Papers by Keyword: Preceramic Polymer

Abstract: Alumina-mullite composites with low shrinkage can be made by reaction bond using mixtures of alumina, aluminum and silicon carbide. In this work, an alternative route is used to produce alumina composites with low shrinkage. Here alumina samples containing additions of 10 and 20 wt% of a preceramic polymer were warm-pressed and treated in the range of 900 -1500°C to produce alumina based composites. The obtained composites were analyzed by linear shrinkage and compared to pure alumina samples sintered at the same temperature range. It were also evaluated the density variation and crystalline phases formed during heat treatment of alumina composites. Results showed that alumina-silicon oxycarbide and alumina-mullite composites were obtained with lower shrinkage than pure alumina samples.

Abstract: SiO₂ nano/submicron-structures have attracted tremendous interest due to their importance in extensive applications. In this work, SiO₂ nano/submicron-rods were successfully synthesized by a simple method through pyrolyzing an amorphous polysilazane preceramic powder with catalyst FeCl₂. The synthesized nano/submicron-rods possess well-defined shape with diameter of 700-900 nm and high purity.

Abstract: Polymer Impregnation Pyrolysis (PIP) is a cost effective technique for obtaining Ceramic Matrix Composites (CMC) modified with nanoparticles. Commercial UBE polymeric precursor (Tyranno polymer VL-100, diluted in xylene) of a SiC ceramic matrix (with 11 wt% O and 2 wt% Ti) was used to infiltrate 100x85x3 mm^{Superscript text}3 SiC felts (Tyranno ZM fibers, diameter 14 microns, 800 filament/yarn, 270 g/m^{Superscript text}2, with 9 wt% O and 1 wt% Zr), applying different pyrolysis procedures. In particular, pyrolysis was performed in two conditions: 1) at 1000 °C for 60 min; 2) at 900 °C for 120 min. A pyrolysis at 900 °C could be more convenient since it can be easily performed in a steel furnace, without a refractory lining. The SiC felts were pretreated by CVD (Chemical Vapour Deposition) in order to deposit a pyrolytic carbon interphase (about 0.1 microns). Impregnation was performed under vacuum, and drying was carried out in an explosion-proof heating oven. Pyrolysis at 900°C was performed in a AISI 310S austenitic steel furnace, under nitrogen flow. Geometric density was monitored during densification. Mechanical characterisation (bending tests at room temperature, following UNI EN 658-3:2002) was performed after 11 PIP cycles. The results were used to compare the influence of pyrolysis temperature on densification.

Abstract: A low cost, easily processable, multi layered functionally graded ceramic coating of thickness ~1000 µm was developed to protect metallic substrates from the risk of thermal oxidation. It consists of a four layered functionally graded coating consisting of aluminide layer followed by an intermediate zirconia as thermal insulative layer, an alumina layer serving as buffer layer with the outer most high emissivity layer to provide the required emissivity. All the layers were brush coated and the specimens were cured at 150°C. The solar absorptivity and emissivity of the coating was found to be 0.82 and 0.88 respectively. 15CDV6 plate of 150 x 150 x 5 mm was coated with multilayer thermal barrier coating of thickness ~ 1 mm. The coated sample was subjected to a heat flux of 8.5 W/cm² for 1035 secs to evaluate the thermo-responsive behaviour of the coating. Maximum back wall temperature measured was 299°C. The coating methodology is simple compared to complicated plasma techniques which can be applied on complex shaped substrates and all operations are carried out at low temperatures, below 150°C ensuring no deterioration of structural properties of the substrate.

Abstract: UV curable vinylic polymethylsilane (v-PMS) was synthesized by a facile one-step Wurtz route, the products’ molecular structures were characterized by FT-IR and the mechanism of the chemical reactions was discussed in detail. The as-synthesized product was a promising candidate for the fabrication of micro ceramic structures for microfluidics by softlithography techniques.

Abstract: A processable polyborazine precursor was synthesized via substitution reaction and deamine condensation reaction by using methylamine, propylamine and B-trichloroborazine (TCB) as the starting materials. The chemical composition, structure and properties of the obtained polymer were investigated using element analysis, FT-IR, NMR, XPS and TG. The results indicated that the backbone of the polymer was approximate linear arrangement of boron-nitride ring molecules with B-N(CH₃)-B structure as bridge bond, meanwhile minor amount of methylamino and propylamino remained in polyborazine as the pendant or end group. The melting point, number average molecular weight and polydispersity index of polyborazine was 90°С , 1002 and 1.355, respectively. The polyborazine showed good processability and polyborazine precursor fibers with diameter of 10~15 µm could be obtained by melt-spinning in N2 atmosphere. The ceramic yield of the polyborazine at 1000°С in nitrogen atmosphere was 53.2 wt%, which suggested that the as-achieved polymer can be used as a precursor to boron nitride ceramic fibers and membranes.

Abstract: This paper describes a method of catalyst-assisted pyrolysis of preceramic precursors for the synthesis of Si-based nanostructures. Through tuning the processing conditions, clustered SiOx amorphous nanowires, bundled single-crystalline Si3N4 nanowires, Y-shape and network-like branched single-crystalline Si3N4 nanostructures, aligned single-crystalline Si3N4 nanowires, and so on, have been synthesized. The results reveal that it is a simple and easy-controlling method, and has potential for mass production of nanomateirals of controlled morphology.

Abstract: The self-healing ceramic coating against oxidation for carbon/carbon composite was fabricated via preceramic polymer pyrolysis process using polysilazane as preceramic and MoSi2, B4C powders as fillers. By means of SEM and XRD, the phase compose and microstructure of coating were characterized, and preliminarily study on its anti-oxidation ability and thermal shock resistance were conducted. The results showed that, the coating is composed of the resisting oxidation layer and the sealing layer. The thickness of the coating is about 50μm, and the coating is uniform and densified. Good contact at the interfaces is visible on the SEM photograph. At 1300°C temperature, the thermal shock resistance test was conducted 50 times, the weight loss rate was 2.12%. In range of 1200°C~1500°C, the anti oxidation ability of the coating is good.

Abstract: In this study, two-dimensional (2D) ordered macroporous SiCN ceramics were prepared by infiltrating sacrificial colloidal silica (SiO2) templates with the low molecular weight preceramic polymer, polysilazane. This was followed by a thermal curing step, pyrolysis at 1250°C in a N2 atmosphere, and finally the removal of the templates by etching with diluted HF. In particular, 100large-scale monolayer silica crystals were prepared on the cleaned Si substrates by spin coating. Two-dimensional SiCN ordered pore arrays were fabricated by a solution-dipping template strategy.