Polymer-derived SiCN Ceramics Research Articles

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.

Wave absorbing performance of polymer-derived SiCN(Fe) ceramics

Abstract Ferreous polymer-derived SiCN ceramics (PDCs-SiCN(Fe)) were successfully synthesized by precursor conversion method. The phase composition and microstructure of the obtained products were investigated by XRD, SEM and TEM. The result indicated a mainly amorphous structure with a few α-Fe,and graphite phases in the gaps. The conductivity, electromagnetic parameters and wave absorbing properties of the prepared ceramics were studied at the frequency band from 2 to 18 GHz. The reflectance of PDCs-SiCN(Fe) was about −10 dB (> 90% absorption) in the range of 4–16 GHz with 25 wt% nano-Fe 2 O 3 addition at the thickness 8 mm. The maximum value of dielectric and magnetic loss angle tangent was about 1.5, indicating PDCs-SiCN(Fe) ceramics possessed good absorption performance.

Continuous regulation from fully dense to high porosity within polymer-derived SiCN ceramics

Porosity-controllable SiCN ceramics were prepared using a simple partial crosslinking method, which is promising for preparing bulk polymer-derived ceramics with different dimensions. Continuous regulation from fully dense to high porosity within polymer-derived SiCN ceramics was realized via secondary crosslinking of partially crosslinked polysilazane (PSN). A fully dense SiCN ceramic was obtained by shaping the single partially crosslinked PSN (PC-PSN) followed by pyrolysis. PC-PSN was obtained by controlling the crosslinking reaction, and it retained a strong ability to cross link further. SiCN ceramics with different porosities were prepared by mixing PC-PSN and completely crosslinked PSN (CC-PSN) at different proportions. The size and density of the pores can be controlled via the proportion of CC-PSN in a mixed powder. The resultant SiCN ceramic has a porosity that ranges from 0% to 21.32%, which is validated to ensure the component integrity of SiCN ceramics with different dimensions due to the release of residual stress during pyrolysis.

Electromagnetic and wave absorbing properties of Fe-doped polymer-derived SiCN ceramics

Fe-doped polymer-derived SiCN ceramics were synthesized by a precursor conversion method with different contents of nano-Fe2O3.

Improved dielectric properties of PDCs-SiCN by in-situ fabricated nano-structured carbons

In order to enhance dielectric properties of polymer derived SiCN ceramics (PDCs-SiCN), nano-structured carbons were in-situ fabricated in PDCs-SiCN by pyrolysis of ferrocene-modified polysilazane. Microstructure evolutions, dielectric and microwave absorption properties of PDCs-SiCN decorated with nano-structured carbons were investigated. Nano-structured PDCs-SiCN ceramics are composed of carbon nanowires as well as interpenetrating graphene-like free carbons, SiC nano-crystals, Si3N4 nano-crystals and amorphous SiCN. Relative complex permittivities of nano-structured PDCs-SiCN increase with increasing ferrocene contents and annealing temperatures. Free carbons in PDCs-SiCN play a dominating role on the improved dielectric properties. Polarization loss is the primary dielectric loss. Loss tangent of PDCs-SiCN exceeding 0.7 is obtained when free carbons are only 2.57% in weight. Nano-structured PDCs-SiCN exhibit good microwave absorption property. The reflectivity is smaller than −14dB in the whole X band when material is composed of both impedance and microwave absorption materials.

Preparation and Electrochemical Performance of Modified Polymer-Derived Sicn Ceramics As Anode Material for Lithium Ion Batteries

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Fabrication of Macroporous SiCN Ceramics from Mixed Polysilazanes

Macroporous polymer-derived SiCN ceramics are fabricated directly by mixing polysilazane precursors followed with crosslinking and pyrolysis. Two kinds of polysilazanes namely polyvinylsilazane and polyhydrosilazane are mixed, crosslinked by 2, 2-Azo-bis-isobutyronitrile to form resins before pyrolyzed to form ceramics in argon flow at 1000°C. The density of the SiCN ceramic is 1.65 g/cm3 with corresponding porosity of 30 % compared to dense SiCN ceramics. SEM images show that the ceramics possess high porosity and homogeneous honeycomb-like macropores of ~2 μm. The porous SiCN exhibits good mechanical property with Vicker hardness of 11-13 GPa under a load of 0.2 kg.

Lithium dynamics in carbon-rich polymer-derived SiCN ceramics probed by nuclear magnetic resonance

We report 7Li, 29Si, and 13C NMR studies of two different carbon-rich SiCN ceramics SiCN-1 and SiCN-3 derived from the preceramic polymers polyphenylvinylsilylcarbodiimide and polyphenylvinylsilazane, respectively. From the spectral analysis of the three nuclei, we find that only the 13C spectrum is strongly influenced by Li insertion/extraction, suggesting that carbon phases are the major electrochemically active sites for Li storage. Temperature (T) and Larmor frequency (ωL) dependences of the 7Li linewidth and spin-lattice relaxation rates T1−1 are described by an activated law with the activation energy EA of 0.31 eV and the correlation time τ0 in the high temperature limit of 1.3 ps. The 3/2 power law dependence of T1−1 on ωL which deviates from the standard Bloembergen, Purcell, and Pound (BPP) model implies that the Li motion on the μs timescale is governed by continuum diffusion mechanism rather than jump diffusion. On the other hand, the rotating frame relaxation rate T1ρ−1 results suggest that the slow motion of Li on the ms timescale may be affected by complex diffusion and/or non-diffusion processes.

Dielectric and microwave absorption properties of polymer derived SiCN ceramics annealed in N2 atmosphere

Abstract Porous SiCN ceramics were successfully fabricated by pyrolysis of a kind of polysilazane. The effects of annealing temperature on the microstructure evolution, direct-current electrical conductivity, dielectric properties, and microwave absorption properties of SiCN in the frequency range 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, SiC, Si3N4 and free carbon nanodomains are gradually formed in the SiCN. Both the SiC and free carbon nanodomains lead to the increases of the complex relative permittivity and loss tangent of SiCN. With the increase of the annealing temperature, the average real permittivity, imaginary permittivity and loss tangent increase from 4.4, 0.2 and 0.05 to 13.8, 6.3 and 0.46, respectively. The minimum reflection coefficient and the frequency bandwidth below −10 dB for SiCN annealed at 1500 °C are −53 dB and 3.02 GHz, indicating good microwave absorption properties.

Composite materials based on polymer-derived SiCN ceramic and disordered hard carbons as anodes for lithium-ion batteries

New composite materials based on polymer-derived SiCN ceramics and hard carbons were studied in view of its application as anodes for lithium-ion batteries. Two kinds of composites were prepared by pyrolysis of the preceramic polysilazane (HTT1800, Clariant) at 1000°C in Ar atmosphere mixed with hard carbons derived from potato starch (HC_PS) or with a hard carbon precursor, namely potato starch (PS), denoted as HTT/HC_PS and HTT/PS composites, respectively. Thermal gravimetric analysis suggests possible reactions between the preceramic polymer and the carbon precursor. The HTT/PS composites contain higher amount of oxygen and appear to be more homogeneous than that of the HTT/HC_PS composite. Raman analysis confirms the presence of highly disordered carbon in the composites by the appearance of the well-pronounced D band at 1347cm−1. The materials are amorphous with a significant fraction of single graphene sheets as confirmed by X-ray diffraction. The HTT/PS composite exhibits a high-recovered capacity (434mAhg−1 when charging with a current of 36mAg−1) and outstanding cyclability for 400 cycles even at high current rates (90mAhg−1 when charging with 3600mAg−1). These properties make the composite a candidate anode material for high power energy devices.

Carbon-rich SiCN ceramics as high capacity/high stability anode material for lithium-ion batteries

Two classes of preceramic polymers, namely polysilazane and polysilylcarbodiimide, with branched and linear molecular structure were pyrolyzed at 1100 °C under argon atmosphere. The resulting nanostructured polymer-derived SiCN ceramics were characterized by means of elemental analysis, X-ray diffraction, scanning electron microscopy and Raman spectroscopy. All investigated ceramics are amorphous and contain a disordered free carbon phase of 2–2.5 nm in size. Electrochemical characterization reveals that the polysilazane-derived electrodes demonstrate higher capacity and stability during subsequent lithium insertion/extraction with different currents than those of the polysilylcarbodiimide-based electrodes. The highest lithium extraction capacity of 724 mA h g−1 is recovered for the sample derived from branched polysilazane whereas the best polysilylcarbodiimide-derived sample recovers 612 mA h g−1. Moreover, the polysilazane-derived samples deliver a higher fraction of capacity recovered below 1.5 V. The electrochemical performance is found to be dependent on the molecular structure (silazane vs. silylcarbodiimide) of the preceramic polymer, while there is no effect associated with the amount of branching (silsesquiazane vs. silazane and silsesquicarbodiimide vs. silylcarbodiimide). The influence of “micropore activity” and oxygen content on the electrochemical performance of polymer-derived silicon carbonitrides is addressed.

Polymer-derived SiCN and SiOC ceramics – structure and energetics at the nanoscale

The complex nanodomain structure and energetic features of polymer-derived SiCN and SiOC ceramics have been investigated by multinuclear 1D and 2D solid-state NMR, micro-Raman, SAXS, XRD, HRTEM and oxidative high temperature oxide melt solution calorimetry, respectively. Structural models consistent with all these lines of evidence put emphasis on interconnected fractal domains stabilized by the mixed-bond-structure and interface bonding between nanodomains. The structural and thermodynamic role of hydrogen in the stabilization of interfacial bonding between the nanodomains in polysilylcarbodiimide-derived SiCN is reported as well.

Prevention of Solid Electrolyte Interphase Damaging on Silicon by Using Polymer Derived SiCN Ceramics

A new composite anode material for lithium ion batteries, based on nano-silicon particles dispersed in a polymer-derived ceramic (PDC) matrix, was produced, characterized and electrochemically analyzed via cyclic voltammetry (CV). For this purpose a commercial preceramic polymer , namely the polysilazane HTT 1800, was mixed with silicon nano powder and pyrolyzed at 900, 1100, 1300 and 1500 °C. It was found that in comparison with pure silicon, the composite presented an enhanced electrochemical stability during lithium insertion/extraction. Moreover, dispersing the silicon into the ceramic matrix avoids a continuous energy loss related to the formation of a solid electrolyte interphase (SEI). By means of cyclic voltammetry measurements we found that for the composites synthesized at temperatures exceeding 1000 °C, no more losses were observable during subsequent insertion/extraction once the stable SEI was formed.

Catalytic formation of carbon phases in metal modified, porous polymer derived SiCN ceramics

A novel method for the simultaneous formation of catalytic active metal nanoparticles, multiwall carbon nanotubes (MWCNTs) and/or turbostratic carbon and porous M@SiCN (M = Fe, Co, Pt, Cu, Ag, Au) ceramics during pyrolysis of metal modified polysilazanes and polyethylene (PE) particles as sacrificial filler is described. The thermal decomposition of the polyethylene leads not only to the generation of the porosity but also to an in situ reduction of the metal compounds to the metal nanoparticles, due to the reductive atmosphere. Depending on the metal, carbon nanotubes as well as turbostratic carbon were formed in different amounts, due to the chemical vapor deposition (CVD) like conditions. The resulting carbon phases, ceramics and metal nanoparticles were investigated using the combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) measurements, giving evidence for the presence of the carbon phases and the metal particles.

Analysis of polyethylene-particle filled SiCN precursor and the resulting porous ceramics with emphasis on using micro computed tomography

Porous, polymer derived SiCN ceramics are synthesized from a solid polysilazane (PSZMS) and an ultra-high molecular weight polyethylene (PE) as sacrificial filler. Common analytical methods for the investigation of the thermal decomposition processes and the formed (macro) porosities, such as thermogravimetry and mercury porosimetry respectively, were supplemented by micro computed tomography (µ-CT) measurements. Due to its non-destructive nature, this method enables the comparative analysis of both, the in the first step formed preceramic duromers, to distinguish between the different polymers, as well as the resulting porous ceramics. The via µ-CT determined values for the two polymer phases after cross-linking are in a good agreement with the used original amounts of PE and PSZMS. In contrast the values for the porosity resulting from the PE filler and for the formed SiCN ceramic after pyrolysis differ in greater extent due to the limiting resolution that can be achieved by applying µ-CT measurements.

Defect structure in lithium-doped polymer-derived SiCN ceramics characterized by Raman and electron paramagnetic resonance spectroscopy

Lithium-doped polymer-derived silicon carbonitride ceramics (SiCN:Li) synthesized at various pyrolysis temperatures, have been investigated by means of multifrequency and multipulse electron paramagnetic resonance (EPR) and Raman spectroscopy in order to determine different defect states that may impact the materials electronic properties. In particular, carbon- and silicon-based 'dangling bonds' at elevated, as well as metallic networks containing Li0 in the order of 1 microm at low pyrolysis temperatures have been observed in concentrations ranging between 10(14) and 10(17) spins mg(-1).

Metastable SiCN glass matrices studied by energy-filtered electron diffraction pattern analysis

Bulk SiCN glasses, derived from polymer precursors with tailored architectures, were characterized upon pyrolysis at 1000°C and subsequent annealing at 1400°C and 1540°C. The characterization tools employed were high-resolution transmission electron microscopy (HRTEM) imaging and energy-filtered selected area electron diffraction (EF-SAED). Main emphasis of this work was to verify as to whether the intrinsic amorphous structure of polymer-derived bulk materials exhibits different structural features upon pyrolysis, that depend on the functionalities of the pre-ceramic polymer, and whether such characteristic features are maintained upon high-temperature anneal. HRTEM imaging did not reveal any pronounced variation between the different SiCN glass structures after the organic–inorganic transition at 1000°C or even upon heat treatment at 1400°C. However, EF-SAED analysis showed differences in the amorphous structure of the four polymer-derived SiCN glasses studied even upon thermolysis at 1000°C. The evolution of the electron diffraction intensity profiles of the EF-SAED patterns after subsequent thermal treatment at elevated temperatures showed clear evidence of structural rearrangements within the SiCN glass networks upon annealing at temperatures exceeding the pyrolysis temperature. The experimental results provide evidence that the metastable glass structure of polymer-derived SiCN ceramics depends on the starting polymer employed and, in addition, that the intrinsic glass architecture affects the thermal stability (onset of crystallization) of the amorphous matrix.

Oxygen diffusion through Al-doped amorphous SiO2

Oxygen (O) diffusion through pure and aluminum (Al)-doped amorphous silica is investigated by using secondary ion mass spectrometry to profile the diffusion of an18O tracer. The oxides are formed by the thermal oxidation of polymer-derived SiCN and SiAlCN ceramics. The authors demonstrate that a small amount of Al dopant can significantly inhibit both the interstitial and network diffusion of O. The activation energy of O network diffusion for Al-doped silica is two times higher than that for pure silica. The results are discussed in terms of the modification of Al doping on the network structure of the otherwise pure silica.