Silicon oxycarbide glasses and glass‐ceramics: “All‐Rounder” materials for advanced structural and functional applications

Silicon oxycarbides can be considered as being carbon‐containing silicates consisting of glass networks in which oxygen and carbon share bonds with silicon. The carbon‐for‐oxygen substitution in silicate glass networks has been shown to induce significant changes in the network connectivity and consequently strong improvements in the properties of the silicate glass network. For instance, SiOC glasses exhibit Young's moduli, hardness values, glass transition, and crystallization temperatures which are superior to those of vitreous silica. Moreover, the silicon oxycarbide glass network exhibits unique structural features such as reduced mass fractal dimension and nano‐heterogeneity, which significantly affect and/or dictate its properties and behavior. In the present Review, a consideration of the current state of the art concerning the synthesis, processing, and various structural and functional properties of silicon‐oxycarbide‐based glasses and glass‐ceramics is done. Thus, the synthesis of silicon oxycarbides starting from macromolecular precursors such as polysiloxanes or alkoxysilanes‐based sol‐gel systems as well as current advances related to their processing will be critically reviewed. In addition, various structural and functional properties of silicon oxycarbides are presented. Specific emphasis will be put on the intimate correlation between the molecular architecture of the precursors and the structural features and properties of the resulting silicon oxycarbides.     

Additive manufacturing of polymer-derived ceramic matrix composites

This study presents a fabrication method and identifies processing bounds for additively manufacturing (AM) ceramic matrix composites (CMCs), comprising a silicon oxycarbide (SiOC) ceramic matrix. A digital light projection printer was used to photopolymerize a siloxane-based preceramic resin containing inert ceramic reinforcement. A subsequent pyrolysis converted the preceramic polymer to SiOC. Particle reinforcements of 0 to 40% by volume in the green state were uniformly dispersed in the printed samples to study their effects on pyrolysis mass loss and shrinkage, and CMC notch sensitivity and strength. Both particle and whisker reinforcements toughened the glassy SiOC matrix (1 MPa m^1/2), reaching values >3 MPa m^1/2. Bending strengths of >300 MPa (>150 MPa (g cm⁻³)⁻¹) and a Weibull modulus of 10 were measured on AM samples without surface finish. We identified two pore formation mechanisms that placed processing bounds on sample size and reinforcement volume fraction. Methods for increasing these bounds are discussed. With properties commensurate to traditionally processed technical ceramics, the presented process allows for free-form fabrication of high-performance AM CMC components.     

Dielectric and microwave absorption properties of polymer-derived TiC/SiC/SiOC multiphase ceramics in X band

Polymer-derived TiC/SiC/SiOC ceramics were prepared using tetrabutyl titanate (TBT)-modified polysiloxane (PSO) as precursor. The effects of heat treatment temperature and TBT content in precursor on the microstructure, phase composition, and microwave absorbing properties of TiC/SiC/SiOC ceramics were investigated. The crystallinity of the ceramics increases with the increase of heat treatment temperature. With the increase of TBT content, the TiC content of the ceramics increases and the SiC content decreases. When the TBT content ranges from 1 to 5 wt.%, the increase of TBT content has little effect on the real part of the dielectric constant of TiC/SiC/SiOC ceramics. When the TBT content is 7 wt.%, the imaginary part of the dielectric constant of the ceramics changes. For TiC/SiC/SiOC ceramic obtained from the pyrolysis of PSO-TBT precursor with 7 wt.% TBT, the dielectric constant is within the target electromagnetic parameters. Therefore, it has an effective absorption bandwidth of 4.2 GHz, covering the entire X band, showing an excellent microwave absorbing performance.     

Effect of anionic substitution on the high temperature stability of polymer-derived SiOC glasses

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 (SiC_0.22O_1.57 vs SiC_0.07O_1.86) 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.     

UV-curable inorganic precursors enable direct 3D printing of SiOC ceramics

3D ceramic parts are of great interest for various applications including aerospace, defense, electronics, photonics, and biomedical. Yet, additive manufacturing of ceramics is challenging due to their poor machinability. Herein, two approaches based on the chemical modification of silicon resins to obtain UV-curable preceramic precursors of SiOC are described. The dual functionality of the synthesized resins acting both as preceramic precursor and as photopolymerizable entity under UV light is exploited. A set of characterization techniques has allowed the investigation of the mechanisms involved in the synthesis of the inorganic SiOC precursors according to the following approaches: (1) blend of the silicon resin with photoactive monomers and (2) synthesis of a single source UV-curable preceramic silicon resin. A correlation between the nature of the precursor and the properties of the derived SiOC is analyzed. From a technological point of view, the materials can be fabricated as dense or crack-free porous customized objects with low mass loss and optimal surface quality.     

Structural and thermodynamic analysis of metal filler incorporations in SiaOb(M)cCd polymer derived ceramics: Ta,Hf, Nb

This work systematically investigates the thermodynamic stability of Si_aO_b(M)_cC_d structures derived from polymeric precursors incorporating metal fillers: Ta, Nb, and Hf, at 1200 and 1500°C. Structural characterization of the polymer derived ceramics (PDCs) employs X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Enthalpies of formation relative to crystalline components (metal oxide, silica, silicon carbide, and graphite) are obtained from thermodynamic measurements by high temperature oxide melt solution calorimetry. The enthalpies of formation (∆H°_{f, comp}) of Ta-1200, Hf-1200, Nb-1200, Ta-1500, Hf-1500, and Nb-1500 specimens are −137.82 ± 9.72, −256.31 ± 8.97, −82.80 ± 9.82, −182.80 ± 7.85, −292.54 ± 9.38, −224.98 ± 9.60 kJ/mol, respectively. Overall incorporation of Hf results in most thermodynamically stable structures at all synthesis temperatures. Si_aO_b(M)_cC_d specimens employing Nb fillers undergo the most stable structural evolution in this temperature range. The results indicate strong thermodynamic drive for carbothermal reduction of metal oxide domains. Incorporation of Ta provides the greatest stabilization of SiO₃C mixed bonding environments. Ultimately, the choice of metal filler influences composition, structural evolution, and thermodynamic stability in PDCs.     

Making lightweight SiC(rGO,xMoSi2) bulk ceramics via polymeric precursor route

Unique properties of MoSi₂ open new opportunities for preparing bulk polymer-derived ceramics (PDCs) displaying favorable structural-functional capabilities. Herein, an ingenious production route via re-pyrolysis process of ball-milling-induced rigid SiC(rGO, xMoSi₂)_p fillers/flexible polycarbosilane-vinyltriethoxysilane-graphene oxide (PCS-VTES-GO, PVG) precursors blends is proposed to obtain in situ formed SiC(rGO, xMoSi₂) bulk PDCs. Interestingly, the possible dense β-SiC/SiOxCy/C_free(rGO, xMoSi₂) framework suffers load and tiny microsized pores relaxes stress, which is beneficial to providing optimized hardness and fracture toughness, ceramic yield, and linear shrinkage. Attractively, MoSi₂ prominently enhances thermal and electrical conductivities of the products owing to increased continuity and compactness. To the best of our knowledge, lightweight SiC(rGO, 20%MoSi₂) bulk PDCs own brilliant ceramic yield (92.13%), liner shrinkage (6.69%), hardness (10.34 GPa), fracture toughness (4.35 Mpa·m^1/2), and thermal conductivity (8.57 W·m^–1·K^–1), opening potential emerging uses in aerospace fields.     

The effects of adding nano-alumina filler on the properties of polymer-derived SiC coating

In this paper, SiC coating was prepared using the polymer-derived ceramic method; then the effect of nano-alumina as a filler material was studied. First of all, polycarbosilane(PCS) was dissolved in xylene; after that, different amounts of nano-alumina particles were added to the solution. The coating was deposited on the alumina substrate using the dip-coating method; this was followed by sintering at 1200℃. The phase content and microstructure of the samples were studied by X-ray diffraction and scanning electron microscope methods, respectively. Nanohardness, Young's modulus, and coating adhesion were investigated by a nanoindentation method. The sheet resistance was evaluated using the four-point probe technique; also, the wear resistance of the coating was studied by applying the pin-on-disk method. It was found that the addition of the nano-alumina filler up to 20 wt% drastically improved the adhesion and wear resistance of the SiC coating.     

Stereolithography-based additive manufacturing of polymer-derived SiOC/SiC ceramic composites

In order to overcome challenges typically encountered during additive manufacturing of ceramics via the polymer precursor route, a novel polymer-derived SiOC/SiC composite system suitable for advanced geometric designs achievable by lithography-based ceramic manufacturing was established. The photoreactive resin system filled with 20 wt% SiC exhibits suitable viscosity characteristics, adequate stability against sedimentation, and a fast photocuring behavior. After printing and pyrolytic conversion, SiC particulates were well-dispersed within the polymer-derived SiOC matrix. A direct comparison with the unfilled polysiloxane-based resin system showed that the addition of particulate SiC increases handleability, reduces shrinkage, and significantly increases critical wall thicknesses up to 5 mm. The biaxial Ball-on-Three-Balls testing methodology yielded a characteristic strength of 325 MPa for SiOC/SiC composites. The results highlight the high potential of particle-filled preceramic polymer systems toward the fabrication of high-performance SiC-based materials by lithography-based additive manufacturing.     

烧结气氛对合成MgAl2O4-Ti(C,N)复合陶瓷的影响

以金属铝粉、钛白粉和轻烧MgO细粉为原料,通过设计100%焦炭粒(简称C气氛),10%钛白粉 90%焦炭粒(简称TC气氛),以及10%硅微粉 90%焦炭粒(简称SC气氛)3种埋粉条件下的高温烧结还原性气氛,采用X射线衍射仪(XRD)、扫描电镜(SEM)和微区电子探针分析(EPMA)等手段,研究了铝热还原氮化法(1600℃3h)制备MgAl2O4-Ti(C,N)复合陶瓷在不同烧结气氛下的相组成和显微结构的变化。结果表明在不同气氛下,烧后试样的主要物相均为MgAl2O4和Ti(C,N),Ti(C,N)可能会固溶氧,气氛对Ti(C,N)的影响较大。和单纯埋炭气氛下相比,在TC气氛下有助于Ti(C,N)的生成,但晶粒细小;在SC气氛下不利于Ti(C,N)的生成,且有玻璃相存在。     

Oxidation resistance of ZrB2-based monoliths using polymer-derived Si(Zr,B)CN as sintering aid

The focus of the present work is the investigation of the influence of polymer-derived ceramics, used as sintering aids for preparing ZrB₂-based monoliths, on their high-temperature oxidation behavior. For the preparation of the monoliths, ZrB₂ powder was coated with polymer-derived SiCN, SiZrCN, or SiZrBCN and subsequently densified via hot-pressing at temperatures as low as 1800°C. To investigate the oxidation kinetics, thermogravimetric analysis (TGA) was performed at 1300°C in synthetic air with exposure times of 50 and 100 h. A detailed study of the materials oxide scale and subsurface microstructure was conducted using optical microscopy, electron probe microanalysis, scanning electron microscopy, and X-ray diffraction. The experimental findings were compared to thermodynamic equilibrium calculations using the CALPHAD method, which led to a better understanding of the oxidation mechanism. In comparison to the literature data of ZrB₂–SiC, the results show improved oxidation resistance for all three investigated materials. The formation of gaseous species during oxidation, in particular CO, CO₂, B₂O₃, and SiO, within the oxide scale of the monoliths was rationalized via CALPHAD calculations and used to explain the oxidation behavior and kinetics and also the formation of bubbles in the subsurface region of the oxidized specimens.     

Investigations on electrical characteristics of (PbO)30(CuO)x(As2O3)(70-x) glass ceramics

This study is mainly focused on dielectric properties of lead arsenate glasses crystallized with different concentrations of CuO over continuous ranges of frequency (3 Hz −100 kHz) and temperature (300–633 K). The glasses were prepared by melt quenching technique and were heat treated for prolonged time for ceramization. Prepared samples were characterized by XRD, SEM and DSC techniques. SEM studies indicated that the samples are composed of small crystal grains of the size varying from 0.2 to 1.0 µm cemented with the residual glass phase. XRD studies indicated CuAs₂O₄, Pb₂Cu₇(AsO₄)₆ and Cu₂O are the main crystal phases developed during the crystallization. Optical absorption studies confirmed the presence of copper ions in Cu⁺ valence state in addition to Cu²⁺ state and the fraction of Cu⁺ ions is found to increase with the content of CuO. The optical band gap exhibited increasing trend with CuO content. IR spectral studies indicated an increase of degree of polymerization of the glass network with the CuO content. The observed variations of dielectric parameters with frequency, temperature and CuO content are discussed using different polarization mechanisms. The dielectric relaxation effects exhibited by the loss tangent and the electric moduli are analyzed using graphical method and observed relaxation effects are attributed to the complexes of divalent copper ions with oxygens. The impedance diagrams indicated increase of bulk resistance of the samples with increase of CuO content. The ac conductivity exhibited a decreasing trend with increase of CuO content. The conduction phenomenon is explained using polaron hopping between Cu⁺ and Cu²⁺ ions. The temperature independent part of the conductivity is explained using quantum mechanical tunneling (QMT) model. Finally, it is concluded that the insulating strength of the material increased with CuO content and such materials may be useful as electrical insulators in the low temperature region     

Mechanical behaviour of reaction processed SiC ceramics from artificial precursor from plant

Mechanical behaviour of SiC ceramics synthesized from two artificial precursors from plants – coir fibreboard and bamboo pulp fibreboard – was studied and the mechanical properties – flexural strength, Young's modulus, fracture toughness and hardness – of the ceramics synthesized from the two kinds of artificial precursors were compared. The effect of processing of the artificial precursors, as reflected in the microstructure of the ceramics synthesized from them, was taken into consideration in the analysis of the mechanical property data; these data could also be possible to be explained by the empirical models of fracture mechanics. The results of the study established the possibility of application of the investigated SiC materials as structural ceramics.     

用有机前驱体制备Si3N4／纳米SiC复相陶瓷的研究

本研究成功地用有机前驱体引入纳米ＳｉＣ粒子制备出Ｓｉ３Ｎ４／纳米ＳｉＣｐ复相陶瓷。研究了制备工艺和有机前驱体加入量对材料性能及显著结构的影响，并对材料显微结构特点与强韧化机制进行了分析讨论。     

Enhanced microwave-absorption properties of polymer-derived SiC/SiOC composite ceramics modified by carbon nanowires

Novel microwave-absorbing SiOC composite ceramics with dual nanowires (carbon nanowires (CNWs) and SiC nanowires) with high performances were fabricated by using the polymer-derivation method and heat treatment in Ar atmosphere. The introduction of CNWs in the amorphous SiOC ceramics promotes the ceramic crystallization into SiC nanoparticles and SiC nanowires at lower annealing temperatures, which leads to multi-phases and multiple nano heterogeneous interfaces. The distinctive architectures largely increase the interfacial and dipole polarizations of the composite ceramics. The CNWs/SiC/SiOC composite ceramics exhibit excellent microwave-absorption properties in the Ku band (12.4–18 GHz). The minimum reflection coefficient (RC) is -24.5 dB at a thickness of 1.8 mm, while the maximum effective absorption bandwidth (EAB, the corresponding frequency band in which RC is smaller than -10 dB) is 4.8 GHz at a thickness of 1.9 mm, which make the CNWs/SiC/SiOC composite ceramics promising electromagnetic-wave-absorbing materials.     

Structure and energetics of SiOC and SiOC‐modified carbon‐bonded carbon fiber composites

The incorporation of SiOC polymer‐derived ceramics into porous carbon materials could provide tailored shapeable, mechanical, electrical, and oxidation‐resistant properties for high‐temperature applications. Understanding the thermodynamic and kinetic stability of such materials is crucial for their practical application. We report here the dependence of structures and energetics of SiOC and SiOC‐modified carbon‐bonded carbon fiber composites (CBCFs) on the pyrolysis temperature using spectroscopic methods and high‐temperature oxide melt solution calorimetry. The results indicate that a SiOC ceramic pyrolyzed at 1200°C and 1600°C is energetically stable with respect to an isocompositional mixture of cristobalite, silicon carbide, and graphite by 4.9 and 10.3 kJ/mol, respectively, and more energetically stable than that pyrolyzed at 1450°C. Their thermodynamic stability is related to their structural evolution. SiOC‐modified CBCFs become energetically less stable with increasing preparation temperature and concomitant increase in excess carbon content.     

Processing and 3D printing of SiCN polymer-derived ceramics

Preceramic polymer resins are attractive for the 3D printing of net-shaped ceramic components. Recently various processes have been demonstrated for 3D printing of polymer-derived ceramics (PDCs). Ultimately in these processes, the process outcomes strongly depend on the process parameters. In particular, for PDCs the ceramic density, and ceramic yield are affected by the catalyst concentration and cross-linking duration. Here, we use thermal analysis and FTIR to quantify the interrelation of the process parameters on the process outcome for polysilazanes and demonstrate 3D printing of PDC components based on the best-identified process parameters. The results of this work can be used as guidelines for future additive manufacturing of PDCs.     

Influence of B-site compositional homogeneity on properties of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3-based piezoelectric ceramics

The effect of B-site compositional homogeneity on microstructure, piezoelectric properties and dielectric behaviour of lead-free piezoelectric ceramics, (K_0.44Na_0.52Li_0.04) (Nb_0.86Ta_0.10Sb_0.04)O₃, is investigated. The B-site compositional homogeneity is evaluated by using an intermediate precursor obtained by solid state reaction between adequate amounts of Nb₂O₅, Ta₂O₅ and Sb₂O₅, calcined at 1350 °C and attrition milled. The B-site precursor powder is mixed with alkaline carbonates to synthesize perovskite powders and, finally, sinter piezoceramics. X-ray diffraction and Raman spectroscopy reveal the formation of a perovskite phase, although tetragonal tungsten-bronze structure is detected as minor secondary phase. Ceramics processed by using B-site precursor show different crystalline structure as a function of sintering conditions or K/Na ratio. The B-site precursor route produces thus lower piezoelectric properties, but the control of alkali volatilization by using sintering powder bed resulted in a relevant decrease of dielectric losses that favours the d₃₃ enhancement.     

Polymer‐derived SiCN cellular structures from replica of 3D printed lattices

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.     

Micro-/nanostructure evolution of C/SiFeO(N,C) polymer-derived ceramic papers pyrolyzed in a reactive ammonia atmosphere

SiFeO(N,C)-based ceramic papers were prepared via a one-pot synthesis approach by dip-coating a cellulose-based paper template with a polymeric perhydropolysilazane precursor modified with iron(III)acetylacetonate. The preceramic composites were subsequently pyrolyzed in ammonia atmosphere at 500, 700, and 1000°C, respectively, and the characteristics of the three resulting ceramic papers were comparatively investigated. Scanning electron microscopy revealed that in each sample, the morphology of the template is successfully transferred on the ceramic system, with the cellulose-derived fibers being converted to elemental carbon encased by a SiFeO(N,C) coating. Electron transparent cross-sectional samples for transmission electron microscopy (TEM) were prepared from the ceramic papers, employing a standard ultramicrotomy slice cutting procedure, allowing for a detailed characterization of their in situ generated micro-/nanostructure as well as occurring crystalline phases. TEM imaging and diffraction revealed that depending on pyrolysis temperature a different microstructure with a distinct phase assemblage is generated in the polymer-derived ceramic papers. Crystallization from the polymer precursor starts with the precipitation of wüstite (Fe_(1-x)O) nanoparticles at 700°C inside the ceramic coating and secondary ε-Fe_xN at the fiber-coating interface. Upon pyrolysis at 1000°C however, the sample primarily accommodates metallic α-iron nanocrystals that impart ferromagnetic characteristics to the ceramic paper. The results show that the template-assisted polymer-derived ceramic route is a feasible approach in the production of complex ceramic compounds with fibrous paper-like morphology. By adjusting the pyrolysis temperature, microstructure and phase composition of the ceramic paper can be conveniently tailored to the needs of its respective application.