Bamboo-like Structure Research Articles

Periodic disruptions induced by high repetition rate femtosecond pulses on magnesium-oxide-doped lithium niobate surfaces

In this paper, we demonstrate the periodic disruption formation on magnesium-oxide-doped lithium niobate surfaces by a femtosecond fiber laser system with wavelength and repetition rate of 1040 nm and 52 MHz, respectively. Three main experimental conditions, laser average power, scanning speed, and orientation of sample were systematically studied. In particular, the ablation morphologies of periodic disruptions under different crystal orientations were specifically researched. The result shows that such disruptions consisting of a bamboo-like inner structure appears periodically for focusing on the surface of X-, Y- and Z-cut wafers, which are formed by a rapid quenching of the material. Meanwhile, due to the anisotropic property, the bamboo-like inner structures consist of a cavity only arise from X- and Z-cut orientation.

Fabrication of Ti Porous body with Improved Specific Surface Area by Synthesis of CNTs

이 논문은 2015년도 정부 (미래창조과학부)의 재원으로 한국 연구 재단의 지원을 받아 수행된 연구임(No.2012R1A2A2A02046179). 본 연구는 2015년도 산업통상자원부의 재원으로 한국에너지기술평가원(KETEP)의 지원을 받아 수행한 연구 과제입니다(No. 20154030200900).

Controlling the Formation of Nanocavities in Kirkendall Nanoobjects through Sequential Thermal Ex Situ Oxidation and In Situ Reduction Reactions.

Controlling the porosity, the shape, and the morphology of Kirkendall hollow nanostructures is the key factor to tune the properties of these tailor-made nanomaterials which allow in turn broadening their applications. It is shown that by applying a continuous oxidation to copper nanowires following a temperature ramp protocol, one can synthesize cuprous oxide nanotubes containing periodic copper nanoparticles. A further oxidation of such nanoobjects allows obtaining cupric oxide nanotubes with a bamboo-like structure. On the other hand, by applying a sequential oxidation and reduction reactions to copper nanowires, one can synthesize hollow nanoobjects with complex shapes and morphologies that cannot be obtained using the Kirkendall effect alone, such as necklace-like cuprous oxide nanotubes, periodic solid copper nanoparticles or hollow cuprous oxide nanospheres interconnected with single crystal cuprous oxide nanorods, and aligned and periodic hollow nanospheres embedded in a cuprous oxide nanotube. The strategy demonstrated in this study opens new avenues for the engineering of hollow nanostructures with potential applications in gas sensing, catalysis, and energy storage.

Synthesis of boron nitride nanotube films with a nanoparticle catalyst

Abstract Boron nitride nanotube (BNNT) films were synthesized by combining ball milling and thermal chemical vapor deposition (CVD) using nano-Fe 3 O 4 as a catalyst. The as-produced BNNTs have a bamboo-like structure and have a diameter in the range of 50∼200 nm with an average length of more than 40 μm. Moreover, BNNT nanojunction structures were synthesized. The structure and morphology of the BNNTs were characterized by XRD, SEM, TEM and HRTEM. The possible growth mechanism of BNNTs and BNNT nanojunction structures were proposed. Though the BNNT films were observed, out of our expectation, BNNTs with thin tube wall and small average diameter have not been achieved, and this could be mainly ascribed to the aggregation of the nanoparticle catalyst, resulting in greater catalyst particles during the process of BNNT growth. This result will provide a promising approach to obtain the desired shape of BNNTs and produce branched junctions of BNNTs.

Nitrogen-doped carbon nanofibers on expanded graphite as oxygen reduction electrocatalysts

A single-step chemical vapor deposition method using simple gaseous precursors was employed to grow nitrogen-doped carbon nanofibers from Fe and Ni particles on the surface of expanded graphite (N-CNF/EG). Due to the high electronic conductivity of the expanded graphite the N-CNF/EG could be used as electrocatalysts without the need for harsh purification procedures. Electrochemical testing showed that the N-CNFs grown from Fe exhibited a notable activity for the oxygen reduction in both acidic and alkaline electrolyte, in addition to demonstrating a high durability with a well-preserved catalytic activity after 1600 cycles in O2-saturated 0.5 M H2SO4. Physicochemical characterization revealed the formation of N-CNFs with a bamboo-like structure, encapsulated Fe particles and high pyridinic nitrogen content. The combination of high ORR-activity, an easily scalable synthesis approach and a highly conductive support material makes N-CNF/EG a promising oxygen reduction catalyst for low temperature fuel cells.

Fabrication of High Strength and Ductile Stainless Steel Fiber Felts by Sintering

Stainless steel fiber felts are important porous stainless steel products for a variety of industry applications. A systematic study of the sintering of 28-µm stainless steel fibers has been conducted for the first time, assisted with synchrotron radiation experiments to understand the evolution of the sintered joints. The critical sintering conditions for the formation of bamboo-like grain structures in the fiber ligaments were identified. The evolution of the number density of the sintered joints and the average sintered neck radius during sintering was assessed based on synchrotron radiation experiments. The optimum sintering condition for the fabrication of high strength and ductile 28-µm-diameter stainless steel fiber felts was determined to be sintering at 1000°C for 900 s. Sintering under this optimum condition increased the tensile strength of the as-sintered stainless steel fiber felts by 50% compared to conventional sintering (1200°C for 7200 s), in addition to much reduced sintering cycle and energy consumption.

Cyclic degradation in bamboo-like Fe–Mn–Al–Ni shape memory alloys — The role of grain orientation

In the present study the cyclic deformation behavior within differently oriented grains in Fe–34.8Mn–13.5Al–7.4Ni (at.%) shape memory polycrystals featuring a bamboo-like structure was investigated. In cyclic tensile tests up to 50cycles, the degree of degradation in pseudoelasticity was evaluated and contributing elementary mechanisms are discussed. The results reveal rapid cyclic degradation in the bamboo-like samples. The unexpected stabilization of parent phase in reverse transformed areas and the proceeding activation of new martensite variants in subsequent cycles were found to be the prevailing degradation mechanisms. Dislocation activity is found to be the most detrimental factor.

Bamboo‐Like Nitrogen‐Doped Carbon Nanotubes with Co Nanoparticles Encapsulated at the Tips: Uniform and Large‐Scale Synthesis and High‐Performance Electrocatalysts for Oxygen Reduction

In recent years, various non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) have been extensively investigated. The development of an efficient and simple method to synthesize non-precious metal catalysts with ORR activity superior to that of Pt is extremely significant for large-scale applications of fuel cells. Here, we develop a facile, low-cost, and large-scale synthesis method for uniform nitrogen-doped (N-doped) bamboo-like CNTs (NBCNT) with Co nanoparticles encapsulated at the tips by annealing a mixture of cobalt acetate and melamine. The uniform NBCNT shows better ORR catalytic activity and higher stability in alkaline solutions as compared with commercial Pt/C and comparable catalytic activity to Pt/C in acidic media. NBCNTs exhibit outstanding ORR catalytic activity due to high defect density, uniform bamboo-like structure, and the synergistic effect between the Co nanoparticles and protective graphitic layers. This facile method to synthesize catalysts, which is amenable to the large-scale commercialization of fuel cells, will open a new avenue for the development of low-cost and high-performance ORR catalysts to replace Pt-based catalysts for applications in energy conversion.

Enhanced Fischer–Tropsch synthesis performance of iron-based catalysts supported on nitric acid treated N-doped CNTs

Iron-based catalysts supported on N-doped CNTs (NCNTs) treated by various concentrations of nitric acid for Fischer–Tropsch synthesis (FTS) were investigated. An improved catalytic performance for the iron catalyst supported on acid treated NCNTs was obtained and the suitable nitric acid concentration was 10M. The physiochemical properties of the NCNTs and the corresponding catalysts were characterized by BET, TEM, XRD, XPS, TGA and H2-TPR. The acid treatment removed the impurity and amorphous carbon, damaged the bamboo-like structure and increased the number of oxygen-containing functional groups and graphitization degree on the NCNTs. The more iron particles located inside the channels of NCNTs, the better catalytic FTS performance due to high dispersion and reducibility.

Morphological control of N-doped carbon nanotubes and their electrochemical properties

The N-doped carbon nanotubes (CNTs) with different microstructures have been fabricated via the pyrolysis of melamine catalyzed by iron chloride. The morphologies of N-doped CNTs were tailored by the molar ratio of melamine and iron chloride and calcined temperature. The pure CNTs with bamboo-like structure are formed at 700°C when the molar ratio of melamine and iron chloride is 4, and its BET surface and specific capacitance are higher than that of samples calcined at 600°C or 800°C. The BET surface and capacitance of samples increase with increasing the molar ratio of melamine and iron chloride, and the maximum BET surface area and specific capacitance are 298m2g−1 and 67Fg−1, respectively.

Vertically aligned N-doped CNTs growth using Taguchi experimental design

The Taguchi method with a parameter design L9 orthogonal array was implemented for optimizing the nitrogen incorporation in the structure of vertically aligned N-doped CNTs grown by thermal chemical deposition (TCVD). The maximization of the ID/IG ratio of the Raman spectra was selected as the target value. As a result, the optimal deposition configuration was NH3=90sccm, growth temperature=825°C and catalyst pretreatment time of 2min, the first parameter having the main effect on nitrogen incorporation. A confirmation experiment with these values was performed, ratifying the predicted ID/IG ratio of 1.42. Scanning electron microscopy (SEM) characterization revealed a uniform completely vertically aligned array of multiwalled CNTs which individually exhibit a bamboo-like structure, consisting of periodically curved graphitic layers, as depicted by high resolution transmission electron microscopy (HRTEM). The X-ray photoelectron spectroscopy (XPS) results indicated a 2.00at.% of N incorporation in the CNTs in pyridine-like and graphite-like, as the predominant species.

AlSi2P nanotubes: a theoretical study

The geometry, stability, and structural properties of single-walled AlSi2P nanotubes were investigated by the first-principles density functional calculations. Considering different arrangements of AlP and SiSi units, three types of zigzag (ZZ-1, ZZ-2, and ZZ-3) (n, 0), n = 3, 4, and 5, and two types of armchair (AC-1 and AC-2) (n, n) AlSi2P nanotubes, n = 4, 5, and 6, were constructed. The present calculations show that, in general, there is a tendency toward the formation of alternate –Si–Si– and –Al–P– chains in the most stable AlSi2P nanotubes. Due to the high tendency of P and Si atoms to adopt sp 3 hybridization, silicon and phosphorus atoms somewhat pop outward, while Al atoms prefer stable sp 2 hybridization and form planar structure on the tube surface. Such structural features in the optimized geometry of the AlSi2P nanotubes lead to amazing structures such as the periodic bamboo-like structure of the ZZ-1(n, 0) AlSi2P nanotubes and “polygonal” cross section in the ZZ-3(n, 0) and AC-1(n, n) AlSi2P nanotubes. It is expected that the high point charges calculated on Al atoms, ranging from +0.839 to +1.021, and P atoms, ranging from −0.561 to −0.949, establish an overall charge transfer on the surface of nanotubes. AlSi2P nanotubes

Damage evolution in pseudoelastic polycrystalline Co–Ni–Ga high-temperature shape memory alloys

Due to its transformation behavior, Co–Ni–Ga represents a very promising high temperature shape memory alloy (HT SMA) for applications at elevated temperatures. Co–Ni–Ga single crystals show a fully reversible pseudoelastic shape change up to temperatures of 400°C. Unfortunately, polycrystalline Co–Ni–Ga suffers from brittleness and early fracture mainly due to intergranular constraints. In the current study, different thermo-mechanical processing routes produced various microstructures which differ in grain size and texture. A bicrystalline bamboo-like grain structure results in the highest reversible transformation strains and excellent cyclic stability. Moreover, solution-annealed and hot-rolled conditions also showed cyclic stability. Using in situ high-resolution electron microscopy, the elementary processes, which govern the microstructural evolution during pseudoelastic cycling were investigated and the mechanisms that govern structural and functional degradation were identified. The observations documented in the present work suggest that the formation of the ductile γ-phase on and near grain boundaries as well as the activation of multiple martensite variants at grain boundaries are beneficial for improved cyclic performance of polycrystalline Co–Ni–Ga HT SMAs.

Bamboo shaped carbon nanotube supported platinum electrocatalyst synthesized by high power ultrasonic-assisted impregnation method for methanol electrooxidation and related density functional theory calculations

Carbon nanotubes (CNTs) with special forms may exhibit novel properties, which are different compared to conventional carbon nanotubes and may open a way for future applications. The nanotubes with a bamboo-like structure due to nitrogen doping have been well studied because of the increased attention for their bamboo shaped structure associated properties. In this work, bamboo shaped carbon nanotubes (BCNTs), synthesized by chemical vapor deposition (CVD) method, were investigated for direct methanol fuel cells (DMFC). Small sized platinum nanoparticles (Pt NPs) were dispersed onto BCNT surface through a high power ultrasonic-assisted impregnation method in a short time. The structure and nature of the resulting Pt NP coated BCNT composites (Pt/BCNTs) were characterized by Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Energy dispersive spectroscopy (EDS). Density functional theory (DFT) calculations of Pt NPs coated on different structural CNTs showed that inclusion of nitrogen improved the catalytic performance of carbon nanotubes toward methanol electrooxidation. High electrochemical activity of BCNTs supported Pt NPs was demonstrated by the electrochemical studies.

Structural, Magnetic and Mechanical Properties of Oligocrystalline Ni-Mn-Ga Shape Memory Microwires

In single-crystalline Ni-Mn-Ga alloys, high reversible strain (∼10%) induced by an external magnetic field due to twin variants reorientation can be achieved. However, twin boundary motion is hindered by grain boundaries in polycrystalline Ni-Mn-Ga alloys, resulting in a low strain output. In this work, oligocrystalline Ni-Mn-Ga microwires with near bamboo-like grain structure (1-3 grains across the wire diameter of 30-80μm) were prepared by melt-extraction technique and subsequent annealing treatment. Single twin variant, with twin lamella thickness up to 6μm, spans across the entire diameter of the microwire. The oligocrystalline microwires are of ferromagnetic nature at ambient temperature. Twining boundary motion was demonstrated by tensile test performed on a Q800 DMA where different twining stresses corresponding to various grain orientations in a single microwire were detected. A maximum strain of ∼1.57% was obtained and related to the orientation of a subset grains. The martensite twin variant orientations after tensile test were observed using a SEM.

Relationship between grain size and ductile-to-brittle transition at room temperature in Fe–18Mn–0.6C–1.5Si twinning-induced plasticity steel

The effect of grain size on tensile properties, particularly elongation (El), was systematically investigated using cold-rolled and annealed Fe–18Mn–0.6C–1.5Si (wt.%) twinning-induced plasticity (TWIP) steels. Whereas both the yield (YS) and ultimate tensile strengths (UTS) decreased with grain coarsening, the El increased up to a grain size of approximately 18μm, and then sharply decreased beyond the critical grain size. This unusual reduction in El in coarse-grained specimens was caused by the transition of the fracture mode from ductile to brittle. The transition in fracture stemmed from a change in the deformation mechanism from mechanical twinning to strain-induced ε-martensitic transformation. The ε plates provided nucleation sites and propagation paths for cracks, causing quasi-cleavage brittle fracture in coarse-grained specimens. The cracks growing along the {101¯0} or (0001) plane were bent by ε plates with different plane orientations and were blocked by γ-austenite plates. When the local stress in the γ plate near the crack tip was high, new ε-martensitic plates formed within the γ plate, enabling the crack to pass through the γ plate, and changed the crack path. The quasi-cleavage fractured surface consisted of (101¯1), (101¯2), and (21¯1¯1) planes. Ridges of bamboo-like structures observed on the fractured surface formed by the variation in crack path occurring at the ε plates with different orientations. The Si-added TWIP steel exhibited a superior combination of UTS and El of more than 70,000MPa %.

Synthesis of highly crystalline multilayers structures of 10BNNTs as a potential neutron sensing element

Highly crystalline multilayers structures of 10B-enriched boron nitride nanotubes (10BNNTs) are synthesized via a simple technique at 1200°C. Field emission scanning electron microscopy images show randomly aligned 10BNNTs with some cotton like morphologies. Transmission electron microscopy indicates highly crystalline nature of the BNNTs with internal bamboo-like structures. X-ray photon spectroscopy spectrum confirms Boron and Nitrogen elemental components of 10BNNTs, whereas Raman spectroscopy reports a peak at 1390(cm−1) that relates to E2g mode of 10BNNTs. The synthesized 10BNNTs can effectively be used as a neutron sensing element in a solid state neutron detector with a 100% suggested efficiency.

Preparation and Characterization of SiC/SiC Micro Ceramic Matrix Composites

SiC continuous fiber reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) and SiC fiber reinforced SiC micro ceramic matrix composites (SiCf/SiC micro-CMCs) were prepared through polymer impregnation pyrolysis (PIP) progress, then the structures, mechanical properties and forming mechanisms of SiCf/SiC CMCs were investigated. Results showed that volume shrinkage of SiC matrix occurred, and pyrolysates with bamboo-like layered structure were attached to the surface of SiC multifilament gradually. Furthermore, the mechanical properties of SiCf/SiC micro-CMCs were consistent with SiCf/SiC CMCs, so the PIP process parameters for the preparation of SiCf/SiC CMC might be optimized primarily through SiCf/SiC micro-CMCs, which could enhance the design and preparation efficiency of SiCf/SiC CMCs obviously.Key word: polymer impregnation pyrolysis progress, micro ceramic matrix composites, forming mechanism

촉매 화학기상증착 공정에서 온도구배 설정을 통한 타이타늄 기판에서의 CNT 성장 거동

In this study, modified catalytic chemical vapor deposition (CCVD) method was applied to control the CNTs (carbon nanotubes) growth. Since titanium (Ti) substrate and iron (Fe) catalysts react one another and form a new phase (<TEX>$Fe_2TiO_5$</TEX>) above <TEX>$700^{\circ}C$</TEX>, the decrease of CNT yield above <TEX>$800^{\circ}C$</TEX> where methane gas decomposes is inevitable under common CCVD method. Therefore, we synthesized CNTs on the Ti substrate by dividing the tube furnace into two sections (left and right) and heating them to different temperatures each. The reactant gas flew through from the end of the right tube furnace while the Ti substrate was placed in the center of the left tube furnace. When the CNT growth temperature was set <TEX>$700/950^{\circ}C$</TEX> (left/right), CNTs with high yield were observed. Also, by examining the micro-structure of CNTs of <TEX>$700/950^{\circ}C$</TEX>, it was confirmed that CNTs show the bamboo-like structure.

Synthesis of microcellular polypyrrole in a unidirectional freeze-dried polystyrene template and the conversion to microcellular carbon via morphology-retaining carbonization

The facile fabrication of polypyrrole (PPy) foams via vapor phase polymerization using microcellular polystyrene, prepared by unidirectional freeze-drying, as a template was demonstrated. Remarkably, the PPy foams maintained the porous morphology of the template, which exhibited a bamboo-like honeycomb channel structure. In addition, the PPy foams were carbonized at 800°C with iodine doping and the carbon products showed similar morphologies to that of the precursor. The carbon foams were characterized by Raman spectroscopy and XRD analysis and were confirmed to be amorphous carbon with a nitrogen content of about 7%.