Preparation of Long Alumina Whiskers

采用氟化铝高温水解化学气相反应合成氧化铝晶须。通过改变载气种类和流量、H_2O分压、反应温度和时间等重要因素进行合成。采用一步合成法在衬底上升华氟化铝并存款氧化铝晶须时，在1200 ℃的反应温度下，晶须可以生长到约20 mm长，晶须从管壁向中心垂直于载气流动方向生长，由于在晶须尖端发现了液滴状沉淀物，晶须的生长被认为是气-液-固（VLS）机制，液滴中含有钾和钠元素作为杂质，它们在高温下形成熔体，晶须的拉伸强度用Iwanaga等（1992）的方法和本研究的方法测量。晶须的强度与其横截面积有明显的相关性，当横截面积为10 m^2时，强度可达几个GPa。然而，目前的数值比Levitt（1966）报告的数值低10 ~ 10^2。通过XPS化学分析，发现晶须中含有一定量的氟，认为这是导致强度低的主要原因。因此，在更高的反应温度下使用两步合成方法来研究晶须的合成，以避免在晶须中引入氟。以莫来石陶瓷为基体，加入金属铁作为杂质，通过这种方法得到了晶须。但与前一种合成方法相比，所得晶须长度较短，产率较低，需要进一步寻找第二种合成方法的最佳实验条件。

Alumina whiskers were synthesized by chemical vapor reaction of alumium fluoride hydrolyzing at high temperatures. Synthesis was performed with changing important factors such as the species and flow rate of sarrier gas, partial gas pressure of H_2O,and temperature and time of reaction. When one step synthetic method to sublimate aluminum fluoride and deposit alumina whiskers on a substrate, the whiskers can be grown up to around 20 mm in length at reaction temperature of 1200ﾟC.The whiskers grew from wall to center of the reaction tube perpendicular to the flow direction of the carrier gas.Since droplet like precipitate was found on the tip of the whiskers, the whiskers were considered to grow by the vapor-liquid-solid (VLS) mechanism.The droplet contained potassium and sodium elements as impurities and they formed melt at high temperature.Tensile strength of the whiskers was measured by the Iwanaga et al. (1992)method and present study's method. The strength showed apparent dependence for the cross section area of the whiskers and was several GPa at cross section area of 10 mum^2. However, the present values were 10 to 10^2 lower than those reported by Levitt(1966). The present whiskers were found to contain some content of fluorine by XPS chemical analysis and was considered to be the main cause for low strength. Therefore, synthesis of whiskers under further higher reaction temperature was examined using two step synthetic method to avoid incorporation of fluorine in the whiskers. Whiskers were obtained by this method when adding iron metal as an impurity and using mullite ceramics as a substrate. However, the whiskers obtained were shorter in length and lower in yield than the former synthetic method and it was necessary to find further optimum experimental conditions for the second synthetic method.