Preparation of Long Alumina Whiskers

以氟化铝为原料，采用高温化学气相反应法制备了氧化铝晶须。通过改变反应气体的种类和流量、水气分压、反应温度和反应时间等重要因素进行合成。当一步合成方法升华氟化铝并在衬底上沉积氧化铝晶须时，在1200ﾟC的反应温度下，晶须可以生长到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。由于晶须顶端存在液滴状沉淀物，因此认为晶须是通过气-液-固(VLS)机制生长的。液滴中含有K和Na元素作为杂质，在高温下形成熔体。(1992)方法和本研究的方法。强度与晶须的横截面积有明显的相关性，在横截面积为10微米/平方厘米时，强度为几Gpa。但目前的值比Levitt(1966)报道的低10~10^2。XPS化学分析发现，目前的晶须中含有一定含量的氟，这被认为是导致强度低的主要原因。因此，为了避免在晶须中掺入氟，采用两步合成法研究了在更高的反应温度下合成晶须。以莫来石陶瓷为衬底，加入金属铁作为杂质，用该方法获得了晶须。然而，与前一种合成方法相比，得到的晶须长度较短，产率较低，需要为第二种合成方法寻找进一步的最佳实验条件。

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.