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Ralf Riedel - One of the best experts on this subject based on the ideXlab platform.

  • influence of sic silica and carbon silica interfaces on the high temperature creep of Silicon Oxycarbide based glass ceramics a case study
    Advanced Engineering Materials, 2019
    Co-Authors: Christina Stabler, Hans-joachim Kleebe, Ralf Riedel, Daniel Schliephake, Martin Heilmaier, Tanguy Rouxel, Masaki Narisawa, Emanuel Ionescu
    Abstract:

    In the present study, the high-temperature creep behavior of three SiOC glass ceramics with different phase compositions are compared by the authors. All three SiOC glass ceramics have a vitreous silica matrix in common, but comprise different homogeneously dispersed phases 1) only spherical beta-SiC nanoparticles (sample denoted hereafter SiC/SiO2), 2) only high-aspect ratio sp(2)-hybridized carbon (i.e., C/SiO2), and 3) both phases (SiC and segregated carbon, i.e., C/SiC/SiO2). Compression creep experiments are performed at temperatures in the range between 1100 and 1300 degrees C and true stresses of 50 to 200 MPa. The determined activation energy for creep of the SiOC glass ceramics of around 700 kJ mol(-1) is independent of the phase composition. A stress exponent value of approximately 2 indicates an interface-controlled deformation mechanism. All SiOC glass ceramics exhibit significantly higher creep viscosities than that of vitreous silica. Surprisingly, the spherical beta-SiC nanoparticles have a higher impact on the effective creep viscosities of SiOC as compared to that of the high-aspect ratio segregated carbon phase. It is concluded that this originates from the beta-SiC/silica and C/silica interfaces, which have different effects on the creep behavior of Silicon Oxycarbide-based glass ceramics.

  • Silicon Oxycarbide glasses and glass ceramics all rounder materials for advanced structural and functional applications
    Journal of the American Ceramic Society, 2018
    Co-Authors: Christina Stabler, Emanuel Ionescu, Magdalena Graczykzajac, Isabel Gonzalojuan, Ralf Riedel
    Abstract:

    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.

  • High-Temperature Raman Spectroscopy of Nano-Crystalline Carbon in Silicon Oxycarbide
    MDPI AG, 2018
    Co-Authors: Felix Rosenburg, Emanuel Ionescu, Norbert Nicoloso, Ralf Riedel
    Abstract:

    The microstructure of segregated carbon in Silicon Oxycarbide (SiOC), hot-pressed at T = 1600 °C and p = 50 MPa, has been investigated by VIS Raman spectroscopy (λ = 514 nm) within the temperature range 25–1000 °C in air. The occurrence of the G, D’ and D bands at 1590, 1620 and 1350 cm−1, together with a lateral crystal size La < 10 nm and an average distance between lattice defects LD ≈ 8 nm, provides evidence that carbon exists as nano-crystalline phase in SiOC containing 11 and 17 vol % carbon. Both samples show a linear red shift of the G band up to the highest temperature applied, which is in agreement with the description of the anharmonic contribution to the lattice potential by the modified Tersoff potential. The temperature coefficient χG = −0.024 ± 0.001 cm−1/°C is close to that of disordered carbon, e.g., carbon nanowalls or commercial activated graphite. The line width of the G band is independent of temperature with FWHM-values of 35 cm−1 (C-11) and 45 cm−1 (C-17), suggesting that scattering with defects and impurities outweighs the phonon-phonon and phonon-electron interactions. Analysis of the Raman line intensities indicates vacancies as dominating defects

  • the thermal conductivity of polymer derived amorphous si o c compounds and nano composites
    Journal of the American Ceramic Society, 2016
    Co-Authors: Aleksander Gurlo, Emanuel Ionescu, Ralf Riedel, David R Clarke
    Abstract:

    Silicon Oxycarbide glasses can be produced over a range of Si–O–C compositions by the controlled pyrolysis of polymer precursors. We present measurements of the thermal conductivity of a Silicon Oxycarbide glass after two different heat treatments and two Si–O–C nano-composites, hot-pressed at 1600°C, up to 1000°C and compare them to fused silica, amorphous carbon, and SiC. The temperature dependence of their thermal conductivities is similar to other amorphous materials. The presence of low volume fractions of nanoparticles of hafnia (4.5 v/o) or zirconia (7.4 v/o) dispersed within the amorphous matrix only modifies the conductivity slightly, consistent with a simple Maxwell model, and does not affect the temperature dependence of the thermal conductivity above room temperature.

  • Silicon Oxycarbide nano Silicon composite anodes for li ion batteries considerable influence of nano crystalline vs nano amorphous Silicon embedment on the electrochemical properties
    Journal of Power Sources, 2014
    Co-Authors: Jan Kaspar, Hans-joachim Kleebe, Magdalena Graczykzajac, Stefan Lauterbach, Ralf Riedel
    Abstract:

    Abstract Silicon Oxycarbide/nano-Silicon composites (SiOC/nSi) are prepared by mixing of nano-sized Silicon, either crystalline (nSi_c) or amorphous (nSi_a), with commercially available polyorganosiloxane RD-684a and subsequent pyrolysis. The influence of the type of nano-Silicon, namely crystalline vs. amorphous, on the electrochemical properties and performance is analyzed and correlated with the corresponding composite microstructure. In the case of crystalline nano-Silicon, a high reversible capacity of 905 mAh g −1 is registered, whereas that for amorphous nano-Silicon embedment reaches 704 mAh g −1 . However, regarding the cycling stability, SiOC/nSi_c shows a significant capacity fading upon continuous cycling, related to SiOC matrix failure. The host phase is not able to accommodate the arising mechanical stresses upon Si grain expansion and contraction when alloying/dealloying with Li. SiOC/nSi_a on the contrary, demonstrates a stable cycling performance for up to 100 cycles. This excellent performance is explained by the enhanced matrix integrity of the compound, rationalized by a smaller size of the embedded crystallized Si grains and an intrinsically enhanced electrical conductivity due to the formation of SiC.

Emanuel Ionescu - One of the best experts on this subject based on the ideXlab platform.

  • influence of sic silica and carbon silica interfaces on the high temperature creep of Silicon Oxycarbide based glass ceramics a case study
    Advanced Engineering Materials, 2019
    Co-Authors: Christina Stabler, Hans-joachim Kleebe, Ralf Riedel, Daniel Schliephake, Martin Heilmaier, Tanguy Rouxel, Masaki Narisawa, Emanuel Ionescu
    Abstract:

    In the present study, the high-temperature creep behavior of three SiOC glass ceramics with different phase compositions are compared by the authors. All three SiOC glass ceramics have a vitreous silica matrix in common, but comprise different homogeneously dispersed phases 1) only spherical beta-SiC nanoparticles (sample denoted hereafter SiC/SiO2), 2) only high-aspect ratio sp(2)-hybridized carbon (i.e., C/SiO2), and 3) both phases (SiC and segregated carbon, i.e., C/SiC/SiO2). Compression creep experiments are performed at temperatures in the range between 1100 and 1300 degrees C and true stresses of 50 to 200 MPa. The determined activation energy for creep of the SiOC glass ceramics of around 700 kJ mol(-1) is independent of the phase composition. A stress exponent value of approximately 2 indicates an interface-controlled deformation mechanism. All SiOC glass ceramics exhibit significantly higher creep viscosities than that of vitreous silica. Surprisingly, the spherical beta-SiC nanoparticles have a higher impact on the effective creep viscosities of SiOC as compared to that of the high-aspect ratio segregated carbon phase. It is concluded that this originates from the beta-SiC/silica and C/silica interfaces, which have different effects on the creep behavior of Silicon Oxycarbide-based glass ceramics.

  • Silicon Oxycarbide glasses and glass ceramics all rounder materials for advanced structural and functional applications
    Journal of the American Ceramic Society, 2018
    Co-Authors: Christina Stabler, Emanuel Ionescu, Magdalena Graczykzajac, Isabel Gonzalojuan, Ralf Riedel
    Abstract:

    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.

  • High-Temperature Raman Spectroscopy of Nano-Crystalline Carbon in Silicon Oxycarbide
    MDPI AG, 2018
    Co-Authors: Felix Rosenburg, Emanuel Ionescu, Norbert Nicoloso, Ralf Riedel
    Abstract:

    The microstructure of segregated carbon in Silicon Oxycarbide (SiOC), hot-pressed at T = 1600 °C and p = 50 MPa, has been investigated by VIS Raman spectroscopy (λ = 514 nm) within the temperature range 25–1000 °C in air. The occurrence of the G, D’ and D bands at 1590, 1620 and 1350 cm−1, together with a lateral crystal size La < 10 nm and an average distance between lattice defects LD ≈ 8 nm, provides evidence that carbon exists as nano-crystalline phase in SiOC containing 11 and 17 vol % carbon. Both samples show a linear red shift of the G band up to the highest temperature applied, which is in agreement with the description of the anharmonic contribution to the lattice potential by the modified Tersoff potential. The temperature coefficient χG = −0.024 ± 0.001 cm−1/°C is close to that of disordered carbon, e.g., carbon nanowalls or commercial activated graphite. The line width of the G band is independent of temperature with FWHM-values of 35 cm−1 (C-11) and 45 cm−1 (C-17), suggesting that scattering with defects and impurities outweighs the phonon-phonon and phonon-electron interactions. Analysis of the Raman line intensities indicates vacancies as dominating defects

  • the thermal conductivity of polymer derived amorphous si o c compounds and nano composites
    Journal of the American Ceramic Society, 2016
    Co-Authors: Aleksander Gurlo, Emanuel Ionescu, Ralf Riedel, David R Clarke
    Abstract:

    Silicon Oxycarbide glasses can be produced over a range of Si–O–C compositions by the controlled pyrolysis of polymer precursors. We present measurements of the thermal conductivity of a Silicon Oxycarbide glass after two different heat treatments and two Si–O–C nano-composites, hot-pressed at 1600°C, up to 1000°C and compare them to fused silica, amorphous carbon, and SiC. The temperature dependence of their thermal conductivities is similar to other amorphous materials. The presence of low volume fractions of nanoparticles of hafnia (4.5 v/o) or zirconia (7.4 v/o) dispersed within the amorphous matrix only modifies the conductivity slightly, consistent with a simple Maxwell model, and does not affect the temperature dependence of the thermal conductivity above room temperature.

  • corrosion behavior of Silicon Oxycarbide based ceramic nanocomposites under hydrothermal conditions
    International Journal of Materials Research, 2012
    Co-Authors: Christoph Linck, Emanuel Ionescu, Benjamin Papendorf, Dagmar Galuskova, Duŝan Galusek, Pavol ŝajgalik, Ralf Riedel
    Abstract:

    Abstract Silicon Oxycarbide-based ceramic nanocomposites (SiOC, SiZrOC and SiHfOC) were prepared by means of hot pressing techniques and their behavior upon hydrothermal corrosion at moderate temperatures (up to 250°C) was investigated. The results indicated linear corrosion behavior for all samples. The corrosion rates of the SiOC ceramic materials were found to be remarkably lower than those of Silicon carbide and comparable to values reported for Silicon nitride. Furthermore, SiZrOC and SiHfOC were found to show improved resistance with respect to the non-modified SiOC, due to a unique synergistic effect: whereas zirconia/hafnia act as “reinforcing” phases with respect to hydrothermal corrosion (due to their extremely low solubility in water under the testing conditions), the Silicon Oxycarbide matrix protects the MO2 phase from a corrosion-induced t-MO2 → m-MO2 phase transformation. Consequently, the prepared Silicon Oxycarbide-based materials exhibit high potential for applications which require high...

Hans-joachim Kleebe - One of the best experts on this subject based on the ideXlab platform.

  • influence of sic silica and carbon silica interfaces on the high temperature creep of Silicon Oxycarbide based glass ceramics a case study
    Advanced Engineering Materials, 2019
    Co-Authors: Christina Stabler, Hans-joachim Kleebe, Ralf Riedel, Daniel Schliephake, Martin Heilmaier, Tanguy Rouxel, Masaki Narisawa, Emanuel Ionescu
    Abstract:

    In the present study, the high-temperature creep behavior of three SiOC glass ceramics with different phase compositions are compared by the authors. All three SiOC glass ceramics have a vitreous silica matrix in common, but comprise different homogeneously dispersed phases 1) only spherical beta-SiC nanoparticles (sample denoted hereafter SiC/SiO2), 2) only high-aspect ratio sp(2)-hybridized carbon (i.e., C/SiO2), and 3) both phases (SiC and segregated carbon, i.e., C/SiC/SiO2). Compression creep experiments are performed at temperatures in the range between 1100 and 1300 degrees C and true stresses of 50 to 200 MPa. The determined activation energy for creep of the SiOC glass ceramics of around 700 kJ mol(-1) is independent of the phase composition. A stress exponent value of approximately 2 indicates an interface-controlled deformation mechanism. All SiOC glass ceramics exhibit significantly higher creep viscosities than that of vitreous silica. Surprisingly, the spherical beta-SiC nanoparticles have a higher impact on the effective creep viscosities of SiOC as compared to that of the high-aspect ratio segregated carbon phase. It is concluded that this originates from the beta-SiC/silica and C/silica interfaces, which have different effects on the creep behavior of Silicon Oxycarbide-based glass ceramics.

  • Silicon Oxycarbide nano Silicon composite anodes for li ion batteries considerable influence of nano crystalline vs nano amorphous Silicon embedment on the electrochemical properties
    Journal of Power Sources, 2014
    Co-Authors: Jan Kaspar, Hans-joachim Kleebe, Magdalena Graczykzajac, Stefan Lauterbach, Ralf Riedel
    Abstract:

    Abstract Silicon Oxycarbide/nano-Silicon composites (SiOC/nSi) are prepared by mixing of nano-sized Silicon, either crystalline (nSi_c) or amorphous (nSi_a), with commercially available polyorganosiloxane RD-684a and subsequent pyrolysis. The influence of the type of nano-Silicon, namely crystalline vs. amorphous, on the electrochemical properties and performance is analyzed and correlated with the corresponding composite microstructure. In the case of crystalline nano-Silicon, a high reversible capacity of 905 mAh g −1 is registered, whereas that for amorphous nano-Silicon embedment reaches 704 mAh g −1 . However, regarding the cycling stability, SiOC/nSi_c shows a significant capacity fading upon continuous cycling, related to SiOC matrix failure. The host phase is not able to accommodate the arising mechanical stresses upon Si grain expansion and contraction when alloying/dealloying with Li. SiOC/nSi_a on the contrary, demonstrates a stable cycling performance for up to 100 cycles. This excellent performance is explained by the enhanced matrix integrity of the compound, rationalized by a smaller size of the embedded crystallized Si grains and an intrinsically enhanced electrical conductivity due to the formation of SiC.

  • polymer derived Silicon Oxycarbide hafnia ceramic nanocomposites part ii stability toward decomposition and microstructure evolution at t 1000 c
    Journal of the American Ceramic Society, 2010
    Co-Authors: Emanuel Ionescu, Hans-joachim Kleebe, Benjamin Papendorf, Ralf Riedel
    Abstract:

    This study presents first investigations on the high-temperature stability and microstructure evolution of SiOC/HfO2 ceramic nanocomposites. Polymer-derived SiOC/HfO2 ceramic nanocomposites have been prepared via chemical modification of a commercially available polysilsesquioxane by hafnium tetra (n-butoxide). The modified polysilsesquioxane-based materials were cross-linked and subsequently pyrolyzed at 1100°C in argon atmosphere to obtain SiOC/HfO2 ceramic nanocomposites. Annealing experiments at temperatures between 1300° and 1600°C were performed and the annealed materials were investigated with respect to chemical composition and microstructure. The ceramic nanocomposites presented here were found to exhibit a remarkably improved thermal stability up to 1600°C in comparison with hafnia-free Silicon Oxycarbide. Chemical analysis, X-ray diffraction, FTIR, and Raman spectroscopy as well as electron microscopy (SEM, TEM) studies revealed that the excellent thermal stability of the SiOC/HfO2 nanocomposites is a consequence of the in situ formation of hafnon (HfSiO4), which represents a concurrent reaction to the carbothermal decomposition of the SiOC matrix. Thus, by the annealing of SiOC/HfO2 materials at 1600°C, novel HfSiO4/SiC/C ceramic nanocomposites can be generated. The results presented emphasize the potential of these materials for application at high temperatures.

  • polymer derived Silicon Oxycarbide hafnia ceramic nanocomposites part i phase and microstructure evolution during the ceramization process
    Journal of the American Ceramic Society, 2010
    Co-Authors: Emanuel Ionescu, Hans-joachim Kleebe, Benjamin Papendorf, Fabrizia Poli, Klaus Müller, Ralf Riedel
    Abstract:

    Polymer-derived SiOC/HfO2 ceramic nanocomposites were prepared via chemical modification of a commercially available polysilsesquioxane by hafnium tetra (n-butoxide). The ceramization process of the starting materials was investigated using thermal analysis and in situ Fourier-transformed infrared spectroscopy and mass spectrometry. Furthermore, solid-state NMR, elemental analysis, powder X-ray diffraction, and electron microscopy investigations were performed on ceramic materials pyrolyzed at different temperatures ranging from 800° to 1300°C, in order to obtain information about the structural changes and phase evolution thereof. The hafnium alkoxide-modified precursor was shown to convert into an amorphous single-phase SixHfyOzCw ceramic at temperatures up to 800°C. By increasing the temperature to 1000°C, amorphous hafnia begins to precipitate throughout the Silicon Oxycarbide matrix; thus, monodisperse hafnia particles with a diameter of <5 nm are present in the ceramic, indicating a homogeneous nucleation of HfO2. At temperatures ranging from 1100° to 1300°C, crystallization of the hafnia nanoprecipitates as well as phase separation of the SiOC matrix occur. The chemical modification of the preceramic precursor with hafnium alkoxide can be considered as a promising method for the preparation of SiOC/HfO2 nanocomposites with well-dispersed hafnia nanoparticles.

Rishi Raj - One of the best experts on this subject based on the ideXlab platform.

  • on the thermodynamically stable amorphous phase of polymer derived Silicon Oxycarbide
    Scientific Reports, 2015
    Co-Authors: Rishi Raj
    Abstract:

    A model for the thermodynamic stability of amorphous Silicon Oxycarbide (SiCO) is presented. It builds upon the reasonably accepted model of SiCO which is conceived as a nanodomain network of graphene. The domains are expected to be filled with SiO2 molecules, while the interface with graphene is visualized to contain mixed bonds described as Si bonded to C as well as to O atoms. Normally these SiCO compositions would be expected to crystallize. Instead, calorimetric measurements have shown that the amorphous phase is thermodynamically stable. In this article we employ first-principles calculations to estimate how the interfacial energy of the graphene networks is favorably influenced by having mixed bonds attached to them. We analyze the ways in which this reduction in interfacial energy can stabilize the amorphous phase. The approach highlights how density functional theory computations can be combined with the classical analysis of phase transformations to explain the behavior of a complex material. In addition we discover a two-dimensional lattice structure, with the composition Si2C4O3 that is constructed from a single layer of graphene congruent with Silicon and oxygen bonds on either side.

  • preparation of ultrathin walled carbon based nanoporous structures by etching pseudo amorphous Silicon Oxycarbide ceramics
    Journal of the American Ceramic Society, 2006
    Co-Authors: Raquel Penaalonso, Gian Domenico Soraru, Rishi Raj
    Abstract:

    Etching polymer-derived Silicon-Oxycarbide ceramics with hydrofluoric acid creates nanoporous structures of specific surface areas as high as 600 m2/g. The change in composition upon etching shows the removal of silica, not carbon. The structure remaining after etching is postulated to consist of a scaffolding of graphene networks with their surfaces decorated with mixed bonds of tetrahedral Silicon bonded to both oxygen and carbon (SiOmC4−m, where m=1, 2, or 3). The pores existing within such scaffoldings are presumed to have been filled with SiO2 tetrahedra, which are removed by etching. The measurement of the average pore size and pore volumes permits us to estimate the width of the graphitic domain walls, δW, left behind by the etching process. The smallest value of δW, which corresponds to the specimen with the highest surface area, is approximately 1 nm, which is about equal to the total width of one graphene layer and two SiOmC4–m tetrahedra, one on either side of the graphene sheet. This highest surface area specimen is also believed to have the largest size of the silica domains in the unetched samples. In specimens with smaller domains, the etching is only partially successful in removing the silica, presumably because their small size hinders the access of the etchant to the silica tetrahedra. The above behavior is found for samples with low to moderate carbon content. In one sample with a very high carbon content, the etching process removes some carbon as well as silica.

  • the role of carbon in unexpected visco an elastic behavior of amorphous Silicon Oxycarbide above 1273 k
    Journal of Non-crystalline Solids, 2005
    Co-Authors: A Scarmi, Gian Domenico Soraru, Rishi Raj
    Abstract:

    Abstract Polymer-derived Silicon Oxycarbide is ostensibly amorphous, yet we show that it exhibits high temperature anelasticity that has so far been seen only in crystalline structures. The time dependency, and the magnitude of strain recovery upon unloading, are non-linear. Indeed the pattern is reminiscent of polymers where viscoelasticity arises from the time dependent but recoverable movement of one-dimensional carbon chains that are intertwined and cross-linked to some degree. In analogy we propose that viscoelasticity in this polymer-derived ceramic arises from an intertwined network of two-dimensional graphene sheets .

  • pyrolysis kinetics for the conversion of a polymer into an amorphous Silicon Oxycarbide ceramic
    Journal of the American Ceramic Society, 2002
    Co-Authors: Gian Domenico Soraru, Luca Pederiva, Jerome Latournerie, Rishi Raj
    Abstract:

    We present experimental and analytical results for the pyrolysis reactions underlying the conversion of a cross-linked polymer into an amorphous ceramic material. The activation energies, obtained from thermogravimetric data, and chemical analysis of the volatiles by mass spectroscopy are used to identify the reaction pathways. The reaction is determined to be first-order, which is consistent with its solid-state nature. The magnitude of the weight loss is analyzed to calculate the number of molecular sites in the polymer that participate in the reaction. The experiments were conducted on a polymer made from silsesquioxanes that convert into Silicon Oxycarbide ceramics on pyrolysis. The results show that <2.5% of the Silicon atoms in the polymer are removed as volatile silanes, and less than one-half of the carbon atoms are lost as methane. These results are a first step in understanding the molecular basis for the ceramic yield, as well as the evolution of the nanostructure as the material changes from an organic into a ceramic state by reactions that can occur at <850°C.

Riedel Ralf - One of the best experts on this subject based on the ideXlab platform.

  • Microstrain-range giant piezoresistivity of Silicon Oxycarbide thin films under mechanical cyclic loads
    'Elsevier BV', 2022
    Co-Authors: Ricohermoso Emmanuel, Klug Florian, Schlaak Helmut, Riedel Ralf, Ionescu Emanuel
    Abstract:

    In the present study, thin-film strain gauge element arrays were prepared based on large-area Silicon Oxycarbide thin films and lithographic deposition of structured electrodes. The individual strain gauge elements were systematically investigated concerning their piezoresistive behavior at ambient temperature and shown to possess giant piezoresistivity with gauge factors in the range of 3–5 × 10³. This has been correlated with the large charge carrier mobility in the Silicon Oxycarbide thin films (i.e., 186 cm(2) x V(-1) x s(−1) as well as with a unique phase composition and morphology thereof, consisting of high-conductivity carbon-rich segregations homogeneously dispersed within a Silicon Oxycarbide-based matrix. The studied strain gauge elements were evaluated in both cyclic tensile and compression load modes and showed excellent reversibility and short response times. The process capability of the strain gauge elements has been statistically assessed and revealed good robustness and replicability which may be further improved. The present work provides a robust and highly reproducible manufacturing process for an ultrasensitive strain gauge prototype and thus points towards a great potential concerning the use of Silicon Oxycarbides in MEMS-related applications

  • Electrically conductive Silicon Oxycarbide thin films prepared from preceramic polymers
    'Wiley', 2022
    Co-Authors: Ricohermoso Emmanuel, Klug Florian, Riedel Ralf, Schlaak, Helmut F., Ionescu Emanuel
    Abstract:

    This work focuses on Silicon Oxycarbide thin film preparation and characterization. The Taguchi method of experimental design was used to optimize the process of film deposition. The prepared ceramic thin films with a thickness of c. 500 nm were characterized concerning their morphology, composition, and electrical properties. The molecular structure of the preceramic polymers used for the preparation of the ceramic thin films as well as the thermomechanical properties of the resulting SiOC significantly influenced the quality of the ceramic films. Thus, an increase in the content of carbon was found beneficial for the preparation of crack-free thin films. The obtained ceramic films exhibited increased electrical conductivity as compared to monolithic SiOC of similar chemical composition. This was shown to correlate with the unique hierarchical microstructure of the SiOC films, which contain large oxygen-depleted particles, mainly consisting of highly graphitized carbon and SiC, homogeneously dispersed in an oxygen-containing amorphous matrix. The matrix was shown to also contain free carbon and to contribute to charge carrier transport between the highly conductive large particles. The ceramic thin films possess electrical conductivities in the range from 5.4 to 8.8 S/cm and may be suitable for implementation in miniaturized piezoresistive strain gauges

  • Compressive thermal stress and microstructure-driven charge carrier transport in Silicon Oxycarbide thin films
    'Elsevier BV', 2021
    Co-Authors: Ricohermoso Emmanuel, Klug Florian, Schlaak Helmut, Riedel Ralf, Ionescu Emanuel
    Abstract:

    This work correlates the charge carrier transport mechanism of Silicon Oxycarbide-based thin films with their morphology and thermal stress. Segregation of highly-graphitized carbon-rich, oxygen-depleted C/SiC areas homogeneously dispersed within an oxygen-rich C/SiOC matrix was seen on the 500 nm-SiOC thin films. Compressive biaxial stress induced by the mismatch with the Si-substrate thermal expansion coefficient was calculated at 109 MPa. Through Hall measurements, p-type carriers were shown dominating the SiOC film similar to monolithic samples. Thin films and monoliths have comparable carrier concentrations while the carrier mobility in SiOC thin films was 2 magnitudes higher than that of monolithic samples and is considered a consequence of the compressive thermal stress acting on the film. Improved conductivity of 16 S cm-1 is measured for the SiOC thin film sample which is assumed considering the enhanced carrier mobility alongside the reduced percolation threshold ascribed to the phase-separated morphology of the thin film

  • Effect of Ca and B incorporation into Silicon Oxycarbide on its microstructure and phase composition
    'Wiley', 2019
    Co-Authors: Xie Fangtong, Riedel Ralf, Gonzalo-juan Isabel, Breitzke Hergen, Fasel Claudia, Trapp Maximilian, Buntkowsky Gerd, Kleebe Hans-joachim, Boccaccini, Aldo R., Ionescu Emanuel
    Abstract:

    Ca and/or B modified Silicon Oxycarbides were synthesized via pyrolysis of suitable polysilsesquioxane‐based single‐source precursors. Their polymer‐to‐ceramic transformation was investigated with thermogravimetric analysis, coupled with in situ evolved gas analysis. The prepared Silicon Oxycarbides were investigated with respect to their crystallization behavior, network architecture and chemical compositions. The network connectivity in Silicon Oxycarbides can be affected / tuned upon using two different “tools”: (i) firstly, the use of network modifiers, such as Ca in our study, leads to a slight depolymerization of the network via generation of a small amount of Q3 sites; (ii) secondly, the modification of Silicon Oxycarbide with B/Ca leads to a decrease of the carbon content in the network and thus to a significant decrease of its connectivity. By using these two different effects, the network connectivity in Silicon Oxycarbides can be finely tuned

  • Influence of SiC/Silica and Carbon/Silica Interfaces on the High-Temperature Creep of Silicon Oxycarbide-Based Glass Ceramics: A Case Study
    'Wiley', 2019
    Co-Authors: Stabler Christina, Riedel Ralf, Kleebe Hans-joachim, Schliephake Daniel, Heilmaier Martin, Rouxel Tanguy, Narisawa Masaki, Ionescu Emanuel
    Abstract:

    In the present study, the high-temperature creep behavior of three SiOC glass ceramics with different phase compositions are compared by the authors. All three SiOC glass ceramics have a vitreous silica matrix in common, but comprise different homogeneously dispersed phases: 1) only spherical beta-SiC nanoparticles (sample denoted hereafter SiC/SiO2), 2) only high-aspect ratio sp(2)-hybridized carbon (i.e., C/SiO2), and 3) both phases (SiC and segregated carbon, i.e., C/SiC/SiO2). Compression creep experiments are performed at temperatures in the range between 1100 and 1300 degrees C and true stresses of 50 to 200 MPa. The determined activation energy for creep of the SiOC glass ceramics of around 700 kJ mol(-1) is independent of the phase composition. A stress exponent value of approximately 2 indicates an interface-controlled deformation mechanism. All SiOC glass ceramics exhibit significantly higher creep viscosities than that of vitreous silica. Surprisingly, the spherical beta-SiC nanoparticles have a higher impact on the effective creep viscosities of SiOC as compared to that of the high-aspect ratio segregated carbon phase. It is concluded that this originates from the beta-SiC/silica and C/silica interfaces, which have different effects on the creep behavior of Silicon Oxycarbide-based glass ceramics