New route to synthesize preceramic polymers for zirconium carbide

A preceramic polymer, polyzirconosaal （PZSA）, was synthesized by the ligand-exchange reaction between polyzirconoxane （PZO） and salicyl alcohol （SA）. The precursor was air-stable and exhibited excellent solubility and rheology. These properties are useful for the processing of C/C-ZrC composites v/a precursor infiltration and pyrolysis （PIP） process. The polymer to ceramic conversion was investigated by TG, XRD and TEM. Nanosized ZrC was formed by pyrolysis of this precursor at 1300 ℃ in argon with ceramic yield of 57.8%.

One pot synthesis of a soluble polymer for zirconium carbide

A new polymer,polyzirconoxanesal（PZS）,is synthesized from the reaction of zirconium oxychloride octahydrate （ZrOCl;-8H;O） with acetylacetone（Hacac） and salicyl alcohol（SA） by one-pot protocol.The polymer is soluble in common organic solvents,such as ethanol,methanol,acetone,tetrahydrofuran and chloroform,and exhibits rheology with viscosity of 100- 300 mpa s at 25℃.These properties are suitable for uses in fabrication of ceramic matrix fiber composites.Pyrolysis of this polymer at 1300℃in argon provides nanosized ZrC with spherical morphology and size of 20-100 nm.

Synthesis and Pyrolysis of Soluble Cyclic Hf-Schiff Base Polymers

Soluble Hf-containing polymers are significant processable precursors for the fabrication of ultra-high temperature ceramics.In this work,cyclic Hf-Schiff base polymers were synthesized via direct polymerization of hafnium alkoxide and bis-salen monomers.The defined structure and molecular weight of the polymers were characterized by NMR spectroscopy,gel permeation chromatography and MALDI-TOF mass spectroscopy.The feed ratio of monomers regulated the molecular weight and solubility of the polymers.This synthetic strategy features simple operation under ambient conditions,efficient reaction with high yield and cyclic polymers as the main products.The Hf-Schiff base polymers were converted to HfC/C materials after pyrolysis under argon at 1600°C,which was identified by XRD measurements,elemental analyses and Raman spectroscopy.This work will inspire more precise and efficient synthesis and applications of metallopolymers.

Fabrication of high performance 3D SiO_(2)/Si_(3)N_(4) composite via perhydropolysilazane infiltration and pyrolysis

Perhydropolysilazane,a low viscosity preceramic polymer with good wettability and high char yield,was used to fabricate three-dimensional silica fiber reinforced silicon nitride matrix composites through the repeated infiltration-curing-pyrolysis cycles.With the increase of the pyrolysis temperature from T_(1),T_(2)to T_(3),the density of the composites increased all through,but the flexural strength showed a maximum value at T_(2)followed by a sharp decrease.The composite prepared at T_(2)exhibited a good ceramization of the preceramic polymer,a high flexural strength of 144.9 MPa and excellent dielectric property.The high performance of the composite resulted from the good state of the silica fibers,controlled fiber/matrix interfacial microstructures and high-purity dense silicon nitride matrix.

Generic principles of crack-healing ceramics

Ceramic materials able to heal manufacture or damage induced microstructure defects might trigger a change in paradigm for design and application of load bearing ceramics.This work reviews thermodynamic and kinetic aspects governing the regeneration of solid contact able to transfer stress between disrupted crack surfaces in ceramics.Major crack healing processes include perturbation of crack-like pores followed by sintering of isolated pores,as well as reaction with an environmental atmosphere and filling of the crack space with an oxidation product.Since thermally activated solid state reactions require elevated temperatures which may exceed 1000℃,processes able to trigger crack healing at lower temperatures are of particular interest for transferring into engineering applications.Generic principles of microstructure modifications able to facilitate crack repair at lower temperatures will be considered:(i)acceleration of material transport by grain boundary decoration and grain size reduction,and(ii)reduction of thermal activation barrier by repair filler activation.Examples demonstrating crack healing capability include oxidation reaction of low energy bonded intercalation metal from nano-laminate MAX phases and catalyzed surface nitridation of polymer derived ceramics containing repair fillers.

Additive Manufacturing of Ceramics from Liquid Feedstocks

In this review,we summarize the research activities carried out by our research group at the University of Padova on the additive manufacturing of ceramics from liquid feedstocks.Particularly,we evaluate the use of preceramic polymers,geopolymers,and sol-gel solutions.We mainly focus on processing with liquid feedstocks because they have some advantages with respect to slurry-based feedstocks in which powders are present.Particularly,lower viscosity,enhanced transparency,and lack of scattering and sedimentation are advantageous features for vat photopolymerization processes,whereas the absence of particulates reduces clogging problems at the nozzle for extrusion-based processes.Simultaneously,preceramic polymers and geopolymers have some limitations in terms of the range of ceramic compositions that can be obtained;sol-gel solutions are intrinsically unstable,whereas printed objects suffer from drying issues.Nevertheless,we successfully produced high-quality parts using a variety of additive manufacturing techniques,some of which(e.g.,volumetric additive manufacturing)have been proposed for the fabrication of ceramic components for the first time.