Effect of Piezoelectric Vibration on the Humidity Sensing Behavior of Nanoporous Alumina

陽極處理氧化鋁其表面具有奈米級的孔洞大小，可以用作優良的溼度感測材料。在過往的文獻中，對於孔洞氧化鋁表面之吸附水層受溫度之影響已經被大幅度的報導，但是卻很少有人探討過孔洞氧化鋁吸附水層受到機械式的振動，對於其性質的影響。在本實驗中，我們將研究孔洞氧化鋁溼度感測的性質受機械振動之影響為何。 利用製備出一個由孔洞氧化鋁與壓電陶瓷的複合結構，來驅使壓電陶瓷產生振動，對孔洞氧化鋁施加機械式的壓電振動，觀察是否有驅散表面吸附水層的效果。本實驗首先量測在無壓電振動下，從低溼度到高溼度的阻抗頻譜與電容值，證實了孔洞氧化鋁具有良好感測環境溼度變化的能力。接著在四個代表性的溼度下 (RH90 %、RH80 %、RH70 %、RH60 %) 施加壓電振動，並再次量測阻抗頻譜與電容值，結果發現在高溼度下，由於表面吸附相當大量的水層，壓電振動無法去影響這麼多的水層，反而是干擾了在低頻量測時的電荷傳遞，當壓電振幅越大，電荷傳遞受影響的程度也越大。在低溼度下，由於此時表面吸附的水層相對較少，壓電振動可以有效的驅散部分的吸附水層，使得在高溼度受振動情況下，得到與低溼度時類似的性質，且驅散水層的效果會隨著壓電振幅的增加而更加明顯。 孔洞氧化鋁溼度感測器在低頻量測時具有較佳的溼度感測敏感度，當施加壓電振動，對於低溼度的感測能力明顯提升，但相對的也會破壞感測的線性行為。 當長時間施加壓電振動後，試片本身會產生疲勞現象，導致整體的阻抗值變大，這是實際使用此複合結構時，必須考慮到的問題。Nanoporous anodic aluminum oxide (AAO) has attracted great attention due to its potential application in humidity sensing. The effect of raising temperature on the adsorbed water layer at AAO surface was widely studied. In this study, the effect and mechanism of piezoelectric vibration on the adsorbed water layer at the AAO surface, which were not reported before, has been investigated by impedance spectroscopy (IS) and capacitance measurement. This is achieved by building a composite structure consisting of an AAO layer and a piezoelectric ceramic layer. At high relative humidity (RH) values, due to the large amount of water covered on AAO surface, the piezoelectric vibration only disturbs the conduction in low measuring frequencies. In contrast, at low RH values, the piezoelectric vibration mainly repels part of the adsorbed water layer at the AAO surface, producing an impedance curve similar to that of a lower RH under no piezoelectric vibration. AAO humidity sensor exhibits a good sensing sensitivity at low measuring frequencies. When the piezoelectric vibration is applied, the sensitivity at low RH values sharply increases, but the linearity is lost. Under a long period of piezoelectric vibration, the AAO surface experiences a fatigue phenomenon, an issue that needs to be considered during its application