Preparation method of silicon nitride fiber containing boron

The invention relates to a preparation method of silicon nitride fiber containing boron. Firstly, high-activity Si-N fiber is prepared by converting polycarbosilane non-melting fiber, then chemical introduction of a B element is realized by utilizing H2 elimination reaction between excessive N-H in the high-activity Si-N fiber and high-chemical activity boron compound gas, and then nitridation is performed in ammonia gas, so as to prepare the silicon nitride fiber containing boron. Compared with the prior art, the silicon nitride fiber containing boron, which is prepared by adopting the method, has the advantages of high boron content, uniformity in distribution, high nitrogen content and high fiber strength. The preparation method has the advantages of simple process and low cost and is suitable for large-scale production, and an existing decarburization-nitridation system does not need to be changed.