摘要
由先驱体高分子转化制备高性能陶瓷材料的技术具有成型容易、反应温度低、分子结构可调等优点,目前已广泛应用于以SiC陶瓷纤维为代表的各种SiC陶瓷材料的制备。有机高分子先驱体可溶可熔的特点使得该法在低维陶瓷材料的制备工艺中具有其他制备方法无可比拟的优势。先驱体高分子可通过熔融和溶液两种方式成型,其中溶液成型不需高温操作和惰性气体保护,具有广泛的应用空间,因此有必要对先驱体溶液的性质进行研究。
本文以含铝碳化硅纤维的先驱体聚铝碳硅烷(PACS)为研究对象,从基本热力学参数入手,探索了PACS的溶解性能,并采用浊点滴定法绘制三元相图来反映PACS/溶剂/非溶剂的相分离情况。在此基础上对PACS/四氢呋喃/乙醇体系的沉淀性能进行了深入研究。通过比较将沉淀剂加入溶液(Prec-Solu)和将溶液加入沉淀剂(Solu-Prec)两种沉淀方式对沉淀结果的影响,发现Solu-Prec方式操作简单,得到的PACS沉淀粒径小、颗粒分散性好、颗粒间分子量分布均匀,但采用此方式沉淀需要适当延长搅拌时间来实现体系的平衡。研究表明,PACS四氢呋喃溶液的沉淀过程可分为三个阶段:首先是线性大分子的沉淀,随后支化大分子发生沉淀,在沉淀后期中小分子才能被沉淀出来。
对PACS溶液沉淀性能的基础研究和工艺探索说明采用沉淀的方法对PACS粗料进行纯化是可行的。与减压蒸馏法得到的PACS精料(Dis-PACS)相比,沉淀纯化产物(Pre-PACS)具有低分子量部分含量较低,陶瓷产率较高等优点,更适合制备高性能的SiC陶瓷材料。
对以上两种PACS精料流变性能的研究表明,由于两者分子量和分子量分布的不同,在相同温度下Dis-PACS的表观粘度比相同软化点的Pre-PACS要小。因此,要提高Pre-PACS的纺丝稳定性必须在传统Dis-PACS的基础上适当提高纺丝温度。Pre-PACS经熔融纺丝、空气不熔化和高温烧成可以制备性能优良的Si-C-Al-O纤维。
The polymer-derived technology has been widely used to prepare various advanced ceramic materials because it offers many advantages such as easy to shaping, low processing temperature and tailorable ceramic compositions. The preceramic polymer can be shaped by means of either melting or solution. The shaping with polymer solution can be carried out at room temperature without inert gases. It is necessary to study the properties of preceramic polymer solution because it has wide applications
In this paper, polyaluminocarbosilane (PACS), precursor of SiC(Al) ceramic fiber, was considered as the starting point of the study. Basic thermodynamics parameters of PACS solution were determined to investigate the solubility of PACS in various solvents. The phase separation behavior of PACS / solvent / precipitant systems can be elucidated by ternary phase diagrams constructed by cloud point titration. Instead of using the conventional precipitation method (adding precipitant into solution, or Prec-Solu), PACS was precipitated by means of dropping PACS solution into precipitant (Solu-Prec). With the later method, PACS precipited in the form of fine dispersed white particles with small size. The molecular weight among these particles is relatively uniform. In the process of precipitation for PACS in THF solution, linear macromolecules precipitate at the beginning, followed by branched molecules and then low molecular weight species.
Basic studies and technological investigations of the precipitation behavior of PACS solution indicate that the purification of PACS by precipitation is an effective method. Comparing with the conventional PACS purified by distillation (Dis-PACS), the PACS purified by precipitation (Pre-PACS) is composed of less low molecular weight species, that leads to a higher ceramic yield. This is an advantage to prepare SiC ceramic materials with higher properties.
The rheological property of Pre-PACS was investigated with comparison of Dis-PACS. For both samples with similar melting temperature, the apparent viscosity of Pre-PACS is higher than that of Dis-PACS at any specific temperature because of the differences of the molecular weight and the distribution. Therefore, the spinning temperature of Pre-PACS should be higher than conventional Dis-PACS. Si-C-Al-O fiber with excellent properties can be prepared by melting spinning, air curing and pyrolysis with Pre-PACS.
引文
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