Polycarbosilazane Precursors Research Articles

Recent Advances in Preceramic Polysilazane Precursors in Additive Manufacturing

Si3N4/SiCN ceramics have been extensively explored for applications in the aerospace, mechanical engineering, and biomedical fields. Recently, there has been significant focus on the additive manufacturing (AM) of polymer-derived ceramic (PDC) technology for fabricating Si3N4/SiCN ceramics. The chemical structure and composition of the preceramic polymer precursors have a crucial influence on the performance of ceramic products. In this paper, recent advances in the use of polysilazane and polycarbosilazane precursors in AM are reviewed and an outlook for future development is presented. The findings of this study could spark inspiration and reflection regarding AM applications and synthetic technology. It is believed that the development of PDCs in ceramic fabrication will become more versatile and application-oriented and provide more freedom in the design of high-performance ceramics.

Polycarbosilazane passivation on graphite foil used as gasket seal

Gasket seals are often used in industry and laboratories where a leak-proof installation is needed in order to avoid loss of products or dangerous materials. Many of products transported inside tubes are at relatively high temperatures excluding polymeric gaskets. In the same line, many of transported materials contain solvents which can attack polymer sealings, therefore limiting their use. An alternative is to use graphite joints as sealings. These joints are a sandwich of graphite foil and stainless steel mesh as forming core. The problem that raises using graphite in contact with steel is that at temperatures of about 500 °C an interdiffusion of carbon on the steel structure occurs which produces adhesion of the graphite gasket on the metallic flange. Therefore this adhesion increases the time to change each gasket, since rests of previously adhered graphite has to be removed from the flange. In order to avoid the adhesion of the graphite on the flange, polycarbosilazane precursor was used as protective finishing on the graphite foil surface. After thermal transformation of the polymer into the corresponding PDC finishing, it acts in two manners: It avoids the direct contact between the carbon and the steel and it allows the sealing of liquids and gases. Adhesion tests were done and showed that the foils passivated with PDCs did not adhere to the steel flange. Moreover, the production methods and products are compatible to industrial environment and processes. The results found here show that the time to change the gasket in industry can be clearly reduced by using the PDC finishing on graphite gasket.

Thermal behavior of polymer-derived ceramics. IV. SiCNO fibers from an oxygen-cured polycarbosilazane

Fibered SiCNO-based ceramics from a polycarbosilazane precursor (copolymer) are studied, then compared to other ceramic materials previously prepared and described in other papers. The emphasis is upon the structural and textural features as observed by high-resolution transmission electron microscopy and their evolution with an increasing heat-treatment temperature up to 1600°C. Both argon or nitrogen atmospheres were used. Specifically, the peculiar behavior of the material during the amorphous to crystalline transition until degradation is revealed. The consequences on the nucleation and the growth of the SiC grains, and the electrical and mechanical properties are discussed. Faserverstärkte SiCNO-Keramiken, hergestellt aus einem neuen Polykarbosilizanvorläufer (Copolymer) wurden untersucht und mit anderen Keramiken verglichen. Der Schwerpunkt liegt auf der Untersuchung der Struktur und Textur mittels hochauflösender Transmissionselektronenmikroskopie und ihrer Entwicklung mit zunehmender Wärmebehandlungstemperatur bis zu 1600°C in einer Argon- oder Stickstoffatmosphäre. Speziell das eigentümliche Verhalten des Materials während des Übergangs vom amorphen in den kristallinen Zustand bis zur Degradation konnte aufgeklärt werden. Die Auswirkungen auf die Keimbildung und das Wachstum der SiC-Körner und die elektrischen und mechanischen Eigenschaften werden diskutiert. On étudie des céramiques de géométrie filamentaire appartenant au système SiCNO et élaborées à partir d'un précurseur copolymère de type polycarbosilazane. L'accent est mis sur les caractéristiques structurales et texturales révélées par microscopie électronique par transmission à haute résolution, et sur leur évolution au cours d'un traitement thermique croissant jusqu'à; 1600°C. En particulier, le comportement du matériau durant la transition amorphe-cristal jusqu'à dégradation s'avère très spécifique, et ses conséquences sur la nucléation puis la croissance des cristaux de SiC, ainsi que sur les propriétés électriques et mécaniques sont discutées

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Si-C-N-O monofilaments were spun from a molten new polycarbosilazane (PCSZ) precursor, cured under oxygen within the temperature range 90-230°C and submitted to a pyrolysis treatment in argon up to 1600°C. The curing step was studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and infrared (i.r.) analysis. The effect of the curing conditions on the weight loss, composition, structure and tensile properties was characterized after pyrolysis. The main reaction occurring during the curing step is the oxidation of Si-H bonds.

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Oxidation tests have been performed on model Si-C-N-O filaments prepared from a new polycarbosilazane (PCSZ) precursor, according to a spinning-oxygen-curing-pyrolysis procedure. Thermogravimetric analyses have been run under a flowing atmosphere of dry oxygen (P=100 kPa) at 1000<T<1400°C. The oxidation treatment results in the growth of a silica layer (passive oxidation regime) and simultaneously the release of carbon oxides and nitrogen, with an overall weight increase. At a given test temperature, the kinetics of growth of the silica scale obeys a parabolic law. The parabolic rate constant is thermally activated with an apparent activation energy of 170 kJ mol -1 .

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Si-C-N model filaments almost free of oxygen have been prepared from a novel PCSZ precursor by melt-spinning, γ-ray curing and pyrolysis under pure nitrogen (or argon) at a temperature θp as high as 1600°C. The organic-inorganic conversion of the precursor takes place at 450<θ p <850°C. It yields an amorphous filament whose composition is close to SiC 0.93 N 0.46 (with less than 2 wt % O). No significant change in composition and microstructure occurs up to about 1400°C.

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Melt-spun/oxygen-cured polycarbosilazane monofilaments were pyrolysed under an argon or nitrogen atmosphere at pyrolysis temperatures, θ p , up to 1600°C. The organic/inorganic transition took place at 500<θ p <850°C with a first weight loss and diameter shrinkage (evolution of CH 4 and H 2 ). It yielded an amorphous Si-C-N-O material which underwent only minor changes (e.g. densification) when θ p was raised to about 1400°C. In contrast, beyond 1400°C, major changes occurred which depended on the pyrolysis atmosphere.

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis and thermal crosslinking of polysilasilazane (PSSZ) copolymers. Their pyrolysis under inert gas, producing Si-C-N ceramics, was investigated up to 1600 °C by analyses performed on the solids (elemental analysis; EPMA; TGA, density; 1H, 13C and 29Si solid state NMR, i.r. XRD, electrical conductivity) and analyses of the evolved gases (gas chromatography and mass spectrometry). From 250 to 450 °C, a first strong weight loss was observed, which was due to the formation and elimination of low-boiling-point oligomers. The weight loss closely depends on the cross-linking degree of the ceramic precursor resulting from the PSSZ/PCSZ conversion. Then, the organic/inorganic transition took place between 500 and 800 °C, proceeding via evolution of gases (mainly H2 and CH4) and yielding a hydrogenated silicon carbonitride. This residue remained stable up to 1250 °C although it progressively lost its residual hydrogen as the temperature was raised. Then, crystallization occurred between 1250 and 1400 °C, yielding β-SiC crystals surrounded by free-carbon cage-like structures. Finally, above 1400 °C, the remaining amorphous Si-C-N matrix underwent a decomposition process accompanied by nitrogen evolution and a second substantial weight loss. At 1600 °C, the pyrolytic residue was a mixture of β-SiC and free carbon. So, the amorphous silicon carbonitride resulting from the pyrolysis of PCSZ precursors was found to be appreciably more thermally stable than the previously reported Si-C-O ceramic obtained by pyrolysis of polycarbosilane precursors.

Preparation of silicon carbide‐silicon nitride fibers by the pyrolysis of polycarbosilazane precursors: A review

AbstractThe development of silicon carbide‐silicon nitride (SiC‐Si3N4) fibers by the pyrolysis of polycarbosilazane precursors that was carried out in this laboratory is reviewed. Precursor resin, which was prepared by heating tris(N‐methylamino)methylsilane or tris(N‐methylamino)phenyl‐silane to about 520°C, was drawn into fibers from the melt and then made unmeltable by humidity conditioning at 100°C and 95 percent relative humidity. The humidity‐treated precursor fibers were pyrolyzed to ceramic fibers with good mechanical properties and electrical resistivity. For example, SiCSi3N4 fibers derived from tris(N‐methyl‐amino)‐methylsilane had a tensile rupture modulus of 29 × 106 psi and electrical resistivity of 6.9 × 108 Ω‐cm, which is 1012 times greater than a value obtained for graphite fibers.

Thermogravimetric analysis of silicon carbide-silicon nitride polycarbosilazane precursor during pyrolysis from ambient to 1000° C

Thermogravimetric analysis data are presented on the unmeltable polycarbosilazane precursor of silicon carbide-silicon nitride fibers, over the room temperature-1000 C range in a nitrogen atmosphere, in order to establish the weight loss at various temperatures during the precursor's pyrolysis to the fiber material. The fibers obtained by this method are excellent candidates for use in applications where the oxidation of carbon fibers (above 400 C) renders them unsuitable.

Preparation of silicon carbide–silicon nitride fibers by the controlled pyrolysis of polycarbosilazane precursors

AbstractExperiments to produce polycarbosilazane resin and high‐strength silicon carbide–silicon nitride (SixNyCz) fibers as well as resin/fiber characteristics are reported. Polycarbosilazane resin was drawn into fibers from the melt and subsequently treated and pyrolyzed into SixNyCz fibers. These materials are characterized by high tensile modulus (29 × 106 psi for 0.4‐mil diameter) and high electrical resistivity (6.9 × 108 Ω·cm for 0.6‐mil diameter).