三元碲化鉍銻電化學還原行為與微結構控制及熱電性質

Abstract

熱電材料可將電能與熱能交互轉換，可利用溫差發電，也可通電產生溫差以冷卻或控溫，在塊材與薄膜熱電材料的應用上，皆已有產業技術領導廠商，唯獨厚膜熱電材料尚未出現明顯技術領先者，而相較於蒸鍍、濺鍍等真空製程，電化學沉積法具備低成本、成膜速度快等特性，對於開發熱電厚膜元件有相當大的優勢，本研究將探討各電鍍參數與三元低溫熱電材料碲化鉍銻還原特性和微結構之效應。 Bi0.5Sb1.5Te3熱電材料的電鍍由於物種彼此間的還原電位差異大，很難同時兼顧鍍層微結構的緻密性與成分比例，而藉由電化學循環伏安分析可檢測錯合劑濃度對BiIII、SbIII和TeIV離子還原行為的影響。透過氯離子(Cl−)與BiIII、SbIII和TeIV離子的錯合可調控還原電位，將三者間還原峰位置的差異由287 mV減少至150 mV。在0.35 M鹽酸，1 mM BiIII + 10 mM SbIII + 7.5 mM TeIV + 0.1 M酒石酸+ 0.5 M氯化鈉的鍍液條件於−90 mV下定電位電鍍10分鐘可獲得Bi0.49Sb1.45Te3.06鍍層，電鍍初期鍍層結構緻密，但由於電極表面離子濃度逐步消耗，還原最終由擴散控制主導，使鍍層結構和表面粗糙度隨電鍍時間增加而變得鬆散。 藉由脈衝電鍍工作週期的調控，可控制電極表面與鍍液界面的離子濃度，在電鍍電位−90 mV、休鍍電流0 A、工作週期91%、0.09 Hz、脈衝次數480次時，可獲得厚約4.5 μm，球狀的Bi0.45Sb1.19Te3.36鍍層，但隨著脈衝次數增加，鍍層球狀顆粒的沉積速率不一，導致鍍層結構變得鬆散，此結構的變化可對應到循環伏安掃描結果中成核圈的逐漸變小直到完全消失。鍍層粗糙度上升會使電阻率上升以及功率因子下降，但並未對Seebeck係數造成明顯影響，最佳的功率因子出現在脈衝次數216次，2.0 μm厚的Bi0.45Sb1.22Te3.33鍍層，其室溫下的Seebeck係數和功率因子分別為+150 μV/K和150 μW/m∙K2。在鍍層和金底材界面處觀察到一碲含量富集層，為獲得緻密碲化鉍銻電鍍鍍層的良好熱電性質，需考慮其電阻與未被補償的歐姆壓降。

Thermoelectric (TE) materials can transfer electricity to heat, and vice versa, which can be applied to thermoelectric power generator or cooler. The applications of thermoelectric materials are generally divided into bulk, thick film, and thin film. The manufacture processes of bulk and thin film are much well developed than those of thick film. Compared with the vacuum systems, such as evaporation and sputtering, electrochemical deposition can fabricate thick film at lower cost and higher deposition rate. The main object of this dissertation is to develop the co-deposition process of ternary Bi0.5Sb1.5Te3 thick film TE materials. The obstacle of co-deposition Bi0.5Sb1.5Te3 is the large reduction potential difference among BiIII, SbIII, and TeIV, which makes microstructure and composition not be controlled at the same time. The complexation of Cl− with BiIII, SbIII, and TeIV reduces the difference of position of cathodic peaks from 287 mV to 150 mV. Bi0.49Sb1.45Te3.06 deposit was obtained by potentiostatic deposition at −90 mV for 10 min in the solution containing 1 mM BiIII, 10 mM SbIII, 7.5 mM TeIV, 0.1 M tartaric acid, 0.5 M NaCl, and 0.35 M HCl. However, deposit roughness became larger with continued electroplating when the reduction reaction turned into diffusion control. The ions concentration at electrode/electrolyte interface can be controlled by the parameters of pulsed electrodeposition. A compact Bi0.45Sb1.19Te3.36 deposit with a thickness of around 4.5 μm and spherical morphology was obtained by pulsed deposition at −90 mV deposition potential, 0 A resting current, 91% duty cycle, 0.09 Hz for 480 cycles. As the deposition cycle was increased, electroplating of p-type deposit proceeded on selected existing grains, resulting in nodular and dendritic grains developed on top of a compact under layer. Meanwhile, the nucleation loop in cyclic voltammograms disappeared. The loose structure associated with nodular and dendritic p-type grains resulted in an increased resistance and decreased power factor, but slightly influenced the Seebeck coefficient. The p-type Bi0.45Sb1.22Te3.33 film with optimal thermoelectric properties was obtained via pulsed deposition with a thickness of around 2 μm and had a Seebeck coefficient of +150 μV/K and a power factor of 150 μW/m·K2 at room temperature. The presence of a distinct Te-rich layer was observed at the p-type film/substrate interface and the electric resistance and uncompensated ohmic drop caused by this layer need to be considered for the electrodeposition of a compact p-type Bi-Sb-Te film with optimal TE properties.