降溫速率對於強誘電性液晶在N*-SmC*相轉變配向之影響

The Influence of Cooling Rate on the Alignment of the Ferroelectric Liquid Crystal in N*-SmC* Phase Transition

謝富程(Fu-Cheng Sie)

指導教授 ： 吳俊傑 ； 陳殿榮

國立臺北科技大學/機電學院/光電工程系所/碩士(2006年)

摘要

具有N*-SmC*相轉移之強誘電性液晶(ferroelectric liquid crystal, FLC)材料，可經由電場輔助配向達到均勻配向的效果。在我們先前的研究當中，已成功的利用強誘電性液晶(FLC)在固定降溫速率下(-1°C/min)，經由在N*-SmC*相轉移的同時，施加一交流電場，當所施加電場頻率在兩種不同的頻率區段時會有均勻的配向。而這兩種頻率區段分別為低頻區段與高頻區段。
由於整塊強誘電性液晶盒由N*相轉變為SmC*相的過程需花費時間。當配向頻率位於低頻區段時，如果所施加的頻率夠低，讓相轉變時間約等於正半週期或負半週期的電場，而產生同一極化方向的分子，如此一來所有的液晶分子都分佈於同一方向，產生單一區塊的形成。當配向頻率為高頻區段時，如果所施加的頻率夠高，在相轉變時間已感應正負電場非常多次，而產生兩種不同區塊的形成，如果這兩種區塊面積大小約相同且區塊夠小混的夠均勻，巨觀來看，其配向效果也會類似單一區塊的形成。很明顯的，配向頻率的區段受限於降溫時間的長短。
因此本論文研究降溫速率對配向頻率區段的影響。由實驗數據可得知，若減緩降溫速率，則相轉變的時間將拉長，因此相變過程中對於同一頻率所感應到正負電場次數也將越多次，越有效的達到兩種均勻區塊的形成，即可將原本的高頻區段往下做擴展；反之，若降溫速率為快速降溫時，則頻率區段的擴展是位於低頻部分，與慢速降溫的情況相反。

關鍵字

強誘電性液晶 ； 配向 ； 降溫速率

並列摘要

Ferroelectric liquid crystal material with N*-SmC* phase transition can be aligned uniformly by applying an electric field. In our pervious research, we successfully found the FLC cell forms a uniform alignment at two particular ranges of AC frequency and at a cooling rate (-1°C/min) during N*-SmC* phase transition. The two ranges are the low frequency portion and the fast frequency portion.
When the FLC cell cooled to the N*-SmC* phase transition temperature, it takes a transition time period to form the SmC* phase in the whole cell. For the low frequency portion, if the AC frequency is slow enough such that half of the period was close to the transition time, the FLC molecules can be aligned nearly in the same direction and have the same polarization direction, and then form a mono-domain alignment. For the high frequency portion, the AC frequency is fast enough such that positive and negative electric fields switch many times in the transition time, and then two types of domains form. The two types of domains have almost the same area and are mixed so well as to like a mono-domain. Obviously, the range of AC frequency is limited and depends on this transition time. To clarify this dependence, we varied the cooling rate of FLC cell. Then, we investigated the influence of cooling rate on the alignment and AC frequency by observing the microscopic texture and measuring the electro-optical characteristics.
As the cooling rate decreases, the transition time of SmC* phase becomes longer, the electric field at fixed frequency switches more times during the transition time. Hence, the high frequency portion which let the two types of domains mix well can be expanded. Moreover, the probability of forming a single direction of polarization becomes less, so the low frequency portion will be reduced. When the cooling rate increases, the transition time of SmC* phase becomes shorter, the electric field at fixed frequency switches less times during the transition time. Therefore, the low frequency portion which makes the FLC molecules have single direction of polarization will be expanded. Additionally, some frequencies switch less and less times in the transition time and can not mix the two types of domain well; hence the high frequency range will be reduced.

並列關鍵字

N*-SmC* ； ferroelectric liquid crystal ； alignment ； cooling rate

參考文獻

Pikin,Ferroelectric and Antiferroelectric Liquid

[2] Sven T. Lagerwall, M. A. Osipor, S. A.

[3] W. L. McMillan, Phys. Rev. A8 (1973) 1921.

[5] S. T. Lagerwall and I. Dahl, Mol. Cryst. Liq. Cryst.,

114, 151 (1984)