Bi-Te-Se三元熱電厚膜製備及性質研究

Abstract

在中低溫範圍，碲化鉍合金系統是效率最好的熱電材料。透過添加第三元素硒，可以形成三元n型熱電材料，其中以Bi2Te2.7Se0.3化學組成熱電轉換效率最高。面對微型化時代，傳統塊材尺寸有其限制，而薄膜材料受限於製程昂貴及前驅物毒性，較為困難工業化；厚膜製備提供了可能的前景，其中電鍍製程具有成膜速率快、不需真空設備、與微機電元件整合等優點。三元n型電鍍製程中，相對於硝酸系統，鹽酸系統較少被討論。本研究將探討不同電鍍參數對鍍層之影響，並量測熱電性質，且於微機電元件上進行電鍍。 由循環伏安掃描結果可知氯離子對於三種離子之影響，分別使鉍離子及硒離子還原峰值電位向負偏，使碲離子還原峰值向正偏。不論在硝酸或鹽酸系統中，均可發現添加Se造成還原峰值電位提前，表示有pure underpotential deposition發生。 板材電鍍部分，在鹽酸系統下，經三小時與六小時定電位電鍍後，可分別獲得27與43 μm平坦緻密的三元n-type鍍層，成分分別為Bi1.88Te2.80Se0.32及Bi1.89Te2.82Se0.29。XRD結果顯示三元鍍層結晶性優於二元系統，TEM影像顯示由奈米等級顆粒堆疊，且具有特殊層狀結構。因導電底材會影響電阻率量測，故鍍層先經由冷鑲埋樹脂翻模後，再進行熱電材性質之量測。本研究之電阻率及載子遷移率優於文獻，但鍍層成分稍微偏離計量比，造成Seebeck係數不足；比較緻密與鬆散鍍層，可發現緻密鍍層之電阻、載子遷移率、Seebeck係數均優於鬆散鍍層。 由於微機電元件幾何形狀與板材不同，影響電鍍過電位，使鍍層形貌與成分皆不同於板材電鍍所設計之結果。經調控電鍍液之配比，成分可符合計量比，定電位電鍍三小時可獲得20 μm三元n型緻密鍍層。

Thermoelectric materials can convert heat into electricity, and vice versa. Bi2Te2.7Se0.3 is considered to be one of the most efficient n-type thermoelectric materials near room temperature. The current tendency to miniaturization has provoked interest in thick film thermoelectric devices. Among the methods to deposit thick films, electrochemical deposition is a promising method due to its vacuum-free system, higher deposition rate, and complete integration with MEMS. In fact, nitric acid system was studied well, so further investigation of hydrochloric acid system has to be conducted. In this research, electrodeposition of n-type Bi2Te2.7Se0.3 was studied, followed by the measurement of thermoelectric properties and the preparation of MEMS devices. Firstly, in order to understand the differences between the nitric acid and the hydrochloric acid system, cyclic voltammetry was conducted. According to the results, with the introduction of chloride ions, reduction potential for BiIII and SeIV would shift toward negative direction; a positive shift of reduction potential for TeIV was observed. Furthermore, by adding selenium ions in the binary system, the reduction peaks shifted toward positive direction. Secondly, compact deposits of Bi1.88Te2.80Se0.32 with 27 μm and Bi1.89Te2.82Se0.29 with 43 μm were obtained, respectively, after potentiostatic deposition for 3 and 6 hours. Thirdly, deposits were flipped from substrates by epoxy resin prior to the measurement of thermoelectric properties. The resistivity and mobility of the deposits were better than other researches, but slight deviation of composition may lead to low Seebeck coefficient. Last but not least, compact and stoichiometric n-type deposits with 20 μm were successfully produced on MEMS.