在垂直磁場下溫度對磁性流體薄膜中磁束結構之影響

柯雅華

指導教授 ： 洪姮娥

國立臺灣師範大學/理學院/物理學系/碩士(2001年)

摘要

由於磁性流體薄膜中磁束結構在外加垂直磁場下，會受到外在溫度的影響，因此產生不同的結構變化。為了更加了解溫度對磁束結構演化的影響，我們首先探討在不同溫度下，觀察磁性流體薄膜中磁束結構隨外加磁場強度的演化，以了解溫度高低如何影響磁性流薄膜中磁束的結構。然後我們固定在某一個初始狀態下，開始改變磁性流體薄膜的溫度，觀察磁束結構在升降溫過程中之變化情形，最後我們改變不同起始狀態，觀察磁束結構隨溫度之變化。
首先我們觀察在不同溫度下，結構隨磁場之靜態演化情形，我們發現當溫度越高時，磁性粒子聚集成磁束所需要的臨界磁場強度越強，且磁束形成有序結構的臨界磁場也將越高。這是因為溫度越高時，磁性粒子的動能越大，因此不易聚集成磁束。且我們在H-T圖上建立結構相圖，以了解溫度與磁性流體薄膜在外加垂直磁場下結構變化的關係，由H-T圖可得知臨界磁場與溫度幾乎成線性關係。
繼而在T = 26 °C下，固定初始條件，觀察磁性流體薄膜中磁束結構在升降溫過程中之演化，並且升溫至不同溫度後開始降溫至T = 26 °C，觀察在降溫過程中，結構的演化是否有所差異。由此我們可以知道隨著溫度升高，當溫度升高至某一臨界溫度時，磁束中磁性粒子開始溶解到載液內，而在降溫的過程中，載液中的磁性粒子會沿著磁束表面附著。
當我們已了解六角有序結構在升降溫過程中結構演化行為後，我們可藉由改變各種初始條件如磁場強度、薄膜濃度、薄膜厚度及磁增率等，以達到不同的初始狀態，觀察在升溫過程中有序結構之演化情形，並且找出結構發生變化的臨界溫度最高時的初始狀態。

關鍵字

溫度 ； 磁性流體 ； 磁束結構

並列摘要

With the contribution of the thermal energy to the formation of magnetic columns in the magnetic fluid films under perpendicular magnetic fields, various structural evolutions can be resulted at various temperatures. In this work, we clarify the role of the thermal energy in the structural formation in the magnetic fluid film by observing the structural evolution.
First, the effect of the temperature on the structure formation in the magnetic fluid film subjected to perpendicular magnetic fields is studied. It was found that a higher magnetic field strength is required to make the magnetic particles agglomerated at a higher temperature. These results are attributed to the enhancement in the kinetic energy of the magnetic particles at higher temperatures and more magnetic particles can disperse into the liquid carrier. Thus, a higher magnetic field strength is needed to achieve a certain structural pattern in the magnetic fluid film under a higher temperature. To illustrate the relationship between the effect of temperature T and of the magnetic field H on the structure formation in the magnetic fluid film, a phase diagram is constructed for the structure pattern in the H-T configuration.
Secondly, the evolution of the initially ordered structure in the magnetic fluid film was investigated for the heating/cooling process. We raised the temperature from the initial temperature T = 26 °C to different finial temperatures (heating process), then reduced the temperature backward to 26 °C (cooling process). During the heating process, the magnetic particles start melting into the carrier until the temperature is raised up to a critical temperature. Under the cooling process, the magnetic particles in the carrier condensed to the existed columns. And also, different structure evolutions under the cooling process were observed for various finial temperatures.
Finally, we examined the initial-state effect on the column melting behavior during the heating process. The various initial states can be achieved by adjusting the control parameters, such as the magnetic field strength, the sweep rate of the field, the concentration of the magnetic fluid, the film thickness, and the initial temperature. It was found that the column distance of the initially ordered structure and the intra-structure of columns dominate the structure evolution in the magnetic fluid with temperature.

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