, komposisi unsur dan pemalar kekisi telah disiasat. Kesan pengaloian ke atas σ, kepekatan pembawa (p), kelincahan pembawa (µH), S dan faktor kuasa (S2σ) telah juga disiasat secara mendalam. Bagi sampel Bi2Se3xTe3(1-x), penggantian Se bagi Te telah melaraskan kepekatan p ke satu nilai optimum 1.25×1020 cm-3 bagi x = 0.2. Nilai maksimum S2σ yang dicapai ialah 2.85 mW/mK2 pada 403 K berasaskan peningkatan S bagi sampel Bi2Se0.6Te2.4 (176.3 µV/K) . Meningkatkan penggantian Bi bagi Sb di dalam sampel Bi2xSb2(1-x)Te3 menyebabkan pengurangan p dan σ, dan peningkatan S. Satu S2σ maksimum bernilai 4.96 mW/mK2 diperolehi pada 373K bagi sampel Bi2Se0.6Te2.4. Rod heksagonal baru telah dicerap pada permukaan jongkong Bi0.4Sb1.6Te3 dan tersebar dengan peningkatan kandungan Se di dalam sampel Bi0.4Sb1.6Se3xTe3(1-x). Pengaloian Se menyebabkan pengurangan p dan σ, dan peningkatan S. Nilai maksimum yang diukur bagi S2σ ialah 7.47 mW/mK2 pada 373K bagi sampel Bi0.4Sb1.6Se2.4Te0.6. Daripada tiga set di atas, S2σ yang tinggi diperolehi bagi sampel jenis-p Bi0.4Sb1.6Se2.4Te0.6, Bi0.4Sb1.6Te3 dan Bi2Se0.6Te2.4. Lebih kurang 70% unsur Te telah ditambah ganti ke dalam komposisi Bi2Se0.6Te2.4 bagi menghasilkan aloi jenis-n. Dua modul TEG 9 p-n pasangan telah difabrikasi. Modul pertama (D1) berasaskan p-Bi0.4Sb1.6Te3 dan n-Bi2Se0.6Te2.4, sementara modul kedua (D2) berasaskan p-Bi0.4Sb1.6Se2.4Te0.6 dan n-Bi2Se0.6Te2.4 bagi menentukan modul dengan Pmax yang tinggi. Modul D2 telah menjana Pmax (88.49 mW) yang lebih besar daripada modul D1 (39.4 mW) pada suhu sisi panas TH = 523 K dan perbezaan suhu ΔT bernilai 184 K. Hasil daripada ini, modul TEG (D3) 18 pasangan berasaskan p-Bi0.4Sb1.6Se2.4Te0.6 dan n-Bi2Se0.6Te2.4 telah difabrikasikan. Modul ini menjana Pmax sehingga 273.18 mW, iaitu lebih besar daripada apa yang dijana oleh modul D1 dan D2 di bawah keadaan terma yang sama.