Influence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> buffer layer on catalyst morphology and spinnability of carbon nanotube arrays

Carbon nanotube (CNT) fibers spun from vertically aligned CNT arrays hold great promise in promoting CNT’s practical applications. Their production and properties strongly depend on the spinnability of the arrays. Herein, we study the influence of Al₂O₃ buffer layer on catalyst morphology and the spinnability of CNT arrays. Long and vertically aligned CNT arrays have been obtained from a wide range of Al₂O₃ buffer layer thickness, but the spinnable ones have only derived from a narrow range of the thickness. It is further found that the Al₂O₃ buffer layer can regulate the size and size distribution of the catalyst particles through balancing surface diffusion and inter-layer diffusion. Small, dense, and uniform-distributed nanoparticles are fingerprinted as the optimal catalyst for growing long and spinnable CNT arrays and can be obtained at a proper thickness of buffer layer. By using a tailored tri-layered Fe/Al₂O₃/SiO₂ catalyst, the obtained CNT arrays could reach a height of 500–800 µm and are highly spinnable.     

Synthesis of ultra-long super-aligned double-walled carbon nanotube forests

The pre-treatment (catalyst reduction with H2) time effect on the carbon nanotube (CNT) growth is reported. The total CNT height, the initial growth rate, the diameter, the number of walls, and the alignment in the CNT forests change with the catalyst reduction time. Densely packed, vertically super-aligned, double-walled CNT (DWCNT) forests with 9 mm height were synthesized in 10 hrs. We find that the density and the size of catalysts plays an important role in the alignment of the DWCNT forests, which is evidenced by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy.     

Optimizing reaction condition for synthesizing spinnable carbon nanotube arrays by chemical vapor deposition

Compared with the ordinary vertically aligned carbon nanotube (VACNT) arrays, the carbon nanotubes in spinnable VACNT arrays have better alignment, higher density, and narrower diameter distribution. The synthesis of spinnable VACNT arrays is sensitive to the reaction condition and the repeatable prepared of spinnable VACNT arrays still need improvement. In this paper, spinnable VACNT arrays were grown by chemical vapor deposition from C₂H₂/Ar using Fe coated on Si wafers as a catalyst. With the aim of improving the yield and reproducibility of spinnable VACNT arrays, the reaction conditions were systematically investigated. The growth kinetics of VACNT arrays was also investigated. The rate of growth of VACNT arrays can reach 465 μm/min at the initial growth stage and the activation energy of VACNT array growth is determined to be 112.2 kJ/mol. Meanwhile, a collective growth model for the evolution of spinnable VACNT arrays is also proposed.     

Study on micro-patterning process of vertically aligned carbon nanotubes (VACNTs)

Vertically aligned carbon nanotubes (VACNTs) have drawn significant attention by the researchers because of their nanometric size and favorable material properties. Patterning of CNT forests in the micrometric domain is very important for their application in the area of microelectromechanical system (MEMS). For the first time this paper reports, detailed experimental investigation on a post growth μ-patterning process of VACNT forests. The micromechanical bending (M2B) process was locally applied at the targeted area in order to change the alignment of VACNT forests. Interestingly, the VACNT forest was transformed from typical black body absorber to reflective mirror as the M2B process was applied. Several parameters were identified that govern the resultant patterns such as rotational spindle speed, lateral bending speed, step size, tool morphology, and total depth of bend. Optimization of the parameters was carried out experimentally to obtain the best surface roughness and integrity of the microstructure. A minimum average surface roughness of Ra = 15 nm was achieved with 2000 rpm spindle speed, 1 mm/min bending speed and 1 µm step size.     

柠檬酸-醋酸盐溶胶-凝胶过程制备3Y-TZP纤维的可纺性研究

以ZrOCl2·8H2O为原料,通过柠檬酸盐-醋酸盐溶胶-凝胶法制备了3Y-TZP可纺性溶胶。醋酸/ZrOCl2、柠檬酸/ZrOCl2的物质的量比分别是1.5~2.0和0.3~0.5时,溶胶具有可纺性。用理论计算、IR和流变性分析考察了可纺性溶胶中化合物的类型。结果表明,溶胶中主要是羧酸与锆低聚物形成的桥合双齿配位结构,使溶胶在制备过程中保持稳定并具有可纺性。     

Optimizing control of Fe catalysts for carbon nanotube growth

One must control the size distribution of catalyst Fe nano-particles (NPs) very carefully if one is to have any chance of growing "super-aligned" carbon nanotube (CNT) forests which can be spun directly into yarns and pulled directly into long sheets. Control of the Fe Nps size is important during all phases, including: the catalyst deposition, annealing and forest growth. As a result, it is important to understand how NPs are affected by various experimental factors as well as how those catalyst NPs then cause the growth of the forests. This paper focuses on two key experimental factors: The as-deposited thickness of the Fe catalyst film and the use of hydrogen gas (H2) during anneal and growth. We found that the sheet resistance (Rs) of as-deposited Fe films is directly related to the average film thickness and can be used to estimate whether the films can catalyze the growth of super-aligned forests. The height of the CNT forests decrease with decreasing Rs, but only slowly. More importantly, CNTs grown on the largest and the smallest Rs films are less aligned. Instead, they are more curled and wavy due to the Fe NP dynamics. The use of Hydrogen (H2) affects the formation of Fe NPs from the as-deposited film as well as their composition during the forest growth. We find that the addition of H2 to a CNT forest growth process at 680 degrees C (C2H2/He [30/600 sccm]) increases the CNT alignment substantially. H2 can also reduce iron-oxides which otherwise would impede the formation of NPs. As a result, H2 has multiple roles: besides its chemical reactivity, H2 is important for catalyst reconstruction into NPs having a proper size distribution as well as surface density.     

Mechanical property enhancement of aligned multi-walled carbon nanotube sheets and composites through press-drawing process

A solid-state drawing and winding process was done to create thin aligned carbon nanotube (CNT) sheets from CNT arrays. However, waviness and poor packing of CNTs in the sheets are two main weaknesses restricting their reinforcing efficiency in composites. This report proposes a simple press-drawing technique to reduce wavy CNTs and to enhance dense packing of CNTs in the sheets. Non-pressed and pressed CNT/epoxy composites were developed using prepreg processing with a vacuum-assisted system. Effects of pressing on the mechanical properties of the aligned CNT sheets and CNT/epoxy composites were examined. Pressing with distributed loads of 147, 221, and 294 N/m showed a substantial increase in the tensile strength and the elastic modulus of the aligned CNT sheets and their composites. The CNT sheets under a press load of 221 N/m exhibited the best mechanical properties found in this study. With a press load of 221 N/m, the pressed CNT sheet and its composite, respectively, enhanced the tensile strength by 139.1 and 141.9%, and the elastic modulus by 489 and 77.6% when compared with non-pressed ones. The pressed CNT/epoxy composites achieved high tensile strength (526.2 MPa) and elastic modulus (100.2 GPa). Results show that press-drawing is an important step to produce superior CNT sheets for development of high-performance CNT composites.     

CVD growth of carbon nanotubes on thin-film Ni20Ti35N45 alloy catalyst

The possibility of forming carbon nanotube (CNT) arrays on a Ni–Ti–N catalytic alloy with low nickel content by chemical vapor deposition (CVD) is demonstrated. Adding nitrogen to the Ni–Ti alloy composition favors the formation of TiN compound and segregation of Ni on the surface, where it produces a catalytic effect on the CNT growth. It is found that, using CVD from acetylene gas phase at a substrate temperature of 650°C, a CNT array of 9-µm height can be grown for 2 min.     

Tuning Array Morphology for High‐Strength Carbon‐Nanotube Fibers

Vertically aligned carbon‐nanotube arrays are synthesized by chemical vapor deposition. Carbon‐nanotube fibers are directly spun from the obtained nanotube arrays and then tested mechanically. A strong correlation between the array morphologies and the mechanical properties of the fibers is observed: well‐aligned arrays yield fibers with much higher performance, while wavy and entangled arrays give poor fiber properties. More importantly, such array morphologies could be controlled by introducing hydrogen or oxygen during the nanotube synthesis. By simply switching the growth condition from 150 ppm oxygen addition to 2% hydrogen addition, the nanotube array changes from the wavy morphology to the well‐aligned morphology, and correspondingly the tensile strength of the resultant fibers could be increased by 4.5 times, from 0.29 GPa for the fibers spun from the oxygen‐assistance‐grown nanotube arrays to 1.3 GPa for the fibers spun from the hydrogen‐assistance‐grown nanotube arrays. The detailed effects of hydrogen and oxygen on the nanotube growth, especially on the growth rate and the array spinnability, are extensively studied. The formation mechanism of the different morphologies of the nanotube arrays and the failure mechanism of the nanotube fibers are also discussed in detail.     

Preparation and properties of silicon oxycarbide fibers

Silicon oxycarbide fibers have been prepared from vinyl trimethoxysilane (VTMS) by a modified sol–gel method and with secondary cellulose acetate (SCA) as the fiber-forming aid. Its main advantage over a normal sol–gel fiber processing is that the spinning dope remains spinnable for a long period of time. The effect of the pre-hydrolysis of VTMS on the dope spinnability is studied. At H₂O/VTMS = 4, the resultant sol transforms into gel very quickly, unsuitable to obtain a spinnable dope; at H₂O/VTMS = 2, too much un-reacted VTMS exists in the sol, making the extruded fiber difficult to solidify; at H₂O/VTMS = 3, a dope with good spinnability and stability, and thus high ceramic yield is obtainable. Pyrolysis at 1,000 °C in argon/5% hydrogen results in silicon oxycarbide fibers with the maximum tensile strength (940.0 MPa), moderate Young’s modulus (63.2 GPa) and high carbon content (33.2%).     

High-capacity cathode for lithium-ion battery from LiFePO4/(C + Fe2P) composite nanofibers by electrospinning

Lithium iron phosphate/(carbon and ferrous phosphide) (LiFePO₄/(C + Fe₂P)) composite nanofibers are successfully synthesized by electrospinning and subsequent heat-treatment. We develop a novel aqueous solution with excellent spinnability, which contains dihydrogen phosphate, ferric citrate, and polymer. Heat-treatment temperature makes great effect on the product morphology and the formation of Fe₂P phase. The Fe₂P phase appears when the temperature increases to 750 °C, while too high treatment temperature of 850 °C results in the destruction of fibrous morphology. The LiFePO₄/(C + Fe₂P) composite nanofibers synthesized using the temperature of 750 and 800 °C show high capacity for lithium-ion battery, possibly attributed to the small fiber diameter and good conductivity enhanced by Fe₂P and carbon species.     

Morphological and phase evaluation of Al/15 wt.% BN nanocomposite synthesized by planetary ball mill and sintering

In this study, Al/15 wt.% BN nanocomposite was fabricated by high energy ball milling, cold compaction, and sintering process. Then the effect of milling process on the morphology change and phase evaluation was studied. The Aluminum (45 µm) and hexagonal boron nitride (70 nm) powders were milled using planetary ball mill under a pure argon atmosphere for various times of 75, 150, 300 and 600 min. The as-milled powders were consolidated at 400 MPa pressure and sintered at 600 °C for 60 min. The morphology change evaluation studied by using scanning electron microscopy (SEM) showed that the equiaxed composite powders were obtained after a short time milling process of 150 min. The steady state condition between welding and fracturing occurred by continuous milling up to 600 min. The results of X-ray diffraction patterns and differential scanning calorimetric (DSC) revealed that the BN particles completely decomposed to boron and nitrogen and dissolved in the Al matrix after 300 min of ball milling and the AlN and AlB₂ as in-situ phases were formed after sintering at temperatures above 580 °C.     

Microwave-absorbing properties of silver nanoparticle/carbon nanotube hybrid nanocomposites

Silver (Ag) nanoparticles fabricated by chemical reduction process were grafted onto the surface of carbon nanotubes (CNTs) to prepare hybrid nanocomposites. The Ag/CNT hybrid nanomaterials were characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The Ag/CNT hybrid nanomaterials were then loaded in paraffin wax, and pressed into toroidal shape with thickness of 1 mm to evaluate their complex permittivity and complex permeability by scattering parameters measurement method in reflection mode using vector network analyzer. The reflection loss of the samples was calculated according to the transmission line theory using their measured complex permittivity and permeability. The minimum reflection loss of the Ag/CNT hybrid nanocomposite sample with a thickness of 1 mm reached 21.9 dB (over 99 % absorption) at 12.9 GHz, and also exhibited a wide response bandwidth where the frequency bandwidth of the reflection loss of less than ?10 dB (over 90 % absorption) was from 11.7 to 14.0 GHz. The Ag/CNT hybrid nanocomposite with thickness of 6 mm showed a minimum reflection loss of ~?32.1 dB (over 99.9 % absorption) at 3.0 GHz and was the best absorber when compared with the other samples of different thickness. The reflection loss shifted to lower frequency as the thickness of the samples increased. The capability to modulate the absorption band of these samples to suit various applications in different frequency bands simply by manipulating their thickness indicates that these hybrid nanocomposites could be a promising microwave absorber.     

Height independent compressive modulus of vertically aligned carbon nanotube arrays

The compressive modulus of dense vertically aligned multiwalled carbon nanotube (CNT) arrays synthesized by chemical vapor deposition was investigated using an optically probed precision-loading platform. For CNT arrays with heights ranging from 15 to 500 microm, the moduli were measured to be about 0.25 MPa and were found to be independent of array height. A continuum mechanics model based on multimode buckling guided by the wavy features of CNT arrays is derived and explains well the measured compressive properties. The measured compressive modulus of the CNT arrays also satisfies the "Dahlquist tack criterion" for pressure sensitive adhesives, which was previously observed for these vertically aligned CNT arrays (Zhao, Y., et al. J. Vac. Sci. Technol., B 2006, 24, 331-335).     

Synthesis and characterization of zinc stannate thin films prepared by spray pyrolysis technique

The zinc stannate thin films were synthesized by simple and inexpensive spray pyrolysis technique on the glass and fluorine doped tin oxide coated conducting glass substrates. The as deposited films were further annealed at 500 °C temperature for 12 h. The structural optical and morphological characterization of as prepared and annealed films was carried out by XRD, UV–Vis spectroscopy, SEM and AFM techniques respectively. The structural analysis shows that films are polycrystalline and crystallized in cubic inverse spinel crystal structure. SEM studies show that grain size increases after annealing and exhibits spherical morphology. AFM study shows that roughness is higher for the post annealed film. Further the samples were tested for testing their applicability for dye sensitized solar cells. The as prepared, annealed and CNT doped samples exhibits photoconversion efficiencies 2.7, 2.8 and 3.1 % respectively.     

Preparation of Y-TZP ceramic fibers by electrolysis-sol-gel method

Polycrystalline yttria stabilized tetragonal Zirconia (T-ZrO₂) fibers were obtained by pyrolysis of gel fibers using zirconium oxychloride octahydrate as raw material. The spinnable zirconia sol was prepared by electrolyzing the zirconium oxychloride octahydrate solution in the presence of acetic acid and sugar (sucrose, glucose or fructose), in which the molar ratio of CH₃COOH/ZrOCl₂ · 8H₂O and sugar/ZrOCl₂ · 8H₂O was in the range of 1.0–4.0 and 0.2–0.4, respectively. The relation of spinnability to the shape of colloidal particle was discussed. The as-prepared zirconia fibers sintered at different temperatures show smooth and crack-free surface with the diameter of 5–10 μm. Slow heating rate below 600 °C and then sintering at 1,400 °C for 30 min were necessary to obtain the dense tetragonal zirconia ceramic fibers, the particles composed the fibers had the size of ∼150 nm.     

Fabrication of PtNi bimetallic nanoparticles supported on multi-walled carbon nanotubes

Platinum/nickel bimetallic nanoparticles supported on multi-walled carbon nanotubes (xPtNi/CNTs) were synthesised. The fabrication process includes the chemical modification on the graphene surface of CNTs by acid treatment and the subsequent deposition of Pt or PtNi bimetallic nanoparticles with different compositions of Pt (x = 100, 90, 80 and 70 wt%). The deposition was carried out using ethylene glycol as a reducing agent in the polyol method or using poly(amidoamine) dendrimer as a platform and sodium borohydride as a reducing agent to load the metal nanoparticles on the CNT surface. The structures of the produced PtNi/CNT nanoparticles were investigated by ultraviolet absorption spectra, X-ray diffraction (XRD) and the composite ratio consisting of 70 wt% of metal content and 30 wt% of CNTs was confirmed by the thermogravimetric analysis. The morphology and the phase identification of the produced PtNi/CNT nanoparticles were investigated by high-resolution scanning electron microscope, transmission electron microscope and XRD measurements. It was observed that the deposited Pt and PtNi bimetallic nanoparticles on the surface of CNTs had average particle sizes of 2–16 nm, when they were prepared from the polyol method. On the other hand, the PtNi/CNT nanoparticles prepared by using a dendrimer as an intermediate had a smaller particle size and more uniform size distribution of the quantum dot size ranged from 2 to 4 nm.     

Synthesis and photocatalytic performance of the electrospun Bi2Fe4O9 nanofibers

Bi₂Fe₄O₉ nanofibers were successfully synthesized by an electrospinning method combined with a sol–gel process. The as-spun nanofibers were annealed at different temperatures ranging from 500 to 700 °C and a pure orthorhombic phase was obtained at 700 °C. The thermo-decomposition behavior, structure, morphology, optical property, and the specific surface area of the nanofibers were characterized by thermogravimetry and differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy, UV-vis diffuse reflectance spectroscopy and photoluminescene spectroscopy, and specific surface analyzer, respectively. The results indicated that the diameter and morphology of the fiber changed with different calcination temperatures. Moreover, the results of UV-vis diffuse reflectance spectroscopy revealed that the Bi₂Fe₄O₉ nanofiber could be a photocatalyst under a visible light irradiation and the bandgap value was determined to be 2.1 eV based on the Kubelka–Munk theory. The photocatalytic activity of the obtained nanofibers was evaluated by the degradation of methyl orange. A favorable degradation rate of 45 % was obtained for the sample annealed at 600 °C under the illumination of visible light for 3 h and an enhanced efficiency up to 70 % with recycling stability could be obtained with the aid of H₂O₂ for the pure-phase sample annealed at 700 °C. These results demonstrated that the electrospun Bi₂Fe₄O₉ nanofibers could be a promising visible light photocatalyst.     

Growth and properties of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin films prepared by multiple metallic layer stacks as a function of sulfurization time

In this paper, we report the two stage growth of Cu₂ZnSnS₄ (CZTS) thin films as a function of sulfurization time. First, magnetron sputtered metallic precursors were deposited sequentially (Zn/Cu/Sn/Cu) over rotating glass substrates held at 230?°C. Later, the sputtered precursors were heat treated at 500?°C in the ambiance of sulfur for various time durations in the range, 10–120 min. The sulfur treated samples were examined using various analytical tools to understand the role of sulfurization time on the CZTS growth and properties. From composition and structural analysis, Zn/Cu/Sn/Cu precursors sulfurized for shorter duration (10 and 20 min) revealed severe deficiency of sulfur that resulted in several metallic, bi-metallic and metal sulfide phases. With the increase of sulfurization time to 30 min, sulfur incorporation was enhanced and reached stoichiometric ratio (~50% S) for CZTS growth, however, samples were poorly crystalline in nature and consisted of prominent Cu_2?xS phase as well. The Zn/Cu/Sn/Cu precursors sulfurized for 60 min exhibited prominent CZTS phase without Cu_2?xS phase. Further, rise in sulfurization time to 120 min enabled drastic improvement in crystallinity of CZTS phase. Raman mapping over 60 µm × 60 µm for these films confirmed the homogeneous phase growth of CZTS. XPS study revealed the oxidation states of Cu¹⁺, Zn²⁺, Sn⁴⁺ and S^2? in CZTS films. The optimized films showed high absorption coefficient of 10⁵ cm^?1 with an optical band gap of 1.51 eV. These films showed leaf like grain morphology with high mobility and low resistivity of 18.2 cm²/V-s and 0.7 Ω-cm, respectively.     

Characterizations of contact and sheet resistances of vertically aligned carbon nanotube forests with intrinsic bottom contacts

Comprehensive studies on the sheet and contact resistances of vertically aligned carbon nanotube (CNT) forests with as-grown bottom contacts to the metal layer have been conducted. Using microfabrication and four distinct methods: (1) the transfer length method (TLM), (2) the contact chain method, (3) the Kelvin method, and (4) the four point probe method, we have designed multiple testing devices to characterize the resistances of CNT-forest-based devices. Experimental results show that devices based on stripe-shaped CNT forests 100 μm in height and 100 μm in width have a sheet resistance of approximately [Formula: see text]. The corresponding specific contact resistance to the molybdenum layer is roughly 5 × 10(4) Ω μm(2). Consistency of the results from the four different methods validates the study. After two months of storage of the CNT forest samples in open air, less than 0.9% deviations in the resistance values were observed. We further demonstrated one application of CNT forests as an NH(3) gas sensor and measured 0.5 ppm of sensing resolution with a detection response time of 1 min.