Papers by Keyword: Polycarbosilane

Abstract: The paper is devoted to the research results dealing with the development of heat-resistant composite cellularmaterial, where polymer matrix is based on10% polycarbosilanesolution in xylene, hollow microspheres and fibrous mineral material are used as fillers. In this study, the used hollow microspheres are corundum and aluminosilicate microspheres, and fibrous filler is quartz fiber. According to the experiments, the best results refer to thecomposite foam samples, containing hollow corundum microspheres as filler. The developed composite foam is characterized with low thermal conductivity and density, good physical and mechanical properties,when polymer binder content was 20-70% wt., corundum hollow microspheres - 20-60% wt., and quartz fiber material - 10-50% wt. Thus, the developed composite foam might be recommended for the applicationin high-tech industries, including heat and power engineering, nuclear power and aircraft construction as high-temperature thermal insulation material.

Abstract: A ZrB₂-SiC multiphase ceramic was fabricated with a precursor pyrolysis method by using zirconium boride organic precursor and polycarbosilane. In order to study the pyrolysis process of complex phase ceramics, the pyrolysis process of the two precursors was studied respectively at the beginning. The precursor and the mixture with different weight ratios both pyrolyzed at 800°C. ~1500°C. under argon atmosphere. The specimens were characterized by thermo-gravimetric analysis, infra-red, X-ray diffraction. It was found that the pyrolysis products of PCS remained amorphous below 1000°C. and converted into crystallization with temperature increasing. ZrO₂ as the intermediate pyrolysis product of ZrB₂ precursor initiate convert to t-ZrO₂ from m-ZrO₂ at 1200°C.. Finally, all kinds of ZrB₂-SiC multiphase ceramic are successful obtained via pyrolysis of precursor after 1500°C. And the 40wt%ZrB₂ precursor initial carbothermal reduction temperature below the ZrB₂ precursor reactive temperature and the crystal degree of SiC is higher than pyrolysis product of PCS.

Abstract: Dynamic Time Sweep of Rotational Rheometer has been Used to Study the Variation in Rheological Properties of Polymers with Time. Polycarbosilane (PCS) is a Solid Oligomer at Room Temperature and Becomes Melt at its Softening Temperature. Bubbles are Unavoidably Produced by Gasification of Low Molecular Weight Part of PCS, which Significantly Disturbs the Subsequent Rheological Measurement. however, the Rheological Data of PCS Melt Cannot Be Repeated on Conventional Dynamic Time Sweep Test even after Reducing the Bubble. in this Work, a Series of Oscillatory Rheological Measurements were Carried out by Temperature Control to Improve the Data Reliability. the Axial Force Data of PCS Melt were Manually Recorded and Compared before and during the Test, which Reflected the Response of PCS Melt to Temperature Change. the Results Confirmed that the Shear Stress of PCS Melt was Readily Affected by Temperature Alteration. the Data Repeatability of the Rheological Test was Evidently Improved for PCS Melt with the Temperature Control.

Abstract: Rotational rheometer has been widely used in studying rheological properties of materials. However, the conventional rheological measurements may not be applicable for some non-traditional materials with characteristic properties. Polycarbosilane (PCS) is an oligomer and in the form of solid state at room temperature. A large amount of air bubbles are evolved during the heating process of PCS, which is extremely harmful for rheological measurements of PCS melt and detrimentally influences the accuracy of rheological data. In this work, a series of PCS raw samples were pretreated with different heating processes by the advanced rheometric expansion system at 315°C before the rheological measurements at 295°C. The softening point temperatures and molecular weight distributions of different PCS samples were obtained before and after the preheating processes. The experimental data collected with the in-situ preheating and ex-situ preheating for 1 h were compared.

Abstract: As an important precursor in preparation of silicon carbide ceramic fibers by polymer precursor-derived method, it is very difficult to find the zero shear viscosity through the conventional steady test for the oligomer-like polycarbosilane (PCS) because of the possible oxidation and a prolonged time required to attain a steady state of the samples. In this work, a steady test and a series of transient tests were performed for the PCS melts. The limitation of the steady test is discussed. An averaging method based upon data from the transient tests is developed to determine the zero shear viscosity for the melt. The reliability of the results from the method is discussed and the zero shear viscosity of the pretreated PCS melt was obtained.

Abstract: Foam-type porous silicon carbide (SiC) ceramics without cracks and hollow struts were fabricated using the polymer replica method with polycarbosilane (PCS) and polyurethane (PU) foam as the starting materials. The synthesized porous SiC was analyzed using X-ray diffraction and scanning electron microscopy. The results revealed that a porous SiC ceramic structure was formed with a dense framework at a low temperature of 1200°C. During the heat-treatment process, the PCS experienced an organic–inorganic transformation and then converted to the SiC ceramics. It was determined that the organic–inorganic transformation of PCS, which was the stage of silicon oxycarbide formation, is affected by the curing condition. In this study, the optimum curing condition was determined to be an air atmosphere at 200°C for 7 h.

Abstract: Polycarbosilane (PCS) solution was used to form PCS fibers by electrospinning and curing them by thermal process. The cured PCS fibers were then pyrolized under inert atmosphere to obtain silicon oxycarbide (SiOC) fibers. The PCS solution contained 1.2 g/mL PCS with 30% N, N-dimethylformamide (DMF)/70% toluene. The needle inner diameters used for spinning were 0.5 and 0.3 mm and variation for the applied voltage were 10, 12 and 14 kV. The electrospun PCS fibers were cured at 200°C in oxygen atmosphere for 1 hour and then pyrolyzed at 1000°C in inert atmosphere for 1 hour. Nonwoven SiOC fibers diameter ranging between 3 to 8 µm were analyzed by SEM and EDS. The oxygen embodied on the surface of cured PCS fibers arising during the curing process resulted in the SiOC fibers with larger diameters. Rapid solvent evaporation during the pyrolysis caused the SiOC fibers to have ribbon-shapes.

Abstract: In this work, the influences of composition and microstructure on the mechanical properties and thermal stability of SiC ceramic fibers were investigated. XPS, XRD, SEM, and element analysis were used to analyze the elemental composition and structural morphology. The contents of oxygen and free carbon influence the crystallinity of SiC fibers, which inhibit the grain growth of β-SiC. The reduction of tensile strength of the fibers sintered under temperatures above 1700°C is attributed to the appearance of massive defects on the outer surface of the fibers, which can be overcome by the change of sintering conditions of the pyrolysis fibers.

Abstract: Up to now, many types of inorganic fibers have been developed. The main purpose is to develop composite materials with lightweight and high fracture toughness. Of these, carbon fiber has already established a very big market. By the way, representative oxide fibers (alumina/silica-based fibers) show heat-resistance’s limitation at around 1200°C. In order to improve the heat-resistance, some types of eutectic oxide-fibers have been studied. On the other hand, SiC fibers with both heat-resistance and oxidation-resistance were developed over 30 years ago. After that, lots of improvements have been performed, and finally several types of excellent heat-resistant SiC-polycrystalline fibers, which can be used up to about 1800°C, were developed from polycarbosilane. Using these fibers, lots of applications have been considered in the fields of aerospace, nuclear system, and so on. Furthermore, making the best use of the aforementioned production process, several types of functional ceramic fibers with gradient-like functional surface layers also have been developed. In this paper, of these inorganic fibers, heat-resistant SiC fibers will be addressed along with historical view point on ceramic fibers.

Abstract: SiC nanopowder, polycarbosilane and divinylbenzene mixed slurries were prepared for viscosity measurement, which were used as matrix source of ceramic matrix composites. Results showed that apparent viscosity of the slurries increased with the increase of the content of SiC particles. The slurries with 50 nm SiC particles showed a low viscosity as compared with those slurries with 20 nm or 120 nm SiC particles at the same content of SiC. In particular, when the viscosity of slurry was higher than 30 mPa•s, the slurry could not be used in the test. Three-dimensional carbon fiber (3D-C_f) preforms were infiltrated with the aforementioned slurries. Addition of the nanoSiC powder as the inert filler in the slurries led to reduce the porosity and the infiltration–curing–pyrolysis cycles to manufacture 3D-C_f/SiC composites by the subsequent polymer impregnation and pyrolysis (PIP) process. Characterizations of the composites showed that the maximum flexural strength of specimen in the composites was 326 MPa and its fracture toughness was 10.5 MPa•m^1/2.