Silicon carbide-based foams from direct blowing of polycarbosilane

Macro-cellular porous silicon carbide foams were produced using a polycarbosilane preceramic polymer and a chemical blowing agent (azodicarbonamide). Polycarbosilane (PCS) was mixed with a blowing agent and the mixture was foamed close to the melting point of PCS at 250-260 °C, under nitrogen in order to avoid cross-linking by oxidation. The foamed PCS was then cured under air at 200 °C and pyrolyzed at 1000 °C, leading to the formation of open macro-cellular ceramic components. Porosity ranged from 59 to 85 vol%, and the cell size ranged from 416 to 1455 μm; these values could be modulated by changing the content of blowing agent and foaming temperature. This process is a simple and efficient way to produce silicon carbide-based foam with tailored pore architecture and porosity.     

Densification of a SiC-matrix by electrophoretic deposition and polymer infiltration and pyrolysis process

With the aim to improve the properties of the SiC-matrix in a SiC_f/SiC composite for fusion applications, a new fabrication technique combining electrophoretic deposition (EPD) and polymer infiltration and pyrolysis (PIP) was introduced. By using EPD from a well-dispersed, aqueous suspension we were able to produce SiC green bodies with closely packed particles (>62%TD). This deposition was followed by vacuum infiltration of the green body with pre-ceramic polymer precursor and then pyrolysis and crystallization at 1600 °C. Due to the high initial packing density of the deposits, only a few polymer-infiltration and pyrolysis steps were needed to achieve a relatively high density of the material. The SiC samples fabricated by the combined EPD-PIP process reached the matrix density of ∼86.5% TD and average pore size of ∼90 nm after six consecutive PIP cycles, while high thermal conductivity values (>30 W/mK) were obtained already after one PIP cycle.     

Effect of plasma treatment of polymeric tape carriers on wetting behaviour of aqueous ceramic tape casting slurry

Due to environmental and health aspects, aqueous ceramic slurries are preferred to traditional organic solvent systems in tape casting. An important obstacle associated with the high surface energy of water is poor wetting of aqueous ceramic slurries on polymeric tape carriers. Therefore, we measured the contact angles of an aqueous epoxy-based ceramic slurry on polyethylene terephtalate (PET), polypropylene (PP), polymethyl methacrylate (PMMA), and aluminium-coated polyethylene terephtalate (PET-Al) films and investigated approaches to improving their wetting. We evaluated the effect of plasma treatment of the tape carrier surface on the wetting behaviour and compared it with the effect of adding non-ionic amphiphilic surfactants to the ceramic slurry. The treatment of the tape carrier surface by low-temperature plasma substantially improved the wetting behaviour of aqueous ceramic slurry. The lowest contact angle of 31° was obtained on the PET film. Although the addition of non-ionic surfactants improved both the wetting behaviour of the slurry and the detachment of the polymeric carrier from the ceramic tape even better than the plasma treatment of the carrier surface did, the plasma-treated carriers still present a useful alternative to the surfactants. In the case of the plasma-treated PET carrier the surfactants could be fully eliminated and potential drawbacks related to the use of surfactants could be prevented.     

On the shrinkage during pyrolysis of thin films and bulk components: The case of a hybrid silica gel precursor for SiOC glasses

This paper compares the shrinkage during pyrolysis of a gel precursor as thin film and as bulk sample. The hybrid silica gel, precursor for SiOC glasses, contains Si-CH₃ and Si-H moieties. The shrinkage of bulk samples has been measured with conventional dilatometry. Shrinkage of thin films has been studied for the first time with in situ dilatometry allowing to measure the thickness and the refractive index during pyrolysis. Thin films shrink more compared to bulk samples and the pyrolytic transformation occurs at lower temperature (100-150 °C) compared to the bulk samples.     

Fabrication of honeycomb-structured protein arrays via one-step method

“Inverse emulsion”-“breath figures” (Ie-BF) method was introduced for fabricating porous films with protein arrays decorating the interior of the obtained polymeric pores. Compared with the traditional BF method in which only hydrophobic or amphiphilic components could be used, Ie-BF method created a system with water phase carrying water-soluble protein before solution casting, which makes it possible to obtain patterned protein arrays within the BF structure in one-step. The stability of the inverse emulsion system is an intractable problem solved by high shearing emulsification. Experimental parameters, including the concentration of the matrix polymer, water/oil ratio of the IE, amount of both emulsifier and protein in IE, were evaluated for establishing an optimal condition to prepare fine BF arrays by following the Ie-BF method. Proteins tagged with fluorescein isothiocyanate were added into the emulsion to cast films, and pores efficiently enriched by proteins were revealed by the images of fluorescence microscope, indicating the successful preparation of protein arrays. Among all the approaches of establishing patterned structures of biomacromolecules, Ie-BF shows promising potential in future applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47084.     

A procedure to evaluate the resistance to biological colonization as a characteristic for product quality of ceramic roofing tiles

Ceramic roofing tiles suffer deterioration through time due to environmental exposure. Biological colonization affects the appearance and integrity of building materials, such as roofing tiles. The resistance to biocolonization represents an important property affecting the product quality of ceramic roofing tiles. While natural colonization of roofing tiles by organisms is a progressive, heterogeneous, and slow process, laboratory assessment of this phenomenon requires a sensitive procedure that can be carried out within a reasonable period of time. Different microorganisms have been evaluated and the use of phototrophs, specifically the cyanobacterium Oscillatoria, presented several advantages such as good adherence, homogeneous growth on surfaces, and the chlorophyll-autofluorescence which can be used for a sensitive detection. Colonization by Oscillatoria on roofing tiles was assessed by measuring the autofluorescence of cells. This study proposes the use of specific cyanobacterial cells and a simple method for monitoring biofilm formation and biological colonization of roofing tiles.     

Preparation of SiC nanowires-filled cellular SiCO ceramics from polymeric precursor

SiC nanowires-filled cellular SiCO ceramics were prepared using polyurethane sponge as a porous template infiltrated with silicone resin by pyrolysis at 1400 °C under Ar atmosphere. The pyrolysis temperature was an important parameter affecting the formation of SiC nanowires. The as-prepared sample obtained at 1000 °C was composed of SiCO glasses and turbostratic carbon. The SiCO ceramic was further converted into SiO₂ crystals and amorphous carbon by pyrolysis at 1200 °C. With the increasing pyrolysis temperature, SiC nanocrystals embedded in the non-crystalline SiCO matrix were observed. Furthermore, the SiC nanowires were formed in the pores of the SiCO ceramic. The diameters of the SiC nanowires are in the range 80–150 nm and the lengths are up to several tens of micrometers. The growth mechanism of the nanowires was supported by the vapor-solid mechanism.     

Optimizing inkjet printing process to fabricate thick ceramic coatings

This article describes the use of Taguchi optimization and ANOVA techniques on inkjet printing process to determine optimal parameters for fabrication of thick ceramic coatings over glass substrates. Stable nanoparticle suspensions are synthesized through high energy milling of precursor powders with adequate quantities of binder and suspending solvent. Most often, inkjet printing process is being used for developing fine and thin layers (<10 µm). However, an attempt is made to fabricate thick ceramic films by varying only IJP process parameters and without multiple layer deposition, thereby reducing efforts in ink synthesis and processing time of coated substrates. Three parameters of IJP were varied for developing a model that was used for precisely predicting the printed layer thickness under varying process parameters. ANOVA technique showed that open time interval in combination with nano particle concentration in the ink could potentially lead to thick coatings. The higher volume % of solvent in the diluted suspension ink under the influence of substrate heating contributed significantly to coffee stain effect with irregular surface coatings. However, increasing the concentration of nanoparticles in the diluted ink resulted in substantial improvement in thickness of the layer with simultaneous control of coating defects.     

Thermal conductivity studies on Si/SiC ceramic composites

Ceramic heat exchangers are increasingly used in many nuclear power plants. Silicon carbide has been treated as a promising material for heat exchanger application since it has good thermal conductivity and corrosion resistance. In this work, four different types of Si/SiC ceramic composites were prepared by liquid silicon infiltration technique. Thermal conductivities of these ceramic composites at different temperatures are measured by the laser flash thermal conductivity method. Results show that the presence of free carbon and voids are notably affecting the thermal conductivity of these materials.     

Synthesis, characterization, and wear and friction properties of variably structured SiC/Si elements made from wood by molten Si impregnation

We have synthesized pre-shaped SiC/Si ceramic material elements from charcoal (obtained from wood) by impregnation with molten silicon, which takes place in a two-stage process. In the first process, a porous structure of connected micro-crystals of β-SiC is formed, while, in the second process, molten Si totally or partly infiltrates the remaining open regions. This process forms a dense material with cubic (β-)SiC crystallites, of which the majority is imbedded in amorphous Si. The synthesis of preshaped “sprocket” elements demonstrates that desired shapes of such a dense SiC/Si composite ceramic material can be achieved, thus suggesting new industrial applications.The structure and composition of numerous as-synthesized samples were characterized in detail by using a wide range of techniques. Wear and friction properties were also investigated, with polished samples. The properties found for the present samples are very promising for abrasive applications and for new generation brake systems.     

Determination of the porosity and pore size distribution of SiC ceramic foams by nuclear methodologies

Abstract

Abstract

Two nuclear methodologies were used for porosity and pore size distribution determination of SiC ceramic foams. Thirty samples were analysed, six of each one of the following pore densities: 30, 45, 60, 80 and 100 pores per inch (ppi). The two nuclear techniques employed were X-ray microtomography and gamma ray transmission. For the microtomography technique, the spatial resolution of the images was 32?μm. For the gamma ray transmission methodology, a NaI (Tl) scintillation detector and Am-241 radioactive source were used. The gamma transmission technique was precise for porosity determination in relationship to the nominal values supplied for the sample manufacturer. The 30 and 45?ppi samples analysed by the microtomography technique present average porosities equivalent to the nominal porosity, and the other samples present an average of 4·6% smaller values. The 30 and 45 ppi sample two-dimensional images show voids inside the structural solid material of the ceramic.     

Ag/Ag2S–TiO2 composite film formed using dual phase Ag/Ag2O layer as a sulfurized layer for enhanced photocatalytic activity

The Ag/Ag₂S–TiO₂ composite films were synthesized through vulcanizing sputtering Ag/Ag₂O dural-phase template capping layer on the TiO₂ film. After hydrothermal vulcanization using thiourea as a sulfur precursor, the initially decorated Ag₂O layer transformed into the Ag₂S. The surface morphology of the continuous block structure of the Ag/Ag₂S composite layer gradually shrinks to form irregular island grains dispersed on the surface of the composite film, with vulcanization reaction duration increased from one to 3 h. The light absorption ability of the Ag/Ag₂S–TiO₂ composite film was enhanced substantially compared to the TiO₂ film because of the surface plasmon resonance effect from the Ag particles and the narrow energy band of the Ag₂S. Among various surface morphologies of the Ag/Ag₂S–TiO₂ composite films, the irregular island Ag/Ag₂S grains-decorated TiO₂ composite film presents superior photoelectrochemical and photocatalytic degradation performances than the composite films consisting of continuous block and spider-web structure of the Ag/Ag₂S decorated layers. The synergistic effects of surface plasmon resonance, Ag pathway between the semiconductors, and Z-scheme charge transfer mechanism explain the high-efficiency photocatalytic activity of the Ag/Ag₂S–TiO₂ composite films.     

Self-assembled Protein Fibril-metal Oxide Nanocomposites

Protein aggregation is commonly associated with the onset and development of neurodegenerative disorders, including Alzheimer's, Parkinson's and other forms of pathological disorders. While this phenomenon has historically been studied in the context of its relevance to human health, over the past decade significant research effort has focused on utilizing amyloid-like protein assemblies as building blocks for the development of functional biomaterials and a number of protein-based functional materials have been demonstrated. Here we extend this concept by synthesizing hybrid organic/inorganic microcapsules containing metal-based NPs and protein nanofibrils as a nanocomposite. To this effect, we exploit the propensity of lysozyme to self-assemble into amyloid nanofibrils and their functionalization by carboxyl-modified Fe₃O₄ NPs. We use a microfluidics-based approach to control the micron scale moprhology of the newly formed nanocomposites. Our results illustrate the potential ofthis strategy as a platform for fabricating microcapsules from nanofibril-inorganic NPs hybrid materials.     

Effects of oxidation curing and sintering temperature on the microstructure formation and heat transfer performance of freestanding polymer-derived SiC films for high-power LEDs

Effects of oxidation cross-linking and sintering temperature on the microstructure evolution, thermal conductivity and electrical resistivity of continuous freestanding polymer-derived SiC films were investigated. The as-received films consisting of β-SiC nanocrystals embedded in amorphous SiO_xC_y and free carbon nanosheets were fabricated via melt spinning of polycarbosilane (PCS) precursors and cured for 3 h/10 h followed by pyrolysis from 900 °C to 1200 °C. Results reveal that nanoscale structure (β-SiC/SiO_xC_y/C_free) provides an ingenious strategy for constructing highly thermal conductive, highly insulating and highly flexible complexes. In particular, the 3 h-cured films sintered at 1200 °C with satisfying thermal conductivity (46.8 W m^?1 K^?1) and electrical resistivity (2.1 × 10⁸ Ω m) are suitable for the realization of high-performance substrates. A remarkable synergistic effect (lattice vibration of β-SiC nanocrystals and close-packed SiO_xC_y, free-electron heat conduction of β-SiC and free carbon, and supporting role of oxygen vacancy) contributing to thermal conductivity improvement is proposed based on the analysis of microstructure, intrinsic properties and simulations. Eventually, the SiC films without additional dielectric layers are directly silk-screen printed with high-temperature silver paste and used as heat dissipation substrates for high-power LED devices via chip-on-board (COB) package. The final devices can emit bright light with low-junction temperature (52.6 °C) and good flexibility owing to the mono-layer SiC substrate with low thermal resistance and desirable mechanical properties. This work offers an effective approach to design and fabricate flexible heat dissipation ceramic substrates for thermal management in advanced electronic packaging fields.     

Passivation effect on the surface characteristics and corrosion properties of yttrium oxide films undergoing SF6 plasma treatment

This study investigates the structures, compositions and fluorocarbon-plasma etching behaviors of yttrium oxyfluoride (YOF) passivation films fabricated on sputter-deposited yttrium oxide (Y₂O₃) by high-density SF₆ plasma irradiation. High-resolution transmission electron microscopy and nano-beam electron diffraction confirmed a YOF passivation film containing multiple phases of (104) and (006) crystal planes was formed on the fluorinated Y₂O₃ surface. X-ray photoelectron spectroscopy revealed few changes in the chemical compositions and surface roughness of the YOF passivation film after fluorocarbon plasma etching, confirming the chemical stability of the SF₆ plasma-treated Y₂O₃ sample. The etching depth was ～20% lower on the SF₆ plasma-treated Y₂O₃ film than on the commercial Y₂O₃ coating. These results showed that the SF₆ plasma-treated Y₂O₃ films have an excellent erosion resistance properties compared to the commercial Y₂O₃ coatings.     

Surface oxidation behavior in air and O2-H2O-Ar atmospheres of continuous freestanding SiC films derived from polycarbosilane

In this contribution, thermodynamic computational calculations firstly carried out on Ar-Si-C-O/Ar-Si-C-O-H database demonstrate that passive oxidation is main reaction of continuous freestanding SiC films in both air and 14%H₂O/8%O₂/78%Ar atmospheres. SiC films were subsequently annealed at 1300?°C, 1400?°C and 1500?°C for 1?h in air and O₂-H₂O-Ar atmospheres. Results suggest that modulus, hardness and resistivity decrease whereas crystallite size of β-SiC and α-cristobalite increase with elevated annealing temperature. In particular, hardness of wet oxidized samples is lower than that of air oxidized ones. Additionally, their oxidation kinetics models were also established and verified by annealing at 1200?°C in air and wet oxygen for different time from 1?h to 100?h. Oxidation of continuous freestanding SiC films is identified to follow parabolic oxidation kinetics, and water could effectively enhance the oxidation rates. It is revealed that SiO₂ layer can protect SiC films from further oxidation, and their thickness increases with prolonged annealing time. In this study, a dense and uniform SiO₂ layer with a thickness of 1.1–1.6?µm was produced for sacrificial and passivation layer based on suitable thermal oxidation process (annealing at 1000?°C for 5?h in O₂-H₂O-Ar environment). Interestingly, fast diffusion paths in this oxide layer could effectively accelerate oxidation process of SiC films. These obtained achievements would promote further applications of SiC films on microelectromechanical systems (MEMS) devices in harsh environments.     

Growth-temperature-dependent microstructure and ferromagnetic properties of perovskite manganite-based heterostructures

Oxide heterostructures composed of ferromagnetic La_0.7Ba_0.3MnO₃ (LBMO) spacers and paramagnetic LaNiO₃ (LNO) spacers were grown on SrTiO₃ (0 0 1) substrates at various substrate temperatures. The X-ray diffraction (XRD) patterns confirm the formation of the heterostructure at a growth temperatures of 400–600 °C. A high substrate temperature of 700 °C caused serious atomic intermixing between the LBMO and the LNO spacers of the heterostructure, further degrading the crystalline quality of the heterostructure. The ferromagnetic LBMO spacer in the heterostructure is under biaxial compressive stress. The XRD results reveal that the lattice of the heterostructure was elongated parallel to the c-axis at a growth temperature of 400–600 °C. Experiments that involve XRD, atomic force microscopy and X-ray photoelectron spectroscopy provide structural information on the designed heterostructures. The observed magnetization characteristics of the heterostructures show that lattice strain and the quality of the surface and the interface of heterostructures affect the Curie temperature of the manganite layers.     

Preparation and tribological properties of graphene/TiO2 ceramic films

Single and multiple-layer TiO₂ (GT) ceramic films with graphene nanoplatelets were obtained by sol-gel method. X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), UV–vis diffuse reflection spectroscope and atomic force microscope (AFM) were used to investigate the structure. Interestingly, the GT ceramic films were decorated by graphene nanoplatelets on surface and at interlayer. After heat treatment, graphene kept non-oxidized. The tribological properties of GT ceramic films were investigated using a reciprocating ball-on-plate configuration. The GT ceramic films show excellent antifriction and abrasion resistant properties that the minimum frictional coefficient approximates to 0.1. With more graphene quantity, less frictional coefficient and larger abrasive resistance were observed. When the graphene quantity keeps equally, the frictional coefficient decreases with layers of ceramic film. At high temperature, the frictional coefficient shows a decrease tendency, which reaches the minimum at 100 °C.     

Plasma etching as a surface engineering technique for SiC/SiC composites to improve joint strength

Silicon carbide fiber reinforced silicon carbide (SiC/SiC) composites have unique properties that make them suitable for demanding applications. These materials have to be joined with other materials or with the same type of material to produce the final component. This work investigates the effectiveness of a fluorine-based plasma process on SiC/SiC composites as a surface engineering technique to manufacture a brush-like texturized surface. The studied pre-treatment induced an interlocking effect at the composite/joining material interface, thus improving the joint strength of the joined components.Several plasma conditions were considered and the most promising one (CF₄, 20 sscm, 200 W, 30 min) was selected to assess the effect on the mechanical performance of SiC/SiC brazed with a commercial brazing alloy (Cusil-ABA®). The selected plasma pre-treatment led to a 55% increase in the apparent shear strength of the joined SiC/SiC composites.     

CFD modeling and optimal design of SiC deposition on the fuel combustion nozzle in a commercial CVD reactor

This work presents a new CFD model developed to investigate the SiC film growth over a fuel combustion nozzle in the commercial hot-wall CVD reactor. A detailed 3D model consisting of the compressive transport models and the reaction mechanism to account for the chemistry of the gas-phase and surface reactions are developed. The velocity, temperature, and concentration profiles inside the reactor are predicted numerically. The multispecies transport and its interplay with the gas and surface reactions are understood to operate the reactor effectively. The natural convection and hydrodynamics stability of the flow is investigated using various dimensionless groups (Re, Pr. Pe, and Gr/Re²). It has been observed that the buoyancy-driven flow is dominant at a large Reynolds number (Re) and Gr/Re² ratio. This results in flow recirculation, which ultimately deteriorates the film uniformity. A sensitivity analysis is performed to identify how critical parameters affect the deposition process. Besides, the optimisation of the CVD reactor is also served by combining the support vector machine, a versatile supervised machine learning method, and the Nelder-Mead algorithm to improve the film quality. Our results demonstrate that the present methodology effectively obtains optimal process conditions, significantly enhancing the film performance.