膨润土的提纯及其在二硅化钼发热元件中的应用

Purification of bentonite and its application in MoSi_2 heating elements

为了提高二硅化钼发热元件的高温使用性能,首先,通过添加不同量(质量分数分别为0. 2%、0. 4%、0. 6%、0. 8%、1. 0%和1. 2%)的磷酸铵,采用湿法对钙基膨润土进行提纯试验;然后,以提纯膨润土为结合剂,经配料、练泥、挤出成型、干燥后,在氢气保护气氛中分别于1 450、1 500、1 550和1 600℃保温2 h烧成制成二硅化钼发热元件试样,检测试样的体积密度和显气孔率,并进行SEM和EDS分析;最后,对二硅化钼发热元件试样和商品二硅化钼发热元件在1 750℃保温2 h条件下进行使用对比试验。结果表明:1)添加1. 0%(w)的磷酸铵湿法提纯后,膨润土的Fe_2O_3、K_2O、Na_2O含量和灼减大幅降低,蒙脱石含量提高到87. 6%(w)。2)以提纯膨润土为结合剂、在1 550℃保温2 h烧成的发热元件的显气孔率和体积密度分别为0. 83%和5. 73 g·cm^(-3),致密度较高,其断裂方式以穿晶断裂为主。3)在1 750℃保温2 h使用后,以提纯膨润土为结合剂制备的发热元件表面保护膜完整,而采用未提纯膨润土为结合剂生产的商品发热元件表面保护膜破损严重。

This work aims at improving the high temperature performance of MoSi2 heating elements.Firstly,calcium-based bentonite was purified by adding different amounts of ammonium phosphate(0.2%,0.4%,0.6%,0.8%,1.0%and 1.2%,by mass)by wet method.Then,MoSi2 heating element specimens were prepared using the purified bentonite as the binder,batching,muding,extruding forming,drying,and firing in hydrogen protective atmosphere at 1 450,1 500,1 550 and 1 600℃for 2 h,respectively.The bulk density and apparent porosity of the specimens were analyzed by SEM and EDS.Finally,a comparative test between the developed MoSi2 heating element specimen and commercial MoSi2 heating elements was carried out at 1 750℃for 2 h.The results show that:(1)after wet purification with 1.0%of ammonium phosphate,the contents of Fe2O3,K2O,Na2O,and loss on ignition of bentonite decrease greatly,the content of montmorillonite increases to 87.6 mass%.(2)For the specimens using the purified bentonite as the binder,the apparent porosity and bulk density of the heating element fired at 1 550℃for 2 h is 0.83%and 5.73 g·cm^-3,respectively,the density is high,and the fracture mode is mainly transgranular fracture.(3)For the specimens fired at 1 750℃for 2 h,the surface protective film of heating element using purified bentonite as binder is complete,while the surface protective film of the commercial heating element produced by unpurified bentonite is seriously damaged.