Microstructure and mechanical properties of hot-pressed carbon nanotubes compacted by spark plasma sintering

Bulk carbon nanotube samples were prepared by spark plasma sintering. The as-prepared bulk carbon nanotube material exhibited brittle fracture similar to that of common ceramics. Its fracture toughness was around 4.2 MPa m^1/2 while flexural strength was 50 MPa due to the weak bonding between carbon nanotubes. Obvious carbon nanotube bridging was found during the development of the crack induced by an indenter, which provides a possibility of carbon nanotube tough material.     

Silica, carbon and boron nitride monoliths with hierarchical porosity prepared by spark plasma sintering process

Silica SBA-15, carbon CMK-3, boron nitride (BN), the latter synthesized from the first two compounds as templates, are mesoporous materials in the form of powders. They have a high specific surface area and an important mesoporous volume. The porosity is organized with the hexagonal symmetric space group p6mm. For selected applications, it could be interesting to preserve these characteristics with materials in a well-defined shape at a macroscopic scale (few millimeters to centimeter). Spark plasma sintering (SPS) is a well-known technique which allows to prepare monoliths with relatively mild conditions. The SPS technique has been used on these mesoporous powders without charge or with a uniaxial charge and at temperatures of 600 °C, 800 °C for silica, 1100 °C, 1300 °C for carbon and 1600 °C, 1700 °C for boron nitride during 1–5 min. The nitrogen adsorption/desorption isotherms reveal that the obtained monoliths present high specific surface area (300–500 m²/g) and important mesoporous volume. The coexistence of interconnected mesoporosity and macroporosity (with volume’s close value) was observed by SEM and TEM, while the XRD and TEM characterization show that the mesoporosity organization is partially preserved.     

Fabrication and mechanical properties of multi-walled carbon nanotubes doped AlN ceramics prepared by spark plasma sintering

In this study, multi-walled carbon nanotubes (MWCNTs) are uniformly dispersed in aluminium nitride (AlN) powders, and the MWCNTs-doped AlN ceramics are sintered at 1500 °C with a holding time of 5 min by spark plasma sintering using Y₂O₃ as the sintering additive. The effects of the MWCNTs content on the microstructure and mechanical properties of the as-obtained ceramic composites are investigated. The results reveal that many submicron pores are generated when protecting the structure of the CNTs, thereby reducing the density of the AlN ceramic. However, the gradual filling of the grain gap may compensate for the strengthening after CNT doping. The relative density and hardness reach the maximum values of 89.6% of the theoretical density and 7.0 ± 0.3 GPa, respectively, at the doping amount of 2.5 wt%.     

Synthesis and spark plasma sintering of sub-micron HfB2: Effect of various carbon sources

The present work describes a simple process to synthesise HfB₂ powder with sub-micron sized particles. Hafnium chloride and boric acid were used as the elemental sources whilst several carbon sources including sucrose, graphite, carbon black, carbon nanotubes and liquid and powder phenolic resin were used. The carbon sources were characterised using thermogravimetric analysis and transmission electron microscope. The mechanism by which the structure of the carbon source used, affects the size and morphology of the resultant HfB₂ powder was studied; the HfB₂ powders were characterised using X-ray diffraction and scanning and transmission electron microscopy. The powder synthesised using powder phenolic resin had a surface area of 21 m² g⁻¹ and a particle size distribution between 30 and 150 nm. This was sintered using SPS to a relative density of 94% of theoretical density (TD) at 2100 °C and 50 MPa pressure without the help of any sintering aids.     

Synthesis of dense NiZn ferrites by spark plasma sintering

Dense NiZn ferrites were fabricated by spark plasma sintering (SPS) at 900 °C and 20 MPa in short periods. The powder was densified to 98% of the theoretical density by the SPS process. The SPS disks exhibited a higher saturation magnetization (Ms), up to 272 emu/cm³, than did the disks sintered by the conventional process. A higher coercivity (Hci) was obtained when the green bodies were sintered by the SPS process for 5 min. A modest holding time is essential to obtain fine grain and uniformity in the SPS process. Secondary crystallization, inhomogeneous microstructure and intragranular pores were found as a result of the rapid sintering and relatively longer holding time in the SPS process. Infrared (IR) spectra were also measured in the range from 350 to 700 cm⁻¹ to study the efforts of the SPS process on NiZn ferrites.     

Nucleation stability of diamond nanowires inside carbon nanotubes: A thermodynamic approach

Aiming at synthesis diamond nanowires, a simple thermodynamic approach was performed with respect to the effect of nanosize-induced additional pressure on the Gibbs free energy of critical nuclei to elucidate diamond nucleation inside carbon nanotubes upon chemical vapor deposition, based on the carbon thermodynamic equilibrium phase diagram. Notably, these analysis showed that the diamond nucleation would be preferable inside a carbon nanotube due to the effect of surface tension induced by the nanosize curvature of the carbon nanotube and diamond critical nuclei, compared with diamond nucleation on the flat surface of a silicon substrate. Meanwhile, the metastable phase region of diamond nucleation would be driven into a new stable phase region in the carbon thermodynamic equilibrium phase diagram by the effect of nanosize-induced additional pressure. Eventually, we predicted that carbon nanotubes would be an effective path to grow diamond nanowires by chemical vapor deposition.     

Diamond growth by carbon ion implantation of diamond

Medium energy (5–25 keV) ¹³C⁺ ion implantation into diamond (100) to a fluence ranging from 10¹⁶ cm⁻² to 10¹⁸ cm⁻² was performed for the study of diamond growth via the approach of ion beam implantation. The samples were characterized with Rutherford backscattering/channelling spectroscopy, Raman spectroscopy, X-ray photoemission spectroscopy and Auger electron spectroscopy. Extended defects are formed in the cascade collision volume during bombardment at high temperatures. Carbon incorporation indeed induces a volume growth but the diamond (100) samples receiving a fluence of 4 × 10¹⁷ to 2 × 10¹⁸ at. cm⁻² (with a dose rate of 5 × 10¹⁵ at. cm⁻² s⁻¹ at 5 to 25 keV and 800 °C) showed no He-ion channelling. Common to these samples is that the top surface layer of a few nanometers has a substantial amount of graphite which can be removed by chemical etching. The rest of the grown layer is polycrystalline diamond with a very high density of extended defects.     

Transport properties of hot-pressed bulk carbon nanotubes compacted by spark plasma sintering

Electrical and thermal transport properties of the carbon nanotube bulk material compacted by spark plasma sintering have been investigated. The electrical conductivity of the as-prepared sample shows a lnT dependence from 4 to 50 K, after which the conductivity begins to increase approximately linearly with temperature. A magnetic field applied perpendicularly to the sample increases the electrical conductivity in the range of 0-8T at all testing temperatures, indicating that the sample possesses the two-dimensional weak localization at lower temperatures (?50 K), while behaviors like a semimetal at higher temperatures (?50 K). This material acts like a uniform compact consisting of randomly distributed two dimensional graphene layers. For the same material, the thermal conductivity is found to decrease almost linearly with decreasing temperature, similar to that of a single multi-walled carbon nanotube. Magnetic fields applied perpendicularly to the sample cause the thermal conductivity to decrease significantly, but the influence of the magnetic fields becomes weak when temperature increases.     

Shock-induced phase transition of oriented pyrolytic graphite to diamond at pressures up to 15 GPa

In studies of shock-induced phase transition of ordered pyrolytic graphite to a diamond-like phase, the lowest transition onset pressure was observed at 19.6 GPa. The phase transition in that case was considered to be martensitic. In the present study ordered pyrolytic graphite with voids between particles was loaded at pressures up to 15 GPa using a planar shock wave propagating along the basal plane of the graphitic crystal structure. As a result, both diamond-like carbon and diamond were observed in the postshock sample. The phase transition of graphite to diamond was assumed to occur by the release of distortional energy stored in the graphite particles, that is, diffusional-controlled reconstructive mechanism, on the basis of the data by high resolution electron microscopy together with electron energy loss spectroscopy.     

Role of carbon nanotubes on mechanical properties of SiC-B4C-Ni hybrid composites fabricated by reactive spark plasma sintering

The present study was conducted to investigate the microstructural and mechanical properties of SiC-45 vol%B₄C-10 vol%Ni and SiC-45 vol% B₄C-10 vol%Ni-5vol% CNT in-situ composites fabricated through reactive spark plasma sintering (SPS) method at 1650 °C for 5 min. The phase and microstructural characterization of the composites were evaluated utilizing x-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Ultimately, microhardness, flexural strength, and fracture toughness of the composites were measured. Densification behavior of the samples revealed that the initial shrinkage of the composites took place at about 750 °C, which is related to the reaction between Ni and SiC phases. XRD results confirmed the formation of Ni₂Si phase. The relative density of 98.3 ± 0.32% was achieved for the SiC-B₄C-10Ni sample; this value was 97.8 ± 0.62% for the SiC-B₄C-10Ni-5CNT sample. The flexural strength of 362 ± 23 MPa was achieved for the SiC-B₄C-10Ni sample; this value was 369 ± 15 MPa for the SiC-B₄C-10Ni-5CNT sample. The comparison between the fabricated composites concerning the fracture toughness indicated further fracture toughness of the SiC-B₄C-10Ni-5CNT sample (5.82 ± 0.32 MPa m^1/2) than that of the SiC-B₄C-10Ni sample (3.35 ± 0.56 MPa m^1/2). Formation of micro-cracks, crack path deflection, and CNT bridging was the main toughening mechanisms in the SiC-B₄C-10Ni-5CNT sample.     

Onion-like carbon deposition by plasma spraying of nanodiamonds

A deposit of carbon nanoparticles based on an onion-like structure was fabricated from detonation nanodiamond powders by a novel plasma spraying process, electromagnetically accelerated plasma spraying (EMAPS). EMAPS was able to transform nanodiamonds to onion-like structured carbon within 300 μs through a thermal graphitization process in which the temperature of the particles would be in the range of 2700-4500 K. Synthesized onion-like carbon nanoparticles were spherical or polyhedral. The G-band in the UV-Raman spectra of the produced deposits was found to be a superposition of a characteristic band of well-formed carbon onions at 1571 cm⁻¹ and the G-band of defective carbon onions at 1592 cm⁻¹. The availability of a plasma spraying process for developing solid lubricant coatings incorporating nanodiamond and onion-like carbon was demonstrated.     

Induction plasma synthesis of fullerenes and nanotubes using carbon black–nickel particles

The existence of fullerenes (as allotropes of carbon) was established in the mid-1980s and during the last 15–18 years, systematic efforts have been devoted to improve the methods of their synthesis, including plasma-based system methods. The work presented here is focused on the investigation of fullerenes synthesis, using a radio frequency plasma reactor. The main objectives were to explore the use of induction plasma technology for the synthesis in-continuo of carbon fullerenes and to predict their formation conditions through conduct of theoretical studies. Thus, a thermodynamic study was carried out to predict the equilibrium composition of fullerenes produced at several combinations of operating conditions. Additionally, a statistical factorial design experiment, employing four factors at two levels, was also developed, in order to study the influence of the system’s operating parameters on the eventual C₆₀ fullerene yield. The results obtained showed that the reactor pressure, the electrical power and the raw material feed rates all have an important effect on the synthesis of fullerenes. The highest C₆₀ concentration in the products was found to be about 7.7 wt.%. Various other carbon nanostructures, such as nanotubes and nano-onions, were also successfully produced.     

Structural changes in soot particles induced by diode laser irradiation

The effects of laser radiation on soot in a vacuum were studied by Raman spectroscopy and transmission electron microscopy. It was found that defective carbon onions could be formed simply by irradiation of focused common diode lasers with intensity much lower (10³-10⁶ W/cm²) than those used in the previous work. A modified Melton model was introduced to analyze the formation of onions. The results show a threshold value of laser intensity (5.49 × 10³ W/cm²), above which sublimation and rearrangement play a dominant role in the formation of onions. Such onions can also be formed with laser intensities below the threshold since neighboring graphene layers may merge and grow by capturing interstitial carbon atoms. The deposited soot is damage resistant to the diode laser radiation in air due to the rapid formation of carbon onions.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

Carbon nanotube/titanium carbide sol-gel coated zirconium diboride composites prepared by spark plasma sintering

With combination of a powder processing technique and a sol-gel process, carbon nanotube/titanium carbide coated zirconium diboride matrix composite was fabricated. Zirconium diboride (ZrB₂) powders were coated with a functionalized carbon nanotubes (CNTs) mixed titanium carbide (TiC) sol-gel precursor. As the results suggests, the carbothermal reduction produced nanosized TiC grains at the surface of the ZrB₂ particles with a homogenous distribution of CNTs. The densification of the CNT/TiC coated ZrB₂ matrix composite was achieved via 1900?°C spark plasma sintering(SPS). The TiC grains and the CNTs were primarily concentrated in the grain boundaries of the ZrB₂ and showed the pinning effects that restrained the growth of ZrB₂ grain. The TiC grain diffusion in the sintering coarsened the grains from nanosizes to 1–2?µm, which improved the densification of the ZrB₂. Due to the difference in coefficient of thermal expansion, CNTs bridged the gaps between the TiC and the ZrB₂ matrix, which formed a weak-bonding interface. The major toughening mechanism found was crack deflection via the TiC grains on the ZrB₂ matrix.     

Rapid and low temperature spark plasma sintering synthesis of novel carbon nanotube reinforced titanium matrix composites

Multi–walled carbon nanotube (MWCNT) reinforced titanium matrix composites were synthesized using a spark plasma sintering method at a low sintering temperature of 550 °C. The effects of the weight fraction of MWCNTs on the microstructures and the mechanical and thermal properties of the composites were investigated. No reaction products were detected in the composites, indicating that the MWCNTs in the composites maintained their structural integrity after sintering, and thus, because of their advantageous properties, could reinforce the titanium matrix. As a result, the compressive strength of the composite containing 0.4 wt.% MWCNTs reached 1106 MPa, which was an increase of 61.5% compared to that of pure titanium under at the same conditions. In addition, the results revealed that compressive strength of the bulk compacts increased initially and then decreased with an increase in weight fraction of MWCNTs. However, compressive strain of the sintered composites continued to fall at a slow rate. The microhardness and thermal diffusivity of the composites rose steadily with an increasing content of MWCNTs. When the weight fraction of MWCNTs in the composites exceeded 0.8%, the compressive strength of the composites declined significantly due to the increasing aggregation of the MWCNTs.     

The biocompatibility of carbon nanotubes

Carbon nanotubes (CNT) are well-ordered, high aspect ratio allotropes of carbon. The two main variants, single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) both possess a high tensile strength, are ultra-light weight, and have excellent chemical and thermal stability. They also possess semi- and metallic-conductive properties. This startling array of features has led to many proposed applications in the biomedical field, including biosensors, drug and vaccine delivery and the preparation of unique biomaterials such as reinforced and/or conductive polymer nanocomposites. Despite an explosion of research into potential devices and applications, it is only recently that information on toxicity and biocompatibility has become available. This review presents a summary of the performance of existing carbon biomaterials and gives an outline of the emerging field of nanotoxicology, before reviewing the available and often conflicting investigations into the cytotoxicity and biocompatibility of CNT. Finally, future areas of investigation and possible solutions to current problems are proposed.