In order to meet the requirements of high temperature structural materials, silicon carbide fibers have evolved from the initial high oxygen content, rich free carbon and low crystallinity (CG-Nicalon) to third generation products with near stoichiometry, low oxygen content, and high crystallinity ( Hi-Nicalon type S and Tyranno SA3). Studies of nuclear power projects in the United States and Japan have shown that the use of third-generation silicon carbide fibers in SiCf/SiC composites greatly improves the structural and performance stability under neutron irradiation conditions. Therefore, in order to develop SiCf/SiC composite materials for nuclear use in China, it is necessary to solve the large-scale production technology of third-generation silicon carbide fibers, which is also crucial for applications under aerospace and other extreme conditions.
Continuous SiC fibers refer to fibers whose fiber product length exceeds 500 meters. High-performance continuous SiC fiber meets the demanding requirements of high-performance CMC: fine diameter, oxidation resistance, high temperature resistance, creep resistance, and corrosion resistance; can be in an atmosphere of not less than 1300°C in air and not less than 1600°C in an inert atmosphere Stable use; fiber strength up to 1960 ~ 4410MPa, modulus up to 176 ~ 400GPa. At present, the research and production of continuous silicon carbide fibers in the international and domestic markets are quite lacking. The high-end applications in this field, including the development of nuclear energy materials, are basically monopolized by Japan's Toyo Carbon and Ube Industries. If China hopes to get rid of the import of key materials in the advanced nuclear energy system in the future, it must carry out systematic and long-term research and development of this strategic material.
The silicon carbide (SiC) fibers are prepared by a precursor conversion method, and the fiber spinning is usually performed by a melt spinning process. Polycarbosilane precursors (PCS) are brittle polymers, usually have poor spinnability, and due to the oxygenophilic nature of the precursors, there is a high requirement for the melt spinning process. It is generally believed that the fiber precursor should have characteristics such as fine and uniform diameter, less hairiness, and good bundling properties. To meet these characteristics, raw materials, processes, and equipment must be matched with each other.
1) Raw materials: Control of melt viscosity of PCS precursors is the key to melt spinning, and the molecular weight and distribution of precursors and the degree of molecular chain branching directly affect the melt viscosity. The precursor viscosity is very sensitive to temperature and the range of spinnable temperatures is narrow. If the temperature is too high or too low, problems such as poor melt spinnability and poor fiber quality will result.
2) Process and equipment: High-quality continuous strands are a prerequisite for the preparation of continuous silicon carbide fibers, while PCS strands are brittle fibers that break or break under very little external force. Therefore, the process of bundling, oiling, and winding the yarn must be strictly controlled. In addition, due to the philophilic nature of the PCS precursor, melt spinning must be performed in an inert atmosphere.
3) As the PCS strand breaks or breaks under a small external force, how to unwind the continuous filaments obtained by spinning to carry out the subsequent process is an important process for preparing continuous silicon carbide fibers. In order to achieve this goal, it is necessary to use a suitable oil agent in the spinning process to improve the bundled and uniform properties of the fibers and to reduce fiber defects such as broken filaments and broken filaments.
In view of the speciality of PCS precursor melt spinning, the silicon carbide fiber research team of the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, has repeatedly adjusted the spinning process, airtightness of the equipment, temperature control system, bundling device, and winding device. Make renovations and improvements. After repeated exploration, a 1-250-diameter silicon carbide precursor melt spinning test line has been built. There are two sets of pilot-spinning and pilot-scale spinning and spinning equipment; now 250 holes of fiber have been prepared, and the continuous length can be unwound, laying the foundation for the subsequent continuous silicon carbide fiber preparation. The silicon carbide obtained by melt-spinning is brittle and has poor operability; in order to maintain the initial shape of the fiber and prevent the fiber from melting and spinning during high-temperature firing, the fiber must be infusible to form a crosslinked structure of the fiber. Fibers do not melt when processed at high temperatures and increase the ceramic yield of the fiber.
However, the infusibilized fibers are basically disorderly stacked in the fiber axis and are organic fibers and have poor mechanical properties. High-temperature firing is a process in which an organic PCS fiber is converted into an inorganic SiC fiber in an inert high-temperature environment. In the process of inorganicization, fibers are progressively dehydrogenated and crosslinked, decomposed in the side chains of the molecular chains, small molecule overflows, and β-SiC grains are formed and gradually grown, and high-performance SiC fibers mainly composed of β-SiC grains are obtained.
The currently used methods of infusibilization include air infusibility, radiation cross-linking, and chemical vapor crosslinking. Among them, the air non-melting method is the earliest method for the crosslinking of continuous silicon carbide fibers. The disadvantage of this method is that a crosslinked structure is formed by the direct reaction of O in the air with the Si-H of the fiber. This kind of cross-linking method introduces a large amount of oxygen into the fiber, which reduces the performance of the fiber in a high-temperature environment. However, this method is simple in process, less in environmental pollution, and operability is strong. Under normal experimental conditions, cross-linking of fibers can be achieved. Therefore, there are many applications in the development of silicon carbide fibers. Radiation crosslinking is the use of high-energy particles or electromagnetic waves to form a cross-linked structure of SiC precursor, which is characterized by the introduction of oxygen during the crosslinking process, can effectively reduce the oxygen content of SiC fibers, improve the high temperature performance of the fiber in the air. However, the irradiation cross-linking method is expensive, the process is complex, and the environment is demanding. To achieve a suitable degree of cross-linking requires high-dose irradiation, and the cost is high, which restricts its application in the development of SiC fibers. Chemical vapor crosslinking involves placing the PCS precursor in a reaction atmosphere and performing a cross-linking reaction at a specific temperature.
The SiC fiber research team has established a test line for the insolubilization, high-temperature firing and sintering of silicon carbide fibers with the help of the accumulated fiber development experience on the carbon fiber project. Mainly adopts air-crosslinking for PCS fiber precursors, and radiation-induced cross-linking is the auxiliary incompatibility. The process of air-insolubilization is mainly the process of increasing the weight of Si-H groups in the fiber precursor to form a cross-linked structure. How to increase the degree of fiber cross-linking and effectively control the oxygen content is the key to infusibilization. Experiments have found that the oxygen content of the fiber has a linear relationship with the degree of Si-H bond reaction and fiber weight gain. These factors are not only related to the precursor, but also determined by the infusibilization process, such as temperature, heating rate, processing time, and atmosphere. Flow, etc. Therefore, the process conditions of the infusibilization must be strictly controlled. Through continuous exploration, we have basically grasped the key factors of the quality of infusibilized fibers and achieved controllability. At present, it has been equipped with small-scale pilot and pilot air infusible equipment, sintering equipment, drafting and wire collection devices. In view of the high-temperature firing of fibers, the research team adjusted the firing temperature and drawing rate based on short fibers, and tried a one-step method and a multi-step firing process. At present, the process of burning infusibilized fibers has been initially determined. Silicon carbide short fibers with a tensile strength of 2.1 GPa and a modulus exceeding 300 GPa were prepared by infusible treatment and high-temperature firing.
PCS infusible fiber will shrink significantly at high temperature firing, with a shrinkage rate of 23%. Therefore, it is necessary to maintain the continuity of the fiber and prevent the decrease of the mechanical properties of the fiber due to rapid shrinkage (caused by fiber internal defects). The important part of SiC fiber. Researchers started from the shrinkage rate of fiber, combined with the characteristics of the drafting device, continuously optimized the drawing speed and firing temperature, and modified the drafting device. At present, the preparation of continuous silicon carbide fibers has been realized.
The SiC fiber research team began to undertake the task of developing third-generation silicon carbide fiber from the beginning of 2015. After more than one year of hard work, it independently developed spinning equipment. Important advances have been made in the development of continuous silicon carbide fibers, and breakthroughs have been made. Prepared, melt-spun, infusible to fire the entire technical route. The next step will be to further improve the melt spinning technology, in-depth study of the structure change of the small diameter silicon carbide fiber in the infusible treatment, sintering process, improve the process, and achieve the preparation of high performance continuous silicon carbide fiber. At the same time, basic research will be conducted on the control of the structural components of the spinning precursor, the physical and chemical processes of the precursor silicon carbide ceramic, the fiber phase composition, and the irradiation evaluation.
Hose Reel Cart,Portable Hose Reel Cart,Commercial Hose Reel Cart,Industrial Hose Reel Cart
NINGBO QIKAI ENVIRONMENTAL TECHNOLOGY CO.,LTD , https://www.hosereelqikai.com