SiOC Ceramic Monoliths with Hierarchical Porosity

SiOC glass monoliths possessing hierarchical porosity were produced by a one-pot processing method. Periodic mesoporous organosilica (PMO) particles were embedded into a foamed siloxane preceramic polymer. After pyrolysis at 1000°C in inert atmosphere, open celled, permeable SiOC ceramic monoliths with a high amount of pores, ranging in size from hundred of micrometers to a few nanometers, were obtained. The components possessed a specific surface area of 137 m²/g, indicating the retention of most of the mesopores after the pyrolytic conversion of the PMO precursor particles. These fillers converted to truncated rhombic dodecahedral SiOC mesoporous micron-sized grains, homogeneously distributed throughout the SiOC cellular matrix. The produced porous ceramics possessed compression strength of about 1.7 MPa, which is adequate for their use in several engineering applications.     

Highly porous macro- and micro-cellular ceramics from a polysilazane precursor

Micro- and macro-cellular SiCN and SiOCN foams were produced via two different routes by using a polysilazane preceramic polymer. In the first route, a mixture of partially cross-linked polysilazane and poly(methylmetacrylate) microspheres, used as sacrificial fillers, was warm pressed and subsequently pyrolyzed to create micro-cellular foams. In the second route, liquid polysilazane was mixed with a physical blowing agent and the blend was cured and pyrolyzed, leading to the formation of macro-cellular ceramics in a one-step process. Ceramic components of different morphology and characteristics, depending on the processing method adopted, were fabricated. The foams had a mostly interconnected porosity ranging from 60 to 80 vol% and possessing a compressive strength in the range 1–11 MPa. Some oxygen contamination was found in the foams obtained using the sacrificial fillers, probably because of the adsorbed humidity on their surface. The polymer derived ceramic (PDC) route is an efficient and cost effective way to produce SiCN-based foams possessing tailored pore architecture and properties suitable for high temperature applications.     

Polysiloxane‐Derived Ceramics Containing Nanowires with Catalytically Active Tips

By direct foaming of a Pt‐containing polysiloxane precursor, macroporous ceramics were generated by pyrolysis at 1400°C under nitrogen or argon. The growth of nanowires was induced via a vapor–liquid–solid mechanism in which the Pt particles acted as deposition site for the decomposition gases released upon pyrolyzing the preceramic polymer. SEM, HR‐SEM, TEM/EDX, and XRD investigations revealed that pyrolysis under argon atmosphere leads to short SiC nanowires of only a few micrometers length and under nitrogen atmosphere Si₃N₄ nanowires evolved, with length of several 10 μm. In both cases the tips of the nanowires mainly consisted of PtSi. In contrast to samples pyrolyzed at 600°C, the components after higher temperature pyrolysis showed moderate‐specific surface areas of 55–67 m²/g. In CO oxidation experiments, a good catalytic activity was found for the Pt silicide particles, suggesting that despite their relatively large size, their location at the tips of the nanowires affords them good reactivity.     

Processing of polymer-derived,aerogel-filled,SiC foams for high-temperature insulation

Porous polymer-derived ceramics (PDCs) are outperforming materials when low-density and thermal inertia are required. In this frame, thermal insulating foams such as silicon carbide (SiC) ones possess intriguing requisites for aerospace applications, but their thermal conductivity is affected by gas phase heat transfer and, in the high temperature region, by radiative mechanisms. Owing to the versatility of the PDC route, we present a synthesis pathway to embed PDC SiC aerogels within the open cells of a SiC foam, thus sensibly decreasing the thermal conductivity at 1000°C from 0.371 W·m⁻¹K⁻¹ to 0.243 W·m⁻¹K⁻¹. In this way, it was possible to couple the mechanical properties of the foam with the insulating ability of the aerogels. The presented synthesis was optimized by selecting, among acetone, n-hexane, and cyclohexane, the proper solvent for the gelation step of the aerogel formation to obtain a proper mesoporous colloidal structure that, after ceramization at 1000°C, presents a specific surface area of 193 m²·g⁻¹. The so-obtained ceramic composites present a lowest density of 0.18 g·cm⁻³, a porosity of 90% and a compressive strength of 0.76 MPa.     

Hydrothermal synthesis of potassium–sodium niobate powders

Potassium–sodium niobates (K_xNa_1−xNbO₃, 0 < x < 1, KNN) were hydrothermally synthesized under varying alkaline ratios (K⁺/Na⁺), total hydroxide concentration, reaction temperature, and time. Compositional surveys were developed by using Rietveld analyses derived quantitative volume fractions. The data demonstrated that phase pure KNN synthesis can be achieved by reacting the niobium source with the hydroxide solution having 6 M total hydroxide concentration, cation ratio (K⁺/Na⁺) of above 6 at temperatures ≥200°C for 24 h. Dissolution–precipitation events through intermediate products including hexaniobates were postulated as a plausible formation mechanism. It was shown also that the single-phase KNN approaching the morphotropic phase boundary (MPB) could be obtained by further incorporation of sodium ions into the crystal via post-annealing at 800°C/2 h, following the hydrothermal synthesis.     

Solvothermal Synthesis of Acmite Conversion Coatings on Steel

Acmite (NaFeSi₂O₆) films were formed on steel coupons via solvothermal reaction of silica, sodium hydroxide, and 1, 4‐butanediol in an autoclave under autogenous pressure. Systematic variation in processing variables led to homogenous coatings comprised of pinacoidal acmite grains with an average grain size of ~33 μm. The coatings were produced on the steel coupons from reactant conditions of 0.635 m SiO₂, 2.546 m NaOH, and 3.087 m 1,4‐butanediol for 72 h at 240°C.     

Fabrication of ceramic components with hierarchical porosity

This article reviews different methodologies for the fabrication of monolithic ceramic components possessing multiscale porosity, i.e., with pores ranging from a few nanometers to several hundred microns. Two main strategies have been discussed: (a) the assembling of micro/mesoporous materials into components possessing also macropores; (b) the addition of micro/mesoporosity to macroporous, cellular monoliths. Both routes include one-pot and multi-step processing routes, and yield components with different properties in terms, for instance, of specific surface area values, mechanical strength, and permeability to fluids. The wide range of processing approaches available enable the fabrication of components with very varied morphology, suitable for a variety of industrial applications.     

Closed porosity ceramics and glasses

In the last three decades, considerable effort has been devoted to obtain both open and closed porosity ceramics & glasses in order to benefit from unique combination of properties such as mechanical strength, thermal and chemical stability at low-relative density. Most of these investigations were directed to the production and the analysis of the properties for open porosity materials, and regrettably quite a few compositions and manufacturing methods were documented for closed porosity ceramics & glasses in the scientific literature so far. This review focuses on the processing strategies, the properties and the applications of closed porosity ceramics & glasses with total porosity higher than 25%. The ones below such level are intentionally left out and the paper is set out to demonstrate the porous components with deliberately generated closed pores/cells. The processing strategies are categorized into five different groups, namely sacrificial templating, high-temperature bonding of hollow structures, casting, direct foaming, and emulsions. The principles underlying these methods are given, with particular emphasis on the critical issues that affect the pore characteristics, mechanical, thermal and electrical properties of the produced components.     

Polymer-derived ceramic molten metal filters

Journal of Materials Science - This paper reports the synthesis and the performance of polymer-derived ceramic filters for molten metal filtration. Two different filter types were studied: foam...     

Reactive Hydrothermal Liquid‐Phase Densification (rHLPD) of Ceramics – A Study of the BaTiO3[TiO2] Composite System

A densification process called reactive hydrothermal liquid‐phase densification (rHLPD), based on principles of hydrothermal reaction, infiltration, reactive crystallization, and liquid‐phase sintering, is presented. rHLPD can be used to form monolithic ceramic components at low temperatures. The densification of barium titanate–titania composite monoliths was studied to demonstrate proof of concept for this densification model. Permeable, green titania (anatase) compacts were infiltrated with aqueous barium hydroxide solutions and reacted under hydrothermal conditions in the temperature range 90°C–240°C. The effects of reaction time and temperature on the conversion of titania (anatase) into barium titanate were studied. Utilizing a 72 h reaction at 240°C between l.0 M Ba(OH)₂, an anatase (TiO₂) powder compact, and a corresponding Ba/Ti ratio of 1.5, it was possible to crystallize a composite 95 wt% (88 mol%) BaTiO₃ and 5 wt% (12 mol%) TiO₂. The composite had a relative density of ~90% with a compressive strength of 172 ± 21 MPa and a flexural strength of 49 ± 4 MPa.