Crack-free silicate bioceramics from preceramic polymers

As a case study, the present paper illustrates an innovative processing method employing a preceramic polymer containing different fillers, which can be used to manufacture various ceramic components for biomedical applications. Crack-free wollastonite (CaSiO₃) ceramics were successfully produced, with high phase purity, after heating at 900°C in air starting from a silicone resin containing CaCO₃ micro-sized particles as ‘active filler’. As ‘passive filler’, wollastonite preceramised powders as well as commercially available wollastonite fibres were added. Their presence reduces the gas evolution occurring due to the decomposition of the calcium carbonate active filler and the polymer-to-ceramic conversion, reducing the stresses that generate in the component during heating. The resulting samples exhibited improvements in terms of the morphology and the mechanical strength, with respect to samples not containing any passive fillers, without significant modification of the final phase assemblage.     

Effects of low-temperature treatment on the properties of commercial preceramic polymers

Low-temperature, pre-gelation, thermal treatment of ceramic precursors is a common preliminary step in the polymer infiltration-and-pyrolysis-based processing of ceramic matrix composites (CMCs). A variety of polymer properties can be modified using this approach, the most important being molecular weight distribution, viscoelastic state, solidification behavior, and ceramic yield. In this work, the effects of thermal treatment on the processing properties of two commercial preceramic polymers, StarPCS™ SMP-10 and KiON Ceraset® PSZ-20, have been investigated. It was determined that the volatilization, molecular chain scission, and polymerization phenomena occurring during treatment, lead to increased viscosity, higher molecular weight, lower temperature of solidification onset, and improved ceramic yield. A wide range of tailorable viscoelastic states was obtained, and allowed the formation of continuous polymer filaments. Understanding the nature and effect of these modifications on the processing state can improve the current state of CMC processing, and open novel processing routes for constituent development.     

Fabrication of porous Al2O3-based ceramics using ball-shaped powders by preceramic polymer process in N2 atmosphere

Porous Al₂O₃-based ceramics were successfully fabricated using ball-shaped powders by preceramic polymer process in N₂ atmosphere. These results showed that the amorphous Si-O-C ceramics were formed on the surface of ball-shaped Al₂O₃ particles by the pyrolysis of the silicone resin during sintering in N₂ atmosphere, which played a role in connecting the Al₂O₃ particles by forming the sintering necks. When the sintering temperatures increased from 1100 °C to 1600 °C, the formed Si-O-C ceramics still existed in the amorphous state and had no crystallization. Interestingly, the amorphous β-SiC formed at 1300 °C and its amount gradually increased with further increasing temperatures. The linear shrinkage rate of the samples varied from 0.49% to 0.73% and the weight loss rate increased from 2.01% to 10.77%. The apparent porosity remarkably varied with the range of 24.9% and 34.5%, as the bulk-density varied from 2.66 to 2.47 g/cm³. The bending strength gradually increased from 9.36 to 22.51 MPa with increasing temperatures from 1100 °C to 1500 °C, however, the bending strength remarkably decreased at 1600 °C, which was attributed to the comprehensive function of the high porosity, broken Al₂O₃ particles and weak connection between Al₂O₃ particles in the samples.     

3D printing of SiC ceramic: Direct ink writing with a solution of preceramic polymers

3D structured SiC ceramics with varying feature sizes (100–400?μm) were achieved by direct ink writing of polycarbosilane (PCS)/n-hexane solution. The rheological properties of the PCS solution and printing parameters were tailored for optimum writing behaviour. The integrity and clear surface of the filaments indicated the printing ability of forming the self-supporting features along with the rapid evaporation of solvent. As-printed 3D structured PCS was processed by oxidative crosslinking and pyrolysis and converted to SiC ceramic. Although strong shrinkage occurred during the pyrolysis, SiC ceramic maintained the original 3D structure. Both proper viscoelasticity of printable solutions and the homogeneous shrinkage in the pyrolysis determine the integrity and feature characteristic of 3D structured SiC using direct ink writing preceramic polymer.     

Bioinspired cross-linking of preceramic polymers via metal ion coordination bonding

Metal coordination chemistry is common in biology, with many proteins containing metal ion-based structural motifs or catalytic centers. An interesting example of nature’s use of coordinate bonding to create a proteinaceous structural material is found in the Alitta virens marine worm jaw. Inspired by Zn⁺² cross-linked polymers in A. virens, we have extended coordination cross-linking to preceramic polymers. Commercially available Si-based backbones functionalized with pyridyl-moieties were introduced to metal ions to initiate cross-linking. Metal coordination offered a single knob to tune multiple characteristics of the material before and after pyrolysis. The loading of the metal ions into the polymer impacted the viscoelastic properties, thermal stability, and ceramic yield. Pyrolysis yielded a ceramic representative of the metal ion cross-linker (e.g., forming TiC/SiC nanocomposites).     

Digital light processing of ceramic components from polysiloxanes

Additive manufacturing using photocurable polymers is one method to answer the increased demand of ceramic structures with complicated morphology by fabricating ceramic parts with high resolution and good surface quality. We introduce here a new method to fabricate SiOC ceramic structures by utilizing a simple physical blend between two different preceramic polysiloxanes, one providing photosensitive acrylate groups while the other one a high ceramic yield. Different blend ratios have been realized and respectively optimized concerning the printing additives and setting times to fabricate exact replications of highly complex polysiloxane structures by Digital Light Processing. After pyrolysis, a uniform, homogenous shrinkage was observed yielding dense, pore- as well as crack-free SiOC ceramics. By adjusting the ratio between the different polysiloxanes, parameters such as the ceramic yield, shrinkage, chemical composition and resolution after pyrolysis could be tailored in a wide range of values.     

Fabrication and performances of preceramic polymer-based high-temperature High emissivity coating

High emissivity coatings on nickel-based superalloy, with good infrared radiating ability and good high temperature resistance, were prepared at room temperature, using preceramic polymer cured at room-temperature as coating former and CeO₂ and B₄C as passive high emissivity fillers. The influences of high temperature and wind channel test on the microstructure and thermal performance of the high emissivity coating were investigated in detail. The high emissivity coating has good thermal stability and no cracking and flaking after heating at 1100 K and the wind tunnel test. The emissivity of the coating reached 0.85−0.92 between room temperature and 1100 K. The high emissivity coating on the nickel-based alloy can make the back temperature of the nickel-based alloy decrease from 686 to 646 ℃.     

Ceramic foams and micro-beads from emulsions of a preceramic polymer

A commercially available solid silicone resin was dissolved in a solvent and emulsified via stirring in the presence of water and surfactant to form three different types of emulsions, namely water-in-oil (w/o), water-in-oil-in-water (w/o/w) and oil-in-water (o/w), by following different preparation procedures. After curing, thermosets possessing different morphologies, ranging from highly porous (monolithic) foams to porous micro-beads and solid micro-beads, formed. The samples kept their shape upon pyrolysis, and resulted in ceramic foams (via w/o) and porous micron sized (∼200 μm) spherical particles (via w/o/w) having more than 80 vol% of total porosity, while with o/w emulsification solid SiOC ceramic particles with an average diameter of ∼100 μm formed. Both surfactant and water altered the IR spectra for emulsion-derived thermoset samples, in comparison to the pure cured resin, but upon pyrolysis similar amorphous ceramics were obtained from all samples.     

Fabrication of dense B4C-preceramic polymer derived SiC composite

B₄C-SiC composites were fabricated via the preceramic polymer (PCP) route combined with pressure-assisted sintering. Fully dense bodies were achieved by controlling surface oxide on B₄C powder and pyrolysis conditions for PCP coated powder. We elucidate i) the microstructure and phase developments observed in the process of fabricating dense B₄C-PCP derived SiC composites and ii) the mechanical properties and crack deflection behavior of dense bodies. The incorporation of PCP derived SiC to B₄C decreases hardness due to the lower hardness value of SiC compared to B₄C and the residual carbon accompanied by SiC formation. Instead, the PCP derived SiC improved indentation fracture toughness. The main toughening mechanism supposed is a combination of crack impeding by SiC grains and crack deflection within SiC grains, likely due to the presence of subgrains or layered microstructure in the PCP derived SiC grains.     

Powder bed fusion of polyamide powders combined with different preceramic polymers infiltration and pyrolysis to produce complex-shaped ceramics

The fabrication of a wide range of polymer-derived ceramic parts with high geometric complexity through a novel hybrid additive manufacturing technique is presented in this article. The process that we introduced in a previous work uses the powder bed fusion technology to manufacture high porous polymeric preforms to be then converted into ceramics through preceramic polymer infiltration and pyrolysis. The cellular architectures of a rotated cube (strut-based) and a gyroid (sheet-based) with 25 mm diameter, 44 mm height and 67 % of geometric macroporosity were generated and used for the fabrication. The complex structures were 3D printed and polycarbosilane, polycarbosiloxane, polysilazane and furan liquid polymers were used to produce SiC, SiOC, SiCN and glassy carbon, respectively. Despite a linear shrinkage of about 24 %, the parts maintained their designed complex shape without deformations. The significant advantages of the proposed method are the maturity of powder bed fusion for polymers with respect to ceramic additive manufacturing techniques and the possibility to fabricate net-shape complex ceramic parts directly from preceramic precursors.     

Modeling the pyrolysis of preceramic polymers: A kinetic study of the polycarbosilane SMP-10

A material model was developed to predict changes in mass, density and thus volume of cured preceramic polymers for CMC matrices as they pyrolyze into ceramics. Because part warpage and delaminations are most likely to occur when matrix strain rates and strain rate gradients are the highest, the ability to accurately predict changes in a matrix material’s volume is essential to determining the processing conditions that will efficiently minimize composite scrap rates. Experimental and model analysis of the SiC forming polycarbosilane, SMP-10, revealed that volume shrinkage is initially driven by mass loss, is quickly dominated by density’s contribution, and has both temperature and time at temperature dependencies, where density is not a simple function of mass yield. While material density is rarely reported in the open literature, the ability to predict changes in density is essential to accurately predicting the volume yield of preceramic polymers used in ceramic matrix composites.     

Effect of preparation temperature on the formation of Sr2CeO4 phosphor particles in the spray pyrolysis

Sr₂CeO₄ phosphor particles were prepared by spray pyrolysis at various preparation temperatures. The effect of preparation temperatures on the morphology, crystal structure and photoluminescence characteristics of the post-treated Sr₂CeO₄ phosphor particles was studied. Phase pure Sr₂CeO₄ phosphor particles were not produced by spray pyrolysis without post-treatment. The optimum post-treatment temperature to produce the Sr₂CeO₄ phosphor particles with high photoluminescence intensity was 1,000 °C in spray pyrolysis. The spherical morphology of the as-prepared particles obtained at high preparation temperatures above 1,400 °C had maintained after post-treatment at 1,000 ‡C. The relative photoluminescence intensities of the Sr₂CeO₄ phosphor particles varied with the preparation temperatures in the spray pyrolysis. The as-prepared particles obtained by spray pyrolysis at preparation temperatures below 1,400 °C converted into phase pure Sr₂CeO₄ phosphor particles after post-treatment at 1,000 ‡C. The optimum preparation temperature of the as-prepared particles was 1,400 °C to produce the Sr₂CeO₄ phosphor particles with spherical shape and high photoluminescence intensity in the spray pyrolysis.     

Fabrication of weaved ceramic mesh from green microfibers based on cross-flow microfluidics

Microfibers-weaved ceramic filters are in increasing demand due to their high filtration efficiency, stable structure and remarkable ceramic properties in water and gas treatment. To date, it remains a challenge to find an effective way to weave ceramic microfibers. This work demonstrates a novel strategy that combines gel-casting and in situ cross-flow microfluidic molding to fabricate highly flexible and shape-retentive green microfibers and then weaves them before sintering. By tailoring the curing temperature and flow rate, the diameter and surface morphology of the microfiber is accordingly tuned. Besides mesh structure, the microfibers can also be weaved into more complex three-dimensional structures such as dragonfly knot, Chinese knot, etc. Benefitting from the widely used solution system and microfluidic method, this system can serve as a general and stable platform for preparing microfibers-weaved ceramic filters made from different materials, thus holds great potential in a wide range of working conditions.     

Synthesis of a soluble preceramic polymer for ZrC using 2-hydroxybenzyl alcohol as carbon source

A preceramic polymer for ZrC was successfully synthesised by chemical reaction between zirconium oxychloride (ZrOCl₂·8H₂O) and 2-Hydroxybenzyl alcohol via a one-pot route. The molecular structure, thermal properties and pyrolysis behaviour of the precursor were investigated. The results indicated that the precursor might be Zr–O–Zr chain polymer with 2-Hydroxybenzyl alcohol as ligand. The precursor was air-stable and exhibited excellent solubility in common organic solvents. The conversions from precursor to ZrC powders were investigated by TG-DTA, X-ray diffraction, Scanning electron microscope, TEM and Raman spectrum. The precursor underwent a thermal decomposition in four steps, and ZrC powders were formed at 1300°C via carbothermal reduction reaction of ZrO₂ and carbon in argon with ceramic yield of 63.0%. The ZrC particles were fine and exhibited irregular polyhedron morphology with average size in the range of 100–300?nm.     

Joining of SiCf/SiC using polycarbosilane and polysilazane preceramic mixtures

Polycarbosilane (PCS)/polysilazane (PSZ) preceramic mixtures with weight ratios of 100/0, 75/25, 50/50, 25/75, and 0/100 were used as a filler for the joining of SiC_f/SiC to obtain high purity SiC at the joining region. SiC_f/SiC was fabricated by the electrophoretic infiltration of a SiC-based matrix phase into Tyranno SA3 SiC fabrics followed by hot-pressing at 1750?°C under 20?MPa for 2?h in an Ar atmosphere. Microstructural analysis confirmed a sound join without cracks after joining at 1750?°C for 2?h under a pressure of 10?MPa. SiC was the only phase remaining at the joint when PCS was used, while a small amount of Si₃N₄ along with the main SiC were observed in the join using PSZ. The flexural strengths of the butt-joint SiC_f/SiC were 120 and 137?MPa for the samples joined using a pure PCS and PSZ at 1750?°C, respectively, whereas those joined using the mixture fillers showed relatively lower strength.     

Synthesis of hierarchical porous silicon oxycarbide ceramics from preceramic polymer and wood biomass composites

Hierarchical porous SiOC ceramics were successfully prepared using a polysiloxane preceramic polymer mixed with wood biomass by annealing at different temperatures under Ar atmosphere. These SiOC ceramics display a trimodal pore size distribution in the micro-, meso- (micropores + mesopores, 1.7–14 nm) and macro-size scale (1–15 μm). The mesopores and micropores mainly originate from the formation of large amounts of SiC crystals and nanowires, graphite-like microcrystallites, and nm-scale pores of ray parenchyma cells and pits of the wood biomass. The SiOC sample prepared at a higher temperature processes the specific surface area up to 180.5 m²/g. The specific surface area, pore volume and average pore width of the samples can be controlled by adjusting the pyrolysis temperature.     

Chemical modification of preceramic polymers: Their reactions with transition metal complexes and transition metal powders

Conclusions This research has demonstrated that a variety of chemistries can be carried out with preceramic polymers that in general are characterized by the presence of an abundance of reactive functional groups. Such chemistries can serve to upgrade a given preceramic polymer by catalytic or stoichiometric processes; they can be used to from new and useful hybrid polymers from the original preceramic polymer, as shown in the present work and also in some of our previously published work; they can, by their pyrolysis in the presence of metal powders, act as chemical reagents that deliver the elements of interest for reaction with the metal to give useful ceramics. Thus the preparation of a preceramic polymer is not the end of the chemistry in the monomer-to-polymer-to-ceramic conversion, but rather it presents many possibilities for further chemistry.     

Fabrication of ceramic bioscaffolds from fly ash cenosphere by susceptor-assisted microwave sintering

Cenospheres (CS) are ceramic hollow microspheres and have been used to prepare composite foams for applications such as medical implants. However, its potential standalone application in the biomedical field is not fully explored. Herein, a susceptor-assisted microwave (SMW) sintering approach was used for producing CS foam scaffolds. Owing to the hybrid heating mechanism offered by the SMW process, sintering of the low-dielectric cenospheres was realized. We found that sintering was initiated at a lower temperature (1100 °C) compared to conventional heating (1250 °C) as reported in the literature, probably due to the lower activation energy required by SMW sintering. The physical and compositional properties of the sintered CS specimens were examined, and in vitro studies were performed. The as-fabricated CS foam possessed minimal effect on cell viability. Cells migrated and adhered well within the pores of the specimens, which indicates the potential of the CS as scaffold materials for cell engineering applications.     

Digital light processing fabrication of mullite component derived from preceramic precusor using photosensitive hydroxysiloxane as the matrix and alumina nanoparticles as the filler

By taking advantage of the low sintering temperatures of the preceramic polymers, stereolithography printed mullite components derived from preceramic polymer precursor containing alumina particles can be sintered at low temperatures. However, due to their high specific surface, nano alumina particles are difficult to be dispersed into the photocurable polysiloxane. Herein, to prepare mullite slurry, a photosensitive hydroxysiloxane was employed as the preceramic polymer matrix while γ-Al₂O₃ nanoparticle was added as the active filler. The introduction of photocurable hydroxysiloxane not only improved the homogeneity and rheological properties of mullite slurry but also shorted the ionic diffusion distance of Si-ion and Al-ion during the sintering process. Therefore, 3D mullite preceramic precursor stereolithography printed from hydroxysiloxane-Al₂O₃ slurry was endowed with a low sintering temperature around 1400 °C. During the sintering process of preceramic precursor, sintering aid AlF₃ can participate in the reaction and further promote the formulation of mullite grains.     

Structure and properties of ceramic fibers prepared from organosilicon polymers

This paper is a review of structure and properties of ceramic fibers derived from organosilicon polymers, with emphasis on the author's research. Ceramic fibers are prepared from organosilicon polymers by melt-spinning, cross-linking, and pyrolysis. Desirable polymeric precursors display the following properties: high char yield of desired composition, thermal stability at melt-spinning temperature, stable rheology, high purity and freedom from particulate impurities, and ability to undergo rapid cure (cross-linking). Ceramic fibers in the Si-C-O or Si-C-N-O systems display a rich nanostructure consisting of some or all of the following metastable phases: (1) an amorphous, continuous siliconoxycarbide or siliconoxycarbonitride phase; (2) dispersed carbon nanocrystallites; (3) dispersed -SiC or Si₃N₄ nanocrystallites; and (4) closed, globular nanopores. The crystalline phases increase in volume fraction and crystallite size as stoichiometry approaches the crystalline composition and as pyrolysis temperature increases. The Si-C-N-O fibers are amorphous. Pore size increases and total pore volume decreases with increasing pyrolysis temperature. Considerable variation in ceramic fiber composition can be achieved by varying cure conditions and pyrolysis atmosphere. Polycrystalline SiC fibers can be produced by pyrolysis above 1600°C. Fiber diameters range from 7 to 20 µm. Elastic moduli vary from 140 to >420 GPa (20 to >60 Msi) and are controlled by composition, nanostructure, and fiber density. Fiber densities range from 2.2 to >3.1 g/cm³. Tensile strengths range up to 5 GPa (700 ksi) and are Griffith flaw-controlled.This review is from the Second International Topical Workshop, Advances in Silicon-Based Polymer Science.