The fate and role of in situ formed carbon in polymer-derived ceramics
Introduction
Si-based advanced ceramics (e.g., SiC, Si3N4) that provide a good combination of high strength, superior hardness, outstanding oxidation resistance as well as excellent thermal and chemical stability are one of the most important classes of advanced materials. Conventionally, the Si-based ceramic parts are manufactured by the powder processing technology, including powder synthesis, powder processing (e.g., milling and mixing), shaping and sintering [1]. However, the powder technologies are not suitable for fabricating ceramic fibers, crack-free coatings/films, ceramic matrix composites, ceramic nanocomposites as well as dense ceramic monoliths obtained at relatively low processing temperatures (e.g., 1100–1300 °C).
In the early 1960s, organosilicon polymers have been proposed for the preparation of Si-based ceramics via a polymer-to-ceramic transformation process, and the resulting ceramics are denoted as PDCs [2], [3]. The potential of such preceramic polymers for materials science was not recognized until the first practical application reported by Veerbeck [4], [5], Winter [6], and Yajima [7], [8], [9], [10] in 1970s, i.e., the manufacture of small-diameter Si3N4/SiC-based and SiC-based ceramic fibers via thermolysis of polyorganosilicon precursors. Since then, numerous preceramic polymers for the synthesis of Si-based ceramics have been developed [11], [12], [13], [14], [15], [16], [17], [18]. The transformation of polymer to ceramic upon pyrolysis provides an important opportunity to develop several novel Si-based advanced ceramics, including coatings/films [19], [20], [21], [22], small-diameter fibers [7], [23], ceramic matrix composites [24], [25], [26], [27], dense monoliths obtained at relatively low temperatures (1000–1300 °C) [15], [28], [29] as well as non-oxide ceramics stable at temperatures up to 2000 °C [18], [14]. Moreover, the PDC route enables facile producing of Si-based ternary (e.g., SiCN, [30], [31], [32], [33], [34] and SiOC [35], [36], [37], [38], [39]), quaternary (e.g., SiBCN [14], [40], [41], [42], [43], SiBCO [44], [45], [46], SiCNO [47], [48], [49], [50], SiAlCN [51], [52], [53], [54], and SiAlCO [55], [56], [57], [58]) or even pentanary (e.g., SiHfBCN [59], [60], [61], [62] and SiHfCNO [63], [64], [65], [66]) ceramics, which are difficult to be fabricated by other methods so far [18], [67]. Consequently, PDCs attracted increasing attention in the past decades, and the number of publications still grows dramatically (Fig. 1). Particularly, the number of publications clearly denoted as “PDCs” in the past 10 years (2009–2018) is around 1600, which is more than that of all the publications before 2009.
Owing to the promising structural and functional properties as well as their ability for being shaped upon various processing techniques, PDCs have found fundamental and technological interest for a variety of applications in several key fields, such as high-temperature resistant materials for structural applications (e.g., ceramic fibers [9], [68], [69], [70], ceramic matrix composites [25], [60], [71], [72], environmental/thermal barrier coatings [21], [73], [74], [75], joining materials [76], [77] and ceramic foams/aerogels [78], [79], [80], [81]), ceramic heaters (e.g., glow plugs) [82], heat exchangers [83], electromagnetic absorbing and shielding applications [84], [85], [86], [87], microelectromechanical systems (MEMS) [88], [89], [90], [91], [92], [93], photoluminescent applications [94], [95], [96], [97], energy and environmental applications [98], [99], [100], [101], [102], [103], tribological applications (e.g., brakes for motorbikes) [104], [105], [106], [107], [108], sensing materials [109], [110], [111], [112] and biomedical components [113], [114], [115]. The SiOC ceramics are even regarded as an “All-Rounder” materials suitable for several advanced structural and functional applications [116]. The typical applications of the PDCs are shown in Fig. 2, and more information can be found in some other review articles and books [13], [18], [78], [82], [89], [116], [117], [118], [119].
Recently, numerous novel ceramic nanocomposites with tailored microstructures and properties were successfully synthesized using the PDC approach. The structural and functional properties of the polymer-derived ceramic nanocomposites can be controlled at the nanoscale level by molecular design of the preceramic precursors [120], [121], [122]. Moreover, polymer-derived porous ceramics, ceramic nanowires/nanobelts as well as ceramic micro-lattice, honeycomb cellular and sub-micrometer 3D complex architectures fabricated by additive manufacturing of preceramic precursors have been reported as well [123], [124], [125], [126], [127]. In addition, a few Si-free PDCs have also been developed in the past, such as polymer-derived BN [128], [129], [130], [131], [132], BCN [133], [134], [135], ZrO2 [136], transition metal nitrides [137], [138], [139] as well as ultrahigh temperature ceramics (e.g., TiC [140], [141], [142], ZrC [141], [142], [143], ZrB2 [144], [145], ZrC/ZrB2 [146], HfC [141], [147] and HfTaC2 [148]). Detailed information regarding the history and developments of PDCs can be found in some other review articles and books published from 1984 to 2019 [18], [67], [82], [89], [116], [118], [119], [120], [127], [149], [150], [151], [152], [153], [154], [155], [156], [157], [158], [159], [160], [161], [162]. In the present review, we focus on highlighting the fate and role of in situ formed carbon in Si-based PDCs. The Si-free PDCs also contain in situ formed free carbon, but they will not be discussed here because the free carbon shows similar fate and role to that within the Si-based PDCs [140], [142], [143], [147], [162].
Within the research field of PDCs, a phenomenon associated with the polymer-to-ceramic transformation process, namely the in situ formation of carbon (so called “free carbon”) in the generated silicon-based ceramic matrix, has been widely reported [7], [41], [163], [164], [165], [166], [167]. After several decades of research, it is believed that the presence of free carbon must be a common event for any PDCs, even with the C/Si ratios as low as 0.7 [17], [163], [168], [169]. As shown in Fig. 1, a large amount of publications (>300 articles) are directly related to free carbon, and the number of publications increases with increasing of PDC-related publications. Moreover, the free carbon is found to act as an independent entity in the PDCs rather than to link to the surrounding medium (except for the mixed bonds at the interface when pyrolyzed at lower temperatures). Different from the “carbidic carbon” in which the carbon atoms are directly bonded to Si atoms forming a sp3-hybridization, the carbon atoms in the free carbon are not directly bonded to Si or other atoms but bonded to each other, forming a sp2-hybridization. Therefore, it is called “free carbon”, “segregated carbon”, “sp2-carbon” or sometimes “excess carbon”.
The free carbon stems from the hydrocarbon groups (e.g., alkyl, vinyl, allyl, phenyl and benzyl groups) that are attached to the backbone of the preceramic precursors (i.e., the R1 and R2 groups in Fig. 3). After pyrolysis, hydrogen atoms are removed preferentially leaving behind excess carbon in the ceramics [170], [171], [172]. Accordingly, there is a strong relationship between the molecular structure of the preceramic polymer and the amount and distribution of free carbon within the PDCs, which was proved by numerous studies [173], [174], [175], [176]. Firstly, G.T. Burns et al. investigated the effects of the hydrocarbon groups (i.e., R groups in Fig. 3) on the chemical composition of SiCN- and SiOC-based ceramics [170], [177], [178], [179]. The results reveal that the saturated and unsaturated R groups have totally different effects on the free carbon content of PDCs. For the saturated R groups (e.g., methyl, ethyl, propyl and isobutyl), the free carbon content in the PDCs is relatively low and it is independent on the molecular structure and number of carbon atoms on the side groups. In contrast, the unsaturated R groups (e.g., allyl, vinyl, phenyl groups) lead to higher free carbon contents with a variable range of amounts. Generally, the free carbon content in the ceramics increases as the number of carbon atoms in the R group increases. For instance, the free carbon content in the ceramics follows the trend of allyl < 3-butenyl < phenyl < benzyl [170]. Analogous phenomenon was analyzed by Mera et al. in a series of SiCN-based ceramics derived from poly(silylcarbodiimides) with different substituents (H, phenyl, methyl or vinyl) [176]. Hurwitz et al. also demonstrated that the free carbon content of SiOC-based ceramics increases with increasing concentration of phenyl groups in the preceramic polymers [180]. E. Bouillon et al. studied the free carbon content in several SiC-based ceramic fibers prepared from a series of functional polycarbosilanes. They claimed that the amount of free carbon within the PDCs strongly depends on the thermal stability of the hydrocarbon linkages, rather than on the percentage of the total carbon content of the polymer [181].
Furthermore, the thermal-treatment conditions (temperature, atmosphere and duration) also play an important role in the fate of free carbon within the PDCs [174], [175], [182], [183]. These correlations will be carefully discussed in the following sections.
Formerly, the free carbon was thought to be detrimental to the mechanical properties (e.g., tensile strength, hardness, and Young’s modulus) and high-temperature properties (e.g., thermal stabilities and oxidation resistance) of the final PDC products [165]. Consequently, many efforts have been made to reduce the content of free carbon, for instance, adjusting the molecular structure of preceramic polymers or pyrolyzing the polymers under reducing atmosphere (e.g., H2 and NH3) [174], [175], [184], [185]. However, in recent years, this view has been completely revised due to the fact that, under certain conditions, several carbon-rich PDCs exhibit better resistance toward crystallization, decomposition and oxidation than those with lower free carbon content [165], [166], [167], [176], [178], [179], [180], [186], [187]. In addition, the free carbon also has been proven to be beneficial for a number of other structural and functional properties of the PDCs, such as electrical conductivity [188], [189], [190], [191], [192], [193], [194], [195], electromagnetic properties [196], [197], [198], electrochemical properties [37], [199], [200], piezoresistivity [201], [202], [203], corrosion resistance [184], [204], tribological properties [104], [106], [205], creep resistance [206], [207], [208], as well as high-temperature anelastic behavior [209]. Accordingly, in the past decades, the fate and role of the free carbon on the structural and functional properties of PDCs is one of the most intriguing questions in this field of research. Numbers of meaningful studies have been reported. However, there is still a need to fully understand its generation and structural evolution under different thermal-treatment conditions, interactions with the Si-based ceramic matrix and how it affects the properties for the sake of tailoring the performance of PDC materials upon adjusting the amount, morphology, grain size, crystallinity and distribution of the free carbon [165]. This article, for the first time, is to review the research results regarding the fate and role of in situ formed free carbon within the PDCs in the past decades in order to provide a systematic information for the researchers to further investigate and understand the free carbon as well as the PDCs.
Section snippets
General synthetic routes for Si-based preceramic polymers
Synthesis of the preceramic precursors is a crucial step in fabrication of PDCs because not only the chemical composition but also the phase composition and microstructure of obtained ceramics (including the status of the free carbon) are strongly affected by the molecular structure of the preceramic polymers. As a result, the structural and functional properties of PDCs can be effectively adjusted by design of the precursors at the molecular level.
An overview of synthesis routes for the most
Processing of PDCs
Fabrication of PDCs typically comprises the following steps: (1) synthesis of preceramic precursors using appropriate monomers; (2) shaping and crosslinking at low temperatures (100–400 °C); (3) polymer-to-ceramic transformation (i.e., ceramization) via pyrolysis at temperatures ranging from 400 to 1400 °C [18], [160]. Most PDCs after the polymer-to-ceramic transformation are amorphous. The (poly)crystalline PDCs can be obtained by subsequent annealing of the amorphous ceramics at elevated
Overview
As mentioned above, the amorphous PDCs will undergo a devitrification processes when they are subject to elevated temperature conditions, leading to the formation of amorphous multiphase systems via redistribution reactions of the chemical bonds (i.e., phase separation) and, subsequently, to the nucleation and coarsening of nanocrystals (i.e., crystallization). Particularly, the free carbon phase after phase separation is subjected to a graphitization process, which plays an important role in
Energetics consideration
The energetics of PDCs have been studied recently via a experimental calorimetry conducted in oxidative molten oxide solvents in order to know their thermodynamic properties [428], [429], [430], [431]. The thermodynamic behavior of a material is a macroscopic manifestation of their atomic, nano- and micro-scaled structure and bonding. Particularly, the enthalpy of formation reveals the number, type, and strength of chemical bonds as well as the short-, mid- and long-range order within the
Mechanical properties
From the view point of history, the potential of PDCs was recognized due to its possibility for producing SiC-based ceramic fibers with high thermo-mechanical performance [4], [5], [6], [7], [8], [9], [10]. After decades of development, the tensile strength, Young’s modulus and the high-temperature stability of SiC-based fibers have been improved significantly by means of tuning the chemical/phase composition of fibers by carefully controlling the chemical composition of precursors and
Electrical properties
Electrical properties of the PDCs have been widely investigated since the early discovery of these new materials due to their functional applications for high-temperature microelectromechanical systems (MEMS), hash-environmental sensors, micro glow plugs as well as electrode materials for lithium-ion batteries and pacemakers [58], [91], [92], [265], [367], [524], [525], [526], [527], [528], [529], [530], [531], [532], [533]. The intrinsic room-temperature DC electrical conductivity of PDCs
Ceramic fibers
As mentioned at the start of the article, the potential of the PDCs for materials science was not recognized until people manufactured small-diameter ceramic fibers via thermolysis of polyorganosilicon precursors in 1970s [4], [5], [6], [7], [8], [9], [10]. Up to now, the ceramics fibers (mainly SiC-based fibers) is still the most successful commercial application of the PDCs. For example, the SiC-based fibers have been applied as heat-resistant materials and as reinforcement for polymer matrix
Conclusions and outlook
PDCs as a class of advanced materials with exciting structural and functional properties have received increasing attention in the scientific community of materials science in the past 50 years. Their amorphous and crystallized states are characterized by interconnected nanodomain architectures and multiphase polycrystalline nano/microstructures, respectively, which dominate their structural and functional properties as well as the resultant applications. The in situ formed free carbon as the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Qingbo Wen thanks the Profile Area of Technische Universität Darmstadt “From Material to Product Innovation (PMP)” and the “Career Bridging Grant” for financial support. Zhaoju Yu thanks the National Natural Science Foundation of China (No 51872246) and the Alexander von Humboldt Foundation for financial support. Ralf Riedel acknowledges the long-term financial support by the German Research Foundation (DFG, Bonn, Germany) via a variety of funded individual projects in the past 25 years
Dr. Qingbo Wen is a postdoc researcher in Prof. Riedel’s group at the Technische Universität Darmstadt in Germany, where he received his PhD degree in Materials Science in 2017. He got his master’s degree (2012) and bachelor’s degree (2009) in Environmental Engineering at Hunan University in China. One of his research interests is focused on polymer-derived ceramics (PDCs), particularly on the development of advanced ceramic nanocomposites with tailor‐made chemical/phase compositions and
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Dr. Qingbo Wen is a postdoc researcher in Prof. Riedel’s group at the Technische Universität Darmstadt in Germany, where he received his PhD degree in Materials Science in 2017. He got his master’s degree (2012) and bachelor’s degree (2009) in Environmental Engineering at Hunan University in China. One of his research interests is focused on polymer-derived ceramics (PDCs), particularly on the development of advanced ceramic nanocomposites with tailor‐made chemical/phase compositions and microstructures for structural and functional applications in harsh environment (e.g., ultrahigh-temperature ceramics/composites for thermal protection systems, high-temperature electromagnetic wave absorbing/shielding materials as well as electrode materials for high-temperature fuel cells). Another research interest is devoted to ultrahigh-pressure and high-temperature synthesis of novel transition metal nitrides and their solid solutions. Up to now, Dr. Wen has published more than 30 papers in peer‐reviewed journals and filed four patents as first/corresponding author or co-author.
Prof. Zhaoju Yu received her PhD degree in Polymer Chemistry and Physics in 2004 at the Wuhan University, China. She is presently Full Professor and Deputy Director of Key Laboratory of High Performance Ceramic Fibers at the College of Materials, Xiamen University, China. Prof. Yu was a Humboldt Research Fellow (Awarded on 15.12.2016) and guest professor in the frame of an international scientist exchange program with the research group “Disperse Feststoffe” at the Materials and Geosciences Department of the Technische Universität Darmstadt. Since 2019, she is Associate Editor of the journal of Ceramics International. Her research fields cover ceramic nanocomposites, ceramic fibers and silicon-based polymer materials. In particular, her research work is focused on i) advanced Si-based ceramic nanocomposites containing nanocarbon phases by molecular approach; and ii) correlation of the molecular structure of preceramic polymers with the microstructure and advance functional properties (such as electrical, dielectric, catalytic, electromagnetic properties) of the ceramic materials obtained therefrom. She has currently published more than 60 scientific papers in peer‐reviewed journals, gave more than 30 Invited Conference Presentations and authored 11 granted patents.
Prof. Riedel got a PhD degree in Inorganic Chemistry in 1986 at the University of Stuttgart. After a Postdoc period at the Max-Planck Institute für Metallforschung in Stuttgart, he became Full Professor at the Institute of Materials Science at the Technische Universität Darmstadt in 1993. He is an elected member of the World Academy of Ceramics, Fellow of the American Ceramic Society, the European Ceramic Society as well as Fellow of the School of Engineering at The University of Tokyo in Japan. Prof. Riedel was awarded with the Gold Medal for Merits in Natural Sciences and with an honorary doctorate of the Slovak Academy of Science as well as with the Gustav Tammann Prize of the German Society of Materials Science (DGM). In 2009, he received an honorary Professorship at the Tianjin University in Tianjin, China. He was Guest Professor at the Jiangsu University in Zhenjiang and at the Xiamen University in China. Recently, Prof. Riedel received the Innovation Talents Award of Shaanxi Province, China at the Northwestern Polytechnical University in Xían. He is Guest Professor at the University of Tokyo in the group of Prof. Ikuhara, Japan and was awarded with the International Ceramics Prize 2020 for “Basic Science” of the World Academy of Ceramics. HeHe is Editor in Chief of the Journal of The American Ceramic Society and of Ceramics International. His current research interest is focused on two research areas, namely i) molecular synthesis of advanced structural and functional ceramics for ultra-high temperature and energy-related applications as well as ii) ultrahigh pressure materials synthesis.