Polymer-derived SiCN Ceramics Research Articles

Tomographic microscopy of functionally graded polymer-derived SiCN ceramics with tunable gradients

Polymer-derived ceramics (PDCs) are a very attractive class of materials due to their excellent properties such as resistance to high temperatures and harsh environments, adjustability of mechanical and functional behavior, and compatibility with a broad range of shaping techniques. The liquid nature of the silicon-containing polymeric precursors and the composition tuning via addition of fillers set the stage for the creation of functionally graded ceramics (FGCs). Polysilazane-based precursors with and without divinylbenzene (DVB) addition for carbon concentration increase are sequentially cast in three different custom molds. DVB concentration, casting order, and thermal processing conditions are varied to study their influence on the interface nature and composition gradient of the functionally graded ceramic parts which are obtained after pyrolysis at 1000–1400 °C. Synchrotron-based tomographic microscopy was applied for high-resolution 3D visualization of the parts, allowing clear distinction of the dissimilar regions and approximate quantification of the composition gradients tunable from submicrometer to millimeter transition lengths. Achieving spatially defined electrical conductivity contrasts in monolithic silicon carbonitride parts, an LED attached onto a FGC plate outlines a promising use case for harsh environment applications.

Enhanced electromagnetic wave absorption properties of ZIF-67 modified polymer-derived SiCN ceramics by in situ construction of multiple heterointerfaces

Enhanced electromagnetic wave absorption properties of ZIF-67 modified polymer-derived SiCN ceramics by in situ construction of multiple heterointerfaces

Enhanced microwave absorbing properties of Y2O3 modified PDC SiCN ceramics with heterogeneous amorphous interface

Polymer derived SiCN ceramics (PDC SiCN) are the key high-temperature matrix materials for structural and stealth integrated ceramic matrix composites (CMC). However, low crystallization degree and homogeneous microstructure of PDC SiCN ceramics results in the lack of effective microwave attenuation mechanism. Hence, a new kind of nonmagnetic metal oxide (Y2O3) was in-situ formed in SiCN ceramics to enhance the conductivity loss as well as the polarization loss. Y2O3 modified PDC SiCN ceramics (SiCN-Y2O3 ceramics) were fabricated by pyrolysis polysilazane with different Y(NO3)3 content at 900 ºC. For the SiCN-Y2O3 ceramic, Y2O3 nanoparticles were dispersed in SiCN ceramic uniformly forming a large number of Y2O3-SiCN heterogeneous amorphous interface, defects and dipole polarization to enhance its microwave absorbing properties. Due to the multiple polarization loss mechanism, SiCN-Y2O3 ceramic with 10 wt% Y(NO3)3 content achieved excellent microwave absorption performance at a wide thickness range with the microwave absorbing efficiency more than 90% in the whole X band, and the RCmin could reach − 50.2 dB at the thickness of 3.5 mm.

Introducing MWCNTs conductive network in polymer-derived SiCN ceramics for broadband electromagnetic wave absorption

In this work, a series of SiCN (MWCNTs) composite ceramics were prepared through polymer derivation method by adjusting the mass ratio of multiwalled carbon nanotubes (MWCNTs). On this basis, the influence of the addition amount of MWCNTs on reflection loss (RL), effective absorption bandwidth (EAB), electromagnetic parameters, direct current conductivity (σ), impedance matching parameter (Z) and attenuation coefficient (α) of composite ceramics was studied, the percolation threshold of MWCNTs addition amount was analyzed, and the electromagnetic wave (EMW) absorption performance and absorption mechanism of composite ceramics when the MWCNTs addition amount was fixed at the percolation threshold were discussed. The analysis shows that the conductive network composed by MWCNTs can significantly improve the EAB of ceramic samples. When the addition amount of MWCNTs reached 10 wt%, a conductive percolation state was achieved inside composite ceramics. At this time, the sample showed the optimal comprehensive EMW absorption performance: when the thickness was 1.7 mm, the minimum reflection loss (RLmin) was −21.7 dB, and its EAB reached 4.7 GHz. Compared with many similar magnetic SiCN-based composite ceramics or carbon-rich SiCN-based composite ceramics previously reported, the SiCN (MWCNTs) composite ceramics prepared in this work showed an obviously improved EAB under the condition of reduced thickness.

Multiscale 2D/3D microshaping and property tuning of polymer-derived SiCN ceramics

Polymer-derived ceramics exhibit excellent properties and are compatible with many shaping techniques due to their liquid precursors. We present a fast and pressureless process for the fabrication of SiCN. Using varied amounts of the filler divinyl benzene, defect-free monolithic disc samples are obtained at high yields. Their electrical conductivity is adjustable across 10 orders of magnitude, flexural strength is improved up to 1.7 GPa, and cytocompatibility is demonstrated. This processing route is applied to a new multiscale microshaping method combining the advantages of two-photon polymerization and casting. The parts’ general shape is defined by KOH-etched silicon molds whereas individual freeform microfeatures like a 3D QR code are implemented through sacrificial 2PP photoresin microstructures added to the mold. The green body is pyrolyzed directly in the mold, whereby the photoresin decomposes and the ceramic part with the submicrometer resolution features imprinted releases itself from the mold undamaged due to ∼30% shrinkage.

Effect of morphology of C-rich silicon carbonitride ceramic on electrochemical properties of sulfur cathode for Li-S battery

Porous conducting materials represent a promising support for the immobilization of sulfur in the cathode of lithium–sulfur (Li-S) batteries. Herein, we provide an easy and scalable procedure for the preparation of such cathodes. This strategy consists of an infiltration of sulfur under solvothermal conditions at 155 °C into pores of carbon-rich silicon carbonitride (C-rich SiCN). Porous polymer-derived SiCN ceramics possess a unique combination of high electronic conductivity and robust, stress accommodating mechanical properties. The impact of the initial porosity and elemental composition of SiCN ceramics on the electrochemical performance of SiCN-S composites is addressed in this work. It is shown that material pyrolyzed at 1000 °C revealing a mesoporous character in line with the presence of a free carbon phase dispersed in SiCN demonstrates the best electrochemical stability and the highest capacity (more than 310 mAh/g over 40 cycles) at a high sulphur content of 66 wt.%.

Influence of DVB as linker molecule on the micropore formation in polymer-derived SiCN ceramics

In this work, the formation of transient microporosity during the polymer-to-ceramic conversion in polymer-derived ceramics was studied using a commercially available poly(vinyl)silazane precursor which was modified with divinylbenzene (DVB) as linker molecule during crosslinking. After pyrolysis treatments between 400 and 700 °C, the resulting materials not only showed distinct changes in elemental composition and structural features upon introduction of the linker molecule, but also a shift in micropore size, e.g. shifting from 0.84 nm to 0.70 nm after pyrolysis at 600 °C. Due to hindrance of transamination reactions in the low-temperature region, the nitrogen content in linker-containing samples was significantly higher, leading to a different composition of micropore-forming entities and, in case of the system studied, to smaller pores. These findings are a first step towards the clarification of the mechanisms leading to the pore formation in PDCs during pyrolytic conversion, which is essential for their use in prospective applications.

Microwave absorption properties of SiCN ceramics doped with cobalt nanoparticles

Polymer-derived SiCN ceramics containing cobalt was prepared with SiCN ceramics as matrix and cobalt nanoparticles as doping phase. Phase composition, Raman analysis, electromagnetic parameters and microwave absorption properties of SiCN ceramics with different cobalt content pyrolyzed at 1100 ℃ and cobalt content of 2 wt% but different pyrolysis temperature were carried out. The microstructures and magnetic properties of SiCN ceramics with cobalt content of 2 wt% and pyrolysis temperature of 900 ℃ were analyzed. The results show that some cobalt particles can react with carbon to form magnetic Co3C particles, which is one of the reasons for the magnetism of the sample. The good dielectric property of SiCN ceramics matches the magnetic property, which makes the material have excellent microwave absorption property. When the cobalt content is 2 wt% and the pyrolysis temperature is 900 ℃, the sample has the best microwave absorption performance. The minimum RL reaches − 10.9 at about 15 GHz, and the effective absorption bandwidth (RL < − 10 dB) is 3.3 GHz with a thickness of 3 mm. When the thickness became 6 mm, the minimum RL of the sample can reach − 11.8 dB and the effective absorption bandwidth (RL < − 10 dB) is 4.2 GHz, showing the excellent microwave absorption performance of materials we obtained.

Processing and thermal characterization of polymer derived SiCN(O) and SiOC reticulated foams

Highly porous polymer-derived SiCN(O) and SiOC ceramics with low thermal conductivity were developed by replicating polyurethane (PU) foams. The PU templates were impregnated with polysilazane or polysiloxane precursor, followed by pyrolysis at different temperatures (1200 °C - 1500 °C) yielding SiCN(O) or SiOC ceramic foams, respectively. The swelling and cross-linking behavior of the used precursors had a significant impact on the morphology of the prepared foams. The samples had bulk densities ranging from 0.03 g.cm-3 to 0.56 g.cm-3 and a total porosity in the range from 75 to 98 vol%. Fourier transform infrared (FT-IR), Raman spectroscopy, X-ray diffraction (XRD) were employed to follow the structural evolution together with morphological characterization by scanning electron microscopy (SEM). The obtained ceramics were thermally stable up to 1400 °C, and the linear thermal expansion coefficient values of the porous SiCN(O) and SiOC components in the temperature range from 30 to 850 °C were found to be ~1.72 x 10-6.K-1 and ~1.93 x 10-6.K-1, respectively. Thermal conductivity (λ) as low as 0.03 W.m-1 K-1 was measured for the SiCN(O) and SiOC foams at room temperature (RT). The λ of the ceramic struts were also assessed by using the Gibson-Ashby model and estimated to be 2.1 W.m-1 K-1 for SiCN(O), and 1.8 W.m-1 K-1 for SiOC.

Hierarchical carbon nanowires network modified PDCs-SiCN with improved microwave absorption performance

Abstract In order to optimize the dielectric performance of polymer derived SiCN ceramics (PDCs-SiCN), carbon nanowires (CNW) were deposited in SiCN by catalytic chemical vapor deposition (CCVD). Microstructure evolutions, dielectric property and electromagnetic (EM) wave absorption capacity of CNW/SiCN were investigated. Results show that carbon nanowires had plentiful pits/defects on their roughened surface and formed hierarchical network in SiCN which benefited the impedance match and generated strong conductivity and polarization loss, enhancing the absorption ability of CNW/SiCN. When CNW accounted for 5.61 wt%, RCmin reached −51 dB with EAB of 3.0 GHz at 2.7 mm in thick, showing excellent microwave absorbing performance. The favorable microwave absorption ability could be ascribed to three aspects including enhanced conductivity loss derived from the excellent conductivity of CNW, polarization loss generated by defects, and multiple reflection loss enhanced by hierarchical network. By comparing the variation tendency between defect concentration and electrical conductivity in CNW/SiCN, it is rational to conclude that the conductivity loss dominated the dielectric loss while the polarization loss and repeated multi-reflection simultaneously worked. This work can be further extended to study regarding the effect of heat-treatment temperature since CNW have the potential to promote the crystallization process of amorphous PDCs thereby improving their dielectric properties.

PIP process greatly influencing the microstructure and electrical conductivity of polymer-derived SiCN ceramics

Precursor infiltration and pyrolysis (PIP) technique is widely used to densify porous ceramics. In this paper, PIP technique was used to fabricate dense polymer-derived SiCN ceramic bulks from porous SiCN preforms. Electrical conductivity and microstructure of dense SiCNs and the evolution with pyrolysis temperature were studied. Results were compared with that of porous SiCNs which were prepared by powder consolidation route without PIP process. Raman analysis showed that graphitization level of free-carbon in dense SiCNs is higher than that in porous SiCNs. As a consequence, the electrical conductivity of dense SiCNs was much higher than that of porous SiCNs. Free-carbon in porous SiCNs underwent a sp3-to-sp2 transition with increasing pyrolysis temperature throughout the temperature range of 1000 °C–1400 °C, whereas the sp3-to-sp2 transition was completed before 1200 °C in dense SiCNs. Main structural evolution of free-carbon in dense SiCNs was the growth of nanocrystalline graphite (nc-graphite) into ordered graphite when pyrolyzed at temperatures higher than 1200 °C. The growth activation energy of sp2 cluster size vs. pyrolysis temperature in the nc-graphite growth stage was found to be higher than that in the sp3-to-sp2 transition stage. The activation energy of conductivity increasing vs. pyrolysis temperature of the porous SiCNs was higher than that of the dense SiCNs, indicating that the electrical conductivity is more sensitive to the sp3-to-sp2 transition process than the sp2 cluster growth. This paper indicates that processing route can have great influence on the microstructure and electrical property of PDCs.

Electrical conductivity change induced by porosity within polymer-derived SiCN ceramics

Polymer-derived ceramics (PDCs) which is synthesized by pyrolysis of organosilicon polymers have recently attracted an increasing attention on the electrical property with possible application in Li-ion batteries, micro electro mechanical systems (MEMS) and harsh-environmental sensors. In this paper, the chemical composition, nanostructure evolution and the electrical conductivity variation which are induced by porosity are explored systematically in polymer-derived SiCN ceramics. The SiCN ceramics with 0, 4.82%, 8.94%, 16.81% and 21.32% porosities were prepared by mixing the polymer precursors with different crosslinking degree. There are significant changes in the content of N element, free carbon, crystallization SiC and the morphology of free carbon with the increasing porosity. As the porosity increases, the electrical conductivity of the SiCN ceramics decrease gradually from 0.015 to 0.003 S cm−1. By comparing the relationship between the conductivity and the porosity from experiment and model prediction (only intrinsic porosity considered), it can be concluded that the chemical composition, nanostructure evolution dramatically decrease the conductivity of SiCN ceramics with 56.41% maximum decline, and the decrease of the free carbon content is the most significant aspect influencing the electrical conductivity of SiCN ceramics. Our findings can realize the regulation of the nanostructure and electrical conductivity in polymer-derived SiCN ceramics by the control of porosity with a new insight on the design of PDCs functional materials.

Electromagnetic properties and microwave absorption performances of nickel-doped SiCN ceramics pyrolyzed at different temperatures

Abstract Nickel-doped polymer derived SiCN ceramics (PDCs-SiCN (Ni)) were successfully prepared via a polymer-derived method from polysilazane and nickel oxide, and pyrolyzed at 900°C-1400 °C. The effects of pyrolysis temperatures on phase composition, microstructure and microwave absorption properties of PDCs-SiCN (Ni) were investigated. Results showed that the polymer-to-ceramic conversion was completed at around 800 °C. Besides carbon nanotubes were formed in the pore area of SiCN matrix through the catalysis of Ni. The electromagnetic (EM) parameters varied with the different pyrolysis temperatures. When the PDCs-SiCN (Ni) pyrolyzed at 1200 °C, the minimum reflection loss value reached −48.5 dB at 15.1 GHz, exhibiting the best EM wave absorption properties.

The influence of carbon materials on the absorption performance of polymer-derived SiCN ceramics in X-band

Carbon-containing polymer-derived SiCN ceramics (PDCs-SiCN-C) were successfully fabricated with multi-layer graphene (MLG) and multiwalled carbon nanotubes (MWCNTs) as additives at 1100 °C. The effects of MLG and MWCNTs on the microwave absorption properties of PDCs-SiCN-C ceramics were analyzed. The imaginary permittivity and loss tangent of SiCN-MLG and SiCN-MWCNTs were about 3.4, 0.67 at 11.2 GHz and 3.1, 0.57 at 10.6 GHz, respectively. The minimum reflection loss of SiCN-MLG and SiCN-MWCNTs at 3 mm was − 54 dB and − 48 dB with the effective absorption bandwidth (RL ≤ −10 dB, >90% absorption) about 1.5 GHz and 0.9 GHz in X-band.

Microwave absorbing performance of polymer-derived SiCN (Ni) ceramics prepared from different nickel sources

A series of nickel-containing polymer-derived SiCN ceramics (PDC-SiCN (Ni) ceramics) were successfully prepared from different nickel sources following a polymer-derived method. The scanning electron microscopy (SEM) and transmission electron microscope (TEM) were used to peruse the microstructure of the ceramics. The electromagnetic and microwave absorption properties of PDC-SiCN (Ni) ceramics in the 2–18 GHz range were investigated. Results showed that the Ni nanoparticles in ceramics were dispersed in the amorphous SiCN matrix and the synergetic effect of dielectric and magnetic loss enhanced electromagnetic attenuation of the materials. The optimal reflection loss (RL) was found for the PDC-SiCN (Ni) ceramics with Ni powders as nickel source, being −78.6 dB at 17.1 GHz when the thickness was 3.5 mm and the bandwidth of RL below −10 dB (90% absorption) was 1.6 GHz. It meant that the PDC-SiCN (Ni) ceramics might exhibit a promising prospect as microwave absorbing materials.

Electromagnetic wave absorption properties of nickel-containing polymer-derived SiCN ceramics

Nickel-containing polymer-derived SiCN ceramics (PDC-SiCN (Ni) ceramics) were prepared with different adding amounts of Ni nanopowders by a polymer-derived method. The compositions, microstructure, electromagnetic (EM) properties and microwave absorption properties of the PDC-SiCN (Ni) ceramics in the 2–18 GHz frequency range were investigated. Results showed that the Ni nanoparticles were distributed in the SiCN amorphous matrix. Moreover, the electromagnetic properties could be tuned by varying the Ni adding amounts. The best absorption properties with a maximum reflection loss (RL) of − 39 dB at 17.2 GHz was obtained in the PDC-SiCN (Ni) ceramics with 10 wt% adding amount and the effective absorption width (RL< −10 dB) was 1.7 GHz with a thickness of 2 mm. Thus the synergetic effect of both the dielectric and magnetic loss endowed the PDC-SiCN (Ni) ceramics with excellent microwave absorption performance.

A Variable Temperature X- and W-Band EPR Study of Fe-Doped SiCN Ceramics Annealed at 1000, 1100, and 1285 °C: Dangling Bonds, Ferromagnetism and Superparamagnetism

Polymer-derived SiCN ceramics, annealed (also referred to as pyrolyzed) at 1000, 1100, and 1285 °C, and doped with Fe(III) acetylacetonate, are investigated by electron paramagnetic resonance (EPR) from 4 to 120 K at X-band (9.425 GHz). In addition, the SiCN ceramic, annealed at 1100 °C, was studied by EPR at 300 K at W-band (93.96 GHz). There was observed a significant increase in EPR linewidth due to dangling bonds (g = 2.001) below 20 K at X-band. The low-field X-band FMR line (g ≈ 12) indicated the presence of ferromagnetic Fe5Si3 crystallites. There were found two EPR lines due to carbon-related dangling bonds: (1) those present as defects on the surface of the free-carbon phase (as sp2 carbon-related dangling bonds with g = 2.0011) and (2) those present within the bulk of carbon phase (as sp3 carbon-related dangling bonds with g = 2.0033). On the other hand, the intense low-field EPR signal observed at X-band was not observed at W-band. As well, there was observed splitting of the single broad EPR signal observed at g = 2.05 at X-band into two signals at W-band at g = 1.99 and g = 2.06, due to two different Fe-containing superparamagnetic nanocrystallites. Two new EPR signals, not observed at X-band, were observed at W-band, namely at g = 2.28 and g = 3.00, which are also due to g∥ of these superparamagnetic nanocrystallites.

Fabrication of a thin double-layer thermistor based on DVB-modified polymer-derived SiCN ceramics

Polymer-derived ceramics (PDCs) are considered to be promising candidate materials for high-temperature sensor applications. Even though the PDCs have been studied for decades, the reliable PDC-based thin film sensors have not been produced, because ceramic films are too fragile to be handled without a substrate, but the huge shrinkage during polymer-to-ceramic conversion makes it difficult to fabricate PDC-based sensing elements on conventional substrates such as Al2O3. In this study, a thin polymer-derived ceramic double-layer thermistor was developed, using a layer with a lower resistivity as the sensing element and a layer with a higher resistivity as the substrate. The difficulty in PDC-based film sensing elements fabrication on a substrate was consequently resolved, because both the sensing elements and the substrate sustained a similar shrinkage during pyrolysis. The proposed double-layer thermistor was fabricated through step-growth photopolymerization and co-pyrolysis of two similar polymer precursors: commercially available polysilazane for the high-resistivity substrate and DVB-modified polysilazane for the low-resistivity sensing part. The DVB-modified polysilazane showed a similar shrinkage during pyrolysis, whereas the resistivity of the pyrolytic product was five orders of magnitude lower. The obtained thermistor exhibited a negative temperature coefficient in the temperature range from the room temperature to 650 °C, and the resistance of the thermistor decreased smoothly as the temperature increased. The resistance-to-voltage conversion was conducted with a voltage divider equipped with a fixed resistor and the output voltage was observed to increase smoothly along with temperature. The current sensor configuration based on PDCs was reported for the first time, and is much closer to applicable sensors.

Interface characteristics of polymer-derived amorphous SiCN ceramics investigated by impedance spectroscopy under direct current bias

The interface characteristics of polymer derived ceramics determine their dielectric properties, useful for sensing applications. In this contribution, the interface characteristics of polymer-derived amorphous SiCN ceramics were studied using direct current (dc) bias superimposed on alternating current (ac) impedance spectroscopy. Double Schottky barrier model was applied to determine the interface characteristics. Potential height and donor concentration at the interface were estimated to 0.99eV and 2.91 × 1018cm−3, respectively. Interface density was recorded as 2.69 × 1013cm−2. Interface resistance reduced as bias rose, resulting in increased dielectric loss. Interface capacitance also declined as bias increased, thus reducing dielectric constant. These changes were related to potential barrier height and width of barrier region. Overall, these findings look promising and should benefit further understanding on interfacial polarization processes of polymer-derived ceramics.

The dielectric and microwave absorption properties of polymer-derived SiCN ceramics

The polymer-derived SiCN ceramics were synthesized at different annealing temperature (900∼1400°C). The XRD, SEM, FT-IR, Raman and XPS were used to analyze the phase composition and microstructure. The result indicated that the crystallization degree and content of free carbon gradually improved with the increase of annealing temperature. The resistivity, dielectric and microwave absorption properties of the samples were studied at 2∼18GHz. The resistivity decreased gradually as the annealing temperature rose. The dielectric constant of sample decreased with the increase of frequency in 1∼5MHz. The existence of free carbon could improve the dielectric properties of polymer-derived SiCN ceramics at high frequency. The reflectance of the sample synthesized at 1100°C was below −10dB (>90% absorption) in a wide frequency range of 6∼16GHz and the maximum value of dielectric loss angle tangent was about 0.6 at 16GHz.