Pulsed electric field induced diamond synthesis from carbon nanotubes with solvent catalysts

Spark plasma sintering (SPS) was used to synthesize diamond from multiwalled carbon nanotubes (MWCNTs) with Fe35Ni powders as solvent catalysts. The MWCNTs/Fe35Ni mixtures were spark plasma sintered at various conditions. The microstructures and diamond phase were analyzed by using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscope techniques. Experimental results showed that the diamond crystals can be synthesized from MWCNTs/Fe35Ni by using the SPS at lower temperature of 1200 °C under very low pressure of 70 MPa. Well-crystallized cubic diamonds consisted of mono-crystals and poly-crystals exhibiting particle sizes ranged from 10 to 40 μm. The Fe35Ni catalysts achieved an effective enhancement for diamond conversion from MWCNTs during the SPS. A model was also proposed to describe the diamond growth and revealed as a layer-by-layer growth mechanism.     

OH and COOH functionalized single walled carbon nanotubes-reinforced alumina ceramic nanocomposites

Alumina ceramics reinforced with 1 wt.% single-walled carbon nanotube (SWCNT) were fabricated via spark plasma sintering (SPS) of composite powders containing carboxyl (COOH) or hydroxyl (OH) group functionalized single-walled carbon nanotubes. The samples were SPS’ed at 1600 °C under 50 MPa pressure for holding time of 5 min and at a heating rate of 4 °C/s. The effects of CNT addition having different surface functional groups on microstructure, conductivity, density and hardness were reported. It was shown that nanotube addition decreased the grain sizeof alumina from 3.17 μm to 2.11 μm for COOH-SWCNT reinforcement and to 2.28 μm for COOH-SWCNT reinforcement. The hardness values of the composites are similar for all samples but there is 4.5 and 7.5 times increase in electrical conductivity with respect to monolithic alumina for COOH-SWCNT and OH-SWCNT, respectively. It was also shown by TEM and FEG SEM observations that transgranular fracture behaviour of alumina was changed to mostly intergranular fracture mode by the addition of both types of CNTs which may be due to location of CNTs along the grain boundaries. A significant grain size reduction in alumina is considered toresult fromthe suppressing effect of CNTs during sintering.     

Fabrication of ultrafine-grained alumina ceramics by two different fast sintering methods

The preparation of ultrafine-grained alumina ceramics by the fast sintering technique Self-propagating High-temperature Synthesis plus Quick Pressing (SHS-QP) method and spark plasma sintering (SPS) technique was reported. The effects of different heating rates (SHS-QP-1600 °C/min, SPS-200 °C/min) on the preparation of ultrafine structure were compared. The densification and grain growth as a function of sintering time and temperature were discussed. Within a short sintering time (<3 min), the full-dense alumina with ultrafine-grained structure was obtained by SHS-QP at 1550 °C under 100 MPa. By SPS, the sintering temperature was lower (1200 °C) than that of SHS-QP. The differences in densification parameters were explained by analyzing the thermodynamics of sintering process.     

Preparation of AlON ceramics via reactive spark plasma sintering

Al₂O₃ and AlN powder mixtures were used to synthesise AlON ceramics using the reactive spark plasma sintering (SPS) method at temperatures between 1400 and 1650 °C for 15-45 min at 40 MPa under N₂ gas flow. AlON phase formation was initiated in the samples sintered above 1430 °C, according to the X-ray analysis. The complete transformation of the initial phases (Al₂O₃ and AlN) into AlON was observed in the samples that were spark plasma sintered at 1650 °C for 30 min at 40 MPa. A high spark plasma sintering temperature together with a low heating rate produced a greater amount of AlON formation at a constant process time. The densification, microstructure and mechanical properties of the produced ceramics were analysed. The highest hardness value was recorded to be 16.7 GPa, and the fracture toughness of the sample with the highest AlON ratio was measured to be 3.95 MPa m^1/2.     

Synthesis of high-purity bulk Ti2AlN by spark plasma sintering (SPS)

Single-phase bulk Ti₂AlN was prepared by spark plasma sintering (SPS) at 1200 °C of Ti/Al/TiN powders in stoichiometric proportion. Investigated by X-ray diffraction (XRD) of samples and the sintering process parameter, the reaction procedure could be analyzed. Scanning electron microscopy (SEM) and electron probe micro-analysis (EPMA) coupled with energy-dispersive spectroscopy (EDS) were utilized to investigate the morphology characteristics. When sintered at 1200 °C, Ti₂AlN phase was well developed with a close and lamellated structure. The distribution of Ti₂AlN grains was homogeneous.     

Studies on ionic conductivity of stabilized zirconia ceramics (8YSZ) densified through conventional and non-conventional sintering methodologies

Densification studies of 8 mol% yttria stabilized zirconia ceramics were carried out by employing the sintering techniques of conventional ramp and hold (CRH), spark plasma sintering (SPS), microwave sintering (MWS) and two-stage sintering (TSS). Sintering parameters were optimized for the above techniques to achieve a sintered density of >99% TD. Microstructure evaluation and grain size analysis indicated substantial variation in grain sizes, ranging from 4.67 μm to 1.16 μm, based on the sintering methodologies employed. Further, sample was also sintered by SPS technique at 1425 °C and grains were intentionally grown to 8.8 μm in order to elucidate the effect of grain size on the ionic conductivity. Impedance spectroscopy was used to determine the grain and grain boundary conductivities of the above specimens in the temperature range of RT to 800 °C. Highest conductivity of 0.134 S/cm was exhibited by SPS sample having an average grain size of 1.16 μm and a decrease in conductivity to 0.104 S/cm was observed for SPS sample with a grain size of 8.8 μm. Ionic conductivity of all other samples sintered vide the techniques of TSS, CRH and MWS samples was found to be ∼0.09 S/cm. Highest conductivity irrespective of the grain size of SPS sintered samples, can be attributed to the low densification temperature of 1325 °C as compared to other sintering techniques which necessitated high temperatures of ∼1500 °C. The exposure to high temperatures while sintering with TSS, CRH and MWS resulted into yttria segregation leading to the depletion of yttria content in fully stabilized zirconia stoichiometry as evidenced by Energy Dispersive Spectroscopy (EDS) studies.     

Temperature and catalyst effects on the production of amorphous carbon nanotubes by a modified arc discharge

Large amounts of amorphous carbon nanotubes (ACNTs) were prepared with Co-Ni alloy powders as catalyst in hydrogen gas atmosphere by a modified arc discharging furnace which can control temperature during the electric arcing process. The experimental results indicate that the cooperative function of temperature and catalyst plays an important role in the soot production rate and the relative ACNT purity. When temperature increases from 25 °C to 700 °C, the soot production rate increases from around 1 g/h to 8 g/h, the best relative ACNT purity at 600 °C can reach up to 99% compared to the room temperature sample. Without catalyst, only plate graphite is formed at 25 °C and very few carbon nanotubes are found when temperature increases to 600 °C. TEM, SEM, HRTEM and XRD analysis showed that the as-prepared carbon nanotubes are almost amorphous. The soot production rate is 8 g/h and diameter range of amorphous carbon nanotubes is about 7-20 nm, respectively.     

Densification and properties of AlN ceramic bonded carbon

To obtain light and tough materials with high thermal conductivity, AlN ceramic bonded carbon (AlN/CBC) composites were fabricated at temperatures from 1600 to 1900 °C in a short period of 5 min by the spark plasma sintering technique. All AlN/CBCs (20 vol% AlN) have unique microstructures containing carbon particles of 15 μm in average size and continuous AlN boundary layers of 0.5-3 μm in thickness. With an increase in sintering temperature, AlN grains grow and anchor into carbon particles, resulting in the formation of a tight bonding layer. The AlN/CBC sintered at 1900 °C exhibited a light weight (2.34 g/cm³), high bending strength (100 MPa), and high thermal conductivity (170 W/mK).     

Thermal conductivity of combustion synthesized β-sialons

To the first time, thermal conductivities of spark plasma sintered β-sialons (Si₃Al₃O₃N₅) procured from combustion synthesis (CS) with no sintering additive were measured by the laser flash method at room temperature. A full densification occurred when these materials were sintered at 1600 °C with a simultaneous pressure of 50 MPa. XRD analyses indicated that single phase β-sialons were formed after SPS though the combustion synthesized β-sialon powders had considerable amounts of silicon impurities. Thermal conductivity values increased with sintering temperature and attained a maximum of 5.49 W m⁻¹ K⁻¹ for fully densified β-sialons sintered at 1700 °C for 10 min.     

Densification behavior and related phenomena of spark plasma sintered boron carbide

In this work, boron carbide ceramics were sintered in the temperature range of 1400–1600 °C by spark plasma sintering (SPS). The influence of sintering temperature, heating rate, and holding time on the microstructure, densification process and physical property was studied. The heating rate was found to have greater influence than that of the holding time on the microstructure and the densification of boron carbide. The optimal sintering temperature was 1600 °C under the heating rate higher than 100 °C/min. The relative density, flexural strength, Vickers hardness and fracture toughness of the sample synthesized at 1600 °C were 98.33%, 828 MPa, 31 GPa and 2.66±0.29 MPa m^1/2, respectively. The densification mechanism was also investigated.     

The effect of feedstock and process conditions on the synthesis of high purity CNTs from aromatic hydrocarbons

High purity multi-walled carbon nanotubes were synthesized from aromatic hydrocarbons (benzene, toluene, xylene and trimethyl benzene) using ferrocene as the source of Fe catalyst. Screening studies of aromatic feeds at 675 °C, residence time of 14 s and Fe/C atom ratio of 1.07%, resulted in feedstock carbon conversion of 20-31%, CNT yield of 19.8-30.5%, and catalyst yield of 5.3-8.3 (g CNT/g catalyst). While the quality of the CNTs as determined by TGA, SEM, TEM and Raman spectroscopy, were high and comparable for different feedstocks; their carbon conversion, CNT yield and catalyst yield differed noticeably. A process optimization study for toluene feed showed that carbon conversion of more than 39%, CNT yield of 38.7% and catalyst yield of 18.3 can be achieved at temperature of 800 °C, Fe/C atom ratio of 0.47%, and residence time of 10-20 s.     

Effect of 1 wt% LiF additive on the densification of nanocrystalline Y2O3 ceramics by spark plasma sintering

Densification of nanocrystalline cubic yttria (nc-Y₂O₃) powder, with 18 nm crystal size and 1 wt% LiF as a sintering additive was investigated. Specimens were fabricated by spark plasma sintering at 100 MPa, within the temperature range of 700-1500 °C. Sintering at 700 °C for 5 and 20 min resulted in 95% and 99.7% dense specimens, with an average grain size of 84 and 130 nm, respectively. nc-Y₂O₃ without additive was only 65% dense at 700 °C for 5 min. The presence of LiF at low sintering temperatures facilitated rapid densification by particle sliding and jamming release. Sintering at high temperatures resulted in segregation of LiF to the grain boundaries and its entrapment as globular phase within the fast growing Y₂O₃ grains. The sintering enhancement advantage of LiF was lost at high SPS temperatures.     

Carbon nanotubes and onions from carbon monoxide using Ni(acac)2 and Cu(acac)2 as catalyst precursors

New catalyst precursors (copper and nickel acetylacetonates) have been used successfully for the synthesis of carbon nanotubes and onion particles from carbon monoxide. Catalyst nanoparticles and carbon products were produced by metal-organic precursor vapour decomposition and catalytic disproportionation of carbon monoxide in a laminar flow reactor at temperatures between 705 and 1216 °C. Carbon nanotubes (CNTs) were formed in the presence of nickel particles at 923-1216 °C. The CNTs were single-walled, 1-3 nm in diameter and up to 90 nm long. Hollow carbon onion particles (COPs) were produced in the presence of copper particles at 1216 °C. The COPs were from 5 to 30 nm in diameter and consisted of several concentric carbon layers surrounding a hollow core. The results of computational fluid dynamics calculations to determine the temperature and velocity profiles and mixing conditions of the species in the reactor are presented. The mechanisms for the formation of both CNTs and COPs are discussed on the basis of the experimental and computational results.     

Fabrication and characterization of sintered TiC-SiC composites

Some TiC-SiC composites with different SiC volume contents (0, 10, 25 and 50%) are prepared by spark plasma sintering (SPS). The relationship between density, grain growth and temperature is studied in order to fabricate dense and nano-sized TiC-SiC composites. A sintering by SPS at 1800 °C during 5 min allowed to form TiC-SiC composites with relative density above 95% and an homogeneous distribution of TiC (grain size from 270 to 900 μm) and nano-sized SiC. With the increasing of SiC volume contents, Vickers hardness and fracture toughness are improved; thermal conductivity at room temperature is increased whereas at high temperature it is reduced. In future studies, those materials will be irradiated to characterize monolithic TiC and TiC-SiC composites behaviour under irradiation.     

Phase analysis and wear behavior of in-situ spark plasma sintered Ti3SiC2

In-situ synthesis of dense near-single phase Ti₃SiC₂ ceramics from 3Ti/SiC/C/0.15Al starting powder using spark plasma sintering (SPS) at 1250 °C is reported. Systematic analysis of the phase development over a range of sintering temperatures (1050–1450 °C) suggested that solid state reactions between intermediate TiC and Ti₅Si₃ phases lead to the formations of Ti₃SiC₂. The effect of starting powder composition on phase development after SPS at 1150 °C was also investigated using three distinct compositions (3Ti/SiC/C, 2Ti/SiC/TiC, and Ti/Si/2TiC). The results indicate that the starting powder compositions, with higher amounts of intermediate phase such as TiC, favor the formation of Ti₃SiC₂ at relatively lower sintering temperature. Detailed analysis of wear behavior indicated that samples with higher percentage of TiC, present either as an intermediate phase or a product of Ti₃SiC₂ decomposition, exhibited higher microhardness and better wear resistance compared to near single phase Ti₃SiC₂.     

Controlling the diameter distribution of carbon nanotubes grown from camphor on a zeolite support

Single-wall and multi-wall carbon nanotubes (SWNTs and MWNTs, respectively) of controlled diameter distribution were selectively grown by thermal decomposition of a botanical hydrocarbon, camphor, on a high-silica zeolite support impregnated with Fe-Co catalyst. Effects of catalyst concentration, growth temperature and camphor vapor pressure were investigated in wide ranges, and diameter distribution statistics of as-grown nanotubes was analyzed. High yields of metal-free MWNTs of fairly uniform diameter (∼10 nm) were grown at 600-700 °C, whereas significant amounts (∼30%) of SWNTs were formed at 850-900 °C within a narrow diameter range of 0.86-1.23 nm. Transmission electron microscopy and micro-Raman spectroscopy reveal that camphor-grown nanotubes are highly graphitized as compared to those grown from conventional CNT precursors used in chemical vapor deposition.     

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.     

Online BET analysis of single-wall carbon nanotube growth and its effect on catalyst reactivation

A system combining pulse chemical vapor deposition (CVD) reaction and cryogenic gas sorption (BET) measurement capabilities was designed to allow the sequential synthesis and online analysis of single-wall carbon nanotubes (SWNTs). Cooling treatment in liquid nitrogen (77 K) during BET measurement was found to be efficient for restoring catalysts when deactivation occurs after carbon deposition. By this treatment, the methane conversion could be enhanced by up to seven times, such as from 5.8% to 42.6% mol. When the temperature changes from 850 °C to 77 K, the metal particles on the tip of nanotubes might contract and be separated from the graphite layer of the nanotubes, leading to more active sites on metal particles being exposed. The single point BET analysis of SWNT has been tested as an efficient method for the rapid online analysis of SWNTs produced by CVD.     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

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.