The effect of carbon nanotubes introduction on the mechanical properties of reaction bonded boron carbide ceramics

The influence of carbon nanotubes (CNTs) on the mechanical properties and structure formation during reactive sintering of B₄C materials with Si addition was studied. Upon infiltrating the B₄C structure with molten silicon, a non-porous composite was formed with a density of 2.45-2.55 g/cm³ and a hardness of 22-27 GPa. The formation of highly dispersed B-C-Si phases was observed in the interphase of adjacent B₄C particles due to the incorporation of Si into B₄C structures. These phases increase the bonding strength between B₄C particles. In spite of the fact that the addition of 1-5 wt% Multi-Walled Carbon Nanotubes decreases the green density of the compacts, the flexural strength of the infiltrated material significantly increased. The improvement of the strength of ceramics modified with MWCNTs was interpreted in terms of the formation of thin flattened SiC crystals at the interfaces between B₄C and B-C-Si particles, which strengthen the interfaces between ceramic particles.     

Probing the band structure of hydrogen-free amorphous carbon and the effect of nitrogen incorporation

Amorphous carbon and carbon nitride bottom gate thin film transistors have been fabricated, which show bulk carrier field effect mobilities around 10⁻³ (cm² V⁻¹ s⁻¹) which is orders of magnitude higher than the previously reported values with p-channel devices at high electric fields between source and drain. The incorporation of nitrogen atoms into the amorphous carbon films deposited by pulsed laser deposition was studied using a wide range of techniques in order to understand the role of nitrogen in the conduction mechanism at high fields. The density of the states (DOS) was measured with several techniques such as electron energy loss spectroscopy, scanning tunnelling spectroscopy, and ultraviolet photoelectron spectroscopy, whereas the joint density of states (JDOS), corresponding to the transitions of electrons from the valence to the conduction bands, were obtained by optical transmittance and photothermal deflection spectroscopy. These measurements when combined can provide unparallel data on the shape and magnitude of the energy band states which are crucial to the understanding of the materials properties and thus opto-electronic applications for these thin films. In this report, the conduction mechanism will be discussed with a band diagram drawn based on the experimentally obtained DOS and JDOS measurements.     

添加物对氮化硼材料性能的影响

研究了引入锆英石和氧化铝等添加物的热压氮化硼材料的抗氧化性、热震稳定性及力学性能。结果表明，向氮化硼中引入锆英石和氧化铝后，虽然未能按预期设想生成莫来石，但却对提高氮化硼材料的抗氧化性、热震稳定性及力学性能有利。     

Comparison of glassy carbon and boron doped diamond electrodes: Resistance to biofouling

Carbon based electrodes are widely used for in vivo and in vitro electrochemical studies. In particular, monoamine neurochemistry has been investigated using carbon microfibre electrodes. Similarly, glassy carbon (GC) is the preferred material for many biochemical applications, such as electrochemical detection in chromatography. More recently, boron doped diamond (BDD) has been utilized for biosensing, as its carbon sp³ structure is expected to provide better resistance to analyte fouling. However, the main factor limiting the use of electrochemical sensors for biological studies is the effect of the biological matrix. Indeed, in vivo or in situ measurements expose the sensor to a complex matrix of proteins, which adsorb on the sensing surface and interfere with the electrochemical measurements.Here, we compare the performance of three carbon based electrodes: GC, GC with low surface oxides and BDD. The redox species ruthenium(III) hexaammine (outer-sphere), ferrocyanide (surface sensitive) and the biologically significant dopamine have been investigated in protein and blood-mimicking matrices. Cyclic voltammetry and electrochemical impedance spectroscopy have been used to examine the effect of spectator molecules and reaction products on electrode mechanisms.Our results show that BDD generally exhibits the best performance for most conditions and reactions and should therefore be preferred for measurements in biologically fouling environments. Furthermore, surface oxides seem also to improve resistance of the GC electrode to biofouling.     

Incorporation of boron and nitrogen in carbon nanomaterials and its influence on their structure and opto-electronical properties

The presence of hexagonal boron nitride in the initial C:BN:Ni:Y₂O₃ mixture in the arc-discharge process leads to the modification of carbon nanostructures and to the creation of B_xC_yN_z entities. BN-incorporation into these carbon nanostructures (single-wall nanotubes and carbon flakes) has been analyzed by high resolution transmission electron microscopy and electron energy loss spectroscopy. The optical absorption spectroscopy measurements have revealed a monotonous increase of the bandgap value of the synthesized nanotubes with the increase of the content of BN phase in the initial mixture of the synthesis compounds. Furthermore, a narrowing of the nanotube diameter distribution in favor of small diameters takes place in the presence of BN during the synthesis process.     

Influence of particle size and glassy carbon content on the thermal,mechanical, and electrical properties of PHBV/glassy carbon composites

Glassy carbon (GC) constitutes a promising carbonaceous material that can be employed as an antistatic agent in the development of antistatic packaging used in the electronics industry. Thus, the present work aims at developing biodegradable and antistatic packaging from poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) reinforced with different GC contents (0, 0.5, 1.0, 2.5, and 5.0 wt%) and different GC particle size (<45 μm and > 75 μm) using extrusion processing. differential scanning calorimetry analysis shows that the addition of GC decreases the degree of crystallinity (X_c) of PHBV, which can explain the reduction of its stiffness. Impedance spectroscopy results reveal that the use of GC contents greater than 2.5 wt% (GC > 75 μm) makes possible the obtainment of a material with an electrical resistivity small enough to be used in antistatic packaging. It can be concluded that GC is an interesting alternative of antistatic agent for electrically dissipative packaging.     

Effects of thermal annealing and Si incorporation on bonding structure and fracture properties of diamond-like carbon films

The effects of thermal annealing and Si incorporation on the structure and properties of diamond-like carbon (DLC) films were investigated. As-deposited DLC film (DLC) and Si incorporated DLC film (Si-DLC), both with and without thermal annealing, were analyzed for bonding structure, residual stress, film thickness, elastic modulus and fracture properties using Raman spectroscopy, wafer curvature, nanoindentation, four-point bend fracture testing, and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy clearly showed that thermal annealing of DLC films promotes more sp² bonding character, whereas Si incorporation into the films promotes more sp³ bonding character. Interfacial fracture energies, film hardness and elastic modulus, and residual film stress were all found to vary strongly with the degree of sp³ bonding in the DLC film. These changes in mechanical properties are rationalized in terms of the degree of three dimensional inter-links within the atomic bond network.     

Frictional properties of diamond-like carbon, glassy carbon and nitrides with femtosecond-laser-induced nanostructure

This paper reports macro and micro frictional properties of DLC, TiN, CrN films and GC substrate of which surfaces are nanostructured with femtosecond (fs) laser pulses. The friction coefficient μ of the nanostructured surface was measured at a usual load with a ball-on-disk friction test machine. The results have shown that carbon materials of DLC and GC provide lower values of μ than TiN and CrN, and μ of DLC and TiN measured with a hardened steel ball decreases with an increase of the laser pulse energy. On the other hand, μ of nanostructured surfaces of thin films monotonously increases with an increase in laser pulse energy, which was measured with a micro-scratch test at an ultralight load of 1.5 mN utilizing a diamond tip. The friction coefficient of the GC substrate irradiated at a low fluence around the ablation threshold has shown a lower value than that of the non-irradiated surface.     

The effect of microwave heating on the microstructure and the mechanical properties of reaction-bonded boron carbide

Reaction-bonded boron carbide composites were fabricated by both microwave (under Ar/10% H₂) and conventional heating (under vacuum or Ar/10% H₂). Silicon carbide (SiC) formation occurred in all cases and was slightly favored in the case of microwave heating under Ar/H₂. The resulting microstructures were influenced by the heating process and atmosphere; the SiC existed in the form of needles with conventional heating under vacuum. SiC small polygonal grains were present after microwave heating under Ar/H₂. Both the atmosphere and the electromagnetic field influence the SiC morphology. Despite this difference, the hardness and toughness of composites obtained by both heating techniques were similar.     

Modification of glassy carbon properties under low energy proton irradiation

Glassy carbon plates were irradiated with 15 keV H⁺ ion-beam in the fluence range of 1 × 10¹⁶–3 × 10¹⁸ ion cm⁻². The influence of ion irradiation on surface morphology and topology was examined by scanning electron and atomic force microscopy. Structural changes were monitored by Raman spectrometry, while changes of wettability and the content of surface oxygen complexes were examined by contact angle measurements and temperature programmed desorption. Elastic recoil detection analysis was applied for determination of hydrogen concentration profiles in irradiated samples. Cyclic voltammetry was used for the assessment of the electrochemical properties of modified glassy carbon electrodes. It was concluded that there is critical fluence range (2 × 10¹⁷–5 × 10¹⁷ ion cm⁻²) inside of which significant changes of glassy carbon properties occur.     

Analysis of the structure of a glassy carbon using the fourier transform technique

The structure of a heat-treated (~3000°C) glassy carbon produced from a polymer of furfural alcohol has been studied by the Fourier inversion of scattering intensities obtained using AgKα₁ radiation. Using Cauchy's distribution, novel theoretical expressions have been developed for the Fourier transforms that take into account the effects of distortion and defects in the lattice in addition to termination effects. The cosine transform of the (002) reflections showed that stacking of layers is extensive but faulty, mean spacing between faults being ~ 21 Å. The mean interlayer spacing was found to be $\text{3.42 ± 0.03}\text{A}\text{?}$, the value of 0.03 Å representing the semi-quartile range of the mean interlayer spacing. A detailed analysis of the Fourier transforms of the (hk) reflections revealed that the sample studied is made up of distorted hexagonal rings. The distortion is severe enough to make it very difficult, if not to prohibit, precise definition of the two-dimensional lattice. Three different lattices (two involving quinoidal rings) have been found to explain the observed transforms equally well. The transform yielded a mean defect-free distance in the layers of ~ 86 Å. Compared to 37 Å indicated by the line-width of the (11) reflections, it is clear that distortion effects outweight the defect or layer size effects in the observed profile of the (11) reflections. In the absence of equations for intensity profiles of random layered lattices that take into account the distortion effects, the Fourier transform procedure outlined in this study constitutes the only method that permits an authentic structural analysis.     

The effect of boron nitride nanosheets on the mechanical and thermal properties of aluminum nitride ceramics

The boron nitride nanosheets (BNNSs)/aluminum nitride (AlN) composites were prepared by hot press sintering at 1600°C. The microstructure, mechanical properties, and thermal conductivity of the samples were measured, and the effect of adding BNNSs to AlN ceramics on the properties was studied. It is found that the addition of BNNSs can effectively improve the mechanical properties of AlN. When the additional amount is 1 wt%, the bending strength of the sample reaches the maximum value of 456.6 MPa, which is 23.1% higher than that of the AlN sample without BNNSs. The fracture toughness of the sample is 4.47 MPa m^1/2, a 68.7% improvement over the sample without BNNSs. The composites obtained in the experiment have brilliant mechanical properties.     

The effect of boron and aluminum additions on the microstructure of arc-melted boron carbide

In this work, B₄C, B₄C + 5 at.% Al, B₄C + B, and B₄C + B + 5 at.% Al were arc melted, and the resultant solid products were characterized. Results from x-ray diffraction and scanning electron microscopy showed that adding Al alone in B₄C did not result in Al doping; adding Al and B in B₄C led to Al doping. Al-doping also changed the surface energy of boron carbide in the liquid state, thus altered the wettability. Transmission electron microscopy revealed that stacking faults are more likely to form in the Al-doped sample, especially in the regions where the Al concentration is high.     

Effect of iodine incorporation on the electrical properties of amorphous conducting carbon films

In this work, the influence of iodine incorporation on the electrical properties of amorphous conducting carbon films, prepared by the vapor phase pyrolysis of maleic anhydride, is reported and discussed. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies reveal structural changes in the system. The anomalous behavior in the electrical properties of the intercalated system at low temperatures is investigated. The system shows a positive temperature coefficient of resistance (TCR) at low temperatures, which suggests reasons for the induced ordering of the system at low temperatures with the iodine incorporation. Also, a systematic increase in the conductivity of the sample is observed. The crossover temperature depends on the disorder in the system. The results indicate the possibility of metal-insulator (M-I) transition as a function of preparation temperature, iodine concentration and magnetic field.     

The effect of pre-carbonization on the properties of PAN-based carbon fibers

A continuous stabilization and two-stage carbonization process was used to prepare polyacrylonitrile (PAN)-based carbon fibers, The effect of pre-carbonization (300 to 550°C) on the final properties and microstructure of carbon fibers was measured. Experimental results using an X-ray diffractometer indicated the presence of a less ordered structure at 2Θ from 5 to 18° in the pre-carbonized fibers and the final carbon fibers. This study found that the pre-carbonization process strongly affects the microstructure of the resulting carbon fibers. The results also showed that a suitable pre-carbonization was very conducive to improvement in tensile strength or in Young's modulus of the final carbon fibers. When the final carbon fiber was pre-carbonized at 300 and 550°C, respectively, these fibers had a higher tensile strength and higher Young's modulus than carbon fibers pre-carbonized at other conditions.     

The effect of lithium-intercalation on the mechanical properties of carbon fibres

Carbon fibres (CFs) can be used as lightweight structural electrodes since they have high specific tensile stiffness and ultimate tensile strength (UTS), and high lithium (Li)-intercalation capability. This paper investigates the relationship between the amount of intercalated Li and the changes induced in the tensile stiffness and UTS of polyacrylonitrile-based CF tows. After a few electrochemical cycles the stiffness was not degraded and independent of the measured capacity. A drop in the UTS of lithiated CFs was only partly recovered during delithiation and clearly larger at the highest measured capacities, but remained less than 40% at full charge. The reversibility of this drop with the C-rate and measured capacity supports that the fibres are not damaged, that some Li is irreversibly trapped in the delithiated CFs and that reversible strains develop in the fibre. However, the drop in the strength does not vary linearly with the measured capacity and the drop in the ultimate tensile strain remains lower than the CF longitudinal expansion at full charge. These results suggest that the loss of strength might relate to the degree of lithiation of defectives areas which govern the tensile failure mode of the CFs.     

Effects of silicon incorporation on composition,structure and electric conductivity of cubic boron nitride thin films

We have achieved in-situ Si incorporation into cubic boron nitride (c-BN) thin films during ion beam assisted deposition. The effects of silicon incorporation on the composition, structure and electric conductivity of c-BN thin films were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and electrical measurements. The results suggest that the content of the cubic phase remains stable on the whole with the incorporation of Si up to a concentration of 3.3 at.%, and the higher Si concentrations lead to a gradual change from c-BN to hexagonal boron nitride. It is found that the introduced Si atoms only replace B atoms and combine with N atoms to form Si–N bonds, and no evidence of the existence of Si–B bonds is observed. The resistance of the Si-doped c-BN films gradually decreases with increasing Si concentration, and the resistivity of the c-BN film with 3.3 at.% Si is lowered by two orders of magnitude as compared to undoped samples.     

Analysis of the structure of a glassy carbon using a profile matching technique

Using equations that take into account the strain in random-layer lattices, calculated intensity profiles expected from completely resonating and quinoidal hexagonal rings have been compared with the experimental data. Completely resonating but distorted rings have been found to be predominant. The mean defect-free distance and the strain coefficient (Cauchy type) obtained from the Fourier transform of the intensities permitted an excellent match between the observed and calculated intensity profiles. It is concluded that glassy carbon is made up of a folded network of layers and the folds induce a considerable strain in the lattice.