Closed porosity ceramics and glasses Vakifahmetoglu, Cekdar; Semerci, Tugce; Soraru, Gian Domenico
Journal of the American Ceramic Society,
20/May , Volume:
103, Issue:
5
Journal Article
Peer reviewed
Open access
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.
Preceramic polymers were proposed over 30 years ago as precursors for the fabrication of mainly Si‐based advanced ceramics, generally denoted as polymer‐derived ceramics (PDCs). The polymer to ...ceramic transformation process enabled significant technological breakthroughs in ceramic science and technology, such as the development of ceramic fibers, coatings, or ceramics stable at ultrahigh temperatures (up to 2000°C) with respect to decomposition, crystallization, phase separation, and creep. In recent years, several important advances have been achieved such as the discovery of a variety of functional properties associated with PDCs. Moreover, novel insights into their structure at the nanoscale level have contributed to the fundamental understanding of the various useful and unique features of PDCs related to their high chemical durability or high creep resistance or semiconducting behavior. From the processing point of view, preceramic polymers have been used as reactive binders to produce technical ceramics, they have been manipulated to allow for the formation of ordered pores in the meso‐range, they have been tested for joining advanced ceramic components, and have been processed into bulk or macroporous components. Consequently, possible fields of applications of PDCs have been extended significantly by the recent research and development activities. Several key engineering fields suitable for application of PDCs include high‐temperature‐resistant materials (energy materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food‐ and biotechnology, etc.), or functional materials in electrical engineering as well as in micro/nanoelectronics. The science and technological development of PDCs are highly interdisciplinary, at the forefront of micro‐ and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers. Moreover, several specialized industries have already commercialized components based on PDCs, and the production and availability of the precursors used has dramatically increased over the past few years. In this feature article, we highlight the following scientific issues related to advanced PDCs research: (1) General synthesis procedures to produce silicon‐based preceramic polymers. (2) Special microstructural features of PDCs. (3) Unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines. (4) Processing strategies to fabricate ceramic components from preceramic polymers. (5) Discussion and presentation of several examples of possible real‐life applications that take advantage of the special characteristics of preceramic polymers. Note: In the past, a wide range of specialized international symposia have been devoted to PDCs, in particular organized by the American Ceramic Society, the European Materials Society, and the Materials Research Society. Most of the reviews available on PDCs are either not up to date or deal with only a subset of preceramic polymers and ceramics (e.g., silazanes to produce SiCN‐based ceramics). Thus, this review is focused on a large number of novel data and developments, and contains materials from the literature but also from sources that are not widely available.
A method is described here for preparing lightweight and low cost polymer-derived silicon carbide foam by impregnation of preceramic polymer in polyurethane foam. A series of silicon carbide foams ...with various density (0.035–0.35g/cm3), porosity (87–98%) and thermal conductivity (0.05–0.12W/m·K) were prepared using allylhydropolycarbosilane as a base material. Surface analysis shows that the resultant foams have an open and interconnected porous structure. Phase analysis shows that it has cubic crystal structure. There was no evidence of cracks or damage even after treating the silicon carbide foam with concentrated hydrofluoric acid for 12days. From the oxidation resistance experiment, it can be concluded that the silicon carbide foam is stable up to 1500°C in air and 2000°C in argon.
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•Lightweight and low cost silicon carbide foam is prepared by polymer derived ceramic route.•Silicon carbide foam shows interconnected open cell structure made of solid strut.•It has good structural stability at very high temperature.•It is stable in reactive environments.
Silicon oxycarbide (SiOC) glasses in the form of thin, dense, and crack‐free samples were fabricated according to the polymer pyrolysis route starting from cross‐linked polysiloxane. The amount of ...free carbon in the final SiOC materials was varied in the range 18‐60 vol%. The mechanical properties of the SiOC glasses were measured by nanoindentaion technique and revealed that both the Young's modulus and the hardness decrease with increase in the free carbon content and follow a simple rule of mixtures model.
Polymer‐derived SiOC‐C composites are typically obtained through pyrolysis of a polysiloxane precursor in inert atmosphere. Recent studies have shown that novel SiOC microstructures and compositions ...can be obtained when the pyrolysis is carried out in a reactive environment, as CO2, which leads to a selective oxidation of the Si─C bonds leaving a microstructure constituted by a nano‐dispersed sp2 carbon phase within an SiO2 matrix. However, little is known about the reaction mechanisms between CO2 and the preceramic polymer to date. In this work, we investigated the pyrolysis of a methyl‐silsesquioxane in reactive (CO2) and inert (Ar or He) atmosphere by combining TG/MS and FT‐IR analysis. The results showed that CO2 starts to react with the preceramic polymer from ≈750°C when the Si─CH3 groups start to form Si─CHx‐Si units. The reaction breaks the Si─C bond increasing the amount of the free carbon phase and releasing water vapor, detected by MS, even at temperatures exceeding 900°C. At higher temperatures (≈950°C), CO2 reacts with the free carbon phase leading to a weight loss and the formation of CO.
In this study the high temperature stability (crystallization and decomposition) of two silicon oxycarbide glasses with a similar amount of free carbon (8.3 vs 9.6 wt%) but different content of Si‐C ...bonds (SiC0.22O1.57 vs SiC0.07O1.86) is presented. The two SiOC glasses are obtained from the same precursor (2 µm methyl‐silsesquioxane spheres) via pyrolysis at 1100°C in inert (Ar) or reactive (CO2) atmospheres. Further annealing in Ar flow at temperatures above 1100°C and up to 1500°C is performed and the samples are characterized by Fourier Transformed Infrared Spectroscopy (FT‐IR) and X‐ray diffraction (XRD). For comparison purposes the same precursor was annealed in air flow to obtain SiO2 and its high temperature evolution is also studied. Results suggest that the onset for the carbothermal reduction is not dependent on the amount of Si‐C bonds. Moreover, contrary to what is usually reported in the scientific literature, silica phase present in the SiOC glasses does not show, in the same experimental conditions, superior crystallization resistance compared to pure silica glass.
In comparison with metals and polymers, ceramics and/or carbon are more difficult to process into well‐defined cellular architectures (e.g., cubic, tetrakaidecadehron, etc.) using Additive ...Manufacturing techniques. The present work reports a simple method for generating complex and precise SiCN ceramic lattices using a preceramic polymer and applying the replica approach to structures fabricated using stereolithography of plastic materials, with the associated ease of fabrication. Three‐dimensional printed plastic lattices impregnated with a polysilazane were converted to SiCN by pyrolysis at 1000°C in inert atmosphere. In spite of the high amount of mass loss (~58%) and volume shrinkage (~65%), the impregnated structures did not collapse during pyrolysis, leading to highly porous (total porosity ~93 vol%) components possessing suitable strength for handling and potential use as lightweight components.
Polymer‐derived ceramics (PDCs) are a class of advanced materials obtained by pyrolysis in a controlled atmosphere of an organosilicon polymer. Their functional as well as mechanical properties ...originate from the peculiar nanostructures developed during the pyrolysis. Herein, we investigate the formation of transient microporosity in a model PDC (methyl‐silsesquioxane) obtained in Ar, Ar–5%H2, CO2, and air. It is shown that a common evolution can be detected up to 700°C. At this temperature, the structure of the material in terms of chemical bonds is marginally changed (only redistribution reactions take place), but the medium‐range order is clearly modified moving the system to a more disordered state (detected by small angle x‐ray scattering SAXS) and causing the formation of a large amount of open micropores sized at about 1.2–1.7 nm. In the 700–800°C range, the proper ceramization starts causing the formation of a new class of small (around 1 nm) open micropores. These partially annihilate at 900°C in Ar and Ar–H2 (i.e., in the second part of the ceramization process), whereas they totally collapse in CO2 due to the formation of a more silica‐like SiOC (less polymerized and viscous). Finally, SAXS points out the persistence of relatively large closed nanovoids of about 1 nm at 1250°C for the samples treated in Ar and Ar–H2. These might explain anomalies in terms of density, elastic modulus, and thermal conductivity of this class of ceramics as reported in the literature.
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.
Details of our study showing a polyurethane foam and its SiC replica, a preceramic aerogel and their composite featuring different aerogel colloidal microstructures as the synthesis solvent is n‐hexane, acetone, or cyclohexane. On the top right, the preceramic aerogels pore size distribution shows the solvent effect; on the bottom right, thermal conductivity of the aerogel/foam composites at high temperature is displayed.
Mechanical properties of polymer‐derived ceramics are usually measured on samples pyrolyzed in inert atmosphere. Here, we report the hardness and elastic modulus of SiOC and SiCN pyrolyzed in both ...inert (Ar) and reactive (CO2) atmosphere. The external surface of the specimens exposed to the pyrolysis gas was characterized by Vickers microhardness measurements and infrared spectroscopy. The elastic modulus was evaluated by three‐point bending tests on thin (150‐200 µm) and dense specimens. Polished sections of the SiOC samples were prepared to study, by energy‐dispersive X‐ray spectroscopy (EDXS) and nanoindentation, how the elemental composition, hardness, and elastic modulus vary from the surface toward the bulk. For both compositions, pyrolysis in CO2 leads to a strong decrease in the hardness and elastic modulus. The hardness of both the samples pyrolyzed in CO2 approaches the typical value for fused silica, suggesting that CO2 selectively breaks the Si–C and Si–N bonds and leads to the formation of a silica‐like network. EDXS and nanoindentation reveal that the modification induced by the CO2 flow extends below the surface at least for a thickness of about 30 µm.
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