Ya- Li

Silazane derived ceramics and related materials

This review highlights the synthesis, processing and properties of non-oxide silicon-based ceramic materials derived from silazanes and polysilazanes. A comprehensive summary of the preparation of precursor compounds containing Si–N–Si units, including commercially available materials, is followed by the discussion of various processing techniques. The fabrication of dense bulk ceramics in the Si/E/C/N systems is reported which involves cross-linking of the polymeric ceramic precursor followed by a polymer-to-ceramic transformation step. The cross-linked precursor can be milled, compacted and pyrolysed to form dense, additive-free, amorphous silicon carbonitride monoliths or polycrystalline composites which withstand oxidation in air at 1600°C. Furthermore, an overview is given on the fabrication of silazane derived powders and coatings involving chemical vapour deposition (CVD) methods utilising volatile precursors. Fibre spinning and fibre properties, as well as other processing techniques like infiltration of preforms, the preparation of porous ceramics and joining are briefly discussed. A state of the art of the mechanical properties of polymer derived amorphous Si/C/N and Si/B/C/N ceramics with respect to hardness as well as high-temperature creep and oxidation resistance is summarised. Finally, some important aspects of industrial applications will be considered. The review is in part based on our own work related to the polysilazane derived ceramics, but will also cover a comprehensive state of the art including the published literature in this field.

Continuous Multilayered Carbon Nanotube Yarns

2010 WILEY-VCH Verlag Gm Carbon nanotubes (CNTs) have ultrahigh strength, high electrical conductivities, high thermal conductivities, electric field emissions, gas sensitivities, and other functional properties. These outstandingmechanical, physical, andmultifunctional properties of CNTs, in combination with their unique 1D nanostructures with high specific areas, allow for a wide range of potential applications such as structural fibers, composites, multifunctional fabrics, and devices. The fabrication of CNTs into a continuous multifunctional CNT yarn is an important step towards these macroscopic applications. Several processes are under development to fabricate macroscopic CNT fibers, including wet spinning of CNTs from polymer dispersions or acid dispersions, dry spinning from aligned CNT matrices, and direct spinning from chemical vapor deposition (CVD) reactions. While the development of a continuous and weavable pure CNT yarn remains a major challenge in the fabrications, CNT yarns so far obtained from the different processes are monolithic in structure, although a hollow yarn was demonstrated from a wet drawing process. One the other hand, CNT sheets or films have been fabricated by drying CNT dispersions, drawing [19,20] or infiltrating of CNT arrays, and by CVD spinning. These 2D CNT assemblies have demonstrated applications as catalyst supports, molecular sieves, infiltrators, conductors, electromagnetic shields, capacitors, and artificial tissues. If CNTs can be made into continuous yarns with a layered structure, they will combine the weavable property of fibers and the structural characteristic of films and can be adopted for numerous applications. In the present work, we report the fabrication of a novel continuous yarn of CNTs with a multiple-layer structure by the CVD spinning process. The yarn consists of multiple monolayers of CNTs concentrically assembled in seamless tubules along the yarn axis. This layered structure is assembled from CNTs produced in a gas flow from the CVD reaction with a mixed acetone and ethanol carbon source. The development of a water-densification and spinning process allows us to spin the CNT yarn continuously with a yarn length of over several kilometers and a yarn quality close to conventional textile yarns. The CNTyarn can be controlled to be either hollow or monolithic with compacted or detached CNTmonolayers by controlling the spinning process. This layered multifunctional CNT yarns combine superior mechanical properties, electrical conductivities, and surface structures, and have potential applications as structural fibers, composites, woven fabrics, catalyst supports, energy storage materials, artificial tissue, and so on. The fabrication of a CNT yarn by the CVD spinning process relies on the assembly of CNTs in the gas flow by van der Waals interactions. The CNTs assemble in the gas flow when produced in a sufficiently high yield with a high purity such that interaction occurs. Their assembly is, therefore, greatly dependent on the chemistry of the carbon sources. The assembly of CNTs in the gas flow forms a continuous sock-like CNT integrate, which can be mechanically spun out into a CNT yarn. This process was first demonstrated with ethanol as a carbon source and a twisting spinning performed inside the reactor. Recent work reports that a CNT yarn spun from this process possesses ultrahigh strength after densification of the yarn with an acetone vapor. The spinning of multilayered CNT yarns in this work is based on our discovery that CNTs can self-assemble into a multilayered CNT ‘sock’ in the gas flow when a mixture of acetone and ethanol is used as the carbon source. The synthesis was conducted by the injection of the carbon source dispersed with ferrocene and thiophene into a heated gas-flow reactor in flowing hydrogen (Fig. 1a). The CNT layers in the sock, which can be clearly seen in the gas flow, are continuous, concentric, and discrete (Fig. 1b). The CNTsock was initiated from the upper gas flow and produced continuously, traveling downstream with the gases. The assembly of the CNTs is believed to be associated with the interaction of gas molecules, which drive the CNTs towards the outer circumference of the gas flow where they interact mechanically. The formation of the multilayer structure may result from the higher concentration of CNTs in the gas flow because the CNT yield ( 240mg h ) from the mixed carbon source is double that from ethanol alone ( 110mg h ). The layered CNT sock was densified with water after it came out of the reactor. This is realized by connecting a water tank to the end of the CVD reactor (Fig. 1a). The water tank simultaneously encloses the gas-flow system and provides a soft connection between the reactor and air. This configuration allows the CNT assembly to be drawn out continuously from the high-temperature, hydrogen-containing reactor into the open air in a safe and controlled manner. The CNT sock shrinks immediately into a fiber upon arriving at the water surface (Fig. 1c). The fiber is directed around a rotator in the water and is pulled out into air from the other side (Fig. 1d). It is then directed onto the second spool that rotates in acetone for washing and

Electrochemical performance of graphene nanosheets and ceramic composites as anodes for lithium batteries

A nanocomposite anode material for lithium batteries is designed and fabricated by the insertion of graphite nanosheets (GNS) into the ceramic network of silicon oxycarbide (SiOC) ceramics for the development of structurally and electrochemically stable lithium batteries. The GNS forms a layered phase in the SiOC ceramic network from the self-assembly of graphite oxides (GO) introduced in a polysiloxane precursor through thermal transformations after pyrolysis. The composite anode (GNS/SiOC) exhibits an initial discharge capacity attaining 1141 mAh g−1. The discharging capacity decreases in the first eight cycles and stays at 364 mAh g−1 in the following cycles. This reversible discharging capacity is higher than that of a graphite reference (328 mAh g−1) and a SiOC monolithic. Correlating the discharge capacities to the material compositions and structures suggest that the interface between SiOC and GNS contributes to the enhanced capacity of the composite anode, in addition to those from GNS and SiOC. Further increasing the electrochemical performance is possible by the increase of the amount of GNS in the composite.

Synthesis of Monodisperse Spherical Silicon Dicarbodiimide Particles

Spherical particles of silicon dicarbodiimide (SiC2N4, see Figure) are synthesized here via the reaction of tetrachlorosilane (T) and bis (trimethylsilyl)carbodiimide (B) using pyridine as a catalyst and toluene or tetrahydrofuran as solvent. It is shown that the morphology of the thus obtained monodisperse particles strongly depends on the molar ratio of the reagents and the type and amount of solvent used.

Structures and characterizations of bundles of collapsed double-walled carbon nanotubes

The performance of carbon nanotube fibers (CNTFs) significantly depends on the packing styles of carbon nanotube (CNT) bundles. Revealing the structures and characterizations of CNT bundles is contributive to understanding the structures, properties and even the formation of CNTFs during chemical vapor deposition (CVD) processing. In this paper, bundles consisting of collapsed double-walled carbon nanotubes (CDWNT) in continuous CNTFs fabricated from CVD processing were characterized and analyzed by transmission electronic microscopy (TEM) and x-ray diffraction (XRD). TEM observations show that the continuous CNTFs are composed of CDWNT-bundle units. CDWNT-bundle units of 10-20 nm in thickness contain near numbers of collapsed tubes. The degree of collapse of the CDWNTs varies with their location in the bundle and their own diameter. CDWNT-bundle units pack side by side or face to face, assembling into super-bundles with diameters of 200-300 nm. XRD patterns show that three novel and strong peaks appear at 10°-15°, 21.3° and 23.7°, respectively, corresponding to CDWNT two side pores (10°-15°) and CDWNT layers (21.3° and 23.7°), which indicates the collapsed tube structures in CNTFs are common characterizations. Finally, a collapse mechanism is discussed from the observation and analysis.

Precise unzipping of flattened carbon nanotubes to regular graphene nanoribbons by acid cutting along the folded edges

Precise unzipping of carbon nanotubes (CNTs) is achieved using flattened carbon nanotubes (F-CNTs) as the starting material and using acid to cut along the folded edges, as these high energy sites are preferentially attacked to yield regular graphene nanoribbons (GNRs). All-bilayer GNRs with narrow uniform width (8–12 nm) and straight edges are fabricated by unzipping flattened double-wall CNTs (F-DWCNTs) using mixed acids of H2SO4 and KMnO4. Transmission electronic microscopy observations confirm the localized and directional unzipping of the F-DWCNTs and the formation of regular bilayer GNRs. The oxidation temperature and acid concentration control the degree of oxidation, the extent of unzipping, and the exfoliation of F-DWCNTs from their bundles. Under certain conditions, assemblies of interconnected GNRs are formed by unzipping F-DWCNT bundles and partial exfoliation. The precise unzipping of flattened CNTs provides a reliable and scalable process for fabricating regular GNRs with controlled structures and morphologies, as demanded for applications that use them as structural, functional and electronic materials.

Synthesis of highly uniform silica-shelled carbon nanotube coaxial fibers from catalytic gas-flow reactions viain situ deposition of silica

Masses of highly uniform silica-shelled carbon nanotube (CNT) coaxial fibers are synthesized in a single step by the catalytic gas-flow reaction method. This method involves the generation of CNTs in the gas flow from the injection catalytic gas flow reactions and the in situ deposition of silica over the gas dispersed CNTs via the decomposition of a polysiloxane downstream of the CNT flow. Silicone grease consisting of polydimethylsiloxane (PDMS) was used as the precursor for silica. The coaxial fibers produced by this process are highly uniform, with each fiber containing a CNT core enclosed in a uniform silica shell. The growth of silica from PDMS over CNTs is efficient, with ∼50 wt% of PDMS converted into silica. The key controlling factor of the growth of the coaxial fibers is the hydrogen flow, which is required for both the growth of CNTs and the formation of silica from the polysiloxanes. The highly localized deposition of silica over the CNTs with the composition of Si/O in silica close to that of PDMS is likely associated to the condensation from Si–O chains decomposed from PDMS. The silica phase of the coaxial fiber resulting from this process is highly oxygen deficient and, as the result, the coaxial fibers emit strong photoluminescence under ultraviolet excitation. Meanwhile, the silica shell phase is highly hydrogenated, which permits the uniform bonding of nanocrystallines on the fibers. The present process provides an effective means to fabricate high quality silica-CNT coaxial fibers for potential functional applications.

Manipulation of individual double-walled carbon nanotubes packed in a casing shell.

Controlled placement of carbon nanotubes is important for carbon-based nanodevice assembly. However, it is difficult to manipulate individual nanotubes because of their extremely small dimensions. Ultra-fine tubes are often in the form of bundles and are hard to efficiently move on a surface due to the strong adhesion among themselves and between the tubes and the substrate. This paper presents a novel manipulation approach of individual double-walled carbon nanotubes encased in a thick amorphous carbon shell. With an atomic force microscope, we are able to freely displace the nanotubes within a casing shell, and unpack it from the shell on a silicon surface. The theoretical analysis demonstrates that the unpacking process is determined by the difference of the static friction between the shell and the substrate and the resistance force between the shell and the embedded nanotube.