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Magnetic carbon nanotubes (CNTs)/alumina (Al2O3) nanocomposites were synthesized by the direct growth of CNTs on alumina by chemical vapor deposition (CVD) process. X-ray diffraction studies show that the crystalline structure of alumina nanoparticles remains preserved during the CVD process. Electron microscopy studies reveal the uniform distribution of CNTs within the alumina matrix. CNTs of average diameter 20 nm are spread around alumina particles of size 120 nm. The proportion of CNTs in alumina composites is determined to be approximately 10% from the thermogravimetric analysis. CNT-reinforced alumina nanocomposites demonstrate a significant improvement of the magnetic properties as compared with pristine alumina powder. The diamagnetic property of alumina nanopowder is transformed into a strongly pronounced ferromagnetic response upon the incorporation of CNTs. The saturation magnetization increased by ~ 2500%, and a pronounced coercivity of 68 Oe was observed in the nanocomposites. It is suggested that the encapsulation of cobalt inside the CNTs constrains the magnetic moments to result in higher saturation magnetization. Hence, the CNTs/Al2O3 system synthesized in this work can be employed to engineer novel ceramic composites with enhanced magnetic properties.

Alumina is an essential ceramic insulator, while carbon nanotubes (CNTs) have intriguing mechanical, thermal, electrical, and magnetic properties. We report on the synthesis of CNTs/Al2O3 nanocomposites using the chemical vapor deposition (CVD) process using iron as a catalyst. The as-prepared composite's structure and morphology are characterized by transmission electron microscope, scanning electron microscope, X-ray diffractometer, thermogravimetric analyzer, and Mossbauer spectroscopy. The experimental results show that these composites contain CNTs of an average diameter of 40 nm distributed homogenously within the alumina matrix. The percentage of CNTs within the composites is at 7%. The CNT/Al2O3 nanocomposites demonstrate a dramatic enhancement of ferromagnetic properties during vibrating sample magnetization measurements. As compared to the magnetization of 0.03 e.m.u./g at an applied field of 1.5 T for the Fe-catalyzed Al2O3, the saturation magnetization of CNT/Al2O3 is 1.78 e.m.u./g, which represents an increment of ∼5800%. The coercivity of CNTs/Al2O3 composites increased from 0.036 T at 300 K to 0.285 T at 5 K. This study provides a relatively simple approach to blending the magnetic properties with potential thermoelectric materials.

Applications such as energy storage and field emission devices demand direct growth of vertically aligned carbon nanotube (VACNT) arrays on conducting metal substrates, which has remained a challenge for a long time. Here, we report a controlled growth of VACNT arrays on a stainless steel (SS) substrate via dc plasma enhanced chemical vapor deposition without the addition of a metal catalyst layer. TEM and AFM examinations revealed the occurrence of nano-hills formed on the SS surface during heating process in an NH3 environment. This is a key step for uniform growth of VACNTs. The VACNTs have uniform length and diameter, good alignment, and uniform coverage over the substrate surface, superior to reported results. The particle on the tip of most of the VACNTs was found to be single crystalline Fe metal, although a few other VACNTs were found to have an alloy of Fe, Ni, Mn, and Cr at the tips. This highlights the dominating catalytic behavior of Fe over Ni, Cr, and Mn, which are the elemental composition of the nano-hills. The high quality VACNTs may find unique applications such as electron field emitters and dense nanoscale anode materials for lithium ion batteries.

Graphene, with its unique 2D nanostructure and excellent electrical, thermal, and mechanical properties, is considered an alternative to carbon nanotubes in nanocomposites. In this study, we present a one step approach for the deposition of iron oxide (Fe2O3) nanoparticles onto graphene sheets through solution mixture. The morphology, crystallinity, and magnetic properties of as-synthesized composites were investigated. It was shown that highly crystalline Fe2O3 nanoparticles were densely and uniformly coated on graphene surface. Magnetic measurements reveal that, as compared to weak diamagnetism of pristine graphene, graphene-Fe2O3 nanocomposites display ferromagnetic behavior with coercivity of 101 Oe, saturation magnetization of 12.6 emu g(-1), and remanent magnetization of 3.13 emu g(-1) at room temperature. The enhanced magnetic performance was attributed to the homogeneous dispersion of Fe2O3 nanoparticles in graphene matrix and such nanocomposites are promising materials for applications in magnetic media and energy storage.

The comprehension of fundamental electrochemical behavior and sodiation mechanism is critical for the design of high-performance electrode materials for sodium-ion (Na-ion) batteries. In this paper, the electrochemical sodiation process and microstructure evolution of individual Co9S8-filled carbon nanotubes (CNTs) have been directly visualized and studied using in situ transmission electron microscopy. Upon the first sodiation, a reaction front propagates progressively along the filling nanowire, causing the filled CNT to inflate. The filled CNTs behave differently depending on their structures and the magnitude of the sodiation voltage. For a Co9S8-filled CNT with an open end, the sodiated Co9S8 filler shows a substantial axial elongation of 120.8% and a small radial swelling due to the extrusion of CNT walls. In contrast, the closed CNT shows a major radial expansion of 40.6% and a small axial elongation because of the mechanical confinement of the carbon shells. After sodiation, the spacing between th...

Randomly-oriented carbon nanotubes (CNTs) and vertically-aligned CNTs have been synthesized by a thermal chemical vapor deposition (CVD) process and a plasma enhanced CVD process, respectively, on stainless steel substrates without any external catalyst. Surface topography studies reveal that polishing and chemical etching result in favorable catalytic conditions for nucleation and growth of CNTs. Scanning electron microscopy and transmission electron microscopy observations reveal the growth of CNTs with catalyst particle at the tips. In comparison to randomly-oriented CNTs, vertically-aligned CNTs demonstrate better field emission properties with lower turn-on electric field of ∼2.0 V/μm, lower threshold electric field of ∼3.2 V/μm, and a 2.5-fold increase in the field enhancement factor. The vertical alignment of the emitters benefits the emission process by reducing the screening effect and streamlining the path of ejected electrons directly onto the anode. Vertically-aligned CNTs on conducting substrates are promising emitters in cold cathode vacuum electronics because of their direct contact with the substrate and efficient performance at low operating voltages.

Carbon nanotube (CNT)-encapsulated metal sulfides/oxides are promising candidates for application as anode materials in lithium ion battery (LIB), while their electrochemical behavior and mechanism still remain unclear. A comprehensive understanding of the lithiation mechanism at nanoscale of this type of composites will benefit the design and development of high-performance LIB materials. Here, we use Co9S8/Co nanowire-filled CNTs as a model material to investigate the lithium storage mechanism by in situ transmission electron microscopy. For a Co9S8/Co nanowire-filled closed CNT, the reaction front propagates progressively during lithiation, causing an axial elongation of 4.5% and a radial expansion of 32.4%, while the lithiated nanowire core is still confined inside the CNT. Contrastingly, for an open CNT, the lithiated Co9S8 nanowire shows an axial elongation of 94.2% and is extruded out from the open CNT. In particular, a thin graphite shell is drawn out from the CNT wall by the extruded lithiated Co9S8. The thin graphite shell confines the extruded filler and protects the filler from pulverization in the following lithiation-delithiation cycles. During multiple cycles, the Co segment remains intact while the Co9S8 exhibits a reversible transformation between Co9S8 and Co nanograins. Our observations provide direct electrochemical behavior and mechanism that govern the CNT-based anode performance in LIBs.

We report the synthesis of periodic arrays of carbon nanotubes (CNTs) with different densities on copper substrate by employing nanosphere lithography (NSL) and plasma enhanced chemical vapor deposition. At a growth pressure of 8 torr and temperature of 520 °C, vertically aligned bamboo-like CNTs were formed with a catalyst particle on the tip. Electrical properties of CNTs with different densities were investigated for the possible applications in field emission (FE). The investigation of FE properties reveals a strong dependence on the density of CNTs. Experimental results show that NSL patterned low density CNTs exhibit better field emission properties as compared to the high density CNTs. Low-density CNTs exhibit lower turn-on and threshold electric fields, and a higher field enhancement factor. The high density of CNTs results in the deterioration of the FE properties due to the screening of the electric field by the neighboring CNTs.

We fabricated vertically aligned carbon nanotube (CNT) point emitters directly on Cu wires with flat or sharp tips. The nanostructures and field emission properties of the two kinds of emitters were investigated. Both emitters showed vertically aligned, multi-walled bamboo-liked CNT structures and demonstrated good field emission properties. The emitter on the Cu wire with a flat tip (F-tip) attained an emission current of 70 μA at 4200 V whereas the emitter on the Cu wire with a sharp tip (S-tip) reached an emission current of 65 μA at 3700 V. Both of them demonstrated excellent field emission stability for 40 h at emission currents of about 40 μA and 10 μA for the F-tip and S-tip emitters, respectively.

Vertically aligned carbon nanotube (CNT) array was synthesized directly on stainless steel (SS) substrates with or without chemical etching or catalyst deposition via plasma enhanced chemical vapor deposition method. The effect of chemical etching on the SS substrates resulted in CNTs with uniform diameter and length. Electron field emission properties of CNT arrays synthesized on pristine SS and Ni/Cr coated SS substrates were measured. The results indicated that CNTs synthesized on pristine SS substrate exhibited lower turn-on electric field, lower threshold electric field, and a higher field enhancement factor as compared to the CNTs synthesized on Ni/Cr coated SS. The robust contact between the CNTs and the SS substrate resulted in the improved stability during the field emission.

Carbon nanotubes (CNTs) have become one of the most interesting allotropes of carbon since the discovery of multi-walled carbon nanotubes (MWNTs) by Iijima [1] in 1991. It took almost 2 more years until Iijima and Ichihashi [2] and Bethune et al. [3] synthesized simultaneously single-walled carbon nanotubes (SWNTs). Ever since, steady progress has been made to successfully synthesize vertically and horizontally aligned arrays of CNTs over a wide range of substrates by employing different techniques. CNTs have shown promising mechanical, electrical, optical, and thermal properties, rendering their applications in new structural and functional materials, electrical circuitry, energy storage, drug delivery, and many other devices of the future generation.

Anisotropic diamagnetism of pristine graphite and graphene is well known. Here, evidence of significant induced ferromagnetism in multilayer graphene (MLG) decorated with ferrimagnetic Fe3O4 particles is reported. This MLG-Fe3O4 nano-composite was prepared by a one-step ultrasonic treatment at 75 °C in the surfactant sodium dodecyl-benzene-sulfonate. To verify the phase structure and morphology of the composite, X-ray diffraction, scanning and transmission electron microscopy, scanning tunneling electron microscopy, and Raman spectroscopy were employed. Room temperature data of magnetization versus magnetic field showed that the saturation magnetization MS = 58.6 emu/gm for pristine Fe3O4 increased to MS = 158.4 emu/gm for a 1:1 composite of Fe3O4 to MLG. These results lead to induced MS = 253 emu/gm in MLG resulting from its proximity to Fe3O4. Similar experiments on Fe3O4 to single walled carbon nanotubes (SWNT) composite did not show any induced magnetism in SWNT.

We observed that nanowires of Fe grown in the lumens of multi-walled carbon nanotubes required four times higher magnetic field strength to reach saturation compared to planar nanometric thin films of Fe on MgO(100). Nanowires of Fe and nanometric thin films of Fe both exhibited two fold magnetic symmetries. Structural and magnetic properties of 1-dimensional nanowires and 2-dimensional nanometric films were studied by several magnetometery techniques. The θ-2θ x-ray diffraction measurements showed that a (200) peak of Fe appeared on thin film samples deposited at higher substrate temperatures. In these samples prepared at higher temperatures, lower coercive field and highly pronounced two-fold magnetic symmetry were observed. Our results show that maximum magnetocrystalline anisotropy occurred for sample deposited at 100 °C and it decreased at higher deposition temperatures.