利用共鍍催化金屬與不同間距高度比之奈米碳管柱列改善氣體游離式感測器之特性研究

Abstract

氣體游離式感測器是一種以氣體分子各自獨特的物理特性來分辨不同氣體的元件，傳統上，氣體游離式感測器受限於過大的結構（如火焰游離式感測器及光游離式感測器）、危險的高電壓操作並其伴隨而來的高功率消耗等因素。因此在本篇論文的實驗中，吾人嘗試利用奈米碳管較低的功函數、尖銳的特點以及在適當電場下能獲得極佳之游離待測氣體能力與穩定性等來改善氣體游離式感測器。 在本篇論文實驗的起頭，首先會討論不同表面型態的奈米碳管薄膜所造成的氣體崩潰特性之差異。由無定向之碳管薄膜的量測結果發現，其崩潰電壓十分不穩定且在高電壓區域的誤差有將近100伏特的變動。這些結果被認為與其表面碳管的長度不一有很大的關係。因為相對來說，均勻垂直之碳管薄膜就有較穩定的氣體崩潰特性。但是，對於這兩種表面型態不同的碳管薄膜來說，在經過穩定性測試的高電壓處理之後，它們的崩潰電壓漂移的情形都十分嚴重。無定向之碳管薄膜在經過1000次重複的穩定性測試之後，其崩潰電壓由起初的365V上升到605V，相當於上升了68%。而均勻垂直之碳管薄膜在經過相同1000次重複的穩定性測試之後，其崩潰電壓則由395V上升到575V，也就是上升了45%。並且我們從掃描式電子顯微鏡圖中可觀察發現，崩潰電壓上升的主要原因跟碳管在高電壓下會有被拔除與燒結的現象有關。 因此，為了增進奈米碳管氣體游離式感測器之穩定性，吾人嘗試以鈷-鈦催化劑金屬共鍍的方式來改善碳管與基板之間的附著力及接觸阻抗。並且由實驗的結果可發現，以此方式合成之碳管薄膜確實有更穩定的崩潰特性，在經過同樣1000次重複的穩定性測試之後，其崩潰電壓僅由375V上升到435V，只上升了16%，與先前兩種碳管薄膜比較起來可說大有改進。 另外，為了改善氣體游離式感測器的功率消耗，減低其操作電壓是首先需要研究的。在這部份，則使用不同間距高度比的奈米碳管柱列來探討在多少的間距高度比下有最理想的表面電場分佈，以期達到有最好的拉電子能力並可最早達到氣體崩潰；也就是有最低的崩潰電壓。在實驗中，吾人嘗可在量測結果的統整中發現，在間距高度比約2.91附近有最低的崩潰電壓。因此此理想的陣列間距高度比可應用於降低氣體游離式感測器的操作電壓以及功率消耗。 接下來，這些理想化過後的碳管柱陣列被使用來探討在不同氣體環境下的氣體游離特性。這些不同的氣體因為具有不同的平均自由路徑、游離能及再結合率，因此會有各自獨特的Paschen’s curve。利用這些Paschen’s curve並加上適當地選擇氣體壓力與間距的乘積值，則可製作出既操作在低電壓，又能有足夠寬的間隔來分辨不同氣體的崩潰電壓。最後，吾人探討不同比例的氬氣、二氧化碳與一般空氣混和之後的崩潰電壓變化：以間距高度比為2.91碳管柱陣列為例，當二氧化碳在空氣中的比例到達15%時，則崩潰電壓上升會60V，當氬氣在空氣中的比例到達11%時，則崩潰電壓會下降100V。

Gas ionization sensors are physical devices that work by fingerprinting the ionization characteristics of distinct gases. Conventional ionization sensors were limited by the huge and bulky architecture (ex: FID, PID), risky high-voltage operation and high power consumption. In this thesis, carbon nanotubes (CNTs) with relatively low work function, extremely sharp nanotips, and structural and chemical stability under high electrical field were therefore used to improve these issues of gas ionization sensors. In the beginning of this thesis, the effects on gas breakdown characteristics of different surface morphology of CNTs film are presented. For the Random oriented CNTs film, the variations of the breakdown voltages are especially large at high voltage region and their error bars in the high voltage region are as wide as 100 volts. These variations are associated with the nonuniformity of the CNTs’ length. On the other hand, the gas breakdown characteristics of the Uniform CNTs film were relatively stable from the measurement results. However, for both of the two samples, the shift-up of their breakdown voltages (Vbr) were fairly severe after the high-voltage process in stability tests. One could find that the Vbr of the Random oriented CNTs film lifts up from 365V to 605V after 1000 cycles, i.e., 68% increase. And for the Uniform CNTs film, it lifts up from 395V to 575V after 1000 cycles, i.e., 46% increase. Observed from the SEM images, the pull-off and evaporation of CNTs resulted from the high local electric field difference were considered as the main reason for the shift-up of breakdown voltages. In order to acquire a better stability in the CNTs gas ionization sensor, the improvement of the adhesion and the contact resistance between CNTs and substrate under high electric field was obtained using Co-Ti co-deposited catalyst structure. The Vbr of the CNTs film synthesized from Co-Ti co-deposited catalyst lifts up from 375V to 435V after 1000 cycles, i.e., only 16% increase, which is much more reduced than that of the first two conventional CNTs film. In addition, to improve the issue of high power consumption, pillar arrays of vertical aligned CNTs bundles with different spacer height ratios (R/H) were utilized to investigate the optimal local electrical field on the nanotubes that has the most efficient field emission, namely, the earliest gas breakdown and lowest breakdown voltage. In this thesis, the lowest breakdown voltages were approached by changing H while maintaining R and the optimal R/H ratio was around 2.91. This optimal R/H ratio would lessen the high operating-voltage and thus improve high power consumption issues of the ionization sensors. Next, the optimized samples were exploited to explore their gas ionization characteristics under different gases environment. From the experiment, dissimilar trends of Paschen’s curve for distinct gases was obtained due to that different gas molecules have different mean free path, ionization energy and recombination rate. With a proper selection of the p×d product value, CNT gas ionization sensor can not only operate under low voltages but also provide enough space to distinguish between different gases. Finally, the breakdown voltages of Ar and CO2 gases in mixture with air as a function of concentration were investigated. Take the R/H = 2.91 optimized patterned sample for example. It was found that the Vbr increases 50V as the concentration of CO2 in the mixture with air reaches 15 %, and decreases 100V as the concentration of Ar in the mixture with air reaches 11 %.