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、流速、h2o分压、反应温度和反应时间等重要因素进行合成。采用一步合成法将氟化铝升华并在衬底上沉积氧化铝晶须，在1200℃的反应温度下，晶须可以长到20 mm左右。晶须垂直于载气的流动方向，从反应管的壁向中心生长。由于在晶须尖端发现了类似液滴的沉淀，因此认为晶须是通过气-液-固（VLS）机制生长的。液滴中含有钾和钠元素作为杂质，在高温下形成熔体。晶须的拉伸强度采用Iwanaga等人（1992）的方法和本研究的方法进行测量。强度与晶须的横截面面积有明显的关系，在横截面面积为10 μ m^2时，强度为几个GPa。然而，当前值比Levitt（1966）报告的值低10到10^2。通过XPS化学分析，发现晶须中含有一定量的氟，认为这是导致晶须强度低的主要原因。因此，在进一步提高反应温度的条件下，采用两步法合成晶须，以避免在晶须中掺入氟。以莫来石陶瓷为衬底，加入铁作为杂质，得到了晶须。但与前一种合成方法相比，得到的晶须长度较短，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.