High-Temperature Raman Spectroscopy of Nano-Crystalline Carbon in Silicon Oxycarbide

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

Effect of the Content and Ordering of the sp² Free Carbon Phase on the Charge Carrier Transport in Polymer-Derived Silicon Oxycarbides

The present work elaborates on the correlation between the amount and ordering of the free carbon phase in silicon oxycarbides and their charge carrier transport behavior. Thus, silicon oxycarbides possessing free carbon contents from 0 to ca. 58 vol.% (SiOC/C) were synthesized and exposed to temperatures from 1100 to 1800 °C. The prepared samples were extensively analyzed concerning the thermal evolution of the sp2 carbon phase by means of Raman spectroscopy. Additionally, electrical conductivity and Hall measurements were performed and correlated with the structural information obtained from the Raman spectroscopic investigation. It is shown that the percolation threshold in SiOC/C samples depends on the temperature of their thermal treatment, varying from ca. 20 vol.% in the samples prepared at 1100 °C to ca. 6 vol.% for the samples annealed at 1600 °C. Moreover, three different conduction regimes are identified in SiOC/C, depending on its sp² carbon content: (i) at low carbon contents (i.e., <1 vol.%), the silicon oxycarbide glassy matrix dominates the charge carrier transport, which exhibits an activation energy of ca. 1 eV and occurs within localized states, presumably dangling bonds; (ii) near the percolation threshold, tunneling or hopping of charge carriers between spatially separated sp² carbon precipitates appear to be responsible for the electrical conductivity; (iii) whereas above the percolation threshold, the charge carrier transport is only weakly activated (Ea = 0.03 eV) and is realized through the (continuous) carbon phase. Hall measurements on SiOC/C samples above the percolation threshold indicate p-type carriers mainly contributing to conduction. Their density is shown to vary with the sp² carbon content in the range from 10¹⁴ to 10¹⁹ cm⁻³; whereas their mobility (ca. 3 cm²/V) seems to not depend on the sp² carbon content

Dehnungsmessung an piezoresistiver polymerabgeleiteter Keramik

Polymerabgeleitete Keramik mit piezoresistiven Eigenschaften bestehen aus Siliconoxycarbide-Nanocomposite (SiOC/C). Im Gegensatz zur kommerziellen (piezo-)resistiven Materialien weist SiCO/C Nanokomposite eine hohe Druckempfindlichkeit und eine hohe Temperaturfestigkeit über von 400°C. Wegen seiner keramischen Eigenschaften kann SiOC/C als Volumenkörper modelliert werden und benötigt deshalb keinen zusätzlichen Verformungskörper. Durch Variation des Volumenan teils des segregierten Kohlenstoffs xc um die Perkolationsgrenze (xc = 10...20 vol%) kann die Empfindlichkeit gezielt beeinflusst werden. Die Druckempfindlichkeit wird im Biege- und Stauchversuch systematisch untersucht und miteinander verglichen

High-Temperature Raman Spectroscopy of Nano-Crystalline Carbon in Silicon Oxycarbide

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

Dynamic Load and Temperature Behavior of a PEFC-Hybrid-System

A dynamic model of a polymer electrolyte fuel cell hybrid system has been developed within MATLAB-SIMULINK. Components are modeled using electrochemical and mass transport as well as heat transfer equations. The implemented equations describe the steadystate as well as the dynamic operation of the system with sufficient accuracy, although considerable simplifications have been made for the stack and the peripheral components to keep model complexity and computing time low. Emphasis is given to the operation limits of the PEFC system, notably the conditions for trouble-free operation at different loads and high or low ambient temperature. The potential of the simulation as system optimization tool and efficient operation guide are demonstrated. Model validation was accomplished by experiments on a homemade 150 W portable system including a Ni-MH accumulator for 300 W peak power output

Effect of the Content and Ordering of the sp2 Free Carbon Phase on the Charge Carrier Transport in Polymer-Derived Silicon Oxycarbides

The present work elaborates on the correlation between the amount and ordering of the free carbon phase in silicon oxycarbides and their charge carrier transport behavior. Thus, silicon oxycarbides possessing free carbon contents from 0 to ca. 58 vol.% (SiOC/C) were synthesized and exposed to temperatures from 1100 to 1800 degrees C. The prepared samples were extensively analyzed concerning the thermal evolution of the sp(2) carbon phase by means of Raman spectroscopy. Additionally, electrical conductivity and Hall measurements were performed and correlated with the structural information obtained from the Raman spectroscopic investigation. It is shown that the percolation threshold in SiOC/C samples depends on the temperature of their thermal treatment, varying from ca. 20 vol.% in the samples prepared at 1100 degrees C to ca. 6 vol.% for the samples annealed at 1600 degrees C. Moreover, three different conduction regimes are identified in SiOC/C, depending on its sp(2) carbon content: (i) at low carbon contents (i.e., <1 vol.%), the silicon oxycarbide glassy matrix dominates the charge carrier transport, which exhibits an activation energy of ca. 1 eV and occurs within localized states, presumably dangling bonds; (ii) near the percolation threshold, tunneling or hopping of charge carriers between spatially separated sp(2) carbon precipitates appear to be responsible for the electrical conductivity; (iii) whereas above the percolation threshold, the charge carrier transport is only weakly activated (E-a = 0.03 eV) and is realized through the (continuous) carbon phase. Hall measurements on SiOC/C samples above the percolation threshold indicate p-type carriers mainly contributing to conduction. Their density is shown to vary with the sp(2) carbon content in the range from 10(14) to 10(19) cm(-3); whereas their mobility (ca. 3 cm(2)/V) seems to not depend on the sp(2) carbon content

UV Raman spectroscopy of segregated carbon in silicon oxycarbides

Polymer-derived silicon oxycarbides exhibiting ≤1 and 10 vol.% of segregated carbon finely dispersed within a glassy SixOyCz matrix have been investigated by UV Raman spectroscopy using a laser excitation of 4.8 eV (λ = 256.7 nm). Carbon exists as amorphous sp2–sp3 bonded component in SiOC/C (≤1 vol.%) pyrolyzed at 1100°C in H2, including C–C single bonds, polymeric chains and small polycyclic aromatic hydrocarbons (PAHs). The formation of nanocrystalline carbon at T > 1400°C is seen in the Raman spectra of SiOC/C (≤1 vol.%) and SiOC/C (10 vol.%) by the appearance of the G band of graphite. Tempering at 1600°C increases the degree of order within the carbon phase. However, the slight narrowing of the G peak with processing temperature (by about 5%) indicates still not well-crystallized carbon: the Raman results can be best explained by turbostratic carbon (with a lateral size La of ≈2 nm) and do not support the model description in literature as a network of single layer graphene

How good would the conductivity of graphene have to be to make single-layer-graphene metamaterials for terahertz frequencies feasible?

Various terahertz metamaterial devices and concepts involving graphene have been introduced in the literature, however, graphene is either a functional add-on to resonators made from metals with a high electrical conductivity, or it is studied as arrays of relatively simple plasmonic stripes or disks, made from single- or multi-layer graphene. Graphene is never the resonator material of more complex structures such as split-ring resonators because its conductivity is too low. However, for electromagnetic chemical sensors, even a moderate conductivity may be adequate since the response of the metamaterial can be strongly modified by the adsorption of molecules, not only by a change of the dielectric environment, as for conventional metamaterials, but also via a direct change of the conductivity. Here, we consider a prototypical split-ring-resonator consisting of a single layer of patterned graphene on a dielectric, and investigate by simulations its terahertz reflectivity response. The crucial material parameters for device performance are the charge carrier density, controlled by the Fermi energy, and the Drude scattering time. We find that metamaterial behavior becomes interesting if the Drude scattering time of 0.1 ps of standard graphene could be raised to the theoretically accessible value of 0.4–0.5 ps