Effect of acrylic acid additive on electric conductivity of polymer-derived amorphous silicon carbonitride

The effect of acrylic acid additive on the electric conductivity of amorphous SiCN derived from polymeric precursor was studied. The conductivity showed to follow the Arrhenius dependence on pyrolysis temperature, but with much smaller activation energy, as compared to the unmodified SiCN. Structural analysis using Raman and XPS revealed that the size of the free-carbon clusters within the AC-modified SiCN changed with pyrolysis temperature, but the sp²-to-sp³ ratio remained almost the same. The reason for the effect of AC on the carbon cluster was speculated. The mechanisms governing the conductivity behavior of the AC-modified SiCN were discussed.     

Electric conductivity and microstructure evolution of polymer-derived SiAlCO ceramics

The electric conductivity and microstructure of polymer-derived SiAlCO ceramic were studied. The conductivity increased drastically with increasing pyrolysis temperature and exhibited a typical Arrhenius dependence on pyrolysis temperature with the activation energy of ~7.15 eV. The microstructure was analyzed by XRD, Raman spectroscopy and XPS. Unlike previous reported PDCs, the increase in the conductivity of the SiAlCO cannot be explained by the structural changes of the free carbon phase. It is speculated that the increased conductivity is possibly due to the redistribution of C–O bonds within the free carbon phase.     

High-temperature electrical properties of polymer-derived ceramic SiBCN thin films fabricated by direct writing

The in situ temperature monitoring of hot components in harsh environments remains a challenging task. In this study, SiBCN thin-film resistance grids with thicknesses of 1.8 μm were fabricated on alumina substrates via direct writing. Owing to their dense microscopic morphology and extremely high graphitisation level, the produced SiBCN films exhibited large high-temperature oxidation resistance and electrical conductivity. The resistance–temperature, stability, and repeatability characteristics of these films were examined in an aerobic environment at temperatures up to 800 °C. The obtained results revealed that the thermistor resistance decreased monotonously with increasing temperature from room temperature to 800 °C. The SiBCN film resistance variations observed during repeated temperature cycling in the regions of 505–620 °C and 610–720 °C were 0.09% and 1.7%, respectively. The high cyclability and stability of the SiBCN thin film thermistor suggested its potential applicability for the in situ temperature monitoring of hot components in harsh environments.     

Effect of the pyrolysis atmosphere on the mechanical properties of polymer-derived SiOC and SiCN

Mechanical properties of polymer-derived ceramics are usually measured on samples pyrolyzed in inert atmosphere. Here, we report the hardness and elastic modulus of SiOC and SiCN pyrolyzed in both inert (Ar) and reactive (CO₂) atmosphere. The external surface of the specimens exposed to the pyrolysis gas was characterized by Vickers microhardness measurements and infrared spectroscopy. The elastic modulus was evaluated by three-point bending tests on thin (150-200 µm) and dense specimens. Polished sections of the SiOC samples were prepared to study, by energy-dispersive X-ray spectroscopy (EDXS) and nanoindentation, how the elemental composition, hardness, and elastic modulus vary from the surface toward the bulk. For both compositions, pyrolysis in CO₂ leads to a strong decrease in the hardness and elastic modulus. The hardness of both the samples pyrolyzed in CO₂ approaches the typical value for fused silica, suggesting that CO₂ selectively breaks the Si–C and Si–N bonds and leads to the formation of a silica-like network. EDXS and nanoindentation reveal that the modification induced by the CO₂ flow extends below the surface at least for a thickness of about 30 µm.     

Growth of multi-morphology amorphous silicon oxycarbide nanowires during the laser ablation of polymer-derived silicon carbonitride

Multi-morphology amorphous SiOC nanowires were successfully prepared within the interfacial interstices between the unaffected SiCN ceramic and the bracket during the laser ablation of polymer-derived SiCN ceramic in a low-pressure argon atmosphere. Laser irradiation experiments were performed using a continuous-wave CO₂ laser, and the gas source for the growth of amorphous SiOC nanowires was provided by the laser ablation of the SiCN ceramic. X-ray photoelectron spectroscopy shows that the amorphous SiOC nanowires possess a SiO₂ dominated nanostructure, and the formation of amorphous SiOC nanowires is attributed to the good diffusivity of CO in SiO₂. The morphologies of the amorphous SiOC nanowires include straight nanowires, beaded nanowires, helical nanowires, and branched nanowires, and these are determined by the flowing state of the reactant gases, the laser power, and the surface morphology of the SiCN ceramics. Each amorphous SiOC nanowire with specific morphology can be uniformly distributed in separate regions, which makes it possible to control the growth of amorphous SiOC nanowires in different morphologies.     

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.     

Competitive effect of KOH activation on the electrochemical performances of carbon nanotubes for EDLC: Balance between porosity and conductivity

This work is focused on the competitive effects on the performance of the electric double layer capacitors (EDLCs) between porosity increase and simultaneous conductivity decrease for KOH-activated carbon nanotubes (CNTs). A series of the CNTs have been activated with KOH to enhance their surface areas for application in EDLCs. The microstructure of the activated carbon nanotubes (ACNTs) is characterized with N₂ adsorption, transmission electron microscopy (TEM) observation and electric conductivity measurement. Their electrochemical performances are evaluated in aqueous KOH electrolyte with galvanostatic charge/discharge, cyclic voltammetry, and ac impedance spectroscopy. It is found that the KOH activation enhances the specific surface area of the CNTs and its specific capacitance but decreases its electric conductivity and the rate performance in EDLC. By controlling the activation of the CNTs to balance the porosity and conductivity, ACNTs with both high capacitance and good rate performance are obtained.     

基体炭种类对C/C复合材料电导率的影响分析

使用炭毡为增强体分别制备了热解炭基、树脂炭基、沥青炭基和热解炭/树脂炭双基体、树脂炭/沥青炭双基体C/C复合材料,比较研究了复合材料的电导率与不同先驱体含量的关系。结果表明,不同前驱体C/C复合材料电导率有较大的差异,热解炭基C/C复合材料的电导率是沥青炭基C/C复合材料和树脂炭基C/C复合材料电导率近3倍,热解炭和沥青炭双基体C/C复合材料的电导率符合简单并联混合法则,树脂炭和沥青炭双基体C/C复合材料的电导率随树脂炭质量分数的增加而减小。     

Effect of demineralization and heating rate on the pyrolysis kinetics of Jordanian oil shales

The effect of mineral matter content on the activation energy of oil shale pyrolysis has been studied. Kerogen was isolated from raw oil shale by sequential HCl and HCl/HF digestion. Oil shale and kerogen samples were pyrolyzed in a Thermogravimetric Analyzer at different heating rates (1, 3, 5, 10, 30, and 50 °C/min) up to a temperature of 1000 °C. Total mass loss of all oil shale samples remained almost constant irrespective of the heating rate employed, whereas it decreased with the increase of heating rate for kerogen (74.5 to 71.4%). From the pyrolysis profile activation energy (Ea) was found to vary between 70 and 83 kJ/mol for oil shale, while 82-112 kJ/mol has been determined for isolated kerogen. An increase of both Ea and pre-exponential factor was observed with an increasing heating rate. It is concluded that the mineral matter in oil shale enhances catalytic cracking as is evident from the reduced Ea values of oil shale compared with those for kerogen.     

Investigation of the electric conductivity and the electromagnetic interference shielding efficiency of SWCNTs/GNS/PAni nanocomposites

This work demonstrates the fabrications and characterizations of polyaniline (PAni) composites containing single-walled carbon nanotubes (SWCNTs), graphite nanosheets (GNS), or hybrid fillers (SWCNTs/GNS). The characterization of microstructure, examination of fracture surface morphologies, and measurement of electric conductivity and electromagnetic interference shielding efficiency (EMI SE) were performed. It was found that both the electric conductivity and the EMI SE increase with filler loading, and the nanocomposites filled with 1.0 wt.% SWCNTs/GNS possessed the highest electric conductivity of 16.2 S/cm and total EMI SE of 27.0 dB. The experimental results also show that absorption is the primary mechanism of EMI SE for all of the loadings and fillers.     

炭砖的导热性能及其影响因素的研究

应用计算机运控的激光热导仪,对不同化学成分、密度和热历史的炭砖的导热系数进行了对比分析和研究,提出了一个由导热性能数据判断炭砖可继续使用的新方法。     

AC conductivity,electric modulus analysis and electrical conduction mechanism of RbFeP2O7 ceramic compound

In this work, the diphosphate compound, RbFeP₂O₇, was prepared by the conventional solid-state reaction. The X-ray diffraction pattern revealed that the sample presents a single phase, that crystallizes in the monoclinic structure with a P2₁/C space group. Impedance analysis was performed using the equivalent circuit model, and, it indicated the presence of intra- and inter-granular contribution. Furthermore, the electrical conductivity, the dielectric properties and the relaxation behavior of this material were studied in detail using the impedance spectroscopy technique, in a frequency ranging from 200 Hz to 5 MHz at several temperatures. The temperature dependency of frequency exponent 's' shows that the correlated barrier-hopping model (CBH) is the most responsible mechanism for AC conduction in the investigated compound. In terms of CBH model, the values of maximum barrier s height, the hopping distance and the density of localized states are determined and discussed. A correlation between electrical and structural properties was also discussed.     

Enhanced electric conductivity of polymer‐derived SiCN ceramics by microwave post‐treatment

The effect of microwave treatment on the electric conductivity and structure of a polymer‐derived SiCN ceramic is studied. It is found that the conductivity of the microwave‐treated sample is about 40 times higher than that of the conventional heat‐treated one at the same temperature and dwell time conventionally. The X‐ray diffraction patterns show that both samples are amorphous without obvious crystallization. Raman analysis reveals that the microwave‐treated sample exhibited a narrower full width at half maximum and upper‐shift of G peak. X‐ray photoelectron spectroscopy spectra show that there is a significant sp³‐to‐sp² transition of free carbon in the microwave‐treated sample. These results suggest that the microwave‐treatment can induce a distinct structure evolution of the free carbon, which contributes to the remarkable enhancement of the conductivity of the sample.     

Effect of electric current on fracture behavior of GaN piezoelectric semiconductive ceramics

Herein, three-point bending testing of an edge-cracked specimen and finite element analysis were used to determine the fracture toughness of GaN piezoelectric semiconductive ceramics, and the effect of an applied electric current on the fracture properties of the GaN ceramics was investigated. The results indicated that an electric current has a significant effect on the fracture behavior of piezoelectric semiconductive ceramics. Specifically, the application of a relatively low electric current density (1.67 × 10⁴ A m⁻²) increased the fracture toughness by 36.8%, while further increases in electric current density reduced the fracture toughness. In addition, the electric current can lead to the ductile fracture of brittle GaN ceramics. Finally, the mechanisms behind these results were systematically analyzed. These findings are useful for designing more reliable GaN electromechanical devices.     

Influence of electric field and current on the strength of depoled GaN piezoelectric semiconductive ceramics

By using three-point bending tests, the effects of an applied DC electric field and current on the strength of depoled GaN piezoelectric semiconductive ceramics are investigated. Under combined mechanical-voltage-electrical current loading, the corresponding stress and electric fields and carrier distribution in specimens are analyzed based on the finite element method. It is shown that, when an electric field of 0.95 kV cm^–1 is applied, the bending strength decreases by 14.7% and then, remains unchangeable with further increase of the electric field. In contrast, the bending strength decreases from 11.5 to 8.5 MPa as the applied electric current increases from 0 to 5 × 10⁴ A m^–2. The results imply that there is a strong correlation between the bending strength and electric field or current for piezoelectric semiconductive ceramics.     

Effect of electric current on high temperature flow behavior of 8Y-CSZ ceramics

High temperature tensile behavior under a DC electric field/current was examined in 8 mol% yttria-stabilized cubic zirconia (8Y-CSZ) polycrystals at the furnace temperature of 1000 °C. For the DC power density higher than the critical value of W_c ≈ 100–200 mW/mm³, flash event similar to that of the powder sintering occurred even in the bulk 8Y-CSZ though the W_c is higher than that of the powders. When below the W_c, the DC current increased the sample temperature due to Joule heating, but did not change the rate of the deformation. When above the W_c, the flash event enhanced the rate of the deformation by several times through the grain boundary sliding mechanism, which is similar to that without the DC electric field/current. The enhanced deformation cannot be explained only by the increment of the sample temperature and is likely to occur by the current-enhanced diffusional processes due to the flash event.     

Influence of carbon nanotubes on thermal and electrical conductivity of zirconia-based composite

Carbon nanotubes (CNTs) are widely used in ceramic-matrix composites (CMC) as a filler. An individual carbon nanotube exhibits extremely high thermal conductivity, however, the influence of CNTs on the thermal conductivity of CMCs is moderate. In contrast, even a small quantity of CNTs significantly increases the electrical conductivity of CMCs. The present paper studies this contradictory influence for ZrO₂-CNTs composites with 3, 5, 10 and 20 vol% multi-wall carbon nanotubes (MWCNTs). Their thermal and electrical conductivity was studied by the laser flash method and electrochemical impedance spectroscopy. The analysis reveals that the moderate influence of MWCNTs on the thermal conductivity of composites originates from the similar thermal conductivity of MWCNTs in a bundle and zirconia. On the other hand, the substantial difference in the electrical conductivity of MWCNTs and zirconia leads to an exponential increase in the electrical conductivity of the ZrO₂-CNTs composite even with small additions of nanotubes.     

Influence of Mn doping on electrical conductivity of lead free BaZrTiO3 perovskite ceramic

Mn doped barium zirconate titanate lead free ceramic with formula BaZr_.045(Mn_xTi_1-x) _.955 O₃ for x?=?0.00, 0.01, 0.02 and 0.03 has been prepared by solid-state reaction method. The single phase tetragonal structure was confirmed by X-ray diffraction (XRD) pattern using Rietveld refinement. Scanning electron microscopy (SEM) shows an inhomogeneous distribution of randomly oriented grains with some voids. Electrical conductivity of Mn doped BZT ceramics was studied using impedance analyzer in the temperature range 493K–673K over wide frequency window. The dispersion behaviour in electrical conductivity obeys Jonscher's double power law for Mn free compound, while it follows single power law for Mn doped compositions. Various parameters viz. dc conductivity (σ_dc), pre-exponential factor (A), frequency exponent (s) and activation energy (E_a) have been estimated by the theoretical fitting of experimental data. The reciprocal temperature dependence of dc conductivity follows the Arrhenius law and specifies thermally activated conduction mechanism. The obtained value of activation energy (E_a) evidences the conduction mechanism is induced by the migration of oxygen vacancies and oxide ions. All samples shows Negative temperature coefficient of resistance (NTCR) and hence exhibit semiconducting behaviour. The environmental friendly lead free ceramic can be exploit to design advanced materials and suitable for the fuel cell electrolyte/electrode applications.     

Effect of boron content on the microstructure and electromagnetic properties of SiBCN ceramics

Electromagnetic wave (EMW) absorbing materials have excellent potential for various applications in civil engineering and the military. In this study, siliconboron carbonitride (SiBCN) ceramics with excellent EMW absorption capability and oxidation resistance were obtained by adjusting the boron content. The results revealed that the graphite crystallite size in the SiBCN ceramics increased from 3.42 to 3.78 nm, whereas the thickness of the oxide layer decreased from 16.6 to 8.2 μm. The highest electrical conductivity and permittivity for the SiBCN ceramics were obtained when the boron content was 5%. The minimum reflection loss was ?35.25 dB at 10.57 GHz and a ceramic thickness of 2.0 mm. At a temperature of 600 °C, the SiBCN ceramic exhibited excellent EMW attenuation ability; particularly, the minimum reflection loss reached ?29.18 dB at 9.65 GHz and a ceramic thickness of 2.5 mm. The superior EMW absorption properties of the SiBCN ceramics at high temperatures can be ascribed to the synergistic effect of relaxation and conductivity. The results suggest that boron could enhance the transformation of amorphous carbon into crystalline graphite and increase the number of heterointerfaces and conductive paths. This work provides a method for obtaining SiBCN ceramics with excellent EMW absorption properties.     

Effects of mechanical deformations on the structurization and electric conductivity of electric conducting polymer composites

Structural changes proceeding in composites under the effect of various mechanical deformations (stretching, compression, shear, etc.) affect the structure of an electrical conducting system. Mechanical stresses, induced by deformation of composite materials during deformation, affect both the molecular and supermolecular structure of polymers. As a consequence, they also affect a substructure bound to it and composed of filler particles. It is evident that in the case of conducting polymer composites, mechanical deformations should reflect electric conductivity of materials. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 601–621, 1999