電流輔助燒結碲化鍺鉛合金之熱電性質研究

Thermoelectric properties of (Ge,Pb)Te alloys prepared by current-assisted sintering method

許誌祐(Chih-Yu Hsh)

指導教授 ： 廖建能

國立清華大學/工學院/材料科學工程學系/碩士(2019年)

摘要

碲化鍺屬於中高溫型熱電材料，近年，Ge0.87Pb0.13Te已被證明在GeTe為基底的熱電系統中具有優異的熱電性質，這歸因於碲化鍺本身良好的電性，以及鉛摻雜後產生的多尺度聲子散射中心，包括點缺陷、奈米級析出物和晶界等。現今Ge0.87Pb0.13Te研究的製備手法大多以水淬獲得合金後，再以高溫500~550℃熱壓或火花電漿燒結的方式將合金的粉末燒結成錠，其中火花電漿燒結具有較優異性質。然而部分研究火花電漿燒結的樣品性質卻難以穩定重現，另外電流效應對Ge0.87Pb0.13Te內部的晶體缺陷和微結構的影響也尚未完全了解。本研究據此目的分為兩部分探討：其一，為達到樣品製備的穩定性，探討燒結前熱處理對後續燒結的可能影響；其二，以低於常見燒結溫度的400℃進行電流輔助燒結，並探討電流在燒結中扮演的角色。實驗首先以「水淬」與「退火」製程製得之合金差異對後續熱壓燒結之影響進行探討，並分析討論「燒結前熱處理的差異」以及「燒結前熱處理差異對後續熱壓燒結的影響」。研究結果指出相較於水淬製程，退火製程製得之合金成分較為單一，使得後續熱壓燒結過程中較缺乏擴散驅動力，其母相組成仍留有較多Pb原子而有較低的載子濃度。試片室溫的載子濃度相較水淬熱壓樣品從5.54  1020 cm-3下降到4.28  1020 cm-3；同時因粉體間擴散較慢而使燒結密度較水淬樣品低，導致其載子遷移率由42 cm2/Vs 微幅下降到36 cm2/Vs。整體而言，儘管退火熱壓電阻率的上升，退火熱壓樣品載子濃度的下降，但較高的Seebeck係數可使Ge0.87Pb0.13Te材料在室溫的熱電功率因子由6.35 W/cmK2提升到6.75 μW/cmK2。另外，本實驗基於上述退火熱壓燒結之結果，進一步藉由電流輔助燒結，以相同於熱壓燒結的溫度製備Ge0.87Pb0.13Te樣品，嘗試探討電流對內部原子的影響及熱電傳輸性質的差異。研究結果指出電流輔助燒結過程中不僅引入了大量的焦耳熱，還促進了燒結過程中Ge0.87Pb0.13Te化合物的原子擴散，使系統整體更接近完全熱平衡狀態。與退火熱壓樣品相比，電流輔助燒結樣品室溫的載子濃度從4.28  1020 cm-3下降到3.85  1020 cm-3，Seebeck係數因此微幅提升；載子遷移率則因為燒結密度提升(由94.4%大幅提升至98.0% (理論密度))，導致其值由36 cm2/Vs 大幅上升到65 cm2/Vs；晶格熱導率則因大量〖Pb〗_Ge錯位缺陷的出現而由1.29 W/m·K大幅下降至0.46 W/m·K。最後，對於Ge0.87Pb0.13Te之高溫傳輸行為，本研究亦藉由變溫XRD，分析熱壓燒結與電流輔助燒結樣品的相變化溫度區間差異，並由相圖估算材料的相變化溫度區間隨成份均質度變化而改變之情形。整體而言，電流輔助燒結之Ge0.87Pb0.13Te因Seebeck係數的些微提升和電阻率進一步的降低，熱電功率因子可在773K達到34.6 W/cmK2、熱電優值達到1.5。

關鍵字

熱電材料 ； 碲化鍺-碲化鉛系統 ； 碲化鍺相變化 ； 低溫熱壓燒結 ； 電流輔助燒結 ； 電流焦耳熱效應

並列摘要

Germanium telluride (GeTe) is a class of thermoelectric materials suitable for operation at middle-high temperatures. Recently, it has been demonstrated that Ge0.87Pb0.13Te exhibits superior thermoelectric properties among various Ge-Te based compounds, which is attributed to superior electronic performance of GeTe matrix and the presence of multi-scale phonon scattering centers including point defects, nano-precipitation and grain boundaries in the material. Nowadays, most of Ge-Te based compounds prepared through consolidation of compound powders by hot-pressing technique or spark plasma sintering (SPS) method at the temperature around 500 ~ 550°C. The samples prepared by SPS usually had better properties. However, the properties of the SPS’ed samples are somewhat difficult to reproduce. In addition, how electric current affects lattice defects and microstructure of Ge0.87Pb0.13Te compound is still not fully understood. This study is divided into two parts: First, we intend to develop a pre-sintering heat treatment to achieve the sample stability during subsequent sintering and thermal operation. Second, we investigate the current effect on thermoelectric transport properties of Ge0.87Pb0.13Te compound prepared by current-assisted sintering method. In first part, we explored the effect of pre-sintering "water quenching" and "annealing" heat treatment on the properties of hot-pressed samples. The results indicated that the composition of the alloy prepared with the pre-sintering annealing process is relatively uniform, which makes more Pb atoms remain in crystal lattice during hot-pressing. Hence, the room-temperature carrier concentration of pre-sintering annealed sample is 4.28×1020 cm-3compared to the water-quenched sample with a value of 5.54×1020 cm-3. The carrier mobility decreases slightly from 42 cm2/V·s. to 36 cm2/V·s due to lower mass density. Although the resistivity slightly increased, the thermoelectric power factor of pre-sintering annealed sample still increased from 6.35 μW/cm·K2 to 6.75 μW/cm·K2 at room temperature owing to increased Seebeck coefficient. In the second part, we investigate the current effect on the thermoelectric properties of the samples prepared by current-assisted sintering. The results indicate that the electric current not only introduces extensive joule heating but also motivates atomic diffusion in the Ge-Te compounds during the sintering process. The carrier concentration of the electrically sintered sample decreases from 4.28×1020 cm-3 to 3.85×1020 cm-3, and hence the Seebeck coefficient is slightly increased. The carrier mobility is sharply increased from 36 cm2/V·s to 65 cm2/V·s due to the enhanced mass density after sintering, increasing from 94.4% to 98.0% of the theoretical density. The lattice thermal conductivity dropped sharply from 1.29 W/m·K to 0.46 W/m·K due to a large amount of 〖Pb〗_Ge anti-site defects. In addition, we have analyzed different temperature dependent thermoelectric properties of hot pressed and current-assisted sample. The phase change phenomena of the sintered Ge0.87Pb0.13Te samples are examined by the x-ray diffraction analysis. To sum up, the thermoelectric power factor of electrically sintered Ge0.87Pb0.13Te sample can reach 34.6 μW/cm·K2 and a zT value of 1.5 at the temperature of 773K. The improvement is mainly attributed to the increased Seebeck coefficient and the decreased resistivity of the electrically sintered samples.

並列關鍵字

Thermoelectric material ； GeTe-PbTe system ； GeTe phase transition ； Low-temperature hot-pressed sintering ； Current-assisted sintering method ； Joule heating effect caused by current

參考文獻

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