Electrically conductive and thermally stable SiC-TiC containing nanocomposites via flash pyrolysis

Flash pyrolysis, which combines conventional pyrolysis with flash sintering, was first conducted to produce polymer derived SiC-TiC nanocomposites. Pre-pyrolysis at 800℃ allows the conversion from titanium isopropoxide (TTIP) modified polysiloxane to an amorphous SiTiOC ceramic. The subsequent application of an electric field gives rise to the formation of turbostratic carbon and creates Joule heating to obtain a sample internal temperature of ~1400℃. The precipitation of β-SiC, TiC, as well as titanium oxides is realized upon carbothermal reduction of extensively phase separated SiO₂ and TiO₂ with carbon. Increasing TTIP content embodies the nanocomposites with prominent electrical percolation behaviors. The electrical transport of the synthesized ceramics follows an amorphous semiconductor mechanism. High thermal stability in air is guaranteed, thanks to the in-situ formed TiC nanocrystals and preferentially reduced amorphous carbon. Flash pyrolyzed nanocomposite with a Ti:Si molar ratio of 0.20 exhibits the highest electrical conductivity (0.696 S/cm) and minimum mass change (~2%) at 1000℃, serving as a competitive candidate for electro-discharge machining (EDM) applications or self-standing conducting devices that must withstand high temperature conditions.     

Vibrational dynamics of non-crystalline glassy alloys

The vibrational dynamics of three Ca-based non-crystalline alloys viz. Ca₇₀Mg₃₀, Ca₇₀Zn₃₀ and Ca₆₀Al₄₀ have been studied at room temperature in terms of the phonon eigen frequencies of longitudinal and transverse modes, employing three theoretical formulations given by Hubbard-Beeby (HB), Takeno-Goda (TG) and Bhatia-Singh (BS). Five local field correction functions viz. Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used for the first time in the present investigation to study the screening influence on the aforesaid properties. The pseudo-alloy-atom (PAA) model is applied for the first time instead of Vegard’s Law. Long wavelength limits of the phonon modes are used to investigate the elastic and thermal properties of the system. The low temperature specific heat is also calculated from the elastic limit of the phonon dispersion curves (PDCs). The present findings of the PDCs of Ca₇₀Mg₃₀ glass are found in fair agreement with available theoretical and experimental data.     

Abnormal Graphitization Behavior in Near-Surface/Interface Region of Polymer-Derived Ceramics

The in situ free carbon generated in polymer-derived ceramics (PDCs) plays a crucial role in their unique microstructure and resultant properties. This study advances a new phenomenon of graphitization of PDCs. Specifically, whether in micro-/nanoscale films or millimeter-scale bulks, the surface/interface radically changes the fate of carbon and the evolution of PDC nanodomains, promotes the graphitization of carbon, and evolves a free carbon enriched layer in the near-surface/interface region. Affected by the enrichment behavior of free carbon in the near-surface/interface region, PDCs exhibit highly abnormal properties such as the skin behavior and edge effect of the current. The current intensity in the near-surface/interface region of PDCs is orders of magnitude higher than that in its interior. Ultrahigh conductivity of up to 14.47 S cm⁻¹ is obtained under the action of the interface and surface, which is 5–8 orders of magnitude higher than that of the bulk prepared under the same conditions. Such surface/interface interactions are of interest for the regulation of free carbon and its resultant properties, which are the core of PDC applications. Finally, the first PDC thin-film strain gauge that can survive a butane flame with a high temperature of up to ≈1300 °C is fabricated.     

Pressureless synthesis of fully dense and crack-free SiOC bulk ceramics via photo-crosslinking and pyrolysis of a polysiloxane

This paper presents the pressureless preparation of fully dense and crack-free SiOC ceramics via direct photo-crosslinking and pyrolysis of a polysiloxane. Elemental analysis revealed the presence of high levels of carbon in the SiOC ceramics. Thus, the samples showed the highest content (78-86 mol%) of segregated “free” carbon reported so far. XRD investigations indicated that the materials prepared at 1100 °C were X-ray amorphous, whereas the sample prepared at 1400 °C contained a turbostratic graphite-like phase and silicon carbide as crystalline phases, as additionally confirmed by TEM and Raman spectroscopy. Vickers hardness was measured to be 5.5-8.6 GPa. The dc resistivity of the prepared material at 1100 °C was 0.35 Ω m, whereas the ceramic pyrolyzed at 1400 °C showed a value of 0.14 Ω m; both values are much lower than those of other known SiOC materials. This latter feature was attributed to the presence of a percolating carbon network in the ceramic.     

Preparation and temperature-resistance characteristics of novel dense SiAlCN ceramics

The compact green bodies, prepared via a novel solid-liquid mixing method of precursors, were successfully pyrolyzed to obtain the dense bulk SiAlCN ceramics at 1000 °C. It can be seen from their SEM that they have uniform and dense microstructure, indicating that this method can be used to prepare bulk ceramics. In order to verify that they can be used as sensor heads, their temperature-resistance characteristics and repeatability were tested. The results show that the conductive mechanism belongs to Arrhenius's Tailed-State and Extended-State in the temperature range of 500–650 °C and 650–930 °C, respectively. And it shows that SiAlCN ceramics can be used as the sensor heads for high-temperature sensors.     

Tape casting of polysiloxane-derived ceramic with controlled porosity and surface properties

Tape casting has been applied to produce porous hybrid and SiOC ceramic tapes using ceramic precursors and commercially available polysiloxanes as polymeric binders. SiC particles of two different mean sizes (4.5 or 6.5?μm) were used as inert fillers to prevent shrinkage and increase mechanical stability. Macroporosity was adjusted by varying the azodicarbonamide (ADA) content from 0 to 30?wt.%. Decomposition of the polysiloxanes at 600?°C resulted in the generation of micropores with high specific surface area (187–267 m²?g^?1) and a predominant hydrophobic behavior. At 1000?°C mainly meso/macroporosity were observed (SSA: 32–162 m²?g^?1) accompanied by increased hydrophilicity. The influence of ADA content, SiC size, and pyrolysis temperature on open porosity (2.5–37%), average pore size (<0.01–1.76?μm), surface characteristics, and flexural strength (10.5–121?MPa) were investigated. The porous tapes with different surface characteristics and controlled structure are highly promising for applications involving membrane processes, particularly microfiltration systems (0.1–10?μm).     

Molecular design for thermally stable SiZrBOC structure from single source precursor with high ceramic yield

A new type of high-temperature-resistant SiZrBOC ceramics was prepared by sol-gel method using polymethyl-hydro siloxane (PMHS), boric acid (B(OH)₃), and n-propyl zirconate (Zr(OPr)₄) as raw materials. After high-temperature pyrolysis, the SiZrBOC precursor was transformed into a crystalline ceramic material with a yield of 89.5 wt%. Fourier infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) were applied to characterize the polymer-ceramic conversion process and thermal behavior of ceramic precursors. According to the results, the addition of boron elements led to the formation of Si-O-B links in the system. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to study the phase composition and microstructure of SiZrBOC ceramics. Finally, the oxidation test at 1200 °C revealed that SiZrBOC ceramics with a boron/zirconium molar ratio of 2.5:1 exhibited the best oxidation resistance at a weight gain of 0.4 wt% only.     

Phonon dispersion in alkali metals and their equiatomic sodium-based binary alloys

In the present article, the theoretical calculations of the phonon dispersion curves (PDCs) of five alkali metals viz. Li, Na, K, Rb, Cs and their four equiatomic sodium-based binary alloys viz. Na_0.5Li_0.5, Na_0.5K_0.5, Na_0.5Rb_0.5 and Na_0.5Cs_0.5 to second order in a local model potential is discussed in terms of the realspace sum of the Born von Karman central force constants. Instead of the concentration average of the force constants of pure alkali metals, the pseudo-alloy-atom (PAA) is adopted to directly compute the force constants of the four equiatomic sodium based binary alloys and was successfully applied. The exchange and correlation functions due to the Hartree (H) and Ichimaru-Utsumi (IU) are used to investigate the influence of the screening effects. The phonon frequencies of alkali metals and their four equiatomic sodium-based binary alloys in the longitudinal branch are more sensitive to the exchange and correlation effects in comparison with the transverse branches. The PDCs of pure alkali metals are found in qualitative agreement with the available experimental data. The frequencies in the longitudinal branch are suppressed rather due to IU-screening function than those due to static H-screening function.     

High surface area SiC(O)-based ceramic by pyrolysis of poly (ethylene glycol) methacrylate-modified polycarbosilane

In the present work, a high surface area SiC(O)-based ceramic powder was synthesized upon thermal transformation of a polymer-derived macromolecular precursor, which was obtained by the chemical modification of a allylhyldrido polycarbosilane with poly(ethylene glycol) methaacrylate under argon environment. The pyrolysis of developed precursor led to the formation of amorphous and high surface area SiC(O)-based ceramic powder with in situ generated micro/meso-porosity. The specific surface area of the obtained powders depends on the processing temperature. It decreases from 363 to 122 m²/g as the pyrolysis temperature increases from 600 to 1200°C, respectively. Furthermore the promising samples were fabricated using pressing technique, which led to crack-free SiC(O) monoliths on subsquent heat treatment. The present study also emphasizes the potential of produced SiC(O) ceramic powder to support NiO catalyst. The impregnation method were used to produce high surface area NiO@SiC(O) ceramic powder (NiO as a catalyst; SiC(O) as a catalyst support) for further catalytic applications. Interestingly, the distribution of the NiO was shown to strongly depend on the oxygen content present in the SiC(O) matrix. Thus, larger oxygen contents induce homogeneously distributed flower-like NiO catalyst onto SiC(O).     

Dielectric properties of polymer-derived ceramic reinforced with boron nitride nanotubes

The electrical and dielectric properties of boron nitride nanotubes (BNNTs) reinforced ceramic composites using the polymer-derived ceramic (PDC) processing route were investigated in this work. The electrical resistivity of the pristine PDC increases from 10⁶ to 10⁸ Ω m after the addition of BNNTs. When the BNNT loading was increased to 5 wt%, the average real relative permittivity of the PDC decreased from 2.94 to 2.80, while the quality factor (Q) of the PDC increased from 134.40 to 176.77. The BNNTs can increase the Q factor of the PDC due to the reduction in the porosity cause by the introduction of the BNNTs. Further increasing the BNNT content decreases the real relative permittivity of the nanocomposites and increases the Q factor at high frequency. The average real relative permittivity decreases to 2.29, while the average Q factor increases to 208.60 when the BNNT content is increased to 30 wt%. The dielectric loss after the addition of high fraction of BNNTs can be explained by the Lorentz resonance relaxation process. Results of this work showed that PDC-BNNT nanocomposites are satisfactory electromagnetic transparent materials when the BNNT fraction is less than 10 wt%.