| 摘要: | 熱電材料是一種能將熱能與電能交互轉換的材料，可以用來製作熱電致冷器或是熱電發電器。而TAGS-85是中溫段的熱電材料，操作溫度約300℃至500℃。日前要提高熱電材料的性質通常以增加Seebeck係數、增加導電率、降低熱傳導係數，而TAGS-85本身具有高Seebeck係數和非常低的熱傳導係數，非常適合用來當熱電發電器的材料。
本論文材料主要為TAGS-85((AgSbTe2)15(GeTe)85)，是一個以GeTe為主要合金與AgSbTe2混合的偽二元合金，將合金預熔之後以水淬、空冷、爐冷三種不同降溫速率獲得的合金，理論上水淬的結晶粒徑最小而爐冷最大。之後將合金磨成粉，粒徑為200μｍ，再以粉墨熱壓的方式製作樣品，熱壓溫度分冸使用350℃、400℃、450℃、500℃，再分冸持壓十分鐘、二十分鐘、三十分鐘。熱壓後的樣品，以X-ray繞射分析材料的結晶性、以光學顯微鏡和場發射掃瞄式電子顯微鏡觀察材料的表面型態、以硬度測試器量測樣品的硬度。在室溫至500℃進行Seebeck係數、導電率隨溫度變化的測量。
Seebeck係數隨著熱壓時間的增加而減少，在熱壓溫度450℃有最大值，導電率與合金的冷卻速率有較大的關係。
Thermoelectric material is a material which can transform heat to electric or transform electric to heat. It can be a thermo cooler or thermo generator. The related compounds (AgSbTe2)1-x(GeTe)x (known collec- tively by the acronym of their constituent elements as TAGS-X, where x designates the mole fraction of GeTe) were first reported in the1960s and have since been successfully deployed in radioisotope thermoelectric generators for deep space and remote applications. The composition (AgSbTe2)0.15(GeTe)0.85 (TAGS-85) was found to have the best combination of thermal and electrical transport properties and mechanical stability. The key feature of TAGS-x for thermoelectric applications is the very low thermal conductivity for the compositions TAGS-80 and TAGS-85.
The performance of materials for either of the above mentioned applications is governed by the thermoelectric figure of merit Z=S2/ρκ where S is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. The dimension less figure of merit, ZT, can be obtained by multiplying Z by the absolute temperature (T). Higher Z values lead to more efficient materials and ultimately to more highly efficient devices. The thermal conductivity is too hard to get, so the Power factor = S2/ρ, also can be the thermoelectric figure of merit.
In my study I select TAGS-85 to be my research material, as the composition is varied from AgSbTe2 to GeTe, the transport properties vary smoothly, except for an anomalous double minimum in thermal conductivity at 80 and 85% GeTe. Other researchers have reported ZT values as high as 1.7 in the TAGS-80 composition, however, its inferior mechanical strength led to more widespread use of TAGS-85, (AgSbTe2)0.15(GeTe)0.85, as the preferred composition.
In many studies, they have different method to product the samples, such as different pressure, ingot cool down rates, different hot-press temperatures, hot-press time, particle size. So I use 200μm particle size, hot-press to product samples, the sample size is 8mm×8mm×15mm, the pressure is 38Mpa, I change hot-press temperature 350℃, 400℃, 450℃ 500℃, and hot-press time is 10min, 20min, 30min, and the ingot cool down rate have three rates, water quenching, air quenching, furnace quenching.
The Seebeck coefficients is relationship with hot-press time 10min>20min>30min, the Seebeck coefficients at hot-press temperature 450℃ have the highest value, the electrical resistivity with water quenching > air quenching > furnace quenching.
Why the Seebeck coefficients relationship with hot-press time? I think because of the grain boundary, the energy gap relationship with Seebeck coefficients, the electric need more energy to through the gap so the Seebeck coefficients have high value. |
