先驱体转化法制备氮化硼纤维研究

发布时间：2018-04-26 19:23

本文选题：先驱体转化法 + 氮化硼　；参考：《国防科学技术大学》2011年博士论文

【摘要】：随着新型高马赫数导弹天线罩对材料防热、承载和透波性能的要求不断提高,现有的透波纤维增强陶瓷基复合材料已经成为天线罩技术发展的瓶颈。氮化硼(BN)纤维具有工作温度高、高温烧蚀率低、工作性能稳定、介电常数和节电损耗小等优点,是新型陶瓷基透波复合材料增强体的理想候选材料。在此背景下,本文开展了聚合物先驱体转化法制备BN纤维的基础研究。在温和条件下分别以共取代和分步取代法合成了三种不同结构、可溶可熔的聚硼氮烷先驱体,合成过程均不需极低温度,简化了操作,降低了成本。分别采用NMR、FTIR、XPS、TGA和元素分析等手段对三种先驱体进行了表征。由三氯化硼和甲胺合成了PTMB,PTMB的分子中含有B3N3六元环、C-H、B-N、N-H和C-N等化学键;PTMB可溶于甲苯和二甲苯等溶剂,典型的PTMB的元素组成为(wt%):B(24.5), C(32.8), N(29.9)和H(10.2),其化学式可表示为BC1.20N0.94H4.5。在NH3中的处理可以提高PTMB的陶瓷产率,在NH3中的无机化可以除去碳元素;失重主要发生在600℃以下,900℃基本完成无机化;PTMB的分子结构和较低的陶瓷产率不适宜用于制备BN纤维,但可能用作制备其它BN材料;经1600℃处理的产物为t-BN,该BN在空气中900℃时的失重小于3.0wt%,表现出了较好的抗氧化性能。由三氯环硼氮烷和异丙胺合成的PTPiAB具有一定的熔融加工性能,PTPiAB的分子中含有B3N3六元环、C-H键、B-N键、N-H键和C-N键等特征结构;PTPiAB可溶于甲苯、二甲苯等常见有机溶剂;典型的PTPiAB的化学组成为(wt%):B (12.4)、N(35.8)、C(37.2)、H(9.3),其化学式可表示为BC2.7N2.2H8.1。1000℃时,PTPiAB在NH3中的陶瓷产率低于在Ar中的陶瓷产率;在NH3中的失重主要发生在800℃以下,800℃先驱体中的有机基团基本消失,接近无机化;NH3的除碳效果明显,600℃时已有94.0wt%的碳被除去;1400℃以上为晶粒长大的主要区间,1800℃热解产物的(002)晶面间距为0.334nm,密度为2.03g·cm-3,在空气中900℃以下的增重小于0.3wt%,抗氧化性能随结晶程度的增加而增强。由正丙胺/甲胺和三氯环硼氮烷合成了PPAB。通过调控原料的比例,控制合成温度和时间等参数,能得到可以熔融的PPAB先驱体,软化点随聚合反应温度的增加和保温时间的延长而升高;优化的合成工艺参数为n-PA?MA?TCB=2?1?1(摩尔比),聚合温度为150~170℃,保温时间为5~7h,所合成PPAB的软化点为90~100℃;典型的PPAB含有B、N、C和H元素,元素组成为(wt%):B(22.28)、C(23.24)、N(44.75)、H(7.78),其化学式可近似表示为BC0.94N1.55H3.75;PPAB分子中含有N-H、C-H、B-N和C-N等化学键,B3N3六元环通过-B-(NCH3)-B-连接;典型PPAB的数均分子量为1002(相对于聚苯乙烯标样),重均分子量为1359,分散系数为1.50,可溶于甲苯和二甲苯等溶剂,在Ar中1000℃的陶瓷产率约为50wt%。对三种先驱体进行的熔融挑丝实验表明,PPAB具有更好的成丝性,更适合用于制备BN纤维。PTMB和PTPiAB可用于制备其它形式的BN材料。对PPAB在空气中的稳定性研究表明,PPAB在空气中极易水解,水解程度随温度、湿度和时间的增加而增大;PPAB对少量的O2不敏感,水解主要是由于H2O引起的;PPAB的水解引入了大量氧元素,生成B-O键,形成不溶不熔的三维网状结构,不能熔融加工;水解的PPAB在Ar中的主要热解产物为BN和B2O3。比较了以PTPiAB和PPAB两种不同分子结构先驱体所制备BN的异同,结果表明:得到的BN均具有近化学计量比,不含碳杂质,具有类似的组成和结构;两种BN的结晶程度不同,具有对称结构的分子单体TPiAB得到的BN结晶程度更高,氧化性能优于由非对称结构单体PAB得到的BN。考察了PPAB的软化点、纺丝温度和纺丝压力等工艺参数对熔融纺丝的影响,当软化点在93~112℃,纺丝温度高于软化点20~70℃,纺丝压力为0.4~0.6MPa,可以得到连续长度大于200m的PPAB纤维。 PPAB纤维在NH3中的不熔化过程伴随着凝胶含量变化、失重和碳含量下降,碳含量的下降主要由烷基基团的脱去引起,C-N键未参与反应;合适的不熔化工艺为,以0.5℃/min的升温速率升温至70~80℃并保温80min,由此得到的PPAB不熔化纤维的凝胶含量大于90wt%。 PPAB在N2中热解得到含碳的BN,而在NH3中热解会脱除碳元素,得到近化学计量比的BN;在N2中的陶瓷产率高于NH3中陶瓷产率,NH3中的热解产物具有更好的结晶性能、抗氧化性能和介电性能。采用NH3作为热解气氛,考察了温度、NH3浓度、保温时间和升温速率等参数对PPAB不熔化纤维无机化过程的影响和无机化纤维组成、结构的影响,结果表明:在800℃基本完成无机化,在NH3/N2=1:1(vol)的混合气氛中即可实现脱碳。较优的无机化工艺为:在NH3浓度为50vol%的NH3/N2热解气氛中,以4℃/min的升温速率升至800℃并保温2h,由此得到的无机纤维碳含量小于0.5wt%。对无机化纤维进行高温处理有利于获得性能更优异的BN纤维,研究了处理温度对纤维结晶性能、密度、力学性能以及抗氧化性能的影响规律。结果表明:1200℃以下是纤维密实性增加的主要阶段, 1800℃得到BN纤维具有较好的抗氧化性,密度为1.92g·cm-3,(002)晶面间距和晶粒尺寸分别为0.337nm和6.50nm,拉伸强度为850MPa,在频率为10GHz处的介电常数为3左右、损耗角正切为10-3量级。
[Abstract]:With the requirements of the new high Maher number missile radome to prevent heat, load and wave performance, the existing permeable fiber reinforced ceramic matrix composites have become the bottleneck of the development of the radome technology. Boron nitride (BN) fiber has high working temperature, low high temperature ablative rate, stable working performance, low dielectric constant and power saving loss. In this context, the basic research on the preparation of BN fibers by polymer precursor conversion method is carried out in this context.
Three different structures, soluble and melted polyboranane precursors were synthesized by CO substitution and stepwise substitution under mild conditions. The synthesis process was not very low, the operation was simplified, and the cost was reduced. The three precursors were characterized by NMR, FTIR, XPS, TGA and elemental analysis respectively.
PTMB is synthesized from three boron chloride and methylamine, and the molecules of PTMB contain chemical bonds of six membered rings of B3N3, C-H, B-N, N-H and C-N, and PTMB can be dissolved in toluene and xylene. The typical PTMB elements are (wt%): B (24.5), C (32.8), 29.9 and 10.2. The carbon element can be removed by the inorganic chemical production in NH3; the weight loss is mainly below 600 C, and the inorganic is basically completed at 900 C; the molecular structure of PTMB and the lower ceramic yield are not suitable for the preparation of BN fiber, but it may be used for the preparation of other BN materials; the product treated at 1600 C is t-BN, and the weight loss of the BN in the air is less than 3 .0wt% shows good antioxidant properties.
PTPiAB, synthesized by three chloro-boranonanane and isopropyl amine, has certain melting processing properties, PTPiAB molecules contain B3N3 six membered ring, C-H bond, B-N bond, N-H bond and C-N bond, and PTPiAB can be dissolved in the common organic solvents such as toluene, xylene, etc.; typical PTPiAB is composed of (wt%): B (35.8), N (37.2), 9.3), and its chemistry When the formula can be expressed as BC2.7N2.2H8.1.1000 C, the ceramic yield of PTPiAB in NH3 is lower than that in Ar; the weight loss in NH3 is mainly below 800 C, and the organic groups in the 800 C precursor are basically disappearing and close to the inorganic; the carbon removal effect of NH3 is obvious, and the carbon of 94.0wt% has been removed at 600 C, and the grain growth above 1400 degrees C is the grain growth. In the main section, the (002) surface spacing of the pyrolysis product at 1800 C is 0.334nm, the density is 2.03g cm-3. The weight gain below 900 C in the air is less than 0.3wt%, and the oxidation resistance increases with the increase of the degree of crystallization.
The PPAB precursor which can be melted can be obtained by controlling the proportion of the raw material and controlling the parameters such as temperature and time by controlling the proportion of the raw material and controlling the temperature and time of the synthetic PPAB.. The softening point increases with the increase of the polymerization temperature and the prolongation of the heat preservation time. The optimized synthetic parameters are n-PA? MA? TCB=2? 1? 1 (mole ratio) and polymerization temperature The softening point of the synthesized PPAB is 150~170 C and 5~7h, and the softening point of the synthesized PPAB is 90~100 C; the typical PPAB contains B, N, C and H elements, and the element composition is B (22.28), C (44.75), 7.78. The average molecular weight of the typical PPAB is 1002 (relative to the standard polystyrene), the weight average molecular weight is 1359, the dispersion coefficient is 1.50, the solvent is soluble in toluene and xylene, and the yield of ceramics at 1000 C is about 50wt%. in Ar.
The experiment of three kinds of precursor melt wire pick - up shows that PPAB has better filaments and is more suitable for the preparation of BN fiber.PTMB and PTPiAB for the preparation of other forms of BN material.
The stability of PPAB in air shows that PPAB is very easy to hydrolyze in the air, and the degree of hydrolysis increases with the increase of temperature, humidity and time; PPAB is insensitive to a small amount of O2, and the hydrolysis is mainly caused by H2O; the hydrolysis of PPAB has introduced a large number of oxygen elements to form a B-O bond, forming an insoluble and unmelted three-dimensional network structure that can not be fused added. The main pyrolytic products of the hydrolyzed PPAB in Ar compare the similarities and differences of BN produced by two different molecular structural precursors of PTPiAB and PPAB, and the results show that the obtained BN has a near stoichiometric ratio, no carbon impurities, similar composition and structure, and the two BN has different crystallinity and a symmetrical structure of molecular single. The crystallization degree of BN obtained by bulk TPiAB is higher and the oxidation property is better than that of BN. obtained by asymmetric structure monomer PAB.
The effects of the softening point of PPAB, spinning temperature and spinning pressure on the melt spinning are investigated. When the softening point is at 93~112 C, the spinning temperature is higher than the softening point 20~70 C, the spinning pressure is 0.4~0.6MPa, and the continuous length of PPAB fiber with a continuous length greater than 200m is obtained.
The melting process of PPAB fiber in NH3 is accompanied by the change of the gel content, the weight loss and carbon content decrease, the decrease of carbon content is mainly caused by the removal of the alkyl group, the C-N bond is not involved in the reaction; the appropriate melting process is that the heating rate of 0.5 C /min is heated to 70~80 and 80min, thus the gel of the PPAB non melting fiber is obtained. The content is more than 90wt%.
PPAB is pyrolyzed in N2 to get carbon containing BN, and the pyrolysis of NH3 will remove the carbon element and get a near stoichiometric BN; the ceramic yield in N2 is higher than the ceramic yield in NH3, and the pyrolysis products in NH3 have better crystallization properties, antioxidant properties and dielectric properties.
The influence of temperature, NH3 concentration, heat preservation time and heating rate on the inorganic process of PPAB fiber and the inorganic fiber composition and structure are investigated by using NH3 as the pyrolysis atmosphere. The results show that the inorganic chemical can be accomplished basically at 800 C and can be decarbonization in the mixed atmosphere of NH3/N2=1:1 (Vol). The better inorganic chemical industry can be achieved. The art is: in the NH3/N2 pyrolysis atmosphere of 50vol% with the concentration of NH3, the temperature rising to 800 C at 4 centigrade /min and thermal insulation 2h, the carbon content of the inorganic fiber is less than 0.5wt%.
The high temperature treatment of inorganic fibers is beneficial to obtaining better performance of BN fibers. The effects of temperature on the crystalline properties, density, mechanical properties and antioxidant properties of the fibers are studied. The results show that the main phase of fiber density increases below 1200 C, and the BN fiber has better oxidation resistance and density at 1800. For 1.92g. Cm-3, (002) the crystal space and grain size are 0.337nm and 6.50nm respectively, the tensile strength is 850MPa, the dielectric constant at the frequency of 10GHz is about 3, and the loss angle tangent is 10-3.

【学位授予单位】：国防科学技术大学
【学位级别】：博士
【学位授予年份】：2011
【分类号】：TQ343

【引证文献】